Morphology of a hydrogen-bonded LC polymer prepared by photopolymerization-induced phase separation under an isotropic phase

A hydrogen-bonded LC polymer was prepared by photopolymerization of an LC blend composed of 4-(6-acryloyloxyhexyloxy)benzoic acid (A6OBA) and 4-hexyloxy-4′-cyanobiphenyl (6OCB), containing small amounts of an inhibitor and photoinitiator, at two different temperatures in an isotropic phase. To elucidate the factors determining the morphology of the obtained polymer (poly(A6OBA)), we chose two irradiation temperatures: one in the LC temperature range of the polymer, the other in the isotropic range. We investigated structures of the polymers by optical microscopy and scanning electron microscopy. SEM images showed that the film obtained at the lower temperature consisted of randomly extended fibers having a diameter of ca. 1.0 μm and some branches, whereas the film prepared at the higher temperature was composed of polymer particles with a diameter ca. 1.5 μm. By comparing these results with those of an earlier experiment in which we obtained macroscopically oriented LC fibers by photopolymerization under the LC phase of the blend, we infer the following; (i) the presence of an LC phase in the resulting polymer itself during photopolymerization is necessary for it to form fibrous morphology and (ii) the LC ordering field present prior to photopolymerization is not indispensable for the fibrous morphology but it is for the macroscopic orientation and reduction of the branches in the fibers.     

Influence of sample thickness on fracture behaviour of a polyketone and a polyketone-rubber blend

The influence of sample thickness on the fracture behaviour of an aliphatic polyketone and a blend of this polymer and 10 wt% core-shell rubber was studied. The sample thickness was varied from 0.1 to 8 mm. The skin morphology was studied by SEM. The fracture behaviour was studied on single edge notch specimen at a high strain rate (30 s⁻¹) in the temperature range of −40 to 120 °C. The fracture stress, fracture strain and fracture energies were determined. The temperature development in the notch area was followed with an Infra Red camera. The cavitation of the rubber particles was studied on tensile bars with a laser setup.With decreasing specimen thickness the fracture energies increased strongly and the brittle-ductile transition shifted to lower temperatures this both for the aliphatic polyketone and the polyketone-rubber blend. The deformation in these materials in accompanied with a strong temperature increase in the deformation zone. The addition of rubber particles decreases the sensitivity towards the thickness. However, in very thin samples the cavitation of the rubber particles is more difficult and the rubber toughening effect decreases. The strong thickness effects on the fracture toughness indicate for both the homo polymer and the blend indicate that data from a standard test with 4 mm thick samples are not representative for thin walled applications.     

Influence of temperature-induced coalescence effects on co-continuous morphology in poly(ε-caprolactone)/polystyrene blends

This paper demonstrates that temperature-induced coalescence effects during melt mixing have a major influence on the concentration range of dual-phase continuity and an order of magnitude effect on the co-continuous microstructure phase size. A detailed study on the effect of temperature of blending on the morphology of immiscible poly(ε-caprolactone)/polystyrene blends is presented. Polycaprolactone (PCL) and polystyrene (PS) are blended in a batch mixer at 50 and 5 min for various temperatures. The continuity of the phases is obtained by selective extraction of each phase and the microstructure is analyzed using image analysis on SEM micrographs and mercury intrusion porosimetry. It is observed that the blending temperature has only a small effect on the morphology up to a PS or PCL composition of about 20 or 30%. However, beyond that composition the effect is dramatic. The microstructure of the 50/50 blend demonstrates a phase size (d_v) of 8.5 μm at 230 °C and 1.1 μm at 155 °C. Furthermore, the concentration range of co-continuity is broadened from 50-65%PS at 230 °C to 30-70%PS at 155 °C. The results at lower concentrations indicate that the temperature has little effect on the overall deformation/disintegration process, which appears to be due to compensating effects. For example, for PS in PCL, shear stress increases significantly at lower temperatures, but is counterbalanced by an increase in the viscosity ratio, elasticity of the phases and an increase in interfacial tension. Beyond a volume fraction of 0.30, the effect of temperature on coalescence plays a dominant role on the final morphology. It is shown in this paper that the observed morphology effects are controlled by the merging stage of coalescence. The data indicate the significant potential of mixing temperature as a tool for the morphology control of co-continuous polymer blends.     

