Journal of CompositesVolume 2013 (2013), Article ID 462914, 11 pageshttp://dx.doi.org/10.1155/2013/462914

Research Article

Influence of SiC/Si₃N₄ Hybrid Nanoparticles on Polymer Tensile Properties

Vijaya K. Rangari, M. Yousuf, and Shaik Jeelani

Department of Materials Science and Engineering, Tuskegee University, Tuskegee, AL 36088, USA

Received 3 February 2013; Accepted 6 September 2013

Academic Editor: Zhiping Luo

Copyright © 2013 Vijaya K. Rangari et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Nanostructured silicon carbide (SiC)/silicon nitride (Si₃N₄) hybrid nanoparticles exhibit a high-potential for reinforcement of polymers. In the present investigation, silicon carbide (-SiC) nanoparticles (~30 nm) were sonochemically coated on acicular silicon nitride (~100 nm  nm) particles to increase the thermal and mechanical properties of Nylon-6 nanocomposite fibers. To produce Nylon-6/(SiC/Si₃N₄) nanocomposite fibers, we have followed a two-step process. In the first step, SiC nanoparticles were coated on Si₃N₄ nanorods using a sonochemical method and Cetyltrimethylammonium Bromide surfactant. In the second step, the SiC coated Si₃N₄ hybrid nanoparticles were blended with Nylon-6 polymer and extruded in the form of nanocomposite polymer fibers. The nanocomposite fibers were uniformly stretched and stabilized using a two-set Godet roll machine. The diameters of the extruded neat Nylon-6 and SiC/Si₃N₄/Nylon-6 nanocomposite fibers were measured using a scanning electron microscope and then tested for their tensile and thermal properties. These results were compared with the neat Nylon-6 polymer fibers. These results clearly indicate that the as-prepared nanocomposite polymer fibers are much higher in tensile strength (242%) and Young’s modulus (716%) as compared to the neat polymer fibers.

1. Introduction

It has always been high interest for the researchers to study the changes in various properties of polymers with the addition of reinforcements. Polymers such as Nylon-6, polypropylene, and LDPE when mixed with an appropriate percentage of nanoparticles as filler materials show significant improvements [1–3]. Many studies have been reported on the structural, chemical, and thermal characterization of nylon [4–7]. Among thermoplastic polymers, Nylon polymers are widely used materials due to low material cost, low density (approximately 12.5% the weight of bronze, 14.3% the weight of cast iron, and 50% the weight of aluminum), wide range of available properties, corrosion resistance, compound customization, insulation qualities, and good load bearing capacity [8]. Many attempts have been made to improve the properties of Nylons with the use of rubber, silica, clay, hydroxyapatite carbon nanotubes, and fibers [9–14]. In these studies, the polymers are blended with appropriate proportions of various shapes and sizes of nanoparticles using various processing techniques to improve mechanical and thermal properties. These materials have been successfully employed in defense, aerospace packing, and electronic and automotive industries. Hence, the improvement of their properties such as mechanical, thermal, electrical, and optical has always been a major point of consideration. Silicon nitride is considered to be a promising ceramic because of its exceptional chemical and mechanical properties, especially at high temperature [15]. Nanostructured silicon nitride particles exhibit a high-potential for the reinforcement of polymers. This covalently bonded material in its larger form exhibits high strength and high toughness and has been used in a number of industrial applications, such as engine components, bearings, springs, high temperature automobile components, and cutting tools [16].

