Permeation and sorption of CO2 through amine-contained polyurethane and poly(urea-urethane) membranes

Abstract

As an absorbent of CO2, tetraethylenepentamine (TEPA) and/or N-methyldiethanolamine (MDEA) was introduced into the hard segment as chain extenders of the series urethane polymer (PU), urea–urethane polymer (PUU), and segmented urethane/urea–urethane copolymer (SPU) based on 4,4′-diphenylmethane diisocyanate (MDI) and either poly(ethylene glycol (PEG)-400 or -600. The obtained polymers thus contained a nearly equal weight composition of both soft and hard segments and were prepared as polymer membranes for permeation and sorption of CO2. The properties of the polymer membranes were characterized using a Fourier transform IR spectrophotometer, thermal gravimetric analysis, and rheometric measurement. The permeation and sorption isotherms as a function of temperature and pressure as well as the activation energy of permeability and diffusivity and the enthalpy change of the solution were measured. The test temperatures were carried out above and below the Tgs of soft-segment domains or the Tgh of hard-segment domains. The steady-state permeability (P) and diffusion coefficients (D) obtained ranged from 1.01 to 12.9 barrer and 1.04 to 4.04 cm2/s, respectively, and the solubility coefficients (S), from 0.529 to 3.43 cm3(STP)/cm3 polymer-atm at 10 atm and 35°C. The TEPA-containing polymer membranes showed a smaller P and D but a larger S than did MDEA-containing ones. The membranes comprising PEG-600 exhibit the values of P, D, and S at about 11.5-, 2.5-, and 4.5-fold the PEG-400 ones, respectively. For SPU membranes, above Tgs or Tgs, the pressure dependencies of P followed the modified free-volume model. On the other hand, below Tgs, they exhibited a minimum permeability at a certain pressure caused by the plasticization action of sorbed CO2. The sorption isotherms of CO2 indicated that the membranes comprising PEG-400 can be described by a dual-mode sorption model below Tgs. Also, the SPU polymer membranes obied the Henry's law above Tgs as well as Tgh. The characteristic constants of the sorption model were also determined and are discussed. © 1996 John Wiley & Sons, Inc.