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Enhanced visco-elastic and rheological behavior of epoxy composites reinforced with polyimide nanofiber
Nano-Structures & Nano-Objects 21 (2020) 100421
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Enhanced visco-elastic and rheological behavior of epoxy composites reinforced with polyimide nanofiber ∗
B.D.S. Deeraj a , Harikrishnan R. b , Jitha S. Jayan c , Appukuttan Saritha c , Kuruvilla Joseph a , a
Department of Chemistry, Indian Institute of Space Science and Technology, Kerala, 695547, India Department of Chemistry, Indian Institute of Science Education and Research, Mohali, Punjab, 140306, India c Department of Chemistry, School of Arts and Science, Amrita Vishwa Vidyapeetham, Kerala, India b
article
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Article history: Received 5 August 2019 Received in revised form 2 December 2019 Accepted 19 December 2019 Keywords: Polyimides Polymer nanofiber Electrospinning Rheology
a b s t r a c t High performance epoxy composites are now a days a must in several industrial applications. In the present work electrospun polyimide (PI) nanofibers with excellent thermal and mechanical properties was used as a reinforcement in epoxy matrix via a simple mechanical mixing followed by thermal curing method. Well defined electrospun nanofibers of aromatic polyimide (PI) were successfully prepared from electrospinning Poly (amic acid) (PAA) and subsequent thermal treatment. The fiber morphology was analyzed using Transmission electron microscopy (TEM) and Atomic force microscopy (AFM). PI/epoxy nanocomposites with different PI loadings were prepared using chopped PI mats. The dynamic mechanical performance of these PI/epoxy composites was investigated to determine the influence of PI fibers in reinforcing the epoxy matrix. The fracture toughness of these composites displayed a note worthy improvement of 20 % at 1 w% loaded samples and the surface of the fractured samples was investigated by Scanning electron microscope. The rheological properties of these systems show a tremendous increase in the storage and loss modulus when compared to neat systems. In addition flow models were employed to model the rheological data and the comparison was made with the experimental data. © 2020 Elsevier B.V. All rights reserved.
1. Introduction Epoxy, a thermosetting polymer is the most studied material owing to its good mechanical performance, chemical and wear resistance and wide range of applications [1–5]. The curing reaction of epoxy takes place after the addition of the hardener. Curing causes irreversible physical and chemical changes in both the epoxy and the hardener. During curing, the molecules form crosslinks with each other and grow as a three-dimensional network finally forming a solid epoxy resin. The resulting cured systems are usually fragile, brittle and highly resistant to the propagation of cracks [6–11]. The low impact resistance and poor resistance to crack propagation limit their use in advanced applications such as aerospace industry. Hence, improving the fracture toughness is an indispensable requirement in epoxy based systems. Incorporating rubbery particles, such as amino-terminated acrylonitrile butadiene and carboxyl-terminated acrylonitrile butadiene, to improve the fracture toughness of epoxy resins is quite known. However, the modification by the incorporation of rubbery inclusions with a low glass-transition temperature (Tg ) results in ∗ Correspondence to: Indian Institute of Space Science and Technology, Trivandrum, Kerala, 695547, India. E-mail address: [email protected] (K. Joseph). https://doi.org/10.1016/j.nanoso.2019.100421 2352-507X/© 2020 Elsevier B.V. All rights reserved.
a decrease in both the modulus and Tg of the resultant cured systems. Thus, engineering thermoplastics, such as polyamide, poly(methyl methacrylate), poly(ether sulfone), poly(ether ether ketone), poly(ether imide), and polyimide, are used as advanced modifiers for epoxy resins to overcome this problem [12–16]. Other strategies like nanoparticle grafted systems [17], electrospun nanofibers [18] and block copolymers inclusions [19] in epoxy to enhance its performance has also been investigated. Polyimides (PIs) are an advanced class of superior-performance polymers possessing cyclic imide group along with stiff aromatic groups in the chain backbone. These, Aromatic polyimides have been broadly investigated due to their excellent mechanical properties, thermal stability, combined with their good chemical resistance and dielectric properties. Its excellent properties like heat and chemical resistance in addition to relatively facile processability, enables it to be used in electronic devices, hot gas filtration applications for spacecraft and in satellites. The spectrum of its applications can be increased by the formation of micro and nanofibers [20–22]. For spinning PI fibers many traditional methods including dry jet wet spinning has been used earlier [23–26]. A typical way of preparing PI fibers by wet spinning process of PAA solution and thereafter by thermal imidization was reported [24]. They reported that the prepared PAA fibers have ultimate stress and
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modulus of 268 MPa and 4.1 GPa, respectively. However, after imidization reaction at temperature of 350 ◦ C for about 30 min in nitrogen atmosphere, the resultant PI fibers displayed a stress and modulus of 399 MPa and 5.2 GPa, respectively. Usually, Conventional PI fibers have fiber diameter in several hundred micrometer range and are utilized for preparing protective clothing, exhaust gas filters and aircraft interior components [27–29]. In case of energy storage/conversion, environmental protection and allied fields, PI fibers with very small diameters are necessary [30,31]. For developing nanostructured inclusions many techniques like template synthesis, self assembly, phase separation etc., are available. But now-a-days, Electrospinning is one of the most widely chosen tool for preparing nonwoven fabrics of submicron or nano-scale dimension. Electrospinning is a quite simple, relatively inexpensive technique and has easy scale up ability. The unique features of such spun nanofibers are one-dimensional morphology, high porosity, high surface-to-area ratio, small pore size between depositing fibers of the electrospun mats, and great possibility for fiber functionalization [32–34]. Owing to these unique properties, electrospun polymer fibers are progressively being used as reinforcement in the polymer matrices [18,35,36]. Epoxy resin with polyimide is one of the important systems being investigated for high temperature applications [37,38]. The use of polyimide electrospun nanofibers as a reinforcement in epoxy matrix is a good idea to develop high performance composites. Polyimide, because of its excellent mechanical and thermal stability forms a stiff system with epoxy matrix, thereby improving the life of the composite system. Moreover the fibers also possess the additional benefit of the improved aspect ratio and better interfacial surface area resulting from their nano sized structure which leads to better adhesion between the filler and the matrix. In this work, chopped Electrospun Polyimide nanofiber mats were incorporated in DGEBA matrix at various loadings and epoxy nanocomposites were developed. The Dynamic mechanical properties were evaluated to investigate the reinforcing capability of these nanofiber mats in the embedded epoxy composites and this data was modeled using Einstein, Guth and Quemada models. The rheological properties of the nanofiber mat/Epoxy systems were tested to find the effect of the PI nanofiber mat on the flow behavior of the systems. These flow properties were modeled with power’s and Casson’s models. The fracture toughness of the PI nanofiber reinforced epoxy systems were tested and compared with the virgin epoxy systems. FTIR was used to study the structural details of PAA before and after imidization reaction. TGA was done to study the thermal stability and char yield of the PAA systems. 2. Experimental details 2.1. Preparation of polyimide electrospun fibers R Polyamic acid (intermediate of classical polyimide Kapton⃝ ) with weight average molecular weight (Mw ) 10000 KDa was used for the formation of polyimide. The PAA was filled in a 10 ml syringe (needle specification: 0.8 mm×40 mm) and electrospun fibers were prepared in a horizontal electrospinning setup. The processing conditions optimized were 11 KV acceleration voltage, 1000 rpm collector speed, 15 cm distance and 1 ml/h flow rate. These electrospun mats were collected on aluminum foil. These mats were thermally treated at 70 ◦ C for 1 h, 120 ◦ C for 1 h, 180 ◦ C for 1 h and 220 ◦ C for 1 h for imidization process to occur and form the polyimide. Thus prepared mats were further characterized and used for the preparation of the epoxy composites.
