Plastics have become an important aspect of our everyday lives. Synthetic plastics such as polyethylene (PE), polypropylene (PP), polystyrene (PS), polyethylene terephthalate (PET), and polyvinyl chloride (PVC) share a major portion of commodity plastics. The environmental concerns and limited fossil fuel resources compel industrial sectors and scientists to find alternatives to current petroleum-based plastics. Biodegradable plastics are generating significant interest in various industries because of their true potential for reducing the dependence on fossil- based resources and their related environmental impacts (Avérous & Pollet, 2012). The bioplastics and biopolymers market is projected to grow at a compound annual growth rate of 12% between 2016 to 2021 (MarketsandMarkets, 2016). The growth of the market is attributed to the stringent environmental regulations across the globe, compelling the manufacturer to reduce the carbon content in their products, and fluctuations in the price of petroleum, which forces companies to search for a stable source of raw materials. The production and applications of bioplastics, nevertheless, are still limited due to their high production cost and poor performance characteristics as compared to the commodity plastics (MarketsandMarkets, 2015; L Shen, Haufe, & Patel, 2009). Polylactides are the leading bioplastics, which have been looked upon as sustainable alternatives to petroleum based plastics over the last decade (Babu, O’Connor, & Seeram, 2013; Erickson & Winters, 2012; Li Shen, Worrell, & Patel, 2010; Williams & Hillmyer, 2008). These polymers are biodegradable aliphatic polyesters derived from renewable resources, such as corn and starch, and has generated much interest due to its mechanical properties and favorable economics (Auras, Lim, Selke, & Tsuji, 2010). However, PLA suffers from inferior impact toughness and thermal stability when compared to the conventional polymers and this limits its wider applications. Previously, PLA was only used for biomedical applications but there is a strong increase in the number of research papers and patent applications pertaining to the modification of PLA in order to develop it as a high performance plastic. PLA must meet and even surpass the mechanical properties expected from petroleum based polymers such as high impact polystyrene (HIPS), acrylonitrile butadiene styrene (ABS), and high impact polyamides in order to successfully replace these plastics in value added applications. Different strategies have been mentioned in the literature to improve
the poor toughness and impact strength of PLA such as copolymerization, plasticization and blending with other bioplastics (Anderson, Schreck, & Hillmyer, 2008; H. Liu & Zhang, 2011; Rasal, Janorkar, & Hirt, 2010).. Generally, polymer blending is considered as an important route to overcome the mechanical property deficiencies of PLA since it is the most versatile and cost- effective approach which is easily adapted to the classic compounding capabilities. PLA has been blended with different polymers to prepare materials with the desired set of properties (Anderson et al., 2008; Paul & Bucknall, 2000).
A remarkable research has been reported on binary polymer blends with a matrix/dispersed or co- continuous phase morphology, nevertheless, understanding the development of complex morphologies in multiphase polymers is still a major challenge in the field of polymer blending. To achieve the desired properties, we need to acquire a good knowledge of structure-property relationships in such systems. Generally, for a multiple phase system two distinct broad category of wetting regimes are possible: complete wetting and partial wetting (de Gennes, Brochard- Wyard, & Quere, 2004; Torza & Mason, 1970). Complete wetting is the most stable thermodynamic state where one phase segregates the two other phases from each other in a ternary system. In contrast, partial wetting is the thermodynamically favorable structure where all phases form a three phase contact and none of the phases completely spreads at the interface of the two others. The majority of studies of ternary immiscible polymer blends deal with complete wetting behavior in which interfacial dynamics can result in a broad range of phase separated structures. It has been shown that through control of composition, interfacial dynamic, and viscoelastic properties of components of a multiphase polymer blend, encapsulated structures can be converted to multiple percolated structures such as the transition of core-shell droplets to tri-continuous structure in a ternary blend (S Ravati & Favis, 2010a, 2010b; Reignier & Favis, 2000a; J. H. Zhang, Ravati, Virgilio, & Favis, 2007). Ravati and Favis (S Ravati & Favis, 2010a) conducted a detailed composition-morphology analysis of binary, ternary, quaternary, and quinary model systems and generated multiple percolated and onion-type structures. The formation of partially wet droplets has just begun to be reported in ternary and quaternary systems (Horiuchi, Matchariyakul, Yase, & Kitano, 1997; Virgilio, Marc-Aurele, & Favis, 2009). Virgilio et al. (Virgilio, Marc-Aurele, et al., 2009) were first to detail the generation of an entirely novel close-packed droplet array of polystyrene (PS) at the interface of high density polyethylene (HDPE) and polypropylene (PP).
