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Linear polysulfides : their characterisation and degradation pathways
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Mahon, Andrea (1996) Linear polysulfides : their characterisation and degradation pathways. PhD thesis, University of Warwick.
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Official URL: http://webcat.warwick.ac.uk/record=b1402901~S15
Abstract
Linear polysulfides (LPs) are widely used in high performance sealants. The typical structure of the polymer
is:
H(SC2H40CH2OC2H4S)nH.
We have completely characterised the complex components that make up commercially used LPs, by means of electrospray ionisation mass spectrometry (ESI-
MS), combined with collision induced dissociation (CID). ESI has proved to be an invaluable mass spectroscopic technique in the characterisation of linear
polysulfides.
The individual spectra are well-resolved, enabling conclusions to be reached about the presence of variant mers
(usually
associated with additional oxymethylene or
oxyethylene units or a monosulfide linkage,
or some combination of these), and about
the identity
of end-groups and nature of the repeat unit.
We have
also
been able to
use ESI-MS to study the degradation reactions of
LP
pre-polymers.
We have
obtained spectra of
LPs
using matrix assisted
laser desorption
ionisation time-of-flight mass spectrometry,
MALDI-TOF-MS), but these are not as
informative
as spectra obtained by ESI-MS. Field desorption
mass spectrometry
(FD-MS) has also
been successful in
characterising LP
samples, but this technique is
less
generally accessible and much more time-consuming than either
ESI-MS
or
MALDI-TOF-MS.
LPs degrade thermally and photochemically
by two competing
degradation
pathways:
i)
a
free radical autoxidation mechanism, and
ii) a hydrolysis mechanism.
UV
photolysis or pyrolysis of
LP
pre-polymers
in the presence of air or
oxygen resulted
in the development
of carbonyl groups
detectable by IR
and
13C
NMR
spectroscopy, while
2-D NMR
studies and the highly
characteristic
field
positions of the NMR
resonances show the carbonyl group to be due to a
formate
ester.
All thin film
samples of cured
LP, irrespective
of the LP
structure and curing
agent,
including ZL-2264 (with
an extra
CH2 linkage in the repeat unit), also
produce this carbonyl absorption at
1725
cm-1
when exposed to UV irradiation. The
formate
ester arises as a result of a conventional
free
radical mechanism of
autoxidation; attack at the C-H bond
adjacent to the ether oxygen atom
leads to a
hydroperoxide
and
hence an alkoxyl radical,
followed by
scission to give the
formate
ester.
The
study of
liquid
exudates formed
when
TBBP-
and
HDDA-
cured
LP
cast
blocks
are exposed to prolonged periods of
heat
or
UV irradiation
also show the
development of carbonyl bands at
1724 cm-1 in their IR
spectra, again indicating that
degradation is
occurring at
least in
part via a
free
radical mechanism.
ESI-MS
studies of
degraded pre-polymer samples and
liquid
exudates show
that, in
parallel with the degradation
route
involving
oxygenation of a methylene
group, there is
a
hydrolysis
mechanism involving initial
cleavage of the formal
group
to release formaldehyde, followed by
secondary reactions to give other products
detected in the ESI-MS
spectrum of the photolysate/pyrolysate. This
mechanism is
supported by the extremely slow
degradation of
ZL-2264,
which
has no
formal
group.
The hydrolysis
mechanism is the principal route
for the degradation
of
LP
pre-
polymers and
TBHP-
and
HDDA-
cured
LP block
samples, while
LPs
cured using
MnO2
and
NaB03 degrade
almost exclusively via the free
radical autoxidation
pathway.
Item Type: | Thesis (PhD) | ||||
---|---|---|---|---|---|
Subjects: | Q Science > QD Chemistry | ||||
Library of Congress Subject Headings (LCSH): | Sulfur compounds, Sealing compounds, Sulfides -- Spectra, Sulfides -- Deterioration | ||||
Official Date: | October 1996 | ||||
Dates: |
|
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Institution: | University of Warwick | ||||
Theses Department: | Department of Chemistry | ||||
Thesis Type: | PhD | ||||
Publication Status: | Unpublished | ||||
Supervisor(s)/Advisor: | Kemp, T. J. (Terence James), 1938- ; Lee, Tim | ||||
Sponsors: | University of Warwick ; Morton International Ltd. | ||||
Extent: | xxvii, 268 leaves : ill., charts | ||||
Language: | eng |
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