Co-reporter:Adam E. Imel;Sahar Rostom;Wade Holley;Durairaj Baskaran;J. W. Mays
RSC Advances (2011-Present) 2017 vol. 7(Issue 25) pp:15574-15581
Publication Date(Web):2017/03/06
DOI:10.1039/C7RA00871F
The diffusion properties of nanoparticles in polymer nanocomposites are largely unknown and are often difficult to determine experimentally. To address this shortcoming, we have developed a novel method to determine the tracer diffusion coefficient of soft polystyrene nanoparticles in a linear polystyrene matrix. Monitoring the interdiffusion of soft nanoparticles into a linear polystyrene matrix provides the mutual diffusion coefficient of this system, from which the tracer diffusion coefficient of the soft nanoparticle can be determined using the slow mode theory. Utilizing this protocol, the role of nanoparticle molecular weight and rigidity on its tracer diffusion coefficient is provided. These results demonstrate that the diffusive behavior of these soft nanoparticles differ from that of star polymers, which is surprising since our recent studies suggest that the nanoparticle interacts with a linear polymer similarly to that of a star polymer. It appears that these deformable nanoparticles mostly closely mimic the diffusive behavior of fractal macromolecular architectures or microgels, where the transport of the nanoparticle relies on the cooperative motion of neighboring linear chains. The less cross-linked, and thus more deformable, nanoparticles diffuse faster than the more highly crosslinked nanoparticles, presumably because the increased deformability allows the nanoparticle to distort and fit into available space.
Co-reporter:Halie J. Martin;B. Tyler White;Christopher J. Scanlon;Tomonori Saito
Soft Matter (2005-Present) 2017 vol. 13(Issue 46) pp:8849-8857
Publication Date(Web):2017/11/29
DOI:10.1039/C7SM01533J
With a growing variety of nanoparticles available, research probing the influence of particle deformability, morphology, and topology on the behavior of all polymer nanocomposites is also increasing. In particular, the behavior of soft polymeric nanoparticles in polymer nanocomposites has displayed unique behavior, but their precise performance depends intimately on the internal structure and morphology of the nanoparticle. With the goal of providing guidelines to control the structure and morphology of soft polymeric nanoparticles, we have examined monomer starved semi-batch nano-emulsion polymerizations that form organic, soft nanoparticles, to correlate the precise structure of the nanoparticle to the rate of monomer addition and crosslinking density. The synthesis method produces 5–20 nm radii polystyrene nanoparticles with tunable morphologies. We report small angle neutron scattering (SANS) results that correlate synthetic conditions to the structural characteristics of soft polystyrene nanoparticles. These results show that the measured molecular weight of the nanoparticles is controlled by the monomer addition rate, the total nanoparticle radius is controlled by the excess surfactant concentration, and the crosslinking density has a direct effect on the topology of each nanoparticle. These studies thus provide pathways to control these 3 structural characteristics of the nanoparticle. This research, therefore provides a conduit to thoroughly investigate the effect of structural features of soft nanoparticles on their individual properties and those of their polymer nanocomposites.
Co-reporter:Brian Morgan
Soft Matter (2005-Present) 2017 vol. 13(Issue 15) pp:2773-2780
Publication Date(Web):2017/04/12
DOI:10.1039/C6SM02631A
Device efficiency in key organic electronic devices such as organic photovoltaics, field transistors, and light emitting diodes has long been known to be closely tied to the conformation of the conjugated polymer chains which make up the active layers. Our previous results show that light exposure can have a profound effect on the structure and assembly of these optoelectronic materials in solution. In order to advance our understanding of the role which solvent quality plays in this phenomenon, we have further studied the modulation of these illumination dependent structural changes on the key benchmark conjugated polymers P3HT and MEH-PPV as a function of solvent quality over a wide range of polymer solubilities. Analysis of this data indicates that use of poorer conjugated polymer solvents ultimately results in larger absolute alterations to polymer conformation, denoting the crucial role which solution thermodynamics plays in this generic effect. This discovery opens the door to controlling final device morphology through careful manipulation of solvent composition during solution based device casting techniques, moving our efforts closer to the development of a powerful, non-destructive, and tunable method for light-driven control of polymer conformation in novel light-responsive organic materials.
Co-reporter:Brian Morgan, Mark D. Dadmun
Polymer 2017 Volume 108() pp:313-321
Publication Date(Web):13 January 2017
DOI:10.1016/j.polymer.2016.11.056
•The impact of illumination on the gelation of a conjugated polymer is studied.•The experiment is completed with and without in-situ white light illumination.•SANS and USANS analysis quantifies structural evolution during gelation.•Light exposure clearly effects structure and assembly over observed length scales.•Aggregate formation is hindered by the presence of white light exposure.Studying the gelation process of conjugated optoelectronic polymers has often been employed as a means of better understanding the final morphology and assembly in active layers of organic electronic devices due to the correlation between the experimentally observed sol-gel transition and many common solution based fabrication techniques. The nature of the percolated network structures formed through the molecular assembly that occurs during this gelation directly affects device performance in conjugated polymer based active layers. Thus, precise knowledge of the evolution of structures during gelation provides crucial information that is needed to rationally improve device performance by directing the assembly during processing. Additionally, observing the effects of environmental factors such as ambient light exposure upon the gelation process will direct efforts towards improving universally overlooked facets of the typical fabrication procedure. Thus, we have conducted a series of ultra small angle and small angle neutron scattering experiments to probe the temperature-driven gelation process of the conjugated photoactive polymer poly(3-hexylthiophene-2,5-diyl) (P3HT) in both the presence and absence of white light. Analysis of the resultant scattering data shows that the gelation process consists of the creation and steady growth of cylindrical aggregates formed by the agglomeration of free chain P3HT. Furthermore, clear differences in the gel structure and assembly between illuminated and non-illuminated gels are observed across multiple length scales, pointing towards an optically-induced variation in the gelation process. Our results indicate that simple white light exposure sharply retards the growth of conjugated polymer microstructures, which clearly suggests that ignoring illumination conditions throughout organic electronic fabrication processes risks producing inconsistent and non-reproducible active layer architectures and ultimately endangers dependable device performance.Figure optionsDownload full-size imageDownload high-quality image (210 K)Download as PowerPoint slide
Co-reporter:Neiko P. Levenhagen, Mark D. Dadmun
Polymer 2017 Volume 122(Volume 122) pp:
Publication Date(Web):28 July 2017
DOI:10.1016/j.polymer.2017.06.057
•A new way to improve the isotropy of parts prepared by fused deposition modeling is presented.•The addition of LMW additives leads to improved mechanical properties and more isotropic parts.•Improvements are a function of increased surface contact of adjacent filaments and increased interfilament entanglement.•The low crystallinity of the polymers presented shows little to no impact on the interfacial properties.Parts prepared by the fused deposition modeling (FDM) 3D printing process suffer from poor interfacial adhesion between layers. This is due to poor diffusion of the very large and slow polymer chains across the inter-filament interface. To address this issue, we have developed the use of a bimodal blend of poly(lactide) (PLA) comprised of a series of synthesized low molecular weight PLA components (8.5 k, 50 k, and 100 k) added to a commercially available PLA (220 k). Tensile testing results indicate that when the LMW additive is of a sufficient length, the maximum stress and modulus in the part printed orthogonal to the print head (transverse) is significantly improved. More specifically, this behavior is observed where increased diffusion and increased entanglement of chains across adjacent layers occurs. The extent of crystallization at various stages of processing is also analyzed and indicates no correlation between the mechanical properties obtained and extent of crystallinity.Download high-res image (221KB)Download full-size image
Co-reporter:Thusitha Etampawala, Mehran Tehrani, Ali Nematollahi, Lilin He, Mark Dadmun
Organic Electronics 2017 Volume 51(Volume 51) pp:
Publication Date(Web):1 December 2017
DOI:10.1016/j.orgel.2017.08.030
