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ABSTRACT RHEOLOGICAL PROPERTIES OF A MODEL SOFT SOLID NANOCOMPOSITE SEPTEMBER 2018 VIJESH A. TANNA B.S. UNIVERSITY OF ILLINOIS URBANA-CHAMPAIGN M.S. UNIVERSITY OF MASSACHUSETTS AMHERST Ph.D. UNIVERSITY OF MASSACHUSETTS AMHERST Directed by: Prof. H. Henning Winter The fabrication and physical properties of polymer/clay nanocomposites has received a great deal of interest in both academic and industrial settings. Clay is a natural 2D mineral comprised of stacks of platelets with high aspect ratios held together through electrostatic interactions. Typically, polymer/clay composite are found to have the best physical properties when these clay sheets are randomly dispersed, exfoliated, throughout the polymer matrix. However, achieving this state is non-trivial due to the strong electrostatic forces holding the clay sheets together. Previous work has shown by mixing an end-functionalized, hydroxyl or carboxyl terminated, polybutadiene with clay, exfoliation can be achieved with mild annealing, which our group has termed the “self-exfoliation” process. The exfoliation of clay leads to the polybutadiene/clay composite forming a soft solid physical gel, presenting a model soft solid to study a material near its gel point. The work presented here relies on using the self-exfoliation process as a model material platform for 4 projects: (1) yielding of the soft solid physical gel, (2) understanding the effect of clay platelet size on the composite’s linear viscoelastic properties, (3) modification of the polymer to tune its interactions with the clay filler and (4) crosslinking of the polymer matrix to create a clay filled thermoset. In the first project, the yielding behavior of the polybutadiene/clay physical gel was studied. Shear yielding was performed by exposing samples to large amplitude oscillatory shear (LAOS) above their yield stress/strain. These large stresses/strains decrease the composite’s modulus and increase its characteristic relaxation times causing an irreversible softening and reduced the material’s internal connectivity. Samples were exposed to SAOS-Deformation-SAOS (SDS) sequences in which deformations were applied by increasing the deformation strain amplitudes and duration, followed by linear viscoelastic characterization through small amplitude oscillatory shear (SAOS). The flow-induced structural changes first began to occur at the same stress/strain values as the onset of non-linearity in traditional SAOS to LAOS (StL) stress amplitude sweeps. Yielding was found to be a strain activated process since the onset of non-linearity is independent of both frequency and temperature with respect to strain, not stress, amplitude. Finally, SDS measurements were performed at increasing deformation times and showed that these flow-induced structural changes require time to occur and soften the material through a reverse gelation type process. In the second project, we investigated the role of platelet size on the viscoelasticity of the same nanocomposite. Here two liquid polybutadienes served as matrix fluids, a non-functionalized polybutadiene (PB), which is a non-interacting liquid, and a carboxyl terminated polybutadiene (CTPB), which caused the clay to exfoliate. Un-exfoliated clay particles were suspended in PB and the liquid mixture was treated with high-intensity chaotic flow in a planetary mixer to reduce the clay platelet size. After performing the size-reduction, clay particles were then exfoliated through the addition of CTPB. The exfoliated clay in the 50/50 PB/CTPB polymer blend, still formed a physical gel but with a lower modulus when platelet size was reduced. Samples were then yielded which caused the already soft physical gel to soften even further by again irreversibly reducing its internal connectivity. The samples comprised of large platelets softened but remained solid while nanocomposites made up of smaller platelets underwent a solid to liquid transition due to yielding. The third project explored performing post-polymer modification on polybutadiene to vary the location and number of functional groups on the matrix. Prior work on the self-exfoliation process had been limited to commercially available end-functionalized polybutadiene. Thiol-ene click chemistry was used to vary the hydroxyl groups content in the polymer matrix. The addition of hydroxyl groups increased the polymer’s glass transition temperature and as a direct consequence, increased the modulus and other linear viscoelastic functions. Composites were formed by adding clay to the synthesized polymers. Interestingly it was found that exfoliation and physical gelation still occurred when having only a single hydroxyl group per chain (on average). While the composites fabricated with the highly grafted polybutadiene exhibited a lower modulus and remained in a swollen intercalated state due to a lack of mobility. The findings presented here describe the self-exfoliation process as two separate steps: intercalation, driven by the presence of functional groups, and exfoliation, driven by having enough overall mobility in the system. In the final project, a polybutadiene/clay thermoset was developed using the self-exfoliation process. Sample fabrication was designed using a one-step mixing process to allow clay exfoliation to occur first via mild thermal annealing followed by UV activated chemical crosslinking of the polybutadiene’s double bonds. By achieving a high crosslinking density, the composite's storage modulus was shown to increase by several orders of magnitude after crosslinking. The effects of clay were found to be significant only at high frequencies, in which the response was dominated by the elasticity of the exfoliated sheets. By comparing the rheological and swelling properties of the filled and unfilled thermosets, we also proved that clay prevents a small number of crosslinks from forming.
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