Entangled high polymers in the melt or semidilute solution exhibit motion dependent on the timescale. This motion may be characterised in terms of the " tube model " in which the random coil polymer under investigation is enclosed in a tube formed by its neighbours. At the shortest timescale, motion consists principally of segment reorientation. The topology of the tube implies that some residual anisotropy will exist in
this motion3. On the next higher timescale reptative displacements around tube bends occur, thus causing a fluctuation in the direction of residual orientation. On the longest timescale, final correlation loss occurs by tube renewal.
T Ip is the relaxation time for a spin system to come to thermal equilibrium in a transverse RF field. It is sensitive to components of the motion at the RF Larmor frequency. This frequency is low and adjustable ( 102 to 1Q5 Hz ) and extends the regime accessible to Field cycling TI experiments4. T Ip therefore provides access to the intermediate and long timescale fluctuations in entangled polymers. It is a major conclusion of this work that reptation and tube renewal effects can be directly observed.
The BPP theory of relaxation25 has been extended to TIp for three proton spins in a methyl group. Results of a relaxation study in two polymer melt systems, namely polydimethylsiloxane and polyethylene oxide are presented. In the latter case the results
are compared with T Ip dispersions made on Polyethylene melts13. The experimental data is seen to follow the theoretical predictions made by Kimmich3,4.