Surface modification of polyimide fibers for high-performance composite by using oxygen plasma and silane coupling agent treatment

Abstract

In this study, the oxygen plasma and silane coupling agent composite treatment was used to modify the polyimide fiber surface to improve the interfacial properties. The oxygen plasma treatment introduced active groups on the fiber surface, which facilitated the grafting of silane coupling agent to the fiber surface. The surface morphology and chemical composition of fibers were characterized by scanning electron microscope and X-ray photoelectron spectroscopy. The results showed that after plasma treatment, the etching spots on the fiber surface increased with the plasma treatment time, and the surface O atom content, O/C ratio and C–O(H) bond ratio reached the highest value at 27 min plasma treatment. After the composite treatment, the surface Si atomic content reached the highest value after 27 min plasma pretreatment. Moreover, polyimide/polyamic acid unidirectional reinforced composites were prepared. In polyimide/polyamic acid composites, the interfacial shear strength of polyimide fibers first increased and then decreased with plasma treatment time, both in oxygen plasma treatment and in composite treatment, and increased by up to 36.98% and 61.68% respectively compared. In addition, the transverse tensile strength of polyimide/polyamic acid composites increased by 103.73% after composite treatment compared with the pristine specimens.

Keywords

Polyimide fiber silane coupling agent surface characteristics interfacial shear strength

With the increasing demand for high-performance composite materials used in aviation and electronics, research on polyimide (PI) with excellent properties has been accelerated. PI fiber is a kind of high-performance reinforcement material because of its light weight, high strength, radiation resistance, excellent mechanical properties and dimensional stability, excellent chemical and thermal properties, and special dielectric properties.^1

–7 Compared with other high-performance resin matrix composites, the polyimide/polyamic acid (PI/PAA) unidirectional reinforced composites can be prepared to the same thickness as films,^8,9 while also having the high performance of PI fibers, so they have been widely used in aerospace, electronics, integrated circuit packaging, and other fields.

In PI/PAA composites, unidirectional PI fibers can greatly improve the mechanical properties and reduce the coefficient of thermal expansion of the composites in the fiber arrangement direction. However, there is a low interfacial performance between PI fibers and the matrix due to the low surface energy and chemical inertia of PI fibers,^10
–12 which is an important factor leading to the low transverse tensile properties of PI/PAA composites,^13,14 and hinders the application of PI fiber reinforced composites. It is worth noting that the existing PI fiber modification methods have various problems, such as insufficient modification effect, pollution, and so on. Therefore, there is an urgent need for an efficient, environmentally friendly, and economical method to improve the interfacial properties between PI fiber and PAA resin, thereby obtaining high-performance composites.

Recently, in order to break through the limitation of poor interfacial adhesion of high-performance fibers, a variety of technologies have been applied, such as alkali treatment, thermal treatment, plasma treatment, silane coupling agent (CA) treatment, ozone treatment, atomic O₂ treatment, dopamine treatment, and carbon nanotubes treatment.^{15

–26} However, alkali treatment and thermal treatment will destroy the structure of PI fibers. Surface functionalization with polymer membranes can improve the interfacial properties between fiber and resin without impairing the mechanical properties of fibers, such as dopamine, silane CA,²⁷ and different random copolymers.²⁸ Liu et al.²⁹ deposited hybrid polymer membrane (TA-APTES) hybrid membrane on the surface of poly(p-phenylene-2, 6-benzobisoxazole) (PBO) fibers and grafted with epoxy-terminated polysesquisiloxane (POSS), thereby increasing the single fiber pull-out strength of poly (p-phenylene-2, 6-benzobisoxazole) (PBO) fibers/bisphenol A dicyanate ester (BADCy) micro-composites to 3.8 MPa, increased by 31.0%. Zhou et al.¹² used silane CA to modify the surface of two kinds of PI fibers, the interfacial shear strength of PI fiber/epoxy resin composites increased by up to 17.3% and 8.3%, respectively. Among these materials for surface treatment, silane CA has the lowest cost and can be directly obtained through commercial purchase without being self-made. This technology has been widely used in the surface treatment of carbon fibers and glass fibers, and recently it has also been applied to the surface treatment of PI fibers.^30,31

Similar to the action principle of a random copolymer (P(S-co-BCB-co-MMA)) for improving the interfacial properties between fiber and resin,²⁸ silane CA on the one hand interacts with the active groups on the surface of PI fibers, and on the other hand forms an interpenetrating polymer network with polymer matrix. When the silane CA is added to the mixture of water and alcohol, the alkoxysilane groups in the silane CA hydrolyze and generate silanol groups. Silanol groups can react with hydroxyl groups on the fiber surface and undergo condensation to form covalent bonds, thus forming a silane CA layer on the fiber surface.^21,32,33 When the composite is formed, the organic functional groups in silane CA can form an interpenetrating polymer network with the polymer matrix, which increases the interfacial adhesion with the resin. An interpenetrating polymer network refers to a mixture of two physically crosslinked polymers.

