Study of surface conditions and shear flow of LCP melts in nanochannels through molecular dynamics simulation

The rheological properties and phase orientation of liquid crystalline polymer (LCP) melts flowing in a nanochannel with different surface roughness are investigated by molecular dynamics (MD) simulations. The molecular chains of LCPs are depicted by a newly developed molecular model named GB-spring-bead model, which has proved to be efficient and accurate in studying the phase transition behaviors of semi-flexible main chain LCPs [Yung KL, He L, Xu Y, Shen YW. Polymer 2005;46:11881 [1]]. The surfaces are modeled as rough atomic serrated walls whereby the roughness is characterized by the period and amplitude of serrations. Simulation results have shown that the surface roughness affects greatly the rheological properties and phase orientations of LCP melts in a nanochannel (the distance between the upper wall and the lower wall is 12.8 nm). As the amplitude of serration increases, the shear viscosity of LCP increases nonlinearly while its orientational order parameter decreases. When the serration amplitude reaches a certain magnitude, a phase transition (from nematic to isotropic phase) happens, which increases the viscosity of the nano LCP flow drastically. On the other hand, the influence of serration period on the shear viscosity and orientational order parameter is not so obvious relatively. Findings in this study provide very useful information in the injection molding of plastic products with nanofeatures.     

Phase behaviors of comb-like liquid crystalline polysiloxanes containing fluorinated mesogenic units

Several liquid crystalline polysiloxanes (Pa-Pf) bearing fluorinated mesogenic units were synthesized using poly(methylhydrogeno)siloxane, 4′-(4-undec-10-enoyloxy-benzoyloxy)-biphenyl-4-yl 4-fluoro-benzoate and 4′-propionyloxy-biphenyl-4-yl 4-allyloxy-benzoate, and effect of fluorinated mesogenic units on phase behaviors of the fluorinated LC polysiloxanes was studied as well. The samples Pa and Pb showed single nematic mesophase when they were heated and cooled, but Pc, Pd, Pe and Pf exhibited both smectic and nematic phases. The glass transition temperature and smectic A-nematic mesophase transition temperature of polymers increased slightly with increase of fluorinated units in the polymer systems, but mesophase-isotropic phase transition temperature decreased slightly. In XRD curves, the intensity of sharp reflections at low angle increased with increase of fluorinated mesogenic units in the polymers' systems, indicating that the longer spacer and the fluorophobic effect of fluorinated mesogenic units could lead to a significant stabilization and even to modifications of smectic mesophases.     

Phase identifications and monotropic transition behaviors in a thermotropic main-chain liquid crystalline polyether

A thermotropic liquid crystalline (LC) polyether was synthesized by condensation polymerization of 1-(4-hydroxyphenyl-2-(2-methyl-4-hydroxypheyl)ethane) and α,ω-dibromononane, and abbreviated as MBPE-9. Multiple phase transitions were found via differential scanning calorimetry (DSC) upon heating and cooling. Based on wide-angle X-ray diffraction (WAXD) and polarized light microscopy (PLM) experiments, two monotropic LC nematic (N) and smectic phases and three crystalline (orthorhombic K₁, triclinic K₂ and triclinic K₃) phases were identified. A study on kinetics showed that only when the N phase formation rate was faster than the crystallization rate of K₁ phase which directly grew from the melt, the presence of the N phase could accelerate both the overall crystallization and the linear crystal growth rates of K₁ phase. In a narrow temperature region, two distinct crystal growth rates could be observed: one was directly from the isotropic melt, and another was from the N phase.     

Manipulating ordered structure of ionic liquid crystalline polymers through tuning the alkyl spacer length

The fabrication of ordered structure is very important for the applications of ionic liquid crystalline polymers (ILCPs) or polymerized (ionic liquid crystal) (PILC). In this paper, we reported a facile approach to manipulate the ordered structure of ILCPs through adjusting the length of alkyl spacer. We designed and synthesized a series of ILCPs contained imidazolium, poly(2,5-bis{[m-(4-butoxy-4′-imidazolium phenyl) m-alkyl] oxy carbonyl} styrene bis (fluoroborate) salts) (denoted as P_4-m−BF₄, m represents the number of carbon in the alkyl spacers and m = 2, 4, 6, 10) via radical polymerization. Combined differential scanning calorimetry (DSC), polarized light microscopy (PLM), X-ray scattering, and two-dimensional wide-angle X-ray diffraction (2D WAXD), we found that the ordered structure of this ILCPs can transfer from smectic A phase (SmA) to hexagonal columnar (Φ_H) phase with the increase of spacer. Furthermore, these results were confirmed by reconstructed relative electron density map using fast fourier transform algorithm (FFT). The result revealed that the increase of alkyl spacer would affect the interaction between ions and side chain, and induce the packing of the side chains. It was evident that the alkyl spacer played an important role in the constructing of ordered structure and offered a new method to fabricate different ordered structure of ILCPs.     

