Tailoring the toughness of sustainable polymer blends from biodegradable plastics via morphology transition observed by atomic force microscopy

Highlights

• The super-toughened blends are fabricated using only biodegradable components.

• Unique AFM is used to identify the morphology evolution in reactive extrusion.

• Core-shell PBAT-PBS structures are formed in the ternary blends.

• Phase transition and improved compatibility work together to the super-toughness.

Abstract

Super-toughened polylactide/polybutylene succinate/polybutylene adipate terephthalate (PLA/PBS/PBAT) ternary blends can be prepared by reactive extrusion in the presence of peroxide. However, the high peroxide contents always lead to lowered biodegradability and decreased flowability of the prepared materials. In this study, a strategy to prepare such super-toughened PLA ternary blends with a small amount of peroxide, by simultaneous phase transition and interfacial compatibilization, was demonstrated. Firstly, based on thermodynamic predictions, a core-shell PBS/PBAT structure was successfully prepared that worked as a toughening agent for PLA. Secondly, the viscosity ratio between PLA and PBS/PBAT was adjusted via the introduction of a small amount of peroxide because of the different reactive priority between peroxide and the polymers. Via transferring the co-continuous PBS/PBAT to droplet structure with optimum size and improving the compatibility between PLA and core-shell PBAT-PBS, two different super-toughened PLA ternary blends with high impact strength and melt strength were successfully prepared. The resulting ternary blends showed super toughness with significantly improved notched impact strength (~530 J/m). The mechanism behind the morphology transformation is discussed based on the phase inversion theory with the help of relaxation time spectrum analysis and atomic force microscopy (AFM). The studies show that both the morphology transition of the different components and improved compatibility make contributions together to the improved mechanical properties. The research provides a new approach to improving the toughness of the polymer blends in reaction extrusion by tailoring the viscosity ratios between different components. The ternary blends developed are expected to be used widely in industrial applications.

Introduction

Arising from the increasing concerns on the environment problems caused by the non-biodegradable petroleum-based plastics, developing sustainable polymeric materials from biobased and biodegradable plastics has drawn a great deal of interest [1]. Although there is a great boost in the global bio-plastic market with a report compound annual growth rate (CAGR) of 12.5%, it is only 4.25% of the whole plastic market by 2020 according to the market prediction [2]. This is because the daily application of the bio-plastics is greatly limited by their lower mechanical properties, poorer thermal stable performance, as well as cost competitive compared to the traditional petroleum-based plastics, such as widely used polypropylene (PP), acrylonitrile butadiene styrene (ABS), etc. Polylactide (PLA), as one of the most consumed bioplastics, exhibits biocompatibility, biodegradability, high stiffness, transparency but extremely low toughness. The inherent brittleness greatly limits its applications in our daily life [3]. Developing biodegradable PLA with comparable mechanical properties, especially with high toughness, is believed to be able to expand the market share of the biodegradable plastics in the world.

To develop fully biodegradable polymeric materials with high mechanical performance via blending technology, the phase dispersion state and compatibility or interfacial adhesion between different biodegradable components are critical [4]. It is well known that improved compatibility can improve the properties, such as mechanical, rheological and thermal properties, of the final blends dramatically [5,6]. Therefore, various methods have been applied to improve the compatibility, and thus decrease the interfacial tension of the polymeric components in blends, aiming to maximize the role of each component. The methods include ex-situ or in-situ co-polymer introduction [7], dispersion of nano-particles on the interface [8], etc. Compared to improved compatibility, the influence of the dispersion state on the performance of the final blends is more complex. Earlier reported research has revealed that toughness could be achieved in polymer-rubber blends when the interparticle distance was smaller than the critical values, even without interfacial chemical bonding [9]. This finding was attributed to the fact that van der Waals adhesion of about 1000 J/m² is enough to provide the minimum adhesion required for rubber toughening. Undoubtedly, improved adhesion can improve the dispersion of the rubber via decreasing the mean diameter, to realize smaller interparticle distance at a fixed rubber amount. However, the dispersion of the toughening phase in the blends seems more important and critical compared to the interfacial chemical bonding. Apart from the classic dispersion states of blends such as sea-island, co-continuous or interpenetrating, other special morphologies such as thin laminae phase have been reported to influence the material properties dramatically [10,11].

