Poly(lactic acid)/carbon nanotube nanocomposites with integrated degradation sensing
Graphical abstract
Introduction
As environmental pollution and limited availability of fossil resources have become more serious than ever before, poly(lactic acid) (PLA), which is available from agricultural renewable resources by a combination of fermentation and polymerization, has been of great interest as a sustainable, environmental-friendly and biodegradable thermoplastic polymer. In recent years, reduced production costs of PLA extended the end-use applications of PLA from biomedical materials [1], [2], [3] to commodities [4], [5], [6] such as packaging films, bottles, textile fibres, and high performance engineering plastics for automobile parts. These materials hold very strong future promise for potential applications as a high-performance bio-based plastic. However, in order to be successful in more demanding engineering applications the important issue regarding degradation during the product's lifetime needs to be addressed. It is for this reason that monitoring of degradation levels during usage could prove to be of a vital interest.
Numerous hydrolytic degradation tests have been performed on PLA in order to simulate its process of degradation in the human body (T ∼37 °C) [7], [8], [9], [10], [11], [12] and in natural media such as soil or compost (25 °C < T < 58 °C) [13], [14], [15], all reporting that PLA can be hydrolysed to give low molecular weight water-soluble oligomers. It has been found that degradability can be modified significantly by changing the microstructure of the PLA [16], or by blending with other polymers, additives, plasticizers and often inorganic fillers [17], [18]. However, to the best of our knowledge, so far there is no report on the development of a degradation monitoring system, which would give on-line information regarding structural safety during the products lifetime, while at the same time reducing inspection and/or maintenance costs.
Insertion of conductive nanoparticles, such as carbon black [19], metal powder [20], or carbon fibre [21], within insulating polymer matrices generates a new species of smart materials termed “conductive polymer composites” (CPCs). The high electrical conductivity together with their large aspect ratio make carbon nanotubes (CNTs) particularly outstanding candidates as multifunctional fillers for CPCs. Polymer/CNT composites have been considered as sensing materials for various stimuli, including temperature [22], gases [23], vapour [24], mechanical stress and strain Refs. [25], [26], pH [27], and liquids [28]. Generally, the underlying mechanism is that the introduced external stimuli results in a deformation of the CNT percolation network, thus leading to a change in electrical conductivity of the composites.
In present work, we pioneered the use of the evolution of electrical resistivity as a means to monitor PLA degradation. As the morphology of the polymer changes during degradation, it results in a change of the filler network, thus leading to a change in electrical resistivity of the nanocomposites. Therefore, through the evolution of the electrical signal during PLA degradation, we will be able to correlate changes in electrical resistivity with degradation levels of the polymer.
Two different mediums were used to understand the degradation behaviour. Phosphate-buffered solution (PBS) is usually used to simulate in-vivo conditions, while water is more related to environmental conditions. Various techniques were performed to study the hydrolytic degradation and morphological changes of PLA.
Section snippets
Materials
Multi-walled carbon nanotubes (MWNTs) (NC7000®) were supplied by Nanocyl S.A., Belgium. MWNTs were used as received without purification. Ingeo® PLA 3051D was purchased from RESINEX, United Kingdom. Phosphate-buffered saline (PBS) powder was obtained from Sigma–Aldrich Chemical Co.
Sample preparation
MWNTs were melt-blended with PLA using a DSM X'plore 15 Mini-extruder (The Netherlands), at 180 °C and 100 rpm for 3 min to prepare masterbatch containing 15 wt.% MWNTs. This masterbatch was then diluted with neat PLA
Hydrolytic degradation and morphological changes
The Mn values of PLA films before and after hydrolysis are plotted in Fig. 1 as a function of hydrolysis time. The Mn of the PLA films decreases linearly with hydrolysis time in both mediums, which means that the hydrolysis rates are constant corroborating previous observations [30]. Accordingly, complete hydrolysis of PLA, in similar conditions, takes place over 80 days during three stages, each at different hydrolysis rates. The first stage, during which only the amorphous phase is
Conclusions
A degradation sensor based on PLA/CNTs has been successfully prepared. Nanocomposites with a CNT loading around the percolation threshold gave the highest sensitivity and strongest signal change. The observed increase in conductivity is due to an increased CNT network density after partial removal of the amorphous phase of the semi-crystalline polymer matrix. Such an in-situ degradation monitoring system would give on-line information regarding structural safety and makes it a good candidate
Acknowledgements
The research leading to these results has received funding from the European Union's MATERA+ program (project HIGHBIOPOL) via the support from the DG06 (Région Wallonne) and the Technology Strategy Board (TSB) (United Kingdom). The authors would like to thank Nanocyl S.A. (Belgium) for supplying the CNTs used. Fang Mai would like to acknowledge financial support through the China Scholarship Council (CSC).
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