INTRODUCTION

Generally binary polymer blends display two types of morphologies: matrix/dispersed phase and co-continuous structures. The formation of these morphologies depend on the blend composition, the rheological and interfacial properties of the components, and processing conditions [1]. Over the last 15 years, co-continuous morphologies, have received significant attention since they have the potential to significantly broaden the application range of polymer blends [2,3]. The interfacial stabilization of the co-continuous morphology in an immiscible polymer blend is crucial since its highly interconnected nature renders it inherently unstable and it can coarsen rapidly during annealing or reprocessing [4,5]. Common approaches to modify the interface are the addition of copolymers as compatibilizers [4,6] or reactive blending [7,8] based on the in situ formation of block/graft copolymer at the interface. More recently, nonofillers [9–11] and Janus polymeric nanoparticles [8,12] have also been used to compatibilize/stabilize the co-continuous morphology of polymer blends. These partially wet particles have shown a potential to stabilize the interface. Multicomponent blends, such as ternary and quaternary polymer blends, have also attracted increasing attention in recent years as they allow for the generation of diverse morphologies. Generally two types of wetting regimes are identified in three phase systems: complete wetting and partial wetting [13]. These wetting behaviors can result in a variety of complex phase structures such as core-shell and tri-continuous structures in the case of complete wetting [14,15] and multiple stacked morphologies in the case of partial wetting [16].

The equilibrium morphology in ternary blends can be predicted by spreading coefficients as proposed in Harkins spreading theory [17–20]. Three spreading coefficients (l) are defined based on the interfacial tensions (g) between components where three negative spreading coefficients predict partial wetting and one positive and two negative spreading coefficients predict complete wetting. Figure 6.1 schematically demonstrates these two cases in a ternary blend with the corresponding coefficients. The theory has been examined and proven to be quite reliable in the analysis of the morphology of a variety of ternary blends [21–23]. Although a number of studies

have examined the formation of complete wetting in ternary polymer blends [24–27], very few have reported on the partial wetting case [28–30]. In one of the first detailed studies on the partial wetting of polymers, Virgilio et al. [29] reported the self-assembly of polystyrene (PS) as segregated droplets at the interface of high density polyethylene (HDPE) and polypropylene (PP) through a partial wetting mechanism.

The correlation between the morphology and mechanical properties of multiphase polymer blends has been well established and the control of morphology can be used to access a wide range of mechanical property sets [31–37]. It has been shown that changing from a matrix/dispersed structure to a co-continuous morphology can significantly improves the impact and tensile properties in different binary blends [3,31]. Co-continuous polylactide (PLA)/acrylonitrile- butadiene-styrene (ABS) compatibilized by styrene-acrylonitrile-glycidyl methacrylate copolymer (SAN-GMA) can improve impact strength and elongation at break [32]. Significant improvements in the impact strength and tensile properties have been obtained through the formation of core-shell structures in ternary blends [24,33–35]. Luzinov et al. [24] observed improvements in the tensile properties of core-shell polymers (PE/styrene-butadiene rubber) dispersed in a matrix of PS and correlated it to stress transfer through the rubbery shell to the core. In another work, the toughening mechanism in ternary PP/PA6/polyethylene-octene elastomer (POE) blends was analyzed through dilatometry experiments [36]. The results showed that that the volume strain decreases with increasing (PA6+POE) content up to 60 wt% which implies a significant increase in shear yielding

Figure 6.1. Schematics demonstrating complete and partial wetting morphologies in ternary polymer blends of minor component of A and two major components of B and C with the corresponding spreading coefficients.

in the PP matrix. The observed enhanced toughness was attributed to the development of the intermediate completely wet phase of POE at the interface of the PP/PA6 system. High impact strength, with significantly enhanced heat resistance, was reported in quaternary blends of PLA, polycarbonate (PC), styrene-ethylene/butylene-styrene (SEBS), and ethylene methyl acrylate– glycidyl methacrylate (EMA-GMA). In that case, dispersed SEBS droplets effectively dilated the highly continuous PLA and PC matrices due to an effective compatibilization effect of EMA-GMA at the interface [37].

PLA and polyamide 11 (PA11) are both renewable materials which are promising alternatives to petroleum-based polymers [38–40]. PLA possesses high strength and stiffness, and excellent processability, but its broader application is limited due to the brittleness and low heat deflection temperature (HDT) [38,39]. On the other hand, PA11 is an engineering plastic with high thermal stability, high impact and chemical resistance, and excellent dimensional stability [40]. Biobased PA11 has been melt blended with PLA to improve its properties [41–44]. Despite the strong interfacial interactions between PLA and PA11 [45,46], very limited enhancements in the impact strength and elongation at break have been reported [41,42]. However, the formation of the co- continuous morphology for that system has been shown to significantly increase the HDT [43,44]. To date, it is still not clear how the incorporation of partially wet droplets at the interface of a co- continuous system can influence the mechanical properties in a ternary system. In this work, four different components, poly(butylene succinate) (PBS), poly(butylene adipate-co-terephthalate) (PBAT), ethylene methyl acrylate (EMA), and EMA-GMA will be examined to assess their potential to assemble as partially wet droplets at the interface of co-continuous PLA/PA11. The morphology of the blends and the effectiveness of the third component in the compatibilization of the co-continuous PLA/PA11 morphology will also be examined. The notched Izod impact strength and tensile properties of the blends will be evaluated and will be related to the morphology and thermal properties of the materials.