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Microstructural Characterization of Short Glass Fiber and PAN Based Carbon Fiber Reinforced Nylon 6 Polymer Composites.

INTRODUCTION

Now-a-days, fiber reinforced polymer (FRP) composite materials are considered as the better substitutes for metallic/ceramic orthopedic implants for load bearing applications [1]. The FRP composite material has higher structural efficiency at lower weights as compared to metallic structures [2]. FRP composite materials are being used in several orthopedic applications, such as in bone fixation plates, hip joint replacement, bone cement, and bone grafts [3], FRP composite implants offer several benefits over traditional metallic/ceramic orthopedic implants. They exhibit high biocompatibility, long-term reliability, noncorrosion, and good bonding between the bone and implant [4].

The polymers used for orthopedic applications to replace or restore damaged function should be biocompatible, nontoxic, and noncarcinogenic [5]. The most commonly used polymers for the orthopedic implants in the medical field are poly(methyl methacrylate) (PMMA), polyetheretherketone (PEEK), polysulfone (PS), polypropylene (PP), polyethylene (PE), polybutylene terephthalate (PBT), polyethersulfone (PES) [6], etc. Recently, the semicrystalline nylon 6 matrix have been widely used as a biomaterial for load bearing orthopedic bone plates due to its high strength, stiffness, and good biocompatibility. The fiber reinforced nylon 6 resins have shown high temperature performance and excellent insulation properties [6]. The composites made from E-glass fibers and PAN-CFs exhibits superior mechanical properties. However, the major problem associated with the untreated E-glass fiber is low interfacial adhesion between the fibers and thermoplastic matrix. This can be overcome by treating the surface of E-glass fiber with aminopropyltriethoxy silane (APS) coupling agent [7].

The carbon fibers are the thin filament materials containing over 92 wt% of carbon [8], exhibit light weight, high strength, stiffness, temperature resistance, and chemical resistance, when compared to most of the metals and ceramics [9-11]. The carbon fibers have multiple potential advantages in developing high-strength biomaterials with a density close to bone for better stress transfer and to enhance tissue formation. The carbon FRP composites have also been studied for different biomedical applications, such as total hip replacement, spine surgery and internal fixation implants [12, 13].

Generally, carbon fibers are manufactured from three precursor materials viz., rayon, mesophase pitch, and polyacrylonitrile (PAN). Among the three types of available carbon fibers, PAN based carbon fibers (PAN-CFs) have shown high degree of molecular orientation and thermally stability, when subjected to heat treatment [8].

In order to improve the mechanical properties of composite materials, it is necessary to have a good interfacial bonding between the fiber surface and polymer resin. The interfacial behavior between CFs and matrix mainly depends on the CF surface [14]. The PAN-CFs are normally surface treated by air oxidation and coated with epoxy, polyester, and urethane for better adhesion [14]. The surface treatments and the coating of PAN-CF increases the surface polar energy and produce carboxyl, hydroxyl, and carbonyl groups on the surface of the fiber and hence improve the bonding between the fiber and matrix [15], AS4C (surface treated and epoxy coated) PAN-CF has shown highest adhesion level due to better stress transfer between fiber and matrix and hence increases the mechanical strength of the composites made from them [16].

From the detailed literature survey, it has been found that the effect of free volume on the mechanical properties of SGF and PAN-CF reinforced nylon 6 polymer composites are nowhere reported. Therefore, in the present study, authors made an attempt to correlate the free volume and mechanical properties of nylon 6/SGF and nylon 6/PAN-CF reinforced polymer composites for load bearing orthopedic applications. This is done by making use of one of the well-established, sophisticated tools viz., positron annihilation lifetime spectroscopy (PALS). The measured free volume parameters by PALS may not directly provide the changes occurring at the interface of the polymerfiber composites. To quantify the extent of interaction at the interface of nylon 6/SGF and nylon 6/PAN-CF reinforced polymer composites, along with FTIR studies, hydrodynamic interaction parameter (h) has also been evaluated using mechanical properties. The hydrodynamic interaction parameter (h) arises by the excess friction generated at the interfaces of polymer/ fiber due to the intramolecular repulsion between the polymeric chains of the constituents [17]. The mechanical properties viz., tensile strength (TS) and Young's modulus (YM) of nylon 6/ SGF and nylon 6/PAN-CF reinforced polymer composites have been measured by universal testing machine (UTM) as per ASTM-D 256 model. These mechanical properties are correlated with the free volume properties studied by PALS.

THEORY

Hydrodynamic Interaction Parameter

The concept of hydrodynamic interactions in polymer solution was first introduced by Zimm [17] in the year 1956. The hydrodynamic interaction is the mechanical interaction generated due to the excess friction between the constituents of the polymeric chains. The friction between the polymeric chains of the composites is not only due to the intermolecular interactions, but also due to the intramolecular repulsion between them [17]. The relaxation process can be characterized by a relaxation time (t), which is related to the viscosity ([r.sub.i]) and YM (a) by the relation [18].

