Studies on the influence of molecular weight and isotacticity of polypropylene on the formation of mesomorphic phase

Polypropylene (PP) blends with various molecular weight and isotacticity were prepared through solution blending and subjected to rapid melt quenching. Structural changes in the PP matrix during mesomorphic phase formation were measured by FTIR spectroscopy and wide‐angle X‐ray diffraction measurements. The blends with different molecular weight and isotacticity provided the pathway to understand their influence on mesomorphic phase formation. It is observed that low molecular weight PP with low isotacticity forms mesomorphic phase, whereas high molecular weight and low isotactic PP does not lead to the formation of mesomorphic phase. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Mesophase structure of some flexible chain polymers

The mesophase structures of some flexible chain polymers, which do not contain mesogenic groups, but are able to form thermodynamically equilibrium mesomorphic states, were studied by means of the X-ray analysis, the differential scanning calorimetry, and other methods. The mesophase polyethylene in bicomponent polymer blends, mesomorphic ultra-high molecular weight polyphosphazenes, and membrane polyvinylorganosilanes were investigated. The scheme of polymer phase transitions is discussed. Besides conformational disordering, the step-by-step loss of different levels of the positional order (i.e. the multiple-step character of melting) of flexible, semiflexible, and stiff chain polymers is deduced and illustrated by examples from the authors' experience and from the literature.     

丁二烯气相聚合产物分子量分布和粒径分布模型研究

对非均相催化的丁二烯气相聚合,基于聚合物多层模型,考虑催化剂颗粒间活性位初始浓度和粒径分布对聚合物分子量分布和粒径分布的影响,建立了聚合物分子量分布和粒径分布的数学模型。模拟了反应温度、催化剂颗粒间活性位初始浓度和粒径分布等因素的影响,结果表明。随着温度升高,聚合物颗粒平均粒径变小,粒径分布变窄,聚合物分子量变小,分子量分布变宽;催化剂颗粒间的活性组分负载越均匀,聚合物分子量越大,分子量分布和粒径分布越窄;随着催化剂平均粒径变大,聚合物分子量变小,分子量分布变宽,不存在催化剂颗粒粒径分布和聚合物颗粒粒径分布间的复制现象。模型模拟结果与实验结果吻合较好,可用于预测丁二烯气相聚合产物的分子量、分子量分布和粒径分布。     

Mesomorphic polyblends of poly(p-phenylene terephthalamide) and poly(4,4′-terephthanilide adipamide)

A simultaneous dissolution of two lyotropic polyamides, poly(p-phenylene terephthalamide) (PPTA) and poly(4,4′-terephthanilidc adipamide) (PTAd), in a concentrated H₂SO₄ (1 to 1.5% wt) produced an isotropic single phase solution. The crystallinity of PPTA/PTAd blends confirmed formation of a mesomorphic polyblend to the molecular level. With increasing concentration, solutions of these PPTA/PTAd blends in 100% H₂SO₄ showed a sequential phase change typical of a single lyotropic polymer. Further, the ternary solution exhibited a wider biphase range than both binary solutions of PPTA/100% H₂SO₄ and PTAd/100% H₂SO₄. Dry-jet wet spinning of anisetropic solution of this ternary composition with a PTAd weight fraction less than 0.5 enabled us to obtain PPTA/PTAd blend fibers. Including a PTAd weight fraction of 0.1-0.15 reduced both the orientation angle and fibrillation. The particular blend fiber with a PTAd weight fraction of 0.15 exhibited a synergistic effect on the mechanical properties.     

Shear induced molecular alignments of a side-chain liquid crystalline polyacetylene containing biphenyl mesogens

Mesomorphic properties of a side chain liquid crystalline polyacetylene, poly(11-{[(4′-heptyloxy-4-biphenylyl)carbonyl]oxy}-1-undecyne) (PA9EO7), are investigated using polarized optical microscope, X-ray diffraction, and transmission electron microscope. Polymer PA9EO7 forms enantiotropic smectic A and smectic B phases. It also exhibits an additional high order smectic phase, a sandwich structure consisting of different molecular packing of biphenyl mesogenic moieties from that of alkyl spacers and terminals, when it is prepared from its toluene solution. Shearing the polymer film at its smectic A phase generates banded texture with the alignment of the backbones parallel to the direction of shear force. While at its high order smectic phase, the mesogen pendants of the polymer are arranged parallel to the direction of shear. The different mesomorphic behaviors arise from different molecular alignments influenced by the fluidity.     

