Growth rate of polyethylene single crystals at high pressure

The growth kinetics of extended-chain single crystals (ECSC) of polyethylene are studied. The results of an expertment on the growth rate and the structure of the high-pressure form suggest that the segments in the crystal slide along their own axes very easily and that the ends of the segments form uneven contour surfaces so as to minimize the boundary free energy. On such a secondary nucleus model, we assume that the elementary step of the growth is the “creeping in” of a repeating unit linked directly to a crystalline segment through the boundary surface of the nucleus. We have the normal growth rate V ∝= T exp (- G * /3 k T ), where Δ T is the degree of supercooling and G * is the least free energy of formation of a critical nucleus. By fitting this relation with the experimental one, we obtain G * =134/Δ T (kJ/mol) at 300 MPa, a value 1/14 of that at 0.1 MPa.