3.1 Effects of ultrasonic-alkali treatment on morphology and structure of BN-OH nanosheets
BN-OH nanosheets were prepared through the combined use of ultrasound and alkali treatments, aiming at constructing continuous thermal paths onto to the target substrate of cotton surface. As shown in Fig. 2A1, the few-layered BN-OH nanosheet can be observed due to its ultimately thin shapes. Meanwhile, some of the stacked BN-OHs with single or a few layers in a small size were also detected, exhibiting a lateral size of the aggregation state less than 350 nm (Fig. 2A2) (Tian et al., 2019). Stability of the BN suspensions treated with ultrasound and alkali were also examined, and the observed appearances after storing for different days are presented in Fig. 2B. Both of the pristine dispersion and the ultrasound-treated exhibited stratified appearance after standing for 3 days, while the BN-OH nanosheets dispersion with ultrasound-alkali process kept uniform appearances within 3 days and slightly stratified until 5 days. Figure 2C illustrates the particle sizes of the above-mentioned three BN suspensions, which are mainly distributed in the ranges of 400–1900 nm, 200–700 nm, and 200–700 nm, respectively. The result is in good agreement with that in Fig. 2B, supporting the effective exfoliation of BN during the treatment (Zhu et al., 2014).
Characteristic structures of different BN samples are depicted in Fig. 2D, two strong FTIR bands at approximately 1343 cm− 1 and 807 cm− 1 are observed for the three samples, mainly attributed to the in-plane stretching and the out-of-plane bending mode of B-N (Xiao et al., 2015). Furthermore, a characteristic peak at 3440 cm− 1 appears in the spectra of BN treated with ultrasound-alkali process, revealing the successful introduction of hydroxyl groups into the nanosheets. According to the results in Fig. 2, it can be inferred that ultrasound mainly peels off BN to form nanosheets, while alkali treatment might introduce more hydrophilic hydroxyl groups exposed on the nanosheet surfaces.
It is well-known that BN has good thermal conductivity, thus it can be used to endow the BN-based composites with improved thermal conductivity. In our experiment, optical and infrared test systems were constructed to examine the thermal properties of the prepared BN-OH nanosheets. As the model of textiles, the filter paper with and without BN were used, the surface temperature of each was recorded every 5 s upon heating at 40°C, and the results are shown in Figure S1. When the filter paper was directly placed on the graphite heating plate, energy from the heating plate might be absorbed, which is similar to that the body continuously transfers a certain amount of heat from skin surface to the clothing. It can be seen that the surface temperature of filter paper containing BN-OH was a bit higher than that of the neat filter paper (about 0.7°C) no matter how long it was placed, indicating that BN-OH achieved a good heat transfer effect. To further investigate the effect of BN-OH concentration on the thermal conductivity, an experimental device was constructed (Fig. 6A) (Y. Yang et al., 2021), and the change in the internal temperature of the fabric was monitored by using the probe to evaluate the thermal conductivity of BN-OH. The results in Figure S2 reveals that the fabric immersed in 10 g/L of BN-OH nanosheets displays the highest internal temperature and the best thermal conductivity. This can be explained as that the heat conduction paths can hardly be efficiently formed with low content of BN-OH, owing to the lack of close overlaps between the nanosheets. In contrast, excessive dosage of BN-OH may block the small gaps between warp and weft yarns, which will lead to unacceptable blocking of heat convection. Thus, 10 g/L BN-OH as the optimal dosage was selected for the subsequent processing.
3.2 Morphology and structure of the prepared temperature-regulating cotton fabric
To investigate the thermal response of PNIPAM, the transmittances of the polymer aqueous solutions were measured at different temperatures. Figure 3A reveals that the transmittance of the polymer aqueous solution starts to decrease at 30°C and reaches the minimum at 32°C. Considering that the LCST of a thermo-responsive polymer reflects the temperature at which the transmittance of an aqueous polymer solution is 50%,(Ma et al., 2010) the PNIPAM solution exhibited a transparent appearance at temperatures lower than 32°C, then turned to be milky white at a temperature higher than LCST. Figure 3B shows the change in the sample length of PNIPAM hydrogel, it shrinks from 3.5 to 2.2 cm when the temperature rises from room temperature up to 50°C. Figure 3C shows the structure and temperature response mechanism of PNIPAM, PNIPAM shows a stretched coil structure at low temperature, due to the hydrogen bonds and van der Waals forces from the hydrophilic interaction of amide bonds of PNIPAM and water molecules. With the increase of temperature, the polymer formed a compact colloidal structure owning to the entropy of the polymer system, and the hydrophobic interactions between molecules both increased.
