Reinforcement of nylon 6 with functionalized silica nanoparticles for enhanced tensile strength

and modulus

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Nanotechnology 19 (2008) 445702 (7pp) doi:10.1088/0957-4484/19/44/445702

Reinforcement of nylon 6 withfunctionalized silica nanoparticles forenhanced tensile strength and modulusHassan Mahfuz1,8, Mohammad Hasan2, Vinod Dhanak3,Graham Beamson4, Justin Stewart1, Vijaya Rangari5, Xin Wei6,Valery Khabashesku7 and Shaik Jeelani5

1 Department of Ocean Engineering, Florida Atlantic University, Boca Raton, FL 33431, USA2 Department of Mechanical and Industrial Engineering, University of Toronto,ON, M5S 3G8, Canada3 Physics Department, University of Liverpool and Daresbury Laboratory,Warrington WA4 4AD, UK4 National Centre for Electron Spectroscopy and Surface Analysis (NCESS),STFC Daresbury Laboratory, Warrington WA4 4AD, UK5 Tuskegee University’s Center for Advanced Materials (T-CAM), Tuskegee, AL 36088, USA6 Department of Chemistry, Texas Southern University, Houston, TX 77004, USA7 Smalley Institute for Nanoscale Science and Technology, Rice University, Houston,TX 77005, USA

E-mail: hmahfuz@oe.fau.edu

Received 11 June 2008, in final form 29 August 2008Published 30 September 2008Online at stacks.iop.org/Nano/19/445702

AbstractPristine and functionalized silica (SiO2) nanoparticles were dispersed into nylon 6 and drawninto filaments through melt extrusion. The loading fraction of particles in both cases was1.0 wt%. Fourier transform infrared (FTIR) studies revealed that reinforcement of pristine silicananoparticles enhances the bond strength of each of the three basic bonds of nylon 6 namely,hydroxyl, amide, and carbonyl. As a result, the improvement over neat nylon in strength andmodulus was 36% and 28% respectively, without any loss of fracture strain (80%). A silanecoupling agent was then used through wet chemical treatment to functionalize silicananoparticles. Functionalization induced an additional covalent Si–O–Si (siloxane) bondbetween silica particles and nylon backbone polymer while the enhancement in the basic bondswas retained. FTIR and x-ray photoelectron spectroscopy (XPS) studies confirmed theformation of the siloxane bond. This added chemical bond resulted in 76% and 55%improvement in tensile strength and modulus, and still retained 30% fracture strain. Calculationof the upper bound on Young’s modulus indicates that one can reach within 5% of the boundwith pristine silica particles, but it is exceeded by 15% when particles are functionalized.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

Nylon’s toughness, low coefficient of friction, and goodabrasion resistance make it an ideal replacement for a widevariety of applications replacing metal and rubber. The amidegroups of nylon are very polar, and can hydrogen bond with

8 Author to whom any correspondence should be addressed.

each other. Because of these, and because the nylon backboneis so regular and symmetrical, nylons are partially crystalline,and they make very good fiber [1–4]. Nylons are made from amonomer, such as a cyclic amide called lactam. For example,Caprolactam is a lactam with six carbon atoms and nylon madefrom Caprolactam is called nylon 6. When nylon is spun intofibers, the long chain-like macromolecules line up parallel to

0957-4484/08/445702+07$30.00 © 2008 IOP Publishing Ltd Printed in the UK1

Nanotechnology 19 (2008) 445702 H Mahfuz et al

Figure 1. (a) Silane coupling agent and functional groups. It has two classes of functionality, X is a hydrolyzable group typically alkoxy,acyloxy, halogen, or amine. Following hydrolysis, a reactive silanol group is formed. (b) Formation of a Si–O–Si (siloxane) bond on thesubstrate surface. Newly formed silanol can condense with other silanol groups available at the silica surface to form a siloxane linkage.

each other. The amide groups on adjacent chains then formstrong bonds with each other called hydrogen bonds. Thesehydrogen bonds hold the adjacent chains together, makingnylon yarn strong. When nylon 6 polymerizes, the amide linkpresent in Caprolactam (starting monomer for nylon 6) opensup and the molecules join up in a continuous chain, providingan ideal mechanism for interacting with nanoparticles. Onthe other hand, silica particles are formed by strong covalentbonds between silicon and oxygen atoms by sharing theirelectron pairs at the p orbitals. In addition, the surfacebound OH groups on silica surfaces offer an opportunity toform stable bonds with nylon or any functional group duringpolymerization.