[tau] = [eta]/[sigma] (1)

The hydrodynamic interaction parameter (h) is related to the relaxation time ([tau]) by the equation

[tau] = [pi][[eta].sub.s][ra.sup.2]/2kT(1 - h [square root of 6/[pi]]) (2)

where k is Boltzmann constant and T is the absolute temperature, r and a are radius and the length of the fibers, respectively. From Eqs. 1 and 2, hydrodynamic interaction parameter (h) can be written as

h = 2kT - [sigma][pi][ra.sup.2]/2kT [square root of 6/[pi]] (3)

and h is more negative for composites having higher YM [19, 20].

EXPERIMENTAL

Materials

Nylon 6. Nylon 6 having density of 1.14 g/cm3 was procured from Gujarat Polyfilms Private Limited Company, Gujarat, India, under the trade name CAPLON. The chemical structure of nylon 6 is as shown in Fig. 1.

E-Glass Fiber. E-glass fibers with a mean diameter of 16 pm and density of 2.60 g/[cm.sup.3] supplied by Nippon Electric Glass, Taiwan, were used as reinforcement.

Silane Treatment of E-Glass Fibers. APS (0.6%) was mixed well with ethanol/water in the ratio 6:4 and kept for 1 h. The pH of the solution was maintained between 3.5 and 4 to complete the hydrolysis of silane by the addition of acetic acid. The E-glass fibers were dipped in the above solution for 1 h. In order to obtain the homogeneous dispersion of silane into the fiber surface, the mixture was continuously mixed for another 30 min. The length of the SGF after mixing is 3 mm and the thickness of the coating is about 0.14 mm. The treated E-glass fibers were then dried at 100[degrees]C for about 5 h in an air circulating oven to evaporate ethanol completely. The mechanism of suface treatment can be found elsewhere [7]. The chemical structure of E-glass fibers are suface treated with APS is as shown in Fig. 2a.

Carbon Fiber. PAN-CF having 6,000 (6K) filaments per tow, supplied by Hexcel Corporation under the trade name Hextow are used in the present study. PAN-CF grade AS4C is surface treated by air oxidation and coated with epoxy resin for better adhesion with the polymer matrix. The chemical structure of surface treated and epoxy coated PAN-CF is as shown in Fig. 2b.

Preparation of FRP Composites

Nylon 6/SGF Reinforced Polymer Composites. Nylon 6 and glass fiber (GF) were dried at 80[degrees]C for 4 h and at 120[degrees]C for 2 h, respectively. Nylon 6 granules and 3 mm short glass fibers (SGF) of various loading (10, 20, 30, and 40 wt%) were mixed in a Haake co rotating twin screw extruder (diameter = 40 mm, L/D =16) operating at a barrel temperature ranging from 235[degrees]C to 240[degrees]C at the feed rate of - 1 kg/h and a screw speed of 280 rpm. The extruded composites were subsequently pelletized by a granulator. The reinforced granules were then molded on a 75 Ton injection molding machine with an injection pressure from 75 to 125 M Pa. The mold temperature was held at 105[degrees]C and the molded specimens were allowed to cool at room temperature. The molded specimens were then cut into rectangular strips and used in the measurement of mechanical properties, viz., TS and YM as per ASTM standards and other experimental investigations.

Nylon 6/PAN-CF Reinforced Polymer Composites. Nylon 6 granules and 4 mm length PAN-CFs of various loading (10, 20, 30, and 40 wt%) were mixed in a Brabender (Plasticorder, Keltron CMEI, MODEL-16 CME SPL, East Germany) operating at a temperature profile ranging from 235[degrees]C to 240[degrees]C with screw speed of 20 rpm. The composite obtained from Brabender was then pressed into sheets using a compression molding machine (Compression Molding Press, Santec, India) with an injection pressure 15 MPa and the mould temperature at 190[degrees]C. After curing, the molds were cooled to room temperature before releasing the pressure for demolding. The sheets were then cut into rectangular strips and used in the measurement of mechanical properties, viz., TS and YM as per ASTM standards and other experimental investigations.

