Impact of foaming air on melting and crystallization behaviors of microporous PLA scaffolds

Poly(lactic acid) (PLA) is a green synthetic polymer which has many excellent properties useful for various applications. In this study, PLA scaffolds were fabricated at 2.0-6.0 MPa saturation pressures by using a solvent-free solid-state air gas foaming technique. Differential scanning calorimetry analysis was used to investigate the melting behavior and the mechanism of isothermal crystallization kinetics of these PLA scaffolds. Kinetics theories, such as Avrami analysis which was established for crystal growth studies of synthetic polymers, are for the first time utilized to investigate the air gas foamed scaffolds. Results showed that 6.0 MPa scaffolds had a 3D spherulitic crystal growth kinetics which is different from the raw PLA and 3.0 MPa foams. The experimental results also proved that two types of crystals: defective α′ and stable α coexisted in the PLA foams, and the contents of these two crystals were varied at different isothermal crystallization temperatures. Compared with the raw PLA, the crystallinities of PLA foams increased slightly after isothermal crystallization. However, the air gas molecules also hindered the crystallization rates of PLA foams. In addition, single crystals or perfect large crystals with α-form can be produced at a high isothermal crystallization temperature, such as 110 °C.