Microwave processing of bio-degradable polymers : ring opening polymerisation of ε-caprolactone

• Main Author: Kamaruddin, Mohd Johari
• Published: University of Nottingham 2012
• Subjects: 547.28
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.576490

Microwave-assisted ring opening polymerisation (ROP) of ε-caprolactone (ε-cap) has been investigated to quantify the influence of microwave heating on polymerisation reactions in order to provide a basis for the design of a continuous process (potential for scale up). This thesis presents a detailed examination of the dielectric property behaviour and a kinetic study of Rap of ε- cap catalysed by a stannous octanoate/benzyl alcohol system, in both conventionally and microwave heated systems. The data from the latter section was then used to define the factors which resulted in the empirically observed cycle time reductions. A dielectric property study was performed across a wide range of frequencies and temperatures on the precursors (reactants), the polymer and model polymerisation mixtures (i.e. specifically constructed polymer in monomer solutions containing varying quantities of poly(ε-caprolectone) (PCL) with different target degrees of polymerisation) in order to relate quantitatively their dielectric properties to microwave heating and polymerisation mechanisms. An analysis of the results concluded that heating mechanism of the polymerisation mixtures in a microwave field was dominated by the dipole reorientation mechanism that controlled by the dielectric properties of monomer, where the monomer was the major component (>90 % volume/volume) as well as the component with highest dielectric loss and dissipation factor. The values of the dielectric properties of the polymerisation mixtures have been shown to decrease with both temperature and monomer conversion to polymer. However, importantly it was observed that the dielectric constant was independent of the resultant number average molecular weight and dispersity of the polymeric material in the mixture at a specific microwave frequency. Consequently, the penetration depth value of polymerisation mixtures at a specific frequency will increase with the progress of the polymerisation reaction. The penetration depth of mixtures at 2.45 GHz was noted to increase from ~0.58 cm (20 QC, 0 % conversion) to ~8.4 cm (150 QC 100 % conversion). However, it is clear from this small penetration depth limits that there is little potential to achieve the successful scale up of a microwave-assisted ROP of ε-cap in batch mode at 2.45 GHz. This is because this will require the use of a reactor which is larger than the two penetration depth figures derived above. As a result, this will lead to inhomogeneous bulk temperature distribution within the polymerisation mixture and irreproducible chemistry. However, a fast heating rate based on a high value of dissipation factor and dielectric loss of the polymerisation mixtures shows potential to enable the Rap to be completed in a few seconds that may allow the polymerisation to be transferred to a continuous flow process. In so doing, small diameter tubular reactors can be employed hence removing this penetration depth issue. In addition, based on the fact that number average molecular weight is independent of the dielectric constant, the direct correlation between dielectric constant and the progress of the polymerisation (i.e. induction period, conversion and polymerisation end-point) were quantified in this study, so that the dielectric constant can be used as a parameter to monitor the progress of ROP of ε-cap in real time. A kinetics study on ROP of ε-cap system has been carried out in both microwave and conventionally heated conditions. Both heating methods have shown comparable pseudo living control characteristics for ROP progress below 80 % conversion. Conventionally, it was shown that the polymerisation reaction time to reach 80% conversion was a combination of (a) an induction period in which the true catalytic species in formed in-situ and (b) the actual polymerisation (or propagation) time. This study has shown that, at the temperatures and power input levels used, the ROP of ε-cap using microwaves has shown reduced cycle time compared to that produced by conventional thermal method. Furthermore, it has been demonstrated that this enhancement is as a result of the reducing/eliminating of induction time in the initiation step and that this reduction is due to a faster heating rate and a higher reaction temperature in the microwave experiments. The faster heating rate and higher temperature achieved in the microwave-assisted ROP was contributed to by high values of dielectric loss and dissipation factor of the polymerisation mixture. More importantly, microwave-assisted ROP of ε-cap has been shown to offer the potential for such processes to be scaled to a continuous process based on the ability of process to be completed in a few seconds. In addition, the importance of process variables such as temperature, catalyst concentration and microwave power level that contribute to enhance the reaction rate and reduce the processing time were quantified in this study. Due to these interactions it is concluded that there is a requirement for dynamic impedance matching and continuous on-line process control and monitoring in order to control and monitor the polymerisation temperature, microwave power, progress of the polymerisation and quality of polymers produced.