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The Enhanced Dyeability of Aromatic Polysulfonamide Fibers Using [gamma]-ray Irradiation-Induced Graft Polymerization.

INTRODUCTION

Aromatic polysulfonamide (PSA), belonging to the family of heat resistant polymers, has resistance to a wide range of organic solvents, excellent thermal stability, flame resistance with low levels of smoke generation (limit oxygen index > 33%), and a low dielectric constant [1]. Due to its unique properties, PSA fibers have extensive applications in many fields including high-temperature gas filtration, flame-retardant protection, and electrical insulation [2,3]. Although used as flame retardant textiles, the dyeability of PSA fibers is an important issue that is worthy of concern. Unfortunately, similar to other heat resistant fibers with an aromatic or heteroaromatic structure, PSA fibers are difficult to be dyed with conventional dyeing techniques due to their low dye up-take and poor color fastness [4,5], For these kind of heat resistant fibers, although there are some particular dyeing sites such as amide groups (-CO-NH-) on the molecular backbone, the extremely high glass transition temperature ([T.sub.g] 280[degrees]C-380[degrees]C) [6] restricts the movement of molecular chains and hinders the diffusion of dyes from the fiber surface into the interior.

To improve the dyeability of fibers with high glass transition temperature, significant studies have been carried out to develop different dyeing techniques. The technique of dope dyeing, the most typical, has the benefits of energy savings and emission reduction [7,8]. Heat resistant fibers (such as meta-aramid fibers) produced by dope dyeing exhibit fairly good color fastness as well as low color variability and have been commercialized by some companies such as DuPont, TELJIN, and TayHo. Nonetheless, it is not diverse in color, because only a small amount of pigments can withstand high stretching temperatures (>350[degrees]C) during the fiber forming process. In order to increase the amount of colors and maintain the color fastness to light, washing or high temperature of the textiles, an extensive amount of research has been conducted to improve the limited swelling of fibers with the help of special dyeing auxiliaries such as carriers (e.g., acetophenone, methyl benzoate) or swelling agents containing dimethyl formamide, dimethyl sulfoxide and so on, which contribute to the ability of cationic dyes or disperse dyes to enter the internal space of the fibers [9-11]. However, inevitable deficiencies such as decreases in tensile strength or dimensional stability as well as difficult removal of the auxiliaries usually can be observed. Highly thermostable disperse dyes can also be selected to dye PSA fibers at high temperature/ high pressure conditions, but utilizing these does not resolve the issues of low dye uptake and poor dye fastness [12]. In addition, the long dyeing time and high dyeing temperature are not energy-efficient processes.

In view of the deficiencies and limitations of traditional dyeing methods, grafting functional monomers onto synthetic fibers induced by irradiation technologies can be considered one of the most effective ways to improve or change the dyeability to various dyes [13-15]. It has been reported that for polyolefin fibers with no dye receptor on the polymeric backbone, like polyethylene (PE) fibers and polypropylene fibers, attachment of functional groups (e.g., acrylic acid [AA], 2-Hydroxy Ethyl Methacrylate) lead to improved dyeability of the fibers with various dyes [16,17]. Huang et al. [15] make the PET fabric dyeable with a cationic dye by grafting AA onto the fabric via continuous UV photografting, which causes the increased electrostatic interactions between the anionic dyeing sites and cationic dyes. It has also been reported that although aromatic heat resistant polymers (such as poly[paraphenylene terephthalamide] fiber [PPTA] and meta-aramid fiber) have a few dye receptors on the molecular chains, it seems to be difficult for the dyestuffs to diffuse into the free volume of the fiber due to their high glass transition temperature. Hirogaki et al. [18] investigated the modification of the PPTA fiber by Electron beam (EB)-induced graft polymerization with AA and methyl acrylate (MA). The dyeability as well as color fastness of the grafted PTTA fiber improved through formation of ionic bonds between the AA in grafted polymer chains and the cationic dye in addition to hydrophilic interactions between the MA in the grafted polymer chains and hydrophilic matrix of the dye. Kim et al. [19] found that aramid fabrics after UV irradiation grafted with dimethylaminopropyl methacrylamide (DMAPMA) showed remarkably enhanced dyeability to the reactive dyes, which is strongly dependent on the covalent bond formation between the dyes and the secondary amino groups in the grafter DMAPMA.

