The surface modification of chitosan aerogels can be carried out in two methodologies: “grafting from” and “grafting to/onto” (Boinovich & Emelyanenko, 2008; Zdyrko & Luzinov, 2011). The first approach, “grafting from”, consists of the polymerization of monomers on the initiator molecules attached to the carrier surface. As part of the “grafting to/onto” approach, the end groups of the pre-synthesized polymers react with complementary functional groups on the modified surface (Hadjichristidis, 2002; Mochalova et al., 2006).
The copolymers based on glycidyl methacrylate and (fluoro)alkyl methacrylates interact with functional amine and hydroxyl groups of chitosan due to the presence of reactive oxirane groups in the composition. In this case, the aerogel surface layer will represent macromolecular chains of chitosan with randomly distributed branches of the graft copolymer (Figure 1). It has been established in ref. (Tanaka & Kakiuchi, 1963) that this modification process is mechanically similar to the curing reactions of compounds that contain reactive epoxy groups. The interaction of amino and oxirane groups is carried out using the conditions of acid–base catalysis (Tanaka & Kakiuchi, 1963, 1964), with non-catalytic behavior also being possible at temperatures of 130–150°C (Galy J., Sabra A., 1986). The glycidyl methacrylate-based copolymers were previously dissolved in methyl ethyl ketone, and chitosan aerogels were immersed in the modifier solution. The most effective (Evgeny Bryuzgin et al., 2020; Evgeny V. Bryuzgin et al., 2017) concentration of the modified solution for obtaining superhydrophobic properties is 3 wt.%.
Chitosan-based film materials were subjected to IR spectroscopy studies to confirm the grafting of the GMA- and AlMA-based copolymers. The IR spectra of the modified samples (Figure 2) were distinguished by the band at 1728 cm−1. This peak corresponds to stretching vibrations of the carbonyl group in esters and was a consequence of the presence of poly(GMA-co-AlMA/FMA) graft copolymers on the chitosan film surface. The progress of the reaction in the amino group was evidenced by the reduction in intensity of the absorption band of NH2 groups at 1584 cm−1. Similar curves were obtained for other modifiers.
Poly(GMA-co-HFIM) was used as a modifier to determine the presence of graft copolymers in the bulk of the aerogel, which was due to the presence of an indicator fluorine atom in its composition. Based on the results of the X-ray microanalysis of the fracture area of the chitosan aerogel, fluorine was identified in the amount of 0.51 wt.% (Figure 3).
SEM images (Figure 4) show the morphology of aerogel fractures before and after modification. The resulting chitosan aerogels have high porosity with a pore size of 100–200 µm.
The pore walls are 600–700-nm-thick film formations (Figure 4c). Pore parameters were retained as a result of the modification; therefore, the pores were not filled with the modifying copolymer.
The modifiers selected for the study (Figure 1) can be ranged according to the increase in hydrophobicity subject to their HLB (hydrophilic–lipophilic balance) values (Table 1).
Based on the positive HLB values (Table 1) for poly(GMA-co-HeMA) and poly(GMA-co-DMA) copolymers (2.025 and 0.125, respectively), it can be assumed that the hydrophobicity of chitosan modified by them will either not change at all or will change only slightly at the initial instant.
It is known from ref. (Boinovich & Emelyanenko, 2008) that the use of hydrophobic agents on smooth surfaces, such as films, enables achievement of contact angles no more than 120°. Therefore, the hydrophobic properties of materials can be increased by the multilevel roughness of chitosan aerogels and further surface modification with GMA- and AlMA/FMA-based copolymers. Studies have shown (Figure 5) that grafting of poly(GMA-co-AlMA/FMA) copolymers onto the surface of chitosan films using 3% modified solutions allows hydrophobicity characterized by contact angles up to 114°. Regarding aerogels, treatment with GMA- and AlMA/FMA-based copolymers allows a superhydrophobic state with contact angles up to 154°.
