Tentative Confinement of Ionic Liquids in Nylon 6 Fibers: A Bridge between Structural Developments and High-Performance Properties.

A reversible confinement of ionic liquid (IL) among the amide segments has been carried out for the preparation of high-modulus and high-strength aliphatic semicrystalline nylon 6 fibers. In this research work, the suppression or the weakening of the hydrogen bonds during the conventional low-speed melt spinning process is followed by a hot-drawing stage and a subsequent IL extraction of the IL out of the 2% wt IL-confined fibers and an immediate thermal stabilization process for the improvement of the properties of the pristine nylon 6 fibers. The resulted crystal structural developments of the IL-confined fibers are attributed to ultimate molecular orientations, which have contributed to the developments of the overall fiber properties. Here, the influences of the IL on the γ and the α crystal phases, the γ-α transition, the morphological properties, and the tensile properties are investigated. The FTIR reported, experimentally, additional peaks at 1237 cm-1 for the γ crystal phase and at 1417 and 1476 cm-1 for the α crystal phase, in conformity with the theoretical computations. The XRD demonstrated that the conventional low-speed melt spinning can successfully be used to prepare as-spun IL-confined fibers having highly improved properties. The so prepared as-spun IL-confined fibers are found to have a γ phase structure that has a small crystal size and high crystal perfections. Fortunately, the γ-to-α crystal phase transition for the IL-confined nylon 6 fibers can be acquired during the hot-drawing stage (stress-induced phase transformation). Furthermore, the IL extraction process followed by a thermal stabilization process, interestingly, has led to significant increases in both of the tensile strengths and the tensile moduli of the reverted nylon 6 fibers. The values that are found are 8.46 cN/dtex for the tensile strength and 39.09 cN/dtex for the tensile modulus. The structure-property relationships between the IL-confined and the reverted nylon 6 fibers have also been discussed.

Introduction

There are two routes for the preparation of high-strength and high-modulus polymeric fibers: using the stiff molecules, which do not tend to fold up into lamellae (such as aromatic amides, aromatic esters, and diazoles polymers) or, on the other hand, using the ultra-high orientation of the conventional flexible molecular chains by preventing them from folding to lamellae or to transform their folded structure (already formed) to an extended molecules.[1] In particular, aliphatic polyamide nylon 6 fibers are known as very important semicrystalline fibers, and they have been used in textiles and in technical applications as well.

Structurally, nylon 6 fibers exhibit two polymorph phase structures, namely, a γ crystal phase (parallel hydrogen-bonded chain) and an α crystal phase (anti-parallel hydrogen-bonded chain). Indeed, the preparation of high-strength and high-modulus properties of the nylon 6 fibers is essentially dependent on the fibers’ crystalline structures and their developments, namely due to the developments of the γ phases into α phases’ structures. In general, the γ phase forms by three methods: the chemical treatments (iodine complexation treatment of the spun-yet fibers,[2−4] metal halide, montmorillonite clay (MMT) treatments,[5,6] and rare-earth halide[7,8] complexation), the physicomechanical treatments (the high-speed melt spinning[9−16] that arises from the orientation-induced crystallization process[17,18]), and the physical treatments (the low crystallization temperatures or high cooling rate from the melt state). The nylon 6 will undergo γ-to-α crystal-phase transitions upon the drawing of the low-speed-spinning as-spun fibers, the annealinga of the high-speed-spinning[19] & the chemical-complexed fibers and the physical treatments (mainly crystallization at high temperatures or a low cooling rate from the melt state).

One may notice that many attempts have been made by using spinning (solution,[20,21] dry-jet-wet,[22] and horizontal isothermal bath[23]), drawing (vibrating and zone annealing[24], incremental drawing[25] and various heating methods[26]), and reversible complexation (temporal counter-ion[27−30]) processes to improve the tensile properties of polyamide fibers. Additionally, Yu et al.[31] have obtained symmetrical twin crystals along the hydrogen bond directions of the nylon 6 nanorods that have provided only an α phase structure. Some researchers considered the preparation of nanocomposites based on nylon 6 and nanomaterials, such as carbon nanotubes (CNTs),[32] nanoclay (MMT),[6] and graphene.[33] Recently, superheated water has also been considered for polyamide dissolution,[34−38] nanocomposite,[39] and revisable shielding processes.[40]

