| Poly(acrylic acid) (PAA) with carboxyl groups bonding on each chain is highly hydrophilic, and will ionize into hydrogen ions and carboxyl anion in PAA aqueous solution. Because of the repulsion force between two carboxyl anions, PAA chains are extended into the relaxation state in solution, which makes carboxyl groups exposed in water and form hydrogen bonding with water. As a result, PAA solution will make contribution to the researches on the thermodynamic behavior of different states of water in hydrophilic polymer/ water mixtures. Poly (acrylic acid) with different molecular weights (Mn=5000~70,000) were prepared by reversible addition-fragmentation chain transfer (RAFT) polymerization with water as the solvent, and S, S'-bis(α,α'-dimethyl-α"-acetic acid)-trithiocarbonate (TRIT) as the chain transfer. Gel permeation chromatography (GPC) analysis indicates that PAA we have prepared possesses narrow molecular weight distribution, whose polydispersity index PDI was controlled between 1.0 and 1.1.The thermodynamic behavior of different states of water in PAA/ water mixtures has been studied by DSC. The results show that different polymer molecular weights have significant effect on the thermodynamic properties of water in PAA aqueous solution. The melting points and enthalpies of water in PAA solution vary with different polymer molecular weights. With the increase of PAA molecular weights, free water content proportion in total water decreases. However, the freezable bound water has changed with a random pattern. It indicated that for freezable bound water, hydrogen bonding ability in PAA/water mixtures could be not the only factor of the interaction between polymer and water, the polymer chain with different length can effect geometrically the crystallization of water. The amount of non-freezable water in mixtures vary with different molecular weights of PAA. There existed a critical Mn nearly 55000, where the amount of non-freezable water in mixtures will reach a maximum. |
PDF Full Text Request