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Morphology and enhanced compatibility of immiscible polymers via specific interactions
Abstract
This work describes the phase behavior and morphology of otherwise immiscible polymer blends that contain small numbers of specific interactions. The experimental results are explained in terms of a new model for phase separation, termed the "ionic crosslink model". Sulfonated polystyrene in both the acid and zinc-neutralized forms was blended with either ethyl acrylate/4-vinylpyridine copolymers or styrene/4-vinylpyridine copolymers. The blends were investigated using differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA), and optical microscopy. At substitution levels of 2%, 5%, and 8% the blends are macrophase separated, microphase separated, and phase mixed, respectively. Microscopy shows that the macrophase separated blends exhibit smaller, more uniform sized domains compared to the unfunctionalized blend. The experimental results are qualitatively consistent with the proposed "ionic crosslink model" for phase separation, in which the chains between ionic groups phase-separate due to an unfavorable free energy of mixing, but the presence of ionic interactions restricts the size of the domains. Aggregation of the ionic groups within the blends was examined using DMTA and small angle x-ray scattering (SAXS). Viscoelastic measurements show the existence of a high temperature loss peak, similar to the peak observed in ionomers. However, the temperature of the transition is depressed relative to the parent ionomers due to internal plasticization. The presence of ionic aggregates was confirmed by calculating the average network functionalities. Activation energies for the high temperature transition are related to the relative strengths of the interactions, which is consistent with the transition being due to motion of the ionic groups. SAXS measurements show that the "ionic peak" present in ionomers is destroyed upon blending. The combined DMTA and SAXS results are consistent with intraparticle scattering models for ionomer morphology and are inconsistent with interparticle scattering models. Tensile properties show enhanced toughness and strength due to the presence of specific interactions. The improved properties are attributed to two factors: the presence of interactions which enhance the interfacial adhesion between phase separated domains and the presence of ionic aggregates which act as filler particles. Examination of freeze-fractured surfaces shows evidence for improved interfacial adhesion and enhanced formation of crazes.
Subject Area
Polymer chemistry
Recommended Citation
Douglas, Elliot P, "Morphology and enhanced compatibility of immiscible polymers via specific interactions" (1993). Doctoral Dissertations Available from Proquest. AAI9316643.
https://scholarworks.umass.edu/dissertations/AAI9316643