Among all of the processes made for preparing nylon 6 fibers, melt spinning is found to be the most convenient, eco-friendly, and cheapest way to produce nylon 6 fibers[41] despite the fact that the formation of the crystal phases depends on the crystallization conditions.[42] On the other hand, ionic liquids (ILs) are molten salts at room temperature that exhibit versatile properties.[43,44] The ILs have melting temperatures below 100 °C, and they are thought to be fluids. In fact, after all, these ionic liquids are salts,[45] and the term “ionic liquid” may have seemed like an oxymoron in the past.[44] These, eco-friendly IL solvents, by considering their physicochemical properties, replaced the volatile solvents,[45−51] the volatile bulky, and the viscous plasticizers and lubricants.[52−54] Much applicable efforts on the potential of ILs in terms of biological functional materials[55] and polymeric research field[56−58] have been carried out. Moreover, it has been reported that: the electro-spun nylon 6 nanofibers can be fabricated by 1-butyl-3-methylimidazolium hexafluorophosphate [BMIM][PF6],[59] the nylon 6 injection-molded extrudate can be prepared by 1-butyl-3-methylimidazolium chloride [BMIM][Cl],[60] the nylon 6 films can be prepared by 1-vinyl-3-butylimidazolium chloride [VBIM][Cl],[61] and the polyeamide 6-steel contact’s lubricant can be prepared by 1-hexyl-3-methylimidazolium hexafluorophosphate [HMIM][PF6][62] ionic liquids. To the best of our knowledge, no research works have been reported in the development of the structural and the molecular orientations of the melt-spun nylon 6 fibers so as to prepare high-modulus and high-strength aliphatic semicrystalline polyamide fibers.

In this article, the effects of temporary reversible imidazolium bromide IL confinement processes on the crystal structure and the molecular orientations of nylon 6 fibers have been investigated for the production of the aliphatic polyamide fibers of high-quality performance properties. Furthermore, the structural, morphological, and mechanical properties of the neat, the IL-confined, and the reverted as-spun and drawn nylon 6 fibers have been examined by using the techniques of the Fourier transform infrared spectroscopy (FTIR), the wide-angle X-ray diffraction (WAXD), the scanning electron microscopy (SEM), and the measuring of the mechanical properties.

Results and Discussions

Crystals’ Structural and Orientational Developments

The confinement process of the ILs in the nylon 6 fibers has caused a phase separation of nylon 6 molecular chains that has led to improvements in the molecular orientations. As a consequence of these crystal microstructural developments, the overall properties of the nylon 6 fibers are highly improved. Surprisingly, for the confinement of IL into nylon 6 fibers, the maximum value of the molecular extension (draw ratio) is found to be 4.5 even for different IL contents and at different drawing temperatures of 120, 140, and 160 °C. However, this maximum draw ratio for the IL-confined nylon 6 fibers is higher than that for the unconfined (neat) nylon 6 fibers, which is 4.0, as shown in Figure .

Figure 1

Maximum draw ratio of the neat and IL-confined nylon 6 fiber.

Fourier Transform Infrared Spectroscopy (FTIR)

The FTIR spectra for different samples of imidazolium-type ionic liquids, different samples of as-spun neat and different samples of as-spun IL-confined nylon 6 fibers are scanned in the range of 4500–600 cm–1 and the results are shown in Figure . However, the as-spun neat nylon 6 fibers presented significant absorption bands at 3298, 3088.5, 2932.8, 2860, 1639.2, and 1545.6 cm–1 assigned to the hydrogen-bonded N–H stretching, aromatic C–H stretching vibration, asymmetric CH2 stretching vibration, symmetric CH2 stretching vibration, hydrogen-bonded C=O stretching vibration, and N–H bending vibration, respectively. Meanwhile, the FTIR spectra of the ILs witnessed an absorption band at 2050 cm–1. Actually, as in Figure , all of the FTIR spectra in the IL-confined nylon 6 fibers exhibited the typical absorption band of the ILs around 2050 cm–1. This observation reveals that the ILs are successfully incorporated into nylon 6 fibers via the melt spinning process. Moreover, in the polyamide nylon 6 fibers, the hydrogen bonds existed between the carbonyl (C=O) and the amine (N–H) sides of the amide group.

Figure 2

FTIR spectra of the as-spun neat and IL-confined nylon 6 fibers.

Furthermore, as shown in Figure and Table , after the IL confinement into the nylon 6 fibers, the FTIR absorption peaks’ wave numbers of the hydrogen-bonded C=O and the N–H stretching vibrations’ wave numbers of the amide group are slightly reduced from 1639.2 to 1637.7 cm–1 and from 3298 to 3296.6 cm–1, respectively, as a result of the IL incorporation into the nylon 6 fibers. This result reveals that the strong interaction of the ILs and the nylon 6 molecules took place in the two sides of the amide group, namely the IL cations interact with the carbonyl sides (C=O) of the amide group, whereas the IL anions interact with the amine sides (N–H) of the amide group of the nylon 6 fibers. It is observed that there is strong coordination between the ILs and the nylon 6 molecules and also there is strong coordination between the cations and the anions of the ions of the ILs. On the other hand, the absorption band wave numbers of the N–H bending vibration increased from 1545.6 to 1546.5 cm–1 after IL confinement. The shifting of the N–H vibrations to higher values, obviously, indicates that the formations of hydrogen bonding between the ILs and the nylon 6 molecules took place.