Polyimide blend alignment layers for control of liquid crystal pretilt angle through baking

Polyimide films were used for liquid crystal (LC) alignment layers to control LC pretilt angles over the full range (8°-90°). The pretilt angles could be controlled using polyimide films prepared from polyamic acid for vertical LC alignment and using polyimide blend films prepared from two types of polyamic acids, one for vertical LC alignment and the other for planar LC alignment, by changing the baking times ranging from 40 to 180 min at 230 °C. The polyimide blend film could control the pretilt angle better than the polyimide prepared from just one polymer component. The LC alignment behavior was well correlated with the wettability of the polyimide films due to the fragmentation of the long alkyl side group on the polyimide surfaces by the baking process.     

Preparation of physical gel consisting of syndiotactic polystyrene and poly(ethylene glycol)

Syndiotactic polystyrene (s-PS) was blended with poly(ethylene glycol) (PEG) in 1,2-dichloroethane (DCE) solvent. The mixture became a homogeneous solution at 155 °C depending on the composition ratio of PEG to DCE. When the solution was cooled at the rate of 5 °C/min to room temperature, a thermoreversible gelation was occurred. Wide angle X-ray diffraction (WAXD) measurements revealed that the polymer chain of s-PS in the obtained gel was crystallized with a helical conformation, while that in the non-gelated sample was done with an all-trans planar zigzag conformation.After drying gelated samples at 70 °C for 24 h, a novel polymer blend type of the physical gel consisting of s-PS and PEG was obtained. Dynamic mechanical analysis (DMA) revealed that the physical gel had a high modulus and a long elastic plateau in the temperature range of −80-270 °C.     

Ductile-brittle transition controlled by isothermal crystallization of isotactic polypropylene and its blend with poly(ethylene-co-octene)

The correlation between crystalline morphology development and tensile properties of isotactic polypropylene (iPP) and its blend with poly(ethylene-co-octene) (PEOc) was investigated to study the ductile-brittle transition (DBT) in fracture modes. The sample processing strategy and the scientific observations have never been reported previously. The samples were first isothermally crystallized at 130 °C, 123 °C or 115 °C for a wide range of crystallization times, and then quenched to 35 °C for characterization. It was found that the crystallization conditions including crystallization temperature and time governed the crystalline morphology and even the tensile properties of iPP and the iPP/PEOc (80/20) blend. The lower the crystallization temperature, the shorter the crystallization time was needed for the occurrence of DBT, and the sharper the transition would be. The addition of the elastomer component delayed the DBT occurrence for the iPP/PEOc blend in terms of the crystallization time, owing to the fact that the existence of PEOc domains between the iPP lamellar stack regions or at the iPP spherulitic boundaries enhanced the ductility of the blend. The X-ray diffraction results displayed the oriented and destroyed crystalline structure characterizing the ductile fracture, while unoriented structure describing the brittle failure. The DBT is closely related to the crystal perfection, and factors such as the crystallization temperature and time and the compositions have been proven to be significant variables in determining the DBT occurrence.     

Influence of composition on the linear viscoelastic behavior and morphology of PP/HDPE blends

In this paper the influence of temperature and composition on the dynamic behavior and morphology of polypropylene (PP)/high-density polyethylene (HDPE) blends were studied. The blend composition ranged from 5 to 30 wt% of dispersed phase (HDPE) and the temperatures ranged from 180 to 220 °C. The interfacial tension between PP and HDPE at temperatures of 180, 200 and 220 °C was obtained from fitting Palierne's emulsion model [1] to the experimental data of PP/HDPE blends with different compositions and from the weighted relaxation spectra of PP/HDPE blends with different compositions, following Gramespacher and Meissner [2] analysis. The interfacial tension between PP and HDPE as inferred from the rheological measurements was shown to depend on PP/HDPE blend composition. However, the results indicated that there is a range of PP/HDPE blend composition for which interfacial tension between PP and HDPE is constant. Considering these values, it was shown that interfacial tension between PP and HDPE decreases linearly with increasing temperature.     