Measurements

Positron Annihilation Lifetime Measurements. Positron annihilation lifetime spectra of PAN-CF, nylon 6 matrix, nylon 6/ SGF, and nylon 6/PAN-CF reinforced polymer composites using a positron lifetime spectrometer. The positron lifetime spectrometer consists of a fast-fast coincidence system with Ba[F.sub.2] scintillators coupled to photo multiplier tubes type XP2020/Q with quartz window as detectors. The detectors were shaped to conical to achieve better time resolution. The two identical pieces of a sample under study were placed on either sides of a 17 [mu]Ci [sup.22]Na positron source, prepared on a pure Kapton foil of 0.0127 mm thickness. This sample source sandwich was placed between the two detectors of PALS to acquire lifetime spectrum. The positron lifetime spectrometer was operated at 220 ps time resolution. All lifetime measurements were performed at room temperature and two to three positron lifetime spectra with more than a million counts under each spectrum were recorded in a time of 2-4 h [21]. Consistently reproducible spectra were analyzed into three lifetime components with the help of the computer program PATFIT-88 [22] with proper source and background corrections. Source correction term and resolution function were estimated from the lifetime of well-annealed aluminum using the program RESOLUTION [22]. Three Gaussian time resolution functions were used in the present analysis of positron lifetime spectra of as received nylon 6 and nylon 6/SGF and nylon 6/PAN-CF reinforced polymer composites for different SGF and PAN-CF concentration. The o-Ps lifetime [[tau].sub.3] is related to the free volume hole size by a simple relation given by Nakanishi et al. [23], which was developed on the basis of theoretical models originally proposed by Tao [24] for molecular liquids and later by Eldrup et al. [25]. In this model, positronium is assumed to be localized in a spherical potential well having an infinite potential barrier of radius [R.sub.0] with an electron layer in the region R<r<R0. The relation between [tau.sub.3] and the radius R of the free volume hole or cavity is

[lambda]= 1/[[[tau].sub.3]= 2P=2[1 - (R/[R.sub.0]) + (1/2[pi])Sin (2[pi]R/[R.sub.0])][ns.sup.-1] (4)

where [R.sub.0] = R + [DELTA]R and [DELTA]R is an adjustable parameter. By fitting Eq. 4 with [[tau].sub.3] values for known hole sizes in porous materials like zeolites, a value of [DELTA]R = 1.657 A was obtained. The free volume radius R has been calculated from Eq. 4 and the average size of the free volume holes ([V.sub.f]) is evaluated as

[V.sub.f] = 4/3[pi]R3 (5)

The fractional free volume or the free volume content ([F.sub.v]) can then be estimated as

[F.sub.v] = [CV.sub.f][I.sub.3] (6)

where C is the structural constant, whose value is taken as 0.0018 [[Angstrom].sub.3], [V.sub.f] is the free volume hole size, and [I.sub.3] is the o-Ps intensity.

Mechanical Property Measurements. Mechanical properties of E-glass fibers and AS4C PAN-CF. The mechanical properties of these fibers are normally probed by nano indentation method. Li et al. [26] and Cao et al. [27] have conducted extensive studies on mechanical properties of E-glass fibers and carbon fibers by using nano indentation method. According to them the change in elastic modulus of E-glass fibers does not affect the hardness and elastic modulus. They also reported that as the E-glass fibers and PAN-CFs in fiber form cannot be measured using conventional mechanical testing technique, the nano indentation is a very powerful method for studying the mechanical properties of these short fibers [26, 27]. The manufacturers specifications of mechanical and physical properties of E-glass fibers and AS4C grade PAN-CF are given in Table 1.

Mechanical properties of nylon6IE-glass fibers and AS4C nylon 6/ PAN-CF reinforced composites. Mechanical properties of nylon 6/ SGF and nylon 6/PAN-CF composite were measured according to ASTM D638 standard using UTM, model 4302 H50 KM, 50KN, Hounsfield, UK to measure TS and tensile modulus. A dog bone-shaped nylon 6/SGF and nylon 6/PAN-CF composite specimens were prepared according to ASTM D638. All the samples were dried at 80[degrees]C for 15 h prior to testing. The test specimens having dimension of length 60 mm, width of 12 mm, and thickness of 3 mm were loaded between two manually adjustable grips of a 60 KN computerized UTM with an electronic extensometer and the surrounding temperature is 35[degrees]C. A tensile test specimen placing in the testing machine with testing speed at 5 mm/min and load was applied until it fractures. Due to the application of load, maximum load of the specimen is recorded. The test was repeated thrice and the average values of the TS and YM of the composites are reported.

X-ray Diffraction Studies. X-ray diffraction spectra of PANCF, nylon 6 matrix, nylon 6/SGF, and nylon 6/PAN-CF reinforced polymer composites were recorded using Rigaku Mini Flex 11 diffractometer with Ni filtered Cu-K[alpha] radiation of wavelength 1.5406 [Angstrom], with graphite monochromator. X-ray difftraction spectra of as received PAN-CF, nylon 6 matrix, nylon 6/SGF and nylon 6/PAN-CF reinforced polymer composites for different fiber concentration were taken in a glass sample holder. The X-ray scans were recorded in the 2[theta] range from 6[degrees] to 60[degrees] with a scan speed of 5[degrees]/min in steps of 0.02[degrees]. The working voltage and current were 30 kV and 15 mA respectively. The crystallinity was evaluated by the ratio of the area under the crystalline peaks to the total area using PEAKFIT4.1 software.

