Thermal behavior of isotactic polypropylene freeze-extracted from solutions with varying concentrations

Isotactic polypropylene (iPP) with narrow molecular mass distribution was freeze-extracted from n-octane solutions with varying concentrations. The recovered samples were characterized by differential scanning calorimetry. It is found that the sample recovered from the very dilute solution exhibits the higher non-isothermal crystallization temperature, faster isothermal crystallization rate, and smaller Avrami index. And there should exist a critical concentration corresponding with the critical overlap concentration proposed by de Gennes in the polymer solutions. In the solution well below the critical concentration, the iPP chains were isolated from each other, resulting in an acceleration of melt crystallization for the recovered samples. It seems that the chain entanglement is a barrier to the melt crystallization of polymer.     

Prediction of poly(oxymethylene) non-isothermal crystallization behaviour from isothermal data

The spherulite growth, nucleation-related,K _g, parameter values obtained from isothermal data (by DSC or optical microscopy) and two other adjustable parameters (the spherulite growth rate preexponential factor and the Avrami's or Tobin's exponent,n) have been used with Nakamura's and Tobin's modified non-isothermal equations to model the kinetics of polymer non-isothermal crystallization. Malkin's model was also tested, for comparison. It is shown that, for polymers that crystallize on cooling almost entirely at temperatures higher than the maximum growth rate temperature, this Tobin's-like non-isothermal model accurately describes the experimental behaviour with only 2 parameters.     

Effect of ZnO nanofillers treated with triethoxy caprylylsilane on the isothermal and non-isothermal crystallization of poly(lactic acid)

The crystallization of PLA-silane surface-treated ZnO nanocomposites was investigated by DSC and compared to that of neat PLA. Several modes of crystallization were considered: isothermal and non-isothermal cold crystallization and also isothermal and non-isothermal melt crystallization. The kinetics of cold crystallization were studied using different methods, namely the Avrami and Ozawa-Flynn-Wall models, to calculate activation energies and kinetic constants. In contrast to what is typically observed when the foreign particles are added in a polymer matrix, the silane surface-treated ZnO delayed the crystallization of PLA and made it more difficult to start. The nucleation activity of the ZnO nanoparticles, ?, was calculated and found to be greater than 1 (? = 1.7). This indicated that ZnO played an anti-nucleating role in the crystallization of PLA nanocomposites. This effect has been linked mainly to the interactions between the silane groups onto the surface of nanoparticles and PLA macromolecules. These interactions which reduce the mobility of polymer chains have been evidenced by rheological experiments.     

一种研究聚合物非等温结晶动力学的方法

作者基于多年对聚合物结晶动力学方面研究的工作积累,联合Avrami方程和Ozawa方程,提出了一种研究聚合物非等温结晶动力学的新方法.该方法既克服了使用Ozawa方程所获得的数据点过少,常常出现非线性,不能获得可靠的动力学参数的缺点,又克服了使用经Jeziorny修正的Avrami方程所获得的表观Avrami指数无法准确预测非等温过程成核生长机理的缺点.该方法已成功用于多种聚合物体系,被国内外学者引用数百次,已成为研究聚合物非等温结晶动力学一种有效方法.     

Non-isothermal crystallization of polyvinylalcohol-co-ethylene

Non-isothermal crystallization of polyvinylalcohol-co-ethylene with different ethylene contents was studied. Several models were used to predict the crystallization behavior of these materials under non-isothermal conditions at a constant cooling rate. Kinetic parameters determined from isothermal date were employed. Experimental data were in accordance with model prediction at low cooling rate and relative degree of crystallization lower than 0.8, but it did not fit at high cooling rate. Kinetic parameters obtained by using a non-linear regression method, i.e, Kamals model and Dietzs modification, were able to describe better the non-isothermal crystallization behavior of the studied materials. The full model, that takes into account the induction and growth of the crystal during cooling under non-isothermal conditions was used to obtain a continuous cooling transformation diagrams for polyvinylalcohol -co-ethylene. Finally, non-isothermal models, coupled with the proposed expressions for induction time and kinetic constant, were used to represent the development of crystallinity during the processing of the polymer.     

