A nanoscale microphase separated but crystallizable block in the presence of a non-crystallizable block provides a unique environment for study of confinement effects on crystallization. Typically, depending on whether the Tg (glass transition temperature) of the amorphous block is higher or lower than the Tc (crystallization temperature) of the crystallizable block, the crystallization is confined or there is breakout. Most studies to date of a crystallizable block copolymer (BCP) involve either spheres, lamellar, or cylindrical morphologies. Here, we explore a very different environment, namely the tubular bicontinuous cubic microdomain structures such as the Double Diamond (DD) (Pn3m) and Double Gyroid (DG)(Ia3d) in polystyrene-b-polydimethylsiloxane (PS-PDMS) block copolymers. PDMS undergoes rapid crystallization at Tc of ~ - 75 C and melts at Tm ~ - 42 C. From differential scanning calorimetry (DSC), simultaneous small-angle and wide-angle scattering (SAXS/WAXS) at Brookhaven National Laboratory (NSLS-II 12-ID SMI) PDMS within these complex geometries exhibits about 50-60% crystallinity. There is also a surprising ~100X increase in the diffuse low q scattering after cooling to ~ –30<sup>o</sup>C presumably due to large density fluctuations from within the much greater thermally contracting PDMS domains due to the mechanical resistance of the 3D glassy PS and/or due to the emergence of more densely packed PDMS crystallization precursors. We investigate the influence of the shape on the 3D size, shape, and continuity of the PDMS crystals, and the degree of crystallinity. Preferred orientations of the PDMS crystallites along specific directions are examined with the help of real space Slice & View Scanning Electron Microscopy (SVSEM) tomography and reciprocal space SAXS/WAXS data using a microfocused beam.

Creating structures within structures: PDMS crystallite thickness and orientation in PS-PDMS block copolymers with PDMS tubular network morphologies via reciprocal and real space studies

ACS Spring 2022

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