2.2. Preparation of polyimide/epoxy nanocomposites The electrospun mat was manually chopped into 0.5 mm × 0.5 mm pieces. These chopped pieces were incorporated in DGEBA epoxy (Trade name: GY 250) at weight percentages of 0, 0.5, 1, 2, and 3 respectively by mechanical mixing. Later, sufficient amounts of polyamidoimadazoline hardener (Trade name: AR 140) was added to the system (epoxy to hardener ratio is maintained to be 2:1) and kept for thermal curing (60 ◦ C for 4 h, 80 ◦ C for 1 h). After curing, the final products were tested and the properties were evaluated. The schematic representation of PI/Epoxy composites were presented in Fig. 1. 3. Characterization techniques The morphology of the nanofibers produced was observed by SEM, TEM and AFM. The samples were gold coated to make them conducting for SEM. These fibers were deposited on the copper grid and the TEM images were taken using HRTEM. For AFM, mica sheet was used as the substrate (Aligent 5500 SPM non-contact mode). The samples were tested in ATR mode from wave number 4000 cm−1 to 600 cm−1 in a Perkin Elmer Fourier transform infrared spectrometer. The visco elastic properties of PI loaded epoxy thermosets and neat epoxy was analyzed using a DMA analyzer from temperature 30 ◦ C to 150 ◦ C at a ramp rate of 2 ◦ C and the modulus and damping factor was plotted. The specimen size maintained was 30 × 5 × 1 mm3 and testing was done in tension mode. Fracture Toughness of the epoxy samples was performed using universal testing machine (Instron 5984) as per standards (ASTM D 5045). The fracture toughness was described in stress intensity factor (KIC ), as per the equation: KIC = f (x) =
L f (x) Bw 0.5 ( ) 6x0.5 [1.99 − x (1 − x) 2.15 − 3.93x − 2.7x2 ]
(1 + 2x) (1 − x)1.5
(1) (2)
where, X is in between 0 and 1. L is the load; B is the specimen thickness; w is the specimen width; a is the length of the crack and x = a/w . The flow characteristics of the neat epoxy and chopped fiber incorporated epoxy systems were tested using Modular Rheometer (MCR 102) using parallel plate mode. The storage modulus and loss modulus were investigated with respect to angular frequency from angular frequency 0.01 rad/s to 200 rad/s. The viscosity and shear stress of the samples were measured using the same set up with-in the angular frequency range of 1 rad/s to 200 rad/s. The thermal stability of neat epoxy and PI loaded epoxy samples were tested from a temperature range of RT to 800 ◦ C by employing a heating rate of 20 ◦ C/min in presence of nitrogen. 4. Results and discussion 4.1. Morphology The PAA solution was filled in a 10 ml disposable syringe and an electrical voltage was applied between the syringe needle and the drum collector. At a particular voltage, the repulsive force of the droplet exceeds the surface tension holding it. Hence it forms a Taylor cone at the needle tip, which subsequently tends to form fibers and moves towards the collector to satisfy the surface tension. The TEM and AFM images of the PAA electrospun nanofibers were taken and are depicted in Fig. 2. The fiber diameters were measured using the available image processing software’s and the diameter range was found to be between 70 nm to 500 nm. By, measuring the diameters of fibers at different places, the average fiber diameter was found to be about 280 nm.
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Fig. 1. Schematic diagram for preparing electrospun fibers and epoxy composites.
Fig. 2. (a) TEM and (b) AFM images of electrospun PAA fibers.
Fig. 3. (a) Optical microscope and (b) SEM images of PI fibers after imidization.
The obtained PAA electrospun mat was then kept in an air oven and subsequent thermal treatment was given at pre-mentioned temperatures for the imidization reaction to occur and thus converting PAA to PI fibers. The SEM micrographs and optical microscopic images of PI fibers are presented in Fig. 3. 4.2. Fourier transform infrared spectroscopy FTIR spectra clearly reveals that, thermal treatment of poly(amic acid) at 70 ◦ C for 1 h, 120 ◦ C for 1 h, 180 ◦ C for 1 h and 220 ◦ C for 1 h results in the transformation of electrospun poly(amic acid) fibers to electrospun polyimide fibers (Fig. 4).
