Thermomechanical processing environment and morphology development of a thermotropic polymer liquid crystal

We have studied a longitudinal polymer liquid crystal consisting of poly(ethylene terephthalate) (PET) and p‐hydroxybenzoic acid, namely PET/0.6PHB, where 0.6 is the mole fraction of the second component. The material was injection molded with systematic variations of the melt and mold temperatures and injection flow rate using design of experiments based on a Taguchi orthogonal array. Thermomechanical environment defined by local melt temperatures and shear rates and stresses imposed during processing was estimated by computer simulations of the mold‐filling phase. The morphology of the moldings was characterized by optical and scanning electronic microscopy, wide‐ and small‐angle X‐ray scattering, and differential scanning calorimetry. An analysis of variance approach identified the significant processing variables and their contributions to variations of morphological parameters. The processing environment affects strongly the melt viscosity, and there is a strong thermo‐mechanical coupling. The result is a complex multilaminated and hierarchical microstructure, whose morphological features are very sensitive to the processing conditions. Relationships between local thermomechanical variables (rather than global ones) and the morphological parameters are established. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Interface morphology and phase separation in polymer-dispersed liquid crystal composites

It is widely appreciated that electro-optic activity in polymer-dispersed liquid crystals (PDLCs) depends on separation of the polymer and liquid crystal (LC) phases. Since the phase structure develops in a non-equilibrium system, the morphology of the LC domains depends on the details of the chemical and physical processes active during domain formation. The nature of the interface between the polymer and liquid crystal phases is of particular interest. This work discusses the two-phase morphology in an acrylate-based system that develops during polymerization-induced phase separation (PIPS). Using small-angle X-ray scattering (SAXS) and ultra-small-angle X-ray scattering (USAXS), we find that interfaces in PDLCs developed from an acrylate-based recipe are more disordered than generally appreciated. Information gained from SAXS and USAXS is compared to data from scanning electron microscopy (SEM) and transmission electron microscopy (TEM). To elucidate the apparent discrepancies between imaging and scattering, we investigated the effects of SEM sample preparation. We observe significant alteration of the interface morphology due to the leaching of the LC phase.     

Effect of thermal history on the morphology of thermotropic liquid crystalline copolyesters based on PET and PHB

Morphological studies were carried out on thermotropic liquid crystalline copolyesters based on poly(ethylene terephthalate) (PET) and para-hydroxybenzoic acid (PHB), where PHB content varied from 30 mole percent up to 80 mole percent. The technique of chemical etching, using n-propylamine as the etchant, coupled with scanning electron microscopy was utilized to obtain structural information. Scanning electron microscopy results on chemically etched, compression moulded films show that selective chemical etching of the PET rich regions occur. This indicates that the morphology of the copolymers is hetergeneous in nature. Further support regarding a hetergeneous morphology was obtained by transmission electron microscopy. A morphological model has been proposed based on these results. The observance of non-equilibrium behaviour associated with amorphous PET regions (as seen from d.s.c. measurements) also strongly indicates the presence of a phase rich in PET and thus supports the non-homogeneous morphological model. Thermal analysis of these copolyesters suggests that the chain structure is non-random and this is an agreement with results published by Wunderlich et al. The glass transition temperature typically associated with PET is present and remains constant in all copolymers compositions where PET is the continuous phase. Further, the melting temperatures obtained experimentally are higher than the values predicted by theory for random copolymers and the melting endotherms are relatively narrow. These observations also indicate a non-random chain structure. Structural studies conducted on films compression molded at different temperatures show that morphological rearrangement occurs at higher temperatures with the formation of domains of the order of 10 microns.     

Estimation of surface diffusion coefficient in liquid phase adsorption

An estimation procedure of surface diffusion coefficient, D_s, in liquid phase adsorption was proposed. The procedure is based on a restricted diffusion model, in which D_s is correlated with molecular diffusivity by considering a restriction energy due to an adsorptive interaction between adsorbates and adsorbents. In some adsorption systems, D_s of different adsorbates could be calculated with an error less than about 50% from only one datum of each adsorption equilibrium constant. Irrespective of temperature, the procedure, can be applied for the estimation of D_s even in a wide range of D_s of about 4 orders of magnitude.     

