Lorraine F. Francis

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Name: Francis, Lorraine

Organization: University of Minnesota , USA

Department: Department of Chemical Engineering and Materials Science

Title: (PhD)

TOPICS

Material Chemistry

Polymer

Polyimide

Chemicals
References

Modulus- and Surface-Energy-Tunable Thiol–ene for UV Micromolding of Coatings

Co-reporter:Yuyang Du, Jun Xu, John D. Sakizadeh, Donovan G. Weiblen, Alon V. McCormick, and Lorraine F. Francis

ACS Applied Materials & Interfaces July 26, 2017 Volume 9(Issue 29) pp:24976-24976

Publication Date(Web):June 29, 2017

DOI:10.1021/acsami.7b06339

Micromolding of UV-curable materials is a patterning method to fabricate microstructured surfaces that is an additive manufacturing process fully compatible with roll-to-roll systems. The development of micromolding for mass production remains a challenge because of the multifaceted demands of UV curable materials and the risk of demolding-related defects, particularly when patterning high-aspect-ratio features. In this research, a robust micromolding approach is demonstrated that integrates thiol–ene polymerization and UV LED curing. The moduli of cured thiol–ene coatings were tuned over 2 orders of magnitude by simply adjusting the acrylate concentration of a coating formulation, the curing completed in all cases within 10 s of LED exposure. Densely packed 50-μm-wide gratings were faithfully replicated in coatings ranging from soft materials to stiff highly cross-linked networks. Further, surface energy was modified with a fluorinated polymer, achieving a surface energy reduction of more than a half at a loading of 1 wt %, and enabling tall (100 μm) defect-free patterns to be attained. The demolding strengths of microstructured coatings were compared using quantitative peel testing, showing its decrease with decreasing surface energy, coating modulus, and grating height. This micromolding process, combining tunability in thermomechanical and surface properties, makes thiol–ene microstructured coatings attractive candidates for roll-to-roll manufacture. As a demonstration of the utility of the process, superhydrophobic surfaces are prepared using the system modified by the fluorinated polymer.Keywords: high throughput; mechanical properties; peel test; replica molding; surface microstructures; surface modification; thiol−ene;

Effect of Nanocrystal Size and Carbon on Grain Growth during Annealing of Copper Zinc Tin Sulfide Nanocrystal Coatings

Co-reporter:Bryce A. Williams, Michelle A. Smeaton, Nancy D. Trejo, Lorraine F. Francis, and Eray S. Aydil

Chemistry of Materials February 28, 2017 Volume 29(Issue 4) pp:

Publication Date(Web):February 3, 2017

DOI:10.1021/acs.chemmater.6b05058

Polycrystalline films were prepared by annealing coatings cast from colloidal dispersions of Cu2ZnSnS4 (CZTS) nanocrystals in sulfur vapor. This nanocrystal dispersion-based route is a promising potential low-cost approach for production of low-cost thin-film solar cells. We studied the effects of nanocrystal size, sulfur pressure, and carbon concentration on the microstructure development and grain growth during annealing. Coatings prepared from dispersions of CZTS nanocrystals with an average diameter of either 5 or 35 nm were annealed for 10–60 min at 600 °C in 50 or 500 Torr of sulfur. The CZTS nanocrystal size influenced both the rate and mechanism of grain growth. When coatings composed of 5 nm nanocrystals are annealed, abnormal grain growth forms micrometer-scale CZTS grains on the surface of the coating. In contrast, when CZTS coatings composed of 35 nm nanocrystals are annealed, grains grow uniformly via normal grain growth. Grain growth rates increased with sulfur pressure regardless of the nanocrystal size. The presence of carbon, originating from ligands used to stabilize nanocrystal dispersions, enhances abnormal grain growth, but too much carbon eventually inhibits all grain growth. On the basis of these observations, we propose a mechanism for microstructure development during annealing of CZTS nanocrystal coatings in sulfur. While much research effort has been expended on the reduction of carbon from nanocrystal coatings prior to sulfidation or selenization by means of ligand exchange or preannealing treatments in the belief that reduced carbon concentration aids CZTS microstructure development and solar cell efficiencies, this work indicates that carbon plays a more complex and significant role in CZTS grain growth than previously assumed: carbon may be beneficial or even required for rapid grain growth during sulfidation.

Scalable, Self-Aligned Printing of Flexible Graphene Micro-Supercapacitors

Co-reporter:Woo Jin Hyun;Ethan B. Secor;Chang-Hyun Kim;Mark C. Hersam;C. Daniel Frisbie

Advanced Energy Materials 2017 Volume 7(Issue 17) pp:

Publication Date(Web):2017/09/01

DOI:10.1002/aenm.201700285

Graphene micro-supercapacitors (MSCs) are an attractive energy storage technology for powering miniaturized portable electronics. Despite considerable advances in recent years, device fabrication typically requires conventional microfabrication techniques, limiting the translation to cost-effective and high-throughput production. To address this issue, we report here a self-aligned printing process utilizing capillary action of liquid inks in microfluidic channels to realize scalable, high-fidelity manufacturing of graphene MSCs. Microstructured ink receivers and capillary channels are imprinted on plastic substrates and filled by inkjet printing of functional materials into the receivers. The liquid inks move under capillary flow into the adjoining channels, allowing reliable patterning of electronic materials in complex structures with greatly relaxed printing tolerance. Leveraging this process with pristine graphene and ion gel inks, miniaturized all-solid-state graphene MSCs are demonstrated to concurrently achieve outstanding resolution (active footprint: <1 mm2, minimum feature size: 20 µm) and yield (44/44 devices), while maintaining a high specific capacitance (268 µF cm–2) and robust stability to extended cycling and bending, establishing an effective route to scale down device size while scaling up production throughput.

