Thermal and dielectric properties of powder coatings based on carboxylated polyester and β-hydroxyalkylamide

Thermal transitions, relaxations and the crosslinking reaction in varnishes and pigmented paints based on carboxylated polyester and β-hydroxyalkylamide (Primid) were studied by differential scanning calorimetry (DSC), temperature modulated DSC (TMDSC), thermogravimetry (TGA) and dielectric analysis (DEA). The effect of the Primid content on the glass transition of the powder coatings before and after crosslinking was analysed. The melting and flow regions were studied by TMDSC and correlated with the loss factor, which was measured by DEA. A comparison of the DSC and TGA scans showed that the crosslinking reaction was masked by the vaporisation of water formed during esterification. The resulting endothermic reaction made it difficult to analyse the crosslinking by DSC. Nevertheless, it was possible to detect the crosslinking reaction by the dielectric measurement of the loss factor and the conductivity.     

Application of an extended Tool–Narayanaswamy–Moynihan model.Part 2. Frequency and cooling rate dependence of glass transition from temperature modulated DSC

In the first part of this paper a Tool–Narayanaswamy–Moynihan-model (TNM) extended by non-Arrhenius temperature dependence of the relaxation time was applied to describe results from temperature modulated DSC (TMDSC). The model is capable to describe the features of the heat capacities measured in TMDSC scan experiments in the glass transition region of polystyrene (PS). In this part the model is applied to bisphenol A-polycarbonate (PC). Both aspects of glass transition, vitrification as well as the dynamic glass transition are again well described by the model. The dynamic glass transition above T_g can be considered as a process in thermodynamic equilibrium. The non-linearity parameter (x) of the TNM model is not needed to describe complex heat capacity as long as the dynamic glass transition is well separated from vitrification. Under such conditions the relation between cooling rate (q₀), and the corresponding frequency (ω) can be found from the two independently observed glass transitions. Fictive temperature and the maximum of the imaginary part of complex heat capacity are used for comparison here. The measurement as well as the TNM-model confirm the relation derived from Donth's fluctuation approach to glass transition, ω=q₀/aδT, where a=5.5±0.1 (confirmed previously experimentally as 6±3) and δT is mean temperature fluctuation of the cooperatively rearranging regions (CRRs).     

Understanding the effect of chain entanglement on the glass transition of a hydrophilic polymer

In this article, we report an interesting phenomenon of the glass transition temperature (T_g) deviation of a hydrophilic polymer. Polyacrylamide (PAL) samples with different extents of chain entanglement were prepared by spray drying and solution casting. We found that the glass transition temperature increases as the extent of chain entanglement decreases upon the sub‐T_g annealing. The water content in the PAL matrix is found with no direct correlation to T_g. However, the observation of a faster diffusion process of water in the disentangled PAL matrix offers an evidence of a faster relaxation process of disentangled PAL molecules. The T_g increase of the disentangled PAL samples is believed to be associated with the increased molecular interaction during the chain relaxation process upon the sub‐T_g annealing. A macroscopic evidence is the fact that the density of the hot‐laminated samples increases as the extent of chain entanglement decreases. A thermodynamic analysis suggests that enthalpy more than entropy favors an elevated T_g of a disentangled hydrophilic polymer upon the sub‐T_g annealing. We believe that this research provides new understanding of T_g of the hydrophilic polymers, which are being extensively used in bio‐related studies. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Factors influencing the glass transition of DGEBA-anhydride epoxy resins

A number of chemical and physical factors influence the glass transition behaviour of anhydride-cured diglycidyl ether of bisphenol A (DGEBA) type epoxy resins. Several of these were investigated using differential scanning calorimetry to assess the nature of the glass transition, the transition temperature and the glass- and liquid-phase specific heats. The influence of molecular weight and chemical functionality, cooling rate and silica fillers were investigated for unreacted epoxy prepolymers. Two anhydride-cured systems were also considered and the influence of cure with and without silica fillers and the effect of phase separation and cooling rate were investigated.     

