Structure-property relationships in physical polymer gels

• Main Author: Sing, Michelle Kay
• Other Authors: Bradley D. Olsen.
• Format: Others
• Language: English
• Published: Massachusetts Institute of Technology 2017
• Subjects: Materials Science and Engineering.
• Online Access: http://hdl.handle.net/1721.1/111330

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2017. === Cataloged from PDF version of thesis. === Includes bibliographical references. === Associating polymer networks are included in a large number of materials where it is important to control and tune the rheological response of the material, particularly including responses to temperature, shear, or changes in the surrounding environment. Knowledge of the relationship between the kinetics of the end-groups that form these associations and the mechanical behavior of the network can provide valuable insight into the design of these sorts of materials. This thesis first looks at a simple theoretical approach for predicting the mechanical behavior of these transient networks as a function of the association kinetics. By using a modified form of the Smoluchowski equation to track the full probability distribution of looped, bridged, and dangling chain conformations in the network, it is possible to see relationships between the rate at which chains associate and dissociate and the presence of non-monotonic flow behaviors. Dangling chain tumbling due to the internal flux of the system under shear resulted in decreased stress through the ability to then form looped chains. The Smoluchowski equation can be adjusted to its Langevin form for Brownian Dynamics simulations of chain ends as a function of location and conformation. This modified formalism makes it so that it is not only striaghtforward to extract network relaxation times as a function of relative kinetics, but it is also possible to incorporate multiple bonds within a single association such that the association has a constant bond energy regardless of the number of bonds. The network relaxation can be grouped into two cases - one where a dissociated dangling chain is able to fully relax prior to association, and the other where the chain can only relax a fraction of the way prior to reassociation. These two cases result in different numbers of characteristic network relaxation times that change with increasing bond number. The second part of this thesis focuses on experimentally investigating how block and sequence architecture affect the deformation behavior and kinetics of thermoresponsive, dual-associating block copolymer systems. These systems are comprised of thermoresponsive elastin-like polypeptide endblocks (ELPs) fused to a polypeptide containing self-associating a-helical domains. Above the critical gel concentration, these protein fusions form disordered spherical nanostructures in solution. In shear flow, the rate of network deformation and rearrangement could be increased or decreased by changing block architecture, temperature, or concentration of the system. Protein fusions containing the standard ELP sequence underwent a liquid-like rearrangement of the nanostructure to accommodate the shear stresses associated with flow. Performing an amino acid substitution in the ELP endblock further affected the kinetics of rearrangement and resulted in multiple orders of magnitude increases in material toughness. This thesis has provided an increased understanding of how tailoring the properties of endblock associations can affect the mechanical behavior of the bulk material. Through increased understanding of how the properties of associating groups affect the macroscopic material properties, it is possible to better inform materials design for end-use applications. === by .Michelle Kay Sing === Ph. D.