3.1. Characterization of the PHEVAH polymer
18.104.22.168H-NMR analysis of the polymer
Chemical structure of the new monomer HEVAH and the polymer PHEVAH synthesized was examined by 1H-NMR analysis. The 1H-NMR spectra were shown in Fig. 1. The protons belonging to vinyl groups displayed as "b" and "c" in the molecular formula of the monomer appeared at 4.36 ppm ad 4.59 ppm, respectively. CH2 groups, indicated by "a" in the molecular formula, appeared at 1.53 ppm in the spectrum. CH2 groups (e) close to the N-H group were revealed at 1.67 ppm, and CH2 groups (f) close to the C = O group were revealed at 2.57 ppm. The peak revealed at 7.25 ppm in the spectrum of the monomer belongs to the N-H group (d). While the proton in the alcohol group was seen at 0.9 ppm, the CH2 groups near the alcohol group (g) were revealed at 3.59 ppm. As seen from the spectrum of the polymer, the peaks of the monomer were displaced in the spectrum of the polymer. Additionally, it was determined that the peaks of vinyl groups did not completely disappear, but their density decreased significantly. The peak that appears at 4.80 ppm in the spectrum of the polymer was the characteristic peak of dimethylsulphoxide used as a solvent for analysis. The findings showed the PHEVAH polymer synthesis was successful even though the synthesized polymer sample containing monomer residue.
3.1.2. Molecular weight of PHEVAH Polymer
The number average molecular weight of the PHEVAH polymer was determined as 4000 g/mole. According to the value, the synthesized polymer was concluded to have low molecular weight.
3.1.3. LCST point of the PHEVAH polymer
The temperature-sensitive polymers are insoluble in water at temperatures above their LCST because of increasing interaction between hydrophobic groups in their molecules. Interaction between their hydrophobic groups causes the phase separation in their aqueous solutions and their solution becomes to be turbidity (Xiao et al. 2017). The temperature at which the aqueous solution of the temperature-sensitive polymer began to cloudy is accepted as its LCST. In the study, LCST values of the PHEVAH polymers were determined by analyzing the images of their aqueous solutions taken at temperatures in the range of 30°C and 40°C. As seen from Fig. 2, turbidity of polymer solution started almost 34–35°C and its LCST value was determined as 34°C, which is close to human body temperature (Demirbağ et al. 2017).
3.2. Characterization of the PHEVAH polymer incorporated fabric
3.2.1. Grafting yield of the PHEVAH Polymer
The amount of polymer applied to the fabric structure was determined as grafting yield. The values were calculated as 11.43% for CF-PHEVAH-3 fabric and 13.34% for CF-PHEVAH-5 fabric. The increase in the polymer concentration applied to the fabric slightly increased the amount of polymer transferred to the fabric.
3.2.2. SEM micrograhs of the PHEVAH incorporated fabrics
The SEM images of the untreated fabric CF-PHEVAH-3 and CF-PHEVAH-5 were given in Fig. 3. According to the SEM images, the surface of the fabrics treated with polymer was covered. However, the coating on the CF-PHEVAH-3 fabric surface was not homogeneous and contained polymer fractions associated with lower polymer concentration. The fiber surfaces on the CF-PHEVAH-5 fabric were smoother and homogeneously coated and the polymeric coating substantially filled the spaces among the fibers.
3.2.3. Temperature-sensitive wetting property of the PHEVAH polymer incorporated fabrics
The temperature-sensitive hydrophility of the fabrics were examined their sinking behavior in the water and the wetting time test. Both untreated fabric and polymer applied fabrics sank in water with 20°C and exhibited hydrophilic character (Fig. 4). Unlike the untreated fabric, polymer applied fabrics suspended on the surface of water at 40°C. Decreasing adhesion force between the PHEVAH polymer and water molecules caused to exhibit the fabric hydrophobic character. This finding was approved by wetting time test. There was no effect of the change in temperature on the wetting time of the untreated fabric and it was determined as 0 s at both temperatures. However, wetting time of the PHEVAH-5 fabric rose from 3 s at 20°C to 47.80 s [with a standard deviation of 3.90] at 40°C, and its hydrophilic character changed to hydrophobic as a function of the increasing temperature. A similar change was also confirmed for fabric with a lower concentration of polymer applique. The wetting time of the CF-PHEVAH-3 fabric was measured as 3.40 s at 20°C and 31.00 s [with a standard deviation of 7.20] at 40°C (Demirbağ-Genç et al. 2019).