Effect of particle size distribution on stress development and microstructure of particulate coatings

Co-reporter:Yan Wu

Journal of Coatings Technology and Research 2017 Volume 14( Issue 2) pp:455-465

Publication Date(Web):2017 March

DOI:10.1007/s11998-016-9866-5

The role of pigment particle size distribution on stress and microstructure development was studied for coatings prepared from aqueous suspensions of ground calcium carbonate (GCC) and latex binder. Stress development was monitored using a modified beam deflection technique under controlled environment. Microstructure was characterized by scanning electron microscopy (SEM) and cryogenic SEM. For coatings containing only GCC particles and no latex, a wide particle size distribution resulted in a significant particle size gradient in the cross-sectional microstructure and irregular stress development. With latex addition, uniform microstructures were observed in coatings with either wide or narrow GCC particle size distribution. GCC/latex coatings prepared using GCC with a wide particle size distribution developed a higher stress than those prepared using GCC with a similar average particle size but a narrow particle size distribution. The higher stress is related to the particle packing that results in smaller pore sizes and larger capillary pressures that drive compaction. In coatings prepared with the same GCC particles but different latex binders, the stress and cracking behavior of the coating depends on the latex properties.

Synergistic Toughening of Epoxy Modified by Graphene and Block Copolymer Micelles

Co-reporter:Tuoqi LiSiyao He, Andreas Stein, Lorraine F. Francis, Frank S. Bates

Macromolecules 2016 Volume 49(Issue 24) pp:9507-9520

Publication Date(Web):December 9, 2016

DOI:10.1021/acs.macromol.6b01964

Binary composites formed by individually mixing exfoliated graphene oxide modified with amine-terminated poly(butadiene–acrylonitrile) (GA) and a spherical micelle forming poly(ethylene oxide)-b-poly(ethylene-alt-propylene) (OP) diblock copolymer with a thermoset epoxy, and the associated GA/OP/epoxy ternary composites, were prepared and studied as a function of the molecular weight Mc between cross-links. The rigid GA filler dispersed well in the cured epoxies as established by transmission electron microscopy (TEM). The toughening efficacy of GA alone was found to depend strongly on the modifier concentration and the matrix cross-link density with an optimal 1.7-fold increase in the critical strain energy release rate (GIc) over the neat epoxy obtained with a 0.04 wt % loading in the most lightly cross-linked (Mc = 6100 g/mol) material. Addition of 5 wt % OP to this epoxy resin enhanced GIc by a factor of 12. Combining the hard GA and soft OP modifiers at the same loading levels (0.04 and 5 wt %, respectively) resulted in 18 times the GIc of the unmodified material, a 31% improvement over the effect anticipated by simple addition of the fracture properties of the binary composites. Decreasing Mc to 700 g/mol eliminated this synergistic effect while reducing the overall improvement in GIc to just 3 times that of the neat epoxy. Topological features on the fracture surfaces, imaged using a scanning electron microscope (SEM), suggest that the synergistic toughening of the GA/OP/epoxy ternary composite involves concurrent mechanisms operating on different length scales, including micelle cavitation and graphene debonding, resulting in simultaneous shear yielding, crack pinning, and crack deflection.

High-Resolution Patterning of Graphene by Screen Printing with a Silicon Stencil for Highly Flexible Printed Electronics

Co-reporter:Woo Jin Hyun;Ethan B. Secor;Mark C. Hersam;C. Daniel Frisbie

Advanced Materials 2015 Volume 27( Issue 1) pp:109-115

Publication Date(Web):

DOI:10.1002/adma.201404133

All-Printed, Foldable Organic Thin-Film Transistors on Glassine Paper

Co-reporter:Woo Jin Hyun;Ethan B. Secor;Geoffrey A. Rojas;Mark C. Hersam;C. Daniel Frisbie

Advanced Materials 2015 Volume 27( Issue 44) pp:7058-7064

Publication Date(Web):

DOI:10.1002/adma.201503478

Wettability Contrast Gravure Printing

Co-reporter:Heng Zhang;Alexer Ramm;Sooman Lim;Wei Xie;Bok Yeop Ahn;WeiChao Xu;Ankit Mahajan;Wieslaw J. Suszynski;Chris Kim;Jennifer A. Lewis;C. Daniel Frisbie

Advanced Materials 2015 Volume 27( Issue 45) pp:7420-7425

Publication Date(Web):

DOI:10.1002/adma.201502639

Formation of Copper Zinc Tin Sulfide Thin Films from Colloidal Nanocrystal Dispersions via Aerosol-Jet Printing and Compaction

Co-reporter:Bryce A. Williams, Ankit Mahajan, Michelle A. Smeaton, Collin S. Holgate, Eray S. Aydil, and Lorraine F. Francis

ACS Applied Materials & Interfaces 2015 Volume 7(Issue 21) pp:11526

Publication Date(Web):May 19, 2015

DOI:10.1021/acsami.5b02484

A three-step method to create dense polycrystalline semiconductor thin films from nanocrystal liquid dispersions is described. First, suitable substrates are coated with nanocrystals using aerosol-jet printing. Second, the porous nanocrystal coatings are compacted using a weighted roller or a hydraulic press to increase the coating density. Finally, the resulting coating is annealed for grain growth. The approach is demonstrated for making polycrystalline films of copper zinc tin sulfide (CZTS), a new solar absorber composed of earth-abundant elements. The range of coating morphologies accessible through aerosol-jet printing is examined and their formation mechanisms are revealed. Crack-free albeit porous films are obtained if most of the solvent in the aerosolized dispersion droplets containing the nanocrystals evaporates before they impinge on the substrate. In this case, nanocrystals agglomerate in flight and arrive at the substrate as solid spherical agglomerates. These porous coatings are mechanically compacted, and the density of the coating increases with compaction pressure. Dense coatings annealed in sulfur produce large-grain (>1 μm) polycrystalline CZTS films with microstructure suitable for thin-film solar cells.Keywords: aerosol-jet printing; copper zinc tin sulfide; nanocrystal; nanoparticle coating; spray coating; thin-film solar cell;

Screen Printing of Highly Loaded Silver Inks on Plastic Substrates Using Silicon Stencils

