Combined effect of nucleating agent and plasticizer on the crystallization behaviour of polylactide

Abstract

Different plasticized and nucleated polylactide (PLA) systems were prepared and characterized. Two PLA with different l-lactic acid contents (96 and 99.5%) were plasticized with dioctyl adipate (DOA) and nucleated by talc, ethylene bis(stearamide) (EBS), or d-lactic acid-based PLA (PDLA). Crystallization behaviour was studied by differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and polarized optical microscopy (POM). The combination of plasticizer and nucleating agent was proved to be a very effective approach to improve crystallization velocity of different PLA matrices. Within the studied crystallization temperature range, faster crystallization rates were achieved at lower temperatures. WAXS results indicated the coexistence of α and α′ crystals in all studied systems, except those which showed very low crystallization degrees. Avrami exponent remained constant at around n ≈ 3 for all systems, suggesting equivalent three-dimensional spherulitic growth behaviour regardless crystallization temperature, nucleating agent, and the stereochemistry of the matrix used. Usually, injection-moulding process, where molten polymer is under high pressure, is used for PLA polymer processing. To analyze the effect of pressure on the crystallization process, pressure volume temperature (PVT) measurements were carried out on the systems that showed the fastest crystallization process under atmospheric pressure by DSC. Results showed that the crystallization process was considerably accelerated under pressure.