Combining multiple treatment methods can gain the advantages of different treatment methods. Tang et al.²⁸ applied CeO₂ and P(S-co-BCB-co-MMA) treatment to PBO fiber, the CeO₂ treatment improved the UV resistance and surface roughness of the fiber, and the P(S-co-BCB-co-MMA) treatment improved the interfacial properties between fiber and resin by forming polymer membrane on the fiber surface.When the concentration of Ce_0.8Ca_0.2O_1.8 nanoparticles was 0.6 wt%, the single fiber pull-out strength of modified PBO fiber/modified BADCy resins composites was 4.6 MPa, increased by 48.4%. Therefore, O₂ plasma treatment and silane CA treatment are combined in this study to better improve the interfacial properties between PI fiber and PAA resin. The low content of hydroxyl groups on the surface of PI fibers leads to the lack of adhesion between silane CA and PI fibers, which leads to the limited improvement of fiber interfacial properties by this simple method. As a simple, efficient treatment method without other by-products, O₂ plasma treatment can obtain more polar functional groups such as hydroxyl groups on the surface of PI fibers,^35,36 which facilitates the grafting of silane CA to the fiber surface. According to our previous work,³⁷ O₂ plasma treatment itself can also improve the interfacial properties of PI/PAA composites by improving the surface roughness of fiber, thus further improving the effect of composite treatment.

In this study, PI fibers were treated with O₂ plasma at various times, and then treated with silane coupling agent 3-aminopropyltriethoxysilane (KH-550). The morphologies, surface chemical compositions, interfacial shear strength (IFSS) and mechanical properties of PI fibers under different treatment conditions were investigated and theoretically analyzed. In addition, the transverse tensile properties of PI/PAA composites prepared from O₂ plasma and silane CA composite treated PI yarns were significantly improved.

Materials and method

Materials

The high-performance PI yarns with a linear density of 500D were provided by Jiangsu Shino New Materials Company, and the average diameter of single fiber inside the yarn was 9.52 μm. Acetone (CH₃COCH₃, 99.5%), ethanol (C₂H₅OH, 95%), and silane coupling agent (C₉H₂₃NO₃Si, 98%) were purchased from China Pharmaceutical Group Co. Ltd, China. N-methylpyrrolidone (NMP) (C₅H₉NO, 99.5%) was purchased from Shanghai Adamasi Reagents Co. Ltd, China. PAA (dissolved in NMP at a ratio of 18 wt%, and synthesized by 1,2,4,5-benzenetetracarboxylic anhydride (PMDA) and 4,4’-Oxydianiline (ODA)) was provided by China Friend Special New Materials Company. All PAA solutions used were diluted to 10 wt% with NMP and stirred for 2 h, then evacuated in a vacuum oven for 15 min to remove air bubbles. PI yarns were soaked in acetone for 12 h, washed with deionized water five times, and dried in an oven at 60°C for 12 h before use. PI fibers used throughout all experiments were disassembled from the cleaned PI yarns.

Preparation

Surface modification of PI fibers with O2 plasma

The surface treatment procedure of PI fibers is shown in Figure 1. In order to increase the hydroxyl group content and roughness, PI fibers were modified by O₂ plasma treatment at different times. First, the PI fibers were wound around a plastic hollow cylinder to ensure all the fibers were treated uniformly and thoroughly. Then, the O₂ plasma treatments were carried out in a machine of VP-T3 low-temperature plasma treatment equipment (Guangzhou SunJune Instrument and Equipment Co., China). The treatment time was controlled at 18, 27, 36 min at 40 liter per minute (LPM) gas flow rate and high gear power. After the plasma treatment, the fibers were sealed in clean plastic bags immediately for further experiments. No significant difference between pristine and treated fibers was observed by visual inspection. In this article, PX will be used to represent the fibers treated with O₂ plasma, where P represents O₂ plasma treatment and X represents the number of minutes of plasma treatment.

Surface modification of PI fibers with silane CA

The silane CA solution of 4 wt% was prepared by adding silane CA to 95% ethanol.²⁷ Meanwhile, the pristine PI fibers and PI fibers treated with O₂ plasma for 18, 27, and 36 min were wound and fixed on a rectangular frame, and then soaked in the silane CA solution for 6 h at 25°C. Afterwards, the fibers were washed three times with deionized water and dried in an oven at 100°C for 4 h. No significant difference between pristine and treated fibers was observed by visual inspection. In the following text, composite treatment will be used to refer to O₂ plasma and silane CA composite treatment, and S-PX will be used to represent the O₂ plasma and silane CA composite treated fibers, where S-P represents composite treatment and X represents the number of minutes of plasma treatment.

