Structure of hydrophilic polymer networks formed by copolymerization/crosslinking reactions
Abstract
Free radical copolymerization/crosslinking reactions of hydrophilic methacrylates with small amounts of the dimethacrylate crosslinking agents were investigated. The products of these reactions are three dimensional network "hydrogels" which have a wide range of applications, including controlled release, and environmentally sensitive materials. The evolution of polymer network structure was investigated theoretically and experimentally. A statistical model of copolymerization/crosslinking reactions was developed to provide information about the formation of network structure. The important chemical reaction features such as intramolecular cyclization, and termination by multiple mechanisms were incorporated into the statistical description. Expressions for structural averages including molecular weights, the critical conversion for gelation, the number of elastically active chains, and the molecular weight between crosslinks were obtained using a generating function technique. Experimental profiles of the reaction rate as a function of time were obtained using differential scanning calorimetry. The magnitude of the gel effect was found to depend on the crosslinker concentration, dilution, and pendent chain length. More detailed experimental analysis was performed using $\sp{13}$C NMR. Well resolved $\sp{13}$C NMR spectra of the reaction system were obtained both before and after the gel point. An experimental procedure using $\sp{13}$C labelled initiator and crosslinking agent was developed to investigate the post-gel reaction regime. The $\sp{13}$C labelling of the reactants present in small amounts enhanced their NMR peaks allowing reliable integration to be performed. The $\sp{13}$C NMR studies indicated that the primary polymer chain length increased during the course of the reaction. The formation of environmentally sensitive hydrogels by incorporating two complementary complexing species in the same network was investigated theoretically. A statistical description of the complexation of free and graft oligomers with complementary polymers was developed. The conformation of a complexed oligomer was described by sequences of repeating units in trains, tails, and loops. Simulation results indicate that the conformational averages for the ungrafted case asymptotically approach those for the grafted case as the segmental binding free energy, polymer concentration, and oligomer chain length increase.
Degree
Ph.D.
Advisors
Peppas, Purdue University.
Subject Area
Chemical engineering
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