Vacuum-Pumping Induced\nCrystallographic Anisotropy\nof Poly(dimethylsiloxane) Oil under Capillary Confinement

Investigating the low-temperature\ncrystallization behavior of liquid polysiloxanes poses a significant\nchallenge. To address this issue, a custom low-temperature chamber\nwith specialized sample holders was meticulously constructed and integrated\nwith a laboratory X-ray diffraction apparatus, enabling time-resolved\nwide-angle X-ray diffraction (WAXD) measurements of linear poly(dimethylsiloxane)\n(PDMS) oil and cross-linked PDMS film during cooling and heating.\nUnder vacuum pumping and capillary confinement, the linear silanol-terminated\nPDMS oil demonstrates significant crystalline orientation at low temperatures\nwhen the crystalline index surpasses 10%, with the recorded 2D diffraction\npatterns showing heterogeneous azimuthal intensity distributions.\nThe presence of crystallographic anisotropy is strongly influenced\nby the experimental vacuum pumping rather than the millimeter-scale\ncapillary confinement. Unlike the strain-induced alignment of crystalline\nplanes observed in stretched PDMS film, the vacuum-pumping-induced\ncrystallographic anisotropy in linear PDMS oil measured within a capillary\nonly results in a minimal or partial lamellar orientation. This alignment\nwill cause crystalline chain segments to pack parallel to the incident\nX-ray beam direction and fold within the <i>a-c</i> plane.\nThese findings are expected to enhance low-temperature X-ray analysis\ntechniques for liquid samples and facilitate future crystallographic\ninvestigations of polysiloxane materials.