Article pubs.acs.org/IECR
Facile Preparation of Hyperbranched Polysiloxane-Grafted Aramid Fibers with Simultaneously Improved UV Resistance, Surface Activity, and Thermal and Mechanical Properties Hongrui Zhang, Guozheng Liang,* Aijuan Gu,* and Li Yuan Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, Department of Materials Science & Engineering, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, P. R. China ABSTRACT: Simultaneously overcoming the poor UV resistance and surface inertness of aramid fibers while maintaining their excellent mechanical and thermal properties is a challenge. New grafted Kevlar fibers (HSi-g-KFs) were facilely prepared by in situ synthesizing hyperbranched polysiloxane with double bonds and epoxy groups on Kevlar fibers (KFs). As the molar ratio of water to silane was adjusted from 1.1 to 1.4, the surface morphology of HSi-g-KFs successively changed from unconnected dots to condensed dots and to a compact coating of hyperbranched polysiloxane. Compared with KFs, all HSi-g-KFs were found to have remarkably improved surface wettability and UV resistance. After 168 h of UV irradiation, the retentions of the modulus and break extension of the HSi-g-KFs were as high as 95−97%. In addition, the HSi-g-KFs were found to have much higher thermal stabilities than KFs. These attractive results demonstrate that the method proposed herein is a new and facile approach for preparing high-performance aramid fibers for cutting-edge industries. surfaces of aramid fibers to improve the surface properties27,28 and has seldom considered improving the UV resistance. To date, only a few works aimed at improving the UV resistance of aramid fibers have been published. Typically, ZnO- or TiO2-nanoparticle-embedded acrylic coatings are produced on the surface of Kevlar fibers to protect fibers from the harm of UV irradiation.29,30 As there is no covalent bond between the surface of the Kevlar fibers and the coating, the coating tends to fall off when the fibers are exposed to an external force. On the other hand, the UV resistance of the modified fibers is closely related to the thickness of the coating,31 and fibers with a 20-μm-thick coating were found to have better UV resistance than those with a coating of 10 μm. However, note that the diameter of the original Kevlar fibers is only 12 μm; hence, the presence of a 20-μm-thick coating tends to hinder the outstanding performance of original fibers. Therefore, simultaneously improving the surface activity and UV resistance of aramid fibers is still a challenge. The target of this research was to make some progress on this interesting topic. In the research reported herein, hyperbranched polysiloxanegrafted Kevlar fibers were designed and in situ prepared through the cohydrolysis and condensation of γ-methacryloxypropyltrimethoxysilane (MPS) and γ-glycidyloxypropyltrimethoxysilane (GPTMS). The influence of the preparation parameters on the structure and integrated properties (including surface properties, mechanical behavior, and thermal properties) of the grafting-modified fibers were systematically investigated. Some attractive results were obtained, and the nature behind them was intensively studied.
1. INTRODUCTION Outstanding integrated performances, especially super fatigue resistance, high strength and modulus, and good thermal and chemical resistance, make aramid fibers among the best competition for meeting the harsh requirements of many cutting-edge fields including space and aviation, electronics, tanks, bulletproof products, and so on.1−6 However, it is known that aramid fibers have two troublesome problems. The first is poor surface activity. Note that about 70% of the total amount of aramid fibers have been used to fabricate polymeric composites;7,8 however, the inert surfaces of aramid fibers are difficult to wet with organic resins,9−11 so the resultant composites do not show expected performances because of poor interfacial adhesion between the fibers and matrix.12,13 The second shortcoming is poor UV resistance. As the majority of the products made of aramid fibers are mainly serviced outdoors, they should have very good weathering resistance;14,15 however, UV radiation could deteriorate the structure of aramid fibers and their mechanical properties.16−18 Many efforts have been made to overcome the two critical disadvantages of aramid fibers. However, the majority of works have focused on improving the surface activity using many techniques, such as chemical etching,19,20 chemical grafting,21−23 plasma modification,24,25 and γ-ray irradiation.26 However, chemical etching seriously harms the structures and properties of aramid fibers, whereas plasma modification and γray irradiation are expensive and promote aging behavior. Chemical grafting has often been employed because of its great potential in achieving expected performance by adjusting the types of grafting material. Liu et al. grafted epoxy chloropropane onto the surfaces of Kevlar fibers (commercial aramid fibers) and found that the surface energy was enhanced by 31.5%.14 So far, however, chemical grafting has mainly focused on grafting some small-molecule compounds onto © 2014 American Chemical Society
Received: Revised: Accepted: Published: 2684
