Effect of Stuctural Inhomogeneity in Early Stage on Phase Transition Behavior in PNIPAAm Hydrogels and Solutions

• Main Authors: Che-Min Lin, 林哲民
• Other Authors: Shiaw-Guang Hu
• Format: Others
• Language: zh-TW
• Published: 2009
• Online Access: http://ndltd.ncl.edu.tw/handle/76958028133928555382

碩士 === 國立臺灣科技大學 === 高分子系 === 97 === PNIPAAm [Poly(N-isopropyl acrylamide] hydrogels and solutions were prepared by free radical polymerization with NMBA (N,N′-methylene-bis-acrylamide) crosslinker. The stress-strain data of hydrogels fitted with the slip-link model was used to calculate the densities of chemical crosslink and the densities of physical entanglement for hydrogels. We calculated the effective crosslink densities (ve) by swelling test data and stress-strain data. The correlation lengths (ξest) of hydrogels were calculated from the thermodynamic equilibrium equation. We used dynamic laser light scattering to measure the dynamic correlation lengths (ξDLS) of hydrogels and solutions, and compared them with the estimated correlation lengths of hydrogels. We used UV-VIS spectrophotometer to measure the transmittance (Tr) versus time with various heating rates from 30 to 45℃ for hydrogels and solutions. The phase transition temperatures (Ts,0; i.e., lower critical solution temperature) and the relaxation times (τ∞) of early stage spinodal decomposition are determined from data of transmittance versus time in the critical point and the initial stage, respectively. The polymer volume fractions (ν2) and elastic modules (G) are increased with increasing the feed ratio of NIPAAm to NMBA in hydrogels. The physical entanglements and chemical crosslinks are increased with increasing the feed ratio. The ratios of physical entanglement to chemical crosslink do not vary much with feed ratios. The effective crosslink densities, Flory-Huggins interaction parameters between crosslinked polymer and water (χ) and molecular weights between crosslinked points (Mc) are increased with increasing the polymer volume fractions. The estimated correlation lengths (ξest) of hydrogels were calculated from ν2 and Mc, which were decreased with increasing the polymer volume fractions. The dynamic correlation lengths (ξDLS) are bigger than the estimated correlation lengths for hydrogels. This difference can be attributed to the inhomogeneity that caused by the end chains or physical entanglements in the hydrogels. The exponent values of power relations between dynamic correlation lengths (ξDLS) and polymer volume fractions for hydrogel and solution are -0.301 and -0.399, respectively. However, the dynamic correlation lengths for hydrogels are bigger than solutions. The phase transition temperatures are decreased with increasing the polymer volume fractions in two kinds of samples. The phase transition temperatures for hydrogels are higher than solutions. The phase transition temperatures are increased with increasing the chemical crosslink for hydrogels. The phase transition temperatures for both are linearly increased with increasing the dynamic correlation lengths. The relation between phase transition temperatures and polymer volume fractions can be used to calculate the enthalpies of phase transition of hydrogel and solution, in which the enthalpy of hydrogel is larger than solution. The exponent values of power relations between relaxation times and polymer volume fractions for hydrogel and solution are -0.338 and -0.580, respectively. However, the relaxation times for hydrogels are smaller than solutions. The exponent values of power relations between relaxation times and dynamic correlation lengths (ξDLS) for hydrogel and solution are 1.057 and 1.143, respectively, with very minor difference. The dynamic correlation lengths (ξDLS) and phase transition rates of hydrogels are less affect by polymer volume fractions than solutions, because of the restriction by crosslink networks of hydrogels. The phase transition rates of hydrogels are faster than solutions, because the energies needed in causing phase transition of hydrogels are lower than solutions. The exponent values of power relations between relaxation times and reduced temperature for hydrogel and solution are 1.419 and 7.629, respectively, whereas solution behavior is deviated far away from Ising model with an exponent of 1.28.