Polymer Degradation and Stability 96 (2011) 2064e2070
Contents lists available at SciVerse ScienceDirect
Polymer Degradation and Stability
journal homepage: www.elsevier.com/locate/polydegstab
Cross-link network of polydimethylsiloxane via addition and condensation
(RTV) mechanisms. Part I: Synthesis and thermal properties
Mohamad Riduwan Ramli, Muhammad Bisyrul Hafi Othman, Azlan Arifin, Zulkifli Ahmad*
School of Materials and Mineral Resources Engineering, Engineering Campus, Universiti Sains Malaysia, Seri Ampangan, 14300 Nibong Tebal, Penang, Malaysia
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 August 2011
Accepted 2 October 2011
Available online 6 October 2011
A series of highly cross-linked polysiloxane was synthesised via hydrosilylation and condensation
reaction. Structural identification using Fourier Transform Infrared (FTIR) and 1H-NMR confirmed their
chemical structures. Their thermal and, mechanical properties, and crystallinity, were analysed and
related to the level of cross-link density. These systems displayed elevated thermal and hardness
properties at an increased cross-link density. Furthermore, the level of crystallinity was reduced as
displayed by XRD analysis. Along with this observation, the calculated fractional free volume (FFV)
showed a decreasing trend leading to the ‘densification’ effect. It was envisaged that the linear polysiloxane chain segments aligned parallel to each other in a triclinic crystal system to generate a crystalline domain. The spacing between these stacking chains was found to be about 7.2 Å as measured from
simulated XRD pattern.
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Polysiloxane
Cross-link network
Thermal
Crystallinity
Fractional free volume
X-ray diffraction
1. Introduction
Cross-linked polydimethylsiloxane (PDMS), or silicone elastomer, is of major interest to researchers because of its unique
properties. It is generally produced by synthesising reactive PDMS
prepolymers, which are subsequently cross-linked to give a highly
tough and durable elastomeric material. Crosslinking or network
formation of silicone polymer can be obtained by three typical
routes: condensation reaction (moisture cure), addition reaction
(hydrosilylation cure), or radical reaction, which is normally performed at higher temperatures [1e3].
Crosslinking of polysiloxane via condensation reaction affording
thermally stable materials as been reported recently [4e7]. Han
et al. [6] studied the effect of cross-link density towards the thermal
stability of room temperature vulcanization PDMS by incorporating
it with polymethylmethoxysiloxane (PMOS) via hydrolytic
condensation reaction. Highly cross-linked PMOS phases were
formed in situ and the average cross-link density increased as the
loading of PMOS was increased. They suggested that dense PMOS
phases could reduce the pyrolysis of PDMS at elevated temperature.
Chen et al. [7] investigated the thermal degradation, thermooxidative stability and mechanical properties of hybrid PDMS
with an octavinyl-Polyhedral oligomeric silsesquioxanes (POSS)
* Corresponding author. Tel.: þ604 599 5099; fax: þ604 594 1011.
E-mail addresses: riduwanramli85@yahoo.com (M.R. Ramli), zulkifli@eng.usm.
my (Z. Ahmad).
0141-3910/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymdegradstab.2011.10.001
derivative prepared by hydrolytic condensation. They reported that
the improvement in thermal properties could be attributed to the
effective three-dimensional network structures resulting from the
structure of the octavinyl-POSS derivative. Furthermore, the
improvement of mechanical properties could be attributed to the
synergistic effect of the structure of three-dimensional multi-arm
cross-linker vinyl-POSS derivative and the perfect distribution of
the vinyl-POSS derivative. Meanwhile the addition reaction of
polysiloxane provides a more stable material at elevated temperatures, and was developed for rapid processing and fast curing rate
of deep section part [8,9]. The two-part heat curable system, which
consists of the vinyl and hydride reactive functional groups, and the
addition of a platinum catalyst hydrosilylation cure, provides a fast
cure system without any by-product. Furthermore, a longer shelf
time, one-component system was developed by premixing a scavenging agent such as oximatosilanes, carbamatosilane or aminosilanes [10]. These scavenging agents react with excess hydroxyl
groups whether from methanol, silanol, or water, and would not
react with the alkoxy groups to prematurely cross-link the PDMS
system. A high flame retardancy effect could be accomplished by
a cross-linked structure performed under platinum catalysed
hydrosilylation reaction as suggested by Hayashida et al. [11] by
suppressing the thermal decomposition. The condensation reaction
of PDMS however, requires a longer curing time [12,13]. On the
other hand, the addition reaction of PDMS in a one-component
system requires less mixing and provide a problem-free production
material but requires an elevated curing temperature [1,3].
