可分散性纳米SiO2/尼龙1010复合材料的结晶行为

采用熔融共混法制备了可分散性纳米SiO2/尼龙1010复合材料.通过X射线衍射和差示扫描量热法对尼龙1010及其复合材料的结晶行为进行了研究.结果表明,SiO2的加入作为结晶的异相成核点,使复合材料的结晶温度升高,熔融温度降低,使其结晶度稍有降低;SiO2的加入并没有改变尼龙1010的结晶形态,只是使尼龙1010的晶格尺寸发生了一定程度的改变.     

Comprehensive Thermal Characteristics of Different Cultivars of Flaxseed Oil (Linum usittatissimum L.)

The aim of this study was to describe the thermal properties of selected cultivars of flaxseed oil by the use of the differential scanning calorimetry (DSC) technique. The crystallization and melting profiles were analyzed depending on different scanning rates (1, 2, 5 °C/min) as well as oxidative induction time (OIT) isothermally at 120 °C and 140 °C, and oxidation onset temperatures (Ton) at 2 and 5 °C/min were measured. The crystallization was manifested as a single peak, differing for a cooling rate of 1 and 2 °C/min. The melting curves were more complex with differences among the cultivars for a heating rate of 1 and 2 °C/min, while for 5 °C/min, the profiles did not differ, which could be utilized in analytics for profiling in order to assess the authenticity of the flaxseed oil. Moreover, it was observed that flaxseed oil was highly susceptible to thermal oxidation, and its stability decreased with increasing temperature and decreasing heating rate. Significant negative linear correlations were found between unsaturated fatty acid content (C18:2, C18:3 n-3) and DSC parameters (OIT, Ton). Principal component analysis (PCA) also established a strong correlation between total oxidation value (TOTOX), peroxide value (PV) and all DSC parameters of thermo-oxidative stability.     

Crystallization and melting behavior of a main-chain thermotropic copolyester

The crystallization and melting behavior of a main-chain thermotropic copolyester has been investigated by differential scanning calormetry (DSC). The effect of annealing time and temperature on the transition temperatures and enthalpies has been evidenced. Two melting peaks are observed and the first one clearly develops on annealing. Hypotheses are suggested about the crystallization mechanism.     

超支化聚酯对聚乙二醇非等温结晶行为的影响

利用差示扫描量热法(DSC)、偏光显微镜(POM)以及傅里叶变换红外(FTIR)光谱研究了一种超支化聚酯(HBP)对聚乙二醇(PEG)非等温结晶行为的影响,用Ozawa法、Jeziorny法和莫志深法对非等温结晶动力学进行了分析.结果表明:PEG和HBP/PEG的非等温结晶过程可用Ozawa和莫志深动力学方程描述,与Jeziorny动力学方程不符;超支化聚酯的加入改变了PEG的结晶成核和生长机理,对PEG的结晶有一定的延缓作用.超支化聚酯中的羰基与PEG的端羟基形成的氢键以及超支化聚酯自身较大的分子体积和高度支化的结构所导致的位阻效应是超支化聚酯延缓PEG结晶的主要原因.     

Crystallization regime characteristics of exfoliated polyethylene/vermiculite nanocomposites

Polyethylene (PE)–vermiculite (VMT) nanocomposites containing 0.5, 2, 4, 6, and 8 wt % clay loadings were fabricated via direct melt compounding in a twin‐screw extruder. Crystallization kinetics was investigated by means of polarized optical microscopy and differential scanning calorimetry. Moreover, the kinetics of the spherulitic growth of PE–VMT nanocomposites was evaluated with the Lauritzen–Hoffman (LH) secondary nucleation theory. The results showed that the PE–VMT nanocomposite exhibits crystallization regime characteristics. The nucleation constants for regimes I and II were determined from the slope of the LH plots. The fold surface energies (σ_e) of the PE–VMT nanocomposites were estimated from these slopes. The σ_e values were found to decrease with an increasing VMT content, up to 2%, in regime I. Further increasing the VMT content resulted in a slight increase of the σ_e values. In regime II, the σ_e values of the nanocomposites were generally lower than that of pure PE. These results demonstrated that the exfoliated silicate layers acted as effective nucleation sites for the secondary nucleus of the nanocomposites. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 253–263, 2005     