Structures and phase transformations of odd-numbered asymmetric main-chain liquid crystalline polyesters

Two new asymmetric odd-numbered main-chain liquid crystalline (LC) polyesters (BPE-Cn) were synthesized through the condensation polymerization of A-B type asymmetric α,ω-carboxylic acid-hydroxyl terminated monomers with odd-numbers of methylene units in the spacers (BPCA-Cn-PmOH, where n=7 and 9). The phase behaviors and structures of these odd-numbered BPE-Cn samples were characterized using the combined techniques of differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD), selected area electron diffraction (SAED) and polarized light microscopy (PLM). One-dimensional (1D) powder WAXD results at different temperatures combined with DSC results revealed that during cooling these two polyesters enter a low-ordered LC phase before developing a crystalline phase at lower temperatures. Based on 2D WAXD of oriented fiber patterns, the low ordered LC phase of odd-numbered BPE-Cn was identified to be a smectic A (SmA) phase, which was constructed by each chemical repeating unit along the fiber drawn direction. The crystalline structures were determined to be monoclinic unit cells (K_M). However, BPE-C7 possessed a γ=87°, while BPE-C9 had a β=100°. These crystalline structure identifications were also confirmed by SAED from single crystals. Two-chain packing models of the K_M phases with four chemical repeating units were proposed on the basis of the experimental diffraction patterns. The different structures were attributed to the interplay between the mesogenic group ordering propensity and the chain constraints dictated by the methylene spacer and the meta-linkage at the end of mesogen. In the ordered phases of these two odd-numbered BPE-Cn polyesters, aromatic mesogenic groups are more or less parallel to each other along the chain with small angles of deviation from the c-axis of the crystal lattice, different from those large zigzag conformation structures in even-numbered BPE-Cn crystals. Crystallographic calculations of these two unit cell structures indicated that the neighboring chains in the crystals are translated along the c-axis (in the case of BPE-C9) or twisted away from the b-axis (in the case of BPE-C7) in order to incorporate both of the bend attributed to the odd-numbered methylene spacers and of the configurational meta-linkage at the end of mesogens. The simulated diffractions based on these calculations qualitatively agreed with the experimental observations. The phase identifications were also supported by the observed texture changes in PLM.     

Energy‐based predictive criterion for LCP fibrillation in LCP/thermoplastic polymer blends under shear

This article relates the fibrillation of liquid crystalline polymer (LCP) under shear in its blend with a thermoplastic polymer (TP) to the relative rate of energy utilization in the LCP and TP phases. The development of a criterion based on the energy relationship for predicting LCP fibrillation in the blend is discussed. The formation of LCP fibers in the blends of LCP with polycarbonate (PC), polyethylene naphthalate (PEN), high‐density polyethylene (HDPE), polypropylene (PP), and silica‐filled polypropylene (PP) was studied to validate the criterion and to demonstrate its applicability. For all the blends, viscosity data were obtained by using a capillary rheometer, which was subsequently used to estimate the rate of energy utilization in the LCP and the matrix phases. The predictions based on the proposed criterion were verified through the morphological investigations carried out on the extrudates obtained from the same capillary experiments. The energy‐based criterion was easy to implement, could account for the effect of variable LCP concentration and fillers in the blend, and could provide reliable predictions for a variety of LCP/TP blends. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 3314–3324, 2003     

