聚合物球晶生长过程模拟

结晶性聚合物在熔融态冷却过程中,往往伴随着球晶的生成,球晶的密度和尺寸分布直接影响到制品的热力学、光学及力学性能。文中研究了聚合物冷却过程中球晶的形态学演变;模拟了球晶在生长过程中的形态演化过程;考察了成核诱导时间、成核密度、生长速率和碰撞对球晶形态变化的影响;比较了等温和非等温条件下聚合物的结晶过程。研究结果表明,在等温结晶过程中随着温度的降低,晶粒越来越细化,结晶程度越来越大,球晶碰撞边界为直线;非等温结晶过程中由于温度一直在降低,不断出现新的球晶,晶体大小不一,碰撞后边界为曲线。     

Isothermal crystallization kinetics and morphology of biodegradable poly(3-hydroxybutyrate-co-4-hydroxybutyrate)

Isothermal crystallization kinetics and morphology of poly(3-hydroxybutyrate-co-4-hydroxybutyrate) [P(3HB-co-4HB)] with different 4-hydroxybutyrate (4HB) molar fraction were investigated by differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD), and polarized optical microscopy (POM). The results show that the crystallization mechanism and crystal structure of P(3HB-co-4HB) copolymers are the similar as those of poly(3-hydroxybutyrate) (PHB). While the equilibrium melting point and crystallization rate decrease with the increase of 4HB molar fraction. Banded spherulites are observed in neat PHB and P(3HB-co-4HB) copolymers, and morphology is influenced apparently by the crystallization temperature and 4HB unit.     

Nucleation,growth and morphology of poly(hydroxybutyrate) and its copolymers

Measurements of the melting temperatures, growth rates and nucleation rates of meltcrystallized polyhydroxybutyrate (PHB) and two poly(hydroxybutyrate-co-hydroxyvalerate) (PHB/HV) copolymers are reported for crystallization over a wide range of temperature. Examples are shown of the spherulitic morphologies obtained. From the variation in melting point with crystallization temperature values for the equilibrium melting points of PHB and PHB/HV are obtained. Growth and nucleation rate measurements are analysed using secondary nucleation theory to obtain approximations to surface energies within the crystals. PHB and PHB/7% HV are described well by secondary nucleation theory. PHB/23% HV exhibits more complex crystallization and melting behaviour, which suggests temperature-dependent exclusion of HV units from the crystals.     

Solution crystallization of polyethylene at high temperatures

Single crystals of polyethylene have been grown from solution in various solvents at temperatures between 70 and 120° C. This represents an overlap in crystallization conditions with those used for melt growth, where substantial isothermal thickening is known to occur during growth. The crystal thicknesses have been measured by Raman spectroscopy. Values of the equilibrium dissolution temperature and fold surface free energy are calculated for each solvent and the results analysed using the kinetic theory. Variations in crystal properties with time of crystallization are also investigated. A specific dependence of fold length on supercooling has been found to apply over the whole temperature range, consistent with predictions by the kinetic theories of crystallization in spite of changes in morphology which are incompatible with assumptions underlying the theoretical model. No evidence for isothermal thickening has been observed, except possibly for a small marginal effect at the highest temperature of 120° C investigated, over the same temperature range where melt crystallized material shows the effect prominently. Crystals grown at all temperatures displayed a rise in dissolution temperature with time which could be associated with an increase in surface perfection. All these findings have wider implications for our picture of polymer crystallization and crystal structure which are discussed here. A further, explicit, correlation with melt crystallization is deferred to a subsequent publication.     

成核剂对PET结晶行为的影响

采用差示扫描量热仪(DSC)研究了PET及添加成核剂后PET的非等温结晶过程,选用7种成核剂和4种复合成核剂进行了实验对比研究.结果表明添加任何一种成核剂后PET体系的结晶峰温移向高温,半结晶时间缩短,结晶速率常数增大,结晶度提高.其中有机类成核剂对PET结晶改善效果较无机类显著,但结晶效果最好的是成核机理为化学成核的高分子类成核剂和复合成核剂.本文还采用了热台偏光显微镜(POM)对PET晶体形貌进行了研究,发现添加有机成核剂后晶体细微而且密集.     

