Thermal behavior of syndiotactic E-1,2-poly(3-methyl-1,3-pentadiene)

Syndiotactic E-1,2-poly(3-methyl-1,3-pentadiene) was synthesized with the catalyst system Fe(bipy)₂Cl₂-MAO. The thermal stability and kinetic parameters of degradation were determined by thermogravimetric analysis. The isothermal crystallization kinetics were described by means of the Avrami equation, which suggested a three-dimensional growth of crystalline units, developed by heterogeneous nucleation, followed by a secondary crystallization stage. Syndiotactic E-1,2-poly(3-methyl-1,3-pentadiene) isothermally crystallizes from the melt according to regime II of crystallization described by Lauritzen-Hoffman secondary nucleation theory. Non-isothermal crystallization kinetics were elaborated using different approaches. The equilibrium melting temperature was calculated. The kinetic and thermodynamic data were compared with those obtained from syndiotactic 1,2-poly(1,3-butadiene), which is the first example of 1,2 polydienes family.     

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.     

On the kinetics of melting and crystallization of poly(l-lactic acid) by TMDSC

The crystallization and melting process of poly(l-lactic acid), PLLA, is investigated by temperature modulated differential scanning calorimetry, TMDSC. The sample is cooled from the melt to different temperatures and the crystallization process is followed by subjecting the material to a modulated quasi-isothermal stage. From the average component of the heat flow and the application of the Lauritzen–Hoffman theory two crystallization regimes are identified with a transition temperature around 118 °C. Besides, the oscillating heat flow allows calculating the crystal growth rate via the model proposed by Toda et al., what gives, in addition, an independent determination of the transition temperature from modulated experiments. Further, the kinetics of melting is studied by modulated heating scans at different frequencies. A strong frequency dependence is found both in the real and imaginary part of the complex heat capacity in the transition region. The kinetic response of the material to the temperature modulation is analyzed with the model proposed by Toda et al. Finally, step-wise quasi-isothermal TMDSC was used to investigate the reversible surface crystallization and melting both on cooling and heating and a small excess heat capacity is observed.     

Die erlangungsfähigkeit des gleichgewichtes in systemen,die aus flüssigkeit und wenigstens aus einer festen phase gebildet sind

The difficulties associated with the precise determination of the freezing temperature of different systems containing one, two, or several solid phases are discussed. The “thermal sensitivity test” is suggested for testing whether the precision with which the thermodynamic liquid-solid phase equilibrium, is restored is quite satisfactory.The static method makes it possible to introduce or to remove very small quantities of heat and to observe whether or not the restoration of the phase equilibrium takes place. If a kinetic method is applied, the “thermal sensitivity” of the system may be tested by comparing the solidification and the melting curves determined at different crystallization or melting velocities.     

Nonisothermal cold‐crystallization kinetics of poly(trimethylene terephthalate)

The nonisothermal cold‐crystallization kinetics and subsequent melting behavior of poly(trimethylene terephthalate) (PTT) were investigated with differential scanning calorimetry. The Avrami, Tobin, and Ozawa equations were applied to describe the kinetics of the crystallization process. Both the Avrami and Tobin crystallization rate parameters increased with the heating rate. The Ozawa crystallization rate increased with the temperature. The ability of PTT to crystallize from the glassy state at a unit heating rate was determined with Ziabicki's kinetic crystallizability index, which was found to be about 0.89. The effective energy barrier describing the nonisothermal cold‐crystallization process of PTT was estimated by the differential isoconversional method of Friedman and was found to range between about 114.5 and 158.8 kJ mol^?1. In its subsequent melting, PTT exhibited double‐melting behavior for heating rates lower than or equal to 10 °C min^?1 and single‐melting behavior for heating rates greater than or equal to 12.5 °C min^?1. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4151–4163, 2004     

快速扫描量热法研究聚对苯二甲酸-1,4-环己烷二甲醇酯的结晶和熔融行为

采用快速扫描量热法(FSC)结合传统的差示扫描量热仪(DSC)考察了聚对苯二甲酸-1,4-环己烷二甲醇酯(PCT)聚酯在接近玻璃化转变(T_g)和熔融温度(T_m)范围(100~270 ℃)的结晶和熔融行为。 较大过冷度时PCT聚酯结晶较快,FSC有效地抑制降温过程结晶的发生,而较低过冷度下传统DSC可以避免样品降解对实验结果的影响,二者的结合能很好地对PCT聚酯结晶动力学进行测量,实验结果表明在175 ℃时结晶速率最快。 并且利用Flash DSC对等温结晶温度下形成的片晶熔点进行加热速率的相关测量,在熔融动力学建模的基础上进行校准,以确定零加热速率下片晶的熔点。 Hoffman-Weeks方程中T_m与结晶温度(T_c)的线性关系与T_c=T_m的交点给出了PCT晶体的平衡熔融温度$T_m^o$为315 ℃。     

Stop-and-return DSC method to study fat crystallization

Differential scanning calorimeters have frequently been used to study the isothermal crystallization kinetics of fats and oils. In some circumstances (e.g. start of crystallization during cooling to the crystallization temperature, crystallization in emulsion) this straightforward approach is not applicable. This paper describes an indirect DSC method for determination of the crystallization kinetics under these ‘difficult’ circumstances. The principle is to stop the crystallization at different moments during the isothermal crystallization and raise the sample temperature. The amount of heat released is then used as a measure for the amount of crystallization and plotted as function of time. Combination of the stop-and-return method with the direct method may sometimes be used to save on measurement time. Stop-and-return experiments can furthermore be used to gain more insight in the crystallization mechanism based on the fact that different polymorphic forms and fractions have different melting temperatures.     