The photographs for the fabric samples, colored with iodine solution are shown in Fig. 4A. After placing the treated sample in dark for 20 min, the color of the vinylaed cotton significantly faded, while the color of the untreated did not change. Structural characteristics of the cotton fabric were analyzed by FTIR, and the vinylated cotton fabric shows the peak at 1750 cm− 1 corresponding to the C = O stretching vibration, indicating that vinyl groups were successfully grafted onto cotton fibers through the esterification with anhydride. The introduction of double bonds onto the fiber surfaces were also verified by iodine-mediated chromogenic reaction. The infrared spectrum for the sample of Cotton-NIPAM is shown in Fig. 4B, the new strong peaks appear at 1640 cm− 1 and 1545 cm− 1 corresponds the stretching vibration of the amide bond in PNIPAM, verifying the formation of the phase-change polymer.
The intelligent temperature-regulating cotton fabric, containing the phase-change material of PNIPAM and thermal conductive BN-OH nanosheets was prepared, and the surface morphology together with comparison samples are shown in Fig. 4C. The cotton fiber without any treatments exhibits a smooth surface, while some flaky fragments distributed on the surface of the fiber was observed for the sample of Cotton-BN-OH. After enzymatic graft polymerization of PNIPAM onto the vinylated cotton, a thin film of sediment scattered on the fiber surfaces. For the fabric based on the combined use of BN-OH and PNIPAM, the fiber surfaces appeared the composite morphology of BN-OH wrapped by PNIPAM at room temperature, companying with moderate porosity inside of fabric yarns.
Air permeability of the cotton fabrics with different treatments were also examined, and the results are shown in Fig. 4D. For the samples of the untreated and cotton-BN-OH, the air permeability reached 202.16 and 186.12 mm/s, respectively. Meanwhile, no obvious changes in air permeability were detected under the two different temperatures of 40°C and 25°C. After graft polymerization of PNIPAM, air permeability of the fabric sample was slightly reduced compared that of the untreated, mainly owing to the reduced porosity in the composite fabric. Moreover, as shown in Movie S1, a large amount of white smoke emerges from the glass bottle containing HCl covered with cotton-PNIPAM-BN-OH fabric, illustrating that the air permeability of the composite fabric is still maintained at an acceptable level. Meanwhile, the air permeability at high temperature was remarkably higher than that at low temperature. For better understanding the synergistic heat transfer mechanism of BN-OH and PNIPAM, Fig. 4D depicted the mechanism on the above phenomenon. At low temperatures, PNIPAM slowly swells after absorbing water molecules, which accordingly reduces the gap between yarns and to improve the heat insulation effect of the fabric. On the contrary, PNIPAM might shrink at high temperatures, which enlarges the gap sizes between fibers and yarns as well, endowing fabric with improved thermal convection in the vertical direction. The proposed mechanism meets the complex requirements of human body temperature regulation in different environments.
3.3 Thermal conductivity behaviors of the prepared temperature-regulating cotton fabric
Among the three forms of heat transfer pathways (i.e., conduction, convection, and radiation) of thermally regulating textiles, heat conduction plays an important role. When the heat is transferred by conduction, the energy is dissipated outward from the human body through clothing to the outside environment (Gao et al., 2017). Figures 5A1 and 5B1 illustrate the schematic of the thermal conductivity measurement. The determined thermal diffusivities for different cotton fabrics are shown in Figs. 5A2 and 5B2. It can be seen that both BN and PNIPAM might increase the thermal diffusivity of cotton fabrics to some extent, regardless of in vertical and parallel directions. For the sample combinedly treated with PNIPAM and BN-OH, when the ambient temperature was at 40°C, the thermal diffusivities for the sample reaches 1.7 and 1.3 times in vertical and parallel directions, respectively, mainly owing to the synergistic effects of the PNIPAM and BN-OH nanosheets. PNIPAM on the fiber surfaces tended to shrink because of its temperature response, resulting in the formation of heat conduction path and the improvement of the thermal diffusion coefficient accordingly (Fig. 5A3, 5B3). The combined use of PNIPAM and the BN-OH have a synergistic effect in promoting the heat conduction effect of the cotton fabric, realizing the switch between heat conduction and preservation at different temperatures.