Co-reporter:Woo Jin Hyun, Sooman Lim, Bok Yeop Ahn, Jennifer A. Lewis, C. Daniel Frisbie, and Lorraine F. Francis

ACS Applied Materials & Interfaces 2015 Volume 7(Issue 23) pp:12619

Publication Date(Web):June 2, 2015

DOI:10.1021/acsami.5b02487

Screen printing is a potential technique for mass-production of printed electronics; however, improvement in printing resolution is needed for high integration and performance. In this study, screen printing of highly loaded silver ink (77 wt %) on polyimide films is studied using fine-scale silicon stencils with openings ranging from 5 to 50 μm wide. This approach enables printing of high-resolution silver lines with widths as small as 22 μm. The printed silver lines on polyimide exhibit good electrical properties with a resistivity of 5.5 × 10–6 Ω cm and excellent bending tolerance for bending radii greater than 5 mm (tensile strains less than 0.75%).Keywords: plastic substrates; printed electronics; screen printing; silicon stencils; silver ink;

High-Resolution, High-Aspect Ratio Conductive Wires Embedded in Plastic Substrates

Co-reporter:Ankit Mahajan, Woo Jin Hyun, S. Brett Walker, Jennifer A. Lewis, Lorraine F. Francis, and C. Daniel Frisbie

ACS Applied Materials & Interfaces 2015 Volume 7(Issue 3) pp:1841

Publication Date(Web):January 16, 2015

DOI:10.1021/am507539a

A novel method is presented to fabricate high-resolution, high-aspect ratio metal wires embedded in a plastic substrate for flexible electronics applications. In a sequential process, high-resolution channels connected to low-resolution reservoirs are first created in a thermosetting polymer by imprint lithography. A reactive Ag ink is then inkjet-printed into the reservoirs and wicked into the channels by capillary forces. These features serve as a seed layer for copper deposition inside the channels via electroless plating. Highly conductive wires (>50% bulk metal) with minimum line width and spacing of 2 and 4 μm, respectively, and an aspect ratio of 0.6 are obtained. The embedded wires exhibit good mechanical flexibility, with minimal degradation in electrical performance after thousands of bending cycles.Keywords: copper electroless plating; flexible electronics; high-resolution metal lines; imprint lithography; inkjet printing

Sag in drying coatings: Prediction and real time measurement with particle tracking

Co-reporter:Robert K. Lade Jr., Jin-Oh Song, Austin D. Musliner, Bryce A. Williams, Satish Kumar, Christopher W. Macosko, Lorraine F. Francis

Progress in Organic Coatings 2015 Volume 86() pp:49-58

Publication Date(Web):September 2015

DOI:10.1016/j.porgcoat.2015.04.005

•A novel method for monitoring sag in drying coatings is developed.•Lycopodium spores are used to track surface velocity using optical microscopy.•Experimentally measured velocities agree well with those predicted using a model.•A sag regime map, which predicts sag extent from coating properties, is presented.Sag is a coating phenomenon characterized by gravity-driven flow after deposition; excessive amounts of sag can lead to coating defects. In this work, a new method for evaluating and quantifying sag is investigated. The motion of micron-sized Lycopodium spores on an inclined coating surface is tracked during drying, and the resulting surface velocity data is used to determine sag length. This in situ particle tracking method is minimally invasive and permits real time measurements. Measured sag lengths and real time surface velocities in aqueous polyvinyl alcohol solution coatings compare well with a theoretical model. The model is also used to develop a predictive sag regime map, which anticipates the extent of sag given coating properties and process-specific parameters. This map also identifies viable processing windows and aids in intelligent coating design given specific process constraints. The predictions of the sag regime map are compared against experimental sag results from polyvinyl alcohol solution coatings as well as four commercial latex paints, revealing good agreement for coatings with Newtonian or ‘Newtonian-like’ rheologies.

A Self-Aligned Strategy for Printed Electronics: Exploiting Capillary Flow on Microstructured Plastic Surfaces

Co-reporter:Ankit Mahajan;Woo Jin Hyun;S. Brett Walker;Geoffrey A. Rojas;Jae-Hong Choi;Jennifer A. Lewis;C. Daniel Frisbie

Advanced Electronic Materials 2015 Volume 1( Issue 9) pp:

Publication Date(Web):

DOI:10.1002/aelm.201500137

Printing is a promising route for high-throughput processing of electronic devices on large-area, flexible substrates by virtue of its integration into roll-to-roll production formats. However, multilayered electronic devices require materials registration with micrometer-level tolerances, which is a serious challenge for continuous manufacturing. Here, a novel, self-aligned manufacturing approach is introduced that allows precision patterning of multilayered electronic devices by inkjet printing on microimprinted plastic substrates. Materials registration is achieved automatically by sequential deposition of liquid inks into multilevel trench networks on the substrate surface using capillary forces. By creating suitable multitier capillary networks, fully self-aligned fabrication of all the major building blocks of an integrated circuit, including resistors, capacitors, transistors, and crossovers, with excellent yields and performance metrics is demonstrated. The current status of inkjet and imprint technologies suggests that this self-aligned manufacturing strategy can be scaled up to large-area substrates with integration densities greater than 1000 devices cm⁻².

Evaluating sag resistance with a multinotched applicator: correlation with surface flow measurements and practical recommendations

Co-reporter:Robert K. Lade Jr.;Austin D. Musliner

Journal of Coatings Technology and Research 2015 Volume 12( Issue 5) pp:809-817

Publication Date(Web):2015 September

DOI:10.1007/s11998-015-9680-5

Sag is a coating defect that results from excessive, gravity-driven flow after deposition. Accordingly, characterizing resistance to sag is critically important. In this paper, sag resistance predicted using a multinotched applicator test is compared with results obtained using an in situ particle tracking technique that measures surface velocity. Four commercial latex paints dried on substrates inclined at three angles were investigated. The results are used to provide insight into the strengths and limitations of using a multinotched applicator to evaluate sag resistance. For coatings dried on vertical surfaces (90°), the suggested condition for the multinotched applicator, sag lengths found by particle tracking show differences between paints that the multinotched applicator ranked as identical. At smaller angles (e.g., 10°), the resolution of the multinotched applicator test is greatly enhanced owing to a reduction in the shear stress difference between adjacent coated lines. Based on these results, specific recommendations are made for successfully employing a multinotched applicator to evaluate sag resistance based on user-specific goals.