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2.4. Characterizations. Attenuated-total-reflection infrared (ATR-IR) spectra were recorded using a Nicolet 5700 FT-IR spectrometer (Madison, WI) attached to an ATR apparatus; the resolution was 4 cm−1, and the average result of 60 automatic scans from 650 to 4000 cm−1 was output as the test result. Scanning electron microscopy (SEM, Hitachi S-4700, Tokyo, Japan) was employed to observe the morphologies of the fibers. Ten measurements were conducted for each sample, and the average value was taken as the final result. The surface free energy and contact angle of a bundle of fibers were measured through a dynamic contact angle analysis system OCAT 21(DataPhysics Instruments GmbH Co. Ltd., Stuttgart, Germany). A bundle of fibers were cut to 1.5 cm in length and fixed indirectly to a wire hook suspended from the microbalance of the system. The fibers were immersed in the testing liquid by raising the elevating stage at a constant speed of 1 mm/min, and then the dynamic contact angle (θ) was obtained. In our experiment, water (a strong polar solvent with a surface tension of 72.8 mN/m) and ethylene glycol (a weak polar solvent with a surface tension of 48.3 mN/m) were chosen as the testing liquids. The tensile properties of a single fiber were measured using an electronic single-fiber tensile strength tester (YG004N, Nantong Hongda Experiment Instruments Co. Ltd., Jiangsu, China) according to the Chinese Standard GB/T14337-2008. At least 20 samples of each kind of fiber were tested, and the average value of these tests was regarded as the datum for each fiber. All tests were carried out in an environment with a temperature of (20 ± 1) °C and a relative humidity of (65 ± 1)%. Wide-angle X-ray diffraction (WAXD) curves were measured on a Rigaku D/Max (Rigaku Co. Ltd., Tokyo, Japan) diffractometer with a Bragg−Brentano geometry using Cu Kα radiation (λ = 1.5405 Å) and analyzed using the Jade 5.0 software program. The data were collected over the 2θ range from 5° to 80°. The separation of overlapping reflections was performed with a peak-fitting program after various intensity corrections, assuming that every crystalline peak belonged to the Gaussian type, and the amorphous halo was centered at 21°. Thermogravimetric (TG) analysis was performed on a TA Instruments apparatus (SDT 2910, New Castle, DE) in the range from 25 to 800 °C under a nitrogen atmosphere at a flow rate of 50 mL/min and a heating rate of 10 °C/min. The initial degradation temperature (Tdi) is the temperature at which the weight loss of the sample reached 5 wt %. X-ray photoelectron spectroscopy (XPS) was performed using an AXIS-ULTRA DLD X-ray photoelectron spectrometer (Kratos Analytical, Manchester, U.K.) with monochromatized Al Kα radiation from a 250-W X-ray source (hν = 1486.6 eV) having a pressure in the analysis chamber of 10−9 Torr. UV radiation tests were conducted by exposing fibers to a UV lamp (3 kW, 295−360 nm, Wuxi Kelong Test Equipment Co. Ltd., Jiangsu, China) at room temperature with a relative humidity of 60% for different lengths of time (24, 72, 120, and 168 h) following the accelerated photoaging procedure according to Chinese Standard GB/T 14522-93.
2. MATERIALS AND METHODS 2.1. Materials. Kevlar 49 fibers made by DuPont Company, Wilmington, DE. Two silanes (MPS and GPTMS) of analytical grade were purchased from Jianghan Fine Chemical Ltd., Jingzhou, China, and used as received. Other reagents were all commercial products of analytical grade and were used as received. 2.2. Preparation of Amino Kevlar Fibers. Kevlar 49 fibers were immersed in acetone, petroleum ether, and deionized water in sequence and then heated to reflux temperature for 3 h to eliminate organic impurities on the fiber surface. The fibers were then dried in a vacuum oven at 80 °C for 12 h to obtain clean fibers, which are denoted as KFs. Nitrification medium was prepared by blending four acids in a volume ratio of 40:2:370:100 (fuming nitric acid/concentrated sulfuric acid/acetic anhydride/glacial acetic acid). The reducing agent (0.55 g of sodium borohydride) and the buffer reagent (0.12 g of potassium dihydrogen phosphate and 0.36 g of dipotassium hydrogen phosphate) were dissolved in 200 mL of tetrahydrofuran to form a restoring medium. KFs were immersed in the nitrification medium at 10 °C for 6 h, washed with deionized water, and then dried. After that, the fibers were reduced by the restoring medium at room temperature for 24 h. The resultant fibers were pretreated fibers, which have amino groups, and are denoted as mKFs. The amount of NH2 groups on mKFs was detected using the potentiometric titration method. Specifically, 3.0 g of mKFs was immersed in 1000 mL of HCl solution (0.001 mol/L) with stirring. Taking the pH glass electrode as the indicator electrode and the saturated calomel electrode as the reference electrode, 0.001 mol/L NaOH standard solution was used to titrate excess HCl solution and simultaneously record the corresponding pH value. Close to the end of titration, a corresponding pH value was recorded by each additional 0.05 mL of titration liquid. The content of NH2 groups was calculated using the equation NH 2 (%) =
(C1V1 − C2V2)M r × 100% m
(1)
where C1 and C2 (mol/L) are the concentrations of the standard HCl and NaOH solutions, respectively; V1 is the volume of standard HCl solution; V2 is the volume of standard NaOH solution consumed; m is the quantity of mKFs, and Mr is the molecular weight of NH2. As a consequence, the amount of NH2 groups on the mKFs was calculated to be 0.04 wt %. 2.3. Preparation of Hyperbranched PolysiloxaneGrafted KFs. One gram of mKFs and 1 mol of GPTMS were placed into a reactor, which was maintained at 80 °C for 5 h. Then, 5 mol of ethanol, 1 mol of MPS, and 2.2 mol of H2O were added into the reactor, into which HCl was added dropwise until the pH value was 4−5. After that, the reactor was heated to 60 °C and maintained at that temperature for 5 h. Subsequently, the fibers were removed from the reactor, washed successively with plenty of ethanol and water, and then dried at 80 °C for 12 h to obtain grafted fibers, denoted as HSi1-g-KF. Similarly, using the same procedure except that the molar ratio of water to silanes (the sum of GPTMS and MPS) (denoted as R) was 1.2, 1.3, and 1.4, fiber samples denoted as HSi2-g-KF, HSi3-g-KF, and HSi4-g-KF, respectively, were obtained.
