It will depends, probably there is no easy solution for that, what you can do is to do a approximation, you know the diameter of the fiber and the length you can just calculate the perimeter of a cylinder and consider that your surface area. Another thing that you can do, that again is not easy is to use microtomography, if the pores in your fibers are big enough you will know the area that is filled with the fiber material and the empty space in that way you can have a better approximation. We have used Microtomography to calculate the porosity of electrospinning fibers and it worked very well, probably in your way it would be very useful too.

https://www.researchgate.net/publication/291422043_Polycaprolactone_Fibers_with_Self-Assembled_Peptide_Micro_Nanotubes_a_Practical_Route_Towards_Enhanced_Mechanical_Strength_and_Drug_Delivery_Applications

Article Polycaprolactone Fibers with Self-Assembled Peptide Micro/ N...

AFM based techniques may be appropriate for this kind of sample.

Using the appropriate AFM image analysis software (e.g. gwyddion) you can have an estimation of the projected surface area and surface area computed by simple triangulation.

http://gwyddion.net/documentation/user-guide-en/statistical-analysis.html

Good luck to your activities.

If you can't use conventional BET, it is possible to estimate a specific surface area by visualization of surface. Use SEM, TEM or AFM (if the surface isn't too rough).

Good luck!  

https://www.researchgate.net/publication/276415611_Nanostructuring_of_the_carbon_macrofiber_surface

Article Nanostructuring of the carbon macrofiber surface

Thank you all very very much for your answers. I will try to look at the surface and estimate the SSA as you propose. This was exactly what I was expecting and I will do what the three of you suggest so I appreciate your help and the time you took to answer my question.

Thank all of you very much again.

Guillermo

Dear Guillermo,

Fibres has a low surface area, below 1m2/g or around it. And it is proved that Nitrogen Sorption technique is not that good at low surface area range, below and around 1m2/g they recommend to use Krypton because using nitrogen is insufficient.

I recommend to you the IGC technique (Carrier gas technique), which uses the same BET principle as other sorption techniques. It is a valid and alternative method for BET Surface Area analysis.

I attached our paper in this subject.

Best wishes,

Anett

Article Inverse Gas Chromatography for Natural Fibre Characterisatio...

Find the diameter of the fiber. Then calculate the perimeter considering the fiber as a cylindrical shape. You'll get an approximated result.