Vacuum-Pumping Induced Crystallographic Anisotropy of Poly(dimethylsiloxane) Oil under Capillary Confinement

Abstract

Investigating the low-temperature crystallization behavior of liquid polysiloxanes poses a significant challenge. To address this issue, a custom low-temperature chamber with specialized sample holders was meticulously constructed and integrated with a laboratory X-ray diffraction apparatus, enabling time-resolved wide-angle X-ray diffraction (WAXD) measurements of linear poly(dimethylsiloxane) (PDMS) oil and cross-linked PDMS film during cooling and heating. Under vacuum pumping and capillary confinement, the linear silanol-terminated PDMS oil demonstrates significant crystalline orientation at low temperatures when the crystalline index surpasses 10%, with the recorded 2D diffraction patterns showing heterogeneous azimuthal intensity distributions. The presence of crystallographic anisotropy is strongly influenced by the experimental vacuum pumping rather than the millimeter-scale capillary confinement. Unlike the strain-induced alignment of crystalline planes observed in stretched PDMS film, the vacuum-pumping-induced crystallographic anisotropy in linear PDMS oil measured within a capillary only results in a minimal or partial lamellar orientation. This alignment will cause crystalline chain segments to pack parallel to the incident X-ray beam direction and fold within the a-c plane. These findings are expected to enhance low-temperature X-ray analysis techniques for liquid samples and facilitate future crystallographic investigations of polysiloxane materials.