The thermal conductivity behaviors of the composite cotton fabrics were also examined by detecting the dynamic temperature changes during heating and cooling. As shown in Fig. 6A, the treated cotton fabric was placed in an oven and a refrigerator, respectively, and the temperatures inside the wrap composite fabrics were recorded in real time by a probe. As can be seen Fig. 6B, when the fabric samples were placed at 55°C, the cotton fabric containing PNIPAM and BN-OH exhibits remarkably higher heat transfer efficacy than others except for cotton-BN-OH. While storing at 4°C, the temperature inside the center of the composite fabric rose more slowly (Fig. 6C), revealing the certain heat preservation of the fabric at low temperatures.
The differences in thermal conductive performances for the cotton fabrics were further explained in Fig. 6D. For the composite cotton fabric, the NIPAM polymer covering fiber surfaces swells at low temperatures and accordingly reduces the air permeability, which endows the fabric with better insulation effects. Comparatively, high temperature leads to a shrinkage of phase-change material and improved air permeability, companying with the formations of thermal conductive path with overlapping nanosheets.
3.4 Application performances of the temperature-regulating cotton fabric
For the composite cotton fabric, the NIPAM polymer covering fiber surfaces swells at low temperatures and accordingly reduces the air permeability, which endows the fabric with better insulation effects. Comparatively, high temperature leads to a shrinkage of phase-change material and improved air permeability, companying with the formations of thermal conductive path with overlapping nanosheets.
Static cooling effects of the prepared cotton textiles in actual application were examined according to the reported methods (Gao et al., 2017). Cotton fabrics with different treatments were placed onto the back of hand, respectively, then the temperature distribution of each sample was recorded by an infrared camera (Fig. 7A). The results indicated that the outer surface temperature of cotton-PNIPAM-BN was a bit higher than that of others, which is close to the normal temperature of human skin (Fig. 7B), revealing that effective cooling effect was achieved. Similar results are depicted in Fig. 7C, the sample of cotton-PNIPAM-BN-OH has more encouraging dynamic cooling effect than the other samples. Therefore, the heat generated by the human body can be effectively transferred via using cotton-PNIPAM-BN-OH fabric.
Other wearability performances for the composite cotton fabric were also concerned. Figure 8 shows the results of UV resistance ability, whiteness, wettability, and mechanical behavior. The combined use of BN-OH and PNIPAM imparted an excellent protective effect to the composite fabric, and the UPF reaches approximately 53. Meanwhile, introduction of PNIPMA and BN-OH did less impacts to the color appearance to the cotton fabrics, and all of the measured Hunter whiteness were at approximately 83%.
Wettability of fabric samples were measured, and expressed as wetting height and the static water contact angle (WCA) at room temperature. For the composite fabric of cotton-PNIPAM-BN-OH, although a slight decrease in the wettability is observed in Fig. 8C, the wetting height still meets the basic requirement of textile wettability (approximately 8 cm). The photograph in Fig. 8D shows the WCA of different samples, the water droplet on the BN-cotton fabric maintained a spherical shape for less than 1 s before collapsing, and the spherical shape of the droplet collapsed altogether within 3 s on the PNIPAM-cotton fabric, and maintained a spherical shape for 4 s after contacting with cotton-PNIPAM-BN-OH fabric. The slight decrease in the hydrophilicity can be explained that the amide bond in the PNIPAM structure is able to combine with water molecules to form intermolecular hydrogen bonds at low temperatures, and the ability of amide bonds to bind water is weaker than that of hydroxyl groups. When the cotton fabric is modified, the hydroxyl groups of the internal fiber structure cannot directly contact the outside because of the tight arrangement of PNIPAM chains. Water molecules are required to pass through the PNIPAM layer to enter the fiber. However, thanks to the tight structure and the relatively weak ability of the amide bond to bind water, the process of water molecules entering the interior of the modified cotton fabric is relatively slow. (Bai et al., 2019) The results in Figs. 8E reveal that introduction of BN-OH and PNIPAM also increases the bending rigidity of the fabric to some extent, mainly owing to the extra component phase-change polymer and nanosheets.