Effects of phase behavior on mutual diffusion at polymer layers interface

Using oscillation mode of rheology and theoretical calculation, we have observed for the first time the crossover of mutual diffusion coefficient, D_m, 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 D_m 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, E_d, derived from the Arrhenius relation of D_m 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.     

Effect of block copolymers of various molecular architecture on the phase morphology and tensile properties of LDPE rich (LDPE/PS) blends

The emulsification efficiency of three different block copolymers consisting of hydrogenated polybutadiene (HPB) and polystyrene (PS), i.e. a pure diblock , a tapered diblock and a triblock copolymer has been compared in low density polyethylene/polystyrene (LDPE/PS) blends rich in polyethylene. The comparison relies upon the ability of these potential interfacial agents to stabilize fine phase dispersion and to promote good interfacial adhesion. Based on the phase morphology, the ultimate tensile properties and the dynamic viscosity of the modified blends, the tapered diblock copolymer is clearly the most efficient emulsifier. For instance a plateau is observed in the property-copolymer content dependence when 2 wt% tapered diblock are used compared to ca. 5 wt% in case of the pure diblock. In contrast, no plateau is observed when the triblock copolymer is used. This is assumed to result from a less quantitative localization of these two copolymers i.e. the pue diblock or the triblock at the LDPE/PS interface.     

Influence of phase morphology and crystalline structure on the toughness of rubber-toughened isotatic polypropylene blends

In the work, ethylene–octene copolymer (POE) toughened isotatic polypropylene (PP) blends with different phase morphologies and crystalline structures were successfully fabricated and their influence on the toughness of PP blends was discussed. POE domains not only played the role of stress concentration to induce stress field around them, but also acted as stress deliverer to transmit stress to the deep into the PP matrix. Elongated domains transmitted stress better, but spherical ones induced a larger stress field. As for the crystalline structure of PP matrix, compared with the “bundle-like” β-crystals, well-developed β-spherulites could induce a larger stress field. The toughness of the blends with different combinations of phase morphology and crystalline structure was also discussed. Only the blend with elongated POE domains and well-developed β-spherulites could achieve super-high toughness. To achieve this goal requires simultaneous optimization of nucleating agent content, POE content and composition, and processing conditions. This work provides a good example to better understand the influence of phase morphology of rubbers and crystalline structure of matrix in rubber-toughed polymer system.     

Effect of compatibilizers on mechanical properties and morphology of in-situ composite film of thermotropic liquid crystalline polymer/polypropylene

An in-situ composite film of a thermotropic liquid crystalline polymer (LC3000)/polypropylene (TLCP/PP) was produced using the extrusion cast film technique. The compatibilizing effect of thermoplastic elastomers, styrene-ethylene butylene-styrene (SEBS), maleic anhydride grafted SEBS (MA-SEBS), and maleic anhydride grafted polypropylene (MA-PP) on the mechanical properties and morphology of the TLCP/PP composite films was investigated. It was found that SEBS provided a higher value of tensile modulus than MA-SEBS, which in turn was higher than MA-PP, despite the expected stronger interaction between the MA chain and TLCP. The observation of the morphology under optical and scanning electron microscopes suggested that all three compatibilizers helped improve the dispersion of the TLCP fibers and increased the fiber aspect ratio to a different extent. The fractured surface of the specimens showed more fiber breakage than pull-out when a compatibilizer was added, which suggested the improvement of interfacial adhesion. The surface roughness of fibers with an added elastomeric compatibilizermay also provide mechanical interlocking at the interface. It is suggested that the increase in the viscosity ratio of TLCP/PP due to the added elastomeric compatibilizer, SEBS and MA-SEBS, compared with the thermoplastic compatibilizer, MA-PP, is more effective in improving the composite mechanical properties.     