FTIR Studies. The FTIR spectra of PAN-CF, nylon 6, nylon 6/ SGF, and nylon 6/PAN-CF reinforced polymer composites were recorded using FTIR machine model (Spectrum Two 94012). All the spectra were run at ambient temperature using KBr disk method at a wave number range of 4,000-500 [cm.sup.-1].

RESULTS AND DISCUSSION

Positron Lifetime Results of Nylon 61'SGF Reinforced Polymer Composites

Positron lifetime studies reveal that the free volume properties of FRP composites are strongly affected by the amount and type of fiber used for reinforcement [21]. As we are interested on free volume hole size and their concentration of SGF and PAN-CF reinforced nylon 6, only the third lifetime component viz., o-Ps lifetime ([[tau].sub.3]), o-Ps intensity ([[tau].sub.3]) are derived from PATFIT program are reported here. Fig. 3a-c are the plots of o-Ps lifetime [[tau].sub.3] (ns), free volume hole size [V.sub.f] ([[Angstrom].sub.3]), o-Ps intensity [I.sub.3] (%), and fractional free volume [F.sub.v] (%) as a function of SGF loading, respectively. The o-Ps lifetime ([tau.sub.3]) and its intensity ([[tau].sub.3]) of nylon 6 matrix are 1.941 ns and 18.73%, respectively.

From Fig. 3a, it is observed that the o-Ps lifetime ([tau.sub.3]) of nylon 6/SGF reinforced polymer composites increases gradually as a function of SGF wt% and about 75 ps (pico seconds) increase occurs from 1.941 to 2.016 ns and exhibits positive deviation from linear additivity relation. This is reflected in the increased free volume hole size ([V.sub.f]) from 92.05 to 99.12 [[Angstrom].sub.3] at 40 wt% of SGF. This is because the o-Ps is preferentially formed and localized within the free volume holes in amorphous polymers. However, in the semicrystalline polymers, o-Ps may be formed within the interfacial free volumes, at vacancy type defects at the crystalline or crystalline-amorphous interface regions. PALS parameters are the average values taking into account of the different dimensions of holes related to the phases and interfaces present in the materials [21].

The probability of o-Ps formation is assumed to be proportional to the number of low electron density regions in which oPs can trapped [28].The o-Ps intensity (/3) as a function of SGF wt% in nylon 6/SGF reinforced polymer composites is as shown in Fig. 3b. The o-Ps intensity ([[tau].sub.3]) correlates with the amount of free volume or the number of voids in the sample. The reduction of 3.39% o-Ps intensity (/3) at 40 wt% of SGF is seen in Fig. 3b. The decreased o-Ps intensity is due to the decreased probability of o-Ps formation (o-Ps inhibition) and show positive deviation from linear additivity relation [29].

The fractional free volume ([F.sub.v]) derived from the product of free volume hole size ([V.sub.f]) and o-Ps intensity ([[tau].sub.3]) would rather represents the overall changes in free volume of the system. On the basis of free volume parameters, Kelly and Buech in the year 1961 derived the composition dependent free volume additivity of the two component system at all temperature [30]. According to this, the fractional free volumes of FRP composites are given by

[F.sub.v] = [W.sub.1][F.sub.v1]] + [W.sub.2][F.sub.v2] (7)

where [F.sub.v] is the fractional free volume of the composite, [F.sub.v]l, [F.sub.v2] and [W.sub.1] and [W.sub.2] are the fractional free volumes and weight fractions of component 1 and component 2, respectively.

Figure 3c shows the variation of fractional free volume ([F.sub.v]) calculated according to the Eq. 6 as a function of SGF wt%. From the Fig. 3c, it is observed that, the fractional free volume ([F.sub.v]) decreases as a function of SGF wt% and deviates positively from the linear additivity relation at 10 to 40 wt% of SGF. The positive deviation indicates the formation of interface due to the less interaction between the polymeric chains of nylon 6 and SGF [29].

Positron Lifetime Results of Nylon 61PAN-CF Reinforced Polymer Composites

The free volume data derived from the PALS results of nylon 6/PAN-CF reinforced polymer composites as a function of fiber loading are shown in Fig. 3d-f. As the filler concentration increases in nylon 6/PAN-CF reinforced polymer composites, the o-Ps lifetime ([tau.sub.3]) increases and show about 87 ps increase from 1.941 to 2.028 ns and the corresponding free volume hole size ([V.sub.f]) from 92.05 to 100.26 [[Angstrom].sub.3] at 40 wt% of PAN-CF. However, the variation of o-Ps lifetime ([tau.sub.3]) in nylon 6/PAN-CF reinforced polymer composite shows negative deviation from linear additivity relation.

