Rheological properties of crystallizing polylactide: Detection of induction time and modeling the evolving structure and properties

Abstract

Linear viscoelastic properties of polymer melts are highly sensitive to any structural changes, including molecular weight changes and the formation and growth of crystallites. Here, we make use of this sensitivity to study the homogeneous crystallization of polylactide. As this polymer is rather quickly susceptible to thermal degradation even at moderate temperatures, it is essentially impossible to study homogeneous crystallization in the absence of degradation. Thus, the evolution of complex viscosity of a polylactide due to thermal degradation in the absence of crystallization was studied. A simple empirical model is used to characterize the variation of complex viscosity due to thermal degradation and to determine the induction time of homogeneous crystallization at wide range of degrees of supercooling. Next, the evolution of complex viscosity due to crystallization was measured at several temperatures. Based on the results, a phenomenological model describing the viscosity evolution during homogeneous crystallization is proposed and validated. Finally, the linear viscoselastic data in the early stages of crystallization are shown to be consistent with gelation due to the formation of a network of tie molecules between spherulites. V C 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 812-822, 2010

A New Approach to Supertough Poly(lactic acid): A High Temperature Reactive Blending

“…PLA, being a slow crystallizing polymer having a relatively long induction time of crystallization and a low spherulitic growth rate, does not experience noticeable crystallization during cooling from the melt at a 10 °C min -1 cooling rate, unless a nucleating agent or interface having sufficient nucleating activity is present. [39] For the binary system under investigation, the PLA/EBA-GMA interface can play the role of nucleator facilitating crystallization during cooling ( Figure 5). The small amount of chain extender does not allow it to form a separate phase.…”

Rheological and thermal properties of polylactide/organic montmorillonite nanocomposites

“…PLA has good mechanical properties, thermal plasticity, and biocompatibility, thus being a promising polymer for various end‐use applications 4–6. However, some drawbacks, such as brittle nature and a lack in toughness, poor thermal stability, and melt strength, are disadvantageous in practical processing especially in the film extrusion industry.…”

Viscoelasticity as a tool to investigate the early stage of the quiescent crystallization process of poly(lactic acid)

“…When embryos start to emerge, some polymer chains are engaged in the embryos making the embryos assume a multiarm-like structure [22], while other chains that are not involved in the embryos still maintain their isotropic configurations as in a normal amorphous melt. The polymer chains of the former category occupy a volume fraction of f. Therefore, the rate of the formation of an embryo should be proportional to the volume fraction of the amorphous melt, 12f , with an embryo formation rate constant of k 1 .…”

“…Another approach is to define the induction time as the moment when an abrupt upturn in the viscosity curves is perceived [20,21]. Yuryev and Wood-Adams [22] modeled the viscoelasticity of the melts by an exponential decay and defined the induction time by the deviation of the properties from the model when a monotonic increase in the viscoelasticity was brought in by crystallization. Chen et al [23] observed that the normal force was more sensitive to the onset of crystallization than viscosity and defined the induction time as the time when the normal force was twice its initial value.…”