PRESSURE-DEPENDENCE OF MELTING TEMPERATURES IN BRANCHED POLYETHYLENE UP TO 2 GPA

The melting temperatures (T-m) of branched polyethylene (B-PE) have be en measured over the pressure range up to 2 GPa by use of a high-press ure X-ray diffraction apparatus. The T-m-P curve for B-PE is expressed as (T-m/T-m,T-o)(c)=(P+P-o)/a where T-m,T-o is the T-m at atmospheric pressure, c=4.788, P-o=280.2MPa and a=P-o +0.1 MPa. The T-m of B-PE i ncreases with increasing pressure monotonically, and T-m=310 degrees C at 2 GPa. The X-ray diffraction peaks corresponding to the spacing d = 3.84 Angstrom for the (110) lattice plane of the orthorhombic B-PE c rystal do not change with increasing temperature below 1.85 GPa but d values for the (200) lattice plane increase with increasing temperatur e. The thermal properties of a B-PE melt crystallized at various high pressures above 1.0 GPa have been measured at atmospheric pressure, an d four endothermic peaks are observed for the sample melt crystallized at 800 MPa. The entropies of fusion Delta S-F Of B-PE and linear poly ethylene (L-PE) at pressures up to 200 MPa have been determined based on the Clausius-Clapeyron equation and P-V-T data by O. Olabisi and R. Simha (Macromolecules, 1975, 8, 206), and Delta S-F for B-PE is much smaller than for L-PE. (C) 1998 Elsevier Science Ltd. All rights reser ved.