Table 1

FTIR Spectra Band Assignments of the IL-Confined Nylon 6 Fibers

sample	NH stretching	C=O stretching	N–H bending	asymmetric CH2 stretching	symmetric CH2 stretching
neat nylon 6	3298	1639.2	1545.6	2860	2932.8
nylon 6–1% IL	3297	1638.7	1545.4	2860	2932.7
Nylon 6–2% IL	3296.1	1638.2	1546.5	2859	2931.7
Nylon 6–5% IL	3296.6	1637.7	1545.9	2859.5	2932.3

In the drawn fibers, however, the absorption band wave numbers of the N–H bending vibrations of the neat and the IL-confined nylon 6 fibers have decreased due to polymorph conversions and due to the deformation increment (drawing ratio) as illustrated in Figures and 4 and in Figures S1 and S2 in the Supporting Information below. The strong band of the γ phase structure of the as-spun neat nylon 6 fibers recorded a backward shift from the wave number 1545.69 cm–1 to the lower wave numbers 1539.91, 1538.46, and 1543.76 cm–1 due to the polymorphs’ conversions to α phases structures (for the maximum drawing ratio of 4.0) at the drawing temperatures of 120, 140, and 160 °C, respectively, whereas, the γ phase structure of the as-spun IL-confined nylon fibers recorded a significant shift to lower wave numbers from 1547.14 to 1538.46 and to 1538.94 cm–1 due to the polymorphs’ conversions to α phases structures (for the maximum drawing ratio of 4.5) at the drawing temperatures of 140 and 160 °C, respectively. The reduction of the N–H bending vibration is an evidence for hydrogen bond formation of the anti-parallel molecular chains (α phase).

Figure 3

FTIR spectra for the as-spun and the drawn IL-confined nylon 6 fibers, with a drawing temperature of 140 °C to different drawing ratios. FTIR spectra at band regions of (a) 4000–600, (b) 1500–1100, and (c) 1100–900 cm–1, respectively.

Figure 4

FTIR spectra for the as-spun and the drawn IL-confined nylon 6 fibers, with a drawing temperature of 160 °C to different drawing ratios. FTIR spectra at band regions of (a) 4000–600, (b) 1500–1100, and (c) 1100–900 cm–1, respectively.

Obviously, the FTIR spectra between the 1500 and 800 cm–1 frequency regions are attributed to the two polymorphs[63] (γ and α phases) of the neat and of the 2% IL-confined nylon 6 fibers.

Focusing is made here on the specific crystal structures’ bands of the FTIR spectra for the as-spun 2% IL-confined nylon 6 fibers and also on its drawn fibers at the temperatures 140 and 160 °C. For the γ crystal phases structures of the as-spun 2% IL-confined nylon 6 fibers, the absorption peaks in the range 1100–900 cm–1 appear at wave numbers of 1073.69, 975.34, and 916 cm–1, while it is seen that in the wave number range 1500–1100 cm–1, the absorption bands appear at wave numbers 1437.21, 1300.29, and 1237.13 cm–1 as shown in Figure b,c and Figure b,c. The band 1118.53 cm–1 arises from the amorphous phase structure of the fibers. Moreover, the α phase structure of the drawn 2% IL-confined nylon 6 fibers (DR = 4.5 and drawing temperature of 140 °C) exhibited absorption peaks at 1029.34, 958.95, and 928.57 cm–1. The bands at 1475.78, 1417.45, and 1200 cm–1 are ascribed to the α crystal phases structures. It is seen that the α phase structure of the drawn 2% IL-confined nylon 6 fibers (DR = 4.5 and drawing temperature of 160 °C) has obtained absorption peaks at 1029.34, 958.95, and 929.05 cm–1 as illustrated in Figures S3 and S4. Similarly, these bands have been reported by Murthy et al.[64] for high α content fibers. The absorption bands at 1475.76, 1416.96, and 1200 cm–1 are attributed to the α crystal phases’ structures.

The FTIR results obtained here are in good agreement with previous experimental results reported by other authors. Moreover, these results are in good conformity with the theoretically calculated results that are based on the density functional theory (DFT) computational method of nylon 6 polymorph spectra.[65] The FTIR spectra for IL-confined nylon 6 results have shown additional peaks at 1237 cm–1 for the γ phase and peaks at 1417 and 1476 cm–1 for the α crystal phase structure. These peaks are theoretically calculated to take place at 1234 cm–1 for the γ phase structure and at 1416 and 1478 cm–1 for the α crystal phase structure. On the other hand, the bands at 1170.12 and 1121.90 cm–1 are attributed to the reference bands and to the amorphous phase structure, respectively.

