Study of thermal and mechanical behavior by analyzing reinforcement effect of graphene nanoplatelets on polyamide-66 composite system developed via melt-mixing technique

In the present study, graphene nanoplatelets (GNP) reinforced polyamide 6,6 (PA-66) composite was studied to investigate the thermal and mechanical properties of PA-66/GNP composite. The composites were developed by varying wt% (1, 3, 5, and 10 wt%) of GNP loading using a co-rotating, intermeshing, twin-screw extruder via melt-mixing and injection molding process. In order to understand the thermal and mechanical behavior of PA-66/GNP composite, various thermal (TGA and DSC) and mechanical (tensile, impact, and flexural) tests were carried out. The FTIR spectral analysis was done to identify the presence of different functional groups in the PA-66/GNP composite, indicating the strong enough Vander-Waals interaction between the PA-66 matrix and GNP filler contents. The TGA result shows a significant enhancement in the thermal stability of the composite by increasing wt% of GNP. The DSC analysis exhibits a significant reduction in enthalpy of fusion (∆Hm) and a decrease in the degree of crystallinity with increasing wt% of GNP, reflecting a depressed form of α-crystalline structure. Further, the significant growth in tensile modulus and tensile strength were identified under the mechanical performance of the PA-66/GNP composite. An increasing trend in tensile modulus and tensile strength characteristics was observed, and tensile modulus exhibited an enhancement of ∼96% than pure PA-66 at 10 wt% of GNP. Also, the tensile strength is found to be ∼16% higher than that of pure PA-66 matrix. Similarly, the impact test result shows a decreasing trend in impact strength on increasing wt% of GNP reinforcements, indicating the restriction to the molecular mobility due to improved brittle behavior. Further, the flexural modulus is found to be increased by ∼28% at 10 wt%, and the flexural strength is found to have an enhancement of ∼9% at 3 wt% than pure PA-66 polymer matrix of GNP loadings, respectively. The influence of GNP filler content reinforced PA-66 composite on the thermal and mechanical properties is found to be noteworthy.


