The Kinetics of Melting Crystallization of Poly-L-Lactide

The influence of non-isothermal melt crystallization on thermal behavior and isothermal melt crystallization kinetics of poly-L-lactide (PLLA) were investigated by differential scanning calorimetry (DSC), polarizing micrograph (POM) and x-ray diffraction (XRD). Crystallization performed at lower cooling rates (2°C·min^?1) is accompanied by a variation of the kinetics around 118°C. The glass transition temperature of PLLA decreases with increase of cooling rate, and the crystallinity at the end of crystallization increases with decreasing cooling rate. The size of PLLA spherulites increases with a decrease in the cooling rate, and PLLA becomes almost amorphous cooled at rapid rate (>10°C·min^?1). PLLA exhibits an Avrami crystallization exponent n = 3.01±0.13 in isothermal crystallization in the range from 90°C to 140°C. According to Hoffman-Lauritzen theory, two crystallization regime are identified with a transition temperature occurring at 118°C, and the value of K_g(II)/K_g(III) is 2.17 [K_g(II) = 6.025 × 10⁵K², K_g(III) = 1.307 × 10⁶ K²].     

高流动抗冲PP的结晶特性及结晶动力学

采用X射线衍射仪、差示扫描量热仪和偏光显微镜研究了5种高流动抗冲聚丙烯(PP)树脂的结晶特性和结晶动力学。结果发现：国产试样的结晶度与晶粒尺寸明显不同于进口试样;在等温结晶条件下。总结晶速率常数和总结晶速率随结晶温度的增加而下降,而结晶半值期增加;在所研究的温度范围内。所有试样的Avrami指数基本为2～3的非整数;在133℃以下,5^#试样的球晶生长速率明显高于其他试样,但在相同结晶温度时,国产试样的总结晶速率大于进口试样。5种试样在抗冲击性能上的差异归因于它们在结晶特性和结晶动力学方面的不同。     

Thermal evolution and crystallization kinetics of potassium-based geopolymer

In this paper crystallization kinetics of potassium-based geopolymer (Si/Al = 2.5, denoted as KG2.5) upon heating are investigated by differential thermal analysis (DTA) under non-isothermal conditions at various heating rates. From 35 to 700 °C KG2.5 shows a significant weight loss due to both evaporation of free water and condensation of hydroxyl groups. Crystallization of KG2.5 starts at a low temperature of ∼960 °C according to XRD and DTA analysis. KG2.5 also exhibits a low onset temperature of viscous sintering stage and on heating to 950 °C the sintering is completed. In the DTA graphs, the exothermic peaks which are caused by leucite crystallization shift to higher temperatures with increasing heating rate. The activation energy value of crystallization for leucite is found to be 455.9 kJ/mol and the corresponding Avarami constant is 3.89 indicating the three-dimensional crystal growth mechanism.     

Crystallization kinetics of two polyesters with different main chain rigidity

The crystallization kinetics of two polyesters, poly(p-phenylene sebacate) (PPS) and poly(p-phenylene isophthalate) (PPI), have been investigated by DSC. The nonisothermal data indicate that the relative crystallization rate of PPI was slower than that of PPS due to the more rigid nature of the PPI the chains. The isothermal crystallization data were analyzed by the Avrami equation, 1 − X_(t) = exp(−ktⁿ). PPS exhibited Avrami exponent (n), values of about 4, indicating that its isothermal crystallization followed a process of homogeneous nucleation, spherical growth and a growth type of interface control. PPI exhibited Avrami exponent values of about 3 indicating its isothermal crystallization followed a process of homogeneous nucleation, a possible disc growth geometry and an interface control growth type. Possibly, the bending structure and rigid nature of PPI forced its growth to follow a three dimensional growth during crystallization.     

