预取向半晶态高分子片晶结构形成微观机理及其应力-应变响应特性的分子动力学模拟

通过分子动力学模拟方法对不同预取向聚乙烯醇熔体(polyvinyl alcohol,PVA)形成的半晶态高分子熔体形成核结晶及拉伸过程中的应力-应变响应特性进行了系统地研究.模拟结果显示预取向度高的PVA熔体对应更快的成核动力学.通过追踪全trans伸直链长度(dtt)、成核原子维诺体积(V)和中心对称参数(S)等序参量在不同取向度熔体下的等温成核与结晶演化过程,给出了PVA熔体成核路径及形成半晶态的分子构象;通过对形成的半晶态高分子结构进一步分析,发现随着熔体取向度的增加,晶体和无定型对应的取向度也会增加,但是当应变剪切大于5时,其对应的结晶度、晶体和无定型取向度不再发生变化;通过对无定型区链结构的定义与分析,可知取向度越高的熔体对应越高的Tie链数目,随着熔体取向度的增加,Loop链的数目也会减小;通过恒速拉伸应力测试可知,所形成半晶态高分子结构力学响应会随着取向度及Tie链数增加而增加,当取向及Tie链数目相同时,应力-应变曲线形状大小也基本保持一致.     

Isothermal Crystallization Kinetics and Melting Behavior of a Luminescent Conjugated Polymer,Poly(9,9-Dihexylfluorene-Alt-2,5-Didodecyloxybenzene)

A study of the isothermal crystallization behaviors of poly(9,9-dihexylfluorene-alt-2,5-didodecyloxybenzene) (PF6OC12) was carried out using differential scanning calorimetry (DSC). The crystallization kinetics under isothermal conditions could be described by the Avrami equation. The Avrami exponent n ranges from 3.43 to 3.71 for PF6OC12 at crystallization temperatures between 100.0°C and 90.0°C, indicating a three-dimensional spherical crystal growth with homogeneous nucleation in the primary crystallization stage for the isothermal melt crystallization process. In the DSC scan, after the isothermal crystallization, multiple melting behavior was found. The multiple endotherms could be attributed to melting of recrystallized materials produced originally during different crystallization processes. According to the Arrhenius equation, the activation energy was determined to be 211.29 kJmol^?1 for the isothermal melt crystallization of PF6OC12.     

Nature of Shear-Induced Primary Nuclei in iPP Melt

Although observations of molecular processes in the formation of primary nuclei prior to actual crystallization are beyond the detection limits of current instrumentation, we attempted to probe the nature of primary nuclei in sheared isotactic polypropylene (iPP) polymer melt. In situ rheo-SAXS (small-angle X-ray scattering) and -WAXD (wide angle X-ray diffraction) experiments using synchrotron radiation were carried out to evaluate the effects of an addition of a high molecular weight atactic polypropylene (aPP) (5 wt%), which is compatible with the iPP matrix but does not crystallize, on the evolution of oriented structures in the sheared iPP melt and its crystallization kinetics. It is unlikely that the aPP chain segments can be incorporated into iPP nuclei or crystal; hence, its addition effects, if any, would be seen only in the amorphous melt prior to crystallization. The results showed stonger orientation and improved crystallization kinetics in the iPP/aPP blend compared to pure iPP. Observations that the presence of long chains of an amorphous polymer aid in nucleation and crystallization kinetics of iPP, combined with our previous synchrotron results of sheared iPP melts at high temperature (165°C), lead us to conclude that primary nuclei in iPP most likely consist of liquid-crystalline or mesomorphic bundles of aligned chain segments prior to the formation of crystals.     

The Melting Behavior of Poly(Ethylene Terephthalate)/SiO2 Nanocomposites

The melting and recrystallization behavior of poly(ethylene terephthalate) (PET)/SiO₂ nanocomposites after isothermal crystallization from the melt was studied by step-scan differential scanning calorimetry (SDSC). The influences of SiO₂ contents, crystallization temperature, and crystallization time on the melting process were examined. Two melting endotherms (in the SDSC C_P.A curves, reversible part) and one recrystallization exotherm (in the SDSC C_P.IsoK curves, irreversible part) of PET/SiO₂ nanocomposites after isothermal crystallization were observed during the melting process. All are ascribed to the melting–recrystallization mechanism. The low-temperature endotherm is attributed to the melting of initial crystals formed during the isothermal treatment and the high temperature endotherm to the melting of recrystallization materials. The reason why more recrystallization happened with the increase of SiO₂ content is given and the process of recrystallization was described in detail. The effects of crystal perfection and recrystallization were minimized by increasing crystallization temperature and time.     

