Polymer composition by FT-IR?

Hi All,

I am trying to employ FT-IR (ATR) for comparing the real composition against the theoretical composition of a copolymer series. I have synthesized the homopolymers of monomers X and Z and then the copolymers at different theoretical composition (5, 10, 25 and 50 mol% of monomer Z) of both monomers.

Then I got the spectra of all samples employing the ATR technique and identified the following peaks:
- (1461cm-1) present in all samples/spectra's, which I have used as an internal reference.
- (985cm-1) present in spectra of homopolymer X and the copolymers. I have assigned it as a characteristic peak from the monomer X.
- (1104cm-1) present in spectra of homopolymer Z and the copolymers. I have assigned it as a characteristic peak from the monomer Z.

Finally I plotted the absorbance ratio of “monomer Z” (1104/1461) and “monomer X” (985/1461) against the theoretical composition. The final plots are non-linear ones – the “monomer Z” one shows a negative deviation from the linear behavior and the “monomer X” one shows a positive deviation.

Is this a correct approach and can I suggest that the copolymers are "monomer X" richer?

Thanks!