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Polymer International 40 (1996) 63-71
Proton NMRStudies of Molecular-Level
Mobility in Vulcanised and Epoxidised
Natural Rubber
Robin K. Harris,* Barry J. Say
Department of Chemistry, University of Durham, South Road, Durham DH1 3LE, UK
& Soon Ng
Department of Chemistry, University of Malaya, Pantai Valley, 50603 Kuala Lumpur, Malaysia
(Received 12 October 1995; revised version received 21 November 1995; accepted 22 December 1995)
Abstract: Proton spin-lattice relaxations in the laboratory and rotating frames,
together with transverse relaxation (free induction decay), have been measured
for natural and epoxidised rubber, plus a series of vulcanised samples. The data
have been analysed by fitting procedures to give characteristic times which are
discussed in terms of the mobility of the various parts of the samples, and of their
compositions (including the effects of parafin wax and oil additives). Interpretation has been aided by recording free induction decays following a period of
spin-locking. Carbon-13 magic-angle spinning spectra have also been measured.
K e y words: NMR, relaxation, rubber, vulcanisation, epoxidisation, additives,
mobility.
INTRO D UCTlON
mation on network structures has been obtained, viz.
accelerated sulphur vulcanisation results in primarily
polysulphidic structures with sulphurisation allylic to
the double bond, cis-trans isomerisation is common,
and small-to-moderate levels of double-bond migration
can O C C U ~ . ~
In this research a proton NMR relaxation study has
been carried out on NR, carbonxarbon crosslinked
NR, and epoxidised NR (ENR 50), as well as various
vulcanised rubbers, cured with and without paraffin
additives. Produced by the chemical modification of
NR,6 ENR is now an established commercial polymer
with a unique set of properties (high strength due to an
ability to undergo strain crystallisation, an increased
glass transition temperature and an increased solubility
parameter) which are reflected in the vulcanisates with
increased oil resistance, enhanced adhesive properties, a
high degree of damping, and reduced gas p e r m e a t i ~ n . ~
The paraffin additives in the ENR formulation provide
ozone protection. The chemical structures of NR and
ENR are shown below.
Sulphur vulcanisation of rubber results in the formation
of a network structure consisting of crosslinks, network
chains (macromolecular segments bound at each end by
a crosslink) and chain ends (macromolecular segments
bound at one end only to a crosslink).' The relation
between density of crosslinks and density of network
chains depends on the crosslink functionality and the
occurrence of chain entanglements. The proportion of
chain ends increases when chain scission occurs during
vulcanisation.'*2 Both dicumyl peroxide (DICUP) as a
curative3 and gamma irradiation4 produce a network in
natural rubber (NR) that contains only carbon-carbon
crosslinks and is accompanied by cis-trans isomerisation, double-bond migration and main-chain scission.
As expected, crosslinking induces motional restrictions
in the polymer chain. From various experimental techniques, especially solid-state I3C NMR, definitive infor-
* To whom correspondence should be addressed.
63
Polymer International 0959-8103/96/$09.00 0 1996 SCI. Printed in Great Britain
R . K . Harris, B. J . Say, S . N g
64
I
Y
NR
IS
8
ENR
A list of possible structures occurring upon sulphurisation of cis-polyisoprene' is shown below.
I
5,
I
(hans)
The sites of sulphurisation in ENR are the same as in
NR, i.e. at the methyl and methylene carbons allylic to
the double bond.g The double bonds may be isolated,
having epoxide groups on either side, or have one adjacent epoxide group, the latter situation being twice as
frequent as the former in the case of ENR 50.
Earlier 'H NMR studies of crosslinked rubber used
linewidths to show the existence of motional
restriction^,^.'^ low-frequency interactions in the kHz
range,' and moderately rapid segmental motions."
