Investigations of the Viscoelastic Properties of Thin Polymer Films by Electromechanical Interferometry

Picosecond acoustic transmission measurements. I. Transient grating generation and detection of acoustic responses in thin metal films

The technique of impulsive stimulated thermal scattering is extended to backside measurement of acoustic wave packets that have propagated through thin metal films following their generation by pulsed optical excitation, heating, and thermal expansion at the front side. The acoustic transmission measurement at the backside substantially isolates the acoustic responses from thermal and electronic responses of the metal film that often dominate acoustic reflection signals measured from the front side, and permits straightforward measurement of the acoustic response generated by optical excitation at a substrate-thin film interface. It can thus better distinguish among different factors that limit the bandwidth of the acoustic wave packet, an issue of concern in the measurement of high frequency responses. The paper that follows demonstrates the application of the backside measurement to a study of high frequency structural relaxation in the glass-forming liquid glycerol.

Electrically driven deformations of nematic gels

The electrically driven deformations of side chain nematic networks swollen by nematic solvents (nematic gels) have been investigated. The strains of freely suspended gels between electrodes were measured as a function of field strength (E) . The deformation of the gels composed of a network and solvent with identical signs of dielectric anisotropy (Delta epsilon) is dominated by the electrically induced alignment of the nematogens. As a result, the stretching direction is variable according to the sign of Delta epsilon: The gel with positive or negative Delta epsilon is elongated parallel or normal to the field axis, respectively. The maximum strain among the samples examined is as large as 20% at E approximately equal to 0.5 MV/m. The gels composed of a network and solvent with opposite signs of Delta epsilon are compressed along the field axis since the electrostrictive effect becomes dominant because of a large reduction in the mesogen alignment effect due to the discord in the director directions of the constituent nematogens. The gels in the isotropic phase show compressive strains along the field direction in proportion to E2 purely originating from electrostriction, independently of the sign of Delta epsilon. The nematic gels are quickly deformed within a second upon field application, while the shape recovery after field removal requires a finite time on the order of 10(3) s, which reflects the structural relaxation in the polydomain texture from the oriented to the random state. The influences of elastic modulus as well as network nematicity on the electrical deformation are also examined.

Electromechanical properties of an ultrathin layer of directionally aligned helical polypeptides

The electromechanical properties of a monomolecular film of poly-gamma-benzyl-L-glutamate (PBLG) 15 nanometers thick grafted at the carboxyl-terminal end to a flat aluminum surface were measured. The field-induced change in film thickness, dominated by a large inverse-piezoelectric effect, demonstrates that the "grafting-from" technique forces the chains into a parallel arrangement. The mechanical plate modulus of the film as determined by electrostriction agrees with the theoretical prediction for a single PBLG molecule along the chain axis. The experiments show that ultrathin polypeptide layers with large persistent polarization can be fabricated by the grafting approach.