The effect of molecular weight of hyperbranched epoxy resins with a silicone skeleton on performance

Article abstract of DOI:10.1039/c3ra00023k

Hyperbranched epoxy resins with a silicone skeleton (HERSS) obtained by us through an environmentally-friendly synthetic method have shown a prominent comprehensive performance in modifying the diglycidyl ether of bisphenol-A (DGEBA). However, controlling

Full text of DOI:10.1039/c3ra00023k

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The effect of molecular weight of hyperbranched epoxy
resins with a silicone skeleton on performance3
Cite this: RSC Advances, 2013, 3,
9522
Daohong Zhang,*a Enbin Liang,a Tingcheng Li,a Sufang Chen,b Junheng Zhang,a
Xinjian Cheng,a Jiliang Zhoua and Aiqing Zhanga
Hyperbranched epoxy resins with a silicone skeleton (HERSS) obtained by us through an environmentally-
friendly synthetic method have shown a prominent comprehensive performance in modifying the
diglycidyl ether of bisphenol-A (DGEBA). However, controlling the performance of the HERSS is still a
challenge. In this paper, we report the preparation of three other kinds of HERSS with various molecular
weights via hydrosilylation, in order to study the effect of the molecular weights of the HERSS on the
degree of branching (DB), the viscosity and the mechanical performance, the thermal properties and the
micro-phase separation behavior of cured HERSS–DGEBA composites, which contain the HERSS prepared
recently. The degree of branching and viscosities of the HERSS are 0.71–0.84 and 103.5–697.4 mPa s,
respectively. An increase in the molecular weight of the HERSS results in a decrease in the DB and an
appearance of a maximum viscosity. Accordingly, the mechanical properties of the cured HERSS–DGEBA
composites, including the tensile, flexural and impact strength, increase first and then decrease, and their
glass transition temperatures decrease weakly. Both the average diameters of 3.62–4.75 mm and the
distribution of the islands in the ‘‘sea–island’’ structure of all the cured composites show an increase.
Compared with those of DGEBA, the tensile, flexural and impact strength could be increased by about
76.4–88.6%, 25.3–36.0% and 78.4–92.1%, respectively, but their glass transition temperature was impaired
only by about 6.3–6.8%.
Received 3rd January 2013,
Accepted 3rd April 2013
DOI: 10.1039/c3ra00023k
www.rsc.org/advances
HTDE, HTME and HTBE, which could toughen and reinforce
DGEBA at the same time.2,11 An in situ reinforcing and
1. Introduction
toughening mechanism was presented2 first, and was further
confirmed through researching unsaturated HBP–linear unsa-
turated polyester resins composites12 and hyperbranched
polyphosphate ester–DGEBA composites.13
Hyperbranched epoxy resins (HERs), as an important class of
hyperbranched polymers, generally have a better solubility and
lower viscosity than linear polymers with a similar molar mass,
and are produced using a facile synthetic processes.1–4 More
importantly, HERs as thermosetting polymers can not only
toughen commercial epoxy resins and but can also show an
excellent comprehensive performance.2 Typical HERs, includ-
ing Boltorn E1/E25–8 and epoxy-functionalized hyperbranched
block copolyethers with a core–shell structure,9 can increase
by several folds the fracture toughness of the commercial
diglycidyl ether of bisphenol-A (DGEBA) using only 5–20 wt%.