Influence of specimen thickness on isothermal crystallization kinetics. A theoretical analysis

In the DSC technique, isothermal crystallization experiments are usually performed on thin flat specimens, but their interpretation generally uses theories developed for an unbounded volume. In this paper, isothermal crystallization of spherical entities in the volume limited by two parallel infinite planes is considered. Our model, derived from Avrami's theory, gives an analytical expression for the transformed volume fraction as a function of time. It is shown that the influence of thickness becomes important when thickness becomes of the order of or smaller than the average spherulite radius. The main effects of a decreasing thickness are a slower crystallization kinetics and a decrease in the Avrami exponent. These results can be used to interpret experimental data obtained in isothermal polymer crystallization.     

Crystallization Kinetics of Glassy As2Se3

Isothermal crystallization of bulk As₂Se₃ glass was studied in temperature range 270-360°C. Johnson-Mehl-Avrami (JMA) equation describes the crystallization process in the whole temperature range. By means of analysis of JMA equation the temperature dependence of kinetic exponent n was found, its value changes from 3.8 to 1.9 with increasing temperature. The relationship between the value of n and crystal morphology was briefly discussed. Furthermore the value of apparent activation energy E was determined as well as melting enthalpy. Temperature dependence of crystal growth rate was also determined. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermal Properties of N-ethyl-N-phenyl-Dithiocarbamates and Their Influence on the Kinetics of Cure

The thermal properties (in the temperature range of 100–250°C) of N-ethyl-N-phenyldithiocarbamatecomplexes of Zn(II), Co(III), Ni(II), Cu(II) and Pb(II) and their influence on the kinetics of cure have been studied by differential scanning calorimetry (in nitrogen). It was found that Zn(II), Co(III) and Pb(II) dithiocarbamates melted without further effects, while the melting of Ni(II) and Cu(II) dithiocarbamates is accompanied with decomposition. From the kinetic point of view, the dithiocarbamates decrease the values of the reaction order and the values of rate constants follow this order (with respect to the metal ion): Zn(II)<Cu(II)<Pb(II)<Ni(II)<Co(III).This revised version was published online in November 2005 with corrections to the Cover Date.     

Isoconversional Analysis of the Nonisothermal Crystallization of a Polymer Melt

The advanced isoconversional method can be used to determine effective activation energies of the nonisothermal crystallization of polymer melts. The application of this method to differential scanning calorimetric data on the crystallization of poly(ethylene terephthalate) yields an activation energy that increases with the extent of crystallization from –270 to 20 kJ·mol^–1. The variation is interpreted in terms of accepted crystallization models.     

Detection of Beef Body Fat and Margarine in Butterfat by Differential Scanning Calorimetry

In this study differential scanning calorimetry (DSC) melting and crystallization curves of butterfat, beef body fat (BBF) and margarine were formed by cooling gradually from 70 to –40°C. Then margarine and BBF were added to butterfat at the rates of 5, 10 and 20% in order to investigate their curves. When BBF or margarine was added to butterfat, 1. and 2. peak areas increased in crystallization curves of butterfat with 2. peak being more discernible. Results obtained show that DSC technique could be used in order to determine adulteration of butterfat. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermal dependence of nucleation and growth rate in polypropylene by non isothermal calorimetry

Crystallization of polypropylene at a constant cooling rate is studied by differential scanning calorimetry. The results are interpreted using Ozawas theory, which allows the determination of the type of nucleation. In the studied polypropylene, a transition between a sporadic in time and an instantaneous nucleation is found at about 122 °C. Furthermore, an extension of Ozawa's theory is proposed to predict the thermal dependence of the spherulite growth rate. The results are consistent with those obtained by direct microscopic observation.     

