SURFACE STUDY OF CROSS-LINKING GRADIENTS IN PHOTOPOLYMERS BY ACOUSTICMICROSCOPY

Acoustic techniques use the propagation of ultrasonic waves a few micr ons under the sample surface and their interaction with the material s tructure to characterize sample surface properties. The aim of this wo rk is to apply these investigation methods to display cross-linking gr adients created in photopolymers by a reticulation process induced by UV light, This method has first been tested on materials exposed throu gh a test target mask. Using 600 and 90 MHz focused lenses, we got fin e 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 gr adual distribution of elastic properties, created via the UV exposure through an apodizing filter with a progressive radial transmission fun ction. This gradient has not been detected with the previous acoustic imaging system, but using the complementary technique of acoustic sign ature, a quite similar profile has been retrieved from measurements of the longitudinal lateral velocity on small volumes close to the mater ial surface, This sample has then been cut off in two parallel cross-s ections to follow, via surface investigation, variations induced insid e the material. This study has demonstrated a softening of the cross-l inkings 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 dis tribution, which agrees with the chemical theories predictions about t he reticulation process, Acoustic attenuation measurements are also pr oduced as a possible complementary information on structural changes i n photopolymers.