Biocompatible and biodegradable polylactide (PLA) composites with supertough mechanical property and sufficient flame retardancy were fabricated by employing a facile approach involving reactive blending of PLA and ethylene-acrylic ester-glycidyl methacrylate terpolymer (EGMA), with the addition of aluminum hypophosphite (AHP) as an effective flame retardant. In consideration of the balance between mechanical property and flame retardancy, the optimal formula was taking a PLA/EGMA 80/20 blend (supertough STPLA) as the matrix and adding 20 wt % of AHP (relative to the mass of STPLA) as the flame retardant, coded as STPLA/20AHP. The mechanical property test showed that for STPLA/20AHP the elongation at break was increased by about 22 times and the notched Izod impact strength was enhanced by approximately 11 times as compared to those for neat PLA. The flame-retardant property test showed that for STPLA/20AHP the limiting oxygen index value reached 26.6% and the UL-94 V0 rating test was passed. Thermogravimetric analysis, microscale combustion calorimetry, and cone calorimeter were further applied to reveal the thermal stability and combustion behaviors of STPLA/xAHP, respectively, where x indicated the mass content of AHP in percentage. The phase separation morphology, dispersion of AHP particles in STPLA matrix, and fracture surfaces and char residues after flame burning were examined by phase contrast optical microscopy and scanning electron microscopy, respectively, which helped comprehend the results obtained from the mechanical property and flame retardancy tests. The supertough STPLA/xAHP, with sufficient flame retardancy as prepared in this work, could have a potential for engineering applications.Topics: Biodegradable materials; Biodegradable materials; Elastic materials; Fire-resistant materials; Mechanical properties; Polyesters; Polymer blends; Polymer morphology; Power; Thermal properties;

In this study, three typical impact-protective materials, D3O, PORON XRD, and DEFLEXION were chosen to explore the dependences of rheological and compression mechanical properties on the internal cellular structures with polymer matrix characteristics, which were examined using Fourier transform infrared spectroscopy, thermogravimetric analyses, and scanning electron microscopy with energy dispersive spectroscopy. The rheological property of these three foaming materials were examined using a rheometer, and the mechanical property in a compression mode was further examined using an Instron universal tensile testing machine. The dependences of rheological parameters, such as dynamic moduli, normalized moduli, and loss tangent, on angular frequency, and the dependences of mechanical properties in compression, such as the degree of strain-hardening, hysteresis, and elastic recovery, on the strain rate for D3O, PORON XRD, and DEFLEXION can be well-correlated with their internal cellular structural parameters, revealing, for example, that D3O and PORON XRD exhibit simultaneously high strength and great energy loss in a high-frequency impact, making them suitable for use as soft, close-fitting materials; however, DEFLEXION dissipates much energy whether it suffers a large strain rate or not, making it suitable for use as a high-risk impact-protective material. The rheometry and compression tests used in this study can provide the basic references for selecting and characterizing certain impact-protective materials for applications.Topics: Distribution function; Materials science; Mechanical properties; Solubility; Thermal properties;

A series of polyborosiloxanes (PBSs) was synthesized by mixing hydroxy-terminated polydimethylsiloxanes (PDMS) and boric acid (BA) in toluene at 120 °C. The molecular masses of selected PDMS precursors were in a wide range, covering from below up to far above the critical entanglement molecular mass of PDMS. The reaction kinetics was followed by using Fourier transform infrared (FTIR) spectroscopy. Unreacted BA was removed from raw PBSs after the reactions. The influence of molecular mass of PDMS precursors on the rheological property of PBSs was explored by dynamic oscillatory frequency sweeps. The results showed that the plateau elastic moduli of PBSs were highly dependent on the molecular mass of PDMS precursors. The plateau elastic moduli of PBSs decreased at first and then increased with increasing molecular mass of PDMS precursors. PBS1 and PBS2 prepared from unentangled PDMS precursors showed sufficient fits by using the two-mode Maxwell model, whereas PBS3 to PBS6 prepared from highly entangled PDMS precursors showed obvious deviations from the two-mode Maxwell model. It could be concluded that the changing trend of plateau elastic modulus of PBSs versus molecular mass of PDMS precursors was determined by the number density of supramolecular interactions (Si–O:B weak bonding and hydrogen-bonding of the end groups Si–O–B(OH)2) and the number density of topological entanglements.

Natural resilin possesses outstanding mechanical properties, such as high strain, low stiffness, and high resilience, which are difficult to be reproduced in synthetic materials. We designed high resilient elastomers (HREs) with a network structure to mimic natural resilin on the basis of two natural abundant polymers, stiff cellulose and flexible polyisoprene. With plasticization via mineral oil and mechanical cyclic tensile deformation processing, HREs show ultrahigh resilience, high strain, and reasonable tensile strength that closely mimic natural resilin. Moreover, the mechanical properties of HREs can be finely tuned by adjusting the cellulose content, providing the opportunity to synthesize high resilient elastomers that mimic different elastic proteins, such as elastin.

In this study a series of long chain branched polycarbonates (LCB-PCs) was prepared from linear PC precursors through gamma radiation with addition of a difunctional monomer, divinyl benzene (DVB) of varying amounts. The topological structures of the linear PCs and LCB-PCs were measured by size-exclusion chromatography coupled with a multiangle light scattering detector (SEC-MALLS) and rheology. SEC-MALLS measurements showed that the topological structures of the PCs were changed with the introduction of LCB structures. Rheological measurements showed that the LCB structures of the LCB-PCs contributed to enhancements of storage modulus, complex viscosity, shear-thinning behavior, and the deviation of phase angle, δ, from the “standard” curve of linear PCs in the δ − |G*| plots (van Gurp–Palmen plot). Modulated differential scanning calorimetry (MDSC) measurements showed that the glass transition temperature, Tg, of LCB-PCs decreases with increasing LCB degree, which is attributed to the combined effects of both increased branching density and free chain ends. LCB structures introduced into PCs did not apparently weaken the tensile properties of the PC materials, whereas the notched Izod impact strength could be obviously improved by the LCB structures. More importantly, the influence of LCB structures on the environmental stress cracking (ESC) behavior of LCB-PCs was explored and the results indicated that the ESC properties of LCB-PCs could be significantly enhanced with the introduction of LCB structures, which makes the application of LCB-PCs as the window view materials for helmets worn by astronauts while in outer space a possibility.