Preparation of specimens for single fiber pull-out test

Specimens for single fiber pull-out test refers to a composite of PI fiber partially embedded in ellipsoidal PAA resin. The PI fibers were straightened and fixed on the metal frame, and a PI yarn adhered with 10 wt% PAA solution was then attached vertically to the fibers for several seconds to generate micron-scale resin droplets on the fiber. Then the specimens were cured by thermal imidization according to the following process: 80°C/1 h, 120°C/1 h, 160°C/1 h, 200°C/1 h, 250°C/1 h, 300°C/1 h, and finally cooled to room temperature.^38
–40 During the curing process, the influence of active groups on the fiber surface can be ignored.³⁴

Preparation of PI/PAA composites

The PI/PAA composites were prepared by the casting method with 10 wt% PAA solution and pristine or treated PI yarns. PI yarns used therein were cleaned and treated with O₂ plasma and silane CA in the same way as PI fibers. In preparation, first, PI yarns were arranged in parallel at a spacing of 2 mm and fixed on a glass plate in a single layer. Second, the PAA solution was slowly poured onto the glass plate by a coating technology, followed by the same thermal imidization process as in the preparation of specimens for single fiber pull-out test. The average thickness of the composite with PI yarns is 0.25 μm, and the volume ratio of fiber to composite is about 20%.

Characterizations

Surface morphology

The surface morphologies of the pristine and treated PI fibers, as well as the transverse tensile fracture of PI/PAA composites were observed by a scanning electron microscope (SEM; HITACHI S-4800, Tokyo, Japan) with an acceleration voltage of 5 kV. All specimens were gold plated before observation.

Surface chemical composition

The surface chemical compositions of the pristine and treated PI fibers were examined by X-ray photoelectron spectroscopy (XPS; ESCALAB 250, Thermo Electron VG Scientific, Waltham, Massachusetts, USA). The X-ray source was Al-Ka (1486.6 eV), the pass energy was set at resolution 20 eV, and the examination was carried out under the pressure between 10⁻⁷–10⁻⁸ Pa. The C1s peak was deconvoluted by XPS-PEAK software.

Interfacial properties

The interfacial properties of the pristine and treated PI fibers were analyzed by the values of IFSS, which were measured on an XQ-2 tensile tester (Shanghai Xusai Instrument Co., Shanghai, China) by single fiber pull-out test. Excellent interfacial properties can effectively transfer loads between fiber and resin and give full play to the load bearing capacity of fibers, ultimately improving the mechanical properties of composites, and the single fiber pull-out test is commonly used to evaluate the interfacial properties between fiber and resin. As shown in Figure 2, during the test, one end of the fiber was held by the upper chuck of the tensile tester, while the other end was passed through a gap formed by two glass sheets, and then the test was carried out at a crosshead speed of 20 mm/min with a gauge length of 20 mm. At least 46 specimens with the embedded length of 120–200 μm were tested for each sample, and the average value of IFSS was calculated. The IFSS between PI fiber and PAA resin can be calculated according to Equation (1):

IFSS = \frac{F_{\max}}{π d l}

(1)

where F_max is the maximum load, d is the average diameter of the fiber, and l is the length of the fiber embedded in the resin.

Mechanical properties

The mechanical properties of the pristine and treated PI fibers and PI/PAA composites were tested by XQ-2 tensile tester and a microcomputer control electron universal testing machine (MTS Systems Co. Ltd, China) respectively. The tensile properties of the pristine and treated PI fibers were tested at a crosshead speed of 10 mm/min with a gauge length of 20 mm. At least 50 specimens were tested for each sample and averaged.

As shown in Figure 3, PI/PAA composites were subjected to transverse tensile tests. Referring to the ISO 527-3-2018 standard, PI/PAA composites prepared from the pristine or treated PI yarns were tested at a crosshead speed of 5 mm/min with a gauge length of 50 mm, and in a size of 100 mm × 20 mm. At least six specimens were tested for each sample and averaged.

