Coherence and Deformation of Lamellar Crystals after Annealing

Abstract

A striking change in coherence is obtained when single-crystal platelets of linear polyethylene are recrystallized by annealing at temperatures between 120°C and the melting point, 137°C. Instead of showing micaceous cleavage, the sintered crystal mat is ductile and malleable in shear, yet still will not flow under compression. Annealing provides enough toughness to the crystal mat to allow a deformation which elongates it several-fold. Hot calendering allows a much greater elongation than cold rolling and produces a tough, crackly, clear film. The sintered aggregate can be easily hand drawn hot to extremely high draw ratios, often above ×40. The product has very good tensile properties when tested as a fiber. A two-way hot calendering operation produces a film with outstanding tear strength and poor puncture and impact strengths. The coherence caused by the annealing of the platelets is analogous to the coherence developed in powder metallurgy or in sintering of ceramic bodies. In polymer crystals, annealing has been shown to cause the molecules to refold into longer fold periods, leaving holes. These holes provide the growing room for the refolding of chains from adjacent platelets; a cooperative intermeshing of chains occurs. This creates one large crystal aggregate which has a homogeneous distribution of sites of weakness. It is proposed that many annealing treatments of bulk crystalline polymers cause this intermeshing to occur, and a new interpretation is at hand for the well-known relationship of strength vs annealing temperature.