A series of asymmetric biodegradable poly(l-lactide) (PLLA)/poly(d-lactide) (PDLA) blends with low PDLA compositions was prepared using a solution blending method. The formation of stereocomplex (SC) crystallites in PLLA/PDLA blends was evidenced by differential scanning calorimetry (DSC), as indicated by the melting point of SC crystallites being about 50 °C higher than that of PLLA homocrystallites. Isothermal crystallization kinetics under shear conditions at the specific high temperature of 160 °C for the PLLA/PDLA blends was investigated using polarized optical microscopy (POM) and rheometry. It was found that the crystallization process of PLLA in the blends was greatly accelerated under shear conditions due to the existence of SC crystallites and the crystallization kinetics of PLLA was promoted with increasing shear rate or shear time. The crystalline morphology remained spherulitic with the spherulitic growth rates unaltered at the applied shear conditions, and the accelerated crystallization kinetics could be attributed to the significantly enhanced nucleation density, for which the extra number of activated nuclei was correlated to shear as a kinetic model to assess the effects of shear on isothermal crystallization kinetics of PLLA/PDLA blends containing SC crystallites. The discrete Maxwell relaxation time spectra at the applied isothermal crystallization temperature of 160 °C were used to obtain the reptation and Rouse times of PLLA chains with high molecular masses. Even though the PLLA chains might be orientated under the applied shear, the relaxation time of the blends was still too short to induce any orientated crystal nuclei.Keywords: Crystallization; Nucleation; Rheology; Shear; Spherulite; Stereocomplex

A significant enhancement in isothermal crystallization kinetics of biodegradable polylactide (PLA) in its immiscible blends can be accomplished through blending it with a comb-like copolymer. PLA was blended with poly(ethylene glycol) methyl ether acrylate (PEGA) and poly[poly(ethylene glycol) methyl ether acrylate] (PPEGA, a comb-like copolymer), respectively. The results measured from phase contrast optical microscopy (PCOM) and differential scanning calorimetry (DSC) indicate that PLA and PEGA components are miscible, whereas PLA and PPEGA components are immiscible. The study of crystallization kinetics for PLA/PEGA and PLA/PPEGA blends by means of polarized optical microscopy (POM) and DSC indicates that both PEGA and PPEGA significantly increase the PLA spherulitic growth rates, G, although PLA/PPEGA blends are immiscible and the glass transition temperatures of PLA only have slight decreases. PPEGA component enhances nucleation for PLA crystallization as compared with PEGA component owing to the heterogeneous nucleation effect of PPEGA at the low composition of 20 wt%, while PLA crystallization-induced phase separation for PLA/PEGA blend might cause further nucleation at the high composition of 50 wt%. DSC measurement further demonstrates that isothermal crystallization kinetics can be relatively more enhanced for PLA/PPEGA blends than for PLA/PEGA blends. The “abnormal” enhancement in G for PLA in its immiscible blends can be explained by local interfacial interactions through the densely grafted PEGA side chains in the comb-like PPEGA, even though the whole blend system (PLA/PPEGA blends) represents an immiscible one.

More dominant shear flow effect with different shear rates and shear time with assistance of added carbon nanotubes (CNTs) of low amounts on the crystallization kinetics of isotactic polypropylene (iPP) in CNT/iPP nanocomposites was investigated by applying differential scanning calorimetry (DSC), polarized optical microscopy (POM), and rheometer. CNTs were chemically modified to improve the dispersity in the iPP matrix. CNT/iPP nanocomposites with different CNT contents were prepared by solution blending method. The crystallization kinetics for CNT/iPP nanocomposites under the quiescent condition studied by DSC indicates that the addition of CNTs of low amounts significantly accelerates crystallization of iPP due to heterogeneous nucleating effect of CNTs, whereas a saturation effect exists at above a critical CNT content. The shear-induced crystallization behaviors for CNT/iPP nanocomposites studied by POM and rheometry demonstrate the continuously accelerated crystallization kinetics with assistance from added CNTs, with increasing CNT content, shear rate, and shear time, without any saturation effect. The changes of nucleation density for CNT/iPP nanocomposites under different shear conditions can be quantified by using a space-filling modeling from the rheological measurements, and the results illustrate that the combined effects of added CNTs and shear flow on the acceleration of crystallization kinetics are not additive, but synergetic. The mechanisms for the synergetic effect of added CNTs and shear flow are provided.Keywords: nucleation density; rheology; shear-induced crystallization; spherulitic growth rate

Composite thermoplastic elastomers (CTPEs) of magnetic copolymer-grafted nanoparticles (magnetite, Fe3O4) were synthesized and characterized to generate magnetic CTPEs, which combined the magnetic property of Fe3O4 nanoparticles and the thermoplastic elasticity of the grafted amorphous polymer matrix. Fe3O4 nanoparticles served as stiff, multiple physical cross-linking points homogeneously dispersed in the grafted poly(n-butyl acrylate-co-methyl methacrylate) rubbery matrix synthesized via the activators regenerated by electron transfer for atom transfer radical polymerization method (ARGET ATRP). The preparation technique for magnetic CTPEs opened a new route toward developing a wide spectrum of magnetic elastomeric materials with strongly enhanced macroscopic properties. Differential scanning calorimetry (DSC) was used to measure the glass transition temperatures, and thermogravimetric analysis (TGA) was used to examine thermal stabilities of these CTPEs. The magnetic property could be conveniently tuned by adjusting the content of Fe3O4 nanoparticles in CTPEs. Compared to their linear copolymers, these magnetic CTPEs showed significant increases in tensile strength and elastic recovery. In situ small-angle X-ray scattering measurement was conducted to reveal the microstructural evolution of CTPEs during tensile deformation.Keywords: ARGET ATRP; cross-linking; glass transition temperature; magnetite; network;

Nanostructured materials have attracted tremendous attention in past decades owning to their wide range of potential applications in many areas. In this study, novel conductive composite thermoplastic elastomers (CTPEs) were fabricated by using a copolymer-grafted multiwalled carbon nanotube (MWCNT) composite thermoplastic elastomer filled with varied amounts of unmodified MWCNTs as additional nanofillers. Rheological measurements and electrical conductivity tests were performed to investigate the viscoelasticity and electrical percolation behavior of these CTPEs, respectively. The incorporation of unmodified MWCNTs can significantly increase the electrical conductivity of these CTPEs, and the electrical conductivity percolation threshold was determined to be 0.34 wt %. The macroscopic mechanical properties of these CTPEs can be conveniently adjusted by the content of unmodified MWCNTs; for example, the strain-hardening behavior can be significantly enhanced with the incorporation of unmodified MWCNTs. This design concept can be generalized to other conductive composite elastomeric systems.