Facile Method for Fabricating Flexible Substrates with Embedded, Printed Silver Lines

Co-reporter:Ankit Mahajan, Lorraine F. Francis, and C. Daniel Frisbie

ACS Applied Materials & Interfaces 2014 Volume 6(Issue 2) pp:1306

Publication Date(Web):December 9, 2013

DOI:10.1021/am405314s

Insertion, curing and delamination is presented as a simple and scalable method for creating flexible substrates with embedded, printed silver lines. In a sequential process, aerosol-jet printed silver lines are transferred from a donor substrate to a thin reactive polymer that is directly adhered to a flexible substrate. Due to the unique ability of the aerosol jet to print continuous lines on a low energy surface, a 100% transfer of the printed electrodes is obtained, as confirmed by electrical measurements. Moreover, the root-mean-square roughness of the embedded electrodes is less than 10 nm, which is much lower than that for their as-printed form. The embedded electrodes are robust and do not show a significant degradation in electrical performance after thousands of bending cycles.Keywords: aerosol-jet printing; embedded silver lines; flexible electronics; flexible epoxy coating; printed electronics; roll-to-roll manufacturing;

Microstructure and performance of block copolymer modified epoxy coatings

Co-reporter:Tuoqi Li, Michael J. Heinzer, Erica M. Redline, Feng Zuo, Frank S. Bates, Lorraine F. Francis

Progress in Organic Coatings 2014 Volume 77(Issue 7) pp:1145-1154

Publication Date(Web):July 2014

DOI:10.1016/j.porgcoat.2014.03.015

•Two distinct diblock copolymers were synthesized as modifiers for an epoxy resin.•Modifiers self-assembled into spherical micelles in cured bulk epoxy and coatings.•Toughened bulk thermosets showed a fivefold increase in the fracture toughness.•Modified epoxy coatings have a greatly enhanced abrasive wear resistance.•Block copolymer modification did not sacrifice other desired properties.The effects of two diblock copolymers, poly(ethylene-alt-propylene)-b-poly(ethylene oxide) (PEP–PEO) and poly(1,2-butadiene)-b-poly(2-vinyl pyridine) (PB–P2VP) on the mechanical properties of epoxy coatings were studied. Both modifiers self-assembled into spherical micelles of 10–20 nm diameter in cured bulk epoxy. This morphology was preserved in 15 μm thick coatings; however, micelle segregation to the coating/substrate interface was also observed. The critical strain energy release rate, G1c, of bulk thermosets was enhanced by up to fivefold with the addition of block copolymers. Likewise, the abrasive wear resistance of thin coatings increased with modifier inclusion. The results showed that at 5 wt.% of loading, block copolymers were able to impart a 40% increase in abrasive wear resistance to modified coatings over neat ones. Block copolymer modifiers did not sacrifice the modulus and glass transition temperature of bulk thermosets and coatings, or the hardness and transparency of coatings.

Stress development and film formation in multiphase composite latexes

Co-reporter:Kyle Price;Wenjun Wu;Kurt Wood

Journal of Coatings Technology and Research 2014 Volume 11( Issue 6) pp:827-839

Publication Date(Web):2014 November

DOI:10.1007/s11998-014-9606-7

Designed appropriately, multiphase soft-core/hard-shell latex particles can achieve film formation without the addition of a coalescing aid, while preserving sufficient film hardness. Achieving optimal performance in these materials requires an understanding of how particle morphology affects film formation and stress development in the film. In this study, soft-core/hard-shell latex particles with different shell ratios, core and shell glass transition temperatures (Tgs), and particle sizes (63–177 nm) were synthesized using a two-stage emulsion polymerization. The film formation behavior of the composite particles was investigated with cryogenic scanning electron microscopy, atomic force microscopy, and measurements of the minimum film formation temperature (MFFT). Results show that film formation was enhanced for particles with thinner hard shells, smaller particle size, and a smaller difference in Tg between the core and shell polymers. For example, the MFFT decreased and the particle deformation increased for particles with thinner shells and smaller particle sizes. Stress development during drying was characterized using a cantilever beam bending technique. A walled cantilever design was used to monitor stress development without the complication of a lateral drying front. The film formation behavior and stress development correlated well with practical paint properties like scrub resistance and gloss.

A Raman spectroscopic method to find binder distribution in electrodes during drying

Co-reporter:Hideki Hagiwara;Wieslaw J. Suszynski

Journal of Coatings Technology and Research 2014 Volume 11( Issue 1) pp:11-17

Publication Date(Web):2014 January

DOI:10.1007/s11998-013-9509-z

Lithium ion batteries are used extensively in electronic devices as well as hybrid and electric vehicles. The anode electrode layer in the battery can be fabricated by coating an aqueous dispersion of carbon, binder, and additives, and then drying. During manufacturing, the distribution of the binder through the coating thickness can become nonuniform, which compromises the properties and performance of the battery. In this study, a quantitative method to analyze the binder distribution in the electrode during drying was established. A drying apparatus with an integrated analytic balance and surface-temperature measurement was used to prepare specimens. At specific time points during drying, specimens were removed from the apparatus, quickly frozen, and then freeze-dried. Raman spectroscopy was then used to measure the binder concentration at different points through the cross section of the freeze-dried electrode coating. Scanning electron microscopy was also used to explore the changing microstructure qualitatively. Using a model electrode formulation, the method demonstrated different binder distributions for electrodes dried at 150°C under airflow and room temperature, 20°C, with no airflow. The results also showed continued changes in distribution in the interior of the coating as drying continued.