Koenig et al. have investigated' by solid-state I3C
NMR relaxation techniques the effect of crosslinking on
the molecular dynamics, and the results are summarised
as follows. The V-curves in the plot of nT, versus 1/T
for all polyisoprene
carbon signals
and for the detectable
- signals arising from the network unit (carbons in polyisoprene units containing sulphur bridges, transpolyisoprene and cyclic sulphide structures) exhibit
similar general trends with increased curing time,
namely curve broadening, shift of the minima to higher
temperatures, and increase of the Tlminvalues. All the
backbone carbons show quantitatively similar effects of
crosslinking on the spin-lattice relaxation. In lightly
crosslinked NR, motional restrictions are more pronounced for the methyl group than for the main-chain
carbons. The experimental data suggest that crosslinking affects isotropic motion much more than librational motion but does not influence rotational motion.
The data also indicate qualitatively that concepts of
multiple correlation times and/or a distribution of
correlation times are applicable.
Krejsa & Koenig have shown14 that in accelerated
sulphur-vulcanised cis-polyisoprene, the proton T2
relaxation process exhibits biexponential behaviour,
consistent with a slow motion related to crosslinked
regions and a fast motion related to mobile regions. The
correlation time difference between these regions is
roughly an order of m a g n i t ~ d e . ' ~
Nine samples have been studied in this research. The
basic materials are a natural rubber (NR) and the epoxidised derivative ENR 50. Samples of NR which underwent vulcanisation with 2.5 and 0-3 parts of sulphur to
TABLE 1. Composition of vulcanisates (in parts by weight) and glass transition temperatures
Constituent
NR
ENR 50
Zinc oxide
Stearic acid
Sulphur
Permanax TO"
Santoflex 13'
MOR"
CBSd
DICUP"
TMTD'
Paraffin wax
Paraffin oil
T,("C)
'
VNR-I
100
VNR-II
100
4
1
2.5
0.5
4
1
0.3
0.5
0.6
3.0
CNR
VENR
JB/27/4
JB/27/5
JB/27/6
100
5
2
1.2
100
5
2
1.5
3
3
100
5
2
1.5
3
3
100
5
2
1.5
3
3
1.5
1.5
1.5
100
4
1
0.5
2.0
3.0
2.0
-28.5
4
5
-25.8
4
10
-30.8
4
20
-30.9
Permanax TO (antioxidant) : Poly-2,2,4-trimethyl-I ,2-dihydroquinoline.
Santoflex 13 (antiozonant) : N - ( I ,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine,
MOR : 2-Morpholinothiobenzothiazole.
CBS: N-Cyclohexylbenzothiazole-2-sulphenamide.
DICUP: Dicumyl peroxide. This causes direct C-C crosslinking of the polyisoprene NR chains.
TMTD: Tetramethylthiuram disulphide.
POLYMER INTERNATIONAL VOL. 40, NO. 1. 1996
Molecular-level mobility of vulcanised and epoxidised natural rubber
100 parts of rubber (VNR-I and VNR-I1 respectively),
together with a C-C crosslinked material (CNR), have
also been examined. Sample VNR-I1 probably contains
mainly monosulphide links between polyisoprene
chains, whereas VNR-I will have polysulphide links.
The epoxidised rubber was also subjected to vulcanisation (sample VENR, with 1.2 parts of sulphur per
hundred parts of ENR 50). The final three vulcanised
epoxidised samples contain paraffin wax, plus oil in
various proportions. All materials except NR and ENR
50 contained a number of additives (see Table 1).
The aim of this study is to evaluate via NMR relaxation the effects of epoxidisation, vulcanisation and of
wax and oil additives on the mobility regimes of natural
rubber. Three types of proton relaxation have been
measured, viz. spin-lattice relaxations in the laboratory
and rotating frames (TI and TIPrespectively) and transverse relaxation (via free induction decays). These processes were mathematically analysed in the case of TIP
and FID to yield multicomponent fits. Carbon-13
magic-angle spinning spectra were also acquired as
checks on sample composition.
EXPERIMENTAL
Preparation of materials
The samples of natural rubber and vulcanisates were
obtained from the Rubber Technology Centre of the
Rubber Research Institute of Malaysia, Kuala Lumpur.
They were prepared according to standard procedures'
and the composition of each sample is shown in Table
1. The materials were not specially made for this work,
and therefore contain a range of constituents as for
commerical use (see Table 1). However, the only difference between the three samples containing parafin wax
is in the proportion of paraffin oil. The natural rubber
(NR) is of SMR-L grade (Standard Malaysian
Rubber-light coloured). Sample ENR 50 is 50 mol%
epoxidised. The cure temperature was 140°C for all vulcanisations, and the minimum cure time was 60 min
(except for sample CNR, for which it was 80min).
