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-acr ylic acid block copolymer micelles has been examined by a multiquantum NMR relaxation study supported by H-1 high resolution and MAS spectra and a SAN S and DLS study of the physical structure of the micellar systems. It is sh own that poly(2-ethylhexyl acrylate)-block-poly(acrylic acid) (PEHA-PAA) an d its sodium salts of a composition EHA(53)AA(279) form micelles in water o r D2O with a PEHA core radius of about 4.7 nm, containing about 40 copolyme r molecules according to SANS. The apparent core radius increases with lowe r neutralization degree of the PAA units reaching 6.7 nm in the purely acid ic form. This difference is shown by NMR to be due to an increasing layer o f a mixed interface containing units of both PEHA and PAA. The overall radi us of the micelle in the form neutralized at least to 10 mol % has been est imated to be 24 nm by SANS, the hydrodynamic radius obtained by DLS in a fu lly neutralized form was 28 nm, and the rotational hydrodynamic radius esti mated from H-1 MAS and static NMR was 26-27 nm. The mostly acidic form of P EHA-PAA tends to form aggregates which can be dispersed by ultrasound. Usin g NMR spectra with various degrees of T-2-filtration, dipolar interaction c anceling by pulse sequence, MAS under various spinning rates, and signal sh ape analysis under varied viscosity of the medium and mobility of the shell blocks, a varying restriction or hindrances of segmental motion both in th e inner part of the shell and in the micellar core can be demonstrated. Usi ng transverse and rotating frame relaxation in varying regime and processin g of data, approximate spectra of the transverse relaxation times for the s ide-group methyl and methylene signals can be reconstructed. According to t hem, 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 swoll en into the core and polymerized by gamma-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 mobi lity equivalent to that in bulk PEHA at the same temperature. The results a re discussed in terms of resistance against motional core-shell group excha nge and osmotic pressure on the core-shell interface.