Crystallization of irradiated polyethylene

This study compares the effects of radiation dose on the isothermal and non-isothermal crystallization of LLDPE, LDPE and HDPE by differential scanning calorimetry (DSC). It includes qualitative comparison of the non-isothermal data and quantitative calculations of Avrami parameters for crystallization rate and nucleation mode. The isothermal crystallization allowed the observation of the changes in the crystallization rate, related to the decrease in the crystallization temperature caused by the crosslinking of the polymer. It was also observed by the non-isothermal crystallization, the development of crystallites of very different sizes in the polymer.     

Crosslinking kinetics of polyethylene with small amount of peroxide and its influence on the subsequent crystallization behaviors

Crosslinking reactions of high density polyethylene with low peroxide concentrations ranging from 0.1 wt% to 1.0 wt% at temperatures of 170, 180 and 190 ° C were monitored by rheological measurements. A critical gel forms at the peroxide concentration of 0.2 wt%, where the transition from long chain branching generation to crosslinking network formation could occur. Rheokinetics of crosslinking can be fitted well by Ding-Leonov's model. The curing rate k_2 at the earlier stage exhibits about 3 times acceleration per 10 °C with increasing temperature, while the equilibrium modulus G′ at the fully cured stage is almost independent of temperature. Influences of crosslinking on the subsequent crystallization behaviors were detected by DSC measurements. Above the critical gel concentration, crystallization is largely retarded as evidenced by the lower crystallization temperature Tc and crystallinity X_c due to the network formation. The secondary crystallization valley located at the temperature near 80 °C can be observed above the critical concentration, which becomes more evident with the increasing peroxide concentration and curing temperature. This phenomenon provides another evidence of crystallization retardation by the crosslinking network.     

The kinetics of homogeneous and two-step nucleation during protein crystal growth from solution

Many experimental reports for the kinetics of crystal nucleation and growth, from an isothermal solution, point to a sigmoidal-like behavior for the process. Here we consider three different nucleation models from the literature and show that all lead to sigmoidal or sigmoidal-like behavior for the kinetics of nucleation. A two-step nucleation process is known to occur within certain supersaturated protein solutions, and it is demonstrated in this report how the sigmoidal law yields kinetic information for the two-step and homogeneous nucleation modes. We propose here that two-step solute-rich associates form in the solution around seed nuclei that are already present at or near the point in time when the solution is prepared. Using this hypothesis, we are able to model the time-dependent volume of the two-step phase per unit volume of solution and show that this compares well with reported experimental data. A kinetic model is given for the proposed process, which leads to a sigmoidal rate law. Additionally, a relation between the initial and final nuclei densities and the induction time is derived. As a result of this study, the conclusion is that two-step activity increases with increasing initial supersaturation or increasing salt concentration.     

聚丁二酸丁二醇酯的自成核结晶行为

利用差示扫描量热仪(DSC)研究了自成核对聚丁二酸丁二醇酯(PBS)的结晶行为的影响. 研究结果表明, PBS的有效自成核温度处理区间为118～120 ℃. PBS经自成核处理后结晶温度提高, 可以在100~118 ℃温度区间内迅速结晶. 同时, 研究了自成核处理后样品在100～104 ℃范围内的等温结晶行为、动力学过程及熔融行为. 结果表明, 随着等温结晶温度的升高, 结晶速率变慢, 熔融曲线出现多重熔融峰. Hoffman-Weeks方程分析结果表明, 自成核处理对PBS的平衡熔点没有影响. Avrami等温结晶动力学方程适合分析自成核处理样品的等温结晶动力学过程, 获得其动力学参数K与n, 其中n值偏大的原因在于自成核的样品结晶生长点增多. 根据Arrhenius方程, 计算获得PBS自成核处理后等温结晶活化能为-286 kJ/mol.     