The peak at ∼1627 cm−1 can be accredited to the absorption of carbonyl stretching (C==O) of the amide group and the peak at ∼1505 cm−1 is due to C–NH stretching vibration. The peak around ∼3500 cm−1 is associated with N–H and O–H stretching in polyamic acid. The vanishing of this peak after imidization indicates successful cyclization of N–H and O–H to the corresponding imide group. Now, considering the polyimide spectra, the peak at ∼1780 cm−1 and ∼1722 cm −1 corresponds to C==O symmetric stretching vibration and C==O asymmetric stretching vibration respectively. The peak at ∼1372 cm−1 can be credited to C==N stretching vibration, and ∼725.5 cm−1 peak can be due to C==O bending. In the Figure, the prepared PI Electrospun mat is
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Fig. 5. Variation of storage modulus of PI nanofiber reinforced epoxy composites with temperature. Fig. 4. FTIR spectra of fibers (a) polyamic acid, (b) electrospun polyimide and (c) casted polyimide.
also compared with casted PI film and both have identical FT-IR spectra, which confirms the formation of Polyimide conclusively. The FTIR spectrum of epoxy resin, hardener and cured epoxy was also tested and are presented in Fig. S1. From the FTIR spectra of the hardener, the peak obtained at about ∼3291 cm−1 confirms the presence of N–H group in it. The peak at ∼1650 cm−1 confirms the amide linkage (C==O–N–H) group, hence the structure of polyamide hardener can be confirmed easily. The peak at ∼1609 cm−1 is the characteristic peak of C–N in imidazoline ring. Peaks at ∼2984 and ∼2893 are due to C–H stretching vibrations. The structure of the hardener is depicted in Fig. S2. The peak obtained at about ∼2968 cm−1 for epoxy resin is due to the C–H stretching of the oxirane group existing in the compound. The Peaks obtained at ∼1247 cm−1 and ∼916 cm−1 respectively are due to C–O–C stretching of ether and C–O stretching in oxirane thereby confirming the epoxide linkage in the epoxy resin. The peak at ∼831 cm−1 corresponds to stretching vibration of C–O–C of ether whereas the ∼1511 cm−1 peak is due to aromatic C–C stretching. The broad peak obtained at ∼3500 cm−1 for the neat epoxy is due to the presence of –OH group in the system. The peak obtained at ∼3291 cm−1 in the cured epoxy shows the presence of N–H group in it which in turn leads to the confirmation of the curing process. Peaks obtained at ∼1650 cm−1 and ∼1609 cm−1 confirms the amide and C–N linkage in the epoxy system due to the reaction between the hardener and the resin. The reaction is depicted in the Fig. S2. Visco-elastic properties and modeling The storage modulus of neat epoxy and PI nanofibers incorporated samples were analyzed and the graphs are presented in Fig. 5. We can observe that the dynamic storage modulus of PI fiber incorporated composites increase with loading percentage and tan delta undergoes a positive shift. When compared to neat epoxy system, the storage modulus of PI loaded samples exhibit a hike at low temperatures until the Tg of PI. After this point the modulus tends to decrease. This increase in the modulus is indicative of the reinforcement effect of the nanofiber mat in epoxy systems. These mats while dispersed in epoxy, constitute an isotropic stiffening system and so contribute to the high strength of the composites. The effect of the nanofiber mat loading showed a constructive positive effect in the mechanical performance of the composites. This increase is due to the effective stress transfer, good interfacial surface area and reinforcing
strength of the mat. In case of tan delta, the positive shift in the peak is due to the change in cross-linking density of the systems because of the presence of PI mats. The Tg of neat epoxy was found to be 92.7 ◦ C, while PI loaded epoxy composites were found to be 104.1 ◦ C, 106.6 ◦ C and 108 ◦ C respectively at 1, 2 and 3% loading. The visco elastic properties of the PI loaded epoxy composites was modeled using Einstein, Guth and Quemada models to assess the change in storage modulus of the epoxy matrix with increase in loading (in Fig. 6). Einstein proposed a simple theoretical model to estimate the modulus of filler reinforced polymer matrices. It is mathematically denoted as Ec Ep
( ) = 1 + 2.5Vf
(3)
Where, Ec and Ep are the storage modulus of resultant composite and matrix and Vf is the reinforcement volume fraction. This model is applied to polymeric systems reinforced with less concentration. Guth equation for fiber reinforced polymeric systems is expressed by the equation Ec Ep
( ) = 1 + 2.5Vf + 14.1Vf2
(4)
This equation takes into account the interactions between the particles even at higher concentration The Quemada equation for polymeric systems is given by Ec Ep
= 1/(1 − 0.5K6 Vf )2
(5)
This equation uses a variable that implies the interaction between particles and the geometry (variable, K6 = 2.5). The experimental values were found to be higher than the theoretical model values. Fracture toughness In epoxy thermosets, fracture toughness is one of the major challenges that need to be addressed. Cured epoxy being highly brittle tends to break with the slightest crack. This drawback of epoxy systems need to be overcome for its industrial applications and can be done by the introduction of suitable reinforcements. The PI nanofiber mat loaded epoxy samples at different compositions were tested and the stress concentration intensity is depicted in Fig. 7. The fractured samples were also observed using SEM and the images are illustrated in Fig. 8. From Fig. 7, we can
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Fig. 6. Visco-elastic properties comparison between experimental values and theoretical models (at room temperature).
low loadings to toughen epoxy systems. So, further loadings were not tested as the experimental trend shows a decrement in the toughness. Rheological properties and modeling The storage and loss modulus of the neat epoxy and PI mat incorporated epoxy systems were estimated with respect to fiber content and are presented in Fig. 9. As per the Figure, we can observe an increasing trend in modulus with respect to the fiber loading. The result conclusively proves that these nanofiber mats have a good influence in flow dynamics of the systems. Fig. 10 depicts the change of viscosity with shear rate. We can observe from the Figure that viscosity depends on filler loading. The difference in viscosity is more significant in the lower frequencies than at higher frequencies. Usually flow models are employed to estimate the type of flow followed by the system based on the experimental data. Here, PI/Epoxy rheology experimental data was modeled using available power’s and Casson’s models. Fig. 7. Variation of stress intensity factor with PI loading (w %).
