GC_FID_Siloxanes_Difficulty

[Jumpshooter]

I am aware that the optimal way to qualify and quantify the linear polymer siloxane is via gel permeation chrom; however, I am not set up for that test method. Thus, I had begun to investigate if the moiety could be chromatographed via GC FID.
For comparative test samples I used: 1) tetra-cyclic siloxane (four ring structure of Si-O-Si covalently bonded to pairs of methyl groups at the four corners of the ring structure (smaller mol. wt.); 2) linear polymer siloxane (Si-O-Si covalently bonded to methyl groups at the termini and to C-H along the backbone of the polymeric repeating unit (large molecular weight).
Chrom was conducted using HP-5 (30x.32x.25) with oven = 75C_10C/min_300C_hold 5min; Helium carrier with inlet psi = 10.
Det = 300C; total run time = 23 min.
Results:
1) the tetracyclic siloxane showed a well-resolved peak at 4.02 mins and displayed first order kinetics with increasing conc.
2) injection of the linear polymer was a flat line with No peaks.

FTIR analysis showed that both tetracyclic siloxane and linear PDMS had the signature peak at 1260 cm-1 which is represented the methyl-Si vibrational deformation. This peak also displayed first order kinetics for both siloxanes with R2 = 0.998.

Certainly there are C-H bonds in the PDMS that once ''broken" would produce a mV signal.

Please help me: Why was no peak observed on GCFID for the linear silox?

[JI2002]

What's the MW and B.P. of linear siloxane?

[Jumpshooter]

Well, that is the "rub" in this paradox: the dow corning manufacturer does not list the Mol wt of the linear silox. I can only presume that is at least 300 units as there are multiple repeating (n) units of the Si-O-Si with CH's abutting them. The n I presume is at minimum 10.
The boiling point is not known at this time.
Yes, I had surmised that the linear silox may not be volatizeable at the Injector temp of 250C. But hey ?

[Jumpshooter]

Will devise an extraction process that involves selective derivatization of the cleaved Si-O.
This may help to at least qualitatively evaluate the pdms linear polymeric. The capiillary column with HP5 phase out to work.
I believe that there is residual Si O on my jet.

[Peter Apps]

HI Jumpshooter

You need to be sure that the vapour pressure of your heavier polymer is in the GC range before you do anything else, ten silicons is equivalent to 27 carbons before you add any of the side chains.

You might also want to consider the likely impact on your analysis of the siloxane phase in the column bleeding siloxanes that might interfere with the analysis.

Peter

[Jumpshooter]

Thanks Peter:

Well, yes, I had considered the "bleed" profile; I was hoping that the bleed profile would not be superimposed on the linear pdms polymer elution.
To date, I cannot see any peaks at all.

OK, I need to continue with your computation of # Carbons per Siloxane.

By what reasoning did you deduce this stochiometry? Thank you sir.

[Peter Apps]

I just looked at the atomic weights of carbon and silicon - nothing fancy but it gets you into the ball park.

Peter

[Jumpshooter]

OK, I follow your calculation and thanks Peter.
However, this analytical objective of specifically quantifying linear PDMS via GC_FID continues to pose ostensibly insurmountable barriers. To date a gc run on the linear PDMS pure standard material (50 mg PDMS dissolved in 10 mL hexane) resulted in no peaks. Why this result manifests is unresolvable?

[Peter Apps]

Hi Jumpshooter

Work out the MW, or get the boiling point of the lightest and heaviest siloxanes in the mix. Then get the alkanes that have similar properties and see if they give you peaks. If not the GC system is not up to the job, if they do give peaks then the problem is specific to the siloxanes.

Peter

[Jumpshooter]

I've worked out the Mol.Wts for the various monomers, dimers, trimers and assorted oligomers that the polymeric PDMS could degrade into. Interestingly, the monomeric units will resolve via GC FID; however, the polymeric unit will not resolve using this method.
Regretfully, I must concede that my GCFID method is inappropriate for identification and much less quantitation of polymeric pdms.