A
UV-induced structural changes and mechanical reinforcement in benzoyl-modified chitosans: A comparative physicochemical studyAli Bahadori *a, Mohammad Taghi Taghizadeh b
aDepartment of Physical Chemistry, Faculty of Chemistry, Urmia University, Urmia, 5756151818, Iran, a.bahadori@yahoo.com
bDepartment of Physical Chemistry, Faculty of Chemistry, University of Tabriz, Tabriz, Iran
Abstract
Chitosan and functionalized chitosans with 3, 4-dihydroxybenzoyl groups (CS-DHBA) and 3,4, 5-trihydroxybenzoyl groups (CS-THBA) were exposed to ultraviolet (UV) irradiation. The physicochemical and mechanical properties of chitosan (CS), CS-DHBA, and CS-THBA, including chemical structures, molecular weights, polydispersity index, intrinsic viscosity, and mechanical properties before and after UV-irradiation, were investigated and compared. The results showed that the molecular weight of chitosan decreased from 782,401 to 522,012 g/mol with increasing UV irradiation time. In contrast to chitosan, the molecular weight of CS-DHBA and CS-THBA increased to 826,187 g/mol and 845,310 g/mol, respectively, with increasing UV irradiation time. In addition, the polydispersity index (Mw/Mn) of chitosan decreased from 7.41 to 5.62 after 12 h of UV irradiation, but the polydispersity index of CS-THBA increased from 9.84 to 9.94. The surface morphology and structural analysis of UV-irradiated chitosan, CS-DHBA, and CS-THBA by SEM, FT-IR, and XRD confirmed that the chemical structure of irradiated chitosan was not significantly changed. In contrast, the chemical structures of irradiated CS-DHBA and CS-THBA were altered. The tensile strength (TS) of chitosan films decreased after 12 h of radiation, but UV-irradiated CS-DHBA and CS-THBA showed a significant adhesive capacity and enhanced TS.
Keywords:
Chitosan
Benzoyl chitosans
UV-irradiation
Molecular weight
Mechanical properties
Introduction
A
Chitosan (CS) is a functional and linear aminopolysaccharide of glucosamine (GlcN) and N-acetylglucosamine (GlcNAc) units and is prepared by partial deacetylation of chitin extracted from the exoskeleton of crustaceans such as shrimps and crabs, as well as from the cell walls of some fungi Muzzarelli and Martin [1]. Chitosan has a special set of interesting properties, including non-toxicity, biocompatibility, biodegradability, and non-antigenicity [2, 3]. These properties make chitosan an attractive biopolymer for applications across various fields, including medical applications, pharmaceuticals, textiles, wastewater treatment, biotechnology, cosmetics, the food industry, agriculture, and environmental protection [4–6]. However, the application of chitosan has been limited because of its high molecular weight (HMW), water-insolubility, and high viscosity of its solutions [7]. The presence of hydroxyl and amino groups provides active sites for chitosan derivatization, thereby improving its physicochemical properties, such as solubility, antimicrobial activity, and adhesive properties [8]. The most commonly used methods for modifying chitosan are N-substitution, O-substitution, and free-radical graft copolymerization [9]. The modified chitosan can be applied in various fields, such as pharmaceutical fields, due to its biocompatibility and non-toxicity [10, 11]. The modified chitosan can also serve as a potential adsorbent for metal ions and dye removal [12, 13]. Recently there has been an increasing amount of research based on different methods such as enzymatic hydrolysis [14–16], acid hydrolysis [17, 18], oxidative degradation [19–21], ultraviolet (UV) irradiation [22–24], gamma radiation [25–27], ultrasonic degradation [28–30] and microwave irradiation [31] to degrade chitosan into low molecular weight (LMW) units. Degradation of polysaccharides using irradiation techniques has received attention due to their technological effectiveness in producing low-molecular-weight oligomers [32–34]. The basic advantages of radiation degradation of polymers include the ability to promote changes reproducibly and quantitatively, without the introduction of chemical reagents or the need to control temperature, environment, or