Introduction
The reinforcement of nanoscale or microscale fillers to enhance the specific material properties has triggered the development of composite materials and has suggested the path for enormous research opportunities in science and engineering [1].Nanoscale fillers such as carbon nanotubes (CNT), nano-clays, nanodiamonds, and graphene have been recognized as ideal multi-functional filler materials [2,3], but the transfer of their potential properties depends upon the better dispersion of these fillers on the matrix material.The better dispersion leads to a more significant interaction of fillers to the host matrix material.These conditions, which may be achieved by solution mixing and in situ polymerization processes, are typically challenging to achieve by melt compounding despite the fact that it is simpler, more economical, and the most industrially scalable technology.In this instance, the high melt viscosity and predominance of filler-filler interactions over filler-matrix interactions reflect technological hurdles that must be met and overcome to expand the large-scale manufacturing of nanocomposites and the scope of their practical applications.
Graphene stands out among the aforementioned nanofillers for its competitiveness, particularly in terms of cost-effectiveness [1].Fundamentally, graphene, a two-dimensional (2D) unique structure with SP 2 carbon atoms arranged in a honeycomb structural pattern, is a single layer of graphite, which can be produced at high purity from graphite flakes, an abundant natural resource.Graphite and graphene platelet carbon fillers appear more significant than other traditional additives for enhancing the conductivity network in composite materials [4].
The platelet-shaped graphene layers (commonly known as graphene nanoplatelets (GNP)) exhibit better reinforcement effects than any other carbon-based nano/micro-sized fillers [5].The GNP has been recognized as one of the potential carbon fillers with high thermal conductivity, superior mechanical properties, and excellent electronic transport properties that provide a significant increase in the structural and functional properties at low loadings for polymer-based composite materials [6].Over time, the easy procurement of graphene has shifted the researcher's focus, already engaged in the studies of polymer composites with nanoclays or CNTs, towards the graphene-based composites for which the dispersion issues are successfully tackled through functionalization [7,8], thermal treatments [9,10] or the use of surfactants [11].
Further, the mechanical, physical, and chemical characteristics of polymers are ultimately determined by the structure and organization of the macromolecules [12].Therefore, one essential requirement for comprehending the macroscopic characteristics of polymeric materials is the precise characterization of molecular order [13].Polyamide-6,6 (PA66) is one of the most widely used engineering thermoplastic polymers that possess crystalline structure in which the crystals melt at high temperatures, and this makes them suitable and promising candidates for the application requires high thermal stability, high strength stiffness properties, chemical and abrasion resistance properties.PA66 offers significant hardness and good dimensional stability [14].The outstanding mechanical properties in terms of resistance and toughness have made PA-66 suitable for employing widely in a variety of industrial sectors, particularly the automobile industry, where the PA-66-based materials are greatly met the ongoing efforts to replace metal parts with plastic components along with decreased in weight and cost [1].However, the poor thermal stability, low electrical conductivity, and the high percentage of shrinkage of polyamides (PAs) with growing market requirements limit the utilization of polymer materials.Therefore, the reinforcement of nanofillers such as carbon-based nanoparticles meet the market demand.In this regard, the development of new material by the inclusion of the graphene nanoplatelets due to its high thermal, mechanical, and electrical properties along with high surface area [15] seems to be a promising filler to develop an ideal composite structure [16,17], and have been given more attention [18,19].Also, the poor dispersion and poor binding affinity with matrix material directly affect the reinforcement efficiency resulting in a poor interfacial interaction between the graphene and polymer matrix.
In this frame, several investigations have been carried out that have paid significant attention to the development of a composite system by modifying PA-66 through the integration of graphene nanoplatelets as a filler material [20][21][22].In this regard, Lin et al [23] have reported their investigation on the mechanical, morphological, and thermal properties of GNP-filled PA66 nanocomposite prepared via the melt blending technique.The investigation found a significant enhancement in thermal conductivity with 50 wt% GNP loading, which was further decreased by the addition of Al 2 O 3 and replacing GNP.However, the mechanical strength is found to be increased.Also, when graphite was introduced to replace the GNP, the thermal conductivity was found to remain unchanged with decreased mechanical strength.Fukushima et al [24] investigated the mechanical, thermal, and electrical properties of GNP-loaded PA6 composites prepared via injection and compression molding.Their findings indicated that adding 20% more GNPs to a PA6 matrix increased the flexural modulus by more than 400%, but that nano clay composites had higher flexural strength values.This suggested that the GNPs' surface conditions were not optimal for PA6, which led to low-strain debonding of the particles.In subsequent experiments, the same group found electrical percolation thresholds for xGnP-1, xGnP-15, and xGnP-100 of around 7, 10, and 5 vol%, respectively.These findings imply that when the aspect ratio of the conductive fillers increases, the percolation threshold of the composite falls because increased particle-to-particle contact is made possible [25].Similarly, GNP/PA6 nanocomposite developed by Mayoral et al [26] via twin-screw extrusion and melt mixing technique.The investigation revealed how particle diameters affected the characteristics of composites.The crystallinity of the GNP/PA6 composites dramatically increased by 110%-120% after the addition of 20% GNPs.When the screw speed rises, the GNPs dispersion grows.The crystallinity also improves by 3%-5% as the screw speed rises from 50 to 200 revolutions per minute.
Similarly, in another experimental investigation, Ong et al [27] developed a GNP-filled PA11 composite via the twin-screw extrusion technique and the melt compounding method.The thorough analysis found that the nanofiller addition significantly improved the conductivity of the composite material with just little mechanical property losses.However, it is worth mentioning that the degree of dispersion and agglomeration that develops during the composite preparation is influenced by the varied wt% of GNP filler loading with different sizes and thicknesses, which results in a high-stiffer nanocomposite.In a pertinent investigation, Harekrushna et al [28] took into account the two distinct GNP filler classes (H25 and M25-grade) with comparable diameters (25 μm) and various thicknesses of 6-8 and 15 nm.According to the study, the interfacial adhesion between the GNP and PP matrix is improved by the thinner GNP, which further reduces agglomeration.Reduced thickness of GNP compared to nanocomposites developed from H25-grade GNPs, results in a high degree of dispersion.Also, the reduction in sheet thickness increases the effectiveness of filler reinforcement.Also, increased surface area for M25-grade GNP reinforced composite with lower GNP sheet thickness enhances the capacity to prevent fracture propagation, resulting in an increase in strength.The study also reveals that the M25-graded GNPs reinforced composite is stiffer than the H25-graded GNPs composite, but the stiffness is significantly reduced.A similar study reported by Kalaitzidou et al [29] examines the possibility of employing exfoliated graphite nanoplatelets, or xGnP (with ∼10 nm thick, ∼1 μm diameter) graphene sheets as reinforcement in polypropylene, or PP via melt mixing and injection molding technique were used to develop xGnP-PP nanocomposites.Compared to the other reinforcements in vol.% used, it is concluded that the smaller aspect ratio xGnP has the strongest influence on the mechanical properties of PP at loadings up to 5 vol.%.This is due to the compatibility of the exfoliated graphite nanoplatelets with the PP matrix and the exceptional mechanical properties of xGnP.Furthermore, in another study reported by Um et al [30], four different grades of GNP, such as H100, M25, M5, and C750, with their average particle diameters of 150μm, 25μm, 5μm, and 2μm along with their respective surface area have been considered.The study shows that the dispersion of large-sized GNP particles with a greater number of layers (H100 and M25 grades) causes a reduction in mechanical properties due to inferior dispersion and aggregation of larger particles.Furthermore, the larger-sized particles exhibit void formation that develops pathways for corrosive agents to diffuse into the composite.The smaller-sized GNP particles in PU/M5 and PU/C750 composites show a uniform dispersion without void formation.Further, the smaller-sized GNP particles with uniform dispersion increase the surface area, providing an efficient pathway that can suppress the penetration of corrosive agents.
Further investigations are still required to understand the influence of GNP filler content on the performance of PA-66 composites.Hence, the current study focuses on the development of PA-66/GNP composite via melt-mixing technique using a co-rotating intermeshing twin-screw extruder.The synergistic effect of 1, 3, 5 and 10 wt% GNP reinforced PA66 composite was studied on the thermal and mechanical properties of PA-66/GNP composites.

