Effect of the application of low shear rates on the crystallization kinetics of PLA

Abstract

Flow-induced crystallization can effectively increase the crystallinity content in poly (lactic acid) parts, and this can be extremely significant since an increase of crystallinity for this material dramatically enhances many properties among which the heat resistance. This work analyzes nucleation and growth rate of spherulites formed during crystallization process of PLA at different temperatures and shear rates. The application of a shear flow results in an enhancement of the nucleation phenomena, with the formation of a new population of nuclei, and an improvement of the crystals growth already at low values of shear rates. Furthermore, it is possible to find a dependence of the equilibrium melting temperature upon the Weissenberg number.

Crystallization of Polylactic Acid with Organic Nucleating Agents under Quiescent Conditions

“…The crystallinity of PLA can be enhanced by employing methods such as isothermal annealing [17][18][19][20][21], polymer blending [4,[22][23][24][25][26], and strain-induced crystallization during processing [27][28][29][30][31][32][33][34][35]. Isothermal annealing at temperatures of 85-115 • C for an extended time has been reported to initiate and develop the crystalline domains for injection-molded amorphous PLA samples [21,28].…”

Thermal Stability of Shear-Induced Crystal Nuclei of Poly(l-lactic acid)

“…For initially mildly sheared PLLA (center plot), qualitatively similar results are observed, while for PLLA sheared at a rate of 2 s –1 (right plot), crystallization in the cooling scan shifts to an even higher temperature of 160 °C due to the higher efficiency (not necessarily higher number) of shish-like crystallization precursors/oriented melt-structure to allow crystal growth at higher temperature. Note that at this stage of research, a possible increase of the crystal-growth rate, as suggested in the literature, , which would also contribute to a higher crystallization temperature, is out of focus in this work. Furthermore, the data of Figure confirm the general trend that on increasing T max , the number (or nucleation efficacy, which is controlled by the structure/morphology of the nuclei) of both self-nuclei and oriented crystallization precursors decreases, however, with the latter surviving to an about 5 K higher temperature at an identical melt-annealing time of 1 min.…”

“…precursors/oriented melt-structure to allow crystal growth at higher temperature. Note that at this stage of research, a possible increase of the crystal-growth rate, as suggested in the literature, 64,65 which would also contribute to a higher crystallization temperature, is out of focus in this work. Furthermore, the data of Figure 7 confirm the general trend that on increasing T max , the number (or nucleation efficacy, which is controlled by the structure/morphology of the nuclei) of both self-nuclei and oriented crystallization precursors decreases, however, with the latter surviving to an about 5 K higher temperature at an identical melt-annealing time of 1 min.…”