Investigations On The Compatibilizing Effects Of Reactive Comb Polymers In PLLA/ABS Blends

Polylactic acid(PLLA) has attracted increasing attention in recent years because it is a typical biosourced material with reasonable price and unique properties such as high strength, high stiffness, resistance to fats and oils. However, the slow crystallization rate, low heat resistance and brittleness of PLLA are main drawbacks for its wide application. This work is aiming at improving the toughness and heat resistance of PLLA by reactive blending PLLA with acrylonitrile-butadiene-styrene copolymer(ABS). We designed and synthesized a reactive comb polymer(RC) as the compatibilizer for PLLA/ABS blends. The RC contains epoxy groups and long PMMA side chains which are randomly distributed along its backbone. The main research contents and results are as follows:(1) We synthesized a PMMA type reactive comb polymer(RC) by grafting-through method. At the same time, a PMMA type reactive linear polymer(RL) was synthesized by copolymerizing methyl methacrylate(MMA) and glycidyl methacrylate(GMA) monomers. The molecular structures of the RC and RL polymers have been investigated and confirmed by GPC, FT-IR and 1H NMR methods.(2) We compared the compatibilizing efficiency of RC and RL on immiscible PLLA/MABS(a type of transparent ABS that is short for MABS)(50/50) blend. It was found that the in-situ formed double-comb structure of RC-g-PLLA(RC graft PLLA copolymer) has a stronger interfacial adhesion and higher interfacial stability than the in-situ formed single-comb structure of RL-g-PLLA(RL graft PLLA copolymer) in the PLLA/MABS interface, indicating a better compatibilizing efficiency of RC than RL for the PLLA/MABS(50/50) blend.(3) The morphologies and properties of a series of PLLA/MABS/RC blends compatibilized by RC were investigated. It was found that RC compatibilized blends with different PLLA/MABS composition ratios showed drastically improved elongation at break and tensile toughness compared to the neat PLLA and neat MABS. The morphological investigation indicated that the disperse phase sizes in RC compatibilized PLLA/MABS blends are much smaller than those blends that without RC compatibilizing. What’s more, the morphology of the PLLA/MABS/RC blends converted gradually from dispersed phase into co-continuous phase with the increasing of MABS content in the blends, and the PLLA/MABS/RC(30/70/3) blend showed a typical co-continuous structure. The rheological studies of the PLLA/MABS blends compatibilized by RC showed enhanced melt elastic properties. The PLLA/MABS/RC 50/50/3 and 30/70/3 blends displayed an elastic “net-work” in their rheological behaviors at low frequency sweeps, indicating the high compatibilizing efficiency of RC for PLLA/MABS.(4) The molecular structure effects of ABS on the morphology and properties of PLLA/ABS/RC blends were studied. Two different types of ABS have been used in the blends. It was found that the MSAN phase of MABS has better compatibility and stronger intermolecular interaction with RC than the SAN phase of TABS(a kind of universal ABS that is short for TABS) so that RC plays more efficiency in compatibilizing PLLA/MABS(50/50) blend. In addition, because of its poor compatibility and weak intermolecular interaction with SAN phase in TABS, the in-situ formed RC-g-PLLA copolymer during reactive blending included some SAN compositions and the redundant RC-g-PLLA copolymers were pulled into the PLLA matrix to form “micelles” under the shearing force. The process follows up the “phase erosion” mechanism.(5) Processing condition effects on the structure and properties of PLLA/TABS/RC blends were investigated. On one hand, increasing the processing shearing rate enhances the miscibility between TABS and RC, but leads to the degradation of PLLA. Therefore, no improvement was observed in the physical performance of the PLLA/TABS blends compatibilized by RC. On the other hand, the lower processing temperature induces a better compatibility between the long PMMA side chains with the SAN of TABS, and RC takes a effective role as compatibilizers locating at the interface. Thus the high performance of PLLA/TABS alloys were obtained by processing at low temperature. We attributed the better compatibilizing effects at low temperature to the LCST phase behavior of PMMA and SAN.