Composites Science and Technology 75 (2013) 62–69

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Composites Science and Technology

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Effect of interphase and strain-rate on the tensile properties of polyamide 6reinforced with functionalized silica nanoparticles

Haiwen Gu a, Yangbo Guo b, Siew Yee Wong a, Chaobin He a,c, Xu Li a,⇑, V.P.W. Shim b,⇑a Institute of Materials Research and Engineering, A�STAR (Agency for Science, Technology and Research), 3 Research Link, Singapore 117602, Singaporeb Impact Mechanics Laboratory, Department of Mechanical Engineering, Faculty of Engineering, National University of Singapore (NUS), Singapore 117576, Singaporec Department of Materials Science & Engineering, Faculty of Engineering, National University of Singapore (NUS), Singapore 117576, Singapore

a r t i c l e i n f o a b s t r a c t

Article history:Received 30 January 2012Received in revised form 4 December 2012Accepted 6 December 2012Available online 20 December 2012

Keywords:A. Particle-reinforced compositesA. NanoparticlesB. InterphaseB. Mechanical properties

0266-3538/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.compscitech.2012.12.004

⇑ Corresponding authors. Tel.: +65 6874 8421; fax: +65162228; fax: +65 6779145 (V.P.W. Shim).

E-mail addresses: x-li@imre.a-star.edu.sg (X.(V.P.W. Shim).

The effects of the polymer/nanofiller interphase as well as the strain-rate on the tensile behaviors of poly-amide 6 (PA6) nanocomposites were studied. Two types of nano-silica with different surface modification(hexamethyldisilazane and 3-aminopropyltriethoxysilane, denoted as HMDZ and APTES) were used as fil-ler. The tensile responses of all nanocomposites, under strain-rates of 1 � 10�3 and 3 � 102 s�1, wereinvestigated. A Hopkinson Split Tensile Bar (SHTB) device was successfully applied to study the highspeed tensile properties. The experimental results showed that nanocomposites tend to illustrate obvi-ously increased tensile strength and dropped ductility under high speed loading in comparison withlow speed cases. On the other hand, it is observed that the incorporation of amine-functionalizedAPTES–silica can yield simultaneous enhancement of both stiffness (modulus/strength) and ductility(failure strain) under high strain-rate. Further interfacial analysis using X-ray photoelectron spectroscopy(XPS) and Fourier transform infrared spectroscopy (FTIR) indicated that covalent binding of PA6 withamine-featured nano-silica constructs a strong and tough interphase and is the major contributor tothe enhanced mechanical properties. Dynamic reinforcing mechanism was speculated as: the cova-lently-bonded nano-silica may inhibit the propagation of micro-cracks and even pin cracks, thus moreenergy can be absorbed and dissipated effectively.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

High-performance polymer nanocomposites containing rigidinorganic particulates (RIPs) have attracted much interest aslight-weight engineering materials for many modern applications,for example, various automobile components and electronic equip-ments over the past decade [1,2]. It is widely accepted that themechanical performance of polymer nanocomposites dependsgreatly on the dispersion of RIP and the nature of the interphasebetween the polymer matrix and the nanofillers [3,4]. Appropriatesurface modification on RIP can generate a larger interfacial adhe-sion region and strong interaction with the matrix, resulting inimprovement of the material properties [5,6].

Nano-SiO2 is one of commonly used RIP nanofillers in thermo-plastic polymer composites [7,8]. In recent years, the influence ofinterface structure in polyamide/nano-SiO2 systems on resultingmaterial properties has received increasing attention [9–11].Through in situ polymerization, with silica nanoparticles pre-

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65 6872 7528 (X. Li), tel.: +65

