The Effect of mild CO2 Treatment on Thermal Properties of Poly(L-lactic acid): An Experimental Study

The influence of mild CO2 treatment on thermal properties of poly(L-lactic acid) (PLLA) is discussed in this article. A slowly crystallizing PLLA with 4% D-isomer was treated with CO2 at room temperature and moderate pressures (up to 6.0 MPa) for short times, not longer than 20 min. These mild treatments are sufficient to develop ε-form (PLLA/CO2 complex), which evolves to α"-mesophase after desorption of CO2. The amount of mesophase depends on the pressure of CO2 and the sorption times, and significantly affects the thermal properties of PLLA. Although the mesophase melts immediately above the glass transition, it affects subsequent cold crystallization. The influence of CO2 sorption time and pressure on the cold crystallization kinetics of PLLA is quantified here, and it is found that only a few minutes of treatment with low-pressure CO2 is sufficient to enhance the crys- tallization rate. After longer exposure to CO2, α-crystals also develop, especially under high CO2 pressure, hence proper adjustment of these parameters can allow tuning the structure and thermal properties of PLLA. The results demonstrate the possibility of producing CO2-induced mesophase at room temperature using very short sorption times, thus introducing a fast, green and cost-effective way to tailor the crystallization kinetics of PLLA.