Ultrasonic investigation of glass transition dynamics in polyurethane systems Pierre-Yves Baillif, Mohamed Tabellout, Jacques R. Emery Groupe Ultrasons - U.M R. 651.5 de Chimie et Physique des Mattfriaux Polymkes Universita!du Maine, Av. 0. Messiaen, 72085 Le Mans Cedex 9, France Email : ultrason@univ-lemanxfi Abstract: Ultrasonic techniques are used in order to probe the high frequency dynamical properties of polyurethane gels. The relaxation processes found are compared with those obtained by low frequency measurements(rheology) and show that the same dynamics is probed and linked to a relaxation which is related to the glass transition. INTRODUCTION Because they are considered as model networks, polyurethane systems are of interest for the determination of structure - properties relationships in polymeric systems. The aim of this study is to obtain the high frequency dynamical properties of these systems for a better understanding of relaxations associated with the glass transition. The principal one, the so-called a relaxation, appears on a large frequency scale and is linked to cooperative movements involving a few monomer units of the polymer chain. Ultrasonics is then a performing tool able to complete the description of this dynamical process. EXPERIMENTALS Polyurethane gels are prepared by polycondensation mechanism between poly(propylene glycol) trio1 with hexamethylene diisocyanate. In order to study the glass transition dynamics during gel formation, quenched samples corresponding to different reaction extent are obtained by varying the stoichiometric ratio r. This allows to simulate the different steps of the network formation. Three triols with different molar masses (260, 700 and 6000g/mole) are used as precursors in order to apprehend the influence of the macromolecular chain size on the systems dynamics. Ultrasonic measurements are made at a fixed frequency (4MHz), using burst longitudinal waves. Transit time and amplitude of ultrasonic signal are compared to a reference. This allows the determination of absolute values for velocity (v) and absorption (aus) of the ultrasonic wave in the medium versus temperature. Longitudinal moduli M are calculated for each system using (I) equations 1. M’z pv2 M”4pv3- 0 (1) RESULTS AND DISCUSSION Velocity and absorption are obtained and the associated moduli, showing a relaxational behavior on the experimental temperature range, are presented on figure (a) and (b) for systems based on trio1 700. A stoichiometric dependence is found and characterized by a temperature shift of M’ and M” curves with r. This shit? is related to glass transition temperature (Ts) difference between each non stoichiometric system. The observed difference in T, is related with an increase of crosslink density in gel forming system. It is also found that an increase in r leads to a broadening of M” and then of the associated relaxation function. The characteristic relaxation times are taken as the maxima of imaginary part of the modulus M” versus temperature. They are compared to those obtained by rheological low frequency measurements (DMTA, RDA) 1943