observe that 0% nanofiber mat loaded samples (neat epoxy) have a KIC of 1.06 ± 0.37 MPa.m1/2 . This increased to 18% and 20% at 0.5 w% and 1 w% respectively. Upon further increase in loading of 2 w% and 3 w%, the toughness decreased gradually. At low loadings, the crack propagation is obstructed by the incorporated PI nanofiber mat and different crack propagation mechanisms like crack deflection, fiber pull out and fiber bridging can be observed. But at higher loadings, the system becomes highly brittle and hence fracture toughness decreases. So, at higher concentrations the PI loaded epoxy composites might have developed ‘‘secondary cracks’’ that are formed parallel to the mat surface, thereby making the entire composite brittle and so, the toughness decreases. Similar decrease in fracture toughness was observed in the PMMA nanofiber/Epoxy systems [39]. Hence based on the results we can state that PI nanofiber mats being brittle can only be used at
Powers Model In this model, the Shear stress and shear rate are plotted on logarithmic axis and it expresses shear thinning and thickening behavior of fluid [40]
σ = Kγ n
(6)
Where σ and γ are shear stress and shear rate respectively, n is the flow index parameter which describes the tendency of fluid to shear thin, and k is consistency coefficient. When n < 1, the fluid is thinning in nature and when n > 1, the fluid is thickening in nature. The power law model holds better in the low shear condition Applying logarithms on both the sides of Eq. (6): log σ = log K + n log γ
(7)
Eq. (7) can be expressed as, log η = log K + (n − 1) log γ Where, η is the viscosity.
Fig. 8. Fractured surface topography of Polyimide/epoxy composites at (a) 1 w% and (b), (c) 3 w% PI loading.
(8)
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Fig. 9. Variation of rheological properties, (a) storage modulus & (b) loss modulus with PI loading (w %).
Fig. 10. Variation of viscosity with respect to shear rate.
Fig. 11. Powers law rheological model fitting.
The Powers law fit data of neat epoxy and loaded samples are depicted in Fig. 11. The parameters obtained are tabulated in Table 1. Cassons model The Casson rheological model is used to estimate the flow of visco-elastic fluids. Mathematically, it is expressed as [40,41] 0.5 σ 0.5 = K0c + Kc0.5 γ 0.5
(9)
Where σ is shear stress, γ is shear rate, koc is Casson yield stress (Pa.s), kc is Casson plastic viscosity in mPa.s. The Casson model fitting data of neat epoxy and fiber loaded samples are presented in Fig. 12 and the parameters obtained are presented in Table 2. Thermogravimetric analysis The TGA profiles of PAA and electrospun PI are presented in Fig. 13. From the profiles, a clear difference can be observed, PI was mostly stable until 550 ◦ C, and after complete degradation quite high char yield is present. However, in case of PAA, most of the degradation takes place at low temperature, which is accompanied with a small peak at 550 ◦ C and very less char yield at the end. The stability of Polyimide incorporated epoxy samples was tested and are presented in Fig. 14. From the TGA thermograms, it is clear that PI mats affect the thermal stability of epoxy only marginally and further, there is an improvement in the char yield compared with the neat cured epoxy.
Fig. 12. Cassons law rheological model fitting.
Conclusion Electrospun nanofibers of polyimide were successfully prepared by imidization of poly (amic acid) electrospun mat. PI nanofiber mat/epoxy composites were prepared at different mat
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Table 1 Powers law parameters. Sample
Slope
Neat epoxy (0% PI) PI 1% PI 2% PI 3%
0.94501 0.44949 0.29772 0.31892
Intercept
± ± ± ±
0.01123 0.02262 0.02692 0.04006
0.80268 1.74934 2.32942 2.52892
± ± ± ±
Regression co-efficient 0.01496 0.03013 0.03586 0.05337
0.99634 0.93807 0.82351 0.70577
Table 2 Cassons law parameters. Sample
Slope
Neat epoxy (0% PI) PI 1% PI 2% PI 3%
2.33069 1.83234 1.50431 1.42839
± ± ± ±
9.15054E−4 0.12928 0.17866 0.35548
Intercept
Regression co-efficient
0.07434 ± 0.00443 4.80946 ± 0.62557 12.39152 ± 0.86452 17.54646 ± 1.72012
1 0.88489 0.72887 0.3681
fibers on the visco elastic flow properties of the composite. The TGA of the systems were done and thermal stability was analyzed and it is observed that PI loading improved thermal stability of epoxy composites. Thus polyimide nanofiber mat reinforced epoxy composites could be utilized for advanced applications at relatively low loadings of PI nanofiber mats. Thus the developed PI incorporated epoxy composites are having advanced performance than neat epoxy samples and has potential use as structural composites and in circuit boards. Data availability All the data presented in the study is available with the corresponding author upon request. Declaration of competing interest
Fig. 13. TGA thermograms of PAA and PI nanofibers.
No author associated with this paper has disclosed any potential or pertinent conflicts which may be perceived to have impending conflict with this work. For full disclosure statements refer to https://doi.org/10.1016/j.nanoso.2019.100421. CRediT authorship contribution statement B.D.S. Deeraj: Conceptualization, Methodology, Investigation, Writing - original draft. Harikrishnan R.: Investigation. Jitha S. Jayan: Writing - review & editing. Appukuttan Saritha: Writing - review & editing. Kuruvilla Joseph: Conceptualization, Writing - review & editing. Acknowledgment The authors deeply acknowledge Indian Institute of Space Science and Technology (IIST) for the financial support Appendix A. Supplementary data Supplementary material related to this article can be found online at https://doi.org/10.1016/j.nanoso.2019.100421.
