Detection of “memory” effects in polyethylene by magnetic susceptibility

Publisher Website
Google Scholar
Abstract
The relevance of diamagnetic susceptibility as a tool for the structure analysis of solid high polymers is stressed in the light of some new examples. The present results complement previous data and offer new aspects on the diamagnetic investigations of longchain hydrocarbons, especially polyethylene (PE). The molecular susceptibility is proportional to the average number of repeat units in the chain. The proportionality factor defines an intermolecular constant μk which characterizes different physical states. This was found to be 2.5 × 10−6 for the liquid and 3.5 × 10−6 cgs for the crystalline state of paraffins and polyethylene (solution‐crystallized). For melt‐crystallized material, μk, approaches the typical value of the liquid paraffin in agreement with previous results. Such a low μk is probably related to the increased disorder of the paracrystalline lattice domains, in contrast to the more ordered microparacrystallites in the so‐called “single crystals,” where μk = 3.5 × 10−6. In single crystals of branched PE, μk approaches 2.5 × 10−6 with increasing branching ratio. Like paraffins in the gaseous state, molten PE, with chains longer than 1000 Å, has μk = 0. If the solution‐crystallized material is molten for 10 min and thereafter cooled, μk retains the original value 3.5 × 10−6 cgs characteristic of the crystalline state. Hence, solution‐crystallized polyethylene apparently possesses a kind of “memory.” Such a “memory” can, nevertheless, be partly destroyed when molten PE is stirred for 10 min and then quenched. Aggregates of solution‐precipitated crystals with 3% branching concentration give μk = 2.9 ± 0.2 × 10−6 in good agreement with x‐ray diffraction data. Finally, experimental details on the magnetic measurements are critically discussed, and various aspects of improvements for further investigations are also described.

This publication has 10 references indexed in Scilit: