Preparation And Properties Of Poly(Lactic Acid)/Epoxidized-Elastomer Composites

Poly(lactic acid)(PLA)is a bio-based plastic with good biodegradability,biocompatibility,mechanical strength and processing properties,so it has broad application prospects in the fields of packaging,biomedicine,textile and garment.However,some disadvantages such as poor toughness,slow crystallization rate,poor heat resistance and poor hydrolysis resistance severely limits its application.Therefore,this thesis aims to simultaneously improve the toughness and heat resistance of PLA.Modification of PLA by reactive melt-blending with epoxidized-elastomer(EVM-GMA)led to excellent toughness and heat resistance,which is expected to broaden the application range of PLA.The main research contents are as follows:First,poly(L-lactide)(PLLA)/EVM-GMA composites were prepared by reactive blending.The reaction degree and compatibility between the PLLA matrix and the EVM-GMA elastomer in the composites were controlled by processing conditions such as temperature to optimize the phase structure and mechanical properties of the composites.The higher the blending temperature,the higher the reaction degree between EVM-GMA and PLLA,that is,more EVM-g-PLLA copolymers were generated in situ.It is more beneficial to improve the compatibility between PLLA and EVM-GMA.However,the excessive temperature would cause thermal degradation of PLLA matrix.The elongation at break(Eb)and notched impact strength(NIs)of the PLLA/EVM-GMA composites prepared at the optimum temperature(230℃)are 176%and 61.2 kJ/m~2,respectively,which are 28-fold and50-fold higher than these of PLLA,respectively.The best comprehensive performance was obtained when the EVM-GMA content is 20 wt%,and the PLLA/EVM-GMA(80/20)composite after annealing possesses both excellent toughness(Eb>60%,NIs>60 kJ/m~2)and heat resistance(HDT>90℃).On this basis,a new method to control the structure and properties of polymer composites by interfacial stereocomplexation was established.The EVM-GMA/PDLA masterbatches comprising EVM grafted poly(D-lactide)copolymers(EVM-g-PDLA)were firstly prepared,and the masterbatches were then melt-blended with PLLA matrix to obtain PLLA/(EVM-GMA/PDLA)composites.During blending,both free PDLA and grafted PDLA of EVM-g-PDLA copolymers collaborated with PLLA matrix to form stereocomplex crystallites in situ.It not only enhanced the interfacial adhesion between PLLA and EVM-GMA but also increased the melt viscosity and crystallization rate of PLLA matrix.Consequently,the PLLA/(EVM-GMA/PDLA)composites achieved a cocontinuous-like structure and ultra-high toughness;for example,the notched impact strength was 84-fold higher than that of PLLA.Moreover,the composites show simultaneously excellent toughness(Eb>80%,NIs>70 kJ/m~2)and heat resistance(HDT>100℃)after a simple annealing.To further improve the heat resistance of the PLA-based composites,a new method to prepare high heat-resistant PLLA/PDLA/EVM-GMA(PLLA/PDLA=1/1)composites by melt-blending and two-step annealing was established.The influence of the second step annealing process on the stereocomplex crystallinity was studied in detail.The optimum condition for the second step annealing is 200℃ for 20 min,and the stereocomplex crystallinity of the composites reached about 50%under this annealing condition.It was found that EVM-g-PLA copolymers were generated in situ during the melt-blending of PLA and EVM-GMA,which improved the compatibility between PLA and EVM-GMA,resulting in high toughness of PLLA/PDLA blends.When 20 wt%EVM-GMA was added,the elongation at break and notched impact strength of the PLLA/PDLA/EVM-GMA composite reached to 48%and 65.5 kJ/m~2,respectively,which are 15-fold and 18-fold higher than these of PLLA/PDLA(50/50),respectively.The PLLA/PDLA/EVM-GMA composites also exhibit more excellent heat resistance(HDT>140℃),chemical resistance and hydrolysis resistance due to the high content of stereocomplex crystallites formed in the two-step annealing treatment.