Kinetics of photopolymerization-induced phase separation and morphology development in mixtures of a nematic liquid crystal and multifunctional acrylate

Photopolymerization behavior and reaction kinetics for a series of multifunctional acrylate monomer(s) and eutectic liquid crystal blends were investigated with particular emphasis on determination of the reaction rate coefficients for propagation and termination steps of photopolymerization. Reaction rate coefficients were determined via real-time infrared spectroscopy and compared with those obtained by photo-differential scanning calorimetry. Effects of various parameters such as LC concentration, light intensity, and monomer functionality on the kinetics were investigated. Phase transition temperature versus composition phase diagrams were established by means of optical microscopy and differential scanning calorimetry for mixtures of triacrylate/liquid crystal (LC) before photopolymerization and after exposing to ultra violet (UV) irradiation under various reaction times. The snapshot phase diagram of the reacting mixtures exhibited isotropic gel, isotropic liquid + nematic, and narrow pure nematic coexistence regions. These coexistence regions were further confirmed by morphological changes of the polymer dispersed liquid crystal films as functions of temperature and concentration using polarized optical microscopy.     

Effect of thermotropic copolyesteramide on the properties of polyamide‐66/liquid crystalline copolyester composites

Liquid crystalline copolyester‐polyamide 66 (LCPES/PA66) composites compatibilized by liquid crystalline copolyesteramide (LCPEA) were prepared by injection molding. The LCPES employed was a commercial copolyester, Vectra A950, and the LCPES was a semiflexible thermotropic copolyesteramides based on 30 mol% of p‐amino benzoic acid (ABA) and 70 mol% of poly(ethylene terephthalate) (PET). Thermal analysis, mechanical characterization, and morphological investigations were conducted on the blends. The dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC) tests showed that LCPEA is an effective compatibilizer for the LCPES/PA66 composites. The mechanical measurements showed that the stiffness, tensile strength and Izod impact strength of the in‐situ composites are improved by adding LCPEA because of the compatibilization and reinforcement to LCPES/PA66 composites. However, the properties improvement vanished when LCP content reached 10 wt%. The drop weight dart impact test was also applied to analyze the impact fracture characteristics of these composites. The results showed that the maximum impact force (F_max), crack initiation and propagation energy all improved with the addition of a small percent of LCPEA. From these results, it appeared that LCPEA prolongs the time for crack initiation and propagation. It also increases the energies for crack initiation and propagation, thereby leading to toughening of the LCPES/PA66 in‐situ composites. Finally, the correlation between the mechanical properties and morphology of the composites is discussed.     

Measurement of intraparticle diffusion in reversed phase liquid chromatography

The intraparticle diffusion coefficient was measured using a method based on the fitting of a set of experimental chromatographic profiles to the lumped pore diffusion model. For this purpose, both the analytical solution of the model in the Laplace domain and a numerical method were used. There was an excellent agreement between the results given by the two methods. These results are compared to those obtained by moment analysis of the same set of chromatographic profiles and by the determination of the intraparticle diffusion coefficient from the second central moment of these bands. Nearly identical results were obtained with these two independent methods. The values of the intraparticle diffusion coefficient, D_e, for rubrene in pure methanol was found to be by the modeling method and by the moment analysis method. These values increase with increasing water concentration, to 1.10×10⁻⁶ and , respectively, in a methanol/water solution and to 1.63×10⁻⁶ and , respectively, in a solution.These results confirm the validity and the consistency of the lumped pore model and the moment analysis theory. They show that both approaches describe correctly the mass transfer kinetics in the particles of packing material during the chromatographic process. Systematic determinations of the intraparticle diffusion coefficient can now be undertaken and the influence of various experimental parameters on this important property of packing materials can be investigated.     

热致液晶共聚酯60PHB/PEN的合成及其表征

合成了对乙酰氧基苯甲酸与聚萘二甲酸乙二醇酯的热致液晶共聚酯(简称60PHB/PEN)。通过元素分析、红外光谱、DSC、热台偏光显微镜以及广角X射线衍射对其进行了结构、液晶性表征。实验表明,合成的共聚酯确系高度无规的PEN、PHB共聚酯,在某个温度范围内呈现向列型液晶的典型特征。     

The external electric field effects on the morphology of phase separation in a mixture of liquid crystal/flexible polymer