In order to utilize the extraordinary strength andstiffness of carbon nanotubes in bulk materials, severalresearchers [5–11] have recently infused CNTs into textilepolymeric precursors and attempted to align the acicularparticles along the length of the drawn filament. Infusion wascarried out either through a liquid route using sonication, or adry route followed by melt mixing in an extruder. Alignmentof CNTs in the filament was enforced by extrusion or spinningfollowed by stretching. The resulting composites either inconsolidated or filament form have no doubt demonstratedimproved mechanical and thermal properties. While it wasencouraging to see phenomenal improvements in strength andstiffness [12], it was also observed that the improvement wasat the cost of sacrificing a significant amount of fracture strainwhich is not attractive for nylon. In an attempt to improveupon the fracture strain, spherical silica nanoparticles, in placeof CNTs, were chosen in the present work. However, theimprovements in mechanical properties with spherical silicaparticles were somewhat less than what we found with CNTs.Functionalization of silica particles was then introduced andit proved to be very effective in enhancing the modulus andstrength.

2. Functionalization of silica nanoparticles

We used organosilanes to modify the surface of silicananoparticles. Organosilanes have the ability to incorporate

both organic- and inorganic-compatible functionality withinthe same molecule. The inorganic compatibility comes fromthe alkoxy groups attached to the silicon atom. This bondis hydrolytically unstable and in the presence of moisturehydrolyzes to an intermediate Si–OH bond which thencondenses with surface bound OH groups on inorganic surfacesto form stable Si–O–Si bonds. The molecular structureof organosilane and its interaction with silica substrates isdepicted in figure 1. The final result of reacting an organosilanewith a substrate is to utilize the substrate to catalyze chemicaltransformations at the heterogeneous interface, ordering theinterface region, and modifying its partition characteristics.Most importantly, it includes the ability to affect a covalentbond between nylon and SiO2 particles.

The silane coupling agent used in this investiga-tion was procured from Gelest (Gelest Inc., East SteelRoad, Morrisville, PA 19067) and it was a diaminefunctional Silane–Trialkoxy (SIA05910), N-(2-aminoethyl)-3-aminopropyltriethoxysilane.

Functionalization of silica particles was performed in thefollowing manner. The weight (g) of silica nanoparticlesthat would be necessary for dispersion into nylon was firstestimated. The total surface area of silica surface was thencalculated from the known specific surface area of silica(440 m2 g−1) and the estimated weight. Since one gram ofsilane was capable of modifying approximately 358 m2 ofinorganic surface, we could easily estimate the amount ofsilane needed for functionalization. Silane was then addedthrough mechanical stirring into a starting mixture of 95%ethanol and 5% water to yield a 2% final concentration ofsilane. Mixing was allowed for a few minutes for hydrolysisand silanol formation. In the next step, nanoparticles wereadded by stirring them into the silane solution for about 2–3 min. After the particles settled at the bottom, the solutionwas decanted. Ethanol and water were dried out by heatingthe particles in a furnace at around 110 ◦C. If particles wereagglomerated, a ball mill was used to bring them back topowder form. Silica nanoparticles were now ready to bedispersed into nylon.