Ozawa方程处理PET非等温结晶动力学过程研究

采用Monte Carlo法计算机模拟在预先成核条件和不同降温速率下PET非等温结晶曲线,将模拟的结果用Ozawa方程及改进的Ozawa方程进行处理。结果表明,用Ozawa方程解析的Avrami指数与理论值相近,用改进的Ozawa方程可直接得到结晶速率常数,求出的线生长速率与理论值基本吻合。     

Crystallization kinetics and nucleation activity of silica nanoparticle-filled poly(ethylene 2,6-naphthalate)

Silica nanoparticle-filled poly(ethylene 2,6-naphthalate) (PEN) composites were melt-blended to improve the mechanical and rheological properties of PEN. The melt viscosity and total torque values of the composites were reduced by the silica content. The crystallization exothermic peak shifted to a higher temperature, and the overall crystallization time was reduced by increasing the silica content. Non-isothermal crystallization kinetics was analyzed using the Ozawa and Avrami theories, and a combined method. The combined method was successful in describing the non-isothermal crystallization of these composites. The crystallization activation energy calculated using Kissinger's method was reduced, and the spherulite growth rate increased, with increasing silica content.A study of the nucleation activity, which indicated the influence of the filler on the polymer matrix, revealed that the fumed silica nanoparticles had a good nucleation effect on PEN.     

Non-Isothermal Crystallization of Bovine Milk Fat

The crystallization kinetics of milk fat were studied under non-isothermal and simulated adiabatic conditions using pulsed NMR spectroscopy. Isothermal experiments confirmed that when milk fat is shock cooled to below the α melting point it crystallizes in two steps due to the different crystallization kinetics of α and β′ modifications. In non-isothermal experiments, the fat samples were heated early during the plateau between steps to a temperature above the α melting point and β′ crystals formed more rapidly than in isothermal conditions. Fresh α crystals are believed to melt and form lamellar units containing triglyceride molecules with high degrees of isomorphism and these units can accelerate the nucleation and growth rates of β′ polymorph crystals. The crystallization behavior changed when the heating occurred late in the plateau and the α crystals are believed to have demixed, which allowed them to transform to β′ crystals directly in the solid state. Under simulated adiabatic conditions the rate of β′ crystallization was increased by a factor of 2–3 over the isothermal case. These findings were used to infer approaches to process difficult fat blends in scraped-surface heat exchanger plants.     

DTA测定玻璃析晶活化能的方法研究

ＤＴＡ测定玻璃析晶活化能的方法研究董大奎，冯喆（上海建材学院，２００４３３）ＳｔｕｄｙｏｎＤｅｔｅｒｍｉｎｉｎｇＣｒｙｓｔａｌｌｉｚａｔｉｏｎＡｃｔｉｖａｔｉｏｎＥｎｅｒｇｙｏｆＧｌａｓｓｂｙＤＴＡ￥ＤｏｎｇＤａｋｕｉ；ＦｅｎｇＺｈｅ（Ｓｈａｎｇｈａ...     

Crystallization kinetics of palm stearin in blends with sesame seed oil

This study investigates the crystallization kinetics of palm stearin (PS), a palm oil fraction, in blends with sesame seed oil. The results indicate that the crystallization behavior of PS in sesame oil is mainly associated with the crystallization of tripalmitin. Therefore, crystallization of blends of 26, 42, 60, and 80% (wt/vol) PS in sesame oil was described by equations developed for simpler systems (e.g., Fisher and Turnbull equation). The isothermal crystallization, melting profile, and fitting of the kinetics of nucleation to the Fisher and Turnbull equation showed that the 26, 42, and 60% PS/sesame oil blends crystallized mainly in the β₁′ polymorph state. In contrast, the 80% blend crystallized in two different polymorph states (i.e., β₁′ at T⪯307.6 K and β₁ at T≽308.2 K). The data indicated that, in spite of the higher concentration of PS in the 80% PS/sesame oil system, crystallization in the β₁ state required more free energy for nucleation (δG _c ) than β₁′ crystallization in the 26, 42, and 60% PS/sesame oil. At the low cooling rate used (1 K/min) it was observed that, for a particular PS blend, the higher the effective supercooling the higher the viscosity of the oil phase and the smaller the induction time of crystallization (T_i). Additionally, the β₁′ crystals from PS, developed at the highest effective supercooling investigated, were smaller than the β₁ crystals obtained at lower effective supercooling.     