3. RESULTS AND DISCUSSION 3.1. Design and Preparation of Modified Fibers. As described previously, poor surface activity and UV resistance are two critical shortcomings of aramid fibers; therefore, grafted 2685
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Scheme 1. Mechanism for Preparing HSi-g-KFs
Figure 1. SEM images of (A) KF and (B−F) modified fibers [(B) mKF, (C) HSi1-g-KF, (D) HSi2-g-KF, (E) HSi3-g-KF, and (F) HSi4-g-KF]. 2686
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and hydrogen-bond formation43 take place among adjacent SiOH groups, so that the polymer size increases. This is why the morphology of the grafted fibers changes with the variation of the R value. On the other hand, when the surface of the fiber is fully coated with hyperbranched polysiloxane, a further increased molecular size cannot be obviously reflected, so the appearance of HSi3-g-KF is similar to that of HSi4-g-KF. The grafting yield of HSi-g-KF was calculated according to the change in the weight of fibers before and after modification as
KFs should have sufficient amounts of active groups and UVresistant chains. Polysiloxane is famous for its excellent UV resistance because the energy of a SiO bond (446 kJ/mol) is obviously higher than the UV irradiation power (314−419 kJ/mol),32,33 whereas the energy of a CC bond is only 358 kJ/mol. Therefore, producing a layer consisting of SiO bonds will be good for improving the UV resistance of aramid fibers. However, linear polysiloxane is difficult to prepare on the surface of fibers, and the functionalization of linear polysiloxane is not easy. During the past decade, hyperbranched polymers, as a new class of three-dimensional macromolecules, have gained great attention owing to their unique structure and properties. One significant advantage of hyperbranched polymers is that they can be designed to have many functional termination groups, supplying great potential for chemical modifications,34,35 and are easy to synthesize through a one-pot process.36,37 Therefore, we have proposed a method to prepare new KFs by grafting hyperbranched polysiloxane with functional groups on the surface of KFs. Epoxy groups are among the most popular groups owing to their high activity with many sorts of chemical groups.38 GPTMS has epoxy rings, so it was selected to prepare hyperbranched polysiloxane. Note that, if only GPTMS were used to take part in hydrolysis and condensation, then the amount of CC bonds in the resultant hyperbranched polysiloxane would be very high. Compared with CC, C C conjugative bonds have a higher bond energy and a higher capacity to absorb UV wavelengths,39 resulting in better UV resistance.40,41 If a silane that contained a CC bond were also used to take part in the formation of hyperbranched polysiloxane, then the resultant hyperbranched polysiloxane would have better UV resistance than the hyperbranched polysiloxane derived from GPTMS. Based on these considerations, two silanes were utilized to in situ synthesize hyperbranched polysiloxane on the surface of KFs. It is known that Kevlar fibers have very large length/ diameter ratios, and even if the dosage of mKFs is small, they account for a large space. Because mKFs had to be immersed in the solution containing GPTMS and MPS to achieve a complete reaction, the content of silanes should be far in excess of mKFs. The mechanism is shown in Scheme 1. 3.2. Characterizations of HSi-g-KFs. 3.2.1. Morphologies. Figure 1 shows SEM images of KF, mKF, and HSi-g-KF materials . The surface of KFs is very smooth (Figure 1a), whereas that of mKFs is somewhat rough (Figure 1b); however, the four HSi-g-KF materials show obviously different surface morphologies. Specifically, there are some dots on the surface of HSi1-g-KF (Figure 1c), whereas many dots are densely dispersed on the surface of HSi2-g-KF (Figure 1d). These dots cannot be seen on the surface of HSi3-g-KF; instead, a rough and uneven coating appears (Figure 1e), as for HSi4-g-KF (Figure 1f). As these fibers were prepared through the in situ formation of hyperbranched polysiloxane (Scheme 1) with different R values, the structures of the hyperbranched polysiloxanes are responsible for these different morphologies. Our previous research indicated that, as the molar ratio of water to MPS or GPTMS increases, especially when the ratio reaches 1.3 or higher, the molecular weight of the corresponding hyperbranched polysiloxane substantially increases;42 in addition, more methoxy groups are hydrolyzed to form SiOH groups, and then condensation polymerization
yield (%) =
M − M0 M0
(2)
where M is the weight of HSi-g-KF and M0 is the weight of mKF. As shown in Table 1, the grafting yield increased as the R value increased. As stated above, an increased molar ratio of Table 1. Grafting Yields of HSi-g-KFs fiber
yield (%)
HSi1-g-KF HSi2-g-KF HSi3-g-KF HSi4-g-KF
1.86 2.13 2.37 2.21
water is beneficial for obtaining hyperbranched polysiloxane with a high molecular weight. For preparing HSi-g-KFs, one end of the hyperbranched polysiloxane is chemically bonded to the fibers, so the fibers play a significant steric effect on the hydrolysis and condensation of silanes. When the molecular dimension of hyperbranched polysiloxane is large, the steric effect becomes obvious; hence the grafting yield will increase to the maximum value as the molar ratio of water increases. 3.2.2. Chemical Structures. Figure 2 shows the ATR-IR spectra of original and modified fibers. The spectrum of mKFs
Figure 2. ATR-IR spectra of KF and HSi-g-KF materials.
