Differential Scanning Calorimeter Thermograms Research Articles

Longitudinal growth of polymer crystals from flowing solutions. VI. Melting behavior of continuous fibrillar polyethylene crystals

AbstractThe melting behavior of continuous fibrillar crystals of high‐molecular‐weight polyethylene has been investigated. The macrofibers were grown from dilute solutions in xylene subjected to Couette flow in the temperature range between 103 and 118.5°C. The thermograms, as determined by differential scanning calorimetry, exhibit three melting endotherms with peak temperatures at 141, 150.5, and 159.5°C after extrapolation to zero scan speed. All peaks were found to be strongly superheatable. Reduction of fiber length, in particular by etching with fuming nitric acid, led to the disappearance of the melting peaks at 150.5 and 159.5°C. The remaining peak at 136°C appeared not to be superheatable. The heat of fusion of the fragmented fibers was 69.8 cal/g. Wide‐angle x‐ray diffractograms taken on a macrofiber while gradually heated at a rate of 0.35°C/min at constant length showed that the triclinic phase present in the fiber disappeared at 130°C and that the orthorhombic cell transformed into the hexagonal modification at 150°C. This hexagonal phase was still observable at 180°C. The retractive force developed on heating at constant length displays first a slight decrease followed by a maximum at 150°C. Beyond the latter temperature the stress decays abruptly corresponding to the temperature at which fracture of the fiber could be observed visually. From all these observations it is inferred that the first melting endotherm in the differential scanning calorimeter (DSC) thermograms arises from the melting of unconstrained fibrillar crystal regions which are able to shrink during fusion. Moreover, the melting of lamellar overgrowths on the elementary fibrils on shish‐kebab type may contribute to this endotherm. The second melting endotherm at about 150°C is associated with the transformation of the orthorhombic into the hexagonal lattice in constrained parts of the sample. This latter “rotator” phase allows slippage of the polymer chains past each other, giving rise to stress relaxation. The third endotherm arises from melting of this hexagonal phase and the heat take‐up connected with the formation of higher energy gauche states upon randomization of the chains in the melt. Almost smooth, fully constrained fibrillar crystals grown at high temperature absorb more than 15.5 cal/g during this process, indicating that the polymer chains in such fibers must be highly extended.

Supercooling and Nucleation in Liquid Crystals

Abstract A study of the supercooling of the isotropic to nematic transition in the homologous neniatogens p-azoxyanisole (PAA) and heptyloxyazoxybenzene (HOAOB) is reported. Differential scanning calorimeter thermograms are consistent with a small supercooling ∼0.01°K. Observations of the nematic-isotropic phase boundary by optical microscopy reveals the development of a dendritic-like growth front with increasing rate of transformation to the nematic phase. This is also consistent with a supercooling -0.01°K. The boundary surfaces induce nematogenic alignment and therefore act as heterogeneous nucleation sites. Nucleation theory is used to show that if heterogeneous nucleation could be eliminated, the observable supercooling would be no more than 0.1(T o - T *), where T o is the nematic-isotropic transition temperature and T * is the pseudo-second order transition temperature.

Application of thermal analysis

The evaluation of molding compounds is generally based on the performance of encapsulated semiconductor devices under specific sets of environmental conditions. Testing under these conditions may require several months. To reduce the evaluation time to a few days, tools and techniques which enable some prediction of encapsulation material’s behavior on the basis of certain characteristics, have been developed by thermal analysts techniques. The instrumentation consists of a DuPont 990 Analyzer equipped with a Differential Scanning Calorimeter (DSC) and a Thermo-gravimetric Analyzer coupled to a Gas Chromatograph. Other commercial equipment is available for this testing. Thermo-gravimetric analysis was used to evaluate the thermal stability of four (4) types of epoxy molding compounds. The types and relative amounts of organic constituents are determined by gas chromatography analysis of the degradation products. Differential scanning calorimeter thermograms are used to characterize the behavior of molding compounds during polymerization. The first exotherm, which is characteristic of cross polymerization, is used for determining shelf life, pelietizing, preheating, and molding process conditions. The second major exotherm reaction, which is characteristic of the degradation mechanism, is used to evaluate the effect of post curing conditions and, in addition, is used to identify the type of resin used in the formation of the moldings compounds.

Investigation of conditions of preparation of oriented films of alpha-Ag 2Se : L Constantinescu et al, Rev Roum Phys, 18 (10), 1973, 1197–1201 ( in French)

Thermal crystallisation studies of (Cu2Se)x(As2Se3)1 − x glasses, where 0.12 ≤x≤0.5 (Cu at% of 5 to 25) is is carried out using a differential scanning calorimeter (DSC). On heating, glasses with 5 and 10at% of Cu show a single glass transition temperature and a single crystallisation temperature with a small shoulder. Glasses with Cu content of 15, 20 and 25at% exhibit the phenomenon of double glass transition and double crystallisation. Isothermal annealing of the glasses at their respective crystallisation temperature(s) results in phase separation. The CuAsSe2 compound is the primary crystallisation phase in all the glasses. The thermodynmically stable Cu3AsSe4 compound is an additional primary crystallisation phase in the glasses with 15, 20 and 25 at% of Cu. The monoclinic As2Se3 compound is the secondary crystallisation product in all the glasses investigated. The presence of these crystalline phases in the devitrified glasses is supported by the multiple melting endotherms observed in the DSC thermograms. An attempt is made to correlate the experimental results with the formal valence shell model.

