Abstract: The present invention is directed to a sheet product and to a method of forming the sheet product suitable for use as a battery separator. The method comprises forming a mixture of a thermoplastic polymer and fluid having a high vapor pressure, shaping the mixture into a sheet material and subjecting the sheet material to stretching/fluid vaporization at high temperature to form an intermediate material having a ratio of percent fluid to percent polymer crystallinity of between 0.15 and 1 followed by a second stretching/fluid vaporization at a lower temperature while removing a portion of the remainder of the fluid from the sheet. The resultant sheet is annealed and remainder of fluid removed to form a sheet product having a thickness comprising a stratified structure of small and larger pore layered configuration across its thickness.