Binodal and spinodal curves: a simulation for various high polymer mixtures

Flory's equation-of-state theory has been used to predict the lower critical solution temperature behaviour of polymer—polymer mixtures. The spinodal phase boundary of numbers of high molecular weight polymer mixtures have been previously simulated using this theory. In this paper a procedure for simultaneous predictions of the binodal and the spinodal curves by equating the chemical potential of each component in the mixture is presented. The method is tested for five different mixtures. The effects of the binary and pure component state parameters on the simulated curves are discussed and the simulated phase diagrams are compared with the experimental cloud point curves. It is found that in most cases the results are more consistent with the cloud point curve being closer to the spinodal curve than the binodal.     

Flow-induced shish-kebab precursor structures in entangled polymer melts

Flow-induced crystallization has long been an important subject in polymer processing. Varying processing conditions can produce different morphologies, which lead to different properties. Recent studies indicated that the final morphology is in fact dictated by the initial formation of crystallization precursor structures (i.e. shish-kebabs) under flow. In this article, factors that affect the shish-kebab formation in entangled polymer melts are systematically reviewed, including the concept of coil-stretch transition, chain dynamics, critical orientation molecular weight, phase transition during shish and kebab formations. In particular, recent experimental results from in situ rheo-X-ray studies and ex situ microscopic examinations have been presented to illustrate several new findings of flow-induced shish-kebab structures in polymer melts. (1) The shish entity consists of stretched chains (or chain segments) that can be in the amorphous, mesomorphic or crystalline state. (2) The kebab entity mainly arises from the crystallization of coiled chains (or chain segments), which seems to follow a diffusion-control growth process. (3) A shish-kebab structure with multiple shish was seen in the ultra-high molecular weight polyethylene (UHMWPE) precursor. Based on the above results and recent simulation work from other laboratories, a modified molecular mechanism for the shish-kebab formation in entangled melt is presented.     

Determination of the enthalpy of solid phase transition for isotactic polypropylene using a modified DSC technique

This paper investigates the enthalpy of solid phase transition of isotactic polypropylene from the mesomorphic phase to the monoclinic crystal form using DSC. iPP fibres with either monoclinic crystal form or mesomorphic phase were obtained by varying the conditions of melt spinning. XRD and mechanical tests were used to confirm the crystal forms. It is found that conventional DSC techniques fail to detect the transition enthalpy; a technique using silicon oil as the thermal medium was then employed to determine the transition enthalpy, which is of 3% the melting enthalpy for crystallites with monoclinic form, or 6.1 J/g on 100% crystallinity basis.     

Solid-state complexes of poly(N-vinyl-pyrrolidone) and dodecyl benzene sulfonic acid: Self-assembled structure and thermal properties

Solid-state complexes of flexible polymers with long-tail surfactants (surf) exhibit mesomorphic phase and microphase-separated lamellar morphology consisting of alternating polymer and surf layers. The alkyl tails in the surf layers are highly stretched and closely packed due to the excluded volume effect. In this study, the self-assembled structure of a new polymer(surf) complex system based on complexation between poly(N-vinyl pyrrolidone) (PVP) and dodecyl benzene sulfonic acid (DBSA) was investigated by optical microscopy and small-angle X-ray scattering (SAXS). Both mesomorphic phases and microphase-separated lamellar morphology were observed in PVP(DBSA) complexes. The thicknesses of polymer and surf layers were determined from the one-dimensional correlation function. Both types of thickness are dependent on complex composition. The interplay between chain conformational entropy and interaction energies (repulsive and attractive) is suggested to control the self-organized structure in the complexes. Complexation with DBSA reduced the thermal stability of PVP, which was in contrast with poly(ethyleneimine)-DBSA complexes reported previously.     

Molecular weight effects on phase transitions of side-chain liquid crystalline poly(vinyl ether)s with terminal alkoxybiphenyl groups

Two vinyl ether monomers with alkoxybiphenyl groups, 2-(4′-ethoxy-4-biphenyloxy)ethyl vinyl ether (EtOVE) and 2-(4′-hexyloxy-4-biphenyloxy)ethyl vinyl ether (HexOVE), were polymerized by living cationic polymerization reactions using either the hydrogen iodide/iodine (HI/I₂) or the hydrogen iodide/zinc iodide (HI/ZnI₂) initiator systems, or both. These initiators yield polymers with narrow molecular weight distributions (MWDs) and for comparison broad MWD polymers were also prepared by using the BF₃OEt₂ initiator. The thermal properties and phase transitions of these polymers were determined by differential scanning calorimetry, by visual observations of samples on a hot stage on a polarizing microscope, by polarized light transmission intensity measurements and by wide angle X-ray diffraction analysis. The polymer from EtOVE, P(EtOVE), with a weight average molecular weight, , of less than about 8000 formed both smectic and nematic liquid crystalline, LC, phases after one heating cycle. In contrast, the polymer from this monomer which had an of more than about 8000 exhibited only a nematic LC phase. For the polymer from HexOVE, P(HexOVE), both the narrow and broad MWD samples showed only a nematic LC phase over the range from 2600 to 7300. The phase transitions of both types of polymers are discussed in relation to the molecular weight and MWD of the samples. The effect of the terminal group attached to the biphenyl group in the polymer is considered in relation to its possible steric effects.     

