Influences of polyhedral oligomeric silsesquioxanes (POSSs) containing different functional groups on crystallization and melting behaviors of POSS/polydimethylsiloxane rubber composites

Article abstract of DOI:10.1039/c4ra07242a

In this article, three kinds of polyhedral oligomeric silsesquioxanes (POSSs) - octamethylsilsesquioxane (OMS), octaphenylsilsesquioxane (OPS) and heptaphenylhydrogensilsesquioxane (H-POSS) - were successfully synthesized. Then, POSSs were incorporated into polydimethylsiloxane (PDMS) rubber through solution blending followed by open two-roll mill blending with curing agent. Finally, the blends were cured with a plate vulcanizing press and the effects of POSSs on crystallization and melting behaviors of PDMS were investigated. DSC tests indicated that crystallinity (X_c) of the OMS/PDMS composite was lowered, while X_cand the melting temperature (T_m) of the OPS/PDMS composite could be significantly enhanced when 20 wt% OPS was incorporated. OPS was proved to play a role as a nucleating agent in the crystallization of PDMS, but its nucleation mechanism was different from H-POSS which was previously studied by us. The crystals of OPS were flake-like with huge surfaces which provide templates for PDMS crystals to grow. However, due to the grafting of H-POSS onto PDMS chains, H-POSS played a role as physical crosslinking points to reduce the mobility of the PDMS chain segments. This journal is

Full text of DOI:10.1039/c4ra07242a

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Influences of polyhedral oligomeric silsesquioxanes
(POSSs) containing different functional groups on
Cite this: RSC Adv., 2014, 4, 41364
crystallization and melting behaviors of POSS/
polydimethylsiloxane rubber composites
Dian Zhang, Yunhui Shi, Yufeng Liu and Guangsu Huang*
In this article, three kinds of polyhedral oligomeric silsesquioxanes (POSSs)—octamethylsilsesquioxane
(OMS), octaphenylsilsesquioxane (OPS) and heptaphenylhydrogensilsesquioxane (H-POSS)—were
successfully synthesized. Then, POSSs were incorporated into polydimethylsiloxane (PDMS) rubber
through solution blending followed by open two-roll mill blending with curing agent. Finally, the blends
were cured with a plate vulcanizing press and the effects of POSSs on crystallization and melting
behaviors of PDMS were investigated. DSC tests indicated that crystallinity (X
c
) of the OMS/PDMS
composite was lowered, while X and the melting temperature (T ) of the OPS/PDMS composite could
c
m
be significantly enhanced when 20 wt% OPS was incorporated. OPS was proved to play a role as a
nucleating agent in the crystallization of PDMS, but its nucleation mechanism was different from H-POSS
which was previously studied by us. The crystals of OPS were flake-like with huge surfaces which
provide templates for PDMS crystals to grow. However, due to the grafting of H-POSS onto PDMS
chains, H-POSS played a role as physical crosslinking points to reduce the mobility of the PDMS chain
segments.
Received 17th July 2014
Accepted 26th August 2014
DOI: 10.1039/c4ra07242a
www.rsc.org/advances
though the effect may be not that striking owing to the aggre-
1
. Introduction
21
gation of POSS, whereas incorporating POSS into polymer
22
The past decades have witnessed the appearance and popularity chains is more complicated but might achieve an ideal result.
of polyhedral oligomeric silsesquioxane (POSS), owing to its
Attention of many researchers has been paying to the inu-
1,2
23–27
particular chemical structure and properties. As POSS has a ences of POSS on crystallization of polymers.
Fern ´a ndez
28
well-dened cage-like three-dimensional structure where the et al. found that both poly(3-caprolactone) (PCL) and POSS in
inside Si–O–Si core is surrounded by an organic surface and the telechelic POSS–PCL–POSS hybrid polymer were able to crys-
size of the POSS cage is as small as 1.5 nm, it is regarded as the tallize: POSS crystallized before PCL crystallization, and the
3
smallest silica particle. POSS is usually represented by the crystal of POSS conned the crystallization of PCL, hindering
formula (RSiO1
groups which can be reactive or nonreactive.