Over the years, a significant progress has been made in the dispersion of acicular nanoparticles in polymer matrices by surface modification and other techniques. While the alignment of acicular or rod shape particles in polymeric fiber and manufacturing advanced macroscopic structures remain a major challenge. Most commonly used techniques for alignment are wet spinning, [17–19] magnetic alignment, [19] electro spinning [15, 20, 21], and melt processing [22–24]. Nylon is one such widely studied polymer which shows good improvements in mechanical properties when rod-shaped and spherical-shaped nanoparticles are aligned in the preferred direction in the polymer matrix. One of the widely used methods for such an alignment is melt extrusion process. To align the nanoparticles while extruding, a number of aspects should be taken into consideration such as the starting materials and chemical compositions, mixing techniques, type of extruder, material loading process, extrusion temperature, material residence time within the extruder, the die and its orifice shape and size, extrusion rate, extrusion direction, surrounding air temperature and its speed, fibers cooling type and process, and finally the filament draw ratio and winding speed. A proper combination of such factors leads to the production of fine fibers; otherwise, it may cause ambiguity. In our previous studies, we have shown an alignment of Si₃N₄ nanorods in the Nylon-6 polymer through the melt extrusion process [24]. Recent developments in synthesis of silicon based nanoparticle have been resulted in different shapes and sizes of silicon nitride nanoparticles. Among other nanoparticles, the spherical and whisker morphologies are studied extensively. Eblagon et al. [25] have developed an industrial woodcutting inserts from Si₃N₄-based ceramic matrix composites (CMC) and compared with tungsten carbide (WC). It was reported that Si₃N₄/30 wt.% SiC gives the best balance between fracture toughness and wear resistance. Wang and Huang [26] reported that the alignment of whisker could improve the sinterability of the SiC (w)/Si₃N₄ composites without lowering the density. It was also reported that the whisker-oriented alignment led to considerable anisotropy of linear shrinkage and sintering rate of SiC (w)/Si₃N₄ compacts during the sintering. In general, a stable and uniform suspension of acicular nanoparticles in the polymer matrix is required to obtain a fine dispersion and proper alignment of hybrid nanoparticles within the fibers, which are instrumental for the enhancement of structural, thermal, and electrical properties of PNC fibers [23, 24]. In the present study, a sonochemical synthesis technique is used to coat SiC nanoparticles on Si₃N₄ nanorods. The sonochemical effects of ultrasound arise from acoustic cavitations involving the formation, growth, and collapse of bubbles in liquid, which generates localized hot spots having a temperature of roughly 5000 K, pressure of about 20 MPa and very high cooling rates of about 10⁷ K s⁻¹ [24, 25]. These extreme conditions can drive chemical reactions such as oxidation, reduction, dissolution, and decomposition. The sonochemical effects on nanoparticles in ethanol have been well studied [27–29].

In the present study, the SiC nanoparticles were coated on Si₃N₄ using ultrasonic irradiation in presence of ethanol. The as-prepared SiC coated Si₃N₄ nanorods were reinforced with Nylon-6 polymer fibers through the melt extrusion process. Thermal, mechanical, and microscopic analyses were carried out and compared with their neat counterparts.

2. Experimental Procedures

2.1. Materials

The Si₃N₄ ( nm) nanorods were obtained from Nanostructured & amorphous Materials, Inc. SiC (30 nm) nanoparticles were purchased from MTI corporation. Commercial Grade UBE Nylon P1011F (Nylon-6, commercial name: Polyamide 6) was procured from UBE America. The physical properties of these materials are presented in Table 1. The density of Nylon-6 is 1.09–1.19 g/cm³ while the melting point is around 115–225°C. The mechanical property data as provided by the supplier are tensile strength 70–80 MPa, tensile elongation at break 100%, and tensile modulus 1–3 GPa.

Table 1: As-received material properties.

Type
Morphology
Size
Manufacturer
Density (g/cm³)
Color

Nylon-6
Irregularly shaped
100–500 m
UBE America, Inc
1.09–1.19
White

Si₃N₄
Rod-shaped (Alpha, 99%)
100 × 800 nm
Nanostructured and amorphous Materials, Inc
Bulk 0.03–0.05 True 3.44
Light gray

