Release characteristics of an azo dye from poly(ureaurethane) microcapsules

The time course for the transfer of azo dyes from the microcapsule core of dioctylphthalate to the dispersing medium of methanol through a poly(ureaurethane) (PUU) membrane was measured for various average size microcapsules with a size distribution. The dye release curves are represented by a stretched exponential function C(t)=C(eq)[1-exp [-(t/tau(eff))(alpha)]], where C(t) is the concentration of the dye in methanol, C(eq) that at the equilibrium state, and t the time. The exponent alpha decreased by increasing the variance of the size distribution of microcapsules. The effective time constant tau(eff) was expressed by tau(eff)=c(0)/kappa(alpha), where and kappa(alpha) are the mean square of microcapsule radius and a correction for the size distribution, respectively. The characteristic proportional constant, c(0), was determined as 2.6+/-0.2 min/microm(2). From the value of the constant, the diffusion coefficient of the azo dye in the PUU membrane was evaluated as 2.6 x 10(-12) cm(2)/s.