Three kinds of poly(DMC-co-MA) copolymers were prepared via free radical copolymerization of DMC and MA with a variety of DMC/MA mole ratios (Fig. S1). Fig. 1 compares the 1H-NMR spectra of the poly(DMC-co-MA) copolymers. They exhibit characteristic signals at 1.01 ppm (a), 3.19 ppm (d) and 4.42 ppm (g), which are attributed to the methyl proton in RCH3, methyl groups in RCH2-N(CH3)3, and methylene in RCOOCH2-, respectively. While, the peaks corresponding to the methylene groups in RCH2-N(CH3)3 (e) and RCOOCH3 (f) are overlapped at 3.79 ppm. By integrating the NMR peaks (e, f and g) of PD10M1, PD50M1 and PD100M1, the molar ratio of the DMC and MA units in the copolymers was estimated. As shown in Table S1, they are in good agreement with design.
As shown in Fig. 2, PMA, PDMC and PD100M1 all show FTIR characteristic peak at 1727 cm−1, corresponding to the ester C=O bond. PD100M1 and PDMC show characteristic peaks at 1483 cm−1 and 1152 cm−1, which are assigned to the CH3-N+
bond (Duan et al. 2020) and C-N bond (Xu et al. 2020b), respectively. The antibacterial cotton fabrics were prepared according to the schematic diagram shown in Fig. 3. Carboxymethyl chitosan (CMC) was grafted onto cotton fiber via an esterification reaction of the CMC carboxyl groups with the hydroxyl groups of cellulose on the fiber surface (CMC-Cotton). Then the poly(DMC-co-MA) copolymers were further linked on the CMC-Cotton fabrics to form poly(DMC-co-MA)-CMC-Cotton via the reaction of the ester groups with the amino groups of CMC to form amide bonds. It was speculated that the CMC molecules tend to react with the MA unites rather than with the quaternary ammonium ionic DMC unites if a considerable number of CMC amino groups were positively charged and then repelled by the cationic ones. As a comparison, the PD100M1 copolymers were linked on cotton fabrics to form the PD100M1-Cotton fabrics.
Scanning electron microscopy (SEM) was employed to study the cotton fabrics surface coated with poly(DMC-co-MA) (Fig. 4). Significant changes were hard to find in low-magnification SEM images (Fig. 4c, f, i and l) after the modification, indicating that the PD100M1 copolymers and CMC were mainly coated on the fiber surface but not occupied the voids between the fibers. The high-magnification SEM images reveal that the surface modifications changed the surface morphology significantly. Original cotton fabrics surface are wrinkly (Fig. 4a-c), but the modified fabrics exhibit a wrinkle-free surface morphology like a coating layer. To further explore the coating structure, we wetted the modified fabrics and freeze-dried them for high-magnification SEM observation. The SEM images shown in Fig. 4e, h and k indicate that the fiber surfaces become smoother after the freeze-drying treatment. This result runs counter to the common senses that freeze-drying of a hydrogel leads to a porous morphology. Undoubtedly, the grafted copolymers render a hydrophilic layer on the fiber surfaces because of the large amount of the cationic DMC units in the copolymer. Therefore, we speculate that the grafting copolymer layer is so thin that unable to cover large water droplets.
Figure 5 shows the ATR-FTIR spectra of PD100M1-Cotton, PD100M1-CMC-Cotton and the untreated fabrics. The three fabric samples show FTIR peaks at 3338 cm−1 and 2896 cm−1, which correspond to the -OH (Xu et al. 2017) and -CH2- groups of cellulose, respectively. The modified cotton fabrics show new peaks at 1723 cm−1 belonging to the stretching of the C=O (ester groups). The PD100M1-CMC-Cotton fabric shows a characteristic peak corresponding to -CONH- (amide groups) at 1661 cm−1, suggesting that the copolymer of PD100M1 are grafted onto the CMC-Cotton fabric via the amide bonds that formed via the reactions between the copolymer and CMC.
XPS is a powerful way for analysis of chemical structure of material surfaces. Here, we collected the XPS spectra of the treated fabric surfaces to analyze their chemical compositions and environments of the surfaces to a depth of tens of nanometers. Fig. 6a and b compare the wide-range XPS spectra of the cotton fabrics before and after PD100M1 modification. As shown in Fig. 6a, by comparing with the XPS spectrum of original cotton fabric (shows only C 1s and O 1s signals at 285.7 eV and 533.0 eV), additional N 1s (at 400.4 eV) (Xu et al. 2018) signal is found in the XPS spectra of PD100M1-CMC-Cotton fabric (Fig. 6a and b), and the elemental information of the surficial layer obtained by the XPS scans further confirmed the existence of nitrogen and chlorine (Table S2). We further examined their C 1s high resolution XPS spectra (Fig. 6c and d) to analyze the chemical structures of the coating on the surface of cotton fibers. The high-resolution C 1s peak of original fabric can be deconvoluted into three peaks at 284.3 eV (C-C), 286.2 eV (C-OH), and 287.2 eV (C-O-C) (Fig. 6c). In contrast, PD100M1-CMC-Cotton fabric exhibits additional peaks at 287.8 eV and 285.7 eV (Fig. 6d), corresponding to the C=O and CH3-N+ bonds, respectively (Duan et al. 2020). Together with the FTIR analyses (Fig. 6), the XPS results demonstrate the PD100M1 molecules grafting onto the cotton fiber surface. The poly(DMC-co-MA)s linked on cotton fabrics were estimated by an adsorption test of methyl orange. Having been immersed in a methyl orange solution, the modified fabrics will adsorb methyl orange molecules by ionic effect. The reduced concentration of methyl orange was measured using UV–Vis spectrophotometry. As shown in Fig. S2 and Table S3, for one kilogram cotton fabric, 0.579 g, 0.394 g and 0.237 g of PD100M1, PD50M1 and PD10M1 were grafted, respectively.