)
(n $ 6) where R's are organic functional the crystal formation of PCL. By applying time-resolved SAXS/
.5 n
29
4–11
WAXS, Heeley et al. discovered that long linear alkyl-chain
Just because of such unique structure, POSS is generally substituented POSS molecules acted as nucleating agents which
incorporated into polymers and various properties of the increased the crystallinity, crystallization kinetics and inu-
resulting composites are expected to be modied, say enced the nal lamellar morphology of polyethylene (PE). Thus,
mechanical properties, thermal stability, dielectric properties, they suggested such POSS compounds be potential llers for
1
2–18

ammability, viscosity during processing and so on.
In the nanocomposites in order to improve physical properties of the
30
compounding process, POSS with nonreactive R's can only host polymer. Nevertheless, Joshi et al. proposed that the
physically blend with polymers, while that with reactive R's can nucleation effect of POSS intensely depended on its dispersion
chemically bond with polymer chains through graing, state in polymer matrix. Explicitly, only octamethyl-POSS
19,20
copolymerizing or crosslinking.
Virtually, the modication dispersing at the molecular level could act as nucleating agents
efficiency of POSS strictly depends on its incorporation method. for high-density PE, while the POSS nanocrystals did not affect
In detail, simple physical blending is a more convenient way the crystallization process.
Although the effects of POSS on crystallization of plastics
were deeply studied, its inuences on rubbers did not widely
College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials catch the eyes of researchers. Among all the rubbers, as most
Engineering, Sichuan University, Chengdu 610065, People's Republic of China. E-mail:
resembles POSS in structure, silicone rubber is the most
guangsu-huang@hotmail.com
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31
frequently referred. Liu et al. indicated that POSS crystals
existed in POSS/silicone rubber composites prepared at low
temperatures, while POSS could dissolve in rubber matrix in
composites prepared at high temperatures. Nevertheless, since
silicone rubber crystals at a temperature far below room
temperature, its crystallization behaviors were not investigated
detailedly.
32
We concluded in our previous study that heptaphenylhy-
drogensilsesquioxane (H-POSS) could accelerate the crystalli-
zation of polydimethylsiloxane (PDMS) rubber by lowering the Scheme 1 The synthesis of OMS.
mobility and inducing the ordered arrangement of PDMS chain
segments. In recent work, we found the inuences of POSSs on
PDMS were not invariable. Factually, the crystallization behav-
tetrahydrofuran successively, the powder was put into a vacuum
oven at 70 C for 48 hours. At last, 5.89 g pure OMS was gathered
iors of POSS/PDMS composites tremendously relied on variety
and incorporation method of POSS.
ꢀ
with a yield of 83.6% through recrystallization. FTIR (KBr,
In present work, three kinds of POSSs were synthesized and
POSS/PDMS composites were prepared through solution
blending and open two-roll mill blending. Then the crystalli-
zation and melting behaviors of the composites were compared
by differential scanning calorimetry (DSC). In order to reveal the
origins of differences in crystallization and melting behaviors of
the composites, the microstructures of POSSs and the
composites were investigated using scanning electron micros-
copy (SEM), X-ray diffraction (XRD) and polarizing optical
microscopy (POM). The results showed that the crystallization
mechanisms of POSS/PDMS composites differ when distinct
POSSs were incorporated.
À1
33
cm ): 2971 (nC–H), 1410 (uC–H), 1116 (nSi–O–Si), 1269.71
2
9
(
nSi–C); Si NMR (C
Octaphenylsilsesquioxane (OPS) (Scheme 2): OPS was
3
D
6
O, ppm): À67.03 (Si–CH
3
).
