Liquid-solid transition in a model hard sphere system of block copolymer micelles

Dynamic viscoelasticity and concentration dependence of micelle-gel transition of styrene and N-tert-butylacrylamide diblock copolymer solutions

We present dynamic viscoelastic studies of styrene and N-tert-butylacrylamide block copolymers (S(278)NtBAM(517) and S(93)NtBAM(252)) in 1-octanol as a function of copolymer concentration at 15, 25, and 35 degrees C. Dilute solutions of these diblock copolymers in 1-octanol, a selective solvent for N-tert-butylacrylamide units, yield spherical micelles as evidenced by transmission electron microscopy (TEM). Dynamic light scattering (DLS) is used to determine hydrodynamic radius of the micelles as a function of temperature (15, 25, and 35 degrees C). Dilute solutions of these block copolymers behave as viscoelastic fluids in the low frequency range. At higher concentrations, these copolymer solutions form glass-clear gels. Rheological measurements show that these block copolymer solutions exhibit power-law behavior at the gel point (G'(omega) approximately G''(omega) approximately omega(n)). In this paper, we discuss the formation and properties of critical gel states of S(m)NtBAM(n) in 1-octanol based on (1) concentration dependence of micelle-gel transition, (2) determination of critical state by rheology, and (3) temperature dependence of critical gel concentration and material parameters.

Structure and rheology of aqueous micellar solutions and gels formed from an associative poly(oxybutylene)-poly(oxyethylene)-poly(oxybutylene) triblock copolymer

The structure and shear flow behaviour of aqueous micellar solutions and gels formed by an amphiphilic poly(oxybutylene)-poly(oxyethylene)-poly(oxybutylene) triblock copolymer with a lengthy hydrophilic poly(oxyethylene) block has been investigated by rheology, small angle neutron scattering (SANS) and small-angle X-ray scattering (SAXS). SANS revealed that bridging of chains between micelles introduces, in the micellar solution, an attractive long-range component which can be described through a potential of interaction corresponding to sticky soft spheres. The strength of the attractive interaction increases with increasing concentration. Rheology showed that the dependence of the storage modulus with temperature can be explained as a function of the micellar bridging, micellisation and phase morphology. SAXS studies showed that the orientation adopted by the system in the gel phase under shear is similar to that previously observed by us for the gel phase of a poly(oxyethylene)-poly(oxybutylene) diblock copolymer with a long poly(oxyethylene) chain, suggesting that the micellar corona/core length ratio and not the architecture of the block copolymer influences the alignment of the gel phase under shear.

Dynamic light scattering study of the dynamics of a gelled polymeric micellar system

The dynamics of the E(92)B(18)/water system are studied by dynamic light scattering (DLS) in the liquid, soft gel, and hard gel phases. Both the liquid and the soft gel phases are micellar phases, although the structural order is higher in the soft gel phase than in the liquid phase. The hard gel phase corresponds to a face-centered cubic arrangement of micelles. DLS results show that the dilute liquid phase is characterized by a single characteristic time tau(1) associated with the diffusion of the micelles. In addition, a second characteristic time tau(2) associated with the presence of micellar clusters in the system is identified in the concentrated liquid and in the soft gel phases. According to these results, DLS suggests that the structure of the soft gel phase comprises micellar clusters coexisting with micellar fluid, in good agreement with hypotheses from our previous work. The dynamics of the system slows down as the hard gel phase is approached and a plateau is observed in the DLS correlation function. The structure of the hard gel is "softened" upon increasing temperature and/or decreasing concentration.