The Phase Diagram Measurements of Polyether-Poly(L-lactic acid)Diblock Copolymers in Aqueous Solutions by Rheology, Dynamic Light Scattering and Flory-Huggins Mean Field Theory

• Main Authors: XIAN-ZHE LEE, 李賢哲
• Other Authors: Shiaw-Guang Hu
• Format: Others
• Language: zh-TW
• Published: 2018
• Online Access: http://ndltd.ncl.edu.tw/handle/nghkk6

碩士 === 國立臺灣科技大學 === 材料科學與工程系 === 106 === Monomethoxy Poly(ethyl glycol)-Poly(L-lactic acid)(mPEG-PLLA) diblock copolymers with fixed mPEG length with degree of polymerization 113 and various PLLA with degree of polymerization from 35 to 98 were synthesized. The molecular weights were determined by gel permeation chromatography (GPC). The critical gelation concentration (CGC) was measured by tube-inverting method. We calculated the power-law exponents for the CGC versus the degree of polymerization. The phase transition temperatures were determined by rheological measurement. We used Flory-Huggins mean field theory and phase transition data to predict the phase diagrams. The exponent n of power relationship of degree of polymerization can be calculated according to the critical gel temperatures on the phase diagrams. The critical micelle concentration (CMC) of diblock copolymer can be measured by fluorescent probe to assess the existence of the micelle phase and we used dynamic light scattering (DLS) to analyze the phase behavior in water solutions. The experimental results showed that when the degree of polymerization ratio of hydrophilic chain to hydrophobic chain was less than 1.15, the diblock polymer couldn’t exist a gel state but precipitated. There was an exponential relationship, with a negative exponent, between the critical gel concentration and degree of polymerization. It meant the critical gel concentration decreased with increasing the hydrophilic length. We used mean field theory to calculate the interaction parameters between copolymers and water. The values increased with increasing hydrophobic segment length. It meant the interaction of copolymers and water decreased with increasing hydrophobic segment length. We found the experimental results were fit well with the predicted phase diagrams. When it was applied on some higher ratio of the hydrophobic segment length to the hydrophilic segment length copolymers, the predicted phase boundary fit poorly for the experimental data around the critical point. Rheological measurement and tube-inverting method couldn’t be used to measure the phase transition at low copolymer concentrations. We used mean field theory to predict the phase boundary even if at low concentrations, but couldn’t predict the precipitated state at high temperatures. This work found that the micelle phase formed at extremely low concentrations. We observed the effect of concentrations and temperatures on the phase behavior by DLS. The correlation length at high concentrations increased with increasing concentrations and decreased with increasing temperatures. By contrast, the correlation length at low concentrations decreased with increasing concentrations and decreased with increasing temperatures. We calculated the enthalpy of phase transition and the enthalpy decreased with increasing the degree of polymerization of hydrophobic segment. In conclusion, we suggest that the phase behavior at low concentrations was controlled by micelles aggregation and the phase behavior at high concentrations was controlled by colloidal particle clusters.