Optimization of Aerosol Jet Printing for High-Resolution, High-Aspect Ratio Silver Lines

Co-reporter:Ankit Mahajan, C. Daniel Frisbie, and Lorraine F. Francis

ACS Applied Materials & Interfaces 2013 Volume 5(Issue 11) pp:4856

Publication Date(Web):May 9, 2013

DOI:10.1021/am400606y

Aerosol jet printing requires control of a number of process parameters, including the flow rate of the carrier gas that transports the aerosol mist to the substrate, the flow rate of the sheath gas that collimates the aerosol into a narrow beam, and the speed of the stage that transports the substrate beneath the beam. In this paper, the influence of process parameters on the geometry of aerosol-jet-printed silver lines is studied with the aim of creating high-resolution conductive lines of high current carrying capacity. A systematic study of process conditions revealed a key parameter: the ratio of the sheath gas flow rate to the carrier gas flow rate, defined here as the focusing ratio. Line width decreases with increasing the focusing ratio and stage speed. Simultaneously, the thickness increases with increasing the focusing ratio but decreases with increasing stage speed. Geometry control also influences the resistance per unit length and single pass printing of low-resistance silver lines is demonstrated. The results are used to develop an operability window and locate the regime for printing tall and narrow silver lines in a single pass. Under optimum conditions, lines as narrow as 20 μm with aspect ratios (thickness/width) greater than 0.1 are obtained.Keywords: aerosol jet printing; focusing ratio; high resolution; operability window; printed electronics; printed silver lines;

Magnetic Microrheology of Block Copolymer Solutions

Co-reporter:Jin Chul Kim, Myungeun Seo, Marc A. Hillmyer, and Lorraine F. Francis

ACS Applied Materials & Interfaces 2013 Volume 5(Issue 22) pp:11877

Publication Date(Web):October 28, 2013

DOI:10.1021/am403569f

The viscosity of poly(styrene)-b-poly(lactide) [PS-b-PLA] solutions in a neutral solvent was characterized by magnetic microrheology. The effect of polymer concentration on the viscosity of the block polymer solutions was compared with that of the PS and PLA homopolymers in the same solvent. The viscosity of PS-b-PLA solution, unlike the homopolymer solutions, showed a steep increase over a narrow concentration range. The steep rise was concomitant with microphase separation into an ordered cylindrical microstructure as determined by small-angle X-ray scattering. Hence microrheology proved effective as a means of characterizing the order–disorder transition concentration. During an in situ drying experiment, changes in local viscosity through the depth of a block copolymer solution were characterized as a function of drying time. Early in the drying process, the viscosity rose steadily and was uniform through the depth, a result consistent with steadily increasing and uniform polymer concentration. However, later in the drying process as the overall polymer concentration approached that required for microphase separation, the viscosity of the polymer solution near the free surface became an order of magnitude higher than that near the bottom of the container. The zone of high viscosity moved downward as drying proceeded, consistent with a microphase separation front.Keywords: block copolymer dynamic viscosity; block copolymer self-assembly; in situ drying; magnetic microrheology; microphase separation; PS-b-PLA; solvent removal; solvent vapor annealing; THF;

Effects of freezing and thawing on the microstructure of latex paints

Co-reporter:Manish Mittal, John A. Roper III, Catheryn L. Jackson, Gregory G. Monaghan, Lorraine F. Francis

Journal of Colloid and Interface Science 2013 Volume 392() pp:183-193

Publication Date(Web):15 February 2013

DOI:10.1016/j.jcis.2012.10.008

Freeze–thaw (FT) cycles can aggregate particles in aqueous paint suspensions. To understand the mechanism of particle aggregation, cryogenic scanning electron microscopy (cryoSEM) was used to visualize the microstructure after the freezing and thawing steps of the FT cycle. After the freezing step, cryoSEM images show that the microstructure contains ice crystals and particle-rich regions. Adding propylene glycol, a FT stabilizing additive, leads to formation of larger ice crystals. After thawing, the dispersion structure revealed by cryoSEM shows that the particles redisperse only in the paint with the highest amount of propylene glycol. The other paints contain clusters that are different from the particle-rich regions found after the freezing step. Increasing the thawing rate leads to a more dispersed microstructure even in the absence of propylene glycol. Analysis of the cryoSEM results shows that particle aggregation into these clusters occurs during the thawing stage, and slow thawing conditions lead to more aggregation. The cryoSEM results reported here are used to propose a mechanism of aggregation of particles in the paint.Graphical abstractHighlights► We used cryoSEM to study the microstructure of paint suspensions at different time points in a freeze–thaw (FT) cycle. ► In a FT cycle, particle aggregation occurs during the thawing process. ► Slow thawing conditions enhance particle aggregation. ► We postulate a mechanism based on the kinetics of aggregation during the individual steps in the FT cycle.

Depthwise Viscosity Gradients in UV-Cured Epoxy Coatings

Co-reporter:Jin-Oh Song;Alon V. McCormick

Macromolecular Materials and Engineering 2013 Volume 298( Issue 2) pp:145-152

Publication Date(Web):

DOI:10.1002/mame.201100362

Coating of Open Cell Foams

Co-reporter:Heng Zhang, Wieslaw J. Suszynski, Kumar Varoon Agrawal, Michael Tsapatsis, Saleh Al Hashimi, and Lorraine F. Francis

Industrial & Engineering Chemistry Research 2012 Volume 51(Issue 27) pp:9250-9259

Publication Date(Web):June 19, 2012

DOI:10.1021/ie300266p

The interior surfaces of three-dimensional open cell foams were coated by a combination of dip coating and spin coating. Glycerol/water solutions were used as model Newtonian liquids, and the coating processes were studied on open cell carbon foams with 10 or 30 pores per inch (PPI). The amount of liquid retained in the foam structures after dip coating increased with withdrawal speed and coating viscosity, as expected from the conventional understanding of dip coating onto nonporous substrates such as flat plates and rods. However, the liquid retention and hence average coating thickness increased with surface tension, a result counter to the observation with coating onto nonporous substrates. Pockets of liquid were observed after dip coating and results with coatings of alumina suspension showed that after drying, the trapped liquid can block pore windows. Spinning the foams after dip coating resulted in uniform liquid distribution and uniform coatings. Foams were placed in a special apparatus and rotated using a commercial spin coater. The liquid layer thickness decreased with spinning time and rotational speed, and increased with the liquid viscosity, results consistent with spin coating theory. The coating thickness after spinning was not affected by the initial dip coating procedure. The dip and spin process was also used to create γ-alumina and zeolite coatings, which are of interest for catalysis applications.