additives [35]. In the scientific literature, the effects of ultraviolet (UV) and gamma irradiation on the physicochemical properties and degradation of chitosan have been investigated [ 36–41]. The proposed mechanism for photodegradation involves the following three-step process: (1) formation of carboxylic groups, (2) production of the hydroxyl groups, (3) formation of carboxylic groups in the presence of oxygen [42]. In recent years, the influence of UV irradiation on the mechanical, thermal, and surface properties of polymeric blends containing chitosan has been studied and several researchers have investigated the structural modifications in chitosan-based systems induced by UV irradiation [43–51]. For example, Meynaud et al. (2023) evaluated the impact of UV irradiation on the structural integrity and bioactivity of low-molecular-weight powdered chitosan for biopesticide applications. They reported that despite minor spectral shifts suggesting potential carbonyl group formation, no significant structural degradation was observed [52]. While the UVinduced degradation of native CS has been extensively studied, the influence of UV irradiation on the benzoyl-functionalized chitosans unexplored. In this study, chitosan was functionalized with 3,4-dihydroxybenzoyl (CS-DHBA) groups and 3,4,5-trihydroxybenzoyl (CS-THBA) groups, and were exposed to UV irradiation of its derivatives. Then the physicochemical and mechanical properties of UV-irradiated samples, including chemical structures, molecular weights, polydispersity index (Mw/Mn), intrinsic viscosity, and mechanical properties were characterized and analyzed by means of by gel permeation chromatography (GPC), scanning electron microscopy (SEM), Fourier-transform infrared (FT-IR) and X-ray diffraction (XRD) patterns and systematically compared.Experimental
Materials
High-molecular-weight chitosan from crab shells with an average molecular weight of approximately 800 kDa according to the manufacturer's specification was obtained from Aldrich Chemical Co. (Buchs, Switzerland). 3,4-Dihydroxybenzoic acid (DHBA, 98%) and 3,4,5-trihydroxybenzoic acid (THBA, 99%) were purchased from Merck Co. (Darmstadt, Germany). N-(3-dimethylaminopropyl)-N-ethylcarbodiimide hydrochloride (EDC), N-hydroxysuccinimide (NHS, 98%) obtained from Sigma-Aldrich Chemicals (Buchs, Switzerland). All other chemicals were of analytical reagent grade and were purchased from Merck. All solutions were prepared using double-distilled water.
Preparation of CS-DHBA and CS-THBA
The CS-DHBA and CS-THBA were prepared by a method similar to that reported in our previous work [53]. Briefly, a 10 g/L chitosan solution was prepared in 1% acetic acid solution. DHBA (0.192 g), EDC (0.24 g), and NHS (0.04 g) were dissolved in 1% aqueous acetic acid solution (20 mL). This solution was added dropwise to a 10 g/L chitosan solution (20 mL), and the obtained mixture was stirred for 24 h at room temperature. For the synthesis of CS-THBA, THBA (0.216 g), EDC (0.24 g), and NHS (0.04 g) were dissolved in 1% acetic acid solution (20 mL). This solution was added dropwise to a 10 g/L chitosan solution (20 mL), and the obtained mixture was stirred for 24 h at room temperature.
UV Irradiation Procedure
Ultraviolet degradation of chitosan, CS-DHBA, and CS-THBA was carried out in the solid state using a pair of UV lamps (Philips, 8 W) that emit in the 200–300 nm wavelength range, with a maximum at 254 nm. Sample films of CS, CS-DHBA, and CS-THBA were prepared by spreading 40 mL of the viscous solution onto a slide glass (7 × 15 cm) and drying for 24 h under vacuum at room temperature. The thickness of these films was measured to be 500 µm by a micrometer (Mitutoyo Manufacturing). Films were placed directly under the lamp. UV exposure time was continued for 12 h in a dark box under air.
Characterization Techniques
1
H-Nuclear Magnetic Resonance (
1
H-NMR) spectroscopy
1H-NMR spectra of chitosan and modified chitosans were recorded on a Bruker 400 MHz spectrometer (Fällanden, Switzerland) at room temperature. Samples were dissolved in D2O containing one drop of DCl [53].
