Preparation of Functional Stimuli-responsive Polyamide 6 Fabric with ZnO Incorporated Microgel Priprava funkcionalne odzivne tkanine iz poliamida 6 z mikrogelom z vključenim ZnO

A new functional, stimuli-responsive microgel coating on polyamide 6 (PA6) fabric with simultaneous temperature (T)and pH-responsive moisture management, UV protection and photocatalytic self-cleaning properties was prepared by applying poly-(N-isopropylacrylamide)/chitosan (PNCS) microgel in combination with ZnO nanoparticles to PA6 fabric. Two diff erent application procedures were used: (i) the application of PNCS previously functionalized with a ZnO nanoparticles suspension and (ii) the application of PNCS with the subsequent application of a ZnO nanoparticles suspension. The coatings were fabricated on the untreated PA6 fabric as well as on fabric previously modifi ed by the silica matrix created by the sol-gel precursor iSys MTX in order to increase the adsorption ability and uniformity of the coating. The chemical and morphological properties of the coated PA6 samples were determined by SEM and FTIR analyses. Stimuli responsiveness and functional properties were assessed by the moisture content, water vapour transmission rate, water uptake, UV protection and photocatalytic self-cleaning measurements. The results show that the application procedure as well as the pretreatment of PA6 fabric greatly infl uenced the properties of the coating. Accordingly, the most appropriate procedure included the creation of a silica matrix on the PA6 fi bres followed by the application of PNCS and subsequent application of ZnO. In this case, the modifi ed PA6 fabric exhibited high T and pH responsiveness due to the swelling/de-swelling activity of the PNCS microgel, as well as good UV protection with UPF equal to 18.8 and photocatalytic self-cleaning properties that are even higher than those in the case of the one-component coating with ZnO.

Stimuli-responsive microgels are water-insoluble three-dimensional cross-linked polymers that can absorb and desorb large quantities of water, changing their volume by swelling or de-swelling [1][2][3]. Microgels can be responsive to diff erent external stimuli, i.e., temperature, pH, ionic strength, light and electric fi eld [1]. For textile functionalization, the most widely studied systems are temperature and pH responsive microgels due to their physiological signifi cance. Th e most important temperature-responsive polymer used for the formation of microgels is poly(N-isopropylacrylamide) (pNI-PAAm), which is known to exhibit a lower critical solution temperature (LCST) that is closest to the surface temperature of human skin (32-36 °C) [3][4][5]. Among pH-responsive polymers, the polysaccharide biopolymer chitosan is the most popular because of its biocompatibility and biodegradability [1,6]. A hydrogel with dual functionality, pH and temperature responsiveness can be created by combining pNIPAAm and chitosan polymers. In textiles, the incorporation of pNIPAAm/chitosan microgel (PNCS) was mostly applied to cellulose, but rarely to polyester or other textile fi bres, using the pad-dry-cure method [7][8][9][10][11][12][13][14][15][16]. To increase the coating durability, diff erent cross-linkers were used in combination with the PNCS microgel, or they were applied to the fi bres in the pretreatment process to increase their adhesion ability. Accordingly, diff erent amounts of 1,2,3,4-butantetracarboxylic acid (BTCA) and PNCS were applied to cotton fabric to evaluate the liquid management properties and washing fastness of the coating [7,[9][10][11]. It was found that the PNCS microgel clearly infl uenced the water retention capacity of the fi bres, which was directly dependant on temperature and pH. It did not appear that the presence of BTCA infl uenced the responsive behaviour of the PNCS [11]. Water retention capacity (WRC) increased by increasing the amount of PNCS, even more in the presence of BTCA. Th e PNCS cross-linked with BTCA also showed dual-responsiveness on cotton fabric aft er fi ve repetitive washings [7]. Th e modifi cation of fibres with PNCS microgel impaired intrinsic properties but improved textile quality and wicking, especially at temperatures above the LCST [12]. When the PNCS microgel was embedded into the silica matrix previously created on the polyester fabric using a sol-gel technology, it enhanced the stimuli responsiveness of the fabric in comparison to that modifi ed by PNCS without the presence of the silica matrix [14]. Furthermore, air, argon or nitrogen plasma was also used for cotton surface activation before the application of the PNCS, achieving better incorporation of microgel onto the fi bres [13]. To enhance the PNCS microgel functionality, controlled-release and bio-barrier formation antimicrobial agents have already been successfully incorporated into its structure by our research group and applied to cotton fi bres [8,17]. Th e concept of the present research was to create new functional properties that will complement the thermoregulation and moisture management for comfort improvement of textiles by preparing the PNCS microgel with UV protection and self-cleaning properties. To this end, ZnO was chosen as a very promising candidate that could provide these properties and thus contribute to the added value of the PNCS microgel coating. Namely, according to literature, ZnO is well known as an excellent UV protection and antimicrobial agent in diff erent applications on textile substrates [18][19][20][21][22][23][24][25]. Th e ZnO nanoparticles were also incorporated in diff erent hydrogels, i.e., N,N-methylenebisacrylamide [19], pNIPAAm [20], β-chitin [21] and chitosan [22,25]. Th e fi lm produced by incorporating ZnO nanoparticles into a pNIPAAm hydrogel had a homogeneous distribution of nanoparticles and high antimicrobial properties with no cytotoxicity towards a mammalian cell line [20]. Th e incorporation of ZnO nanoparticles into chitosan fi lm enhanced the antimicrobial activity against E. coli and S. aureus]. Other properties were also improved, such as UV-blocking and swelling behaviour in different pH solutions [25]. In this research, ZnO nanoparticles (NPs) were incorporated into PNCS microgel and applied to polyamide 6 (PA6) fabric with the aim to create pH-and temperature-responsive fabric with UVprotective and self-cleaning properties. Two diff erent functionalization procedures were used. In the fi rst procedure, the ZnO NPs were incorporated into the PNCS microgel, and both components were applied to the PA fabric together. In the second one, PA6 fabric was fi rst coated by the PNCS microgel, followed by the application of ZnO NPs. Furthermore, to increase the adsorption ability of PA6 fabric, the silica matrix was fi rst created on the fi bres in which the PNCS and ZnO NPs were incorporated by both the functionalization procedures. Based on the determination of stimuli responsive, UV protective and self-cleaning properties of the coated fabric, the most eff ective functionalization procedure was chosen.

