R. H. Peters

High-density polyethylene: thermal history and melting characteristics

Abstract The melting characteristics of high-density polyethylene have been shown to be dependent on molecular weight and molecular weight distribution as well as on annealing and crystallizing history. The phenomenon of double melting endotherms is explained on the basis of a molecular weight fractionation process occurring during annealing and crystallization in which there is preferential crystallization of low or high molecular weight species according to the temperature. Reference is made to other polymers exhibiting similar behaviour.

Melting and transition phenomena in some polyester-urethanes

A series of poly(ethylene adipates) with molecular weights between 1800 and 15 000 were prepared, and were extended by reaction with di-isocyanates. In one series of extended polymers the different polyesters were all extended with tolylene di-isocyanate whilst in a second series a polyester of molecular weight 3000 was extended with 4 different di-isocyanates. The glass transition temperatures and melting points of all polymers were studied. The variation of Tg of homopolymers with molecular weight was not adequately represented by the usual equations and it was concluded that the OH end groups are tied into the structure. On the other hand the variation of Tm with molecular weight obeyed the Flory equation and leads to a value for the heat of fusion similar to that obtained by Edgar and Ellery by the copolymer method. The observed values of Tg and Tm of the copolymers did not agree with values calculated by random copolymer theory. In the case of melting points it is likely that the discrepancy arose from the disparate nature of the ethylene adipate, crystallizable, units and the di-isocyanate, non-crystallizable, units; the latter distorted the structure so badly that a number of neighbouring crystallizable units were prevented from crystallizing.

Gelation in the reactions of aliphatic di-isocyanates with triols

Gelation in reactions of Niax triols with hexamethylene and decamethylene di-isocyanates has been studied. The gel points occur at extents of reaction in excess of those predicted by Flory-Stockmayer theory. The excess, intramolecular reaction increases with dilution and with decreases in the molecular weights of the reactants. Kilbs theory of gelation does not explain the results, but a semi-quantitative explanation is found in terms of a combination of Frischs and Kilbs theories. In this, λ, the intramolecular branching parameter of Frisch, is given by the equation λ = const.v−32 (OH0 + NCO0)−1, with v the number of bonds in the smallest ring which can form.

Characterization of the aminolytic product of poly(ethylene terephthalate)

Abstract Various analytical methods were used in addition to weight loss to characterize the products resulting from the aminolysis of both conventional (regular laboratory) poly(ethylene terephthalate) fibre and commercial, basic-dyeable Dacron T92. The results obtained from infra-red spectroscopy and scanning electron microscopy together with elemental analysis of products resulting from progressive degradation of the polymer showed that the corresponding amide of the polyester was ultimately formed on complete degradation. Whilst the weight-loss curves showed considerable variation with varying pretreatment, the result obtained showed little selectivity on the part of the aminolysis and thus was not suitable for estimating crystallinity because low molecular weight crystalline products are deposited and actual weight loss passed through a maximum with time.

Crystallization phenomena in some polyester — urethanes

Abstract A density balance has been used to measure the crystallization isotherms of poly(ethylene adipate) and copolymers which were prepared by extending the polyester with four different diisocyanates. The occurrence of extensive secondary crystallization processes precluded satisfactory interpretation in terms of the Avrami theory and further analysis could only be based on the half-time for crystallization. This parameter has the advantage that it is determined directly and is independent of any theoretical analysis. The presence of very small concentrations of diisocyanate units can have a profound effect on the rate of crystallization if they are very different structurally from the parent polyester, e.g. a molar concentration of about 5% of 2,4-tolylene diisocyanate increases the half-time by a factor of 20 whereas the same concentration of hexamethylene diisocyanate has an insignificant effect. The temperature dependence of the half-times is discussed in relation to several theories of crystallization.

Morphology of cold-drawn high-density polyethylene fibres

Abstract Drawn fibres of high-density polyethylene, and their crystalline residues obtained by nitric acid digestion of the amorphous phase, exhibit single- or double-peaked melting endotherms depending on the degree of crystallinity of the fibre, the draw ratio and, in the case of some of the crystalline residues, whether or not they have been melted once or twice. The melting behaviour of the drawn fibres and their crystalline residues has been accounted for in terms of the rupture of a fraction of the original lamellar structure and the growth of a new crystalline structure.