Cellulosic nanofibers filled poly(β-hydroxybutyrate): Relations between viscoelasticity of composites and aspect ratios of nanofibers

Highlights

• Percolation thresholds of composites reduce with increased aspect ratios of fibers.

• Stress-scaling is independent on the geometry or size of fibers during start-up flow.

• Hysteresis energy-scaling is independent on the loading and aspect ratios of fibers.

Abstract

Dispersion states are vital for fibrous nanocelluloses to be used as reinforcements for polymers, which is highly dependent on geometry of nanocelluloses. Three types of nanocelluloses with various fiber aspect ratios were used to prepare target composite samples with poly(β-hydroxybutyrate) in this work. Viscoelasticity/elastoplasticity were used as probes to detect the flexibility-morphology relations of nanocelluloses in polymer. Cellulose nanocrystals (aspect ratio = 8) were rigid in polymer, retaining their rod-like shape, whereas bacterial celluloses (aspect ratio = 600) fully flexible, forming closely networked structure, and cellulose nanofibers (aspect ratio = 70) semi-flexible, dispersing as loosely flocculated clusters. Owing to these differences, the viscoelastic flow and elastoplastic deformation of three kinds of composites differed from one another. The strain-scaling and hysteresis work-scaling behaviors were then used to establish relaxation scale-structure correlations of target samples. This work provides interesting information around regulating the dispersion of nanocelluloses in polymer composites by tailoring aspect ratios of nanocelluloses.

Introduction

One-dimensional nanocelluloses, including cellulose nanocrystals (CNCs), cellulose nanofibers (CNFs) and bacterial celluloses (BCs) have generated much attention in recent years. This types of nanocelluloses show fibrous structure with high degree of crystallinity, and hence possess extraordinary transverse/axial moduli, and mechanical strengths. Their surface properties are easily tuned because of good reactivity of their surface hydroxyl groups. Accordingly, they have been developed as promising fibrous reinforcements for polymers (Oksman et al., 2016), especially to be used to prepare composites with biodegradable aliphatic polyesters such as poly(β-hydroxybutyrate) (PHB), poly(ε-caprolactone) (PCL), poly(butylene adipate-co-terephthalate) (PBAT), and polylactide (PLA), etc. (Abdul Khalil et al., 2012) This kind of polymer composites show green or sustainable features because both the matrices and fillers are biodegradable or partially/fully biomass-derived, and hence have attracted much interest and been extensively studied so far (Clarkson et al., 2020).

The reported studies on the nanocelluloses filled aliphatic polyesters mainly focused on establishing hierarchical structure-property relationships, for instance, optimizing dispersion of nanocelluloses to tailor barrier property or degradation rates of matrices (Ding et al., 2015; Fang et al., 2019; Fox et al., 2014; Geng et al., 2018), strengthening phase adhesions between celluloses and polyesters to improve mechanical performance of nanocomposites (Fang et al., 2019; Geng et al., 2018; Kiziltas et al., 2016; Lai et al., 2020; Re et al., 2018; Xu et al., 2016; Ying et al., 2018), regulating surface properties of celluloses to tune their nucleating roles during crystallization of matrices (Chen et al., 2015; Chen, Wu, et al., 2017; Eriksson et al., 2018; Wang et al., 2020; Xu, Lv, et al., 2017; Xu, Wu, et al., 2017), and using nanocelluloses as the rheological modifiers to cater to the requirements of processing flow and melt strength of composites (Bagheriasl et al., 2016; Eriksson et al., 2018; Meree et al., 2016; Safdari et al., 2016; Safdari et al., 2017; Wang et al., 2018; Wang et al., 2020), etc. The polyester-nanocellulose interactions are key to correlate structures and properties of this kind of green composites (Xu et al., 2016), and the chemical modifications of nanocellulose surfaces are the effective ways to improve two-phase adhesions.

It is well known that the interfacial shear strengths are influenced by both chemical and physical aspects for the fiber-reinforced polymer composite (FRPC) (Thomason, 2007; Thomason et al., 1996; Thomason & Vlug, 1996). In other words, the length of fiber is also very important. Sufficient length is an essential prerequisite for the fibrous fillers to act as effective reinforcements in discontinuous fibers filled polymer composites (Thomason, 2007). However, overlength of fibers is against their dispersion in polymer matrices because the long fibers, especially those one-dimensional nanofibers (with high aspect ratio (length-to-diameter)), are easily self-entangled, forming clusters even at low concentrations, instead of being dispersed homogeneously in matrices (Keshtkar et al., 2009; Lu et al., 2021). This decreases effective interfacial area. Accordingly, it is reasonable to propose that the nanofibers with different lengths might present fully different fiber morphologies in polymer matrices at the same physicochemical environments. From perspective of suspension rheology, those fibers reveal different dilute-to-semi-dilute transitions in matrices (Dinh & Armstrong, 1984; Djalili-Moghaddam & Toll, 2006), and the aspect ratio relates their flexibility with final dispersion state closely.

As for the nanocelluloses filled biodegradable polyesters, it is clear that the length of nanocelluloses is also crucial to final performance of nanocomposites. Disclosing the flexibility-length-morphology relations of nanocelluloses in the matrix is meaningful, therefore, favoring tailoring hierarchical structures and properties of this kind of green nanocomposites. In this work, nanocellulose filled PHB was used as the target system for the study. Three types of cellulosic nanofibers with various aspect ratios, including CNC (short fiber), CNF (moderate-length fiber) and BC (longer fiber), were used for comparison. The rheological properties of these three sample systems were explored then, aiming at using viscoelastic responses as probes to detect the relations between flexibility of nanofibers and their final dispersion morphology. This work provides useful information on the structural design of green composites based on the fibrous nanocelluloses filled biodegradable polyesters.