Synthesis of PNCS microgel
Th e synthesis of the PNCS microgel via free radical polymerization was performed according to the procedure described by Lee et al. [26]. In 300 mL of distilled water and 3 mL of glacial acetic acid, 1.0 g of chitosan was dissolved and placed in a fl ask and degassed with nitrogen for 30 minutes. Next, 7.0 g of NiPAAm and 0.21 g of a cross-linking agent N,N'methylenebisacrylamide were added with intense stirring (320 rpm), and the temperature of the reaction mixture was raised to 50 °C. Aft er 40 min, 0.9 g of ammonium persulfate was added to initiate polymerization, which caused the reaction mixture to turn turbid. Th e reaction proceeded in a nitrogen atmosphere at 50 °C for 3 h. Subsequently, the reaction mixture was dialysed against bi-distilled water using a Spectra/Por 4 membrane (Fisher Scientifi c) for 10 days to remove impurities and unreacted monomers.

Preparation of the ZnO nanoparticles suspension
Th e ZnO solution (3% concentration) was prepared by dispersing ZnO powder in bi-distilled water. Th e solution was stirred for 15 min while 1 mL/L CH 3 COOH 30% was added dropwise. Th en, the solution was sonicated for 30 min.

Functionalization of PNCS microgel and its application to the polyamide 6 fabric
Prior to the deposition of the functionalized microgel, a polysiloxane matrix was applied to the PA6 fabric to assure an even distribution of the functionalized PNCS microgel particles. For this purpose, an aqueous solution of a reactive organic-inorganic binder iSys MTX was used in a concentration of 15 g/l. Th e latter was applied by the pad-dry-cure pro cedure, which included full immersion of the fabric sample, wringing by 60% wet pick up, drying for 1 minute at 100 °C and curing at 150 °C for 5 minutes. Functionalization of PNCS microgel was obtained using two approaches. In the fi rst approach, the microgel was functionalized directly in the suspension and applied to the PA6 fabric. In this case, the suspension of the microgel was heated to 50 °C (above the LCST of poly-NiPAAm). Th e expelled water was removed and replaced with the same amount of ZnO NPs suspension. Aft erwards, the obtained suspension was cooled to 20 °C to stimulate the microgel swelling and thus absorb the ZnO NPs into the PNCS microgel. Th e process of heating and cooling was repeated twice. Th e functionalized microgel was then applied to the PA6 fabric, using the paddry procedure, which included the full immersion of the fabric sample into the corresponding suspension, wringing by 60% wet pick up and drying for 1 minute at 100 °C.
In the second approach, the microgel suspension was fi rstly applied to the PA6 fabric, using the paddry process and the same parameters as described in the fi rst approach. During drying, the fabric samples were exposed to hot air which resulted in the de-swelling of the PNCS microgel particles. Drying was followed by an immediate immersion of the