Fig. 14. TGA thermograms neat epoxy and PI loaded epoxy thermosets.
loadings. The morphology of the fibers was confirmed by TEM and AFM images. The completion of imidization reaction was confirmed by FTIR and the corresponding peaks. The DMA results showed an increase in the modulus for the PI mat loaded samples than neat epoxy composites at room temperature. The fracture toughness of the epoxy systems exhibited a 20% increase at 1% loading and thereafter decreased with increase in loading. The dynamic rheology studies reveal the increased effect of these
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Enhanced visco-elastic and rheological behavior of epoxy composites reinforced with polyimide nanofiber ∗
B.D.S. Deeraj a , Harikrishnan R. b , Jitha S. Jayan c , Appukuttan Saritha c , Kuruvilla Joseph a , a
Department of Chemistry, Indian Institute of Space Science and Technology, Kerala, 695547, India Department of Chemistry, Indian Institute of Science Education and Research, Mohali, Punjab, 140306, India c Department of Chemistry, School of Arts and Science, Amrita Vishwa Vidyapeetham, Kerala, India b
article
info
Article history: Received 5 August 2019 Received in revised form 2 December 2019 Accepted 19 December 2019 Keywords: Polyimides Polymer nanofiber Electrospinning Rheology
a b s t r a c t High performance epoxy composites are now a days a must in several industrial applications. In the present work electrospun polyimide (PI) nanofibers with excellent thermal and mechanical properties was used as a reinforcement in epoxy matrix via a simple mechanical mixing followed by thermal curing method. Well defined electrospun nanofibers of aromatic polyimide (PI) were successfully prepared from electrospinning Poly (amic acid) (PAA) and subsequent thermal treatment. The fiber morphology was analyzed using Transmission electron microscopy (TEM) and Atomic force microscopy (AFM). PI/epoxy nanocomposites with different PI loadings were prepared using chopped PI mats. The dynamic mechanical performance of these PI/epoxy composites was investigated to determine the influence of PI fibers in reinforcing the epoxy matrix. The fracture toughness of these composites displayed a note worthy improvement of 20 % at 1 w% loaded samples and the surface of the fractured samples was investigated by Scanning electron microscope. The rheological properties of these systems show a tremendous increase in the storage and loss modulus when compared to neat systems. In addition flow models were employed to model the rheological data and the comparison was made with the experimental data. © 2020 Elsevier B.V. All rights reserved.
1. Introduction Epoxy, a thermosetting polymer is the most studied material owing to its good mechanical performance, chemical and wear resistance and wide range of applications [1–5]. The curing reaction of epoxy takes place after the addition of the hardener. Curing causes irreversible physical and chemical changes in both the epoxy and the hardener. During curing, the molecules form crosslinks with each other and grow as a three-dimensional network finally forming a solid epoxy resin. The resulting cured systems are usually fragile, brittle and highly resistant to the propagation of cracks [6–11]. The low impact resistance and poor resistance to crack propagation limit their use in advanced applications such as aerospace industry. Hence, improving the fracture toughness is an indispensable requirement in epoxy based systems. Incorporating rubbery particles, such as amino-terminated acrylonitrile butadiene and carboxyl-terminated acrylonitrile butadiene, to improve the fracture toughness of epoxy resins is quite known. However, the modification by the incorporation of rubbery inclusions with a low glass-transition temperature (Tg ) results in ∗ Correspondence to: Indian Institute of Space Science and Technology, Trivandrum, Kerala, 695547, India. E-mail address: [email protected] (K. Joseph). https://doi.org/10.1016/j.nanoso.2019.100421 2352-507X/© 2020 Elsevier B.V. All rights reserved.
a decrease in both the modulus and Tg of the resultant cured systems. Thus, engineering thermoplastics, such as polyamide, poly(methyl methacrylate), poly(ether sulfone), poly(ether ether ketone), poly(ether imide), and polyimide, are used as advanced modifiers for epoxy resins to overcome this problem [12–16]. Other strategies like nanoparticle grafted systems [17], electrospun nanofibers [18] and block copolymers inclusions [19] in epoxy to enhance its performance has also been investigated. Polyimides (PIs) are an advanced class of superior-performance polymers possessing cyclic imide group along with stiff aromatic groups in the chain backbone. These, Aromatic polyimides have been broadly investigated due to their excellent mechanical properties, thermal stability, combined with their good chemical resistance and dielectric properties. Its excellent properties like heat and chemical resistance in addition to relatively facile processability, enables it to be used in electronic devices, hot gas filtration applications for spacecraft and in satellites. The spectrum of its applications can be increased by the formation of micro and nanofibers [20–22]. For spinning PI fibers many traditional methods including dry jet wet spinning has been used earlier [23–26]. A typical way of preparing PI fibers by wet spinning process of PAA solution and thereafter by thermal imidization was reported [24]. They reported that the prepared PAA fibers have ultimate stress and
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modulus of 268 MPa and 4.1 GPa, respectively. However, after imidization reaction at temperature of 350 ◦ C for about 30 min in nitrogen atmosphere, the resultant PI fibers displayed a stress and modulus of 399 MPa and 5.2 GPa, respectively. Usually, Conventional PI fibers have fiber diameter in several hundred micrometer range and are utilized for preparing protective clothing, exhaust gas filters and aircraft interior components [27–29]. In case of energy storage/conversion, environmental protection and allied fields, PI fibers with very small diameters are necessary [30,31]. For developing nanostructured inclusions many techniques like template synthesis, self assembly, phase separation etc., are available. But now-a-days, Electrospinning is one of the most widely chosen tool for preparing nonwoven fabrics of submicron or nano-scale dimension. Electrospinning is a quite simple, relatively inexpensive technique and has easy scale up ability. The unique features of such spun nanofibers are one-dimensional morphology, high porosity, high surface-to-area ratio, small pore size between depositing fibers of the electrospun mats, and great possibility for fiber functionalization [32–34]. Owing to these unique properties, electrospun polymer fibers are progressively being used as reinforcement in the polymer matrices [18,35,36]. Epoxy resin with polyimide is one of the important systems being investigated for high temperature applications [37,38]. The use of polyimide electrospun nanofibers as a reinforcement in epoxy matrix is a good idea to develop high performance composites. Polyimide, because of its excellent mechanical and thermal stability forms a stiff system with epoxy matrix, thereby improving the life of the composite system. Moreover the fibers also possess the additional benefit of the improved aspect ratio and better interfacial surface area resulting from their nano sized structure which leads to better adhesion between the filler and the matrix. In this work, chopped Electrospun Polyimide nanofiber mats were incorporated in DGEBA matrix at various loadings and epoxy nanocomposites were developed. The Dynamic mechanical properties were evaluated to investigate the reinforcing capability of these nanofiber mats in the embedded epoxy composites and this data was modeled using Einstein, Guth and Quemada models. The rheological properties of the nanofiber mat/Epoxy systems were tested to find the effect of the PI nanofiber mat on the flow behavior of the systems. These flow properties were modeled with power’s and Casson’s models. The fracture toughness of the PI nanofiber reinforced epoxy systems were tested and compared with the virgin epoxy systems. FTIR was used to study the structural details of PAA before and after imidization reaction. TGA was done to study the thermal stability and char yield of the PAA systems. 2. Experimental details 2.1. Preparation of polyimide electrospun fibers R Polyamic acid (intermediate of classical polyimide Kapton⃝ ) with weight average molecular weight (Mw ) 10000 KDa was used for the formation of polyimide. The PAA was filled in a 10 ml syringe (needle specification: 0.8 mm×40 mm) and electrospun fibers were prepared in a horizontal electrospinning setup. The processing conditions optimized were 11 KV acceleration voltage, 1000 rpm collector speed, 15 cm distance and 1 ml/h flow rate. These electrospun mats were collected on aluminum foil. These mats were thermally treated at 70 ◦ C for 1 h, 120 ◦ C for 1 h, 180 ◦ C for 1 h and 220 ◦ C for 1 h for imidization process to occur and form the polyimide. Thus prepared mats were further characterized and used for the preparation of the epoxy composites.
