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The association of a monodisperse model associative polymer, hydrophobically end-capped poly(ethylene oxide), denoted C12EO460C12, in aqueous solution has been studied. The macroscopic properties have been investigated by rheological methods and correlated to properties on the microscopic level, as revealed from fluorescence and light scattering. The C12EO460C12 polymer associates in water through its hydrophobic end groups and gives rise to a sharp increase in viscosity at about 2 × 10-2 g mL-1 due to formation of large aggregates or networks. The aggregation starts, however, at much lower concentrations. Already at 0.5 × 10-2 g mL-1 hydrophobic domains large enough to dissolve pyrene have formed. The aggregation process is very gradual without a distinct cmc. An estimate of the aggregation number of the hydrophobic domains obtained from fluorescence is about 15−30 end groups per micelle. The distribution of relaxation times from DLS shows three important diffusive modes in the concentration range 3 × 10-3 to 5 × 10-2 g mL-1. A fast mode in the dilute region is attributed to unimers or small oligomeric aggregates of C12EO460C12. At concentrations near the point where the viscosity rises an intermediate mode appears. This mode is considered to reflect the cooperative motions of the formed network. In the whole concentration range studied a slow mode is present. In the dilute region this mode dissappears completely upon addition of salt and is therefore attributed to the PEO interchain interaction. In the high-concentration region, however, this mode is probably also influenced by the aggregation of the hydrophobic ends of C12EO460C12. The validity of these results is discussed and compared with a model considering the gain/loss of free energy upon formation of aggregates (loops, oligomeric aggregates, and micelles).

group^.^-^ We could not obtain evidence for such secondary aggregation in CHAPS.Thus, GFC can provide very accurate C, values for CHAPS, which make possible the determination of aggregation numbers and stepwise aggregation constants. These results will be helpful J . Phys. Chem. 1991Chem. , 95, 1850Chem. -1858 for applications of CHAPS in membrane biochemistry. Furthermore, the present approach &n be used for other compounds, such as bile salts, drugs, nucleosides, and dye^.^-'^ The odd-even alternation observed in the aggregation number dependence of stepwise aggregation constants is a novel phenomenon, which has been revealed by the determination of very accurate V, values.The properties of aqueous solutions of low molecular weight triblock copolymers of PEO/PPO/PEO have been investigated, mainly by dynamic and static light scattering. At low concentrations (C < 10%) and temperature (<25 O C ) the relaxation time distributions from dynamic light scattering show the coexistence of the monomer (RH = 18 A), micelles (RH = 80 A), and micellar aggregates in relative proportions which depend critically on temperature and concentration. Micelles are formed at about C = 5% at 25 O C . At 40 "C and above micelles are present at all concentrations used (C > 0.3%). At infinite dilution the hydrodynamic radius of the micelles is approximately constant over the temperature range 15-50 OC. At finite concentrations the apparent micellar radius increases with increasing temperature. The growth into asymmetric particles with increasing concentration is stronger as demonstrated by ultracentrifugation and combining static and dynamic light scattering data. At higher concentrations, a solidlike gel is formed at a well-defined temperature as shown by oscillatory shear measurements. It is characterized by a dynamic correlation length which decreases monotonically with increasing concentration to about 20 A.

In dilute water solutions of a PEO-PPO-PEO triblock copolymer, polarized dynamic light scattering measurement data reveal a sphere-to-rod transition in the shape of the micelles at a temperature close to 70 OC. This transition is anticipated from theory and was also inferred from earlier small-angle neutron scattering measurements. Regularized inverse Laplace transformation of the polarized dynamic light scattering data typically showed a fast rotational diffusion component of low amplitude in addition to a major slow component from translational diffusion. Good agreement was obtained between the rotational diffusion coefficient estimated from the polarized measurements and the value measured directly using depolarized dynamic light scattering. From the rotational diffusion coefficient a length of the P-85 rodlike micelle can be estimated, via Broersma's expressions for the rigid rod, as 1050 A in the dilute limit. This corresponds to a 1engtWdiameter ratio of 7. Intensity autocorrelation functions were modeled on the measured dynamic light scattering data using Pecora's theoretical expressions for the time correlation function in terms of the dynamic form factors. Good agreement between theory and experiment was found at the lowest concentrations. Concentrations c > 0.05 7% correspond to the semidilute regime where interparticle friction effects preclude determination of the length.

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