View
7
Download
0
Category
Preview:
Citation preview
1
Poly(sodium 4-styrenesulfonate) modified graphene for reinforced biodegradable
poly(ε-caprolactone) nanocomposites
Ming Wang1, Xiao-Ying Deng1, An-Ke Du2*, Tong-Hui Zhao1, Jian-Bing Zeng1,*
1College of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715,
PR China.
2Chongqing Academy of Science and Technology, Chongqing 401123, China
Email: jbzeng@swu.edu.cn (J.-B. Zeng) and castduke@163.com (A.-K. Du), fax/tel: +86-23-
68254000.
Electronic Supplementary Material (ESI) for RSC Advances.This journal is © The Royal Society of Chemistry 2015
2
Abstract: Homogeneous and stable water dispersion of graphene nanosheet (GNS) was
prepared through a non-covalent functionalization technique by ultrasonic processing of GNS
in the presence of poly(sodium 4-styrenesulfonate) (PSS) as the modifier. The dispersion was
then used to compound with poly(ε-caprolactone) (PCL) through solution coagulation to
fabricate PCL/GNS nanocomposites. Scanning and transmission electron microscopes
observations indicated that PSS modified GNS dispersed uniformly in and showed strong
interfacial adhesion with the PCL matrix when the loading of GNS was not more than 0.5
wt%. While when the loading of GNS increased to 1.0 wt%, aggregation morphology of GNS
in PCL matrix was detected. The crystallization temperature of PCL, as investigated by
differential scanning calorimeter, increased apparently by incorporation of PSS modified
GNS. Investigation on isothermal crystallization kinetics of neat PCL and its composites
indicated that the crystallization of PCL was accelerated considerably. Addition of only 0.05
wt% GNS caused a 5.8 times improvement in crystallization rate compared to neat PCL. The
crystallization mechanism almost kept unchanged. The improvement in crystallization rate
was ascribed to the enhanced nucleation density by incorporation of PSS modified GNS, as
evidenced by polarizing optical microscope (POM). Tensile tests manifested that both the
tensile strength and the Young’s modulus of PCL were reinforced and increased gradually
with increasing GNS loading within 0.5 wt%, meanwhile the elongation at break did not
reduced but increased slightly. While when the loading of GNS increased to 1.0 wt%, the
tensile strength and elongation at break reduced considerable due to the aggregation of GNS
with increasing loadings. Dynamic mechanical analysis indicated that the storage modulus of
PCL was reinforced in the full temperature range by incorporation of PSS modified GNS.
3
Introduction
Graphene, as a one-atom-thick two-dimensional layer of sp2-bonded carbon, shows a lot of
unique properties with many property parameters measured in experiments superior to any
other material and even some reaching theoretical limits1. For example, the room-temperature
electron mobility is 2.5×105 cm2 V-1 s-1 2, approaching to theoretical limit of ~2×105 cm2 V-1
s-1 3; the Young’s modulus and intrinsic strength are 1 TPa and 130 GPa4, respectively, very
close to the predicted values5. Besides, graphene shows very high thermal conductivity6,
complete impermeability to any gas7, room-temperature quantum hall effect8, room-
temperature ferromagnetism9, as well as many other supreme properties. Those properties
make it highly attractive for numerous applications such as printable and flexible electronics,
high-frequency transistors, photodetectors, optical modulators, supercapacitors, batteries,
biomaterials, etc1.
The incorporation of graphene into polymer matrix to form polymer nanocomposite
represents an important application of this unique nanomaterial, as it has the potential to
reinforce numerous properties of or impart some novel functionalities to matrix polymers10, 11.
Therefore, extensive works have been done to incorporate graphene into various polymers
and investigate the properties of the formed composites12-16. The dispersion state of graphene
plays a vital role in the final properties of the composites. Small addition could significantly
reinforce many properties of host polymers if in which graphene dispersed uniformly.
However, polymer composites with well dispersed graphenes are hard to achieve because
graphenes have a pronounced tendency to agglomerate in polymer matrices due to the strong
π-π interactions between graphene nanosheets.13 So, to improve and stabilize the dispersion
4
state of graphene in host polymers constitutes the greatest challenge in graphene based
polymer nanocomposites.
The efficient way to prevent graphenes from aggregation is to weaken the π-π interactions
via either chemical modification or non-covalent functionalization17. Chemical modification,
also known as covalent functionalization, involves chemical reactions between modifiers and
graphene or its derivatives that contain reactive functional groups, such as graphene oxide18-21
and reduced graphene oxide22, 23. The presence of surface grafted modifiers with different
natures could disturb the π-π interactions among graphene sheets thus facilitate dispersion of
graphene in various solvents or polymer matrices13. It is however worth noting that the
structural regularity of graphene is usually interrupted during chemical modification, which
thus reduces the performances of graphene, for example, lowering electrical conductivity24.
