Effects of shape anisotropy on microstructural evolution of diblock copolymers
This dissertation discusses the effects of shape anisotropy on the evolved microstructure of diblock copolymers at various levels. Due to chain connectivity and microphase separation, the diblock copolymers self-assemble into ellipsoidal grains of lamellar and cylindrical morphologies. A grain-structure related phenomenon, Excluded Volume Epitaxy (E.V.E.) is explored in Chapter 2. E.V.E. is a local, inter-grain azimuthal orientational correlations effect, which results from a combination of sporadic nucleation and impingement of growing anisotropic shaped grains. Due to E.V.E., the ellipsoidal grains have a propensity for similar orientations in a local neighborhood, despite complete absence of global orientation in the sample. Simulations and experiments have verified this effect.
The Avrami kinetics of anisotropic shaped grains is discussed in Chapter 3. Traditionally Avrami equation is used to model the growth kinetics of volume filling isotropic shaped grains. The probabilistic nature of Avrami kinetics produces a coupling between the grain shape and Poisson distribution. The Poisson-shape coupling remains latent for isotropic grains but becomes operative for anisotropic grains in random orientations scheme leading to inhibited growth kinetics. For unidirectional orientation of anisotropic grains, the growth kinetics remains uninhibited due to absence of Poisson-shape coupling. For two-dimensional case in simultaneous and continuous nucleation regimes, the inhibited kinetics scales as L1/2 where L is the shape anisotropy.
The blends of highly shape anisotropic nanoclay, montmorillonite (MMT) and lamellar poly(styrene-b-isoprene), PS-PI are discussed in Appendix A. Annealing and cooling is sufficient to produce long-range lamellar order at 1 wt % clay loading. However at 5 wt % clay loading, shear force is additionally required. This system reveals the effect of shape anisotropy on evolution of long-range order in clay-block copolymer blends.
The effect of shape anisotropy of polymer chain in the context of rod-coil block copolymer is explored in Appendix B. Poly(styrene-b-1,3-cyclohexadiene), PS-PCHD self-assembles into core-shell cylinder structure, seen rarely in AB diblock copolymers. A rod-coil free energy model that incorporates liquid crystalline splay distortion energy in the rod phase is presented. In this model, the PCHD block is treated as rod to explain the stability of the core-shell cylinder structure.