Moreover, the FTIR spectra have shown absorption bands at 621.94 and 693.78 cm–1 attributed to the γ phase. Also, they have shown absorption bands at 687.51 (at the drawing temperatures of 140 °C) and 686.55 cm–1 (at the drawing temperatures of 160 °C) for the α crystal phase structure. The absorption band shown at 728.97 cm–1 obtained from the CH2 rocking of the amide chain of the as-spun and drawn IL-confined nylon 6 fibers. These observations are slightly in agreement with the results of Arimoto[3] and Matsubara and Magill[66] for the γ–α phase transition at lower-frequency infrared spectra of the nylon 6 film.

Surprisingly, despite the obvious confinement of the IL among the amide molecules, the IL-confined nylon 6 fibers exhibited a slight shift (1 cm–1) for the FTIR wave length of the hydrogen-bonded N–H stretching band. The same observation has been reported by Tian et al.[60] and Zheng et al.[61] for the imidazolium cation. The absence of such a wave number shift has been reported by Li et al.[59] as well. However, the imidazolium cation has strong affinity that can compete with the C=O interaction to withdraw the hydrogen from the N–H of the amide group. This slight shift is contrary to 60 cm–1 for the lanthanide[7,8] and 40 cm–1 for the calcium chloride.[67] This behavior may be due to the plethora of the network hydrogen bond interactions between the amide groups and IL and also the formation of the new hydrogen bond between the N–H and the anion. It may be concluded that the ILs diffused in the rigid amorphous fraction (RAF) due to the conventional melt spinning process.

Crystallographic Structures of the IL-Confined Fibers

The crystal structural changes and the molecular orientations developments of the as-spun and the drawn neat and the IL-confined melt-spun nylon 6 fibers are investigated by the XRD method.

Crystal Structure

The nature of the crystalline structures of the as-spun and the drawn nylon 6 fibers exhibit, basically, γ and α crystalline peaks at (h00) and (00l) reflection planes for equatorial scanning, respectively. The as-spun fibers primarily take γ phase structures with parallel hydrogen-bonded and twisted chains, which develop a pseudo-hexagonal unit cell structure corresponding to a “kinked” or pleated hydrogen-bonded sheet structure.[4] However, after fiber deformation, the oriented molecules took an α crystal phase of anti-parallel hydrogen-bonded and fully extended chains, which exhibits a monoclinic unit cell structure that corresponds to extended zigzag conformation.[15]

Here, the crystal structures of the neat and the IL-confined nylon 6 fiber are examined by the XRD technique, and the results are shown in Figures –7. The as-spun neat nylon 6 fibers entail a mixture of crystalline structures of γ and α phases due to the rapid crystallization and the formation of hydrogen bonds once the extrudate molten is cooled down. These crystal structures obtain diffraction peaks centered at 11.05 and 21.35° with the interplanar spacings (d-spacing) of 8.00 and 4.16 Å, respectively, for the γ phase and peaks centered at 20.44 and 22.66° with the interplanar spacings of 4.34 and 3.92 Å, respectively, for the α crystal phase structure. On the other hand, after the IL confinement, the XRD pattern of the as-spun 2% IL-confined nylon 6 fibers obtained a rich γ phase, which gives two crystalline peaks at (100) and (001) reflection planes with interplanar spacings of 7.82 and 4.12 Å for two-theta reflection angles of 11.30 and 21.55°, respectively, as shown in Figure . It should be pointed out that besides the high-spinning speed melt spinning,[16] the complexation of the iodine,[14,29] lithium chloride,[27] calcium chloride,[68,69] lithium bromide,[70] or yttrium[7] and the solution crystallization[71] with specific solvents, IL confinement is also able to form γ phase for the nylon 6 fibers as a single-ordered phase.

Figure 5

XRD equatorial diffraction pattern of the as-spun neat and IL-confined nylon 6 fibers.

Figure 7

Equatorial diffraction patterns of (a) as-spun neat and drawn neat nylon 6 fibers drawn to different drawing ratios at a drawing temperature of 160 °C and (b) as-spun and the drawn 2% IL-confined nylon 6 fibers drawn to different drawing ratios at a drawing temperature of 160 °C.