Changes in Chemical Composition of Flaxseed Oil during Thermal-Induced Oxidation and Resultant Effect on DSC Thermal Properties

To investigate the changes in chemical composition of flaxseed oil during thermal-induced oxidation and the resultant effect on thermal properties, samples with different oxidation levels were obtained by being heated at 180 °C for two hours and four hours. The oxidation degree was evaluated using peroxide value (PV), extinction coefficient at 232 nm and 268 nm (K₂₃₂ and K₂₆₈), and total polar compounds (TPC). Using chromatography, the fatty acid profile and triacylglycerol (TAG) profile were examined. Differential scanning calorimetry (DSC) was used to determine the crystallization and melting profiles. Thermal-induced oxidation of flaxseed oil led to a significant increase (p < 0.05) in PV, K₂₃₂, K₂₆₈, and TPC, but the relative content of linolenic acid (Ln) and LnLnLn reduced dramatically (p < 0.05). TPC derived from lipid degradation affected both crystallization and melting profiles. Statistical correlations showed that the onset temperature (T_on) of the crystallization curve was highly correlated with K₂₃₂, TPC, and the relative content of LnLnLn (p < 0.05), whereas the offset temperature (T_off) of the melting curve was highly correlated with the relative content of most fatty acids (p < 0.05). This finding provides a new way of rapid evaluation of oxidation level and changes of chemical composition for flaxseed oils using DSC.     

Thermal characterization of polycarbonate/polycaprolactone blends

In this work, we prepared blends of bisphenol A polycarbonate (PC) and poly(ϵ‐caprolactone) (PCL) in a wide composition range by melt mixing and solution mixing. Two different molecular weights of PCL were used (nominally, 10.000 g/mol, PCL10, and 80.000 g/mol, PCL80). The thermal behavior of both systems was studied via differential scanning calorimetry under dynamic and isothermal conditions. The blends were miscible in the entire composition range in the liquid and amorphous states, as indicated by the single glass‐transition temperature (T_g) exhibited by both the PC/PCL10 and PC/PCL80 blends. The compositional variation of the T_g was accurately described by the Fox equation for the PC/PCL80 blends, whereas slight deviations from this equation were exhibited by the PC/PCL10 blends. For blend compositions containing 40% or more PCL, either one or both blend components crystallized. Crystallization occurred during cooling from the melt or during subsequent heating in the form of cold crystallization. Although PCL crystallization was reduced and its crystallization rate decreased with the addition of PC, PCL was a very effective macromolecular plasticizer for PC, to the extent that crystallization during the scan was detected for some blend compositions. Isothermal crystallization experiments allowed the determination of equilibrium melting points (T) by the Hoffman–Weeks extrapolation method. A T depression was found for both PCL and PC components as the content of the other blend component was increased. The Avrami equation was closely obeyed by both blend components during the isothermal overall crystallization kinetics up to crystalline conversion degrees of 60–70% and with values of Avrami indices ranging from 3 to 4, depending on the crystallization temperature employed. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 771–785, 2001     

Effect of Cooling Fluid Flow Rate on the Estimation of Conversion by Calorimetry in a Lab-Scale Reactor

Significant information about polymerization reactions carried out in lab-scale reactors is lost because sampling is not always possible due to the high viscosity, heterogeneity of the reaction medium or pressurization of the reactor. Thus, monitoring these reactions through calorimetry technique could be very valuable. Nevertheless, standard lab-scale reactors can present a relatively high residence time of the cooling fluid in the jacket and significant heat loss of the jacket to the surroundings. In the present work, the effect of the cooling fluid flow rate on the estimation of conversion through isothermal and isoperibolic calorimetry during a batch emulsion polymerization was investigated. Results show that the estimation of conversion through isothermal and isoperibolic calorimetry was not significantly affected by the cooling fluid flow rate using heat flow calorimetry. Nevertheless, when employing the energy balance of the jacket and the estimation of the global heat exchange coefficient between the jacket and the surroundings to estimate conversion (heat balance calorimetry) better results were obtained for lower cooling fluid flow rates.     

非等温结晶对PLLA的热行为和形貌的影响

将聚L-乳酸(PLLA)熔化非等温熔融结晶, 采用DSC、POM、SEM等技术研究了降温速率对PLLA的热行为和形貌的影响. PLLA在低降温速率(2 ℃·min-1)下的结晶在118 ℃伴随有结晶机制的转变. 玻璃化温度和结晶度随着降温速率的降低而增大. 随着降温速率的降低, 球晶尺寸增大, 当降温速率为10 ℃·min-1 时, PLLA 为无定型材料. 采用模压成型的方法并控制降温速率制备了具有球晶结构的条状PLLA 生物材料, 与高降温速率下制备的PLLA相比,低降温速率下获得的具有球晶结构的PLLA材料的断面更光滑和致密, 但脆性增强.     

