Small-angle X-ray scattering (SAXS) was used to determine the packing of micelles in cubic phases formed by poly(oxyethylene)−poly(oxybutylene) (EB) diblock copolymers in water. SAXS with large amplitude oscillatory shear was used to identify structures formed by the highly asymmetric molecules E96B18, E184B18, E315B17, and E398B19 (where the subscripts denote the number of repeats). At a constant temperature (20 °C) and concentration (10 wt %), we find that the two copolymers with shorter hydrophilic blocks form face-centered cubic (fcc) gels, whereas the two copolymers with longer corona-forming blocks form body-centered cubic (bcc) gels. SAXS also confirmed that the sols formed at lower copolymer concentrations are micellar liquids. Our results for the gel structures are in accord with the observation that micelles with relatively short coronas behave as hard spheres, and pack in a fcc structure, whereas micelles with large coronas act like soft spheres and pack in a bcc array. This is confirmed by assembling a phase diagram as a function of copolymer asymmetry and concentration using results from the four copolymers discussed here and a series of gels of other EB diblocks studied previously by us.

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Nineteen oxybutylene/oxyethylene/oxybutylene triblock copolymers [Bn/2EmBn/2, E ) oxyethylene, OCH2CH2; B ) oxybutylene, OCH2CH(CH2CH3)] were prepared and characterized. Twelve of the copolymers microphase-separated in the melt. Investigation of this microphase-separation behavior using small-angle X-ray scattering (SAXS) yielded values of the domain spacing (d-spacings) in the ordered phases and of the temperature of the order-disorder transition (TODT). In several cases the ordered phase structure was deduced from a combination of 1D and 2D SAXS and rheology. Values of TODT for the triblock copolymers were ca. 100 °C lower than those for EmBn diblock copolymers of identical composition and chain length but 30 °C higher if compared with diblock copolymers of half the triblock length. Values of the d-spacings indicated that the triblock copolymers were 10% more stretched than corresponding diblock copolymers. Determination of the Flory-Huggins parameter (χ) for the diblock and triblock systems gave identical results. The experimental results are compared with the prediction of mean-field theory.

Copolymers E96B18, E184B18, E315B17, and E398B19 (E = oxyethylene unit, B = oxybutylene unit) were synthesized and characterized by gel permeation chromatography (for distribution width) and 13C NMR spectroscopy (for absolute molar mass and composition). Dynamic and static light scattering was used to determine micellar properties in dilute aqueous solution at three temperatures (25, 40, and 50 °C): that is, hydrodynamic radius and hydrodynamic expansion factor, mass-average molar mass, and thermodynamic expansion factor. At a given temperature, the values of the radii and expansion factors increased as E-block length was increased, whereas values of the micelle association number decreased. The tube-inversion method was used to define the mobile−immobile (hard gel) phase boundary. At room temperature (20 °C), hard gels were formed in the concentration range 3.5−8 wt % copolymer depending on E-block length, the lowest value being for the copolymer with the longest E block (E398B19).

Copolymers B 20 E 430 , B 20 E 510 , and B 20 E 610 (B ) oxybutylene repeat unit, E ) oxyethylene repeat unit, subscripts indicate chain length in repeat units) were synthesized and characterized by gel permeation chromatography (for distribution width) and 13 C NMR spectroscopy (for absolute molar mass and composition). Dynamic and static light scattering were used to determine micellar properties in dilute aqueous solution: e.g., micelle association numbers and radii. A tube-inversion method was used to define the mobile-immobile (hard gel) phase boundary. For copolymer B 20 E 610 , immobile gels form at concentrations as low as 2.9 wt %. Rheological measurements of dynamic modulus and yield stress served to characterize the gel properties and to confirm the phase boundaries. The results are combined with those from previous work on related block copolymers to obtain scaling relationships for the dependence of micellar and gel properties on E-block length, thus enabling prediction of the requirements for and properties of very dilute aqueous gels. The validity of treating the micelles as hard spheres is discussed.

The phase behavior of an oxyethylene/oxybutylene diblock copolymer (E 22 B 7 ) in aqueous solution was defined using three properties: yield stress (by rheometry and tube inversion), dynamic modulus (by rheometry), and structure (by small-angle X-ray scattering). The boundary of immobile, structured gel (hard gel) was detected by all three methods, giving satisfactory agreement. A structural transition within the hard gel, from bcc to hex, was detected by rheometry and SAXS. Two types of soft gel were detected and assigned to disordered phases containing either spherical or cylindrical micelles. Comparison with results for other diblock E m B n copolymers showed a regularity of behavior influenced largely by oxyethylene-block length, with oxybutyleneblock length having a minor but significant role.

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The effect of steady or oscillatory shear on the orientation of a face-centered cubic micellar phase formed by a poly(oxyethylene)-poly(oxybutylene) diblock copolymer in an aqueous salt solution has been investigated using small-angle X-ray scattering (SAXS). Steady shear was found to orient the mesophase into a polydomain structure with the hexagonal close-packed (hcp) planes both parallel and perpendicular to the shear plane. The hcp layers were found to be randomly stacked along the shear gradient direction. Different flow mechanisms were also observed as the shear rate was increased. At γ ) 5 s -1 a sliding mechanism of the 2D hcp layers stacked perpendicular to the shear gradient direction was identified. This produced an increase in domain spacing compared to that for an unoriented gel that is consistent with a transition from an fcc powder to sliding hcp layers. At γ ˘) 50 s -1 a partial reorientation of the {111} planes oriented perpendicular to the shear plane was observed. Upon cessation of shear, melted grains recrystallized in a distinct orientation of an fcc crystal with a [110] direction parallel to the shear. Partial melting of grains with {111} planes oriented parallel to the shear plane only occurred at γ ˘) 250 s -1 . The effect of strain amplitude of oscillatory shear was investigated at constant shear rate on samples preoriented by steady shear. The behavior noted for steady shear, specifically, partial reorientation of {111} planes was only observed in the limit of very large strain amplitudes.