Surface study of cross-linking gradients in photopolymers by acoustic microscopy

Acoustic techniques use the propagation of ultrasonic waves a few microns under the sample surface and their interaction with the material structure to characterize sample surface properties. The aim of this work is to apply these investigation methods to display cross-linking gradients created in photopolymers by a reticulation process induced by UV light, This method has first been tested on materials exposed through a test target mask. Using 600 and 90 MHz focused lenses, we got fine acoustic images with resolution respectively around 4 mu m and 25 mu m, where the abrupt variations of elastic properties produced by the optic filter are easily detectable. Next, we tried to locate a more gradual distribution of elastic properties, created via the UV exposure through an apodizing filter with a progressive radial transmission function. This gradient has not been detected with the previous acoustic imaging system, but using the complementary technique of acoustic signature, a quite similar profile has been retrieved from measurements of the longitudinal lateral velocity on small volumes close to the material surface, This sample has then been cut off in two parallel cross-sections to follow, via surface investigation, variations induced inside the material. This study has demonstrated a softening of the cross-linkings gradients close to the surface, responsible for the absence of contrast on previous acoustic images. Moreover, the evolution of the velocities measured inside this sample has shown a special volumic distribution, which agrees with the chemical theories predictions about the reticulation process, Acoustic attenuation measurements are also produced as a possible complementary information on structural changes in photopolymers.