Methodology and experimental details 2.1. Materials
The present research is well focused on the polymer compound constituted by (unfilled natural, PXR-01 NC) Polyamide-6,6 supplied by Next Polymers Ltd. under the trade name Celanese with a density of 1.14 g cm −3 , which has been utilized as the matrix material and graphene nanoplatelets were purchased from Signa-Aldrich Chemical Pvt.Ltd. (USA) has been introduced as reinforcing phase.The latter, having a density of 2.2 g cm −1 , is available in the form of fine particles with a particle size of 5 μm.

Melt compound preparation
The raw ingredients were extruded in a co-rotating twin screw extruder (Haake Minilab, Germany).After initially being dried in a hot air oven for 8 h at 90 °C temperature.First, the dried PA-66 granules were fed from the hopper to die by maintaining the screw speed and screw temperature at 125 rpm and 275 °C, respectively.It was allowed to melt for 2 min, and then GNP filler was fed to melt mixing for another 3 min in the die.
The melt mixing compound was then collected in the cylinder, in the form of compounded filaments, having a cylinder temperature of 325 °C.The collected melt compound was then fed into the injection molding machine (Haake Minijet, Germany), by maintaining the mold temperature at 75 °C, and an injection pressure of 500 bar was applied for 10 sec.Several samples were formulated by injection molding technique with the abovementioned processing parameters.

