Title: Photochemical Crosslinking Reactions in Polymers
Description: The post-synthesis modification of polymer properties has very broad applications in industry. It is employed to produce products that are impossible to directly synthesize, modify biomolecules for medical use, and provide compounds for industrial and academic research. Modifying the polymer molecular weight distribution through crosslinking is one of the simplest methods of achieving the desired properties. The crosslinking of polymers to form gels has been used for decades in the automotive industry to produces tires. More recently, polymer crosslinking has been applied to environmental cleanup, wound healing materials, consumer products, artificial organs, self-healing coatings, and the micro-patterning of surfaces. Photocrosslinking using additives is one of the safest and most robust methods as it allows precise control of the reaction in space and time.This thesis explores the photocrosslinking of polymers and relates it to structure and mechanism: the properties and intermiscibilities of four crosslinker chemistries and five polymers are rationally designed in Chapter 3.

The influence of additive functionality on the reaction is explored in depth with a single additive chemistry in Chapter 4. The mechanism and reaction location is examined in Chapter 5, and a survey of the efficacy of additional crosslinking chemistries is performed in Chapter 6. Multiple polymers and additives are examined in Chapter 3 and their reactions and mechanisms are examined to predict efficacy and utility. It is shown that multiple reaction chemistries allow crosslinking, and that the manipulation of functionality and polymer allows the exploration of specific reaction mechanisms. The differential refractive indices of the polymers are measured by experiment, and the intermiscibility of polymer-additive systems are calculated using group contribution techniques. A hydrogen abstraction induced radical crosslinking mechanism is explored in depth in Chapter 4. Benzophenone-derived additives are used to study photocrosslinking in thin films while varying multiple parameters: the irradiation time, additive to polymer molar ratio, additive functionality, and polymer mobility.

Bi-functionality is found to increase the density of radicals in glassy and rubbery systems. The macroradical recombination and scission reactions are modeled and shown to conform to experiment. Analysis of the model results shows that the functionality of the additive is only important above a molar ratio threshold. Below the threshold combination reactions are binary and there are no macroradical bridging reactions in the bi-functional system. Above the threshold the density of radicals is so high that the combination reactions are pseudo first order and macroradical bridging causes differences in the behavior of mono- and bi-functional additive systems. The changes in the molecular weight distributions with reaction extent are tracked using size exclusion chromatography (SEC) with multiple detectors. Chapter 5 studies the hydrogen abstraction reaction of Chapter 4 using electron paramagnetic resonance (EPR) to confirm the predicted reaction location. Spin-trap experiments demonstrate that radicals primarily form on the expected tertiary carbon, confirming the hydrogen abstraction mechanism employed in Chapter 4.

Unexpected peaks in the EPR spectra point towards a potentially new reaction between benzophenone and the spin-trap. The EPR experiments are also used to verify that no other radical reactions are occurring in the systems of Chapter 4. The experimental space is expanded in Chapter 6 to other polymers and other crosslinking chemistries. Full characterization of all the potential reactions was impossible, but many polymer-additive combinations are shown to react in the predicted ways. Certain crosslinker chemistries and functionalities combine to allow the study of macromolecule combination without scission and these chemistries are suggested for further use in the experimental study of the early stages of crosslinking and gelation. This thesis finds that at low molar ratios, the additive functionality does not matter; functionality is only important at high molar ratios of additive to polymer due to high radical density and a transition to pseudo-first order kinetics. The expectations in Chapter 3 are shown to be accurate and the choice of additive chemistry and polymer allows the preferential selection of desired reactions.

Hydrogen abstraction can be forced to occur either from the pendant group or the chain backbone, leading to systems in which chain scission is not possible and combination can be exclusively selected. Non-radical based crosslinking chemistries can also be used to produce crosslinks without risk of chain scission. These findings have a wide applicability in many fields and implications for the improved design of radical-based crosslinking systems.