Compatibilization of Immiscible PA6/PLA Nanocomposites Using Graphene Oxide and PTW Compatibilizer for High Thermal and Mechanical Applications

The aim of this work is synthesis a novel nanocomposite containing Polylactide (PLA) and polyamide 6 (PA6) reinforced with graphene oxide (GO) and poly ethylene-butyl acrylate-glycidyl methacrylate) (PTW) compatibilizer during solvent-based method. For this purpose, GO was added to the nanocomposite with 0.1, 0.3, 0.5, 0.7 and 1 phr. Morphology, rheology and mechanical properties of nanocomposites were studied with scanning electron microscopy (SEM), transmission electron microscopy (TEM) and (DMTA) which showed rougher fracture surface due to the presence of compatibilizer and an increase in the amount of graphene oxide and better dispersion of graphene oxide. The results of experimental and theoretical studies of mechanical properties showed that increasing the concentration of graphene oxide in the presence of PTW improved the tensile strength, impact strength and tensile modulus in the PA6/PTW/PLA system. The study of rheological properties showed an increase in storage modulus and complex viscosity, which also confirmed the role of PTW compatibilizer in better GO dispersion. So, PA6/PTW/PLA is a good candidate for mechanical and high thermal applications.


Introduction
There is various type of polymers which is used in modern lives and industries such as automotive, bioengineering, biomedical, thermal, mechanical and so on [1][2][3][4][5].Polylactide (PLA), also known as polylactic acid, is a biodegradable, semi-crystalline, or amorphous biopolymer made from renewable natural sources [6][7][8].In fact, PLA is a type of aliphatic thermoplastic polyester that has hydroxyl and carboxyl end groups [9].Over the past two decades, due to its remarkable properties such as renewability, biocompatibility, high transparency and modulus, biodegradability and the ability to replace oil-based polymers, it has received much attention [10][11][12][13].These properties have made PLA commercially available.PLA is suitable for a variety of applications such as absorbable sutures, food and beverage packaging, surgical implants, bone regeneration substrates, porous scaffolds for the growth of nerve tissue and textiles, like many other biodegradable polymers [14,15].Despite its convenient properties and applications, the complexity of using PLA in high value-added or durable applications is limited.These limitations are due to low melt strength, inherent brittleness, slow crystallization kinetics, low impact strength and heat resistance, and instability during processing [16][17][18].In durable applications, materials must have properties that make them capable of long-term use in structural applications over a wide range of temperatures under mechanical stress in difficult chemical and physical environments.Therefore, in order to use PLA, its disadvantages must be corrected [19,20].Many studies have been done to overcome these disadvantages and expand the range of applications of PLA, including the mechanisms of toughening, copolymerization, formation of the spatial complex of PLA, the addition of nanoparticles and nucleating agents and blending with polymers [11,12,21].Among all these approaches, polymer blending has been considered more due to the practicality of this method in making new materials with various and custom properties as well as its costeffectiveness [10,22,23].The use of blends is very common in industry, because blending is a low-cost method for combining the properties of blended components into single materials [22].Blending PLA with engineering polymers is an effective way to overcome most of its limitations [23,24].In recent years, there has been much research on PLA blends with engineering polymers such as polycarbonates [25], polymethyl methacrylate [26], polyesters [27] and polyamides (PAs) [7,28,29].Among engineering polymers, polyamides have received the most attention because the polylactic acid blending with them enhances many of the weak properties of this polymer.Improved properties such as ductility, impact strength and low heat resistance help PLA to enter the consumer market for durable and valuable applications [30][31][32].Polyamides are a class of engineering polymers in which repeating units are bonded together by amide bonds.Polyamides have good dimensional stability and barrier properties, high heat distortion temperature and ductility, chemical resistance, impact resistance and abrasion resistance, which makes them suitable for blending with PLA.In general, PLA/PA blends are immiscible, but of a compatible nature.This compatibility goes back to the hydrogen bonds between the PA amide and ester PLA groups.So far, different types of polyamides have been blended with PLA, including PA6, PA11, PA6-10, PA10-10, PA6-6 and PA12 [33,34].PA6, commonly known as nylon 6, is a valuable thermoplastic and is widely used as a synthetic fiber in construction materials, food packaging, and engineering resins.PLA/PA blending results in blends with poor mechanical properties, as both polymers are brittle.At