Preparation and physicochemical characteri̇zation of hydrogels and graphene oxide based nanocomposites

In the first part of this work, poly (N-ethylacrylamide) (PNEAM) gels were synthesized. Yttrium tri-fluoro- methanesulfonate [Y(OTf)3] is used as lewis acid, N,N/-methylenebisacrylamide (BIS, Aldrich) is used as cross-linker, ammonium persulfate (APS) is used as initiator, N,N,N/,N/-tetramethylethylenediamine (TEMED) is used as catalist. The effect of the composition of solvent to the synthesized gel were investigated. Methanol-water mixture were used as solvent. Tacticity were determined by changing the mole fraction of methanol in this mixture. The mole fraction of methanol xm=0, 0.06, 0.13, 0.21, 0.31, 0.43 were prepared. Tacticity of the synthesized gels were prepared in presence as well as in the absence of 0.1 M Y(OTf)3. The gels synthesized in the absence of Lewis acid are atactic and in the presence of the same are isotactic. SEM micrograph reveals that the hydrogels synthesized in the presence of Lewis acid are more porous than the gels prepared in the absence of Lewis acid. Swelling ratios of the hydrogels decrease with the increase in the temperature and Lewis acid gels (LA) show higher swelling ratio value than non Lewis gels (NLA). Deswelling rate of the hydrogels prepared in methanol-water mixture in presence of Lewis acid is faster than the hydrogels prepared in absence of Lewis acid. Moreover, increase of the tacticity of PNEAM chain in the gel, deswelling kinetics of the gels decreased. Reswelling kinetics of the LA gels faster than non LA gels and increase of the tacticity of PNEAM chain in the gel, reswelling kinetics of the gels increased. All these results have been explained on the basis of the formation of highly porous hydrogels with higher isotactic PNEAM chain segment owing to the faster polymerization rate in methanol-water mixture in the presence of Lewis acid towards PNEAM chain segment in the PNEAM hydrogel. Yields of polymer gels synthesized absence of LA were in between 61 to 98% and yields gradually decreased with increase in the methanol fraction in the synthesis solvent. Yields of polymer gels synthesized presence of LA were in between 80 to 98% and higher yields are observed for the gels synthesized in the solvent mixtures having lesser methanol fraction. The number average molecular weight (Mn) of the homopolymers showed no particular trend and varied in between 1.28 × 105 gmol-1 and 4.38 × 105 gmol-1. Polydispersities (Mw/Mn) were also wayward 1.49 to 1.99. Isotacticities of the polymers prepared in absence of lewis acids are remained constant. Swelling ratio value at 20 oC the of the gel prepared at pure water in absence of Lewis acid (X0) has been found 12.8. This value is smaller than the gel prepared in presence of 0.06 mole fraction of methanol (run X0.06, 13.4). As the Xm value 0.13 (14.8), 0.43 (18.0), 0.21 (16.7) ve 0.31 (17.2) increases equilibrium swelling ratio in water at 20 oC has increased. The equilibrium swelling ratio in water at 20 oC of the PNEAM gels in absence of LA follows the order: X0.43>X0.31> X0.21>X0.13>X0.06>X0. Swelling ratios (Ws/Wd) of the hydrogels at 85 oC are almost same for all the gels and comes in between 2 and 3. In general, for all gels, swelling ratio values decreases with increase temperature. The equilibrium swelling ratio in water at 20 oC of the PNEAM gels in presence of LA follows the order: X0.43>X0.31> X0.21>X0.13>X0.06>X0. The synthesis solvent composition has very little effect on the deswelling rate of the non LA gels as very minimum difference observed in the deswelling rates with time. The equilibrium swelling ratio in water at 20 oC of the PNEAM gels in presence of LA follows the order: X0>X0.06> X0.13>X0.21>X0.31>X0.43. The synthesis solvent composition has not much effect on the reswelling rate of the gels prepared in absence of LA like deswelling kinetics. The reswelling rate of the hydrogels prepared in presence of 0.1 M Lewis acid increased in the order X0LA< X0.06LA< X0.13LA < X0.21LA< X0.31LA < X0.43LA. We can observed that gels prepared in presence of 0.1 M LA is highly porous whereas gels prepared in absence of LA is nonporous. In the second part of this work, macroporous, dual stimuli responsive, stereoregular, poly(N-isopropylacrylamide) (PNIPAM) gels were prepared in 1:1 methanol-water mixture. Yttrium tri-fluoro- methanesulfonate [Y(OTf)3] is used as lewis acid, N,N/-methylenebisacrylamide (BIS, Aldrich) is used as cross-linker, ammonium persulfate (APS) is used as initiator, N,N,N/,N/-tetramethylethylenediamine (TEMED) is used as catalist. FTIR, SEM, DSC, RAMAN, XRD, TGA analysis were used in the charcterization of the products. The swelling ratios of the gels gradually decrease with increasing temperature. Gels prepared in the methanol-water mixture (1:1 v/v) are more porous than the conventional gel prepared in water. The absorbance peaks at 1045, 1379, 1613, 1725 and 3147 cm-1 can be attributed to C–O stretching (epoxy or alkoxy), O–H, stretching (carboxyl), C = C assigned to skeletal vibrations of unoxidized graphite domains, C = O in carboxylic acid and carbonyl moieties, and O–H broad coupling stretching (hydroxyl), respectively. The major mass loss for GO occurred at around 200 oC. This means that GO has synthesized. In the XRD models, the characteristic diffraction peak of GO was observed at about 2θ= 11.85o,while one broad peak appeared at around 2θ=20.50o and is ascribed to the diffraction peaks of pure PNIPAM. The pattern was similar for the gel prepared with of 0.1 M LA. But for the stereoregular PNIPAM-GO nanocomposites, the characteristic peak of GO at 2θ= 11.85o is absent in XRD the pattern of all nanocomposites. Instead, three new peaks appeared at 2θ=21.36, 23.76, and 26.60o respectively. There were not significant changes observed with the variation of the amount of GO. Swelling ratio of the different gels is compared as a function of pH ranging from 2-8 at 25 oC. The swelling ratio gradually increased for all the nanocomposite gels prepared in the presence of GO, up to pH 6 and then decreased. But the, swelling ratio of the gels prepared in the absence of GO remained almost constant throughout. For the gels prepared in the absence of GO due to the absence of such ionisable groups in the system, the swelling ratio of the gels remains constant throughout the pH. Tramadol hydrochloride is used for drug release experiments. Almost 90 % drugs are released from the corresponding drug-loaded gel within 60 min.