Scanning Electron Microscopy (SEM).
The effect of different degradation times on the film morphology was studied using a scanning electron microscope (Philips XL30) operated at 15 kV. The instrument was equipped with an INCA system and an energy-dispersive X-ray detector. Samples were placed in copper sample holders using double-sided carbon tape and then gold-plated before analysis.
Fourier-Transform Infrared (FTIR) spectroscopy
FTIR spectra of CS, CS-DHBA, CS-THBA, and their irradiated samples were recorded on
a Shimadzu-4300 spectrometer (Tokyo, Japan). The FT-IR absorbance was recorded over 600–4000 cm− 1. The samples were mixed with analytical-grade KBr at a weight ratio of 5:200 mg at room temperature.
X-Ray Diffraction (XRD)
X-ray diffraction patterns of the films were measured using a Siemens XRD-5000 diffractometer (Aubrey, TX, USA) with a CuKa = 1.54 target at 40 kV and 50 mA at 25°C. Dried films were cut into rectangular pieces (1 cm × 1 cm), adhered to the matrix, and scanned from 2θ = 0° to 40°.
Gel Permeation Chromatography (GPC)
GPC was used to measure the molecular weights of CS, CS-DHBA, and CS-THBA, and to qualitatively assess the decrease in molecular weight of these samples after ultrasonic degradation. GPC measurement was carried out on a Shimadzu 6-A GPC instrument equipped with a refractive index detector at 30°C using connected columns (TSK G4000-PWxl and G3000-PWxl) with 0.1 mol/L CH3COONa/0.2 mol/L CH3COOH buffer pH 4.8 solution as an eluent at a flow rate of 1 mL/min.
Viscometry
The intrinsic viscosities of the original samples and their irradiated sample solutions at different times were measured using the capillary viscometer (Setavic Kinematic Viscometer, London, England) at 25°C. The internal capillary diameter was 0.05 mm. Efflux times were calculated for sample solutions (ts) and the solvent (t0). Measurement of efflux times was repeated two times, and the average efflux time was then converted to the ratio of ts/t0, which is proportional to relative viscosity, and specific viscosity, of the sample solution:
1
The intrinsic viscosity Values can be related to the specific and relative viscosity, By the Huggins and Kramer equations [54]:
2
Mechanical properties
Mechanical properties, including tensile strength (TS) and elongation at break (E), were measured with an Instron Universal Testing Machine (Instron Corp, Canton, MA, USA) following the ASTM Standard Test Method D 882 − 91(ASTM, 2003). For tensile testing, 200 mg of the original CS, CS-DHBA, CS-THBA, and their irradiated samples after 12 h were combined with 1 mL deionized water and mixed with a spatula until a viscous, homogeneous paste was obtained. Within about 1–2 minutes, the adhesive materials to be examined were applied. Films were cut into 20 mm-wide and 50 mm-long strips and mounted between the grips. The initial grip separation was set to 30 mm, and the crosshead speed to 5 mm/min. Measurements represent an average of at least eight samples.
Results and Discussion
1
H-NMR studies of CS-DHBA and CS-THBA
The 1H-NMR spectra of CS, CS-DHBA, and CS-THBA were studied in our previous work. In brief, the 1H-NMR spectra of CS-DHBA and CS-THBA, in addition to the protons of the repeating unit of chitosan, aromatic ring protons (H-arom) appear at 6.8 (H-8), 7.5 (H-7,9), and 7.2 (H-7',9'), respectively [53, 54]. These results indicated that the chitosan contained 3,4-di- and 3,4,5-trihydroxy-benzoyl residues.
