Differential scanning calorimetric studies on phase transition of glucose and cellulose oligosaccharides

The phase transitions of D-glucose and an homologous series of cellulose oligosaccharides up to cellotetraose have been studied using a differential scanning calorimeter. A detailed comparison of our experimental results with those reported by other workers has been made in order to derive precise information on melting and glass transitions of carbohydrates. Particular attention has been given to the static values of melting temperature of D-glucose and cellobiose. It has also been found that cellulose oligosaccharides which have more than three anhydroglucose units decompose below melting temperature.     

Differential scanning calorimetric examination of melamine–formaldehyde microcapsules containing decane

The main objective of this study was to investigate the composition of microcapsules and the degree of curing of melamine–formaldehyde (MF) resin, which formed a shell of microcapsules, by the use of differential dynamic calorimetry (DSC). For this purpose, decane was chosen as core material. The microencapsulation of decane with MF resin was carried out at different temperatures and pH values. The temperature and pH value were kept constant during the process. The composition of the microcapsules and the degree of curing of the shell material were studied during and after the microencapsulation process. DSC analysis, in combination with scanning electron microscopy analysis, was revealed as an effective tool for the investigation of the microencapsulation process with MF resin. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

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。     

Effect of water on the glass transition temperature of hydrophilic polyurethanes

The depression of the glass transition temperature by water was studied in a set of polyurethanes in which the soft segment consisted of polyethylene oxide (sample I) or a block copolymer of poly(propylene oxide) terminated with poly(ethylene oxide) in various proportions (sample set II). DSC measurements were made at two added water contents for each type of sample and at various temperatures. The T_g reduction appeared to be governed solely by the nonfreezing bound water and was much larger in sample I than in samples of set II. The more limited effect on the T_g of set II samples is attributed to restricted mobility arising from coupling of the short terminal poly(ethylene oxide) to rigid hard-segment units. Therefore, the data for sample I are preferred as a test of the predictive relations for the T_g depression. On this basis, it appears that the simple Fox mixing equation is more reliable than is the available free volume approach, which required unrealistically high values of the thermal expansion coefficient for water to fit the data for sample I. © 1993 John Wiley & Sons, Inc.     

Dynamic mechanical studies of the glass transition temperature of photopolymerized multifunctional acrylates

Summary The glass transition temperatures of the highly crosslinked networks formed from photopolymerizing-the multifunctional monomers trimethylol propane triacrylate (TrMP TrA), pentaerythritol tetraacrylate (PETeA), and dipentaerythritol monohydroxy pentaacrylate (DPEMHP eA) were determined using a dynamic viscoelastometer. All glass transition temperatures, Tg, were in excess of 150°C indicating the highly crosslinked structure of the resulting networks. The curves of the damping factor as a function of temperature were broadened peaks spanning up to 100°C due to the extreme heterogeneity of these structures. Evidence of trapped radicals was also observed as cured samples subjected to a thermal pretreatment had higher Tg than those without the thermal pretreatment.     

Estimation of frequency from a temperature scanning rate in differential scanning calorimetry at the glass transition of polystyrene

A transform formulation of a heating rate β in differential scanning calorimetry (DSC) into frequency ω at the glass transition of polystyrene is proposed by using Duhamel’s theorem, a superposition integral based on a linear response theory.A heat capacity spectroscopy in a frequency range of eight orders of magnitudes for glass transition of polystyrene, including the results of temperature wave analysis and all the literature values, is well fitted with a Vogel-Fulcher-Tamman-Hesse (VFTH) curve. The origin of the discrepancies is discussed.     

Effect of cooling rate and frequency on the calorimetric measurement of the glass transition

The glass transition of thermoplastics of different polydispersity and thermosets of different network structure has been studied by conventional differential scanning calorimetry (DSC) and temperature modulated DSC (TMDSC). The cooling rate dependence of the thermal glass transition temperature T_g measured by DSC, and the frequency dependence of the dynamic glass transition temperature T measured by TMDSC have been investigated. The relation between the cooling rate and the frequency necessary to achieve the same glass transition temperature has been quantified in terms of a logarithmic difference Δ=log₁₀[|q|]−log₁₀(ω), where |q| is the absolute value of the cooling rate in K s⁻¹ and ω is the angular frequency in rad s⁻¹ necessary to obtain T_g(q)=T(ω). The values of Δ obtained for various polymers at a modulation period of 120 s (frequency of 8.3 mHz) are between 0.14 and 0.81. These values agree reasonably well with the theoretical prediction [Hutchinson JM, Montserrat S. Thermochim Acta 2001;377:63 [6]] based on the model of Tool–Narayanaswamy–Moynihan with a distribution of relaxation times. The results are discussed and compared with those obtained by other authors in polymeric and other glass-forming systems.     