However, the tensile strength, flexural strength and glass
transition temperature are usually impaired to some extent. In
2006, we synthesized several novel HERs with low viscosities of
350–5000 cP and a low epoxy equivalent weight,10 including
A lot of methods have been used to synthesize HERs,
including proton transfer polymerization,14–16 ATRP,17 graft-
ing an epoxy group to hyperbranched polymers1,10,18–26 with
epichlorohydrin (ECH), esterification27 and hydrosilyla-
tion.28,29 These endeavors gained much success in preparing
HERs in labs, but only a minority of HERs could be supplied
commercially, such as Boltorn E1 and E2 (Perstorp Corp.,
Sweden) and our HyperT E1, E2, E3 and E4 series (HyperT
Resins, China). It is still very difficult to produce HERs with
wide applications on a large scale due to the following facts. (1)
A great deal of organic solvents are used during the synthetic
processes, such as DMF.27 (2) Tedious processes involving the
synthetic epoxy monomers14–16 are needed. (3) A large amount
of water is wasted to wash and remove the byproduct
NaCl.1,10,18–26 (4) The low glass transition temperature and
thermal decomposition temperature of the cured HER
composites hamper their applications. (5) The solid29 or high
viscosity liquid14–16 properties of some HERs means that they
aKey Laboratory of Catalysis and Materials Science of the State Ethnic Affairs
Commission & Ministry of Education, South-central University for Nationalities,
Wuhan, Hubei Province, 430074, China. E-mail: zhangdh27@163.com;
Fax: +86-27-67842752; Tel: +86-27-67842752
bKey Laboratory for Green Chemical Process of Ministry of Education, Wuhan
Institute of Technology, Wuhan, Hubei 430073, China
Electronic supplementary information (ESI) available. See DOI: 10.1039/
3
c3ra00023k
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Fig. 1 The synthetic schemes for the production of AHRSS and HERSS.
should be diluted with a thinner or solvent during application
and can not be applied under solvent-free conditions.
more information about hyperbranched epoxy resins, but will
also be helpful in selecting appropriate HERSS with certain
properties for applications.
To develop an excellent performance and economic method
to prepare HERs, in our recent paper,30 we adopted an
environmentally-friendly technique to prepare novel hyper-
branched epoxy resins with a silicone skeleton (HERSS). Every
reaction during the preparation of the HERSS involves only
one solvent and affords a high yield (.90%), which is
important when producing economically and environmentally
a heterogeneous catalyst that can be recycled 7 times. The
resulting HERSS were used to modify DGEBA and showed an
excellent toughening and reinforcing function. With the
incorporation of 9 wt% HERSS, the impact, flexural and
tensile strength of the DGEBA is increased by about 92.5%,
36.0% and 88.6%. It is very important to be able to control the
performance of the HERSS due to the fact that the different
application fields of the HERSS have various requirements,
such as viscosity, heat-resistance, strength, chemical structure,
etc. Therefore, based on our recent work,30 in this article, we
focus on preparing HERSS with different molecular weights
and we investigate the effects of molecular weight on the
chemical properties and the mechanical and thermal perfor-
mances, including the degree of branching, the viscosity, the
tensile, flexural and impact strength, the glass transition
temperature, the micro-phase separation size and structure
during curing, etc. These results obtained will not only provide
2. Experimental
2.1 Materials
Allylglycidyl ether (AGE) and other materials were purchased
from commercial sources and used without further purifica-
tion unless noted. Tetrahydrofuran (THF), 1,1,3,3-tetramethyl-
disiloxane (TMDS), allyl chloride (AC) and phenyltrichlorosi-
lane (PTCS) were purified by distillation. AC, TMDS, AGE and
PTCS were dried over 24 h with 4 Å molecular sieves before
use. The heterogeneous halloysite supported platinum catalyst
(Pt–Halloysite) with 0.4 wt% of Pt was prepared as described in
our previous paper.31 DETA–AN was used as a curing agent,
which was prepared by an addition reaction between acryloni-
trile (AN) and diethylene triamine (DETA)2 at an equal molar
ratio.