PHB／PPO共混体系的相容性和结晶

采用溶液共混方法对聚β－羟基丁酸酯／聚氧化丙烯共混体系的相容性和结晶行为进行了研究，ＤＳＣ，ＰＯＭ，ＷＡＸＤ等实验结果表明，该体系不是相容体系。但在特殊的组成比下有相容的第三相存在。ＰＰＯ对ＰＨＢ的结晶结构无影响，但对ＰＨＢ的结晶速率和结晶完整性有很大的作用。     

Structure development in PP/CaSO4 composites: Part I Preparation of the filler by an in situ technique

The structure, growth and morphology of calcium sulphate prepared in situ of a polymer has been investigated using polyethylene oxide as the growth medium. The structure was predominantly the dihydrate type with monoclinic configuration for these samples as compared to the anhydrite type with orthorhombic structure obtained in commercial samples. The crystal morphology consisted of sharp needle shape, having aspect ratio of more than 10. The concentration of the polymer used had a profound effect on the crystal size and its distribution. The size distribution became narrow and the average crystal size reduced with the increase of polymer concentration. Highly oriented crystals could be obtained with application of, small shearing force on the samples. These results have been explained on the basis of polymer mediated crystallization and large interaction between the filler and the matrix. © 1998 Chapman & Hall     

Conteol of Crystal Growth in Drug Suspensions: 1) Design of a Conteol Unit and Application to Acetaminophen Suspensions)

Abstract

A monitor system is described for the control of particle growth by crystallization in real pharmaceutical suspensions, based on the measurement of drug concentration in the liquid phase in contact with the drug crystals. The control unit consists of a thermostated vessel containing the drug suspension and a monitoring circuit including a dedector (i.e. refractive index, UV absorption). The concentration of the liquid supernatant is recorded in parallel with the actual temperature. Typical concentration-time curves indicate any dissolution or crystallization if temperature cycling (ΔT ± 10K) is applied on the suspensions.

It is demonstrated by acetaminophen crystals that after decreasing the temperature the crystal growth appears significantly impeded even by very small amounts of PVP (3 ppm, mol mass 180,000). The polymer did not influence the rate of dissolution of the crystals at higher temperature.

Surfactants reduce the protective action of PVP on crystal growth, in particular anionic surfactant which neutralize the protective action totally.

Crystal growth can be successfully inhibited by substances, which are irreversibely adsorbed to the crystal surface by specific interactions with their functional groups and a polymer structure of high molecular mass.     

Aqueous film coating to reduce recrystallization of guaifenesin from hot-melt extruded acrylic matrices

Objectives: This study investigated the effect of aqueous film coating on the recrystallization of guaifenesin from acrylic, hot-melt extruded matrix tablets. Methods: After hot-melt extrusion, matrix tablets were film-coated with either hypromellose or ethylcellulose. The effects of the coating polymer, curing and storage conditions, polymer weight gain, and core guaifenesin concentration on guaifenesin recrystallization were investigated. Results: The presence of either film coating on the guaifenesin-containing tablets was found to prolong the onset time of drug crystallization. The coating polymer was the most important factor determining the delay in the onset of crystallization, with the more hydrophilic polymer, hypromellose, having a higher solubilization potential for the guaifenesin and delaying crystallization for longer period (3 or 6 months in tablets stored at 40°C or 25°C, respectively) than the more hydrophobic ethylcellulose, which displayed a lower solubilization potential for guaifenesin (crystal growth on tablets cured for 2 hours at 60°C occurred within 3 weeks, whereas uncoated tablets displayed surface crystal growth after 30 minutes). Crystal morphology was also affected by the film coating. Elevated temperatures during both curing and storage, incomplete film coalescence, and high core drug concentrations all contributed to an earlier onset of crystal growth.     

Studies of the effect of molding pressure and incorporation of sugarcane bagasse fibers on the structure and properties of poly (hydroxy butyrate)

Poly (hydroxy butyrate) (PHB) is a biodegradable polymer that can be obtained from both renewable and synthetic resources. There have been many attempts to improve its structure and properties by different methods. This paper, while mentioning briefly PHB and sugarcane bagasse fibers, focuses on the effect of compressive/molding pressure on its structure and thermal properties with incorporation of sugarcane bagasse fibers, with and without steam explosion treatment. Thermal behavior (thermogravimetry and dynamic mechanical analysis), X-ray diffraction, Fourier transform infrared spectroscopy, optical microscopy methods were used to understand the changes in PHB resulting from the pressure and incorporation of fibers while scanning electron microscopy is used to understand the morphology of both the fiber and PHB.     

