Rheology and Structures of Aqueous Gels of Diblock(Oxyethylene/Oxybutylene) Copolymer E₂₂B₇

Abstract

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.

Association Properties of a Diblock Copolymer of Ethylene Oxide and Styrene Oxide in Aqueous Solution Studied by Light Scattering and Rheometry

“…The transition from sol to soft gel is assumed to occur when micellar aggregates reach a percolation threshold yielding sufficient structure to cause an increase in modulus and, at a suitable frequency, the dynamic storage modulus to exceed the loss modulus. [7,53,54] More complex behaviour is expected if the micelles are subject to a sphere-to-cylinder transition, [34,35] and the present results for the soft gel region are indicative of spherical micelles across the whole temperature range.…”

“…[35] High-T destabilisation of hard gel in triblock copolymer (E m P n E m ) systems has been assigned on experimental grounds [34,45] (with some support from theory) [46] to a spherical-to-cylindrical micellar transition, with consequent release of packing constraints. It is known that the Figure 11.…”

“…In related experiments on aqueous micellar systems, for example on micellar solutions of copolymer E 22 B 7 , [35] gels with high storage modulus have been assigned to cubic phases formed from packed spherical micelles with body-centred symmetry by means of small-angle X-ray scattering experiments, while lower-modulus gels formed on increasing the temperature were similarly assigned to packed cylindrical micelles with hexagonal symmetry. It is possible that the low-T transition has a similar origin, but this can only be certain when structural information is available.…”

Aqueous Gels of Triblock Copolymers of Ethylene Oxide and 1,2‐Butylene Oxide (Type BEB) Studied by Rheometry

“…The lower values of storage modulus recorded for the copolymer with the shorter E-block length reflects the smaller intermicellar separation (centre to centre) required for bridging and the smaller effective hard-sphere volume occupied by the micelles, the latter effect of E-block length being much as found for micellar solutions of diblock EB and triblock EBE copolymers. [26,32] As noted in Concentration and Temperature Dependence of Modulus, the smaller effective hardsphere volume increases the minimum concentration for gelation from 9 wt.-% (B 10 E 227 B 10 …”

“…These results are consistent with those for related non-bridging copolymer systems: an increase in hydrophobic-block length is known to increase the stability of the micelles (and so the gel) in water, a good solvent at low temperatures. [32] The sol-gel diagram for copolymer B 12 E 114 B 12 was not defined with the same precision. [17] However, the minimum concentration for gel formation of micellar solutions of copolymer B 12 E 114 B 12 was approximately 15 wt.-%, compared with 9 wt.-% for copolymer B 12 E 227 B 12 , consistent with a shorter E-block length, i.e.…”