Equilibrium and nonequilibrium morphologies of polymer blends

• Main Author: Kielhorn, Lars
• Language: ENG
• Published: ScholarWorks@UMass Amherst 1999
• Subjects: Polymers|Materials science|Chemical engineering|Chemistry
• Online Access: https://scholarworks.umass.edu/dissertations/AAI9920617

Several extensions to the phase behavior of binary polymer blends were investigated. Complicated and intricate phase diagrams were found in ternary block copolymer/homopolymer blends as microphase separation competes with macrophase separation. Composition fluctuations were accounted for in the Hartree approximation and the phase diagrams are an extension to mean-field theory. The Lifshitz point, a mean-field multicritical point at the boundary between macrophase separation and microphase separation, is destroyed by the inclusion of composition fluctuations. The kinetics of phase separation in ternary block copolymer/homopolymer mixtures was modeled using the time-dependent Landau-Ginzburg (TDGL) theory calculating the expansion coefficients from molecular parameters. The expulsion of diblock copolymers from the interior of developing domains and subsequent accumulation at the domain interfaces was observed. The simulations suggest that for higher block copolymer content saturation of the interface with block copolymer is responsible for the formation of separate copolymer-rich domains at later stages. Since the diblock copolymer acts as a compatibilizer between the two homopolymers, phase separation kinetics are slowed down. In order to study the viscoelastic effect of polymers, phase contrast optical microscopy and time-resolved small-angle light scattering was used to study the relaxation behavior of a high molecular weight polybutadiene/polyisoprene. Upon the cessation of steady-state shear anisotropic morphologies relaxed by a variety of ways depending on the previously applied shear rate. Application of sufficiently high shear rates resulted in the fastest relaxation of the anisotropy. After passing through a dynamic scaling regime, the largest domains were formed. The TDGL formalism with a convective flow was used to simulate such relaxation behavior. By incorporating only surface tension some of the basic features of the relaxation behavior were recovered. Finally the phase separation in a binary blend in the vicinity of a patterned surface was studies using the TDGL equation. For a short range potential long-range “checkerboard-like” oscillations normal to the surface were observed as transient states, the extension of which strongly depended on the noise. Spinodal decomposition is observed for films as thin as one spinodal wavelength. For thinner films the surface imposes its periodicity on the blend if the surface interaction is sufficiently strong.