Undoubtedly, the grafting of selective groups on fiber substrates has a direct impact on the functional performance of fibers. Nevertheless, one must consider the potential effect of irradiation grafting on the multilayer structure of the fibers from chemical structure to macrostructure, which in turn may alter their desirable properties such as mechanical strength, thermal stability and so on. For example, Poncin-Epaillard et al. [20] observed that degradation occurred within the PPTA fiber or films through amide scission when subjected to irradiation at certain conditions. Xing et al. [21] found that grafting MA on ultra-high-molecular weight polyethylene fibers had no influence on the orthorhombic crystalline phase, but they disordered the monoclinic crystalline and intermediate phase, especially at a higher degree of grafting, which led to a decrease in tensile strength and modulus of grafted fibers. Ting et al. [22]. pointed out that vinylbenzyl chloride (VBC)-grafted nylon-6 fibers could undergo structural changes in terms of transformation of crystalline structure from the less-stable [gamma]-form to more-stable yet smaller [alpha]-form during grafting process. Besides, the grafted moiety was mainly incorporated in the non-crystalline region of the fibers without causing a major crystalline disruption. Only the amendable VBC-grafted nylon-6 fibers retained favorable properties.

Generally, high-performance fibers containing rigid rod-like backbones such as PPTA have a high-crystallized and high-oriented structure. This kind of material is less functionalized after irradiation but a high radical concentration was found, which mainly resulted from the limitation of monomer diffusion to reactive sites [20]. In contrast, it is known that PSA molecules are flexible and have a random conformation in solution due to the insertion of sulfonyl (-S[O.sub.2]-) groups into the completely aromatic polymer chain detracting from the effectiveness of conjugation [23,24]. The random conformation of PSA in solution hinders the formation of ordered structures like PPTA [25] and polybenzobisoxazole [26] during solution processing. In this situation, it is of interest to gain knowledge about the response of PSA fibers with a large amorphous region to irradiation grafting. In this study, AA, selected as the hydrophilic monomer, was grafted copolymerized onto the PSA fibers to interact with a cationic dye or disperse dye. The object of this work is to investigate the dyeing behavior and to evaluate the comprehensive performance of PSA fibers after y-ray irradiation-induced graft polymerization, in terms of properties such as mechanical behavior, thermal properties, and hygroscopicity. The structural information of grafted PSA fibers in different scales can be characterized by Fourier transform infrared spectrometry (FTIR), small-angle X-ray scattering (SAXS), wide-angle X-ray scattering (WAXS), and by using a scanning electron microscope (SEM) to determine the cause of performance changes of the PSA fibers. Some possible pathways to develop a new dyeing method of PSA fibers are suggested.

EXPERIMENTAL

Materials

The aromatic PSA fibers (~15 [micro]m in diameter) prepared by wet spinning technology were supplied by Shanghai Tanlon Fiber Co., Ltd, China. AA (reagent-grade) was used as grafting monomers without further purification. Deionized water was added to dissolve AA during irradiation-grafting process. Cationic blue XBL and disperse red 60 were used for dyeing. The chemical structure of PSA fibers, monomer (AA), and dyes showed in Scheme 1.

Irradiation grafting

The grafting reaction was carried out under simultaneous irradiation (Co-irradiation). The PSA fibers were immersed in the AA monomer solution and then exposed to y-ray radiation at a dose of 10 kGy for 17 h under nitrogen atmosphere. The right amount of Fe[Cl.sub.3]-7[H.sub.2]O was used to inhibit the self-polymerization of AA in the solution. The treated PSA fibers was washed with water at 100[degrees]C and methanol at 60[degrees] C for each 2 h to remove any untreated monomer and homopolymer. The various grafting yield (degree of grafting) around 30%-60% of PSA fibers were obtained by adjusting the concentration of AA solution. The degree of grafting was calculated using the following equation:

Degree of grafting (%) = [(Wg - [W.sub.0]) x [W.sub.0]] X 100.