shows the characteristic absorption of stretching vibrations (3500 and 3410 cm−1) and bending vibration of NH2 (1600 cm−1). The bending vibration peak usually has a coupling effect with the framework vibration of the benzene ring, so the intensity of the bending peak is decreased. Compared with the spectra of KFs and mKFs, that of each grafted fiber shows two new absorption peaks at 1483 and 907 cm−1, assigned to the 2687
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due to the introduction of NH2 groups onto the surfaces of mKFs. In addition, the spectra of the HSi-g-KF materials show more peaks assigned to CO, CC, and C Si, reflecting that hyperbranched polysiloxane is present on the surfaces of the fibers and the hyperbranched polysiloxane is derived from two kinds of silanes. Note that all functional groups of HSi3-g-KF and HSi4-g-KF come from the hyperbranched polysiloxane on their surfaces, so they have similar relative contents of functional groups except that HSi4-g-KF has higher relative contents of CC and CO than HSi3-g-KF, meaning that the polycondensation of MPS is more sensitive to the R value, and a larger amount of MPS was grafted on the surface of HSi4-g-KF than on the surface of HSi3-g-KF. This phenomenon is easy to understand by comparing the dependence of the molecular weight of the resultant hyperbranched polysiloxane on the molar ratio of water to single silane. In detail, as the molar ratio of water to single silane changed from 1.1 to 1.4, the weight-average molecular mass (M w ) of the resultant hyperbranched polysiloxane based on MPS increased from 2654 to 11475,42 whereas that originating from GPTMS increased from 7380 to 9326 (Figure 5). 3.2.3. Crystalline Structures. It is known that aramid fibers consist of a skin layer and a core part.44 Whereas the skin layer is generally noncrystalline, the core part tends to exhibit a high degree of order and crystallinity,45 so it was necessary to detect the crystal structures of HSi-g-KF materials. Figure 6 presents typical WAXD patterns of original and grafted fibers, and the corresponding crystalline parameters, including the Bragg angle (2θ), the full-width at half-maximum (fwhm), the weightaverage size of a crystallite perpendicular to its diffracting planes (Lhkl), and the relative crystallinity (crystalline index, CI), of these fibers are summarized in Table 4. It can be seen that all grafted fibers have almost same 2θ, fwhm, and CI values as KFs; moreover, the shape and position of the diffraction peaks of the former are also the same as those of the latter, so the original and grafted fibers have similar crystal structures. In other words, the grafting process of the fibers affects only the outside of the KFs while maintaining the inside structure. This indicates that the method for preparing grafted fibers is a gentle approach. Note that the intensities of the diffraction peaks for HSi-gKFs are lower than those for KFs. This is attributed to the fact that the intensity of the diffraction peak is related to the percentage of crystal, whereas the grafted hyperbranched polysiloxane on the surface of the fibers is noncrystal, so the crystalline percentage decreases, leading to reduced intensities of the diffraction peaks. 3.3. Properties of HSi-g-KF Materials. 3.3.1. Tensile and Thermal Properties. Effectively overcoming the disadvantages of original aramid fibers without deteriorating their outstanding performance is an essential precondition, but one that has proved to be a great challenge.46,47 The mechanical and thermal properties are two outstanding properties of KFs, so we first discuss these properties for evaluating the effectiveness of the modification method developed herein. The mechanical properties of a fiber are usually characterized by the tensile properties including tenacity, break extension, energy to break, and modulus.48,49 As shown in Figure 7, the tensile strength of mKFs is about 95−90% of that of KFs; this is an expected result because chemical modification usually deteriorates the mechanical properties of Kevlar fibers.50−52 Interestingly, HSi-g-KFs have similar or even higher overall tensile properties than KFs. The improved tensile properties
characteristic absorption peaks of CC and epoxy groups, respectively. Therefore, it is reasonable to state that hyperbranched polysiloxane exists on the surface of the fibers. On the other hand, as the grafting reaction takes place only on the surface of the fibers, hyperbranched polysiloxane comprises a small content of modified fibers, so the absorption peaks are relatively weak. XPS was used to characterize the chemical structures of the surfaces of HSi-g-KFs. Wide-scan spectra of KFs, mKFs, and HSi-g-KFs are shown in Figure 3, and the corresponding
Figure 3. XPS spectra of KF and HSi-g-KF materials.
