Date of Award
Doctor of Philosophy (PhD)
The kinetics of free-radical polymerization with and without crosslinking have been studied both theoretically and experimentally using methyl methacrylate (MMA) and ethylene glycol dimethacrylate (EGDMA) as a model system.
The mechanisms involved in the auto-acceleration of polymerization rate due to both chain physical entanglement and/or chemical crosslinking of polymerchains have been further elucidated using on-line electron spin resonance techniques (ESR). It was found that the observed radical trapping effect is associated with the conversion rate auto-acceleration phenomenon and that there exist two radical populations during the auto-acceleration period. The polymerization system is therefore heterogeneous with respect to the radical environment. The trapped radical fraction has been estimated by both post-effect measurement of radical concentration and deconvolution of the ESR signal. Crosslinking has been found to significantly enhance auto-acceleration and radical trapping. The radical concentrations found using large concentrations of crosslinker reach levels as high as 10⁻³ molar. The stability of the radicals trapped on the polymer network has been studied at temperatures up to 180ºC to assess the potential of the so-called reactive gels for various applications.
Radical concentrations versus time were measured by ESR for the copolymerization of MMA/EGDMA over the entire ranges of monomer composition and conversion. Together with conversion measurements, this provides a set of comprehensive kinetic data suitable for the estimation of individual rate constants for propagation, termination and also the initiation efficiency. It was found that these kinetic rate constants and the initiation efficiency fall simultaneously and dramatically when the polymerization system approaches its glass state. The free-radical gelation of MMA/EGDMA system under adiabatic conditions has also been investigated. These kinetic data permits severe testing of existing kinetic gelation models and suggest the need for further effort to develop more realistic models. For the theoretical work done in this thesis. the effort was directed to the development of more realistic kinetic models. Progress has been made in the following aspects: (1) A general method was developed to acount for chain-length dependent bimolecular terminatioll ofpolymer radicals for linear chain systems. With this method, there is no need to propose the full form of the bivariate distribution function which accounts for the dependence ofthe termination constanton the chain lengthsofterminatingpolymerradicals. (2) The Tobita-Hamielec kinetic model for free-radical polymerization with crosslinking was re-examined. The governing moment equations using the pseudo-kinetic rate constant method has been derived to include gel population and extended for asymmetric divinyl monomers.Anewsemi-statistical gelationmodel wasderived. Thismodel clearly illustrates the influenceofcrosslinkdensity distributiononotheraspectsofpolymernetworkformation. Problems in post-gelation modelling were addressed and discussed in terms of further development to provide a more realistic kinetic model. (3) A comprehensive kinetic model for the chemical modification of prepolymer in reactive processing using free-radical initiator has been derived. This model equationswhen solved give the full molecularweight distribution at different degrees of chain scission, long chain branching, crosslinking and grafting (processing time, type and level of initiator and additive). A general scheme for kinetic parameter estimation bas been also suggested.
Zhu, Shiping, "Advances in Free-Radical Polymerization Kinetics" (1991). Open Access Dissertations and Theses. Paper 3582.