Cocontinuous blends are used in a number of applications, including porous media for filtration, dessicant entrained polymers and substrates for drug delivery devices. A major drawback of these materials is that they are thermodynamically unstable, which implies that their morphology evolves into coarser structures when they are above their glass transition (or melting) temperatures. The mechanisms involved in the coarsening process are not fully understood yet.
Three aspects of the coarsening process were addressed in this work: (1) thorough characterization of the microstructure during coarsening via the implementation of novel 3D imaging techniques, (2) modification of interfacial properties via addition of block copolymers in order to hinder the coarsening, and (3) determination of the connection between morphology and viscoelastic response during the coarsening process.
Laser scanning confocal microscopy (LSCM) was used to image fluorescently labeled polystyrene (FLPS)/styrene-ran-acrylonitrile copolymer (SAN) blends. A methodology to obtain 3D micrographs of the blends and analyzed them to extract information of the geometry (size and local curvatures), topology (connectivity) and anisotropy (normal vectors) was implemented. From the analysis of the time evolution of the size and local curvature of the interface, two regimes of coarsening were identified: an early regime, where the characteristic size grows linearly with time and the interface evolves in a self-similar manner, and a late regime where the surface growth is neither linear nor self-similar. The measured decrease of the interface curvature was used to explain this regime transition.
Symmetric diblock copolymers (BC) made of polystyrene (PS) and polymethyl methacrylate (PMMA) were used to compatibilize the blends. A dramatic decrease in the rate of coarsening was observed after adding just 1\% of BC. The stabilization was particularly good for BC with an intermediate molecular weight. This result was explained with a theory based on the equilibrium between micelle dissolution and formation and adsorption and desorption of BC from the interface.
Small amplitude oscillation measurements were performed on both the compatibilized and the non-compatibilized blends. A noticeable extra contribution to the elastic modulus was observed for all the blends with bigger values for the compatibilized blends. A decrease in the elasticity was observed during the coarsening of the blends. This was explained by the decrease in interfacial area. This decrease was slower for the blends with BC, which evinces stabilization of the microstructure.
In an attempt to find a quantitative relation between morphology and rheological response of these materials, a simplification for small deformation to the Doi-Ohta model [Doi, M.; Ohta, T., J. Chem. Phys., 95, 1242] was proposed. A key parameter in Doi-Ohta's theory is the degree of anisotropy. In this work a method to compute the anisotropy tensor from the analysis of the 3D images was developed. Predictions from the simplification fit rheological data of blends with low interfacial tension in the late regime of coarsening, but failed in the early stage and for high interfacial tension blends.
University of Minnesota Ph.D. dissertation. December 2009. major: Chemical Engineering. Advisor: Chris Macosko. 1 computer file (PDF); xiv, 225 pages, appendices A-D, + 2 computer files ((JPEG); red-green separation 3-D images titled 25-75Blend and 50-50Blend, + 1 computer file (MOV); 28 sec. col. computer animation, titled 3DImage
Lopez-Barron, Carlos Rene.
On the coarsening of immiscible polymer blends with cocontinuous morphologies..
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