Determination of Spherulite Growth Rates of Poly(L-Lactide) Using Combined Isothermal and Non-Isothermal Procedures

A combination of isothermal and non-isothermal methods was used to measure spherulite growth rates (G) of poly(l-lactic acid) (PLLA). G was determined during cooling at a constant rate by monitoring the growth of a spherulite radius as a function of temperature. Coupling with a self-nucleation procedure permitted to highly enlarge the temperature range where G data can be obtained within a single experiment. Very good agreement existed between data gained with the proposed procedure and those obtained with the traditional isothermal method. Further enlargement of the temperature range was provided by using tailored temperature programs. After the first growing crystals had appeared, the samples were quenched in liquid nitrogen to stop crystal growth, then a conventional isothermal measurement was performed. Again the thermal treatment imposed on the sample did not affect the measured value of G. The use of combined isothermal and non-isothermal methods coupled with self-nucleation permitted to obtain G data of PLLA in a very wide temperature range. Spherulite growth rates of PLLA were analyzed with the Hoffman and Lauritzen theory, which allowed to detect a regime II–III transition at 120°C.