It is worth noting that the hot drawing of the as-spun neat fibers develops the two γ phases into two α phases upon increasing the deformation ratio. The XRD diffraction patterns of the drawn neat nylon 6 fibers at temperatures of 140 and 160 °C are shown in Figures a and 7a, respectively. Meanwhile, the X-ray diffraction peaks reveal that the γ crystal phase still existed in the XRD pattern of the drawn neat nylon 6 fibers when they are drawn at drawing temperatures of 120, 140, and 160 °C for various draw ratios. This observation revels that the γ-to-α crystal phases’ transition is incomplete (i.e., partial transition). On the other hand, in the case of the XRD pattern of the drawn 2% IL-confined nylon 6 fibers (for the maximum drawing ratio of 4.5 at drawing temperature of 160 °C), it is only observed with two α crystalline peaks at two-theta positions of 20.45 and 23.44° with d-spacings of 4.34 and 3.79 Å at (200) and (002) reflection planes, respectively. Interestingly, the XRD diffraction patterns of the drawn IL-confined nylon 6 fibers disclose complete γ-to-α crystal phase transitions (i.e., a complete vanishing of the γ phase structure) as shown in Figures b and 7.

Figure 6

Equatorial diffraction patterns of (a) as-spun neat and drawn neat nylon 6 fibers drawn to a drawing ratio of 4.0 at a drawing temperature of 140 °C and (b) as-spun and the drawn 2% IL-confined nylon 6 fibers with a drawing temperature of 140 °C to different drawing ratios.

The (200) peak of the IL-confined nylon 6 fibers, in contrary to the case of the neat ones, exhibits a peak intensity lower than that of the (002) peak due to the interaction of the ILs with the amide group through the hydrogen bonds. Further examinations show that the draw ratio of 4.5 yields a (200) peak intensity higher than that in the case of the draw ratio 4.0 as a result of molecular alignment and formation of hydrogen bonds between the opposite segments. However, the (002) peak of the IL-confined nylon 6 drawn fiber has shown a slight shifting of two-theta to a higher value. This shift indicates that the developments of the changes on the crystalline region of the IL-confined nylon 6 fibers are significant.

Crystal Orientations

The development of the molecular orientations is, essentially, the leading structure characteristic, which controls and improves the overall properties of the nylon 6 fibers.[72] Obviously, increasing the α crystalline phase, along the fiber axis, at the expense of decreasing the γ crystalline phase upon the ultimate drawing ratio leads to high-crystallinity, high-strength, and high-modulus semicrystalline polyamide nylon 6 fibers. Basically, the molecular orientation of the semicrystalline nylon 6 fibers increases upon drawing in both the crystalline and the amorphous regions. However, the crystal orientations of the IL-confined nylon 6 fibers are mainly depending on the efficiently of pulling out the nylon’s molecules from the folded lamellar crystal into well-oriented crystals without the breaking of the taut molecules (and the chain backbone). Apparently, the confinement of the ILs into the nylon 6 molecules inhabits the crystallization via hydrogen bond interactions. In the crystal orientation, the full width at half maximum (FWHM) of the azimuthal angle decreases due to the increase in the deformation when both types of the nylon 6 fibers are changed from as-spun fibers to drawn ones. In contrast to the neat drawn nylon 6 fibers, as seen from Figures S5 and S6, the crystal orientations of the IL-confined fibers obtain a higher degree of orientation with a concomitant increase in crystallinity. Moreover, for the same drawing ratio, each of the Herman’s orientation functions of the neat and of the IL-confined fibers take different values at different temperatures (in other words, the values of the Herman’s orientation function vary with temperature for the same fixed drawing ratio for each of the neat and the IL-confined nylon 6 fibers), as shown in Figure .

Figure 8

Herman’s orientation function of the neat and IL-confined fibers versus the drawing ratio at drawing temperatures of 120, 140, and 160 °C.

However, the values of the angle between the molecular chains and the fiber direction decrease significantly as a result of the molecular orientations and show small values for the IL-confined fibers smaller than those for the neat fibers. The degree of the crystal orientations, the crystallinity, and the orientation angles for the neat and the IL-confined nylon 6 fibers are tabulated in Table and Tables S1 and S2 and are illustrated in Figures and 10. Indeed, these results confirm that the orientations of the unconfined (neat) nylon 6 molecules depend on the molecular mobility when increasing the drawing temperature from 120 to 160 °C. Despite the fact that the crystals orientations increase with increasing the drawing ratio, these last above mentioned results indicate that the effects of the drawing temperatures are also significant in this respect. As seen in Figure , the values of the molecular orientations for both of the neat and the IL-confined nylon 6 fibers are, obviously, reduced as the drawing temperature is increased to 160 °C. This finding is in agreement with Park and co-workers’ findings.[73]

Table 2

Crystalline Structural Properties of the As-Spun, Neat, and IL-Confined Nylon 6 Fibers at Various IL Contentsa

		apparent crystal size (ACS) (Å)
samples	crystallinity (%)	100	001	⟨cos2ϕhkl⟩	⟨cos2β⟩	degree of orientation (fc)	orientation angle
neat nylon 6, as-spun fibers	50.28	32	81	0.88	0.76	0.640	29.33
nylon 6–1% IL as-spun fiber	53.32	20	39	0.88	0.76	0.638	29.42
nylon 6–2% IL as-spun fiber	50.23	19	36	0.861	0.723	0.584	31.78
nylon 6–5% IL as-spun fiber	57.44	22	20	0.863	0.726	0.589	31.58

Some of the as-spun fibers exhibit a mixture of diffractions at (002) and (001) and (200) and (002) reflection planes.

