Abstract
Abstract In this study, a series PLLA triblock copolymers and random copolymers were synthesized for adjusting various properties of PLLA. Expect to increase the application potential of biodegradable PLLA by introducing these polymers, including PTMEG, aliphatic PC, PET and PBT, respectively. For electrical application purpose, we also introduced graphene oxide(GO) and graphene(rGO and TRG) into biopolymers, PVA and PLLA, respectively. In the part of ABA triblock copolymers, a series of ABA triblock copolymers of PLLA and aliphatic polycarbonate (PCLA) have been synthesized and thoroughly characterized. PLLA and aliphatic PC are miscible and its Tg is increased with increasing PLLA content. The crystallinity of PLLA is decreased with increasing PC content. These tendency might be used for adjusting the transparency of PLLA. According to the results of biodegradability test, we found that the PLLA in PCLA triblock copolymers is still biodegradable and the degradation is starting from the amorphous region. A series of ABA triblock copolymers of PLLA and PTMEG(TMLA) have been also synthesized and thoroughly characterized. In enzymatic degradation, through the manipulation of PTMEG content in the triblock copolymer, the hydrolysis rate of PLLA segment by enzyme can be well controlled. The enzymatic degradation of PLLA segment decreases with increasing PTMEG content in the copolymer, while the weight loss percentage of the PLLA segment increases exponentially with time, indicating that the enzymatic degradation of PLLA segment is a diffusion-controlled process. Besides, the random copolymer, PET/PLLA and PBT/PLLA also had been synthesized and thoroughly characterized. The series of random copolymers display a long-term biodegradation, indicating that the introduction of PBT or PET with random form could retard the biodegradation of PLLA. In the part of graphene/biopolymer composites, GO/PVA composites were prepared and thoroughly characterized. Through a convenient effective method, uniform dispersed reduced graphene oxide(rGO)/PVA composites could be obtained by swelling the PVA matrix with a reducant solution. The electrical conductivity of the composite increases with increasing rGO content, while a sharp increase happens as the rGO content beyond 10wt%. Such an increase was evidenced by the formation of rGO network so as to diminish the electrical resistance by the interconnection structure of rGO. In our studies, the conductivity of rGO/PVA film increases from 6.04×10-3 S/m to 5.92 S/m as the rGO content increases from 4wt% to 14wt%. A series of poly(L-lactide) (PLLA)/thermally reduced graphene oxide (TRG) composites (GLLA) were prepared via the in situ ring-opening polymerization of lactide, with TRG as the initiator; after their preparation, the composites were characterized. By using a more effective method of synthesis, the thermal stability, crystallization rate, and electrical conductivity of PLLA were increased. The starting temperature for the thermal decomposition of PLLA was increased from 173oC to 211oC via the introduction of 2.00wt% TRG sheets. At a TRG content of 2.00wt%, the chemical interaction between the PLLA and the TRG sheets was strong enough to increase the nucleation rate and the overall crystallization rate of the PLLA. The electrical conductivity of the GLLA composites increased with increasing TRG content. Typical insulating-conductive percolation behavior was observed for TRG contents between 1.00 and 1.50wt%, and the electrical conductivity of the PLLA was improved by 12 orders of magnitude in the GLLA composites, from 7.14×10-14 S/m for neat PLLA to 1.63×10-2 S/m for GLLA with 2.00wt% of TRG sheets. These results demonstrate a straightforward means of preparing GLLA composites that perform satisfactorily in terms of their electrical conductivity and their thermal stability. In conclusion, the Tg, crystallinity and biodegradation rate of PLLA could be adjusted or controlled by copolymerized with other polymers. Besides, introduction of GO and TRG could enhance the thermal stability of biopolymer PVA and PLLA respectively and endowed the biopolymers(PLLA and PVA) with an outstanding electrical conductivity. To sum up, these improvement raised the applied potentials of PLLA, successfully.