•Neutron scattering determines the morphology of PEDOT:dPSS with solvent doping.•Morphological changes correlate well to enhanced electrical performance.•PEDOT:PSS films consists of two-phase structure at both micron and nm length scales.•Improved performance is commensurate with better ordering and connectivity of PEDOT.Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is used in a broad range of organic electronics devices, where their performance can be dramatically enhanced by solvent processing. The morphological changes that occur with solvent processing, as well as the impact of deposition technique, are not well understood, but this understanding is crucial to rationally tune the performance of PEDOT:PSS. In this study, the change in the morphology and conductivity of spray-coated PEDOT:PSS films with pre- and post-deposition processing in the presence of polar solvents is investigated by means of neutron scattering. Deuterium labeling of PSS is used to distinguish between the PEDOT and PSS phases. Changes in the morphology of PEDOT:deuterated PSS (PEDOT:dPSS) films with the addition of 5% dimethyl sulfoxide (DMSO) to the pre-deposition PEDOT:dPSS solutions enhances the conductivity by ca. 60-fold, and is further enhanced by ca. 2-fold by soaking the films in ethylene glycol (EG). Neutron scattering reveals that the morphology of the spray-coated PEDOT:dPSS film consists of a two-phase structure on the micron to nanometer length scales, where more locally ordered PEDOT-rich fibril-like domains are dispersed on the smallest length scale. Upon DMSO addition, the PEDOT:dPSS domain sizes decrease dramatically. However, with soaking in EG, the sizes of the local domains increase isotropically. Thus, the enhanced electrical conductivity of the spray-coated PEDOT:dPSS with solvent doping is dominated by morphological changes, and not changes in the PEDOT:dPPS composition in these spray-coated films.Download high-res image (281KB)Download full-size image
Co-reporter:Adam E. Imel, Amit K. Naskar, and Mark D. Dadmun
ACS Applied Materials & Interfaces 2016 Volume 8(Issue 5) pp:3200
Publication Date(Web):January 12, 2016
DOI:10.1021/acsami.5b10720
Carbon fiber produced from lignin has recently become an industrial scalable product with applications ranging from thermal insulation to reinforcing automobile bodies. Previous research has shown that mixing 1–2 wt %, of poly(ethylene oxide) (PEO) with the lignin before fiber formation can enhance the properties of the final carbon fibers. The research reported here determines the impact of adding PEO to a lignin solution on its assembly, focusing on the role of the lignin structure on this assembly process. Results indicate the addition of PEO anisotropically directs the self-assembly of the hardwood and softwood lignin by lengthening the cylindrical building blocks that make up the larger global aggregates. On the other hand, results from an annual lignin exhibit a shapeless, more complex structure with a unique dependence on the PEO loading. These results are consistent with improved carbon fibers from solutions of lignin that include PEO, as the local ordering and directed assembly will inhibit the formation of defects during the carbon fiber fabrication process.Keywords: carbon fiber; lignin; neutron scattering; poly(ethylene oxide) ; solution assembly
Co-reporter:Thusitha Etampawala, Derek L. Mull, Jong K. Keum, David M. Jenkins, and Mark Dadmun
Crystal Growth & Design 2016 Volume 16(Issue 3) pp:1395
Publication Date(Web):January 12, 2016
DOI:10.1021/acs.cgd.5b01509
The kinetics of the formation of novel porous metal organic nanotubes, [Ag2(4,4′-(1,4-(xylene)diyl)bis(1,2,4-triazole) (NO3)2]·NMP, was investigated by means of ex-situ time-resolved small-angle X-ray scattering (SAXS) and scanning electron microscopy (SEM). The SAXS results were modeled using the Gualtieri model, which decouples the nucleation and growth processes giving additional insight into the crystal formation mechanism. The results show that the semirigid 4,4′-(1,4-(xylene)diyl)bis(1,2,4-triazole) ligand (L) binds with silver ions, adopting a seesaw geometry to form a polydisperse isotropic framework immediately after mixing the ligands and metal ions. In addition, the SEM imaging demonstrates that the microcrystals grow anisotropically, with nucleation along the edge of the 3D aggregate. These combined data demonstrate that the growth of this MONT occurs in two steps: a rapid formation of an isotropic porous structure immediately after mixing the reactant, which then develops anisotropically as the aggregates of nanorods grow in a preferred direction. The anisotropic growth of the crystal is autocatalytic and determined by the rate of nucleation of new growth sites on the crystals. Moreover, the results of this analysis elucidate, for the first time, the exact order of the competing processes that occur in the synthesis of these MONTs, showing that their anisotropic growth occurs on the initial 3D aggregate and appears to be directed by the interplay between the surface energies that exist during the MONT formation process. This insight is crucial to the use of crystal engineering to guide the crystal formation processes in MONTs to targeted structures, properties, and applications.
Co-reporter:Brian Morgan and Mark D. Dadmun
Macromolecules 2016 Volume 49(Issue 9) pp:3490-3496
Publication Date(Web):April 20, 2016
DOI:10.1021/acs.macromol.6b00527
The importance of chain structure in conjugated polymer-based material active layers and its relation to device efficiencies in OPVs, organic field transistors, OLEDs, and other devices has been well established. However, the influence that the absorbance of the light inherent to these devices might have on the conjugated polymer structure is not well understood. Herein, we employ small-angle neutron scattering to investigate structural changes occurring in solutions of poly(3-hexylthiophene-2,5-diyl) with exposure to white light. Results indicate significant decrease in both Kuhn length (b) and radius of gyration (Rg) of the polymer upon illumination, coupled with a drop in the second virial coefficient (A2). We explain this phenomenon through a chain collapse model, proposing that the interaction of light with the polymer backbone alters its thermodynamic interactions with and solubility in the surrounding solvent. The presence of such an effect, which we observe in several conjugated polymers, introduces the possibility of a powerful, nondestructive, and tunable method for controlling polymer conformation in solution. This in turn opens a path to develop a broad range of new light-responsive materials, in that a variety of conjugated polymers could be used as the stimuli-responsive material. Additional implications include the identification of the importance of illumination in the reproducible fabrication of organic electronic active layers from conjugated polymer inks.
Co-reporter:Cameron S. Lee;Wen Yin;Adam P. Holt;Joshua R. Sangoro;Alexei P. Sokolov
Advanced Functional Materials 2015 Volume 25( Issue 36) pp:5848-5857
Publication Date(Web):
DOI:10.1002/adfm.201501707
Poly(3-hexyl thiophene) (P3HT) is widely regarded as the benchmark polymer when studying the physics of conjugated polymers used in organic electronic devices. P3HT can self-assemble via π–π stacking of its backbone, leading to an assembly and growth of P3HT fibrils into 3D percolating organogels. These structures are capable of bridging the electrodes, providing multiple pathways for charge transport throughout the active layer. Here, a novel set of conditions is identified and discussed for P3HT organogel network formation via spin coating by monitoring the spin-coating process from various solvents. The development of organogel formation is detected by in situ static light scattering, which measures both the thinning rate by reflectance and structural development in the film via off-specular scattering during film formation. Optical microscopy and thermal annealing experiments provide ex situ confirmation of organogel fabrication. The role of solution characteristics, including solvent boiling point, P3HT solubility, and initial P3HT solution concentration on organogel formation, is examined to correlate these parameters to the rate of film formation, organogel-onset concentration, and overall network size. The correlation of film properties to the fabrication parameters is also analyzed within the context of the hole mobility and density-of-states measured by impedance spectroscopy.
Co-reporter:Dilru R. Ratnaweera, Dipendu Saha, Sai Venkatesh Pingali, Nicole Labbé, Amit K. Naskar and Mark Dadmun
RSC Advances 2015 vol. 5(Issue 82) pp:67258-67266
Publication Date(Web):30 Jul 2015
DOI:10.1039/C5RA13485D
Recently, there has been a growing interest in developing value added uses for lignin, including the utilization of lignins as a precursor for carbon materials. Proper understanding of the association behavior of lignins during solution processing provides important structural information that is needed to rationally optimize the use of lignins in industry in a range of value added applications. In these experiments, we follow the assembly of lignin molecules from a variety of sources in dimethyl sulfoxide, a good solvent for lignins, using small angle neutron scattering. In order to mimic industrial processing conditions, concentrations of lignins were kept above the overlap concentration. At small length scales, short lignin segments with ∼4–10 monolignol units associate to form rigid rod-like/cylindrical building blocks, where the number of repeat units in a cylindrical segment decreases with increasing lignin concentration. These cylindrical building blocks associate to form aggregates with low cross-linking densities and a random coil or network like structures from highly branched lignin structures. The degree of branching of the base lignin molecule, which varies with source, plays a crucial role in determining their association behavior. The overall sizes of the aggregates decrease with increasing concentration at low cross-linking densities, whereas the opposite trend is observed for highly branched lignins.