2.2 Synthesis of AHRSS and HERSS
The synthetic scheme for the production of hyperbranched
epoxy resins containing a silicone skeleton (HERSS) with
various molecular weights is shown in Fig. 1, where HERSS-1,
HERSS-2, HERSS-3 and HERSS-4 refer to HERSS containing 3,
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6, 12 and 24 molar epoxy groups, respectively. First,
phenyltriallylsilane (PTAS) was prepared in a reaction between
allylmagnesium chloride (AMC) and phenyltrichlorosilane
(PTCS), and 1,1,3,3-tetramethyl-disiloxane-propylglycidyl ether
(TDPGE)30 was synthesized from the addition of equal molar
amounts of TMDS and allylglycidyl ether (AGE). Next, four
of the AHRSS and HERSS were measured using a Brookfield
DV-III+ Viscometer with a RV-61 spindle at 298 K and 20 rpm.
The no-notch impact strength of the cured samples (80 mm
6 10 mm 6 4 mm) was determined using a XJJ-5 simple-
supported-beam impact testing machine (ChengDe Precision
Testing Machine Co., Ltd, China) at room temperature (298 K)
according to ASTM D256-81. The tensile strength of the
dumbbell samples (75 mm 6 5 mm 6 5 mm) and the flexural
strength of the samples (80 mm 6 10 mm 6 4 mm) were
measured using a WSM-20KN testing machine according to
ASTM D368-91a with a loading rate of 10 mm min21 and ASTM
D790M-92 by a WSM-20KN using a three-point blending mode,
respectively.
kinds of allyl-ended hyperbranched resins containing
a
silicone skeleton (AHRSS) with various molecular weights
were obtained by a hydrosilylation between PTAS and 1,1,3,3-
tetramethyl-disiloxane (TMDS) in various molar ratios using
halloysite-supported platinum as the catalyst. Finally, four
kinds of HERSS were synthesized by a similar hydrosilylation
between the four kinds of AHRSS and TDPGE.30
2.2.1 The synthesis of AHRSS. Preparation of AHRSS-1 (B3).
PTCS (30.0 g, 0.14 mol) was added dropwise into a three-
necked flask containing an AMC (0.85 mol)–THF (500 mL)
solution at 273–278 K with stirring. The reaction continued for
about 24 h, and then an ammonium chloride saturated
aqueous solution (100 mL) was added into the flask to
terminate the reaction. After the resulting solid was filtered,
the solvent was removed under a pressure of 5 mmHg at 343 K
to yield 31.25 g (96.49%) of a yellowish transparent liquid
(AHRSS-1).
A scanning electron microscope (SEM) (XL-30FEG, Philip)
was used to examine the morphology of the fractured surface.
A thin section of the fractured surface was cut and mounted on
an aluminum stub using a conductive (silver) paint and was
sputter-coated with gold prior to fractographic examination.
SEM micrographs were obtained under conventional second-
ary electron imaging conditions, with an accelerating voltage
of 20 kV.
Dynamic mechanical thermal analysis (DMTA) was per-
formed on the cured samples (30 mm 6 10 mm 6 4 mm)
using
a Q800 measurer (TA Instruments, USA) over a
Preparation of AHRSS-2. A mixture of PTAS (9.14 g, 0.04 mol)
and THF (10 mL) was added dropwise into a three-necked flask
containing TMDS (4.03 g, 0.03 mol), THF (15 mL) and Pt–
Halloysite (9.0 g) as the catalyst equipped with a stirrer, a
cooler and a thermometer. The reaction continued for 12 h at
about 338–343 K. Then, the catalyst was filtered and the
solvent was removed under a pressure of 5 mmHg at 338–343
K to yield 12.99 g (98.67%) of a yellowish transparent liquid
(AHRSS-2).
temperature range of 298 K to 473 K at the rate of 5 K
min21 and at a frequency of 1 Hz. Thermogravimetric analyses
(TGA) were performed using a thermogravimetric analyzer
(NETZSCH TG209) at a heating rate of 10 K min21 with a
nitrogen flow of 20 mL min21
.
3. Results and discussion
AHRSS-3 and AHRSS-4 were also prepared by a similar
synthetic process to that of AHRSS-2.