Crystallization kinetics of poly(hexamethylene succinate)

Isothermal and nonisothermal crystallization kinetics of polyester 64 have been investigated by means of differential scanning calorimetry and optical microscopy. The Avrami analysis has been performed to obtain the kinetic parameters of primary crystallization. These indicate a three-dimensional spherulitic growth on heterogeneous nuclei for the isothermal crystallization, whereas an sporadic nucleation becomes dominant in the nonisothermal crystallization. The maximum crystallization rate of polyester 64 was deduced to take place at a temperature close to −3 °C. Polarizing light microscopy showed that spherulites with a negative birefringence are formed during isothermal crystallization, whereas transmission electron microscopy indicates that the b crystallographic axis is aligned parallel to the spherulitic radius.     

甲壳胺的结晶动力学

给出了制备非晶甲壳胺的新方法，并由Ｘ衍射曲线和偏光显微镜照片及ＤＳＣ曲线得到证实。由ＤＳＣ测得玻璃化温度为８２℃，结晶温度为１５２℃，并研究了其结晶动力学。     

Non-isothermal crystallization kinetics of exfoliated and intercalated polyethylene/montmorillonite nanocomposites prepared by in situ polymerization

Polyethylene/montmorillonite (PE/MMT) nanocomposites, one intercalated sample with higher MMT content and one exfoliated sample with lower MMT content, were prepared by in situ polymerization using MMT-supported metallocene as catalyst. Non-isothermal crystallization behaviors of these two nanocomposites were investigated and compared. The exfoliated sample exhibits higher crystallization temperature (T_c) than the neat PE, showing nucleation effect of MMT. The intercalated sample has lower T_c than the neat PE due to the confinement of MMT. It is observed that the intercalated sample has longer induction period and faster overall crystallization rate, indicating co-existence of suppression and nucleation effects in this sample. The Avrami plots show that the crystal growth of PE in the intercalated sample is two-dimensional, while it is three-dimensional in the exfoliated sample. The crystallization activation energy of the intercalated sample is slightly smaller than that of the exfoliated sample.     

Crystallization kinetics of polypropylene with hyperbranched polyurethane acrylate being used as a toughening agent

The crystallization kinetics of polypropylene (PP) with hyperbranched polyurethane acrylate (HUA) being used as a toughening agent was studied by isothermal and nonisothermal differential scanning calorimetry (DSC). The presence of a small amount of HUA (2-7%) remarkably influences the crystallizability of PP. An addition of HUA leads to an increase in the number of effective nuclei, thus resulting in an increase of crystallization rate and a stronger trend of instantaneous three-dimensional growth. For isothermal crystallization, Avrami exponents were determined to be about 2.97 for pure PP and 3.51 for the HUA/PP blend containing 5% HUA (HUA-PP). The half crystallization time (t_1/2) of pure PP was measured to be 8.43 min, while being 3.28 min for HUA-PP at the crystallization temperature of 132 °C. The nonisothermal crystallization kinetics of HUA/PP blends was analyzed by Avrami, Ozawa and Kissinger methods. It has also been proved that an addition of HUA could increase the crystallization rate of PP. Moreover, the crystallization activation energies of pure PP and HUA-PP were estimated by Kissinger and Friedman methods.     

Crystallization kinetics,morphology, and mechanical properties of novel poly(ethylene succinate-co-octamethylene succinate)

The crystallization kinetics, morphology and mechanical properties of a novel poly(ethylene succinate-co-octamethylene succinate) (PEOS) copolyester with 82 mol% ethylene succinate (ES) units and 18 mol% octamethylene succinate (OS) units, and its homopolymer poly(ethylene succinate) (PES) were extensively investigated. The glass transition temperature, cold crystallization peak temperature and melting point of PEOS are around −24, 47.5, and 80.5 °C, respectively. The Avrami equation was used to analyze the isothermal melt crystallization kinetics of PEOS and PES. They display the same crystallization mechanism, and PEOS crystallizes slower than PES at the same degree of supercooling. The spherulitic growth rates of PEOS and PES exhibit a bell shape within the investigated crystallization temperature range, with the crystallization regime transition temperature of PEOS being lower than that of PES. In addition, PEOS has high thermal stability and good mechanical properties.     