Where [W.sub.0] denotes the weight of the blank PSA fibers and [W.sub.g] presents the weight of the fibers after treatments.

The PSA fibers prepared with various degree of grafting such as 0, 30%, 45%, 56% were denoted as PSA, PSA-gf-30% AA, PSA-gf-45% AA, PSA-gf-56% AA, respectively.

Characterization

The stress-strain test of a single fiber was carried out using a XQ-2 tensile tester with a gauge length of 20 mm and an extension rate of 20 mm/min. At least 50 samples were tested for each sample. The tenacity and elongation at break were calculated averagely.

The hygroscopicity of PSA fibers was characterized by moisture regain values (MRV) and contact angle. MRV of fibers were determined with the method described in DIN 54351. A proper amount of fibers, conditioned in a standard atmosphere of 65% RH at 20[degrees]C for a minimum of 48 h, were weighed, dried in an oven at 105[degrees]C for 4 h, and reweighed. Moisture regain was calculated using the following equation:

MRV (%) = [([W.sub.C] - [W.sub.D])/[W.sub.D]].

Where, [W.sub.C] denotes the weight of conditioned fiber, [W.sub.D] presents the weight of dried fiber.

Contact angle measurements of the fibers were conducted on an OCA 40 Micro contact angle instrument at ambient temperature. Distilled water was used and the volume of the sessile drop was maintained as 10 nL in all cases using a micro-syringe. For accuracy, measurements were repeated 15 times of each sample.

The flame retardant of the fibers was judged according to the burning behavior and burning time by igniting the fibers.

The volume resistivity of the fibers was evaluated using a XR-1A electric resistance tester in a standard atmosphere of 65% RH at 20[degrees]C.

Dyeability of the PSA fibers graft polymerized with AA was evaluated from the degree of coloration with cationic Blue XBL and disperse Red 60, respectively. The dye solution was prepared at 2% owf (% on the weight of the fibers), and its pH was controlled within 5-5.5 by adding aqueous acetic acid solution. A bundle of PSA fibers was dyed with dye solution at a bath ration of 150:1 at 95[degrees]C for 3 h using a thermostat water bath cauldron. After the dyeing, the dyed PSA fibers were taken out, rinsed in hot and in cool water and later air-dried at room temperature. From the optical density obtained, the concentration of the residual liquor was determined from the calibration curves obtained for the cationic dyes or disperse dyes and hence the amount of dye uptake was calculated. Color fastness to washing, sunlight and high temperature was evaluated from the degree of color fading in the dyed fibers by using washing test at 60[degrees]C and 100[degrees]C for 30 times, light test for 40 h and high temperature test at a temperature ranging from 160[degrees]C to 250[degrees]C for 5 min.

Thermomechanical properties of PSA fibers were determined on a Hitachi thermal analyzer (TMA/SS 7100). A single fiber was clamped into the sample holder with a small initial stress in order to keep the fiber straight. The strain of fiber at constant stress (~5 MPa) was measured ranging from 40[degrees]C to 400[degrees]C at 5[degrees]C/min. The dynamic mechanical (DMA) behavior of the PSA fibers was analyzed using TA Q800 V7.5 instrument to obtain the loss factor (Tan [delta]). The frequency applied was 1 Hz over a temperature range from 40[degrees]C to 400[degrees]C at a 5[degrees]C/min in the temperature-frequency sweeping mode.

The surface morphology and cross-section of the PSA fibers was observed using a SEM (Hitachi S-3000 N). The samples were pasted on a sample-carrier, and then sputtered with gold prior to observation. For the cross-section of specimens, PSA fibers were cryogenically broken using fiber slicer under liquid nitrogen.

The FTIR spectra of PSA fibers before and after irradiation-grafting treatment were determined using a Nicolet spectrometer (Nexus 670, Thermo Fisher, USA) equipped with a diamond ATR cell. All the spectra were collected from the dry samples using 16 scans with a resolution of 4 [cm.sup.-1] and in the wave-number range from 500 to 4,000 [cm.sup.1].