丙烯酸改性卤锑阻燃聚丙烯的结晶性能

研究了丙烯酸(AA)改性Sb2O3,聚丙烯(PP)母料制备的不同用量卤锑阻燃PP。阻燃PP的差示扫描量热法结果表明,阻燃剂在PP中存在明显的异相成核作用,使PP结晶温度提高,Sb2O3对PP的异相成核作用比十溴二苯醚明显。在无引发剂过氧化二异丙苯(DCP)存在下,AA对阻燃PP中PP结晶温度的影响取决于阻燃剂的用量;在DCP存在下,PP的结晶温度不随AA用量增加而改变。DCP用量增加导致PP严重降解,结晶温度与熔融温度降低。     

Isothermal crystallization kinetics of polypropylene by differential scanning calorimetry. I. Experimental conditions

Reliable isothermal crystallization kinetic studies can be achieved by differential scanning calorimetric techniques only under restricted conditions. In the case of isotactic polypropylene, our results indicate that those conditions are met in the range of 3°C below the isothermal crystallization temperature T_c. Experimentally, it is only in this range when the complete crystallization peak can be registered by the DSC and depicted in a thermogram. Just around this temperature interval, the Avrami exponent n = 3 for bulk crystallization, whereas for any other test the isothermal temperature T_it, nonisothermal conditions prevail and the Avrami exponent deviates from the expected value. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 970–978, 2004     

Evaluation of tripalmitin crystallization in sesame oil through a modified avrami equation

Tripalmitin (TP) crystallization in sesame oil solutions (0.98, 1.80, and 2.62%, wt/vol) was investigated by utilizing a modification of the Avrami equation. The modified equation retains the original correspondence to the nucleation process (i.e., n) and crystal growth and simply corrects the value of the crystallization rate constant (z) by eliminating the influence of n. The energy of activation (E _a ) values for TP crystallization in sesame oil solution, calculated with the modified z, were quite similar to those calculated with the reciprocal of time required to achieve 50% of TP crystallization (t _F =0.50⁻¹). However, E _a values calculated with z from Avrami’s original equation were quite different from those obtained with t _F =0.50⁻¹. Thus, z and E _a values calculated through the Avrami equation yield erroneous results, especially when comparing crystallization processes having different magnitudes of n, as in this study. Additional analysis that considered the viscosity of the TP oil solutions concluded that, at equal supercooling conditions (e.g., 22.0–22.5), the magnitude of z and E _a became more dependent upon the crystal growth process as oil viscosity decreased. In contrast, as viscosity of the oil phase increased, the main crystallization process, evaluated through z and E _a′ was nucleation. Furthermore, within the supercooling interval achieved at the temperatures utilized, the increase in supercooling at constant viscosity conditions (e.g., 5.25–5.5 dynes/cm²) would produce a higher degree of nucleation without an appreciable effect on TP crystal size. The results obtained indicate that investigating the effects of supercooling, molecular diffusion (i.e., viscosity) and TP concentration on the magnitude of z and E _a during TP crystallization in sesame oil requires a multiple variable statistical approach.     

聚合物型成核剂对HDPE结晶行为的影响

以丙烯酰氯、对羟基苯甲酸和对羟基苯甲醚为原料,采用酰化反应制备了一种乙烯基单体,并采用自由基聚合反应合成出聚合物型成核剂聚4-甲氧基-4′-丙烯酰氧苯甲酸苯酯(PMAPAB);研究了PMAPAB对高密度聚乙烯(HDPE)结晶和熔融行为以及晶体形态的影响,并分析了HDPE的非等温结晶行为。结果表明：该聚合物用量为0．25％,1．5％时,均能促进HDPE的结晶,提高HDPE的结晶速率及结晶度,降低其晶粒的尺寸并使其分布更为均匀,当PMAPAB用量为0．25％时,能达到最佳的成核效果。     