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Nanotechnology 19 (2008) 445702 H Mahfuz et al

Figure 2. Tensile tests of single filaments. The test was carried outon a Zwick-Roel TC-FR 2.5 TN Materials Testing Machine using a50 N load cell. An individual filament was attached to the crossheadsby special 8201 wedge grips. These grips eliminated slippage andstress concentration at the grips. The gage length for the filamentswas 75 mm. The tests were performed at a constant crosshead speedof 1.27 mm min−1. The diameter of each length of filament wasmeasured using scanning electron microscopy (SEM) at severalpoints along the length since it was not constant for any of thematerials, and varied by as much as 5%. An average area estimatedfrom several locations was used in the stress calculations. Thediameter of the filament in each category also varied but was around170 µm. Machine compliance was taken into consideration perASTM D3379-75 while calculating the modulus. About 15individual filaments were tested for each category of materials.

3. Filament extrusion

In the present investigation, we dry mixed SiO2 nanoparticles(pristine/functionalized) with nylon 6 powder. The amountof nanoparticle loading was 1 wt%. The dry-mixed powderwas then melted in a single screw extruder which wasfollowed by distributive mixing, extrusion, stretching, and heatstabilization to continuously draw SiO2-reinforced filaments.In parallel, control filaments were also extruded using identicalprocedures. The dry mixing of nylon powder with SiO2

was performed in a mechanical blender for 3 h. The SiO2

nanoparticles were procured from MTI Corporation (2700Rydin road, unit D, Richmond, CA 94804). The nanoparticleswere 15–20 nm in diameter. Commercial grade nylon 6was procured from UBE Industries, Ltd (UBE building 3-11 Higashi-shinagawwa 2-chome, Shinagawa-Ku, Tokyo 140,Japan). A mechanical crusher was used to produce micron-sized powders of nylon which were then used in the mechanicalblender for mixing with SiO2 nanoparticles. The density ofnylon 6 was 1.07 g cm−3 with a melting point of 215 ◦C. Inorder to eliminate moisture, the nylon and SiO2 mixture wasplaced into a cylindrical drying chamber.

Hot air was supplied to the chamber through an insulatedflexible tube using a vortex blower. The dryer was operatedfor 24 h with the temperature set at 90 ◦C. Prolonged heatingaccompanied with a vortex flow broke up large agglomerates of

Table 1. Tensile moduli and strength from single filament tests.

E-modulus Gain/ Tensile Gain/Material (GPa) loss (%) strength (MPa) loss (%)

Neat nylon 6 1.10 ± 0.10 — 210 ± 10 —SiO2-nylon 6 1.41 ± 0.15 +28 285 ± 8 +361% SiO2-silane 1.70 ± 0.17 +55 370 ± 25 +76Nylon 6

silica particles if any were left after the mechanical blending.Once the mixture was dried, it was extruded through a WayneYellow Label Table Top Extruder. Five thermostaticallycontrolled heating zones were used to melt the admixture priorto extrusion. The die zone consisted of a circular plate, a 10 cmlong steel tubing with an inner diameter of 4 mm, and the dieitself. A distributive mixing of the silica nanoparticles withnylon was enforced through the use of a circular plate withmultiple orifices [9, 11]. A specially designed die was usedin the process. The die configuration generated two distinctflow regimes that significantly affected the distribution of theparticles. After extrusion, filaments were solidified by passingthem through chilled water maintained at approximately 10 ◦C.In the next step, filaments were stretched using a tension-adjuster (Godet), and heat stabilized using the Wayne YellowJacket Stabilizing unit. The heater temperature was set at110 ◦C, and the filament travel per minute (FPM) was adjustedin the Godet stations to allow continuous drawing of filaments.Finally, the filaments were wound on a spool using a filamentwinder at a winding speed of about 70 rpm. Several of thesespools were produced.