In this article, the morphologies of phase separation in the mixtures of small molecular liquid crystal and flexible polymers with or without being subjected to the external electric field are preliminarily studied by polarized light microscope. It is found that the characteristic “Swiss cheese” morphology can always be observed for the case of zero field strength. For the case of 1.2 V/μm field strength, it is found that the external electric field make the binodal curves of the phase diagrams shift to higher temperature and the “Swiss cheese” morphology can never be observed. However, the domain growth rate is highly accelerated due to the fact that the electric field oriented liquid crystal phase excludes the polymer coils strongly. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 250–258, 2002     

Investigation of liquid water in gas diffusion layers of polymer electrolyte fuel cells using X-ray tomographic microscopy

In polymer electrolyte fuel cells (PEFCs), condensation of water within the pore network of the gas diffusion layers (GDL) can influence the gas transport properties and thus reduce the electrochemical conversion rates. The use of X-ray tomographic microscopy (XTM), which allows for a resolution in the order of one micrometer is investigated for studying ex situ the local saturation in GDL's. The strength of XTM is the high spatial resolution with simultaneous contrast for water and carbon, allowing for non-destructive 3D-imaging of the solid and the contained water. The application of this method for imaging the ex situ water intrusion into the porous network of GDLs is explored using absorption and phase contrast methods. It is shown that the inhomogeneous filling behavior of GDL materials can indeed be visualized with sufficient resolution. For Toray paper TGP-H-060 the local saturation was measured as function of the water pressure. The results, evaluated in 1D, 2D and 3D show a liquid water retention effect at the denser layers near the surface. A comparison with established capillary pressure functions is presented. Altogether, the results show the potential of the XTM-method as a tool for studying the liquid water behavior in PEFC on a microscopic scale.     

Phase separation of the liquid crystal in the cholesterin phase

In the ethyl-cyanoethyl cellulose [(E-CE)C]/acrylic acid [AA]/4-cyano-4'-n-heptyl biphenyl (TCB) solutions, 7CB acted as a diluting reagent when it was added into the (E-CE)C/AA cholesteric liquid crystalline solutions. The dispersed phase with 250–300nm in size appeared after the AA in the isotropic (E-CE)C/AA/7CB solutions with the 7CB concentration (C_7CB) between 27.4 – 41.2wt% was polymerized at 24°C. There was the dispersed phase rich with 7CB in the (E-CE)C/AA/7CB cholesteric liquid crystalline solutions when C_7CB was 43.0wt% at 24°C. The temperature at which the anisotropic phase in the dispersed phase transformed to the isotropic phase in the solutions was increased after the polymerization of the AA, but the transition temperature of the dispersed phase in both the solutions and the films was lower than that of the pure 7CB. Received: 6 October 2000/Revised version: 18 December 2000/Accepted: 28 December 2000     

Compatibility in binary blends of thermotropic liquid crystal polymers

The compatibilization by transesterification of aromatic thermotropic liquid crystal polyesters (Ar-TLCP's) and TLCP's containing aliphatic ethylene-terephthalate (ET) linkages has been studied. Three Ar-TLCP's were mixed with two p(ET/HBA)'s, each having different extents of blockiness, to form a series of binary blends (Bin-TLCP's). The morphology and physical properties of the resultant Bin-TLCP's depend on the block structures, compositions, viscosities, and susceptibility to transesterification of the components. In this work, the influence of TLCP block structure and transesterification on the compatibility of the components of the Bin-TLCP's is described.     

Effect of counter current gas phase on liquid film

Liquid film flow is very important in many industrial applications. However, there are few reports about its characteristics on structured packings. Therefore, in this paper, liquid film phenomena were investigated experimentally to exploit new approaches for intensifying the performance of the structured packings. All experiments were performed at room temperature. Water and air were the working fluids. The effect of counter current gas phase on the liquid film was taken into consideration. A high speed camera, a non-intrusive measurement technique, was used. It is shown that both liquid and gas phases have strong effects on film characteristics. In the present work, liquid film width increased by 57% because of increasing liquid flow rate, while it decreased by 25% resulting from the counter current gas phase.     

