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

Abstract

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 °C. With low filler contents (10 and 25 phr), two different multistage kinetic pathways, namely, (i) amorphous−mesophase-α′−α forms and (ii) amorphous−mesophase-β′−β−α forms, are first observed during SIC of PDMS at −45 and −50 °C, respectively. However, with higher filler contents (40 and 55 phr), only α and β 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 α′−α and β′−β are reversible and controlled by stress. Under preset strains, cooling results in the emergence of β, β′, α′, and α 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. β′ and α′ are the independent crystal structures which occur before the SIC of β and α forms, respectively, rather than intermediate transient states for β−α 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.

Strain Rate Dependence of Stretch-Induced Crystallization and Crystal Transition of Poly(dimethylsiloxane)

“…Previous cyclic tensile tests of PDMS 49,50 have shown that after the stress is unloaded, the crystal transitions of α−β, β−β', and α−α' are observed. Obviously, it is indicative of a relative stability β' < β < α and α' < α with increasing the stress.…”

“…Similarly, we have drawn the nonequilibrium SCT diagram in the strain rate−temperature space. Because the SCT of β−α has been discussed thoroughly in our previous work, 49 here we would pay attention to the SCT of α'−α at different strain rates. The effects of the strain rate on the SCT mechanism have been elucidated.…”

“…The procedure and sample preparation method are the same as those in the previous work. 49 PDMS-SiO 2 nanocomposite films of thickness 1 mm were prepared.…”

“…Four SIC/SCT kinetic pathways, that is, amorphous−α'−α, amorphous−α, amorphous−β'−β−α, and amorphous−β−α, appeared during the stretching in different temperature ranges. 49 The SCT of α'−α, β'−β, 49 and β−α 50 is a reversible process controlled by the stress. The split azimuthal peaks of β' and α' forms are related to the tilt of chain's axis away from the stretching direction (SD).…”

“…The split azimuthal peaks of β' and α' forms are related to the tilt of chain's axis away from the stretching direction (SD). 49 Although α' and β' forms have been suggested to be high entropy and low order, their thermodynamic stabilities and kinetic competition in the strain rate−temperature space have not been studied yet. If the strain rate could tune their stabilities and kinetic pathway as expected, it would modify the strain−temperature phase diagram of the SIC/SCT.…”