Dear Ramhari,

please attach your XRD pattern;

you appearently forgot to attach it.

Thanks in advance.

But I think, you know that broad XRD peaks are due to local atomic disorder ( PS  in general is known to have one broad peak; thus beeing quite amorphous) and that narrow peaks  are due to local order of atoms.

There might be a change of atomic order (at least at the surface of your sample) during your treatment or a damage of your molecules; or partly melting and setting up stress when cooling down. Stress will cause smaller peaks in polymers due to arrangement of chain molecules into  local regular arrays of molecules etc...

You may explain a little bit more in detail what you have done  what chemical changes you expext during treatement.

---air plasma.... temperature treatment...oxydation/burning...

If not really familiar with XRD in polymer science please have a look at the link:

http://www.polymertechnology.it/bacheca/NanocompositeForm/page5/files/rx_2.pdf

Remark for other readers: I just saw the XRD pattern in a similar question of Ramhari:

https://www.researchgate.net/post/what_is_the_reason_for_this_x_ray_diffraction_peaks_of_polystyrene_sample