High pressure polyethylene molecular dimensions as estimated from solution properties of whole polymers

Solution light scattering, osmotic pressure, and viscosity measurements at elevated temperatures in non‐ideal solvents were performed to characterize the M̄w's, M̄n's, mean‐square end‐to‐end distances, and second virial coefficients, of seventeen whole polymer samples of high pressure polyethylene (HPPE)and of five fractions of one of these whole polymers. The data were treated to eliminate the effects of excluded volume and of polydispersity on the molecular dimensions. As a measure of unperturbed chain extension, values of (〈R̄02〉wM̄w)1/2 and (〈R̄02〉w/〈R̄0f2〉)1/2 were calculated, where 〈R̄0f2〉 refers to a free rotational polymethylene chain. To estimate the extent of long‐chain branching, values of 〈R̄0f2〉 and [η]θ were calculated from literature equations for low pressure polyethylenes having molecular weights corresponding to those of the high pressure samples. This study confirms previous investigators' observations that HPPE polymers may have extremely broad molecular weight distributions. Considerably extended molecular configurations were observed in a number of instances. Clear evidence from the fractions studied indicates that branching increases with increasing molecular weight in a given polymer. It is considered that serious undercorrection occurs in 〈R̄02〉 for very broad distribution samples due to the distribution function used. This factor plus conflicting hydrodynamic theories do not permit quantitative evaluation of extent of branching; however, variation from essentially unbranched (long‐chain)to highly branched HPPE samples was observed.