Title

Stimuli-Responsive Polymers

Date of Award

2010

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Photochemical Sciences

First Advisor

Douglas Neckers, PhD

Second Advisor

Thomas Kinstle, PhD (Committee Member)

Third Advisor

John Cable, PhD (Committee Member)

Fourth Advisor

Sheryl Coombs, PhD (Committee Member)

Abstract

Stimuli-responsive polymers are of both fundamental and commercial interest. Such polymers respond dramatically to external stimuli such as changes in light, pH, temperature, mechanical force, electric field, or solvent composition. Hydrogels and organogels that can respond to external stimuli by changing their microstructures from collapsed to expanded as well as by reversibly returning to their initial state with or without the stimuli are important. Herein, three stimuli-responsive polymers have been explored. They are Photoresponsive oil sorbers, Photo- and thermo- responsive hydrogels, and photo- and electro- responsive hydrogels.

A photoresponsive oil sorber with a hydrophobic, photoresponsive core and shell has been synthesized via suspension polymerization. Lauryl acrylate (LA), isodecyl acrylate (IDA) and tert-butylstyrene (tBS) were used as monomers, 4-(methacrylamino)azobenzene (Azo-M) was used as a photoresponsive monomer, and bis(methacryloylamino)azobenzene (Azo-CL-M) was employed as a photoresponsive surface cross-linker. The oil sorber prefers non-polar solvents. It absorbed 15 times its dry weight in toluene, 19 times its dry weight in chloroform and 16 times its dry weight in dichloromethane. Rapid and photoresponsive desorption of solvent (86% of solvent expulsed in 30 minutes) was characteristic. The new oil sorber is an excellent gasoline absorber rapidly increasing its body weight in its presence. And also, it is less dense than water, and can potentially be utilized for cleaning oil spills on water.

A dual-stimuli responsive hydrogel with a thermoresponsive backbone and photochromic pendant groups was synthesized via free radical polymerization using N-isopropylacrylamide (NIPAAm) as main monomer, 9'-Methacryloyloxy-1,3,3-trimethyl-spiro[indoline-2,3'-[3H]naphth[2,1b][1,4]oxazine] (MSp) (MSp) as photochromic monomer, the crosslinker N,N'-methylenebisacrylamide (BIS) and the initiator 2,2'-azobisisobutyronitrile (AIBN) in dimethylsulfoxide (DMSO). The polymers are dual responsive, in that poly(N-isopropylacrylamide) [PNIPAAm] responds to temperature changes while the pendant spironaphthoxazines respond to light. Irradiation enhanced the water absorption of the polymers while increases in temperature decreased it. The irradiated PNIPAAm-MSp showed best water absorption at 0 °C (Q = 3.25) while water desorbed at higher temperatures (35 °C; Q = 0.30); where Q is the amount of water absorbed by a gram of dry polymer.

Photoresponsive bending of rod shaped PNIPAAM-MSp hydrogels under a DC electric field have been investigated. The excited state formed by the absorption of light dissociated at the spiro C-O bond of the spironaphthoxazine moeity and generated charges in the polymer. These then aligned with the electric field to cause bending of the polymer. In the absence of light, the bent polymers reversibly straightened. The photoresponsive bending became faster in salt solutions and the direction of bending switched with the changing polarity of the electric field. No actuation was observed without photoirradiation.