Effect of soluble polymer binder on particle distribution in a drying particulate coating

Co-reporter:Felix Buss, Christine C. Roberts, Kathleen S. Crawford, Katharina Peters, Lorraine F. Francis

Journal of Colloid and Interface Science 2011 Volume 359(Issue 1) pp:112-120

Publication Date(Web):1 July 2011

DOI:10.1016/j.jcis.2011.03.054

Soluble polymer is frequently added to inorganic particle suspensions to provide mechanical strength and adhesiveness to particulate coatings. To engineer coating microstructure, it is essential to understand how drying conditions and dispersion composition influence particle and polymer distribution in a drying coating. Here, a 1D model revealing the transient concentration profiles of particles and soluble polymer in a drying suspension is proposed. Sedimentation, evaporation and diffusion govern particle movement with the presence of soluble polymer influencing the evaporation rate and solution viscosity. Results are summarized in drying regime maps that predict particle accumulation at the free surface or near the substrate as conditions vary. Calculations and experiments based on a model system of poly(vinyl alcohol) (PVA), silica particles and water reveal that the addition of PVA slows the sedimentation and diffusion of the particles during drying such that accumulation of particles at the free surface is more likely.Graphical abstractDrying regime map demonstrating role of soluble polymer on particle distribution in a drying coating: E – evaporation, D – diffusion, and S – sedimentation.Highlights► A model predicts particle and polymer distribution in a drying coating. ► Results are summarized in maps showing regions of particle accumulation. ► Addition of binder slows particles, favors particle accumulation at the surface. ► Experimental results corroborate model predictions.

Effect of water chemistry on calcium carbonate deposition on metal and polymer surfaces

Co-reporter:Zhen Wu, Jane H. Davidson, Lorraine F. Francis

Journal of Colloid and Interface Science 2010 Volume 343(Issue 1) pp:176-187

Publication Date(Web):1 March 2010

DOI:10.1016/j.jcis.2009.11.031

The formation of calcium carbonate on the surfaces of polypropylene and copper tubes was studied by exposing tubes to a laminar flow of room temperature distilled water, supersaturated with respect to calcite. Three water chemistries were used: a control (pH 9.3), one with lower supersaturation but similar pH as the control, and one with higher pH (pH 11) but similar supersaturation as the control. The accumulation of calcium carbonate with time was characterized along with the microstructure and the crystal structure of the deposits. On both tube materials, the amount of calcium carbonate deposited per unit surface area increased with time. At any given time, the accumulation was significantly less when the low supersaturation water was used. For all three water chemistries, more calcium carbonate formed on polyproplyene as compared with copper, with the greatest difference between the two noted for the higher pH water. The deposits consisted of discrete particles and particle clusters, the number and size of which increased with time. Calcite was the dominant polymorph for deposits on copper. On polyproplyene, the deposits were a mixture of calcite, vaterite and aragonite with aragonite dominating except at low supersaturation. A simple model for the mass accumulation of particulate-based deposits with time is presented and the factors responsible for the differences between materials are discussed.

Dip coating of charged colloidal suspensions onto substrates with patterned wettability: Coating regime maps

Co-reporter:Timothy D. Reynolds, Sreeram K. Kalpathy, Satish Kumar, Lorraine F. Francis

Journal of Colloid and Interface Science 2010 Volume 352(Issue 1) pp:202-210

Publication Date(Web):1 December 2010

DOI:10.1016/j.jcis.2010.08.028

Aqueous dispersions of silica nanoparticles were dip coated onto Si substrates that contained patterned wettability. The patterns were prepared by photolithography and consisted of groups of hydrophilic lines (5–100 μm wide) separated by hydrophobic areas (5–100 μm wide). Coating were made from two aqueous silica dispersions: a cationic dispersion in which particles have positive surface charge, and an anionic dispersion in which particles have negative surface charge. Coating morphology, thickness, and pattern quality were characterized. For a pattern containing 25 μm wide hydrophilic stripes separated by equally wide hydrophobic spaces, coating regime maps were created to show the effect of process variables on pattern features and morphology. Within the map there is a critical concentration for both dispersions, above which uniform stripes are formed and below which a segregated non-uniform structure results. Coatings prepared at withdrawal rates of 0.1 mm/s or lower resulted in a monolayer of coatings in the case of cationic silica and no deposition in the case of anionic silica. A maximum withdrawal rate was also found; above a critical speed, excess liquid is entrained and results in nonuniformity in the bottom of the pattern. The physical origin of the regimes and differences between the two types of particles are discussed.Graphical abstractCoating regime map for silica coating on a chemically patterned substrate..Research highlights► Dip coating of silica nanoparticle dispersions onto chemically patterned substrates. ► A ‘coating regime map’ shows effect of variables on morphology. ► Uniform particle patterns created with more concentrated suspensions (>10 wt.%). ► Silica with positive surface charge, cationic silica, forms monolayer at low rates.

Structure and property development during solidification of VOC compliant automotive refinish clearcoats

Co-reporter:K. Jindal, D. Bhattacharya, L.F. Francis, A.V. McCormick, L.T. Germinario, C. Williams

Progress in Organic Coatings 2010 Volume 67(Issue 3) pp:296-301

Publication Date(Web):March 2010

DOI:10.1016/j.porgcoat.2009.10.030

A model volatile organic compound (VOC) compliant two component (2K) acrylic polyol refinish clearcoat was prepared and used to study the effects of a cellulose-based additive on drying, curing, rheology and stress development at room temperature. Results were compared with commercially available VOC compliant and traditional non-compliant clearcoats. Most of the drying of the low VOC coatings occurred before appreciable (20%) crosslinking. Tensile stress developed in the same timeframe as drying and then relaxed over a longer time scale. All coatings had relatively low levels of peak stress (<0.25 MPa) compared to other polymer coatings. Model low VOC coatings prepared with the additive had higher peak stresses than those without the additive. In addition, rheological data showed that the additive resulted in greater viscosity buildup after application. Both effects are attributed to the high glass transition temperature of the additive (∼70 °C).