From Fig. 3e, it is observed that about 4.5% decrease of o-Ps intensity ([[tau].sub.3]) at 40 wt% of PAN-CF loading, when compared to as received nylon 6 matrix and exhibit negative deviation from linear additivity. The fractional free volume ([F.sub.v]) deviates negatively from linear additivity between 10 and 40 wt% of PANCF reinforced nylon 6 polymer composite. The observed negative deviation in fractional free volume ([F.sub.v]) is attributed to the induced molecular packing due to the chemical interaction between the polymeric chains of nylon 6 polymer matrix and PAN-CF [29].

X-ray Diffraction Results

X-ray diffraction pattern of FRP composites contain both sharp and diffused peaks. The sharp peaks correspond to the crystalline regions and diffused peaks for amorphous regions of polymer samples. XRD spectra of PAN-CF, nylon 6 matrix, nylon 6/SGF, and nylon 6/PAN-CF reinforced polymer composites of different wt% of SGF and PAN-CF loading are shown in Figs. 4 and 5. The X-ray diffraction pattern of PAN-CF exhibits the sharp crytsalline peaks at 2[theta] = 39[degrees], 45[degrees] and nylon 6 matrix at 2[theta] = 21[degrees], 27[degrees], 39[degrees], and 45[degrees], respectively. Nylon 6 is a semicrystalline polymer, its crystallinity is calculated by the ratio of the area under the crystalline peaks to the total area using computer program PEAKFIT4.1 gives 30.90%. The variation of crystallinity of nylon 6, nylon 6/SGF, and nylon 6/PAN-CF reinforced polymer composites as a function of SGF and PAN-CF loading are shown in Table 2. The crystallinity of nylon 6/SGF reinforced polymer composite decreases and reaches to 24.62% at 10 wt% of SGF. After 10 wt% of SGF, the crystallinity of nylon 6/SGF reinforced polymer composite increases and shows 28.71% at 40 wt% of SGF. The reduced crystallinity below 10 wt% of SGF indicates the increased amorphousity in nylon 6/SGF reinforced polymer composites. After 10 wt% of SGF, the crystallinity of nylon 6/ SGF reinforced polymer composite increases due to the fact that the short glass fiber surfaces act as nucleating agents upon crystallization [31]. The similar kind of results is found for date palm wood flour/glass fiber reinforced hybrid composites of recycled polypropylene system [32].

From Table 2, the crystallinity of nylon 6/PAN-CF reinforced polymer composite also increases with increasing CF loading. The increased CF loading could act as nucleating agent and enhances the crystallization of polymer. The carbon fiber reinforcement increases the nucleation density in the matrix, resulting in the enhanced crystallization ability of molecular segments [33].

Hydrodynamic Interaction Parameter Results

The hydrodynamic interaction is the mechanical friction generated between the constituents of the polymeric chains. The friction between the polymeric chains of the composites is not due to the intermolecular interactions, but it is due to the intramolecular repulsion between the polymeric chains of the constituents. In such circumstances, the hydrodynamic interaction parameter (h) attains large negative values. Suppose, if the friction is less at the interface due to the lack of interactions between the polymeric chains of the constituents, each polymer forms its own domain in the system and results to the reduced contacts between the polymer chains. For such system hydrodynamic interaction parameter (h) becomes positive [17].

The variation of hydrodynamic interaction parameter (h) of nylon 6/SGF and nylon 6/PAN-CF reinforced polymer composites as a function of fiber loading evaluated by Eq. 3 are shown in Fig. 6. The hydrodynamic interaction parameter (h) in nylon 6/SGF and nylon 6/PAN-CF reinforced polymer composite decreases as a function of SGF and PAN-CF loading and show minimum value at 40 wt% of fiber loading. However, the value of (h) parameter of nylon 6/SGF reinforced polymer composites (-1.926 X [10.sup.20]) is more negative than nylon 6/PAN-CF reinforced polymer composites (-4.084 X [10.sup.19]). Therefore, one would expect high friction at the interface due to the intramolecular repulsion between the polymeric chains of SGF, PAN-CF, and nylon 6 matrix. The fractional free volume obtained experimentally show negative deviation in nylon 6/ PAN-CF reinforced polymer composites at 10 to 40 wt% PANCF and show positive deviation for SGF reinforced nylon 6 polymer composites from the linear additivity relation. This suggests that the hydrodynamic interaction in nylon 6/SGF reinforced polymer composites is more compared to nylon 6/PANCF reinforced polymer composites.