4. Tensile response

Representative stress–strain responses from filament tests areshown in figure 2. Improvements in strength and stiffnessare listed in table 1. It is observed that the gains in tensilestrength and Young’s modulus are around 36% and 28%,respectively with pristine silica infusion. It is also noticed thatthe fracture strain of the filament in this case is still around80% and the load–deformation behavior of the nanophasedsystem is very similar to that of the neat system. If oneconsiders 0.2% yield strength, the gain is almost in the identicalrange. When silica particles are silated (functionalized), theenhancement in strength and modulus increases by 76% and55%, respectively as seen in table 1 and figure 2. It is alsonoticed that there is a loss of ultimate strain but it is still around30% which is considerably higher than for other fibers. Ifwe now calculate the upper bound on Young’s modulus basedon micromechanical theories [13], we find it to be 1.48 GPa.The experimental value of 1.41 GPa as shown in table 1 forsilica-reinforced nylon is within 5% of the upper bound. Whensilica particles are functionalized, the modulus increases to1.7 GPa, exceeding the upper bound by 15%. A similar patternin the case of strength was observed in an earlier work bySumita et al [14]. They predicted the strength of thin filmsmade from nylon 6 reinforced with silica and glass fillers usingempirical formulations which would give the upper bound ofthe composite strength as the strength of the unfilled matrix.

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Figure 3. SEM images of cross-sections of filament samples:(a) neat nylon 6, (b) nylon 6 with silica, (c) nylon 6 withfunctionalized silica.

But they found that the strength of the composite when filledwith 40 nm silica particles exceeded the neat nylon 6 strength,i.e. the upper bound, by about 16%. The loading of fillers was,however, 20.0 wt%—quite different from 1.0 wt% used in thepresent investigation.

Enhancement in stiffness and strength can be visualizedfrom various aspects. Crystallinity of nylon 6 as determined bydifferential scanning calorimetry (DSC) was around 24% [13].With the addition of functionalized silica particles crystallinityincreased to 29%. We have also noticed a corresponding shiftin the crystalline temperature from 189 to 196 ◦C. Similarchanges in crystalline temperature have been reported fornylon 66 when acid dyes were added [15]. Since changes inper cent crystallinity in our case were small, their effect onthe mechanical properties was modest. Similar observationshave been made in [16] where changes in the crystallinityof nanocomposites were not very evident with the additionof different types of silica particles. It was also indicatedthat improvement in mechanical properties was mainly due

to the effect of nanofillers. The increase in stiffness is nodoubt contributed by the higher stiffness (nylon 1.1 GPa, silica72.3 GPa) of the silica reinforcement. One order higherstiffness of silica would surely influence the stiffness of thecomposites simply from the rule of mixture concept.

Although the volume fraction of silica is small, itscontribution to stiffness is indeed substantial. The otherreason for an increase in stiffness may be explained fromthe viscoelastic approach as the morphological structure ofpolymer is changed due to inclusion [17]. As the filler particlesare surrounded by the shell of matrix there will be a relativeincrease in the particle diameter which effectively increasesthe particle volume fraction in the composite, which in turninfluences the stiffness. It is also known [18] that an increase inparticle diameter can be related to an increase in loss modulusof the composite which allows determination of the compositemodulus [13]. The key to such improvement will, however,depend on the dispersion of particles within the body of thematrix.

The SEM images of the surface morphologies of thefilament cross-section for neat nylon 6 and nylon 6 filledwith silica and functionalized silica nanoparticles are shownin figure 3. It is seen in figure 3 that in comparison with aquite rough surface of neat nylon 6 (figure 3(a)) the surfaceof the silica-filled sample (figure 3(b)) looks smooth, showingthe embedded SiO2 nanoparticles (outlined by arrows). Theiraverage size is quite large (about 200 nm) which indicatesclustering. In contrast, the image of nylon 6, filled withfunctionalized silica particles, shows the dispersion of thelatter in the form of 20–40 nm size individual nanoparticles,as shown by several arrows in figure 3(c). This improveddispersion can be attributed to aminosilane treatment of thesilica surface and interaction of terminal amino groups on thefunctionalized silica surface with nylon 6 functional groups.

The improvement in strength can be explained from adifferent perspective as it is dictated by the fracture processrather than by the deformation behavior. This fracture processis mostly governed by the interfacial adhesion between thematrix and the filler particles which essentially controls theload transfer to the filler.