SiC
Spherical
~30 nm
MTI corporation
1.09–1.19
Black

2.2. Sonochemical Synthesis of SiC/Si₃N₄ Hybrid Nanoparticles

Sonochemical coating of SiC nanoparticles on rod shaped Si₃N₄ was carried out using a high intensity Sonics Vibra Cell ultrasonic liquid processor (Ti-horn, 20 kHz, 100 W/cm²). A 1 gm of Si₃N₄, 250 mg of SiC nanoparticles, and 250 mg of CTAB (Cetyl Trimethyl Ammonium Bromide) surfactant in a 60 mL ethanol were mixed well and transferred the reaction mixture to a stainless steel ultrasonic cell jacketed with a circulation of thermostated liquid to maintain a constant temperature of 10°C during the ultrasonic reaction. This reaction mixture was irradiated with ultrasonic horn for 3 hrs in an inert atmosphere of nitrogen gas. The resulting reaction mixture is separated from ethanol by using a high speed Allegra 64 R centrifuge at a speed of 10,000 rpm and 10°C. The precipitate was washed at least 5 times with ethanol to remove the CTAB. The resulting hybrid nanoparticles of SiC/Si₃N₄ were kept in a vacuum oven for overnight at room temperature to dry the particles and these particles were used for analysis and reinforcements in Nylon-6 fibers.

2.3. Melt Processing

SiC-coated rod-shaped Si₃N₄ particles and Nylon-6 powder were carefully measured in the ratio of 1 : 99 by weight. The dry mixing of Nylon-6 powder with SiC/Si₃N₄ particles was carried out in a mechanical blender in a circulating cold base container (5°C) at a speed of 2300 rpm. The mixing process was paused for 6 minutes after 3 minutes of continuous mixing and then resumed for further mixing. This was done to avoid temperature rise, which may result in softening the Nylon-6. This technique is repeated 20 to 30 times until uniform mixing was observed. The effective time for which the mixture was blended was between 75 to 90 minutes. At this time the mixture took a light gray appearance and it seemed that the SiC/Si₃N₄ was thoroughly mixed with the Nylon-6. The same process was repeated for rod-Si₃N₄ and SiC nanoparticle mixing with Nylon-6 polymer to ensure proper blending. The reinforcement of higher concentrations of hybrid nanoparticles was not studied because of the problems encountered in extrusion of polymer fibers due to particle agglomeration.

The mixture of Nylon-6 and Si₃N₄ nanoparticles was then further air dried in a dryer for 12 hrs and extruded using a Wayne Yellow Label Table Top single screw extruder. The extruder has a 19 mm diameter screw, which is driven by a 2 HP motor via toothed timing belt for smooth speed reversal and encased inside the barrel. Thermostatically controlled five heating zones were used to melt the mixture prior to extrusion; three of which were occupied by the barrel zone and the rest of the two were by the die zone. The heaters inside the barrel zone were placed at the feed hopper side, center, and die zone side of the barrel and set at a temperature of 226.67°C, 235°C, and 243.33°C, respectively. The purpose of these three heaters is to maintain gradually increasing temperature in the molten mass. The process begins when the nanophased dry-mixed powder is poured through the feed hopper to get inside the barrel zone of the extruder. As soon as it reaches the barrel zone, Nylon-6 starts melting due to high barrel temperature. At this stage, the whiskers particles are randomly distributed within the liquid matrix. The outer surface of the screw is designed to maintain a close fit with the barrel inner surface. As a result, the molten mass cannot escape from the screw surface and are trapped inside the screw segment. As the extruder screw rotates, this molten matrix is slightly mixed with the nanoparticles, conveyed in a spiral pattern and finally reaches the screw end which is located immediately before the die zone. The screw end is shaped in such a way that it allows the flowing mass to escape through a narrow opening at high velocity. As the screw rotation is continued, the high velocity liquid experiences an enormous shear force. The shear force at this high temperature can alter the chemical and thermal properties of the nylon 6 and also the orientation of rod-shaped and spherical particles. It is assumed that the rod shape/spherical particles are partially aligned at this stage. The partially aligned rod shape particles containing liquid nylon 6 now enter the die zone which is constructed with a circular plate, 10 cm long steel tubing with 4 mm inner diameter, and the die itself. The two heaters at this zone are set at a temperature of 245°C to maintain constant temperature of the flowing mass. One of the heaters is placed after the circular plate and the other one is placed immediately before the die. The circular plate is 15 mm in diameter and contains about 20 orifices, each of which is 2 mm in diameter. As the bulk materials are passing through the plate, they are disintegrated into several branches, and then combined again. This ensures a distributive mixing of the nanoparticles with Nylon-6. Now partially aligned nanoparticles containing liquid Nylon-6 are passed through the 10 cm long steel tube and arrive close to the die. It should be noted that in order to ensure proper alignment of fiber, the size of the die plate and the diameter of the die opening are very critical. Hence, a special type of die was used in this process, which has a converging inlet and a narrow outlet (1 mm). This die configuration generates two distinct flow regimes that highly affect the fiber alignments. First, the converging die inlet introduces a converging flow pattern, which in turn aligns the fibers along the stream line direction. Second, the narrow orifice allows the flow pattern to transform into shear flow as it enters the narrow orifice. The shear flow produces additional fiber alignment due to differential shear rate along the boundary layer that orients the fibers in the direction of the flow.

