Picosecond Laser-Induced Transient Grating Probe of the Mechanical Properties of High-Modulus Poly(p-phenylenebenzobisoxazole-2,6-diyl)

The picosecond laser-induced transient grafting technique was used to determine the elastic constants of a predominantly uniaxial film of a rigid-rod polymer, poly(benzobisoxazole). By adjusting the grating angle, ultrasonic phonons in the frequency range 0.4-2 GHz were generated and their in-plane speed in various directions was measured. The speed was found to be independent of the phonon frequency in the range of frequency studied. The general Christoffel equation was used to fit the observed anisotropy of the acoustic velocity. This fit conveniently yielded various elastic moduli demonstrating the application of picosecond laser-induced transient grating methods for obtaining both longitudinal and shear components of elastic constants for an anisotropic medium.