Preparation and Characterization of Hydrophilic Wetting-Modified Polyamide Fibers

To improve the moisture absorption and air permeability of polyamide (PA) fibers, the modified fibers with porous structure were fabricated by melt spinning using polyethylene glycol (PEG) as the pore-forming agent due to its high solubility in water. (e effects of pores caused by different PEG contents on the structure and properties of modified fibers were analyzed by electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and water contact angle. (ese results indicate that the porosity of modified fibers increases with PEG content increasing. Moreover, the formation of pores obviously affects the crystal forms of modified fibers. (e alpha crystal gradually decreases and the gamma crystal increases with the porosity increasing. Furthermore, with respect to the modified fibers obtained from 47.33% PEG content, its water absorption increases from 3.78% of pure PA to 19.76% and its contact angle decreases from 116° to 85°. In addition, due to the interaction of hydrogen bond occurring between PA and PEG during the spinning process, the elongation at break, rupture work, and initial modulus of modified fibers were improved when the porosity was not more than 4.23%.


Introduction
e polyamide (PA) is an important polymeric material and widely applied from industrial areas to civil fields, especially sportswear, because of its light weight, softness, high elastic resilience, good wear resistance, excellent chemical resistance and mechanical properties. Nowadays, it has become the largest synthetic fiber after the polyester in the clothing industry. However, due to its poor moisture absorption and air permeability, the property of wearing comfort is not excellent. To solve this problem, there are many modified methods executed such as blending modification [1][2][3], copolymerization modification [4][5][6], surface modification [7][8][9], special-section modification [10,11] and postfinishing [12][13][14]. For example, Pappas et al. [15] functionalized the PA surface by the addition of new reactive chemical groups to improve its wettability. Miao et al. [16] found that the rate of oil and water absorption all decreased significantly after the treatment by an ionized mixture of gases for nylon fabric, which was well-agreed with the research of Samanta K K team [17]. Generally speaking, there exist advantages and disadvantages for each method. As for the copolymerization modification, its complex process increases production difficulty and cost. e surface modification, especially plasma treatment, is much complicated and difficult to industrialize. With respect to the special-section modification, the manufacture of the spinneret equipment is complex and requires high precision.
ere exists poor durability for the postfinishing modification. Comparing with the above mentioned methods, the blending modification is relatively popular and simple, thus, it is usually applied to prepare the modified fibers in chemical fiber industry. Moreover, there are many literatures on PA blending modification [18][19][20].
In recent years, with the further application of the ultrafiltration membrane, the phase inversion method through adding suitable additives is increasingly paid attention to improve membrane performances and many additives can be employed [21][22][23]. e polyethylene glycol (PEG), which is nontoxic, nonirritant and highly soluble in water, is usually used as a polymeric additive to enhance the pore connectivity and permeability of membranes during the fabrication of solution casting. Yang et al. [24] blended PEG into polystyrene-block-poly (acrylic acid) to adjust the pore size and permeability of isoporous membranes. ey found that when the membrane permeability reached the largest, there existed a balance between the micro-and macrophase separation. Some researchers investigated the effects of PEG with different molecular weights on the membrane formation and water permeation [25,26]. In view of this, the hydrophilic wetting PA fibers can be obtained by the phase inversion method through nonsolvent additives.
To our best knowledge, although this phase inversion method through adding some additives to enhance the pore connectivity and permeability of materials was usually applied in membrane fabrication, there were relatively few literatures to improve the hydrophilic wettability and air permeability of PA fibers. An attempt was made to investigate the feasibility using PEG as the pore-forming agent to fabricate modified fibers with pore structure by melt spinning. e amount of PEG on the porosity was discussed, and the effects of porosity on the structure and performances of modified fibers were characterized.

Preparation of Modified Fibers.
e blending method of melt spinning was employed to obtain modified fibers. e schematic diagram shown in Figure 1 mainly consisted of the following steps.
Mixture preparation: firstly, PEG 2000 was melted in a water bath at 90°C. en granular PA was added and stirred by a mechanical stirrer for 5 h. In order to research the effects of the porosity on the structure and properties of modified fibers, a series of PA/PEG blends was prepared with a PEG content of 0%, 8.85%, 18.79%, 28.25%, 38.66% and 47.33%, respectively.
Blended fibers preparation: the blends were melt-spun into fibers by a single screw spinning machine at 235°C-285°C with a winding speed of 1 m/min.
Modified fibers preparation: the melt-spun fibers by PA and PEG were immersed into distilled water at room temperature to remove the PEG which was highly solule in water. Meanwhile, to ensure complete removal of PEG, the sample was taken out at a regular interval (30 min) and weighted after drying. e modified fibers were obtained when the mass no longer changed after weighing 5 times continuously. Moreover, the porosity of modified fibers depended on the PEG content during the preparation of modified fibers. In general, high PEG content corresponded to large porosity in fibers.

