Novel thermoreversible gelation behavior of aqueous solutions of ABA-type triblock copolymers composed of the central polyethylene oxide (PEG) block and two poly(D,L-lactic acid-co-glycolic acid) side blocks was found. Phase transition characteristics, such as critical gel concentration (CGC) and lower and upper critical gel temperature (CGT), are closely related to the molecular structure of the triblock copolymers. The CGC and the lower CGT both increases with increasing PEG/PLGA molecular weight ratio. Increasing the GA content in PLGA block induces a somewhat higher CGC. The copolymer forms micelles with a PLGA loop core and a PEG shell in water. Also grouped micelles are identified seemingly due to the bridging of two micelles sharing two PLGA blocks of a block copolymer chain. As the temperature increases the association of micelles increases, which results in gelation. The ABA-type copolymers exhibit a relatively low CGC (<10%) and low sol-gel transition temperatures compared to BAB-type copolymers. As the temperature increases further gel-sol transition is observed, which would result from the shrinkage of micelles with temperature increase. The hydrodynamic size of the micelles is monitored by dynamic laser scattering, and a possible gelation mechanism was suggested.
Poly(l-lactide)-block-poly(ethylene oxide)-block-poly(l-lactide) triblock copolymers (PLLA-b-PEO-b-PLLA) were
fractionated in terms of the number of LLA units by liquid
chromatography at the critical condition (LCCC) of PEO
block. The fractionated samples were identified using
MALDI-TOF mass spectrometry. The dependence of the
LCCC retention of the diblock and triblock copolymers
on the degree of polymerization of PLLA block(s) follows
Martin's rule very well. Unlike the case of PEO-b-PLLA
diblock copolymer reported earlier (Lee, H.; et al. Macromolecules
1999, 32, 4143), however, a splitting of the
elution peaks containing the same number of LLA units
was found. The peak splitting was ascribed to the different
length distributions of PLLA blocks at the two ends of the
PEO block. From the relative intensities of the peaks, the
split peaks were assigned to different isomeric structures
of the PLLA blocks. From these results we conclude that
the interaction of the triblock copolymers with the stationary phase is affected by the distribution of the interacting
blocks at the two ends of the center PEO block, in addition
to the total number of LLA units in the triblock copolymer.
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