测试方法对聚合物玻璃化温度的影响

选取环氧树脂固化物为样品,分别采用DSC法和DMA法测定了其玻璃化转变温度。结果表明,玻璃化转变温度随测定方法和条件的不同而变化,因此,在给出某种材料的玻璃化转变温度时,必须注明测定方法和条件。     

温度调制热分析技术对预吸热峰的研究

以快速的冷却速率冷却熔体获得的金属玻璃通常远离平衡状态。如果在适当温度经过合适时间退火后,在热流曲线上玻璃转变温度之前会产生明显的预吸热峰。使用常规差示扫描量热法(DSC)和温度调制差示扫描量热法(TMDSC)对Pd₄₀Ni₁₀Cu₃₀P₂₀和Au₄₉Ag_5.5Pd_2.3Cu_26.9Si_16.3两种金属玻璃进行研究。结果表明,预吸热峰从玻璃态到过冷液态是一个连续演变过程,当预吸热峰出现在玻璃转变温度之下时,预吸热峰向高温移动过程中对玻璃化转变过程没有影响。然而,当预吸热峰出现在玻璃化转变温度以上时,玻璃转变开始温度会随着预吸热峰值的增加而增加,因此金属玻璃的动力学稳定性得到了提高。实验结果对理解预吸热峰从玻璃态到过冷液态过程中的稳定变化具有重要意义。     

A study about the structure of vulcanized natural rubber/styrene butadiene rubber blends and the glass transition behavior

A study of the thermal behavior of cured elastomeric blends of natural rubber (NR) and styrene butadiene rubber (SBR) prepared by solution blending in toluene is presented. Binary blends with different compositions of NR/SBR were produced using a conventional cure system based on sulfur and TBBS (n-t-butyl-2-benzothiazole sulfonamide as accelerator. The compounds were vulcanized at 433 K up to an optimum time of cure determined by rheometric tests. From swelling tests, the crosslink densities of the compounds were obtained and compared with those obtained in similar blends prepared by mechanical mixing. The results were analyzed in terms of the disentangling of the chain structures of the SBR and NR phases and the achieved cure state of the blend. Using differential scanning calorimetry, the glass transition temperature T_g of each blend was measured. In most compounds, the value of T_g corresponding to each phase of the blend was determined, but in some blends a single value of T_g was obtained. The variation of T_g with the composition and cure level in each phase was analyzed. On the other hand, a physical mixture of two equal parts of NR and SBR vulcanized was measured and the results were compared to those of the NR50/SBR50 cured blend. Besides, to analyze the influence of the network structure, pure NR and SBR unvulcanized samples were measured. On the basis of all the obtained results, the influence of the interphase formed in the blend between SBR and NR phases is discussed. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Evaluation of aluminosilicate glass sintering during differential scanning calorimetry

The aim of this work is to investigate a difference in heat flow observed in the differential scanning calorimetry curves when aluminosilicate glasses are analyzed. Glasses with nominal composition 56.21 SiO₂ 18.65 Al₂O₃ 25.14 MgO (wt%) were produced by the conventional melting process. Glass frits were milled and sieved in the range of 45–63 µm. The material was analyzed by X-ray fluorescence spectrometry, X-ray diffraction, differential scanning calorimetry (DSC), and differential thermal analyses (DTA). The particle size distribution was determined by laser diffraction technique. The microstructures of the samples were observed by scanning electron microscopy after removing them from the DSC sample holder. The density was determined by He picnometry. The difference in heat flow in the DSC curves is assigned to the sintering process occurring during the heating cycle, which was confirmed by the neck formation in the particle interface, DSC signal variation in isothermal measurements, no change in the heat flow when monolith specimen are analyzed, and in subsequent DSC analysis after cooling. The concurrent crystallization was also determined.     

差示扫描量热法测定聚丙烯腈纤维玻璃化转变温度

本文采用差示扫描量热法,两段升温模式,以10℃/min加热速度,从30℃加热到150℃,测定了4个聚丙烯腈纤维样品的玻璃化转变温度,探讨了加热方法和样品量试验条件,得到了准确可靠的玻璃化转变温度。     

Reconstruction of histograms of the glass transition temperature of free radical copolymers from DSC thermograms

