Rotations of nitroxide radicals in condensed polymeric systems

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An analytical expression was developed to explain the effect of molecular weight (M w ) upon the effective rotational frequencies (v eff) of different nitroxide spin probes incorporated in natural rubber. It was found that the effective activation energy (E) of probe rotation decreases with increasingM w and approaches the true activation energy (E 0) of segmental relaxations of polymer chains. Thev eff values of different probe radicals can be reduced to a single master curve by means of the equation: $$\frac{{v_{eff} }}{{exp\left( {\frac{{E_0 e^{ - M_W /M_S } }}{{RT}}} \right)}} = v_0 \exp \left( { - \frac{{E_0 }}{{RT}}} \right)$$ whereE 0 = 7.8 kcal mol−1,v 0 = 2 x 1013 s −1 andM s = 114 g mol−1. The value ofM s was interpreted as the smallest rotating unit of polymer chain which corresponds to a segment of about 1.7 monomer units. This leads to the interpretation that the glass transition process involves a coordinated motion of a group of about 30 short polymer segments. As a consequence of increasing intramolecular mobility of large radicals they can be regarded as flexible chains of more or less freely rotating links. Since the intermolecular coupling of the radical rotations with segmental rotations of polymer chains increases with increasingM w it is proposed that large flexible nitroxide probes describe best the segmental relaxations of polymers.