Graduation Year

2021

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Mechanical Engineering

Major Professor

Nathan Gallant, Ph.D.

Co-Major Professor

Ryan Toomey, Ph.D.

Committee Member

David Murphy, Ph.D.

Committee Member

David Rogers, Ph.D.

Committee Member

Alex Volinsky, Ph.D.

Keywords

Controlled-release, Network, Polymer, Ring Opening Polymerization, Synthesis, Thiol-ene

Abstract

Polymeric molecular structure consists of repeating units bonded together. Mechanicalproperties can be altered without affecting chemical makeup by altering the number of these units. Small molecules can be introduced and/or polymers can be modified to form bonds between molecular chains. Cross-linking, as this is called, also introduces mechanical variation with minimal effects on chemical composition. Lastly, polymer chains reorient themselves in response to intermolecular forces. This temperature dependent response is known as crystallization. Although chemistry is unaltered, mechanical properties can depend highly on the percent of the sample that is crystallized.

Cross-linking is known to enhance the mechanical properties of amorphous materials, but its impact on crystallization and degradation is less understood. To investigate this, a novel, photocross-linkable form of poly(lactic acid) known as ɑ,ɷ-ene functionalized poly(lactic acid) was synthesized using organic catalysts. Direct polymerization of vinyl end caps resulted in disordered network formation. Ordered networks of different functionalities were also prepared using near click thiol-ene chemistry.

Swelling was found to be proportional to the cross-link density independent of network topology as predicted by the Flory-Rehner equation. Higher functionality networks, however, were more effective at disrupting the formation of crystalline regions. The formation of spherulites restricted by cross-links reduced melt temperatures similar to secondary crystal structures in disordered networks. Melt temperatures of the higher functionality networks at the same cross-link density were further reduced. Though network topology has little effect on swelling, the location and type of cross-links can lead to variation in crystallinity and melt temperature.

Controlled degradation of bio- and eco-safe poly(lactic acid) have the potential to revolutionize non-degradable single use plastics and drug delivery. Crystallization, a wellestablished parameter for controlled degradation, is more effectively exploited in cross-linked samples. Cross-linked samples resist changes to crystallization better than their uncross-linked counterparts. Here, studies show, that the variable degradation rates resulting from increased crystallization in uncross-linked samples can be remedied by the use of cross-links. Further, the networks prepared in this work are made from a modular, photo-cross-linkable form of PLA. Various molecular weights can be cross-linked together to provide another mode of degradative control.

Beyond the controlled degradation demonstrated in this work, the PLA oligomers developed provide a significant advantage over other controlled release polymer systems. Currently, drug incorporation requires the development of new chemistries or the use of lengthy complicated procedures. ɑ,ɷ-ene functionalized poly(lactic acid), however, can be dissolved in chloroform and vortexed with water based drug solutions. Exposure to UV light crosslinks the polymers locking the medicines into the matrix. Additional tests for homogeneity, controlled release, and medical efficacy are still required before successful implementation.

The stabilized crystallinity and the ease of drug “mixing” are two advantages of ɑ,ɷ-ene functionalized poly(lactic acid). Other than that, it is quite difficult to compare the degradation of one controlled release system to another. Though excellent work has been done showing the effects of both polymeric and environmental conditions on degradation, the lack of a standard control makes quantitative comparison from experiment to experiment difficult. For example, molecular weight, chemical additives, crystallinity, and temperature have all been shown to alter degradation characteristics effectively. However, the control in each of these experiments were different. Control samples between experiments have different dimensions, crystallinity, and molecular weights. Even experiments emphasizing in vivo use, use different environmental conditions (some use water, some use simulated body fluid, and others simply set pH). Each work on its own demonstrates an important avenue of degradative control, but without a standard reference, only qualitative conclusions between experiments can be made.

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