Composition fluctuations in a non-critical binary polymer blend studied by ultrasonic and light scattering experiments

Abstract

A non-critical mixture of polyethylene glycol (PEG600, M = 600 g mol⁻¹) and polypropylene glycol (PPG1000, M = 1000 g mol⁻¹) was investigated by ultrasonic and light scattering experiments in the one-phase region. The mass fraction of polypropylene glycol in the non-critical mixture was y_PPG = 0.365. The explored temperature and frequency range of the ultrasonic experiment was 0.1 ≤ T − T_P ≤ 17.3 K and 0.4 MHz ≤ f ≤ 30 MHz (T_P: phase separation temperature). The frequency dependence of the ultrasonic attenuation of the non-critical mixture shows a relaxation behaviour typical for composition fluctuations. The data can be analysed by the dynamic scaling theory of Bhattacharjee and Ferrell which was formally extended to the non-critical case using the concept of a pseudospinodal temperature. The characteristic time scale of the concentration fluctuations is described by a frequency ω_D = 2D/ξ² where D is the mutual diffusion coefficient and ξ is the correlation length. In the frame of the pseudospinodal concept the temperature dependence of the characteristic frequency is expressed by ω_D = ω₀ε^zν with ε = (T − T_PS)/T_PS (T_PS: pseudospinodal temperature, ε: reduced temperature, ω₀: critical amplitude, zν: critical exponent). The temperature dependence of the frequency ω_D was determined by the ultrasonic spectra. From this data, using mean field exponents, a value of ω₀ = 9.4 MHz was estimated which is comparable to that of the critical mixture. The description of the ultrasonic data with mean field exponents is justified by the results of dynamic light scattering.