M.R. Ramli et al. / Polymer Degradation and Stability 96 (2011) 2064e2070
2065
Table 1
Feed molar ratio of hydride terminated PDMS which have been synthesised.
a
b
Mw/Mn cIntrinsic
Molar bMn
Mw
Ratio, r (g/mol) (g/mol)
Viscosities
Monomer I Monomer II
(dL/g)
(EC)
(D4)
Samples Concentration (mmol)
A
B
C
D
a
b
c
67.6
67.6
67.6
67.6
20.0
10.1
5.4
2.7
0.30
0.15
0.08
0.04
3858
4697 1.22
5866
8206 1.40
11,104 14,838 1.34
18,971 28,672 1.51
0.0122
0.0404
0.0673
0.1495
Calculated based on molar ratio monomer I and monomer II, r ¼ [EC/D4].
Measured using gel permeation chromatography (GPC).
Obtained from Ubbelohde solution viscometer using toluene as the solvent.
This work attempted to incorporate both mechanisms to yield
a novel elastomeric product having synergistic thermal and
mechanical properties. It comprised of investigation into the
thermal and physical properties of cured PDMS at varying crosslink densities. The degree of cross-link density was designed
based on a series of linear prepolymers prepared as the starting
materials. The effect of cross-link density on the level of crystallinity was evaluated and the simulated crystal structure of the
polysiloxane was generated based on the XRD pattern.
2. Experimental
2.1. Materials
Octamethylcyclotetrasiloxane (D4), trifluoromethane sulfonic acid
(triflic acid), Platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane
complex (Karstedt’s catalyst) and vinyltrimethoxysilane 98% were
obtained from ALDRICH. 1,1,3,3-tetramethyldisiloxane was purchased
from Fluka. The solvents such as the toluene and diethyl ether were
obtained from J.T. Baker and Merck respectively. All materials were
used as received.
2.2. Synthesis of a,u-dihydrido-polydimethylsiloxane (PDMS-1)
Octamethylcyclotetrasiloxane, I (D4) (20 g) and 1,1,3,3tetramethyldisiloxane II (2.7 g, 20.0 mmol) were charged into
a round bottom flask and purged with nitrogen gas. The temperature was increased to 55 C and triflic acid (0.2 g) was slowly added
into the reaction. The reaction was equilibrated at 55 C for 72 h.
The mixture was dissolved in diethyl ether then neutralized by
repeatedly being washed with deionized water. The solution was
dried over anhydrous magnesium sulphate for 1 h and subsequently filtered through glass wool. Then, diethyl ether and
unreacted monomer were distilled out under reduced pressure at
80 C for 2 h. A clear liquid form was obtained with over 90 wt%
yield. Four pre-polymer with different molar ratio of I and II were
synthesised (Table 1). 1H-NMR (CDCl3): d ¼ 4.7 (m, 2H, SiH);
d ¼ 0.1e0.05 (m, 6H, SiCH3). IR y: 2962, 2907, 2127, 1412, 1258, 1079,
1011, 911, 864, 788, 699 cm 1.
Fig. 1. Repeat unit for polysiloxane network used to determine the FFV. The n values
for samples RTV-A ¼ 51, RTV-B ¼ 78, RTV-C ¼ 149, RTV-D ¼ 256.
2.3. Synthesis of a,u-bis-(trimethoxysilane)-polydimethylsiloxane
(PDMS-2)
Vinyltrimethoxysilane IV (4.58 ml, 30 mmol), toluene (10 ml)
and Karstedt’s catalyst (0.075 ml) were charged into a 50 ml twoneck round bottom flask equipped with a dropping funnel.
PDMS-1 III (10 g) was charged into the dropping funnel and the
temperature was increased to 50 C. PDMS-1 was slowly dropped
into the reaction mixture, which was maintained at a temperature
of 50 C. The temperature was then increased to 75 C for 2 h. After
completion of the reaction, the solution was cooled to room
temperature and the remaining unreacted monomers were
removed using rotary evaporator at 90 C for 3 h to give a clear
liquid form with over 90 wt% yield. 1H-NMR (CDCl3): d ¼ 3.5 (s, 18H,
eSi(OCH3)3); 1.1 (m, 4H, (CH3O)3SiCH2e); 0.6 (d, 4H,
e(CH3)2SiCH2e); 0.1e0.05 (6H, eSi(CH3)2e). IR y: 2962, 2907, 2839,
1412, 1258, 1191, 1079, 1013, 788, 699 cm 1.