5. Filament fracture process

The fracture process in nylon filament has three distinctfeatures. First is the formation of a V-notch, sometimes calleda razor notch, at the beginning of the fracture process [19–21].As the stress on the filament is increased, this is followed by asegment of slow crack growth region characterized by ductilecleaving across a straight front from the edge of the V-notch,as seen in figure 4. The third stage is a fast crack growth regionfeatured by a large structural difference of the surface, giving amelted and rough appearance (figures 4(a) and (b)). This roughportion or the third stage fracture surface is associated with thecatastrophic failure of the filament. A gaping hole is noticedin figure 4(a) which has evidently grown from a materialflaw originated during the melt extrusion process. The threeregions are identifiable with distinct boundaries especially infigures 4(b) and (c). These boundaries are somewhat merged

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Figure 4. SEM micrographs of fractured surfaces: (a) neat nylon 6, (b) nylon 6 with silica, and (c) nylon 6 with functionalized silica. Thefractured samples were collected after tensile tests of individual filaments in each category. SEM analysis was carried out in a JEOL JSM5800 scanning electron microscope. in all cases, samples were placed on a carbon double-sided tape and coated with gold to prevent chargebuild-up during the electron absorption of the specimen. Three to ten kilovolt accelerating voltage was applied to get to the desiredmagnification.

due to the presence of multiple V-notches in the case ofneat nylon (figure 4(a)). These V-notches are formed due totransverse cracks originating from the surface craze as seen inhuge numbers in figure 4(a). The number of such surface crazereduces significantly when we look at the silated sample. It isalso noticed that the size of the V-notch decreases and the notchsurface gets smoother as one moves from neat to nanophasedsamples. The size of the V-notch is usually determined by thespeed of the propagating transverse crack and the relaxationrate of the fractured part. If crack propagation is higherthan the relaxation rate of the material, it would force anundeveloped V-notch. In that sense i.e. from the size of the V-notch, it seems that crack propagation was highest in the silatedsamples. Also, the smoother surface is another indication offaster crack growth. Then what caused the higher strengthof the nanophased samples? One explanation comes from thesurface area of the third region. At the instant before failure the

entire breaking load is supported by the third segment whichhas considerably larger area in the case of figures 4(b) and (c)as compared to figure 4(a).

A larger area would require a larger force to break. Theother reason is that in the nanophased samples, the initiation ofthe fracture process (at the first stage) is delayed. This delaycan occur due to increased bonding between the particle andpolymer as we will show in the next section, and due to asmoother outer surface with much fewer surface crazes, as seenin figures 4(b) and (c). The bonding characteristics of neat andnanophased nylon were next characterized by FTIR and XPSstudies.

6. FTIR characterization

In order to determine the development of various functionalbonds during polymerization, FTIR spectroscopy was per-formed. FTIR is most useful for identifying chemical bonds

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Figure 5. FTIR spectroscopy—(a) neat nylon 6, (b) nylon 6 with 1 wt% silica, and (c) nylon 6 with 1 wt% functionalized silica. FTIR peaksof all the samples were obtained using a Thermo-Nicollet Nexus 670 FTIR spectrometer. Samples were cut into small pieces and directlyplaced on the attenuated total reflectance (ATR) accessory’s diamond cell. The spectrometer was run at 4 cm−1 resolution step for 128 scans.Before the actual run, the machine was run without the sample to eliminate the environmental effects. Data were acquired and analyzed byusing OMNIC software.

(a) (b)

Figure 6. XPS spectra of pristine and silated particles. A thin layer of each of the powder samples was mounted on a standard stainless steelsample stub using double-sided adhesive tape. Spectra were recorded at an electron take-off angle of 45◦. Charge compensation was achievedusing a VG Scienta FG300 low energy electron flood gun with the gun settings adjusted for optimum spectral resolution. The x-ray sourcepower was 2.8 kW. For survey spectra the spectrometer pass energy was 150 eV and the entrance slit width of the hemispherical analyzer was1.9 mm, whereas for region spectra values of 150 eV and 0.8 mm were used. Under these conditions the overall spectrometer resolution was0.55 and 0.35 eV, respectively.

that are either organic or inorganic, and its absorption spectrumis almost like a molecular fingerprint. An FTIR experimentwith nylon 6 and with functionalized silica nanoparticles isshown in figure 5.

