Munehisa Yasuniwa

X-Ray Studies of Polyethylene under High Pressure

The phase diagram, growth processes of extended-chain crystal and high pressure phase, and the temperature dependency of lattice parameter in polyethylene have been studied under high pressure up to 6000 kg/cm2 using a new high pressure X-ray diffraction apparatus. The high pressure phase diagram obtained by micro-DTA measurement agrees with that by X-ray measurement. The crystallization process of the high pressure phase seems to be a homogeneous and diffusion controlled two dimensional growth. The growth process of the extended-chain crystal is very rapid, and the direct formation of the extended-chain crystal without passing through the high pressure phase is clarified. The temperature dependency of the lattice parameters of the extended-chain crystal and the high pressure phase at 5000 kg/cm2 were obtained, and the shrinkage of the c-axis in the high pressure phase which shows only a sharp (100) diffraction pattern is discussed with the aid of dilatometric data.

X-ray and DSC studies on the melt-recrystallization process of poly(butylene naphthalate)

Melt-recrystallization in the heating process of poly(butylene naphthalate) (PBN) was studied with X-ray analysis and differential scanning calorimetry (DSC). DSC melting curve of an isothermally crystallized sample showed double endothermic peaks. With increasing the temperature, wide-angle X-ray diffraction (WAXD) patterns of the sample were obtained successively. Crystal structure did not change during the double melting process. The X-ray diffraction intensity decreased gradually in the temperature region up to about 200 °C, and then increased distinctly before steep decrease due to the final melting. This increase is interpreted as a proof of recrystallization. The temperature derivative curve of the diffraction intensity was similar to the DSC melting curve.

High‐pressure DTA of poly(butylene terephthalate), poly(hexamethylene terephthalate), and poly(ethylene terephthalate)

Pressure effect on the melting behavior of poly(butylene terephthalate) (PBT) and poly(hexamethylene terephthalate) (PHT) was studied by high-pressure DTA (HP-DTA) up to 320 and 530 MPa, respectively. Cooling rate dependence on the DSC melting curves of the samples cooled from the melt was shown at atmospheric pressure. Stable and metastable samples were prepared by cooling from the melt at low and normal cooling rates, respectively. DTA melting curves for the stable samples showed a single peak, and the peak profile did not change up to high pressure. Phase diagrams for PBT and PHT were newly determined. Fitting curves of melting temperature (Tm) versus pressure expressed by quadratic equation were obtained. Pressure coefficients of Tm at atmospheric pressure, dTm/dp, of PBT and PHT were 37 and 33 K/100 MPa, respectively. HP-DTA curves of the metastable PBT showed double melting peaks up to about 70 MPa. In contrast, PHT showed them over the whole pressure region. HP-DTA of stable poly(ethylene terephthalate) (PET) was also carried out up to 200 MPa, and the phase diagram for PET was determined. dTm/dp for PET was 49 K/100 MPa. dTm/dp increased linearly with reciprocal number of ethylene unit. The decrease of dTm/dp for poly(alkylene terephthalate) with increasing a segmental fraction of an alkyl group in a whole molecule is explained by the increase of entropy of fusion.

Stepwise annealing of poly(butylene terephthalate)

Annealing of poly(butylene terephthalate) (PBT) was studied by differential scanning calorimetry (DSC) and small angle X-ray scattering (SAXS) measurement. A PBT sample was annealed at a recrystallization temperature where recrystallization occurs with a maximum rate in the heating process of the sample. In the subsequent annealing steps, the annealed sample was annealed repeatedly at the recrystallization temperatures, and the stepwise annealing sample was obtained. Peak melting temperature (Tm) and sharpness of DSC peak of the stepwise annealing sample increased with the annealing step. A high melting-temperature sample was obtained in a short time, and Tm increased up to 238.5°C which is higher than all the Tm values that appear in the literature. The long period calculated from SAXS curves of the stepwise annealing sample increased with the annealing step. The increase of crystallite size and perfection of the crystal in the stepwise annealing process is suggested. Annealing experiment indicated that T°m should be higher than about 235°C. Tm increased linearly with the annealing temperature of the final step in the stepwise annealing (Ta). The equilibrium melting temperature (T°m) for PBT was estimated to be 247°C by the application of a Hoffman–Weeks plot to the relation between Tm vs. Ta.

Lamellar thickening of polyethylene under high pressure

Single crystal mat ( SCM ) samples of polyethylene ( PE ) were prepared from dilute solution of p-xylen, then they were annealed at pressures of 200 and 500 MPa. Lamellar thickness of the original and annealed SCM samples was measured by small-angle X-ray scattering method. Orientation of the molecular chain in those SCM samples was investigated by wide-angle X-ray diffraction pattern. From these X-ray measurements, annealing temperature dependence of the lamellar thickness, i.e., lamellar thickening, under high pressure was obtained. Melting process of the SCM samples was also investigated at 200 and 500 MPa by high pressure differential thermal analysis. Then correspondence between the lamellar thickening and the melting process was studied. The lamellar thickness increases markedly with approaching to the melting temperature of the orthorhombic crystal even in the high pressure region where the high pressure phase (hexagonal phase) appears. The annealing temperature dependence curve of the lamellar thickness at 200 MPa can be superimposed on the curve at 500 MPa by shifting the curve along the temperature scale by 47 K. Large scale lamellar thickening occurs in the orthorhombic crystal phase in the high pressure region. The formation process of extended-chain crystal is discussed.

Melting and Crystallization of Solution Crystallized Ultra-High Molecular Weight Polyethylene under High Pressure

Differential thermal analysis of a ultra-high molecular weight polyethylene (UHMW-PE, Mv; 2.7×106) crystallized from a 0.1 wt% decalin solution was performed at pressures from about atmospheric pressure to 600 MPa. In the high pressure region such as 600 PMa, thick lamellae similar to extended-chain crystal are formed very rapidly in the heating process. The rapid lamellar thicknening is suggested to be due to the decrease of enthanglements. Its phase diagram was determined. The triple point in the phase diagram was about 370 MPa. The high pressure phase region of the solution crystallized sample is between those of as-polymerized powder sample and bulk sample which is melt-crystallized one after kneading in the melt. The change of phase diagram among those samples may be due to differences in entanglement density.

Melting and crystallization of ultra-high-molecular-weight polyethylene in a mixture with tetracontane under high pressure

Abstract The melting and crystallization processes of ultra-high-molecular-weight polyethylene (UHMW PE) in a mixture with tetracontane (TC) were studied at elevated pressures up to 500 MPa by high-pressure differential thermal analysis. At atmospheric pressure, melting-point depression of UHMW PE occurs with increasing TC content, but it does not occur at 500 MPa. The phase transition to the hexagonal phase of UHMW PE is impeded by the addition of TC, and does not occur below a weight fraction of PE equal to 0.4 at 500 MPa. The content of extended chain crystals (ECC) of UHMW PE formed by high-pressure crystallization decreases with TC, and below a weight fraction of PE equal to 0.35, ECC do not form even on crystallization at 500 MPa.