synthesized through a method named high temperature-reux-
hydrolysis in which deionized water was charged at a temper-
ature higher than the boiling point of water. By means of reux,
water took part in the reaction circularly, thus the reaction time
was shortened and the yield was elevated. The denite
process is as follows: under nitrogen atmosphere, phenyl-
triethoxylsilane (28 g, 0.14 mol), sodium hydroxide (0.56 g, 0.01
mol) and distilled toluene (150 mL) were charged into three-
necked ask equipped with a reux condenser and a magnetic
ꢀ
stirrer in oil bath. When the mixture was heated to 110 C,
2
. Experimental
deionized water (4 g, 0.22 mol) was charged into the ask
dropwise. Aer reux for 72 hours, a kind of white powder was
obtained by ltration. Then it was washed with anhydrous
methanol three times followed by being dried in a vacuum oven
2.1. Materials
In this study, POSSs and poly(dimethylsiloxane) (PDMS) rubber
having 0.20% vinyl substituents with molecular weight of
ꢀ
at 70 C for 48 hours. Finally, through recrystallization, 12.13 g
À1
528 000 g mol were all synthesized in our laboratory. Methyl-
À1
OPS was collected and the yield was 81%. FTIR (KBr, cm ):
triethoxylsilane was supplied by Mingtian chemical, Liyang,
China. Phenyltriethoxylsilane was purchased from Xiangqian
chemical, Nanjing, China. Platinum(0)-1,3-divinyl-1,1,3,3-tetra-
methyldisiloxane (Pt(dvs)) was obtained from Tansoole,
Shanghai, China. 2,5-Dimethyl-2,5-di(tert-butylperoxy)hexane
1
6
136 (nSi–O–Si), 3073 (nC–H of phenyl), 998, 1029, 1431 (Si–Ph),
97 (bC–H of phenyl), 742 (gC–H of phenyl), 428 (sSi–O–Si), 501
29
(dSi–O–Si); Si NMR (C D O, ppm): À75.90 (Si–Ph).
3
6
In addition, the syntheses and characterizations of hepta-
phenylhydrogensilsesquioxane (H-POSS) and PDMS have been
described previously.
(DBPH) was provided by Dongyang chemical, Haian, China.
32
Anhydrous methanol, tetrahydrofuran, acetone, ethanol,
toluene, potassium hydroxide and sodium hydroxide were all
obtained from Kelong chemical, Chengdu, China. All the
2
.3. Preparation of POSS/PDMS composites
reagents were used as received except toluene, which needed to OMS/PDMS and OPS/PDMS composites were all prepared in
be distilled before use.
the same way. POSS and PDMS were dissolved in toluene
2.2. Syntheses and characterizations of POSSs
Octamethylsilsesquioxane (OMS) (Scheme 1): Methyltriethoxyl-
silane (17.8 g, 0.1 mol), potassium hydroxide (0.56 g, 0.01 mol)
and anhydrous methanol (250 mL) were charged into a three-
necked ask equipped with a reux condenser and a magnetic
stirrer. When potassium hydroxide was dissolved completely,
deionized water (14 g, 0.77 mol) was charged into the ask
ꢀ
dropwise. Then the mixture was heated to 60 C and reuxed for
72 hours. Aer that, a kind of white powder was obtained by

ltration. Having been washed with ethanol, acetone and Scheme 2 The synthesis of OPS.
