Skip to Main Content
 

Global Search Box

 
 
 
 

ETD Abstract Container

Abstract Header

PHYSICALLY CROSSLINKED HYDROGELS: IMPACT OF INTERFACES AND STRESS ON STRUCTURE AND PROPERTIES

Wiener, Clinton G, Wiener

Abstract Details

2017, Doctor of Philosophy, University of Akron, Polymer Engineering.
The methods to control hydrogels’ toughness, ultimate stress, and fracture energy have received attention by researchers in recent years. The toughness and fracture energy can be increased by incorporating energy dissipating mechanisms. Physical crosslinks that can break and reform during stressing provide a simple method to modulate hydrogel mechanics. Hydrophobic physical crosslinks are a well suited for increasing toughness and fracture energy. This work investigated a physically crosslinked hydrogel composed of a random copolymer containing hydrophilic segments [N,N-dimethylacrylamide or N-isopropylacrylamide] and hydrophobic segments [2-(N-ethylperfluorooctane sulfonamido)ethyl acrylate (FOSA)] that form a network by the hydrophobic aggregation of FOSA segments into nanodomains. This system provides a model system for tough physically crosslinked hydrogels. How physical crosslinks impact swelling in laterally confined thin films, how the physical hydrogel and nanodomains deform in stress relaxation, and how absorbed water is altered due to confinement between hydrophobic nanodomain crosslinks were studied. Unlike the significantly reduced swelling of chemically crosslink thin film gels, the nature of the physical crosslinks’ allows rearrangements in laterally confined thin films. These rearrangements allow the thin films to obtain an equilibrium swelling ratio similar to bulk. The osmotic pressure of the hydrophilic chain swelling induces these rearrangements. The result of this is the ability of physically crosslinked thin films ability to overcome thin film swelling constraints. An equilibrium water content/distribution can be obtained by rearrangement of the network. The route by which physical crosslink domains breakup and reform was identified by performing 1D elongation during small angle neutron scattering with contrast matching (CM-SANS). The stress induced relaxation time was found to span 5 orders of magnitude when fit with seven Maxwell elements. From the CM-SANS measurements, the relaxation times of the physical crosslinks and the interconnecting hydrophilic segments were found to correlate with distinct macroscopic stress relaxation times. The CM-SANS measurements also revealed that the domain breakup occurs primarily by the pullout of the FOSA segments only after the interconnecting swollen segments have become significantly strained. Upon relaxation of the physical crosslinks, the physical domains display a spring-like rebound effect, increasing their spacing, and hydrophilic chain stretching, in the opposite direction in which the network is strained macroscopically. This strain-transverse domain spacing increase only relaxes after the physical crosslinks have reformed. The effect of the hydrophobic moieties, which are impermeable to water, on the absorbed water behavior was studied. The fraction of supercooled water measured at 200K increases with increasing the copolymer FOSA fraction as found by differential scanning calorimetry. Hydrogels’ primarily supercool water by binding to hydrophilic segments, but here the hydrophilic fraction decreases with increasing FOSA content. The cause of this was found to be the result of water nanoconfinement. Probing the dynamics of the absorbed water with neutron scattering revealed that as the FOSA fraction is increased, water mobility below 240K is higher than in bulk supercooled water. This demonstrates that the nanoconfinements increase the supercooled water mobility. These physical crosslinks provide a route to supercool water in soft tissue-like hydrogels.
Bryan Vogt (Advisor)
Robert Weiss (Advisor)
Nicole Zacharia (Committee Chair)
Matthew Becker (Committee Member)
Bi-min Newby (Committee Member)
260 p.

Recommended Citations

Citations

  • Wiener, Wiener, C. G. (2017). PHYSICALLY CROSSLINKED HYDROGELS: IMPACT OF INTERFACES AND STRESS ON STRUCTURE AND PROPERTIES [Doctoral dissertation, University of Akron]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=akron1498033274841434

    APA Style (7th edition)

  • Wiener, Wiener, Clinton. PHYSICALLY CROSSLINKED HYDROGELS: IMPACT OF INTERFACES AND STRESS ON STRUCTURE AND PROPERTIES. 2017. University of Akron, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=akron1498033274841434.

    MLA Style (8th edition)

  • Wiener, Wiener, Clinton. "PHYSICALLY CROSSLINKED HYDROGELS: IMPACT OF INTERFACES AND STRESS ON STRUCTURE AND PROPERTIES." Doctoral dissertation, University of Akron, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=akron1498033274841434

    Chicago Manual of Style (17th edition)