DSC Study of Cold Crystallization Process Characterizing the Stereocomplexed PLA Compounds ^†

The European strategy for plastics in a circular economy [1] underlines the importance of using renewable polymers for sustainable applications. The possibility of increasing the durability of the polylactic acid by stereocomplexing at melt compounding was studied. The purpose of the paper was to study the dependence of the cold crystallization processes on the Differential Scanning Calorimeter (DSC) cooling rate. Types of polylactic acid (PLA) with different molecular weights and dextro sequence content were stereocomplexed. To identify the dependence of the melting and crystallization characteristics on the type of polymers and DSC recording conditions, the compounds were studied, after the elimination of each compound’s thermal history. The recordings were made on a DSC 3 Mettler Toledo device. On the DSC thermograms, besides the melting and crystallization processes, two additional exothermic ones were observed (Figure 1). According to [2], the two additional processes were attributed to the cold crystallization, which probably appeared because the polymers had no time to crystallize (too high of a cooling rate). In this situation, the crystallization continued over time. Another possible explanation can be related to defects of the new appeared crystals [2].

At a cooling rate of 20 °C/min, the crystals do not have enough time to be formed and, therefore, the resultant morphology is an amorphous type. At a 2 °C/min cooling rate the resultant morphology has a semi-crystalline architecture. The semi-crystalline phase content and the type of crystals depend on the intensity of the stereocomplexation phenomenon. The nature of the new exothermal process observed on the DSC thermograms of the stereocomplexed PLA compounds will be clarified in the next experiments.