Co-reporter:Adam E. Imel, Mark D. Dadmun
Polymer 2015 Volume 75() pp:134-140
Publication Date(Web):28 September 2015
DOI:10.1016/j.polymer.2015.08.028
•We monitor the formation of polyacrylonitrile/C60 nanocomposites from solution.•Fullerenes and polymer are individually dispersed in solution prior to deposition.•During film formation, the nanoparticles sequester to the polymer crystal edges.•The fullerene acts similar to residual solvent, altering the PAN crystal structure.The production of polymer nanocomposites from solution consists of the mixing of the polymer and nanoparticle in solution and subsequent evaporation of the solvent. We examine the formation of polyacrylonitrile and C60 fullerene nanocomposites, with a focus on monitoring these two steps. The results of this study indicate that the nanoparticles are individually dispersed with the polymer chains in solution prior to deposition and in the final film. As the solution becomes more concentrated, the nanoparticles are sequestered to the outer edges of the polymer crystals, altering the detected crystal structure. The self-assembled structure of the crystalline polymer is directed by the addition of C60 and manifests itself as a peak in small-angle X-ray scattering on a length scale of ∼150 Å. The results suggest that the non-covalent molecular interactions between C60 and polyacrylonitrile matrix are sufficiently strong to alter the self-assembled morphology of the polymer and the meso- and nanoscale structures in the nanocomposite.
Co-reporter:Thusitha Etampawala, Dilru Ratnaweera, Brian Morgan, Souleymane Diallo, Eugene Mamontov, Mark Dadmun
Polymer 2015 Volume 61() pp:155-162
Publication Date(Web):20 March 2015
DOI:10.1016/j.polymer.2015.01.064
•We monitor the molecular dynamics in miscible conjugated polymer-fullerene blends.•Quasi-elastic neutron scattering provides the local segmental motions.•Two relaxation processes are observed, correlating to the motion of both components.•The presence of the fullerene slows down the local motion of the conjugated polymer.•The local motion of the fullerene is less affected by the presence of the polymer.This work addresses the detailed molecular dynamic behavior of miscible blends of Poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and their pure counterparts by quasi-elastic neutron scattering measurements (QENS). The study provides the measure of relaxation processes on pico-to-nanosecond time scales. A single relaxation process was observed in pure P3HT and PCBM while two relaxation processes, one fast and one slow, were observed in the blends. The fast process was attributed to the dynamics of P3HT while the slow process was correlated to the dynamics of PCBM. The results show that the relaxation process is a balance between two opposing effects: increased mobility due to thermal activation of P3HT molecules and decrease mobility due to the presence of PCBM which is correlated to the percent crystallinity of P3HT and local packing density of PCBM in the amorphous phase. The threshold for the domination of the thermally activated relaxation is between 5 and 9 vol.% of PCBM loading. Two distinct spatial dependences of the relaxation processes, in which the crossover length scale depends neither on temperature nor composition, were observed for all the samples. They were attributed to the collective motions of the hexyl side chains and the rotational motions of the C–C single bonds of the side chains. These results provide an understanding of the effects of PCBM loading and temperature on the dynamics of the polymer-fullerene blends which provides a tool to optimize the efficiency of charge carrier and exciton transport within the organic photovoltaic (OPV) active layer to improve the high performance of organic solar cells.
Co-reporter:Brad Miller, Adam E. Imel, Wade Holley, Durairaj Baskaran, J. W. Mays, and Mark D. Dadmun
Macromolecules 2015 Volume 48(Issue 22) pp:8369-8375
Publication Date(Web):November 12, 2015
DOI:10.1021/acs.macromol.5b01976
The impact of the inclusion of a nanoparticle in a polymer matrix on the dynamics of the polymer chains is an area of recent interest. In this article, we describe the role of nanoparticle rigidity or softness on the impact of the presence of that nanoparticle on the diffusive behavior of linear polymer chains. The neutron reflectivity results clearly show that the inclusion of ∼10 nm soft nanoparticles in a polymer matrix (Rg ∼ 20 nm) increases the diffusion coefficient of the linear polymer chain. Surprisingly, thermal analysis shows that these nanocomposites exhibit an increase in their glass transition temperature, which is incommensurate with an increase in free volume. Therefore, it appears that this effect is more complex than a simple plasticizing effect. Results from small-angle neutron scattering of the nanoparticles in solution show a structure that consists of a gel like core with a corona of free chain ends and loops. Therefore, the increase in linear polymer diffusion may be related to an increase in constraint release mechanisms in the reptation of the polymer chain, in a similar manner to that which has been reported for the diffusion of linear polymer chains in the presence of star polymers.
Co-reporter:Huipeng Chen;Jeff Peet;Sheng Hu;Jason Azoulay;Guillermo Bazan;Mark Dadmun
Advanced Functional Materials 2014 Volume 24( Issue 1) pp:140-150
Publication Date(Web):
DOI:10.1002/adfm.201300862
This manuscript reports the mixing behavior, interdiffusion, and depth profile of 1-[3-(methoxycarbonyl)propyl]-1-phenyl-[6,6]C61 (PCBM):low-bandgap (LBG) polymer thin films that are formed by thermally annealing initial bilayers. The extent of mixing of PCBM is higher in polymers that include the 2,1,3-benzothiadiazole (BT) unit than in polymers that incorporate the 2,1,3-benzooxadiazole (BO) unit. This difference is ascribed to the enhanced mixing behavior of PCBM with the benzothiadiazole functionality than with benzooxadiazole functionality, which is attributed to preferred intermolecular interactions. The increased polymer/fullerene mixing is found to be crucial for optimal device performance. A decrease of polymer/fullerene mixing reduces the donor/acceptor interface, which lowers the probability of exciton dissociation and charge generation. Moreover, low PCBM mixing provides limited pathways for electron transport out of a miscible region, due to long distances between adjacent PCBM in such a miscible phase. This inhibits electron transport and increases the recombination of free charge carriers, resulting in a decrease in short circuit current and device performance. These results further exemplify the importance of the thermodynamic mixing behavior of the polymer:fullerene pair in designing next-generation conjugated polymers for organic photovoltaic (OPV) applications, as this controls the final morphology of the OPV active layer.
Co-reporter:Huipeng Chen;Yu-Che Hsiao;Jihua Chen;Bin Hu;Mark Dadmun
Advanced Functional Materials 2014 Volume 24( Issue 46) pp:7284-7290
Publication Date(Web):
DOI:10.1002/adfm.201401419
One way to improve power conversion efficiency (PCE) of polymer based bulk-heterojunction (BHJ) photovoltaic cells is to increase the open circuit voltage (V oc). Replacing PCBM with bis-adduct fullerenes significantly improves V oc and the PCE in devices based on the conjugated polymer poly(3-hexyl thiophene) (P3HT). However, for the most promising low band-gap polymer (LBP) system, replacing PCBM with ICBA results in poor short-circuit current (J sc) and PCE although V oc is significantly improved. The optimization of the morphology of as-cast LBP/bis-fullerene BHJ photovoltaics is attempted by adding a co-solvent to the polymer/fullerene solution prior to film deposition. Varying the solubility of polymer and fullerene in the co-solvent, bulk heterojunctions are fabricated with no change of polymer ordering, but with changes in fullerene phase separation. The morphologies of the as-cast samples are characterized by small angle neutron scattering and neutron reflectometry. A homogenous dispersion of ICBA in LBP is found in the samples where the co-solvent is selective to the polymer, giving poor device performance. Aggregates of ICBA are formed in samples where the co-solvent is selective to ICBA. The resultant morphology improves PCE by up to 246%. A quantitative analysis of the neutron data shows that the interfacial area between ICBA aggregates and its surrounding matrix is improved, facilitating charge transport and improving the PCE.
