SIMULTANEOUS BIAXIAL STRETCHING OF A BIODEGRADABLE POLYMER

Abstract

Biaxial drawing is an important industrial process for producing thin films with improved properties. Generally, biaxially drawn films have higher tensile strengths and moduli compared to the undrawn films due to the induced orientation of polymer fibrils by the drawing process. In this work, biaxial drawing of a biodegradable polymer - Poly (Œ-caprolactone) (PCL) - was successfully carried out. Biodegradable polymers are gaining more and more research importance in the fields of biomedical science. The study investigated the changes in the properties of PCL after simultaneous biaxial drawing. PCL was preferentially selected over other biodegradable polymers, because of its elastomeric property and ease of processing. Films made from solution-casting and melt pressing, were drawn to a draw ratio of 3 x 3, using a biaxial stretcher that was assembled in-house. A fibrillar morphology predominated in the biaxially drawn samples although there were still presence of undrawn material. This indicates that there is a potential for a higher draw ratio that would allow the development of a complete fibrillar network. The biaxially drawn films from solution-cast samples have a looser network with thicker fibrils compared to melt-pressed films. However, in both cases, these fibrils improved the tensile strengths by 50-160 %. The solution-cast films were more permeable than melt-pressed films. This was observed in both undrawn and biaxially drawn films. Infrared dichroic studies showed a significant amount of fibrils orientation in biaxially drawn films. The results also indicated a well balanced orientation of fibrils in all directions. No difference in orientation was observed between the two heat pressing temperatures. Gas permeability experiments revealed that gas transport mechanism in solution-cast films (undrawn and drawn) was mainly Knudsen flow, indicating diffusion through voids and drawing defects. However, melt pressing eliminated most of the voids in the PCL films and reduced the gas permeability. Oxygen was also found to permeate 3 times faster than nitrogen in the undrawn films. Biaxial drawing increased the gas permeability slightly but selectivity remained fairly constant. The predominant transport mechanism in melt-pressed films was solution-diffusion.