Human skin exhibits highly nonlinear elastic properties that are essential to its physiological functions. It is soft at low strain but stiff at high strain, thereby protecting internal organs and tissues from mechanical trauma. However, to date, the development of materials to mimic the unique mechanical properties of human skin is still a great challenge. Here we report a bioinspired design of nanostructured elastomers combining two abundant plant-based biopolymers, stiff cellulose and elastic polyisoprene (natural rubber), to mimic the mechanical properties of human skin. The nanostructured elastomers show highly nonlinear mechanical properties closely mimicking that of human skin. Importantly, the mechanical properties of these nanostructured elastomers can be tuned by adjusting cellulose content, providing the opportunity to synthesize materials that mimic the mechanical properties of different types of skins. Given the simplicity, efficiency, and tunability, this design may provide a promising strategy for creating artificial skin for both general mechanical and biomedical applications.Keywords: human skin; mechanical processing; mechanical property mimicking; microphase separation; multiphase polymer; nonlinear elasticity;

Supertough biocompatible and biodegradable polylactide materials were fabricated by applying a novel and facile method involving reactive blending of polylactide (PLA) and poly(ethylene glycol) diacylate (PEGDA) monomer with no addition of exogenous radical initiators. Torque analysis and FT-IR spectra confirm that cross-linking reaction of acylate groups occurs in the melt blending process according to the free radical polymerization mechanism. The results from differential scanning calorimetry, phase contrast optical microscopy and transmission electron microscopy indicate that the in situ polymerization of PEGDA leads to a phase separated morphology with cross-linked PEGDA (CPEGDA) as the dispersed particle phase domains and PLA matrix as the continuous phase, which leads to increasing viscosity and elasticity with increasing CPEGDA content and a rheological percolation CPEGDA content of 15 wt %. Mechanical properties of the PLA materials are improved significantly, for example, exhibiting improvements by a factor of 20 in tensile toughness and a factor of 26 in notched Izod impact strength at the optimum CPEGDA content. The improvement of toughness in PLA/CPEGDA blends is ascribed to the jointly contributions of crazing and shear yielding during deformation. The toughening strategy in fabricating supertoughened PLA materials in this work is accomplished using biocompatible PEG-based polymer as the toughening modifier with no toxic radical initiators involved in the processing, which has a potential for biomedical applications.Keywords: biodegradable; mechanical property; morphology; phase separation; poly(ethylene glycol) diacylate; toughening

In this work, we report an elegant design of novel graft copolymers based on two natural abundant biopolymers with opposite physical properties: rigid and hydrophilic cellulose, and flexible and hydrophobic synthesized polyisoprene (analogue of natural rubber), which combines rigidity and flexibility, hydrophobicity and hydrophilicity all in one macromolecule. These cellulose-graft-polyisoprene (Cell-g-PI) copolymers were synthesized via homogenous supplemental activator and reducing agent atom transfer radical polymerization (SARA ATRP). FT-IR, 1H NMR, 13C NMR and TGA measurements demonstrate that Cell-g-PI copolymers are successfully prepared. TEM and DMA results illustrate that phase separation occurs in Cell-g-PI copolymers. Water contact angle measurements verify that their hydrophobicity increases with increasing polyisoprene side chain length. In addition, the core–shell Cell-g-PI nanoparticles in water can be prepared via a self-organized precipitation (SORP) method.

To investigate the effects of long chain branching (LCB) on the crystallization kinetics of polylactides (PLA), a series of long chain branched PLA samples have been prepared, which have been proven to be bimodal of linear PLA and long chain branched PLA with different branching degrees. The crystallization kinetics of these LCB PLA samples have been investigated using polarized optical microscopy (POM) and differential scanning calorimetry (DSC). The POM results show that the spherulitic growth rates of the LCB PLA samples are lower than that of the linear PLA precursor and the PLA spherulitic growth rate decreases with increasing branching degree at each isothermal crystallization temperature. In contrast, the nucleation density increases with increasing branching degree. The crystallization kinetics from the POM observation are analyzed using the Hoffman–Lauritzen theory. In the studied temperature range, both linear PLA and LCB PLA samples crystallize according to the Regime II mechanism. The nucleation constant (Kg) and fold surface free energy (σe) decrease with increasing branching degree, suggesting that the LCB PLA samples have lower free energy barriers for nucleation than the linear PLA precursor. Analysis of the DSC data by the Avrami equation indicates that the crystallization of both the linear PLA and LCB PLA samples follows a three-dimensional crystal growth. The half crystallization time reduces with increasing branching degree and the overall crystallization rates for the LCB PLA samples are higher than that of linear PLA.

The formation of ring-banded spherulites in double-layer semicrystalline polymer films has seldom been a concern. In this study, the nucleation and growth of ring-banded spherulites in poly(ethylene oxide)/poly(L-lactide) (PEO/PLA) double-layer films during isothermal crystallization at various temperatures above the melting point of the PEO layer have been investigated by using polarized optical microscopy (POM). The sequential crystallization of PLA and PEO components in the double-layer films was confirmed by a double quench crystallization procedure. The surface morphologies of ring-banded spherulites were examined by using scanning electron microscopy. The experimental results are compared with those for PLA/PEO blend films with 25 wt% PEO composition. It is interesting to find that the changing trend of band space in ring-banded spherulites at different isothermal crystallization temperatures in PEO/PLA double-layer films is more complex than that in PLA/PEO blend films. For the former ones, the ring-banded spherulites form at the lowest temperature approaching 65 °C and the band space slightly increases from 65 to 90 °C, is absent from 90 to 105 °C, obviously decreases from 105 to 115 °C, and significantly increases from 115 to 135 °C. For the latter ones, the band space only shows a monotonic increase with increasing temperature at above 105 °C. The mechanism for the formation of ring-banded spherulites in the PEO/PLA double-layer film system is proposed. The experimental results may shed some light on explaining the formation mechanism of ring-banded spherulites in semicrystalline polymers.

