Reversible crystallization and the rigid-amorphous phase in semicrystalline macromolecules

The melting and crystallization behavior of poly(L-lactic acid) (PLLA; weight-average molecular weight = 3 × 105) was studied with differential scanning calorimetry (DSC). DSC curves for PLLA samples were obtained at various cooling rates (CRs) from the melt (210 °C). The peak crystallization temperature and the exothermic heat of crystallization determined from the DSC curve decreased almost linearly with increasing log(CR). DSC melting curves for the melt-crystallized samples were obtained at various heating rates (HRs). The double-melting behavior was confirmed by the double endothermic peaks, a high-temperature peak (H) and a low-temperature peak (L), that appeared in the DSC curves at slow HRs for the samples prepared with a slow CR. Peak L increased with increasing HR, whereas peak H decreased. The peak melting temperatures of L and H [Tm(L) and Tm(H)] decreased linearly with log(HR). The appearance region of the double-melting peaks (L and H) was illustrated in a CR–HR map. Peak L decreased with increasing CR, whereas peak H increased. Tm(L) and Tm(H) decreased almost linearly with log(CR). The characteristics of the crystallization and double-melting behavior were explained by the slow rates of crystallization and recrystallization, respectively. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 25–32, 2004

Thermal analysis of the double-melting behavior of poly(L-lactic acid)

The melting behavior of isothermally crystallized poly(butylene succinate) (PBS) has been investigated using differential scanning calorimetry (DSC) and wide-angle X-ray analysis. The samples crystallized between 80°C to 100°C show middle endotherm at the position just before the high exotherm, while the others under 80°C show two endotherms (low and high). From the results of the melting peak vs. crystallization temperature plot, it was suggested that the middle endotherm corresponds to the melting process of the original crystallites and the high endotherms to the melting process of the recrystallized ones. As the DSC heating rate was increased, the peak temperature of the low and middle endotherms increased and that of the high endotherm decreased, indicating that the low endotherm was due to the original crystallites as well as the middle endotherm. Consequently, in the heating scan of PBS, the existence of two kinds of morphologically different crystallites as well as the process of melting and recrystallization becomes evident. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1357–1366, 1999

Melting behavior of poly(butylene succinate) during heating scan by DSC

In this paper, we present results of morphological studies during long time melt crystallization and subsequent melting in poly(aryl ether ether ketone) (PEEK). Morphological changes were monitored via small angle X-ray scattering (SAXS). SAXS data were analyzed via a combination of the correlation and interface distribution functions. Our analysis indicates the following:  (1) The semicrystalline morphology is best described by a three-phase, dual lamellar stack model. Stacks of a finite number of lamellae and interlamellar amorphous layers are separated from each other by interstack regions of amorphous material (liquid pockets). (2) Secondary crystallization occurs via the formation of secondary lamellar stacks within the liquid pockets. Secondary lamellae are thinner than primary lamellae (70 A vs 120 A), and the amorphous layer thicknesses are about 47 A in both stacks. (3) The low endotherm observed during a heating scan is associated with the melting of the secondary lamellae. (4) At room temperatu...

Morphological Changes during Secondary Crystallization and Subsequent Melting in Poly(ether ether ketone) as Studied by Real Time Small Angle X-ray Scattering

Poly(trimethylene teraphthalate) crystal structure and morphology in different length scales

Thermal analysis was performed on poly(butylene terephthalate), PBT, between 210 and 560 K. By combination of experimental heat capacities with computations with an approximate frequency spectrum of 65 group and 19 skeletal vibrations, preliminary recommended ATHAS (1988) heat capacities are proposed for the solid state from 0 to 600 K. The Tarasov parameters used for the computation of the skeletal vibrations were θ1 = 542 K and θ3 = 80 K for crystalline PBT and θ3 = 40 K for amorphous PBT. The glass transition temperature of amorphous PBT was found on efficiently quenched samples to be 248 K, much lower than for semicrystalline PBT where a 310–325 K glass transition temperature is typical. The increase in heat capacity calculated for 100% amorphous samples is 107 J/(K · mol) at 248 K and 77 J/(K · mol) at 320 K. The equilibrium melting temperature is estimated to be 518 K. The unique existence of rigid-amorphous fractions of the semicrystalline polymers is discussed with quantitative data for samples crystallized from the glass and from the melt between 275 and 490 K. The rigidamorphous fraction varies between above 0,9 for cold-crystallized samples to 0,3 for samples crystallized at 490 K. The crystallinity varied from below 0,1 to 0,5. The crystallinity could be separated into four parts, melting at high, medium, and low temperatures, and a part crystallized on cooling after isothermal crystallization. The sequence of crystallization of differently melting crystals was established.

Thermal analysis of poly(butylene terephthalate) for heat capacity, rigid-amorphous content, and transition behavior

High melting polymers of three different types of crystallization behavior were analyzed as to their thermal properties. The heat capacity at constant pressure of liquid poly(butylene terephthalate) (PBT) from 490 to 540 K was measured by scanning calorimetry. Combining with prior measured liquid heat capacity data of poly(oxy-1,4-phenylene) (PO) (358 – 620 K), poly-(thio-1,4-phenylene) (PPS) (540 – 600 K), poly(ethylene 2,6-naphthalenedicarboxylate) (PEN) (400 – 450 K, 530 – 600 K), an addition scheme for liquid heat capacities was constructed. In addition, the thermodynamic functions H and S have been calculated for those five polymers. The absolute, molar, residual entropy for the glassy state at zero kelvin is estimated to be 6,6,15,2, 7,48, 5,70, and 19,9 J/(K · mol) for PPS, PEN, PPO, PO, and PBT, respectively. Another two polymers with similar structure, poly(1,4-oxybenzoic acid) (POB) and poly(2,6-oxynaphthoic acid) (PON), were also studied in their high temperature mesophase states. A test of the addition scheme has been made to predict their heat capacities.

Robert Pan

Papers

Thermal analysis of poly(butylene terephthalate) for heat capacity, rigid-amorphous content, and transition behavior

Thermodynamic properties in the liquid state of high melting polymers containing phenylene and naphthylene groups