用差示扫描量热法测定橡胶填充油的玻璃化转变温度

采用差示扫描量热法(DSC)测定了不同组成的橡胶填充油的玻璃化转变温度(Tg),考察了操作条件对Tg的影响,并分析了测试方法的精密度和重复性.结果表明,试样用量和升温速率对测试结果均影响不大,综合考虑试样用量宜为5～20 mg,升温速率为20℃/min;DSC测试具有良好的精密度和重复性.     

Interaction of epoxy resins with water: the depression of glass transition temperature

The depression of T_g in a number of epoxy-water systems has been investigated calorimetrically. Except where an unbalanced reaction stoichiometry was used, experimental results are in good agreement with theoretical predictions based on equations developed for the composition dependent T_g in polymer diluent-systems. Thermal analysis also demonstrates a complex thermal behaviour on rescanning of samples, attributed to diluent diffusing out of the sample. The importance of this behaviour in the context of solvent-induced physical ageing of epoxy resin composites is discussed.     

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.     

Studies on glass transition temperature during staling of bread containing different monomeric and polymeric additives

Glass transition temperature (T_g) of bread containing different antistaling agents and also of bread superquenched and annealed in a DSC cell and outside, was determined using differential scanning calorimeter (DSC). Increase in T_g during the staling of bread correlated with firming as measured by Instron. Singlet T_g was an indication of miscibility or compatibility of bread components with each other. The lowest increase in T_g was found in bread containing propylene glycol (singlet) followed by glycerol (doublet), maltodextrin (broad), gelatin (singlet), antistaling enzyme (singlet), and polypropylene glycol (doublet) in the order of increase in T_g. Superquenching produced a maximum increase in T_g of bread. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1147–1152, 1999     

Substituent effect on the glass transition temperature of polyoxetanes

Summary The effect of the substituents on the glass transition temperature is analyzed for a series of polyoxetanes, with alkyl substituents in position 3. The results show a regular increase of that parameter with the Van der Waals volume of the linear substituents, although a departure from this trend seems to be observed for poly(3,3-dibutyloxetane), presumably because of the effect of internal plasticization. On the other hand, the increase of T_g for poly(3-terbutyloxetane) is specially high, as usual for branched substituents. The behaviour of the glass transition temperature along the series has been analyzed in terms of interand intramolecular interactions, concluding that the latter factor is the dominant contribution in determining the T_g in this group of polymers.     

Soluble aromatic polyimides with high glass transition temperature from benzidine containing tert-butyl groups

A series of soluble polyimides with high glass transition temperature (T _g ) were prepared from four commercial aromatic dianhydrides (i.e., BPDA, OPDA, 6FDA and BPADA) with a rigid aromatic diamine containing tert-butyl groups (3,3′-di-tert-butylbenzidine) (1). The number-average molecular weights of them were in the range from 4.21?×?10⁴ to 1.25?×?10⁵ with the polydispersity indexes between 2.34 and 3.04 by means of gel permeation chromatography relative to a polystyrene standard. They were able to form transparent, flexible and tough films by polymer solution casting. The transmittance of them was higher than 90 % in a wavelength range of 400–700 nm by UV–vis measurement. The XRD results demonstrated that they were amorphous glassy polymers. Except for polyimide 1-BPADA, the T _g s of the rest were higher than 330 °C, and especially the T _g of polyimide 1-BPDA reached up to 375 °C. They did not show appreciable decomposition up to 500 °C under a nitrogen atmosphere, in other words, the 5 % weight losses of them were in the range from 508 to 523 °C. Most of them exhibited high tensile strength and modulus with a low elongation at break at room temperature. They are potential candidates for high performance materials.     

