Dear Chehat Nour El Houda ,

please see for example:

https://publicspectra.com/Raman/Polyvinyl%20fluoride

See also around Fig. 5 and text around bottom of page 60 of:

https://deepblue.lib.umich.edu/bitstream/handle/2027.42/30156/0000533.pdf;jsessionid=E3B06770D14C21FE89FD585FD61C37DD?sequence=1

The following thesis may also help:

https://etd.ohiolink.edu/acprod/odb_etd/ws/send_file/send?accession=case1055351497&disposition=inline

(uploading may take a while...)

Best regards

G.M.