Study on co[poly(butylene terephthalate‐p‐oxybenzoate)] thermotropic copolyester. II. Thermal behavior and crystalline morphology

The POB/PBT copolyesters, designated B28, B46, B64, and B82, were prepared from p‐acetoxybenzoic acid (PAB) and poly(butylene terephthalate) (PBT). The polymeric products obtained were then ground and subjected to solid‐state polymerization under vaccum for 4 h. The melting and crystallization behaviors of these copolyesters haven been studied by differential scanning calorimetry (DSC). In the DSC scan of the POB‐rich composition, the endothermic peak shows obscurely, and enthalpy of fussion becomes small due to the change in the crystalline morphology from isotropic to anisotropic. In general, the melting point of the copolyester is increased by the solid‐state polymerization reaction. Also, thermogravimetric analysis (TGA) were performed with these samples obtained. It was found that the decomposition temperature (T_d ) is increased as the POB content is increased. Effects of composition and solid‐state polymerization on the decomposition temperature of copolyesters are also discussed. The crystalline morphology of copolyester was investigated with a Zeiss polarized optical microscope. It was found that the POB/PBT copolyesters with 60 mol % POB was shown to be highly anisotropic. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 2363–2368, 2000     

Transesterification-controlled compatibility and microfibrillation in PC-ABS composites reinforced by phosphorus-containing thermotropic liquid crystalline polyester

For polymer/liquid crystal polymer (LCP) blend systems, the in-situ fibrillation of LCP in polymer matrix can result in the self-reinforcement of polymer/LCP composites. How to control the microfibrillation of LCP in matrix is a key to enhance the mechanical properties of composites. In this paper, we investigated the transesterification-controlled compatibility and microfibrillation of phosphorus-containing thermotropic liquid crystalline polyester, poly(p-hydroxybenzoate-co-DOPO-hydroquinone ethylene terephthalate) (PHBDET) in the PC/acrylonitrile-butadiene-styrene copolymer blend (PC-ABS) during the melt processing. A standard mode and temperature-modulated differential scanning calorimetry (DSC and TMDSC) and ¹³C nuclear magnetic resonance (¹³C NMR) were used to investigate the transesterification and compatibility of PHBDET with PC-ABS. Microstructures, rheological and mechanical properties of the composites were also studied via scanning electron microscopy (SEM), dynamic rheological measurement and universal material testing machine. The results showed that the extent of transesterification could influence the compatibility of PHBDET with PC-ABS, and could be controlled by processing temperature and time. The improved compatibility was not always favorable for the microfibrillation of PHBDET in PC-ABS, but a certain extent of transesterification showed a positive influence on the tensile properties of the composites. Therefore, there existed an optimal extent of transesterification, in which the composite could show a good balance of compatibility and tensile properties.     

Liquid phase behavior and its effect on crystalline morphology in the extruded poly(ethylene terephthalate)/poly(ethylene-2,6-naphthalate) blend

Morphology in an extruded poly(ethylene terephthalate)/poly(ethylene-2,6-naphthalate) was investigated using time-resolved light scattering, optical microscope and small-angle X-ray scattering. During annealing at 280 °C, the domain structure via spinodal decomposition preceded, the transesterification followed, and then the transesterification between the two polyesters induced the dissolution of the liquid-liquid (L-L) phase separation, i.e. the homogenization. The annealed specimen for various time periods (t_s) at 280 °C was subjected to a temperature-drop to 120 °C for the isothermal crystallization and then the effects of liquid phase morphology on crystallization was investigated. With t_s, the H_ν (cross-polarization) light scattering patterns exhibited the dramatic change from a four-leaf clover pattern with maximum intensity at azimuthal angle 45° (×-type scattering pattern) to a diffuse pattern of circular symmetry and then a four-leaf clover pattern with maximum intensity at azimuthal angles 0 and 90° (+-type scattering pattern). This suggests that the crystalline structure depends on the level of the block and/or random copolymer produced by the transesterification during annealing. The H_ν scattering patterns reflected differences in the principle polarizability of the crystalline lamellae with respect to the spherulitic radius. On the other hand, the long period L_B, an average distance between two adjacent crystalline lamellae, increased with t_s at 280 °C. The dependence of L_B on t_s was explained by the change in the crystallization rate G.