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Table 1 The ingredients of POSS/PDMS composites
3
. Results and discussion
Toluene
Toluene
3.1. Crystallization behaviors of POSS/PDMS composites
POSS PDMS to dissolve to dissolve
DBPH Pt(dvs)
The crystallization behaviors of POSS/PDMS composites were
respectively characterized by non-isothermal and isothermal
processes, and the results are listed in Table 2. In the non-
isothermal crystallization processes shown in Fig. 1, the
Sample (g)
(g)
POSS (mL) PDMS (mL) (g)
(mL)
M-x
P-x
x
x
x
98-x
98-x
98-x
10x
10x
10x
980-10x
980-10x
980-10x
2
2
2
0
0
200
H-x
evolutions of crystallization temperature (T ) of the composites
c
show distinctions with each other. In detail, the initial addition
of OMS almost exerts no inuence on Tc of OMS/PDMS
composites, then T
than 10 wt% of OMS is included, but 20 wt% OMS leads to an
obvious decrease in T . The situations for OPS/PDMS and H-
POSS/PDMS composites are almost the opposite where, with the
addition of POSS, T of the former falls rstly but rises subse-
quently, while that of the latter increases at the beginning but
decreases by inches aerwards. As non-isothermal crystalliza-
tion takes place during cooling and the temperatures are
c
increases and remains constant when lower
respectively and stirred adequately, and then the two kinds of
solution were mixed directly. Aer being ultrasonic processed
and mechanical stirred, the mixer was placed to vacuum oven
at 100 C for 48 hours to discharge solvent. In two days, the
mixer and predetermined amount of DBPH were mixed with
an open two-roll mill to get a good distribution. Subsequently,
c
ꢀ
c
the product obtained was placed to plate vulcanizing press at
ꢀ
1
65 C and a pressure of 10 MPa for 12 min, followed by post-
ꢀ
substantially lower than room temperature, higher T
c
means
curing at 180 C for 4 hours. Samples were named M-x, P-x and
H-x where M, P and H respectively represented OMS, OPS and
H-POSS were incorporated into the composites, and x meant
the weight percentage of POSS in each composite. The ingre-
dients of the samples are listed in Table 1.
the facilitation of crystallization. Thus, we might assume that
Table
2 Crystallization and melting parameters of POSS/PDMS
composites
ꢀ
ꢀ
Sample
T
c
( C)
t1/2 (min)
T
m
( C)
c
X (%)
2.4. Measurements
PDMS
M-1
M-5
À66.86
À66.94
À66.02
À66.04
À67.21
À68.82
À68.72
À66.51
À64.08
À66.32
À66.76
À67.10
À67.87
4.70
4.87
2.91
2.84
1.73
9.23
6.48
7.55
5.12
2.45
3.22
3.70
4.77
À37.34
À39.14
À38.20
À37.42
À36.85
À39.24
À37.66
À38.61
À38.30
À35.79
À36.60
À37.15
À38.43
62.8
57.4
56.9
55.6
37.5
62.2
58.1
56.7
88.3
63.7
64.2
62.6
58.9
2
.4.1 Differential scanning calorimetry (DSC). The crystal-
lization and melting behaviors of the composites were investi-
gated using a TA Q200 differential scanning calorimeter (DSC)
M-10
whose temperature was calibrated with indium. The tests were M-20
P-1
P-5
P-10
P-20
H-1
performed under nitrogen atmosphere; sample weights were
between 5 and 8 mg. All the samples were rstly isothermal at
ꢀ
3
0 C for 5 min to remove thermal history, then they were cooled
ꢀ
ꢀ
À1
ꢀ
to À80 C at a rate of 10 C min and heated to 30 C at a rate of
ꢀ
À1
ꢀ
1
0 C min . Subsequently, the samples were cooled to À57 C H-5
ꢀ
À1
H-10
H-20
at a cooling rate of 50 C min for isothermal crystallization
until the completion of crystallization, nally they were heated
ꢀ
ꢀ
À1
to 30 C at a rate of 10 C min
.4.2 Scanning electron microscopy (SEM). SEM observa-
.
2
tions of POSS were performed on a JSM-5900LV scanning
electron microscope at a voltage of 20 kV. Before tests, the
vulcanized samples were placed into liquid nitrogen for some
time and then fractured into two pieces to create fresh
surfaces. The samples were then coated with a gold coating
˚

lm of about 100 A thick, examined and photographed in the
microscope.
2.4.3 X-ray diffraction (XRD). XRD spectra analyses were
carried out on the Philips X'Pert Graphics and Identify with Ni-
ꢀ

ltered Cu Ka radiation. The 2q angle ranged from 5 to 45 , and
À1
the scanning rate was 2.4 deg min
.
2.4.4 Polarizing optical microscopy (POM). The morphol-
ogies of the composites were examined using a Leica DMIP
polarizing optical microscope. Before testing, POSS, PDMS and
curing agents were placed between two coverslips. Then the
ꢀ
coverslips were heated to 165 C for 12 min in order to make the
Fig. 1 DSC crystallization exotherms of OMS/PDMS (a), OPS/PDMS (b)
mixers cured.
and H-POSS/PDMS (c) composites.