Synchrotron X-ray measurements were carried out at Shanghai Synchrotron Radiation Facility on beam line (BL14B and BL16B) with an X-ray wavelength of 0.124 nm. A bundle of co-PSA fibers was put on a sample holder with the fiber direction perpendicular to the X-ray beam. Two types of X-ray measurements were performed respectively: WAXS and SAXS. Two-dimensional WAXS and SAXS patterns were acquired using a Mar-CCD (345 and 165) detector. The sample-to-detector distances for WAXS and SAXS were 186.74 and 1,870 mm, respectively. All data analysis (background correction, radical, and azimuthal integration) was carried out using the Xpolar software (Precision works NY) as reported in previous paper [27-31].

RESULTS AND DISCISSION

Irradiation-Induced Grafting Polymerization of PSA Fibers

To discuss the feasibility to improve dyeability of PSA fibers, the grafting polymerization of AA monomers on PSA fibers has been carried out by a simultaneous irradiation technique. Therefore, in this study, the degree of grafting of PSA fibers varied only by changing the monomer concentration. The other experimental variables such as absorbed dose, dose rate and irradiation time was not discussed. To illustrate the response of PSA fibers to irradiation grafting, the multilayer structure of PSA fibers at varying length scales as well as the performance was systematically studied.

Morphology Changes of PSA Fibers Before and After Irradiation Grafting. The morphology of PSA fibers with various degree of grafting presented by SEM images is summarized as shown in Fig. 1. For non-grafted PSA fibers, a small number of grooves along the fibers axis are observed on the surface of fibers, which is mainly formed in the coagulation stage during the wet spinning process. In contrast, the PSA fibers with higher graft yield, the surface of fibers were much rougher, indicating that a heterogeneous grafting layer has been grafted on the surface of fibers. The incorporation of polyacrylic acid layers lead to the average diameter increase of original PSA fibers. There was also a change in cross-section geometry: the non-grafted PSA fibers exhibited a waist circle shape cross-section and approached a circle shape after grafting, especially at higher grafted yield. As we know, for the fibers with higher crystallinity, the graft copolymerization mainly occurred on the surface of fibers, because it is difficult for the monomer to penetrate the fibers. For PSA fibers with large amorphous region, it suggested that AA could also be grafted to the polymeric backbone within the fibers by thorough penetration of the monomer into the polymer, which could swell the fiber and cause a circular cross section.

Microstructure Changes of PSA Fibers Before and After Irradiation Grafting. The microstructure changes of PSA fibers after graft at micro-nano scale were systematically investigated by SAXS as shown in Fig. 2. According our previous studies [31], the equatorial streak of small-angle diffuse scattering for PSA fibers is generally attributed to the presence of fibrils elongated along the fiber axis. Therefore, the size of the fibrils was determined. The radius of the fibrils in cross-section of grafted PSA fibers were calculated according to the method described in our previous work [31].

As listed in Table 1, the size of fibrils in the cross-section exhibited multi-order characters and increased with the increase of grafting yield, which is mainly related to the polyacrylic acid molecules as side chains grafted onto PSA molecular backbone. It is worth mentioning that the existence of side chains on the backbone could increase the free volume within the fibers to some extent and it may favor the penetration of dyes into the fibers during the dyeing process and increase the dye pickup. The average length and misorientation [B.sub.[psi]] of fibril can also be determined by the method of Ruland [30] as shown in Table 1. It can be found that the value of misorientation increased as function of graft yield, indicating a lower fibril orientation formed within the fibers, which may have adverse impact on the strength of fibers. In addition, it is clear that the fibril length increased with the increase of graft yield. This phenomenon can be explained from the following two aspects: On the one hand, partial crosslinking structure may form within the fibers during the irradiation grafting process. On the other hand, it is well known that the scattering is mainly due to electron density difference between the fibrils. As mentioned, the incorporation of polyacrylic acid into the amorphous region after graft may decrease the electron density difference within the fibers, which may also lead to the larger result about the fibril length.

Supramolecular Structure Changes of PSA Fibers Before and After Irradiation Grafting. The supramolecular structure changes at nano-scale of PSA fibers after graft were studied by WAXS. WAXS patterns of the PSA and AA-grafted PSA fibers were shown in Fig. 3. For the pattern of the non-grafted PSA fibers showed a visible equatorial reflections, meridional reflections and off-equatorial reflections. However, the incorporation of polyacrylic acid by grafting to PSA fibers caused a strong diffuse halo in the WAXS pattern, which was a function of graft yield. It mainly results from the dilution of the crystalline structure with the amorphous polyacrylic acid grafted.