Most polymeric materials appear as complex mixtures of macromolecules characterized by distributions of specific properties that are essential to the quality of these products. Among such properties, the accurate determination of the glass transition temperature, and therefore, accurate representation of it, is a key issue. When analyzed using dynamic scanning calorimetry (DSC) techniques, many copolymers exhibit a wide range of temperature over which the glass transition takes place, and the width of the transition region is, therefore, not satisfactorily described by average T_g values, for example those computed from tangent curves drawn on thermograms. This article describes a method that allows us to characterize this spreading of the glass transition region by reconstructing weighted T_g distributions from DSC thermograms. As such an objective might appear as questionable from a strictly physical point of view, the significance of what is meant by “distribution” is specified in the text. A model is proposed that accounts for relaxation phenomena. The approach is validated by examining samples of BuA/Sty emulsion copolymers produced at different overall conversions and compositions, and examining the corresponding histograms of T_g were computed. The results show that accurate and consistent information on the glass transition behavior of the copolymer is obtained, and that the effective distribution is clearly connected with the composition drift in the polymer particles. The proposed algorithm allows one to obtain a maximum amount of information from DSC measurements, and provides a deeper insight into the “history” of complex polymer mixtures. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 357–367, 2000     

Crystallization behaviour of biodegradable poly(ethylene succinate) from the amorphous state

Nonisothermal and isothermal crystallization kinetics of biodegradable poly(ethylene succinate) (PES) from the amorphous state were studied by differential scanning calorimetry (DSC). For the nonisothermal crystallization, there were two crystallization exotherms upon heating from the amorphous state. One major crystallization exotherm located at low temperature corresponded to the real cold crystallization of PES, while the other minor one located at high temperature may correspond to the melt-recrystallization of the unstable crystals formed during the nonisothermal crystallization earlier. Several methods, such as Avrami equation, Tobin equation and Ozawa equation, were applied to describe the nonisothermal crystallization process of PES. Meanwhile, Avrami equation was also employed to study the isothermal crystallization of PES from the amorphous state. Similar to the nonisothermal crystallization the minor crystallization exotherm was also found in the DSC trace upon heating to the melt after the isothermal cold crystallization finished completely, and was attributed to the melt-recrystallization of the unstable crystals formed during the isothermal cold crystallization. Temperature modulated differential scanning calorimetry (TMDSC) was used in this work to investigate the origin of the minor crystallization exotherm located at high temperature, and the TMDSC experiments gave a direct evidence that the origin of the minor crystallization exotherm was from the melt-recrystallization of the originally existed unstable crystals formed through previous crystallization.     

Glass-transition temperatures for soft-contact-lens materials. Dependence on water content

Glass-transition temperatures for three soft-contact-lens (SCL) materials are measured by modulated differential scanning calorimetry as a function of SCL hydration, as determined by thermogravimetric analysis. The SCL materials are: a conventional hydrogel (SofLens^® 38, polymacon) with a low water content at saturation; a conventional hydrogel (SofLens™ One Day, hilafilcon A) with a high water content at saturation; and a siloxane-hydrogel (PureVision™, balafilcon A).Polymacon, hilafilcon A, and balafilcon A turn glassy at 35 °C when their water contents drop below 10.4, 13.5, and 6 wt%, respectively. These water contents correspond to the equilibrium water uptake at 35 °C for polymacon, hilafilcon A, and balafilcon A at relative humidities, RH, of 74, 64, and 57%, respectively. Our results suggest that the outer surface of a soft contact lens worn on the eye may develop a glassy skin when exposed to air at low relative humidity. This glassy skin may alter fluid transport through the soft contact lens, and influence SCL-wear comfort.     

Investigation on curing reaction of phthalonitrile resin with nanosilica and the properties of their glass fiber-reinforced composites

In this work, Phthalonitrile containing benzoxazine (BA-ph) and Bisphenol A based cyanate ester (CE) were chosen as the matrix resin. Various amount of nano-SiO₂ was incorporated into BA-ph/CE and their glass fiber-reinforced composite laminates were fabricated. Curing reaction and processability of BA-ph/CE/SiO₂ blends were studied by differential scanning calorimetry and dynamic rheological analysis. Results showed that BA-ph and CE exhibited good processability and curing reaction of BA-ph/CE was not obviously affected by SiO₂. Scanning electron microscope images of the composites showed that SiO₂ particles were well dispersed in BA-ph/CE matrix. Moreover, SiO₂ could act as physical crosslinking points and diluent in matrix as well as between the glass fibers to improve the mechanical properties of composite laminates. As the results of dynamic mechanical analysis and thermogravimetry analysis, composite laminates possessed satisfactory T_g and good thermal stability. With incorporation SiO₂ particles into matrix resin, dielectric constant and dielectric loss of BA-ph/CE/SiO₂/GF composites were increased and showed frequency dependence.     