Co-reporter:Huipeng Chen;Yu-Che Hsiao;Bin Hu;Mark Dadmun
Advanced Functional Materials 2014 Volume 24( Issue 32) pp:5129-5136
Publication Date(Web):
DOI:10.1002/adfm.201400552
Low bandgap polymer (LBG):fullerene mixtures are some of the most promising organic photovoltaic active layers. Unfortunately, there are no post-deposition treatments available to rationally improve the morphology and performance of as-cast LBG:fullerene OPV active layers, where thermal annealing usually fails. Therefore, there is a glaring need to develop post-deposition methods to guide the morphology of LBG:fullerene bulk heterojunctions towards targeted structures and performance. In this paper, the structural evolution of PCPDTBT:PCBM mixtures with solvent annealing (SA) is examined, focusing on the effect of solvent quality of the fullerene and polymer in the annealing vapor on morphological evolution and device performance. The results indicate that exposure of this active layer to the solvent vapor controls the ordering of PCPDTBT and PCBM phase separation very effectively, presumably by inducing component mobility as the solvent plasticizes the mixture. These results also unexpectedly indicate that solvent annealing in a selective solvent provides a method to invert the morphology of the LBG:fullerene mixture from a polymer aggregate dispersed in a polymer:fullerene matrix to fullerene aggregates dispersed in a polymer:fullerene matrix. The judicious choice of solvent vapor, therefore, provides a unique method to exquisitely control and optimize the morphology of LBG conjugated polymer/fullerene mixtures.
Co-reporter:Huipeng Chen, Jeff Peet, Yu-Che Hsiao, Bin Hu, and Mark Dadmun
Chemistry of Materials 2014 Volume 26(Issue 13) pp:3993
Publication Date(Web):June 11, 2014
DOI:10.1021/cm5015898
Neutron reflectivity experiments are utilized to obtain the miscibility limit of four different fullerenes, bis-PCBM, ICBA, thio-PCBM, and PC70BM, in poly(3-hexylthiophene) (P3HT). The intermixing of P3HT and fullerene bilayers is monitored by neutron reflectivity before and after thermal annealing, providing quantification of the miscibility and interdiffusion of the fullerene within P3HT. These results indicate that the miscibility limit of these fullerenes in P3HT ranges from 11% to 26%, where the bis-adduct fullerenes exhibit lower miscibility in P3HT, which is also verified by small angle neutron scatting (SANS). The in-plane morphology of the P3HT:fullerene mixtures was also examined by SANS, which shows a decrease in domain size and an increase in the specific interfacial area between the fullerene and the polymer with the bis-fullerenes. Correlation of miscibility and morphology to device performance indicates that polymer/fullerene miscibility is crucial to rationally optimize the design of fullerenes for use in organic photovoltaics. Bis-PCBM has a higher open circuit voltage (Voc) than PC60BM with P3HT; however, device performance of bis-PCBM based devices is lower than that of PC60BM based devices. This decrease in performance is attributed to the lower miscibility of bis-PCBM in P3HT, which decreases the probability of exciton dissociation and enhances the recombination of free charge carriers in the miscible region. Moreover, the minimum distance between fullerenes in the miscible region to facilitate intermolecular transport is identified as ∼11 Å.
Co-reporter:Huipeng Chen, Yu-Che Hsiao, Bin Hu and Mark Dadmun
Journal of Materials Chemistry A 2014 vol. 2(Issue 25) pp:9883-9890
Publication Date(Web):07 May 2014
DOI:10.1039/C4TA01436G
Replacing PCBM with a bis-adduct fullerene (i.e. ICBA) has been reported to significantly improve the open circuit voltage (VOC) and power conversion efficiency (PCE) in P3HT bulk heterojunctions. However, for the most promising low band-gap polymer (LBP) systems, replacing PCBM with ICBA results in very poor short-circuit current (JSC) and PCE although the VOC is significantly improved. Therefore, in this work, we have completed small angle neutron scattering and neutron reflectometry experiments to study the impact of post-deposition solvent annealing (SA) with control of solvent quality on the morphology and performance of LBP–bis-fullerene BHJ photovoltaics. The results show that SA in a solvent that is selective for the LBP results in a depletion of bis-fullerene near the air surface, which limits device performance. SA in a solvent vapor which has similar solubility for polymer and bis-fullerene results in a higher degree of polymer ordering, bis-fullerene phase separation, and segregation of the bis-fullerene to the air surface, which facilitates charge transport and increases power conversion efficiency (PCE) by 100%. The highest degree of polymer ordering combined with significant bis-fullerene phase separation and segregation of bis-fullerene to the air surface is obtained by SA in a solvent vapor that is selective for the bis-fullerene. The resultant morphology increases PCE by 190%. These results indicate that solvent annealing with judicious solvent choice provides a unique tool to tune the morphology of LBP–bis-fullerene BHJ system, providing sufficient polymer ordering, formation of a bis-fullerene pure phase, and segregation of bis-fullerene to the air surface to optimize the morphology of the active layer. Moreover, this process is broadly applicable to improving current “disappointing” LBP–bis-fullerene systems to optimize their morphology and OPV performance post-deposition, including higher VOC and power conversion efficiency.
Co-reporter:Cameron S. Lee, Mark D. Dadmun
Polymer 2014 Volume 55(Issue 1) pp:4-7
Publication Date(Web):14 January 2014
DOI:10.1016/j.polymer.2013.11.033
Poly(3-hexyl thiophene) (P3HT) is widely studied as a model material in many electrical and photovoltaic applications, and has become the benchmark polymer when studying the physics of these devices. Despite its extensive use, its precise crystalline structure and thermodynamic characteristics, such as its enthalpy of melting of an ideal crystal, crystalline density, and amorphous density, are not well characterized. This work seeks to provide more certainty in defining these thermodynamic characteristics for regioregular P3HT. This is accomplished by determining the density of rr-P3HT with various thermal histories, and thus percent crystallinity. These densities are correlated to their melting enthalpy melting (ΔHm) via DSC. This relationship estimates that ΔH0 for P3HT is between 37 and 50 J/g, that the density of the amorphous portion of semicrystalline rr-P3HT is 1.094 g/cc, and that the density of crystalline P3HT is 1.12–1.14 g/cc. Interestingly, the density of the amorphous portion of rr-P3HT differs significantly from that of regio-random P3HT. This result indicates that the local packing of the segments differs in regio-random P3HT from that in the amorphous portions of rr-P3HT and that care must be expended when equating the behavior of these two phases.
Co-reporter:Huipeng Chen;Sheng Hu;Huidong Zang;Bin Hu;Mark Dadmun
Advanced Functional Materials 2013 Volume 23( Issue 13) pp:1701-1710
Publication Date(Web):
DOI:10.1002/adfm.201202035
Abstract
The impact of controlled solvent vapor exposure on the morphology, structural evolution, and function of solvent-processed poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) bilayers is presented. Grazing incident wide angle X-ray scattering (GIWAXS) shows that the crystallization of P3HT increases with solvent exposure, while neutron reflectivity shows that P3HT simultaneously diffuses into PCBM, indicating that an initial bilayer structure evolves into a bulk heterojunction structure. Small angle neutron scattering (SANS) shows the agglomeration of PCBM and the formation of a PCBM pure phase when solvent annealing for 90 min. The structural evolution can be described as occurring in two stages: the first stage combines the enhanced crystallization of P3HT and diffusion of PCBM into P3HT, while the second stage entails the agglomeration of PCBM and formation of a PCBM pure phase. The phase separation of PCBM from P3HT is not driven by P3HT crystallinity, but is due to the concentration of PCBM exceeding the miscibility limit of PCBM in P3HT. Correlation of the morphology to photovoltaic activity shows that device performance significantly improves with solvent annealing for 90 min, indicating that both sufficient P3HT crystallization and formation of a PCBM pure phase are crucial in the optimization of the morphology of the active layer.