3.1 The effect of the molecular weights of the AHRSS and
HERSS on their properties
2.2.2 The synthesis of HERSS. Preparation of HERSS-1.
AHRSS-1 (4.57 g), THF (15 mL), TDPGE (14.91 g, 0.06 mol)
and Pt–Halloysite (5.0 g) were added into a three-necked flask,
and the reaction continued for 10 h at temperature between
338 K and 343 K. A yellowish transparent liquid (HERSS-1) of
19.07 g (97.9%) was afforded after the catalyst was filtered and
the solvent, THF, was removed.
The properties of the AHRSS and HERSS obtained through the
above processes are shown in Table 1 and Table 2, respectively.
An increase in the functional (allyl-ended and epoxy-ended)
groups results in an increase in the molecular weight and
molecular distribution of the AHRSS (Table 1) and HERSS
(Table 2). The difference between the theoretical values for
molecular weight and those obtained from GPC becomes
bigger with an increase in the molecular weight, which results
from the branched structure of the AHRSS and HERSS using
linear polystyrene as the standard compound during measure-
ment.
Viscosity is dependent on the hydromechanical volume and
the degree of branching (DB) of the HBPs.10,19,20,32 Generally, a
lower hydromechanical volume or a higher DB in the HBPs
results in a lower viscosity. The viscosities of HERSS-1, HERSS-
2, HERSS-3 and HERSS-4 are much lower than that (15 600
mPa s) of DGEBA, indicating a lower hydromechanical volume
or a higher DB in the HERSS. The finding that the viscosities of
both the AHRSS and the HERSS reach a maximum with an
increase in their molecular weights agrees with the viscosity
properties of dendrimers,32 indicating that they have high
degrees of branching.
HERSS-2, HERSS-3 and HERSS-4 were also synthesized by a
similar synthetic process to that of HERSS-1.
2.3 Preparation of HERSS–DGEBA composites
HERSS–DGEBA composites containing different amounts of
the HERSS were prepared by a similar mixed process described
in our previous paper.30
2.4 Characterization
Infrared spectra were obtained using a NEXUS 470 Fourier
transform infrared spectrometer. 1H and 13C NMR measure-
ments were conducted on an AVANCE III-400 (Bruker) nuclear
magnetic resonance spectrometer. The molecular weights and
the distributions (PDI) were determined using gel permeation
chromatography (PL GPC-50) using a PL-RTLS 15/90 dual-
angle LSD as a detector and THF as the eluent. The viscosities
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Table 1 Properties of the allyl-ended hyperbranched resins with a silicone skeleton
AHRSS
AHRSS-1
AHRSS-2
AHRSS-3
AHRSS-4
Yield/%
96.49
228.4
98.67
1316.6
2131
97.70
3492.9
2391
99.20
7845.5
4082
Theoretical molecular weight/Da
Number molecular weight (
Mn
) from GPC/Da
3612
6846
2.10
4955
17 350
2.08
252.5
10 520
30 175
2.58
Weight molecular weight (Mw) from GPC/Da
Z-molecular weight (Mz) from GPC/Da
Molecular weight distribution (PDI) from GPC
Viscosity/mPa s
18.6
161.4
362.9
tiny difference in structural variation among the three units.
The use of the 1H NMR-minus spectrum for a model
compound with a similar chemical structure is a convenient
strategy to analyze effectively these attributions. It simplifies
the calculation process and allows a precise DB to be
obtained.19 Herein, we designed and synthesized a model
compound tri-(1,1,3,3-tetramethyl-disiloxane)-phenylpropylsi-
lane (A3) in a reaction involving TMDS and PTAS which
underwent a similar synthetic process to that used in the
production of TDPGE. Chemical shifts at d 1.37–1.57 (a, 2H), d
0.91–1.09 (b, 2H) and d 0.60–0.80 (c, 2H) are observed clearly in
the 1H NMR spectrum (Fig. 4) for A3. Compared with the
corresponding chemical shifts of A3, the chemical shifts of all
3.2 Chemical Structures of AHRSS and HERSS
1
The FT-IR, H NMR and 13C NMR spectra of AHRSS-1 (PTAS),
AHRSS-2, AHRSS-3 and AHRSS-4 are shown in Fig. S1, ESI ,
3
Fig. 2 and Fig. S2, ESI, respectively, and their detailed spectra
3
are supplied in the supporting information (Fig. S1, ESI ).