A distribution kinetics approach for crystallization of polymer blends

The cluster distribution approach is extended to investigate the crystallization kinetics of miscible polymer blends. Mixture effects of polymer-polymer interactions are incorporated into the diffusion coefficient. The melting temperature, activation energy of diffusion, and phase transition enthalpy also depend on the blending fraction and lead to characteristic kinetic behavior of crystallization. The influence of different blending fractions is presented through the time dependence of polymer concentration, number and size of crystals, and crystallinity (in Avrami plots). Computational results indicate how overall crystallization kinetics can be expressed approximately by the Avrami equation. The nucleation rate decreases as the blending fraction of the second polymer component increases. The investigation suggests that blending influences crystal growth rate mainly through the deposition-rate driving force and growth-rate coefficient. The model is further validated by simulating the experimental data for the crystallization of a blend of poly(vinylidenefluoride)[PVDF] and poly(vinyl acetate)[PVAc] at various blending fractions.     

Crystallization kinetics and excess free energy of H2O and D2O nanoscale films of amorphous solid water

Temperature-programmed desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS) are used to investigate the crystallization kinetics and measure the excess free energy of metastable amorphous solid water films (ASW) of H(2)O and D(2)O grown using molecular beams. The desorption rates from the amorphous and crystalline phases of ASW are distinct, and as such, crystallization manifests can be observed in the TPD spectrum. The crystallization kinetics were studied by varying the TPD heating rate from 0.001 to 3 K/s. A coupled desorption-crystallization kinetic model accurately simulates the desorption spectra and accurately predicts the observed temperature shifts in the crystallization. Isothermal crystallization studies using RAIRS are in agreement with the TPD results. Furthermore, highly sensitive measurements of the desorption rates were used to determine the excess free energy of ASW near 150 K. The excess entropy obtained from these data is consistent with there being a thermodynamic continuity between ASW and supercooled liquid water.     

Poly(ethylene oxide)/poly(ethyl methacrylate) blends: Crystallization,melting behavior,and miscibility

Results of an investigation of isothermal crystallization and thermal behavior of poly(ethylene oxide)/poly(ethyl methacrylate) (PEO/PEMA) blends are reported. The blend composition and the crystallization temperature strongly influence the crystallization process from the melt and the melting temperature of PEO. The addition of PEMA to PEO causes a depression in the spherulite growth rate, in the overall kinetic crystallization constant, and in the melting temperature. Experimental data on the radial growth rate G and overall kinetic rate constant K_n are analyzed by means of the latest kinetic theory. From this analysis it emerges that the crystallization of pure PEO and PEO in the blend conforms to the regime I process of surface secondary nucleation. The depression of the melting temperature cannot be explained only in terms of a diluent effect due to the compatibility of the two components in the melt. Annealing and morphological effects, dependent on composition and time, must also be taken into account.     

KINETICS OF NON-ISOTHERMAL CRYSTALLIZATION OF POLY (ETHYLENE TEREPHTHALATE) MODIFIED BY POLY (ETHYLENE GLYCOL)

The non-isothermal crystallization kinetics of poly(ethylene terephthalate) (PET) modified by poly (ethlene glycol) (PEG) were determined by DSC. The dual linear regression method was used to evaluate the relationship between the reciprocal of t 1/2 ( the half life of crystallization) and the appropriate temperature variable. The parameters such as the activation energy (Ed) for transport, the equilibrium melting temperature (T_m~0),the nucleation parameter (ψ),themaximum crystallization temperature (T_(e, max)), and the kinetic crystallizability (G) for the copolyesters were obtained. The influence of the PEG content in PET chains on the parameters characterizing crystallization kinetics and crystallization thermodynamics was discussed.     

An Unusual but Consistent View on Flow Induced Crystallization of Polymers

Summary.  There is a considerable difference of more than 40 degrees centigrade between the equilibrium melting point of the α-crystal modification of i-PP and the lower temperature, where the α-spherulites of this polymer melt. The equilibrium melting point represents the temperature, where ideal crystals melt. In these crystals the macromolecules are in a stretched conformation. In contrast, in the spherulites the molecules are contained in lamellae of finite thickness. As a consequence it seems that in the interval between these two characteristic temperatures the nucleation kinetics is very different from the kinetics observed at temperatures below the melting temperature of the spherulites. This observation is of importance because almost all measurements on flow induced crystallization have been carried out below the melting temperature of the spherulites. It can be shown that at these lower temperatures the kinetics of crystallization (including flow induced crystallization) has nothing to do with the classical ideas about sporadic nucleation.     