By contrast, non-covalent functionalization provides a way of improving dispersion of
graphene without disturbing its structure and the electronic network.17, 25, 26 This method
refers to the modification through physical absorption of modifiers onto the surface of
graphene so as to improve its dispersity in different solvents. The modifiers are usually the
substances that are able to form some particular interactions, such as π-π, cation-π, anion-π
interactions, with graphene sheets.17 Surfactants, such as sodium dodecylbenzene sulfonate
(SDBS)27-29, sodium dodecyl sulfate (SDS)30, cetyltrimethylammonium bromide (CTAB)27, 31,
and 4-(1,1,3,3-tetramethylbutyl) phenyl-polyethylene glycol (Triton X-100)32 are the widely
used non-covalent modifiers for graphene filled in polymer composites. In addition to
surfactants, some polymers that can form special interactions with graphene were also used as
non-covalent modifiers. Poly(sodium 4-styrenesulfonate) (PSS) was found to be able to
5
prevent reduced graphene oxide from agglomeration upon reduction by hydrazine33, and
could be used to facilitate preparation of graphene through electrolytic exfoliation of graphite,
as the aromatic rings of PSS could form edge-to-face interaction (π-π interaction) with
graphene surface 34. It seems that PSS modified graphenes were often used to fabricate
multilayer films through layer-by-layer technique35-37 but rarely employed to incorporate into
polymer matrix to form conventional composites38. As PSS modified graphenes are only
dispersible in water, it is a challenge to incorporate such graphemes into water insoluble
polymers.
Biodegradable aliphatic polyesters such as poly(lactic acid) (PLA) and poly(ε-
caprolactone) (PCL) have attracted considerable attentions due to their eco-friendly nature39-
44. However, those polymers have more or less disadvantages that restrict their applications.
Incorporation of graphene into biodegradable polymers provides an effective way of
reinforcing their properties45-49. Some papers focusing on PCL/graphene composites have
been published50-52. However, as the graphenes used in those studies were unmodified ones,
which tended to agglomerate in PCL matrix. Therefore, further efforts are still required to
improve the dispersion of graphene in PCL matrix to present high reinforcing efficiency. In
this paper, we modify graphene with PSS by direct ultrasound of water suspension of
graphene in the presence of PSS, then incorporate PSS modified graphene into PCL to form
composites, and finally investigate the effect of loadings of PSS modified graphenes on the
morphology, crystallization behaviors, and mechanical properties of the composites.
2. Experimental Section
6
2.1. Materials
Poly(ε-caprolactone) (PCL, Esun500C) with molecular weight of 5.0 104 g/mol was ×
purchased from Guanghua Weiye industrial co., LTD (Shenzhen, China). Graphene
nanosheets (GNSs) (NO: XF001W) was procured from XF NANO Materials Tech Co., Ltd.
(Nanjing, China). According to the manufacture claimed, the GNS was prepared by physical
methods and has diameter of 0.5~2 um, thickness of ~0.8 nm, and single layer ratio of ~80%.
Poly(sodium 4-styrenesulfonate) (PSS, MW: 70000) with 21 wt % aqueous solution was
obtained from Micxy Chemical Co., Ltd (Chengdu, China). Tetrahydrofuran (THF) and other
chemicals with AR grades were obtained from Kelong Chemical Co., Ltd (Chengdu, China).
All of the raw materials were used without further purification.
2.2. Modification of GNS with PSS
Modification of GNS with PSS was carried out by ultrasound of GNS water suspension
containing PSS at room temperature for 30 min. The weight ratio of GNS to PSS was 1:5,
and the concentration of GNS in water was 2.0 g L-1. The detailed description for the
modification procedure is as follow: GNS water suspension was prepared by addition of
0.40g GNSs, 9.52g PSS solution, and 190.18g H2O into a 500 mL beaker; the suspension was
then sonicated with a probe sonicator (SCIENTZ-IID, Ningbo China) for 30 min to form a
uniform dispersion. The dispersion was used directly for preparation of PCL/GNS composites.
For comparison, GNSs water dispersion without PSS was also prepared by the similar
processing.
2.3. Preparation of PCL/GNS composites
PCL/GNS composites with GNS loading varying from 0.05 to 1.0 wt% were prepared
7
through a solution coagulation technique. THF was used to dissolve PCL and the
concentration was ~5% (g/vol). Taking preparation of the composite with GNS loading of 0.1
wt% for an example, the procedures were: 9.99g PCL was dissolved in 200 mL THF with
mild stirring at 50 °C for 2 hr; after cooling down to room temperature, 5 g of above prepared
GNS dispersion was dropped into the strongly stirred PCL/THF solution, then excessive H2O
was dropped to coagulate the composites, which were collected by filtration, washed with
ethanol, and dried in a 50 °C air blast oven to evaporate solvents, and finally vacuum dried at
50 °C for 48 hr to remove any residual solvent. For convenience, we abbreviated the
composite to PCL/GNS-x, where x represents the loading of the GNS. For example,
PCL/GNS-0.1 indicates the composite containing 0.1 wt% GNS. For comparison, neat PCL
was also processed with the similar procedures. The sample sheets with thickness of 1 mm
were prepared by hot pressing for further characterization.
2.4. Characterization
The morphologies of original GNS and PSS modified GNS were observed on a JEM-2100
transmission electron microscope (TEM) an accelerating voltage of 200 kV. Samples were
prepared by depositing a drop of the water dispersion onto a copper micro grid and then
vacuum dried at 80 °C for 24 hr.