Three curves are shown in figure 5; (a) neat nylon,(b) nylon with 1 wt% SiO2, and (c) nylon with 1 wt%functionalized (silated) SiO2. Three basic bonds of nylon6, i.e. amide N–H at 3297 cm−1, hydroxyl O–H at 2800–3000 cm−1, and carbonyl C=O at 1637 cm−1 were of primaryinterest. It is seen in figure 5(b) that IR absorbance for each ofthe three basic bonds has increased significantly, characterizedby their sharper and higher peaks. Higher peak and larger

area under the curve corresponds to higher absorption oflight energy required for excitation. In other words, the IRabsorbance is a direct measure of bond strength, indicatingthat SiO2 reinforcement into nylon was responsible for suchan increase. On the other hand, after functionalization of SiO2

particles, i.e. in figure 5(c), it is seen that three basic bondstrengths are maintained, while in addition, a siloxane Si–O–Si bond at 1090 cm−1 is formed which was not seen withfigures 5(a) or (b).

This is what we expected from functionalization;establishing a continuous covalent linkage across the particle(silica) and polymer (nylon 6) interface.

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Nanotechnology 19 (2008) 445702 H Mahfuz et al

The area under the FTIR curves is also a qualitativemeasure of the concentration (number) of the respective bonds.Since there is no change in the frequency, the individual bondstrength has not changed, but their number has increased inthe nanophased samples. This higher concentration is due tothe increase in the number of nucleation sites caused by thepresence of nanoparticles during the polymerization process.

7. XPS studies

XPS spectra of the pristine and treated silica nanoparticleswere recorded using a VG Scienta ESCA300 spectrome-ter [22–24]. The survey spectrum of the pristine nanoparticles(figure 6(a)) shows Si, O, and a minor amount of C, asexpected for pure silica. Whereas the spectrum of thetreated nanoparticles (figure 6(b)) shows Si, O, and significantamounts of C and N, and thus demonstrates the success ofthe functionalization reaction. The elemental quantificationdata are: pristine nanoparticles, Si:O:C = 22.3:72.7:5.0 at.%;treated nanoparticles, Si:O:C:N = 13.4:28.0:51.5:7.2 at.%.The core line binding energies, used as reference are: pristinenanoparticles, Si 2p = 104.0, O 1s = 533.5, C 1s = 285.0;treated nanoparticles, Si 2p = 103.0, O 1s = 532.5, C1s = 285.0, N 1s = 399.4. The Si 2p and O 1s bindingenergies of the pristine nanoparticles are consistent with therange of literature values measured for SiO2 [25] and thereduction in the Si 2p binding energy upon functionalizationis consistent with the reduced O coordination number in asiloxane type environment. The N 1s binding energy of thefunctionalized nanoparticles is consistent with N in an aminetype environment [22].

8. Summary

It is demonstrated that infusion of pristine silica nanoparticlescan modestly increase the strength and modulus of nylonwithout any loss of fracture strain. FTIR analysis has shownthat the mere presence of silica nanoparticles can significantlyinfluence the three primary bonds of nylon 6 resulting in a28% and 36% increase in modulus and strength, respectively.Once the silica particles are functionalized, the enhancementin primary bonds is still maintained, but a new silane bond isformed which improves the modulus and strength even further;by 55% and 76%, respectively. The source of the improvementis traced to: (i) an increase in the concentration of basic bondsnucleated by the presence of nanoparticles, (ii) formation ofsiloxane bond, and (iii) changes in the fracture process induced

by the infused silica particles. Fracture studies have alsorevealed that after functionalization nylon 6 seems to exhibitbrittle failure modes, but still retains high strength with 30%ultimate strain.

Acknowledgments

The authors would like to thank the Office of Naval Research,ONR (grant No. N00014-06-1-0696) and the National ScienceFoundation, NSF (grant No. HRD-976871) for supporting thisresearch.

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