Table 2. Chemical Compositions on the Surfaces of Fibers chemical composition (%) fiber
C 1s
N 1s
O 1s
Si 2p
KF mKF HSi1-g-KF HSi2-g-KF HSi3-g-KF HSi4-g-KF
76.1 70.3 67.0 61.4 57.4 58.6
11.5 15.2 5.7 3.8 − −
12.4 14.5 22.1 28.8 33.3 32.8
− − 5.2 6.8 9.3 8.6
element concentrations are summarized in Table 2. Compared with KFs, mKFs have more N because of the increased NH2 groups on the surfaces of mKFs, whereas HSi-g-KFs have decreased concentrations of N and increased concentrations of Si. This is attributed to the presence of hyperbranched polysiloxane on the surfaces of HSi-g-KFs. Note that no N was detected on the surfaces of HSi3-g-KFs and HSi4-g-KFs; this is because HSi3-g-KFs and HSi4-g-KFs are fully covered with hyperbranched polysiloxane (Figure 1) and the thickness of the coating is larger than the depth that the XPS technique can detect,43 so the element contents of HSi3-g-KF and HSi4-gKF reflect those of the corresponding hyperbranched polysiloxane. The C 1s core-level spectra of KF, mKF, and HSi-g-KF materials are shown in Figure 4, and the corresponding functional groups on the surfaces of the fibers are summarized in Table 3. The C 1s spectrum of KFs can be divided into three separate peaks representing CC, CN, and CO. Similar results also appear in the spectrum of mKFs; however, mKFs have an increased percentage of CN 2688
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Figure 4. C 1s core-level spectra of KF and HSi-g-KF materials.
Table 3. Contents of Functional Groups on the Surfaces of Fibers contents of functional groups (%) fiber
CC
CN
CO
CO
CC
CSi
KF mKF HSi1-g-KF HSi2-g-KF HSi3-g-KF HSi4-g-KF
63.3 56.8 59.7 50.5 41.5 38.6
20.8 27.9 6.1 4.1 − −
15.9 15.3 8.5 7.3 8.8 10.3
− − 18.4 26.4 28.3 27.6
− − 3.1 4.4 9.1 11.4
− − 4.2 7.3 12.3 12.1
all of the grafted fibers almost have similar modulus and break extension values. This is attributed to the physical meaning of each property. As described above, the grafting does not change the crystallinity of the fiber, so the grafted fibers retain almost the original modulus. On the other hand, there are defects on the surface of KFs, and the grafted hyperbranched polysiloxane mends these defects, leading to increased tensile strength. With regard to the energy to break, this property is dependent on
can be attributed to the presence of HSi on the surfaces of the fibers. Specifically, the grafted hyperbranched polysiloxane has a high molecular weight and is chemically bonded with the fibers, so the damage produced during the process for preparing mKFs can be compensated, endowing the grafted fibers with attractive tensile properties, as shown in Figure 7. Figure 7 also shows that different grafted fibers have somewhat different tenacities and energies to break, whereas 2689
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Tdi is often regarded as an index for evaluating the thermal stability of a material.53,54 In either atmosphere, HSi-g-KFs have significantly higher Tdi values than KFs, so the former have much better thermal stabilities. This result is competitive because modified aramid fibers in the literature usually have lower Tdi value than the original fibers.3,55−57 This is because the reported modified fibers were prepared by grafting materials with low molecular weight and poor thermal stability56 or introducing active groups.57 Comparatively, the hyperbranched polysiloxane on the surface of HSi-g-KFs mainly consists of SiOSi chains that have higher thermal stability than the original CC and CN structure in Kevlar fibers, so HSi-g-KFs have increased Tdi and Yc values. For example, in N2 atmosphere, the Tdi and Yc values of HSi3-g-KF are as high as 502 °C and 61.98%, about 37 °C and 11% higher, respectively, than the corresponding values of KFs (Figure 8a, Table 5). The grafting yields of four HSi-g-KF materials are around 2% (Table 1), but the Yc values of HSi-g-KFs in N2 atmosphere are about 5−10% higher than that of KFs, so the improvement of Yc not only results from the residue of hyperbranched polysiloxane but also includes the increased residue of HSi-gKFs itself. This result fully reflects that the hyperbranched polysiloxane on the surface of fibers prevents the fibers from thermal decomposition. A similar conclusion is also obtained from observing the TG and DTG results in air atmosphere (Figure 8b, Table 5). Based on the above discussion, it can be concluded that in situ grafting of hyperbranched polysiloxane on KFs does not deteriorate the outstanding performance of the original aramid fibers; more attractively, HSi-g-KFs have better thermal and mechanical properties than KFs. 3.3.2. Surface Wettability. The greatest shortcoming for the application of Kevlar fibers is their chemical inertness, so it is necessary to evaluate this property of HSi-g-KFs. Dynamic contact angle testing was chosen as an important indicator of surface wettability. The contact angles and surface free energies of HSi-g-KFs and KFs are summarized in Table 6. It is obvious that KFs have a large contact angle with either deionized water or ethylene glycol, suggesting that KFs have poor wettability. In contrast, HSi-g-KF shows an obviously decreased contact angle for deionized water, and similar contact angle for ethylene glycol; hence, the grafted fibers have improved wettability to polar materials. This statement is further confirmed by the remarkably increased surface free energy (γ) of grafted fibers. For example, the γ value of HSi3-g-KFs is 33.2 mN/m, about 1.9 times the value of KFs, meaning that HSi on the surfaces of KFs can overcome the surface tension of a liquid, so the liquid easily spreads and infiltrates into the surface of fibers; in other words, these modified fibers have significantly increased wettability. These attractive results are attributed to the introduction of polar groups, including SiOH and epoxy
Figure 5. Dependence of the Mw value of hyperbranched polysiloxane originating from GPTMS on the molar ratio of water to GPTMS.