Results and discussion

SEM analysis

To study the effect of surface modification on the high-performance PI fibers, the surface morphologies of the pristine and treated fibers were observed by SEM, as shown in Figure 4. The surface of the pristine PI fiber was relatively clean and smooth (Figure 4(a)). After being treated by O₂ plasma for 18 min, some visible but uneven etching spots appeared on the surface of P18 (Figure 4(b)). More etching spots and protrusions can be observed on the surface of P27 (Figure 4(c)). However, when the treatment time reached 36 min, the surface of P36 was damaged by prolonged exposure to the plasma treatment environment, resulting in a further increase in etch marks (Figure 4(d)). These etching spots on the fiber surface increased the surface roughness of the fiber and enhanced the mechanical bonding with PAA resin. Therefore, O₂ plasma treatment can improve the interfacial properties and mechanical properties of PI/PAA composites.

After being treated by O₂ plasma, PI fibers were subsequently treated by silane CA, thus the surface of PI fiber was attached by non-uniform silane CA. The silane CA on the S-P0 surface without O₂ plasma pretreatment was the least (Figure 4(e)), while the silane CA on the surface of S-P18 increased significantly (Figure 4(f)). As the plasma pretreatment time increased to 27 min, the silane CA on the surface of S-P27 reached the maximum (Figure 4(g)), and decreased slightly after 36 min O₂ plasma pretreatment (Figure 4(h)). The diameter of PI fibers did not change after O₂ plasma treatment and silane CA treatment. After the composite treatment, silane CA was grafted on the fiber surface, so the surface of the fiber became uneven and the surface roughness was greatly improved, and the place without silane CA was also rough due to the etching spots brought by plasma treatment. Therefore, the composite treatment can greatly improve mechanical bonding between fibers and PAA resin, thereby improving the interfacial and mechanical properties of PI/PAA composites.

XPS analysis

XPS analysis was used to study the changes of elements and functional groups on the surface of the pristine and treated PI fibers. As can be seen from Table 1, the O atom content and oxygen/carbon (O/C) ratio increased with the increase of plasma treatment time from 0 min (P0) to 18 min (P18) and then to 27 min (P27), but decreased slightly with the further increase of plasma treatment time to 36 min (P36). In these specimens, the content of N and Si atoms remained at a stable value, the presence of Si atoms may be due to the use of silicone-containing conductive tape as the substrate during the test. When the plasma treatment time was 27 min, the O atom content and O/C ratio on the surface of P27 reached the maximum value, which increased from 16.65% to 22.82% and 0.23 to 0.35 compared with P0, respectively. As shown in Figure 5(a), the highly reactive oxygen species in the plasma broke the imide bond and formed various functional groups on the surface of PI fibers, such as –OH, –COOH, etc.^35,41 As a result, the increase of oxygen-containing functional groups on the fiber surface increased the content of oxygen atoms. At the same time, with the formation of N-containing functional groups, the carbon chain in the polyimide became shorter and the content of C atoms decreased, but the content of N atoms did not change significantly. It should be noted that when the plasma treatment time was 36 min, the oxygen-containing functional groups were degraded, and the O atom content decreased due to the excessively high plasma treatment power.

Table 1.

X-ray photoelectron spectroscopy (XPS) atomic concentration of the pristine and treated polyimide (PI) fibers

Samples	Element content (%)				Proportion
Samples	C	O	N	Si	O/C
P0	72.24	16.65	6.08	5.03	0.23
P18	70.96	17.9	5.87	5.26	0.25
P27	65.42	22.82	6.24	5.52	0.35
P36	69.47	19.84	5.01	5.69	0.29
S-P0	67.81	18.56	6.68	6.95	0.27
S-P18	61.26	23.75	4.62	10.37	0.39
S-P27	58.88	24.18	4.92	12.02	0.41
S-P36	58.90	24.11	5.41	11.58	0.41

Figure 6 and Table 2 respectively show the C1s XPS spectrum and functional group content of the pristine and plasma treated sample surfaces. In addition, the binding energies of three carbon-containing groups^21,31 are also listed in the table. When the plasma treatment time increased to 18 min, the C–O(H) bond ratio on the surface of P18 increased significantly, indicating that the C–O(H) bond accounted for the majority of the polar functional groups generated on the fiber surface by O₂ plasma treatment. From P18 to P27, the proportion of C–O(H) bond and COO(H) bond increased while the content of Sp²C decreased. However, when the plasma treatment time reached 36 min, the contents of oxygen-containing functional groups on the surface of P36 decreased, which may be caused by the prolonged plasma treatment.³¹

Table 2.

