Stretch-Induced Intermediate Structures and Crystallization of Poly(dimethylsiloxane): The Effect of Filler Content

Stretch-induced crystallization (SIC) and structural transitions of poly(dimethylsiloxane) (PDMS) filled with different contents of nanosilica were studied with an in situ synchrotron radiation wide-angle X-ray scattering technique during uniaxial tensile deformation at temperatures from -40 to -65 degrees C. With low filler contents (10 and 25 phr), two different multistage kinetic pathways, namely, (i) amorphous-mesophase-alpha'-alpha forms and (ii) amorphous-mesophase-beta'-beta-alpha forms, are first observed during SIC of PDMS at -45 and -50 degrees C, respectively. However, with higher filler contents (40 and 55 phr), only alpha and beta forms are observed during SIC of PDMS. The nonequilibrium SIC structural evolution diagrams of PDMS with different filler contents are constructed in strain-temperature space. Further cyclic tensile experiments demonstrate that the transitions of alpha'-alpha and beta'-beta are reversible and controlled by stress. Under preset strains, cooling results in the emergence of beta, beta', alpha', and alpha forms in sequence with the increase of the preset strains, which precisely follows the structural evolution diagram of SIC for PDMS with the low filler content. beta' and alpha' are the independent crystal structures which occur before the SIC of beta and a forms, respectively, rather than intermediate transient states for beta-alpha transition. These results demonstrate the complexity of SIC with different structural intermediates in PDMS which is influenced not only by temperature and strain but also by the filler content and the kinetic process.