Li), mpespwv@nus.edu.sg

treated by coupling agents, Li et al. achieved simultaneousenhancements in the strength and toughness of polyamide 6(PA6) [12]. The enhanced toughness by RIPs was attributed tothe formation of flexible interphase layer. Zhang et al. measuredthe mechanical properties of polyamide 66 composites filled withdifferent surface-modified nano-SiO2 and tried to speculate theeffects of interfacial structure of nano-SiO2 on mechanical behav-ior of composites [13]. It was concluded based on Fourier trans-form infrared spectroscopy (FTIR) analysis that chemical bondingmay have occurred between polyamide 66 molecules and themodified silica during melt compounding, and thus change boththe tensile properties and the impact strength of the composites.However, the dispersion of nano-SiO2 in matrix and the interfa-cial chemical structure are still waiting for further characteriza-tion. Despite the previous efforts, the effects of interphase onthe mechanical behavior of RIP-filled polymer nanocompositeare still not completely understood. More investigation at micro-scopic scales is needed to correlate the interfacial interactionwith the macroscopic mechanical properties.

In another aspect, the tensile properties of nanosilica-reinforcedpolyamide composites have mostly been examined at normalstrain-rate, i.e. quasi-static tension rate. The elastic modulus andtensile strength under quasi-static tension have been found to be

H. Gu et al. / Composites Science and Technology 75 (2013) 62–69 63

enhanced by the addition of well-dispersed nano-SiO2 [14,15].Since polymers and polymer-based nanocomposites are viscoelas-tic and thus rate-sensitive, the investigation into their dynamic re-sponse under different strain-rate is particularly crucial. Regardingthe strain-rate dependent mechanical properties, dynamicalmechanical analysis (DMA) technique is usually applied in themeasurements where the strain-rate applied was extremely slow[16,17]. In practical applications, however, the materials may expe-rience pretty high strain-rate, and therefore the evaluation of highspeed dynamic mechanical behavior is of great interest. From thedesign point-of-view, it is also very important to have a compre-hensive understanding of the effects of strain-rate on the mechan-ical behavior of composite materials. The Split Hopkinson TensileBar (SHTB) method is the most commonly used test method tostudy mechanical behavior at high strain-rate [18,19]. Chen et al.and Hokka et al. studied the dynamic tensile properties of poly(methyl methacrylate) (PMMA), epoxy [19] and polyamide sheets[20] under strain-rate up to 104 s�1. Glass-filled epoxy [21], graph-ite/epoxy composites [22], and fiber reinforced polymer compos-ites [20,23] were also studied using the SHTB devices. It has beenrarely, however, used to perform high strain-rate tensile tests onRIP reinforced polymer nanocomposite due to difficulties to reachstress equilibrium, high noise-to-signal ratios, a short loading timeand so on, that were caused by the bad uniformity of polymernanocomposite and brittle nature of polymer [20]. Beside the reli-able and systematic investigations of the high strain-rate mechan-ical behavior, the study of mechanism of tensile failure underdifferent strain-rate is also lacking and highly desirable.

In present work, two PA6-based nanocomposites containing sil-ica nanoparticles of similar size (50 nm) but with different surfacemodification (hexamethyldisilazane and 3-aminopropyltriethoxy-silane, denoted as HMDZ and APTES, respectively) were preparedthrough melt compounding. The chemical structure of PA6/nano-SiO2 interphase was characterized by X-ray photoelectron spec-troscopy (XPS) and FTIR successfully. The distribution of thenano-SiO2 in matrix was studied by transmission electron micros-copy (TEM). As main objective of present study, the effects of thepresence of two types of nanoparticle on the mechanical propertiesof PA6 were examined quantitatively at engineering strain-ratesranging from 1 � 10�3 to 3 � 102 s�1, both quasi-static and dy-namic tensile responses. A well-developed SHTB device and testingmethod were utilized to obtain tensile stress–strain data based onour previous research publication [24]. At last, the tensile failuremechanism at microscopic scales was discussed considering the ef-fects of both the strain-rate and the nanofiller–matrix interactionon the performance of PA6/nano-SiO2 composites.