present, nanofillers such as carbon nanotubes, halosites, organic montmorillonites and graphene are very common to enhance mechanical properties such as stiffness and strength [6,35].It has been reported that nanographene sheets (GNP) have the ability to improve the mechanical and thermal properties of PLA/PA blends [36,37].So far, several studies have been conducted on the use of fillers such as nanoscale graphene to improve the properties of polyamides [38].This improvement includes strengthening the mechanical properties, reducing the permeability and better flame retardancy [39][40][41][42][43]. Studies have shown that nanoparticles with high aspect ratio have complex effects such as lamination.In addition to having exceptional properties such as high tensile modulus (1 TPa) and excellent conductivity, GO has an inherent tendency to interact with polymers due to its single-layer laminated structure [10,44].The potential for thermodynamic interactions with polymeric components due to oxygenated groups is that it participates in hydrogen bonds and leads to effective covalent and non-covalent modifications [45].
O'Neill et al. studied the in-situ preparation and identification of PA6/GO and PA6/rGO [46].They reported the involvement of graphene in advancing the nucleation of microcrystals.Azizli et al. investigated the effect of GO on the morphology, thermal and mechanical properties and rheological behavior of 6 polyamide-chloroprene (PA6/ CR) blends in the presence of maleic anhydride grafted to ethylene-propylene-diene rubber (EPDM-g-MA) prepared by melt mixing method.The results showed good dispersion of GO nanosheets in PA6 phase.GO has been shown to increase thermal stability and can act as a nucleating agent in the crystallization process and increase the crystallization temperature.They also found that with increasing graphene content, tensile strength, Young's modulus, stiffness and impact strength increase and elongation-at-break decreases [47].Kelnar et al. investigated the effect of GO nanofillers on PA6/EPR blend and modified PA6.The results showed that the effect of GO on the structure orientation is comparable to the effect of nanoclay, but the mechanism of dynamic phase behavior is different due to the nature of GO and its interactions with polymer components [48].In order to achieve better final properties, it is necessary to increase compatibility.One of the classic ways to increase compatibility is to use compatibilizers.Various compatibilization approaches are used for PLA/PA blends with the addition of compatibilizers, such as polyalkenyl-polyanhydride-imide/amide, maleic anhydride grafted on polyethylene-octene elastomer and compounds based on epoxy and poly(ethylene-co-glycidyl methacrylate-g-styrene-co-acrylonitrile) [6].Wang et al. used maleic anhydride grafted on a polyethylene-octene elastomer as a compatibilizer in the PLA/PA6 blend.The blends showed good dispersion with smaller particle size, proper formability and good strength.They attributed this improvement in properties to the formation of POE-co-PA6 and POE-co-PLA copolymers, which improved compatibility between the components of the blend [48].Poly (ethylene-butyl acrylate-glycidyl methacrylate) (PTW) is a type of terpolymer with epoxy groups (glycidyl groups).These groups react readily with hydroxyl and carboxyl groups when blended.Butyl acrylate segments can provide excellent low temperature properties.Also, PTW is a very significant modifier for impact resistance, because it has the ability of active processing and toughness [49,50].Yang et al. used a new PTW compatibilizer to improve the compatibility and mechanical properties of 1010 polyamide-polypropylene blends (PA1010/PP).They found that the mechanism of PTW compatibilization lies in the chemical interactions between the PA1010 end groups and its epoxy groups during the blending process, as well as between the polypropylene and the PTW ethylene groups.Therefore, the addition of PTW drastically reduces the interfacial tension and size of the region, thus greatly improving the mechanical properties of the blend [51].
Introducing and use of polymeric materials for high temperature and mechanical applications is a challenge in various industries.So far, a composite containing PTW, PA6 and PLA with GO reinforcement has not been synthesized.In the present work, poly (ethylene-butyl acrylate-glycidyl methacrylate) (PTW) has been used as a compatibilizer to increase the integration of two immiscible polymers (PA6 and PLA) during the solvent-based method.Different amount of graphene oxide was used on the nanocomposites as a reinforcement to increase the mechanical properties and boost the nanocomposite characterization.The nanocomposite characterizations were studied theoretically and experimentally.Mechanical properties such as tensile strength, elongation-at-break, fracture stress and Charpy impact strength, thermal properties such as TGA and DMTA and rheological properties as well as morphology, SEM and TEM of blends with different amounts of GO were determined and compared with the results of theoretical models.

