Swelling, compression and tribological behaviors of ...

1128 KORRES ET AL.Figure 4DSC curves of swollen hydrogels.least three types of water in swollen polymers: freewater (water with the same freezing point as bulkwater, i.e., near to 0 C), freezing bound water (waterfreezing at a temperature much lower than 0 C),and nonfreezing bound water (water that does notform ice at all and therefore cannot be detecteddirectly by calorimetry). 26 DSC analysis allows toestimate the amount of freezable (free and freezingbound) water from enthalpies of its phase transitions(melting/crystallization) quantitatively. In this work,the DSC analysis of water-swollen gels was carriedout during the heating process.As no endothermic peaks were observed in theregion below 0 C (Fig. 4), and considering the factthat the detection of different types of freezablewater (and their melting enthalpy) is significantlyaffected by the annealing period before DSC scanning,27 it was suggested that there is no freezablebound water in our gels. So, the phase transitionwas assigned to the melting of free water. Itsamount in the gels was evaluated from the meltingenthalpies using the following equation:W f ¼ DH endoDH ice 100 (10)where DH ice is the enthalpy of fusion of pure water(334 J/g), DH endo is the enthalpy values associatedwith melting peaks of freezable water in gels determinedas an area under the melting peak from DSCdata. 28 The amount of bound water was evaluatedfrom the assumption that equilibrium total watercontent represents the sum of the amount of freezableand nonfreezable water:W b ¼ EWC W f (11)where W b is the amount of nonfreezing boundwater. The results of EWC, enthalpy value, and calculatedamount of free and bound water related tothe weight of the dry polymer in the hydrogels arepresented in Table II.From the data obtained, it can be seen that both Hand H-NB hydrogels contain very high amounts ofwater (more then 99%), which correlates with theirhigh hydrophilicity. However, in spite of the insignificantlylower water content of H-NB gel (99.09 %compared to 99.49% of H-gel, see Table II), its enthalpyof melting is notably lower than that of thereference gel (227 J/g compared to 283.6 J/g for thepristine gel). This indicates a considerably lowercontent of freezable water in the H-NB system (TableII). Because of the presence of ionic functionalgroups of AMPSA, the amount of bound water wasrather high and was found to be 14.66% for the clayfreegel. Modification by 4 wt % NB significantlyincreased the content of bound non-freezable water,i.e., up to 31.41%. It should also be noted that thetemperature of ice melting determined from DSCcurve of the NB-modified gel is notably lower thanthat for the H-gel (4.5 compared to 1 C, see Fig. 4).Such a depression in the melting point demonstratesa more bonded state of freezable water in a claymodifiedhydrogel matrix compared to the parentgel. Accordingly, the water state was influencedmarkedly by the inorganic clay. These results indicatea strong interaction of water molecules with theclay layers, and these interactions were notdestroyed during the hydrogel formation. This factmay significantly influence the mechanical and tribologicalproperties of clay-modified hydrogels.Mechanical properties of the hydrogelsThe stress–strain curves of uniaxial compressiontests of equilibrium swollen at 25 C hydrogels arepresented in Figure 5(a), whereas Figure 5(b) showsthe initial part of experimental curves at small deformations.As seen in Figure 5(a), the incorporation of4 wt % NB increased the ultimate compressionstrength (r) at break from 20 to 35 kPa; however, atthe same time, it slightly reduced the correspondingstrain (from 60 to 50%) compared to the unmodifiedgel. This indicates a significant improvement in thecompressive mechanical properties due to the claymodification. The slight reduction of strain is obviouslydue to the higher network density of NBmodifiedgel, and to the lower amount of water, thatmakes the gel stronger. An additional straightforwardexplanation is that the exfoliated/intercalatedbentonite layers may change their orientation underloading, while enhancing the maximum (or criticalwhere the hydrogel breaks apart) stress.In the swollen state, most hydrogels demonstratea rubber-like elastic behavior with a rapid rearrangementof the polymer segments under externalJournal of Applied Polymer Science DOI 10.1002/app