| Summary: | 碩士 === 國立臺灣科技大學 === 材料科學與工程系 === 103 === This study is methoxypoly(ethylene glycol)-poly(L-lactide) (mPEG-PLLA) diblock copolymers were synthesized by ring-opening polymerization of L -lactide in the presence of mPEG. Copolymers with fixed mPEG length (degree of polymerization= 113) and various PLLA segment lengths (degree of polymerization= 308 and 96). Above all, the copolymers were characterized by using gel permeation chromatography (GPC) and 1H-NMR.
We use fluorescent probe to determine the critical micelle concentration (CMC) of diblock copolymer solutions with various concentrations of sodium chloride or sodium iodide at 25 to 45℃, and calculate subsequently thermodynamic parameters of micellization. Also, the micelle size is measured with dynamic light scattering (DLS) and transmission electron microscopy (TEM). We research the effects of degree of polymerization, electrolyte species and concentrations on the critical micelle concentration, thermodynamic properties of micellization and the particle size of micelles.
Experimental results show that critical micelle concentration increase with rising temperature, and decrease with increasing the degree of polymerization of hydrophobic block chain. The CMC value is lower with the higher concentration of electrolyte solution. It should not be neglected that various CMC are dramatically in the range from 0 to 0.005 M NaI solutions, and CMC tend to be less significant in the range from 0.1 to 0.5 M NaI solutions.
The salting coefficient (ks) is used to signify the effect of salting in solutions of various electrolytes and diblock copolymers. It is indicated that the salting out effect is pronounced as the salting coefficient increases with rising temperature, decreasing the degree of polymerization of hydrophobic block in copolymer. The greater value of salting coefficient on NaI solution means the electrolyte effect on CMC is more pronounced.
Calculated thermodynamic parameters of micellization show that the absolute values of Gibbs energy of micellization (△Gmic, negative value), entropy of micellization (△Smic, positive value) increase but enthalpy of micellization (△Hmic, negative value) decreases with electrolyte concentration. The negative Gibbs energy indicates that micellization process is spontaneous and the micellization is driven by entropy. There are absolute values of △Gmic and △Hmic increasing with increasing the degree of polymerization of PLLA chain. Moreover, △Gmic and △Smic are affected more significantly by sodium iodide than sodium chloride. △Hmic is affected in the opposite way.
DLS experiment reveals the mean size of micelle increases with increasing the electrolyte concentration. The micelle size determined by TEM is smaller than by DLS due to dehydration and shrinkage during drying. The index (a) of scaling in pure water is 0.23 by analyzing scaling relation of micelle size against hydrophobic segment length, which is in the limiting range of 0.16 to 0.67. The effect of NaCl concentration on the micelle size makes the index of scaling increase, indicating the chain of polymer in electrolyte solution tends to be the crew-cut.
|
|---|