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smaller loadings of OMS and H-POSS simplify crystallization of
PDMS, but much more POSS is adverse to that, whereas OPS/
PDMS composites just display the reverse situation.
Through isothermal crystallization, t1/2, the time needed to
reach half of the nal crystallinity which reects the rate of
crystallization, can be obtained. As shown in Fig. 2, the addition
of OPS initially decelerates the crystallization of OPS/PDMS
composites, but accelerates the crystallization of the compos-
ites aerwards. Additionally, with the rise in H-POSS loading,
the crystallization rate of H-POSS/PDMS composites monoto-
nously increases, and until 20 wt% H-POSS is added, H-POSS
shows adverse effect on crystallization of PDMS compared with
neat PDMS. The situations for the above two kinds of compos-
c
ites are consistent with the results of T . Nevertheless, saltation
in t1/2 of OMS/PDMS composites also appears when 20 wt%
OMS is included, but it shis to a lower value, implying the
faster crystallization rate. The result seems contradictory to the
Fig. 3 DSC melting endotherms of OMS/PDMS (a), OPS/PDMS (b) and
results of non-isothermal crystallization. So there is a doubt H-POSS/PDMS (c) composites.
whether various t1/2's will result in differences in melting
behaviors. Thus, it is necessary for us to discover the mysteries
by analyzing the melting behaviors of the composites.
for M-20 is at the cost of drop in X and the crystallization rate is
c
virtually almost unchanged, which is disadvantageous for the
formation of PDMS crystals.
3.2. Melting behaviors of POSS/PDMS composites
The melting behaviors for OPS/PDMS composites are vastly
c
The DSC melting curves of the composites aer crystallizing different. The initial addition of OPS tends to slightly lower X ,
ꢀ
completely at À57 C are displayed in Fig. 3 and the results are which is accompanied by the rise in t1/2, indicating the reduc-
also listed in Table 2.
tion in crystallization rate. Nevertheless, in case 20 wt% OPS is
The case for the sample H-20 t well with our previous added, X suddenly increases by more than 30%, but t1/2 reveals
c
32
conclusion that crystallinity (X ) of H-POSS/PDMS composites an abrupt fading. The promotion effect of OPS in crystallization
c
almost remains constant, and the difference in melting of PDMS is extremely apparent, and thus, similar as H-POSS, we
temperature (T
PDMS crystals.
m
) is only caused by the change in integrity of may consider that OPS plays as nucleating reagent in crystalli-
zation of PDMS. Nonetheless, the distinctions between the two
As to OMS/PDMS composites, T
m
m
shows a persistent kinds of POSSs lie in that H-POSS only enhances T of the
augment with the increase of OMS loading, indicating the more resulting composites by improving the integrity of PDMS crys-
intact of PDMS crystals. However, though Xc of OMS/PDMS tals within a certain POSS loading range, while OPS tends to
composites only shows a slight decrease when OMS is in a lower enhance X
of the composites, regardless of T . Such disparities
m
c
loading, one can observe that X decreases by about 20% when must be closely associated with micro structure of POSS and the
c
2
0 wt% OMS is added. Accordingly, the sharp reduction in t1/2 composites, so we are looking forward to tracing the origins by
exploring the aggregation state of POSS.
3.3. Characterizations of POSSs
Aer being recrystallized, the crystal structures of OMS, OPS
and H-POSS were characterized by scanning electron micros-
copy (SEM) and X-ray diffraction (XRD).
SEM graphs of the three kinds of POSSs are presented in
Fig. 4. One can easily nd that though the size of OMS crystals
varies, it's the smallest among the three. This may be ascribed
to the weaker molecular interactions than OPS and H-POSS
where p–p interactions among phenyls tend to be extremely
strong. Regarding morphologies of POSS crystals, in addition,
34
OMS mostly shows inerratic cubic crystals, OPS demonstrates
ake-like crystals with huge surfaces, and H-POSS crystals seem

as cuboids which have large aspect ratios. In addition, OMS and
OPS have smooth surfaces while that of H-POSS seem much
coarse.