Materials
PA6 with a density of 1.13 g.cm −3 and a melting point of 220 °C, PLA with a density of 1.24 g.cm −3 , PTW compatibilizer with a density of 0.94 g.cm −3 were used in this study.The specifications of these materials are listed in Tables 1  and 2. Dimethyl formamide (DMF) and formic acid were also used as solvent in the preparation of the composites.In this project, Graphite, peroxide (H 2 O 2 ), sulfuric acid (H 2 SO 4 ), hydrazine hydrate and potassium permanganate (KMnO 4 ), hydrochloric acid (HCl), dimethylformamide (DMF), were purchased from Merck (Germany), for the synthesize of GO.
Graphene oxide was synthesized by using Hummers' method [52].Firstly, 450 mg of graphene oxide powder was slowly added in a 750 ml round-bottomed flask under stirring condition and let to achieve a homogeneous yellow-brown solution.An ultrasonic bath was used to obtain uniform GO (150 min).After that, 7 ml hydrazine hydrate was added and continued the reaction for 36 h at 110 °C in an oil bath with water cooled condenser fitted with the round-bottomed flask.A vacuum oven was used to dried the products at 45 °C for 36 h.To break the aggregates and separation of GO sonication was used.

Preparation of PA6/PLA/PTW/GO
To prepare PA6/PLA/PTW/GO nanocomposite, 80 g of PA6, 20 g and 5 g of PTW was first dissolved in 500 mL of formic acid (HCOOH) for 3 h at 60 °C with stirring.In five stages, according to the different amount of GO, the 0.1, 0.3, 0.5, 0.7 and 1 g of GO was dispersed in 200 mL of a mixture of formic acid-dimethylformamide (MDMF) in a ratio of 1:4, by ultrasonication for 4 h at 40 ± 5 °C temperature.The solution was sonicated for 70 min and added to PA6/PLA/PTW/HCOOH solution [53,54].DMF is anti-solvent for PA6/PLA/PTW, so it changes the type of phase separation.PA6/PLA/PTW/ GO nanocomposites float and are easily removed from the surface.After washing the samples in a water coagulation bath, they were filtered in vacuo and ground and mixed to homogenize [53,54].The samples were then washed with deionized water until they reached a neutral pH and finally dried under vacuum for 10 h at 120 °C.The resulting samples were mixed and diluted with PA6/ PLA/PTW to obtain the desired concentrations of GO.After solid mixing, about 4 g of GO-containing samples were obtained and transferred to a discontinuous mixer for melting.The steps for preparing PA6/PLA/PTW/GO nanocomposite are shown in Fig. 1.

Measurement of Mechanical Properties
To measure tensile strength, Iranian-made Hiva tensile tester was used.Tensile test was performed according ASTM D 638.For this purpose, dumbbells of the cured sample with dimensions of 120 × 25 × 25 mm were molded using a 25-ton hydraulic press machine (Defen Bacher, Germany) at a pressure of 160 bar and a temperature of 190 °C with 50 mm/ min crosshead speed.The Charpy impact testing was carried out on the samples.In accordance with the ISO 179 standard, resistance to impact loading and Charpy impact strength were measured at room temperature with an impact tester from Zwick/Roell, Germany.In this method, a notched polymer bar is struck by a pendulum and the energy lost in the fracture is calculated.The velocity of the pendulum's impact was 2.9 m/s.Standard impact test samples were made using injection molding with a back pressure of 160 bars and a temperature of 190 °C from the hopper to the nozzle.

Thermal Properties
The effect of nanofiller on the thermal degradation of nanocomposites was evaluated using a thermogravimetric analyzer (DuPont 951, USA).Under nitrogen flow and a heating rate of 10 °C/min, tests were performed at temperatures ranging from 100 to 800 °C.

Surface Properties Study
The Tescan VEGA-II scanning electron microscope was used to study the morphology and particle size distribution of the dispersed phase in the blends.For this purpose, first all samples were broken down into liquid nitrogen and then placed in tetrahydrofuran solvent for 48 h.The fracture surface of the samples was then coated with gold to make them electrically conductive.To obtain the particle size distribution, 150 holes per SEM image were counted and measured.Then, the obtained data were counted based on the size of the classification and the frequency of each category.To study the dispersion of GO in the polymer blends were studied by using transmission electron microscopy (TEM) model Zeiss EM900 (Italy) using an accelerated voltage of 80 kV.Ultra-thin sections of the prepared nanocomposites were obtained using an ultramicrotome (Leica) equipped with a diamond knife under cryogenic conditions at − 75 °C.