Figure 6. WAXD patterns of KF and HSi-g-KF materials.
both tenacity and break extension, so the energy to break has the most obvious variation. Comparing the tenacities of the four HSi-g-KF materials, the value gradually increases from HSi1-g-KF to HSi3-g-KF, whereas the tenacity of HSi4-g-KF decreases to a value similar to that of HSi2-g-KF. This trend is consistent with the variation of the grafting yield, indicating that HSi-g-KF with more hyperbranched polysiloxane tends to have higher tenacity. Figure 8 presents the TG and DTG curves of original and grafted fibers in nitrogen and air atmospheres; the corresponding parameters, such as the initial decomposition temperature (Tdi), the temperature of the maximum degradation rate (Tmax), and the char yield (Yc) at 800 °C, are summarized in Table 5.
Table 4. Typical Crystalline Parameters of KF and HSi-g-KF Materials 2θ (deg)
Lhkl (nm)
fwhm (deg)
fiber
(110)
(200)
(110)
(200)
(110)
(200)
CI (%)
KF HSi1-g-KF HSi2-g-KF HSi3-g-KF HSi4-g-KF
20.612 20.741 20.454 20.690 20.602
22.207 22.401 22.424 22.529 22.467
2.227 2.515 2.583 2.367 2.545
2.076 2.224 2.132 2.444 2.053
5.132 5.133 5.132 5.133 5.132
4.378 4.379 4.376 4.380 4.376
75.41 75.28 75.18 75.16 75.21
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Figure 7. Tensile properties of fibers.
with the spectrum of KFs, that of UV-KFs (Figure 10) shows three new phenomena: the appearance of a new peak at 1735 cm−1, an increased amplitude of the amide I peak, and a decreased amplitude of the amide II and amide III peaks. These variations indicate that UV irradiation breaks the amido bonds, and then polar groups (for example, carboxyl) are formed with the oxidation of functional groups at the ends of the chains.30 In comparison, UV irradiation does not result in any differences in the ATR-IR spectra of grafted fibers (Figures 2 and 10), further demonstrating that UV irradiation has little effect on the chemical structure of the grafted fibers. This can be attributed to the presence of hyperbranched polysiloxane, which has outstanding UV resistance. Figure 11 presents wide and narrow XPS spectra of UV-KFs and UV-HSi1-g-KFs, and Table 7 summarizes the chemical compositions and contents of functional groups on the surfaces of these irradiated fibers. There are three kinds of elements (C, N, and O) on the surface of KFs, and these elements also appear on the UV-KFs, but the C content decreases significantly from 76.1% to 52.1% (Tables 2 and 7). Meanwhile, the content of O markedly increases from 12.4% to 33.7%, suggesting that UV irradiation creates a functional group on the surface of KFs, which is believed to be COO with a content of 12.1% based on the narrow-scan curves of C 1s for UV-KFs as shown in Figure 11. In the case of HSi1-g-KFs, 168 h of irradiation also caused a decrease of the C content, but the decrease was not significant, only from 67.0% to 60.4%, whereas the Si content increased from 5.2% to 9.1%. It is known that chemical bonds containing Si, such as SiC and SiO, have strong stability under UV irradiation;61,62 moreover, UV-HSi1-g-KFs have a decreased concentration of CO bonds (Tables 2 and 7), so the increased content of Si and decreased content of C
groups, on the surfaces of grafted KFs and the increased surface roughness.58 The γ values of HSi-g-KFs are about 1.5−2.0 times that of KFs, meaning that the grafted fibers have remarkably larger wettabilities than KFs. Note that the γ value consists of polar (γp) and nonpolar (γd) components. Compared with HSi-gKFs, the grafted fibers have increased polar components but decreased nonpolar components, which is reasonable because the presence of polar functional groups on the surfaces of these grafted fibers is responsible for the increased wettability. 3.3.3. UV Resistance. To evaluate the UV resistance of grafted fibers, KFs and HSi-g-KFs were kept under the same UV irradiation conditiond for 168 h, and then SEM images of these irradiated fibers (denoted as UV-KFs and UV-HSi-g-KFs, respectively) were recorded as shown in Figure 9. Both UVHSi3-g-KFs and UV-HSi4-g-KFs were still covered with a coating; some dots on the surfaces of UV-HSi1-g-KFs and UVHSi2-g-KFs became sparse compared with those of HSi1-g-KFs and HSi2-g-KFs (Figure 1). These changes are relatively slight, as long-time UV irradiation creates many grooves on the surface of UV-KFs, preliminarily suggesting that HSi-g-KFs have improved UV resistance. To confirm the influence of UV irradiation on the original and grafted fibers, some effective techniques including ATR-IR and XPS spectroscopies were applied, and HSi1-g-KF was chosen for detection as it had the lowest grafting yield among HSi-g-KF materials. As shown in Figure 2, KFs had some typical peaks at about 3327, 1650, 1540, and 1309 cm−1 that are assigned to amide A (hydrogen-bonded NH stretching when CO and NH are in a trans configuration), amide I (CO group stretching), amide II (combination of CN stretching and NH bending), and amide III (combination of CN stretching and NH bending), respectively.59,60 Compared 2691
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Figure 9. SEM images of (A) UV-KF and (B−E) modified fibers [(B) UV-HSi1-g-KF, (C) UV-HSi2-g-KF, (D) UV-HSi3-g-KF, (E) UVHSi4-g-KF] after 168 h of UV irradiation.