Nano/micro-scale morphologies of semi-interpenetrating poly(ε−caprolactone)/tung oil polymer networks: Isothermal and non−isothermal crystallization kinetics

The isothermal and non-isothermal crystallization kinetics of pure poly(ε−caprolactone) (PCL) and its blends with crosslinked tung oil were investigated as a function of composition, crystallization temperature, and heating rate using differential scanning calorimetric (DSC). The PCL/tung oil semi-interpenetrating polymer networks of different compositions were prepared via cationic polymerization of tung oil in the presence of homogenous solutions of PCL. This unique and relatively new in-situ polymerization and compatibilization blending technique created nano/micro-scale morphologies that cannot be obtained with the traditional melt-processing and/or solvent casting methods. Blends with different miscibility, phase behaviors, and morphologies (miscible, partially miscible, and immiscible) were observed as a function of composition with a constant concentration of boron trifluoride diethyl etherate (BFE) cationic initiator. The morphology of the semi-interpenetrating polymer networks was performed using scanning electron microscopy (SEM). Miscible blends with a single T_g for PCL ≤ 10 wt.%. were observed. While, on the other hand, partially miscible blends with two distinct T_gs and nanoscale morphologies and average particle sizes as small as 100 nm were observed for blends with 20 ≤ PCL wt.% ≤ 30. Immiscible blends with microscale highly interconnected, co-continuous two-phase morphology and two distinct T_gs were detected for 50 wt.% PCL. Both isothermal and non-isothermal crystallization kinetics were strongly influenced by the different miscibility and morphology of the blends. The isothermal and non-isothermal crystallization kinetics of PCL/tung oil blends were analyzed on the basis of Avrami and modified Avrami approaches, respectively. A substantial decrease in the isothermal (longer half time) and non-isothermal (T_m shifted to lower temperature) crystallization kinetics was observed as the concentration of PCL increased in the blends up to 30 wt.% due to the partially miscibility of the blends in this composition range. In a contrast, for 50 wt.% PCL blend, a considerable increase in the crystallization kinetics (isothermal and non-isothermal) was detected due to the highly interconnected, co-continuous two-phase morphology.     

Unified isothermal and non-isothermal modelling of neat PEEK crystallization

A differential generalized Avrami’s law is used to model crystallization kinetic of PEEK in considering that PEEK crystallization results from the contribution of two distinct mechanisms. The form of this equation allows to predict with good accuracy both isothermal and non-isothermal crystallization kinetics. Nevertheless, isothermal model parameters are not entirely satisfactory for predicting non-isothermal crystallization and the identification of kinetic parameters is needed for both isothermal and non-isothermal cases. The results show that the Avrami exponents and Arrhenius activation energies remain constant for both conditions and therefore suggest that these parameters are only material dependent. On the other hand, the other kinetic parameters depend on the crystallization condition and vary with temperature and/or cooling rate.     

Polymer crystallization

Differential scanning calorimetry (dual furnace, null-balance, DSC) and optical microscopy (OM) have been used to study the isothermal crystallization kinetics of poly(oxymethylene)-POM. The non-isothermal crystallization of the same material has also been studied by optical microscopy. A very controversial problem is whether the isothermal kinetic parameters may be applied to describe the non-isothermal crystallization. The results show that the kinetic spherulite growth parameters obtained by non-isothermal optical microscopy are, within the experimental errors involved, the same as those obtained by isothermal optical microscopy or isothermal DSC. The importance of this finding is highlighted.     

