Structure and morphology of ionic polymer networks modified with poly(ethylene glycol)
Abstract
Copolymer networks of poly(acrylic acid) (PAA) grafted with poly(ethylene glycol) (PEG) were synthesized by UV initiated free-radical solution polymerization using tetraethylene glycol dimethacrylate as a crosslinking agent. Methacrylates of poly(ethylene glycol) 2000 and 5000 were synthesized by the dicyclohexylcarbodiimide method to allow for a greater control of the molecular architecture of the copolymer networks. The fraction of ionic groups in the network was also used as a parameter to control the network structure in aqueous solutions with pH ranging from 2.0 to 7.4. A linear correlation length, ξ, of the space available for diffusion was obtained using a small deformation rubber elasticity theory. Intermediate strength polymer-polymer interactions such as hydrogen bonds which can significantly affect the polymer network structure were investigated using infrared spectroscopy and differential scanning calorimetry. Intermolecular (ethylene glycol and acrylic acid) and intramolecular (acrylic acid and acrylic acid) hydrogen bonding interactions were observed in the dry state. While the effect of graft molecular weight on the ratio of inter- and intramolecularly bonded acrylic acid units was minimal, an increase in the relative amount of ethylene glycol and acrylic acid segments in the network resulted in a profound increase in intermolecular associations. Spectroscopic evidence showed that in an acidic environment both inter- and intramolecular associations of acrylic acid groups were dominated by hydrogen bonding interactions of these groups with water. In a basic environment, the carboxylic acid groups ionized which prevented any interactions in the polymer network. Penetrant uptake studies probed the structure of the networks dynamically in low and high pH environment. In an acidic medium, the penetrant uptake followed Fickian type diffusion. In a basic medium, the networks exhibited two stage swelling dynamics controlled by polymer relaxation. A high loading efficiency of a model macromolecular protein into the networks and very fast release kinetics in an in vitro release study were observed.
Degree
Ph.D.
Advisors
Peppas, Purdue University.
Subject Area
Chemical engineering
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