Multiple melting behaviour of poly(ethylene terephthalate)

Multiple melting behavior in isothermally cold-crystallized isotactic polystyrene

Isotactic polystyrene (iPS) is demonstrated to have unique thermal behavior, showing dual reversible crystal melting and irreversible enthalpic relaxation of its rigid amorphous fraction (RAF). Quasi-isothermal temperature-modulated differential scanning calorimetry (TMDSC) and standard DSC were used to study the heat capacity of cold-crystallized iPS. IPS shows two or three endotherms depending upon cold crystallization temperature, Tc. Crystal melting causes the higher temperature endotherm(s), and under quasi-isothermal conditions, we report for the first time observation of dual locally reversible melting endotherms. Quasi-isothermal TMDSC shows that the RAF is established at the crystallization temperature in iPS. Furthermore, we show that the lowest temperature endothermic peak, called the annealing peak, represents the transition of the RAF. For cold-crystallized iPS the annealing peak is an irreversible, enthalpy-involved relaxation of RAF, which transforms solidlike RAF into liquidlike mobile amo...

Heat Capacity Study of Isotactic Polystyrene: Dual Reversible Crystal Melting and Relaxation of Rigid Amorphous Fraction

Melting behaviour of poly (phenylene sulphide): 2. Multiple stage melt crystallization

The rigid amorphous phase of semicrystalline poly(phenylene sulfide) (PPS) has been studied as a function of thermal history using scanning calorimetry, dielectric relaxation, density, and small-angle x-ray scattering (SAXS). Based on the new heat of fusion of perfect crystalline PPS, which is 26.7±0.8 cal/gram, the weight fraction of rigid amorphous phase is shown to be nearly twice as large as previously reported [1]. The mass fraction of the rigid amorphous phase ranges from 0.24 to 0.42 and is dependent upon thermal treatment. We have taken the approach of assuming a three-phase model for the morphology of semicrystalline PPS consisting of crystalline lamellae, mobile amorphous, and rigid amorphous components. Using this three-phase model, we determine that the average density of the rigid amorphous fraction is 1.325 g/cc, which is slightly larger than the density of the mobile amorphous phase fraction and was insensitive to thermal history. From the SAXS long period, the layer thicknesses of the mobile amorphous phase, rigid amorphous phase, and crystal lamellae were estimated. Only the lamellar thickness shows a systematic variation with thermal history, increasing with melt or cold crystallization temperature, or with decreasing cooling rate.

Effects of thermal history on the rigid amorphous phase in poly(phenylene sulfide)

In this study, we report the melting behavior of poly(phenylene sulfide), PPS, which has been cold-crystallized from the rubbery amorphous state. We find that the crystallization kinetics are faster for cold-crystallized PPS than for melt-crystallized material, due to formation during quenching of a short-range ordered, but noncrystalline, structure. We observe that the endothermic response of cold-crystallized PPS at a large undercooling consists of a low temperature endotherm, followed by an exothermic region, and by the main higher melting endotherm. The lower melting peak temperature of cold-crystallized PPS increases as the crystallization temperature increases, but the main upper melting peak temperature remains almost the same. The size of the exothermic region is strongly related to the degree of undercooling, and must be taken into account in order properly to determine the degree of crystallinity of the material prior to the scan. When the crystallization time is varied, we see a systematic decrease in the size of the main endotherm, and an increase in size of the lower melting endotherm. This suggests that a portion of the main endothermic response is due to reorganization during the scan. Annealing will not only increase the degree of crystallinity but also improve the crystal perfection; therefore the ability of an annealed sample to reorganize decreases as the annealing time increases. However, an additional third melting peak is seen when a cold-crystallized sample is annealed at high temperature for a sufficiently long residence time. The existence of the third melting peak suggests that more than one kind of distribution of crystal perfection may occur when PPS has been cold-crystallized and subsequently annealed.

Crystallization and melting of cold-crystallized poly(phenylene sulfide)

Morphology of poly(phenylene sulphide) single crystals grown by a two-stage self-seeding technique

The morphology of poly(phenylene sulfide), PPS, obtained as processed film has been studied using wide angle X-ray scattering (WAXS), scanning electron microscopy (SEM), and differential scanning calorimetry (DSC). A unique morphology has been identified in the original film. The spherulites are very small in size and possess a cylindrical symmetry with their lamellae oriented on the edge. In these processed PPS films, the a-axis of the lattice is preferentially aligned perpendicular to the film plane, while the b and c axes are predominantly in the film plane. In the PPS isothermally crystallized from the melted, original film, there is no such preferential orientation. Thermal analysis of the original and melt crystal-lized PPS shows that while the degree of crystallinity is about the same, the nature of the amorphous phase in the two materials is different. In the original film, we did not observe a heat capacity increment at a glass transition temperature by DSC, indicating that all of the amorphous phase belongs to the category of rigid amorphous phase. In the melt crystallized PPS, a distinct glass transition was seen, though only a portion of the amorphous phase becomes mobile at Tg. The differences in orientation and mobility of the amorphous phase in the original film compared to melt crystallized PPS are explained by the different thermal processing procedures used for the two materials.

Jerry Sengshiu Chung

Papers

Melting behaviour of poly (phenylene sulphide): 2. Multiple stage melt crystallization

Crystallization and melting of cold-crystallized poly(phenylene sulfide)

Morphology of poly(phenylene sulphide) single crystals grown by a two-stage self-seeding technique

Unique orientation of lamellae in film processed poly (phenylene sulfide)

A method for treating residual stress effects in traction structural stress based approach and validation on a welded component