One of the main stability characteristics of superhydrophobic properties is the preservation of the wetting regime at prolonged contact between the droplet and the coating in an atmosphere saturated with water vapor. The resulting coatings based on the GMA- and AlMA/FMA-based copolymers exhibit a stable high and superhydrophobic state (Figure 6). The figure shows that the unmodified aerogel completely absorbed a water drop after 30 s of contact, while the aerogel modified with GMA-LMA copolymer retained a contact angle of approximately 150° for a long time. However, the contact angles remained unchanged with an increase in modifier concentration, which indicated the preservation of the porous structure of the aerogels.
As shown in Figures 6 and 7, the unmodified chitosan aerogel samples showed high absorption capacity, and the water absorption value was 18.92 g/g (Figure 7), which was explained by the presence of hydrophilic groups (-OH, -NH2) in the chitosan.
Modification of the aerogel samples with a poly(GMA-co-HeMA) copolymer led to a significant degradation and decrease in water absorption, which was linearly dependent on the copolymer concentration in the modified solution. The alkyl substituent in the modifier provided the surface with hydrophobic properties due to the displacement of the hydrogen atom in the hydrophilic amino and hydroxyl groups of chitosan and their shielding.
Because all materials used as wound dressings must be autoclaved at a temperature of 200°C and higher, it is necessary to study the thermal stability of the resulting aerogels. Figure 8 shows the results of the thermogravimetric analysis (TG and DTG curves).
As shown in Figure 8 (TG), the weight loss of the samples in the temperature range up to 600°C occurred in several stages. The mass variation in the temperature range up to 200–240°C was due to the desorption of moisture from the surface of the samples and from the bulk as a result of breakdown of hydrogen bonds between the water molecules and polar functional groups of chitosan.
The sample decomposition rate was the highest in the temperature range of 240–300°C (Figure 8, DTG). For the chitosan-based film samples, the rate of weight loss (the maximum decomposition rate) in this range determined from the variation in the peak intensity was 7.2%/min. Moreover, for aerogels, i.e., chitosans cross-linked with glutaraldehyde, namely, samples 2, 3, and 4, the rate of weight loss was two times less at 3.5%/min. Thus, the decomposition rate of spongy materials was lower than that of the chitosan film.
The final stage of decomposition occurred in the temperature range from 300 to 600°C with the formation of coke residue. The weight loss for all samples was approximately 60% of the initial weight (Figure 8, TG). Thus, the decomposition temperature of the resulting samples corresponded to ~250°C, i.e., chitosan-based hydrophobic materials meet the thermal stability criteria.
To study the effect of the lyophilicity of the film materials on the biodegradation time, the samples were subjected to in vitro soil degradation. The study of the decomposition of the chitosan films showed (Figure 9) that the initial chitosan samples with a contact angle of 86±3° showed 90% soil biodegradation within 80–90 days. The weight loss of the samples treated with GMA- and AlMA/FMA-based copolymers with contact angles of 110-120° was 5-10%. This suggests the possibility of decreasing the rate of decomposition of the polysaccharide film materials by modification with GMA- and AlMA/FMA-based copolymers.
According to Figure 9a, the increase in the size of the hydrocarbon radical in the modifying copolymer to C12, as well as the presence of a fluoroalkyl radical, maximally reduced the rate of decomposition of the resulting materials. This can be explained by the use of long hydrophobic alkyl substituents of the modifying copolymers for shielding of hydrophilic amine and hydroxyl groups of chitosan. The dependence of the decomposition rate on the length of the hydrocarbon substituent of the modifier (Figure 9b) corresponds to the HLB values provided in Table 1. However, the rate of decomposition of chitosan-based materials modified with a poly(GMA-co-SMA) copolymer did not correlate with the size of the hydrocarbon substituent (C18). This may be due to the association of long hydrophobic tails of SMA units, which increased the availability of hydrophilic groups of chitosan for interaction with water molecules. This result is in line with a previous publication (Evgeny Bryuzgin et al., 2020), which showed that given comparable molecular weights, the poly(GMA-co-LMA) copolymer has large macromolecular coils in comparison with poly(GMA-co-CMA).