Date of Award

5-2007

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Chemical Engineering

Advisor

Kilbey, II, S. Michael

Abstract

The focus of this research is to understand the kinetic and mechanistic aspects of surface-initiated photoiniferter-mediated photopolymerization (SI-PMP) and exploit the robustness of SI-PMP to synthesize stimuli-responsive polymer brushes. The 'living' characteristics of dithiocarbamate-based photoiniferter-mediated photopolymerization are well documented. However, in this dissertation I show that the growth of poly(methyl methacrylate) (PMMA) brushes by SI-PMP is nonlinear, suggesting loss of radicals during SI-PMP and, in turn, non-living characteristics. Results from kinetic models in conjunction with experimental results suggest that irreversible bimolecular termination reactions are a primary culprit for the loss of radicals during SI-PMP. To overcome this problem of irreversible termination reactions, tetraethylthiuram disulfide (TED), a source of deactivating dithiocarbamyl radicals, was added to the SI-PMP system. Preaddition of TED successfully reduced irreversible termination reactions. Contention of decrease in irreversible termination reactions is further supported by results from reinitiation studies using styrene: reinitiation efficiency, as indicated by the thickness of the added polystyrene block, increases as TED concentration increases. The impact of various photopolymerization conditions on SI-PMP is further investigated by simulating the SI-PMP process using a rate-based model. With this approach the effect of photopolymerization conditions such as light intensity, TED concentration, exposure time and initial photoiniferter concentration on the growth kinetics and reinitiation ability of PMMA layers has been studied in detail. The simulations show that increases in [TED] and decreases in light intensity impact the PMMA layer propagation in similar fashions; these trends are observed in experiments. However, simulations also indicate that the effect of [TED] and light intensity on the reinitiation ability of PMMA layers are significantly different: reinitiation ability increases with increasing [TED], but decreasing light intensity does not improve reinitiation ability. The simulations also show that choice of photopolymerization conditions used during the first polymerization step is critical to the final structure of the polymer brush created upon reinitiation: PMMA layers formed in the presence of TED are more likely to form block copolymers as compared to PMMA layers synthesized without TED and at lower light intensity.
Strategies learned from these simulations and experiments were applied for the synthesis of bi-level, multiresponsive poly(methacrylic acid)-block-poly(N-isopropyl-acrylamide) (PMAA-b-PNIPAM) layers. In-situ multi-angle ellipsometry investigations of these layers demonstrate that these layers respond to changes in pH, temperature and ionic strength. While the individual blocks retain their customary responsive characteristics, the overall swelling behavior of the PMAA-b-PNIPAM layers can be tuned by any number combinations of pH, temperature and ionic strength.
The efforts described in this dissertation, demonstrate not only the robustness of SI-PMP for making a variety of functional polymer brushes, but also the complex links between synthesis, structure and properties of polymer brushes.

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