| Hydrogel is a kind of polymer that has a three-dimensional network structurewhich containing a hydrophilic group. Hydrogels are widely used in health care, drugdelivery and release, bio-separation and detection, sensor technology because of theirunique hydrophilic property and biocompatible. Mechanical properties of traditionchemical cross-linked hydrogels are generally poor which limited the application. Toovercome the shortage, researchers have developed four kinds of novel hydrogelswhich are nanocomposite hydrogels, double-network hydrogels, topologicalhydrogels and macromolecular microsphere composite hydrogels. These kinds ofhydrogels have high mechanical strength, but the complicated synthesis process andnarrow range of monomer selectivity limited their further applications in industrial.Recently, our laboratory has synthesized a novel physical hydrogel which calledhydrophobic association hydrogels (HA-gels). This kind of hydrogel also hasexcellent mechanical properties, swelling properties as well as self-healing nature andenvironmental sensitivity. The advantages of HA-gels are that the preparation processof HA-gels is simple and external cross-linkers aren’t needed. In recent study, we hadstudied some important properties like mechanical properties, transparency ofHA-gels which synthesized by using octylphenol polyoxyethlene (4) acrylate(OP-4-AC) or octylphenol polyoxyethlene (10) acrylate (OP-10-AC) as hydrophobicmonomer. The structure of hydrophobic monomer octylphenol polyoxyethylene (7)acrylate (OP-7-AC) is similar to OP-4-AC or OP-10-AC. Compared with OP-4-AC,The hydrophilic chains of OP-7-AC is longer. Compared with OP-10-AC, OP-7-ACcan’t soluble in water, it can only solubilize in water by SDS and it can’t be used assurfactant. HA-gels prepared by using OP-7-AC as hydrophobic monomer haven’tbeen studied in system. In this paper， HA-gels are synthesized via micellarcopolymerization which use acrylamide (AM) as main monomer, OP-7-AC ashydrophobic monomer, sodium dodecyl sulfate (SDS) as surfactant. The rubber elasticity and viscoelasticity were tested and made a deep research of the networkstructure of HA-gels.We studied the mechanical properties of HA-gels first. The mechanical propertyparameters were tested by tensile text. Tensile strength and fracture energy go up firstthen decrease and elongation at break becomes greater until approaches to a plateauvalue with increasing R (molar ratio of SDS and OP-7-AC). When the R value is3.94,the tensile strength reaches the peak value. With increasing OP-7-AC content, tensilestrength increases, elongation at break and fracture energy decrease. The averagehydrophobic monomer number in each micellar NHwas calculated by equation. At thelimited stretch state of HA-gels, we obtained two parameters by Mooney curves. λinincreases sharply and σindecreases with an increase on the R value. The slope of thestraight line part of Mooney-Rivlin curves is2C2and the intercept on the Y-axis ofMooney curves is2C1, which is the modulus of hydrogel and dropping with anincrease on R value. The effective network chain densityeand the average molecularweight of the chains between cross-linking points Mcwere calculated. The valueegoes up and Mcgoes down with increasing R value. From the results, we indicatedthat the variation of mechanical property parameters was induced by the number ofeffective cross-linking points and the match degree of long and short chains.Increasing OP-7-AC content when SDS is constant, elastic parameter C1showsdecrease and C2goes up. The value ofedecrease while Mcincrease when R value isincreased.Next，the effect of SDS and OP-7-AC contents on the swelling properties ofHA-gels was studied. Swelling dynamics of hydrogels and xerogels have been testedand gel loss has been calculated. Swelling ratio exhibits up-down tendency withincreasing R value and it is least when SDS content is1.6wt%, the network structureis most stable in this case. All hydrogels exhibit “swelling overshoot” and thisphenomenon is more obvious with higher SDS content. After swelling10-12days, theswelling ratio has not significantly change which means the hydrogels have reachedswelling equilibrium state. When we increase OP-7-AC content, swelling ratioincrease and swelling overshoot become more obvious. When hydrogels reach swelling equilibrium state, the equilibrium swelling rate of different SDS contents arealmost no different. The swelling process of xerogels was analyzed in qualitative andquantitative. Diffusion of water molecules plays a major role to the swelling processof xerogels when SDS content is at a low value. The motion of network chains plays aleading role to the swelling process when SDS content is high. The leading role of themotion of network chains is increased when hydrophobic monomer content increases.The diffusion coefficient is at minimum when R value is3.94and it shows decreaseswith increasing hydrophobic monomer content. The variation of swelling curves ofhydrogels and xerogels is basically same and water absorption of xerogels is lowerthan swelling ratio of hydrogels when the content of monomer is same. The gel lossincreases sharply when swelling started. After4-5days, non-gel compositions andexcessive SDS have basically diffused into water and the gel loss isn’t increasedsignificantly. The mass loss of hydrogels increases sharply with the hydrophobicmonomer content increases when R value is same. The gel fraction was calculated andthe apparent swelling ratio was replaced by real swelling ratio. The variation of curvesafter correction is consistent with the original curves.We also studied the effect of monomer content on the mechanical and swellingproperties as well as chain entanglement. Content of AM changes from10wt%to35wt%, SDS and OP-7-AC contents are also changed accordingly. Mechanicalstrength was obtained by tensile test, tensile strength goes up and elongation at breakdrops with increasing monomer content. Modulus was calculated by Mooney theoryand the effective cross-linking density was also counted. With monomer contentincreases, modulus of HA-gels grows. Through the swelling test, we find thatswelling ratio of hydrogels and xerogels decline with monomer content increases. Allhydrogels exhibit “swelling overshoot”. The gel fraction was gained by calculatingthe mass loss of hydrogel, and then we calculated density and column fraction ofhydrogels. Then the average molecular weight of the chains between cross-linkingpoints was calculated. The network chain numbers of hydrophobic associationcrosslinking and physical entanglement in hydrogels were calculated by abovestructure parameters. An iterative method was used to calculate the fitting exponential equation of the number of physical entanglement network chains with volume fractionof hydrogels by gradient descent formula in MATLAB software, and the exponentialfactor was1.822. The equation of entanglement molecular weight was deduced by thefitting equation. We can calculate the entanglement molecular weight of any othermonomer concentrations by this equation.Finally we analyze the stress relaxation behaviors. The results of compressionstress relaxation were fitted by ternary Maxwell model. Fast relaxation of molecularchains is occurred first in hydrogel. After about half an hour, movement of networkchains accompanied by unlock of weak cross-linking points are taken place. Thencross-linking points in hydrogel are disassociation, the relaxation time is very longdue to the high association strength of cross-linking points and the stress can relax tonear zero ultimately. When SDS content increases, the association strength ofcross-linking point is weaken due to hydrophobic segments in micellar become less,resulting in shorter relaxation time. The results of tensile stress relaxation were fittedby dual Maxwell model. The initial stress and relative balance stress exhibit up-downtendency with increasing SDS content. Pre-exponential factor and relaxation timeincrease first and then decrease with an increase on SDS content, which is related tothe association strength of micellar. |
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