Doctoral Dissertations

Date of Award

12-2019

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Chemistry

Major Professor

Alexei Sokolov

Committee Members

Sheng Dai, Brian Long, Joshua Sangoro

Abstract

Supramolecular polymers (also termed as associating polymers), which are connected by non-covalent interactions between polymer chains, have become an increasingly important class of polymers and gained tremendous interest in the last few decades. The co-existence of reversible secondary interactions and covalent bonding makes supramolecular polymers promising candidates for functional materials. Immense effort has been put on the development of chemical structure design, while the understanding of their physical properties is rather limited, especially in the melt state. In this dissertation, we studied the dynamics and viscoelastic properties of H-bonded telechelic associating polymers by tuning the association strength, main chain length, flexibility and polarity. A systematical analysis was conducted by employing a combination of experimental techniques: dielectric spectroscopy, differential scanning calorimetry, rheology and small angle X-ray spectroscopy. We demonstrated that hydrogen-bonding has a strong influence on both segmental and slower dynamics in the polydimethylsiloxane (PDMS) and poly(propylene glycol) (PPG) systems with low molecular weights. The supramolecular association of hydroxyl-terminated PDMS chains leads to the emergence in dielectric and mechanical relaxation spectra of the so-called Debye process traditionally observed in monohydroxy alcohols. Then we investigated telechelic associating PMDS with different hydrogen bonding end groups, e.g. NH2, NHCO-COOH (amide-acid groups). Remarkably, a single species of end group forms two qualitatively different types of associates in PDMS-NHCO-COOH: transient bonds which allow stress release by a bond-partner exchange mechanism, and effectively permanent bonds formed by a phase segregated fraction of end groups which are stable on the timescale of the transient mechanism. In the following work, we studied telechelic PDMS and PPG with three types of H-bonding end-groups possessing different interaction strengths and a non-H-bonding end-group as reference were compared. Unraveling the mechanisms of many molecular processes and structure-dynamics-property relationship in supramolecular polymers is of great importance for both fundamental studies and industrial applications. Findings in this work suggested that the backbone length, flexibility and polarity, the strength and lifetime of the associating groups, and the ratio of characteristic time scales between backbone and chain ends should be considered in the design of associating polymers to achieve the desired properties.

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