Co-reporter:Huipeng Chen, Jihua Chen, Wen Yin, Xiang Yu, Ming Shao, Kai Xiao, Kunlun Hong, Deanna L. Pickel, W. Michael Kochemba, S. Michael Kilbey II and Mark Dadmun
Journal of Materials Chemistry A 2013 vol. 1(Issue 17) pp:5309-5319
Publication Date(Web):25 Feb 2013
DOI:10.1039/C3TA10386B
The impact of various polymeric compatibilizers, including end-functionalized P3HTs and diblock copolymers containing P3HT, on the structure and function of poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) bulk heterojunctions is presented. Careful analyses of small angle neutron scattering curves provide a measure of the miscibility of PCBM in P3HT, the average PCBM domain size, and the interfacial area between PCBM and the P3HT-rich phase in the uncompatibilized and compatibilized systems. Differential scanning calorimetry (DSC) also provides information regarding the changes in the crystallinity of P3HT due to the presence of the compatibilizer. Results show that most compatibilizers cause the domain sizes to decrease and the P3HT crystallinity to increase; however, some cause an increase in domain size, suggesting that they are not effective interfacial modifiers. The correlation of morphology with photovoltaic activity shows that the decreased domain size, increased crystallinity and increased interfacial area do not always result in improved power conversion efficiency (PCE). It appears that the introduction of an insulating molecule at the PCBM:P3HT interface as a compatibilizer results in a decrease in PCE. Thus, the presence of the compatibilizer at this interface dominates the photovoltaic activity, rather than the morphological control.
Co-reporter:Caleb Dyer, Zhe Jiang, Joe Bozell, Timothy Rials, William T. Heller and Mark Dadmun
Soft Matter 2013 vol. 9(Issue 12) pp:3402-3411
Publication Date(Web):12 Feb 2013
DOI:10.1039/C3SM27648A
The miscibility of cellulose ester blends with varying degree of substitution (DS) of acetates along the chain backbone has been investigated using small-angle neutron scattering. The difference in degree of substitution (ΔDS) between the two components in the blend was systematically varied from 0.06 to 0.63 where each blend was found to be a partially miscible, two-phase system. Miscibility between the two components initially decreases as ΔDS of the blends increases. The Flory interaction parameter, χ, concurrently increases with increasing ΔDS as a result of diminishing van der Waals forces between components. The cellulose acetates with lower degree of substitution, which contain more hydroxyl substituents, however, demonstrate greater miscibility even at higher ΔDS. This is interpreted to be the result of favorable hydrogen bonding between blend components that are possible in the presence of more hydroxyl groups. FT-IR data support this interpretation, indicating an increase in hydrogen bonding in a blend having a lower DS component. These results indicate that while an increase in structural differences between cellulose acetate blend components limits miscibility, the presence of hydroxyl groups on the chain promotes mixing. This competition accentuates the significant impact specific interactions have on blend miscibility for these copolymers.
Co-reporter:C. Dyer, P. Driva, S. W. Sides, B. G. Sumpter, J. W. Mays, J. Chen, R. Kumar, M. Goswami, and M. D. Dadmun
Macromolecules 2013 Volume 46(Issue 5) pp:2023-2031
Publication Date(Web):February 19, 2013
DOI:10.1021/ma202650a
The impact of block connectivity on the morphologies of four block copolymers of varying architecture containing polystyrene (PS) and polyisoprene (PI) has been studied. The volume fraction of PS and molecular weight are held constant while varying the architecture from a linear PS–PI diblock copolymer to three different miktoarm star architectures: PS2PI, PSPI2, and PS2PI2. Morphologies of the PS2PI and PSPI2 miktoarm stars are different from those observed for the linear copolymer and dependent on the connectivity of the copolymer blocks. The change in morphology with connectivity indicates that combining two chains at a junction point leads to chain crowding, where subsequent excluded volume effects drive the change in morphology for each sample. The PS2PI2 miktoarm star exhibits the same morphology as the linear diblock but with a reduction in the size of the domains. The extent of the decrease in domain size indicates that chain stretching impacts the formation of this morphology. Experimentally observed morphologies for different chain architectures are generally consistent with three-dimensional self-consistent-field theory simulations, taking into account conformational asymmetry and experimental uncertainty in the copolymer composition. Furthermore, these results generally agree with analytical theory predictions that account for architectural and conformational asymmetry.
Co-reporter:Huipeng Chen, Raghavendra Hegde, J. Browning and M. D. Dadmun
Physical Chemistry Chemical Physics 2012 vol. 14(Issue 16) pp:5635-5641
Publication Date(Web):17 Feb 2012
DOI:10.1039/C2CP40466D
Recent work has shown that poly(3-hexylthiophene) (P3HT) and the surface-functionalized fullerene 1-(3-methyloxycarbonyl)propyl(1-phenyl[6,6])C61 (PCBM) are much more miscible than originally thought, and the evidence of this miscibility requires a return to understanding the optimal morphology and structure of organic photovoltaic active layers. This manuscript describes the results of experiments that were designed to provide quantitative thermodynamic information on the miscibility, interdiffusion, and depth profile of P3HT:PCBM thin films that are formed by thermally annealing initial bilayers. It is found that the resultant thin films consist of a ‘bulk’ layer that is not influenced by the air or substrate surface. The composition of PCBM in this ‘bulk’ layer increases with increased PCBM loading in the original bilayer until the ‘bulk’ layer contains 22 vol% PCBM. The introduction of additional PCBM into the sample does not increase the amount of PCBM dispersed in this ‘bulk’ layer. This observation is interpreted to indicate that the miscibility limit of PCBM in P3HT is 22 vol%, while the precise characterization of the depth profiles in these films shows that the PCBM selectively segregates to the silicon and near air surface. The selective segregation of the PCBM near the air surface is ascribed to an entropic driving force.
Co-reporter:Nathan Henry;David Harper;Mark Dadmun
Macromolecular Chemistry and Physics 2012 Volume 213( Issue 12) pp:1196-1205
Publication Date(Web):
DOI:10.1002/macp.201100633
Abstract
Experiments are designed and completed to identify an effective polymeric compatibilizer for lignin–polystyrene blends. Copolymers of styrene and vinylphenol are chosen as the structure of the compatibilizer as the VPh unit can readily form intermolecular hydrogen bonds with the lignin molecule. Electron microscopy, thermal analysis, and neutron reflectivity results demonstrate that among these compatibilizers, a copolymer of styrene and VPh with ≈20%–30% VPh most readily forms intermolecular interactions with the lignin molecule and results in the most well-dispersed blends with lignin. This behavior is explained by invoking the competition of intra- and intermolecular hydrogen bonding and functional group accessibility in forming intermolecular interactions.
Co-reporter:M. Mutz, D.W. Holley, D. Baskaran, J.W. Mays, M.D. Dadmun
Polymer 2012 Volume 53(Issue 22) pp:5087-5096
Publication Date(Web):12 October 2012
DOI:10.1016/j.polymer.2012.08.029
A study of the impact of the size and shape of a nanoparticle on the evolution of structure and surface segregation in polymer nanocomposite thin films is presented. This is realized by monitoring the evolution of structure with thermal annealing and equilibrium depth profile of a deuterated polystyrene/protonated polystyrene bilayer in the presence and absence of various nanoparticles. For the three shapes examined, sheet-like graphene, cylindrical carbon nanotubes, and spherical soft nanoparticles, the presence of the nanoparticles slowed the inter-diffusion of the polymers in the thin film. The larger nanoparticles slowed the polymer motion the most, while the smaller spherical nanoparticles also significantly inhibited polymer chain diffusion. At equilibrium, the soft spherical nanoparticles, which are highly branched, segregate to the air surface, resulting in a decrease in the excess deuterated PS at the surface. The graphene sheets and single walled carbon nanotubes, on the other hand, enhanced the dPS segregation to the air surface. The graphene sheets were found to segregate to the silicon surface, due to their higher surface energy. Interpretation of these results indicates that entropic factors drive the structural development in the nanocomposite thin films containing the spherical nanoparticles, while a balance of the surface energies of the various components (i.e. enthalpy) controls the thin film structure formation in the polymer-carbon nanoparticle nanocomposites.Graphical abstract
Co-reporter:M. Mutz;Eric Eastwood;Mark E. Lee Jr.
