Interface Affected Polymer Dynamics:  NMR, SANS, and DLS Study of the Influence of Shell−Core Interactions on the Core Chain Mobility of Poly(2-ethylhexyl acrylate)-block-poly(acrylic acid) Micelles in Water

Mobility of side chain groups in the essentially fluid core of acrylate−acrylic acid block copolymer micelles has been examined by a multiquantum NMR relaxation study supported by 1H high resolution and MAS spectra and a SANS and DLS study of the physical structure of the micellar systems. It is shown that poly(2-ethylhexyl acrylate)-block-poly(acrylic acid) (PEH A − P AA) and its sodium salts of a composition EHA 53 AA 279 form micelles in water or D2O with a PEHA core radius of about 4.7 nm, containing about 40 copolymer molecules according to SANS. The apparent core radius increases with lower neutralization degree of the PAA units reaching 6.7 nm in the purely acidic form. This difference is shown by NMR to be due to an increasing layer of a mixed interface containing units of both PEHA and PAA. The overall radius of the micelle in the form neutralized at least to 10 mol % has been estimated to be 24 nm by SANS, the hydrodynamic radius obtained by DLS in a fully neutralized form was 28 nm, and the rotational hydrodynamic radius estimated from 1H MAS and static NMR was 26−27 nm. The mostly acidic form of PEH A − P AA tends to form aggregates which can be dispersed by ultrasound. Using NMR spectra with various degrees of T 2-filtration, dipolar interaction canceling by pulse sequence, MAS under various spinning rates, and signal shape analysis under varied viscosity of the medium and mobility of the shell blocks, a varying restriction or hindrances of segmental motion both in the inner part of the shell and in the micellar core can be demonstrated. Using transverse and rotating frame relaxation in varying regime and processing of data, approximate spectra of the transverse relaxation times for the side-group methyl and methylene signals can be reconstructed. According to them, about a third of the side groups possess a mobility comparable to that in bulk PEHA whereas the remaining part is perceptibly hindered in motion, in particular that near to the core−shell interface. Monomer EHA was swollen into the core and polymerized by γ-irradiation offering thus 0.78−0.57 free PEHA/bound core PEHA. Although this dilution of the core blocks led to some increase in mobility, no chains or their side groups acquired mobility equivalent to that in bulk PEHA at the same temperature. The results are discussed in terms of resistance against motional core−shell group exchange and osmotic pressure on the core−shell interface.