Synthesis And Modification Of Biodegradable Multi-block Copolymers Containing Polylactic Acid Hard Segments

Polylactic acid (PLLA) is an important biobased & biodegradable material and it is most widely used as a commercial biodegradable polymer. It has high modulus and strength, good biocompatibility and processing performance, but also shortcomings likes low crystallization rate and brittleness which limit it for some end applications. Synthesis of block copolymers is an important approach for polymer modification. In previous studies of our group, multi-block copolymers (denoted as PLSA) using PLLA as hard segmentsand poly (butylene succinate-co-adipic acid ester) (PBSA) as soft segments via condensation-chain/coupling method have been synthesized, and their structure and properties have been studied. PLSA multi-block copolymers with adjustable properties from toughened polylactic acid (thermoplastics) to thermoplastic elastomers were obtained. In order to further improve the performance of the multi-block copolymers, modifications from both aspects of hard and soft segments have been conductedin this article. On the one hand, the crystallinity of PLLA segments has been improved in order to obtain better mechanical properties of toughened polylactic acid by adding nucleating agent. On the other hand, the structure/composition of the soft segment have been adjusted/changed to improve flexibility and strength of the soft segment for obtaining PLLA-based thermoplastic elastomers with better performance.Firstly, crystallization of PLLA the prepolymer (Mn=8200 g/mol) in the presence of 1 wt% nucleating agent (NA) was studied to screen proper NA. It is found that 8000Talc, BUr-1 and ZnCC have excellent nucleating effect and can effectively promote crystallization of PLLA prepolymer, improving the crystallization rate, melting enthalpy and crystallization temperature, and therefore appeared to be suitable NA for PLLA-based mult-block copolymers. The three NAs were used for crystalline modification of multi-block copolymer PLSA30 and PLSA70 (PBSA contents were 30 wt% and 70wt%, the number-average molecular weight PLLA hard segment and PBSA soft segment was respectively 8200 g/mol and 3100 g/mol). The results showed that the crystallization of the hard segment in copolymers had been improved significantly by adding the three nucleating agent, and "8000Talc is better than ZnCC and ZnCC is better than BUr-1". In the presence of 8000Talc, the crystallinity degree of PLLA hard segments in PLSA30 and PLSA70 was increased by more than 40%, and smallest crystalsize was also obtained. For PLSA30, the presence of 0.1 wt% 8000Talc or ZnCC does not result in expected strengthening effect (the strength and modulus remained unchanged), but leads to surprising toughening effect:the elongation at break increased to about 200% and the notched impact strength increased to 50 J/m2. For PLSA70, the presence of 8000Talc and ZnCC resulted in increase in the fracture strength (from 14MPa to 16MPa) and elongation at break (from 190% to 330% and 600%), showing simultaneous strengthening and toughening effects.Secondly, PBSA prepolymers with various molecular weights (from 3100 to 9600 g/mol) have been synthesized by adjusting polycondensation time and used to synthesize a series of high molecular weight multi-block copolymers PLSA30 and PLSA70 to investigate the influence of segment molecular weights on crystallinity and mechanical properties of PLSA. The mechanical properties of PLSAs were improveby increasing Mn of PLLA hard segment from 6000 g/mol to 8200 g/mol and Mn of PBSA soft segment from 3100 to 9600 g/mol. The crystallinity of PBSA prepolymer weakened as its molecular weight increases, but that of PBSA segments in PLSA multi-block copolymer, increased instead, especially when PBSA segment content is high (PLSA70). Furthermore, the improved crystallization of PBSA segments played a facilitating role for the crystallinity of PLLA segments. For PLSA30, with increasing of molecular weight of the soft segment, the modulus and yield strength increased, while the breaking strength decreased. For PLSA70, the elongation at break increases with increasing of PBSA molecular weight. The elongation at break of PLSA70-9600 has reached 720%.Finally, flexible amorphous copolyester PBSeATI with low glass transition temperature (Tg-38.0℃) has been synthesized via quadri-copolycondensation based on sebacic acid, adipic acid, terephthalic acid and isophthalic acid (composition 30:30:20:20), and was used to synthesize multi-block copolymers PLSeATI30 and PLSeATI70 with fully amorphous PBSeATI soft segments. Similar to multi-block copolymer PLSA containing crystalline soft segment PBS A, the increasing of molecular weight of PBSeATI segments leads to increase in elastic modulus, yield strength, and fracture strength of PLSeATI30, but decrease in the elongation at breakand therefore embrittlement ofthe material. In contrast, the elongation at break of PLSeATI70 increases with increasing Mn of PBSeATI. In comparison with PLSA30, PLSeATI30 has a higher modulus and yield strength, but lower elongation at break, and in comparison with PLSA70, PLSeATI70 has higher modulus, lower breaking strength and elongation at break. Therefore, the desired modification effect by using completely amorphous soft segment PBSeATI has not been achieved.