Journal of Nanoparticle Research 2012 Volume 14( Issue 11) pp:
Publication Date(Web):2012 November
DOI:10.1007/s11051-012-1237-2
The solubility of boron containing nanoparticles in a variety of solvents is quantified using static light scattering in conjunction with refractometry. Four polyhedral boranes were tested in this work, using refractometry to obtain dn/dc, while static light scattering quantifies A2. A2 obtained from these measurements was then used to calculate χ, the solute–solvent interaction parameter, and the Hildebrand solubility parameter, δ, which provides a quantifiable method to identify good solvents. Of the nanoparticles studied, 1,3-di-o-carboranylpropane is thermodynamically stable in toluene, with a χ less than 0.5, a solubility limit of 2.47 mg/mL, and all solutions remaining clear with no visible particle settling. For all of the particles tested, there was good correlation between the physical observations of the solutions, χ, and δ. For instance, lower values of χ correspond to a smaller radius of gyration (Rg). A list of suitable solvents based on δ is also presented.
Co-reporter:Raghavendra Hegde, Nathan Henry, Ben Whittle, Huidong Zang, Bin Hu, Jihua Chen, Kai Xiao, Mark Dadmun
Solar Energy Materials and Solar Cells 2012 107() pp: 112-124
Publication Date(Web):
DOI:10.1016/j.solmat.2012.07.014
Co-reporter:Earl Ashcraft;Haining Ji;Jimmy Mays;Mark Dadmun
Macromolecular Chemistry and Physics 2011 Volume 212( Issue 5) pp:465-477
Publication Date(Web):
DOI:10.1002/macp.201000557
Co-reporter:Dana Algaier, Durairaj Baskaran, Mark Dadmun
Reactive and Functional Polymers 2011 71(8) pp: 809-819
Publication Date(Web):August 2011
DOI:10.1016/j.reactfunctpolym.2011.03.015
Co-reporter:Wen Yin and Mark Dadmun
ACS Nano 2011 Volume 5(Issue 6) pp:4756
Publication Date(Web):May 12, 2011
DOI:10.1021/nn200744q
Organic photovoltaics (OPVs) have attracted increasing interest as a lightweight, low-cost, and easy to process replacement for inorganic solar cells. Moreover, the morphology of the OPV active layer is crucial to its performance, where a bicontinuous, interconnected, phase-separated morphology of pure electron donor and acceptor phases is currently believed to be optimal. In this work, we use neutron scattering to investigate the morphology of a model OPV conjugated polymer bulk heterojunction, poly[3-hexylthiophene] (P3HT), and surface-functionalized fullerene 1-(3-methyloxycarbonyl) propyl(1-phenyl [6,6]) C61 (PCBM). These results show that P3HT and PCBM form a homogeneous structure containing crystalline P3HT and an amorphous P3HT/PCBM matrix, up to ca. 20 vol % PCBM. At 50 vol % PCBM, the samples exhibit a complex structure containing at least P3HT crystals, PCBM crystals, and a homogeneous mixture of the two. The 20 vol % PCBM samples exhibit behavior consistent with the onset of phase separation after 6 h of thermal annealing at 150 °C, but appear to be miscible at shorter annealing times. This suggests that the miscibility limit of PCBM in P3HT is near 20%. Moreover, for the 50 vol % PCBM sample, the interface roughens under thermal annealing possibly owing to the growth of PCBM crystals. These observations suggest a different morphology than is commonly presented in the literature for optimal bulk heterojunctions. We propose a novel “rivers and streams” morphology to describe this system, which is consistent with these scattering results and previously reported photovoltaic functionality of P3HT/PCBM bulk heterojunctions.Keywords: conjugated polymers; nanocomposites; neutron scattering; organic photovoltaics; phase behavior
Co-reporter:Say-Lee Teh, Dias Linton, Bobby Sumpter, and Mark D. Dadmun
Macromolecules 2011 Volume 44(Issue 19) pp:7737-7745
Publication Date(Web):September 13, 2011
DOI:10.1021/ma200795g
Polymer nanocomposites (PNCs) are materials based on a class of filled plastics that contain relatively small amounts of nanoparticles, which can impart improved structural, mechanical, and thermal properties relative to the neat polymer. However, the homogeneous dispersion of the nanoparticles into a polymer matrix is critical and an impeding factor for the controlled enhancement of PNC properties. In this work, we provide new insight into the importance of polymer chain connectivity and nanoparticle shape and curvature on the formation of noncovalent electron donor–acceptor (EDA) interactions between polymers and nanoparticles. This is accomplished by experimentally monitoring the dispersion of nanoparticles in copolymers containing varying amounts of functional moieties that can form noncovalent interactions with carbon nanoparticles with corroboration through density functional calculations. The results show that the presence of a minority of interacting functional groups within a polymer chain leads to an optimum interaction between the polymer and fullerene. Density functional theory calculations that identify the binding energy and geometry of the interaction between the functional monomers and fullerenes correspond very well with the experimental results. Moreover, comparison of these results to similar studies with single-walled carbon nanotubes (SWNT) indicate a distinct difference in the ability of EDA interactions to improve the dispersion of fullerenes relative to their impact on SWNT. Thus, the polymer chain connectivity, the polymer chain conformation, and size and shape of the nanoparticle modulate the formation of intermolecular interactions and directly impact the dispersion of the resultant nanocomposite.
Co-reporter:S.M. Fontana, M.D. Dadmun, D.H. Lowndes
Thin Solid Films 2010 Volume 518(Issue 10) pp:2783-2792
Publication Date(Web):1 March 2010
DOI:10.1016/j.tsf.2009.10.161
Topographically patterned substrates are known to induce long-range lateral order in spherical diblock copolymers, but it is not clear that similar confinement will also order cylindrical diblock copolymers across the whole surface. The role of graphoepitaxial parameters including trough width and mesa height on the ordering process of cylindrical domains in diblock copolymers thin films is monitored in this study. The quantification of order was achieved by the calculation of an order parameter of the hexagonally packed cylinders. These results demonstrate that graphoepitaxy is an effective method to induce long-range order in cylindrical domain diblock copolymer systems. An increase in order was observed in samples prepared on the mesas and in troughs of widths up to 20 μm, and mesa heights greater than 1.0 but less than 5.0 L0 The role of molecular weight on the kinetics of the ordering process of cylindrical domains in diblock copolymers thin films is also monitored in this study, where ordering is readily observed for lower molecular weight copolymers (number average molecular weight, Mn = 63,000), but not for larger copolymers (Mn = 230,000). The reduction of the rate of formation of long-range order is attributed to the impeded diffusion of higher molecular weight polymers. These results demonstrate that there will exist upper limits on the molecular weights of diblock copolymers that can be used to create nanoscale templates with long-range order, which also translates to an upper limit in pore size and spacing in these templates.