Figure 9

Crystallinity of the as-spun and the drawn neat and IL-confined nylon 6 fibers at drawing temperatures of 120, 140, and 160 °C.

Figure 10

Orientation angle of the as-spun and the drawn neat and 2% IL-confined nylon 6 fibers at drawing temperatures of 120, 140, and 160 °C.

Apparent Crystal Size (ACS) and Crystal Perfection Index (CPI)

The ACS is obtained by using the Scherrer equation along the hydrogen bonds for both the γ and the α phases, mainly, γ(001) and α(200) reflection peaks. Interestingly, the as-spun IL-confined nylon 6 fibers exhibited a reduction in the ACS of the γ(001) reflection plane as a result of the distraction of the hydrogen bonds by the IL confinement into nylon 6 molecules as shown in Figure . Indeed, the ACS decreases with the increase in the IL contents from 0 to 5 wt %. However, the development in the chain packing along the direction of the van der Waals forces, α (002), improves simultaneously with the crystal growth along the direction of the hydrogen bonds, α (200), at the expense of the γ crystal phase due to the drawing of the nylon 6 molecules (see Figure S7). It is worth noting that the as-spun neat nylon 6 fibers showed the existence of the ACS of 24 and 21 Å of α crystal structures at (200) and (002) diffraction peaks, respectively.

Figure 11

Apparent crystal size (ACS) of the as-spun IL-confined nylon 6 fibers.

At the same time, the values of the CPI of the IL-confined nylon 6 fibers obtained substantial improvements compared to those of the neat nylon 6 fibers. The slippage of the nylon 6 molecules in the absence of the hydrogen bonds and under thermal and tension treatments results in the pulling of the folded molecules out from the lamella into oriented molecules without breaking the taut molecules. The CPI values are tabulated in Tables S1 and S2 and are plotted in Figure S8 for both types of nylon 6 fibers. Interestingly, the results have shown some values of the CPI for the IL-confined fibers that are reaching to about 80%. Also, the results have shown that at the same drawing temperature, any value of the CPI for the IL-confined fibers is greater than any value of the CPI for the neat fibers.

Moreover, it is worthy to indicate that the crystallinity, crystal size (ACS), and the crystal perfection index (CPI) increase with the increase in the orientation function for the IL-confined fibers only, but this property will not take place for the neat fibers.

Morphological Structures

The morphological structures, as can be seen from the SEM micrographs in Figure , of the as-spun IL-confined nylon 6 fibers are uniformly melt spun into smooth surfaces, cylindrical-like endless filament, and round cross-sectional shape unlike the awful and the bark old surface of gel-spun lithium salted-nylon 6 fibers.[74] As the IL concentration increases, however, in comparison with the polyethylene, the polypropylene, and the nylon 6,6,[75] the formed fibers (as-spun IL-confined nylon 6 fibers) show only a lamellar structure and do not show spherulites and hedrite structures. This observation revealed that the IL-involved lamellae structure can also be formed by confining the ILs in semicrystalline nylon 6 via a melt spinning process. This result also strongly resembles the IL confinement of the semicrystalline fluorinated copolymer poly(vinylidene fluoride-co-chlorotrifluoroethylene).[76] Cleary, the image of the surfaces and cross sections of the as-spun IL-confined fibers showed good miscibility of the ILs with the resins of the nylon 6 fibers.

Figure 12

SEM micrographs of surfaces and cross-sectional observations of IL-confined nylon 6 fibers with various IL confinement ratios. The surfaces and cross-sectional micrograph of (1) as-spun 1%IL-confined, (2) as-spun 2%IL-confined, and (3) as-spun 5%IL-confined nylon 6 fibers, respectively.

Interestingly, the SEM image of the IL-confined nylon 6 fibers shows some spots on the surfaces of the fibers. This phenomenon can only be sufficiently explained by the coexistence of the IL liquid and the IL solid phases at the interface between a pure bulk IL and solid substrate.[77] Moreover, there exist solid-like and liquid-like interfaces of the IL due to the amphiphilic nature of the ILs, the ion mobility, and the diffusion behavior at the polymer–IL interface. On the other hand, the SEM micrographs of the as-spun and drawn 2% IL-confined nylon 6 fibers upon different drawing ratios at drawing temperatures of 140 and 160 °C elucidate the development of the lamellar structures into fibrillar structures. However, when the drawing ratio increases, the lamellar structures are found to be helpful to increase the potential of the disentanglement of the confined fibers due to the reduction of some of the hydrogen bond interactions caused by the confined ILs. Obviously, upon the hot-drawing processes, the as-spun IL-confined fibers of the lamellar structures have developed into fibrillar structures for the drawn fibers. The fibrillar structures emerged as parallel traces throughout the surfaces of the fibers as a result of the stress-induced orientations (Figure ). Here, the IL-confined nylon 6 fibers had obtained zigzagged-conformation molecules (α phase), which is the form of the fibrillar structure “fully extended chains” during the hot-drawing process. This result is in conformity with the FTIR and XRD results.