FTIR Results

Fourier transform infrared (FTIR) spectrometry is used to study the chemical interaction between silane treated E-glass fiber, PAN-CF, and the polymeric chains of nylon 6 matrix. The FTIR spectrum of silane treated E-glass fiber is as shown in Fig. 7a. The bands at 2,386 and 881 [cm.sup.-1] is due to [CH.sub.2] stretching and Si--OH band [34], The band at 1,484 cmT1 corresponds to -[NH.sub.2] group hydrogen bonded with the hydroxyl group of both silane and the fiber surface indicating the complete adsorption of silane on to the fiber surface [35]. This confirms the grafting of silane on to the fiber surface after the silane treatment. The E-glass fiber treated with silane coupling agent also introduces an organosiloxy group to the fiber surface [36].

FTIR spectra of silane treated SGF reinforced nylon 6 matrix, 10%, 20%, 30%, and 40 wt% of nylon 6/SGF reinforced polymer composites are shown in Fig. 7b-f. The FTIR spectrum of nylon 6 matrix shows the transmittance peak corresponding to the ester group (C=0) at 1,762 [cm.sup.-1]. The band at 1,614 [cm.sup.-1] corresponds to amine (N--H) bending in nylon 6 matrix [37]. The bands at 2,954 and 2,883 [cm.sup.-1] corresponds to [CH.sub.2] symmetric and [CH.sub.2] asymmetric stretching, respectively. The transmittance band at 2,954 [cm.sup.-1] gradually increases from 10 to 40 wt% of SGF loading in nylon/SGF reinforced polymer composites. This increased transmittance indicates that there is no chemical interaction between silane treated E-glass fiber and the polymeric chains of nylon 6 matrix.

The FTIR spectrum of PAN-CF shows the transmittance bands at the wave number range of 3,637-3,201 cm~'(--OH stretch), 2,304-2,238 [cm.sup.-1] (C=N stretch), 1,594 [cm.sup.-1] (C=0), 1,473 [cm.sup.-1] (--[CH.sub.2]), and 1,297-1,000 [cm.sup.-1] (C-O) [38], which is shown in Fig. 8a. The FTIR spectra of nylon 6/PAN-CF reinforced polymer composites with 10 to 40 wt% of PAN-CF loading are shown in Fig. 8b-d. There appears to be the formation of hydrogen bonding between the methylene group of epoxy coated oxidized PAN-CF with the ester group (C=0) and N--H group of nylon 6 matrix. The bond energies of N--H is 391 kJ/ mol, O--H is 467 y/mol, C=0 is 799 kJ/mol, N-H-O is 8 kJ/ mol [39], and C=0--H group is 21 kJ/mol [40]. This indicates that the bond energy and bond dissociation energy of C=0--H is very much greater than N--H--O bond. Therefore, the formation of O--H bond at N--H--O site is more probable compare to C=0--H site [40, 41], which is also evident from the FTIR spectra of nylon 6/PAN-CF and nylon 6/SGF reinforced polymer composites. Due to the formation of chemical bonding at N--H--O sites, the extent of chemical interaction is more in nylon 6/PAN-CF than nylon 6/SGF reinforced polymer composites. This would be one of the reasons for the negative deviation of free volume parameters from linear additivity relation and improved mechanical strength of 40 wt% of nylon 6/PANCF reinforced polymer composites. The possible chemical interactions between epoxy coated PAN-CF and nylon 6 matrix are shown in Fig. 9.

Influence of Free Volume and Crystallinity on the Mechanical Properties of Short Glass Fiber and PAN-CF Reinforced Nylon 6 Composites

The efficiency of a fiber reinforced composite depends on the ability to stress transfer from the matrix to the fiber and hence the TS and YM of the composites [21]. In the present study, mechanical properties viz., TS and YM of nylon 6/SGF and nylon 6/PAN-CF reinforced polymer composites for different fiber loading are measured at ambient conditions and correlated with the free volume parameters.

It was reported that the variation in mechanical properties mainly depends on the load transfer from polymer molecules to fillers due to the interaction of polymeric chains with fillers, filler dispersion, and molecular packing in the polymer matrix. The physical, mechanical, and thermodynamic properties of thermoplastic materials are also correlated to the degree of crystallization and the morphology of the crystalline areas. The increased crystallinity favors improvements in some physicochemical properties, such as higher density, rigidity, melting and glass temperature, dimensional stability, and higher chemical and abrasion resistance [32]. The mechanical strength of FRP composites also depends on the buckling and rotation of fibers incorporated in to the polymer matrix [42]. In general, buckling and rotation reduces the mechanical strength of FRP composites. However, the mechanical strength of both the composites increases upon fiber loading in the present work, the effect of buckling and rotation are not taken in to account.