Plain CS: 1H-NMR (1% DCl in D2O): δ1.9–2 (3H, s, H-Ac), 4.5–4.8 (1H, d, H-1) and 3.2–3.4 (6H, m, H-2,3,4,5,6), Plain CS-DHBA: 1H-NMR (1% DCl in D2O): δ1.9–2 (3H, s, H-Ac), 4.5–4.8 (1H, d, H-1), 3.2–3.4 (6H, m, H-2,3,4,5,6) and 6.8–7.5 (3H, s, H-arom) and Plain CS-THBA: 1H-NMR(1% DCl in D2O): δ1.9–2 (3H, s, H-Ac), 4.5–4.8 (1H, d, H-1), 3.2–3.4 (6H, m, H-2,3,4,5,6) and 7.2 (2H, s, H-arom) [57, 59]. IR (KBr) CS: 1659 cm− 1 (C = O stretching vibration of amid I), 1630 cm− 1 (N-H bending vibration of amine) 1564 cm− 1 (amide II) 1158 cm− 1 (bridge-O-stretching vibration), 1078 cm− 1 (C-O-stretching vibration) and 3449 cm− 1 (O-H and N-H stretch vibration), IR (KBr) CS-DHBA: 1656 cm− 1 (C = O stretching vibration of amide I) and 1552 cm− 1 (N-H bending vibration of amide II) and IR (KBr) CS-THBA: 1642 cm− 1 (C = O stretching vibration of amide I) and 1556 cm− 1 (N-H bending vibration of amide II) [53, 55].
SEM analysis of surface morphology
To investigate the surface morphology of chitosan and its derivative films before and after 12 h irradiation, their SEM images were taken (Fig. 1). As shown in Fig. 1, the smooth surface of the initial chitosan was partially altered to a porous surface after the irradiation process, which was reported previously [56, 57].
Fig. 1
Micrographs of the surface of chitosan before (a) (in 1 µm scale) and after (b) (in 1 µm scale); CS-DHBA before (c) (in 10 µm scale) and after (d) (in 100 µm scale); CS-THBA before (e) (in 300 nm scale) and after (f) (in 300 nm scale) 12 h degradation, respectively.
In the case of CS-DHBA, at the initial time of irradiation, its pristine surface was not noticeably changed, but after eight hours of continuous irradiation, its roughness decreased and pores appeared, as reported in the literature [58]. As inferred from SEM images (Fig. 1e, f), the surface of CS-THBA after eight hours of irradiation becomes very streaky, porous, and has many cracks similar to a sponge.
FTIR analysis of UV-Irradiated CS, CS-DHBA, and CS-THBA
The FT-IR spectra of the un-irradiated UV and UV irradiated CS, CS-DHBA, and CS-THBA that are sampled at the end of 12 h loadings in the irradiation process, as well as initial samples, are shown in Fig. 2.
Fig. 2
FT-IR spectra of initial samples: (a) CS; (b) CS-DHBA and (c) CS-THBA, UV degraded samples: (d) CS; (e) CS-DHBA and (f) CS-THBA at the end of 12 h degradation.
Compared with the FT-IR spectrum of chitosan, the spectrum of CS-DHBA showed new peaks at 1656 cm− 1 and 1552 cm− 1, attributed to the carbonyl stretching vibration of amide I and the N-H bending vibration of amide II, respectively. For CS-THBA, these peaks appeared at 1642 cm− 1 and 1556 cm− 1, respectively [51]. The FT-IR spectrum of the irradiated chitosan was similar to that of the initial chitosan. In the FT-IR spectrum of the irradiated chitosan, the peak at 3449 cm− 1 becomes wider, indicating that the scission of glycosidic bonds leads to the formation of hydroxyl groups, which is manifested as an increase in the ratio of hydroxyl group peak to the reference peak [52]. Simultaneous decrease in the peak ratio of C-O-C groups relative to the reference is also observed. The FT-IR spectra indicated no significant difference in the chemical structure between the irradiated and initial chitosan. Thus, the conclusion can be drawn that the initial chitosan’s monomeric structure still existed wholly in the resulting degraded chitosan with reduced molecular weight [59]. In the irradiated CS-DHBA, the -NH2 bending vibration peak at 1656 cm− 1 shifts to 1696 cm− 1, and for the CS-THBA, the peak at 1642 cm− 1 shifts to 1712 cm− 1. In addition, in the irradiated CS-DHBA and CS-THBA, the shoulder peaks, depending on the O-H and N-H stretch vibration at 3444 cm− 1, disappeared, and new sharp peaks appeared at 3564 cm− 1 and at 3572 cm− 1, respectively. These results showed that ultraviolet irradiation altered the chemical structures of CS-DHBA and CS-THBA.
