Evidence for structure in polyisobutylene from positron lifetimes

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Abstract
The lifetimes and intensities of positrons annihilating in polyisobutylene were determined over a temperature range from − 100 °C through its glass transition to + 20 °C. The glass transition was determined to occur at − 70 °C compared with a value of − 63 °C determined by differential scanning calorimetry. The intermediate and long lifetimes are concluded to be associated with positrons annihilating in the ordered and amorphous regions, respectively, of the polyisobutylene sample usually considered to be completely amorphous under normal conditions. Depending upon the thermal history, it was determined that between 40 and 50% of the polyisobutylene molecules are aligned in some manner at room temperature. This is to be compared with about 30% in cis 1–2 polybutadiene at the same temperature. The existence of this significant amount of structure in polyisobutylene is offered as an explanation for the sluggish response of butyl rubber to stress even at temperatures 80–90 °C above Tg .