Search Results
Thermal properties of polylactides
Effect of molecular mass and nature of lactide isomer
Abstract
A thermal analysis of a series of polylactides (PLA) was carried out based on the number of average molecular mass (M n), and the nature of isomer (D, L and DL). It is confirmed that the glass transition temperature (T g) of PLA increased as a function of molecular mass irrespective of isomer type except sample with a high polydispersity index. The melting temperature (T m) and enthalpy of crystal fusion (ΔH f) of L-isomer increased as the M n was increased from 1100 to 27500. The degree of crystallinity (χc%) increased as a function of molecular mass. However no crystallization peak was detected in the lower molecular mass range (550–1400). The non-isothermal crystallization behavior of the PLA melt was significantly influenced by the cooling rate. Both D and L isomers exhibited insignificant difference in thermal properties and DL lactides exhibited amorphous behavior at identical molecular masses. Change in microstructure showed significant difference between two isomers. Analysis of the FTIR spectra of these PLA samples in the range of 1200–1230 cm−1 supported DSC observation on crystallinity.
nanoparticles is having interesting properties of photo-oxidation response, which has applications in key areas of water splitting and bacterial growth inhibition, therefore kinetics of glass transition temperature and the corresponding thermodynamic transition
Abstract
Photo-initiated polymerisation of dimethacrylate oligomers provide an easy method for producing highly crosslinked polymer networks. The physical properties of the material are dependent on the polymerisation conversion value. The determination of this conversion value is quite difficult on the final product. The first step is to measure a characteristic temperature of the glass transition. The weakness of the DSC glass transition signal makes this measure unrealisable while the DMA tan peak is broad and weak. At the difference of these two thermal analysis techniques, TSDC gives an observable signal and a Tα temperature close to the glass transition temperature region. The bad sample preparation reproducibility observed was attributed to the high conversion rate.
Abstract
Thermal analysis was performed on the anti-HIV agent loviride in order to test its suitability to be processed using hot-melt extrusion. Temperature characteristic parameters of crystallization were determined to quantify the stability of amorphous loviride. The present study has shown that cooling and heating loviride at different rates influenced its thermal stability. At high cooling rates melted loviride did not crystallize during cooling, and formed a glass that recrystallized during reheating. Very low cooling rates resulted in significant decomposition of the drug. The glass transition temperature was found to increase as a function of increasing heating rates and the activation energy for the transition from the glassy to the super-cooled liquid state was relatively high, indicating good stability of the glass.
Abstract
Volumetric thermal analysis of semicrystalline poly(ethylene terephthalate), PET, with different content of crystalline phase was carried out using mercury-in-glass dilatometry. The effect of crystals on the thermal properties of amorphous phase (glass transition temperature, T g, thermal expansion coefficients, α) were determined. At cold-crystallization (106°C, up to 4 h), crystalline content of 2.4–25.3 vol.% was achieved. Increasing content of crystalline phase broadens the glass transition region and increases T g. The change of thermal expansion coefficient during glass transition is lower than that predicted by the two-phase model, which indicates the presence of a third fraction — rigid amorphous fraction (RAF), whose content steadily increases during crystallization. However, its relative portion (specific RAF) is significantly reduced. Further significant decrease in specific RAF appears after annealing at a higher temperature.
Abstract
The thermal degradation of the epoxy system diglycidyl ether of bisphenol A (DGEBA n=0) and m-xylylenediamine (mXDA) containing different concentrations of polyhedral oligomeric silsesquioxanes (POSS) nanoparticles was studied by thermogravimetric analysis in order to determine the influence of both, the POSS concentration and the curing cycle on the degradation process and to compare it with the results for the non modified system. Glass transition temperatures for the same systems were also determined by differential scanning calorimetry. Different behaviors have been observed, depending on the POSS concentration and on the curing selection.
Thermal analysis of the influence of water content on glass transitions
Heat capacities of starches from different origins
Abstract
Starch is an important natural substance in which the water content has a significant influence on its structure and properties. In the present study, the effect of the water content on glass transition temperatures T g and heat capacities C p of wheat, maize and potato starches were investigated by high-sensitivity differential scanning calorimetry (temperature modulated TMDSC and conventional DSC). Thermal analysis measurements were performed on starch samples with different water contents. The exact water mass percentage of each sample was determined using the Karl-Fischer method. The obtained results show that the water content does influence the starch thermal properties in a systematic and measurable trend, the higher the water% the lower the glass transition temperature, and the higher the heat capacity jump during gelatinisation. At this stage possible interpretations of the results are just put forward and should be confirmed through complementary measurements.
Abstract
Plausible effect of clustering of undercooled liquid water (pentamer configuration, icosahedra formation) is discussed showing water continuous but non-periodic patterns and its impact to the either formation of ice-crystals or ice-glass, particularly when making contact with plants. These processes are viable for the cryogenic storage of biological germplasm and subject for thermoanalytical studies aimed to the determination of glass transition temperatures.
Abstract
Thin films of 3,4-epoxycyclohexylmethyl 3',4'-epoxycyclohexane carboxylate were UV irradiated (1.1 J cm-2) under isothermal conditions ranging from 0 to 50C. Under these conditions the polymerization advanced quickly but only to a conversion level of less than 10% before the reaction rate slowed by more than an order of magnitude. This drop off in rate was not caused by the glass transition temperature, T g, reaching or exceeding the reaction temperature, T rxn, since the epoxide's T g remained at least 40C below T rxn. Raising the sample temperature above 60C caused a sharp increase in the conversion level. At 100C conversion exceeds 80% and the ultimate T g approaches 190C. The addition of 10 mass% 1,6-hexanediol, HD, to the epoxy caused the conversion at room temperature to quintuple over the level obtained without the alcohol present. The heat liberated from this alcohol epoxy blend during cure on a UV conveyor belt system caused the sample's temperature to increase by about 100C above ambient whereas the epoxy alone under these conditions only experienced a modest temperature rise of about 26C. If the amount of HD in the blend is increased above 10% the heat of reaction at 23C decreases due to HD being trapped in a nonreactive crystalline phase. Boosting reaction temperatures above 50C melts the HD crystals and yields significantly improved conversion ratios. As the level of alcohol blended with the epoxy is raised its ultimate T g is lowered and when the concentration of alcohol in the blend nears 30 mass%T g drops below room temperature.
Abstract
Results of phase transformations, enthalpy released and specific heat of Ge22Se78–xBix(x=0, 4 and 8) chalcogenide glasses, using differential scanning calorimetry (DSC), under non-isothermal condition have been reported and discussed. The glass transition temperature, T g, is found to increase with an average coordination number and heating rates. Following Gibbs—Dimarzio equation, the calculated values of T g (i.e. 462.7, 469.7 and 484.4 K) and the experimental values (i.e. 463.1, 467.3 and 484.5 K) increase with Bi concentration. Both values of T g, at a heating rate of 5 K min–1, are found to be in good agreement. The glass transition activation energy increases i.e. 102±2, 109±3 and 115±8 kJ mol–1 with Bi concentration. The demand for thermal stability has been ensured through the temperature difference T c–T g and the enthalpy released during the crystallization process. Below T g, specific heat has been observed to be temperature independent but highly compositional dependent. The growth kinetic has been investigated using the Kissinger, Ozawa, Matusita and modified JMA equations. Results indicate that the crystallization ability is enhanced, the activation energy of crystallization increases with increasing the Bi content and the crystal growth of these glasses occur in 3 dimensions.
