Structure, Crystallization and Melting of Linear, Branched and Copolymerized Polyethylenes as Revealed by Fractionation Methods and DSC

Abstract

The influence of chain length and short chain branching on crystallization and melting behaviour, as studied by differential scanning calorimetry (DSC), is discussed for linear polyethylene (LPE), high density polyethylene (HDPE), low density polyethylene (LDPE), homogeneous ethylene-propylene (EP) and ethylene-l-octene (EO) copolymers and heterogeneous ethylene-l-butene and ethylene-1-octene copolymers such as linear low density polyethylene (LLDPE) and very low density polyethylene (VLDPE). Attention is paid to the clarification of molecular structure, in particular the distribution of short chain branching over the chains. Other subjects discussed are the use of various methods of separating polymer chains according to molecular dimension, for example direct extraction (DE) and analytical and preparative size exclusion chromatography (SEC), and the complementary separation according to degree of short chain branching, which is actually a separation according to ethylene sequence length and is based on the crystallizability/dissolvability of chains. In the context of the latter separation, the use of techniques such as analytical and preparative temperature rising elution fractionation (TREF) is discussed. Cross-fractionation, which should preferably comprise a first separation according to molecular dimension followed by a separation according to obtaining insight into the detailed structure of heterogeneous copolymers.crystallizability/dissolvability, is shown to be an important tool in obtaining insigt into the detailed structure of heterogeneous copolymers.