The process is continuous and the composite fibers with constant tension were extruded at a screw speed of 8–10 rpm and feed rate of approximately 80 g/h. Fibers were allowed to travel from the water bath, the water at room temperature of 20°C to a set of tension rollers, winder guide rollers, then wound on a spool using Wayne Desktop Filament Winder at a winding speed of 45 rpm. The process temperature, speed ratio between the winder and extruder screw, the position of the roller guide, and filament travel distance, all these factors control the draw ratio of the process as well as the continuity of the fibers.

2.4. Characterization

The X-ray diffraction (XRD) measurements were carried out using a Rigaku D/MAX 2200, X-ray diffractometer with Cu Kα radiation at 40 kV and 30 mA. The (2θ) angles of diffraction were measured from 10 to 80° in the equatorial direction to understand the crystalline nature of as-received nanoparticles and its hybrid nanoparticles. These samples were prepared by spreading uniformly powder nanoparticles on to a quartz glass sample holder and XRD tests were conducted. Differential scanning calorimetry (DSC) experiments were carried to measure the glass transition temperatures, heat of enthalpy, and melting points of neat Nylon-6 and nanoparticles reinforced Nylon-6 nanocomposite fibers. These experiments were carried out using a Mettler Toledo DSC 822e from 30°C to 300°C at a heating rate of 5°C/min under nitrogen atmosphere.

The as-received and as-prepared nanoparticles shapes and sizes were characterized using JEOL-2010 transmission electron microscope (TEM) and scanning electron microscope (JEOL-5800). The TEM samples were prepared by dropping a colloidal solution of nanopowder in ethanol on to a carbon-coated copper grid, whereas SEM samples were prepared by placing a powder sample on double-sided carbon tape and coated with gold palladium to avoid the sample charging. Single fiber tensile tests were conducted on Zwick-Roell tensile testing equipment with a 20 N load cell. These tests were carried out under displacement control at a crosshead speed of 0.011/s strain rate and gage length of 102 mm (ASTM standard D 3379-75) [30].

3. Results and Discussion

The as-received and hybrid nanoparticles were analyzed using X-ray diffractometer. The technique was used to study the crystallinity and coating of SiC nanoparticles on Si₃N₄ nanorods. X-ray diffraction patterns reveal that the as-received SiC, Si₃N₄, and synthesized SiC/Si₃N₄ nanoparticles are highly crystalline. Figure 1 depicts the powder XRD patterns of (a) as-received SiC, (b) as-received Si₃N₄, and (c) SiC coated Si₃N₄. Figure 1(a) shows the XRD peaks of SiC nanoparticles (b) Si₃N₄ nanorod particles and match very well with JCPDF no. 29–1129 and JCPDF no. 41–0360 respectively. Figure 1(c) shows the XRD peaks of SiC coated Si₃N₄ hybrid nanoparticles and all the peaks are assigned to SiC and Si₃N₄ particles. The 100% peak of SiC nanoparticles is submerged in Si₃N₄ peak and we can clearly see the peak widening ~35 of two theta degrees of SiC/Si₃N₄ hybrid nanoparticles. It should be noted that the peak intensities of Si₃N₄ are also decreasing after coating with SiC nanoparticles on Si₃N₄. These results indicate that the SiC nanoparticles are coated on Si₃N₄. Similar results were also observed by other researchers [31].