Regeneration of activators by electron transfer for atom transfer radical polymerization (ARGET ATRP) was performed to synthesize hybrid materials with multi-walled carbon nanotubes (MWCNTs) grafted by in situ polymerized isoprene (PI, analogue of natural rubber). A series of MWCNT-graft-polyisoprene (MWCNT-g-PI) was prepared via this robust grafting strategy, and the successful grafting was confirmed by FT-IR, 1H NMR and TGA measurements. TGA curves show that MWCNT-g-PI samples have sufficient thermal stability. The increase in glass transition temperature for MWCNT-g-PI samples as compared with neat PI was characterized by DSC. TEM was used to observe the morphologies of MWCNT-g-PI. MWCNT-g-PI samples show much better dispersibility in various organic solvents than pristine MWCNTs. Water contact angle measurement indicates that the hydrophobicity of MWCNT-g-PI samples decreases with increasing chain length of the grafted PI.

Cellulose-graft-poly[poly(ethylene glycol) methyl ether acrylate] (Cell-g-PPEGA) and poly[poly(ethylene glycol) methyl ether acrylate] (PPEGA) comb-like copolymers were synthesized by electron transfer atom transfer radical polymerization (ARGET ATRP). PPEGA/PLA and Cell-g-PPEGA/PLA pairs are both immiscible as indicated by slightly decreasing glass transition temperatures of PLA in the blends measured by differential scanning calorimetry (DSC) and phase separation morphologies observed by a phase contrast optical microscope (PCOM). Cell-g-PPEGA/PLA and PPEGA/PLA double-layer films and neat PLA film were prepared to measure the nucleation and spherulitic growth rates during isothermal crystallization at various temperatures above the melting points of Cell-g-PPEGA and PPEGA layers by using a polarized optical microscope (POM). In contrast to our previous results on the miscible polymer pairs, covering the immiscible molten PPEGA layer can greatly accelerate the spherulitic growth rates for PLA and the Cell-g-PPEGA layer shows a similar but less significant effect. Nevertheless, nucleation density is much higher for the Cell-g-PPEGA/PLA film than for the PPEGA/PLA and neat PLA films. A significant finding is that although the whole double-layer film systems represent phase separated ones, the densely grafted PEGA chains in the comb-like copolymers can simultaneously take action to amplify the segmental mobility of PLA chains through local contacts at interfacial layers between phase separated domains, which significantly enhances the formation of chain folding lamellae of the PLA, resulting in obvious enhancements of the crystallization kinetics.

An easy procedure was applied to prepare high-melt-strength polylactide (PLA) that involves γ-radiation-induced free-radical reactions to introduce a long-chain branched structure onto a linear PLA precursor with addition of a trifunctional monomer, trimethylolpropane triacrylate (TMPTA). The results from size-exclusion chromatography coupled with multiangle laser light scattering (SEC-MALLS) detection indicate that the resultant long-chain branched PLA (LCB PLA) samples have an increased molecular mass and an elevated branching degree with increasing amount of TMPTA incorporated during the irradiation process. Various rheological plots including viscosity, storage modulus, loss tangent, Cole–Cole plots, and weighted relaxation spectra were used to distinguish the improved melt strength for LCB PLA samples. The effect of LCB structure on elongational rheological properties was further investigated. The LCB PLA samples exhibited an enhancement of strain-hardening under elongational flow. The enhanced melt strength substantially improved the foaming performance of the LCB PLA samples.

Considering that multiwalled carbon nanotubes (MWCNTs) can be used as anisotropic and stiff nano-objects acting as minority physical cross-linking points dispersed in soft polymer grafting matrixes, a series of copolymer-grafted multiwalled carbon nanotube composite thermoplastic elastomers (CTPEs), MWCNT-graft-poly(n-butyl acrylate-co-methyl methacrylate) [MWCNT-g-P(BA-co-MMA)], with minor MWCNT contents of 1.2–3.8 wt % was synthesized by the surface-initiated activators regenerated by electron transfer for atom-transfer radical polymerization (ARGET ATRP) method. Excellent dispersion of the MWCNTs in the CTPEs was demonstrated by SEM and TEM, and the thermal stability properties and glass transition temperatures of the CTPEs were characterized by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), respectively. Mechanical property test results demonstrated that the CTPEs exhibit obviously enhanced mechanical properties, such as higher tensile strength and elastic recovery, as compared with their linear P(BA-co-MMA) copolymer counterparts. The microstructural evolutions in the CTPEs during tensile deformation as investigated by in situ small-angle X-ray scattering (SAXS) revealed the role of the MWCNTs, which can provide additional cross-linking points and transform soft elastomers into strong ones.

In this study a unique phenomenon has been found for isothermal crystallization of double-layer semicrystalline polymer films. It is surprisingly found that there exists a speeding of poly(l-lactic acid) (PLA) spherulitic growth rate for poly(ethylene oxide)/poly(l-lactic acid) (PEO/PLA) double-layer films at the late stage of isothermal crystallization, which does not exist for PLA/PEO blend films and neat PLA films. The mutual diffusion between PEO and PLA layers plays the key factor to bring out the observed speeding of spherulitic growth rate. This type of study provides an avenue for understanding the interplay between polymer crystallization and interfacial diffusion in multilayer polymer films, which is not available when employing the polymer blend films.

An easy procedure to introduce a photocrosslinked interface for polypropylene (PP)/maleic anhydride grafted styrene-b-(ethylene-co-butylene)-b-styrene triblock copolymer (mSEBS) thermoplastic/elastomer blends by UV radiation is reported. This procedure involves free-radical crosslinking at the interface between the mSEBS domain phase and the PP matrix phase and in the mSEBS phase domains with benzophenone (BP) as the photoinitiator and triallyl isocyanurate (TAIC) as the crosslinking agent. Compared with the absence of photocrosslinking, the structured PP/mSEBS blends with photocrosslinked interfaces confirmed by rheological measurements demonstrate simultaneous enhancements of toughness and tensile strength because of the enhanced interfacial adhesion and the more rigid mSEBS particle-like phase domains due to photocrosslinking. The tensile strength for the PP/mSEBS thermoplastic/elastomer blends with photocrosslinking increases with increasing mSEBS contents even at high mSEBS contents, and eventually approaches that for neat PP. Furthermore, the ultimate elongation at break increases with increasing crosshead speed, opposite to the general observation in that polymers tend to become brittle with increasing crosshead speed, indicating an enhanced tensile resistance for the UV radiation treated PP/mSEBS thermoplastic/elastomer blends. The procedure reported here is promising for preparing high-performance polymer blends used in high impact fields. For the purpose of comparison, PP/ethylene-propylene-diene rubber (PP/EPDM) blends were subjected to the same studies, however, the above observed simultaneous enhancements of toughness and tensile strength for the PP/mSEBS blends are completely absent for the PP/EPDM blends, simply because the UV radiation-induced interfacial crosslinking between the dispersed EPDM phase domains and the PP matrix phase does not occur.