The concentration of functional groups on the pristine and O₂ plasma treated polyimide (PI) fibers

Samples	Contents of the functional groups of the polyimide fiber surfaces (%)
Samples	Sp²C (284.4 eV)	C–O(H) (285.3 eV)	COO(H) (288.2 eV)
P0	40.16	50.73	9.12
P18	40.31	52.34	7.41
P27	38.35	53.9	7.75
P36	40.62	52.5	6.88

When the silane was added to 95% ethanol, only a portion of the ethoxy groups on the silane CA was replaced by a hydroxide because the hydrolysis of silane CA to silanol was an equilibrium reaction (Figure 5(c)). The plasma-treated fiber surfaces contained C–O(H) bonds, most of which formed amide bonds with amino groups in silanol, and the rest esterified with silanol⁴² (Figure 5(b)). Therefore, the loading amount of saline CA as well as Si atom content is closely related to the C–O(H) bond ratio. Since the O atom content and C–O(H) bond ratio of P27 reached the maximum value, the Si atom content on the surface of S-P27 also reached the highest value, and the silane CA treatment was the most efficient at this time.

After silane treatment, the fibers were grafted with silane CA, and the fiber surface was gradually covered with silane CA. With the increase of the plasma pretreatment time, the Si atom content on the surface of silane CA treated samples first increased and then decreased, which was the same as the trend shown in Figure 7. Among these samples, the Si atom content on the surface of S-P27 reached the highest 12.02%, which was much higher than 5.03% of P0 and 6.95% of S-P0. The Si atom content on the surface of S-P36 decreased because of the decreasing of the C–O(H) bond ratio on the surface of PI fiber. C–O(H) bonds were formed on the surface of PI fiber by O₂ plasma treatment, which was an important factor in the effectiveness of silane CA treatment because C–O(H) bonds can form a strong chemical bond with silane CA on the fiber surface (Figure 5(b)). Therefore, with the increase of C–O(H) bond ratio, the silane CA grafted on the fiber surface increased, and the element content detected by XPS became closer and closer to that of silane CA, which eventually led to the decrease of the content of C atoms and N atoms, and the increase of the content of Si atoms and O atoms. Therefore, a large amount of silane CA was grafted on the fiber surface due to the good interaction between PI fibers and silane CA. When the modified fibers were composited with PAA resin, the silane CA on the fiber surface formed a copolymer network with the resin, and the interfacial and mechanical properties of the composite were improved due to the connection effect of silane CA on the fiber and resin.

IFSS analysis

The IFSS test results of the pristine and treated PI fibers are shown in Figure 8, which reflect interfacial properties of PI/PAA composites. In O₂ plasma treated samples, the IFSS of O₂ plasma treated fibers was higher than that of pristine fibers, as shown in Figure 8(a), the IFSS of P0 was 24.01 MPa, while the IFSS of P18 was 25.06 MPa. When the plasma treatment time reached 27 min, the IFSS of P27 increased to 32.89 MPa, 36.98% higher than that of P0, reaching the maximum value. However, the IFSS of P36 decreased to 30.52 MPa after 36 min plasma treatment. After O₂ plasma treatment, the IFSS firstly increased and then decreased with the increase of plasma treatment time, which was consistent with the trend of O atom content and O/C ratio on the PI fiber surface. The change may be attributed to the increase of polar functional groups on the surface of plasma treated PI fiber, because polar functional groups can form strong chemical bonds with the PAA resin matrix. In addition, with the deepening of the surface etching marks after plasma treatment, the roughness of the fiber surface will also increase, which contributes to the improvement of IFSS.

In Figure 8(b), the IFSS of S-P0, S-P18, S-P27, and S-P36 increased to 30.21, 34.91, 38.82, and 34.94 MPa respectively, showing a trend of first increasing and then decreasing, indicating that composite treatment greatly improved the interfacial adhesion between fiber and resin. Since the silane CA treatment process for PI fibers was the same, the change of IFSS of composite treated samples depended on the length of plasma pretreatment. When the plasma pretreatment time reached 27 min, the IFSS of samples reached the maximum value and increased by 61.68% and 28.50% compared with P0 and S-P0, respectively. These results may be ascribed to the fact that silane CA connected to the PI fiber surface through C–O(H) bonds on the one hand and formed an interpenetrating polymer network with PAA resin on the other hand, so that the fiber and resin were well connected together. During the curing process of PAA resin, silane CA grafted on the fiber can react with PAA resin, and its reaction mechanism is similar to the reaction between C–O(H) bonds and groups in silanol in Figure 5(b). According to the results of XPS analysis, the content of silane CA on the surface of S-P27 also reached the highest, thus the composite treatment effect was the best when the plasma pretreatment time was 27 min.

Compared with plasma treated PI fibers, composite treated PI fibers have greater IFSS. This is because after composite treatment silane CA interacts with fibers through C–O(H) bonds, and forms a copolymer network with PAA resin after thermal imidization, which improves the PI/PAA composites interfacial properties. In addition, the attachment of silane CA and the O₂ plasma pretreatment also increased the surface roughness of the fibers, thereby enhancing the mechanical bonding with the PAA resin.