XRD patterns
Figure 3 shows the X-ray diffraction patterns of CS, CS-DHBA, and CS-THBA before and after UV irradiation. The wide-angle X-ray diffraction patterns of the initial and irradiated chitosan showed their characteristic peaks at 2θ = 10.4 and 20.2°, which coincide with the pattern of the “L-2 polymorph” of chitosan reported previously [60]. Samuels et al. reported that the reflection at 2θ = 10.4° was assigned to crystal forms I, and the strongest reflection appears at 20.2°, which corresponds to crystal forms II [61].
Fig. 3
X-ray diffraction patterns of: initial samples, (a) CS, (c) CS-DHBA, and (e) CS-THBA: UV degraded samples, (b) CS, (d) CS-DHBA, and (f) CS-THBA at the end of 12 h irradiation.
The crystallinity of UV-irradiated chitosan products is remarkably higher than that of the initial chitosan. This rise in crystallinity also happened in enzymatic and ultrasonic depolymerization of chitosan [55, 62, 63]. The obtained results indicated that UV irradiation alters the physical structure of chitosan, mainly due to a decrease in molecular weight to [64]. The X-ray diffraction patterns of CS-DHBA and CS-THBA show only a weak characteristic peak at 2θ = 10.4°, and the specific broad peak for chitosan (2θ = 20.2°) disappeared in the CS-DHBA and CS-THBA [34]. In the irradiated CS-DHBA and CS-THBA, the peak at 2θ = 10.4° disappeared. Thus, the presence of 3,4-dihydroxy- and 3,4,5-trihydroxybenzoyl groups in functionalized chitosan before and after UV degradation decreases the crystallinity of CS-DHBA and CS-THBA, mainly due to changes in the chemical structure of chitosan, an increase in hydrogen bonding, and cross-linking of the irradiated CS-DHBA and CS-THBA, which could be amorphous.
Molecular weight evaluation
The weight-average molecular weight (Mw) and number-average molecular weight (Mn) were obtained as a function of irradiation time by gel permeation chromatography and were summarized in Table. 1.
|
Irradiation Time (h)
|
Mw (g/mol)
|
Mn (g/mol)
|
PD = Mw/ Mn
|
||||||
|---|---|---|---|---|---|---|---|---|---|
|
CS
|
CS-DHBA
|
CS-THBA
|
CS
|
CS-DHBA
|
CS-THBA
|
CS
|
CS-DHBA
|
CS-THBA
|
|
|
0
|
782,401
|
824,152
|
810,134
|
105,558
|
93,009
|
82,289
|
7.41
|
8.86
|
9.84
|
|
3
|
699,194
|
783,761
|
790,461
|
101,142
|
92,512
|
81,407
|
6.91
|
8.47
|
9.70
|
|
6
|
620,843
|
788,680
|
807,563
|
98,768
|
93,163
|
82,153
|
6.28
|
8.46
|
9.82
|
|
9
|
560,359
|
825,186
|
834,431
|
97,403
|
94,061
|
84,201
|
5.75
|
8.77
|
9.90
|
|
12
|
522,012
|
826,187
|
845,310
|
92,884
|
96,826
|
85,041
|
5.62
|
8.53
|
9.94
|
Figure 4 shows the GPC profiles of CS, CS-DHBA, CS-THBA, and their ultraviolet irradiated products at different irradiation times. In the GPC profiles of degraded chitosans, a shift toward higher elution times was observed, indicating depolymerization [59, 65]. In the GPC profiles of irradiated CS-DHBA and CS-THBA, the peaks of the elution curve shifted toward lower elution times with increasing irradiation time.
Fig. 4
The GPC profiles of CS (a), CS-DHBA (b), CS-THBA (c), and their UV irradiated products at different degradation times.