Figure 8. TG and DTG curves of original and grafted fibers in nitrogen and air atmospheres.
Table 5. Typical Thermodegradation Data of Fibers under N2 and Air Atmospheres Tdi (°C)
Tmax (°C)
Yc at 800 °C (wt %)
fiber
N2
air
N2
air
N2
air
KF HSi1-g-KF HSi2-g-KF HSi3-g-KF HSi4-g-KF
465.9 503.0 511.3 502.8 515.0
479.3 500.6 485.7 492.4 508.2
533.6 534.8 541.2 534.7 537.0
581.3 578.7 589.5 585.6 590.3
51.47 56.80 58.04 61.98 60.70
2.933 4.793 7.323 12.24 10.72
Figure 10. ATR-IR spectra of UV-KF and UV-HSi1-g-KF materials.
Table 6. Contact Angles and Surface Free Energies of KF and HSi-g-KF
the changes in the N and O contents were small, meaning that the grafted fibers had good chemical stability. Based on the above discussion, it can be concluded that UV irradiation has obvious affects on both the surface morphology and chemical structure of KFs, mainly resulting from the breaking of amide bonds. However, this phenomenon is greatly changed by the presence of a hyperbranched polysiloxane coating that is chemically bonded on the surface of the KFs. As the properties of a material are greatly dependent on the structure, so these structural differences between KFs and HSig-KFs are reflected in the performance of the two fibers. Figure 12 shows the retentions of tensile properties, including the tenacity, energy to break, modulus, and break extension of fibers after being irradiated for different lengths of time. KFs quickly lost their tensile performances, especially their tenacity and energy to break, even after being irradiated for only 24 h. With the extension of the irradiation time, the tensile performances continuously decreased. After 168 h of
contact angle (deg) fiber KF HSi1-gKF HSi2-gKF HSi3-gKF HSi4-gKF
deionized water
ethylene glycol
γp (mN/m)
γd (mN/m)
γ (mN/m)
91.2 ± 2.1 83.2 ± 2.7
80.9 ± 1.6 81.2 ± 1.3
11.6 24.4
5.9 1.4
17.5 25.8
79.8 ± 2.4
80.8 ± 1.1
30.6
0.8
31.5
77.9 ± 2.2
79.1 ± 1.2
32.4
0.8
33.2
80.2 ± 3.3
80.4 ± 1.3
29.3
0.9
30.1
for UV-HSi1-g-KFs result from the decomposition of the part that was not coated with hyperbranched polysiloxane. Note that 2692
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Figure 11. Wide-scan and C 1s core-level XPS spectra of UV-KF and UV-HSi1-g-KF materials.
Table 7. Chemical compositions and contents of functional groups on the surfaces of irradiated fibers chemical composition (%)
contents of functional groups (%)
fiber
C 1s
N 1s
O 1s
Si 2p
CC
CN
CO
CO
CC
CSi
COO
UV-KF UV-HSi1-g-KF
52.1 60.4
14.2 5.9
33.7 24.6
− 9.1
58.6 59.1
15.1 8.0
14.2 8.2
− 14.5
− 3.4
− 6.8
12.1 −
HSi4-g-KFs exhibited the best performances because each of them had a full coating of hyperbranched polysiloxane and very high content of double bonds.