Cure kinetics,glass transition temperature development,and dielectric spectroscopy of a low temperature cure epoxy/amine system

This article reports a study of the chemical cure kinetics and the development of glass transition temperature of a low temperature (40°C) curing epoxy system (MY 750/HY 5922). Differential scanning calorimetry, temperature modulated differential scanning calorimetry, and dielectric spectroscopy were utilized to characterize the curing reaction and the development of the cross‐linking network. A phenomenological model based on a double autocatalytic chemical kinetics expression was developed to simulate the cure kinetics behavior of the system, while the dependence of the glass transition temperature on the degree of cure was found to be described adequately by the Di Benedetto equation. The resulting cure kinetics showed good agreement with the experimental data under both dynamic and isothermal heating conditions with an average error in reaction rate of less than 2 × 10^?3 min^?1. A comparison of the dielectric response of the resin with cure kinetics showed a close correspondence between the imaginary impedance maximum and the calorimetric progress of reaction. Thus, it is demonstrated that cure kinetics modeling and monitoring procedures developed for aerospace grade epoxies are fully applicable to the study of low temperature curing epoxy resins. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Relationship between cooling rate and crystallization behavior of hydrogenated sunflowerseed oil

The relationship between cooling rate and crystallization behavior of hydrogenated sunflowerseed oil and its thermal properties during melting were investigated by differential scanning calorimetry and x-ray diffraction. At crystallization, temperature peaks and area percents of components with higher melting points were found to be greater when cooling rate was slower. When samples were melted, total enthalpies were similar at all rates. According to the chemical composition of the analyzed samples, the components with higher melting points were found in samples with a higher saturated fatty acid content. Changes in the profile of melting curves might be due to differences in triglyceride intersolubility.     

Phase transitions and glass transition in a hyperquenched silica‐alumina glass

We investigate phase transitions, glass transition, and dynamic behavior in the hyperquenched 69SiO₂–31Al₂O₃ (mol%) glass (SA glass). Upon reheating, the SA glass exhibits a series of thermal responses. Subsequent to the sub‐T_g enthalpy release, the glass undergoes a large jump in isobaric heat capacity (ΔC_p) during glass transition, implying the fragile nature of the SA glass. The mullite starts to form before the end of glass transition, indicating that the SA glass is extremely unstable against crystallization. After the mullite formation, the remaining glass phase exhibits an increased T_g and a suppressed ΔC_p. The formation of cristobalite at 1553 K indicates the dominance of silica in the remaining glass matrix. The cristobalite gradually re‐melts as the isothermal heat‐treatment temperature is raised from 1823 to 1853 K, which is well below the melting point of cristobalite, while the amount of the mullite remains unchanged.     

The glass transition temperature of poly(N-vinyl pyrrolidone) by differential scanning calorimetry

Previously a wide range of values have been reported for the glass transition temperature, T_g, of poly(N-vinyl pyrrolidone), PVP, and it was suggested that lower values are due to variable uptakes of water caused by the hygroscopic nature of the polymer. Now it has been found that there are large variations in T_g, even in carefully dried specimens of PVP. Other factors found to influence T_g are residual monomer and the molecular weight of PVP. Polymers prepared by bulk polymerization, either by γ-irradiation or by heating with 2-azobisisobutyronitrile, have much lower values of T_g than dried ones prepared containing 30% water. The difference is mainly due to depression of T_g by residual monomer which, in the absence of water during polymerization, fails to react completely because of conversion to a glassy state. An unexplained observation is that even when all residual monomer has been removed, polymers prepared by bulk polymerization still have a lower T_g than would be expected from their molecular weight.     

Determination of the transition temperature at different cooling rates and its influence on prediction of shrinkage and warpage in injection molding simulation

In injection molding simulation the phase change from melt to solid state is usually simplified by using a so called transition temperature. In the present work, the transition temperatures of several amorphous and semicrystalline polymers were determined using DSC‐runs at different cooling rates. The transition temperature was described as a function of cooling rate. The obtained transition temperatures of the semicrystalline polymers were used in injection molding simulations with the commercial software package Autodesk Moldflow Insight 2010 to calculate the shrinkage and warpage of box‐shaped test parts. The simulation results were compared with the experimental values of optically measured injection‐molded boxes. The results showed a strong influence of the transition temperatures on the simulation results of a 3D model and a very low influence for a 2.5D model. Transition temperatures obtained at higher cooling rates improved the 3D simulation results for several dimensions. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011