Both the nonisothermal and isothermal crystallization kinetics under the influences of different shear conditions for poly(lactic acid) (PLA) were investigated by rheometry. The nucleation and growth of PLA spherulites during isothermal crystallization with different shear conditions were observed by polarized optical microscopy (POM). Shear-induced nucleation rate enhancements of PLA were studied on the basis of the prerequisite determination of the critical shear rates, for which the stretch of the longest chains (high molecular mass tails) of PLA would be expected. The transitions between different shear flow regimes for shear-induced crystallization of PLA at the temperature of 135 °C were determined by two characteristic Weissenberg numbers on the basis of reptation time and Rouse time for the high molecular mass tails, which were determined through combination of the discrete Maxwell relaxation time spectra of PLA at the reference temperature of 190 °C and the Arrhenius type of temperature dependence for the horizontal shift factor, aT. It was then found that the crystallization process of PLA was greatly enhanced by shear compared to the quiescent condition, and the crystallization kinetics could be accelerated by the increased shear rate and/or shear time. It was more interesting to find that there existed a critical shear time under a certain shear rate, and a further increase in the shear time did not lead to further acceleration of the crystallization kinetics. POM observation indicated that the acceleration of crystallization kinetics was obviously brought about by the enhanced nucleus density under the application of shear and the subsequent spherulitic growth rates kept about constant. Thus, a kinetic model based on directly relating the extra number of activated nuclei promoted by shear to the shear rate was further applied to well predict the effects of shear time on the shear-induced isothermal crystallization kinetics of PLA.Keywords: Crystallization; Nucleation; Poly(lactic acid); Rheology; Shear; Spherulite

The spherulitic growth rates for the lower polylactide (PLA) layer in poly(ε-caprolactone)/polylactide (PCL/PLA), poly(ethylene oxide)/polylactide (PEO/PLA), and poly(ethylene glycol)/polylactide (PEG/PLA) double-layer films during isothermal crystallization at various temperatures above the melting points of PCL, PEO and PEG layers have been measured by using polarized optical microscopy (POM), with the particular results compared with those for neat PLA films. The PCL/PLA, PEO/PLA and PEG/PLA double-layer films were in situ prepared by covering PCL, PEO and PEG films, respectively on PLA films at 180 °C and holding for 5 min before quenching to isothermal crystallization temperatures for POM observations. It is interesting to find that the covering molten PEG layer can greatly accelerate the spherulitic growth rates for the lower PLA layer and a PEO layer shows a similar effect for PEO/PLA double-layer films. However, the effect of the PEO layer is weaker than that of PEG layer for acceleration of PLA spherulitic growth rates in the double-layer films. Covering a molten PCL layer on a PLA layer can only slightly increase the spherulitic growth rate of PLA. Different spherulitic morphologies in the neat PLA film and PCL/PLA, PEO/PLA and PEG/PLA double-layer films can be observed. The miscibility of these polymer pairs was investigated by using differential scanning calorimetry (DSC) and phase contrast optical microscopy (PCOM), which is proposed to play a key role for the observed accelerating effects.

Spherulites of microcrystalline cellulose (MCC) have been prepared by using the vapor precipitation procedure with proper humidity at various temperatures from concentrated microcrystalline cellulose/1-allyl-3-methylimidazolium chloride (MCC/AMIMCl) solutions, for which AMIMCl is an ionic liquid, a good solvent for dissolving MCC. Four different types of MCC spherulites have been investigated by using polarizing optical microscopy (POM), scanning electron microscopy (SEM) and wide-angle X-ray diffraction (WAXD) technique. POM observations reveal four types of MCC spherulites, i.e., negative and positive banded spherulites, and negative and positive non-banded spherulites, depending on MCC concentration and crystallization temperature (Tc). For banded spherulites, both the band spacing and sizes of spherulites evidently increase with increasing Tc for each MCC/AMIMCl solution. The sizes of spherulites increase with increasing MCC concentration at a given Tc. The findings imply that MCC concentration plays a key role in MCC chain reorganizations into positive or negative spherulites, while the crystallization temperature mainly affects the MCC crystalline lamellar twisting for the formation of banded spherulites. SEM observation reveals that the formation of negative and positive banded spherulites is due to different lamellar twisting directions and the formation of non-banded spherulites is due to the formed radiating fibrillar textures. WAXD profiles confirm that all the four types of MCC spherulites formed are in the crystalline form of cellulose II family, exhibiting more intense and sharper diffraction peaks than those of cellulose II family obtained by the dissolution/precipitation process.

Novel bionanocomposite ionogels consisting of an ionic liquid (IL) 1-ethyl-3-methylimidazolium acetate (EMIMAc), microcrystalline cellulose (MCC) and nano-silica (nano-SiO2) particles with high tensile strength and high ionic conductivity have been successfully prepared. Dynamic light scattering (DLS) and transmission electron microscopy (TEM) measurements reveal a homogeneous dispersion of nano-SiO2 in the MCC/nano-SiO2/EMIMAc bionanocomposite ionogels. In order to clarify the influences of added nano-SiO2 on the sol–gel transition process and liquid crystalline phase transition for the MCC/nano-SiO2/EMIMAc systems, the complexes were investigated by dynamic rheological measurements, mechanical tensile property tests and polarized optical microscope (POM) observations. The rheological results indicate that the introduction of nano-SiO2 can induce and accelerate the gelation for the MCC/nano-SiO2/EMIMAc solutions. By adjusting the MCC and nano-SiO2 concentrations, the gel-sol transition temperature and elastic modulus can be well controlled and the optimized values reach 125 °C and 7 × 105 Pa, respectively. The POM results reveal that the addition of nano-SiO2 significantly suppresses the liquid crystalline behavior of ionogels. A more significant result is that the bionanocomposite ionogels exhibit high ionic conductivity in the order of 10−3 S cm−1 at 30 °C. The ionic conductivity of the ionogels increases with increasing temperature and decreasing MCC concentration. The above results demonstrate that the novel bionanocomposite ionogels with high tensile strength are promising for the application as gel polymer electrolytes (GPE) in electrochemical devices.

