Formation of Interpenetrating and Stress Relaxing Polymer Networks with Photo-initiated Cu(I) Catalyzed Azide Alkyne Cycloaddition Chemistry

Abstract

Interpenetrating polymer networks (IPNs) are a class of materials with interwoven polymers that exhibit unique blended or enhanced properties useful to a variety of applications, ranging from restorative protective materials to conductive membranes and hydrophobic adhesives. The IPN formation kinetics can play a critical role in the development of the underlying morphology and in turn the properties of the material. Simultaneous photoinitiation of copper-catalyzed azide–alkyne (CuAAC) and radical mediated methacrylate polymerization chemistries enable the manipulation of IPN microstructure and properties by controlling the kinetics of IPN formation via the intensity of the initiating light. Specifically, azide and alkyne-based polyethylene glycol monomers and tetraethylene glycol dimethacrylate (TEGDMA) were polymerized in a single pot to form IPNs, and the properties were evaluated as a function of the photoinitiating light intensity. Morphological differences as a function of intensity were observed in the IPNs as determined by thermomechanical properties and atomic force microscopy (AFM). At moderate intensities (20 mW/cm2), the TEGDMA network percolates the sample volume, or gels, first and therefore forms the underlying scaffold of the material. At low intensities (0.2 mW/cm2), the CuAAC network can gel first. The selection of light intensity is also shown to determine whether the IPN formation proceeds via simultaneous or sequential CuAAC and methacrylate polymerizations. The ability to switch sequence of gelation and IPN trajectory (simultaneous vs. sequential), affords control over phase separation behavior. Thus, light not only affords spatial and temporal control over the IPN formation but also provides control over their thermomechanical properties, representing a new route for facile IPNs design, synthesis, and application.

To elucidate the mechanism behind the switch in gelation sequence, a kinetic model was employed to analyze the reaction kinetics and auto-acceleration behavior of the photo-initiated CuAAC polymerization and the photo-initiated free radical polymerization of methacrylates. The copper (Cu) content and light intensity were varied and the resulting effects on the reaction rate constants and induction periods for the polymerizations were studied to ultimately elucidate that Cu acts in a dual role: of pushing the CuAAC polymerization forward but of also inhibiting the photo-initiation of both polymerizations. At low intensities, this inhibition is accentuated, and the gel sequence is allowed to switch from methacrylate gelling first to methacrylate gelling second. The thermomechanical consequence of this switch is a disappearance of the tan(δ) loss due to the methacrylate network, indicating that when the methacrylate network gels within a gelled CuAAC network, it interpenetrates the CuAAC network significantly more. Ultimately, the amount of Cu in the system determined the light intensity at which the switch in gelation and the switch in thermomechanical behavior would occur.

Building upon the findings of the previous two studies, the tunability of reaction kinetics, phase separation, and property of the photo-CuAAC-methacrylate IPN (PCM IPN) system in the presence of compatibilizing interactions, such as hydrogen bonding and ionic bonding, was evaluated. Hydroxyl containing azide monomers and ionic, imidazolium-based alkyne monomers were utilized to probe how the range of interpenetration afforded by modulation in light intensity would be affected. The hydrogen bonding monomer exhibited a greater degree of interpenetration than the non-hydrogen bonding monomer. The addition of ionic, di-functional alkyne monomer reduced crosslink density and rubbery modulus without affecting interpenetration. Varying intensity modulated polymerization induction periods and rates, allowing for a change in gelation sequence at sufficiently low intensities. Irradiation schemes were varied to pause or slow down TEGDMA polymerization. In doing so, IPN formation trajectories were achieved, in which the induction period could be altered without affecting polymerization rate and vice versa. Increasing the induction period of TEGDMA increased its interpenetration into the CuAAC network, but not to the same extent as achieved by both, increasing induction period of TEGDMA and reducing the rate at which it is polymerized at. Ultimately, the use of compatibilizing monomers increases the range of interpenetration achievable by variation in light intensity and the interpenetration is not only dependent on gelation sequence but also on the rate at which polymerization occurs. The final study presented in this thesis was the principal study in the pursuit of making photopolymerizable IPNs that can be reprocessed by bond exchange reactions. A photo-CuAAC resin was designed with hydroxyl and ester containing monomers, and it was observed that the resulting network exhibited stress relaxation at elevated temperatures (T>150 °C), most likely by transesterification and ester-ester exchange reactions. Leaching copper (Cu) out of the networks improved stress relaxation performance, indicated internal catalysis driving the likely bond exchange reactions. Further studies are required to elucidate the form of internal catalysis taking place.