GC_FID_Siloxanes_Difficulty
Posted: Fri Oct 15, 2010 4:12 pm
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?
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?