Nuclear magnetic resonance
Proton relaxation measurements were carried out using
a purpose-built spectrometer operating at 60 MHz,
which has been described in a previous publication.''
The glass NMR tube was of 7.5mm 0.d. The samples
were difficult to manipulate at room temperature and
were therefore placed in the tube in a frozen state
(liquid nitrogen temperature), being subsequently
warmed to ambient probe temperature for the NMR
experiments. All measurements were carried out at
298 K f 1 K (the temperature of the house air supply).
POLYMER INTERNATIONAL VOL. 40, NO. 1, 1996
65
Spin-lattice relaxation times (TI) were obtained by the
inversion-recovery method, while spin-lattice relaxation
times in the rotating frame (TIP)were measured by
observing (following a solid echo 90-2-909,) the decay
of initial magnetisation generated by a 90" pulse
(duration 1.5ps) and then spin-locked by an r.f. magnetic field of strength corresponding to 4 0 k H ~ . ' ~ .As
'~
the resulting decays were not single-exponential in the
case of Tip, the times of observation were chosen so as
to afford the best characterisation of the decay, i.e. more
closely spaced in the initial, rapidly decaying part.
Transverse magnetisation relaxation was investigated,
either by observation of the F ID following a solid echo
(in the cases where decay was so rapid as to avoid any
effects of B , inh~rnogeneity)'~,'~
or by a combination
of F I D and Carr-Purcell-Meiboom-Gill (CPMG) meas u re me n t~ ,'~
where there was both a rapidly decaying
part, and longer-lived components ( > 2 ms). All quoted
measurements were carried out with a delay between
successive cycles of the relevant pulse sequence equal to
at least 10 times the longest TI component in order to
ensure full relaxation of all spins. Typically 16 points
were used for Tl plots, 112 for Tlp and 1024 for FIDs,
each point being the average of 8, 16 and 32 co-added
determinations respectively. The B , field was adjusted
to observe the signals as close as possible to resonance
as judged from lack of oscillations on the 'imaginary'
receiver channel. Quadrature data pairs were collected
simultaneously, allowing phase correction of the averaged signals. Data analysis was carried out by nonlinear least squares fitting as described in a previous
publication,'
involving Gaussian, Weibullian and
exponential functions in the case of FIDs. The Weibullian function" used here is exp[-t/T')"]
where n can
take any value: n = 1 gives an exponential, n = 2 a
Gaussian, 1 < n < 2 must be intermediate, and n < 1
looks like an exponential whose time constant increases
with time (whence the term stretched exponential).
All relaxation times are quoted to two significant
figures and fractions to half a percentage point. It must
be remembered that fitting to multiexponentials is an
ill-conditioned situation, and that there is a high covariance between parameters, so the exact values quoted do
not necessarily represent any intrinsic properties of the
system. Care should be taken particularly with the
values associated with the Gaussian components. The
magnitudes of the populations are significant to the
quoted accuracy but the time constants cannot be interpreted to generate second moments.
Carbon-13 spectra were obtained at high resolution
using magic-angle spinning (typical rates c. 3 kHz) and
high-power proton decoupling. A Varian VXR 300
spectrometer operating at 75.43 MHz for carbon and at
ambient probe temperature was employed, together
with a Doty probe accepting 7mm 0.d. rotors. Singlepulse sequences were used, with 90" pulse angles and
recycle delays of either 1 s or 30s. The number of FIDs
R. K . Harris, B. J . Say, S . N g
66
that at 6, = 26.9ppm to C-9 and C-15, with the shoulder between these signals attributable to C-8 (see Ref.
20).
Addition of paraffin wax and paraffin oil produced
further resonances in the aliphatic region, as expected.