In the study, contact angle measurements on the fabrics having different temperatures was also carried out to evaluate their temperature-sensitive wettability behaviors and the results were given in Fig. 5a. The contact angle values of the untreated fabric used as control fabric couldn't measure because of its high hydrophility. The contact angle of the polymer-applied fabrics increased especially between 30–40°C which is around the LCST value (34°C) of PHEVAH polymer. The maximum angle was measured as 64.25° and 49.95° for CF-PHEVAH-3 and CF-PHEVAH-5, respectively. The increase in the contact angles resulting from rising in temperature indicated the wettability decreased. However, it was not high enough to explain the hydrophobic character (θ > 90°) as expected. The reason which the expected high contact angle value could not be reached despite the hydrophilic character of the polymer on the fabric surface was related to the decrease in the surface smoothness of the fabric as a result of the polymer coating. Similar result was obtained for PNIPAM polymer grafted cotton fabric in our previous studies (Demirbağ and Alay-Aksoy 2019; Demirbağ and Alay-Aksoy 2021). Especially with increasing polymer concentration, a smoother surface was obtained, which caused the contact angle of CF-PHEVAH-5 to be lower. Especially with increasing polymer concentration, a smoother surface was obtained, which caused the contact angle of CF-PHEVAH-5 to be lower (Fig. 3).
3.2.4. Determination of the temperature-sensitive water uptake property of the fabrics
Figure 5b shows the water absorption values of the fabrics. In the study, pretreated fabrics (cationized fabric and BTCA applied fabrics) were used as control samples. As seen from the figure, the water uptake values of the pretreated fabrics were not affected by the temperature increase. However, especially at high concentrations, BTCA application significantly reduced the water absorption capacity of fabrics regardless of temperature change (from 110% for cationized fabric to 70% for BTCA 5% concentration). This decrease was a result of the esterification reaction of the free hydroxyl groups interacting with the water molecules in the structure of the cellulose with BTCA (Alay-Aksoy and Genç 2015; Ji et al. 2019).
On the other hand, the sudden decrease in water uptake values of the polymer applied fabrics as a function of the increase in the temperature was observed. The decrease carried out between 29°C and 37°C for the CF-PHEVAH-3 and between 29°C and 33°C for the CF-PHEVAH-5 (Fig. 5b). The lowest water uptake values of the CF-PHEVAH-3 and CF-PHEVAH-5 fabrics at temperatures above LCST were detected as 54.2% and 50%, respectively. In contrast to the pretreated fabrics, the decrease in the absorption capacity of the polymer applied fabric was a result of becoming dominant of hydrophobic groups in the PHEVAH polymer macromolecule above the LCST. This finding indicated the synthesized PHEVAH polymer incorporated cotton fabrics exhibited comparable temperature-sensitive water absorption property to the PNIPAM applied cotton fabric. In our previous study, cotton fabric containing PNIPAM changed the hydrophilic character of PNIPAM around the LCST value. The difference in water absorption ability of low concentration (3%) PNIPAM grafted cotton fabric at temperatures below and above the LCST value was approximately 41%. (Demirbağ-Genç and Alay-Aksoy, 2021) In another study in the literature, the water absorption ability of cotton fabric incorporating 16% concentration poly(2-(2-methoxyethoxy)ethoxyethyl methacrylate-co-ethylene glycol methacrylate decreased by 34.6% (Chen et al. 2017). In this study, this decrease for low concentration PHEVAH polymer-containing fabric was 52%. From these findings, it was concluded that the decrease in the water uptake ability of the fabric due to the increase in temperature was more pronounced compared to other studies.
The water uptake test results after repeated washings were given in Fig. 5c to investigate the durability of the temperature-sensitive properties of the fabrics. As seen from the Fig. 5c, temperature-sensitive water uptake ability of the fabrics was not significantly influenced from repeated washings until 5th wash. The fabrics kept their ability until 5th wash. However, after tenth washing, the difference between water absorption capacity values of the fabrics at temperature above and below the LCST value diminished and their temperature-sensitive water uptake features were weakened.
3.2.5. Determining of the temperature-sensitive water vapor permeability of the fabrics
To investigate the temperature-sensitive water vapor permeability of the fabrics, the water vapor permeability test was performed at 20°C (T < LCST) and 40°C(T > LCST), and the results were presented in Fig. 6. The untreated fabric had the highest water vapor permeability at 20°C. Cationization process and BTCA application caused the decrease in the water vapor transmission of the fabric significantly. Hydrophobic character of the fabric increased by the reduction of the OH groups in the cellulose molecule as a results of the reaction with cationizing agent and BTCA. Additionally, crosslinking of the fibers caused to block the pores of the fabrics resulting hindering the water vapor permeability. However, PHEVAH polymer application on the pretreated fabric caused to increase water vapor permeability of the fabrics. The CF-PHEVAH-3 fabric exhibited statistically the same water vapor permeability as the untreated fabric (p > 0.05). This increasing resulted from the increasing hydrophility of the fabric after polymer application but it limited by fabric pores closed by swelling polymer molecules at higher concentrations (CF-PHEVAH-5).