Measurement of Rheological Properties
The linear viscoelastic properties of the melt of all samples were measured with a dynamic rheometer model 4308 made by Zwick company at a temperature of 220 °C with parallel Fig. 1 View of PA6/PLA/PTW/GO nanocomposite preparation steps [53] plate geometry with a diameter of 25 mm and a distance of 1 mm and the rheological behavior was investigated.Frequency scan test from 0.04 to 600 rad.s −1 was performed for all samples.A strain value of 1% was used to place the material response in the linear viscoelastic region in all tests.

Dynamic Mechanical Thermal Analysis (DMTA)
Perkin Elmer (Pyris Diamond model, USA) was used for DMTA test.The test was performed with 3-point bending method in the temperature range of − 100 °C to 100 °C with a frequency of 1 Hz and a heating rate of 0.5 °C.min−1 .

X-Ray Diffraction (XRD) and Raman Spectroscopy Characterization
X-ray diffraction was used to investigate the crystalline structure of nanocomposite with CU-Kα radiation (λ = 1.54 Å) using the 2θ range of 10-70° with step size of 0.04° and count rate of 50 s per step (Shimadzu, Japan).

Investigation and Evaluation of GO Production and Graphite
GO was produced by using Hummers' procedure.The results of the raw graphite, synthesized GO and XRD patterns of synthesized nanocomposites are presented in Fig. 2. For graphite, a characteristic peak at 2θ = 26.7° is related to the existence of well-arranged layer structure.As shown from this pattern, the characteristic peak of graphite has been transferred to lower degrees (from 2θ = 26.7° to 2θ = 12.8°) which is shows that graphite was fully oxidized into graphene oxide [55] and, is related to the presence of epoxy, -OH and -COOH groups at interlayer platelets in GO and increasing the interlayer of GO [56].
As can be seen, all samples indicate the amorphous structure with crystalline peaks in the same positions which are due to the presence of semicrystalline polymer into the structure.However, a negligible difference was observed.In the XRD spectrum of samples without GO, two peaks at 19° and 24° were appeared, corresponded to the (200) and (202) planes of PA6 polymer [40].A peak at around 2θ = 22.5° shows the crystalline peak of PLA [57].As can be seen, by increasing GO in the nanocomposites, the peak at 2θ = 19° became wider which is related to the presence of more GO in nanocomposites.It should be noticed that the intensity of the characteristic peak at 2θ = 24.5° is depend on spacings between the carbon layers depending on the amount of graphitization or oxidation concentration [58].
PTW is an amorphous polymer that does not show any specific peaks.PTW illustrates a wide peak at around 2θ = 19° which is as same as PA6.So, the peak at 2θ = 19° corresponds to both PTW and PA6.One point that can be figured out from the XRD patterns, is the low-intensity peak of Go in nanocomposites patterns.As shown, the GO has a characteristic at about 2θ = 12° which is not observed specifically at nanocomposites patterns.This is due to the low amount of graphene oxide in nanocomposites (max = 1phr) in comparison with higher amounts of polymers.Because of high intensities peaks of polymers, the GO peaks fall down which is not visible in the patterns.