Morphology, isothermal and non-isothermal crystallization kinetics of poly(methylene terephthalate)

The morphology of crystals, isothermal and non-isothermal crystallization of poly(methylene terephthalate) (PMT) have been investigated by using polarized optical microscopy and differential scanning calorimeter (DSC). The POM photographs displayed only several Maltese cross at the beginning short time of crystallization indicating that some spherulites had been formed. The crystal cell belonged to the Triclinic crystal systems and the cell dimensions were calculated from the WAXD pattern. The commonly used Avrami equation and that modified by Jeziorny were used, respectively, to fit the primary stage of isothermal and non-isothermal crystallization. The Ozawa theory was also used to analyze the primary stage of non-isothermal crystallization. The Avrami exponents n were evaluated to be in the range of 2-3 for isothermal crystallization, and 3-4 for non-isothermal crystallization. The Ozawa exponents m were evaluated to be in the range of 1-3 for non-isothermal crystallization in the range of 135-155 °C. The crystallization activation energy was calculated to be −78.8 kJ/mol and −94.5 kJ/mol, respectively, for the isothermal and non-isothermal crystallization processes by the Arrhenius’ formula and the Kissinger’s methods.     

Crystallization kinetics in relation to polymer processing

Phase distribution of quenched samples has been determined by a deconvolution procedure of WAXS spectra in a wide range of cooling rates. The informations collected together with isothermal and DSC results provide a very wide set of data on the crystallization kinetics of polymers relevant which covers conditions encountered in most polymer processing operations. They have been compared with predictions of a non-isothermal crystallization model assuming two independent and parallel crystallization processes competing during solidification.     

尼龙1218的等温及非等温结晶动力学研究

采用示差扫描量热计DSC考察了一种新型长烷基链偶偶尼龙 尼龙 12 18 自熔体的结晶过程 ,分别利用Avrami方程和Ozawa方程对等温及非等温结晶动力学进行了描述与研究 ,计算了相关的结晶动力学参数 ,得出相应的结晶机理 .最后计算了等温结晶活化能和非等温结晶活化能 ,依此得到烷基链段长度与尼龙结晶过程有密切关系     

Prediction of non-isothermal crystallization parameters for isotactic polypropylene

Non-isothermal crystallization kinetics of isotactic polypropylene (iPP) was simulated with the assumption that the non-isothermal crystallization process was composed of some finite isothermal crystallization processes, while each isothermal crystallization process consists of three main steps—induction, nucleation and crystal growth. In the simulation, induction time was taken into account, allowing one to make predictions on the start of the non-isothermal crystallization of iPP; nuclei density was treated as a function of temperature; the Hoffman–Lauritzen theory was employed to describe the spherulite growth rate varied with temperature, and the relative crystallinity was determined by the equation of Kolmogorov. Finally, model prediction was verified by quantitative comparison between the theoretical results with the experimental results.     

Thermophysical analysis and modeling of the crystallization and melting behavior of PLA with talc

The crystallization kinetics and the melting behavior of PLA and PLA with talc are investigated by dynamic scanning calorimeter and optical microscopy. The polymorphic aspect of PLA was highlighted by analyzing the melting process throughout heating after isothermal crystallization. The melting process of PLA with 5 mass% talc (PLAT5) shows the same thermal transitions as for PLA alone. The thermodynamic melting temperature of PLA and PLAT5 is determined to be 167.7 °C. The effects of the temperature and the cooling rate on the crystallization kinetics of PLA are analyzed. Finally, a simple and efficient protocol is defined to model the isothermal and the non-isothermal crystallization taking into account the polymorphism of PLA. Good agreement is found between the predictions of the proposed model and the experimental results under isothermal and non-isothermal conditions.     

