Flow-induced crystallization of a Poly(Lactic acid): Effect of the application of low shear rates on the polymorphous crystallization

The study of the crystallization process of biodegradable polymers is crucial to predict the final properties of the industrial plastic parts. In this work, the effect of the application of low shear rates on the polymorphous crystallization of the Poly (lactic acid), PLA, has been studied. Results in terms of nucleation and growth rate of crystalline structures in the spherulitic regime revealed a non-monotonous dependence of the growth rate upon the shear rate. DSC and WAXD analyses on samples crystallized at 120 °C allowed to assess that in quiescent conditions the more ordered α phase prevails, but even at low shear rates a predominance of the α′ phase can be found. By increasing the shear rate, the amount of the more ordered α phase increases again. The transition from α′ to α seems to inhibit the spherulitic growth rate, that reaches a relative minimum at γ˙ = 0.2 s−1. Similar observations hold true also at other temperatures in the range 110–130 °C. A further increase in the shear rate leads to a large prevalence of the α-phase, as in the case of quiescent conditions.