Scanning Electron Microscopy (SEM)
SEM was conducted on a JSM-6060 LV (JEOL, Japan). Prior to the SEM analysis, samples were coated with a layer of gold to ensure suffi cient electrical conductivity. SEM micrographs were taken at 1000x and 3000x magnifi cation.

Fourier Transform Infrared Spectroscopy (FT-IR)
FT-IR spectra were obtained on a FT-IR spectrometer (Perkin Elmer, UK). Th e spectra were recorded over the range of 4,000-600 cm -1 with a resolution of 4 cm -1 . An average of 16 spectra were taken per sample.

Moisture content
Temperature responsiveness was assessed by measuring the moisture content (MC) using a Moisture analyser MLB 50-3C (Kern and Sohn, Germany). Studied samples were pre-conditioned at 65% relative humidity at 20 and 40 °C for 24 hours before they were put in a moisture analyser and dried at 50 °C until constant mass was reached. Th e value of MC was determined by the following equation: where m 0 is the mass of the preconditioned sample and m f the fi nal mass of the sample aft er drying. For each sample, fi ve measurements of MC were obtained and presented as the mean value with standard deviation.

Water vapour permeability
Water vapour permeability was determined according to the standard method UNI 4818-26 and performed at 65% relative humidity at 20 and at 40 °C. Th e water vapour transmission rate (WVTR) was calculated using the following equation: where Δm is the change of mass, S is the testing area of the fabric sample (actual value 7 cm 2 ) and t is the time of testing (actual value 24 h). For each sample, three measurements were performed and the mean value was calculated.

Water uptake
To evaluate the dual pH and temperature responsiveness of the PNCS microgel, the samples of known mass were immersed for 30 minutes in the buff er solutions of pH 3 at 20 °C and of pH 10 at 40 °C. Aft erwards, the samples were taken from the buff ers, and their mass was determined. Water uptake (WU) was determined by the following equation: where m w is the weight of the sample taking up water and m o is the initial weight of the sample.
where E λ is the relative erythemal spectral eff ectiveness, S λ is the solar spectral irradiance, T λ is the spectral transmittance of the specimen, and Δλ is the measured wavelength interval in nm. Th e UPF rating and UVR protection categories were determined from the calculated UPF values according to the Australian/New Zealand Standard: Sun protective clothing -Evaluation and classifi cation.

Photocatalytic self-cleaning effi ciency
Th e photocatalytic self-cleaning properties of the studied samples due to the presence of ZnO nanoparticles were determined by measuring the fading of coff ee stains when exposed to UV light. Samples were immersed in a 2.5% coff ee solution and air dried under ambient conditions. Th e photocatalytic activities of the samples were assessed by monitoring the degradation of the coff ee stain aft er 3, 6 and 9 hours aft er the exposure of the samples to artifi cial light at 65 ± 2% relative humidity and 20 ± 1 °C in a Xenotest Alpha (Atlas, USA), equipped with a xenon lamp with an adjustable radiation power region 0.8-2.5 kVA and an expanded range of radiation (300-400 nm). An assessment of the degradation of the stain was performed using the refl ectance measurements, R, of the studied samples on a Datacolor Spectrafl ash SF 600 spectrophotometer (Datacolor, Switzerland) using D 65/10° light. For each sample, ten measurements of the R value were obtained, and the corresponding K/S values were calculated according to the Kubelka-Munk equation [27]: where K S is the ratio of the coeffi cient of light absorption (K) to the coeffi cient of light scattering (S) and R is the refl ectance at 400 nm. Based on the K S values, the stain degradation degree expressed as a ΔK S value was determined according to the literature [27,28]: where K S 1 belongs to the non-illuminated sample and K S 2 to the illuminated sample.