From Fig. 10a and b, it is observed that both TS and YM of nylon 6/SGF reinforced polymer composites increases with the addition of silane treated short glass fiber due to the random distribution of fibers in the polymer matrix [19]. The increased mechanical properties of nylon 6/SGF composites can be explained as follows: the interphase between fiber and polymer matrix plays a key role in determining the mechanical properties of the composites. The efficiency of the stress transfer between fibers and matrix is determined not only by molecular interaction at interface, but also by the properties of the formed interphase, in particular, its thickness and strength. One of the effective ways of creating interphase is fiber surface treatment and film formers or coupling agents. The suface treated short glass fiber by silane coupling agents would improve the interfacial adhesion at the interface of SGF and nylon 6 polymer matrix [7]. At the interface the hydroxyl groups of the silanes and the glass fiber surface can interact through siloxane or hydrogen bonding. This possible chemical interaction is shown in Fig. 2. The adhesion between the silane treated short glass fibers and nylon 6 polymer composite increases the mechanical strength of the composites [20]. The crystallinity of nylon 6/ SGF reinforced polymer composites show increasing trend as a function of fiber loading. The value of hydrodynamic interaction parameter (h) show more negative for nylon 6/SGF reinforced polymer composites. However, the fractional free volume ([F.sub.v]) obtained experimentally show positive deviation due to the formation of interface between SGF and nylon 6 matrix.

The variation of [V.sub.f], TS, and YM as a function of PAN-CF wt% are shown in Fig. 10c and d, respectively. There appears to be the marginal increase in TS and YM values of nylon 6/PANCF reinforced polymer composites from 10 to 40 wt% of PANCF loading. In general the chemical and physical interaction between reinforced carbon fibers and the matrix at the interface determines the mechanical properties of the FRPs. The surface treatment of PAN-CFs increases the surface area, remove the weak surface layer, increases the surface polar energy, and produce carboxyl, hydroxyl and carbonyl groups on the surface of the fiber. The surface of PAN-CFs is treated by the air oxidation and coated with the epoxy resins. The air oxidative treatment increases the oxygen and nitrogen content on the PAN-CFs surface, which leads to better bonding between PAN-CF with the polymer matrix [16]. The epoxy coating on PAN-CFs surface provide a chemical link between the PAN-CF surface and nylon 6 matrix. The improved mechanical properties upon PAN-CFs reinforcing are due to the interpenetration of chains of nylon 6 polymer matrix and PAN-CF and thus the fiber-matrix adhesion [16].

However, the fractional free volume ([F.sub.v]) obtained experimentally show negative deviation in nylon 6/PAN-CF reinforced polymer composite indicates the induced molecular packing due to the chemical interaction between the polymeric chains of nylon 6 polymer matrix and PAN-CF. The crystallinity of nylon 6/PAN-CF reinforced polymer composites also increases as a function of fiber loading. This is due to the close packing resulted between the fiber and matrix due to the strong interaction provides less space for the chain movement. AS4C (surface treated by oxidation and epoxy coated) PAN-CF has shown highest adhesion level due to better stress transfer between fiber and matrix and hence the mechanical properties of the composites [15, 16].

Even though, the hydrodynamic interaction parameter (h) is more negative for nylon 6/SGF than nylon 6/PAN-CF reinforced polymer composites, FTIR results show the formation of hydrogen bonding between methylene groups of epoxy coated oxidized PAN-CF with the ester group and N--H group of nylon 6 matrix. This indicates that the extent of chemical interaction is more in nylon 6/PAN-CF than nylon 6/SGF reinforced polymer composites. Therefore, the observed negative deviation in PALS results of nylon 6/PAN-CF reinforced polymer composites is due to the induced chemical interaction between the polymeric chains of nylon and PAN-CF, not due to hydrodynamic interaction. This further suggests that the chemical interaction is stronger than the hydrodynamic interaction.

CONCLUSIONS

The negative deviation of fractional free volume from the linear additivity relation at 10 to 40 wt% of PAN-CF reinforced nylon 6 composite indicates the induced molecular packing between the polymeric chains of nylon 6 and PAN-CF. The positive deviation in nylon 6/SGF reinforced polymer composites indicates the formation of interface due to the less interaction between the polymeric chains of nylon 6 and SGF. The increased crystallinity of nylon 6/PAN-CF and nylon 6/SGF reinforced polymer composite indicates that short glass fibers and PAN-CFs act as the nucleating agents for the enhancement of crystallinity of the composites. The increased crystallinity of nylon 6/PAN-CF and nylon 6/SGF reinforced polymer composites leads to the increased TS and YM of the composites. The improved TS and YM in nylon 6/PAN-CF reinforced polymer composites is due to the highest stress transfer between nylon 6 and PAN-CF. The FTIR results suggests that observed negative deviation in PALS results of nylon 6/PAN-CF reinforced polymer composites is due to the induced chemical interaction at N--H--O sites. This further suggests that the chemical interaction is much stronger than the hydrodynamic interaction.