2.4. Preparation of highly cross-linked PDMS
Room temperature vulcanization PDMS (RTV-PDMS) was
prepared via hydrolysis followed by condensation reaction of
methoxy groups using triflic acid as catalyst (0.3 phr) under
ambient conditions for 24 h. The alcohol that was produced as
a side product was eliminated from the reaction system using
Dean-Stark apparatus. For comparison, a control sample was
provided by Penchem Industry Sdn. Bhd. The control was PDMS
system cured with the platinum catalyst at 120 C for 2 h. IR y: 2962,
2907, 1412, 1258, 1079, 1013, 788, 699 cm 1.
2.5. Analytical techniques
1
H-NMR spectra were recorded with a Bruker 400 UltraShield
linked to a computer running WIN-NMR software. The frequency
used was 400 MHz for 1H. Experiments were performed in CDCl3
with tetramethylsilane (TMS) as an internal reference. Infrared
spectra were recorded using a PerkineElmer System 2000 Attenuated Total Reflectance (ATR-FTIR) spectroscopy. The spectra were
recorded in 4000e500 cm 1 region using 16 scans.
Table 2
Some physical properties of RTV samples.
Samples
Molar Ratio, r
a
Density, re
(g/cm3)
Hardness
(Shore A)
Optimum Q
RTV-A
RTV-B
RTV-C
RTV-D
0.30
0.15
0.08
0.04
0.985
0.982
0.975
0.971
67
63
60
59
1.99
2.79
3.44
4.08
a
Were taken using pycnometer.
Fig. 2. Synthetic route of highly cross-linked RTV-PDMS.
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M.R. Ramli et al. / Polymer Degradation and Stability 96 (2011) 2064e2070
Fig. 3. Schematic structure of three-dimensional cross-linked PDMS produced.
The swelling test was carried out in toluene and the weights of
swollen samples were measured after being blotted with filter
paper to remove the excess toluene. The optimum weights of
swollen samples were obtained after 24 h and the degree of
swelling (Q) was calculated using Eq. (1) following Mazan et al. [14]
works,
Q ¼
re m2 m1
þ1
rs
m1
(1)
where, Q is the volume swollen ratio, m1 is the mass of dry sample
after elution in toluene, m2 is the mass of swollen sample, rs is the
density of solvent which was taken as 0.87 g/cm3 for toluene and re
is the density of samples which can be seen in Table 2.
The thermal analyses of samples were performed on a DSC 1
Mettler Toledo apparatus with 40 mL aluminium crucible running
Mettler Toledo software. The experiments were run from 140 to
100 C with a ramp of 10 C/min for the first heating. Then the rate
was fixed at 20 C/min during cooling and second heating. The
samples were equilibrated for 5 min at each turning temperature.
Thermal degradation and thermo-oxidative stability were performed using a PerkineElmer 7 thermo-analyser. The samples were
heated in aluminium crucibles from 30 to 800 C at a heating rate of
20 C/min in an air conditioned environment. The X-ray diffraction
(XRD) was carried out on D8 advance Bragg-Brentano configuration
from Bruker with Cu Ka ¼ 1.54056 Å, and the slit size and scan rate
were 0.01 and 10 /min, respectively. Densities of samples were
obtained using XB220A Precisa pycnometer in deionized water and
the hardness was measured using Shore A. Meanwhile fractional
free volume (FFV) was calculated using van Krevelen group
contribution method [15] based on repeat unit of polymer
networks as shown in Fig. 1.
3. Results and discussions
3.1. Synthesis of highly cross-linked RTV-PDMS
RTV-PDMS was prepared following a two-steps synthesis.
Firstly, the preparation of III through ring opening polymerization
[16], and followed by hydrosilylation with IV [17] to obtain V.
Subsequently, the cross-link networks of V were generated via
condensation process of methoxy groups as depicted by Fig. 2.