irradiation, the retentions of the tenacity, energy to break, modulus, and break extension for UV-KFs were 75%, 66%, 84%, and 85%, respectively, whereas the corresponding values for UV-HSi-g-KFs were around 92%, 86%, 95%, and 97%, or about 11−20% higher than those for UV-KFs. In addition, upon short-term irradiation (for example, 24 h), the HSi-g-KF materials almost retained their tensile performances. These attractive data clearly demonstrate that HSi on the surface of KFs can effectively protect the KFs from the damage of longterm UV irradiation. Recall that the difference in mechanical properties between the original and modified fibers is mainly reflected in the tenacity and break extension, as shown in Figure 7, and in fact, the difference is not significant. However, interestingly, after UV irradiation, the retentions of mechanical properties for HSig-KFs were much higher than the corresponding values for KFs. In particular, the modulus and break extension retentions of UV-HSi-g-KFs were as high as 95% and 97%, respectively, and the tenacity retention was 92%, so the HSi-g-KF materials prepared herein have excellent performance advantages. The four HSi-g-KF materials did not exhibit obviously different retentions of their mechanical properties. In comparison, HSi1-g-KFs had the lowest retention in each mechanical property because of their lowest grafting yield and only a small number of double bonds, whereas HSi3-g-KFs and
4. CONCLUSIONS New surface-modified Kevlar fibers with simultaneously improved surface activity and UV resistance were developed by in situ producing hyperbranched polysiloxanes on the surfaces of the fibers. By adjusting the molar ratio of water to silanes from 1.1 to 1.4, the chemical and morphological structures as well as the grafting yield were controlled. The appearance of the obtained fibers changed successively from some dots to dense dots and then to a full coating. When the molar ratio of water to silanes was 1.3, the modified fibers had the highest grafting yield. The surface free energies of the HSi-g-KF materials were found to be much higher than that of KFs owing to the improved surface roughness and increased content of polar functional groups. In addition, HSi-g-KFs exhibit very high retentions of their mechanical properties, showing markedly improved UV resistance. HSi3-g-KFs and HSi4-g-KFs were found to have the best performances, demonstrating that a full coating of hyperbranched polysiloxane is favorable for protect fibers from UV rays. 2693
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Figure 12. Retentions of tensile properties of UV-KF and UV-HSi-g-KF materials. (5) Sun, Y. Y.; Sun, G. Novel Refreshable N-Halamine Polymeric Biocides: N-Chlorination of Aromatic Polyamides. Ind. Eng. Chem. Res. 2004, 43, 5015. (6) Luo, J.; Sun, Y. Y. Acyclic N-Halamine Coated Kevlar Fabric Materials: Preparation and Biocidal Functions. Ind. Eng. Chem. Res. 2008, 47, 5291. (7) Liu, L.; Huang, Y. D.; Zhang, Z. Q.; Jiang, B.; Nie, J. Ultrasonic Modification of Aramid Fiber−Epoxy Interface. J. Appl. Polym. Sci. 2001, 81, 2764. (8) Abu Talib, A. R.; Abbud, L. H.; Ali, A.; Mustapha, F. Ballistic Impact Performance of Kevlar-29 and Al2O3 Powder/Epoxy Targets under High Velocity Impact. Mater. Design 2012, 35, 12. (9) Jin, H.; Wang, Y. Y. Synthesis and Characterization of the Novel Meta-Modified Aramid Fibers with Liquid Crystalline Properties. Polym. Compos. 2012, 33, 1620. (10) Liu, T. M.; Zheng, Y. S.; Hu, J. Surface Modification of Aramid Fibers with New Chemical Method for Improving Interfacial Bonding Strength with Epoxy Resin. J. Appl. Polym. Sci. 2010, 118, 2541. (11) Zhao, J. Effect of Surface Treatment on the Structure and Properties of para-Aramid Fibers by Phosphoric Acid. Fibers Polym. 2013, 14, 59. (12) Li, G.; Zhang, C.; Wang, Y.; Li, P.; Yu, Y. H.; Jia, X. L.; Liu, H. Y.; Yang, X. P.; Xue, Z. M.; Ryu, S. Interface Correlation and Toughness Matching of Phosphoric Acid Functionalized Kevlar Fiber and Epoxy Matrix for Filament Winding Composites. Compos. Sci. Technol. 2008, 68, 3208. (13) Cai, R. Q.; Peng, T.; Wang, F. D.; Ye, G. D.; Xu, J. J. Improvement of Surface Wettability and Interfacial Adhesion of Poly(p-phenylene terephthalamide) by Incorporation of the Polyamide Benzimidazole Segment. Appl. Surf. Sci. 2011, 257, 9562. (14) Hassanin, A. H.; Said, M. A.; Seyam, A. F. M. Composite Porous Membrane for Protecting High-Performance Fibers from UltravioletVisible Radiation. J. Appl. Polym. Sci. 2013, 128, 1297. (15) Lam, J. W. Y.; Häußler, M.; Dong, H. C.; Qin, A. J.; Tang, B. Z. Synthesis of Hyperbranched Conjugative Polymers and Their Applications as Photoresists and Precursors for Magnetic Nanoceramics. Nanocryst. Mater. 2006, 7, 207.
HSi-g-KF materials were found to have high thermal and thermal-oxidation stabilities, overcoming the disadvantages of previously reported modified Kevlar fibers.
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AUTHOR INFORMATION
Corresponding Authors
*Tel.: +86 512 65880967. Fax: +86 512 65880089. E-mail:
[email protected]. *Tel.: +86 512 65880967. Fax: +86 512 65880089. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the Natural Science Foundation of China (21274104), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and the Suzhou Applied Basic Research Program (SYG201141) for financially supporting this project.
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