SPIN-LABEL STUDY OF IMMISCIBLE POLYMERS .3. LOCATION OF CHAIN ENDS IN A BLEND OF POLYSTYRENE AND POLYISOPRENE

Electron spin resonance (e.s.r.) spectra of polystyrene (PS) samples labelled at chain ends or at in-chain sites with nitroxide radicals were recorded over a temperature range. Up to 423 K for the end-labelled PS and 453 K for the in-chain labelled PS, the spectra are essentially of the broad-line type and in both cases the spectral changes are reversed exactly on cooling. Both labelled PS samples were blended with polyisoprene (PIP) (1:1, w/w). The e.s.r. spectra of the blend of the in-chain labelled PS are identical to those of the bulk labelled PS. The e.s.r. spectra of the blend of the end-labelled PS reveal, as previously, that PIP plasticizes PS chain ends. Also, for this blend, the spectral changes produced on heating are not reversed exactly on cooling, and even at room temperature a motionally narrowed component persists. It appears that after heating, the original freeze-dried polymer is reorganized so that more nitroxide chain ends are located in a PIP-rich environment. It is concluded that the e.s.r. experiments confirm the theoretical prediction of Helfand that at thermodynamic equilibrium the interphase in a blend of immiscible polymers contains more than the statistical concentration of chain ends.