Brownian motors in the photoalignment of liquid crystals

Light can change the orientation of liquid-crystal molecules. Usually, the torque that causes the reorientation originates in angular-momentum transfer from the radiation field to the material. If a small amount of dichroic dye is dissolved in the liquid crystal, a light-induced torque can appear essentially without the transfer of angular momentum from light. We show that, in such cases, the dye molecules act as light-driven molecular motors which, via an orientational Brownian ratchet mechanism, transfer angular momentum, which originates at the cell walls, to the liquid crystal. Understanding the details of this mechanism is important for applications ranging from flat-panel displays to optomechanical transducers. Received: 20 October 2001 / Accepted: 14 January 2002 / Published online: 22 April 2002     

Relaxation of the director field in the form of a traveling wave in twisted nematic cells

The orientational relaxation of the director field toward its equilibrium orientation in the form of a traveling wave is investigated. It is shown that the nonlinear relaxation can occur in the form of a traveling wave in a twisted nematic cell under specific conditions for the external electric field and material parameters characterizing the nematic liquid crystal. The calculations of the relaxation processes in the form of traveling waves have demonstrated that these nonlinear processes in the twisted nematic cell proceed more easily when the initial perturbation of the director is induced in the vicinity of one of the bounding cell surfaces.     

Relaxation dynamics of poly(vinylidene fluoride) studied by dynamical mechanical measurements and dielectric spectroscopy

The aim of this study is to analyze the mobility of polymer chains in semicrystalline poly(vinylidene fluoride) (PVDF). PVDF crystallizes from the melt in the α crystalline phase. The transformation from the α phase to the electroactive β phase can be induced by stretching at temperatures in the range between 80 and 140 °C. The spherulitic structure of the crystalline phase is deformed during stretching to form fibrils oriented in the direction of the strain. The amorphous phase confined among the crystalline lamellae is distorted as well and some degree of orientation of the polymer chains is expected. Dynamic-mechanical and dielectric spectroscopy measurements were performed in PVDF films stretched to strain ratios up to 5 at temperatures between 80 and 140 °C. Dynamic-mechanical measurements were conducted between -60 °C and melting and in this temperature range the relaxation spectra show the main relaxation of the amorphous phase (called β-relaxation) and at higher temperatures a relaxation related to crystallites motions (α (c)-relaxation). Although the mean relaxation times of the β-relaxation are nearly equal in PVDF before and after crystal phase transformation, a significant change of shape of the relaxation spectrum proves the effect of chain distortion due to crystal reorganization. In stretched PVDF the elastic modulus of the polymer in the direction of deformation is significantly higher than in the transversal one, as expected by chain and crystals fibril orientation. The recovery of the deformation when the sample is heated is related with the appearance of the α (c)-relaxation. Dielectric spectroscopy spectrum shows the main relaxation of the amorphous phase and a secondary process (γ-relaxation) at lower temperatures. Stretching produces significant changes in the relaxation processes, mainly in the strength and shape of the main relaxation β. The Havriliak-Negami function has been applied to analyze the dielectric response.     

Induction time as a measure for heterogeneous spherulite nucleation: Quantitative evaluation of early-stage growth kinetics

A theory is proposed indicating that the induction time characterizes the nucleation process in polymer crystallization. A microscopic method has been developed to monitor early stages of the spherulite growth. Using the method, based on photometric measurement of the depolarized light intensity during isothermal crystallization from the melt, nonzero nucleation induction time values were found for polypropylene (PP). From comparison of the theory with the experimental results obtained from polypropylene crystallization, it follows that the formation of PP crystal nuclei on admixtures is affected both by the formation of the first crystalline layer and by the size of the critical nucleus.     

A new analytical method for the optimization of thermomechanical conditions in zone drawing and its application to polyethylene

An original method of zone drawing of polymers at constant load and a procedure for the optimization of thermomechanical conditions (stress, temperature) are suggested, allowing high draw ratios and favorable strength values to be obtained. The temperature (or stress) range of necking has been determined in a nonisothermal and in an isothermal regime. The advantage of the method consists in that the increasing orientation in the neck starting from the initiation point and up to fracture allows the morphology and properties to be quickly examined, depending on the varying thermomechanical conditions in different regions of the neck. At a high temperature and low load the mechanism of oriented crystallization from melt becomes operative; in opposite cases, orientation of the solid state takes place. It is shown that in the nonisothermal regime an increased rate of heating allows extreme draw ratios (up to ca. 150) to be obtained, approximately twice as high as those obtained in the isothermal regime. This is interpreted as a gradual improvement of the oriented structure by recrystallization during extension and by a quick fixation on cooling below the zone.     