3
The FT-IR, 1H NMR and 13C NMR spectra of HERSS-1,
HERSS-2, HERSS-3 and HERSS-4 are shown in Fig. S3, ESI ,
3
Fig. 3 and Fig. S4, ESI, respectively, and their detailed spectra
3
are supplied in the supporting information (Fig. S2, ESI ).
The shape and the location of the peaks for the AHRSS and
HERSS shown in Fig. 2, Fig. 3 and Fig. S1–S4, ESI remain
unchanged. This is attributed to the similar chemical structure
of the AHRSS and HERSS, although their molecular weights
show distinct variations in Table 1 and Table 2.
3
3
the hydrogen atoms in the three kinds of units (Fig. S5, ESI )
3
can be obtained as following: d 1.37–1.57 (T2-H, 2H and B2-H,
6H), d 1.34 (L2-H, 4H), d 0.81–1.09 (T3-H, 2H, B3-H, 6H and L3-
H, 4H), d 0.67–0.78 (B1-H, 6H) and d 0.54–0.67 (T1-H, 2H and
L1-H, 4H) (Fig. 4).
3.3 The effect of the molecular weight of the AHRSS on the
degree of branching
The degree of branching (DB) of hyperbranched polymers is
usually calculated by NMR spectroscopy33,34 according to eqn
(1). The DB refers to the ratio of the branched and terminal
units to the total units, i.e.
The relative areas of all these peaks were obtained and are
summarized in Table 3 according to their relationships in the
chemical structures. The degree of branching in AHRSS-2,
AHRSS-3 and AHRSS-4, 0.84, 0.72 and 0.71, respectively,
decreases with an increase in molecular weight by calculation
according to eqn (2). The DB in AHRSS-1 should be 1.00 because
it is composed only of terminal units. The degree of branching
in HERSS-1, HERSS-2, HERSS-3 and HERSS-4 is 1.00, 0.84, 0.72
and 0.71, respectively, and this is due to the fact that their
silicone skeleton has the same structure as that in AHRSS.
DB = (Nb + Nt)/(Nb + Nt + Nl)
(1)
The chemical chains of the AHRSS are composed of three
different structure units (Fig. S5, ESI ), including (1) a
3
branched unit (Nb), (2) a linear unit (Nl) and (3) a terminal
unit (Nt).
It is very difficult to designate directly the attributions of
these hydrogen atoms in range of 0.5–1.7 ppm (Fig. 4) for a
(T1zT2zT3)=2z(B1zB2zB3)=6
DB~
(T1zT2zT3)=2z(B1zB2zB3)=6z(L1zL2zL3)=4
Table 2 Properties of the hyperbranched epoxy resins with a silicone skeleton
HERSS
HERSS-1
HERSS-2
HERSS-3
HERSS-4
Yield/%
97.9
973
99.7
99.5
99.8
Theoretical molecular weight/Da
Number molecular weight (Mn) from GPC/Da
Weight molecular weight (Mw) from GPC/Da
Z-molecular weight (Mz) from GPC/Da
Molecular weight distribution (PDI) from GPC
Epoxy equivalent weight/g mol21
Theoretical epoxy equivalent weight/g mol21
Viscosity/mPa s
2807
2243
5052
14 217
2.25
526
476
223.5
6474
3960
8920
22 714
2.25
556
526
697.4
13 808
6675
17 767
46 317
2.66
588
588
391.4
1810
2097
2561
1.16
417
323
103.5
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Fig. 4 1H NMR spectra of A3, AHRSS-2, AHRSS-3 and AHRSS-4 between d 0.5–
Fig. 2 1H NMR spectra of AHRSS-1,AHRSS-2,AHRSS-3 and AHRSS-4.