Characterization techniques
The FTIR spectral analysis was carried out for the identification of functional groups available in the prepared composites using a Perkin Elmer 100 spectrometer with an ATR sampling accessory.All the spectra were recorded at a resolution of 4 cm −1 with 32 scans in the mid-infrared radiation (MIR) region of 4000 cm −1 to 400 cm −1 .
On samples weighing 3-5 mg sealed in an aluminium crucible, the DSC tests were run by employing the heat-cool-heat method to estimate the melting and crystallizations behavior of the polymer composite at a scanning speed of 10 °C min −1 , under the inert nitrogen condition using the Discovery, DSC-25 (Waters, USA), following a standard step-by-step test protocol that included heating the samples from 25 °C (room temperature) to 300 °C, isothermal stasis at 300 °C for 1 min, cooling back down to room temperature, and then heating up again to 300 °C.In all non-isothermal phases, a thermal rate of 10 °C min −1 was constantly maintained.In the present study, the compounds are melt-compounded prepared composite was compared to the pure unfilled PA66 matrix compounded samples produced under the same circumstances as described in this study.
Further, the DSC thermogram processing is mainly limited to the second heating in order to exclude the any previous thermomechanical history of thermal processing as well as essential to stabilize the material.The heat was computed employing the integrals between the determined onset points of the relevant peaks and the areas under the curves.The average values of the enthalpy of fusion, ΔH f , were used to calculate the percentage of crystallinity (X c ) using the relation given in equation (1).
X c represents the percentage crystallinity; H f D denotes the enthalpy of fusion; and H m 0 D indicates the enthalpy of melting of 100% crystalline material taken as 230 J/g for PA66 [31].
Bond energy is principally responsible for thermal stability.The polymer deteriorates as the temperature rises to the threshold at which the vibrational energy ruptures a bond link [32].In this regard, for in-depth and better understanding of thermal stability characteristics of GNP/ PA66 composite system, the TGA analysis was performed using a Discovery, SDT-650 (Waters, USA) TGA analyzer which enables the recording of the weight loss data of the substance as a function of temperature.The studies were carried out using samples (about 3 mg) in a platinum pan under inert nitrogen environment with nitrogen flowing at a rate of 50 ml/min.The samples were then heated at a rate of 10 °C min −1 from 30 to 800 °C.
Further, the developed composite samples were assessed for the investigation of tensile and flexural characteristics according to the ASTM D638, and ASTM D790 standards respectively.All mechanical testing were conducted using a universal testing machine (UTM Instron-3366) dynamometer based with a load cell capacity of 30 kN and a crosshead speed set to 5 mm min −1 .For flexural tests, the parallelopiped specimen were loaded in a three-point bending test arrangement configuration in a same UTM Instron-3366.For both the tests, the maximum of three samples were tested to ensure the repeatability and accuracy of the test data that have been later provided in terms of mean data and standard deviation.
Izod impact tests were carried out at room temperature using an Izod impact tester IT504 (Tinius Olsen, INDIA) impact tester with 11.224 J hammer according to the ASTM D256.A total of three samples were tested for analyzing the impact strength of the prepared GNP filled PA-66 composite system.Izod impact test was carried out in tension mode (notch opening mode).
The spectral range from 1642-1628 cm −1 ;1520-1537 cm −1 attributes to C=O amide I [35] and stretching of N-H & C-N amide II [36] respectively.CH 2 twist-wag vibration was observed at 1201 cm −1 ; CH 2 twisting was observed at 1446.67 cm −1 .The addition of GNP in PA66 resulted in change of CH stretching from 3316 to 3303 cm-1 indicating Vander-Waals interaction in the blending system [37].With addition of GNP in PA66 revealed a decrease in the intensity of -C=O indicating less hydrogen bonding due to decrease in peak intensity [33].

Thermal properties 3.2.1. Thermogravimetric analysis
The TGA curve of the unfilled PA-66 and PA-66/GNP composite is illustrated in figure 3 and thermal stability property is tabulated in table 1.From the figure, it can be clearly observed that the onset temperature (T onset ) of pure PA-66 started increasing after the addition of GNP as a filler content.In this study, the thermal degradation of GNP-filled PA-66 composite system, pure PA-66 exhibits thermal decomposition at temperatures ranging from 330 to 550 °C, and its 5% and 50% weight loss is detected at the temperatures 294.93 °C and 448.87 °C, respectively.The thermogram delineated in figure 3 exhibits a slight enhancement of the thermal stability of PA-66 due to an increase of varying wt% of GNP filler reinforcement.At 3 wt% of GNP filler loading, the 5% and 50% of weight reduction are found at 353.70 °C.and 450.770 °C with an increase of 33.27 °C and 1.9 °C, respectively.Similarly.At 5 wt% of GNP addition, an increase of 61.28 °C and 2.98 °C was recorded.Further, the 5% weight loss of pure PA-66 may be due to the hydrophilic nature of polyamide that absorbs moisture content and the small wide peak shows the evaporation of the moisture contents.At 3 and 5 wt% of GNP addition an FTIR spectra determination for PA-66, PA-66/ GNP_1%, PA66/ GNP_3%, PA-66/ GNP_5%, PA-66/ GNP-_10% composite system.enhancement in thermal stability can be seen [38][39][40].This higher thermal stability may be due to the formation of a protective layer of GNP over the PA-66 matrix phase that hinders the spread of temperature to the matrix phase and delays the thermal decomposition of the developed PA-66/GNP composite [41].At 1 wt% of GNP addition, no significant weight loss is observed at 50% weight loss of the composite system.This indicates that 1% GNP loading was not reaching at percolation threshold for the PA-66/GNP composite system [42].
The findings in the present work suggest that the Pure PA-66 is thermally stable at ∼ 300 °C temperature.a single-step degradation of the PA-66/GNP composite system was observed that starts at a temperature ranging from 330 °C to 450 °C.The addition of varying wt% of GNP filler gradually increases the onset temperature by 1.067 to 4.892 °C for 50% weight loss than unfilled PA-66.This enhancement in thermal stability may be the result of the maximization of the physico-chemical characteristics due to the incorporation of GNP fillers.Since the gradual GNP addition provides the barrier effect which is more effective in preventing the emission of degraded products, yielding a higher barrier performance [40,43].The previous work identified in the literature survey supports the present results of thermal stability for GNP filler as well as other filler materials-based composite materials.Similar, results have been reported by Shueb et al [32] that exhibit a slight improvement in the thermal stability of the PA-66/GNP composite material.This indicates an increase in the onset degradation temperature by 10 °C.Another study carried out by Russo et al [1] explains the slight improvement in thermal stability due to GNP loading in PA-6/GNP composite.Further, the thermal property of GNP-reinforced polycarbonate (PC) (virgin and recycled) composite was also studied by Wijerathne et al [44].The study shows that thermal stability and glassy transition temperature (Tg) were only significantly impacted by the incorporation of GNP into the PC matrix.The highest GNP loading for recycled PC/GNP composites was 10 wt% (2.42% increase over recycled PC), whereas the highest GNP loading for virgin PC/GNP composites was 1 wt% (2.74% increase over virgin PC).Recycled PC-based composites exhibited lower thermal stability than virgin PC-based composites under the same GNP loading.In another research, Duan and his co-workers [45] investigated the PA-66/modified clay (Mclay) nanocomposite for their properties.The study reported no significant change between the TGA curves of PA-66 and PA-66/mClay nanocomposite, indicating that the mClay enforcement does not play a significant role in PA-66 thermal decomposition, resulting in an increase in carbon residues with increased mClay loading.

