Study On Plasticization Of Poly(L-lactide) And Toughening Of Poly(Butylene Terephthalate)

Poly(L-lactide)(PLA) derived from renewable resources have received considerable attentions due to its good biocompatibility and biodegradability. However, as aliphatic polyesters, the inherent deficiencies of PLA, such as the low crystallization rate, limited dimensional stability and bad mechanical properties, etc. have limited its wide applications. Based on the inherent deficiencies of PLA, small molecular plasticizer and different molecular polymer plasticizer were used to plasticize and toughen PLA, respectively. As an important engineering thermoplastic, poly(butylene terephthalate)(PBT) has been widely used in automobile, electron, electronic appliance and so on due to its good properties like high rate of crystallization, solvent resistance and excellent processing. But its low notched impact strength at room temperature restricted its application in wider areas. Based on the defect of PBT, thermolplastic polyurethane(TPU) and glycidyl methacrylate-functionalized methyl methacrylate-butadiene(MB-g-GMA) core-shell copolymers were used as impact modifiers for PBT, respectively. Main contents and conclusions are as follows:(1) The environment-friendly diethylene glycol monobutyl ether adipate(DGBEA) was employed as a plasticizer, and the PLA/DGBEA blends were prepared by melt compounding. The miscibility, rheological properties, crystallization behavior and mechanical properties were studied. The research finding showed that the PLA/DGBEA blends were partial miscible. The complex viscosity and storage modulus gradually decreased at the whole angular frequency with the concentration of DGBEA increased, indicating that the plasticization effect was achieved. The results from DSC showed that the glass transition temperature of PLA decreased with the addition of DGBEA, because the plasticizer promoted the movement of PLA chain. The PLA/DGBEA blends showed two melting temperature. The double melt phenomenon was attributed to a melt recrystallization-remelting process upon heating. With increasing DGBEA content, the elongation at break and impact strength drastically increased, whereas tensile strength and modulus decreased.(2) The different molecular weight poly(diethylene glycol adipate)s(PDEGA) were employed as plasticizers, and the PLA/PDEGA blends were prepared by melt compounding. The influences of the different molecular weight PDEGA on the mechanical properties of PLA were studied. The research finding showed that the low molecular weight PDEGA(L-PDEGA) could be as a plasticizer which significantly increased the elongation at break, and the high molecular weight PDEGA(H-PDEGA) could be as a tougher which significantly increased the impact strength. Then, L-PDEGA and H-PDEGA were used to plasticize and toughen PLA simultaneously. Results showed that PLA/L-PDEGA/H-PDEGA blends were partial miscible and H-PDEGA dispersed in PLA matrix uniformly. H-PDEGA was effective in lowering the glass transition temperatures as well as in increasing the elongation at break and the impact strength. Compared with neat PLA, the crystallinity of PLA increased with increasing H-PDEGA content. When 20wt% H-PDEGA was added, the elongation at break and the impact strength increased from 5.6% and 3.1kJ/m2 of neat PLA to 272.4% and 68.3kJ/m2, respectively. Additionally, morphological study revealed that the fracture behavior of PLA had been changed from brittle to ductile after H-PDEGA incorporated. The results of rheological analysis showed that the complex viscosity in the melt state of the blends was decreased compared with that of neat PLA.(3) TPU was employed as a toughener, and the PBT/TPU blends were prepared by melt compounding. The miscibility, crystallization behavior, mechanical properties and toughening mechanism were studied. The research finding showed that PBT was immiscible with TPU. As for the PBT/TPU blends, the PBT crystallization peak became broader and shifted to a lower temperature, indicating that the incorporation of TPU retarded the nonisothermal melt crystallization of the PBT matrix. An obvious improvement in toughness of PBT was achieved with addition of TPU. The brittle-ductile transition of the blends was obtained when the TPU content was 20wt%. The morphology of the PBT/TPU blends after tensile and impact tests was investigated, and the corresponding toughening mechanism is discussed. It was found that the PBT showed obvious shear yielding in the blend during the tensile and impact tests, which led to the improvement in toughness of the PBT.(4) MB-g-GMA core–shell copolymer was used to toughen PBT and the PBT/MB-g-GMA blends were prepared by melt compounding. The miscibility, morphology, crystallization behavior, rheological properties, mechanical properties and toughening mechanism were studied. Results showed that compatibilization reactions took place between the carboxyl and/or hydroxyl groups of PBT and the epoxy groups of MB-g-GMA. The PBT/MB-g-GMA blends were partial miscible and MB-g-GMA core–shell particles dispersed in the PBT matrix uniformly. The addition of MB-g-GMA significantly improved the mechanical properties of PBT, and the elongation at break and the impact strength increased with the increase of MB-g-GMA content. When the MB-g-GMA content was 15wt%, the brittle-ductile transition of the blends was occurred and the shear yielding properties of the PBT matrix was the main toughening mechanism. The storage modulus and complex viscosity of the PBT/MB-g-GMA blends increased with increasing MB-g-GMA content at the whole angular frequency, indicating that the intermolecular interaction between PBT and MB-g-GMA in the melt state existed.(5) Epoxy resin(EP) was used to modify the properties of 75/25 PBT/MB-g-GMA blend and PBT/MB-g-GMA/EP blends were prepared by melt compounding. The miscibility, morphology, crystallization behavior, rheological properties, mechanical properties and toughening mechanism were studied. Results show that epoxy resin further improved the miscibility of PBT and MB-g-GMA dispersed in PBT matrix uniformly. However, the added epoxy resin restricted the mobility of PBT macromolecular chains during the growth process of the crystal, which reduced the final crystallinity of PBT. The chemical reaction between PBT and epoxy resin induced the high complex viscosity and storage modulus of PBT/MB-g-GMA blend. With the incorporation of TPU, the crystallization peak of PBT became broader and shifted to a lower temperature, indicating that TPU retarded the crystallization of the PBT. When only 1wt% epoxy resin added,the notched impact strength increased from 37.2kJ/m2 of 75/25 PBT/MB-g-GMA to 75.6kJ/m2 and the shear yielding of the PBT matrix and the cavitations of rubber particles became more obvious.