Total Heat Of Fusion Research Articles

Melting and crystallization behaviors of electrostatic spinning polycaprolactone/polyethylene glycol blends

Polycaprolactone/polyethylene glycol (PCL/PEG) blends were prepared by electrostatic spinning. The melting behavior and isothermal, non-isothermal crystallization kinetics were investigated using differential scanning calorimetry (DSC). The results showed the DSC thermograms of blend samples. With increasing PEG content, the position of the crystallization peak shifted to a higher temperature, while the position of the melting peak shifted slightlys. With increasing the heating rates, the position of the melting peak shifted to a higher temperature and the total heat of fusion also gradually increased. Both PCL and PEG could crystallize. By DSC, the PEG and PCL mutually promote to develop a thicker crystal and more perfect crystallite. This effect of promotion may result from the interaction of the molecular chain period of the two polymers. The Avrami equation described the non-isothermal crystallization kinetics of this blend very well.

Structure Development upon Melt Drawing of Ultrahigh Molecular Weight Polyethylene: Effect of Prior Thermal History

The effect of thermal history on the melt drawing of ultrahigh molecular weight polyethylene (UHMWPE) reactor powder was studied. The samples for drawing were prepared by compression-molding of reactor powder at various temperatures above the melting point (Tm). The drawing temperature (Td) was 150 °C. It was found that the maximum achievable draw ratio at the optimum Td decreased from 60 to 23 when the prior-melt temperature increased from 160 to 230 °C. The highly drawn films exhibited tensile moduli ≤58 GPa and strength ≤0.95 GPa at room temperature. Scanning electron microscopy (SEM) observations of the drawn films, etched by fuming nitric acid, revealed a characteristic “shish kebab” structure, as reported. Consistent with such morphology, differential scanning calorimetry (DSC) showed double melting endotherms at 134 and 143 °C, corresponding to the “kebob” and “shish” components, respectively. The crystallinity evaluated by the total heat of fusion from the double peaks increased steadily with the draw ratio, and the increase was more rapid for the samples that were prior-melted at a higher temperature and/or for a longer time. The formation of “shish” and “kebob” components and their crystal sizes were also significantly influenced by the prior-melting temperature and the elongation ratio. The efficiency of the draw, evaluated from the fraction of the “shish” component and the tensile properties vs draw ratio, was also interpreted from the differences of the prior-melt preparation conditions. The results suggest that the different level of entanglement formation, which was associated with the scale of segmental diffusion, affected significantly the resultant structure and properties.

Improved Purity Measurement Method of Organic Compounds Using Differential Scanning Calorimetry

A study was made of the calculation methods which give an accurate purity value when the purity is measured with a mixture which forms an eutectic mixture. It was revealed by a comparison of the purity values obtained by various measurement methods using DSC that those obtained by the stepwise method are more accurate than those obtained by other methods. Aiming to apply the advantages of the stepwise method to the dynamic method, the following improvements were made. The total heat of fusion was measured inclusive of the eutectic peak. To calculate the melt fraction, the total heat of fusion was regarded as extending up to the inflexion point at the end of the peak. Furthermore, the melt fraction was calculated assuming the heat up to each temperature to be the partial heat of fusion. A van’t Hoff plot was assumed starting with a point 5% below the principal component peak following the end of the eutectic mixture’s peak and ending at the inflexion point. An analysis using the improved method excludes any bias, and the obtained value was highly reliable. A 95mol% pure sample gave a measurement with an error within 0.5mol% or less.

Physical properties of interesterified fat blends

Fat blends, formulated by mixing fully hydrogenated soybean oil with nine different commonly used vegetable oils in a ratio of 1:1 (w/w), were subjected to interesterification (also commonly referred to as rearrangement or randomization) with sodium methoxide catalyst. Fatty acid composition and triacylglycerol molecular species of each fat blend and the interesterified product were determined and correlated with the following physical properties: melting, crystallization characteristics and solid fat content. The differences in the endothermic and exothermic peak temperatures, total heat of fusion and crystallization (β and β′ crystalline content) and solid fat content among the fat blends clearly showed the effect of the composition of each oil on the physical properties. Oils that contained a considerable amount of palmitic acid had a favorable influence on the crystallization and polymorphic form of interesterified fat blends.

Designed experiment for polyethylene crystallization by DSC

A central composite design involving three independent variables was used to define an experimental design matrix of crystallization conditions for polyethylene. The independent variables examined were cooling rate, isothermal crystallization temperature and total time at the crystallization temperature. The crystallization schemes were carried out in the differential scanning calorimeter and the subsequent data from melting thermograms were statistically analyzed in terms of endothermic peak temperatures, peak heights, number of discernible peaks and total heat of fusion. The effect of the independent variables on the well-defined, high temperature endotherm was accurately delineated. However, difficulties were encountered in identifying peaks other than the major peak, and this limited the scope of the statistical analysis.

The effect of heating rate on the melting of polytetrafluoroethylene

AbstractWhen virgin polytetrafluoroethylene is heated at intermediate rates, two melting peaks are observed. As the heating rate is increased, the higher‐temperature peak grows at the expense of the lower‐temperature one without any significant change in the total heat of fusion. It is suggested that the higher‐temperature peak represents a transition to a strained melt which subsequently changes to the more stable equilibrium melt. After recrystallization from the melt, there is only a single melting peak which occurs at a lower temperature than peaks for the virgin polymer. All of these transitions are subject to superheating.

Characterization of Block and Random Ethylene Propylene Copolymers by Differential Thermal Analysis

Abstract DTA furnishes a reliable method for the analysis of physical mixtures and for multisegment and bisegment block copolymers of ethylene and propylene in cases where chain randomness is minor. If a secondary method of analysis which is independent of crystallinity is possible, e.g., use of C14-tagged samples, the DTA method can aid in elucidating the chain structure, including degree of copolymer randomness. Block copolymers and physical mixtures have been found to have thermograms with endothermal minima at approximately 138 and 163° C. The ratio of heights of the two minima has been found to be a function of the ethylene and propylene content for mixtures of homopolymers and for block copolymers without randomnness. Cooling thermograms of block copolymers have been demonstrated to have two exotherms in all cases where the block ethylene content exceeded 10 per cent; the propylene limit was not fixed. Mixed homopolymers do not exhibit two freezing exotherms. The total heat of fusion of the polymer has been found to be a usable measure of chain randomness and block nature. Conditions for observation of single and multiple transitions have been extended. The following conclusions can be drawn from Table I. Multisegment block copolymers are characterized by heats of fusion almost equal to or lower than corresponding physical mixtures. Bisegment block copolymers have heats of fusion equivalent to physical mixtures within experimental error. Cooling curves definitely indicate that samples 3–6 and 3–13 are block copolymers according to Ke's criterion of double exotherms. Sample 3–3 may be inferred to have a large amount of randomness between blocks from its low heat of fusion, which is comparable to that of polymers with known randomness. Samples 3–35 and 3–40 are both block copolymers by the double freezing point technique. The block copolymer identification and the high heat of fusion (almost as large as a comparable physical mixture) indicate long block length and only a few blocks. Very little randomness is present (compare data of Figure 2). Low heats of fusion consistent with short block segments in sequential arrangement are noted for samples 3–30 and 3–31. No confirmation of the block nature can be obtained from freezing point data due to low ethylene content. The displacement from curve in Figure 2 indicates noncrystalline segments in the chain. Multisegment block copolymers are identified from heat of fusion data and freezing point thermograms; note samples 3–32, 3–33, and 3–34. The location of the peak height ratios in Figure 2 indicates the presence of random copolymer with some crystallinity between blocks in all cases.