1.7 ppm.
3.4 The effect of the molecular weight of the HERSS on the
mechanical performance
increase in the molecular weight of the HERSS (Fig. 5). The
tensile strengths of 9 wt% HERSS-1–DGEBA, 9 wt% HERSS-2–
DGEBA, 9 wt% HERSS-3–DGEBA and 9 wt% HERSS-4–DGEBA
are 56.41 MPa, 56.73 MPa, 60.31 MPa and 58.48 MPa,
respectively, all of which are higher than that (31.98 MPa) of
DGEBA by 76.39%, 77.39%, 88.58% and 82.89% (Fig. 5a). The
flexural strengths of the composites are 39.03 MPa, 40.10 MPa,
42.34 MPa and 39.63 MPa, respectively, all of which are higher
than that (31.14 MPa) of DGEBA by about 25.34%, 28.77%,
35.97% and 27.26% (Fig. 5b). At the same time, the impact
strengths of the composites are 3.56 kJ m22, 3.67 kJ m22, 3.83
kJ m22 and 3.66 kJ m22 (Fig. 5c), all of which are higher than
that (1.99 kJ m22) of DGEBA by about 78.44%, 84.10%, 92.08%
and 83.45%. The excellent toughness of the HERSS–DGEBA
composites results from the non-crosslinkable hyperbranched
structure which may absorb energy upon impact.2 At the same
time, the improvement in the tensile and flexural strength is
attributable to the increase in the crosslinking density of the
composites due to the large number of epoxy groups in the
HERSS. However, the crosslinking density and the non-
crosslinkable hyperbranched structure have opposite effects
on the strength and toughness of the composites. A balance
was found with the incorporation of 9 wt% HERSS and a
medium molecular weight of HERSS, which demonstrated not
only outstanding toughness but also excellent tensile and
flexural strength.
One of the major applications of hyperbranched epoxy resins
is to toughen and reinforce commercial DGEBA.2,3,7,24,35 The
novel hyperbranched epoxy resin with a silicone skeleton
(HERSS-3)30 had a remarkable effect on increasing the tensile
and impact strength of the DGEBA. The mechanical perfor-
mance of the HERSS-3–DGEBA composites first increased and
then decreased with an increase in the HERSS-3 content, and
an optimum performance appeared with the incorporation of
9 wt% HERSS-3, which increased the impact, flexural and
tensile strength of DGEBA by about 92.5%, 36.0% and 88.6%,
respectively. HERSS-1, HERSS-2 and HERSS-4 should also have
a similar effect on toughening and reinforcing DGEBA. The
results showing the effect of molecular weight on the
mechanical performances of the HERSS–DGEBA composites
are show in Fig. 5.
In addition to a similar performance change in all of the
HERSS-3–DGEBA composites with an increase in the HERSS
content,30 the mechanical performances of the HERSS/DGEBA
composites also first increase and then decrease with an
3.5 The effect of the molecular weight of the HERSS on the
micro-phase separation behavior of the cured HERSS/DGEBA
composites
Compared with the homogeneous microstructures in the
HTDE–DGEBA,2 HTME–DGEBA,36 HTBE–DGEBA37 and
HTPE–DGEBA1 composites, it is interesting to note that the
clear two-phase ‘‘sea–island’’ structure morphologies of the
fractured surface are essentially different (Fig. 6). An increase
in the molecular weight of the HERSS results in an increase in
the number and volume of ‘‘islands’’ on the surface (Fig. 6).
The volumes of the islands observed in the SEM micrographs
are close to their theoretical values (Table S1, ESI ) calculated
according to their components, indicating that the islands
3
Fig. 3 1H NMR spectra of HERSS-1, HERSS-2, HERSS-3 and HERSS-4.