2.2. Preparation of polyimide/epoxy nanocomposites The electrospun mat was manually chopped into 0.5 mm × 0.5 mm pieces. These chopped pieces were incorporated in DGEBA epoxy (Trade name: GY 250) at weight percentages of 0, 0.5, 1, 2, and 3 respectively by mechanical mixing. Later, sufficient amounts of polyamidoimadazoline hardener (Trade name: AR 140) was added to the system (epoxy to hardener ratio is maintained to be 2:1) and kept for thermal curing (60 ◦ C for 4 h, 80 ◦ C for 1 h). After curing, the final products were tested and the properties were evaluated. The schematic representation of PI/Epoxy composites were presented in Fig. 1. 3. Characterization techniques The morphology of the nanofibers produced was observed by SEM, TEM and AFM. The samples were gold coated to make them conducting for SEM. These fibers were deposited on the copper grid and the TEM images were taken using HRTEM. For AFM, mica sheet was used as the substrate (Aligent 5500 SPM non-contact mode). The samples were tested in ATR mode from wave number 4000 cm−1 to 600 cm−1 in a Perkin Elmer Fourier transform infrared spectrometer. The visco elastic properties of PI loaded epoxy thermosets and neat epoxy was analyzed using a DMA analyzer from temperature 30 ◦ C to 150 ◦ C at a ramp rate of 2 ◦ C and the modulus and damping factor was plotted. The specimen size maintained was 30 × 5 × 1 mm3 and testing was done in tension mode. Fracture Toughness of the epoxy samples was performed using universal testing machine (Instron 5984) as per standards (ASTM D 5045). The fracture toughness was described in stress intensity factor (KIC ), as per the equation: KIC = f (x) =
L f (x) Bw 0.5 ( ) 6x0.5 [1.99 − x (1 − x) 2.15 − 3.93x − 2.7x2 ]
(1 + 2x) (1 − x)1.5
(1) (2)
where, X is in between 0 and 1. L is the load; B is the specimen thickness; w is the specimen width; a is the length of the crack and x = a/w . The flow characteristics of the neat epoxy and chopped fiber incorporated epoxy systems were tested using Modular Rheometer (MCR 102) using parallel plate mode. The storage modulus and loss modulus were investigated with respect to angular frequency from angular frequency 0.01 rad/s to 200 rad/s. The viscosity and shear stress of the samples were measured using the same set up with-in the angular frequency range of 1 rad/s to 200 rad/s. The thermal stability of neat epoxy and PI loaded epoxy samples were tested from a temperature range of RT to 800 ◦ C by employing a heating rate of 20 ◦ C/min in presence of nitrogen. 4. Results and discussion 4.1. Morphology The PAA solution was filled in a 10 ml disposable syringe and an electrical voltage was applied between the syringe needle and the drum collector. At a particular voltage, the repulsive force of the droplet exceeds the surface tension holding it. Hence it forms a Taylor cone at the needle tip, which subsequently tends to form fibers and moves towards the collector to satisfy the surface tension. The TEM and AFM images of the PAA electrospun nanofibers were taken and are depicted in Fig. 2. The fiber diameters were measured using the available image processing software’s and the diameter range was found to be between 70 nm to 500 nm. By, measuring the diameters of fibers at different places, the average fiber diameter was found to be about 280 nm.
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Fig. 1. Schematic diagram for preparing electrospun fibers and epoxy composites.
Fig. 2. (a) TEM and (b) AFM images of electrospun PAA fibers.
Fig. 3. (a) Optical microscope and (b) SEM images of PI fibers after imidization.
The obtained PAA electrospun mat was then kept in an air oven and subsequent thermal treatment was given at pre-mentioned temperatures for the imidization reaction to occur and thus converting PAA to PI fibers. The SEM micrographs and optical microscopic images of PI fibers are presented in Fig. 3. 4.2. Fourier transform infrared spectroscopy FTIR spectra clearly reveals that, thermal treatment of poly(amic acid) at 70 ◦ C for 1 h, 120 ◦ C for 1 h, 180 ◦ C for 1 h and 220 ◦ C for 1 h results in the transformation of electrospun poly(amic acid) fibers to electrospun polyimide fibers (Fig. 4).
