Structures and properties of different low density polyethylenes

Abstract

Low density polyethylenes (LDPE) made by the known high pressure processes show significantly different molecular structures. When the reaction conditions are variable, e.g. in a tubular reactor (resinT) or in a system of two autoclaves with a lower temperature in the first reactor (resinA 2 * ), the polymer shows a narrower molecular weight distribution, but wider distributions of long-chain and short-chain branching compared with a polymer produced at constant temperature and under practically ideal mixing in a stirred autoclave reactor (resinA). The polymerT displays a decrease in long-chain and short-chain branching with growing molecular weight and differs from sampleA in the type of long-chain branching and in the molecular shape.A-type molecules show tree-like branching and nearly globular shapes whereas theT-type molecules are characterized by comb-like branching and consequently have more extended (rod-like but flexible) conformations. These structures may be interpreted in terms of reaction kinetics. The differences in molecular structure lead to changes in the morphology. For example, the bulk density distributions of the polymersT andA 2 * are narrower than that of polymerA although the latter has a much narrower short-chain branching distribution. The morphology (e.g. crystallinity and crystal size) is dominated by the tendency of short sidechains to accumulate in the amorphous phase and by the limited mobility of the molecules in the melt during crystallization. The proportion of short side-chains incorporated in the crystalline phase ranges from about 20% at high molecular weights to about 7% at low molecular weights for the resinsA andT. The physical and technological properties are closely related to molecular structure and morphology. They may be optimized by selecting suitable polymerization conditions, e.g. by use of a new two-autoclave process.