Differential scanning calorimetry
The effect of GNP addition (in the PA-66 polymer matrix phase) on the thermal transition was studied through the DSC technique.Figure 4 compares the thermograms of the composite compounds via single heating curves examined with respect to the unfilled PA-66 polymer matrix.Through DSC traces, the thermal behavior such as melting temperature (Tm), enthalpy of fusion (ΔHm), enthalpy of crystallization (ΔHc), and degree of crystallinity (Xc%)) were obtained and are presented in table 2. The various parameters such as processing technique, thermal conditions, residual stress, moisture contents as well and filler loadings can affect the crystalline structure of polyamide.The DSC scans are capable of exhibiting the change in crystal structure due to the change in the thermal behavior of the PA-66/GNP composite system.The DSC scans were carried out for unfilled PA-66 and PA-66/GNP composite with varying wt% of GNP reinforcements (1%, 3%, 5%, and 10%).
All the curves containing the varying wt.fraction of GNP loading depicting a single endothermic melting peak whose shape and position seem to be slightly influenced by the addition of varying wt% of GNP loadings.The present study exhibits a significant reduction in enthalpy of fusion (ΔH m ) with increasing GNP loadings.This trend also reflects the significant decrease in the degree of crystallinity of the composite system ranging from 34.43% for pure PA-66 to 25.21% for the compounding composite system containing 10 wt% of GNP addition.This trend can be explained by assuming the effective dispersion of the filler contents that arrests the structural organization of the host matrix phase during cooling [1].Further, the present study also highlights that the GNP reinforcement does not alter the melting temperature (T m ) significantly of all composite compositions.This exhibits that PA-66 possesses the highest value of percentage crystallinity (X c ) of 34.431%.Further, the addition of 1 to 3 wt% of GNP loading does not affect the T m , but the decrease in percentage crystallinity is observed from 28.899% to 25.213%.However, the melting T m peak, slightly shifted towards the lower temperature range with increased GNP loadings.This indicates the depressed form of the α-crystalline structure.This may happen due to excess wt% of GNP that arrests the free movement of polymeric chains.
Further, the cooling thermograms, as depicted in figure 4(b), exhibit an increase in onset crystallization temperature (Tm) on a gradual increase in GNP loading percentage.This indicates that GNP is acting as a nucleating agent for PA-66, resulting in an increase in activation energy for crystallization [4,32,46].The tensile tests for the unfilled PA-66 and the prepared GNP/PA-66 composite were carried out to investigate the influence of the GNP filler dispersion in varying wt% over the PA-66 thermoplastic polymer matrix.The tensile properties of a composite system are the result of the distribution and orientation of filler particles in the matrix phase.Figure 5 demonstrates the tensile modulus on the reinforcement of varying wt.% of GNP filler in PA-66 matrix phase.A continuous increasing trend along with higher wt% in the tensile modulus can be clearly seen.The measured values of tensile properties of the prepared composite system are tabulated in table 3. The increasing trend of modulus is due to the arrangement and orientation of macromolecular polymeric chains along the direction of tensile loading of the specimen.Hence, GNP reinforcement provides a stiffening characteristic to the sample along with increasing and varying GNP filler contents.In the present study, PA-66 showed a tensile modulus of ∼1400 MPa, while the highest tensile modulus value of ∼2800 MPa (95.58% higher) was observed for 10 wt% of GNP filler loading.The constrained polymeric chains and higher stiffness of GNPs have more likely increased the stiffness characteristics of the GNP/PA-66 composite system [47,48].In our study, the present developed PA-66/ GNP composite system, the tensile moduli were observed to be increased by 39% at 1% GNP, 53% at 3% GNP, 61% at 5% GNP, and 96% at 10% GNP loadings as compared to the unfilled PA-66 matrix phase, and the maximum tensile modulus value achieved was 2801.979MPa.However, it should be noted that dispersion often worsens as nanoparticle loading increases, suggesting that the improvement may not be due to enhanced dispersion [26].There are very few literatures available on GNPreinforced Polyamide composite.The increase in tensile modulus is well documented in the literature.Thanh et al [49] reported the effect of GNP loading on the mechanical and structural characteristics of PA-66 elastomer nanocomposite.They observed a similar increasing trend in tensile modulus of 2300 MPa at a maximum of 10% GNP loading., but with different processing techniques.The improved tensile properties in the present study are likely due to the enhanced dispersion of nanoplatelets as a result of the deployment of the twin screw extrusion process.The melt compounding of the polymer nanocomposites at increased screw speed decreases the  agglomerate size as well as enhances the distribution and dispersion of nanoparticles in the matrix phase, and also the increased shear forces imparted, which in turn increases the mixing energy input.Therefore, the overall bulk properties of the composite are enhanced [49].
Other similar findings have also been reported by Mayoral et al [26].The study shows a similar increasing trend to our research.Mayoral and his coworkers have studied the GNP/PA-6 composite system produced by melt-compounding techniques via a twin-screw extruder.The study reported an increased tensile modulus from 28% (at 5% GNP loading) to 376% (at 20% GNP loading).Similarly, an agreement of increased tensile modulus has also been reported by King et al [50] .In their study of PC/GNP systems developed by extrusion and injection molding, the tensile modulus increased from 2.2GPa (neat polymer) to 3.5GPa at 8%wt GNP (59% improvement) and 5.9GPa at 15%wt GNP (168% improvement).In another research of an extruded and injection-molded PP/GNP system, Kalaitzidou et al [29] observed the increased tensile moduli of 3 GPa at 8 wt% GNP loading and 5 GPa at 15 wt% GNP loadings.The aspect ratio of the filler and the interaction at the filler-polymer interface have a significant influence in defining the mechanical characteristics of the finished composite specimens, in addition to the aggregation, orientation, and alignment of the nanoparticles inside the polymer matrix phase.
Further, the increasing trend in tensile strength up to 5 wt% of GNP loading can be clearly observed from figure 6.The decrease in tensile strength at higher GNP loading (at 10 wt%) is probably due to the occurrence of agglomeration of GNP filler.The increase in tensile strength is attributed to the proper dispersion of filler, good interfacial adhesion, high aspect ratio, and better interfacial stress transfer as compared to the unfilled PA-66 thermoplastics matrix.At a higher GNP loading of 5 wt%, an increase of tensile strength of ∼16% is observed.This increase is due to uniform stress distribution with better dispersion, appreciable interfacial stress transfer, and good interfacial adhesion of GNP fillers with the matrix phase.The interfacial stress transfer minimizes the presence of a stress concentration center [32].This may also be attributed to the fact that tensile stress at maximum load is governed by the specimen failure, which is caused by damage accumulation.Additionally, GNP fillers may behave as the locations of stress concentration where crack initiation may take place significantly sooner under tensile loading, and ultimately, the failure occurs at a lesser stress value [41].Although the use of interfacial compatibilizer or surface functionalization can be used to compensate for this strength reduction.Such approaches (utilizing coupling agent chemicals) are expensive and difficult, which may not be advantageous for industry-level production at large scale [51].
Further, from figure 7, a decreasing trend in strain at break as a function of GNP loading in the PA-66 matrix phase can be clearly seen.The strain at break is an important characteristic of mechanical testing to analyze the ductile behavior (tensile fracture toughness) of specimens.In this work, at a maximum of 10 wt% of GNP loading, the maximum decrease in strain at break was observed, which suggests the addition of GNP loading at higher wt%, leading the composite samples from ductile to brittle transition phase.This enhancement of higher GNP loading wt% shows the reduction of toughening effects of the composites.This reduced toughening effect is due to the large surface area occupied during the formation of an agglomerate of higher GNP loading [52].The agglomerate does not provide proper adhesion of filler to the matrix phase in the polymeric system.Also, the higher GNP loading generates the restrictive effect in the polymeric chains' mobility, resulting in the reduction in strain at break for the composite system.The present work finds good agreement with previous literature Further, the strain at break is majorly influenced by the filler-matrix interactions, the aspect ratio of fillers, and filler dispersion.Also, any boundary defects developed during the cutting or specimen-finishing process may lead to premature failures [1].