The morphologies for the cryo-fractured surfaces of PCL/GNS nanocomposites were
observed by a XL-30s FEG (Philips, Holland) scanning electron microscope (SEM) with an
accelerating voltage of 5 kV. The fractured surface was sputtered with a layer of gold prior to
observation.
The dispersion of PSS modified GNS in PCL matrix was observed by a JEM-2100F
8
transmission electron microscope with an accelerating voltage of 200 kV. Ultrathin sections
of ca. 70-80 nm in thickness were sliced using a Leica EM FC6 cryo-ultramicrotome.
Thermal and crystallization behaviors of neat PCL and its composites were investigated
by a NETZSCH DSC-214 differential scanning calorimeter. The samples with ~6 mg in
aluminum pans were first melted at 80 °C for 3 min to remove thermal history, then cooled to
-60 °C at a scanning rate of 10 °C/min, and finally reheated to 80 °C at the same scanning
rate. All the operations were carried out under N2 atmosphere. Both the cooling and the
second heating scans were recorded for data analysis.
Isothermal crystallization kinetics was conducted on the NETZSCH DSC-214 differential
scanning calorimeter. The samples with ~6 mg in aluminum pans were first melted at 80 °C
for 3 min to remove thermal history, and then quickly cooled to 44 °C at a cooling rate of 60
°C/min, and finally kept at 44 °C until crystallization completed. The operations were carried
out under N2 atmosphere. The crystallization exothermal curves were recorded for analysis.
Spherulitic morphologies of neat PCL and its composites were investigated by a NIKON
ECLIPSE LV100POL polarizing optical microscope with an HSC621V temperature
controller. The sample films in two microscopic cover glasses were first melted at 80 °C for 3
min to remove any thermal history and then rapidly cooled down to 40 °C and kept at this
temperature until crystallization finished.
X-ray diffraction patterns of neat PCL and its composites were recorded with an X-ray
diffractometer (Philips X’Pert X-ray diffractometer) with Cu Kα radiation. The equipment
was operated at room temperature with a scanning rate of 2 °/min scanning from 5 to 40°.
The mechanical properties neat PCL and its composites were measured on a Sansi
9
CMT6503 Universal Testing Machine at a crosshead speed of 50 mm/min at room
temperature. The dumbbell-shaped specimen with respective width and thickness of 4 and 1
mm were used. The length between the two mechanical grips of the testing machine was 25
mm. At least five specimens were tested for each sample, and the averaged result was
reported.
Thermo-mechanical properties of neat PCL and its composites were tested on a TA DMA
Q800 dynamic mechanical analyzer using a tensile mode. Tests were performed from -70 to
40 °C at a heating rate of 3 °C/min and an oscillation frequency of 1 Hz.
3. Results and Discussion
3.1.Preparation and Morphologies of PSS modified GNS reinforced PCL composites
GNSs were modified with PSS under the aid of ultrasound prior to fabrication of PCL/GNS
composites. The modification was done by probe ultrasonication of GNS/PSS (w/w, 1/5)
water dispersion for 30 min. To make a comparison, water dispersion of original GNS
(OGNS) was also treated with the same procedure. Figure 1a shows the digital photos of
water dispersions of OGNS and PSS modified GNS (MGNS). It seems that homogeneous
water dispersions were obtained for both GNSs. However, the GNS particles adhered to the
wall of bottle revealed the poor dispersion of OGNS in water. In contrast, the wall for the
bottle that contained PSS modified GNS was very cleaning, indicating good dispersion of
modified GNS in water. After placement for 24 hr, original GNS went down to the bottom of
the bottle as shown in Figure 1b, revealing the instability of OGNS water dispersion, which
was ascribed to the aggregation of graphene sheets arisen from their strong π-π interaction. In
10
the case of PSS modified GNS, no obvious precipitate could be observed, indicative of good
stability of the dispersion. The ability of PSS to stabilize GNS water dispersion was attributed
to the formation of π-π interaction between benzene rings of PSS and graphene sheet, which
consequently reduced the interactions among graphene sheets and thus prevented them from
aggregation. The morphologies of OGNS and MGNS were characterized by TEM. Figure 1c
and d shows the TEM images of OGNS and MGNS, respectively. Original GNS showed
typical characteristic of graphene sheets, which aggregated strongly. In addition, many folds
could be found. For PSS modified GNS, although it was hard to detect the surface wrapped
PSS, the aggregation of graphene sheets was reduced significantly and the density of folds
was much lower than that of OGNS, suggesting that the presence of PSS was very efficient to
stop graphene sheets from agglomeration. The homogeneous and stable MGNS dispersion
should be used as a useful material to incorporate GNS into polymer matrix via a solution
compounding technique to achieve a well-dispersed morphology and improved physical
properties.
As PSS modified GNS is only dispersible in aqueous medium due to the hydrophilic
nature of PSS, it seem hard to incorporate it into water insoluble or dispersible polymers due
to their different solubility. However, solution coagulation provides an alternative way of
compounding PSS modified GNS with water insoluble polymers if they can dissolve in a
solvent that is miscible with water. Therefore, we incorporated PSS modified GNS into PCL
matrix which was dissolved in THF. A series of PSS modified GNS filled PCL composites
containing various loadings of GNS were prepared through solution coagulation. For brevity,
the composite was abbreviated as PCL/GNS-x, where x represents the loading of GNS in
11
percentage. For example, PCL/GNS-0.05 represents PSS modified GNS filled PCL
composite that contains 0.05 wt% GNS. Due to the poor dispersity of original GNS in water,
we did not prepare original GNS filled PCL composite.
