Yd. Cheng et C. Jolicoeur, COSOLVENT EFFECTS ON THERMALLY-INDUCED TRANSITIONS OF A BLOCK-COPOLYMER - POLY(ETHYLENE OXIDE)-POLY(PROPYLENE OXIDE) IN AQUEOUS-SOLUTIONS, Macromolecules, 28(8), 1995, pp. 2665-2672
The thermally-induced transitions of a water-soluble block copolymer,
poly(ethylene oxide)-poly(propylene oxide) (MW 12 000, PO/EO ratio 3.4
6:1), were investigated in mixed aqueous-organic solvents using a high
-sensitivity differential scanning calorimeter(DSC). The endothermic t
ransitions of the block copolymer are markedly influenced by the addit
ion of various organic cosolvents, ethylene glycol (EG), enthanolamine
(EA), ethylenediamine (ED), 8-methoxyethanol (2-ME), and 1-propanol (
1-PrOH). The transition temperatures, T-m, are lowered from similar to
30 degrees C in water to similar to 10 degrees C in the mixed solvent
s at relatively low cosolvent mole fraction (X less than or equal to 0
.15); the transition enthalpies, Delta H-m, are simultaneously decreas
ed. Among the five cosolvents, 1-PrOH exhibits the largest effect on T
-m and Delta H-m, whereas ED displays an effect on T-m similar to that
of 1-PrOH but the weakest effect on Delta H-m. T-m versus Delta H-m m
ay be described by a second-order polynomial with a linear term (rho(1
)) and a nonlinear term (rho(2)) Since T-m variations relate to solven
t composition changes, rho(1) and rho(2) may be viewed as reflecting r
espectively the cosolvent/water interactions or affinities and changes
in these interactions in the presence of the copolymer (ternary cosol
vent/water/copolymer interactions). Attempts to correlate the rho(1) v
alues with various parameters of the cosolvents show a qualitative rel
ationship between rho(1) and a solvent hydrophobic parameter (theta) d
erived from the binary water/cosolvent mixture using the Kirkwood-Buff
theory. The trends observed in the transition enthalpies (Delta H-m)
and entropies (Delta S-m) are consistent with the view that the transi
tion is initially of the type sol-micelle and that the hydrophobic eff
ects associated with the PO segment of the copolymer, or with the coso
lvent, largely dominate this process.