Improvement of Mechanical Properties of Polymer-clay Nancomposite Hydrogels by Graphene Oxide

Abstract

Poly (N-isopropylacrylamide) (PNIPAm) hydrogels have been proposed for application as promising biomaterials and stimuli-responsive materials in biomedical fields, but their poor mechanical properties limited their development. Here we report that the mechanical properties of PNIPAm-clay nanocomposite hydrogels were enhanced significantly through in situ polymerization by addition of very low content of graphene oxide (GO). GO sheets were used as both the crosslinking agent and reinforcing agent. Compared to nanocomposite hydrogels without GO, a more than 200% increase in tensile strength and a nearly 400% improvement of Young’s modulus were achieved with the addition of 0.1 wt. % of GO, which suggests an excellent load transfer between the GO and hydrogel matrix. KEYWORD: Graphene oxide; hydrogel; N-isopropylacrylamide International Conference on Industrial Technology and Management Science (ITMS 2015) © 2015. The authors Published by Atlantis Press 1102 with uniform thickness (∼ 1.1 nm) according to cross-section analysis and lateral dimensions ranging from several hundred nanometers to several micrometers. While individual graphene nanosheet is atomically flat with a well-known thickness of ~0.34 nm, GO is much thicker due to the attachment of oxygen-containing functional groups such as epoxy, hydroxyl and carboxyl groups, which disrupt the sp2-bonding nature of carbon atoms and introduce structure defects to form folds on the GO sheets. The results are consistent with those reported in the literature (Haraguchi et al 2006), which proving the graphene oxide is capable of forming uniform dispersion of single thickness sheet. Figure 1. The AFM images of graphene oxide 2.2 The mechanical properties of NC-GO hydrogels 2.2.1 The images of NC and NC-GO hydrogels It can be observed from Fig. 2a and Fig. 2b, compared to NC hydrogel, NC-GO hydrogel has better strength and toughness. GO has high specific surface energy, good hydrophilicity, good mechanical properties, and can be well dispersed in water. Composites consisting of GO and polymer can realize the complementary of the two components which result in mechanical enhancement. Since the excellent mechanical properties of graphene oxide, it is considered desirable reinforcing fillers in polymer composites (Wang et al 2010). In the process of forming the hydrogel, the epoxy, hydroxyl and carboxyl groups on the edge of GO with the polymer chains form dense three-dimensional network structure which is crucial for the improvement of the mechanical properties. Figure 2. The photographs of ClayPNIPAm (NC) hydrogel (a) and NC-GO hydrogel (b) 2.2.2 Mechanical behavior Fig. 3 shows the stress-strain curves of NC hydrogel with different content of GO. One can see that the addition of GO significantly improved the tensile properties of NC hydrogels. The tensile strength and Young’s modulus increased sharply by more than 200% from 50 KPa to 111 KPa (Figure 3a) and by nearly 400% from 5.6 KPa to 20.4 KPa, respectively. The addition of GO made the mechanical property of the hydrogel enhanced dramatically due to the increasing of crosslinking points. On the other hand, the hydrogen bond between epoxy, hydroxyl and carboxyl groups on GO made the hydrogel network more dense which led to the improvement of tensile strength. It was strongly obvious that even a small amount of GO could significantly improve the mechanical properties (Fig. 3b). The content of GO increased from 0.01wt% to 0.1 wt. %, the tensile strength increased with the increasing of GO. Surprisingly, when the content of GO increased from 0.1 wt. % to 0.15 wt. %, the tensile strength decreased on the contrary. This may be caused by excessive crosslinking points of excessive GO, thus affecting the mechanical properties of the entire system. Figure 3. The stress-strain curves of NC2 hydrogel and NC2GO1 hydrogel (a) and NC2-GO hydrogels with different contents of GO (b) (a) (b)