Effect of Isotacticity of Linear Poly(N-isopropylacrylamide) on its Gelation in Benzyl Alcohol

Abstract

Thermoreversible gelation of three different isotactic linear poly(N-isopropylacrylamide) (PNI-PAM)s having meso dyad (m) values 62, 68 and 81% has been observed in benzyl alcohol. All the gels were transparent in nature. SEM image of the dried gels showed fibrillar network morphology. Melting temperature of the gels gradually increased with the increase in the concentration. XRD data of dry polymers and their corresponding dry gels showed shifting in the peak positions. Rheological study showed that stronger gels were formed with increasing isotacticity of PNIPAM while lower isotactic sample exhibited typical polymer melt rheology. The formation of a plunge in the storage modulus as well as in the viscosity plot at the same frequency range indicates the reversible nature of the structure breaking/reformation under frequency sweep. Moreover, the mechanical strength of the gel decreased with increase in temperature. UV-Vis kinetic study also indicated the change in the conformation and aggregation of PNIPAM chains during gelation. Molecular modelling calculation showed that the number of solvent molecules involved in forming gel (polymer-solvent compound) decreased with the increase in the isotacticity of the polymer. Gelation rate of these gels was studied as a function of temperature, concentration and isotacticity using test-tube tilting method. It increased with the increase in the concentration and isoacticity of the polymer, and with the decrease in the temperature. Critical gelation concentration of the gel gradually increased with the decrease in the isotacticity and with the increase in the temperature. All these experimental results indicated that gelation occurs presumably through polymer-solvent compound formation.

Effect of n‐Alkyl Side Chain Length on the Thermal and Rheological Properties of PolyN‐(3‐(alkylamino)‐N‐(3‐(isopropylamino)‐3‐oxopropyl)acrylamide) Homopolymers

“…At low temperatures, the moduli of polyMns ( n = 4–10) are not very much different, but polyM12 is much higher. This may be due to the formation of polymer‐solvent compound [ 31,32 ] through physical network formation via stronger H‐bonding among amide groups or, between amide group‐solvent and hydrophobic interaction among n ‐dodecyl side chains or, between dodecyl side chain and benzyl group of solvent with a decrease in temperature. That can be originated from the limited solubility of polyM12, which would indicate a structure similar to PNIPAM in water above the LCST.…”

“…The general features of these findings are very similar to our previous investigations on PNIPAM in benzyl alcohol under variation of the tacticity. [ 31,32 ] Especially the atactic PNIPAM behaves in a very similar matter to polyMn ( n = 4–10). However, due to the significantly higher molar mass of this series of PNIPAM the data are not quantitatively comparable.…”

“…However, gel has preferably been formed from polyM12 possibly due to the formation of polymer–solvent compound via the formation of greater physical network through considerably stronger hydrophobic interaction among n ‐dodecyl side chains apart from other factors responsible for gelation. [ 32 ]…”