Morphological studies of LC polymer networks prepared by photopolymerization of (LC monomer/LC) blends

The morphology of LC polymer networks prepared by photopolymerization of (LC monomer/LC) blends containing photoinitiator and crosslinker was investigated. For the blends of 4-acryloyloxyhexyloxy-4′-cyanobiphenyl and 4-hexyloxy-4′-cyanobiphenyl, which had the same mesogen, orientation order of LC textures was memorized by photopolymerization, while any structure of LC polymer networks was not observed under an optical microscope because the networks did not phase separate from the low molecular weight LC. However, specific anisotropic phase-separated structures of LC polymer networks were observed for the blends of 4-acryloxloxyhexyloxy-4′-cyanobiphenyl and 4-hexyloxybenzoic acid, which had dissimilar mesogens, on condition that photopolymerization was carried out under the LC phase. If photopolymerization was performed under the isotropic phase conditions, polygonal or continuous phase-separated structures of LC polymer networks were observed for the dissimilar mesogenic blend. These morphologies were strongly dependent on the phase diagrams of (LC monomer/LC) blends and (the corresponding LC polymer/LC) blends. It has been found that the ordering field of LC molecules can give LC polymer networks anisotropic morphologies, which have the long range as the same length scale of LC textures.     

Liquid-liquid phase separation in a polyethylene blend monitored by crystallization kinetics and crystal-decorated phase morphologies

A series of polyethylene (PE) blends consisting of a linear high density polyethylene (HDPE) and a linear low density polyethylene (LLDPE) with an octane-chain branch density of 120/1000 carbon was prepared at different concentrations. The two components of this set of blends possessed isorefractive indices, thus, making it difficult to detect their liquid-liquid phase separation via scattering techniques. Above the experimentally observed melting temperature of HDPE, T_m = 133 °C, this series of blends can be considered to be in the liquid state. The LLDPE crystallization temperature was below 50 °C; therefore, above 80 °C and below the melting temperature of HDPE, a series of crystalline-amorphous PE blends could be created. A specifically designed two-step isothermal experimental procedure was utilized to monitor the liquid-liquid phase separation of this set of blends. The first step was to quench the system from temperatures of known miscibility and isothermally anneal them at a temperature higher than the equilibrium melting temperature of the HDPE for the purpose of allowing the phase morphology to develop from liquid-liquid phase separation. The second step was to quench the system to a temperature at which the HDPE could rapidly crystallize. The time for developing 50% of the total crystallinity (t_1/2) was used to monitor the crystallization kinetics. Because phase separation results in HDPE-rich domains where the crystallization rates are increased, this technique provided an experimental measure to identify the binodal curve of the liquid-liquid phase separation for the system indicated by faster t_1/2. The annealing temperature in the first step that exhibits an onset of the decrease in t_1/2 is the temperature of the binodal point for that blend composition. In addition, the HDPE-rich domains crystallized to form spherulites which decorate the phase-separated morphology. Therefore, the crystal dispersion indicates whether the phase separation followed a nucleation-and-growth process or a spinodal decomposition process. These crystal-decorated morphologies enabled the spinodal curve to be experimentally determined for the first time in this set of blends.     

Intrinsically proton-conducting poly(1-vinyl-1,2,4-triazole)/triflic acid blends

In the present work, proton conductivity in a polymer blend comprising proton solvating heterocycles was examined. Poly(1-vinyl-1,2,4-triazole), PVTri was produced by free radical polymerization of 1-vinyl-1,2,4-triazole and then proton-conducting polymer electrolytes were obtained by blending of PVTri with trifluoromethanesulfonic acid, triflic acid (TA). To promote the intrinsic proton conductivity the percent blending ratio was changed from 25% to 150% with respect to polymer repeat unit. The protonation of aromatic heterocyclic rings was proved with Fourier-transform infrared spectroscopy (FT-IR). Thermogravimetry (TG) analysis showed that the samples are thermally stable up to approximately 300 °C. Differential scanning calorimetry (DSC) results illustrated that the samples are homogeneous and their glass transition temperatures are located within 130-160 °C. The surface morphology of the materials were characterized by scanning electron microscopy (SEM). The proton conductivity of the blends increased with triflic acid concentration and the temperature. In the anhydrous state, the proton conductivity of PVTriTA100 is 2.2 × 10⁻⁴ S/cm at 150 °C and that of PVTriTA150 is approximately 0.012 S/cm at 80 °C which is similar to that of hydrated Nafion^®.     

