Dynamically Vulcanized Biobased PLA/NR TPV With Co-continuous Structure

In order to solve the oil crisis and environmental problems, it is an important trend worldwidely to research and develop biodegradable polymers with super properties and wide applications. Poly(lactic acid)(PLA), as a biodegradable polymer with high modulus, strength and excellent transparency, has attracted great attention because of its biodegradability, renewable and biocompatibility in recent years. However, PLA shows some inherent drawbacks including brittleness and low crystallization rate etc, which limit its wide application. In this article, based on the background of rubber toughen plastic and dynamic vulcanization, studies is presented in an attempt to prepare thermoplastic vulcanizates(TPVs) based on PLA, in order to overcome these drawbacks of PLA and extend its application.For keeping biobased properties of PLA, we selected natural rubber(NR), which is also derived from natural source, as toughening agent for PLA. By using a cross-linking agent, dicumyl peroxide(DCP), and a dynamic vulcanizing technique, a fully biobased PLA/NR TPVs was prepared. It reveals that a 30 wt % of NR content(DP70) shows impact strength and elongation at break of 7.75kJ/m2和 55%;while the impact strength and elongation at break increased to 58.25kJ/m2 and 216% as NR content increased to 35wt%(DP65), being 21 times that of neat PLA approximately, which may be the value for achieving brittle-ductile transition in the present PLA/NR TPVs. The scan electron microscopy(SEM) and dissolution/swell experiments provided the direct proof of the continuous cross-linked NR phase. This new biobased PLA/NR TPVs with the novel co-continuous structure is reported for the first time in the field of dynamic vulcanization, which breaks the traditional understanding for TPVs formed “sea-island” structure with rubber particles dispersed in continuous plastic matrix, and shows promise for development for various functional applications. The much lower viscosity ratio of PLA and NR at processing conditions is believed to be responsible for the formation of co-continuous structure. On the other hand, the results of FTIR measurement suggesting that PLA reacted with NR during melt blending in the presence of DCP, which achieved in situ compatibilization between PLA and NR. And this may be one of the reasons for the highly toughened PLA/NR TPVs.Afterwards, PLA/NR TPVs were prepared through dynamic vulcanization using DCP, sulphur(S) and phenolic resin(2402) as curing agents, respectively. The crosslinked NR phase was found to be a continuous structure in all the prepared blends. The chemical graft reaction occurred in all the curing systems, which improved the interfacial compatibility between PLA and NR. The PLA with 40 wt% NR in DCP and S curing systems showed notched impact strength of 42.5 kJ/m2 and 25.75 kJ/m2, about 16 times and 10 times that of the neat PLA, respectively. Compared with the three curing system, the DCP-induced dynamic vulcanization owned the optimum comprehensive properties. The molecular weight changes of PLA were studied by gel permeation chromatography, and the results suggesting that phenolic resin led to a severe degradation of PLA chains, thus lowered the mechanical properties and thermal stability of the blends. The DCP and S showed a small influence on the degradation of PLA.Based on the co-continuous structure of fully biobased PLA/NR TPVs, we studied the shape memory performance. The PLA/NR TPVs possesses excellent shape fixities(～100%), shape recoveries(>95%), and fast recovery speed(<15 s) at the switching temperature(～60 °C), compared with neat PLA and PLA/NR blend. Recovery speed of the TPVs was enhanced and residual deformation was reduced with the increased crosslink density of NR. The excellent shape memory performance of PLA/NR TPVs was resulted from:(1) unique co-continuous structure of the TPVs, the crosslinked NR phase owns stronger resilience and generates higher SR driving force. The continuous PLA phase acted as a rigid “container” to restrict the deformed rubber phase below its Tg;(2) the improved interface adhension originated from in situ reactive compatibilization during dynamic vulcanization keeps the deformed NR phase in the temporary shape firmly, storing the sufficient elastic resilience;(3) the inherent shape memory performance of PLA.