Mechanical properties

Tensile properties of PI fibers

Figure 9 shows the tensile strength of the pristine and treated PI fibers. The tensile strength of PI fibers decreased with the increase of plasma treatment time due to the surface damage caused by plasma treatment. The tensile strength of P0 was 2.11 GPa, but after plasma treatment for 18, 27, and 36 min, the tensile strength of plasma treated PI fibers decreased to 1.92, 1.85, and 1.73 GPa, respectively. After plasma treatment for 36 min, the tensile strength of P36 was about 18.01% lower than that of P0. After plasma pretreatment for 0, 18, 27, and 36 min, the tensile strength of composite treated PI fibers was 2.14, 1.92, 1.81, and 1.73 GPa, respectively, which also decreased with the increase of plasma pretreatment time. By comparing Figure 9(a) and (b), it can be seen that silane CA treatment has no significant effect on the tensile strength of PI fibers, indicating that silane CA treatment will not damage the mechanical properties of PI fibers.

Transverse tensile properties of PI/PAA composites

The transverse tensile properties of PI/PAA composites prepared by pristine and composite treated PI yarns are shown in Figure 10. Compared with the transverse tensile properties of PI/PAA composites prepared by pristine PI yarns (5.90 MPa), the transverse tensile strength of PI/PAA composites prepared by composite treated PI yarns was significantly improved, reaching 7.70, 8.48, 12.02, and 8.21 MPa, respectively, after 0, 18, 27, and 36 min of plasma pretreatment. After O₂ plasma and Silane CA composite treatment, the transverse tensile strength of the PI/PAA composites can be increased by 103.73% at most due to the improvement of the interfacial properties.

As shown in Figure 11, tensile fracture pictures of PI/PAA composites were taken in order to further study the influence of composite treatment on PI/PAA composites. When PI/PAA composites were subjected to transverse tensile damage, there were two failure modes at the fracture section at the same time: (a) fibers were pulled out and (b) fibers were broken (Figure 11(a) and (b)). When the fiber is completely encapsulated by the resin, the interface performance is good enough, the interface can effectively transfer the load between the fiber and the resin, and give full play to the bearing capacity of the fiber, so the fiber fracture appeared before the interface debonding, which is mode (b).

However, pores are usually present in the interior of the composite,⁴³ and since the yarns used herein are not pre-sized, the porosity may be higher, where the fibers are not fully encapsulated by the resin, as shown in Figure 12. In this case, the interface debonding may appear before the fiber fracture, which is mode (a). When PI/PAA composite is subjected to transverse tensile, the fiber will bend and deform with the resin opposite the pore. At this point, the fiber is subjected to a down-right force F_f at point a, which can form the component force F_l and F_t in the longitudinal and transverse respectively. When the composite is subjected to transverse tension, the cracks first propagate from the vicinity of the pores.⁴⁴ At the interface of the resin with pores, if there is a low interfacial adhesion between the fiber and the resin, interfacial debonding will occur, and the fiber and resin will be separated, resulting in the failure of mode (a) at the cross-section. Immediately thereafter, the stress is transferred to the interface of the void-free resin, and the failure mode (b) appears, which eventually leads to the failure of the composites. Therefore, in the case that the fibers are not fully encapsulated by the resin, the transverse tensile strength of PI/PAA composites is more affected by the interfacial properties. After O₂ plasma and silane CA composite treatment, IFSS and interfacial properties of PI fibers are improved, as a result, the transverse tensile properties of PI/PAA composites are improved.

Conclusions

High-performance PI fibers were treated to improve the interfacial properties by O₂ plasma and silane CA composite treatment. The tensile strength of PI fibers decreased with the increase of plasma treatment time due to the surface damage. When the plasma treatment time was 36 min, the tensile strength of PI fiber was only 1.73 GPa. However, in the second process, silane CA treatment did not reduce the strength of PI fiber. With the optimal O₂ plasma treatment time of 27 min, the PI fiber surface showed widely distributed etching spots, and the O atom content, O/C ratio, and hydroxyl group content reached the maximum value, which made the IFSS of fibers increase by 36.98% compared with that of the pristine PI fibers. When the PI fiber surface was modified by silane CA combined with 27 min O₂ plasma pretreatment, the content of Si atoms reached the maximum value because a large amount of silane CA was grafted on the fiber surface. Meantime, the IFSS of PI fibers increased by 61.68% compared with that of the pristine PI fibers. The improvement of IFSS measured by the single fiber pull-out test is attributed to the formation of strong chemical bonds between polar functional groups on PI fiber surface and the PAA resin matrix after O₂ plasma treatment, and the silane CA layer connecting the fiber and the resin after silane CA treatment. The surface modification method of composite treatment applied to PI yarns can effectively improve the interfacial properties of PI/PAA composites. As a result, the transverse tensile strength of PI/PAA composites was increased by 103.73% compared with the pristine sample. From these results, the combination of O₂ plasma treatment and silane CA treatment is a promising method to improve the interfacial properties of PI fibers which are vital to high-performance fiber-reinforced composites.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was financially supported by the Shanghai Natural Science Foundation of Shanghai Municipal Science and Technology Commission (20ZR1400600), and the Fundamental Research Funds for the Central Universities (grant No. 2232021G-06, 2232020A4-09). This study was also supported by the Shanghai Collaborative Innovation Center of High Performance Fibers and Composites.

