Shear Alignment Mechanisms of Close-Packed Spheres in a Bulk ABA Triblock Copolymer

The alignment and shear-induced microstructural changes of closed-packed spheres observed in a poly(styrene-b-lauryl acrylate-b-styrene) triblock copolymer were quantified with a combination of in situ small-angle X-ray scattering (SAXS) and Fourier-transform (FT) rheology. Disordered spheres produced through compression molding were transformed to close-packed spheres through the application of nonlinear oscillatory shear. FT rheology identified strain amplitude regimes for small-, medium-, and large-amplitude oscillatory shear behaviors (SAOS, MAOS, and LAOS, respectively) at fixed temperature and frequency. The presence of oscillatory shear induced orientational changes in the hexagonally-closed packed (HCP) layers. Increasing the strain amplitude produced faster ordering and higher degree of orientation, quantified through fitting a stretched exponential function to in situ SAXS and FT rheology data. In the MAOS regime, shear thinning behavior was observed, with decrease in the storage modulus (G′) due to the formation of HCP layers that exhibited less resistance to flow. The third-order harmonic (I_3/1) increased with the square of the strain amplitude, hypothesized to originate from the presence of sharp grain interphase boundaries (due to defects), and Chebyshev coefficients indicated intracycle shear thickening and strain stiffening. In the LAOS regime, HCP layers adopted a parallel orientation with a zigzag path sliding mechanism, and both G′ and the loss modulus (G″) decreased with increasing strain amplitude. Interestingly, at higher strain amplitude, shear deordering was observed for the first time in close-packed spheres under oscillatory shear, as evidenced by characteristic signatures in both in situ SAXS and FT rheology data. Structural rearrangements were consistent with the domain dissolution and reformation mechanism.