A series of long-chain branched polylactides (LCB-PLAs) were prepared from a linear PLA precursor by applying gamma radiation with addition of the trifunctional monomer, trimethylolpropane triacrylate (TMPTA) and their topological structures were investigated by size-exclusion chromatography coupled with a multiangle light scattering detector (SEC-MALLS) and rheology. SEC-MALLS measurements show that LCB-PLAs exhibit not only the increased weight-average molecular mass but also a bimodal architecture with a short linear chain fraction and a LCB fraction, as compared with LCB-PLA samples generated by electron-beam radiation, which only show the monomodal macromolecular structure. The introduction of LCB structure contributes to the enhancement of complex viscosity, shear-thinning and storage modulus, and the deviations of phase angle, δ from the “universal” curve of linear PLA in δ(|G*|) plots (van Gurp–Palmen plot). By the analysis of the thermorheological behaviors and determination of activation energies, the bimodal architecture is confirmed. Activation energies around 77 kJ mol−1 lead to an expectation that some linear macromolecules are still present in LCB-PLAs, while the higher activation energies at large phase angles are associated with the existence of the LCB ones. A conclusion with respect to the tree-like topography for LCB-PLAs is drawn from the molecular mass dependences of zero-shear viscosity (η0–Mw plot). An explanation to these findings is provided under the consideration of the radiation dose rate for the gamma radiation. The remarkable modification of PLA topological chain structure contributes to the improvement of the foaming properties of LCB-PLAs.

Thermoplastic elastomers (TPEs) are ever sought using a simple robust synthetic approach. Widely successful first-generation TPEs rely on microphase-separated ABA triblock copolymers (Architecture I). Recent multigraft copolymers represent the second-generation TPEs in which multiple branched rigid segments are dispersed in a rubbery backbone matrix (Architecture II). This paper reports our discovery of the third-generation TPEs that are based on rigid backbone dispersed in a soft grafted matrix. This Architecture III allows the use of random copolymers as side chains to access a wide spectrum of TPEs that cannot be achieved by architecture designs of the first two generations. In this report, random copolymer-grafted cellulose, cellulose-graft-poly(n-butyl acrylate-co-methyl methacrylate) copolymers with only 0.9–3.4 wt % cellulose prepared by activators regenerated by electron transfer for atom transfer radical polymerization (ARGET ATRP), as novel thermoplastic elastomers are investigated.

The effects of long chain branching on the nucleation density enhancements and morphological evolution for polylactide (PLA) materials during shear-induced isothermal crystallization process were thoroughly investigated by using rotational rheometer and polarized optical microscopy (POM). Shear-induced nucleation density enhancements for the long chain branched PLA (LCB PLA) were studied on the basis of the determination of the critical shear rate, for which the stretch of the longest chains of the linear component is expected. The results of shear-induced isothermal crystallization kinetics show that the crystallization process under shear is greatly enhanced compared to the quiescent conditions and the crystallization kinetics is accelerated with the increases in shear rate and/or shear time. LCB PLA crystallizes much faster than linear PLA under the same shear condition. A saturation effect of shear time on crystallization kinetics is observed for both linear PLA and LCB PLA. In-situ POM observations demonstrate that LCB PLA not only possesses higher nucleation density under the identical shear time and a constant lower value of spherulitic growth rate compared with that of linear PLA but also forms the shish-kebab structure after sheared for sufficient time. The quantitative evaluation of the shear-induced nucleation densities from rheological measurements is based on the space-filling model by using the Avrami equation, and the obtained nucleation density values are well consistent with that estimated from POM observations. A saturation of nucleation density under shear can be reached for both linear PLA and LCB PLA. The saturated nucleation density values are higher than that under the quiescent condition by a factor of over 3 orders of magnitude, and the saturated nucleation density value for LCB PLA is more than that for linear PLA by a factor of 1 order of magnitude under the same shear condition. The enhancement of nucleation ability and the morphological evolution from the spherulitic to shish-kebab structures induced by shear flow can be ascribed to the broadened and complex relaxation behaviors of LCB PLA.

The influence of a molten liquid polymer layer on the crystallization of the beneath semicrystalline polymer has been seldom considered. In the study, the nucleation and growth of spherulites for the beneath polylactide (PLA) layer in poly(ethylene oxide)/polylactide (PEO/PLA) double-layer films during isothermal crystallization at various temperatures above the melting point of PEO have been investigated by using polarized optical microscopy, with the particular results compared with that for neat PLA and PLA/PEO blend films. It is interesting to find that the top covering molten PEO layer can greatly accelerate the spherulitic growth rate (G) of the beneath PLA layer. Another significant result is that the temperature for the measurable nucleation and spherulitic growth of PLA in the double-layer films can be eventually pushed down close to the glass transition temperature of neat PLA. The changes of glass transition temperature, Tg, for PEO/PLA multilayer films have been measured by using modulated differential scanning calorimetry and dynamic mechanical analysis, which reveal slight decreases of Tg for PLA layer due to the influence of PEO layer. The layer structures of fractured surface of the double-layer films are analyzed on the basis of the observation from scanning electron microscopy, and the existence of interdiffusion areas with irregular boundary between PEO and PLA layers is the key clue to understanding the significant acceleration of G for PLA. The layer-by-layer film method infers promising applications, which might be considered to well replace the blending method.

In this work the nucleation and growth of spherulites for the below polylactide (PLA) layer in poly(ɛ-caprolactone)/polylactide (PCL/PLA) double-layer films during isothermal crystallization at various temperatures above the melting point of PCL have been investigated by using polarized optical microscopy (POM). It is revealed that two types of spherulitic morphologies are observed in PCL/PLA double-layer films. One is the well defined highly birefringent spherulites, and the other one is the coarse spherulites. It is interesting to find that the spherulitic growth rate of the coarse spherulites is higher than that of the well defined spherulites. It is thought that the coarse spherulites nucleate and grow with the assistance of the interfaces between the PCL and PLA layers, and the well defined highly birefringent spherulites only nucleate and grow in the PLA layer.

