Polylactic acid (PLA) is a biodegradable semi-crystalline thermoplastic polymer that can be used in many applications such as tissue engineering scaffolds and packaging. The crystallinity of PLA is an important factor that affects its process-ability, mechanical strength, and biodegradability. The solid-state foaming of semi-crystalline PLA has been a subject of recent investigations. In this paper, crystallization through out the solid state foaming process was studied. It was found that the crystallization reaches the equilibrium once the gas sorption reaches the equilibrium. There are two main factors that will affect the PLA crystallization: gas sorption during the saturation stage and the heating and stretching during the foaming stage. Within the range of 2 to 5 MPa saturation pressures and 60 to 100 °C foaming temperatures, a maximum crystallinity of approx. 25% was observed in the foamed PLA. Effects of stretching and foaming temperature on crystallinity of foamed specimens were also investigated.

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