ORCID iDs

Huiqi Shao

Jinhua Jiang

References

Chen

, et al. Thermal conductive hybrid polyimide with ultrahigh heat resistance, excellent mechanical properties and low coefficient of thermal expansion. Eur Polym J 2020; 122: 109368.

Yang

Zhao

, et al. Study about mechanical property and machinability of polyimide. Polymers 2018; 10: 173.

Ruan

Zhang

Liao

, et al. Enhancing tribological, mechanical, and thermal properties of polyimide composites by the synergistic effect between graphene and ionic liquid. Mater Design 2020; 189: 108527.

Wang

, et al. Enhanced impact properties of hybrid composites reinforced by carbon fiber and polyimide fiber. Polymers 2021; 13: 2599.

Chang

Zhang

, et al. Effect of pre-imidization on the structures and properties of polyimide fibers. RSC Advances 2015; 5: 69555–69566.

Zhang

Niu

Chang

, et al. High-performance fibers based on copolyimides containing benzimidazole and ether moieties: Molecular packing, morphology, hydrogen-bonding interactions and properties. Polym Eng Sci 2015; 55: 2615–2625.

Wang

Zhang

Niu

, et al. Influences of different imidization conditions on polyimide fiber properties and structure. J Poly Sc 2021; 138: 51189.

Liu

Chambers

Cutts

WD.

Comparison of nanostructure and nanomechanical properties of cast and air sides of polyimide films from different manufacturers. Micron 2021; 145: 103059.

Lian

Lei

Xue

, et al. Janus polyimide films with outstanding AO resistance, good optical transparency and high mechanical strength. Appl Surf Sci 2021; 535: 147654.

10.

Lin

, et al. Interfacial properties of high failure strain polyimide fiber/epoxy composites analyzed by a modified single fiber fragmentation test. Appl Surf Sci 2020; 513: 145869.

11.

Jian

Effect of surface treatment on enhancing interfacial strength of carbon fiber/polyimide composites. J Thermoplast Compos 2020; 35: 708–719.

12.

Zhou

Liu

Dai

, et al. Surface modification optimization for high-performance polyimide fibers. Mater Res Express 2019; 6: 095107.

13.

Canal

González

Segurado

, et al. Intraply fracture of fiber-reinforced composites: Microscopic mechanisms and modeling. Compos Sci Technol 2012; 72: 1223–1232.

14.

Melro

Camanho

Andrade Pires

, et al. Micromechanical analysis of polymer composites reinforced by unidirectional fibres: Part II – Micromechanical analyses. Int J Solids Struct 2013; 50: 1906–1915.

15.

Zeng

Liu

Chen

, et al. π – π interaction between carbon fibre and epoxy resin for interfacial improvement in composites. Compos Part B-Eng 2021; 220: 108983.

16.

Jia

, et al. Interfacial performance of high-performance fiber-reinforced composites improved by cold plasma treatment: A review. Surf Interfaces 2021; 24: 101077.

17.

Lin

Jiang

, et al. Mechanical and electrical properties of graphene-coated polyimide yarns improved by nitrogen plasma pre-treatment. Text Res J 2021; 91: 1627–1640.

18.

Bai

Liu

Qian

Surface characteristics and dyeing properties of polyimide fabric treated with sodium hydroxide. Color Technol 2019; 135: 53–59.

19.

Wang

Gao

Wang

, et al. Surface modification of UHMWPE fibers by ozone treatment and UV grafting for adhesion improvement. J Adhesion 2018; 94: 30–45.

20.

Yao

Gao

Jiang

, et al. Comparison of carbon nanotubes and graphene oxide coated carbon fiber for improving the interfacial properties of carbon fiber/epoxy composites. Compos Part B-Eng 2018; 132: 170–177.

21.