Determination of the copper activity of liquid CuLa alloys

The thermodynamic activities of liquid CuLa alloys were obtained at 1549 K by Knudsen effusion experiments. The experimentally known concentration and temperature dependences of thermodynamic properties of liquid CuLa alloys are explained on the basis of an association model. These results are discussed in comparison with the enthalpy of crystallization and the crystallization temperature as well as the enthalpy of melting and the melting temperature of lanthanum-rich glassy and crystalline alloys respectively.     

Kinetics of polymer crystallization from the melt (calorimetric approach)

Crystallization kinetics for 12 polymers including polyolefins, polyesters, polyurethanes, polysiloxanes was measured by the evolution of heat in a modified Calvet-type calorimeter over wide temperature ranges. The results are analyzed in terms of the Avrami equation and a comparison between calorimetric and dilatometric results is carried out. It is concluded that, although in the majority of cases experimental results do not obey the Avrami equation, for some polymers the agreement is rather good. The Avrami parameter obtained, however, depends on the experimental technique. Possible reasons for this disagreement are discussed. Analysis of the calorimetric crystallization rate in the vicinity of the melting point by using the kinetic theory of crystallization shows that the growth is controlled by surface (two-dimentional) nucleation. Energy parameters for the crystallites were determined and it is shown that the surface energy of the crystallites depends on the molecular structure of the polymer. Temperature dependence of the calorimetric crystallization rate of the polymers for which crystallization rates could be determined above and below the maximum rate are analyzed using a kinetic equation with common approximations for the transport term. The influence of melting conditions on the crystallization rate was studied. The results indicate heterogeneous nucleation in the polymer melt. It is concluded that this may be due both to impurities and to high regularity of macromolecules in the polymer melt.     

On the definition of a Monte Carlo model for binary crystal growth

We show that consistency of the transition probabilities in a lattice Monte Carlo (MC) model for binary crystal growth with the thermodynamic properties of a system does not guarantee the MC simulations near equilibrium to be in agreement with the thermodynamic equilibrium phase diagram for that system. The deviations remain small for systems with small bond energies, but they can increase significantly for systems with large melting entropy, typical for molecular systems. These deviations are attributed to the surface kinetics, which is responsible for a metastable zone below the liquidus line where no growth occurs, even in the absence of a 2D nucleation barrier. Here we propose an extension of the MC model that introduces a freedom of choice in the transition probabilities while staying within the thermodynamic constraints. This freedom can be used to eliminate the discrepancy between the MC simulations and the thermodynamic equilibrium phase diagram. Agreement is achieved for that choice of the transition probabilities yielding the fastest decrease of the free energy (i.e., largest growth rate) of the system at a temperature slightly below the equilibrium temperature. An analytical model is developed, which reproduces quite well the MC results, enabling a straightforward determination of the optimal set of transition probabilities. Application of both the MC and analytical model to conditions well away from equilibrium, giving rise to kinetic phase diagrams, shows that the effect of kinetics on segregation is even stronger than that predicted by previous models.     

Isothermal crystallization behavior of PP and PP-g-GMA copolymer at high undercooling

The study involves synthesis of polypropylene grafted with glycidyl methacrylate (PP-g-GMA) using three different initiators, benzoyl peroxide, dicumyl peroxide and tertiary butyl cumyl peroxide (TBSP). Among the peroxides used, dicumyl peroxide resulted in considerable reduction of molecular weight of the resulting graft copolymer. The melting/crystallization behavior and isothermal crystallization kinetics of PP homopolymer and PP-g-GMA copolymers were studied with differential scanning calorimetry (DSC) at high undercooling (44–60°C). The results showed that the degree of crystallinity and overall crystallization rate of copolymers is greater than that of virgin PP. Among the three initiators used, TBCP exhibited lowest half crystallization time. The isothermal crystallization kinetics of the PP and copolymers was described with the Avrami equation and Sestak-Berggren (SB) equation. The Avrami exponent n of the PP and copolymers were found to be in the range 1.03 to 1.41 at high undercooling conditions employed in this study. The agreement between the values of n calculated from SB kinetics and Avrami equation is satisfactory with few exceptions. The crystallization rate of PP-g-GMA copolymer was found to be more sensitive to temperature. The isothermally crystallized samples showed a single melting peak for PP while a double peak at lower temperature was recorded for PP-g-GMA copolymer samples. The equilibrium melting point was deduced according to Hoffman-Weeks theory. The decrease of recorded for the PP modified with GMA suggests that the thermodynamic stability of the PP crystals is influenced by the chemical interactions.     

稀土顺-1,4-聚丁二烯的分子量及分子缠结对结晶行为的影响

本工作以线膨胀、体膨胀、DSC、扭辫等方法,在较宽的分子量和结晶温度范围内研究了稀土顺-1,4-聚丁二烯(Ln-PB)的分子量与结晶速率,结晶熔点,玻璃化温度及结晶比表面能之间的关系。进一步考察了分子链缠结对结晶速率的影响,并在结晶成核理论的基础上引入链缠结的影响,导出Ln-PB的结晶动力学方程,得到了与实验符合较好的结果。