The peak at ∼1627 cm−1 can be accredited to the absorption of carbonyl stretching (C==O) of the amide group and the peak at ∼1505 cm−1 is due to C–NH stretching vibration. The peak around ∼3500 cm−1 is associated with N–H and O–H stretching in polyamic acid. The vanishing of this peak after imidization indicates successful cyclization of N–H and O–H to the corresponding imide group. Now, considering the polyimide spectra, the peak at ∼1780 cm−1 and ∼1722 cm −1 corresponds to C==O symmetric stretching vibration and C==O asymmetric stretching vibration respectively. The peak at ∼1372 cm−1 can be credited to C==N stretching vibration, and ∼725.5 cm−1 peak can be due to C==O bending. In the Figure, the prepared PI Electrospun mat is
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Fig. 5. Variation of storage modulus of PI nanofiber reinforced epoxy composites with temperature. Fig. 4. FTIR spectra of fibers (a) polyamic acid, (b) electrospun polyimide and (c) casted polyimide.
also compared with casted PI film and both have identical FT-IR spectra, which confirms the formation of Polyimide conclusively. The FTIR spectrum of epoxy resin, hardener and cured epoxy was also tested and are presented in Fig. S1. From the FTIR spectra of the hardener, the peak obtained at about ∼3291 cm−1 confirms the presence of N–H group in it. The peak at ∼1650 cm−1 confirms the amide linkage (C==O–N–H) group, hence the structure of polyamide hardener can be confirmed easily. The peak at ∼1609 cm−1 is the characteristic peak of C–N in imidazoline ring. Peaks at ∼2984 and ∼2893 are due to C–H stretching vibrations. The structure of the hardener is depicted in Fig. S2. The peak obtained at about ∼2968 cm−1 for epoxy resin is due to the C–H stretching of the oxirane group existing in the compound. The Peaks obtained at ∼1247 cm−1 and ∼916 cm−1 respectively are due to C–O–C stretching of ether and C–O stretching in oxirane thereby confirming the epoxide linkage in the epoxy resin. The peak at ∼831 cm−1 corresponds to stretching vibration of C–O–C of ether whereas the ∼1511 cm−1 peak is due to aromatic C–C stretching. The broad peak obtained at ∼3500 cm−1 for the neat epoxy is due to the presence of –OH group in the system. The peak obtained at ∼3291 cm−1 in the cured epoxy shows the presence of N–H group in it which in turn leads to the confirmation of the curing process. Peaks obtained at ∼1650 cm−1 and ∼1609 cm−1 confirms the amide and C–N linkage in the epoxy system due to the reaction between the hardener and the resin. The reaction is depicted in the Fig. S2. Visco-elastic properties and modeling The storage modulus of neat epoxy and PI nanofibers incorporated samples were analyzed and the graphs are presented in Fig. 5. We can observe that the dynamic storage modulus of PI fiber incorporated composites increase with loading percentage and tan delta undergoes a positive shift. When compared to neat epoxy system, the storage modulus of PI loaded samples exhibit a hike at low temperatures until the Tg of PI. After this point the modulus tends to decrease. This increase in the modulus is indicative of the reinforcement effect of the nanofiber mat in epoxy systems. These mats while dispersed in epoxy, constitute an isotropic stiffening system and so contribute to the high strength of the composites. The effect of the nanofiber mat loading showed a constructive positive effect in the mechanical performance of the composites. This increase is due to the effective stress transfer, good interfacial surface area and reinforcing
strength of the mat. In case of tan delta, the positive shift in the peak is due to the change in cross-linking density of the systems because of the presence of PI mats. The Tg of neat epoxy was found to be 92.7 ◦ C, while PI loaded epoxy composites were found to be 104.1 ◦ C, 106.6 ◦ C and 108 ◦ C respectively at 1, 2 and 3% loading. The visco elastic properties of the PI loaded epoxy composites was modeled using Einstein, Guth and Quemada models to assess the change in storage modulus of the epoxy matrix with increase in loading (in Fig. 6). Einstein proposed a simple theoretical model to estimate the modulus of filler reinforced polymer matrices. It is mathematically denoted as Ec Ep
( ) = 1 + 2.5Vf
(3)
Where, Ec and Ep are the storage modulus of resultant composite and matrix and Vf is the reinforcement volume fraction. This model is applied to polymeric systems reinforced with less concentration. Guth equation for fiber reinforced polymeric systems is expressed by the equation Ec Ep
( ) = 1 + 2.5Vf + 14.1Vf2
(4)
This equation takes into account the interactions between the particles even at higher concentration The Quemada equation for polymeric systems is given by Ec Ep
= 1/(1 − 0.5K6 Vf )2
(5)
This equation uses a variable that implies the interaction between particles and the geometry (variable, K6 = 2.5). The experimental values were found to be higher than the theoretical model values. Fracture toughness In epoxy thermosets, fracture toughness is one of the major challenges that need to be addressed. Cured epoxy being highly brittle tends to break with the slightest crack. This drawback of epoxy systems need to be overcome for its industrial applications and can be done by the introduction of suitable reinforcements. The PI nanofiber mat loaded epoxy samples at different compositions were tested and the stress concentration intensity is depicted in Fig. 7. The fractured samples were also observed using SEM and the images are illustrated in Fig. 8. From Fig. 7, we can
B.D.S. Deeraj, R. Harikrishnan, J.S. Jayan et al. / Nano-Structures & Nano-Objects 21 (2020) 100421
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Fig. 6. Visco-elastic properties comparison between experimental values and theoretical models (at room temperature).
low loadings to toughen epoxy systems. So, further loadings were not tested as the experimental trend shows a decrement in the toughness. Rheological properties and modeling The storage and loss modulus of the neat epoxy and PI mat incorporated epoxy systems were estimated with respect to fiber content and are presented in Fig. 9. As per the Figure, we can observe an increasing trend in modulus with respect to the fiber loading. The result conclusively proves that these nanofiber mats have a good influence in flow dynamics of the systems. Fig. 10 depicts the change of viscosity with shear rate. We can observe from the Figure that viscosity depends on filler loading. The difference in viscosity is more significant in the lower frequencies than at higher frequencies. Usually flow models are employed to estimate the type of flow followed by the system based on the experimental data. Here, PI/Epoxy rheology experimental data was modeled using available power’s and Casson’s models. Fig. 7. Variation of stress intensity factor with PI loading (w %).
