Study On Synthesis Of PLA-b-PCL Multiblock Copolymer And Its Low-Temperature Processing By Pressure Induced-Flow

PLA, known as poly(lactic acid), because of its good biodegradable and biocompatible, has been widespread concerned in the scientific and the industrial fields recent years. To substitute the traditional plastic and achieve the industrialization, not only PLA material should be modificated but also the thermal degradation problem should be settled during the processing. Although the properties of PLA were improved in most of modification methods but the problem of thermal degradation of PLA was seldom solved.In this paper, a novel processing method named low temperature processing by pressure-induced flow was introduced to the processing of PLA and its copolymers to overcome the thermal degradation problem. The low temperature processing by pressure-induced flow is a new processing method which polymers could be directly processed at a temperature much lower than the melting point which means it can process polymers without melting them. Thus, compare to the traditional processing methods, the new processing method can effectively avoid polymers' thermal degradation. Simultaneously low temperature processing by pressure-induced flow can conserve energy, reduce auxiliaries and save cost. Owing to the high pressure a special polymer structure was observed which substantially improves the performance of products and expands the field of application of products.Based on the above purposes of the study, this paper is summarized as following aspects:Firstly, a series of PLA-b-PCL triblock copolymers were synthesized by ring opening polymerization of L-lactide, initiated by the hydroxyl terminal groups of the PCL chain, catalyzed by the stannous octanoate. Followed by the chain extension of these PLA-PCL-PLA triblocks, using hexamethylene diisocyanate (HDI) in treated chloroform solution, the corresponding PLA-b-PCL multiblock copolymers were synthsized. The chain extension has obvious effect that the highest growth of molecular weight raises up to 5.1 times and the Mn of the mulutiblock copolymer reaches 100,000.Secondly, the copolymers were characterized by IR, ~1HNMR, DSC, TGA, and GPC, the results showed that the block copolymers were synthesized successfully. Effect of reaction conditions such as reaction time, [OH]/[NCO] value on molecular weight of multiblock copolymers were studied. It showed that the appropriate reaction time was 3h, and the effect of [OH]/[NCO] value on molecular weight of multiblock copolymer presented a trend of ascending on the beginning then descending which means that each multiblock copolymer's molecular weight can reach its maximum when the best [OH]/[NCO] value is given in the chain extension of corresponding triblock copolymer.Thirdly, the PLA-b-PCL multiblock copolymers were processed using the method of low-temperature pressure-induced flow. The original opaque polymer powder was transformed into transparent bars after processing. The effect of PCL contents, processing temperature and processing pressure on liquidity of copolymers, tensile properties and impact properties of bars were studied. The results showed that the ultimate tensile strength decrease as the PCL content increase, and it increase as the processing temperature increase; the effect of PCL content, processing temperature and pressure on impact properties presented a trend of ascending on the beginning then descending; compared to other general plastics, the overall mechanical properties of PLA-b-PCL multiblock copolymer bars have been close to or even reach the standards of some general plastics.Finally, XRD showed that the copolymers were orientated along the flow direction under pressure, the orientation degree increases as the pressure increases. The SEM showed that the copolymers formed a new layered structure which is similar to the armor or the shell in the nature world in the process. It was concluded that the layered structure contributed to the promotion of mechanical properties of copolymers.