Impact test result
The effect of GNP filler content on the measured Izod impact test for PA-66/ GNP composite system as a function of varying wt% of GNP filler can be seen in figure 8.The unfilled PA-66 composite was found to have the highest impact strength of 50.147 +/− 1.463 J/m.From Fig, a decreasing trend in the impact strength can be clearly observed.The synergistic effect of GNP fillers is the result of the reduction of impact strength of the developed composite system.The high impact strength of unfilled PA-66 is due to the good ductile nature of the unfilled PA-66.However, the addition of GNP filler contents continuously decreases the impact strength.This decrease in impact strength is due to the restriction of molecular mobility of the composite system due to the improved brittle behavior on GNP reinforcement [53].At 3 wt% of GNP loading, a slight reduction in impact strength with the value of 39.4477 J m −1 can be observed, whereas on further GNP loadings of 5 and 10 wt%, a  rapid reduction in impact strength to the values of 23.4032 J m −1 and 18.616 J m −1 can be identified, respectively.The loss of impact strength is due to the agglomeration of the GNP fillers at higher wt%, which improved the brittleness along with the proportion to hardness.This may weaken the structure of GNP fillers [54].This reduced impact strength provides a good agreement with previous works carried out in the literature, which shows that the higher loading content of graphene-based filters reduced the impact toughness of thermoplastics [41,[55][56][57].In the present study, we also deduced the reason of reduced impact strength due to brittle fracturing phenomena of the PA66/ GNP composite system due to no sign of plastic deformation of PA-66 matrix was traced.In the present study, we also deduced the reason for reduced impact strength due to brittle fracturing phenomena of the PA66/GNP composite due to no sign of plastic deformation of the PA-66 matrix.The synergistic effect of GNP filler weakens the composite system due to the agglomeration of GNP particles on the addition of higher wt%.This may lead to the probability of more void formation and improper mixing of fillers to the matrix phase.Also, the agglomerated GNP contents induce the microcracks that act as internal notches, resulting in decreased impact strength [58].

