Free-radical polymerisation (FRP) has been one of the most important techniques for producing materials used in a very wide variety of applications and has enhanced the lives of millions of people around the world. However, for many years a number of fundamental questions regarding the key kinetic processes involved in FRP have remained unresolved. In particular, an accurate description of the mechanism for diffusion-controlled bimolecular termination has proven elusive. As a result, conventional modelling tools for FRP have often proven unreliable. The aim of this thesis, therefore, was to accurately study the evolution of the bimolecular termination rate coefficient during free radical polymerisation using a new and more accurate methodology based on ‘controlled/living’ reversible addition-fragmentation chain transfer (RAFT) polymerisation. This was undertaken in order to develop a more precise understanding of bimolecular termination and thereby develop a more reliable modeling approach capable of predicting the rates of reaction and evolution of molecular weight distributions for a wide range of experimental conditions and a wide range of functional monomers. The RAFT-CLD-T (RAFT Chain-Length-Dependent Termination) Method was used to determine accurate values for the conversion and chain-length-dependent termination rate coefficient, kti,i(x), as a function of various parameters. These parameters included the chain size, i, polymer concentration (or conversion, x), chain length size distribution and chain architecture/structure. The accuracy of the RAFT-CLD-T Method was crucial to this work, therefore, an important part of this thesis was devoted to evaluating the reliability of this technique. Below 5 % conversion and above 80 % conversion the method was found to be unreliable due to the effects of chain-length-dependent propagation, high PDI’s and short-long termination. However, between 5 % and 80 % conversion it was found that the method is extremely robust and a series of easy-to-use experimental guidelines were determined for accurately applying the RAFT-CLD-T Method. The effects of chain size, chain size distribution, solution polymer concentration, and matrix architecture were examined for the RAFT-mediated polymerisations of methyl methacrylate (MMA), styrene (STY) and methyl acrylate (MA). It was found that four distinct scaling regimes of termination are observed: (1) a ‘short’ chain dilute solution regime, (2) a ‘long’ chain dilute solution regime, (3) a semi-dilute solution regime and (4) a concentrated solution regime. In dilute polymer solutions, chain-length-dependent power law exponents, ’s, determined during the polymerisation of MMA, STY and MA (where kti,i(x) i-) indicated that termination follows two major scaling regimes with exponents of approximately ~0.5 to 0.6 for ‘short’ chains and and ~0.12 to 0.16 for ‘long’ chains. Importantly, these exponents are in excellent agreement with theoretical predictions for translational and segmental diffusion-controlled termination, respectively. At increasing polymer concentrations, kti,i(x) falls rapidly coinciding with the onset of the gel effect. By comparing results from the RAFT-mediated polymerisations of MMA, STY, MA, and vinyl acetate (VAc) with theoretical models, we found that the onset of the gel effect coincided closely with the theoretical onset of chain overlap. Considerable uncertainty has plagued the evaluation of this phenomenon, but using a difunctional RAFT agent we showed this uncertainty arises from the influence of broad MWD’s on chain overlap and short-long termination. Finally, critical tests of this theory involving the bimolecular termination of linear radicals in solutions of star polymer confirmed that the gel effect coincided with chain overlap. Beyond the gel effect termination slows enormously, passing through the ‘semi-dilute solution’ regime and into the ‘concentrated solution’. In semi-dilute solution, theoretical predictions based on scaling theory (i.e. the ‘blob’ model) were in excellent agreement with results for the polymerisation of PSTY in linear and star polymer solutions, indicating that the solvent quality diminished both with increasing chain length and through the addition of a star polymer matrix. In concentrated solutions, the chain-length-dependent power law exponent increased linearly with conversion. For example, for MMA the chain length dependence of kt in the gel regime scaled as gel = 1.8x + 0.056, suggesting that reptation alone does not describe termination in the concentrated solution. Values of gel for PSTY, MA, and VAc were in similar agreement, indicating that a mechanism intermediate between unentangled and entangled semi-dilute scaling laws applies in the concentrated solution regime. Interestingly, gel values for these monomers were found to decrease with increasing chain flexibility in the order gel(MMA)> gel(STY)> gel(VAc)> gel(MA), suggesting matrix mobility is rate determining in concentrated solutions. Similarly, gel values were also larger in star polymer solutions, coinciding with decreasing matrix mobility. Thus, although it has been commonly believed that polymer chains diffuse via reptation above the gel effect, these results show that this only occurs for solutions containing rigid and/or highly immobile macromolecules and in very high concentrations. To describe these behaviours, a semi-empirical ‘composite kt model’ was also developed to describe kti,i(x) as a function of i and x up to high conversions. We showed that the model is very simple to implement and accurate for modelling a wide range of functional monomers and experimental conditions. In particular, we showed the method was accurate for modelling RAFT-mediated polymerisations of a very wide range of monomers (MA, MMA, and PSTY) and was even accurate for modelling conventional FRP’s. Thus, the model provides a simple, flexible and accurate method for predicting the rate of reaction and evolution of molecular weight distributions across a range of experimental conditions based on accurate kti,i(x) values.
| Identifer | oai:union.ndltd.org:ADTP/290355 |
| Creators | Geoffrey Johnston-hall |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
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