To quantitatively describe the change of crystal structure of PSA fibers after graft, the contribution from amorphous polyacrylic acid was subtracted, and the crystallinity of AA-grafted PSA fibers was calculated based on the peak area of crystal peaks and amorphous phase of PSA fibers, as shown in Fig. 4. It can be found that the crystallinity of grafted PSA fibers had a slight decrease from 46% for non-grafted PSA fibers to 42% for PSA fibers with a degree of grafting around 56%. In addition, the crystal plane of (002) and (100) was chosen to calculate the crystallite size in the axial direction and radial direction of fibers, respectively. It can be found that the crystallite size of PSA fibers after graft remained unchanged (Fig. 5). It is suggesting that the crystal structure of PSA fibers was not significantly disrupted and remained stable after irradiation grafting, while the crystal orientation of grafted PSA fibers decreased as shown in Fig. 5. In a word, the irradiation grafting polymerization mainly occurred in the amorphous region of PSA fibers, which may enlarge that free volume of fibers and promote the dye uptake. This deduction about the improvement of dyeability of PSA fibers will be confirmed in the following section.

The influences of irradiation grafting on the structure changes also can be indirectly reflected by DMA analysis (Fig. 6), which is used to characterize the mobility of molecules in amorphous region. It is obvious that a continuous [alpha] relaxation process can be identified from 300[degrees]C to 400[degrees]C [4], which correspond to the glass transition temperature. It is clear that the peak temperature of [alpha] relaxation of grafted PSA fibers shifted to lower temperature with the increase of grafting yield, suggesting that the AA grafted onto the PSA molecular chains could provide more space for the motion of PSA molecular chain segments. In addition to the [alpha] transition, a broad sub-glass transition ranging from 150[degrees]C to 300[degrees]C appeared for the grafted PSA fibers, especially at higher degree of grafting, corresponding to lower transition temperature, which is mainly relative to the motion of the polyacrylic acid molecular chain and some small segments of PSA like side and end groups. From the above analysis, it provided further evidence that the grafted AA chains could increase the internal space of fibers and enhance the movability of chains, which may contribute to the diffusion of dyes into the fibers based on the dyeability of the fibers.

Chemical Structure Changes of PSA Fibers Before and After Irradiation Grafting. To verify AA grafting onto the PSA fiber, FTIR spectra of non-grafted and grafted PSA fibers were compared (Fig. 7). For non-grafted PSA fibers [4], the characteristic peaks were observed at 789, 836, 1,149, 1,320, and [1,662.sup.-1], corresponding to the bending vibration peaks of m-phenyl and p-phenyl groups, symmetric and asymmetric sulphonamide stretching, amide carbonyl groups, respectively. By comparison, the grafted PSA fiber has additional peaks at 1,710 [cm.sup.-1], corresponding to the C=0 stretching vibration, indicating the AA grafting on the PSA fibers [32]. However, there are still some problems need further discussion, such as the position of grafted chains on PSA molecules, molecular weight of grafted chains as well as the distribution of grafted chains within the fibers.

Inherent Performance Changes of PSA Fibers After Irradiation Grafting. Generally speaking, the essential prerequisite for material modification is that it should not cause the deterioration of its inherent performance characteristics. It is undoubted that the grafting of functional group on the fibers usually has a significant impact on its performance. Therefore, the effect of degree of grafting on the inherent performance of PSA fibers was firstly considered. Table 2 shows a comparison of mechanical properties of the acrylic acid-grafted PSA fibers with different degree of grafting. For the original fiber, the average tensile stress was determined to be 386 MPa. As to the grafted fibers, the tensile stress decreased sharply based on degree of grafting, while the elongation at break increased. For instance, at a degree of grafting around 30%, the residual strength was approximately 90% of the original fiber. Even worse, the tensile stress of PSA fibers with a degree of grafting around 56% decreased to 286 MPa, only having 74% strength retention. The decrease in tensile stress could be attributed to the decrease of molecular orientation of PSA fibers during the irradiation grafting process as mentioned above.

