Limited-supply non-Fickian diffusion in glassy polymers

Abstract

Non-Fickian diffusion of a flame-retardant plasticizer, resorcinol bis(diphenyl phosphate) (RDP), in a glassy poly(ether imide), Ultem TM, was measured with Rutherford backscattering spectroscopy (RBS). Volume fraction versus depth profiles were obtained as a function of time, temperature and externally applied stress in experiments where a limited supply of RDP was initially present on the surface of the Ultem. The profiles of the plasticizer in the glassy polymer in all samples had sharp diffusion fronts with constant volume fraction behind the front. The limited-supply boundary condition requires that the volume fraction, φ, of RDP in the plastized zone decreases as penetration depth increases. Isochronal values of φ decrease with increasing temperature. At long times, φ approaches a value such that the material in the plasticized zone has a glass transition temperature equal to the temperature of the experiment. At 140, 160 and 180°C, φ decreased in direct proportion to the logarithm of time. At 120°C, two regimes of diffusion behaviour were observed in a plot of φ approaches a value such that the material in the diffusion, and long-time low-φ behaviour corresponds to anomalous diffusion. Only the regime of anomalous diffusion was observed at the higher temperatures. Externally applied biaxial tensile or compressive stresses of order 10 to 40 MPa in the plane of the sample had no effect on diffusion normal to the plane in experiments at 120°C lasting 1 or 72h.