It is well-known that the homogeneous dispersion of GNS in polymer matrix plays an
important role in determining final properties of the composite. The morphologies of neat
PCL and its composites were observed by SEM. Figure 2 shows the SEM images of cryo-
fractured surfaces of neat PCL, PCL/GNS-0.05, PCL/GNS-0.5, and PCL/GNS-1.0. The cryo-
fractured surface of neat PCL as shown in Figure 2a was smooth, while those of PCL/GNS
composites became more buckling with increasing loading of PSS modified GNS. However,
it is worth noting that neither apparent agglomeration nor pulling-out of GNS from PCL
matrix was observed when the loading of GNS was not more than 0.5 wt% (Figure 2b and c),
indicating a strong interfacial interaction between PSS modified GNS and PCL matrix, which
could be attributed to the hydrophilic character of PCL and PSS modified GNS. It is worth
noting that although PCL is water insoluble, it is hydrophilic polyester as it can absorb up to
1.0 wt% water molecules53. When the content of GNS increased to 1.0 wt%, the fractured
surface of the composite showed some aggregations due to agglomeration of GNS as shown
in Figure 2d.
TEM was employed to further observe the effect of loadings on the dispersion state of
PSS modified GNS in PCL matrix. Figure 3 shows the TEM images of PCL/GNS-0.5 and
PCL/GNS-1.0. The low magnified image of microtomic specimen showed that GNS
dispersed uniformly in PCL matrix for PCL/GNS-0.5 (Figure 3a). In the case of PCL/GNS-
1.0, some obvious aggregations of GNS could be observed, as shown in Figure 3c. The high
12
magnified TEM images (Figure 3b and d) revealed that GNS showed a characteristic worm-
like morphology with some folds in PCL matrix. Agglomeration of GNS was not found for
PCL/GNS-0.5 (Figure 3b) but detected for PCL/GNS-1.0 (Figure 3d), which is in agreement
with the results observed by SEM.
XRD is a good tool to detect the stacking structure of graphenes in the nanocomposites54.
The diffraction peak for layer-to-layer distance of graphite usually existed at around 2θ of 10°
55, 56. Figure 4 shows the XRD diffractions of neat PCL and its nanocomposites. It is clear that
no diffraction peak can be observed below 2θ of 15° for all samples, which indicate that the
stacking structure of the GNS powders was disordered in the PCL composites55. Neat PCL
showed three typical diffraction peaks at 2θ = 21.27, 21.87, and 23.55°, corresponding to
(110), (111), and (200) planes52, respectively. It is worth noting thatthe composites also
showed the same three diffraction peaks, indicating that the incorporation of PSS modified
GNS does not change the crystal structure of PCL.
3.2. Thermal and crystallization behaviors
DSC was used to study the effect of GNS loadings on the thermal and crystallization
behaviors of PCL/GNS composites. Figure 5 shows the DSC cooling scans and the second
heating scans of the samples at scanning rates of 10 °C/min. All the samples crystallized
during cooling scan. Neat PCL shows a wide crystallization exothermic peak with a
crystallization temperature (Tc) of 25.5 °C. The crystallization exothermic peak was narrowed
by incorporation of PSS modified GNS and shifted to higher temperature range, indicating
improved crystallization rate possible due to enhanced nucleation efficiency with addition of
13
GNS. For the effect of GNS loadings on the crystallization of the composites, it is interesting
to find that the Tc of PCL increased significantly to 33 °C with addition of only 0.05 wt%
GNS, suggesting good nucleation efficiency. Similar improvement in crystallization
temperature was obtained with addition of about ten times (0.5 wt%) amount of unmodified
reduced graphene oxide, as reported by Wang et al50 and zhang and Qiu52. With further
increasing GNS loadings, the Tc increased slightly. The values were 33.3, 34.4, 35.3, and 35.8
°C for PCL/GNS-0.1, PCL/GNS-0.3, PCL/GNS-0.5, and PCL/GNS-1.0, respectively. The
enthalpies of crystallization for all samples were around -60 J g-1. It is noted that glass ∙
transition of PCL could not be observed from the second heating scans, which was ascribed
to the fast crystallization rate of the samples and the limitation for the mechanical
refrigeration system of DSC 214. All the samples showed almost the same melting
temperatures of around 58 °C and the same fusion enthalpies of around 60 J g-1, which ∙
indicated that the incorporation of GNS did not change the crystal structure and the degree of
crystallinity of PCL. From those results, we can conclude that the incorporation of PSS
modified GNS did not change the melting temperature and degree of crystallinity of PCL but
accelerated the crystallization rate of PCL by nucleation.