2. Experimental

2.1. Materials

Neat PA6 with the trade name of Amilan� CM1014 was suppliedby TORAY Corp. (Japan). Nanosilica (CAB-O-SIL� TG-C413) wasacquired from CABOT Corp. (US) and pre-treated with hexamethyl-disilazane (denoted as HMDZ–silica). Pristine nanosilica sol, NYA-COL� DP5480, was obtained from Nyacol NanoTechnologies Corp.(US). Both nanosilicas have an average diameter of 50 nm. 3-ami-nopropyltriethoxysilane and formic acid were purchased from Sig-ma–Aldrich and used as-received.

2.2. Preparation of amine functionalized nanosilica

Nanosilica functionalized with amine groups was synthesizedas follows. In brief, 10 g of DP5480 nanosilica sol (silica content:30 wt.%) was diluted by 360 mL of ethanol to achieve a suspension.

3-aminopropyltriethoxysilane (100 mg) was then dropped into thenanosilica sol, under stirring at 80 �C. After further stirring for 2 h,the treated nanosilica was centrifuged and washed five times usingethanol to remove the excess silane. The pH value of the 3-amino-propyltriethoxysilane-modified nanosilica suspension, denoted asAPTES–silica, was adjusted to 3 by quickly adding dilute hydro-chloric acid solution (0.01 M) to avoid agglomeration of silicacolloid. Subsequently, the treated nanosilica suspension wasspray-dried using a mini spray dryer (Buchi B290) to obtain a finewhite nanopowder, further dried in a vacuum oven at 110 �C for12 h and stored in desiccator before compounding with PA6.

2.3. Fabrication of silica reinforced PA6 composites

Before melt compounding, the two nanosilicas (HMDZ–silicaand APTES–silica) and PA6 pellets were vacuum dried for 12 h at110 and 80 �C, respectively. PA6 and modified silica were pre-mixed using a Brabender� series 50 mixer at 230 �C operating ata blade speed of 100 rpm. Then the primary composites wentthrough a Prism parallel twin-screw extruder (L/D = 16) at 250–265 �C and a screw speed of 200 rpm. Pure PA6 was extruded fol-lowing the same procedure.

2.4. Extraction of nanosilica from composites

After the nanocomposites were fully dissolved by formic acidinto suspensions, the silica was collected through centrifuging.The procedure was repeated five times to ensure there were no freePA6 molecules retained on the silica particles. The extracted silicawas then washed by large amounts of ethanol and water and driedunder vacuum at 110 �C for 24 h. Extracted silica particles from thePA6/HMDZ–silica and PA6/APTES–silica nanocomposites are de-noted as Ex–HMDZ–silica and Ex–APTES–silica, respectively.

2.5. Characterization

The distribution of silica nanoparticles in PA6 was observedusing a high-resolution transmission electron microscope (PhilipsCM300 FEGTEM). Samples for TEM observation were preparedusing a Leica microtome (Model CM 3050S) and cryo-microtomywas used to prepare thin slices for observation at �100 �C in a li-quid nitrogen atmosphere. Fourier transform infrared (FTIR) spec-tra were obtained via a Perkin–Elmer spectrometer set to thetransmission mode to characterize qualitatively the silica surface.A VG ESCA Lab 220i-XL X-ray photoelectron spectrometer (XPS)was used to investigate the surface composition of the nanosilica.A Malvern ZetaSizer Nano ZS analyzer was employed to measurethe Zeta potential value of the treated silica nanoparticles.

2.6. Mechanical property testing

Before mechanical test, neat PA6 and nanocomposites weredried under vacuum for 8 h at 80 �C and then molded into dumb-bell-shaped specimens (following ISO 527-2-5a standards) for qua-si-static tension tests. An Instron Universal Testing Machine wasemployed to apply tension at a rate of 1.5 mm/min (correspondingto an engineering strain-rate of 1 � 10�3 s�1). Dynamic tensiontests were carried out using an aluminium alloy (12 mm diameter)Split Hopkinson Tensile Bar (SHTB) device, as shown in Fig. 1a. Thespecimen deformation was recorded by a conventional digital vi-deo camera operating at 25 fps for quasi-static tests, and a high-speed camera operating at 100,000 fps for dynamic tests. Theimages were then processed using a motion-tracking software(TEMA) to determine the displacement of the markers, from whichstrains were calculated.