(a)

(b)

(c)

Figure 1: XRD patterns of (a) as-received SiC, (b) as-received Si₃N₄, and (c) SiC coated Si₃N₄.

SEM analysis was carried out to study the shape and sizes of as-received Nylon-6 polymer powder and presented in Figure 2(a). The micrograph shows that the Nylon-6 polymer powder is irregular in shape and ~100 to 500 μm in sizes. TEM analysis was also carried out to study the shape and sizes of as-received SiC, Si₃N₄, and extent of SiC coating on Si₃N₄ nanorods. Figures 2(b) and 2(c) show the TEM picture of the as-received spherical SiC and Si₃N₄ nanorods. The sizes measured from the micrographs are ~30 nm in diameter for SiC and 100–500/80 nm in length/width for Si₃N₄. Figure 2(d) depicts the SiC coated Si₃N₄ nanorods. This micrograph clearly shows that the SiC nanoparticles are evenly coated on Si₃N₄ nanorods and the particle sizes of SiC and Si₃N₄ marches very well with the as-received materials. Figure 2(e) represents a high resolution micrograph of one of the SiC nanoparticles coated Si₃N₄ nanorod. This micrograph also shows the lattice spacing of (111) and (100) planes of SiC and Si₃N₄, respectively. The crystal average -spacing of 0.25 nm and 0.68 nm confirms the presence of both SiC and Si₃N₄ together.

(a)

(b)

(c)

(d)

(e)

Figure 2: SEM micrograph of as-received (a) Nylon-6 polymer powder, TEM micrograph of as-received, (b) SiC, (c) Si₃N₄ and (d) SiC coated Si₃N₄, (e) high-resolution micrograph of SiC nanoparticle coated Si₃N₄ nanorod.

SEM analysis was also carried out for as-extruded single fibers to study the smoothness of the fibers and measure the accurate dimensions to use in tensile strength calculations. The SEM images in Figures 3(a), 3(b), 3(c), and 3(d) show the surfaces and diameters of continues single fibers of (a) Neat Nylon-6, (b) SiC/Nylon-6, (c) Si₃N₄/Nylon-6, and (d) SiC-coated Si₃N₄/Nylon-6 polymer fibers. The surfaces of these fibers are almost smooth and uniform, no physical defects were observed along the length of the fiber. The diameters measured for these single fibers are ~100 μm, 85 μm, 80 μm, and 75 μm, respectively for (a) Neat Nylon-6, (b) SiC/Nylon-6, (c) Si₃N₄/Nylon-6, and (d) SiC-coated Si₃N₄/Nylon-6. These diameters were used in calculating the tensile strength and modulus of the single fibers. Figures 4 and 5 display the DSC curves obtained from heating and cooling of (a) Neat Nylon-6, (b) Si₃N₄/Nylon-6, (c) SiC/Nylon-6, and (d) SiC-coated Si₃N₄/Nylon-6 filaments obtained from the extrusion process. These results are also presented in Table 2. In this study, the Tgs were measured as the inflection points of the heat flow curves [32, 33]. Table 2 clearly shows that the Tgs of extruded Nylon-6 has increased from 80°C to 130°C maximum with the reinforcement of 1 wt% rod Si₃N₄ nanoparticles. We have also observed the increase in Tg for the reinforcement of SiC and SiC coated Si₃N₄ nanoparticles from 80°C to 94°C and 126°C, respectively. This increase in Tg is due to the presence of Si₃N₄, SiC, and their SiC/Si₃N₄ hybrid nanoparticles, which may have imposed restrictions in molecular mobility at earlier stages. This effect can also be understood in terms of decreasing free volume of the polymer. From the concept of free volume, it is known that, in the liquid state when free space is high, molecular motion is relatively easy because of the unoccupied volume. As the temperature of the melt is lowered, the free volume would be reduced until there would not be enough free space to allow molecular motion or translation. With the reinforcement of Si₃N₄, SiC and SiC-coated Si₃N₄ nanoparticles, this free space is evidently further reduced [34, 35]. The melting temperature Tm of polymers that can crystallize such as Nylon-6 has been linked with their chemical structure. Three factors influence this link: chain geometry and regularity, chain stiffness, and hydrogen bonding within nylon [36]. It has been reported in the literature that the nanoparticles can act as sites of nucleation sites for polymer crystals [37, 38]. According to Table 2, there is a considerable difference in the melting temperatures of the neat Nylon-6 and Si₃N4, SiC and SiC-coated Si₃N₄ reinforced Nylon-6. This is confirmed by the variation of crystallinity of the neat and the degrees of crystallinity, calculated from melting endotherm (Figure 4) and heats of fusion, which are 6.7%, 64.26%, 71.79%, and 71.45%, respectively, for neat Nylon-6, Si₃N4, SiC, and SiC-coated Si₃N₄ reinforced Nylon-6 nanocomposite fibers, respectively. Similar results were observed in our earlier studies as well [25]. These results are also consistent with X-Ray studies.

