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

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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.

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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

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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

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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

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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).

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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.