Fig. 1. Schematic diagram of (a) Split Hopkinson Tensile Bar (SHTB) device fordynamic tensile tests, and (b) surface chemical structure of HMDZ- and APTES-modified nanosilicas.

Fig. A1. FTIR spectra of pristine HMDZ–silica and HMDZ–silica treated by HClsolution.

64 H. Gu et al. / Composites Science and Technology 75 (2013) 62–69

3. Results and discussion

3.1. PA6/nano-SiO2 interphase characterization

The definition of ‘interphase’ in polymer/nanofiller systemcould be stated as: the interfacial region surrounding the nanofillercomprises mainly the parent polymer chains adsorbed or bondedon the filler surface and has properties different to those of thebulk matrix [25]. The formation and structure of PA6/nano-SiO2

interphase are closely related with the surface chemical modifica-tion on pristine nanoparticles. In the present study, the trimethyl-silyl of HMDZ and the 3-aminopropyl of APTES were introducedonto the nano-SiO2 particles. Their surface chemical structureswere illustrated in Fig. 1b.

In Fig. 2a, the FTIR spectrum of the APTES–silica displayed typ-ical CAH stretching vibration of methylene groups at 2919 and2850 cm�1, indicating the successful attachment of APTES to thesilica. Meanwhile, the spectrum of the as-received HMDZ–silicadid not show obvious vibration of hydroxyl and methyl groups thatwere explained as result of de-protonized effect in basic environ-ment (hydrogen ions of hydroxyl groups were replaced) and thevery low degree of coverage by trimethylsilyl groups, which wasproved later by XPS result. It was found that the vibration of hydro-xyl groups of HMDZ–silica could be retrieved by treating it withhydrochloric acid (Fig. A1 in Appendix A).

2

HMDZ-

Ex-HM(b)

4000 3500 3000 25004000 3500 3000 2500 2000 1500 1000

HMDZ-silica

Pristine nanosilica

2919 2850

3420

(a) APTES-silica

960

Wavenumber (cm-1) Wavenumb

Fig. 2. FTIR spectra of pristine nanosilica, surface-modified silica (APTES–s

Further characterization of interphase was conducted on theblended silica particles extracted from the polymer matrix usingformic acid, which can still preserve the chemical bonding[15,26]. The FTIR spectra of extracted silica were analyzed andcompared with non-blended silica, as shown in Fig. 2b and c. Thereis no obvious difference between the spectra of the blended andnon-blended HMDZ–silica (Fig. 2b), indicating an absence of chem-ical bond. In Fig. 2c, the presence of characteristic vibrations of PA6molecules, such as NAH stretching at 3303 cm�1, methylene CAHstretching at 2943 and 2869 cm�1, amide C@O stretching at1642 cm�1 and amide NH deformation at 1543 cm�1, indicatesclearly the formation of covalent bonds, i.e. amide bonds betweenthe reactive amine groups from APTES and the carboxyl acid endgroups from PA6 during melt compounding.

The XPS spectra of pristine silica, HMDZ–silica, APTES–silica,and extracted silica particles were recorded as shown in Fig. 3.XPS data refer to Table B1 in Appendix B. The survey spectrum ofthe pristine silica (Fig. 3a) indicates Si and O, as expected for puresilica, whereas the spectrum of HMDZ–silica (Fig. 3b) exhibits Si, O,and C atoms, confirming the incorporation of methyl groups on thesilica surface. The signature of N atoms in the spectrum of APTES–silica (Fig. 3c) demonstrates the effectiveness of the functionaliza-tion reaction of introducing amine groups, which concurs with theFTIR characterization results. The degree of coverage of HMDZ andAPTES were estimated to be 1.9% and 3.3% using atom ratio of C/Siand N/Si, respectively. The Si2p and O1s binding energies of parti-cles are consistent with the range of literature values measured for

1000

Ex-APTES-silica

APTES-silica 3303 2943

2869 1642

1543

(c)

000 1500 1000

silica

DZ-silica

4000 3500 3000 2500 2000 1500

er (cm-1) Wavenumber (cm-1)

ilica and HMDZ–silica) and silica extracted from PA6 nanocomposites.