To predict the morphology of polymer blends, various theories are constantly being proposed. Among them, drop breakup and coalescence equilibrium theory during mixing has been successfully applied in many research works to explain the invariant morphology [12]. The classic Taylor's theory [13] proposes that the equivalent droplet size D could be given by equation (1):

$$\text{D}=\frac{4\text{Γ}({\eta }_r+1)}{\dot{\gamma }{\eta }_m(\left(\frac{19}{4}\right){\eta }_r+4)},{\eta }_r<2.5$$

where $\text{Γ}$ is the interfacial stress,${\eta }_r$ is the ratio between the matrix (${\eta }_m$) and dispersed phase (${\eta }_d$) viscosities and $\dot{\gamma }$ is the shear rate.

Although the theory has been developed by other researchers such as Grace [14] and Wu [15], intuitively, the breakup of the dispersed phase droplet is mainly controlled by: i) capillary number, Ca, which is defined as the ratio between matrix viscous stress $\dot{\gamma }$ and interfacial stress $\text{Γ}$; ii) the ratio (${\eta }_r$) between the matrix (${\eta }_m$) and dispersed phase (${\eta }_d$) viscosities. Compared to droplet breakup, coalescence in polymer blends is more significant and complex than expected. The dispersed volume factions [16], matrix phase viscosity [17], interfacial mobility [18], elastic recoil and shear rates [19] all are reported to influence the coalescence process in polymer blends. Influenced by various parameters, the balance between droplet breakup and coalescence determines the final morphology of the polymer blends. Polymer blends normally exhibited different morphologies such as droplets or co-continuous, which greatly depend on the composition ratio of the components [20]. Taking A/B binary polymer blends as an example, the sea-island morphology can occur if B is dispersed as droplets in continuous phase of A. On the other hand, co-continuous structures can happen with equivalent amount of A and B in the A/B polymer blend. The volume fraction (${\text{Φ}}_2$) at which phase transition happens is different for different polymer blends systems and mainly is dependent on the viscosity ratio between the components, which can be simply represented as in equation (2):

$${\text{Φ}}_2=\frac{1}{1+({\eta }_1/{\eta }_2)}$$

where ${\eta }_1$ and ${\eta }_2$ are the viscosities of component 1 and component 2, respectively.

Except the composition ratio, the viscosity ratio, interfacial tension and shear rate during process are also reported to influence the morphology of polymer blends. For blends with the same formulation and processing conditions, the morphology can be changed by adjusting the viscosity ratio and interfacial tension. Therefore, incorporation of pre-made compatibilizer, such as co-polymers or maleic anhydride-grafted polymers [21], or conducting in-situ reactions during the process [22], can be used to stabilize and control the morphology, especially using a high efficiency in-situ reaction. Previous research on the effects of in-situ reaction on the morphology evolution was mainly focused on decrease of the droplet size via stabilizing the interface, however, its effect on tailoring the morphology because of changing the viscosity is lacking in the literature. This results from the difficulty of distinguishing the viscosity ratios between the different components after reactive extrusion.

In this work, a small amount of peroxide was added in the PLA/PBS/PBAT blends, solving the high gel content problem in the previous research [23] which influence adversely the processability and thermal-mechanical properties of the final products, but retaining the super toughness. The peroxide used in this study is 10 times lower compared to our previous research [22]. The effects of both the dispersion phase state and interfacial adhesion on the mechanical and rheological properties of the polymer blends were deeply evaluated. Through our carefully observation on the morphology transition by AFM, the contribution of the phase dispersion state on the toughening of polymer blends via reactive extrusion is highlighted in our work, which is normally neglected in the previous reactive extrusion studies. Our study found that super-toughened PLA ternary blends can be obtained by dispersing the toughening phase into droplets with appropriate size approximately 0.6 μm, along with the improved interfacial adhesion resulting from the co-polymer formed in reactive extrusion.