Process NMR

Overview of Solid-State 29Si NMR Studies of Silicone Gasket Materials


Silicon-29 NMR has a unique ability to probe the detailed solid state chemistry of silicone rubber materials. The chemical shift range of the various silicone chemistry groups spans 120 ppm (at 4.9T this equates to 4800 Hz). This is a large dispersion which allows all chemistry types to be readily observed. In these, studies, however, we have not attempted to identify every chemical type present. We have identified general chemical types and followed their relative concentration from one sample to the next. Both cross polarization and single pulse experiments were performed on magic angle spinning samples (a full explanation of these two complimentary experiments is given below). Optimal experimental conditions were initially obtained and all subsequent samples have been analyzed under identical conditions to facilitate an understanding of the spectral changes from a chemistry as well as molecular mobility standpoint. Set NMR experiment conditions were used because of the time consuming nature of these experiments which typically take around 12 hours for each of the cross polarization and single pulse experiments.

Experimental

At the present time samples are being run on a Varian UnityPlus-200 spectrometer operating at a 29Si frequency of 39.74 MHz. The probe was a Doty Scientific 7 mm Supersonic CP/MAS probe using zirconia and Kel-F end caps. For the single pulse NMR experiments a p/6 pulse of 2 ms was used with a relaxation delay of 4 seconds to facilitate quicker acquisition.The 4 second relaxation delay was obatined from full T1-inversion recovery experiments. Gated proton decoupling was used during FID acquisition. For the cross polarization experiments full contact time array experiments were obtained on the initial samples submitted. Due to the mobility of the polymer backbone the optimum cross polarization contact time for the polymer backbone was around 15 ms with signal lasting until 50+ ms. However, the more rigid structures in the polymer ? such as the silicates, had optimum contact times around 3-5 ms. As a compromise we chose a 6.4 ms contact time which yielded good signal sensitivity for both the polymer and silicate components. Cross polarization inversion recovery experiments yielded a short relaxation delay of 2 seconds. A 1H p/2 pulse of 4.6 ms was used along with gated proton decouplind during FID acquisition. For all samples the same experimental conditions have been maintained. MAS spinning speeds were around 7 kHz to avoid spinning side band coincidence on real signals. Also, to avoid MAS induced modulation of the contact-time, the variable amplitude cross-polarization contact pulse was used.

 

Silicone Chemistry Observed by NMR

The notation in use for silicone chemistry is M,D,T,Q (mono, di, tri and quaternary) denoting the oxygen substitution on the silicon atom. The polymer backbone itself is predominantly D i.e. [(SiO2(CH3)2]n which has a typical resonance frequency around -21 ppm. The termination of the polymer would be an M group (SiO(CH3)3) (found at +4 to +10 ppm) or MOH (SiO(CH3)2OH) (-10 to -15 ppm). Another area of interest in the spectrum is the -20 to -10 ppm region which is partially due to MOH but also due to D type silicon centers that are within 5 monomer units of a termination. Thus, if hydrolysis of the silicone backbone is occuring, this region will increase in intensity as one will now have more silicon centers close to termination points as well as more MOH terminations.

In some gaskets one observes small signals in the -60 to -70 ppm region which is due to T type silicone centers (SiO3(CH3))n, however this is usually not observed. The only other region where one observes signal is in the -80 to -120 ppm region of the spectrum. These silicon centers can only be Q1 (SiO(OH)3), Q2 (SiO2(OH)2), Q3 (Si(OSi)3(OH)) or Q4 (SiO4)types, as only silicons with 4 attached oxygens can appear in this region, any methyl substitution would cause these silicons to appear in the +10 to -70 ppm range of the spectrum. Of relevance to any discussions on silicone polymers it should be noted that Q1 is equivalent to M(OH)3, Q2 is equivalent to D(OH)2, Q3 is equivalent to TOH.

When one looks at the NMR experiments for the certain silicones one does not observe a resonance at +10 to +4 ppm. This indicates that the predominant polymer termination is MOH. Silicate is observed, however, it is not clear if this silicate is a filler for hydrogen bonding crosslinking or actual polymer Q4/Q3/Q2 sites of directly condensed silicates acting as bonded crosslinking agents.

SP-MAS NMR Experiments

In this experiment one quantitatively observes all silicon species in the system allowing a "bulk" silicon type distribution to be calculated. One observes a narrow resonance at ?21 ppm which is due to the silicone polymer backbone (-O-Si(CH3)2-O-)n. Very little signal is observed in the -20 to -10 ppm region indicating that the polymer chains are quite long. In the -80 to -120 ppm region of the spectrum one observes silicon present in silicate that has been added as a cross-linking agent. The hydrogen bonding between the silicone polymer and the Si-OH groups of the silicate add structural integrity to the gasket. It is differences in the silicate silanol (Si-OH) chemistry that may account for changes in compressibility of the gasket during service. Thus, one will observe relative changes in the amount of 29Si signal observed in the -80 to -103 ppm and -10 to -23 ppm regions of the spectrum. This region is where Q3 (Si(Osi)3(OH)), Q2 (Si(Osi)2(OH)2), and Q1 (Si(Osi)(OH)3) groups are found.