Fig. 2 Plots of t1/2 values versus POSS loadings in POSS/PDMS
composites.
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Fig. 4 SEM graphs of OMS, OPS and H-POSS after recrystallization.
Fig. 6 POM graphs of OMS/PDMS composites: M-1 (a), M-5 (b), M-10
(c) and M-20 (d).
Fig. 5 XRD patterns of OMS, OPS and H-POSS.
XRD patterns are illustrated in Fig. 5 to provide further
information about crystal structure of POSSs. Many a sharp
peaks in patterns of OMS and OPS verify their high crystalliza-
35
tion capacities. Furthermore, crystallization peaks of H-POSS
emerge at almost the same degrees as OPS, but peak intensities
of H-POSS are extraordinarily weaker than that of the OPS and
OMS, manifesting the existence of much more defects in H-
POSS crystals which result from the reduction in integrity of
crystals because of the replacement of a benzene ring by a
hydrogen atom.
Fig. 7 POM graphs of OPS/PDMS composites: P-1 (a), P-5 (b), P-10 (c)
and P-20 (d).
3
.4. Dispersions and morphologies of POSSs in PDMS
OPS aggregates become larger and larger. As POM graph of P-20
shown in Fig. 7(d), the diameter of OPS aggregates can be as
matrix
Furthermore, the dispersion states and morphologies of POSS large as 10–15 mm, and also similar to SEM graph of OPS (Fig. 4),
in PDMS matrix are investigated by polarizing optical micros- the aggregates in composite are observed to have ake-like
copy (POM) and XRD.
structures with large surface areas. Regarding H-POSS/PDMS
Fig. 6–8 show POM photographs of OMS, OPS and H-POSS composites, owing to the complete gra of H-POSS onto PDMS
based composites, respectively. It is obvious that OMS, OPS and chains, aggregates don't appear in the sample H-1. What can be
H-POSS all can be dispersed homogeneously in PDMS matrix by seen in POM graph of H-1 are some black dots which may be
solution blending and vulcanization, but their sizes and shapes cavities caused by the migration of H-POSS. However, in case
are different from each other. OMS is hardly seen in the sample more H-POSS is added, because of the p–p interactions among
M-1, but it forms tiny aggregates when more OMS is added, and phenyls, H-POSS begins to aggregate. In addition, with the
most of the aggregates seemingly have inerratic cubic structure, loading of H-POSS increases, the aggregates get larger, whereas
which is comparable with SEM graph of OMS in Fig. 4. And with since the graing reaction destructs of the integrity of H-POSS
the rise of OMS loading, though OMS aggregates in PDMS are crystals, H-POSS aggregates with analogous structures to that in
denser, their sizes almost keep constant. By contrast, OPS Fig. 4 only appear in the sample H-20 where a majority of H-
aggregates in PDMS matrix are much larger and their shapes are POSS is free from PDMS matrix.
multitudinous. When smaller loading of OPS is incorporated,
More details can be obtained from XRD patterns of the
OPS forms little aggregates; but as the loading of OPS increases, composites shown in Fig. 9. A broad diffuse scattering peak at
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can observe that peaks of OMS don't emerge until 20 wt% OMS
is included. Four sharp peaks can be obviously seen in the XRD
patterns of M-20 and the peaks t well with peaks in XRD
patterns of OMS (Fig. 5). Thus, we may conclude that lower than
10 wt% OMS is amorphously dispersed in PDMS matrix, and
OMS crystals only exist in composites with more than 10 wt%
OMS.
3.5. Differences in inuence mechanisms of POSSs on
crystallization and melting behaviors of PDMS
XRD patterns of M-20, P-20 and H-20 show that crystallization of
each kind of POSS is prominent. However, what has been
concluded is that the inuences of each POSS on crystallization
and melting behaviors of PDMS are signicantly distinct.