Novel approach to fibrillation of LCP in an LCP/PP blend

A novel concept of improving shear‐induced fibrillation of liquid crystalline polymer (LCP) in LCP/thermoplastic blend systems was introduced. Silica fillers (SiO₂) were added to an LCP/polypropylene (PP) system to serve as a viscosity thickening agent and to improve the fibrillation of the LCP phase. The formation of LCP fibrils was found to enhance with the incorporation of 5–15 wt % of fillers. The presence of LCP fibrils improved the flow properties of the LCP/PP/SiO₂ composites. It was evident from the rheological and morphological studies that the addition of silica led to an increase of the aspect ratio of the LCP fibrils, which, in turn, should improve their effectiveness as reinforcements and/or toughening agents. Substantial improvement in LCP aspect ratio was achieved by the introduction of hydrophobic SiO₂ fillers in the PP/LCP blends. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 2070–2078, 2002     

Effect of electrical field on polypeptide phase behavior involving a conformationally coupled anisotropic-isotropic transition

Effect of external electrical field on phase equilibrium was reported for poly(γ-benzyl l-glutamate) (PBLG) in a mixed solvent containing denaturant acid where a helix-coil transition coupled anisotropic-isotropic transition exists. The experimental results revealed that the anisotropic phase and helix conformation can be stabilized when an external electrical field is applied. In the presence of electrical field, the anisotropic-isotropic phase boundary shifts to lower polymer concentrations and the helix-coil induced anisotropic-isotropic transition becomes stable in the denaturant acid. The lattice model derived in our previous work was further generalized by implementing a free energy term contributed from external orientational field of dipole type. The calculated phase diagrams have been compared with those experimentally observed. A good qualitative agreement has been achieved.     

Molecular dynamics simulation of liquid crystal formation within semi-flexible main chain LCPs

Molecular dynamics simulations of a semi-flexible main chain LCP (liquid crystalline polymer) have been carried out using a newly developed model named solo-LJ-spring-GB model. The new model represents the molecular chain in the form of GB-spring-LJ-spring-…-LJ-spring-GB sections that simplifies the model and reduces the simulation computation by many times. The new model was evaluated by studying the phase behaviors of semi-flexible main chain LCPs through simulation. The results, such as the spontaneous phase transition from isotropic phase to nematic phase as the system temperature decreases and the odd-even effect of the number of flexible spacers on its thermodynamic properties agree well with other experimental results as well as simulations using the traditional GB/LJ model. The orientational and translational mobilities of mesogenic units in the new model have also been measured and compared with those in the traditional GB/LJ model with very little differences found.     

Short-time Monte Carlo study on the phase transition of a ferromagnetic polymer chain model

Short-time dynamic scaling analysis method is applied to investigate the phase transition of a ferromagnetic Ising chain model on the simple cubic lattice. The phase transition temperature T_c is determined from the time evolutions of magnetization M(t) and square magnetization M²(t) at temperatures near T_c. The resulted T_c is consistent with that obtained from the annealing Monte Carlo method. The short-time dynamic behaviors of magnetizations near the transition temperature are also studied. We find both M(t) and M²(t) show power law decay near T_c after a very short time period τ_mic≈100 Monte Carlo steps.     

Domain evolution and phase transition dynamics of BaTiO3 ferroelectric single crystal under high pressure with various loading methods: Phase field simulation

The microstructure and macroscopic properties of ferroelectric materials at high pressure are of great interest in both the engineering and scientific arenas. The effect ofthe pressure value, loading time (the time taken for the pressure to increase from atmospheric pressure to the highest pressure) and loading direction on the evolution of domains and the ferroelectric phase transition for a BaTiO₃ single crystal was investigated using a phase field approach. It was found that under symmetrical compression loading the pressure loading time affected the phase transition path and rate but did not affect the phase transition pressure or the ultimate stable phase. For example, at room temperature, even when the loading time increased from 1 ns to 10 μs, the phase transition pressure remained stable at 2.1 GPa, but the phase transition time was prolonged. At −70 °C the orthorhombic–cubic phase transition was induced when the loading pressure was 5 GPa and the loading time was 1 ns, whereas the orthorhombic–tetragonal–cubic phase transition occurred when the loading time increased to 10 μs. In addition, it was found that the application of symmetrical pressure tended to reduce the degree of ferroelectricity, while one-dimensional compression favored the ferroelectric phase.     