Coatings characterization
Th e SEM images in Fig. 1 reveal the morphology changes of the PA6 fi bre surface caused by the applied coatings. Whereas the fi bre surface of the PA and PA-Si samples are smooth, indicating that the presence of the silica matrix did not signifi cantly change the topography of fi bres, with respect to the magnifi cation scale used, the application of PNCS increased the structured roughness of the fi bre surface. Bright spots which are clearly visible on the PA(ZnO) sample are attributed to the ZnO particles, which confi rm that smaller to larger agglomerates of ZnO nanoparticles were formed on the fibres. ZnO agglomerates can also be seen in the presence of PNCS, which suggests that the particles were entrapped in the structure of the microgel. Chemical changes of the studied samples were determined using ATR FT-IR spectroscopy (Fig. 2). Sample PA displays absorption bands characteristic for the amide groups, which are present at 1634 cm -1 (amide I), 1535 cm -1 (amide II) and 1262 cm -1 (amide III) [29,30]. Additionally, absorption bands characteristic for the secondary NH group appear at 3291 cm -1 and 3088 cm -1 , as well as CH 2 asymmetrical stretching at 2924 cm -1 and 2854 cm -1 , absorption band characteristic for C=O group at 1744 cm -1 , a set of the relatively weak bends associated with the fi ngerprint of aliphatic polyamides in the region 1500-900 cm -1 and an absorption band at 689 cm -1 , which can be ascribed to the bending motion of the O=C-N group are clearly evident [29]. Similar spectra were obtained for all of the coated samples, although the intensities of the aforementioned bands decreased. Th is phenomenon was most prominent at 2924 cm -1 , 2854 cm -1 and 1744 cm -1 . Absorption bands, ascribed to PNCS and Si coatings were overlapped by those of PA6 [29]. Namely, characteristic bands of PNCS microgel, contributed by the presence of N-H stretching vibrations appear at 1456 cm -1 and the bands characteristic for amide I and amide II appear at 1645 cm -1 and 1535 cm -1 , while siloxane chains in the silica matrix could be seen by the formation of an absorption band at 1085 cm -1 . Due to the overlapping, no additional bands indicating the presence of PNCS and Si coatings could be observed. Hence, ZnO particles could not be detected from the ATR spectra.

Functional properties of the coated samples 3.2.1 Temperature responsiveness
To determine the temperature responsiveness of the PNCS microgel on the samples, moisture content (MC) (Fig. 3) and water vapour transmission rate (WVTR) (Fig. 4) at 20 °C and 40 °C (i.e., below and above the LCST of poly-NiPAAm) were investigated. Th e swelling of the hydrophilic PNCS at 20 °C caused an increase in the MC values for all microgel coated samples, with respect to the PA and PA-Si samples. Increasing the temperature from 20 °C to 40 °C led to the de-swelling of the PNCS microgel, which resulted in water expulsion from the hydrophobic microgel and, consequently, a decrease in the MC values. However, the results in Fig. 3 show that the MC values of the PA(PNCS-ZnO), PA/ (PNCS+ZnO), PA-Si(PNCS-ZnO) samples, gathered at 40 °C were unexpectedly higher, compared to the values of the PA and PA-Si samples. Only the PA-Si(PNCS+ZnO) sample possessed a lower MC value, compared to the untreated PA6 sample. Such behaviour could not be easily explained, although these results suggest that the incorporation of ZnO particles into the microgel slightly hindered the deswelling ability of the PNCS microgel. Th e de-swelling ability was most hindered, when PNCS was previously functionalized with ZnO nanoparticles and subsequently applied onto the PA6 fabric. pores between the fi bres and yarns in the fabric structure enlarged. Without a doubt, the MC results suggest that the coating created by the application of iSys MTX followed by the application of PNCS with the subsequent application of the ZnO nanoparticles suspension has the highest temperature responsiveness.

