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Influence of nano-graphite platelet concentration on onset of crystalline degradation in polylactide composites

https://doi.org/10.1016/j.polymdegradstab.2012.02.010Get rights and content

Abstract

Nano-carbon fillers offer enhanced thermal and mechanical properties to biodegradable polymer matrices. In this work, we study extruded nano-graphite platelet (NGP) loaded polylactide (PLA) using thermal, mechanical, and microstructural analysis techniques. The influence of NGP loading on polymer crystallization, the Young’s modulus, tensile strength, and crystallography of the polymer composite are determined. We establish the optimal NGP loading concentration beyond which agglomeration effects degrade crystalline and structural properties of PLA-NGP composites.

Introduction

Polymer-based composites began in the 1960s as a new concept for advanced materials. By dispersing strong and stiff fibres within polymer matrices, composite materials offering significant structural benefits with respect to weight loss reduction, durability, and mechanical performance were developed. The enhanced performance of polymer composites is a result of interfacial bond chemistry as well as the degree of nano-additive dispersion and the resultant morphology.

Early in 1990s, an important discovery which promised to revolutionize the field of nano-material science was made in the form of carbon-based nano-fillers. Addition of minute quantities of carbon nano-tubes (CNTs) or carbon nano-fibres (CNFs) to polymer resins promised polymer nanocomposites with enhanced thermal, mechanical, and electrical performance. Significant advantages from nano-carbon dispersion were determined when dispersed in the polymer matrix. However, when dispersed in the polymer matrix through viable mixing techniques (e.g., melt blending and solvent casting), these carbon-based nano-fillers are likely to exhibit strong van der Waals’ interaction. This results in agglomeration, and therefore, inhomogeneous composites [1], [2], [3].

Recently, a new breed of cost-effective carbon-based nano-dimensional materials named as nano-graphite platelets (NGP) have emerged. They promise to eliminate the problems associated with nano-clays, CNTs, or CNFs while offering potentially enhanced performance as well as being a low cost alternative [4], [5], [6]. The potential applications of graphene-polymer composites include single molecule gas detection, transparent conducting electrodes, and energy storage devices such as supercapacitors and lithium ion batteries [7]. Moreover, the highest thermal conductivity reported in literature so far belongs to nano-graphene materials (six times higher than copper). In terms of electrical and mechanical properties they are similar to copper and fifty times stronger than steel while having one fourth density of copper [5], [6], [8]. This work explores the influence of such nano-graphitic fillers in composites, and the degree of property enhancement and degradation as a function of loading.

Polylactide (PLA) is a thermoplastic biodegradable polyester. This polymer is currently receiving considerable attention for conventional uses, due to its valuable qualities such as biodegradability, production from renewable resources and its numerous functions including packaging materials, production of fibres and composites for technical applications [9], [10].

PLA is a rigid thermoplastic biodegradable polymer that can be semi-crystalline, crystalline, or totally amorphous, depending on the stereo purity of the polymer backbone. PLA is a unique polymer that in many ways behaves like polyethylene terephthalate, but also performs a lot like polypropylene; a polyolefin. Ultimately, it can potentially be utilised in the broadest range of applications because of its ability to be stress crystallised, thermally crystallised, impact modified, filled, copolymerised, and processed in most polymer processing equipment [11]. Nevertheless, the application of PLA in engineering devices such as solar panels and in mechanical and automotive parts requires enhancements in its properties via combining this polyester matrix with different dispersed phases such as nano-fillers, plasticisers, impact modifiers, and flame-retardants [12], [13].

Section snippets

Materials and methods

For this study, PLA composites with NGP fillers were prepared via melt intercalation mixing process. The NGP loading (or doping) concentration was determined based on the weight percentage of the starting mixture. NGPs and PLA pellets were dry mixed in different compositions (0–10 wt% NGP) prior to melt intercalation process. This was followed by melt blending in a Brabender Twin Screw extruder at 180 °C and 40 rpm mixing condition. Finally, the melt blended pellets underwent compression

Results and discussion

The MDSC analysis was used to determine the glass transition temperature, crystallization characteristics, and melting properties of the NGP-loaded PLA nanocomposites. The results showed no significant change in the glass transition temperature (Tg) for the nanocomposites, which was consistently 59–60 °C. Likewise the melting temperature (Tm) also did not appear to be influenced by the NGP loading, with values of 149–151 °C for all samples. However, the crystallisation properties of the

Conclusions

In summary, we present an investigation of NGP-loaded polylactide polymer composites. The inclusion of nano-graphite platelets is favoured for increased thermal, mechanical, and electrical properties of polymers such as the biodegradable polylactide. We have determined that a 1–5 wt% loading concentration of NGP can lead to increased crystallinity in the polymer matrix and provide improved thermal and mechanical characteristics. Further loading tends to degrade crystalline properties of the

Acknowledgements

MB and SS acknowledge Australian Research Council (ARC) Post-Doctoral Fellowships through DP1092717 and DP110100262, respectively. Infrastructure support from the ARC through LE100100215 (SS) is also acknowledged. The authors thank Prof. Hyoung J. Choi (Inha University) for support with the XRD measurements.

References (17)

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