BET method requires dehydrating the cotton fibers at 120°C under vacuum, therefore removing the intramolecular water molecules and allowing the cellulose chains to collapse. Thus, the internal surface area of the cotton fibers is inaccessible to the N2 molecules. In effect, the dehydration of cotton fiber by vacuum drying promotes formation of interchain hydrogen linkages or cellulose-cellulose linkages which are too strong to be replaced by N2 molecules.

See reference: http://eprints.qut.edu.au/7630/1/7630.pdf

Thank you sir! Florian Glodeanu, Sir then how can i measure surface area? Is there any other technique?

Hi, you might want to look into mercury porosimetry. https://en.wikipedia.org/wiki/Porosimetry