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Author ORCID Identifier

https://orcid.org/0000-0003-0093-1932

Document Type

Campus-Only Access for One (1) Year

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Polymer Science and Engineering

Year Degree Awarded

2019

Month Degree Awarded

September

First Advisor

E. Bryan Coughlin

Second Advisor

David A. Hoagland

Third Advisor

Sarah L. Perry

Subject Categories

Condensed Matter Physics | Polymer and Organic Materials | Polymer Chemistry

Abstract

Ion-containing polymers are a unique class of materials for which strong electrostatic interactions dictate physical properties. By altering molecular parameters, such as the backbone chemical structure, the ion content, and the ion-pair identity, the structure and dynamics of these polymers can be altered. Further investigation of the molecular parameters that govern their structure-property relationships is critical for the future development of these polymeric materials. Particularly, the incorporation of ammonium-based counterions into these polymers offers a facile method to tune their electrostatic interactions and hydrophobicity. Applying this concept, a bulky dimethyloctylammonium (DMOA) counterion was used to modify the organic solubility of styrenesulfonate in order to facilitate its direct solution copolymerization with isoprene. With these poly(isoprene-ran-styrenesulfonate) (P(I-ran-SS)) copolymers the effect of ion content and the counterion identity on the structure and dynamics were evaluated.

In the first project, poly(isoprene-ran-dimethyloctylammonium styrenesulfonate) (P(I-ran-DMOASS)) copolymers with high molecular weights and dimethyloctylammonium styrenesulfonate (DMOASS) compositions ranging between 8 and 40 mol% (30 - 77 wt%) were synthesized via nitroxide-mediated polymerization. Thermal and viscoelastic characterization revealed distinct behaviors for the low (30 - 51 wt%) and high (56 - 77 wt%) DMOASS content copolymers. Three structural regimes were identified: ion clusters (30 wt% DMOASS), continuous ionic phase (56 - 77 wt% DMOASS), and the coexistence of the two (42 - 51 wt% DMOASS). As DMOASS content increased, small angle X-ray scattering revealed a gradual transition from the characteristic ion cluster structure to a smaller, more regular backbone-backbone structure associated with a continuous ionic phase. The ion clusters acted as physical crosslinks and introduced additional elasticity into the low DMOASS content copolymer, while the continuous ionic phase showed restricted flow behavior and the disappearance of a definitive plateau modulus. Dynamic mechanical analysis revealed two distinct Tg’s at intermediate DMOASS content, indicating the coexistence of both structures.

In the second project, the role of counterion sterics on the structure and dynamics of a low glass transition temperature, amorphous P(I-ran-SS) at low ion contents (7 mol%) was investigated using a series of symmetric, tetraalkylammonium counterions with methyl (TMA), ethyl (TEA), propyl (TPA), and butyl (TBA) pendent groups in addition to a sodium cation control. A detailed analysis of the aggregate structure was achieved by fitting the X-ray scattering profiles with a modified hard sphere model. Increasing the counterion sterics from sodium to TEA resulted in slight changes to the aggregates with some ionic groups present in the isoprene matrix. For the more sterically hindered TPA and TBA counterions, considerable disruption of the structure occurs. Using solid-state NMR, dynamic mechanical analysis, and rheology, the effect of the counterion sterics on the copolymer dynamics was determined. The larger counterions exhibited an increase in the dynamic moduli at high frequency while decreasing the dynamic moduli at lower frequencies in addition to possessing faster molecular dynamics. These two observations correspond to the incorporation of more ionic groups into the isoprene matrix and weakening of the dipole-dipole interactions, respectively.

Lastly, binary mixtures of TMA and TBA ammonium counterions were employed in these P(I-ran-SS) copolymers. The P(I-ran-SS) ionomers with TMA:TBA weight ratios of 100:0, 75:25, 50:50, 25:75, and 0:100 were prepared through solution blending. The SAXS profiles and Kinning-Thomas fitting showed only slight structural changes between 100:0 and 50:50, while major modification of the structure appears once the ratio reaches 75:25 and above. The alterations of the structure also indicated a mixed counterion aggregate structure. The linear viscoelastic characterization of the mixed counterion ionomers showed an increase in the polymer dynamics at low frequencies with increasing TBA weight percentages. Additionally, preliminary tensile tests were collected that showed increased mechanical properties with the stronger electrostatic interaction associated with TMA counterions. Thus, the structure and properties of these low Tg, amorphous ionomers can be specifically tuned by using multiple counterions.

Through these studies, the role of both ion content and counterion identity on the structure and dynamics of low Tg, amorphous P(I-ran-SS) copolymers have been elucidated. Furthermore, ammonium-based cations have been shown to offer a versatile means to modify both the ion aggregate structure and interaction strength of an ionomer. Appropriate selection of the pendent groups and mixture of different counterions allow for the properties of the ionomer to be freely tuned.

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