Phase Morphology
Figures 3 and 4 shows the SEM images of the fracture surface of the samples and the average size of the dispersed phase of the PLA in the PA6 matrix respectively.As can be seen from the Fig. 3, the fracture surface of the specimens has been dramatically roughened by the addition of PTW compatibilizer and the increase in GO concentration.This is probably due to the interaction of PA6/PLA/PTW/GO nanocomposite components.Because, as shown in Fig. 3a, the fracture surface of the PA6/PLA sample with a ratio of 80/20 without any compatibilizer and reinforcement, is completely smooth, which indicates the lack of interaction between the two polymers.
In Fig. 3b, the surface of the sample containing the PTW compatibilizer is slightly rougher than the surface of the sample without it, which may indicate greater compatibility of PA6 and PLA.By adding GO nanofiller to PA6/PLA/ PTW sample and increasing its amount, the fracture surface of the samples has become rougher, and the mean diameter of PLA disperse phase reduced, so in the Fig. 4, that for samples (80/20), (80/20/5) and containing 0.1, 0.3, 0.5, 0.7 and 1 phr GO, the mean diameter of PLA disperse phase became 43.9, 36.1, 32.5, 21.6, 16.6, 13.4 and 11.8 μm, respectively, which indicates the creation of an interface and interaction between the components of nanocomposites, which is consistent with the results of some studies [59].It seems that the interface is formed by the formation of hydrogen bonds between the hydroxyl and carboxyl functional groups in GO with the carboxyl functional groups of PLA and the amide functional groups of PA6.Also, the reaction of GO hydroxyl functional groups and epoxy ring in PTW compatibilizer in this nanocomposite can be another factor to create an interface among the components of PA6/PLA/PTW/GO nanocomposites.Figure 5 shows the possible chemical interactions of GO, PA6, PLA and PTW.
In fact, the addition of GO in the presence of a compatibilizer to PA6/PLA can greatly affect the morphology and failure behavior of the specimens.As mentioned, the fracture surface of the samples has become rougher with increasing amount of GO, which indicates the presence of different stiffened phases due to increased interactions between the polymer phases and GO sheets.Higher roughness indicates higher resistance to crack propagation due to more efficient stress transfer from the matrix to GO particles [60].Polydispersity (PD), volumetric mean radius ( R v ) and numerical mean radius ( R n ) were calcu- lated using the following equations: (1) where n i and R i are the number and diameter of the droplets, respectively [61,62].
( Table 2 shows the calculated values of PD, R n and R v using Eqs.(1) to (3) and using J image analysis software.Figure 6 also shows the numerical mean radius of the samples containing the compatibilizer with increasing GO concentration.It can be seen that with increasing concentration of GO, R n values decrease.This is probably due to the compatibility of the components of PA6/PLA/PTW/ GO nanocomposite samples due to the presence of PTW compatibilizer and GO.As mentioned earlier, the possible interaction of the hydrogen bond of the hydroxyl and carboxyl functional groups in GO with the carboxyl functional groups of PLA and the amide groups of PA6 has caused this.In addition, the reaction of the GO hydroxyl functional groups and the epoxy ring in the PTW compatibilizer in these nanocomposites can cause an interface that reduces R n .
Figure 7 compares TEM images for PA6/PLA blend with a ratio of 80/20 containing 1 phr GO in two modes without PTW (Fig. 8a) and with PTW (Fig. 7b).It can be seen that GO in the sample without compatibilizer tends to localize in the dispersed phase of PLA.This is due to the higher density of PLA than the PA6 matrix [62].Accumulation of GO is also observed in the PLA phase.Figure 7b shows the effect of adding PTW to the PA6/PLA matrix.It can be seen that there is a good compatibility between the PLA and PA6 phases and the interface is greatly increased and a continuous phase is seen.This continuous phase indicates the good dispersion of GO nanosheets in the matrix and the formation of exfoliated structures.The good dispersion of GO in this matrix confirms the possible interactions between the hydroxyl functional groups of GO and the epoxy ring in PTW and also the formation of hydrogen bonds between the GO functional groups and the PLA and PA6 functional groups and creation of an interface between the two polymers.These findings are in line with previous findings of researchers [62,63].

Mechanical Properties
Table 3 shows the mechanical properties such as fracture stress, tensile strength, elongation-at-break and Charpy impact strength and the resultant charts are presented in Fig. 8.The table and charts indicate the effect of adding PTW compatibilizer and GO reinforcement to PA6/PLA on the mechanical properties.The mechanical properties of polymer nanocomposites depend on several factors, the most important of which are the inherent mechanical properties of the nanocomposite components, the composition and compatibility of these components, the presence or absence of compatibilizer, the dispersion of reinforcement and the final morphology of the nanocomposite.In fact, the reduction or improvement of the mechanical properties of polymers can be attributed to the interaction of the reinforcement and the compatibilizer with the polymer matrix [61].It has also been shown that the addition of a reinforcement to the polymer matrix enhances its tensile and impact strengths, but reduces elongation-at-break, which is due to the physical structure and stiffness of the nanoparticles [62].As shown in Table 3, the addition of PTW to PA6/PLA in the presence of GO nanoparticles significantly has increased tensile strength, fracture stress, and Charpy impact strength.These properties showed an increasing trend with increasing GO, which seems to be due to the good dispersion of GO in the polymer matrix and the presence of compatibilizer in the blend.This good dispersion was previously confirmed by TEM images.Increasing the properties is related to the high surface area of GO and its interfacial interactions with the polymer matrix in the presence of PTW, leading to the restriction in the mobility of nanocomposite molecular structure and thus reducing elongation-at-break [62].
To understand the mechanical properties of nanocomposites, it is important to compare the experimental results with the standard micromechanical models developed for such systems.Hence, Halpin-Tsai (H-T) equations is used to predict the bulk properties of nanocomposites [64].The Halpin-Tsai equation for composites is as follows [64]: where E C , E L , E T and E m are the composite, longitudinal, transverse and the matrix modulus, respectively.f , m are the volume fractions of filler and matrix, L and T are constants given by: Which, E f and E m are the Young's moduli of filler and matrix respectively, and is filler aspect ratio that equal to the following equation for one dimensional material: That, w and t are the lateral size (under the assumption of with = length) and thickness of nanoparticles, respectively [65].Volume fractions is related to the weight fractions and the densities of each component.GNP density was considered being equal to 2.2 g/cm 3 .The Young's moduli of matrix is experimentally measured by tensile tests on neat matrix, which was equal to 4991 MPa, while Young's moduli of filler value for GNP were taken from the scientific literature [65].