Figure 13

SEM micrographs of as-spun and drawn IL-confined nylon 6 fibers. The SEM micrograph of the (A) as-spun, (B) drawn [DR4.5@HT140 °C] nylon 6 confined-2% IL fibers, and (C) drawn [DR4.5@HT160 °C] nylon 6 confined-2% IL fibers, respectively.

Mechanical Properties

The mechanical properties of the drawn 2 and 5% IL-confined nylon 6 fibers at a drawing ratio of 4.5 and drawing temperatures of 120, 140, and 160 °C are shown in Figure , while Figure S9 illustrates the tensile moduli, the tensile strengths, and the elongations at different drawing ratios and a drawing temperature of 160 °C. However, in comparison with the unconfined fibers, the 2% IL-confined nylon 6 fibers achieved the maximum tensile strength (6.90 cN/dtex) and the maximum tensile modulus (38.60 cN/dtex) at a draw ratio of 4.5 and a drawing temperature of 160 °C. The values of the tensile moduli, tensile strengths, and elongations of the neat and the 2% IL-confined nylon 6 fibers are tabulated in Table below for different drawing temperatures and drawing ratios.

Figure 14

Mechanical properties of the drawn 2 and 5% IL-confined nylon 6 fibers at a drawing ratio of 4.5 and drawing temperatures of 120, 140, and 160 °C.

Table 3

Mechanical Properties of Neat and 2% IL-Confined Nylon 6 Fibersa

	neat nylon 6 fibers			IL-confined nylon 6 fibers
drawing conditions	tensile modulus (cN/dtex)	tensile strength (cN/dtex)	elongation (%)	tensile modulus (cN/dtex)	tensile strength (cN/dtex)	elongation (%)
DR 4.0, HT 140	26.10	4.70	25.40	16.20	5.50	33.00
DR 4.5, HT 140				30.40	6.40	21.10
DR 3.0, HT 160	12.50	3.10	71.40	11.80	3.60	78.00
DR 3.5, HT 160	17.90	3.80	46.60	8.90	4.50	50.50
DR 4.0, HT 160	25.10	4.70	27.90	30.00	5.60	29.90
DR 4.5, HT 160				38.60	6.90	17.90

N.B: DR: drawing ratio; HT: heat temperature (or drawing temperature).

Extraction of the Ionic Liquids and the Structural Stabilization Processes

The reversion of the IL-confined nylon 6 fibers to the pure nylon 6 fibers is carried by extracting out the ILs and applying thermal processes to the resulting fibers, which is the main aim of this piece of research. Thenceforth, the IL-free fibers are the so-called the regenerated fibers, and the thermally stabilized fibers are the so-called the stabilized fibers. Herein, in the amorphous phase, the well-oriented free-IL molecules crystallized, and the amorphous phase converted into a crystalline phase and thus contributed to the crystallinity. It should be noted here that the nylon 6 fibers have undergone thermal stabilization to enhance the crystallinity and to keep permanent the resulting improved developed structures.

After the completion of the IL extraction process, the FTIR, the XRD, the SEM, and tensile testing measurement have been carried out to determine the degrees of the improvements of some of the properties of the emergent pristine nylon 6 fibers. All the different tests done for any of the nylon 6 fibers that emerged from the extraction out of the ILs from the IL-confined nylon 6 fibers confirmed that the extraction process is complete and the resulting fiber is a neat nylon 6 fiber, i.e., the reversion process to the pure nylon 6 fibers from the IL-confined ones is 100% complete.

The FTIR spectra for the IL-confined, regenerated, and stabilized nylon 6 fibers are illustrated in Figures S10 and S11. The FTIR spectrum indicates the reversions of the nylon 6 fibers from their confined form after the crystal structure development. The consequent results have shown the vanishing of the IL spectrum peak at 2050 cm–1 from the regenerated and stabilized fibers. The XRD pattern has revealed that the α crystal phase structure exists in the melt-spun hot-drawn IL-confined, regenerated, and stabilized nylon 6 fibers (see Figure S12a,b). The XRD patterns have also shown sharp peaks at the (200) diffraction plane and the shifting of the (002) diffraction plane to a higher value of the diffraction angle (two-theta), which lead to a significant increment of the values of the crystal perfection index (CPI) of the regenerated and stabilized fibers. The XRD results are tabulated in Table S3. Scheme , below, shows the variations of the orientation factor through all the steps of the formation processes (cf. the sketch made by Ziabicki[78]) for all types of the prepared nylon 6 fibers (including the IL-confined and the reverted nylon 6 fibers).