Fig. 3. XPS spectra of (a) pristine nanosilica; (b) HMDZ–silica; (c) APTES-silica; (d) extracted HMDZ–silica and (e) extracted APTES–silica.

Table B1Elemental quantification data for pristine nanosilica and modified silicas.

Elements Pristine SiO2 HMDZ–SiO2 EX–HMDZ–SiO2 APTES–SiO2 EX–APTES–SiO2

Atom (%) BEa (eV) Atom (%) BE (eV) Atom (%) BE (eV) Atom (%) BE (eV) Atom (%) BE (eV)

C – – 2.1 285 2.5 285/287b 2.4 285 57.4 285/286/288c

N – – – – 0.2 400.0 1.2 399.8 9.2 399.8O 62.6 533.0 60.7 533.3 63.4 532.9 60.5 532.8 23.3 531.6/533.0d

Si 37.4 103.4 37.2 103.6 33.9 103.2 35.9 103.1 10.1 103.4

The binding energies used as references are: pristine nanoparticles, Si2p 103.4 eV, O1s 533.0 eV; HMDZ–silica, Si2p 103.6 eV, O1s 533.3 eV, C1s 285.0 eV; APTES–silica, Si2p103.1 eV, O1s 532.8 eV, C1s 285.00 eV, N1s 399.8 eV.

a BE: binding energy.b 285 eV (C1s C@C/CAH); 287 eV (C1s CASi).c 285 eV (C1s C@C/CAH); 286 eV (C1s CAN); 288 eV (C1s C@O).d 531.6 eV (O1s C@O); 533.0 eV (O1s SiAO).

H. Gu et al. / Composites Science and Technology 75 (2013) 62–69 65

SiO2 [14]. The N1s binding energy of the functionalized nanoparti-cles is consistent with N in an amine-type environment [14].

Compared with non-blended HMDZ–silica, the elemental quan-tification of extracted HMDZ–silica (Ex–HMDZ–silica) shows noobvious change, indicating again that there were no chemicalbonds formed within its interphase layer. On the other hand, thesignificant increase of N and C elements in extracted APTES–silica(Ex–APTES–silica) as shown in Fig. 3e, and a N/C atom ratio ofabout 1:6, which corresponds to the N/C atom ratio in structuralunits of PA6 (ANHCH2CH2CH2CH2CH2COA), indicate the existenceof PA6 molecules linked to the surface of blended APTES–silica. Thevalue of N/Si atom ratio increased greatly from 3.3:100 (of APTES–silica) to 91.1:100 (of extracted APTES–silica), which suggest theformation of very dense PA6 interfacial region. In addition,brand-new C1s(C@O), N1s and O1s(C@O) values peaking at 288,399.8 and 531.6 eV, further substantiate that the interface layercomprised PA6 molecules covalently bonded to the silica surface.

3.2. Strain-rate and interphase dependence of tensile properties

The stress–strain curves for PA6/silica nanocomposites are de-picted in Fig. 4 as under different strain-rate. The resulting dataclearly indicates that all materials show much higher tensile

strengths but significantly lower ductility (failure strain) under dy-namic tension, compared to their quasi-static tensile properties.Maximum tensile strength of pure PA6, for example, increasesfrom 68.3 ± 0.59 to 113.5 ± 5.80 MPa (66% rise), as strain rate wasincreased. The increases of tensile strength for all nanocompositeswere found from 52% to 64%. This typical rate dependence agreeswith expectation for conventional viscoelastic materials [27]:tested under high speed loading, ‘‘soft and flexible’’ polymer mate-rials become relatively ‘‘hard and tough’’ due to the insufficientrelaxation time for polymer chains. To our surprise, however, thecomposite containing APTES–SiO2 (5 wt.%) demonstrates muchbetter performance in high strain-rate regime. Its failure straindrops only 18% at dynamic strain-rate, which is much better thanpure polymer (60% drop) and other composites (45–60% drop).Moreover, it can absorb 13.2 ± 0.28 MJ/m3of strain energy atfailure, which is much higher in contrast to the value of7.5 ± 0.60 MJ/m3 for pure PA6.