Thermal Properties
Figure 10 shows the TGA thermograms of PA6/PLA nanocomposites with different amounts of GO with and without PTW compatibilizer.The highest degradation temperature was observed for sample containing 1 phr GO at 398 °C.According to the Fig. 10, there is the highest amount of water in the sample containing 1 phr GO, so the peak appears at 398 °C.The degradation of the main chain in the pure PA6/ PLA blend without GO and compatibilizer has begun at a lower temperature compared to other composites containing filler and compatibilizer.Due to low oxygen levels, oxidation and thermal degradation occur in the device compartment or in the polymer matrix [66].It seems that GO layers prevent the release of internal heat and small volatile molecules, resulting in higher stability of nanocomposites compared to pure PA6/PLA.Increasing the thermal stability of PA6/ PLA nanocomposites is due to lower oxygen penetration and permeability of degradation products from nanocomposite bulk.In the present work, the Kissinger differential method is used to calculate the activation energy of the degradation of blend according to the following equation [66]: In which, T max is the maximum temperature, n refers to the reaction order and α max is conversion degree.The temperature in this equation represents the point of deviation in the thermal degradation curve and α max is considered at the point of deviation.The slope of the fitting line indicates the activation energy (E a ), which is expressed by the Flynn-Wall-Ozawa integral method [66,67]: In this equation, β is the heating rate, α is the conversion degree, R is the universal gas constant, T is the absolute temperature and g(α) is the alpha integral function, which is calculated as follows [67]: As previously said, the slope of the log β vs. 1/T line represents activation energy.TGA/DTG was performed in accordance with the Flynn-Wall-Ozawa and Kissinger methods.For this purpose, the constant heating rates of 20, 30 and 40 °C/min were applied to determine the activation energy of PA6/PLA and PA6/PLA/GO.Based on these two methods, the activation energy of the second stage of the degradation of samples containing 0.1-1 phr GO was calculated and the results were compared with each other.Unlike the Flynn-Wall-Ozawa method in the Kissinger method, the activation energy evaluation is performed without any information about the reaction mechanism and its conversion degree.While in Flynn-Wall-Ozawa method, activation energy is calculated at different conversion degrees (α = 0.05-0.6).Kinetic analysis of PA6/ PLA/GO containing 1 phr GO using the Kissinger method is shown in Fig. 11a and using the Flynn-Wall-Ozawa method is shown in Fig. 11b at different heating rates.Figure 11c also shows how to modify the activation energy of a sample containing 1 phr of GO.
As shown in Fig. 11a, the activation energy of the PA6/ PLA/GO nanocomposite in the presence of PTW at 1 phr of GO is about 30 kJ/mol.As shown in Fig. 11c, the activation energy increases near α = 0.3 and then decreases sharply.This decrease can be related to GO, which forms an insulating layer on the surface of the nanocomposite and is actually responsible for reducing the activation energy at high conversion percentages [36].Many researchers attribute the improvement in thermal stability of nanocomposites mainly to the better dispersion of GO, which increases the interactions between GO and the polymer.GO sheets create heat barriers and make the polymer matrix more stable.In fact, GO layers prevent the evaporation and escape of volatile components as well as the degradation products created by the thermal decomposition of nanocomposites.This is due to the physical structure and large surface area of GO nanolayers, which act as a barrier to the formation of volatile products and delay degradation [8,68].Table 4 present the thermal properties of nanocomposites.