Crystallization kinetics of PHB/PVAc blends using time resolved dielectric spectroscopy

Crystallization kinetics of poly(hydroxy butyrate), PHB, and its blends with poly(vinyl acetate), PVAc, have been thoroughly investigated using broadband dielectric technique over a wide range of frequencies (10⁻²-10⁵ Hz) as functions of crystallization temperature and blend composition. The dielectric strength of the amorphous segments, Δε, which is directly proportional to the volume fraction of the mobile amorphous phase in the blend decreases exponentially with increasing the crystallization time. However, on the other hand, the dielectric strength of the rigid amorphous segments, Δε_α′, which is related to the percentage of crystallinity in the blend increases dramatically with increasing crystallization time. A great variation in the dynamical constraints of relaxation segments with increasing crystallization time has been observed as a result of different environments, which would lead to a variation in the consistency of the cooperative regions. The value of the dielectric constant, ε′, decreases dramatically with increasing crystallization time, after that it reaches an equilibrium value at the end of the crystallization process. This dramatic decrease in the value of ε′ as a result of crystallization at a given crystallization temperature, was taken as an accurate evaluation for the amount of the amorphous phase that has undergone crystallization considering the theoretical approach of Avrami. The Avrami exponent, n, was found to be crystallization temperature, T_c, independent (n ∼ 3) indicating a three-dimensional crystal growth for pure PHB. The crystallization rate constant, k, increases greatly with increasing T_c due to the high crystallization rate. In the blend the value of n was found to be concentration dependent (n ∼ 1.8-3.2). The different values of n indicate that the shapes of the growing crystals are affected by blend concentration. For n ∼ 1.8, the crystals can either grow sporadically as rods or instantaneously as disks, while for n ∼ 3 a three-dimensional crystal growth takes place.     

Equilibrium melting temperature and spherulitic growth rate of poly(phenylene sulfide)

The equilibrium melting temperature (T_m), through linear extrapolation method (HW) and nonlinear extrapolation method (MX) respectively, and the supercooling dependence of the spherulitic growth rate, in the context of the Lauritzen-Hoffman secondary nucleation theory, of poly(phenylene sulfide) (PPS) have been investigated by differential scanning calorimeter (DSC) and a polarized optical microscope (POM) equipped with a CSS450 shear hot stage. The results show that the T_m value obtained by MX method is more suitable for analyzing the regime transition behavior of PPS than that by HW method. And the regime I/II and II/III transitions can be really observed at T_X = 526 and T_X = 516 K, respectively, while analyzing the secondary nucleation theory of PPS by use of the T_m value obtained by MX method.     

Crystallization of polymers in variable external conditions

A new model of crystallization kinetics in variable external conditions has been developed. The model concerns situations when temperature, pressure, stress, change in time. Compared to earlier models, the present treatment includes transient and athermal effects, proportional to the rate of change of the external conditions. The model can be used for simulation of crystallization in industrial processes (injection molding, fiber spinning, film blowing). The present paper offers general theoretical fundamentals of the model. Applications concerning more specific cases will be published separately.     

Influence of the order of polymer melt on the crystallization behavior: II. Crystallization kinetics of isotactic polypropylene

The crystallization behavior of isotactic polypropylene (iPP) melts with a high order has been carefully examined by differential scanning calorimetry (DSC) and polarized light microscopy (PLM). The experimental results show that the helically ordered iPP melt crystallizes by heterogeneous nucleation with two-dimensional diffusion controlled growth and the Avrami exponent is about 2. The data available both from our DSC and PLM experiments and from the literature indicate that the order of a polymer melt can speed up the crystallization process. When some unmelted materials exist in the ordered melt, the crystallization will become more rapid. Received: 16 June 2000 Accepted: 16 October 2000     

Influence of degree of intercalation on the crystal growth kinetics of poly[(butylene succinate)-co-adipate] nanocomposites