Synthesis, mesomorphic properties and light scattering of polyacrylates liquid crystals

Summary The synthesis and phase behavior of three homopolymers with liquid crystalline side-chains is described. These new homopolymers show a dependence of the phase transition temperatures on the molecular weight. The synthesized material exhibits typical mesomorphism-nematic, smectic A and C. We compare the mesomorphic behavior of the homopolymers of the different liquid crystals polyacrylates. In addition, a dynamic and static light scattering study in different solvents has been undertaken. The hydrodynamic radius as well as the diffusion coefficient of the single polymer molecules could be determined. Moreover, the experimental results suggest the formation of aggregates or clusters in the THF dilute regime. Received: 29 November 1998/Revised version: 2 April 1999/Accepted: 5 April 1999     

Thermally induced phase separation in poly(lactic acid)/dialkyl phthalate systems

Thermally induced phase separation in poly(lactic acid)/dialkyl phthalate systems was investigated. Poly(DL ‐lactic acid) (PDLLA) and poly(L ‐lactic acid) (PLLA) with different molecular weights were used. A series of dialkyl phthalates, with different numbers of carbon atoms in the alkyl chain, were employed as solvents to control the interaction between polymer and solvent. The liquid–liquid phase‐separation temperature of the poly(lactic acid) solutions decreased systematically with a shorter alkyl chain in the phthalate. Based on the interaction between polymer and solvent and the molecular weight of polymer influencing liquid–liquid phase‐separation temperature significantly but crystallization temperature only slightly, proper thermal conditions were employed to investigate competitive phase separation and crystallization in PLLA solutions. Factors that can influence the final morphology of PLLA solutions were examined. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 2224–2232, 2003     

A facile preparation of highly interconnected macroporous poly(d,l-lactic acid-co-glycolic acid) (PLGA) scaffolds by liquid-liquid phase separation of a PLGA-dioxane-water ternary system

Regular and highly interconnected macroporous scaffolds ranging in size from 50 to 150 μm were fabricated from poly(d,l-lactic acid-co-glycolic acid) (PLGA)-dioxane-water ternary systems via thermally induced phase separation (TIPS) without any surfactant or other additives. The effect on scaffold morphology of processing parameters including quenching temperature, polymer concentration, solvent composition and molecular weight, was investigated as a function of quenching time. The cloud-point temperature of the polymer solution was found to depend on polymer concentration, solvent composition, and polymer molecular weight. The water content in the solvent mixture had the greatest effect on the cloud-point temperature. The optimal quenching temperature for preparing macroporous inter-connected scaffolds from a 9 wt% PLGA solution (dioxane-water=87/13, by wt) was less than −7 °C. In low viscosity PLGA solutions, sedimentation of the polymer rich phase occurred due to the segregation of the separated phases under gravity. This led to the formation of scaffolds with irregular and closed pores.     

Polymer molecular weight and chain transfer during the photopolymerization of an aliphatic monoacrylate in a smectic liquid crystal

Polymer/liquid crystal (LC) composites offer a unique opportunity to study polymerizations in ordered media, specifically the potential effect mesophase order can have on polymer properties including molecular weight. To develop successful polymer/LC composites for display applications, it is important to understand the effect of mesophase order on polymer molecular weight in order to optimize the electro-optic (EO) properties of the polymer/LC composite. Polymer molecular weight may be influenced in a LC by changes in polymerization rate as LC order is modulated and by chain transfer. This work focuses on the photopolymerization of an aliphatic monoacrylate monomer, decyl acrylate (DA), both in the ordered LC phases of 8CB as well as in isotropic solutions with LC and co-solvent.When DA is polymerized using the LC as the solvent, enhanced polymerization rates and polymer molecular weights are observed in the highly ordered smectic phase compared to the less ordered nematic and isotropic phases. When conducted strictly in an isotropic environment using a co-solvent with increasing 8CB percentages, a dramatic decrease in the polymerization rate and a significant reduction of the polymer molecular weight is observed, implying degradative chain transfer to the LC. NMR results show that this chain transfer is a result of hydrogen abstraction from the liquid crystals, which leads to the reduction in the polymerization rate with increasing 8CB concentration. The most likely site of hydrogen abstraction is from the benzyl hydrogens of the alkyl chain of 8CB. This chain transfer also plays a role for polymerizations performed in the ordered phases of the LC. Chain transfer appears to be less significant when polymerizations are conducted in the smectic phase due to the anti-parallel association of the LC molecules. When polymerizations occur in the less ordered phases, chain transfer dominates leading to a large reduction in polymer molecular weight and polymerization rate.     