In the case of the sample with most paraffin oil (JB/27/
6) (Fig. lb) there was also a small decrease in linewidth
of the ENR 50 lines (with an incipient splitting of the
signal at 6,
135), and the lines not present in ENR 50
(and therefore assigned to the paraffin oil) were notably
sharp-significantly sharper than for JB/27/4 and JB/
27/5. There is some evidence here that the oil agglomerates do give more mobile domains when present in high
quantities, which is also evident in the proton relaxation
data (see below).
Our relaxation data are reported in Tables 2 and 3.
The latter includes results for pure paraffin wax and
paraffin oil. The first row in Table 3 is the same as the
last in Table 2, and is included for comparison purposes. Spin-lattice relaxation is, in all solid cases, well
represented by a single exponential. Fitting to two
exponentials was attempted in several cases, but the
quality of fit was not sufficiently improved to warrant
usage of the data. Of course, the paraffin oil, being
liquid and therefore having inefficient spin diffusion, did
not have single-exponential Tlrelaxation. The values of
TI for the vulcanised (but not epoxidised) samples do
not significantly differ from that of natural rubber itself,
indicating relatively light crosslinking. However, epoxi-
accumulated for each spectrum varied between 50 and
2000.
RESULTS AND DISCUSSION
The proton-decoupled 13C NMR spectrum of the
natural rubber sample consisted of sharp signals
(linewidths c. 0-5ppm) at chemical shifts 6, = 135.1,
125.6, 32.7, 27.0 and 23.8ppm, assigned to C-4, C-3,
C-1, C-2 and C-5 respectively. There were also a few
peaks of minor intensity.
The spectrum of the epoxidised form (ENR 50) was
more complex and showed substantially broader lines
(Fig. la). A new signal appeared at 6, = 22.8ppm.
There were no discernible differences on vulcanisation
(VENR). Increase in the recycle delay from 1 s to 30s
increased the relative intensities of the lines at 6, =
135.1, 60.3 and 22.8 ppm, presumably because these
correspond to carbons with long spin-lattice relaxation
times. These carbons are assigned to C-7 (quaternary),
C-11 (quaternary) and C-12 (mobile methyl) respectively. The epoxy methyl (C-12) would appear to have a
longer Tl than the isoprenic methyl (C-13) (assumed to
give a signal indistinguishable from C-5) and is therefore presumably more mobile (lower barrier to C-CH,
internal rotation). The signals from the quaternary and
methyl carbons were appreciably sharper than the rest.
The CH, carbons did not give clear single lines; the
peak at 6, = 32.5ppm can be assigned to C-14, and
nA
(a)
I
150
-
I
I
I
I
I
100
I
I
I
I
I
I
I
I
I
I
0
50
6c /PPm
Fig. 1. Carbon-13 magic-angle spinning NMR spectra with high-power proton decoupling at 75.4MHz and ambient probe temperature of (a) epoxidised natural rubber (sample ENR SO), and (b) sample JB/27/6, which contains added paraflin wax and paraflin
oil. Spectrometer operating conditions: Repeated single-pulse operation with 90" pulse angle and 1 s pulse delay; number of
transients 2000 for (a) and 1280 for (b); spin rate 2930 Hz.
POLYMER INTERNATIONAL VOL. 40, NO. 1, 1996
67
Molecular-level mobility of vulcanised and epoxidised natural rubber
TABLE 2. Proton-spin relaxation data for natural rubber and derivative materials
72
Natural rubber
(NR)
Epoxidised natural rubber
(ENR 50)
250
18
8.7
8.7
1.2
0.48
0.21
90
11
3.5
(79.5%)
(20.5%)
(1.5%)
(7.5%)
(42.5%)
(48.5%)
(1 .O%)
(87.5%)
(11 .So/,)
Vulcanised natural rubber,
S, 2.5 pphrb (VNR-I)
81
Vulcanised natural rubber,
S, 0.3 pphrb (VNR-II)
77
59
(1.5%)
12
(91.5%)
3.3 (7.0%)
C-C crosslinked natural rubber
(CNR)
74
60
(2.0%)
14
(92.0%)
2.7 (6.OYO)
290
Vulcanised epoxidised natural rubber
(VENR)
55
4.9
0.57
0.21
(2.0%)
(1 .OYO)
(33.5%)
(63.5%)
1.2 (92.5%) w1 .7
0.23 (7.5%) W0.6
0.29 (11.OYo) E
0.12 (77.0%) E
0.055 (1 2.0%) E
10
(5.0%) E
5.0 (66.5%) E
2.5 (9.5%) E
0.51 (17.0%) E
0.013 (2.0%) G
12
(6.OYO) E
5.8 (63.0%) E
0.34 (28.0%) E
0.014 (3.0%) G
18
(4.5%) E
6
(64.0%) E
0.57 (30.0%) E
0.010 (1 .5%) G
0.42 (2.5%) E
0.12 (47.5%) E
0.051 (45.5%) E
0.017 (4.5%) G
W = Weibullian; E =exponential; G =Gaussian. For the Weibullian the exponent is quoted following the W.