The influence of the degree of intercalation of polymer chains in the two dimensional silicate galleries on the crystallization behavior of poly[(butylene succinate)-co-adipate] (PBSA) is being reported on. The nanocomposites were prepared by melt-blending of PBSA and organically modified montmorillonite (OMMT) in a batch-mixer. Two different types of commercially available OMMTs, with different extents of miscibility of organic modifiers with PBSA, were used, leading to highly delaminated and stacked/intercalated nanocomposite structures as revealed by X-ray diffraction (XRD) patterns and transmission electron microscopy (TEM) observations. The non-isothermal crystallization behavior of PBSA and the nanocomposite samples were studied by differential scanning calorimetry (DSC). Crystal growth kinetics studies showed that when silicate layers are highly delaminated into the PBSA matrix, nucleation behaviors decreased significantly, relative to the stacked/intercalated silicate layers. These observations indicate that the overall crystal growth kinetics retard in delaminated nanocomposites, opposed to increasing in the case of stacked/intercalated nanocomposites. Polarized optical microscopy (POM) observations and light scattering studies indicate that PBSA spherulites are fairly large and more perfectly grown in the case of delaminated nanocomposites, relative to the pure PBSA matrix. The effect of high levels of dispersion of silicate layers in the PBSA matrix on cold crystallization behavior was also studied.     

Crystallization kinetics of virgin and processed poly(lactic acid)

Poly(lactic acid) (PLA) is an emerging material mainly because it can be synthesized from renewable resources and is thus environmentally and ecologically safe. The mechanical properties, above all the thermal resistance of PLA are determined by the crystalline content: the heat deflection temperature of crystalline PLA can reach 100 °C, whereas amorphous PLA loses mechanical properties at temperatures slightly higher than 60 °C. However, PLA has a low crystallization rate, so that after processing it remains mostly amorphous. This characteristic heavily limits the use of PLA for commercial applications. Many studies have been recently published on the crystallization kinetics of PLA. The effect of processing on this feature is however often neglected. In this work, the significance of processing on the crystallization kinetics of a commercial PLA was investigated. Two processing methods were explored: extrusion and injection moulding. The obtained materials, and the starting pellets of virgin polymer, were analyzed by calorimetry in order to obtain the crystallization kinetics. Two protocols were adopted to determine the crystallization rates during cooling from the melt or heating from the solid. The parameters of a kinetic equation were determined for all the materials and protocols adopted and it was thus possible to describe the evolution of crystallinity during heating and during cooling.     

Crystallization kinetics and the fine morphological evolution of poly(ethylene oxide)/ionic liquid mixtures

The overall crystallization kinetics and spherulite morphologies of miscible poly(ethylene oxide)(PEO)/1-butyl-3-methylimidazolium hexafluorophosphate([BMIM][PF_6]) mixtures were studied by differential scanning calorimetry(DSC),polarized optical microscopy(POM) and rheological measurements. The finer crystal structures were further detected by wide angle X-ray diffraction(WAXD) and small angle X-ray scattering(SAXS). Crystallization of PEO is largely suppressed by [BMIM][PF_6] addition especially at higher ionic liquid(IL) concentrations above 20 wt%. Both the overall crystallization rate and the spherulite growth decrease with the increase of IL content and crystallization temperature; however, the crystallization mechanism keeps unchanged as evidenced by the similar Avrami exponent n and WAXD results. The addition of [BMIM][PF_6] could induce more nuclei to some extent, but the induction time of crystallization is evidently prolonged,and a linear to non-linear transition of the spherulite growth(R ∝ t to R ∝ t~(1/2)) can be observed. At higher IL concentration,the spherulite texture changes apparently from particular serrated to branch surface due to the diffusion-controlled growth and the dilution effect, which also as a main factor contributes to the increasing trend of the long period of crystals.     