Table 2: DSC results of neat and nanoparticles infused Nylon-6 nanocomposite fibers.

Sample
(°C)
(°C)
(°C)
Δ(J/g)
% Crystallinity
% Increase

Neat Nylon-6 fiber
80
225.5
187.4
61.8
106.6
6.6

Nylon-6/1 wt% rod-Si₃N₄
130
226.2
191.8
99.1
170.9
64.2

Nylon-6/1 wt% SiC
94
229.2
193.5
103.5
178.4
71.7

Nylon-6/1 wt% SiC-coated rod-Si₃N₄
126
229.2
194.3
103.3
178.1
71.4

(a)

(b)

(c)

(d)

Figure 3: SEM micrograph of as-extruded (a) Neat Nylon-6, (b) SiC/Nylon-6, (c) Si₃N₄/Nylon-6, and (d) SiC coated Si₃N₄/Nylon-6 polymer fibers.

(a)

(b)

(c)

(d)

Figure 4: DSC melting curves of (a) Neat Nylon-6, (b) Si₃N₄/Nylon-6, (c) SiC/Nylon-6, and (d) SiC coated Si₃N₄/Nylon-6 filaments.

(a)

(b)

(c)

(d)

Figure 5: DSC cooling curves of (a) Neat Nylon-6, (b) Si₃N₄/Nylon-6, (c) SiC/Nylon-6, and (d) SiC coated Si₃N₄/Nylon-6 filaments.

3.1. Tensile Response

Tensile tests of single-fiber specimens of the neat Nylon-6 and composites infused with, Si₃N₄, SiC, and SiC-coated Si₃N₄ fibers were carried out to estimate the increase in tensile properties, such as strength and modulus. The tests were performed according to the ASTM standard D 3379-75 [30]. The results of tensile properties of the (a) neat Nylon-6, (b) 1.0 wt% Si₃N₄/Nylon-6, (c) 1.0 wt% SiC/Nylon-6, and (d) 1.0 wt% SiC-coated Si₃N₄/Nylon-6 fibers are shown in Figure 6 and summarized in Table 3. The ultimate tensile strength values for neat Nylon-6, 1.0 wt%, Si₃N₄/Nylon-6, SiC/Nylon-6, and SiC-coated Si₃N₄/Nylon-6 fibers are 248 Mpa, 687 MPa, 371 MPa, and 602, respectively. The modulus is also significantly increased ~716% as compared to the neat Nylon-6.