DMTA Analysis
Figure 12a shows the storage modulus of PA6/PLA nanocomposite samples with compatibilizer and different amounts of GO.As can be seen, the pure blend shows two perfectly thermal transitions for PA6 and PLA at temperatures of 47.6 °C and 78.5 °C, respectively.This indicates an incompatibility between the two polymer phases, but with the addition of the PTW compatibilizer, there is a good  containing PTW compatibilizer, which is increased by increasing the amount of GO as the nanofiller in the blend, so that in larger amounts of GO, the reduction in thermal transition height is noticeable and its temperature is shifted to higher temperatures.Given the possible reactions between the two polymers and the PTW compatibilizer, shown earlier in Fig. 5, this seems to be due to the immobility of the PLA and PA6 chains near the surface of the GO sheets in the presence of PTW compatibilizer [69,70].These results are in good agreement with the results obtained from the temperature-storage modulus (Fig. 12a) with tanδ in Fig. 12b.In this figure, it is quite obvious that the increase in the amount of GO in the PA6/PLA blend in the presence of the compatibilizer has increased the storage modulus, so that in larger quantities this increase is quite noticeable.This indicates the exfoliated structure of GO in the PA6/PLA matrix [69][70][71].Table 5 shows the DMTA properties for all nanocomposite samples.

Rheological Properties
The rheological properties of the filled blends depend on the surface properties and the dispersion state of the droplet phase.Hence, rheological tests are a powerful tool to investigate the dispersion state of nanofillers [59,72].To investigate the effect of GO addition on rheological properties, storage modulus and complex viscosity of nanocomposites were determined.Figure 13a and b shows the complex viscosity (η*) and the storage modulus (G′) of nanocomposites containing the compatibilizer.Earlier TEM results showed that GO was well dispersed in the polymer matrix in the presence of a compatibilizer.As shown in the figure, the addition of GO nanofiller to the PA6/PLA blend increased the complex viscosity at low shear rates.This increase is due to the formation of a GO physical network in the presence of the PTW compatibilizer.At lower frequencies, the increase in complex viscosity can be justified by the amount of material with yield stress [59,72].As can be seen, the yield stress is greatly increased by increasing the amount of GO in the presence of PTW compatibilizer, which means good dispersion of GO nanoparticles in PA6/PLA/PTW.This increase is due to the solid behavior and formation of GO network in the PA6/PLA composite.As shown in Fig. 13, the slope of the G' line for PA6/PLA nanocomposites decreases rapidly, which appears to be due to the formation of a stronger network due to the greater specific surface area of the GO nanofiller [63].Increasing the amount of GO in nanocomposites has increased the melt elasticity.The rheological behavior of nanocomposites varies before and after this point.The following equation is used to calculate the dependence of the volume fraction of GO on the storage modulus [64]: where n is the shear-thinning exponent value, A is the specific exponential factor of the composites, and ω is the angular frequency.In this equation, the degree of exfoliation of GO and its interactions with PA6/PLA are shown with value n.   increasing GO content.It can be seen that the reinforcing effect is greater at higher values of n, which is increased as a function of the amount of GO.This indicates that the GO sheets are oriented in the direction of shearing the nanocomposite.The exfoliation behavior of GO in PA6/PLA nanocomposites is determined by the Carreau-Yasuda model [63]: In this equation, a is the Yasuda coefficient, λ is the relaxation time, m is the dimensionless power index, η 0 is the viscosity at zero shear and σ 0 is the yield stress.Also, increasing the amount of GO in the presence of compatibilizer in nanocomposite causes a sharp increase in limit stress due to the better dispersion and orientation of GO nanosheets in nanocomposite.Relaxation time and zero-shear viscosity show a similar trend in GO loading, especially changing the relaxation time by adding it to nanocomposite.According to Table 6, the increases with increasing GO and the addition of compatibilizer, which is probably due to the segmental dynamics of the   polymer chains and the GO exfoliation and the entrapment of polymer molecules between the GO spaces [63].Zeroshear viscosity and relaxation time also show the same trend as a function of nanofiller content.Increased relaxation time with the addition of GO can be related to the limited mobility of PA6/PLA/PTW chains, which are entrapped between the nanosheets [63,73].As shown in Table 6, as the amount of nanofillers increases, the value of n decreases, indicating a strong dispersion of nanoparticles in the PA6/PLA matrix.It can also be seen that nanocomposites filled with GO show lower stress transfer that is related to the transferred hydrodynamic stress due to the rotational motion of GO nanosheets in the presence of PTW compatibilizer.Therefore, these results are consistent with the linear elastic viscosity observations.In other words, the presence of PTW compatibilizer in the samples increases the hydrodynamic stress.This is due to the excellent dispersion of GO in the PA6/PLA matrix due to the presence of a compatibilizer that facilitates the transfer of stress to the GO nanoparticles [73].
The dependence of the storage modulus on strain in nanocomposites is used to determine their linear elastic viscosity region.The results show that the elastic viscosity region is reduced by adding GO to PA6/PLA.By increasing the amount of GO in nanocomposites, the decrease in the elastic viscosity region can be attributed to the difference in dispersion in the polymer matrix.One of the main factors in the nonlinear behavior of non-polymer nanocomposites is the failure of the three-dimensional filler network in the matrix due to dynamic strains.Therefore, with the formation of a three-dimensional GO network in the PA6/PLA matrix, nonlinear behavior occurs in the smaller strain domain.Also, better dispersion of nanoparticles increases the modulus in the linear region and nonlinear behavior begins at lower strains.The results show that the modulus changes at lower strains are more severe for GO-based nanocomposites in the presence of PTW compatibilizer.

