Effects of blend morphology and layered silicates' localization on the mechanical, thermal and viscoelastic properties of multiphase biopolymeric systems

Abstract

In today's world, petroleum-based polymers are extensively consumed in various applications but their wastes have turned to be a serious environmental concern and their incineration has expedited the global warming. These concerns have motivated the scientists and industries to develop biodegradable polymeric products with satisfactory properties. Polylactide (PLA) is an aliphatic polyester, which is derived from renewable agricultural crops such as cornstarch, sugar cane, cassava etc. Bio-based and biodegradable PLA possesses promising mechanical properties such as high elastic modulus and tensile strength as well as high transparency with good barrier properties. Therefore, PLA could be an attractive alternative to be used in commodity and engineering applications such as plastic utensils, films and packaging, construction and even automotive where petroleum based polymers are widely used. Aside all these advantages, PLA also have some disadvantages like low-melt strength and slow crystallization, which could have an adverse effect on its processability, formability, and foamability. Moreover, its brittleness, low impact strength and low-service temperature could also confine its usage in various applications. In order to improve these drawbacks, it is possible to blend PLA with various biopolymers and synthetic non-compostable thermoplastics. So far, it was shown that blending PLA with thermoplastic polyurethane (TPU) that possesses high impact strength, biocompatibility and durability, is an effective way to improve the toughness and melt properties of PLA. However, PLA and TPU are not compatible polymers and hence the formed blends would be fully immiscble without having a strong interfacial interaction at the interface of PLA and TPU phases. On the other hand, during the melt mixing of a PLA-based blend with TPU as the minor phase, which forms droplet morphology within the PLA matrix, in the competition between the droplet breakup and coalescence of the TPU minor phase, due to their incompatibility the droplet coalesence would be dominent and hence a coarse droplet morphology with poor interfacial adhesion is expected. All these would supresss the final desired properties of the blend. Consequently, it is crucial to improve the phase compatibility and to stabilize the morphology of the minor phase with finer droplet structure. It is know that chain extenders (CE) that are reactive with both polymers could act as compatibilizers by decreasing the interfacial tension between two phases which also refine the droplets and stabilize the morphology of the secondary phase. Among chain extenders, Joncryl has known to be an effective case for polyesters although it has not been properly investigated for PLA/TPU blends. On the other hand, the incorporation of nanoparticles in PLA blends and their interfacial localization could stabilize and refine the droplet morphology while they could also enhance the final strength and stiffness of the blend. In a polymer blend nanocomposite with droplet morphology,nanoparticles could localize at polymer phases or at the interface, and as noted the latter case prevents the coalescence of minor phase. Among nanoparticles, due to their lamellar structures and high aspect ratio, organoclays are inexpensive additives with high barrier properties. However the use of organoclays with PLA increases the degradation of PLA during melt processing, which deteriorates PLAs rheological properties. Therefore, the incorporation of CE together with organoclays is a convenient way to compensate the rheological properties of PLA. The main objective of this study is to improve the compatibility of PLA/TPU blend systems through using Joncryl in order to properly enhance the ductility, toughness, and impact strength of the brittle PLA matrix. Therefore, firstly, the effect of Joncryl CE incorporation on the properties of different PLA/TPU (75 wt.%/25 wt.%) blend systems was investigated. The blends were designed with either ether- or ester-based TPU grades (i.e., TPUether and TPUester) and with amorphous or semicrystalline PLA as the matrix material (i.e., aPLA and scPLA). It was observed that TPUester had a better compatibility with PLA and hence the mechanical properties of the blends with TPUester were improved more remarkably. The rheological results also showed that the CE had a better reactivity with PLA than with TPU grades. Among the TPUs, the CE also showed a better reactivity with TPUester than with TPUether. Therefore, while the PLA and TPUester possessed better compatibility, the CE addition further enhanced the interfacial compatibility of their blends. This dramatically caused the enhancements of the final morphological, rheological, and mechanical properties of their blends. Afterwards the composition of compatibilized PLA/TPU blends with Joncryl CE was optimized to obtain structure with high impact resistance and ductility. In order to determine the effect of composition ratio of PLA/TPU blends on the final properties, aPLA/TPUester blends were prepared with different blending ratios of 95 wt%/05 wt%, 85 wt.%/15 w.t%, 75 wt.%/25 wt.% and 65 wt.%/35 wt.% with incorporating 0.5 wt.% of CE. Joncryl CE provided remarkable decrease in TPU droplet sizes in all cases but toughness and strain at break increased profoundly at 75 wt.%/25 wt.% and 65 wt.%/35 wt% blending ratios. The 75 wt.%/25 wt.% blend was then melt mixed with changing CE amounts of 0.25, 0.5, 0.75, and 1.0 wt.%. It was observed that incorporation of 0.5 wt% CE was optimum content to reach the highest toughness and strain at break values. Finally, scPLA/TPUester blends at weight ratio of 75 wt.%/25 wt.% compatibilized with fixed CE content of 0.5 wt.% were prepared with different loadings of Cloisite 30B (C30B) nanoclay (i.e., 1, 3, and 5 wt.%). PLA/TPU blends with C30B nanoclays were also prepared without compatibilizer for the sake of comparison. The effect of nanoclay content and the synergistic effect of nanoclay and CE on the resultant properties were then analyzed through morphological, rheological, thermomechanical, and mechanical analyses. In all cases it was observed that C30B localized at the interface between PLA and TPU phases as it was expected from thermodynamic calculations. Clay nanoparticles localized at the interface could act as a barrier in order to prevent droplet coalescence. Fine droplet morphology was achieved due to the interface localization of C30B.