Except the rate dependence, mechanical behaviors of PA6 nano-composites also show dependence on quality of the polymer ma-trix/nano-SiO2 interphase. According to previous characterization,a dense PA6 layer covered the APTES-modified nanosilica surfacethrough covalent bond. There is no covalent connection in caseof HMDZ–nanosilica, and the adjacent polymer molecules mainly

(a)

(c)

(b)

(d)

2

4

1

3

2 1

4 3

2

4 3

1

4 2

1 3

2 3

2 1

1

4

4 3 1 2

2

4

1

3

3 4

Fig. 4. Stress–strain curves for neat PA6 and PA6/silica nanocomposites with various silica concentrations. (a–d) Correspond to quasi-static and dynamic tensile conditions,respectively.

66 H. Gu et al. / Composites Science and Technology 75 (2013) 62–69

adhered on the surface via weak-interaction, for instance, hydro-gen bond. Based on our results, it was observed that such differentinterphases do result in varied tensile responses.

Under quasi-static tension, the APTES nano-SiO2 is able to im-prove the elastic modulus of PA6 up to 13%. With the increase ofsilica concentration, the APTES nano-SiO2 tends to enhance thestiffness (modulus and tensile strength) of PA6 at much greater de-gree than HMDZ nano-SiO2 does, as shown in Fig. 5a and b. Simul-taneously, the composites filled with APTES nano-SiO2 showapparently better ductility than HMDZ ones. As seen in Fig. 5d, neatPA6 and composite bearing HMDZ–SiO2 (5 wt.%) fracture at 21.6%and 8.5% (strain values), respectively; while the material withAPTES–SiO2 (5 wt.%) breaks at 15.5%. The strain energy at failure,i.e. the area under the stress–strain curve up to the point of failure,is an indication of the energy absorption capability of a materialunder stress. Although both nano-SiO2 can enhance the tensilestrength, the strain energy at failure in case of APTES–SiO2 filledcomposites is obviously superior (Fig. 5f). Typically, the strain en-ergy at failure of PA6/APTES–SiO2, at silica content of 1, 3 and5 wt.%, increased by 92%, 111% and 106%, respectively, on basisof the values of their counterparts filled with HMDZ–SiO2.

Under dynamic tension, the tensile strength of both compositesshows different trend. Fig. 5c demonstrates that strength of bothcomposites decreases slightly at 1 wt.% silica content, however,the APTES–SiO2 filled composites ascend quickly with increase ofsilica content, in the other side, comparatively slow increase canbe found for HMDZ–SiO2. The similar trend could be observed interms of the failure strain, as shown in Fig. 5e, and the strain en-ergy at failure, Fig. 5g. But it is deserved to notice that only thecomposites containing APTES–SiO2 present better ductility (maxi-mum increment of failure strain: 46%) and more strain energyabsorption (maximum increment: 76%), compared to pure PA6.Moreover, on basis of data of PA6/HMDZ–SiO2 nanocomposites,the values of failure strain energy for PA6/APTES–SiO2 nanocom-posites increased by 278%, 260% and 181% that corresponding todifferent silica content of 1, 3 and 5 wt.%, respectively. Compara-

tively, the introduction of HMDZ–SiO2 is unable to improve themechanical properties of PA6 under both low and high strain-rate,as APTES–SiO2 does (all mechanical data refer to Table C1 inAppendix C).

