Elsevier

Composites Part B: Engineering

Volume 135, 15 February 2018, Pages 25-34
Composites Part B: Engineering

Phase transition and anomalous rheological behaviour of polylactide/graphene nanocomposites

https://doi.org/10.1016/j.compositesb.2017.10.002Get rights and content

Highlights

  • Biodegradable nanocomposites with electrical conductivity were prepared.

  • Electrical percolation threshold was observed in GNP loading range of 6–9 wt% (3.5–5.3 vol%).

  • Phase transition in rheological behaviour of the nanocomposites was found to depend on temperature.

  • An unusual increase in storage modulus with increasing temperature was detected for nanocomposites with 6–9 wt% GNPs.

Abstract

Polylactide (PLA) nanocomposites with graphene nanoplatelet (GNP) contents of 0–15 wt% were prepared and characterised in terms of rheology and electrical conductivity. As expected for rigid nanofillers, GNP incorporation significantly enhanced the viscoelastic properties of the nanocomposites. Furthermore, above a certain GNP concentration the liquid-like melt flow behaviour changed into a solid-like behaviour. However, this rheological percolation threshold was found to decrease from near 9 wt% at 180 °C to below 6 wt% at 220 °C, detected and measured by Van Gurp-Palmen plots and Winter-Chambon method, respectively. It was also found that the viscoelastic properties of some of the nanocomposites increased with increasing temperature, which is in contrast to the rheology of ideal polymeric melts. The Electrical percolation threshold of the PLA/GNP system, which is determined by a sudden increase in DC conductivity and is indicative of establishment of interconnected conductive structures of GNPs within the matrix, was observed to be between 6 and 9 wt% GNPs. Morphological studies also showed physical contact between the platelets at 9 wt% while at 6 wt% they were still separated by the PLA matrix. These observations challenge the attribution of rheological percolation threshold to the formation of a percolating network of only filler particles within the matrix. A percolating network consisting of both polymer chains and filler particles might be the reason for the temperature dependency of rheological percolation threshold in the PLA/GNP system.

Introduction

Synthetic polymeric materials have become indispensable in today's life. They are utilised in a wide range of applications in diverse fields such as packaging, agriculture, food, consumer products, construction materials, automotive, aerospace, and medical appliances [1], [2], [3], [4], [5]. The durability properties which make traditional petroleum-based polymers ideal for some applications such as in the construction and automotive sectors can lead to waste disposal problems, leading to environmental pollution [6]. Consequently, significant amount of research is being conducted in developing biodegradable polymers to replace the persistent non-biodegradable polymers in various products. Polyesters play a predominant role as biodegradable polymers due to their potentially hydrolysable ester bonds [7]. Polylactide (PLA) is a biodegradable aliphatic polyester, derived from renewable resources, with potential to replace the petroleum-derived polymers in a variety of applications such as biomedical, pharmaceutical, and agricultural areas, as well as in engineering applications like electronic and electrical devices [8], [9].

To date, a variety of nanofillers has been used to reinforce and overcome some of the insufficient and inadequate properties of PLA. Materials of natural origins, such as a montmorillonite type of layered silicate compounds have been used extensively due to their abundant availability and reasonable price as well as their effectiveness in enhancing crystallinity, barrier, thermal and mechanical properties of PLA [10]. PLA nanocomposites containing various nanofillers including cellulose [11] and natural [12] fibres have also been studied. However, addition of these nanofillers to the matrix does not result in composites with satisfactory electrical and thermal conductivity. Such shortcomings can be overcome by introducing carbonous nanofillers, such as carbon nanotubes (CNT), carbon nanofibres (CNF) and graphene nanoplatelets (GNP) into the matrix. Various characteristics including mechanical, thermal and electrical properties as well as rheological behaviour of PLA composites embedded with carbon nanotubes [13], [14], [15], [16] and carbon fibres [17], [18], [19] have been reported. A few researchers have also worked on incorporation of expanded graphite [20], GNPs [21], [22], and thermally reduced graphene [23] in PLA. These reports have investigated properties of these systems including their mechanical and rheological properties.