According to the DSC results of OMS/PDMS composites,
Fig. 8 POM graphs of H-POSS/PDMS composites: H-1 (a), H-5 (b), H- OMS shows no nucleation effect on crystallization of PDMS. In
1
0 (c) and H-20 (d).
the sample M-20, OMS forms little aggregates and the packing
density of OMS is extraordinarily high. Thus, the tightly packed
OMS keeps PDMS chain segments from approaching each
other, which results in that PDMS crystals are difficult to take
shape, but the formed crystals are very intact.
Nevertheless, as described above, although both OPS and H-
POSS can act as nucleating reagents to accelerate the crystalli-
zation of PDMS, their inuences are signicantly different: P-20
shows substantial augment in T
while H-POSS within a certain improves T
and T of the composite. OPS crystals are observed to have large
c
, crystallization rate and X
c
,
c
, crystallization rate
m
surfaces from the POM graph of P-20. During crystallization of
PDMS, such surfaces can provide templates for PDMS crystals to
grow and the induction effect can appreciably enhance X of the
c
composite. Whereas, the surfaces of OPS crystals in OPS/PDMS
composites with lower loadings of OPS are not large enough to
induce the crystallization of PDMS and contrarily, OPS crystals
tend to restrict the regular arrangement of PDMS chain
segments. In H-POSS/PDMS, H-POSS in low loadings can gra
with vinyls of PDMS. Once more H-POSS is added, because of
the weaker crystallization capability of H-POSS compared with
OPS, H-POSS crystals couldn't provide enough large surfaces for
Fig. 9 XRD patterns of OMS/PDMS (a), OPS/PDMS (b) and H-POSS/
PDMS (c) composites.
ꢀ
around 11.7 which can be detected in all the patterns corre- PDMS crystals, which is similar as OPS/PDMS composites with
sponds to the amorphous structure of PDMS. Furthermore, a small OPS loadings. The nucleating effect of H-POSS on crys-
small sharp peak at the le side of the broad peak appears upon tallization of PDMS expresses as that the mobility of bonded
5
wt% OPS or H-POSS is added. For H-POSS/PDMS composites, PDMS chains are largely restrained by H-POSS, and the total
36
according to Bragg's equation, as the small peak shis to result is that PDMS crystals become considerable heat-stable
lower angles with the addition of H-POSS, H-POSS crystals tend but the crystallinity almost keeps unchanged.
to be more regular, which is in accordance with POM graphs
shown in Fig. 8. Nevertheless, the peak intensity of H-POSS only
4
. Conclusion
shows a slight increase with the rise of H-POSS loading, which
might stems from the weaker crystallization trend of H-POSS
themselve as described in Fig. 5. For OPS/PDMS composites,
with the addition of OPS, the intensity of peak of OPS increases
ceaselessly because of the aggregation of OPS. What is the most
signicant is that when 20 wt% OPS is added, some new peaks
appear at the right side of the broad peak. The new peaks
correspond well with the smaller peaks of OPS (Fig. 5) and
conrm the high crystallinity of OPS in the composite. The
cases for OMS/PDMS composites are distinctly different. One
Three different polyhedral oligomeric silsesquioxanes (POSSs),
octamethylsilsesquioxane (OMS), octaphenylsilsesquioxane
(
OPS) and heptaphenylhydrogensilsesquioxane (H-POSS) were
synthesized and then incorporated into polydimethylsiloxane
PDMS) rubber, respectively. Aerwards, the inuences of
(
POSSs on crystallization and melting behavior of PDMS, as well
as morphologies of POSSs were investigated. Due to the weakest
POSS–POSS interactions, OMS crystals are much smaller than
the others and the crystal peaks emerge only when 20 wt% OMS
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is added. The small and closely packed OMS crystals are adverse 12 Y. Liu, Y. Shi, D. Zhang, et al., Polymer, 2013, 54(22), 6140–
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2
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41370 | RSC Adv., 2014, 4, 41364–41370
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