3.3. Reinforcing mechanism

What is the mechanism dominating the enhanced mechanicalperformance by the amine-functionalized nanosilica under dy-namic tension? Refer to published work [2], the reinforcement byincorporation of stiff nanoparticles has been articulated as the re-sult of strong interfacial bonding between the parent polymer andthe particles. Typically, the degree of bonding is also influenced bythe dispersion of particles in the polymer matrix. A homogeneousdispersion will increase the effective silica–polymer interfacialarea and benefit strong interaction. In addition, good dispersionalso reduces large and dense silica agglomeration, which tends toact as defects, constituting stress concentrations under appliedloads, which facilitate the triggering of micro-cracks and their pro-gression to material failure.

The pure APTES–silica and HMDZ–silica were spherical withuniform size distribution (see in Fig. D1 in Appendix D). After beingcompounded into PA6, the HMDZ–silica nanoparticles were foundto be uniformly spread throughout the PA6 composite, proved byTEM observations (Fig. 6a–d). In contrast, the PA6/APTES–silicacomposites displayed imperfect dispersion gradually as increaseof silica content, as evidenced by Fig. 6e–g, which show the forma-tion of agglomerates. The agglomeration of the APTES–silica wasdue to its huge surface energy, which is typical for nanoparticles.In the other side, the perfect distribution of HMDZ–silica in PA6may result from its strong electrostatic repulsion between theHMDZ–silica nanoparticles. Zeta potential measurement indicateda negatively charged surface characteristic of the HMDZ–silica(n = �6.49 eV, in ethanol).

Comparing PA6/HMDZ–silica composites to PA6/APTES–silicacomposites, the latter has inferior dispersion but presents better

Fig. 5. Quasi-static and dynamic tensile properties of PA6/APTES–SiO2 and PA6/HMDZ–SiO2 composites as a function of nano-SiO2 concentration. (a, b, d and f) and (c, e and g)correspond to quasi-static and dynamic testing, respectively.

Table C1Mechanical properties of pure polyamide 6 and its nanocomposites reinforced with HMDZ- and APTES–silica, respectively.

Filler Quasi-static Dynamic

Type Content (wt.%) Ea (GPa) rTb (MPa) ef

c (%) efd (MJ/m3) E (GPa) rT (MPa) ef (%) ef (MJ/m3)

No filler 3.25 (±0.074) 68.3 (±0.59) 21.6(±1.47) 13.5 (±0.93) – 113.5 (±5.80) 8.7 (±0.07) 7.5 (±0.60)HMDZ–silica 1 3.30 (±0.040) 71.3 (±0.42) 10.1 (±0.71) 6.2 (±0.57) – 108.9 (±5.80) 3.9(±0.57) 2.7 (±0.64)

3 3.38 (±0.118) 71.6 (±0.85) 9.3 (±0.07) 5.7 (±0.14) – 110.2 (±3.60) 3.7 (±0.93) 2.0 (±0.15)5 3.46 (±0.031) 71.8 (±1.10) 8.5 (±0.51) 5.1 (±0.45) – 117.4 (±4.19) 5.7 (±0.52) 4.7 (±0.85)

APTES–silica 1 3.36(±0.082) 71.4 (±0.83) 18.4 (±0.78) 11.9 (±0.66) – 108.6 (±3.27) 10.5 (±1.20) 10.2 (±1.63)3 3.57 (±0.071) 75.3(±0.69) 17.8 (±0.35) 12.0 (±0.17) – 119.9 (±4.80) 7.8 (±1.07) 7.2 (±1.25)5 3.68 (±0.120) 76.4 (±1.91) 15.5 (±3.04) 10.5 (±2.28) – 123.7 (±0.92) 12.7 (±0.28) 13.2 (±0.28)

a E: elastic modulus.b rT: tensile strength.c ef: failure strain.d ef: strain energy at failure (values in brackets indicate standard deviation).

H. Gu et al. / Composites Science and Technology 75 (2013) 62–69 67

Fig. D1. TEM micrographs of, (a) APTES–SiO2, and (b) HMDZ–SiO2.