GNPs are graphitic nanoparticles with layered structures, which are composed of stacked 2D graphene sheets bonded together with van der Waals forces [24]. With exceptional properties such as high thermal conductivity, superior mechanical properties and excellent electronic transport properties, this new type of nanofiller has attracted significant attention in both academia and industry [25], [26], [27], [28], [29], [30]. Graphene can be used in the new generations of high speed and radio frequency logic devices, electronic circuits, sensors, and solar cells [2], [31]. GNP has also proved to have the potential for reinforcing polymers and developing novel materials with enhanced properties [32], [33], [34], [35], [36]. Recently, researchers in the area of polymeric nanocomposites have shown that GNP embedding can significantly enhance the mechanical properties [37], [38], thermal conductivity [24], [39], dimensional stability [40], thermal stability [41], [42], gas barrier properties [40], and electrical conductivity [43], [44] of polymers.

Graphene-reinforced polymers have broad potential applications in automotive and aerospace, biomedical engineering and biotechnology as well as electronics [2], [6]. Such composites can be used for antistatic coatings, lightening-protection panels, solar panels, thermoelectric materials, electromagnetic interference shielding, and high temperature conducting adhesives [26], [27], [45]. Graphene-based nanocomposites have also been used in biomedical engineering in particular with biodegradable matrices such as PLA for applications such as scaffolds, neuronal cells, and fibrous membranes for tissue engineering [7], [8], [25]. Performance of PLA nanocomposites embedded with graphene nanoplatelets in shielding electromagnetic radiations has been also studied over different radiation frequency ranges [46], [47].

As an important part of nanocomposites characterisation, many researchers have also studied the variations of the melt flow behaviour of GNP-based nanocomposites with different filler loadings. Such rheological studies are important with regard to the processing of these polymeric materials, which exhibit liquid-solid (viscoelasticity) behaviour in the melt phase leading to complications in their processing. Furthermore, rheological studies of filled polymers can be used as a tool for examining their microstructure [25], [48]. Dynamic viscoelastic properties obtained at low frequencies in oscillatory rheometry reveal information about the microstructure and the interactions within the system for instance, the formation of a rigid network of the nanoparticles. At high frequencies, the rheological properties reflect the motions and mechanical resistance of short polymer segments being less affected by the presence of nanoparticles [40].

A variety of nanocomposites containing both low [49], [50], [51], [52], [53] and high [54], [55], [56], [57], [58], [59] concentrations of graphene/graphite have been prepared and their melt rheology has been investigated. Dynamic oscillatory rheometry has shown that the viscoelastic properties of these nanocomposites increase with increasing filler concentration. Frequency sweeps have demonstrated that the effect of filler loading is more significant within the low frequency range compared to higher frequencies. Furthermore, it has been observed that the dynamic moduli, in particular storage modulus, show weaker frequency dependency with increasing filler loading, and above a certain loading they exhibit frequency independency (plateau) over the low frequency region.

Rheological behaviour of a polymeric nanocomposite is governed by its microstructure, which in turn is determined by the various interactions present within the system, namely polymer-polymer, polymer-filler and filler-filler interactions [60]. In rheological investigations of filled polymers, the transition in the viscoelastic behaviour of the systems from liquid-like to solid-like has been of interest to researchers. The filler loading at which this transition occurs is commonly referred to as the percolation threshold. However, considering that temperature can affect the microstructure of nanocomposites and consequently their viscoelastic properties, it is surprising that the effect of temperature has not been taken into account in the rheological investigations of GNP-based nanocomposites. To the best of authors knowledge, our recent papers on rheology of poly (butylene adipate-co-terephthalate)/GNP nanocomposites high [59], [61] are so far the only works studying the effect of measurement temperature on the viscoelastic properties and physical gelation of GNP-based nanocomposites. The present study aims to further the investigation on rheological behaviour of GNP-based nanocomposites by examining the variations of viscoelastic properties of PLA/GNP nanocomposites versus GNP loading and measurements temperature.