Table 3: Tensile properties of neat and nanoparticles infused Nylon-6 nanocomposite fibers.

Type
Strength, (MPa)
% Increase in strength
Modulus (MPa)
% Increase in modulus

Neat Nylon-6
248
—
0.55
—

Nylon + 1 wt% Si₃N₄
450
181
3.36
510.9

Nylon + 1 wt% SiC
371
149
3.28
496.36

Nylon + 1 wt% Si₃N₄/SiC
602
242
4.49
716.36

Figure 6: Tensile response of (a) Neat Nylon-6, (b) Si₃N₄/Nylon-6, (c) SiC/Nylon-6, and (d) SiC coated Si₃N₄/Nylon-6 filaments.

The SiC-coated Si₃N₄ hybrid nanoparticles infused Nylon-6 exhibit a greater stiffness and higher value of Young’s modulus when compared to the individual SiC and Si₃N₄ reinforcements. The main reason may be that the SiC nanoparticles are uniformly adhered to the surface of the Si₃N₄ rods and they provide better interaction with the polymer matrix. The interfacial bonding and the cross liking with the polymer is better exhibited by such coated Si₃N₄ particles. The effect is as if it is the combine synergistic effect of the SiC particles and the Si₃N₄ particles together. This clearly suggests that the sonochemical coating of SiC on Si₃N₄ better hybridization which improves the better alignment as well as better adhesion to the polymer matrix.

As we derive the results from the tensile testing of single filaments of Nylon-6 and the Nylon-6 filaments reinforced with the various fillers, it becomes important that we study and explain the characteristics of the curves obtained. The response of the nylon filaments to the application of the tensile force is recorded in a particular pattern curve. The following is an explanation of what physically happens to the filament during the test. This is clearly explained in the Figure 7 with various stages of particles alignment in the single fiber.

Figure 7: Tensile stress-strain curves a fiber at various stages (a) fiber before subjected to the tensile force, (b) Fiber initial stretching the stage 1, (c) fiber necking the stage 2, of (d) complete stretching and stage 3, complete stretching of the filament length, and diameter reduction.

As shown in Figure 7(a), the fiber is in its original diameter and length before being subjected to the tensile force. Although the single screw extrusion process aligns the rod-Si₃N₄ in the Nylon-6 fiber [24] while stretching and there are some unaligned rod-Si₃N₄ left in the Nylon-6 fiber. When the force is applied to the ends of the fiber (tensile testing), the initial stretching of the fiber starts and is shown as the stage 1 of the fiber stretching in the Figure 7(a). At this initial stage, the tensile response is a linear curve from the origin and is generally used to calculate the modulus. The fiber experiences reduction in its diameter and increases in length until the necking of the fiber starts. The necking of the fiber continues as shown in Figure 7 as stage 2 of the fiber stretching. The diameter of the fiber gets smaller at the necking region. Due to this event, the rod shaped particles (Si₃N₄) are forced to straighten and consequently get aligned in the axial direction. This phenomenon is represented in Figure 7(b). Such reduction of diameter occurs until the complete length of the fiber gets to this uniform diameter. The dip in the curve obtained due to the tensile testing is assigned to stage 2. We predict that all the unaligned rod-shaped particles get aligned throughout the length of the fiber at stage 2 and the fiber shows a higher strength than in stage 1. The final stage, stage 3, as shown in Figure 7 is the fiber resistance to the applied force to it to a higher extent and the dip in the tensile testing curve starts to form linear. An interesting situation to know here is that after stage 2, the Nylon-6 polymer chains are also stretched along with the rod-shaped particles in the direction of loading and entangled polymer chains to resist the tensile force until breakage point. This is the point where the complete failure occurs and measured as ultimate tensile strength. The extent of this failure depends on the polymer used and the percentage loading and aspect ratio of nanoparticles. A perfect combination of these will give strengths which possibly cannot be acquired through the neat polymers without fillers. The above graph (Figure 6) represents the curve obtained when Nylon-6/rod shaped silicon nitride fibers are subjected to tensile test. The three stages of stretching and the failure can be seen in the graph. The stages 1, 2, and 3 occur in the neat systems also, except that the neat systems do not have anything to resist the tensile forces except for their polymer chains. Hence, we can observe that the neat systems dip in the curve and slope of the linearity are far lesser than those of their composite fibers. Interestingly we can also see that the stress-strain curve for SiC/Si₃N₄ Nylon-6 polymer composite fibers in stage 2 is almost insignificant. The reason for this may be the strong interaction between SiC/Si₃N₄ hybrid nanoparticles with Nylon-6 polymer chains where there is highly restricted polymer chain motion.

