Swelling, compression and tribological behaviors of ...

BENTONITE-MODIFIED POLYACRYLATE-TYPE HYDROGELS 1129modulus correlates with crosslink-density of swollennetwork (n c ) as follows:E ¼ RTv p n c (14)Figure 5 Typical DMA compression stress–strain curves(a) and the straight lines fits to the experimental data inthe initial deformation range (represents the elastic modulusE) (b). Note: data presented are linked with the averagefrom 10 experiments for each gel.stresses. In such a state, the mechanical behavior ofgels is dependent mainly on the architecture of thepolymer network. Then, the elastic modulus, E, canbe determined from the slope of the linear dependencein the following equation:r ¼ Eðk k 2 Þ (12)where r is the applied compressive stress, k is therelative deformation:k ¼ L=L 0 (13)where L is the deformed thickness and L 0 is the initialthickness of a sample before deformation. 29 Thefitted lines used for the determination of E as theirslope value are shown in Figure 5(b). The elasticwhere R is the universal gas constant, T is the temperature,v p is the volume fraction of polymer inswollen gel, and n c is the concentration of crosslinkjoints per unit volume. It should be noted that thisapproach of crosslink density determination doesnot consider the molecular structure of the gel andits interaction with the solvent and is concernedonly with the observed macroscopic behavior of thesystem.Comparing the results obtained for the hydrogels’crosslink density by two approaches (using equilibriumswelling (n c Q ) and elastic modulus (n c E ) forcalculations, Table II), it can be seen that the crosslinkdensity determined via elastic modulus is about10 times higher than that determined by swelling.This can be explained by the fact that these twomethods reflect different properties of the hydrogels.As mentioned above, the mechanical approach doesnot take into account the chemical nature of hydrogelnetwork, its affinity to solvent (water here), orelectrostatic forces in ionic hydrogels. However, forthe clay-modified gels during swelling, the interactionof bentonite with water was not taken intoaccount. This interaction may change the Flory-Hugginsparameter v 12 and influence the calculated n c .On the other hand, the modulus of elasticity ismainly influenced by the number of elastically effectivechains of the hydrogel network and, very significantly,by the entanglements and micro-heterogeneitiesof the network, which are typical for this kindof hydrogels. 30 So, from a practical viewpoint, thedetermination of the network density from mechanicaltests is favorable. It should be noted that the valuesof n c , determined from the mechanical tests, arealso much lower compared to the theoretical crosslinkdensity. Similar effects were observed by otherscientists, as well. 21Sliding friction and wearFigure 6 shows the time profiles of the COF of thehydrogels investigated. As it can be seen for H-gel,its COF is very low in spite of the relatively highroughness of the metallic shaft counterpart (TableII). Very low values of COF for AAm-AMPSA-basedhydrogels have already been reported by Gonget al. 12,13 and were attributed to the strong repulsiveforces of negatively charged ionic groups of AMPSA.They also reported that friction coefficient tends todecrease when raising the charge density of a surfaceof ionic hydrogels. 13Journal of Applied Polymer Science DOI 10.1002/app