Enhancement of interfacial adhesion in immiscible polymer blend by using a graft copolymer synthesized from propargyl-terminated poly(3-hydroxybutyrate-co-3-hydroxyhexanoate)

Highlights

• PMMA-g-PHBH was synthesized from Vinyl-PHBH and methyl methacrylate.

• The phase separation size of the PHBH/PMMA blend was lowered by adding PMMA-g-PHBH.

• The improvement in the miscibility of PHBH and PMMA was proved by thermal analysis.

• The toughness of the PHBH/PMMA/PMMA-g-PHBH blend was tuned by their weight ratio.

• PMMA-g-PHBH worked as a compatibilizer of PHBH/PMMA blends.

Abstract

Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH) is a biomass-derived polymer synthesized by the fermentation inside the bacteria, which has excellent flexibility and mechanical toughness. However, the industrial applications of PHBH have been limited because of its poor modulus of elasticity and tensile strength. For improving the mechanical characteristics of PHBH to eventually broaden its industrial applications, it was blended with poly(methyl methacrylate) (PMMA), a rigid polymer. Moreover, blending PHBH was also expected to increase the toughness of PMMA. Nevertheless, PHBH/PMMA blends lack interfacial adhesion due to low miscibility, thus restricting potential applications. This work focusses on enhancing the compatibility of PHBH/PMMA blend by developing a compatibilizer. The synthesis of PMMA-g-PHBH, a graft copolymer, was carried out using methyl methacrylate and vinyl-terminated PHBH derived from propargyl-terminated PHBH. The findings obtained from scanning electron microscopy and thermal analysis showed improvement in the miscibility of PHBH and PMMA upon the inclusion of PMMA-g-PHBH in the blend structure. The mechanical properties of the blends can also be tuned by controlling the weight fraction of PHBH/PMMA/PMMA-g-PHBH. This study proves that PMMA-g-PHBH acts as an effective compatibilizer for PHBH/PMMA blends.

Introduction

In recent times, poly(3-hydroxyalkanoate)s (PHAs) have gained significant attention as a bio-derived, non-toxic and biodegradable plastic which are produced by the fermentation of sugars or plant oils. During the microbial metabolic activity, PHAs are produced in microorganisms as an intercellular storage compound for energy and carbon [1], [2], [3], [4], [5], [6]. Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH) belongs to the category of PHAs, which is a random copolymer consisting of 3-hydroxybutyrate (3HB) and 3-hydroxyhexanoate (3HHx) units [4]. Among the constituent units, the physical properties of PHBH strongly depend on the amount of bulky 3HHx units. Mainly, the mechanical strength and elastic modulus of PHBH are relevant to the fraction of 3HHx units [5], [6]. PHBH also has relatively high flexibility in comparison to other kinds of PHAs such as poly(3-hydroxybutyrate) (PHB) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV). However, Young’s modulus and tensile strength of PHBH are not enough for the industrial applications including food packaging products, and engineering materials.

For expanding the applications of PHBH in daily life, a polymer binary blend is one of the promising strategies, rendering various physical properties to PHBH. Many researchers have reported various PHBH-based blends to adjust its physical properties. For example, polycarbonate [7], poly(vinyl alcohol) [8], poly(ε-caprolactone) (PCL) [9], and poly(L-lactic acid) (PLLA) [10] have been investigated. Poly(methyl methacrylate) (PMMA) is one of the widely employed polymers for the production of organic glass and commodity plastics due to its exceptionally high elastic modulus and rigidity, and could also serve as one of the potential polymers for blending with PHBH since blending rigid PMMA can improve the elastic modulus and maximum strength of PHBH [11], [12]. On the contrary, PMMA has low ductility and toughness, limiting its applications [13], [14]. Therefore, in the present study, it is hoped that the binary blending of PHBH and PMMA will overcome the mechanical weaknesses of individual components including the low elastic modulus of PHBH and brittleness of PMMA. This strategy will also promote the usage of PHBH as a bio-based flexibilizer, thus improving the fragility of polymers.

However, PHBH-based blends generally cause phase separation in the film due to the low miscibility of PHBH with other polymers [14]. Such a phase separation phenomenon in the PHBH/PMMA blend deteriorates the mechanical toughness and ductility of the blends, thereby restricting their industrial applications [15], [16], [17]. Thus, it is necessary to augment the compatibility of the PHBH/PMMA blend which could be achieved by adding additives into the polymeric blend. In such a condition, the incorporation of a compatibilizer is argued to improve the interfacial adhesion between PHBH and PMMA [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26]. For example, as a compatibilizer, the applications of graft copolymers [18], [19], [20], block copolymers [21], [22], Janus particles [23], [24], and surface-modified carbon nanotubes [17], [25], [26] have been reported for the improvement of interfacial adhesion between the polymeric blends. In the present study, attempts have been made for the development of compatibilizer based on graft copolymers, which was prepared through facile methods under mild conditions [18], [19], [20]. Fig. 1 shows the incorporation of a compatibilizer in PHBH and PMMA blend to render good miscibility thus improving the mechanical properties. Moreover, the type of monomers and the reaction conditions are the two governing factors that determine the design of polymeric structures [18], [27]. Recently, end-functionalized PHAs have been used for the synthesis of block copolymers, graft copolymers, and branched copolymers through the various facile processes [28], [29], [30]. Propargyl-terminated PHBH (Propargyl-PHBH), one of the end-functionalized PHAs, is expected to synthesize a graft copolymer via copper-catalyzed azide-alkyne cycloaddition (CuAAC) reaction [31], [32], [33], [34], [35]. In the latest research, Oyama and his coworkers reported that PHBH-b-PCL was prepared from Propargyl-PHBH, used as a compatibilizer for PHBH/PCL blends [21].

In this study, the development of a compatibilizer has been proposed which is synthesized from vinyl-terminated PHBH (Vinyl-PHBH) and methyl methacrylate (MMA) via ‘grafting through’ method to improve the compatibility and enhance the mechanical properties of PHBH and PMMA blend. Vinyl-PHBH was derived from Propargyl-PHBH through the CuAAC reaction. The morphological, thermal and mechanical characterizations of PHBH/PMMA blends with the obtained graft copolymer were investigated to elucidate the physicochemical and structural characteristics and further compared with neat PHBH/PMMA blends. This study presents the novel application of PHBH as an additive both in terms of a flexibilizer and a compatibilizer for polymer blends constituting rigid polymers such as PMMA by using end-reactive PHBH. Furthermore, the obtained results will promote the utilization of bioplastics, thus reducing the environmental burdens caused due to the reckless usage of non-biodegradable plastic materials in the field of material engineering.