In order to study the effect of GNS loadings on the crystallization behaviors of PCL
composites in detail, the isothermal crystallization of neat PCL, PCL/GNS-0.05, PCL/GNS-
0.5, and PCL/GNS-1.0 at 44 °C were carried out and analyzed by Avrami equation. Figure 6a
shows the development of relative crystallinity (Xt) with crystallization time at crystallization
temperature of 44 °C for neat PCL and its composites. Neat PCL finished crystallization in
~45 min. Incorporation of 0.05 wt% GNS, the crystallization developed quickly and finished
14
in ~7 min. When the loading of GNS increased to 0.5 wt%, the time required to complete
crystallization further reduced to 3.3 min. While with further increased loadings of GNS to
1.0 wt%, the time only reduced to 3.2 min. The results indicate that the crystallization rate of
PCL increased significantly with increasing GNS loading within 0.5 wt% while almost
remained with further increasing GNS loading.
The isothermal crystallization kinetics of neat PCL and its composites were analyzed by
the Avrami equation57-59:
1 ‒ 𝑋𝑡 = 𝑒𝑥𝑝( ‒ 𝑘𝑡𝑛)
where Xt represents the relative crystallinity at time t, k is a crystallization rate constant
depending on nucleation and crystalline growth rate, and n is the Avrami exponent which
denotes the nature of the nucleation and growth process. Double logarithm of Avrami
equation gives rise to:
𝑙𝑜𝑔[ ‒ ln (1 ‒ 𝑋𝑡)] = log 𝑘 + 𝑛 𝑙𝑜𝑔 𝑡
A plot of versus log t would give a straight line from which both the rate 𝑙𝑜𝑔[ ‒ ln (1 ‒ 𝑋𝑡)]
constant and the Avrami exponent can be derived. Figure 6b shows the Avrami plots of neat
PCL and its composites at crystallization temperature of 44 °C. The respective n value for
neat PCL, PCL/GNS-0.05, PCL/GNS-0.5, and PCL/GNS-1.0 was 1.83, 2.32, 2.39, and 2.38.
The results reveal that polymeric chains in both neat PCL and its composites adopt a two-
dimensional crystallization growth with a heterogeneous type of nucleation, in agreement
with other studies focused on crystallization kinetics of PCL60-62. The crystallization rate
constants were 3.9 -3, 5.68 -2, 0.31, and 0.37 min-n. It seems unreasonable to 0 × 10 × 10
compare the crystallization rates of the samples from their rate constants since the n values
15
were also changing. The reciprocal of half-time of crystallization (1/t1/2) can be used directly
to describe the overall crystallization rate of the sample. The 1/t1/2 values of neat PCL for
isothermal crystallization at 44 °C was only 0.058 min-1. The overall crystallization rate was
increased by 5.8 times to 0.336 min-1 upon addition of only 0.05 wt% GNS. When 0.5 and
1.0 wt% GNSs were incorporated, the overall crystallization rates were increased by 12.2 and
13.3 times, with the values of 0.709 and 0.769 min-1, respectively.
It is well-known that the crystallization rate of a polymer is dependent on the nucleation
density and crystal growth rate. In this study, the incorporation of GNS is unable to increase
the crystal growth rate of PCL as the layered structure of GNS may confine the mobility of
polymer chains63. Thus the improved crystallization rate could only be ascribed to the
improved nucleation density by incorporation of GNS. POM was used to observe the
crystalline morphologies of neat PCL and its composites. Figure 7 shows the spherulitic
morphology of neat PCL and its composites formed by isothermal crystallization at 40 °C.
Large spherulites with low nucleation density were observed for neat PCL. The size of
spherulites decreased while the number of spherulites increased significantly with addition of
0.05 wt% GNS, indicating significantly improved nucleation density. With further increasing
loadings of GNS to 0.5 and 1.0 wt%, a large number of small-sized crystals were formed the
regular spherulites were hard to distinguished, revealing further increased nucleation density.
From above discussion, we can conclude that incorporation of PSS modified GNS was
capable of accelerating crystallization rate of PCL by improving nucleation density without
changing the crystallization mechanism.
3.3. Mechanical properties
16
GNSs with unique mechanical strength and modulus can be used as ideal fillers for
reinforcing mechanical properties of polymer matrix. The static mechanical properties of neat
PCL and PCL/GNS composites were measured by tensile test to study the effect of GNS
loadings on the yield strength, Young’s modulus, and elongation at break of PCL composites.
The results are summarized in Table 1. The yield strength and Young’s modulus of neat PCL
were 17.2 and 298.2 MPa, respectively. Both the yield strength and the Young’s modulus
showed uptrends with increasing GNS loading within 0.5 wt%, revealing reinforcement. Both
yield strength and Young’s modulus of PCL were improved by ~12 % when the loading of
GNS was only 0.5 wt%. PSS modified graphenes showed better reinforcing efficiency in
yield strength than unmodified reduced graphene oxides (RGOs) with same loading,
incorporation of 0.5 wt% RGOs cause 1.1 MPa in yield strength of PCL, as reported by
Wang et al50. Furthermore, it is worth noting that the elongation at break was not reduced but
also increased when the loading of GNS was not more than 0.5 wt%. The improvement in the
mechanical properties should be ascribed to the good dispersion of PSS modified GNS in
PCL matrix as well as the strong interfacial adhesion between PSS modified GNS and PCL
matrix which is helpful for stress transfer from low strength PCL matrix to high strength
GNS fillers to reinforce the mechanical properties of the composites. When the loading of
GNS increased to 1.0 wt %, although the Young’s modulus were further increased, the yield
strength was reduced slightly and elongation at break was dropped considerable, which might
be ascribed to the aggregation of GNS as discussed in Section 3.1.