Formation of Salt Crystal Whiskers on Porous Nanoparticle Coatings

Co-reporter:Heng Zhang, Zhen Wu and Lorraine F. Francis

Langmuir 2010 Volume 26(Issue 4) pp:2847-2856

Publication Date(Web):October 30, 2009

DOI:10.1021/la902902k

Salt crystal whiskers were grown from aqueous solution on porous nanoparticle silica coatings. Coated substrates were partially immersed in an aqueous potassium chloride solution and then kept in a controlled relative humidity chamber for whisker growth. The salt solution was pulled into the porous coating, reaching a steady level about 1 h after immersion. Crystals with whisker morphologies, typically 2−50 μm in lateral dimension and up to ∼1 cm in length, emerged from the coating surface at a position above the original liquid level. Crystallites pushed upward by attached whiskers indicated a base growth mechanism in which ions are added to the surface of a growing whisker that is in contact with the coating. Sheetlike crystals formed from the base growth of whiskers that had fallen flat onto the porous coating surface. The effects of solution concentration and relative humidity on growth were characterized and used to elaborate the transport phenomena and growth mechanisms. Salt whiskers were also grown on bare substrates immersed in salt solutions containing nanoparticles. In this case, growth occurred below the original contact line on coatings created by convective assembly.

Drying and collapse of hollow latex

Co-reporter:C. M. Cardinal;L. F. Francis;L. E. Scriven

Journal of Coatings Technology and Research 2009 Volume 6( Issue 4) pp:457-469

Publication Date(Web):2009 December

DOI:10.1007/s11998-009-9167-3

Hollow latex particles are used as white pigments for paints and paper coatings. In the coating dispersion, each hollow particle is filled with water. As the coating dries, water vacates the latex, leaving an air-filled void sized to scatter light (~0.5 μm) within each particle. Examinations of dried coatings reveal that hollow particles can collapse, decreasing their light scattering efficiency. Cryogenic scanning electron microscopy (cryoSEM) was used to characterize the microstructure of coatings containing hollow latex during drying. Images suggest latex voids empty after air invades into the coating interstitial space and collapse occurs late in the drying process. The effects of temperature (10–60°C), humidity (20–80%), and binder concentration (0–30 wt%) on particle collapse were also studied through SEM of dried coating surfaces. High drying temperature, high humidity, and low binder concentrations promoted collapse. For hollow latex particles with porous shell walls, temperature and humidity had little effect, whereas binder increased collapse. From these results, a theoretical model is proposed. During drying, diffusion of water from the particle creates a vacuum inside the latex. The vacuum is either relieved by nucleation of a gas bubble from the dissolved air in the water-filled particle or it causes the particle to collapse by buckling.

Ceramic Nanoparticle/Monodisperse Latex Coatings

Co-reporter:Hui Luo, Christine M. Cardinal, L. E. Scriven and Lorraine F. Francis

Langmuir 2008 Volume 24(Issue 10) pp:5552-5561

Publication Date(Web):April 17, 2008

DOI:10.1021/la800050u

Ceramic nanoparticle/monodisperse latex coatings with a nanoparticle-rich surface and a latex-rich body were created by depositing aqueous dispersions of monodisperse latex, ∼550 nm in diameter, and nanosized ceramic particles onto substrates and drying. On the top surface of the dried coating, the latex particles are closely packed with nanoparticles uniformly occupying the interstitial spaces, and along the cross section, nanoparticles fill the spaces between the latex particles in the near surface region; a compacted latex structure, nearly devoid of nanoparticles, lies beneath. Cryogenic scanning electron microscopy images of partially dried coatings at successive drying stages reveal two important steps in forming this structure: top-down consolidation of latex particles and accumulation of nanoparticles in interstitial spaces among latex particles near the surface. A systematic study of the effect of processing conditions, including nanoparticle concentration, nanoparticle size, latex glass transition temperature, and drying conditions, on the final microstructure was carried out. The unique microstructure described above forms when the monodisperse latex is large enough to create pore channels for the transport of nanosized particles and the drying conditions favor “top-down” as opposed to “edge-in” drying.

Transparent, conductive polymer blend coatings from latex-based dispersions

Co-reporter:Jiakuan Sun, William W. Gerberich, Lorraine F. Francis

Progress in Organic Coatings 2007 Volume 59(Issue 2) pp:115-121

Publication Date(Web):1 May 2007

DOI:10.1016/j.porgcoat.2007.01.019

Flexible, transparent and conductive polymer blend coatings were prepared from aqueous dispersions of poly(3,4-ethylenedixoythiophene)/poly(styrenesulfonate) [PEDOT/PSS] gel particles (∼80 nm) and latex (∼300 nm). The stable dispersions were deposited as wet coatings onto poly(ethylene terephthalate) substrates and dried at 80 °C. Microstructure studies using tapping mode atomic force microscopy (TMAFM) indicate that a network-like microstructure formed during drying at 0.03 volume fraction PEDOT/PSS loading. In this network-like structure, the PEDOT/PSS phase was forced into the boundary regions between latex. In addition, migration of the PEDOT/PSS particles towards coating surface is likely during drying of the aqueous dispersions. The addition of a small amount of dimethyl sulfoxide (DMSO) in dispersions altered the distribution of the PEDOT/PSS phase. As PEDOT/PSS concentration increases to 0.15 volume fraction, the coating surface is dominated by the PEDOT/PSS phase. The effect of DMSO on microstructure becomes less apparent as PEDOT/PSS concentration increases. The conductivity of the polymer blend coatings increases in a percolation-like fashion with a threshold of ∼0.02 volume fraction PEDOT/PSS. The addition of DMSO in dispersions enhanced the coating conductivity beyond the threshold by more than two orders of magnitude. The highest conductivity, ∼3 S/cm, occurs at 0.20 volume fraction PEDOT/PSS concentration. The polymer blend coatings have good transparency with only a weak dependence of transparency on wavelength due to the small refractive index difference between filler and matrix.

