Low‐temperature thermal expansivities of polyethylene, polypropylene, mixtures of polyethylene and polypropylene, and polystyrene

Length‐temperature measurements on a series of polymer blends over the whole range of composition from pure polyethylene to pure polypropylene and one set of determinations on a 50:50 copolymer and on polystyrene are evaluated. The total crystallinity of the samples did not exceed 54%. The experimental procedure utilized a linear variable differential transformer without the use of a confining fluid, and the temperature ranged from about +20 to –185°C. A least‐square numerical differentiation procedure based on moving arcs is applied to yield directly the coefficients of thermal expansion as a function of temperature. The linear voltage differential transformer (LVDT) technique can detect transitions in which the change in thermal expansion coefficients is less than 10 −5 °C. −1 . In polypropylene as well as the blends, the principal glass transition is clearly seen in the range observed by others, namely at about –9 to –14°C. Its location varies only slightly with composition at polyethylene contents less than 88 mole‐%. For polyethylene the transition region broadens noticeably. The results are suggestive of two transitions for 0 > T > –40°C. A second transition region is observed for either pure component around –126°C. Its location varies some what with composition. However, our results do not indicate the appearance of an additional transition region characteristic of the mixture. The copolymer exhibits a major transition at –61°C. in good agreement with earlier workers. The thermal expansion decreases again around –150°C. In general our observations concerning transitions below T g are consistent with dynamic results.