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.     

Preparation of porous hollow fiber membranes with a triple‐orifice spinneret

A triple‐orifice spinneret has been applied for the preparation of hollow fiber microfiltration membranes with a high surface porosity. Considering the general rules of diffusion induced phase separation, a low polymer concentration is required at the outer layer to obtain a highly interconnected open‐porous structure. Therefore, by using N‐methylpyrrolidone (NMP) as the external liquid at the outside orifice of the spinneret, a highly porous surface can be obtained. For a polymer solution containing a low molecular weight additive and with an initial concentration close to the cloud point, this technique shows slightly improvement on the pure water and gas fluxes since the major resistance of the membrane is located at the substructure and the inner skin. However, for a solution containing a high molecular weight additive and with an initial concentration far from the cloud point, a porous shell surface is obtained, resulting in a significant improvement in water flux. The effect of various external liquids on the morphology has been investigated as well. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 87: 2151–2157, 2003     

1,3-二(二苯基氯硅烷基)-2,2,4,4-四苯基环二硅氮烷的合成及液晶性表征

用1,3-二(二甲基氯硅烷基)-2,2,4,4-四甲基环二硅氮烷与二苯基二氯硅烷反应,得1,3-二(二苯基氯硅烷基)-2,2,4,4-四苯基环二硅氮烷,用甲苯与正辛烷提纯化合物,熔点200～206℃,收率75%～78%。产品经IR和1H NMR确证了结构,用示差扫描量热法(DSC)与偏光显微镜分析,发现该化合物具有液晶性,属于热致型液晶,显示六方相。经教育部科技查新工作站(L03)2007年11月22日出具的第2007236号《科技查新报告》证实,在国内文献中未见关于1,3-二(二苯基氯硅烷基)-2,2,4,4-四苯基环二硅氮烷液晶性表征的相关报道。     

Rubber-modified epoxies: III. Influence of the rubber molecular weight on the phase separation process

The effect of varying the rubber molecular weight on the reaction induced phase separation taking place in rubber-modified epoxies is reported. Three butadiene–acrylonitrile copolymers with number-average molecular weights of 2030, 3600 and 6050 g mol^?1 were used. The thermosetting polymer was an epoxy based on diglycidylether of bisphenol-A cured with a cycloaliphatic diamine. Increasing the rubber molecular weight showed the following effects: (i) a decrease in the conversion at the cloud point; (ii) a corresponding decrease in the viscosity at the cloud point; (iii) a primary morphology of a low concentration of particles with a large size; (iv) a more complete segregation of the rubber from the matrix leading to a smaller decrease of the glass transition temperature of the system; (v) a decrease in the toughening. The Flory–Huggins lattice model may be used as a simple approximation to describe the phase separation process when varying the rubber molecular weight.     

Crystallization of poly(ethylene terephthalate) (PET) from the oriented mesomorphic form

Yarns of different inherent viscosities, in the range 0.6–1.1 dL/g, spun in industrial plants, and drawn at room temperature to obtain mesomorphic samples, have been characterized. The evolution from the mesomorphic form toward the triclinic crystalline form has also been studied by combined differential scanning calorimetry (DSC), dynamic mechanical analyses (DMA), and accurate wide angle X-ray diffraction experiments. The DMA analysis of the mesomorphic samples allows better resolution of the glass transition and crystallization phenomena, which are superimposed in the DSC scans. The degree of molecular orientation in the mesomorphic samples, and the temperature of crystallization from the mesomorphic form (60–80°C), are essentially independent of the polymer molar mass. © 1994 John Wiley & Sons, Inc.     

聚烯烃微孔膜相分离法成型技术

介绍了聚烯烃微孔膜相分离法成型加工原理、工艺过程、影响因素、性能参数及其应用。其影响因素包括聚合物相对分子质量、成核剂、化合物含量、冷却速率和拉伸条件等。聚合物相对分子质量高可提高薄膜的拉伸强度和化合物含量及孔隙率；成核剂可提高化合物的含量、拉伸成型性能、微晶数量，降低微晶大小，提高孔隙率；采用适宜的冷却速率和拉伸条件才能获得性能良好的微孔膜。