Siloxane modification of polycarbonate for superior flow and impact toughness

Reactive modification of polycarbonate (PC) with a small amount of ultra-high molecular weight polydimethylsiloxane (PDMS) provides an effective route to a novel blend polymer with superior flow and excellent impact toughness. Low temperature impact toughness for such a blend was found to be comparable to polycarbonate copolymers made by interfacial copolymerization of bisphenol A and specialty silicones with phosgene. Interestingly, the blend also showed strong shear thinning behavior and a viscosity that is almost an order of magnitude lower than the starting PC resin. Analysis of the blend composition and blend morphology revealed the presence of both PC-PDMS copolymer and un-grafted siloxane as a dispersed phase in the polycarbonate matrix. The PC-PDMS copolymer provides a compatibilization effect for the stable sub-micron blend morphology in an otherwise immiscible PC-PDMS blend system. Improvement of low temperature ductility (e.g., at −40 °C) by PDMS was thus made possible. The lubricating effect from siloxane and the possibility of fibrillation flow at high shear stress are suspected to be the main reasons for high flow characteristics of these blends.     

Temperature dependence of surface composition and morphology in polymer blend film

Thin films of poly(methyl methacrylate) (PMMA) and poly(styrene-ran-acrylonitrile) (SAN) blend can phase separate upon heating to above its critical temperature. Temperature dependence of the surface composition and morphology in the blend thin film upon thermal treatment was studied using in situ X-ray photoelectron spectroscopy (XPS) and in situ atomic force microscopy (AFM). It was found that in addition to phase separation, the blend component preferentially diffused to and aggregated at the surface of the blend film, leading to the variation of surface composition with temperature. At 185 °C, above the critical temperature, the amounts of PMMA and SAN phases were comparable. At lower temperatures PMMA migrated to the surface, leading to a much higher PMMA surface content than in the bulk. The migration and preferential segregation of a blend component in thin films demonstrated here are responsible for the great difference between in situ and ex situ experimental (not real quenching or annealing) results of polymer blend films, and help explain the slow kinetics of surface phase separation at early stage for blend thin films reported in literature. This is significant for the control of surface properties of polymer materials.     

Effect of spinning temperature and blend ratios on electrospun chitosan/poly(acrylamide) blends fibers

We report the formation of non-woven fibers without bead defects by electrospinning blend solutions of chitosan and polyacrylamide (PAAm) with blend ratios varying from 75 wt% to 90 wt% chitosan using a modified electrospinning unit wherein polymer solutions can be spun at temperatures greater than ambient up to 100 °C. Electrospinning at elevated temperature leads to further expansion of the processing window, by producing fibers with fewer defects at higher chitosan weight percentage in the blends. Effects of varying blend ratios, spinning temperatures, and molecular weights on fiber formation were studied and optimum conditions for formation of uniform non-woven fiber mats with potential applications for air and water filtration were obtained. Uniform bead-less fiber mats with fiber diameter as low as 307 ± 67 nm were formed by spinning 90% chitosan in blend solutions at 70 °C.     

Synthesis of a soluble conjugated copolymer based on dialkyl-substituted dithienothiophene and its application in photovoltaic cells

A soluble conjugated alternating 3,5-didecanyldithieno[3,2-b:2′,3′-d]thiophene-thiophene copolymer was synthesized by palladium(0)-catalyzed Stille coupling reaction. The thermal, absorption, emission, electrochemical, and photovoltaic properties of the polymer were examined. A weight-average molecular weight around 6.2 × 10⁴ and a polydispersity index of 1.8 was estimated for the polymer using gel permeation chromatography. The polymer exhibits good thermal stability with decomposition temperature of 340 °C and glass-transition temperature of 136 °C. The polymer shows strong absorption peaked at 505 nm in diluted solution and 518 nm in thin film with an optical band gap 2.0 eV. The polymer exhibits intense emission located at 550 nm in solution and 603 nm in film. The HOMO and LUMO energies of the polymer were estimated to be −5.4 and −3.4 eV, respectively, by cyclic voltammetry. Polymer solar cells were fabricated based on the blend of the polymer and methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM). The power conversion efficiency of 0.7% was achieved under AM 1.5, 100 mW/cm² using polymer:PCBM (1:4, w/w) as active layer.     

