Oriented chain-extended polyethylene

Simple observations have been made of the fracture behaviour and Young's modulus parallel to c, for drawn polyethylene after annealing at 5·34 kbar between 220 and 260°C. The modulus shows three regions of behaviour, corresponding to the textural changes described in the preceding paper. At first the polymer has extra stiffness associated with drawing, but this has disappeared by 236°C when the average crystal thickness is only of order 500 Å. There follows a region of series behaviour, to 1000 Å thickness, in which the modulus increases according to the proportion of crystalline and interlamellar regions, both of which have constant properties. Beyond this the polymer softens abruptly as chain-extended lamellae replace chainfolded and then stiffens again with further increase of crystal thickness. The maximum overall chain modulus produced is about 40 kbar, essentially determined by the proportion of interlamellar regions, which have a modulus of 3 kbar. Attainment of the theoretical modulus requires poorer crystal interfaces than are given by annealing, coupled with high molecular extension and orientation. Preparation might be practicable by flow crystallisation above 3 kbar. Annealing at above 240°C embrittles whole polymer because of the segregation of shorter molecules between chain-extended lamellae associated with a phase transition, leading to fracture in {001} planes. Ductility is restored by removal of low-melting material.