To understand further details of the tensile test results, we have carried out the SEM analyses of tensile failure fracture surface of (a) Neat Nylon-6, (b) Si₃N₄/Nylon-6, (c) SiC/Nylon-6, and (d) SiC-coated Si₃N₄/Nylon-6 fibers are shown in Figure 8. This Figure shows that the cross-sectional area of the fibers was circular and the failure was initiated at the edge of the fiber. Since the size of rod Si₃N₄ and SiC particles are very small, ranging from 20 to 30 nm in diameter, the pores within the structure could be an indication of nanoparticles integrated within the Nylon-6 composite fiber matrix which detached during fracture, described as particle matrix de-bonding during deformation [39]. The even distribution of pores throughout the cross-section could imply that the nanoparticles domains were evenly distributed through the fracture surface. It is also seen in the micrograph that many craze normal to the tensile direction are found at the side surface of specimen Figures 8(a)–8(d). Craze grows bigger and joins together to form cracks. For neat Nylon-6, craze is minimum compared to the nanoparticles infused Nylon-6 fiber. Maximum craze can be seen for Nylon-6 with SiC/Si₃N₄ hybrid nanoparticles indicating stronger resistance to crack propagation. In general, the fibers stress at fracture is also the ultimate stress. Upon reaching the point of ultimate stress, there was no sign of any further elongation before fracture. This could be attributed to the small dimensions of the fibers that caused instability during the initial stage of neck formation and, hence, fracture to take place easily as in the case of neat Nylon-6.

(a)

(b)

(c)

(d)

Figure 8: SEM micrograph of fracture surface of (a) Neat Nylon-6, (b) Si₃N₄/Nylon-6, (c) SiC/Nylon-6, and (d) SiC coated Si₃N₄/Nylon-6 filaments.

4. Conclusions

SiC/Si₃N₄ hybrid nanoparticles were synthesized using the sonochemical method. Neat Nylon-6, SiC, Si₃N₄, and SiC/Si₃N₄ infused Nylon-6 polymer composite fibers were fabricated using a single screw melt extruder. XRD and TEM studies reveal that SiC nanoparticles were uniformly coated on Si₃N₄ surfaces. The improvement in ultimate tensile strength and elastic modulus is attributed to the alignment of the SiC/Si₃N₄ nanoparticles along the extrusion direction. The increase in thermal stability and crystallinity of SiC/Si₃N₄-infused Nylon-6 PNC is correlated with the better cross-linking between the nanoparticles and the polymer matrix. The in situ 1% SiC/Si₃N₄ Nylon-6 nanocomposite fibers were demonstrated to have an excellent strength when compared to the neat Nylon-6. It is also noteworthy to mention that this sonochemical synthesis technique can be used to synthesize SiC/Si₃N₄ hybrid nanoparticles in bulk quantities. Overall, this technique can be adapted for fabrication of other multifunctional nanomaterials for specific application.

Conflict of Interests

The authors declare that there is no conflict of interests in this paper.

Acknowledgments

The authors would like to thank the National Science Foundation for their financial support through NSF-NSF-RISE #113768 and AL-EPSCoR #1158862 and also the UBE Corporation for providing Nylon-6 polymer for this research.

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