While there have been some previous studies on the effect of GNP loading on the rheological behaviour of PLA/GNP system, none has investigated the effect of temperature. In a series of studies, Sabzi et al. evaluated the differences between the linear [57] and nonlinear [2] rheological behaviours of PLA nanocomposites embedded with two different types of GNPs. Wang et al. [22] investigated the effect of thickness of graphene nanosheets on the percolation networks and transient rheology of PLA composites. In another series of studies, Narimissa et al. evaluated the effects of GNP addition on the linear [21] and extensional [6] rheology of PLA. In all the above mentioned studies, temperature of the measurements was fixed except in Ref. [21], in which time-temperature superposition (TTS) principle was applied to frequency sweep (0.01–100 rad/s) curves of 0–10 wt% GNPs in PLA over a temperature range of 170–200 °C with the reference temperature of 180 °C. However, the master curves presented in Ref. [21] were generated by software and they only extended slightly beyond the upper limit of the frequency range of the measurements and not below the lower limit. Nevertheless, no examination of the data, which were obtained at different temperatures, was provided in that study nor the success of TTS was discussed. However, as it will be seen in the Results and Discussion section of the present paper, temperature has a significant and unique impact on the viscoelastic behaviour of the investigated PLA/GNP system.

In the current work, electrically conductive biodegradable nanocomposites were produced by melt-mixing of PLA as the host matrix and GNPs as the conductive filler. Samples with GNP concentrations of 0–15 wt% were prepared and their rheology was investigated in dynamic oscillatory shear mode. By conducting the measurements at three different temperatures, effect of GNP loading was studied simultaneously with the effect of temperature and the liquid-solid transition behaviour of the system was investigated at each temperature. Furthermore, morphology of the nanocomposites was studied by scanning electron microscopy and the effect of GNP incorporation on electrical conductivity of PLA was also determined. The present study determines, for the first time, effects of GNP concentration and temperature simultaneously on the viscoelastic properties of PLA/GNP nanocomposites and analyses their rheological and electrical percolation thresholds.

Section snippets

Materials

PLA was purchased from NatureWorks LLC. The grade used was 4032D, which exhibits a density of 1.24 g/cm3 and a melting temperature range of 155–170 °C [62]. “M” grade GNP was obtained from XG Sciences (USA) with average thickness of 6–8 nm, surface area of 120–150 m2/g, density of 2.2 g/cm3 and electrical conductivity of 102 and 107 S/m for perpendicular and parallel to the surface, respectively [63].

Preparation of PLA/GNP nanocomposites

Prior to the mixing, PLA pellets and GNPs were dried in an oven at 80 °C overnight in order to

Effect of GNP loading on viscoelastic properties of PLA

Fig. 1 depicts the frequency dependency of the storage (G′) and loss (G″) moduli of PLA/GNP nanocomposites as a function of GNP loading at 180 °C. It is observed that addition of GNPs significantly enhances the moduli of PLA over the entire frequency range. At high frequencies (>10 rad/s), G′ and G″ increase monotonically with increasing GNP loading. On the other hand, at lower frequencies, adding GNPs up to 9 wt% gradually improves the moduli but above 9 wt% a sharp increase is observed in

Conclusions

Effects of GNP loading and temperature on viscoelastic properties of PLA/GNP nanocomposites were simultaneously investigated via frequency sweep tests in the present study for the first time. GNP incorporation significantly enhanced the linear viscoelastic properties of PLA particularly its storage modulus, which showed weaker frequency dependency in the low frequency region with increasing GNP loading. Variations of the rheological behaviour of PLA with increasing GNP concentration were found

Acknowledgment

Authors would like to acknowledge the support received from the Australian Government through a Research Training Program (RTP) scholarship. Authors are also grateful for the support received from the School of Engineering, RMIT University. Authors would also like to acknowledge the Australian Defence Science and Technology group for providing the DC measurement equipment. Authors are also grateful for the support received from the Australian Research Council (ARC) Research Hub for Future

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