Crystallinity and the effect of ionizing radiation in polyethylene. I. Crosslinking and the crystal core

Abstract

An extensive study has been carried out on the effect of ionizing radiation on polyethylene in the form of solution‐grown single crystals to follow up preceding investigations which had indicated that the radiation‐induced effects along the fold surfaces could be significantly different from those within the crystal interior. In this first part of the series, radiation‐induced crosslinking was investigated in the case of “crystal core” material. This material, obtained by removal of the fold surface by oxidative degradation with ozone, consists of isolated foldfree crystal layers of dicarboxylic acids of uniform length, the length itself depending on the fold length of the starting crystal. This “core material” was irradiated by γ rays and the effect of crosslinking was followed by GPC analysis by recording the development of dimer, trimer, etc., peaks in the chromatograms. For a given dose, the fractional amount of crosslinked material is independent of the chain length. This, together with other effects described, provides unambiguous evidence that crosslinking occurs at the chain ends or, in other words, that there is no crosslinking within the interior of the paraffinoid lattice. Further, no permanent scission was observed to occur within the lattice, at least in amounts detectable by the present molecular weight measurements. The obvious significance of these effects for radiation studies on paraffinoid substances in general, beyond those of the present long‐chain dicarboxylic acids, is discussed prior to utilizing them in the study of chain‐folded polyethylene crystals in the following parts of the series.

A 13C NMR Study of End Groups and Stereoregularity Changes Induced by γ Irradiation of Molten Polypropylenes

“…The first difference suggests that sterochemical change only occurs in the amorphous regions, as the ratio correlates well with the fraction of amorphous component in the solid state. This is reasonable when * To whom correspondence should be addressed.considering the conclusions of Keller et al 4 which suggest that neither crosslinking nor chain scission occur within the crystalline regions of polyethylene when subjected to high energy irradiation. The large dose rate effect on the other hand, was explained by a difference in mechanism.…”

“…First neither scission nor crosslinking are thought to occur in the crystalline regions. 4 This approximately accounts for the difference between G(crosslink) values for solid and molten PP's. Secondly, increased backbone motion is likely to cause an increase in the fraction of initial scission events which become permanent.…”

“…considering the conclusions of Keller et al 4 which suggest that neither crosslinking nor chain scission occur within the crystalline regions of polyethylene when subjected to high energy irradiation. The large dose rate effect on the other hand, was explained by a difference in mechanism.…”

Thermally recoverable crosslinked polyethylene

“…It is observed that the plot is linear at low doses but becomes slightly nonlinear at higher doses, as previously observed. 9 The value of radiation dose at gelation r g (i.e., the dose at which s / s Å 2) is calculated for all the polyethylenes and is given in Table III. The radiation dose at gelation is the dose of irradiation at which the first insoluble residue is detected in the irradiated polyethylene.…”

“…On irradiation with A thermoplastic can be melted and reshaped as gamma rays, it predominantly under goes crossdesired by heating it to a temperature above the linking in the amorphous regions. [8][9] The probamelting point of the plastic crystalline domains. bility of free radicals being generated at the adja-…”