|Title:||Polymers as low-temperature thermoplastic splinting material|
Hong Kong Polytechnic University -- Dissertations
|Department:||Department of Applied Biology and Chemical Technology|
|Pages:||xi, 146 leaves : ill. (some col.) ; 31 cm|
|Abstract:||Polycaprolactone (PCL) is a polymer commonly used in making low temperature spliniting material. Crosslinking is a useful approach to modify the properties of PCL and to achieve splinting materials with enhanced performance. Past studies on crosslinking of PCL were however limited and mainly focused on ionizing crosslinking, theological properties and a few mechanical properties. This research is aimed to give a comprehensive and systematic study on how the extent of crosslinking affects properties of PCL. Unlike most of previous studies, a more extensive range of peroxide content, from 0 to 25 wt%, has been investigated. At the same time, this project is also intended to develop standard methods for measuring the fabrication properties of splinting materials, so that objective information can be generated in future for therapists to choose the right splinting material to suit different requirements. The structure of crosslinked PCL samples was characterized by sol-gel test and swelling measurement. It is found that the crosslink density of PCL, represented by gel content and MC, increases with peroxide content. The increase is rapid initially but slows down at high peroxide concentrations. Based on the measured gel content of crosslinked samples, scission reaction is found to occur simultaneously with crosslinking and the probability ratio of scission to crosslinking, calculated by the modified Charlesby equation, is found to be 0.1. The results of infrared spectroscopy (IR) help to understand the mechanism of crosslinking PCL. The decrease in the absorbance of (CH2)5 rocking peak suggests that the crosslinking reaction occurs at CH2 groups. The decrease in the absorbances of C-O and C=O is in accord with a proposed mechanism of scission reaction, in which CO and CO2 gases are evolved. Thermal properties, including the melting temperature, crystallization temperature, glass transition temperature, heat of fusion and crystallinity, were determined by differential scanning calorimetry, whereas thermal stability was measured by thermogravimetry. The thermal properties are strongly influenced by the degree of crosslinking through the change in crystallite size, degree of crystallinity, mobility of chains, etc. On the contrary, the thermal stability is independent of crosslink density. The mechanical properties are important properties relating to the use of splinting materials. The tensile properties, flexural properties and microhardness are all affected strongly by the degree of crystallinity, which is altered by the degree of crosslinking of the material. The methods for measuring fabrication properties, including working temperature, hardening temperature, working time, memory and hardness of polymer melt, have been developed based on DSC and TMA techniques. Working temperature and hardening temperature are found to decrease with increasing crosslink density, while working time, memory and hardness of polymer melt shows an opposite trend. Considering the decrease in mechanical properties and increase in memory and melt hardness upon crosslinking, PCL containing 0.5 to 1 wt% peroxide is recommended for the formulation of splinting materials for general use, including the preparation of delicate splints, while samples with higher than 1 wt% peroxide content are suitable for the preparation of large splints.|
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