Nanohybrids of Nylon 6 with Multi-Walled Carbon Nanotubes: Solvent-Free Polymerization of ε-Caprolactam Under Variable Experimental Conditions

The solvent-free polymerization of epsilon-caprolactam on 1,8-diaminooctane-functionalized multi-walled carbon nanotubes (DA-MWNTs) is proposed as a simple and ecologically friendly approach to the preparation of carbon nanotubes/nylon 6 hybrid materials. The main goal of the present study was to find a minimum temperature resulting in an efficient epsilon-caprolactam polymerization, along with the optimization of the weight ratio of DA-MWNTs to epsilon-caprolactam and reaction time. The effect of temperature was studied in the range of 170 degrees C to 210 degrees C. After the reaction at 170 degrees C, the nanotubes functionalized contained a large amount of unreacted monomer along with ca. 14% of polyamide. Increasing the reaction temperature dramatically reduced the content of epsilon-caprolactam impurity and increased the nylon 6 content to ca. 20%. The reaction time tested was 1, 2, 4, 6 and 8 h. Exposures at less than 4 h were insufficient, where the infrared spectral bands of nylon 6 were barely seen. The reaction time of 6 h was found to be optimal since a more prolonged heating for 8 h did not provide an evident further increase in polyamide content. The effect of stoichiometry was studied by varying the weight ratio of DA-MWNTs to epsilon-caprolactam from 1:0.1 to 1:1. The ratios 1:0.1 and 1:0.2 were too high, since they did not provide the amount of epsilon-caprolactam necessary to form the composites with at least 20% content of nylon 6. Starting with the ratio of 1:0.3, the infrared band intensities qualitatively stabilized and did not show dramatic variations. The use of reagent ratios of 1:0.3 to 1:0.7 might be especially appropriate for preparing the composites targeted to biomedical applications, whereas higher weight ratios are expected to increase the content of undesirable monomer impurities.