In the first step, cationic ring opening polymerization of I was
carried out with an acidic catalyst. The acidic catalyst was preferred
Table 3
1
H-NMR chemical shifts for both PDMS-1 and PDMS-2.
Fig. 4. Transmittance ATR-FTIR spectra of a) PDMS-1, b) PDMS-2, c) RTV-PDMS.
Abbreviation
Species
Chemical shift (ppm)
a
b, g
c, h
d
e
f
^SieH
^SieCH3
eO2SieCH3
^SieOCH3
(CH3O)3SieCH2CH2e
eO(CH3)2SieCH2CH2e
4.7
0.1e0.05
0.1e0.05
3.6
1.1
0.6
M.R. Ramli et al. / Polymer Degradation and Stability 96 (2011) 2064e2070
2067
Fig. 5. 1H-NMR spectrum of PDMS-1 (above) and PDMS-2 (below).
over a basic catalyst due to the sensitivity of hydride (SieH) reactive
groups to the latter which might lead to O-silylation reaction [18].
Siloxane cation was first generated in this system by the reactive
triflic acid which propagates with the other cyclic siloxanes. The
propagation of linear PDMS-I was then terminated by II to obtain
a hydride terminal group. The molecular weight of this linear PDMS
chain was tailored by controlling the mole ratio of the 1,3tetramethyldisiloxane as the end-capper. The molecular weight
for this series is shown as in Table 1. Some physical properties are
shown as in Table 2.
The hydrosilylation process that occurred in the second step
involves the hydrogen of the hydrosilyl group and the vinyl group
from the trimethoxy silane in the presence of Karstedt’s catalyst;
this introduced the tri-methoxysilane group to the terminal of the
chain. The condensation step was finally performed with the active
methoxy groups reacting under atmospheric moisture in the
presence of 0.3 phr of triflic acid. During the first two hours of the
reaction, the methanol-like smell was detected suggesting that the
condensation and hydrolysis of the methoxy groups took place.
Three methoxy groups attached at the terminal end of PDMS-2
affording a three dimensionally cross-linked PDMS structure
(Fig. 3).
The disappearance of methoxy groups represented by CH3eO
and SieOeCH3 peaks at 2839 cm 1 and 1190 cm 1 respectively
unambiguously proves the formation of crosslinking networks as
shown in Fig. 4c.
3.3. Nuclear magnetic resonance
NMR analysis for PDMS-1 and PDMS-2 are given as in Table 3. In
PDMS-1 spectrum, the proton from SieH groups occurred at
d 4.7 ppm (assigned as a in Fig. 5) meanwhile siloxane groups
(proton from methyl attached to siloxane groups) represented by
b and c were assigned mostly upfield at (d 0.1ed 0.05 ppm)
respectively. The chemical shifts for SieH and SieCH3 were
consistent with the previous study [17,23]. The integration ratios of
these two peaks were taken to determine the value of repeating
unit n.
In PDMS-2 spectrum, methoxy proton peak occurred mostly
downfield at d 3.6 ppm as a sharp singlet represented by d (see
3.2. FTIR spectroscopy
Complete formation of SieH bonds in PDMS-I was been
observed after purification based on FTIR spectrum. Fig. 4a shows
the FTIR spectrum of SieH terminated PDMS with two distinctive
bands at 2127 cm 1 and 910 cm 1 stretching represented by A and
B respectively [19e21]. These bands can be used as the preliminary
evidence of the appearance of hydride groups in III.
The disappearance of SieH peaks at 2127 cm 1 and 910 cm 1
indicate that the hydrosilylation process was accomplished in the
system (Fig. 4b). This is reiterated further by the appearance of
symmetrical CH3eO stretching and SieOeCH3 peaks at 2839 cm 1
and 1190 cm 1 respectively. The peak at 2840 cm 1 (sharp) and
1190 cm 1 can be assigned as SieOCH3 groups as has been reported
elsewhere [21,22].
Fig. 6. The swelling ratio of RTV in toluene at 25 C.
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M.R. Ramli et al. / Polymer Degradation and Stability 96 (2011) 2064e2070
Fig. 7. Effect of molar ratio (end capper to monomer ratio) on cross-link density and
degree of swelling of RTV samples.
Table 3) due to the presence of neighbouring electronegative
oxygen. The methylene proton e and f occur at d 1.1 and 0.6 ppm
respectively. For e, the silicon atom bonded to three methoxy
groups which contain three oxygen atoms. The chemical shift
therefore, is slightly downfield compared to f of which the adjacent
silicone atom is attached to two methyl groups and one oxygen
atom.