Scanning Electron Microscopy.
e surface and crosssection morphologies of modified fibers were observed by a scanning electron microscope (SEM, SIGMA500, Carl Zeiss Jena, Germany). e specimens were sputtered with thin gold to avoid electron-charging effects before SEM observation.

Infrared Spectroscopy Analysis.
e chemical structures of modified fibers were characterized by an infrared spectrometer (TENSOR37, Bruker Optics, Germany) using the KBr discs method. e spectra were collected in the wavenumber region of 400-4000 cm −1 with a spectral resolution of 4 cm −1 .

X-Ray
Analysis. X-ray analysis was conducted by an X-ray diffractometer (Ultima-Iv, Rigaku Corporation, Japan) with CuKα radiation to measure the crystal structure of modified fibers. e fiber samples were cut into pieces and the 1.5 cm × 1.5 cm thin sheet was pressed before testing. e step-scan method with 10°/min scanning rate was performed to determine the changes of fibers' crystalline structure.

Porosity Calculation.
A certain amount of modified fibers was weighed (W air ) and then immersed into glycerol for 1 h to absorb sufficiently. After that, it was taken from the glycerol and the surface was dried using oil-absorbing sheets. At last, its quality was weighed again (W G ). e porosity (P) was calculated according to the following formula.
where W air and W G are the quality of modified fibers before and after glycerol absorption. e ρ air (1.293×10 −3 g/cm 3  2 Advances in Polymer Technology (1.2617 g/cm 3 ) and ρ PA (1.14 g/cm 3 ) were the densities of air, glycerol and PA, respectively.

Water
Absorption. e calculation of water absorption was very similar to that of porosity and just the glycerol was replaced by water. Firstly, the quality of modified fibers after PEG removal was weighed (W 1 ). en they were immersed into water to fully absorb and weighed again after the water on the fiber surface was removed (W 2 ). e water absorption (Wa) was calculated by the following formula.
3.6. Wettability. e wettability of modified fibers was measured by the water contact angle measurement with a contact angle meter (OCA2, Dataphysics instruments gmbh,

Advances in Polymer Technology
Germany) using the suspension drop method and the injection volume of water was 0.5μml. e fibers should be closely aligned in a plane before the testing and the water contact angle was the angle of water droplets reaching equilibrium.

Physical
Properties. e fineness of modified fibers was measured by a XD-1 fiber fineness machine made by Donghua University (Shanghai, China). A 20 mm gauge length was employed and 20 tests were carried out for each sample to estimate the average fineness. Mechanical properties of modified fibers were determined by LILY-06ED fiber tensile tester. e gauge length was 20 mm and tensile speed was 20 mm/min. A minimum of 20 tests were carried out for each sample to estimate the average value.

Porosity of Fibers.
e porosity of modified fibers depended on the PEG content and calculated by the formula (1). Figure 2 plotted the porosity of modified fibers obtained from different PEG contents. It indicated that the porosity of modified fibers increased significantly with the increase of PEG content, however, it was much smaller than the blending ratio of PEG. Two possible reasons were that one was the incomplete removal of PEG during the immersion process causing by the interaction between PEG and PA during the spinning. e other was the mass loss of PEG during the spinning for its low melting point.

Surface and Cross-Section Morphologies of Fibers.
e surface and cross section of modified fibers were affected by the pores caused by the PEG removal. Figure 3 showed the surface and cross-section SEM images of modified fibers with 11.81% porosity and pure PA. It demonstrated that the surface and cross section of pure PA fibers were smooth and compact. With respect to the modified fibers, the surface and cross section presented many irregular pores which were resulted from the PEG removal after they were immersed in water.