Ying

Liu

Dan

, et al. The effect of coupling agents on the interfacial properties of wood-fiber-reinforced polyimide composites. Surf Interface Anal 2017; 49: 1232–1237.

22.

Zhang

, et al. Tensile and interfacial properties of polyacrylonitrile-based carbon fiber after different cryogenic treated condition. Compos Part B-Eng 2016; 99: 358–365.

23.

Fan

Zharig

, et al. Modification of tensile, wear and interfacial properties of Kevlar fibers under cryogenic treatment. Compos Part B-Eng 2017; 116: 398–405.

24.

Cao

Wang

Xia

, et al. Preparation and performance of the modified high-strength/high-modulus polyvinyl alcohol fiber/polyurethane grouting materials. Constr Build Mater 2018; 168: 482–489.

25.

Liu

Xia

, et al. Highly tough and strain sensitive plasma functionalized carbon nanotube/epoxy composites. Compos Part A-Appl S 2019; 121: 123–129.

26.

Shao

Marriam

, et al. Quasi-static and dynamic interfacial evaluations of plasma functionalized carbon nanotube fiber. Appl Surf Sci 2019; 465: 795–801.

27.

Tang

Yang

Tang

, et al. Facile functionalization strategy of PBO fibres for synchronous improving the mechanical and wave-transparent properties of the PBO fibres/cyanate ester laminated composites. Compos Part A-Appl S 2021; 150: 106622.

28.

Tang

Fan

Tang

, et al. Calcia-doped ceria hybrid coating functionalized PBO fibers with excellent UV resistance and improved interfacial compatibility with cyanate ester resins. Appl Surf Sci 2021; 569: 151124.

29.

Liu

Fan

Cheng

, et al. Hybrid polymer membrane functionalized PBO fibers/cyanate esters wave-transparent laminated composites. Adv Fiber Mater 2022; 4: 520–531.

30.

Zhou

Liu

Dai

, et al. Surface modification of high-performance polyimide fibres by using a silane coupling agent. Compos Interface 2018; 26: 687–698.

31.

Máková

Holubová

Krabicová

, et al. Hybrid organosilane fibrous materials and their contribution to modern science. Polymer 2021; 228: 123862.

32.

Huang

Kipsang

, et al. Surface modification mechanism of SiC particles using KH5X0 (X = 5,6,7,8,9) silane coupling agents: First principle study. Ceram Int 2021; 47: 25551–25557.

33.

Tao

, et al. Tailoring surface properties and structure of layered double hydroxides using silanes with different number of functional groups. J Solid State Chem 2014; 213: 176–181.

34.

Rasekh

Rafiee

Preparation and properties of polyimide-based nanocomposites containing functionalized Fe3O4 nanoparticles. High Perform Polym 2019; 32: 418–428.

35.

Sun

Liu

, et al. Surface modification of polyimide fibers by oxygen plasma treatment and interfacial adhesion behavior of a polyimide fiber/epoxy composite. Sci Eng Compos Mater 2017; 24: 477–484.

36.

Wen

Meng

Liu

, et al. Surface modification of high-performance polyimide fibers by oxygen plasma treatment. High Perform Polym 2017; 29: 1083–1089.

37.

Lin

Tang

, et al. High-performance polyimide filaments and composites improved by O2 plasma treatment. Polymers 2018; 10: 695.

38.

Unsal

Cakmak

Real-time characterization of physical changes in polyimide film formation: From casting to imidization. Macromolecules 2013; 46: 8616–8627.

39.

, et al. Dielectric thermally conductive boron nitride/polyimide composites with outstanding thermal stabilities via in-situ polymerization-electrospinning-hot press method. Compos Part A-Appl S 2017; 94: 209–216.

40.

Cheng

Dong

Yang

, et al. Synthesis of poly(benzobisoxazole-co-imide) and fabrication of high-performance fibers. Polymers 2017; 133: 50–59.

41.

Naddaf

Balasubramanian

Alegaonkar

, et al. Surface interaction of polyimide with oxygen ECR plasma. Nucl Instrum Meth B 2004; 222: 135–144.

42.

Yang

Deng

, et al. Synthesis and thermal stability of a novel acetylene end-capped silicon-containing polyimide coupling agent with a silane pendant group. High Perform Polym 2019; 31: 1259–1271.

43.

Hagstrand

Bonjour

Månson

JAE.

The influence of void content on the structural flexural performance of unidirectional glass fibre reinforced polypropylene composites. Compos Part A-Appl S 2005; 36: 705–714.

44.

Canal

González

Segurado

, et al. Intraply fracture of fiber-reinforced composites: Microscopic mechanisms and modeling. Compos Sci Technol 2012; 72: 1223–1232.