Film formation from cationic electropaint systems containing resins with different glass transition temperature

Some phenomena in the deposition process of electropainting have not been well elucidated till now. In this paper, to investigate an influence of glass transition temperature (T_g) on film formation, the deposited film was observed with an atomic force microscope and the electrochemistry was investigated, using two kinds of cationic acrylate resin with different T_g (methyl methacrylate system (MMAs): T_g=70°C, and methyl acrylate system (MAs): T_g=5°C). Electrodeposition was performed under constant voltage or current condition.

At constant voltage, the deposition behavior in the two resin systems differed extremely. The MMAs, the resin with high T_g, produced a high resistance film. The MAs, the resin with low T_g, was deposited forming a film at a voltage lower than 20 V. At constant current, the film formation did not result in a rise in voltage. It behaved like a conductive film. When the resin with high T_g was used, particulate deposits were observed by AFM even in the induction period. The resin with low T_g formed flat deposits. These results suggest that paint deposition is initiated once electrolysis of water starts. In addition, there are two types of film formation on the cationic electropainting: high resistance film formation for the resin with high T_g, and ion-permeable film formation for the resin with low T_g. In both cases, film growth occurs at the film/bulk solution interface.     

Absorption and glass transition temperature of adhesives exposed to water and toluene

If adhesives are to be used in practical applications, such as for aerospace or marine components, it is essential that their long-term behaviour when exposed to aggressive environments, natural or man-made, should be known. The objective of this paper is to determine the liquid absorption behaviour of epoxy adhesives under complete immersion in two solvents (water and toluene) at two different temperatures (room temperature and 60 °C) and the effect on their glass transition temperature (T_g). The specimens were periodically removed from their environment for weighing and T_g measurement, the latter by using a rapid method which was developed previously by the authors. The rate of weight gain of the specimen (sorption) and the equilibrium level were higher for the higher temperature immersion than for the low temperature. The equilibrium level reached for toluene was higher than that of water. The values of T_g were significantly influenced by the liquid uptake. Finally, heating to a temperature above the T_g of the adhesive, even in a very short time, was found to significantly influence the results of the liquid uptake.     

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。     

The glass transition temperature of an epoxy resin and the effect of absorbed water

Summary The glass transition temperature, T_g, of a diamino-dipenyl methane (DDM, II) cured diglycidyl ether of bispenol-A epoxy resin (I, n=0) has been determined by proton NMR line width studies to be 410°K. In the presence of 2.5±0.1 w/o absorbed water T_g is reduced to 378°K. Such a large reduction of Tg by such a small amount of water requires specific interactions. It is postulated that segmental motion is increased as a result of the disruption of hydroxyl group hydrogen bonds by the absorbed water molecules.     

丁羟胶玻璃化温度的模拟计算

丁羟胶的玻璃化转变温度(Tg)预测对丁羟推进剂的贮存、运输和使用具有重要的参考意义。根据丁羟胶的主要性质搭建合理的无定形端羟基聚丁二烯(HTPB)结构模型,选用COMPASS力场、在恒温恒压(NPT)系综下,采用分子动力学(MD)方法模拟计算HTPB在不同温度时的比体积。研究结果表明,比体积与温度的关系曲线斜率在Tg处会发生转折;模拟计算得到的Tg为208.00K,采用差示扫描量热(DSC)法实测得到的Tg为194.86K,两种结果在误差允许范围内基本一致,表明MD法可以用来预测丁羟胶的Tg。     

Differential scanning calorimetric survey of brominated PAN, pitch-based and vapor-grown fibers

A differential scanning calorimetric survey of brominated PAN, pitch-based and vapor-grown fibers revealed that only vapor-grown fibers heat-treated to temperatures of 2250°C or above exhibited the phase change at 100°C associated with the two-dimensional melting of bromine, similar to that observed in HOPG. All the pitch-based fibers tested, as well as vapor-grown fibers annealed to 2250°C, showed a substantial decrease in resistivity upon bromination but gave no indication of a phase change, suggesting that the bromine in these fibers does not have the long-range order needed to undergo two-dimensional melting. The brominated PAN fibers and unannealed vapor-grown fibers showed no phase change and no change in resistivity, indicating that these fibers do not intercalate bromine.