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Table 3 The DB in the AHRSS and their calculated data
0.54–0.67
0.81–1.09
T3-H
1.37–1.57
T2-H
d (ppm)
Hydrogen atoms
0.67–0.78
B1-H
1.34
L2-H
DB
T1-H
L1-H
L3-H
B3-H
B2-H
AHRSS-2
AHRSS-3
AHRSS-4
0.93
0.81
0.40
0.65
1.11
1.16
2.22
1.89
3.07
0.93
0.81
0.40
0.65
1.11
1.16
2.22
1.89
3.07
0.65
1.11
1.16
0.93
0.81
0.40
2.22
1.89
3.07
0.84
0.72
0.71
should be mainly composed of cured HERSS. The differences
in the solubility parameters of DGEBA (d 17.91 (J cm
23 0.5) and
HERSS (dHERSS-1, dHERSS-2, dHERSS-3 and dHERSS-4 = 15.06, 15.16,
15.19 and 15.33 (J cm23 0.5
respectively) change their
)
)
,
compatibility during curing and induce micro-phase separa-
tion.30 The average diameters of these ‘‘islands’’ in all four
cured HERSS-1–DGEBA, HERSS-2–DGEBA, HERSS-3–DGEBA
and HERSS-4–DGEBA composites (Fig. 6 and Fig. 7) are 3.62,
4.47, 4.66 and 4.75 mm, suggesting that their diameters
increase with an increase in the molecular weight of the
HERSS. Their diameter distributions also increase as shown by
the fact that the islands in Fig. 6e are more evenly distributed
than those in Fig. 6f.
Dynamic mechanical thermal analysis of materials is an
available tool used to analyze the phase-separation behavior of
composites.2 The relationship between tan d and temperature
in all of the cured systems is shown in Fig. 8. With an increase
in the HERSS-3 content in the composites, the a-relaxation
peak gradually changes from a single to a double peak
(Fig. 8a), suggesting the appearance of phase-separation and
the weak miscibility between the HERSS-3 and DGEBA. The
separation (Tg2 2 Tg1) between the two peaks increases with an
increase in the HERSS-3 content as shown in Table 4, and this
indicates that the degree of phase-separation also increases
which is agreement with the results from our previous paper.30
When the molecular weight of the HERSS is increased, the
separation (Tg2 2 Tg1) of the composites containing 9 wt%
HERSS increases weakly (Fig. 8a and Table 4), implying a
higher degree of phase-separation size between the HERSS and
DGEBA in the HERSS-3–DGEBA and HERSS-4–DGEBA compo-
sites compared to the HERSS-1–DGEBA and HERSS-2–DGEBA
composites. This is attributed to the fact that HERSS-3 and
HERSS-4 with higher molecular weights have a larger molecule
size.
3.6 The effect of the molecular weight of the HERSS on the
thermal performance
Previous results showed that glass temperature (Tg) of the
HERs–DGEBA composites decreased over 20%, however HERs
showed an outstanding toughening and reinforcing function
to DGEBA.2,3,7,24,35 The Tg of the HERSS-3–DGEBA composites
decreases slowly with an increase in the HERSS-3 content.30
When increasing the molecular weight of the HERSS, the Tg1
of the HERSS–DGEBA composites changes a little as shown in
Fig. 8. The Tg1s of 9 wt% HERSS-1–DGEBA, 9 wt% HERSS-2–
DGEBA, 9 wt% HERSS-3–DGEBA and 9 wt% HERSS-4–DGEBA
are 390.2 K, 388.5 K, 388.2 K and 390.5 K, respectively, and
only decrease by about 6.3%, 6.7%, 6.8% and 6.3% compared
with that (416.6 K) of DGEBA, suggesting that the HERSS have
Fig. 5 The effects of the HERSS molecular weights on the (a) tensile, (b) flexural
and (c) impact strength of the HERSS–DGEBA composites.
