Effect of dissolved CO 2 on the crystallization behavior of linear and branched PLA M. Nofar, W. Zhu, C.B. Park * Microcellular Plastics Manufacturing Laboratory, Department of Mechanical and Industrial Engineering, University of Toronto, 5 King’s College Road, Toronto, Ontario, Canada M5S 3G8 article info Article history: Received 25 January 2012 Received in revised form 26 April 2012 Accepted 28 April 2012 Available online 11 May 2012 Keywords: Polylactide Crystallinity Dissolved CO 2 abstract In this study, the crystallization behavior of polylactide (PLA) was investigated in the presence of dis- solved CO 2 using high-pressure and regular differential scanning calorimeter. The isothermal and non- isothermal melt crystallization results showed that increasing the CO 2 pressure decreased the crystal- lization half-time. During isothermal and low-cooling-rate non-isothermal crystallization, a very high crystallinity was achieved at 15 bar CO 2 pressure by facilitating more perfect crystal formation with the plasticization effect of CO 2 while limiting the crystal nucleation rate. At higher CO 2 pressures, a larger number of less close-packed crystals were formed due to chain entanglement, and consequently, the final crystallinity was decreased. The non-isothermal results at high cooling rates showed the total crystallinity decreased for all CO 2 contents, because of less time given for crystallization. Also the effects of the CO 2 pressure and the cooling rate on T c and T g were investigated. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Most polymers and plastics are derived from fossil fuels. After being used, polymer products become waste material in the envi- ronment, where they do not degrade. Consequently, global efforts are being made to create green, biodegradable polymers. New biodegradable and biocompatible polymers that have reasonable properties are receiving increasing attention from researchers with environmental and biomedical points of view. Due to its origin in renewable sources and its biodegradability, polylactide (PLA) is one of the polymers that have received much attention [1e5]. PLA is a promising candidate for a range of applications [6], and it could replace other non-biodegradable polymers such as polystyrene in the areas such as packaging, cushioning, and plastic utensils [7e9]. The crystallization control for PLA is crucial to enhance its heat deflection temperature and mechanical properties since the utility of PLA has been limited because of the low glass transition temperature (T g ) (and a low crystallinity) [7]. Currently, PLA’s slow crystallization behavior is one of its most challenging issues; even in isothermal treatment, it is difficult to achieve sufficient crystal- linity [10]. This structural parameter is paramount in such polymer processes as foaming. PLA’s low melt strength creates a serious challenge to the mass production of PLA products such as PLA foams and, specifically, microcellular foams [11,12]. But it was found that the increase in melt strength of the connected molecules by crystallization improved the foamability of PLA significantly [13,14]. On the other hand, too high crystallinity will decrease the foam’s expansion ability and cell density because of too high stiffness and less gas dissolution. Therefore, an optimal degree of crystallization would be required to achieve a high expansion ratio and a desirable close-cell structure [14e17]. The connected molecules will be able to flow through the slow-growing crystals in the early stage, but these molecules will have much higher melt strength. This will result in gas loss reduction and a lower amount of cell coalescence, and hence an increase in expansion capacity during foaming [17]. Furthermore, according to heterogeneous cell nucleation theory, the local pressure variations around the formed crystals can promote cell nucleation [14,18]. Therefore, the crystallization control can significantly affect the foaming ability while improving the heat-deflection/service temperature [7] and mechanical prop- erties of the final products. Various parameters that enhance the crystallization behavior of PLA have been explored in the literature. These parameters include the following processes: Isothermal annealing at elevated temperatures in a static condition or in a dynamic and continuous extrusion process within a tandem line or an extended die [19e21]; blending with other polymers with a high melt strength and a strong potential to crystallize [22e26]; compounding with inor- ganic or organic additives as crystal nucleating agents [27e31]; adding chain extender (CE) [16,32e36]; using PLAs with low D- content [13], strain-induced [37e39] and gas-induced crystalliza- tion [10,40e42]. As noted, branching would affect the PLA’s crystallization behavior. It improves the melt strength by hindering the chains’ * Corresponding author. E-mail address: park@mie.utoronto.ca (C.B. Park). Contents lists available at SciVerse ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer 0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2012.04.054 Polymer 53 (2012) 3341e3353