Co-reporter:Chang-Uk Lee, Debashish Roy, Brent S. Sumerlin, Mark D. Dadmun
Polymer 2010 Volume 51(Issue 6) pp:1244-1251
Publication Date(Web):11 March 2010
DOI:10.1016/j.polymer.2010.01.033
A facile approach to prepare thiol-terminated poly(styrene-ran-vinyl phenol) (PSVPh) copolymers and PSVPh-coated gold nanoparticles is reported with the goal of creating stabilizing ligands for nanoparticles with controlled hydrophilicity. Dithioester-terminated poly(styrene-ran-acetoxystyrene) copolymers were synthesized via RAFT polymerization using cumyl dithiobenzoate as a chain transfer agent. These copolymers were converted to thiol-terminated PSVPh copolymers by a one step hydrazinolysis reaction using hydrazine hydrate to simultaneously convert dithioester-terminal and acetoxy-pendant groups to thiol-terminal and hydroxyl-pendant groups, respectively. Spectroscopic observations including NMR and IR confirm end- and pendant-group conversion. PSVPh-coated gold nanoparticles were synthesized in the presence of a mixture of thiol-terminated PSVPh and PSVPh copolymers containing disulfides as stabilizing ligands in a water/toluene, two-phase system. The size and size distribution of core gold nanoparticles were determined by TEM and image analysis. The hydrodynamic radius of PSVPh-coated gold nanoparticles was also determined by dynamic light scattering experiment, which confirms the particle analysis by TEM. This procedure provides a facile technique to control the polarity and hydrophilicity of metal nanoparticle surfaces and could prove critical in advancing the control of nanoparticle placement in biological and hierarchically ordered systems, such as diblock copolymers.
Co-reporter:Earl Ashcraft, Haining Ji, Jimmy Mays and Mark Dadmun
ACS Applied Materials & Interfaces 2009 Volume 1(Issue 10) pp:2163
Publication Date(Web):September 16, 2009
DOI:10.1021/am900333y
Difunctional reactive polymers, telechelics, were used to reactively form multiblock copolymers in situ when melt-blended with a blend of polystyrene and polyisoprene. To quantify the ability of the copolymer to compatibilize the blends, the time evolution of the domain size upon annealing was analyzed by SEM. It was found that the most effective parameter to quantify the ability of the copolymer to inhibit droplet coalescence is Kreltstable, the relative coarsening constant multiplied by the stabilization time. These results indicate that intermediate-molecular-weight telechelic pairs of both highly reactive Anhydride-PS-Anhydride/NH2-PI-NH2 and slower reacting Epoxy-PS-Epoxy/COOH-PI-COOH both effectively suppress coalescence, with the optimal molecular weight being slightly above the critical molecular weight of the homopolymer, Mc. The effects of telechelic loading were also investigated, where the optimal loading concentration for this system was 0.5 wt %, as higher concentrations exhibited a plasticizing effect due to the presence of unreacted low-molecular-weight telechelics present in the blend. A determination of the interfacial coverage of the copolymer shows that a conversion of ∼1.5−3.0% was required for 20% surface coverage at 5.0 wt % telechelic loading, indicating a large excess of telechelics in this system. At the optimal loading level of 0.5 wt %, a conversion of 15% was required for 20% surface coverage. The results of these experiments provide a clear understanding of the role of telechelic loading and molecular weight on its ability to reactively form interfacial modifiers in phase-separated polymer blends and provide guidelines for the development of similar reactive processing schemes that can use telechelic polymers to reactively compatibilize a broad range of polymer blends.Keywords: coalescence; coarsening constant; compatibilization; interfacial modification; multiblock copolymer; plasticization; polymer blend; telechelic
Co-reporter:Chang-Uk Lee
Journal of Polymer Science Part B: Polymer Physics 2008 Volume 46( Issue 16) pp:1747-1759
Publication Date(Web):
DOI:10.1002/polb.21510
Abstract
Polymer nanocomposites composed of poly(styrene-ran-vinyl phenol) (PSVPh) copolymers and 5 wt % multi-walled carbon nanotubes (MWNTs) were prepared by three different methods, including melt-mixing and precipitation. The MWNTs were either oxidized to incorporate oxygenated defects or utilized as received. The mechanical properties of the nanocomposites were measured by dynamic mechanical analysis (DMA), and the extent of intermolecular hydrogen bonding between MWNTs and PSVPh was quantified by infrared (IR). Our DMA results suggest that melt-mixing leads to more stable morphologies of the final nanocomposites than precipitation. Additionally, the IR analysis of the nanocomposites indicates melt-mixing can result in the formation of more intermolecular hydrogen bonding between the MWNTs and PSVPh than precipitation, and thus suggests that melt-mixing leads to more reproducible mechanical properties than precipitation. Our DMA and IR results may provide guidelines to realize the desired morphologies and to improve the properties of polymer carbon nanotube nanocomposites by optimizing intermolecular interactions between MWNTs and polymers using processing. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1747–1759, 2008
Co-reporter:Asif Rasheed, Jane Y. Howe, Mark D. Dadmun, Phillip F. Britt
Carbon 2007 Volume 45(Issue 5) pp:1072-1080
Publication Date(Web):April 2007
DOI:10.1016/j.carbon.2006.12.010
The oxidation of carbon nanofibers (CNFs) with various oxidizing agents; 6 M HNO3, KMnO4, RuO4, and a mixture of concentrated H2SO4/HNO3 is studied to determine the reaction conditions that optimizes yield and chemical functionalization. The oxidized nanofibers were examined by scanning electron microscopy, and Raman spectroscopy to characterize the defects caused by oxidation. The amount of acidic sites generated during oxidative treatment was quantified by reacting them with amine terminated octadecane and the subsequent quantification of the products of this grafting process by Fourier transform infrared spectroscopy. The concentrated H2SO4/HNO3 mixture created the maximum number of carboxylic acid groups and the highest amount of defect sites, but had very low yield (∼12%). Six molar of HNO3 generated fewer defect sites and a lower number of –COOH groups, but did exhibit a 70% yield. RuO4 and KMnO4 resulted in similar amounts of overall defect sites, however the yield and amount of –COOH groups for KMnO4 treated CNF were significantly lower than the RuO4 treatment. Overall, oxidation of these CNF with RuO4 provides the best balance of yield and carboxylation.
Co-reporter:Khaled S. Mriziq;Hank D. Cochran
Rheologica Acta 2007 Volume 46( Issue 6) pp:839-845
Publication Date(Web):2007 June
DOI:10.1007/s00397-006-0160-9
The rheological and structural properties of perfluoropolyether (PFPE) lubricant films including viscosity, shear stress, and birefringence were measured at relatively low to extremely high shear rates using a rotational optical rheometer. The viscosity of various films with different thicknesses exhibit Newtonian behavior up to a shear rate 1 × 104 s−1, with a transition to shear-thinning behavior obvious at higher shear rates. Birefringence of these films was also measured for the first time, and these results indicate chain alignment with shear in the shear-thinning regime. The shear rate at which alignment occurs is similar to that of the onset of shear thinning. This correlation between chain alignment and shear thinning provides direct evidence that the ability of PFPEs to lubricate hard drives at high shear rates is a direct consequence of the ability of the applied shear field to align the molecules on a molecular level.
Co-reporter:Asif Rasheed;Phillip F. Britt
Journal of Polymer Science Part B: Polymer Physics 2006 Volume 44(Issue 21) pp:3053-3061
Publication Date(Web):18 SEP 2006
DOI:10.1002/polb.20930
Nanocomposites of styrene and vinyl phenol copolymers, which contain varying (10, 20, and 40 mol %) vinyl phenol content, were prepared with 1 wt % unoxidized, 1 wt % oxidized, and 5 wt % oxidized carbon nanofibers. Dynamic mechanical analysis and differential scanning calorimetry indicate that the composites prepared from oxidized nanofibers exhibit improved thermal and structural properties relative to those prepared from unoxidized nanofibers. The optimum enhancement in the mechanical and thermal properties was observed for the composite containing oxidized nanofibers and the 20% vinyl phenol copolymer. These results are in excellent agreement with our previous work on carbon nanotube–polymer composites and suggest that the presence of intermolecular interactions between the copolymer matrix and nanofibers are responsible for the observed property enhancement. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3053–3061, 2006
Co-reporter:Sriram Viswanathan
Macromolecular Rapid Communications 2001 Volume 22(Issue 10) pp:779-782
Publication Date(Web):10 JUL 2001
DOI:10.1002/1521-3927(20010701)22:10<779::AID-MARC779>3.0.CO;2-U
Guidelines for creating miscible blends containing a liquid crystalline polymer and an amorphous polymer by optimizing intermolecular interactions between the two polymers are presented. It is shown that by controlling the spacing between the functional groups that participate in hydrogen bonding along the amorphous polymer chain, the extent of intermolecular interactions between the two polymers is optimized, and this induces miscibility in the systems studied.