Table 1 and Fig. 7 represent antibacterial activity of the modified fabrics. The PD100M1-CMC-Cotton fabric achieves the best antibacterial effect, showing 100% BR values against both the E. coli and S. aureus. In the present work, CMC acts as a polymer binder to link more poly(DMC-co-MA)s copolymers onto the cotton fabrics.
Table 1
Information and antibacterial rate of the cotton fabrics.
samples
|
BR (%)
|
E. coli
|
S. aureus
|
PD100M1-CMC-Cotton
|
100
|
100
|
PD50M1-CMC-Cotton
|
99.4 ± 0.05
|
99.1 ± 0.25
|
PD10M1-CMC-Cotton
|
98.5 ± 0.35
|
97.2 ± 0.60
|
PD100M1-Cotton
|
85.7 ± 0.45
|
85.2 ± 0.64
|
PDMC-CMC-Cotton
|
84.4 ± 0.47
|
84.0 ± 0.55
|
Because amino group is more reactive than hydroxyl group, introduction of CMC improves the reactivity of the cotton fiber surface, leading to more copolymers grafted onto the cotton fabric. As shown in Table 1, without CMC involvement, the antibacterial capability of PD100M1-Cotton dramatically degraded to less than 90%. On the other hand, PDMC-CMC-Cotton showed weakened antibacterial activity than the PD100M1-CMC-Cotton fabric does, revealing that MA units in the copolymer exert positive effect on antibacterial function. However, incorporation of overmuch MA units into the copolymer makes the DMC constitute reduced, resulting in BR decreases, which were found at PD50M1-CMC-Cotton and PD10M1-CMC-Cotton fabric samples. Antibacterial capability of PD100M1-Cotton dramatically degraded to less than 90%. On the other hand, PDMC-CMC-Cotton showed weakened antibacterial activity than the PD100M1-CMC-Cotton fabric does, revealing that MA units in the copolymer exert positive effect on antibacterial function. However, incorporation of overmuch MA units into the copolymer makes the DMC constitute reduced, resulting in BR decreases, which were found at PD50M1-CMC-Cotton and PD10M1-CMC-Cotton fabric samples.
Figure 8 displays antibacterial durability of the modified fabrics. The BR of PD100M1-CMC-Cotton fabric against E. coli and S. aureus both remained above 98% after 50 laundering cycles (Fig. 8a), whereas the BR rates of PD100M1-Cotton against E. coli and S. aureus dropped to 74.1% and 74.0% (Fig. 8d), respectively. These results suggest that CMC play an important role on the antibacterial durability. In addition, PD50M1-CMC-Cotton and PD10M1-CMC-Cotton fabrics show weakened durability against both E. coli and S. aureus after 50 laundering cycles (Fig. 8b and c). These results mean that the MA units in the copolymer affect the laundering durability of the modified cotton fabrics.
To explain the effect of the MA units on the antibacterial activity and laundering durability, a mechanism was proposed in Fig. 9. It was premised that the grafted poly(DMC-co-MA) copolymers on the cotton fiber surfaces should have suitable movable DMC segments to exert the antibacterial role. Due to the repulsion effect caused by the positively charged amino groups, the DMC units must be of lower reactivity than the MA units when undergoing the aminolysis reaction with the grafted CMC chains, which have a considerable number of positively charged amino groups. As shown in Fig. 9, it is comprehensible that increased DMC units tend to increase the ionic density of the fiber surface, leading to an extension of the grafted poly(DMC-co-MA) copolymers. The charged DMC segments are movable to form more adequate contact with the bacterial cell membrane, exhibiting an enhanced antibacterial function. When compared to PD100M1, PD10M1 and PD50M1 have more MA units. This may result in shorter DMC segments on the fiber surface, constraining their movement, thereby decreasing the contact to bacterial cell membrane, and weakening the antibacterial efficiency of the grafted coating.
Water absorbability and water vapor permeability are typical wearing comfort properties of cotton fabrics. To evaluate the damage of the modification process on the important properties, both water absorbability and water vapor permeability were assessed. As shown in Fig. 10a and 10b, PD100M1-CMC-Cotton fabric shows higher water absorbability (191.4%) and vapor permeability (679.3 g·m−2·d−1) than original fabric. This can be attributed to the hydrophilic coating of poly(DMC-co-MA) copolymer. Moreover, Fig. 10c shows that the fabric flexibility has not been significantly changed by the modification process.