Scheme 1

Orientation Factor Changes in the Formation of the Nylon 6 Fibers

In addition it is observed that, the orientation factor of the IL-confined nylon 6 fibers at the drawing temperature of 160 °C is higher than that at the drawing temperature of 140 °C. The molecular orientation factor for the reverted fibers first decreased during the IL extraction process and finally increased again due to the thermal stabilization process under tension of the reverted fibers. Furthermore, the morphological structures of the IL-confined, regenerated, and stabilized nylon 6 fibers are investigated by the SEM technique. Here, the SEM micrographs of the surfaces have clearly shown and illustrated sweeping of the ILs and the fibrillar structure of the developed nylon 6 fibers as shown in Figure . Concerning the mechanical properties of the emergent pristine nylon 6 fiber, here, the tensile properties are tested at the room temperature for single-strand fibers. The results of the test have shown increase in both of the tensile strength and the tensile modulus as a result of the crystal structure developments. The results of this testing experiment (for the mechanical properties) are tabulated in Table .

Figure 15

SEM micrograph of the IL-confined and regenerated nylon 6 fibers. The SEM micrograph of the (1) drawn nylon 6 confined-2% IL fibers [DR4.5@HT140 oC], (2) regenerated fibers, and (3) stabilized fibers, respectively. The SEM micrograph of the (4) drawn nylon 6 confined-2% IL fibers [DR4.5@HT160 oC] and (5) regenerated fibers, respectively.

Table 4

Mechanical Properties of the Developed Nylon 6 Fibers

samplea	treatment temperature (°C)	tension force (cN)	linear density (dtex)	tensile strength (cN/dtex)	tensile modulus (cN/dtex)	elongation (%)
regenerated nylon 6 fibers, 00z-1#	room temperature	free tension	4.13	7.88	34.15	38.00
stabilized nylon 6 fibers, 1#-100	190	98 cN	3.79	8.35	38.29	32.93
stabilized nylon 6 fibers, 1#-200	190	196 cN	3.76	8.46	39.09	37.83
regenerated nylon 6 fibers, 00z-2#	room temperature	free tension	4.04	7.52	33.73	40.32
stabilized nylon 6 fibers, 2#-100	190	98 cN	3.72	8.39	36.94	35.62
stabilized nylon 6 fibers, 2#-200	190	196 cN	3.65	8.27	29.38	40.45

1#: Reverted from the drawn IL-confined nylon 6 fiber DR4.5 HT140 °C; 2#: Reverted from the drawn IL-confined nylon 6 fiber DR4.5 HT160 °C.

Therefore, in comparison with the melt-spun lithium chloride[27] and the melt-spun calcium chloride[67] -salted nylon 6 fibers, the findings (results) of the abovementioned improvement testing experiments have revealed and proved that the removal of the ILs from the IL-confined fibers followed by thermal stabilization under tension is a significant way for the preparation of high-strength and high-modulus nylon 6 fibers.

Conclusions

In this piece of research, a temporal confinement of the ILs among the amide groups along the molecular chain is employed for partially interrupting the amide (original) hydrogen bonds and forming strong (new) hydrogen bonds between the ILs and the amide groups (C=O and N–H).

The results of the investigations of the effects of the confinement of the ILs on the morphological structures, the mechanical properties, and the thermal behaviors of the nylon 6 fibers during the melt spinning, the hot drawing, and the reversion of nylon 6 fibers and their structure–property relationships are studied.

The structural developments of the IL-confined fibers as a consequence of molecular disentanglements allow the nylon 6 fibers to have high draw ratios, which are higher than those for the neat fibers in the cases of hot-drawing processes. A possible explanation for this improvement may significantly be attributed to the weakening of the hydrogen bonds and to the small size of the ACS of the as-spun IL-confined fibers, which are developed (by hot drawing) to have well-oriented molecules and perfect crystal orientations along the fibers, i.e., the γ-to-α crystal phase transitions for the IL-confined nylon 6 fibers have been found to take place during the hot-drawing stage without the need for annealing.

The FTIR absorption bands are found to be in good agreement with the theoretical values of the nylon 6 polymorphs. Moreover, the confinement of the ILs among the amide groups, IL extraction, and thermal stabilization processes lead to high tensile strengths and tensile moduli of the nylon 6 fibers as well.

Being eco-friendly can be considered as an extra credit to the method of the preparation of pristine nylon 6 fibers by the reversible ILs confinement method.