However, it is still not clear how the incorporation of partially wet droplets can influence the mechanical properties in a ternary system.
Binary blends of PLA with other polymers are among the most widely studied bioplastic systems (H. Bai et al., 2012; Imre et al., 2013; Z. Liu, Luo, Bai, Zhang, & Fu, 2016a; Ojijo, Ray, & Sadiku, 2013; Stoclet, Seguela, & Lefebvre, 2011; Thurber, Xu, Myers, Lodge, & Macosko, 2015; M. Wu, Wu, Wang, Zhang, & Fu, 2014), however, researchers have shown an increased interest in multicomponent blends with PLA (H. Liu, Chen, Liu, Estep, & Zhang, 2010; K. Zhang, Mohanty, & Misra, 2012). High performance polymeric materials with high levels of toughness, mechanical strength, and thermal resistance can be obtained by combining plastics with complementary properties (Yongjin Li & Shimizu, 2009; H. Liu, Song, Chen, Guo, & Zhang, 2011; Luzinov, Pagnoulle, & Jerome, 2000a). They usually exhibit a more balanced performance compared with binary systems where improvement in one property can lead to a substantial decrease in other properties. However, a wide variety of complex morphologies in the study of ternary and quaternary polymer blends have been reported which must be determined and quantified in the case of bioplastic based multicomponent systems. The formation of a variety of partial and complete wet structures has been reported in ternary blends of bioplastics with some novel tunable morphologies (Sepehr Ravati & Favis, 2013b). To the best of our knowledge, little attention has been paid to the detailed characterization and control of the morphology in multiphase bioplastic blends, despite the clear role of phase morphology in controlling the physical properties (Dou et al., 2015; Luzinov, Xi, Pagnoulle, Huynh-Ba, & Jerome, 1999; Sepehr Ravati, Beaulieu, Zolali, & Favis, 2014). What is still unknown are how the development of morphologies such as multiple encapsulated, multiple percolated as well as partially wet structures contribute to the mechanical properties and particularly the toughness of multiple phase bioplastic blends. In addition, the effect of the level of continuity and interfacial interactions of phases are required to be thoroughly investigated in such heterophase systems.
1.2 Objectives
The main objective of this project is to control the morphology in ternary and quaternary PLA- based blends to generate ultratough materials. Thus, the following sub-objectives are envisaged as the main milestones to achieving the main objective of this research:
a) Establish the principal parameters which control partially and completely wet morphological structuring in ternary and quaternary PLA-based blends.
b) Determine the effect of partially wet droplets on the mechanical performance of co-continuous PLA-based systems.
c) Establish the correlation between the tri-continuous structure and mechanical properties of co- continuous PLA-based blends.
d) Develop ultratough materials based on partial and complete wetting in PLA-based blends. In this work, the structuring of morphology in blends composed of the most relevant bioplastics with PLA are investigated. The effect of composition and interfacial dynamics on the morphology development of the ternary and quaternary systems are studied in detail. A simple melt mixing method, either using internal melt mixing or twin screw extrusion, is used to generate new structures based on two wetting behaviors of partial and complete wetting. The effect of these wetting behaviors on mechanical properties, and in particular notched Izod impact toughness, is examined. Attempts are made to understand the relationship between these structures and the mechanical properties.