Uniform to Accelerated Crystal Twisting Transition in Deuterate Polyethylene/Poly（ethylene-alt-propylene） Blend Films

A uniform to accelerated crystal twisting transition is observed in deuterate polyethylene/poly（ethylene-alt- propylene） （d-PE/PEP） blend films. And the band period is a function of initial d-PE concentration, quench depth and annealing time of phase separation. As Keith and Padden suggested, twisting of lamella is due to the unbalanced stress on its both sides, which can supply a satisfying explanation to banded spherulites formed in homogeneous systems. When it comes to d-PE/PEP blend system, in homogeneous 99% d-PE/PEP （weight fraction of d-PE） blend film, the formation of banded spherulite is observed as a result of uniform twisting of ribbon like d-PE lamellae along the radial direction. With the amorphous PEP piling up, it transfers into accelerated edge-on to fiat-on twisting due to crystallization assisted phase separation. The mechanism can be interpreted as following： d-PE molecules must inter-diffuse to the twisting growth front to continue the secondary nucleation and growth process. Meanwhile, the amorphous PEP molecules are rejected and accumulated at the twisting growth front. Once the d-PE lamella begins to twist because of unbalanced stress on both sides, the accumulated rubber phase at the growth front strengthens the unbalance and accelerates the edge-on to flat-on twisting. The concentration wave propagates further away with constant speed, and leads to concentric ring pattern with periodic nonuniform twisting along the radial direction. Since this is a kinetic effect, the band period can be controlled through initial d-PE concentration, quench depth and annealing time of phase separation. Our result shows that crystallization assisted phase separation can modify lamella growth kinetic pathway, thereby assisting concentric ring pattern formation.     

剪切条件下等规聚丙烯的结晶行为

聚丙烯是工业中应用最广泛的聚合物。聚丙烯在静态结晶时生成球晶，剪切后会生成排核，最终长成柱晶或纤维晶；在强剪切条件下会生成串晶。聚丙烯是一种存在多种晶型的聚合物，α型晶是热动力学最稳定的晶型，在一般实验条件下很难得到其他晶型；在剪切条件下聚丙烯会生成β晶-热动力学亚稳晶型。剪切会显著影响聚丙烯的结晶动力学：增加球晶的生长速率，缩短结晶诱导时间，增加活化晶核密度。人们提出了许多模型来解释剪切加速结晶动力学的实验现象，但是都有不足之处。     

Crystallization kinetics and morphology of biodegradable poly(lactic acid) with a hydrazide nucleating agent

The effect of tetramethylenedicarboxylic dibenzoylhydrazide (designated here as TMC) on the nonisothermal and isothermal crystallization behavior of PLA was investigated by differential scanning calorimetry (DSC), polarized optical microscopy (POM) and wide angle X-ray diffraction (WAXD). TMC shows excellent nucleating effect on PLA. With the addition of 0.05 wt% TMC, the crystallization half-time of PLA decreases from 26.06 to 6.13 min at 130 °C. The isothermal crystallization data were further analyzed by the Avrami model. The values of the Avrami exponent of the blends are comparable to that of neat PLA, indicating that the presence of TMC does not change the crystallization mechanism of the matrix. The observation from POM and WAXD measurements showed that the presence of TMC increases significantly the nuclei density of PLA but has no discernible effect on its crystalline structure.     

Isothermal-crystallization kinetics and spherulite growth of aliphatic polyketone/polyamide-6 blends

In this research,the morphologies,isothermal-crystallization kinetics,and spherulite growth of aliphatic polyketone/polyamide-6 blends were studied.A single glass-transition temperature (Tg) was determined,and the composition dependence of Tg for these blends was well described by the Kwei equation.The strong intermolecular interaction between the two polymer components was confirmed by melting-point depression.The isothermal-crystallization kinetics were analyzed on the basis of the Avrami approach.A linear increase in the radii of the spherulites with time was observed for all compositions.All the spherulites continued to grow at nearly identical growth rates.With increasing polyamide-6 content,the size of the spherulites in the polyketone/polyamide-6 blends gradually decreased,and the number of spherulites in the blends increased.