Effect of carbon dioxide sorption on chain and small molecule diffusion in polymers
Compressible and supercritical fluids such as CO2 have gained popularity as a medium for polymer processing. The growing interest in such fluids is due to the attractive physical and chemical properties. Even though CO2 is an effective transient plasticizing solvent for most practical polymers, its limited solubility in them causes most processes to be heterogeneous in nature rendering mass transport as the rate-determining step. Extensive work has been done to understand the thermodynamics of CO 2-swollen polymers with limited attention towards quantifying the mass transport limitation in such systems. The aim of this study is to evaluate the diffusivity of both polymer chains and small molecules in CO2-swollen polymers and relate it to variables such as temperature, pressure, molecular weight, and concentration of solvent sorbed into polymer.
The first-half of the study looks into the diffusivity of polystyrene chains in CO2-swollen polystyrene films. The study is conducted in situ, in real time, using high-pressure neutron reflectivity. Diffusivity is studied as a function of polymer molecular weight, concentration of CO 2 in the polymer film, and temperature. Nearly an order of magnitude enhancement in the diffusivity of polystyrene chains (M = 2 × 105), from 1.62–9.35 × 10−16 cm2/s, is found with a modest increase in the concentration of CO2 in the polystyrene (from 8.9% to 11.3% by weight) at 62°C. This concentration dependence is modeled using the Vrentas-Duda free volume theory. At a constant temperature and CO2 pressure the polystyrene diffusivity scales as M−2.38. The scaling of the self-diffusivity of polystyrene in CO2-swollen polystyrene with T-Tg, where Tg is the glass transition temperature depressed by the presence of the solvent, is also attempted.
The second part of this study looks into the tracer diffusion coefficients of decacyclene, perylene and 9,10-bis(phenylethynyl) anthracene (BPEA) in CO2-swollen polymer films. Real time measurements are conducted using high-pressure fluorescence nonradiative energy transfer (NRET) techniques. Diffusivity is studied as a function of size, shape, temperature and concentration. The diffusivity of the molecules increases markedly upon modest increases in CO2 sorption. For example, decacyclene diffusivity increases from 6.6 × 10−14 cm2/s to 1.4 × 10−12 cm2/s as CO2 sorption increases from 7 to 11 wt %. The concentration dependence of the probe diffusivity is in agreement with the Vrentas-Duda free volume model extended to ternary systems. The analysis indicates that small molecule diffusion in CO2 -dilated films is strongly coupled to polymer segment mobility, but CO2 diffusion is not. The friction factor for the probes decreases with decreasing probe volume. The apparent activation energies for decacyclene diffusion in CO2-dilated films are considerably smaller than those in the polymer melt at equivalent values of T-Tg.