Parameters Calculated:

Silicate Content% of Si atoms present in silicate filler
Q4% of Si atoms present in Q4 structures in silicate- Si(OSi)4
Qn% of Si atoms present in Qn structures in silicate- (Q1, Q2, Q3)
Q4/Qn Ratio Allows relative change in silanol (Si-OH) distribution to be observed.
Percent Polymer% of Si atoms in D and MOH polymer units.

Relative changes in these parameters can be utilized to interpret changes in silicon chemistry caused by coolant exposure and service.

CP-MAS NMR Experiments

This experiment warrants a detailed explanation as the results are not quantitative from a "bulk" silicon chemistry standpoint. The CPMAS experiment utilizes the strong NMR signal that can be generated from protons (H) in the sample. The experiment preferentially observes silicons that are in close proximity to H. However, mobility is also a ?problem? in this experiment. The way the experiment works is that the protons in the sample are polarized initially and magnetization is transferred from the protons to the silicons via their dipole-dipole interaction (similar to the interaction between 2 bar magnets). This interaction weakens the further the H and Si are from each other, and also weakens if there is considerable molecular motion. In the case of these samples this means that in the silicate region of the spectrum one observes an enhancement of the signal due to Si-OH containing species. In the case of the silicone polymer, however, one observes an overall decrease in the signal at -21 ppm due to the -(O-Si(CH3)2-O-)n backbone due to its rapid segmental (rubbery) molecular motions. One observes a large signal (that is hardly observable in the SPMAS spectra) in the -5 to -20 ppm region. This is due to silicone silicons that are at or directly adjacent to cross-linking sites (where there could be a D-silicate bond, or polymer termination sites such as MOH. The fact that they are enhanced by the CP technique indicates that these termination proximate silicons are motionally restricted compared to the rest of the silicone backbone. They represent either strongly hydrogen-bonded regions or chemical cross-links of the type (O2(CH3)2Si-O-Si-O-Silicate) where a defect in the silicone backbone has reacted with a silanol of the silicate filler to form a Si-O-Si bond. This experiment is very powerful when used to observe relative changes in Si-OH chemistry in the silicate region and relative mobility changes in the polymer backbone.

Parameters Calculated:

% Polymer Backbone% of Si observed in mobile silicone backbone
% Restricted Polymer Backbone% of Si present in motionally restricted regions of the silicone backbone (D units in close proximity too cross-linking sites, termination sites (MOH) or adjacent to termination sites
Silicate Content% of Si atoms present in silicate filler
Q4% of Si atoms present in Q4 structures in silicate- Si(OSi)4
Qn% of Si atoms present in Qn structures in silicate- (Q1, Q2, Q3)
Q4/Qn RatioAllows relative change in silanol (Si-OH) distribution to be observed.

As with the SP-MAS calculated parameters one can utilize these parameters to determine changes in silicon chemistry resulting from coolant exposure and service.

Table I

Summary of 29Si NMR Chemical Shift Regions

Chemical Shift
Region (ppm)Silicon Species
+10 to +4polymer termination sites (SiO(CH3)3)M
-5 to -15polymer termination sites (SiO(CH3)3)2OH)MOH
-10 to -20Motionally restricted silicone polymer. Cross-linked and hydrogen- bonded-(O-Si(CH3)2-O-)n as well.
As D Units within 5 monomer units of polymer termination.
-21D Units- Motionally unrestricted silicone -(O-Si(CH3)2-O-)n
-75 to -85Q1 Si(OSi)(OH)3 Silicate Center(M(OH)3)
-85 to -94Q2 Si(OSi)2(OH)2 Silicate Center(D(OH)2)
-94 to -104Q3 Si(OSi)3(OH) Silicate Center(TOH)
-104 to -120Q4 Si(OSi)4Silicate Center(Q)

Discussion
In use one observes that the gasket silicon chemistry changes dependent on additive chemistry and temperature/pressure conditions. When the polymers degrade one observes a general loss of D type signal intensity in the 29Si SP/MAS experiment as well as a corresponding increase in silicate content. One does not typically see changes in Q4 type but instead large changes in Q3 and Q2 content. These changes occur regardless of the presence of silicate in the coolant. This leads one to deduce that the Q3 and Q2 types are being generated by degradation of the polymer itself rather than a change in the chemistry of the silicate that was present in the sample initially. The author is not privy to additives and experimental conditions so he cannot speculate on the effect of silicate and other additives on the speed of the degradation that occurs. At the same time that Q3 and Q2 types are increasing in intensity the CP/MAS experiment clearly shows that there is a large increase in the relative amount MOH types and D types close to terminations (-5 to -20 ppm region). This proves that the exposure to coolants causes a hydrolysis of the Si-O-Si bond. However, it should also be notes that for the Q3 and Q2 types to appear one must also be hydrolyzing the Si-CH3 bonds.

The complimentary nature of the SP/MAS and CP/MAS experiments along with the use of only on set of experimental CP/MAS conditions means that relative changes in the various silicon chemistries can be analyzed to quantify the degree of degradation that a polymer has gone through.

For more information contact John Edwards Tel: +1 (203) 744-5905
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