The thermal stability of PSA fibers after irradiation grafting was investigated by TMA, as shown in Fig. 8. It can be clearly observed that the strain of all the fibers had an abrupt change at the temperature ranging from 300[degrees]C to 350[degrees]C, revealing the start of glass transition of the fibers. Furthermore, the transition region of the fibers tend to shift to lower temperature at higher degree of grafting, indicating that the mobility of the PSA molecular chain segments increased in the amorphous region with the help of AA grafted chains. Even at a temperature between 150[degrees]C and 250[degrees]C far below glass transition temperature of the fibers, the deformation stability was found to decrease with the increase of degree of grafting. Therefore, to maintain the high deformation stability of the fibers, the degree of grafting should be controlled to be as low as possible, for example, <45%, according the current experimental results.

The hygroscopicity of the untreated and grafted PSA fibers determined by moisture regain value and contact angle was shown in Table 3. As expected, the hygroscopicity of the PSA fibers grafted with hydrophilic AA groups significantly enhanced. For example, even if the degree of grafting was 30%, the moisture regain value and contact angle of grafted PSA fibers can still be achieve at 10.41% and 84[degrees], while those of the untreated PSA fibers was only 6.74% and 105[degrees], respectively. Thanks to the improvement of hydrophilicity of the fibers, the grafted PSA fibers exhibit better electrical conductivity with [10.sup.8] [OMEGA]*cm volume resistivity compared with the untreated PSA fibers, which can considerably enhance the antistatic property of the PSA fibers, as listed in Table 3. The effect of graft treatment on the flame retardant property of PSA fibers was also assessed by burning behavior (Table 3). It can be found that the both the untreated fibers and grafted fibers have similar combustion behavior and can be extinguished from the flame within 0.5 s, showing excellent flame retardancy.

In summary, the irradiation grafting treatment seemed to have little adverse effect on the inherent performance of PSA fiber within a proper grafting yield.

Dyeing Behavior of Grafted PSA Fibers

Under the premise of guaranteeing the inherent performance of fibers, the PSA fibers with the degree of grafting around 30% were selected to investigate the dyeability. The cationic dyeing and disperse dyeing was conducted at 95[degrees]C without the help of carrier. The performance of grafted PSA fibers before and after dyeing was firstly compared, as shown in Table 4. Unlike the carrier dyeing technology, it can be found that the bulk properties of the grafted PSA fibers have not changed during the dyeing process, such as the mechanical properties and flame retardancy. This is to say, the performance changes of PSA fibers are mainly determined by irradiation grafting conditions.

The dyeing behavior of grafted PSA fibers was evaluated as presented in Fig. 9. It can be seen from the results that the untreated PSA fibers apparently had no dyeability to the cationic dyes and disperse dyes with very low dye uptake about 10% and 8%, respectively, while the grafted PSA fibers showed considerable improvement in dyeing characteristics under normal pressure with rather high dye uptake around 95% for cationic dyeing and 92% for disperse dyeing.

Cationic Dyeing. For cationic dyeing, the increases in coloration by grafting can be easily understood that the dye molecules interacted with the grafted polymer chains through ionic bonding between the cationic portion of the dye and the anionic portion (-COOH) of the grafted polymer chain. Besides, the hydrophilic grafted polyacrylic acid may be conducive to swelling of PSA fibers, and act as internal plasticizer within the fibers, which can enlarge the free volume of the fibers and contribute to the diffusion of dyes. According to color of the cross section of grafted PSA fibers (Fig. 10), it can be proved that the cationic dyes could penetrate the fibers.