Kinetics of the ordered phase growth at the phase transition from the isotropic to cholesteric state in a main chain liquid crystalline polymer

The optical microscope investigation of the isotropic- ordered phase transition in a main chain LC polymer and subsequent statistical treatment of the microscopic images allowed recognition of three regimes of the ordering: non-stationary regime which is characterized by increasing rate of the droplets growth; stationary regime; within it the ordered phase grows with a constant rate, and the coalescence regime for which the rate of the droplets growth deceases. Linear interpolation of the time dependence of the mean droplets diameter in log–log scales allowed conclude that within the stationary regime of the cholesteric phase growth, log <d> is a linear function of log t, whereas within the coalescence regime, <d> is proportional to t ^1/2 as expected from the theory of phase separation binary liquids. The time of beginning and duration of both growth regimes of the phase transition for the main chain LC polymer exceed those for low molecular weight LC compounds. This is caused by lower mobility of macromolecules in comparison with that for low molecular weight LC compounds.     

Side-chain functionalized liquid crystalline polymers and blends, 10: phase behavior and structure of side-chain liquid crystalline ionomers containing ions of d-metals

Novel side-chain liquid crystalline (LC) ionomers containing d-metals Co(II) and Ni(II) were synthesized and characterized. Both families of the ionomers are characterized by the same influence of charged group content in polymer on their phase behavior. The incorporation of 2-3 mol% of metal ions in the nematic polymer matrix leads to the induction of SmA phase and rise in the clearing point. The peculiarity of their phase behavior in comparison with the earlier investigated LC ionomers with alkaline or alkali-earth metals is the full destruction of the mesophase at the concentration of d-metal higher than 12 mol%. This phenomenon was associated with the well-known ability of the transition metal ions to form various complexes that, in the case of LC ionomers, can negatively influence the ordering of the side mesogenic groups. The proposed structure of the LC ionomers is discussed in comparison with the metallomesogenic polymer systems.     

Phase transition in nylon 6/clay nanocomposites on annealing

The γ→α crystalline phase transition in nylon 6/clay nanocomposite prior to melting was investigated by X-ray diffraction. The phase transition in the nanocomposite took place at 160 °C, 40 °C higher than that of nylon 6 at 120 °C. The transition extent in the nanocomposite was lower than that in nylon 6. This could be caused by the strongly confined spaces between layers, and the favorable environment for the formation of the γ phase in the existence of clay. Besides, the less grown crystallites of the α phase transformed from the γ phase in the nanocomposite began to melt at much lower temperature than its normal melting temperature.     

Morphological transitions of spinodal decomposition in confined geometry

We studied the phase separation of a polymer solution in thin slabs of thickness of micrometers. The material used was an asymmetrical binary system that consisted of 6 wt% semi-crystalline poly(ethylene butene) (or cEB) and 94 wt% of diisodecyl adipate (or DIDA) solvent. The de-mixing mechanism in the confined geometry is very different from that in bulk. Phase separation in bulk usually starts with formation of the solvent holes, and polymer-rich phase becomes a micro-reticulated network. While, in thin films the phase separation starts with formation of the polymer-rich droplets, that behave like Brownian particles. We have observed that there exists a critical thickness L_c below which the size of the droplets decreases drastically. Above the critical thickness, we have observed a phase morphological transformation from droplets to chains, branches, and then to networks, which follows a very similar pattern of a percolation process. There is correspondingly a second critical thickness (>L_c) below which no bi-continuous phase separation takes place.     

Phase transition at subcritical and supercritical conditions of triglycerides methanolysis

The analysis of phase equilibrium between methanol and glycerides during methyl esters of fatty acids (FAME or biodiesel) synthesis at high pressure and temperature is very important for describing the kinetic and process design. It was studied at pressure between 1.1 and 28.0 MPa and temperature from 150 to 270 °C. The transition of phases and composition of identified phases was calculated using RK-Aspen EOS and obtained values were also compared to experimentally determined data at subcritical condition (1.1-4.5 MPa and 150-210 °C).Results of experimental investigation, as well as performed simulation of some specified composition of reaction mixture, showed that system of triglycerides and methanol, at the beginning of reaction (at all analysed conditions except for supercritical state of mixture) is in equilibrium between two liquid phases. During the methanolysis of triglycerides, the phase's distribution was changed accordingly and it highly depends on actual composition of reaction mixture, temperature and pressure. Calculated and measured values indicated that distribution of methanol between the oil phase, the methyl esters, and the glycerol rich phase exists and depends of working condition. As a consequence of fact, that the methanolysis of triglycerides (oil) is mainly realized in the oil-rich phase, at the end of reaction, after all triglycerides are converted into FAME and glycerol, the oil phase disappears. Furthermore, according to the results of phase composition calculation, it was shown that from the beginning to the end of reaction one phase only exists, for methanolysis performed at 270 °C and 20.0 MPa.     