Parts per hundred of rubber.
a
'
TABLE 3. Relaxation data for vulcanised epoxidised samples and paraffin additives
Sample
Paraffin oila
T, (ms)
290
VENR
J 812714
5
270
J 812715
10
240
J 812716
20
230
Paraffin wax
174
Paraffin oilc
160 (27.0%)
65 (73.0%)
a
Parts per hundred of rubber.
' E =exponential; G = Gaussian
Liquid.
POLYMER INTERNATIONAL VOL. 40, NO. 1, 1996
TI,
(ms)
55
(2.0%)
4.9 (1 .OYO)
0.57 (33.5%)
0.21 (63.5%)
39
(3.0%)
4.5 (5.0%)
0.57 (38.0%)
0.21 (54.0%)
51
(5.5%)
4.5 (6.0%)
0.64 (31.5%)
0.23 (57.0%)
52
(12.5%)
4.2 (6.5%)
0.64 (29.5%)
0.23 (51.5%)
19
(45.0%)
8.7 (55.0%)
62
(52.0%)
17
(48.0%)
T* (ms)b
0.42 (2.5%) E
0.12 (47.5%) E
0.051 (45.5%) E
0.017 (4.5%) G
0.43 (8.6%) E
0.12 (49.0%) E
0.046 (34.0%) E
0.016 (8.0%) G
0.74 (9.0Yo) E
0.14 (45.0%) E
0.053 (38.0%) E
0.017 (8.0%) G
0.95 (18.0%) E
0.14 (45.0%) E
0.052 (31 .O?'o) E
0.018 (6.00/,) G
0.22 (3.0%) E
0.015 (97.0%) G
long
R . K . Harris, B. J . Say, S . N g
68
disation causes a significant increase in TI, giving values
within a narrow range for all epoxidised samples.
Rotating-frame relaxation, on the other hand, is clearly
not single-exponential for any of the samples, and we
have fitted the experimental measurements to two, three
or four exponentials, making judgements as to the
optimum number necessary in each case. Figure 2
shows an example. Such judgements are mathematical
and are deliberately not based on any particular physical model. However, it is significant that all epoxidised
samples required four exponentials, whereas the vulcanised (but not epoxidised) materials were satisfactorily
fitted to three, and natural rubber needed only two (as
did paraffin wax).
It is well understood that the analysis of NMR lineshapes or FID forms is far from straightforward.
Although the shape attributed to rigid solids is generally Gaussian, critical examination always shows the
situation to be more complex. In the case where the
motions in the sample conform to a single-exponential
correlation function an exponential FID (Lorentzian
lineshape) is observed, which can be theoretically interpreted. When the motions become more complex, i.e.
either an exponential autocorrelation function is no
longer appropriate or there is a distribution of correlation times, the situation is rather intractable. Various
authors have taken an ad hoc approach to the matter,
suggesting model functions for the FID, while there
have also been attempts to achieve a theoretical prediction for the lineshape in certain systems. Our own
approach has been to use a sum of simple functions to
represent the FID, on the understanding that if such a
sum represents the FID to within experimental error,
then the true functions must be linear combinations of
the fitted functions, also to within experimental error. It
is extremely important that the quoted values (as for the
results given herein) are not taken to correspond with
specific parts of the sample unless there is some corroborating evidence or good theoretical reasoning.
