Synthetic preparations, thermal properties and crystallization characterization of PLLA‐PPG block copolymers/clay nanocomposites

Abstract

In this study, intercalation of the polymer or pre-polymer from solution was used to blend different proportions of polylactic acid-propylene glycol (LPG) copolymers (polypropylene glycols (PPG) of M n : 700, 1000, 2000) and lipophilic montmorillonite (clay) in order to investigate the melting and the crystalline nature of LPG copolymers/clay nanocomposites via a differential scanning calorimeter (DSC). In addition, changes in the intermolecular force and crystal morphology of the nanocomposites under different crystallization conditions were also studied. For the results, it was observed from a thermogravimetric analyzer that increasing the clay content elevated the weight loss temperature. In non-isothermal experiments using a DSC, it was discovered that the melting temperature and crystallization temperature of the LPG copolymers also increased with increasing amounts of added clay. The crystallinity of LPG2000 + 1.5 wt% clay was enhanced by 17.00%; in addition, it was found in the crystallinity study that adding clay slowed down the crystallization rate of the LPG copolymers. Moreover, it was found via X-ray diffractometer (XRD) that the intensity of the diffraction peaks of the 1.5 wt% specimen was stronger than that of the 0.5 wt% specimens. The results imply that copolymers with a longer chain length provide greater space for the crystals to grow, thus making it easier for larger crystals to grow. Conversely, the added clay generates an inhibitory effect in copolymers, reducing the d-spacing (d) in the XRD. Therefore, adding clay would change the crystallization behavior and the morphology of the LPG copolymers. Copyright © 2015 John Wiley & Sons, Ltd.Keywords: polypropylene glycols; montmorillonite; copolymers; nanocomposites; crystallization; morphology INTRODUCTIONPolylactic acid (PLA) is a straight chain thermoplastic aliphatic polyester. The main raw material for its production is starch from corn, wheat, potatoes, and other plants. Lactic acid is derived from fermentation and is then used to produce PLA through condensation polymerization. Under natural microbial action, used PLA gradually decomposes into carbon dioxide, water, and other harmless residual substances. [1][2][3][4][5][6] Polylactic acid is one of the popular green polymeric materials of today and possesses characteristics such as being non-toxic, non-irritating, having good biocompatibility and biodegradability, and good mechanical properties, and so on. These characteristics allow PLA to have potential applications in many disposable items. Changing the scope of application and sources of plastic materials also reduce dependence on petroleum feedstock. However, PLA has the drawbacks of poor flexibility and thermal stability, which restrict its applications outside disposable items or medical uses. [7][8][9][10][11][12] In addition, lactic acid has two different optical isomers, poly-Llactic acid (PLLA) and poly-D-lactic acid (PDLA). Of these two isomers, PLLA gives a better performance. PLLA is soluble in chloroform and dichloromethan...