Comparison between spectra PDMS-1 and PDMS-2 shows that
peak a in PDMS-1 completely disappeared in the PDMS-2 spectrum
suggesting a complete hydrosilylation process had occurred. The
appearance of peak d at d 3.6 ppm indicates the successful incorporation of methoxy groups in the PDMS-2 structure. The formation of methylene at peak e and f in PDMS-2 fully support the
complete hydrosilylation process.
3.4. Cross-link density
Cross-link density of samples were obtained after curing process
at room temperature and were determined using Flory-Rehner’s
Eq. (2),
Fig. 9. TGA curves for RTV samples compared to control sample measured in air
atmosphere condition.
n¼
h
lnð1
V1
V2 Þ þ V2 þ xps V22
1=
V2 3
V2
2
!
i
(2)
where, V1 is molar volume of solvent (106.85 cm3/mol was taken
for toluene [24]), V2 is reciprocal of volume swelling ratio Q, xps is
polymeresolvent interaction coefficient and was taken as 0.465
[24] for siloxane elastomer and toluene interaction.
In Fig. 6, the optimum degrees of swelling for RTV-B, RTV-C, and
RTV-D were achieved after 23 h of swelling test. RTV-A attained its
optimum degree of swelling after only 6 h of swelling test. Degree
of swelling is inversely proportional to the cross-link density since
at higher cross-link density, solvent diffusion in the polymer matrix
would be less efficient. The order of cross-link density would be
RTV-A > RTV-B > RTV-C > RTV-D.
It can be seen that, the cross-link density of RTV samples
increased correspondingly with the increase of molar ratio of endcapper to the monomer as depicted in Fig. 7. At high molar ratio of
end-capper, the molecular weights of the chain will decrease and
subsequently provide a higher bulk concentration for the condensation reaction site with the silane cross-linker. The highest crosslink density was obtained at 0.30 ratio while the lowest value is at
0.04 ratio. The end-capper was subsequently replaced by three
active methoxy groups during the hydrosilylation process. There
are six active methoxy groups per molecule, (i.e. three at each endchain of the molecule), and it was these groups which formed the
cross-link network through the condensation reaction between
chains.
The effect of cross-link densities on hardness were shown in
Table 2. The hardness of samples decreased with decreasing cross-
Table 4
Effect of cross-link density on free volume and glass transition temperature.
Fig. 8. Glass transition temperature (2nd scan) of RTV-PDMS samples run at 20
min.
C/
Samples
Cross-link density
(mol/cm3)
RTV-A
RTV-B
RTV-C
RTV-D
13.48
4.52
2.43
1.51
a
10
10
10
10
4
4
4
4
Calculated using Krevelen method [15].
a
Fractional free
volume (FFV)
0.186
0.187
0.192
0.195
Tg ( C)
108
112
114
119
M.R. Ramli et al. / Polymer Degradation and Stability 96 (2011) 2064e2070
Fig. 10. XRD spectrum for sample RTV-A until RTV-D and the simulated pattern.
link densities. Hardness values of samples produced in this study
were superior to the commercial RTV samples. The commercial
hardness was in the range of 22e40 Shore A [12].
3.5. Thermal properties
Differential Scanning Calorimetry (DSC) was performed on RTVPDMS to reveal the thermal transition in the polymer. The transitions in Fig. 8 were attributed to the glass transition (Tg) of RTVPDMS samples in the range of 108 to 119 C. This is in close
agreement with early studies [25] which reported the Tg of analogous RTV silicone elastomers cured with POSS structures was
between 118 and 120 C. The Tg decreased progressively with
the decreasing cross-link density of the samples. Thus, sample RTVA displayed the highest Tg compared to other samples due to chain
restriction as the result of a high cross-link density. All samples
shows a gradual step drop during the glass transition as due to the
plasticizing effect of the linear siloxane chains which adjoined the
crosslinking points in the network.
Fractional free volume (FFV) of RTV samples were calculated
using group contribution of van der Waals’ volume method as
established by van Krevelen [15]. The values in the range 0.15e0.19
are typical for polymeric systems. RTV-A contains the highest
cross-link density when compared to other three samples, thus,
exhibiting the lowest free volume. It was established from Positron
2069
Annihilation Lifetime Study that a high cross-link density induces
a lower fractional free volume [26]. This is because a high cross-link
density would affect a ‘densification’ between chains.
