Temperature sensitive sol-gel transition behavior of aqueous solution of biodegradable block copolymers

Abstract

In part I, non-toxic, biodegradable star-shaped poly(L-lactide-b-ethylene oxide) (star PLLA-PEO) copolymers were synthesized by the coupling reaction of two reactive precursors, a hydroxy terminated 3-armed poly(L-lactide) (star PLLA) and a-monocarboxy-w-monomethoxy poly(ethylene oxide) (CMPEO). The resulting star-block copolymers were characterized by $^1$H and $^13$C nuclear magnetic resonance, gel permeation chromatography, and differential scanning calorimeter. In the dilute aqueous solutions of star-block copolymers, micellization behavior was investigated by using UV/Visible spectrometer and dynamic light scattering (DLS). Critical micelle temperature was measured by UV/Visible spectrometer using DPH probe. Over specific concentration and specific temperature, unimer to micelle transition occurred. The hydrophobic PLLA block aggregated by hydrophobic interaction, and hydrophilic PEO block surrounded the core. DPH can be solubilized in the micelle core and the absorbance of DPH at 356 nm increases when micelle is formed. The effective diameter of micelle was measured by DLS. The effective diameter was highly affected by temperature. With increasing temperature, micelle size decreased dramatically. In high concentration solution, star-block copolymer showed temperature sensitive sol-gel transition behavior. Over specific concentration, the whole system cannot flow by the packing of micelles. However, with increasing temperature, packing structure was destroyed by decrease of micelle volume due to the dehydration and contract of PEO chain, and the system flew. In the sol-gel transition phase diagram obtained by tube tilting method, the critical gel concentration decreased, the boundary curve shifted to the left, and the gel regions were expanded with increasing the molecular weight of PLLA block and PEO block. In part II, carboxylic acid terminated PHAs were synthesized by bulk polycondensation reaction of 1,6-hexanediol and excess adipic acid at 23...