The liquid crystalline phase behavior and sol–gel transition in halloysite nanotubes (HNTs) aqueous dispersions have been investigated by applying polarized optical microscopy (POM), macroscopic observation, rheometer, small-angle X-ray scattering, scanning electron microscopy, and transmission electron microscopy. The liquid crystalline phase starts to form at the HNT concentration of 1 wt %, and a full liquid crystalline phase forms at the HNT concentration of 25 wt % as observed by POM and macroscopic observation. Rheological measurements indicate a typical shear flow behavior for the HNT aqueous dispersions with concentrations above 20 wt % and further confirm that the sol–gel transition occurs at the HNT concentration of 37 wt %. Furthermore, the HNT aqueous dispersions exhibit pH-induced gelation with more intense birefringence when hydrochloric acid (HCl) is added. The above findings shed light on the phase behaviors of diversely topological HNTs and lay the foundation for fabrication of the long-range ordered nano-objects.

Acid-oxidized multiwalled carbon nanotubes (A-MWCNTs) with a range of reduced aspect ratios (from about 11 to 5.8) were obtained by acid oxidization of MWCNTs in the mixture of HNO3 and H2SO4 for varying periods of 1, 3, 8 and 12 h, respectively. The aspect ratios and surface functionalization of A-MWCNTs were well characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy and thermogravimetric analysis (TGA). Poly(L-lactide)/A-MWCNT composites containing 0.5 wt% A-MWCNTs with a range of reduced aspect ratios were prepared by solution cast. The effects of added A-MWCNTs on the isothermal crystallization kinetics of poly(L-lactide)/A-MWCNT composites were investigated by means of differential scanning calorimetry (DSC), rheology and polarized optical microscopy (POM). It is surprising to find that not only the addition of A-MWCNTs effectively increases the poly(L-lactide) (PLA) crystallization kinetics, but also the nucleation ability of A-MWCNTs for PLA crystallization exponentially increases with the reduced aspect ratio, that is to say, those with lower aspect ratios show much stronger nucleation ability for PLA crystallization than those with higher aspect ratios. The exponentially increased nucleation ability of A-MWCNTs with a range of reduced aspect ratios for PLA crystallization is disclosed.

Thermal annealing-induced enhancements of electrical conductivities at the temperature higher than the melting point of poly(ethylene-co-hexene) matrix for multiwalled carbon nanotubes filled poly(ethylene-co-hexene) (MWCNTs/PEH) composites were investigated by electrical conductivity measurements. Two types of MWCNTs with low and high aspect ratios (4 and 31) were added as fillers into PEH matrix, respectively for comparison study purpose. The morphological changes due to annealing for MWCNTs/PEH composites were observed by SEM. The formation of MWCNT networks in the composites were clearly demonstrated by rheological measurements. It is surprisingly found that the electrical conductivity for MWCNTs/PEH composites with high MWCNT concentrations increases obviously with annealing time of 40 min and the maximum increment approaches about 3 orders of magnitude with annealing time of 120 min. The increase of electrical conductivity of MWCNTs/PEH composites depends on MWCNT content, MWCNT aspect ratio and annealing time. SEM results clearly reveal that micrometer-sized MWCNT aggregates are broken down and more loosely packed MWCNT networks form due to annealing. Different types of networks in the composites are responsible for the evolutions of rheological (MWCNT network and PEH chain-MWCNT combined network) and electrical conductivity properties (tube–tube contacting MWCNT network). The reconstruction of MWCNT network during annealing is attributed to rotational diffusion of MWCNTs in PEH matrix at high temperature and the length of MWCNTs shows significant effect on this. The obvious enhancements of electrical conductivities can be ascribed to the thermal annealing-induced formation of loosely packed more homogeneous networks through non-Brownian motions.Keywords: aspect ratio; carbon nanotube; diffusion; electrical conductivity; network; polyethylene;

The influence of the addition of low amounts of ultrahigh-molecular-weight polyethylene (UHMWPE) on the crystallization kinetics of isotactic polypropylene (iPP) in iPP/UHMWPE blends has been investigated by means of differential scanning calorimetry (DSC) and polarized optical microscopy. During the nonisothermal crystallization process, the primarily formed UHMWPE crystals serve as heterogeneous nucleating agents for iPP nucleation, whereas during the isothermal crystallization process, UHMWPE is in the molten state, iPP nucleation preferentially occurs at the UHMWPE and iPP phase interfaces, and the spherulitic growth rates are not obviously affected. It is particularly interesting to find a critical UHMWPE content (2.5 wt %) in the blends to induce the highest iPP nucleation rate; however, above the critical UHMWPE content, the iPP nucleation rate slows because of aggregation of the UHMWPE component. A delicately designed DSC measurement provides insight into the nucleation mechanism of iPP at the interfaces between the UHMWPE and iPP phase domains. It is proposed that the concentration fluctuations generated from the unstable inhomogeneous phase interfaces in the iPP/UHMWPE blends promote the formation of nuclei, which eventually enhances the nucleation and overall crystallization rates of the iPP component.

Time-resolved simultaneous synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) technique was used to investigate the phase transitions in prequenched mesomorphic isotactic polypropylene (iPP) samples during heating and annealing processes, respectively. For the heating process, it is shown that the mesomorphic-to-monoclinic phase transition is relatively faster for the mesomorphic iPP sample obtained with the high quenching rate than that with the low quenching rate. For the former, the stability of α-monoclinic crystals formed during heating is relatively higher. As for the annealing process, WAXD and SAXS data illustrate that the higher the annealing temperature (Ta), the earlier the mesomorphic-to-monoclinic phase transition occurs. Namely, Ta controls the phase transition rate. Both heating and annealing processes show that the increase of content of α-monoclinic crystal phase is mainly at the expense of the mesomorphic phase, with the content of amorphous phase almost invariable. The isothermal crystallization kinetics for the prequenched mesomorphic iPP sample was analyzed through the Avrami equation, revealing a two-dimensional crystal growth under the diffusion-limited mechanism.