observe that 0% nanofiber mat loaded samples (neat epoxy) have a KIC of 1.06 ± 0.37 MPa.m1/2 . This increased to 18% and 20% at 0.5 w% and 1 w% respectively. Upon further increase in loading of 2 w% and 3 w%, the toughness decreased gradually. At low loadings, the crack propagation is obstructed by the incorporated PI nanofiber mat and different crack propagation mechanisms like crack deflection, fiber pull out and fiber bridging can be observed. But at higher loadings, the system becomes highly brittle and hence fracture toughness decreases. So, at higher concentrations the PI loaded epoxy composites might have developed ‘‘secondary cracks’’ that are formed parallel to the mat surface, thereby making the entire composite brittle and so, the toughness decreases. Similar decrease in fracture toughness was observed in the PMMA nanofiber/Epoxy systems [39]. Hence based on the results we can state that PI nanofiber mats being brittle can only be used at
Powers Model In this model, the Shear stress and shear rate are plotted on logarithmic axis and it expresses shear thinning and thickening behavior of fluid [40]
σ = Kγ n
(6)
Where σ and γ are shear stress and shear rate respectively, n is the flow index parameter which describes the tendency of fluid to shear thin, and k is consistency coefficient. When n < 1, the fluid is thinning in nature and when n > 1, the fluid is thickening in nature. The power law model holds better in the low shear condition Applying logarithms on both the sides of Eq. (6): log σ = log K + n log γ
(7)
Eq. (7) can be expressed as, log η = log K + (n − 1) log γ Where, η is the viscosity.
Fig. 8. Fractured surface topography of Polyimide/epoxy composites at (a) 1 w% and (b), (c) 3 w% PI loading.
(8)
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Fig. 9. Variation of rheological properties, (a) storage modulus & (b) loss modulus with PI loading (w %).
Fig. 10. Variation of viscosity with respect to shear rate.
Fig. 11. Powers law rheological model fitting.
The Powers law fit data of neat epoxy and loaded samples are depicted in Fig. 11. The parameters obtained are tabulated in Table 1. Cassons model The Casson rheological model is used to estimate the flow of visco-elastic fluids. Mathematically, it is expressed as [40,41] 0.5 σ 0.5 = K0c + Kc0.5 γ 0.5
(9)
Where σ is shear stress, γ is shear rate, koc is Casson yield stress (Pa.s), kc is Casson plastic viscosity in mPa.s. The Casson model fitting data of neat epoxy and fiber loaded samples are presented in Fig. 12 and the parameters obtained are presented in Table 2. Thermogravimetric analysis The TGA profiles of PAA and electrospun PI are presented in Fig. 13. From the profiles, a clear difference can be observed, PI was mostly stable until 550 ◦ C, and after complete degradation quite high char yield is present. However, in case of PAA, most of the degradation takes place at low temperature, which is accompanied with a small peak at 550 ◦ C and very less char yield at the end. The stability of Polyimide incorporated epoxy samples was tested and are presented in Fig. 14. From the TGA thermograms, it is clear that PI mats affect the thermal stability of epoxy only marginally and further, there is an improvement in the char yield compared with the neat cured epoxy.
Fig. 12. Cassons law rheological model fitting.
Conclusion Electrospun nanofibers of polyimide were successfully prepared by imidization of poly (amic acid) electrospun mat. PI nanofiber mat/epoxy composites were prepared at different mat
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Table 1 Powers law parameters. Sample
Slope
Neat epoxy (0% PI) PI 1% PI 2% PI 3%
0.94501 0.44949 0.29772 0.31892
Intercept
± ± ± ±
0.01123 0.02262 0.02692 0.04006
0.80268 1.74934 2.32942 2.52892
± ± ± ±
Regression co-efficient 0.01496 0.03013 0.03586 0.05337
0.99634 0.93807 0.82351 0.70577
Table 2 Cassons law parameters. Sample
Slope
Neat epoxy (0% PI) PI 1% PI 2% PI 3%
2.33069 1.83234 1.50431 1.42839
± ± ± ±
9.15054E−4 0.12928 0.17866 0.35548
Intercept
Regression co-efficient
0.07434 ± 0.00443 4.80946 ± 0.62557 12.39152 ± 0.86452 17.54646 ± 1.72012
1 0.88489 0.72887 0.3681
fibers on the visco elastic flow properties of the composite. The TGA of the systems were done and thermal stability was analyzed and it is observed that PI loading improved thermal stability of epoxy composites. Thus polyimide nanofiber mat reinforced epoxy composites could be utilized for advanced applications at relatively low loadings of PI nanofiber mats. Thus the developed PI incorporated epoxy composites are having advanced performance than neat epoxy samples and has potential use as structural composites and in circuit boards. Data availability All the data presented in the study is available with the corresponding author upon request. Declaration of competing interest
Fig. 13. TGA thermograms of PAA and PI nanofibers.
No author associated with this paper has disclosed any potential or pertinent conflicts which may be perceived to have impending conflict with this work. For full disclosure statements refer to https://doi.org/10.1016/j.nanoso.2019.100421. CRediT authorship contribution statement B.D.S. Deeraj: Conceptualization, Methodology, Investigation, Writing - original draft. Harikrishnan R.: Investigation. Jitha S. Jayan: Writing - review & editing. Appukuttan Saritha: Writing - review & editing. Kuruvilla Joseph: Conceptualization, Writing - review & editing. Acknowledgment The authors deeply acknowledge Indian Institute of Space Science and Technology (IIST) for the financial support Appendix A. Supplementary data Supplementary material related to this article can be found online at https://doi.org/10.1016/j.nanoso.2019.100421.
Fig. 14. TGA thermograms neat epoxy and PI loaded epoxy thermosets.
loadings. The morphology of the fibers was confirmed by TEM and AFM images. The completion of imidization reaction was confirmed by FTIR and the corresponding peaks. The DMA results showed an increase in the modulus for the PI mat loaded samples than neat epoxy composites at room temperature. The fracture toughness of the epoxy systems exhibited a 20% increase at 1% loading and thereafter decreased with increase in loading. The dynamic rheology studies reveal the increased effect of these
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