Conductive coatings and composites from latex-based dispersions

Co-reporter:Lorraine F. Francis, Jaime C. Grunlan, Jiakuan Sun, W.W. Gerberich

Colloids and Surfaces A: Physicochemical and Engineering Aspects 2007 Volume 311(1–3) pp:48-54

Publication Date(Web):1 December 2007

DOI:10.1016/j.colsurfa.2007.08.026

Electrically conductive coatings and composites are prepared from aqueous dispersions of conducting particles and polymer latex particles. Relatively small amounts of conductive particles are needed to develop electrical conductivity, because the particulate nature of the latex leads to a segregated network that lowers the percolation threshold. Several nanosized conductive fillers have been studied: carbon black, antimony-doped tin oxide, indium tin oxide and carbon nanotubes. The latex chosen for most studies was either a poly(vinyl acetate-co-acrylic) polydisperse latex, a poly(vinyl acetate) polydisperse latex, or monodisperse poly(vinyl acetate) latex. This paper reviews the effect of particle size, aggregation and aspect ratio on the microstructure and properties of conductive composites and coatings.

Silica nanoparticle dispersions in homopolymer versus block copolymer

Co-reporter:Qiang Lan;Frank S. Bates

Journal of Polymer Science Part B: Polymer Physics 2007 Volume 45(Issue 16) pp:2284-2299

Publication Date(Web):2 JUL 2007

DOI:10.1002/polb.21251

Silica nanoparticles (17 ± 4 nm in diameter) were modified by grafting polystyrene chains to the surfaces using atom transfer radical polymerization (ATRP). The molecular weight of the grafted chains ranged from 8 to 48 kDa. These modified nanoparticles were mixed in solution with poly(styrene) homopolymer (18–120 kDa) and symmetric poly(styrene-b-butadiene) (PS-PB) diblock copolymer (34–465 kDa) and the states of dispersion in the dried composites were characterized by transmission electron microscopy (TEM). In the so-called wet brush limit, when the graft molecular weight equals or exceeds the matrix value, the silica particles form a uniform random dispersion in poly(styrene). Increasing the homopolymer matrix, molecular weight above the graft value results in particle clustering and macroscopic-phase separation. Mixtures of the lamellar forming block copolymer and nanoparticles exhibit a very different trend, with particle clustering at the lower PS-PB molecular weights and dispersion at the highest value. This latter finding is rationalized on the basis of packing constraints associated with lamellar order and the effective particle dimensions, and the degree of solvation at ordering, both of which favor higher molecular weight block copolymers. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2284–2299, 2007

Processing and properties of porous poly(l-lactide)/bioactive glass composites

Co-reporter:Kai Zhang, Yunbing Wang, Marc A. Hillmyer, Lorraine F. Francis

Biomaterials 2004 Volume 25(Issue 13) pp:2489-2500

Publication Date(Web):June 2004

DOI:10.1016/j.biomaterials.2003.09.033

Porous poly(l-lactide)/bioactive glass (PLLA/BG) composites were prepared by phase separation of polymer solutions containing bioactive glass particles (average particle size: 1.5 μm). The composite microstructures consist of a porous PLLA matrix with glass particles distributed homogeneously throughout. Large pores (>100 μm) are present in a network of smaller (<10 μm) interconnected pores. The porous microstructure of the composites was not significantly influenced by glass content (9 or 29 vol%), but silane pretreatment of the glass resulted in better glass incorporation in the matrix. Mechanical tests showed that an increase in glass content increased the elastic modulus of the composites, but decreased their tensile strength and break strain. Silane pretreatment enhanced the increase in modulus and prevented the decrease in tensile strength with increasing glass content. Composites soaked in simulated body fluid (SBF) at body temperature formed bone-like apatite inside and on their surfaces. The silane pretreatment of glass particles delayed the in vitro apatite formation. This bone-like apatite formation demonstrates the composites’ potential for integration with bone.

Electrical and optical properties of ceramic–polymer nanocomposite coatings

Co-reporter:Jiakuan Sun;William W. Gerberich

Journal of Polymer Science Part B: Polymer Physics 2003 Volume 41(Issue 14) pp:1744-1761

Publication Date(Web):11 JUN 2003

DOI:10.1002/polb.10532

Transparent, conductive composite coatings were fabricated from suspensions of poly(vinyl acetate-acrylic) (PVAc-co-acrylic) copolymer latices (50–600 nm) and nanosized antimony-doped tin oxide (ATO) particles (∼15 nm). The suspensions were deposited as coatings onto poly(ethylene terephthalate) substrates and dried at 50 °C. Microstructure studies using field emission scanning electron microscopy and tapping-mode atomic force microscopy (TMAFM) indicated that the latex particles coalesced during drying and forced the ATO particles to segregate into the boundaries between the latex particles. Low phase contrast was observed with TMAFM; this result was consistent with the presence of PVAc-co-acrylic in the ATO-rich phase of the composite. The conductivity of the composite coatings followed a percolation power-law equation, with the percolation threshold between 0.05 and 0.075 volume fractions of ATO and the critical conductivity exponent ranging from 1.34 to 2.32. The highest direct-current conductivity of the composite coatings was around 10⁻² S/cm. The optical transmittance and scattering behavior of the coatings were also investigated. Compared with the PVAc-co-acrylic coating, the composite coatings had lower transparency because of the Rayleigh scattering. The transparency of the composite coatings was improved by a reduction in the coating thickness. The best transparency for the coatings with a direct-current conductivity of approximately 10⁻² S/cm was around 85% at a wavelength of 600 nm. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1744–1761, 2003