Polyethylene glycol-400 (PEG) based polyurethane (PU) and polyacrylonitrile (PAN) semi-interpenetrating polymer networks (SIPNs) (PU/PAN; 90/10, 70/30, 60/40, and 50/50) have been prepared by sequential polymerization method. The prepared SIPNs have been characterized by physicomechanical properties. The microcrystalline parameters such as crystal size (< N >), lattice disorder (g), surface (D-s) and volume (D-v) weighted crystal size of SIPNs have been estimated using wide angle X-ray scattering studies, and quantification of the polymer network has been carried out on the basis of these parameters. The microstructural parameters have been established using Exponential, Lognormal, and Reinhold asymmetric column length distribution functions and the results are compiled. (c) 2005 Wiley Periodicals, Inc.