Besides static mechanical properties, dynamic mechanical properties were also
investigated to evaluate the effect of GNS loading on the storage modulus of the composites.
17
The dynamic storage moduli of neat PCL and its composites were measured by dynamic
mechanical analyzer. Figure 8a shows the dynamic storage modulus (E’) as a function of
temperature for neat PCL and the composites. It can be seen from the storage modulus plots
that all samples showed α-relaxations at around -40 °C (i.e., glass transition temperature), and
corresponding relaxation peak can be observed on the tan delta plots (Figure 8b). Drastic
drop in storage modulus occurred at around glass transition temperature of PCL. It is worth
noting that all PCL/GNS composites showed higher storage moduli than neat PCL in the full
temperature range, indicating reinforcement by incorporation of PSS modified GNS. The
loading of GNS plays an important role in the storage modulus of the composite, which
showed uptrend with increasing GNS loading. For example, the E’ value below glass
transition (~-40 °C) for neat PCL at -60 °C was 2633 MPa. With only 0.05 wt% GNS
incorporated, the value increased to 2883 MPa; and with the GNS loadings increased to 0.5
and 1.0 wt%, the storage modulus further increased to 3129 and 3214 MPa, improving by
18.8% and 22.1%, respectively, compared to neat PCL. The E’ values above glass transition
such as at 0 °C were 600 MPa for neat PCL, which increased to 651, 721, and 830 MPa for
PCL/GNS-0.05, PCL/GNS-0.5, and PCL/GNS-1.0, showing improvement in percentage of
8.5, 20.2, and 38.3%, respectively.
4. Conclusions
Ultrasonication of GNS in the presence of PSS water solution gives rise to a homogeneous
and stable dispersion, which could be then used to compound with PCL through a solution
coagulation technique to prepare PCL/GNS nanocomposites. It was found that GNS
18
dispersed almost uniformly in PCL matrix without apparent aggregation when the loading of
GNS was not more than 0.5 wt% and showed strong interfacial adhesion with PCL. However,
when the loading increased to 1.0 wt%, aggregation of GNS occurred. The crystallization of
PCL was accelerated significantly by incorporation of PSS modified GNS. Addition of only
0.05 wt% GNS caused 5.8 times improvement in overall crystallization rate, which increased
by 12.2 times with GNS loading increased to 0.5 wt%. While with further increasing GNS
loading the improving extent in crystallization rate became very small. The acceleration in
crystallization was ascribed to the significantly improved nucleation density by incorporation
of GNS. The tensile strength and Young’s modulus increased gradually with increasing PSS
modified GNS loading within 0.5 wt%, and the elongation at break did not reduce but
increased, due to the good dispersion and strong interfacial adhesion of the composites. But
when the loading of GNS increased to 1.0 wt%, the tensile strength and elongation at break
deteriorated considerably ascribing to the aggregation of GNS. The incorporation of PSS
modified GNS resulted in reinforced dynamic storage modulus in the full temperature range.
PCL/GNS-0.5 showed ~18% and ~20% improvement in storage modulus below Tg (-60 °C)
and above Tg (0 °C), respectively, compared to neat PCL.
Acknowledgements
This work was supported by Fundamental Research Funds for the Central Universities
(XDJK2015C022).
References
19
1. K. S. Novoselov, V. I. Fal'ko, L. Colombo, P. R. Gellert, M. G. Schwab and K. Kim,
Nature, 2012, 490, 192-200.
2. A. S. Mayorov, R. V. Gorbachev, S. V. Morozov, L. Britnell, R. Jalil, L. A.
Ponomarenko, P. Blake, K. S. Novoselov, K. Watanabe, T. Taniguchi and A. K. Geim,
Nano Lett., 2011, 11, 2396-2399.
3. S. V. Morozov, K. S. Novoselov, M. I. Katsnelson, F. Schedin, D. C. Elias, J. A.
Jaszczak and A. K. Geim, Phys. Rev. Lett., 2008, 100.
4. C. Lee, X. Wei, J. W. Kysar and J. Hone, Science, 2008, 321, 385-388.
5. F. Liu, P. Ming and J. Li, Phys. Rev. B, 2007, 76.
6. A. A. Balandin, Nat. Mater., 2011, 10, 569-581.
7. J. S. Bunch, S. S. Verbridge, J. S. Alden, A. M. van der Zande, J. M. Parpia, H. G.
Craighead and P. L. McEuen, Nano Lett., 2008, 8, 2458-2462.
8. K. S. Novoselov, Z. Jiang, Y. Zhang, S. V. Morozov, H. L. Stormer, U. Zeitler, J. C.
Maan, G. S. Boebinger, P. Kim and A. K. Geim, Science, 2007, 315, 1379-1379.