Analysis of crystallization kinetics of sputtered SmCo based permanent magnetic films

Crystallization kinetics of the sputtered SmCo based permanent magnetic films was investigated by differential scanning calorimeter, x-ray diffraction, and atomic force microscope methods. The results show that the apparent activation energy for crystallization is observed as 173.7 kJ/mol, and the local activation energy for crystallization decreases with increasing crystal phase transformation fraction in non-isothermal crystallization. For isothermal crystallization, the apparent activation energy for crystallization is 159.8 kJ/mol. The local activation energy for crystallization exhibits non-monotonic dependence on the crystal phase transformation fraction. The crystallization mechanism is obtained from the investigation of Avrami exponent and microstructure.     

Morphology and Structure of Poly(p-phenylene Benzobisthiazole) Crystals

The rigid polymer poly(p-phenylene benzobisthiazole) (PBZT) was crystallized from dilute solution. Electron microscopy showed that, on quenching, flat fibrils several nanometers thick were produced. Subsequent heat treatment in a solvent changed the morphology from fibrillar into “segmented ribbon” structure. Isothermal crystallization at a temperature of about 30°C below the dissolution temperature, in general, resulted in aggregation of rod crystals. The polymer chains were oriented normal to the rod crystals. The width of the rod crystal increased with average molecular length, but saturated to a value much smaller than the average molecular length. In the shorter molecular length range, the rod crystals clustered in a fanned-out manner, while with a medium molecular length (ca. 70–120 nm), all rods crystals in a cluster aligned parallel to each other and were of the same length. With longer molecular length (more than ca. 180 nm), the rod growth slowed because of small diffusion constants of molecular chains to the growing face. Based on observation of the morphology and the crystallization process, an isothermal crystallization mechanism is proposed. Because of the rigidity and wide length distribution of polymer chains, the chain ends were inevitably included within the crystals, resulting in crystal defects such as axial shifts, lattice curvatures, and edge dislocations, all of which were observed directly by lattice imaging.     

Effects of draw-down ratio and annealing treatment on structure formation in extruded strands of a thermotropic liquid crystalline copolyester

The molecular orientation, thermal behavior, and crystal lattice structure in extruded strands of a thermotropic liquid crystalline polymer (LCP) were studied with wide-angle x-ray diffraction (WAXD) and differential scanning calorimetry (DSC). The purpose of this work is to elucidate the effects of draw-down ratio and annealing treatment on the structure development in the LCP strands. The crystal orientation function markedly increased with increasing draw-down ratio, but the increase of orientation function saturated at higher draw-down ratio. Annealing treatment below 250°C slightly increased the degree of orientation, whereas the molecular orientation was relaxed by annealing at 270°C. In addition, the thermal properties and crystal lattice structure were sensitive to the annealing treatment. The change of DSC curves with annealing temperature suggested that the initial crystalline texture of as-extruded samples was reorganized into a more ordered structure by the annealing treatment. Draw-down ratio had some effects on the thermal properties. The molecular orientation facilitated the crystallization during annealing.     

Sequential Structure,Crystallization, and Properties of Biodegradable Poly(ethylene Terephthalate-Co-Ethylene Oxide-Co-Lactide) Copolyester

The sequential structure, isothermal crystallization, tensile property, and degradation behavior of poly(ethylene terephthalate-co-ethylene oxide-co-lactide) (ETOLA) copolyester based on melt transesterification of poly(ethylene terephthalate) with poly(ethylene oxide) and oligo(lactic acid) was investigated. The degree of randomness was calculated to be 0.38, showing the incorporation of poly(ethylene oxide) (PEO) blocks into the homogeneous sequences of ethylene terephthalate (ET) and lactide (LA) units. The isothermal crystallization kinetics results revealed that the crystallization activation energy of the copolyester calculated using the Arrhenius’ equation was lower than that reported for poly(ethylene terephthalate) (PET), indicating that the addition of PEO and LA units into PET retarded the crystallization of PET. The copolyester exhibited the same crystal structure at different crystallization temperatures, similar to that of PET homopolymer, based on wide angle X-ray diffraction results. The size of the spherulites of ETOLA increased with crystallization temperature. The increase of crystallization temperature reduced the elongation at break of the copolyesters, as well as the enzymatic degradation.     

