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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 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 silicon 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
crosslinking sites (where there could be a D-silicate bond, or polymer
termination sites such as MOH. Thefact 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 crosslinks of the type (O2(CH3)2SiOSi-OSilicate)
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
.
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 in Qn structures in silicate (Q1, Q2,
Q3) (One
could give individual numbers for Q3, Q2 and Q1)
Q4/Qn Ratio - Allows 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 +4
Polymer termination sites (SiO(CH3)3)
M
-5
to 15
Polymer Termination Sites (SiO(CH3)2OH)
MOH
-10 to 20 Motionally restricted silicone polymer. Cross-linked and hydrogen- bonded (O-Si(CH3)2-O-)n as well
as D units within five monomer units of polymer termination
-21
D Units - motionally unrestricted silicone (O-Si(CH3)2-O-)n
-75
to 85
Q1
Si(OSi)(OH)3
Silicate Center (M(OH)3)
-85
to 94
Q2
Si(OSi)2(OH)2
Silicate Center (D(OH)2)
-94
to 104
Q3
Si(OSi)3(OH)
Silicate Center (TOH)
-104
to 120
Q4
Si(OSi)4
Silicate
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 on this topic please contact:
Manager, Process and Analytical NMR Services
Process NMR Associates LLC,
87A Sand Pit Rd
Danbury, CT 06810, USA
Tel: (203) 744-5905