9. Y. Wang, Y. Huang, Y. Song, X. Zhang, Y. Ma, J. Liang and Y. Chen, Nano Lett., 2009,
9, 220-224.
10. X. Huang, X. Qi, F. Boey and H. Zhang, Chem. Soc. Rev., 2012, 41, 666-686.
11. S. Stankovich, D. A. Dikin, G. H. B. Dommett, K. M. Kohlhaas, E. J. Zimney, E. A.
Stach, R. D. Piner, S. T. Nguyen and R. S. Ruoff, Nature, 2006, 442, 282-286.
12. K. Hu, D. D. Kulkarni, I. Choi and V. V. Tsukruk, Prog. Polym. Sci., 2014, 39, 1934-
1972.
13. T. Kuilla, S. Bhadra, D. Yao, N. H. Kim, S. Bose and J. H. Lee, Prog. Polym. Sci., 2010,
20
35, 1350-1375.
14. Z. Jiang and Z. Qiu, Rsc Adv., 2015, 5, 55486-55491.
15. R. B. Valapa, G. Pugazhenthi and V. Katiyar, Rsc Adv., 2015, 5, 28410-28423.
16. Y. Zhou, H. Xiu, J. Dai, H. Bai, Q. Zhang and Q. Fu, Rsc Adv., 2015, 5, 30912-30919.
17. V. Georgakilas, M. Otyepka, A. B. Bourlinos, V. Chandra, N. Kim, K. C. Kemp, P.
Hobza, R. Zboril and K. S. Kim, Chem. Rev., 2012, 112, 6156-6214.
18. S. Stankovich, R. D. Piner, S. T. Nguyen and R. S. Ruoff, Carbon, 2006, 44, 3342-3347.
19. Y. Sun and C. He, Acs Macro Lett., 2012, 1, 709-713.
20. D. D. Zhang, S. Z. Zu and B. H. Han, Carbon, 2009, 47, 2993-3000.
21. F. Y. Yuan, H. B. Zhang, X. F. Li, H. L. Ma, X. Z. Li and Z. Z. Yu, Carbon, 2014, 68,
653-661.
22. T. Phung Xuan, C. Basavajara, J. K. Kim and D. S. Huh, Polym. Compos., 2012, 33,
2159-2168.
23. P. Ding, S. S. Su, N. Song, S. F. Tang, Y. M. Liu and L. Y. Shi, Carbon, 2014, 66, 576-
584.
24. E. E. Tkalya, M. Ghislandi, G. de With and C. E. Koning, Curr. Opin. Colloid In., 2012,
17, 225-231.
25. J. R. Lomeda, C. D. Doyle, D. V. Kosynkin, W.-F. Hwang and J. M. Tour, J. Am. Chem.
Soc., 2008, 130, 16201-16206.
26. M. Lotya, Y. Hernandez, P. J. King, R. J. Smith, V. Nicolosi, L. S. Karlsson, F. M.
Blighe, S. De, Z. Wang, I. T. McGovern, G. S. Duesberg and J. N. Coleman, J. Am.
Chem. Soc., 2009, 131, 3611-3620.
21
27. K. Zhang, L. Mao, L. L. Zhang, H. S. O. Chan, X. S. Zhao and J. S. Wu, J. Mater. Chem.,
2011, 21, 7302-7307.
28. L. Liu, B. Gao, L. Wu, Y. Sun and Z. Zhou, Chem. Eng. J., 2015, 262, 1187-1191.
29. X. Zhao, Q. H. Zhang, D. J. Chen and P. Lu, Macromolecules, 2010, 43, 2357-2363.
30. M. Moradi, J. A. Mohandesi and D. F. Haghshenas, Polymer, 2015, 60, 207-214.
31. C. F. Matos, F. Galembeck and A. J. G. Zarbin, Carbon, 2014, 78, 469-479.
32. M. E. Uddin, T. Kuila, G. C. Nayak, N. H. Kim, B. C. Ku and J. H. Lee, J. Alloy. Compd.,
2013, 562, 134-142.
33. S. Stankovich, R. D. Piner, X. Q. Chen, N. Q. Wu, S. T. Nguyen and R. S. Ruoff, J.
Mater. Chem., 2006, 16, 155-158.
34. G. Wang, B. Wang, J. Park, Y. Wang, B. Sun and J. Yao, Carbon, 2009, 47, 3242-3246.
35. S. Liu, J. Ou, Z. Li, S. Yang and J. Wang, Appl. Surf. Sci., 2012, 258, 2231-2236.
36. J. Luo, Y. Chen, Q. Ma, R. Liu and X. Liu, Rsc Adv., 2013, 3, 17866-17873.
37. Z. Li, J. Wang, X. Liu, S. Liu, J. Ou and S. Yang, J. Mater. Chem., 2011, 21, 3397-3403.
38. Y. Li, J. Tang, L. Huang, Y. Wang, J. Liu, X. Ge, S. C. Tjong, R. K. Y. Li and L. A.
Belfiore, Compos. Part a-Appl. S., 2015, 68, 1-9.
39. G. C. Liu, Y. S. He, J. B. Zeng, Q. T. Li and Y. Z. Wang, Biomacromolecules, 2014, 15,
4260-4271.