Due to the presence of large error in the quantitative characterization in color strength test for fiber samples, the color fastness of dyeing grafted PSA fibers to washing, light, high temperature was qualitative characterized as shown in Fig. 11. The color fastness to washing of grafted PSA fibers seems to be excellent due to the formation of strong ionic bond between the grafted chains and dye molecules (Fig. 11a). The light color fastness of the fibers was mainly related to the light resistance of dyes, grafted chains and polymer matrix. In this work, a little decrease in light color fastness was found for the dyeing grafted PSA fibers (Fig. lib), suggesting that both the graft monomer and dye with better light aging resistance should be screened in our future work. The high temperature color fastness of grafted PSA fibers was mainly determined by the thermal stability of the grafted polymer and dyes. In the current study, the outstanding high temperature color fastness of grafted fibers was observed for the cationic dyeing fibers below 220[degrees]C as shown in Fig. 11c (the decomposition temperature of Cationic Blue X-BL ~ 280[degrees]C).

Disperse Dyeing. For disperse dyes, there is no active group on the molecule. Thus, it is impossible to form strong interactions between the disperse dyes and the grafted fibers such as the formation of ionic bonds. Because of the hydrophilic character of grafted PSA fibers, the swelling of fibers could produce much more free volume. Therefore, it has an adequate affinity for disperse dyes to diffuse into the fibers, as shown in Fig. 12.

The color fastness of disperse dyeing PSA fibers to washing, lighting and high temperature was also investigated, as shown in Fig. 13. A little color fading after washing was observed for disperse dyeing PSA fibers, especially at 100[degrees]C washing (Fig. 13a). As mentioned above, the hydrophilic grafted polyacrylic acid could act as internal plasticizer to swell the fibers and promote the diffusion of dyes. As such, the disperse dye molecules could also leave the fibers due to the weak interaction between the dye molecules and grafted polymer chains. Similar to cationic dyeing fibers, the decrease in light color fastness was also observed in disperse dyeing-grafted PSA fibers (Fig. 13b). In addition, it also can be found that the disperse dyeing grafted PSA fibers showed an excellent high temperature color fastness below 180[degrees]C (the melt point of Disperse Red 60[degrees]C-185[degrees]C) as shown in Fig. 13c.

CONCLUSIONS

In order to improve the dyeability of PSA fibers, the modification of PSA fibers was conducted using [gamma]-ray irradiation-induced AA graft polymerization. The AA-grafted PSA fibers showed appreciable dyeability to cationic and disperse dyes without the help of a carrier under normal pressure conditions. This is mainly due to the incorporation of polyacrylic acid into the amorphous region within the PSA fibers acting as an internal plasticizer, enlarging the free volume of fibers. The color fastness of the dyed PSA fibers was strongly reliant on the dye category. The irradiation grafting treatment has few adverse effects on the inherent performance of PSA fibers such as the mechanical properties, thermal stability, and flame retardancy within a proper grafting yield.

ACKNOWLEDGMENT

This work was financially supported by the National Natural Science Foundation of China (51773032 and 51273039) and China Postdoctoral Science Foundation (2017 M620125) and the Fundamental Research Funds for the Central Universities (17D110622).

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Jinchao Yu (iD,1) Minglei Wang, (2) Jiangtao Hu, (2) Jianning Wang (iD), (1) Guozhong Wu, (2) Chunfang Qian, (3) Xiaofeng Wang, (3) Yumei Zhang, (1) Huaping Wang (1)

(1) State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai 201620, China

(2) Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China

(3) Shanghai Tanlon Fiber Co. Ltd., Shanghai 201419, China

Correspondence to: Y. Zhang; e-mail: zhangym@dhu.edu.cn

Contract grant sponsor: China Postdoctoral Science Foundation; contract grant number: 2017M620125. contract grant sponsor: National Natural Science Foundation of China; contract grant numbers: 51273039; 51773032. contract grant sponsor: Fundamental Research Funds for the Central Universities; contract grant number: 17D110622.

DOI 10.1002/pen.24974

Published online in Wiley Online Library (wileyonlinelibrary.com).

Caption: SCH. 1. Chemical structure of PSA fiber, graft monomer and dyes.

Caption: FIG. 1. The SEM morphology of PSA fibers with various degree of grafting.

Caption: FIG. 2. SAXS pattern of PSA fibers with various degree of grafting. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 3. WAXS pattern of PSA fibers with various degree of grafting.

Caption: FIG. 4. Crystallinity of PSA fibers with various degree of grafting.

Caption: FIG. 5. Crystallite size and crystal orientation of PSA fibers with various degree of grafting.

