Small-angle neutron scattering investigation of the temperature-dependent aggregation behavior of the block copolymer pluronic L64 in aqueous solution

Small-angle neutron scattering (SANS) was used to investigate the temperatu re dependence of the structure of micelles formed by the poly(ethylene oxid e)-poly(propylene oxide)-poly(ethylene oxide) Pluronic L64 amphiphilic bloc k copolymer in aqueous (D2O) solution. Different models for the form factor and the structure factor have been considered for fitting the SANS scatter ing patterns. By comparing the various models, we found that in the interme diate block copolymer concentration range, e.g., 2.5 wt % Pluronic L64, the micelles are well separated while the intermicellar interaction remains st rong. Thus the more appropriate model was that of core-corona form factor a nd hard sphere structure factor, which allowed the determination of the cor e, micelle, and hard sphere interaction radii, the micelle association numb er, and the solvent volume fraction in the micelle core and corona. Upon an increase of temperature tin the range 35-55 degreesC) the micelle radius i ncreased by approximate to 10%, accompanied by the loss of water in the mic elle core. These trends coincide with the second relaxation process (attrib uted to the micelle core dehydration) observed in recent kinetics studies ( Kositza, et al, Macromolecules 1999, 32, 5539). The SANS data also indicate d a stronger increase of the micelle association number occurring above 47 degreesC, reflecting a strong increase of the intermicellar interactions. T hese results support the attribution of the third relaxation process observ ed in the aforementioned kinetic studies to micelle clustering.