The general form of the FID of the uncured rubber is
best represented (with our set of functions) by a Weibullian (intermediate between a Lorentzian and a
Gaussian) plus a small contribution from a stretched
exponential (i.e. a Weibullian with an exponent of < 1).
This is consistent with our expectations for elastomer.’l
The FIDs of the vulcanised samples show a small
Gaussian component with a time constant which is
appropriate to residual static dipolar interactions, indicating the presence of regions where the molecular
framework is effectively rigid. However, the majority
of the decay in these cases becomes considerably
longer, indicating an increase in mobility for most of the
sample.
Our first postulate in this case is that, along with
crosslinking, some chain scission occurs in vulcanisation. Tl is largely unaffected by this procedure while
Tlp decreases for the bulk of the sample, indicating an
increase in the efficiency of relaxation of 30% to 80%. A
small longer-time component also appears, which may
well be related to the crosslinked regions.
The effect of epoxidisation, on the other hand, is
entirely different. The FID becomes shorter by nearly
an order of magnitude and is best represented as a sum
of three exponentials. This alone would indicate a sig-
W.?+-*P.,-Ce+
d
~
Residuals x 1 6
2 ms
,
/
0% i
.
0 . 4 ms
Fig. 2. Analysis of TIPresults for JB/27/6. Trace (i) shows the acquired data for the free induction decay and (as a solid line) the
longest exponential component. Trace (ii) shows the difference derived from trace (i) and the second longest exponential component. Traces (iii) and (iv) are derived in the same way. In each case the abscissa scale is successively magnified by a factor of five.
The residuals are differences (magnified by a factor of 16) between the acquired data and the sum of the four components. The
ordinate scales are intensities normalised to the zero-time full transverse magnetisation. Estimated errors on the points are within
the size of the symbols used, as can be seen from the plot of the residuals.
POLYMER INTERNATIONAL VOL. 40, NO. 1, 1996
69
Molecular-level mobility of vulcanised and epoxidised natural rubber
nificant reduction in the mobility of the rubber. The TIP
decreases substantially and TI increases. Both these
observations are also consistent with a decrease in the
mobility of the rubber chains. Vulcanisation of this
material produces a small Gaussian component in the
FI D and a long TIPcomponent as in previous examples
of crosslinking.
The remaining samples (Tables 1 and 3) differ in the
amount of paraffin oil which has been blended in. As
the percentage of paraffin oil increases, so does the
amplitude of the ‘tail’ of the FID. Figure 3 shows an
example of our mathematical treatment of the FID. All
these samples have the same amount of paraffin wax
added, and the increase in the Gaussian percentage over
VENR is consistent with the wax retaining the structure
of its ‘solid’. The effect of the paraffin oil on TIPis to
increase the amplitude of the long-time component as
the amount of oil increases.
In an attempt to further elucidate these matters we
carried out a two-dimensional experiment on sample
JB/27/6, acquiring FIDs following various spin-lock
times. The FIDs were analysed as before, using as a
starting point the parameters for the immediately preceding FID. The results of these analyses are presented
in Table 4 and shown graphically in Fig. 4. It is immediately apparent that the time constants of the FID components vary with spin-lock time. This emphasises the
inherent simplification in our model for the FID shape.
However, the components are sufficiently different that
the same qualitative model can be applied to all the
FIDs. Also, it will be noted that while the sums of
squares of residuals are fairly constant down the series,
the values do increase around 4 and 8ms of spin-lock
time. This is probably due to trace signals from ‘lost
components’ which are too small to identify precisely
but which are large enough to affect the SDSQ.
,
,
‘
,’
,’
60% -,’
’ /
40% -” ,’ ,’
80% -,’
,/
/’
’
20% -.’
.
,
/
I
0 . 0 4 ms
,
,
I
A*
0%
?
-
0 . 0 4 ms
Fig. 3. Analysis of the FID components for JB/27/6. The data are displayed as in Fig. 2 except that the timescale is identical for
traces (iii) and (iv).
+
x
rl
20% -
0
- Long E x p o n e n t i a l
- Medium Exponential
- Short Exponential
- Gaussian
2%
1%
10 ms
Fig. 4. Decay of the amplitude of the various FID components for JB/27/6 as a function of spin-lock time. The percentages are
plotted on a logarithmic scale and refer to the initial amplitude of the FID for a zero spin-lock time.