Thermo-XRD and differential scanning calorimetry to trace epitaxial crystallization in PA6/montmorillonite nanocomposites

The physical properties of montmorillonite–polyamide nanocomposites, which in turn determine varied industrial applications are greatly dependent on polymer crystal structure and crystallinity. Thus, control of polyamide crystallization is critical, especially when this polymer adopts different polymorphic structures. To fully characterize the crystallization behavior of this nanocomposite by increasing temperature, this work presents for the first time an innovative approach based on combined new thermo-X-ray diffraction (TXRD) routines for 2D mapping and differential scanning calorimetry (DSC) techniques. Results reveal that: (i) organically modified montmorillonite included in the polymer matrix, composed of polyamide-6, increases the thermal stability of the nanocomposite, and (ii) epitaxial mechanisms explain the nucleation of the thermodynamically less stable γ crystal phase reported in the literature.     

Controlling Homogenous Spherulitic Crystallization for High‐Efficiency Planar Perovskite Solar Cells Fabricated under Ambient High‐Humidity Conditions

The influence of precursor solution properties, fabrication environment, and antisolvent properties on the microstructural evolution of perovskite films is reported. First, the impact of fabrication environment on the morphology of methyl ammonium lead iodide (MAPbI₃) perovskite films with various Lewis‐base additives is reported. Second, the influence of antisolvent properties on perovskite film microstructure is investigated using antisolvents ranging from nonpolar heptane to highly polar water. This study shows an ambient environment that accelerates crystal growth at the expense of nucleation and introduces anisotropies in crystal morphology. The use of antisolvents enhances nucleation but also influences ambient moisture interaction with the precursor solution, resulting in different crystal morphology (shape, size, dispersity) in different antisolvents. Crystal morphology, in turn, dictates film quality. A homogenous spherulitic crystallization results in pinhole‐free films with similar microstructure irrespective of processing environment. This study further demonstrates propyl acetate, an environmentally benign antisolvent, which can induce spherulitic crystallization under ambient environment (52% relative humidity, 25 °C). With this, planar perovskite solar cells with ≈17.78% stabilized power conversion efficiency are achieved. Finally, a simple precipitation test and in situ crystallization imaging under an optical microscope that can enable a facile a priori screening of antisolvents is shown.     

On the lateral habits of polymer single crystals

Single crystals of trans-1, 4-polyisoprene (TPIP) have been grown from dilute solutions in amyl acetate, n-heptane and n-butanol/n-heptane mixtures. Two samples of different molecular weights, and molecular-weight distribution have been used to demonstrate the effect of these molecular parameters on the shape of single crystals. The experiments suggested that crystallization of narrowly distributed polymer chains, irrespective of the molecular weight of the crystallizing species, will produce a hexagonal morphology. This narrow distribution of chains during crystallization can be achieved by starting with a sample of low polydispersity or using a solvent/non-solvent mixture for crystallization. If the chain-size distribution is not controlled, then at later stages of growth the heterogeneous mixture of chains will create a curvature on the {110} faces and produce oval-shaped lamellar platelets. Further, the crystal habits in the present work were found to be related to the solvent, molecular weight and molecular-weight distribution, rather than to the crystallization temperature.     

Morphology, crystallization and dynamic mechanical properties of PA66/nano-SiO2 composites

This article addresses the effect of nano-SiO₂ on the morphology, crystallization and dynamic mechanical properties of polyamide 66. The influence of nano-SiO₂ on the tensile fracture morphology of the nanocomposites was studied by scanning electron microscopy (SEM), which suggested that the nanocomposites revealed an extensive plastic stretch of the matrix polymer. The crystallization behaviour of polyamide 66 and its nanocomposites were studied by differential scanning calorimetry (DSC). DSC nonisothermal curves showed an increase in the crystallization temperature along with increasing degree of crystallinity. Dynamic mechanical properties (DMA) indicated significant improvement in the storage modulus and loss modulus compared with neat polyamide 66. The tan ä peak signifying the glass-transition temperature of nanocomposites shifted to higher temperature.     

聚酯／热致液晶聚合物体系的非等温结晶动力学研究

热致性液晶共聚酯ＰＥＴ／６０ＰＨＢ组分对ＰＥＴ及ＰＢＴ在两种共混体系中的非等温结晶行为的影响用ＤＳＣ方法进行了研究，并用Ｏｚａｗａ方法处理了动力学数据。随共混体系ＬＣＰ含量的增加，ＰＥＴ的Ａｖｒａｍｉ指数ｎ趋于降低而ＰＢＴ的ｎ值趋于增加，表明在非等温结晶条件下，对不同组成的共混物体系有着不同的成核和晶体生长的机理。     

Nucleation behaviour of poly-3-hydroxy-butyrate

Poly-3-hydroxy-butyrate (PHB) is a thermoplastic polyester produced by bacterial fermentation. Because of this bacterial origin PHB is a very pure polymer. This high purity in turn leads to very few (if any) heterogeneous nuclei, which gives a much wider scope for a systematic study of nucleation behaviour and the effect of nucleating agents than was possible before. It is shown that in pure PHB nucleation is sporadic. The nucleation rate may be measured over a temperature range of some 100° C. The nucleation rate data are recorded in the range where homogeneous nucleation is possible and are at least consistent with it. When foreign particles are added the nucleation rate is modified. Two distinct types of behaviour are observed. One may be interpreted quantitatively as the local raising of the crystal melting point due to the constraints of the actual presence of a surface; and the other as being due to epitaxial growth on the foreign surface. Detailed kinetic data are presented to support these conclusions.     