Biodegradable polylactide (PLA) composites added with acid oxidized multiwalled carbon nanotubes (A-MWCNTs) of two different aspect ratios (length to diameter) were prepared by coagulation. The aspect ratios and surface structures of A-MWCNTs were characterized by TGA, Raman, and SEM measurements. The percolation thresholds for gelation in the PLA composites with A-MWCNTs of large and small aspect ratios are 2.5 and 4.0 wt %, respectively, which were determined by a rheological method, and in turn, the rheological result confirms the aspect ratio differences for the added two types of A-MWCNTs in the composites. Isothermal crystallization kinetics of neat PLA and its composites were further investigated by using polarized optical microscope (POM) and differential scanning calorimetry (DSC) to clarify the effects of A-MWCNTs of different aspect ratios and concentrations. The different aspect ratio A-MWCNTs with the same carboxyl group mass percent show substantial effects on PLA crystallization kinetics. Those with smaller aspect ratios enhance nucleation rate for PLA spherulites much more than those with larger aspect ratios. This phenomenon can be attributed to fewer sidewall carboxyl groups on the surfaces of A-MWCNTs with smaller aspect ratios, which provides more nucleation sites for PLA crystallization than those with larger aspect ratios at the same concentration, resulting in faster PLA nucleation rates for the former one.Keywords: aspect ratio; carbon nanotube; crystallization; polylactide;

Composites consisting of polylactide (PLA) and poly(ε-caprolactone) (PCL) filled with acid-oxidized multiwalled carbon nanotubes (A-MWCNTs) were prepared through melt compounding. Phase morphologies of PLA/PCL/A-MWCNT composites with different contents of filled A-MWCNTs and PCL compositions were mainly observed by scanning electron microscope. The results show that A-MWCNTs are selectively dispersed in the PCL phase, regardingless of PCL phase domain sizes. For PLA/PCL/A-MWCNT composites with fixed PLA/PCL ratio of 95/5, the dispersed PCL phase domain sizes in the PLA matrix decrease even though a small content of A-MWCNTs is added, compared with PLA/PCL blend with the same composition, indicating that A-MWCNTs effectively prevent from coalescence of the dispersed PCL phase domains. With filling of 1.0 wt % A-MWCNTs, an interesting change of electrical conductivity for PLA/PCL/A-MWCNT composites is observed, in which the maximum conductivity is observed for PLA/PCL/A-MWCNT composite with PLA/PCL ratio of 60/40. The result is well-explained by the formed cocontinuous phase morphology and effective A-MWCNT content.Keywords: composites; electrical conductivity; multiwalled carbon nanotubes; phase behavior; polylactide;

The isothermal crystallization of isotactic polypropylene (iPP) spherulites was observed under chain entanglements, reduced chain entanglements, and unconfined thick film conditions using polarized optical microscopy (POM). While spherulite translations and rotations were not observed in entangled iPP samples, the growing spherulites in the entanglements-reduced crystallizing melt conspicuously translated and/or rotated, even when their sizes exceeded the sample film thicknesses. Our entanglements-reduced samples were prepared by first slowly crystallizing a commercial entangled iPP in mineral oil (and then extracting the mineral oil with hexane) and by a direct polymerization method, respectively. The purity of the entanglements-reduced iPP samples and the extent of chain entanglements were examined by thermogravimetric analysis (TGA), gel permeation chromatography (GPC), and rheological measurements. Our findings seem to provide new clues into the crystallization of high molecular mass polymers.

Liquid crystalline (LC) phase transition and gel−sol transition in the solutions of microcrystalline cellulose (MCC) and ionic liquid (1-ethyl-3-methylimidazolium acetate, EMIMAc) have been investigated through a combination of polarized optical microscope (POM) observation and rheological measurements. Molecular LC phase forms at the 10 wt % cellulose concentration, as observed by POM, whereas the critical gel point is 12.5 wt % by rheological measurements according to the Winter and Chambon theory, for which the loss tangent, tan δ, shows frequency independence. Dramatic decreases of G′ and G′′ in the phase transition temperature range during temperature sweep are observed due to disassembling of the LC domain junctions. The phase diagram describing the LC phase and gel−sol transitions is obtained and the associated mechanisms are elucidated. A significant feature shown in the phase diagram is the presence of a narrow lyotropic LC solution region, which potentially has a great importance for the cellulose fiber wet spinning.

The phase transition and rheological behaviors of concentrated solutions of microcrystalline cellulose (MCC) in an ionic liquid of 1-allyl-3-methylimidazolium chloride (AMIMCl) have been investigated. Polarized optical microscopy (POM) measurements indicate that the two critical cellulose concentrations for the appearance of biphase and fully anisotropic phase for MCC/AMIMCl solutions are 9 and 16 wt%, respectively. POM and differential scanning calorimetry (DSC) measurements coherently indicate that the clearing temperature, Tc increases with increasing cellulose concentration. Oscillatory shear measurements show that the crossover frequency first moves to lower values and then moves back to higher values with increasing cellulose concentration, which indicates that most cellulose chains are aligned or oriented to reduce chain entanglements when the cellulose concentration is above 14 wt%. From the steady shear measurements, it is surprising to find that the viscosity versus shear rate curves exhibit four flow regions including two plateaus and two shear-thinning regions when the cellulose concentration exceeds 9 wt%. The influences of cellulose concentration and temperature on the first normal stress differences (N1) are analyzed according to the Larson theory. The peak of N1 always appears at the intermediate part of the first shear-thinning region, and the following minimum of N1 appears at the onset of the second shear-thinning region. The viscosity versus shear rate curves only exhibit two flow regions when temperature is above the respective Tc; meanwhile, negative N1 values disappear and N1 increases monotonically. The above results suggest that melting of the liquid crystal domains at high temperature results in the disappearance of the second plateau for the viscosity versus shear rate curves.

Using oscillation mode of rheology and theoretical calculation, we have observed for the first time the crossover of mutual diffusion coefficient, Dm, from high to low temperatures at the multiple layers interface of polymer films. A model which reflects a more realistic terminal state has been proposed to fairly fit the experimental data, by which the mutual diffusion coefficient Dm can be determined. It is substantially found that the diffusion keeps proceeding for the multilayer system at the temperature lower than the critical temperature due to the requirement of a period of time for binodal compositions to reach. Moreover, it is found that the apparent activation energy, Ed, derived from the Arrhenius relation of Dm versus 1/T, increases surprisingly when the welding temperature is below 150 °C, which relates closely to the effects of the phase behavior occurring in the two-phase region of the blend.