A fundamental study of the complex structure-property-processing relationships in interpenetrating polymer networks (IPNs)

Date
2007-01-01
Authors
Nowers, Joseph
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Balaji Narasimhan
Committee Member
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Chemical and Biological Engineering
Abstract

Experimental studies were conducted to qualitatively define the relationships between dilution, temperature, and reaction sequence on the polymerization kinetics of neat monomers, diluted monomers and during interpenetrating polymer network (IPN) formation. The system studied was a thermally initiated cationic polymerization of a difunctional epoxy and the photoinitiated free radical polymerization of a difunctional acrylate. Both reactions are autoaccelerating and quickly become diffusion controlled. The effects of increasing temperature and dilution on the acrylate polymerization rate profiles are similar, leading to reduced polymerization rate and longer polymerization times. The dilution effect on the epoxy polymerization is similar to that of the acrylate. However, unlike the acrylate reaction the epoxy polymerization rate increases strongly with temperature. The pre-existence of one polymer has a significant effect on the polymerization of the second monomer. This effect is larger for the acrylate then for the epoxy polymerization. New kinetic models are needed to capture these complex behaviors.;Samples of the same model system were prepared over the range of compositions and by varying the reaction sequence for physical property and morphology studies. The materials were evaluated by attenuated total reflectance Fourier transform infrared spectroscopy, photo differential scanning calorimetry and modulated differential scanning calorimetry for conversion. Initial and final sample glass transition temperature was estimated from modulated differential scanning calorimetry. Mechanical testing and rheology tests revealed information on the strength and hardness of the materials. Morphology and phase separation was explored via optical microscopy and scanning electron microscopy. As expected, all of the physical properties were dependant on composition. Some of the material properties and the morphology were also dependent on reaction sequence. Differences in glass transition temperatures as high as 75°C were observed at the same composition but formed by different reaction sequence. Correlations can be made between the morphology and material properties with partially phase separated samples exhibiting maximum damping. The experiments indicate that the relationships between phase morphology and physical properties of IPNs are complex and not readily predictable a priori.;Combinatorial methods and informatics were applied to the study of complex property-structure-processing relationships during IPN formation in this model epoxy-acrylate system. PCA of a dataset covering different compositions and process sequences successfully identifies the most unique samples as well as relationships between material properties. The relationships between material properties can be exploited in future investigations by allowing high throughput screening and as a guide for engineering materials. The use of combinatorial methods, high throughput screening, and informatics will lead to accelerated material design.;A new methodology for determining kinetic parameters from thermal analysis has been proposed. The new methodology has the advantages of being very computationally efficient, allows the use of physically meaningful reaction orders, and retains the mathematics of the rate equation. This new methodology is applied successfully to polymerizations of two different chemistries with results that are consistent with literature values.;The kinetics of an epoxy-acrylate simultaneous IPN was studied as a function of dilution, temperature, and reaction sequence. Reaction orders were estimated for the homopolymerizations using a new methodology and were assumed to be constant for the diluted systems and IPN formation. To account for the difference in the reaction rate profile observed during IPN formation, the kinetic rate equation was modified with a diffusion factor, based on both polymer and monomer diffusion. Polymer diffusion is based on point source diffusion into an infinite volume and the monomer diffusion is based on diffusion into a sphere. The best set of kinetic and mass transfer parameters were determined by modeling heat flux during concurrent IPN polymerization and comparing with the observed heat flux. It was found that the epoxy polymerization is largely unaffected by the presence of the other system. In contrast, prior network formation severely diminishes the acrylate reaction. This approach provides a new framework to study diffusion-limited polymerizations during IPN formation.;A roadmap is outlined for developing kinetic models that account for the reacting environment during IPN formation. A qualitative and quantitative framework is defined to engineering IPNs.

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