Swelling And Nonlinear Diffusion Of The PET In The Presence Of Supercritical Fluid Carbon Dioxide

The glass transition temperature(T_g)is one of the most important characteristic parameters of the polymers.It is useful in describing the movement of polymer chains and the plasticization of polymers.A broad rang of applications have been proposed for synthesis and processing polymers with supercritical fluid carbon dioxide(scCO₂),for its strong point of the depression of glass transition temperature and the plasticization of polymers.The applications can be extraction of impurities and fractionation,facilitation impregnation of polymers with various additives,devolatilization,dyeing,and so on.So it is valuable to make clear the glass transition behavior of the polymer in supercritical fluid carbon dioxide.Up to now,the instruments used to detect T_g at high pressure exist some disadvantages,including the complex of instruments and the poor Capacity of Pressurization.In order to solve the problems,one high pressure autoclave with a view window was developed.So the length change of the rectangle PET film immerging in the carbon dioxide can be determinationed via a CCD camera.According to the length change,the volume change of sample can be got.Because the volume changes sharply at glass transition temperature,it is easy to confirm the T_g from experimental results.In our research,the T_g corresponding to different pressure were obtained(0.1/87,5/82,10/78,15/68,20/62,MPa/℃).It has evidences to prove the availability of the new approach to measure the T_g in the presence of supercritical fluid carbon dioxide.The results obtained from experiment were compared with the calculated values from G-D model and Chow model.It shows that the two models only suitable for not high pressure.The depression of glass transition temperature is induced by the plasticization of polymer with small molecules,while the swelling of polymer is ascribed to the diffusion of small molecules in polymer.So the deep understanding of diffusion in polymer is very important.In the past, Fick's law was commonly used to describe such diffusion,but the actual diffusion of small molecules into/out of polymer isn't obey Fick's law nearby T_g,and present a non-linear behavior.So we did a preliminary study of it.