Fig. 6. TEM micrographs of PA6 nanocomposites incorporated with 1, 3, 5 and 9 wt.% HMDZ–SiO2 (a–d); and 1, 3 and 5 wt.% APTES–SiO2 (e–g), respectively.

68 H. Gu et al. / Composites Science and Technology 75 (2013) 62–69

mechanical properties, whereas the former owns superior disper-sion but shows worse tensile performance. Integrated with previ-ous discussion on mechanical behaviors, it suggests that thestrength of particle–matrix binding within the interphase shouldtake precedence over particle dispersion in terms of enhancementof mechanical properties. Though the APTES-functionalized silicashows agglomeration at higher silica loading, the generation of astrong and tough interphase through covalent bonds improvesboth stiffness and ductility of PA6.

The interesting observation of the inversed effects of APTES–sil-ica particles on ductility, i.e. reduction under quasi-static tensionbut enhancement under dynamic tension, inspired our furtherthinking and speculation on the acting mechanism. There aroundthe amine functionalized silica particles are two possible compet-ing effects as external tension applying: firstly, the presence ofnanoparticles induces stress concentrations which facilitate nucle-ation of micro-cracks; on the other hand, the particles obstruct thepropagation of micro-cracks by inhibiting their growing. Underquasi-static loading, deformation tends to concentrate at those ini-tiative micro-cracks instead of being distributed evenly throughoutthe material. Therefore, the initiation of more micro-cracks is de-layed due to the continuing energy dissipation at the deformation

zones. When strain energy around the existing cracks growsgreatly, significant de-bonding and plastic deformation occur nearthe interface. Hence, the weakening effect of nanoparticles prevailsunder quasi-static tension and accounts for the reduction in ductil-ity. However, under dynamic tension with very high loading speed,deformation occurs sufficiently rapidly and its concentrating atcertain domains is reduced. Since deformation is more evenly dis-tributed, more micro-cracks could be generated. Therefore, thedriving force for growing of crack is thus smaller than that underlow rate (quasi-static one). The ‘‘pinning’’ of the crack by theAPTES–silica nanoparticles which owns strong covalent bondingwith polymer matrix becomes possible. As result, under dynamictension, enhancing effect of ductility by APTES–silica nanoparticlespreponderates, thus generating an increase in ductility.

4. Conclusions

In this study, the quasi-static and dynamic stress–strain behav-ior of series of PA6 composites filled with nanosilica has beendetermined. It is revealed that the mechanical properties of PA6composites are dependent on the quality of polymer/nano-SiO2

H. Gu et al. / Composites Science and Technology 75 (2013) 62–69 69

interphase, as well as on the strain-rate applied. The amine-func-tionalized nano-SiO2 (APTES–nanosilica) is capable to enhanceboth the stiffness and the ductility of PA6 under high strain-rateapplication, though its dispersion in matrix is inhomogeneous withagglomeration. Almost twofold more strain energy can be ab-sorbed thereof before material failure, compared to neat PA6. Char-acterization by XPS and FTIR indicates that a dense interphaselayer comprising PA6 chains has formed through covalent bondon the APTES–silica during melt compounding. This strong cova-lent binding interaction is the major contributor to the enhancedperformance of nanocomposites. Under high strain-rate loading,covalently-bonded nanoparticles may inhibit the propagation ofmicro-cracks and even pin cracks. More energy can thus be dissi-pated effectively and material becomes strong and tough simulta-neously. Therefore, an appropriately-engineered interphase in ananosilica reinforced nanocomposite should not only benefit themechanical properties at low-rate loading, but also yield excellentperformance under high speed loading conditions that such poly-mer-based nanocomposites could be exposed to in common prac-tical applications.

Acknowledgement

This work was made possible by an A�Star SERC Project GrantNo. 092 137 0017, for which the authors are grateful.

Appendix A

See Fig. A.1.

Appendix B

See Table B.1.

Appendix C

See Table C.1.

Appendix D

See Fig. D.1.

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