DIFFERENTIAL SCANNING CALORIMETRIC STUDY OF SPONTANEOUS GERMINATION OF BACILLUS MEGATERIUM SPORES BY WATER VAPOR

Freeze-dried Bacillus megaterium IAM 1166 spores humidified with water vapor to a water content of above 30% germinated spontaneously without the addition of any germinant. Differential scanning calorimetric thermogram of the spores thus germinated had absorption peaks at about 100° and 130° other than a large peak due to the evaporation of spore water. Both 100° and 130° peaks were characteristic of spores germinating spontaneously by water vapor and corresponded to those in the standard Ca-DPA precipitate deposited at room temperature. The reflection spectra of germinated spores also agreed with those of the standard Ca-DPA. Since the weight decrease in thermogravi-metric analysis, the water content by the Karl Fischer method, and the elementary analysis of the standard Ca-DPA showed that 1mol of Ca-DPA was bound by 1.5mol of H2O, it was concluded that 1mol of Ca-DPA in germinated spores was hydrated by 1.5mol of H2O. It is also suggested that the macromolecule-Ca-DPA complex in dormant spores might be transformed into a free state of Ca-DPA hydrate by water vapor, produces more Ca-DPA hydrate molecules autocatalytically, and thus triggers the spontaneous germination by setting free more and more functional macromolecules.

Melting Behavior of lrradiated Poly(vinylidene fluoride)

The melting and crystallization behavior of poly(vinylidene fluoride) (PVdF) before and after irradiation has been observed by the use of a differential scanning calorimeter (DSC). Gamma irradiation was carried out with 6 Co at a dose rate of 1 MR/hr at ambient temperature. Electron beam irradiation was carried out with EBG at 2 MeV and at a dose rate of 2MR/pass by intermittent irradiation to prevent a rise in temperature by means of a shuttle conveyor.Melting temperature (T.) and crystallization temperature(T.) were measured at the peak in the DSC thermograms. T. decreased with increaseing radiation dose. PVdF, irradiated in vacuum, gave mostly a simple and sharp peak.On irradiation in air, more complex peaks were observed. These results indicate that the melting behavior is strongly affected by the presence of oxygeR during the irradiation. Depressions of T. and T, were the same fer gamma and electron irradiation in vacuum. On the other hand, T. depression of PVdF by the irradiation in air was smaller than that in vacuum, and a difference in melting peint depression between gamma and electron irradiation was also observed.The peak at higher temperature corresponds to crystalline state where the degradation dqe, to irradiation is mainly preduced by the presence of suMcient oxygen near the film surface. While, the peak at lower temperature corresponds to crystalline phase where crosslinking mainly takes place by irradiation in the absence of oxygen at inner parts of the film similar to the vacuum irradiation, When crosslinks are introduced, defects would be produced in the crystalline lattice. The defects would cause a melting point lowering, the magnitude of which is larger than that produced by the scission of molecular chain in the crystalline phase on irradiation in air.Both heat of fusion and crystallinity decreased with increasing radiation dose. About 1706. decrease of heat of fusion was observed after 200 MR irradiation. On the other hand, X-ray diffraction pro es did not change markedly. The decrease in the crystallinity, determined from X-ray diffraction, was 7fZe.

STUDIES ON ISOTHERMAL CRYSTALLIZATION KINETICS OF POLYVINYL ALCOHOL

Isothermal crystallization of PVA was studied by differential scanning calorimeter (DSC). Sigmoidal shapes of the isotherms for crystallization of PVA obtained at different temperatures of 205°C to 223°C are very similar to one another and are superposable.The value of exponent n in Avrami's equation of PVA is 1.75±0.25 irrespective of degree of polymerization and configuration of PVA. The value of n for mixture of PVA and triethylene glycol is 3.82. This value suggests that spherulites are grown. Half time of crystallization is dependent remarkably on the crystallization temperature and degree of polymerization. Multipeaks are observed in DSC thermogram of PVA film which was stepwise annealed.

Cationic Copolymerization of Tetrahydrofuran with ε-Caprolactone

Cationic copolymerization of tetrahydrofuran (THF) with ε-caprolactone (CL) was carried out with some stable cationic salts. Both monomers are consumed simultaneously, but THF seems twice as reactive as CL, reflecting the high basicity of THF. The copolymerization system seems almost living in nature at higher THF feed and a copolymer of Mn 89000 containing 13.9% of CL was obtained. However, fractionation of the product showed a broad distribution of the copolymer composition. The lower-molecular-weight fraction contained a higher proportion of CL. The X-ray diffraction pattern and differential scanning calorimeter thermogram showed the existence of the crystalline region of poly-THF and poly-CL. Thus the copolymerization may have proceeded by a two-state mechanism yielding a block copolymer.

A Differential Scanning Calorimeter Study of Phase Transitions in Alkali Metal Stearates

Differential scanning calorimeter thermograms have been obtained for the alkali metal stearates from 25 to 200 °C. Four different samples of different thermal history were studied for each soap, and differences in temperature and relative enthalpy of transitions were found to depend on the various sample histories. For lithium stearate good correlation is found between DSC and nuclear magnetic resonance (n.m.r.) results. Rubidium and caesium stearates seem to be less sensitive to thermal history than the sodium and potassium salts.

Liquid‐induced crystallization of poly(ethylene terephthalate)

AbstractAmorphous unoriented poly(ethylene terephthalate) was crystallized at 25°C by various organic liquids. The crystalliznity induced in the amorphous polymer was measured by differential scanning calorimetry and infrared spectroscopy. The ability of liquids to interact with and induced crystallinity in the amorphous polymer was classified on the basis of their solubility parameters. Measurements of the density of liquid‐crystallized 0.8‐mil films of poly(ethylene terephthalate) indicated the presence of extensive internal voids in the semicrystalline polymer matrix. Comparison of differential scanning calorimetric thermograms and infared spectra of heat‐crystalized and liquid‐crystallized polymer indicated significant differences in the polymer morphologies induced by the two crystallization processes.