Above the LCST (at 40°C), water vapor permeability of the fabrics increased because of the evaporation of more water molecules at high temperature compared to the measurement results obtained at 20°C. According to the test results at 40°C, the water vapor permeability of the fabrics treated with BTCA decreased significantly compared to the untreated fabric (p < 0.05) However, polymer application increased the permeability of the pretreated fabrics and tolerated the loss caused by pretreatments. Even, the water vapor permeability of the CF-PHEVAH-3 fabric (5593,00 g/m2/h) increased over the untreated fabric (5104.75 g/m2/h) and the difference was statistically significant. Above the LCST, polymer macromolecules on the fabric returned from swollen state to puckered state and the fabric pores were opened resulting in more water molecules passes through. However, unexpectedly, the water vapor permeability of the CF-PHEVAH-5 fabric was less than the untreated and CF-PHEVAH-3 fabrics. Water vapor permeability is related to porosity and hydrophilicity (Save and Jassal 2005). The smoothly covering the fabric surface by high amount of polymer in the CF-PHEVAH-5 fabric (seen in SEM images) limits the porosity opened by shrinkage of the polymer, resulting in less water vapor transmission.
As a result, the CF-PHEVAH-3 and CF-PHEVAH-5 fabrics exhibited temperature-sensitive water vapor permeability changing their hydrophilic character and porosity as function of temperature. The increasing hydrophilic character and shrinkage of the polymer molecules were key parameter effecting the thermo-responsive water vapor permeability of fabrics. The hydrophilic character of the polymer below the LCST caused to increase the water vapor permeability of the fabrics. The shrinkage of the polymer molecules on the fabric above their LCST increased the porosity of the fabric and accordingly water vapor permeability. In the literature, there was finding that the water vapor permeability of the cotton fabric grafted by poly(vinylcaprolactam-co-hydroxyethyl acrylamide) polymer using BTCA decreased at elevated temperature (Xiao et al. 2017). However, generally the temperature-sensitive polymers including PNIPAM and PVCL and their copolymers provide to increase the water vapor permeability at temperatures above the LCST (Demirbağ and Alay-Aksoy 2019: Demirbağ-Genç and Alay-Aksoy, 2021; Sun et al. 2017). Additionally, in this study it was concluded the low concentration PHEVAH polymer incorporated fabrics exhibited important level higher water vapor permeability property above the LCST compared to untreated fabric. However, other studies concerning thermo-responsive polymer applied fabric reported the water vapor permeability was lower compared to untreated fabric. Here, it was proved the synthesized polymer offers an opportunity to obtain significantly higher water vapor permeability at high temperatures compared to untreated fabric (Sun et al. 2017; Xiao et al. 2017).
3.2.6. Test results of the bending rigidity and tear strength of the fabric
The effect of polymer application on the bending rigidity and tear strength of the fabric was investigated and the test results were given in Table 1. PHEVAH polymer application had no significantly affect the bending rigidity of the fabric (p > 0.05). However, the treatment with BTCA with 5% concentration caused to increase the bending rigidity significantly (p < 0.05) because of limiting the movement of fiber elements cross-linked by BTCA. Nevertheless, the CF-PHEVAH-5 fabric had similar bending rigidity statistically with other fabrics, although the BTCA-5 fabric had statistically highest rigidity. The PHEVAH polymer application resulted in a reduction of the bending stiffness of the fabrics. With this finding, it was concluded that the PHEVAH polymer formed a flexible coating on the fabric surface.
In this study, the effect of polymer application on the fabric tenacity was assessed by tear strength test. Tearing strength is described as the sequential breakage of yarns or groups of yarns. The restricted movement of the yarns in the fabric results in the lower tear strength. On the contrary, loose and open fabric constructions allow yarns to move and group together, resulting in a high tear strength (Eryürük and Kalaoğlu 2015). Additionally, decrease in tear strength may be attributed to acidic pH of the application bath as well as reduced yarn slippage to resist tearing. Normally, bending rigidity and tear strength are related to each other and the increasing bending rigidity causes the yarns to break one by one and reduces the warp tear strength (Bulut and Sular 2015; Kim et al. 2002). However, while the bending stiffness of the PHEVAH-containing fabrics did not increase significantly, the tear strength decreased significantly (p < 0.05). This reduction was associated with reduced yarn strengths due to the acidic pH of the application bath (pH 4).
The bending rigidity and warp tear strength test results
Bending rigidity test results (mg.cm)
Warp Tear strength test results (N)