Caption: FIG. 6. DMA curves of PSA fibers with various degree of grafting. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 7. FTIR spectra of PSA fibers with various degree of grafting. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 8. Thermo-mechanical curves of PSA fibers with various different degree of grafting. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 9. Dyeing behavior of PSA fibers before and after irradiation grafting (degree of grafting ~30%). [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 10. Cross section of cationic dyeing AA grafted PSA fibers (degree of grafting ~30%). [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 11. Color fastness of cationic dyeing AA grafted PSA fibers at different conditions (degree of grafting ~30%) (a) Color fastness to washing, (b) Color fastness to light, and (c) Color fastness to high temperature. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 12. Cross section of disperse dyeing AA grafted PSA fibers (degree of grafting ~30%). [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 13. Color fastness of disperse dyeing AA grafted PSA fibers at different conditions (degree of grafting ~30%) (a) Color fastness to washing, (b) Color fastness to light, and (c) Color fastness to high temperature. [Color figure can be viewed at wileyonlinelibrary.com]
TABLE 1. Results from SAXS analyses

Parameter                      PSA    PSA-gf-30% AA   PSA-gf-45% AA

Fibril radius [R.sub.1](nm)   4.34        5.80            5.63
Fibril radius [R.sub.2](nm)   9.87        10.39           13.16
Fibril radius [R.sub.3](nm)   18.61       19.03           24.70
Fibril length [l.sub.f](nm)    387         413             462
Misorientation                13.13       14.15           15.75
[B.sub.[phi]]([degrees])

Parameter                     PSA-gf-56% AA

Fibril radius [R.sub.1](nm)       6.17
Fibril radius [R.sub.2](nm)       13.35
Fibril radius [R.sub.3](nm)       25.33
Fibril length [l.sub.f](nm)        547
Misorientation                    19.20
[B.sub.[phi]]([degrees])

TABLE 2. Mechanical property of PSA fibers with various different
degree of grafting

Sample           Degree      Diameter     Stress (MPa)   Strength
               of grafting   ([micro]m)                  retention
                   (%)                                      (%)

PSA                --          14.90          386           100
PSA-g-AA 30%       30          16.49          348            90
PSA-g-AA 45%       45          17.65          300            78
PSA-g-AA 56%       56          18.11          286            74

Sample         Elongation
                at break
                   (%)

PSA               24.56
PSA-g-AA 30%      27.23
PSA-g-AA 45%      28.11
PSA-g-AA 56%      30.03

TABLE 3. Hygroscopicity and flame resistance degree of grafting

Sample                           PSA            PSA-g-AA 30%

Degree of grafting (%)            .                  30
Moisture regain (%)             6.74               10.41
Contact angle([degrees])         105                 84
Volume resistivity              >1010               ~108
([OMEGA] x cm)
BURNING behavior           non-combustible    non-combustible
Self-extinguishing time         <0.5                <0.5
form the flame (s)

Sample                      PSA-g-AA 45%      PSA-g-AA 56%

Degree of grafting (%)           45                56
Moisture regain (%)             12.14             12.52
Contact angle([degrees])         75                69
Volume resistivity           [~10.sup.8]       [~10.sup.8]
([OMEGA] x cm)
BURNING behavior           non-combustible   non-combustible
Self-extinguishing time         <0.5              <0.5
form the flame (s)

TABLE 4. The performance of grafted PSA fibers before
and after dyeing (degree of grafting ~30%)

Sample            Stress (MPa)   Elongation at    moisture
                                   break (%)      regain (%)

Grafted               348            27.23          10.41
Cationic Dyeing       345            26.89          10.40
Disperse dyeing       346            26.48          10.37

Sample            Self-extinguishing
                    time form the
                       flame (s)

Grafted                  <0.5
Cationic Dyeing          <0.5
Disperse dyeing          <0.5
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Author:Yu, Jinchao; Wang, Minglei; Hu, Jiangtao; Wang, Jianning; Wu, Guozhong; Qian, Chunfang; Wang, Xiaofe
Publication:Polymer Engineering and Science
Article Type:Report
Date:Mar 1, 2019
Words:6276
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