Dielectric response of polymer films confined between mica surfaces

The thin-film dielectric response of organic films confined within a surfac e forces apparatus (SFA) and also between parallel sheets of atomically smo oth mica is reported for the first time. Analysis is presented to infer die lectric properties of the organic film from the measured capacitance of the total system: sample, and mica sheets intervening between sample and elect rodes. Measurements concerned the frequency dependence of normal-mode diele ctric relaxation of cis-polyisoprene having dipoles aligned in the same dir ection along the chain backbone. We find that in thin-film geometries the p eak frequency, f(peak), of normal mode dielectric loss (epsilon") is modera tely lower than for bulk samples and that, more important, the expected ter minal tail, observed in the bulk sample (epsilon"proportional to f for f < f(peak)), is not observed even at the lowest frequency examined. Thus the s low normal mode distribution is much broader and the terminal relaxation ti me is much longer for chains in the thin layers. These dielectric features are attributed to spatial constraints on global chain motion in the thin la yers and also to adsorption of chains on mica surfaces when the layer thick ness is comparable to the unperturbed chain dimension. Independent measurem ents of shear relaxation, performed using a SFA modified for measurement of dynamical mechanical shear rheology, found a tremendously retarded viscoel astic response relative to bulk samples. There is the possibility that the broad distribution of the dielectric response of individual polymer chains may correspond to the observed retarded viscoelastic relaxation. However, w e cannot rule out the other possibility that the dielectrically detected re laxation of individual chains is still faster than the terminal viscoelasti c relaxation and that the latter thus corresponds to the collective motion of many confined chains. (C) 1999 American Institute of Physics. [S0021-960 6(99)50419-4].