Thermal expansion of isotactic polypropylene Part I: temperature-dependent contributions of thermal expansions in its amorphous and crystalline phases

In this study, the nature of the linear thermal expansion of injection-molded isotactic polypropylene (iPP) was investigated by temperature-variable small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) techniques. The analysis of the temperature-variable SAXS data was based on the Strobl-Schneider method, and the results provided the temperature dependence along the lengths of the long periods of the iPP lamellae and amorphous layers. Based on these values, the corresponding coefficient of linear thermal expansion (CLTE) alpha(am) for the length of the amorphous layer was obtained. The results of temperature-variable WAXS revealed that the iPP had a monoclinic a-form. In addition, the reflection peak corresponding to each crystal plane in the unit cell shifted with increasing temperature, which indicated that the lattice constants, especially the length of each crystal axis, the a-, b-, and c-axes, varied. The CLTE alpha(a), alpha(b), and alpha(c) values corresponding to each a-, b- and c-axis were obtained. Combining aam with aa, ab, and ac and information on the orientation of the iPP crystal, i.e., the mean squared cosine <cos(2)omega(a, b, c)> provided the total CLTE alpha(cr+am), which allowed thermal expansion to occur in both crystalline and amorphous layers. The resulting acr+ am was in good agreement with the bulk CLTE. Furthermore, alpha(am) /alpha(cr+ am) was >0.99. The linear thermal expansion of the iPP was governed only by the linear thermal expansion of its amorphous layer.