Chemical Structure of Fibers.
Infrared spectra are very sensitive to the chemical structure of materials. Figure 4 presented the infrared spectra of PA fibers with different porosities. It demonstrated that there were prominent peaks at 3430, 2920, 2850 and 1640 cm −1 for all fibers, which were the characteristic absorption peaks of PA6 [27]. e strong absorption peak centered at 3430 cm −1 was attributed to the N-H stretching vibration of amide. e absorption peaks located at 2920 cm −1 and 2850 cm −1 were related to the antisymmetric and symmetric stretching modes of   Advances in Polymer Technology methylene, respectively. e strong absorption peak at 1640 cm −1 was associated with the bending vibration of amide I and water [28]. e intensity of these absorption peaks decreased gradually with the porosity increasing except for that of the modified fiber with 17.72% porosity. Moreover, the intensities of absorption peaks located at 1540 cm −1 and 1065 cm −1 , which were caused by amide II and C-N stretching vibration, decreased gradually and until disappeared with the increase of the porosity. In addition, the characteristic absorption peaks of PEG resulting from the C-H bending vibration of methylene and C-O-C symmetric stretching vibration appeared at 1380 cm −1 and 1110 cm −1 in modified fibers [29,30]. e changes of FTIR spectra indicated that there were some reactions occurred between PA and PEG during spinning which led to a little amount of PEG incomplete removal [31]. When the PEG content reached 47.33%, there were more pores in modified fibers and these pores easily absorbed the moisture in the air which resulted in the enhancement of absorption peaks intensity at 3430 cm −1 and 1640 cm −1 .

Crystal Structure of Fibers.
PA is a highly crystalline polymer with various crystalline forms. e existence of pores and a little PEG affected the crystalline structure of modified fibers. e XRD fitting diagram of pure PA fibers was given in Figure 5. ere were three crystallization peaks for pure PA fibers which were alpha crystallization peaks located at 20.9°and 23.11°(peak 1 and 4 as shown in Figure 5) and gamma crystallization peak at 21.35°(peak 2 in Figure 5), as well as an amorphous peak (peak 3 in Figure 5), respectively [32,33]. As shown in Figure 6, the XRD patterns of modified fibers changed significantly and the three crystallization peaks separated obviously because of the formation of pores. Moreover, the intensity of the alpha crystallization peak at 20.9°decreased gradually and the intensity located at 23.11°increased with the increase of the porosity, however, the gamma crystallization peak firstly increased and then decreased, which demonstrated that the existence of pores obviously changed the crystal form and crystal face structures of PA fibers. In addition, although the pure PEG was also a highly crystalline material, there was no the characteristic diffraction peak in XRD patterns. It indicated that the PEG did not form an independent crystalline phase in modified fibers. e reason was that the relatively little amount of PEG could not crystallize, and, furthermore, the reactions between PEG and PA resulted in the absence of free PEG molecular.

Hydrophilic Wettability of Fibers.
e porous structure affected the hydrophilic wettability of fibers which was characterized by the water absorption and contact angle. e water absorption and contact angle of modified fibers with different porosities were shown in Figures 7 and 8. With the porosity increasing, the water absorption increased continuously, and the contact angle decreased gradually. Moreover, when the porosity reached 14.65%, the modified fibers had changed from hydrophobic to hydrophilic. e reason was that the roughness surface of modified fibers caused by pore structure was conducive to improve the hydrophilic wettability of fibers.

Physical and Mechanical Performances of Fibers.
e physical and mechanical properties of modified fibers with different porosities were are listed in Table 1     Advances in Polymer Technology and tensile strength of modified fibers decreased gradually with the porosity increasing; however, the elongation at break, rupture work and initial modulus appeared firstly increasing and then decreasing. e main reason for the increase was that a little amount of PEG played the plasticizing and toughening roles in fibers. In addition, once the porosity exceeded 4.23%, the properties of modified fibers began to deteriorate for the pore increase in fibers leading to the slip and fracture of the molecular chains during the stretching process.

Conclusion
e purpose of this paper was to investigate the effects of the porosity caused by different PEG contents on the performances of PA modified fibers. e results demonstrated that the performances of modified fibers were obviously affected by the porous structure. Due to the existence of pores, the morphologies of surface and cross section and crystal forms of modified fiber distinctly changed. Moreover, the hydrophilic wettability of modified PA fibers increased with the porosity increasing. e modified fibers could be completely transformed into hydrophilic fibers when the porosity was large enough. With the porosity increasing, the fineness and tensile strength of modified fibers decreased gradually, however, the elongation at break, rupture work and initial modulus appeared firstly increasing and then decreasing because of a small amount of PEG residues and pores. When the porosity was not more than 4.23%, the modified fibers with excellent mechanical properties and hydrophilic wettability were capable to be obtained.

Data Availability
Data are available from the corresponding author upon reasonable request.

Conflicts of Interest
e authors declare that they have no conflicts of interest.