Polycarbonate/liquid crystalline polymer blend: Crystallization of polycarbonate

The enhanced crystallization of polycarbonate in the blend of liquid crystalline polymer/polycarbonate/(ethylene-methyl acrylate-glycidyl methacrylate) copolymer (LCP/PC/E-MA-GMA) was studied by wide angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC). The LCP/PC/E-MA-GMA 5/95/5 blends annealed at 200 °C, for 2, 4, 6, and 10 h, present an obvious crystalline structure corresponding to PC crystallization. The PC crystal obtained shows two melting temperature, T_m1 of about 214 °C and T_m2 of about 231 °C, with a total heat of fusion of 29 J/g (annealing time = 10 h). The preliminary results indicate that amorphous PC can be induced to crystallization by the synergistic action of LCP dispersed phase and reactive compatibilizer.     

Preparation and characterization of polylactide/thermoplastic konjac glucomannan blends

In this article, a new degradable thermoplastic konjac glucomannan (TKGM) was synthesized by graft copolymerization of vinyl acetate and methyl acrylate onto konjac glucomannan (KGM). Melt blending of polylactide (PLA) and TKGM has been performed in an effort to improve the processing and comprehensive mechanical properties of PLA and TKGM without compromising its degradability and biocompatibility. The miscibility, processing rheology, phase morphology, thermal properties, interaction, crystallization and mechanical properties of PLA/TKGM blends were investigated in detail. The thermal processing property of PLA/TKGM blend (60/40) was quite close to low density polyethylene (LDPE). As observed from the tan δ curves in dynamic mechanical analysis, all of the blends exhibit a single glass transition over the entire composition range, indicating that the blends were thermodynamically miscible. The TKGM exhibited a relatively broad endothermic peak at around 120 °C, which was lower than that of KGM. And an obvious glass-transition behavior was obtained around 26.6 °C. Furthermore, the PLA/TKGM blend (60/40) had a very high elongation at break of 234.8%, while the tensile strength remained as high as 36.5 MPa. And the PLA/TKGM blend (20/80) resulted in an even greater ductility with an elongation at break of 520.5% as compared with 14.1% for pure PLA. A substantial increase in the non-notched impact strength was also observed with the PLA/TKGM blend (20/80) demonstrating two times the impact strength of pure PLA.     

Crystallization and phase separation kinetics in blends of linear low-density polyethylene copolymers

We have systematically studied the crystallization and liquid-liquid phase separation (LLPS) kinetics in statistical copolymer blends of poly(ethylene-co-hexene) (PEH) and poly(ethylene-co-butene) (PEB) using primarily optical microscopy. The PEH/PEB blends exhibit upper critical solution temperature (UCST) in the melt and crystallization temperature below the UCST. The time evolution of the characteristic morphology for both crystallization and LLPS is recorded for blends at various compositions and following a quench from initial homogenous melts at high temperature to various lower temperatures. The crystallization kinetics is measured as the linear growth rate of the super structural crystals, whereas the LLPS kinetics is measured as the linear growth rate of the characteristic length of the late-stage spinodal decomposition. The composition dependence crystallization kinetics, G, shows very different characteristics at low and high isothermal crystallization temperature. Below 116 °C, G decreases with increasing PEB content in the blend, implying primarily the composition effect on materials transport; whereas at above 116 °C, G shows a minimum at about the critical composition for LLPS, implying the influence of the LLPS. On the other hand, LLPS kinetics at 130 °C is relatively invariant at different compositions in the two-phase regime. The length scale at which domains are kinetically pinned, however, depends strongly on the composition. In a blend near critical composition, a kinetics crossover is shown to separate the crystallization dominant and phase separation dominant morphology as isothermal temperature increases.     