Isoconversional kinetic analysis of the crystallization phases of amorphous selenium thin films

The crystallization kinetics of Se thin films were determined under nonisothermal conditions using a differential scanning calorimetry (DSC) technique. The development of crystal phases by thermal treatment of the film at various temperatures was analyzed by X-ray diffraction. Microstructures were identified by studying the morphology of the films using scanning electron microscopy and atomic force microscopy. This study reveals the simultaneous presence of distinct hexagonal and monoclinic phases; the DSC crystallization curve is formed by overlapping the exothermic crystallization curves of both phases. Two isoconversional methods, the Kissinger-Akahira-Sunose and Vyazovkin methods, were used to determine the variation of the activation energy for crystallization with temperature. The results show that the crystallization activation energy decreases with the extent of crystallization or temperature, which suggests that the examined phases follow multi-step kinetics.     

Atomistic simulations of spinodal phase separation preceding polymer crystallization

Many polymeric materials crystallize when cooled below their melting temperature. Although progress has been made in our understanding of the crystallization process through both experimental and theoretical efforts, these studies have focused mainly on the crystal nucleation and growth mechanism, where critical nuclei are formed from a metastable state during the first stages of crystallization, leading ultimately to the growth of crystal domains. Attention has also been given to the structure during the precrystallization (induction period). A pretransition state occurring before crystallization has been characterized as an unstable phase separation initiated by density and orientational fluctuations. These fluctuations are caused by an increase in the average length of rigid trans segments along the polymer backbone during the induction period. These observations are consistent with the theory proposed in ref. 14 on the isotropic-to-nematic transition of polymer liquid crystals, that is, the parallel ordering of polymers is caused by an increase in chain rigidity. Here we use large-scale computer simulations to investigate melts of polymers in the early ordering stages (induction period) before crystallization. In the ordered domains we identify growing dense regions similar to smectic liquid crystals. Our simulations reveal a 'coexistence period' in the ordering before crystallization, where nucleation and growth mechanisms coexist with a phase-separation mechanism.     

Encoding crystal microstructure and chain folding in the chemical structure of synthetic polymers

The development of robust methodologies to control the solid-state structure of polymeric materials by appropriate design of the macromolecular architecture has a crucial impact on the mechanical properties of these materials. Here, we demonstrate the feasibility of controlling chain folding of polymers by steric interactions only, in contrast to previous attempts aimed at engineering polymer crystallization through hydrogen bonding. In a linear synthetic macromolecule similar to polyethylene, we encoded structural instructions that are translated during a crystallization process to generate a unique, semi-crystalline morphology with structure-controlled crystal thickness of approximately 5 nm that remains constant over a wide temperature range. The molecular code consists of a linear backbone alternating crystallizable, long alkyl sequences of monodisperse sizes separated by short spacers containing side-chains and acting as stops and fold-controlling units. This simple strategy could be used to produce advanced polymeric materials with fine control of the crystalline and amorphous regions.     

On crystallization of polypropylene on atomically flat silicate substrate and relationship to mechanical properties

Phyllosilicates including mica offer one of the easiest methods to obtain atomically flat surface of macroscopic size by cleavage. Thus, thin films of polypropylene crystallized at different pressure and as a function of polypropylene concentration were investigated. The objective was to probe the nucleation and chain organization in thin films on basal plane surface from the view point of relating the nucleation and growth process to the intergallery space between the clay layers. The crystal morphology was altered from leaf-like structure at high undercooling to fully developed spherulites at low undercooling. Furthermore, the lamellae structure change from well-oriented to cross-hatched with increase in the crystallization pressure was indicative of epitaxial growth between polymer matrix and mica surface. The possible epitaxial mechanism is discussed here. The study provides a method to explore the macro- and microstructure of polymer and epitaxy between polymer matrix and inorganic particle surface. The good interfacial bonding favored by epitaxial crystallization is beneficial for toughness.