Self-Reinforcing Composite Based on Ethylene Acrylic Elastomer (AEM)/Liquid Crystalline Polymer (LCP) Blend

Abstract

Blends of ethylene acrylic elastomer (AEM) and thermotropic liquid crystalline polymer (TLCP) have been prepared by melt mixing technique. Processing studies indicated a decrease in the viscosity of the blends with the addition of liquid crystalline polymer (LCP). At lower level of LCP the tensile strength and tear strength increased. However, at higher level of LCP tensile strength values decreased due to insufficient adhesion between two phases. The modulus of the samples increased with the LCP content. The degree of crystallinity increased with increasing LCP content. This improvement in crystallinity is associated with the increase in crystallite size. For the blends, thermal studies indicated, the endothermic signals which were more prominent at all the peak temperatures. The heat of degradation values increased with the LCP content. Scanning electron microscope (SEM) study suggested the fibril formation, which affected the failure mechanism under DMA studies. Storage modulus and loss modulus of the blends increased with increasing LCP content. At above glass transition temperature (Tg) improvement in storage modulus is nearly five times higher than that of the pure AEM.     

Thermal transition behaviors in a liquid crystalline polyesterimide

The crystallization and melting behaviors of the polyesterimide, derived from N,N′-hexane-1,6-diylbis(trimellitimides), 4,4′-dihydroxybenzophenone and p-hydroxybenzoic acid, were investigated by using polarized light microscopy (PLM), differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD). The nematic texture of the polyesterimide was observed on raising temperature to 265 °C, and the nematic phase was found to convert to isotropic melt beginning from about 300 °C, the ordered nematic micro-domains still surviving after 320 °C. Isothermal crystallization of the samples was performed at 180 °C after heating samples at various temperatures in the range of 265-360 °C, and a completed crystallization peak can appear on DSC curves up to the heating temperature of 360 °C in the presence of the nematic phase and the ordered nematic micro-domains. Non-isothermal crystallization of the samples at different cooling rate was carried out, and the melting of the resulting crystals exhibits double endotherms. It is indicated that a fast crystallization in the nematic phase forms relatively more ordered crystals, which melt at higher temperature, and a slow crystallization in the isotropic phase or in the biphasic melt produces poor crystals, which melt at lower temperature. The crystallized polyesterimide was annealed, which has a minor effect on the high-melting peak but leads to a continual shifting of the low-melting peak to higher temperature with increasing annealing temperature or annealing time. WAXD patterns indicated that the structural transform was not found during annealing process.     

Synthesis and characterization of side-chain liquid crystalline polymers and oriented elastomers containing terminal perfluorocarbon chains

Several novel chiral side-chain liquid crystalline (LC) polysiloxane resins containing epoxy groups and mesogenic components have been graft copolymerized by a one-step hydrosilylation reaction with poly(methylhydrogeno)siloxane, an epoxy monomer 2-(allyloxymethyl)oxirane, and chiral fluorinated liquid-crystalline monomers 4′-(4-(allyloxy)benzoyloxy)biphenyl-4-yl 6-(perfluorooctanoyloxy)hexahydrofuro[3,2-b]furan-3-yl adipate and 4′-(4-(undec-10-enoyloxy)benzoyloxy)biphenyl-4-yl 6-(perfluorooctanoyloxy)hexahydrofuro[3,2-b]furan-3-yl adipate. The synthesized epoxy resins are cured using 4,4′-diaminodiphenyl-methane in mesophase state under a magnetic field to obtain crosslinked oriented elastomers. The chemical structures, LC properties and surface morphology of the monomers, the resins and the liquid crystalline elastomers (LCEs) are characterized by use of various experimental techniques such as FTIR, ¹H NMR, EA, TGA, DSC, POM, and X-ray measurements. The mesomorphic properties of the synthesized resins and corresponding oriented elastomers are influenced by the terminal perfluorocarbon chains components effectively. The resins show chiral nematic and chiral smectic C phases (), and are frozen in their corresponding oriented elastomers. The LC phases are verified by X-ray measurements, and the orientational order parameters of the oriented LCEs are calculated as well.