Crystallization of tethered polyethylene in confined geometries

Ordered poly(ethylene)‐poly(vinylcyclohexane) (PE‐PVCH) block copolymers are employed to study the crystallization of tethered PE in confined geometries. The high T_g of the PVCH component of these materials forces PE chains to crystallize in well‐defined geometries dictated by the mesophase structure of the block copolymer. Effects of chain tethering on crystallization are examined through comparison of singly‐tethered PE chains in PE‐PVCH (EV) diblocks and doubly‐tethered PE in PVCH‐PE‐PVCH (VEV) triblocks. Crystallinity is independent of the block copolymer mesophase structure in both the EV and VEV systems, although crystallinity in VEV depends on the molecular weight of the PE block of the copolymer. Melting temperature data indicate that spatial confinement reduces crystallite size in EV and VEV, and that the double tethering of PE chains in VEV reduces crystallite size further through topological constraints. Crystal nucleation and growth depend strongly on the type of microstructure in both EV and VEV block copolymers. Differences in the overall rate of crystallization are correlated with the dimensional continuity of the PE microdomains. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37:2053–2068, 1999     

Kinetic Study on Pyrolysis of Extracted Oil Palm Fiber. Isothermal and non-isothermal conditions

Pyrolysis of extracted oil palm fibers under isothermal and non-isothermal conditions was carried out in a thermogravimetric analyzer. Isothermal curves showed that increasing pyrolysis temperature resulted in a faster pyrolysis and a higher conversion of oil palm fibers into gaseous products. Raw material sizes (below 1.0 mm) had insignificant effects on the isothermal pyrolysis, but the fibers with a size fraction of 1.0 to 2.0 mm resulted in a lesser conversion. Two-step reactions were found in the non-isothermal pyrolysis as evidenced by the presence of two peaks in the derivative thermogravimetry curves. Raw material sizes had no obvious effects on the temperature at which the maximum rate of pyrolysis occurred, but affected the rate of sample mass loss. For the low and high temperature regimes, a three-dimensional diffusion mechanism and a first-order of reaction mechanism respectively were used to describe the non-isothermal pyrolysis kinetics of extracted oil palm fibers. This revised version was published online in July 2006 with corrections to the Cover Date.     

Dependence of Thermal Stability of an Engine Lubricating Oil on Usage Period

Mineral oil type lubricating oils have an important role in reducing engine wear. But after some period, depending on the oil quality, oil replacement is needed since the oil loses its properties. In this paper, an engine oil was tested in a 1300 cc engine car and the change of the thermal stability of the collected portions of oil at different usage periods were experimented by TG/DTG and DSC techniques. Results show that, as usage period increases lubricating oil contains more of the light components that distill at low temperatures. The increase in the peak maximum temperature, T_max, of the DTG, as usage period increases, proposes that the molecular mass of the oil increase, which presumably changes the viscosity characteristics. Moreover as the usage period increases some residue type components, which only can be destroyed at very high temperatures, are produced in oil phase. This revised version was published online in August 2006 with corrections to the Cover Date.     

Polydispersity of ethylene sequence length in metallocene ethylene/α‐olefin copolymers. II. Influence on crystallization and melting behavior

In this work, crystallization and melting behavior of metallocene ethylene/α‐olefin copolymers were investigated by differential scanning calorimetry (DSC) and atomic force microscopy (AFM). The results indicated that the crystallization and melting temperatures for all the samples were directly related to the long ethylene sequences instead of the average sequence length (ASL), whereas the crystallization enthalpy and crystallinity were directly related to ASL, that is, both parameters decreased with a decreasing ASL. Multiple melting peaks were analyzed by thermal analysis. Three phenomena contributed to the multiple melting behaviors after isothermal crystallization, that is, the melting of crystals formed during quenching, the melting‐recrystallization process, and the coexistence of different crystal morphologies. Two types of crystal morphologies could coexist in samples having a high comonomer content after isothermal crystallization. They were the chain‐folded lamellae formed by long ethylene sequences and the bundlelike crystals formed by short ethylene sequences. The coexistence phenomenon was further proved by the AFM morphological observation. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 822–830, 2002