Conclusion
PA6/PLA blend composite was prepared with adding PTW compatibilizer and GO nanofiller at different filler contents.The results showed that the addition of PTW compatibilizer to PA6/PLA blend resulted in better dispersion of GO nanofiller.The study of mechanical properties showed that with increasing the amount of GO, tensile strength, impact strength and tensile modulus increase, but elongation-at-break decreases due to limited movement of polymer chains on the surface of GO.This improvement in properties is due to the better dispersion of GO in the presence of a compatibilizer.This conclusion was confirmed by SEM images in which a rougher fracture surface was observed with increasing GO content.Also, TEM and DMTA results showed better filler dispersion and penetration of polymer chains into GO nanosheets.The study of rheological properties showed an increase in storage modulus and complex viscosity, which also confirmed the role of PTW compatibilizer in better GO dispersion.

Fig. 2
Fig. 2 The XRD results of a Graphite and GO, and b synthesized nanocomposites

Fig. 3
Fig. 3 SEM images of the fracture surface of PA6/PLA samples without and with compatibilizer at different contents of GO (0.1, 0.3, 0.5, 0.7 and 1 phr)

Fig. 4
Fig. 4 SEM images of the mean diameter of PLA disperse phase of PA6/PLA samples without and with compatibilizer at different contents of GO (0.1, 0.3, 0.5, 0.7 and 1 phr)

Fig. 7 Fig. 8
Fig. 7 TEM micrographs of PA6/PLA blend with a ratio of 80/20 containing 1 phr GO: a without PTW and b with PTW

Figure 9
Figure 9 presented the relation between tensile moduli and Go concentration for both experimental and Halpin-Tsai model.As shown, by increasing the GO in to the nanocomposites, tensile moduli increased for both experimental and model.Accordingly, it is evident that experimental line indicates good agreement with Halpin-Tsai model.

Fig. 9 Fig. 10 a
Fig. 9 Comparison of the experimental and theoretical values of the tensile modulus of PA6/PLA blend obtained from proposed models based on the volume fraction of graphene oxide compatibility between the phases, which appears in the diagram as a single peak.Another issue that is quite obvious is the decrease in the tanδ thermal transition height for blends

Fig. 11 a
Fig. 11 a Results of the Kissinger method for PA6/PLA blends.b Results of the Flynn-Wall-Ozawa method at different heating rates for PA6/PLA blends.c Activation energies of PA6/PLA blends in different weight losses obtained by Flynn-Wall-Ozawa method

Fig. 12 a
Fig. 12 a Storage modulus curve in terms of temperature for PA6/ PLA blends.b Tanδ vs. temperature curve for PA6/PLA blends

Table 2
PD, R n and R v values obtained from SEM micrographs for PA6/PLA/PTW/GO nanocomposites

Table 6
shows this value for PA6/PLA composites with

Table 4
Thermal properties of PA6/PLA nanocomposites with varying amounts of GO

Table 6
Results