Flexural test results
Representative plots of flexural modulus and flexural strength for pure PA-66 and PA-66/GNP composite are delineated in figures 9(a) and (b).For flexural test results, the elaboration of each plot has been provided by taking the mean of all five tested samples.From figure 9(a), the flexural stiffness (modulus) seems to be in increasing trend as the wt% of GNP filler increases.The reinforcing effect of GNP fillers produces an increase of ∼28% in flexural modulus at a maximum of 10 wt% of GNP loading.This is due to the homogeneous dispersion of GNP, no agglomerates formed even at higher GNP loading, and better filler-matrix interaction due to sufficient surface area of GNP distributed over the matrix phase [29].At 5 wt% the performance of GNP addition is slightly affected due to proper dispersion but still showing the higher flexural stiffness than unfilled PA-66 polymer.
Similarly, figure 9(b) exhibits an increase in flexural strength due to the lower reinforcement effect of GNP filler.This increment in flexural strength is significant at a maximum of 3 wt% of GNP loading, showing an increase of ∼9% to unfilled pure PA-66 matrix.This is mainly due to the significant dispersion, less aggregation, and retention of GNP reinforcement with the matrix phase during polymer processing.Further, higher GNP loading (5 to 10 wt%) indicates a noticeable reversed tendency due to lower adhesion and insufficient dispersion with poor filler matrix interaction that contains a larger surface area on increased GNP loading [28,29].This may also be attributed to the fact that tensile strength is determined by failure, whereas flexural strength is calculated at maximum strain reached at 5%.Moreover, under flexural loading the upper surface of specimen experiences the compression, and the lower surface experiences tension about the neutral axis of the specimen [41].