The mechanisms of recrystallization after melting in syndiotactic polypropylene and isotactic polystyrene

Several semicrystalline polymers show a recrystallization after melting during a heating scan. We have studied the mechanisms of such recrystallization processes for two different polymers, namely syndiotactic polypropylene (sPP) and isotactic polystyrene (iPS). This was done by monitoring the structure evolution during the recrystallization process and its changes during a subsequent heating scan via time- and temperature-dependent SAXS measurements, respectively. The results of this study showed that the sPP samples exhibited a recrystallization mechanism similar to the multi-stage route found upon initial crystallization of semicrystalline polymers from an entangled melt. Meanwhile, a different recrystallization mechanism was shown by the iPS samples. In this case, the recrystallization process proceeded as a direct growth into the melt in a one-step process. This is the first time we have observed such a mechanism which resembles the picture presented by the classical models for crystallization from an entangled polymer melt. The reason for such different mechanisms may be related to the initial melt state prior to crystallization. It seems as though, when crystallization sets in an entangled polymer melt, it follows the multi-stage route, whereas if the melt is locally disentangled, it proceeds by a direct growth mechanism. Received 23 July 2001 and Received in final form 4 October 2001     

The Role of Poly(phenylene sulfide) Microfibrils in Isothermal Crystallization of Isotactic Polypropylene under Shear

An optical polarizing microscope with a hot shear stage was used for an in‐situ investigation of the influences of poly(phenylene sulfide) (PPS) microfibrils on isothermal crystallization of isotactic polypropylene (iPP) under shear. As the nucleation sites on the PPS microfibril's surface are not able to induce a transcrystalline layer, there are only spherulites generated in a PPS/iPP in‐situ microfirbillar blend in quiescent condition. Applying shear during isothermal crystallization, the crystalline morphology greatly changes. There are fibrillar nuclei induced after steady shear with a shear rate of 5 and 10 s^–1, and these nuclei formed fibrillar crystals after crystallization completion. Two opposite effects coexist in PPS/iPP in‐situ microfibrillar blends during shear‐induced isothermal crystallization; one is the obstructive effect of PPS microfibrils on the iPP molecular chains orientation; the other is the positive effect provided by stress between fiber and matrix, generated by shear, which reduces the potential barrier of crystallization. The results of wide angle x‐ray diffraction (WAXD) show that there are β‐iPP crystals generated in neat iPP and PPS/iPP blends, but that PPS microfibrils have an inhibiting influence on the formation of β‐iPP.     

CO2-induced Crystallization of Isotactic Polypropylene by Annealing Near Its Melting Temperature

CO₂-induced crystallization of isotactic polypropylene (iPP) by annealing had been studied using differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS). The iPP before annealed was in α-form and amorphous states. At lower temperatures by CO₂ isothermal treatments, iPP chains crystallized from the amorphous phase and only one crystal form, i.e., α-form, was observed. At higher temperatures by CO₂ isothermal treatments, both crystallization from the amorphous phase and thickening of existing crystal lamellae were observed. Moreover, light γ-form crystal appeared in the treated iPP. The crystalline lamellar thickness of iPP annealed at different CO₂ pressures had been determined. Using the Gibbs–Thomson plot method, the equilibrium melting temperature was found to be 187.6°C.     

Isothermal Crystallization Kinetics and Melting Behavior of Poly(ethylene terephthalate)/Attapulgite Nanocomposites Studied by Step-scan DSC

The crystallization kinetics of poly(ethylene terephthalate)/attapulgite (AT) nanocomposites and their melting behaviors after isothermal crystallization from the melt were investigated by DSC and analyzed using the Avrami method. The isothermal crystallization kinetics showed that the addition of AT increased both the crystallization rate and the isothermal Avrami exponent of PET. Step-scan differential scanning calorimetry was used to study the influence of AT on the crystallization and subsequent melting behavior in conjunction with conventional DSC. The results revealed that PET and PET/AT nanocomposites experience multiple melting and secondary crystallization processes during heating. The melting behaviors of PET and PET/AT nanocomposites varied in accordance with the crystallization temperature and shifted to higher temperature with the increase of AT content and isothermal crystallization temperature. The main effect of AT nanoparticles on the crystallization of PET was to improve the perfection of PET crystals and weaken its recrystallization behavior.     

Dielectric relaxation in polymers and copolymers of methacrylates and methacrylamides with chromophoric groups in side chains

The dielectric relaxation and molecular motion in polymers and copolymers of methacrylates and methacrylamides with π-conjugated anisodiametric chromophoric groups in the side chains are investigated in the temperature range from ?170 to 120–150°C at frequencies of 0.1–100 kHz. It is shown that the motion of chromophoric groups and relaxation transitions in these systems are similar to those in liquid-crystal polymers with side mesogenic chains. The chromophoric groups execute orientational motion of two types: orientational rotations about the long axes in the glass state and orientational rotations about the short axes at temperatures above T _g . The π-conjugation excludes local motion within the chromophoric group.