Effect of long‐chain branching on some solution properties of polyethylene

Abstract

Two polyethylene samples polymerized by the standard high‐pressure technique employing oxygen as the initiator were resolved into a total of 19 fractions. Also, a Zieglertype polyethylene was separated into 12 fractions. A comparison of the intrinsic viscosities and the number‐average molecular weights of the polyethylene fractions prepared at high pressure over a broad molecular weight range shows that the spatial requirements of the polymer molecules increase anomalously with molecular weight. It is shown that this behavior is not due to increasing frequency of over‐all chain branching. Rather, it is due to the increasing degree of long‐chain branching with molecular weight. A comparison of the intrinsic viscosities of unbranched with branched polyethylene at a number‐average molecular weight of 200,000 indicates there are approximately 4 tetrafunctional branch points per molecule. It is demonstrated that polyethylene solutions can be passed through ultrafine filters without loss of soluble polymer under the proper conditions. Angular light scattering intensity measurements of clarified polyethylene solutions in Tetralin show that unbranched polyethylene has a normal M̄_w/M̄_n ratio, whereas branched polyethylenes have M̄_w/M̄_n ratios from 7 to more than 100. For the more highly branched polyethylenes it is shown that the dissymmetry method seriously undercorrects the weight‐average molecular weight because of nonconformity to a randomly coiled molecule. The M̄_w values of fractions of high‐pressure polyethylene show increasing polydispersity with increasing molecular weight, which is attributed to the influence of long‐chain branching. The magnitude of the polydispersity of these fractions and the parent polymers (high‐pressure) suggests that the degree of long‐chain branching is much greater than previously assumed.