A control from a commercial source was used in determination
of thermal stability to compare with other RTV samples synthesised in this work (Fig. 9). It can be seen that, RTV-A starts to
degrade at 475 C along with the control sample. Meanwhile RTV-B
was degraded at around 450 C followed by RTV-C and RTV-D at
around 375 C. Thus, the thermo-oxidative stability of RTV samples
increased with the cross-link density. Interestingly, the thermal
stability of RTV samples was comparable with the control sample,
although the control sample is the addition system, which is known
to be thermally stabile. This might be due to the high cross-link
network achievable in this system. Further, RTV-A showed the
highest char yield. It has been shown that char yield is indirectly
proportional to the cross-link density in a polysiloxane system. A
linear, uncross-linked polydimethylsiloxane heated in an Ar
atmosphere was completely degraded at 600 C with no residue left
[27].
The decreasing trend of Tg (Table 4) was further investigated by
studying the X-ray diffraction (XRD) spectra. Essentially the XRD
spectra (Fig. 10) show broad halos representing the highly amorphous nature of all the samples. It can be seen however, that the
RTV-A shows broad peaks while RTV-D shows the sharpest peaks.
This observation shows that the highly cross-linked sample of RTVA is less crystalline than the low cross-linked sample of RTV-D. This
can be explained that as the RTV-A has more cross-link network,
chain flexibility is immobilised to certain degree which restrict any
possible chain alignment to form crystalline domains. Alternatively,
as the cross-link densities decreased, the molecular chains have
adequate space to align and rearrange into crystalline domains
thus, giving sharper peaks in XRD. However, the peak’s positions do
not vary much between samples with the change in the cross-link
densities implying that crystal system remains intact. The XRD
analysis, therefore, corroborate further the observed degree of
cross-link density as discuss in the preceding sections.
Simulated crystal structures based on the experimental XRD
pattern were generated using Material Studio v4.4 software using
Forcite module whose energy minimisation was perform under
Dreiding Forcefield. One chain per unit cell was generated and
minimised in an initially constrained triclinic crystal system.
Towards the end of the simulation cycle, this unit cell was relaxed
and no apparent change was detected in either the crystal lattice or
the chain configuration. Thus, it was established that the obtained
unit cell configuration should be correct. Fig. 11 shows the simulated diffraction pattern as compared to the experimental data. The
laterally adjacent chain distance was found to be 7.2 Å. These values
Fig. 11. Simulated crystal structure of crystallite domain of the cured RTV material showing the stereo projection (a) and laterally (b) adjacent chain in a unit cell.
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M.R. Ramli et al. / Polymer Degradation and Stability 96 (2011) 2064e2070
corresponded very well to the main peak positions at 2q ¼ 12.0
from the experimental data. This distance represents the d-spacing
of the linear polysiloxane chain backbone. It is envisaged that the
crystalline domain is formed between the linear polysiloxane
chains which are at close proximity and undergoes chain
arrangement. The polysiloxane region resulting from condensation
is unlikely to form any crystalline structure as expected from
a highly cross-link network.
4. Conclusion
Synthesis of RTV siloxane by a combination of addition and
condensation systems has been presented in this paper. Structure
identifications were established using FTIR and H-NMR methods.
The results showed significant improvement of RTV samples
compared with a high temperature vulcanization (control) sample.
The greatest contribution to this improvement was from high
content of cross-link density and synergistically having combined
thermal properties from both the addition and condensation
system. The swelling test showed that the lower the chain length of
the linear polysiloxane segment, the higher the cross-link density.
A high cross-link network affects an improvement in hardness and
thermal properties. The free volume is also reduced leading to the
effect of ‘densification’ of polymeric chains in the bulk system. The
high cross-link density of this system, however, reduced the level of
crystallinity, which owed much to the chain immobilisation.
Simulated crystalline structure based from XRD analysis established that the crystalline domain was generated from the parallel
alignment of linear polysiloxane backbone in triclinic crystal
system whose inter-chain spacing is about 7.2 Å.
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
Acknowledgement
[20]
The authors thank Universiti Sains Malaysia and the USM
Fellowship Scheme for the financial support of this work by
Research University Grant (No. 814069).
[21]
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