Carbon nanotubes (CNTs) have been gathering much attention due to their high aspect ratio, electrical, thermal and mechanical remarkable properties. CNTs can be especially useful to act as high-strength reinforcement and confer electric conductivity to polymeric composites.
Possible advantages of combining CNTs and polymers are strongly dependent on the ability to disperse and orient CNTs in the filler. Overcoming the aggregation due to van der Waals interactions between individual tubes is one of the major challenges for the development of high-performance CNTs/polymer composites. Consistent dispersion of CNTs over the matrix leads to an efficient load transfer to the CNTs and, in addition, helps to create the network for the electrical conductivity. Several methods have been used to achieve efficient dispersion: dispersion by ultrasonication using a solvent; melt blending process using high temperature and shear forces; functionalization of CNTs; use of surfactants; in situ polymerization.
CNTs can be especially interesting when used in combination with biodegradable and biocompatible polymers, due to the large potential for biomedical applications, especially in bone tissue engineering. Furthermore, it has been shown that CNT-reinforced biocompatible polymer composite can stimulate cell growth and regeneration by transferring the electrical signals to the osteoblasts. Consequently, electrical conductive scaffolds, CNTs/biocompatible polymer, in bone tissue engineering might be a good solution to improve bone regeneration and physical properties of the scaffold. In this work, composites composed by the FDA-approved biocompatible poly(L-lactide) (PLLA) and multi-walled carbon nanotubes (MWNTs) were produced by a melt blending process, using a DSM Xplore micro-compounder. This apparatus is a micro-scale twin-screw compounder, mimicking the mixing behavior of the large twin-screw extruders. Our objective is to study the performance of the micro-compounder to produce PLLA/MWNTs nano-composites in a controllable process without solvents or toxic products involved in method, to be further applied as bone tissue engineering scaffolds.
The melting and blending in the compounder was tested for 2 different mixing times; and a MWNTs loading of 3 %wt was used. In order to evaluate the method, specimens were observed by SEM and subjected to tensile tests (ISO 527-2). SEM analyses have shown that very good dispersions are achieved and, changing mixing time from 5 to 8 min, higher dispersions are obtained. Although for 5 min mixing time few 500 nm agglomerations were observed, 8 min of mixing time were necessary to observe no significant agglomerations,. Mechanical tests, comparing pure PLLA samples with composites, revealed that UTS had not increased significantly after the introduction of CNTs; but Young modulus was 7% higher for PLLA/MWNTs composites with 8 min mixing time. It has been also shown that percentage of elongation at brake was reduced in 50%.
Concluding, it was shown that the method tested is very quick and easy, highly controllable, versatile and feasible. Furthermore, the method allows the production of desirable complex shape and does not require the use of toxic substances, which are essential for the production of biomedical devices. Electrical properties will be also studied in order to verify the practical importance of a good dispersion. Further studies should optimize the process parameters in order to obtain higher mechanical and electrical properties.
Acknowledgements: The work was financially supported by European Commission in the frame of FP6 EST Marie Curie project called Join(ed)T.