Crystallization kinetics of poly(glycolic acid-alt-6-aminohexanoic acid)

The crystallization behavior of a new regular poly(ester amide) constituted by glycolic acid and 6-aminohexanoic acid units under both isothermal and non-isothermal conditions is studied. Differential scanning calorimetry (DSC) is used to monitor bulk crystallization, and subsequently Avrami and Ozawa analyses are applied. A three-dimensional spherulitic growth from heterogeneous nuclei is deduced for isothermal crystallization, whereas higher exponents are obtained for non-isothermal crystallization when an Avrami equation is applied. However, modifications of the Ozawa methodology indicate a crystallization mechanism similar to that of the isothermal process.The maximum crystallization rate is deduced to take place at a temperature close to 91 °C by considering experimental data and theoretical equations with adjusted parameters. The equilibrium melting temperature is determined to be 168 °C by the characteristic Hoffman-Weeks plot. One crystallization regime is detected by using the Lauritzen-Hoffman kinetic theory for isothermal crystallization and also with an isoconversional method applied for non-isothermal crystallization. Activation energy of molecular transport and nucleation constant are close to 1500 cal/mol and 1.81 × 10⁵ K², respectively. Crystal morphology, nucleation, and spherulitic growth rates are also investigated with hot-stage optical microscopy (HSOM).     

低密度聚乙烯/乙丙烯三元共聚(LDPE/EPO)共混体系的结晶动力学

用DSC方法研究了LDPE/EPO共混体系的等温及非等温结晶动力学,对LDPE/EPO共混体系的等温结晶动力学研究表明,共混物是三维生长的异相成核,共混物在各个结晶温度下的结晶过程都是以方式K_g(Ⅱ)进行的.采用联系Avrami方程和Ozawa方程导出的新非等温结晶动力学方程,处理了LDPE/EPO共混体系,得到了非等温结晶过程的一些基本参数,新方程很好地描述了此共混体系的非等温结晶动力学过程.     

表面接枝改性纳米二氧化硅填充聚丙烯的结晶行为

应用差示扫描量热方法研究了纳米二氧化硅 (SiO2 )及其表面接枝改性对聚丙烯 (PP)结晶过程、等温与非等温结晶动力学的影响 ,并研究了上述等温结晶的熔融行为和平衡熔点 .研究发现纳米SiO2 具有明显的异相成核效应 ,能够提高PP的结晶温度、熔融温度、结晶度和结晶速率 ,但降低聚丙烯结晶的完善程度 .粒子的表面接枝处理 ,因改善了粒子与基体的亲和性而有利于粒子成核效应的提高 ,而且此效应尚与粒子的分散相关     

热致液晶共聚酯对聚丙烯结晶的诱导作用

用差示扫描量热法和光学解偏振法研究了热致液晶性芳香共聚酯与聚丙烯共混物的等温和非多温结晶行为.结果表明,这一热致液晶聚合物对聚丙烯结晶有诱导成核和加速作用.当共聚酯含量在2-5％之间时,聚丙烯的结晶速率最快.偏光显微镜的观察揭示出在聚丙烯熔体中原位形成的液晶聚合物微纤诱导了聚丙烯横穿晶的形成.     

Crystallization kinetics of polyethylene/paraffin oil blend sheets formed by thermally induced phase separation with different molecular weights of polyethylene

Polyethylene/paraffin oil blend sheets with different molecular weights of polyethylene were prepared by thermally induced phase separation. Isothermal and non-isothermal crystallization behaviors of blend sheets were investigated by differential scanning calorimetry (DSC). Isothermal DSC curves were analyzed by Avrami equation, whereas non-isothermal DSC curves were analyzed by Jeziorny method and Mo method. Effective activation energy (ΔE) of isothermal and non-isothermal crystallization was calculated by Friedman method. Under isothermal condition, value of n in Avrami equation hovered at 2.1, and lgZ increased with the decrease of crystallization temperature. lgZ and ΔE of blend sheets with a larger molecular weight of polyethylene was smaller than that of blend sheets with smaller molecular weight. Under non-isothermal condition, value of n obtained by Jeziorny method hovered at 2.4, close to n of isothermal condition. lgZ _c increased with the increase of cooling rate and decrease of molecular weight of polyethylene. ΔE of different blend sheets were close to each other. Crystal structures of blend sheets formed under non-isothermal condition were analyzed by X-ray diffraction (XRD) analysis. XRD analysis showed that molecular weight of polyethylene and cooling rate had slight influence on crystal structure and crystallinity of polyethylene/paraffin oil blend sheet.