Dual temperature-and pH-responsiveness
Th e WU results presented in Fig. 5 show the dual temperature and pH responsiveness of the PNCS microgel. At pH value 3 and the temperature of 20 °C, both polyNiPAAm and chitosan within the microgel were in hydrophilic form, thus the microgel reached its fully swollen state. On contrary, at pH 10 and 40 °C, both polyNiPAAm and chitosan were in hydrophobic state and the microgel was in the fully deswollen state. As seen from Fig. 5, the presence of PNCS on the coated samples increased water uptake by 7-12% when samples were immersed in acidic solution at room temperature. Th e highest increase in WU was obtained for the PA-Si(PNCS+ZnO) sample. In contrast, aft er the samples were immersed in an alkaline buff er solution at 40 °C, the WU values did not vary signifi cantly, irrespective of the present coating, meaning that the WU of the coated samples were similar to the WU of the untreated PA sample, when PNCS was hydrophobic. Th ese results are reasonable since PA6 fi bres are hydrophobic by nature and therefore are poorly swollen and wetted in water. Results are contrary to the one obtained in the case of PNCS microgel applied on the hydrophilic cotton fi bres, where the water absorption to the amorphous region of the fi bres could have also occurred, despite the presence of the de-swelled microgel on the fi bre surface [10,17].

UV protection
Th e results of the UV protection properties calculated for the untreated and coated samples ( Table 2) show that the lowest UPF values, equal to 4.79 and 4.68, were obtained for PA and PA-Si samples, respectively, indicating that these samples exhibited insuffi cient UV protection. Th is suggests that no UV active additives were included in the melt spinning process of the PA6 fi bres and that the silica matrix created on the fi bres surface did not act as a UV absorber. Th e UPF of the samples increased in the presence of ZnO, which represented a UV active component in the coating. Accordingly, the UPF value of the PA(ZnO) sample increased to 12.46 which is rated as good UV protection. It is also clearly evident from the results that the fi nishing procedure significantly infl uenced the UV protection properties of the fi bres.

Photocatalytic self-cleaning
Th e results of the coff ee stain degradation as a measure of photocatalytic self-cleaning properties of the coatings due to the presence of ZnO nanoparticles are presented in Fig. 6. It was observed that during the illumination of the samples, the colour of the coff ee stains became lighter because of their photodegradation, which caused a decrease in the K/S values of the samples. According to equation 5, the greater the decrease in the K/S value is, the higher the ΔK/S value is, and the greater is the photocatalytic self-cleaning effi ciency of the coating. Th e degradation rate of the coff ee stain was significantly higher on the coated samples, with respect to the PA sample. Th roughout the whole period of the illumination, the maximal degree of stain degradation was observed in the case of the PA-Si(PNCS-ZnO) and PA-Si(PNCS+ZnO) samples with almost the same ΔK/S values, which were much higher than those obtained for the PA(ZnO), PA(PNCS-ZnO) and PA(PNCS+ZnO) samples. Th is suggests that the presence of the silica matrix had a greater eff ect on the ZnO photocatalytic self-cleaning properties, than the method of incorporation of the ZnO particles into the coating. It is well known that SiO 2 does not exhibit a photocatalytic self-cleaning activity [31], therefore the most reasonable explanation for this phenomenon is that the silica matrix increased the uniformity of the PNCS and ZnO coatings on the PA6 fabric and thus contributed to the behaviour of the coatings.

Conclusion
In this work, coatings consisting of temperatureand pH-responsive PNCS microgel functionalized with ZnO were successfully fabricated on PA6 fabric, using diff erent application procedures. Th e results show that the swelling caused the increase in the MC values for all samples with the microgel coating compared to the untreated PA and PA-Si samples Th e hydrophilic and hydrophobic character of PNCS was clearly shown when the swelling and de-swelling ability of the microgel was investigated in a buff er solution at pH 3 and 20 °C when pNi-PAAm and chitosan components were in hydrophilic form and at pH 10 and 40 °C when polyNiPAAm and chitosan components were in hydrophobic form. Th e presence of ZnO provided the UV protection and photocatalytic self-cleaning properties of the coatings. Th e results also show that the application procedure of the coating greatly infl uenced its stimuli responsiveness and functionality. Th e highest swelling/de-swelling ability with simultaneous good UV protection and the most eff ective photocatalytic self-cleaning properties was obtained for the coating that was created by the application of PNCS to the PA6 fabric previously modifi ed by the silica matrix followed by the subsequent application of ZnO particles to the PNCS.