PET/分子筛复合材料的非等温结晶性能

采用原位聚合法制备了PET／分子筛复合材料并研究了其非等温结晶性能。结果表明：分子筛的加入有明显的异相成核效应,加快了结晶速度,增加了结晶度,减小了晶粒粒径分布;PET及PET／分子筛体系的热结晶峰温随着降温速率的增加而移向低温,半结晶时间、结晶度随降温速率的增加而减小,晶粒粒径分布则增大;分子筛的加入降低了降温速率对PET半结晶时间的影响;随着分子筛用量的增加,半结晶时间t1／2、结晶热焓ΔH、结晶峰半峰宽ΔW都能达到一个较佳值。     

Overall crystallization kinetics of polymorphic propylene-ethylene random copolymers: A two-stage parallel model of Avrami kinetics

The isothermal crystallization of propylene ethylene random copolymers evolves with a simultaneous formation of two polymorphic forms, monoclinic crystals (α form) and orthorhombic crystals (γ form). The relative content of each polymorph changes during crystallization and impacts the kinetics and mechanisms of growth. The content of γ crystals developed at high levels of transformation increases with the concentration of ethylene and with the crystallization temperature. In this work, the overall crystallization kinetics of copolymers with an ethylene content ranging from 0.8 to 7.5 mol% were followed by DSC and analyzed according to classical Avrami kinetics. For most copolymers, fits to single-stage nucleation and growth models were poor. Following structural models for lamellar growth that account for epitaxial γ branching from α surfaces, the experimental data were modeled with a parallel two-stage kinetic model with excellent fits up to ∼70% transformations. The Avrami exponents obtained from the fits are consistent with a linear spherulitic growth from pre-existing nuclei for the α stage and homogeneous nucleation (linear with time) for the γ stage, and strongly support the postulated structural growth model. The rate constants of each stage follow the expected temperature dependence. Attempts made to extract the interfacial free energies for α and γ crystals from the values of the rate constants are discussed.     

Model of cold crystallization of uniaxially oriented poly(ethylene terephthalate) fibers

The differential scanning calorimeter heating curves of uniaxially oriented poly(ethylene terephthalate) (PET) fibers with three peaks were analyzed by using a newly proposed equation. The diffusion-controlled crystallization theory is suitable for describing cold crystallization of uniaxially oriented PET fibers. A crystallization model was proposed based on the kinetic parameters obtained. The model embraces the three sub-processes of crystallization corresponding to different growth geometries. The first sub-process corresponds to the nucleation of ordered molecular segments or the radial growth of preformed nucleus, resulting in the shorter bundle-like entities. The second sub-process corresponds to further growth of the bundle-like crystallites along chain direction, resulting in the longer bundle-like entities. The third sub-process corresponds to the three-dimensional growth of crystallites relating to the random segments, resulting in the spherical entities.     

Effect of Yb doping concentration on crystallization kinetics in yttrium aluminum garnet nano-powders

Different ytterbium concentration–doped yttrium aluminum garnet (Yb:YAG) nanopowders were synthesized using coprecipitation with nitrate salts as the starting materials. The phase evolutions, morphologies, and microstructures of the powders synthesized from various ytterbium-doped precursors were investigated. Ytterbium doping concentration was discovered to have a crucial effect during the YAG phase formation from the precursor. Crystallized Yb:YAG powders were directly obtained at temperatures as low as 900 °C without the formation of any intermediate phase. The crystallization kinetics of the Yb:YAG precursors were analyzed using non-isothermal differential scanning calorimetry. Avarami parameters of 0.97, 1.00, 1.13, and 1.21 were obtained for Yb doping concentrations of 0, 3, 6, and 9 at% respectively, and crystallization activation energies of 1506±40, 1342±36, 1171±31, and 978.1±26 kJ/mol were calculated. The activation energy for YAG crystallization was lower when a high Yb doping concentration was used because the presence of Yb³⁺ prohibited the presence of anionic SO₄²⁻ in the precursors, thus enhancing the elemental diffusion between particles. Both the average grain size and particle size of Yb: YAG decreased when Yb doping concentration was increased, and at various calcination temperatures.