Preparation of highly crystalline carbon nanofibers from pitch/polymer blend

High crystalline carbon nanofibers were prepared by using polymer blend technique. Naphthalene-based mesophase pitch (AR pitch) was dispersed finely in polymethylpentene matrix, spun by using a melt-blown spinning machine, stabilized at 160 °C in an oxygen atmosphere and carbonized at 900 °C in a nitrogen atmosphere. Bundles of the carbon nanofibers with ca. 100 nm in diameter were obtained after removal of polymethylpentene at the carbonization process. No impurity carbon was observed. The carbon nanofibers consisted of fine carbon crystallites with preferred orientation along the fiber axis. After heating to 3000 °C, the carbon crystallites grew drastically to have an interlayer spacing of 0.3367 nm and a crystallite thickness of 56.9 nm, respectively, with remarkable improvement of the preferred orientation of the crystallites. Advantages and disadvantages of the present method were discussed briefly.     

Temperature sensitive poly(N-t-butylacrylamide-co-acrylamide) hydrogels: synthesis and swelling behavior

A series of temperature sensitive hydrogels was prepared by free-radical crosslinking copolymerization of N-t-butylacrylamide (TBA) and acrylamide in methanol. N,N′-methylenebis(acrylamide) was used as the crosslinker. It was shown that the swelling behavior of the hydrogels can be controlled by changing the amount of TBA units in the network chains. Hydrogels immersed in dimethylsulfoxide (DMSO)-water mixtures exhibited reentrant swelling behavior, in which the gels first deswell then reswell if the DMSO content of the solvent mixture is continuously increased. In water over the temperature range of 2-64 °C, hydrogels with less than 40[percnt] TBA by mole were in a swollen state while those with TBA contents higher than 60[percnt] were in a collapsed state. Hydrogels with 40-60[percnt] TBA exhibited swelling-deswelling transition in water depending on the temperature. The temperature interval for the deswelling transition of 60[percnt] TBA gel was found to be in the range from 10 to 28 °C, while for the 40[percnt] TBA gel, the deswelling started at about 20 °C and continued until the onset of the hydrolysis of the network chains at around 64 °C. It was shown that the Flory-Rehner theory of swelling equilibrium provides a satisfactory agreement to the experimental swelling data of the hydrogels, provided that the sensitive dependence of the χ parameter on both temperature and polymer concentration is taken into account.     

Thermotropic liquid crystalline polymer (Rodrun LC5000)/polypropylene in situ composite films: rheology, morphology, molecular orientation and tensile properties

In situ composite films were prepared by a two-step method. First, polypropylene and thermotropic liquid crystalline polymer (TLCP), Rodrun LC5000 (80 mol% p-hydroxy benzoic acid (HBA)/20 mol% polyethylene terephthalate (PET)), were melt blended in a twin-screw extruder and then fabricated by extrusion through a mini-extruder as cast film. Rheological behavior of the blends, morphology of the extruded strands and films, and tensile properties of the in situ composite films were investigated. Rheological behavior of the blends at 295 °C studied using a plate-and-plate rheometer revealed a substantial reduction of the complex viscosity with increasing TLCP content, and all specimens exhibited shear thinning behavior. Over the angular frequency range of 0.6-200 rad/s, the viscosity ratio (dispersed phase to matrix phase) was found to be very low, in the range of 0.03-0.07. Morphologies of the fracture surfaces of the blend extrudates and the film surfaces etched in permanganic solution were investigated by scanning electron microscope (SEM). The TLCP droplets in the extruded strands were seen with a progressive deformation into fibrillar structure when TLCP content was increased up to 30 wt%. In the extruded films, TLCP fibrils with increasing aspect ratio (length to width) were observed with increasing TLCP concentration. Orientation functions of each component were determined by X-ray diffraction using a novel separation technique. It was observed that the Young's modulus in machine direction of the extruded film was greatly improved with increasing TLCP loading, due to the increase in fiber aspect ratio and also molecular orientation.