ACKNOWLEDGMENTS

Authors are grateful to Prof. M. A. Sridhar, DOS, in Physics, Manasagangotri, University of Mysore, Mysuru, for providing the X-ray diffraction spectra used in the present study.

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Munirathnamma Lakkur Munirajappa, Ravikumar Harijan Basavaraju [iD]

Department of Studies in Physics, Manasagangotri, University of Mysore, Mysuru 570006, India

Correspondence to: Ravikumar Harijan Basavaraju; e-mail: hbr@physics. uni-mysore.ac.in

Contract grant sponsor: UGC, India (to L.M.M).

DOI 10.1002/pen.24737

Published online in Wiley Online Library (wileyonlinelibrary.com).

Caption: FIG. 1. Chemical structure of nylon 6.

Caption: FIG. 2. (a) Chemical structure of E-glass fibers surface treated with silane. (b) Chemical structure of AS4C PAN-CF.

Caption: FIG. 3. Variation of positron lifetime parameters viz., o-Ps lifetime [[tau].sub.3](ns), free volume hole size [V.sub.f] ([[Angstrom].sub.3]), o-Ps intensity [I.sub.3] (%), and fractional free volume ([F.sub.v]) as a function of SGF and PAN-CF loading in nylon 6/SGF and nylon 6/PAN-CF polymer composites. The dotted line is drawn to guide the eye. The solid line represents the linear additivity relation.

Caption: FIG. 4. XRD spectra of pure nylon 6, short glass fiber reinforced nylon 6 polymer composites as a function of fiber loading.

Caption: FIG. 5. XRD spectra of pure nylon 6, PAN-CF, PAN-CF reinforced nylon 6 polymer composites as a function of fiber loading.

Caption: FIG. 6. Hydrodynamic interaction parameter of nylon 6/SGF and nylon 6/PAN-CF reinforced polymer composites as a function of SGF and PAN-CF loading.

Caption: FIG. 7. FTIR spectra of silane treated E-glass fiber, SGF reinforced nylon 6 polymer composites for different fiber concentration.

Caption: FIG. 8. FTIR spectra of PAN based CF, PAN based CF reinforced nylon 6 polymer composites for different fiber concentration.

Caption: FIG. 9. Schematic diagram of possible chemical interaction between epoxy-coated oxidized PAN-CF and nylon 6 matrix.

Caption: FIG. 10. Variation of free volume hole size ([V.sub.f]) and mechanical properties of nylon 6/SGF and nylon 6/PAN-CF reinforced polymer composites as a function of fiber loading.
TABLE 1. The properties of E-glass fibers and
PAN-CF used in nylon 6 polymer composites.

Fiber properties                   E-glass fibers   PAN-CF

Filaments diameter ([micro]m)            16            7
Tensile strength (MPa)                 1,720         4,300
Young's modulus (GPa)                    72           230
Density (g/[cm.sup.3])                  2.60          1.8
Average molecular weight (g/mol)       81,300       100,000
Length (mm)                              3             4

TABLE 2. The crystallinity of nylon 6, nylon 6/SGF, and nylon
6/PAN-CF reinforced polymer composites as a function of
SGF and PAN-CF loading.

Nylon 6/SGF composite   Crystallinity (%)   Nylon 6/PAN-CF composite

Nylon 6                 30.58 [+ or -] 2            Nylon 6
Nylon 6 + 10 wt% SGF    24.62 [+ or -] 2     Nylon 6+10 wt% PAN-CF
Nylon 6 + 20 wt% SGF    26.54 [+ or -] 2    Nylon 6 + 20 wt% PAN-CF
Nylon 6 + 30 wt% SGF    27.43 [+ or -] 2    Nylon 6 + 30 wt% PAN-CF
Nylon 6 + 40 wt% SGF    28.71 [+ or -] 2    Nylon 6 + 40 wt% PAN-CF

Nylon 6/SGF composite   Crystallinity (%)

Nylon 6                 30.58 [+ or -] 2
Nylon 6 + 10 wt% SGF    30.16 [+ or -] 2
Nylon 6 + 20 wt% SGF    31.65 [+ or -] 2
Nylon 6 + 30 wt% SGF    33.33 [+ or -] 2
Nylon 6 + 40 wt% SGF    35.65 [+ or -] 2
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Author:Munirajappa, Munirathnamma Lakkur; Basavaraju, Ravikumar Harijan
Publication:Polymer Engineering and Science
Article Type:Report
Date:Aug 1, 2018
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