Probing solvent dynamics in concentrated polymer films with a high-frequency shear mode quartz resonator

Solvent reorientational dynamics in concentrated polymer films have been probed with a new experimental method based on a linear electromechanical model and a AT-cut high-frequency quartz resonator. This method is unique in that the viscoelastic characteristics of a composite resonator comprising the viscoelastic film and the quartz resonator are probed at the frequency of minimum resonator amplitude (f(Ymin)) rather than at f(Ymax), thus permitting measurements under conditions where a linear electromechanical model is most applicable. The method involves measurement of the admittance characteristics of the unloaded quartz resonator and the composite resonator, transformation of the admittance data near f(Ymin) into a linear form that provides accurate determination of the resonant conductance and susceptance, and use of Newton-Raphson numerical iteration to determine the viscoelastic characteristics from these values. This procedure enables real-time investigation of dynamic processes in polymer films, as demonstrated here by the simultaneous determination of the film thickness, storage modulus, and loss modulus during the drying of a spin-coated film containing polystyrene and 2-chlorotoluene solvent, The viscoelastic characteristics are investigated at a resonant frequency near 5 MHz under ambient conditions as the solvent mass fraction continuously decreases from its initial value of 15%. The trends in the measured storage and loss moduli are consistent with a single relaxation process, namely rotational relaxation of the 2-chlorotoluene solvent molecules, The solvent relaxation time increases with decreasing solvent content owing to the increasing influence of the polymer chains on the solvent reorientational dynamics, A plateau in the relaxation time at low solvent content (<2%) suggests the presence of a solvent glass transition. The results demonstrate that shear mode quartz resonators can be used to investigate solvent dynamics in polymer films at high concentrations that are inaccessible by other experimental methods. (C) 1996 American Institute of Physics.