Conclusion
In the present study, the effect of varying wt% (1%, 3%, 5%, and 10%) of GNP filler reinforcement in the PA-66 matrix phase for analyzing their thermal and mechanical behaviors has been successfully studied for the PA-66/ GNP composite system that has been developed via milt-mixing and injection molding technique as per the ASTM standards.The present findings suggest that the thorough mixing of GNP filler in the PA-66 matrix phase exhibits an appreciable mechanical and thermal performance of the prepared composite system.The performance of the PA-66/GNP composite system has been thoroughly investigated via various thermal and mechanical tests that have been summarized as follows: • The TGA result indicated a slight improvement in thermal stability on the inclusion of varying wt% of GNP filler, which was attributed to the formation of a protective layer of GNP fillers in the PA-66 matrix phase.The results show that pure PA-66 is thermally stable at 300 °C.A single-step degradation occurs at a temperature range of 330 °C to 450 °C.The addition of varying wt% of GNP filler gradually increases the onset temperature by 1.067 to 4.892 °C for 50% weight loss than unfilled PA-66.
• Similarly, the DSC result shows a considerable decrease in degree of crystallinity, from 34.43% for unfilled PA-66 to 25.21% for compounded composite with 10wt% of GNP loading, along with a significant drop in enthalpy of fusion (Hm) with increasing GNP loadings.This pattern is related to the filler's efficient dispersion, which, during cooling, stops the host matrix phase's structural organization.Further, the cooling curve shows that with higher GNP loadings, the melting T m peak was slightly shifted to a lower temperature range.This reveals the α-crystalline structure in its depressed state.This could occur because excessive weight percentages of GNP hinder the free mobility of polymeric chains.
• Reinforcing the GNP filler over the matrix phase is found to improve the tensile modulus and tensile strength significantly.PA-66 was found to have a tensile modulus of ∼1400 MPa in the current study, whereas the greatest modulus value of ∼2800 MPa (95.58% greater than unfilled PA-66) recorded after 10% GNP filler loading is remarkable.
• Similarly, the tensile strength of the prepared composite with 5 wt% of GNP loading was found to be increased by ∼16% than unfilled PA-66.Further, the ductile behavior of PA-66 is found to be decreased by higher GNP loadings.the reinforcement of the increased wt% of GNP loading, resulting in the ductile to brittle transition and producing a reduction in the toughening effect of the specimen.
• Further, a significant increase in flexural modulus and flexural strength was observed at varying wt% of GNP loadings.however, maximum flexural strength was identified at 5 wt% of GNP loading.Higher GNP loading leads to a reverse tendency due to lower adhesion and insufficient dispersion with poor filler matrix interaction.

Figures 2 (
Figures 2(a) and (b) depicts at 1633 cm 1 , decrease in band intensity of amide I as hydrogen bonds occurs with amide I functional group.

Figure 2 .
Figure 2. Enlarged FTIR spectral range depicting (a) N-H bonding, and (b) C=O bonding in various wt% of PA/GNP composite system.

Figure 5 .
Figure 5. Tensile modulus of PA-66/ GNP composite as a function of the GNP loadings.

Figure 6 .
Figure 6.Tensile strength of PA-66 and PA-66/ GNP composite system as a function of the GNP loadings.

Figure 7 .
Figure 7. Strain at break of unfilled PA-66 and PA-66/ GNP composite system as a function of the GNP loadings.

Figure 8 .
Figure 8. Impact property of PA-66 and PA-66/ GNP composite system as a function of the GNP loadings.

Figure 9 .
Figure 9. PA-66 and PA-66/ GNP composite system plot as a function of the GNP loadings for (a) Flexural modulus, and (b) Flexural strength.

Table 1 .
Summarized thermal stability data of PA-66 and PA66-GNP composite system.

Table 2 .
Thermal transition properties of PA-66 and PA-66/GNP composite system.

Table 3 .
Tensile Test result of unfilled PA-66 and GNP/PA-66 composite system.