POLYMER INTERNATIONAL VOL. 40, NO. 1, 1996
R. K . Harris, B. J . Say, S . N g
70
TABLE 4. Analysis of free induction decays from sample JB/27/6 following various spin-lock times and a
solid echo
Spin-lock
time (ms)
Long exponential
Amplitude
T2 (ps)
95.6
94.1
91.6
89.8
85.7
84.2
83.1
81.4
71.2
60.6
52.1
37.8
25.3
13.4
8.6
989
101 5
101 7
1043
1085
1092
1093
1098
1239
1411
1451
1480
1604
1505
1366
0.0
0.1
0.2
0.4
0.5
0.6
0.8
1 .o
2.0
4.0
8.0
20
40
80
100
Mid-exponential
Amplitude
245.5
190.2
147.4
89.7
69.8
54.7
35.3
20.6
Short-exponential
T2 (p) Amplitude
133
138
139
141
146
148
140
132
165
103
63.1
27.3
19.7
14.5
5.3
4.1
2.0
T2 ( p )
48.7
47.9
47.7
46.1
44.9
46.0
45.6
52.8
39.0
SDSQ"
Gaussian
Amplitude
35.9
29.2
27.5
23.9
21.6
21 .I
21.3
20.0
16.1
11.7
8.2
5.3
5.1
4.0
3.4
T2 ( p )
16.7
14.9
14.5
13.3
13.4
13.1
13.3
12.7
12.2
10.8
10.2
9.4
9.4
9.4
9.4
139
118
127
120
117
127
126
116
133
345
439
268
150
139
132
SDSQ =Sum of deviations squared for the fitting procedure.
The dependence of the amplitudes for each component on spin-lock time was then analysed as a multiexponential decay. The results are presented in Table 5
and plotted as solid lines in Fig. 4. The long-exponential
FID component does not show a simple behaviour
under spin-locking, but appears to be lost with two time
constants of 51 ms and 2.3ms, which we attribute to
free and restricted paraffin oil, the latter being embedded more thoroughly in the rubber matrix. The two
intermediate components of the FID are lost with submillisecond time constants and are attributed to the
epoxidised rubber. These components form major parts
of the samples which have undergone epoxidisation.
The Gaussian component has a range of decay times.
The longest is longer than any which were observed in
the direct measurement of T I P .The directly observed
longest relaxation time TIPis thus a combination of
relaxation times. The use of this two-dimensional
TABLE 5. Multiexponential spin-locked relaxation
of FID components"
~
FID component
Relaxation characteristics
Amplitude (%)
Gaussian
Short exponential
Medium exponential
Long exponential
a
6.3 (1.2%)
18 (3.3%)
11.4 (2.1Yo)
163.6 (30.2%)
244.6 (45.1YO)
58.3 (10.7%)
36.8 (6.8%)
From the data in Table 4.
Time constant (ms)
170
3.5
0.17
0.22
0.40
51
2.3
approach (FID following spin-locking) allows the better
discernment of the behaviour of the rigid component.
CONCLUSIONS
The molecular-level effects of vulcanisation, epoxidisation and the addition of oil/wax additives have been
investigated by studying the 13C MAS spectra and
proton relaxation times. These provide evidence for
some increase of chain mobility on vulcanisation, possibly associated with chain scission, whereas epoxidisation causes a reduction in mobility. Analysis of free
induction decays following spin-locking enables further
conclusions to be made about the mobility of the
sample, with a small part shown to be highly rigid.
ACKNOWLEDGEMENTS
We are grateful to the UK Science and Engineering
Research Council for the 13C spectra obtained under
the National Service in Solid-state NMR, based at
Durham. We thank Dr A. M. Kenwright for helpful discussions. Grateful thanks are due to Mr Lim Hun So0
and Dr K. Muniandy of the Rubber Research Institute
of Malaysia, Kuala Lumpur, for providing all the
natural rubber samples and vulcanisates used in this
study and for valuable discussions concerning these
materials.
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Molecular-level mobility of vulcanised and epoxidised natural rubber
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