Structure of PS−PEO Diblock Copolymers in Solution and the Bulk State Probed Using Dynamic Light-Scattering and Small-Angle Neutron-Scattering and Dynamic Mechanical Measurements

Abstract

The phase behavior of a low molecular weight (M W = 4000) diblock copolymer of polystyrene and poly(ethylene oxide), PS−PEO, in the bulk as well in aqueous, D2O, solutions has been studied using small-angle neutron-scattering, dynamic and static light-scattering, and rheological methods. At low temperature the pure block copolymer forms a lamellar mesophase driven by the crystallization of PEO. The melting point of PEO, T ≈ 64 °C, is correspondingly accompanied by an order-to-disorder transition. The melt is typified by single-exponential depolarized correlation functions in the dynamic light-scattering spectra, observed in the temperature range 60 to 100 °C. Neutron scattering fails to show concentration fluctuations in the amorphous, disordered phase due to lack of contrast. In aqueous solutions up to roughly 20% polymer concentration, PS−PEO self-associates into spherical micelles with a core size of R c ≈ 56 Å and an interaction radius R hs ≈ 115 Å, corresponding to an aggregation number of 470. The hydrodynamic radius R h = 140 Å obtained from dynamic light scattering is close to but somewhat larger than the interaction radius, as expected. Extended clusters, displaying a small but significant anisotropy, form at above C = 10% and coexist with the single spherical micelles. These clusters may be due to association of spherical micelles or micelles of different form attributed to residual aggregates of PS moities.

Morphology of hybrid polystyrene-block-poly(ethylene oxide) micelles: Analytical ultracentrifugation and SANS studies

“…We believe that the differences are due to the different model chosen to describe the data. To support the validity of our results we note that the average core radius obtained from our analysis for SE1030 at c = 1.0% w/w (25 • C) is 32.7 Å, to be compared with the value of 56 Å obtained by Mortensen et al [43] under the same conditions for the same block copolymer (SE1030). Considering the PS chain, one can estimate a fully extended length of approx.…”

“…The aggregation numbers that we derive from the present characterization of SE1030 solutions are different from those derived in Ref. [43]. We believe that the differences are due to the different model chosen to describe the data.…”

“…6 we report the data sets relative to the concentration and temperature dependence of the SANS pattern for SE0720. It can be appreciated that while a visible structural effect is caused by the concentration, on the other hand, a minor influence is due to the temperature change in the range 25-65 • C. Similar observations can be done for the system SE1030, for which we do not report the whole set of experimental data, as comparable SANS patterns have already been published [43]. In Fig.…”

“…Light scattering investigations on high molecular weight block copolymers (M n > 20,000) [42] highlighted the existence of a bimodal distribution of particles sizes: (a) small (R h ∼ 20 nm) copolymer aggregates (regular micelles) and (b) loose micellar clusters with sizes larger than 60 nm. More recently application of combined PCS and SANS techniques [43] to PS-b-PEO SE1030 aqueous solutions led to the finding that for low concentrations (lower than about 10 wt%) only spherical micelles are present. At higher concentrations, clusters of micelles begin to form.…”

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

“…Unfortunately, values of the association number reported for micelles of St 10 E 68 in water at ambient range from 60 to over 400. [14,17,20] Very high values may derive from the presence of large particles, as frequently reported for samples of E/St block copolymers in dilute aqueous solution. [11,13,20] The lowest value [20] is from sedimentation velocity and so independent of a second population of particles.…”