Co-reporter:Mark D. Dadmun
Macromolecular Theory and Simulations 2001 Volume 10(Issue 9) pp:795-801
Publication Date(Web):19 NOV 2001
DOI:10.1002/1521-3919(20011101)10:9<795::AID-MATS795>3.0.CO;2-Z
The effect of disparity in the reactivity ratios of monomer pairs on the composition distribution and microstructure of the resultant copolymer formed through free-radical polymerization is quantified computationally. This correlation has been determined for the monomer pairs of styrene/methyl methacrylate and styrene/2-vinyl pyridine for a variety of monomer feed ratios. These monomer pairs were chosen as they represent systems that have been utilized to experimentally examine the importance of copolymer architecture on its ability to compatibilize an immiscible polymer blend. Moreover, their respective random copolymers show conflicting results for this examination. The results of this work show that the difference in the reactivity ratios of styrene and 2-vinyl pyridine copolymer (r1 = 0.5, r2 = 1.3) significantly broadens the composition and randomness distribution of the resultant copolymer. This breadth is not easily avoided as it evolves even in the early stages of the copolymerization. Conversely, for the styrene/methyl methacrylate pair, the reactivity ratios are similar (r1 = 0.46, r2 = 0.52) and this results in a copolymer with a narrow composition distribution and sequence distribution dispersion. Stopping the polymerization at early conversion further narrows both distributions. The presented results, therefore, provide fundamental information that must be considered when planning an experimental procedure to evaluate the relative importance of sequence distribution and composition distribution of a random on its application.
Co-reporter:Brian Morgan, Mark D. Dadmun
European Polymer Journal (April 2017) Volume 89() pp:
Publication Date(Web):April 2017
DOI:10.1016/j.eurpolymj.2017.02.026
•SANS probes relationship between illumination and conjugated polymer structure.•Experiments vary light intensity, wavelength, and exposure time.•Results indicate structure fluctuates as a function of total absorbable photons.•Larger number of generated excitons leads to larger structural alterations.Our recent investigations indicate that simple white light exposure holds intriguing potential to alter the structure of conjugated polymer chains. To provide further insight into how to rationally control this process, we have employed small angle neutron scattering to investigate structural changes occurring in solutions of poly(3-hexylthiophene-2,5-diyl) upon exposure to white light. Our previous results indicate significant change in the structure of the polymer upon illumination, an effect we attribute to an alteration in the thermodynamic interactions of the polymer with the surrounding solvent. This work provides additional information to further refine our understanding of this phenomenon by studying the relationship between the magnitude of exciton creation and subsequent structural changes through a series of exposure time, light intensity, and wavelength sensitive experiments. The results demonstrate a clear and consistent relationship between the number of absorbable photons available to the conjugated polymer chain and more powerful alterations in polymer structural parameters, indicating that net exciton generation provides the fundamental driving force for this phenomenon. This points towards the feasibility of employing light exposure protocols as a powerful unobtrusive in-situ method for precise tailoring of optoelectronic active layers over a wide range of organic electronic device fabrication procedures.
Co-reporter:Huipeng Chen, Yu-Che Hsiao, Bin Hu and Mark Dadmun
Journal of Materials Chemistry A 2014 - vol. 2(Issue 25) pp:NaN9890-9890
Publication Date(Web):2014/05/07
DOI:10.1039/C4TA01436G
Replacing PCBM with a bis-adduct fullerene (i.e. ICBA) has been reported to significantly improve the open circuit voltage (VOC) and power conversion efficiency (PCE) in P3HT bulk heterojunctions. However, for the most promising low band-gap polymer (LBP) systems, replacing PCBM with ICBA results in very poor short-circuit current (JSC) and PCE although the VOC is significantly improved. Therefore, in this work, we have completed small angle neutron scattering and neutron reflectometry experiments to study the impact of post-deposition solvent annealing (SA) with control of solvent quality on the morphology and performance of LBP–bis-fullerene BHJ photovoltaics. The results show that SA in a solvent that is selective for the LBP results in a depletion of bis-fullerene near the air surface, which limits device performance. SA in a solvent vapor which has similar solubility for polymer and bis-fullerene results in a higher degree of polymer ordering, bis-fullerene phase separation, and segregation of the bis-fullerene to the air surface, which facilitates charge transport and increases power conversion efficiency (PCE) by 100%. The highest degree of polymer ordering combined with significant bis-fullerene phase separation and segregation of bis-fullerene to the air surface is obtained by SA in a solvent vapor that is selective for the bis-fullerene. The resultant morphology increases PCE by 190%. These results indicate that solvent annealing with judicious solvent choice provides a unique tool to tune the morphology of LBP–bis-fullerene BHJ system, providing sufficient polymer ordering, formation of a bis-fullerene pure phase, and segregation of bis-fullerene to the air surface to optimize the morphology of the active layer. Moreover, this process is broadly applicable to improving current “disappointing” LBP–bis-fullerene systems to optimize their morphology and OPV performance post-deposition, including higher VOC and power conversion efficiency.
Co-reporter:Huipeng Chen, Jihua Chen, Wen Yin, Xiang Yu, Ming Shao, Kai Xiao, Kunlun Hong, Deanna L. Pickel, W. Michael Kochemba, S. Michael Kilbey II and Mark Dadmun
Journal of Materials Chemistry A 2013 - vol. 1(Issue 17) pp:NaN5319-5319
Publication Date(Web):2013/02/25
DOI:10.1039/C3TA10386B
The impact of various polymeric compatibilizers, including end-functionalized P3HTs and diblock copolymers containing P3HT, on the structure and function of poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) bulk heterojunctions is presented. Careful analyses of small angle neutron scattering curves provide a measure of the miscibility of PCBM in P3HT, the average PCBM domain size, and the interfacial area between PCBM and the P3HT-rich phase in the uncompatibilized and compatibilized systems. Differential scanning calorimetry (DSC) also provides information regarding the changes in the crystallinity of P3HT due to the presence of the compatibilizer. Results show that most compatibilizers cause the domain sizes to decrease and the P3HT crystallinity to increase; however, some cause an increase in domain size, suggesting that they are not effective interfacial modifiers. The correlation of morphology with photovoltaic activity shows that the decreased domain size, increased crystallinity and increased interfacial area do not always result in improved power conversion efficiency (PCE). It appears that the introduction of an insulating molecule at the PCBM:P3HT interface as a compatibilizer results in a decrease in PCE. Thus, the presence of the compatibilizer at this interface dominates the photovoltaic activity, rather than the morphological control.
Co-reporter:Huipeng Chen, Raghavendra Hegde, J. Browning and M. D. Dadmun
Physical Chemistry Chemical Physics 2012 - vol. 14(Issue 16) pp:NaN5641-5641
Publication Date(Web):2012/02/17
DOI:10.1039/C2CP40466D
Recent work has shown that poly(3-hexylthiophene) (P3HT) and the surface-functionalized fullerene 1-(3-methyloxycarbonyl)propyl(1-phenyl[6,6])C61 (PCBM) are much more miscible than originally thought, and the evidence of this miscibility requires a return to understanding the optimal morphology and structure of organic photovoltaic active layers. This manuscript describes the results of experiments that were designed to provide quantitative thermodynamic information on the miscibility, interdiffusion, and depth profile of P3HT:PCBM thin films that are formed by thermally annealing initial bilayers. It is found that the resultant thin films consist of a ‘bulk’ layer that is not influenced by the air or substrate surface. The composition of PCBM in this ‘bulk’ layer increases with increased PCBM loading in the original bilayer until the ‘bulk’ layer contains 22 vol% PCBM. The introduction of additional PCBM into the sample does not increase the amount of PCBM dispersed in this ‘bulk’ layer. This observation is interpreted to indicate that the miscibility limit of PCBM in P3HT is 22 vol%, while the precise characterization of the depth profiles in these films shows that the PCBM selectively segregates to the silicon and near air surface. The selective segregation of the PCBM near the air surface is ascribed to an entropic driving force.