40. G. C. Liu, Y. S. He, J. B. Zeng, Y. Xu and Y. Z. Wang, Polym. Chem., 2014, 5, 2530-
2539.
41. M. Okada, Prog. Polym. Sci., 2002, 27, 87-133.
42. Y.S. He, J.B. Zeng, G.C. Liu, Q.T. Li and Y.Z. Wang, Rsc Adv., 2014, 4, 12857-12866.
22
43. S. Kumar, S. Bose and K. Chatterjee, Rsc Adv., 2014, 4, 19086-19098.
44. C. C. Neikirk, J. W. Chung and R. D. Priestley, Rsc Adv., 2013, 3, 16686-16696.
45. Z. Qiu and W. Guan, Rsc Adv., 2014, 4, 9463-9470;
46. X. Jing and Z. Qiu, Ind. Eng. Chem. Res., 2014, 53, 498-504.
47. X. Jing and Z. Qiu, Ind. Eng. Chem. Res., 2012, 51, 13686-13691.
48. H. Wang and Z. Qiu, Thermochim. Acta, 2012, 527, 40-46.
49. H. Wang and Z. Qiu, Thermochim. Acta, 2011, 526, 229-236.
50. B. Wang, Y. Li, G. Weng, Z. Jiang, P. Chen, Z. Wang and Q. Gu, Compos. Sci. Technol.,
2014, 96, 63-70.
51. B. Wang, Y. Zhang, J. Zhang, H. Li, P. Chen, Z. Wang and Q. Gu, Ind. Eng. Chem. Res.,
2013, 52, 15824-15828.
52. J. Zhang and Z. Qiu, Ind. Eng. Chem. Res., 2011, 50, 13885-13891.
53. P. Musto, M. Galizia, M. Pannico, G. Scherillo and G. Mensitieri, J. Phys. Chem. B 2014,
118, 7414-7429.
54. Z. Wang, J. K. Nelson, H. Hillborg, S. Zhao and L. S. Schadler, Adv. Mater., 2012, 24,
3134-3137.
55. X. Wang, H. Yang, L. Song, Y. Hu, W. Xing and H. Lu, Compos. Sci. Technol., 2011, 72
1-6.
56. X. Lu, J. Huang, L. Yang, N. Zhang, G. Jin and J. Qu, Polym. Adv. Technol., 2014, 25
1515-1522.
57. M. Avrami, J. Chem. Phys. 1939, 7, 1103-1112.
58. M. Avrami, J. Chem. Phys., 1940, 8, 212-224.
23
59. M. Avrami, J. Chem. Phys., 1941, 9, 177-184.
60. G. Siqueira, C. Fraschini, J. Bras, A. Dufresne, R. Prud'homme and M. P. Laborie, Eur.
Polym. J., 2011, 47, 2216-2227.
61. L. N. Luduena, A. Vazquez and V. A. Alvarez, J. Appl. Polym. Sci., 2008, 109, 3148-
3156.
62. M. J. Jenkins and K. L. Harrison, Polym. Adv. Technol., 2006, 17, 474-478.
63. H. D. Huang, J. Z. Xu, Y. Fan, L. Xu and Z. M. Li, J. Phys. Chem. B, 2013, 117, 10641-
10651.
24
Tables and Figures
Table 1. Mechanical properties of neat PCL and its composites.
Sample Yield strength(MPa)
Young’s modulus(MPa)
Elongation at break(%)
Neat PCL 17.2 0.2± 298.2 10.4± 530 17±PCL/GNS-0.05 18.6 0.7± 305.0 4.2± 597 30±PCL/GNS-0.1 18.9 1.1± 309.4 18.8± 584 31±PCL/GNS-0.3 19.0 1.0± 324.6 22.6± 558 22±PCL/GNS-0.5 19.4 0.8± 333.5 3.2± 600 10±PCL/GNS-1.0 17.9 1.3± 360.5 4.2± 10 ± 2
25
Figure 1. Digital photos for the as prepared water dispersions of original GNS (OGNS) and
PSS modified GNS (MGNS) (a) and for the dispersions placed for 24 hr (b); TEM images for
OGNS (c) and MGNS (d).
26
Figure 2. SEM images for cryo-fractured surfaces of neat PCL (a), PCL/GNS-0.05 (b),
PCL/GNS-0.5 (c), and PCL/GNS-1.0 (d).
27
Figure 3. TEM images for PCL/GNS-0.5 (a, b) and PCL/GNS-1.0 (c, d) with low
magnification (a, c) and high magnification (b, d).
28
Figure 4. XRD patterns of neat PCL and its nanocomposites.
29
Figure 5. DSC cooling (a) and the second heating scans (b) of neat PCL and its composites at
scanning rate of 10 °C/min.
30
Figure 6. (a) Development of relative crystallinity with time and (b) Avrami plots for
isothermal crystallization of PCL and its composites at 44 °C.
31
Figure 7. Spherulitic morphologies of neat PCL (a), PCL/GNS-0.05, PCL/GNS-0.5, and
PCL/GNS-1.0 formed by isothermal crystallization at 40 °C.
32
Figure 8. Storage modulus (a) and tan δ (b) versus temperature for neat PCL and its
composites.
Recommended