Swelling, compression and tribological behaviors of ...

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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
1130 KORRES ET AL.Figure 6 Tribological behavior of the reference (H, opensymbol) and bentonite-modified hydrgels (H-NB, filledsymbol). Solid curves represent the fits with experimentaldata.The friction of the 4 wt % NB gel was unexpectedlylow, in spite of the significantly lower swellingratio (Table II). A decrease of COF for clay-modifiedhydrogels was also observed by Haraguchi andTakada. 31 This was assigned to dangling of uncrosslinkedpolymer chain ends from exfoliated clay platelets,which affected the interface and reduced thefriction under wet conditions. On the other hand,this behavior may be related to changes in the surfaceproperties of the hydrogels. The presence ofclay nanoparticles at the gels’ surface can rearrangeand alter the surface polarity and morphology. Thisis accompanied by a change in the surface tension,which can reduce the friction. However, this hypothesisrequires further experiments.The specific wear rate (W S ) is remarkably lowerfor the NB-modified hydrogel when compared withthat of the reference (Table II). This suggests a reinforcingeffect of NB due to the strong adsorption ofpolymer chains on the filler surface, and strong H-bond interactions between the hydroxyl groups ofthe bentonite (at its surface) and functional groupsof the copolymer network (carbonyl and aminogroups of AAm and AMPSA, respectively). All thisincreases the apparent crosslink degree. Thus,improvement in the compression mechanical propertiesof NB-modified hydrogel due to the nanofillerreinforcement is associated, at the same time, with ahigher resistance to sliding wear.Creep behaviorIn general, hydrogels are not simply elastic materialsbut behave viscoelastically and mechanical deformationleads to a time dependence on the appliedstress due to the movement of polymer chains segments.This movement induces an internal response,resulting in a time-dependent recovery when theexternal load is removed. Thus, the time dependenceof the applied stress or strain is as important as theprediction of the material’s mechanical response.The investigation of the creep behavior makes thedetermination of the viscoelastic and viscoplasticresponses possible. In this work, the initial and NBreinforcedhydrogels were subjected to creep tests.The total deformation of hydrogels (e) after the loadcyclewas calculated as a ratio of thickness at theend of loading to the initial thickness of the samples.Additionally, the plastic (irreversible) deformation(d) was estimated as the strain value at the end ofthe recovery-cycle. The DMA method was applied ata relatively short load-cycle setting and TMA testswere carried out under several load-conditions withincreased durations for both load and relaxationcycles.Representative DMA creep and relaxation curvesare presented in Figure 7. At the beginning, the samplesresponded elastically to the initial mechanicalload yielding at 30 and 23% strain values for the Hand H-NB gels, respectively. After 20 min of loading,the related strain (e) values reached 33 and 27%for H and H-NB, respectively (Table III). When theload was removed, the strain dropped suddenly to2.5–3% and within 3–4 min the initial thickness ofthe specimens was practically restored and d wasfound to be 0% for both H and H-NB gels. Such fastresponse of hydrogels to load changing conditionswas also observed by other investigators. 32Results of creep tests carried out by TMA at differentload conditions are presented in Figure 8. Thevalue of elastic (reversible) deformation (e) was evaluatedas the difference between e and d at the firstFigure 7 Compression creep and relaxation for thehydrogel samples in DMA. Notes: the applied load was 7kPa before deloading. The duration of both loading anddeloading was 20 min.Journal of Applied Polymer Science DOI 10.1002/app
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