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Melting Point Depression in Biodegradable Polyesters Melting Point Depression in Biodegradable Polyesters by Shona Murphy A thesis submitted to The University of Birmingham for the degree of MASTER OF RESEARCH School of Engineering Metallurgy and Materials The University of Birmingham University of Birmingham Research Archive e-theses repository This unpublished thesis/dissertation is copyright of the author and/or third parties. The intellectual property rights of the author or third parties in respect of this work are as defined by The Copyright Designs and Patents Act 1988 or as modified by any successor legislation. Any use made of information contained in this thesis/dissertation must be in accordance with that legislation and must be properly acknowledged. Further distribution or reproduction in any format is prohibited without the permission of the copyright holder. Synopsis Investigation into the crystallisation kinetics and melting point depression of poly(L-lactide- co-meso-lactide) with approximately 3.3% D-lactide content (PLA 3051D) was undertaken. The rate of crystallisation was too slow for a crystallisation exotherm to be detected by the DSC, therefore hot-stage microscopy was used as an alternative method to characterise the crystallisation behaviour. Light intensity with time during isothermal crystallisation of a thin polymer film (<15µm) was measured. The results were normalised in order to calculate crystallisation half-life (t0.5). From half-life calculations, the optimal crystallisation temperature was found to be 118°C. By replacing the light dependant resistor with a digital camera, the diameter growths of individual spherulites could be measured minute-by-minute. Using the results obtained through hot-stage microscopy, re-processing of PLA could be carried out to restore the original crystallinity at Tmax 118°C. Plots showing % crystallinity with storage time at 118°C indicated a storage time of 2 hours was required to restore the crystallinity to 40%. Studies using the high pressure DSC found that with increasing CO2 pressure up to 60 bar, the melting point decreased from 152 to 142°C . An alternative biodegradable polymer, polycaprolactone CAPA 6800 was investigated with high pressure DSC as a comparison polymer to polylactic acid. Similarly to PLA, the melting point of PCL showed a significant reduction (55 to 40°C) when soaked with increasing pressures of CO2. Acknowledgments I would like to express my sincere gratitude to all who aided me during the time of my MRes. In particular, I would like to thank Dr Mike Jenkins and Dr Gary Leeke for their guidance and support. I would also like to give my appreciation to Frank Biddlestone who has provided me with invaluable technical assistance. Finally, I would like to thank my family, friends and colleagues for their continuous support and encouragement over the past year. Contents 1.0 Introduction .................................................................... 1 2.0 Literature Review ............................................................ 2 2.1 Polymer Morphology........................................................................................ 2 2.2 Polylactic acid (PLA)........................................................................................ 4 2.2.1 PLA meso- copolymers .........................................................................................7 2.2.2 PLA synthesis ........................................................................................................8 2.3 Polycaprolactone (PCL).................................................................................. 11 2.4 Polymer Degradation ..................................................................................... 13 2.5 Supercritical Processing.................................................................................. 16 3.0 Projects Aims .................................................................20 4.0 Experimental.................................................................21 4.1 Materials......................................................................................................... 21 4.1.1 Polylactic Acid (PLA 3051D) ..............................................................................21 4.1.2 Polycaprolactone (PCL CAPA 6800)..................................................................22 4.2 Experimental Methods and Techniques ........................................................ 24 4.2.1 Sample Preparation ............................................................................................24 4.2.2 Differential Scanning Calorimetry (DSC)...........................................................25 4.2.3 High Pressure DSC.............................................................................................30 4.2.4 Hot-stage Microscopy .........................................................................................32 5.0 Results and Discussion ..................................................34 5.1 Initial Characterisation of PLA3051D ........................................................... 34 5.1.1 Characterisation Using Differential Scanning Calorimetry (DSC) ....................34 5.1.2 Characterisation Using Hot-Stage Microscopy..................................................35 5.1.3 Re-crystallisation of PLA3051D .........................................................................42 5.2 Melting Point Depression in PLA 3051D by CO2 ......................................... 45 5.2.1 Establishing The Optimal Experimental Conditions .........................................45 5.2.2 Measurement of Melting Point Depression in PLA............................................49 5.3 PCL CAPA6800 ............................................................................................. 51 5.3.1 Initial Characterisation of PCL...........................................................................51 5.3.2 Characterisation of The Melting Point Depression Using High-Pressure DSC.51 6.0 Conclusions and Future Work...........................................54 7.0 References .....................................................................56 Appendix 1…………………..……………………………………………….. Hot-Stage Microscopy Images………………………………………………………………… Appendix 2…………………………………………………………………….. DSC melting traces of re-conditioned samples………………………………………………... Appendix 3…………………………………………………………………….. Jenkins, M.J.; and Murphy, S.H. A Comparison of the use of FTIR spectroscopy with DSC in the characterisation of melting and crystallisation in polycaprolactone. Thermochimica Acta (submitted) 1.0 Introduction Currently within the EU, approximately 55 million tonnes of polymers are produced which contributes 24% of the global total (230 million tonnes in 2009). Polyethylene (PE-LD, PE- HD, PE-LLD) and polypropylene (PP) account for around 50% of polymer demand with polyvinyl chloride (PVC) being the third largest polymer at 11%. These polymers would typically be used in applications such as packaging, which is the largest end use market segment with a 40.1% share. This is followed by building and construction (20.4%) and Automotive (7.0%) [1]. However, these polymers are difficult to recycle cost-effectively, therefore, the majority are sent to landfill. Those that are not recycled pose a harmful threat to the environment [2], influencing major food outlets and retailers to pursue biodegradable polymer packaging from renewable sources which have a much lower environmental impact [3]. These products can be composted or if left as litter, will degrade. Commercially available biodegradable polymers from renewable sources are typically either polyesters or polysaccharides [4]. Aliphatic polyesters are the more commonly known and studied biodegradable polymers which include Polylactic Acid (PLA), Polycaprolactone (PCL), Polyhydroxyalkanoates (PHAs) and Polyglycolic Acid (PGA). Of the polymers derived from renewable sources, only PLA is produced in sufficient quantities to be a commercially viable solution. PLA is currently extruded where the products are thermoformed into cups, containers or trays to replace polystyrene and PET. However, PLA possesses an inherent thermal degradation during extrusion as it quickly loses its thermal stability when heated above its melting point, this significantly reduces its processing window compared to other polymers. Consequently, manufacturing products from PLA becomes difficult, therefore screw extruders have been modified in order to reduce PLA’s thermal degradation during 1 extrusion. A solution to the limitations of extruding PLA would be to use a supercritical fluid as a solvent. Supercritical carbon dioxide (scCO2) is the preferred choice for these applications, due to its properties of being non-toxic, non-flammable, chemically inert, environmentally safe and inexpensive. Its solubility in many polymers is substantial, being influenced by temperature, pressure and, sometimes, weak interactions with the constituent groups in the polymer. Dissolved CO2 causes a reduction in the viscosity by plasticisation, allowing processing at lower temperatures [5]. For these reasons, melting point depression in biodegradable polyesters using CO2 will be investigated. 2.0 Literature Review 2.1 Polymer Morphology Polymers, also known as macromolecules, consist of significant numbers of ordered monomer repeat units that are covalently bound. These polymers exist as either amorphous or semi-crystalline. Polymers that contain ordered crystalline regions and unordered amorphous regions are known as being semi-crystalline.
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