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Fig. 6 SEM micrographs of the fractured surface of the HERSS/DGEBA composites and DGEBA (a. 9 wt% HERSS-1, b. 9 wt% HERSS-2, c. 9 wt% HERSS-3, d. 9 wt%
HERSS-4, e. 9 wt% HERSS-1, f. 9 wt% HERSS-3).
better heat-resistance properties compared to other hyper-
4. Conclusions
branched epoxy resins reported.2,24
Four kinds of hyperbranched epoxy resins containing a
silicone skeleton (HERSS) with different molecular weights
were synthesized in an environmentally-friendly hydrosilyla-
The thermal decomposition temperature, Ti (initial thermal
decomposition temperature or temperature at 5% weight loss),
of the cured HERSS–DGEBA composites increases slightly (Fig.
S6, ESI and Table S2, ESI ) with an increase in the molecular
3
3
weight of the HERSS. All the composites have a higher thermal
decomposition temperature of about 620 K than previous
results.2,24 The low Ti (506 K) of the cured DGEBA, is attributed
to the dehydration of the hydroxyl groups in DGEBA. Only 9
wt% HERSS could increase remarkably the thermal decom-
position temperature of the cured HERSS–DGEBA composite,
indicating that the HERSS molecular chains obstruct collision
and react with the hydroxyl groups in DGEBA.
Fig. 8 DMTA curves of the cured HERSS–DGEBA composites and DGEBA.
This journal is ß The Royal Society of Chemistry 2013
Fig. 7 The diameters and distribution of the islands in the fractured surface.
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6 R. Mezzenga, L. Boogh and J. A. E. Månson, Compos. Sci.
Technol., 2001, 61, 787.
Table 4 Peak temperatures of the DMTA curves of the cured HERSS–DGEBA
composites and DGEBA
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Composites
Tg1/K
Tg2/K
DT(Tg2 2 Tg1)/K
DGEBA
416.6
390.2
388.2
402.1
391.2
388.5
390.4
387.2
390.5
416.6
410.1
410.7
402.1
411.8
412.3
413.7
412.5
413.5
0
¨
9 J. Frohlich, H. Kautz, R. Thomann, H. Frey and
9 wt%HERSS-1–DGEBA
9 wt%HERSS-2–DGEBA
3 wt%HERSS-3–DGEBA
6 wt%HERSS-3–DGEBA
9 wt%HERSS-3–DGEBA
12 wt%HERSS-3–DGEBA
15 wt%HERSS-3–DGEBA
9 wt%HERSS-4–DGEBA
19.9
22.5
0
20.6
23.8
23.3
25.3
23.00
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methyl-disiloxane (TMDS) and 1,1,3,3-tetramethyl-disiloxane-
propylglycidyl ether (TDPGE). The chemical structure, mole-
cular weight and distribution and degree of branching (0.72–
0.84) of the HERSS were characterized by FT-IR, GPC and
NMR. The mechanical and thermal performances of the cured
HERSS–DGEBA composites were influenced by the molecular
weight of the HERSS. The research results show that the
impact, flexural and tensile strength increased first and then
decreased with an increase in the molecular weight of the
HERSS. The HERSS with a median molecular weight (HERSS-2
and HERSS-3) have the best performance, however their glass
transition temperatures decrease slightly. The average dia-
meter (3.62–4.75 mm) and the distributions of the islands
increase in the ‘‘sea–island’’ structure of all the cured
composites. The relatively low viscosities of the HERSS of
about 103.5–697.4 mPa s are helpful for its applications in
solvent-free resin fields. Controlling the performance of the
HERSS–DGEBA is also useful so that the correct HERSS–
DGEBA can be selected for their application as a functional
material.
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Acknowledgements
The authors gratefully acknowledge the financial support of
the National Natural Science Foundation of China
(No.50903094) and the Young Scientists Foundation of
Wuhan Institute of Technology (Q201202).
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