The molecular weight changes in CS, CS-DHBA, and CS-THBA samples following ultraviolet degradation at different irradiation times are shown in Fig. 5. As shown, the molecular weights of CS decreased with increasing irradiation time, whereas those of CS-DHBA and CS-THBA decreased slowly at the beginning of UV degradation and then increased with increasing irradiation time [23, 24]. In the UV irradiation of CS, CS-DHBA and CS-THBA, the degradation mechanism is attributed to free radical reactions [66, 67].
Fig. 5
Effects of UV irradiation on the molecular weight of CS, CS-DHBA and CS-THBA.
In UV irradiation, energy is absorbed mainly by the polysaccharide and consequently, macroradicals are formed due to the excitation and ionization. The experimental results showed that chitosan average molecular weight reduces from 782,401 to 522,012 g/mol with increasing UV irradiation time. In contrast to CS, the molecular weight of CS-DHBA and CS-THBA increased from 824,152 to 826,187 and from 810,134 to 845,310 g/mol, respectively, with increasing UV irradiation time. The increase of molecular weight of the CS-DHBA and CS-THBA after UV irradiation, is attributed to the formation of macromolecules by polymerization of macroradicals. The proposed mechanism of reaction between macroradicals of CS-DHBA was shown in Fig. 6 [68, 69].
Fig. 6
The proposed mechanism of reaction between macro radicals of CS-DHBA and formation of macromolecules.
Polydispersity Index (PDI) of UV- Irradiated CS, CS-DHBA, and CS-THBA
To study the polydispersity index (Mw/Mn), the Mw/Mn values of CS, CS-DHBA and CS-THBA derived from ultraviolet degradation at different times were calculated. Figure 7 shows polydispersity index of CS, CS-DHBA and CS-THBA at different ultraviolet irradiation times. The polydispersity index of chitosan was obviously decreased with increasing irradiation time. As is shown in Fig. 7, the polydispersity index of chitosan decreased from 7.41 to 5.62 under 12 h UV irradiation times. This result is in agreement with other degradation method findings such as enzymatic hydrolysis [14] and ultrasonic treatment [59], so can conclude that the decrease of chitosan polydispersity is mainly due to the decrease of molecular weight, not to the degradation methods. The polydispersity index of CS-DHBA decreased slowly with increasing irradiation time, from 8.86 to 8.52, but the polydispersity index of CS-THBA rose from 9.84 to 9.94 under 12 h UV irradiation times.
Fig. 7
Polydispersity (Mw/Mn) of CS, CS-DHBA, and CS-THBA products at different UV irradiation times.
Effect of UV irradiation on the viscosity of CS, CS-DHBA, and CS-THBA
The influence of UV irradiation on the intrinsic viscosity of CS, CS-DHBA, and CS-THBA was shown in Fig. 8. In the case of chitosan, as is seen, Decreases with increasing UV irradiation time [39]. Still, the intrinsic viscosity of CS-DHBA and CS-THBA slowly reduced at the beginning of UV irradiation and then increased slowly as irradiation time increased. Relative viscosity, specific viscosity, and intrinsic viscosity The values of chitosan, CS-DHBA, and CS-THBA as a function of UV irradiation time were calculated and summarized in Table 2. As mentioned above, the increase in the intrinsic viscosity of CS-DHBA and CS-THBA after UV irradiation is attributed to the formation of macromolecules via the polymerization of macroradicals [68, 69].
|
Irradiation Time (h)
|
ηr
|
ηsp
|
[η] (mL/g)
|
||||||
|---|---|---|---|---|---|---|---|---|---|
|
CS
|
CS-DHBA
|
CS-THBA
|
CS
|
CS-DHBA
|
CS-THBA
|
CS
|
CS-DHBA
|
CS-THBA
|
|
|
0
|
1.336
|
1.579
|
1.565
|
0.336
|
0.579
|
0.565
|
152
|
249
|
240
|
|
3
|
1.314
|
1.552
|
1.544
|
0.314
|
0.552
|
0.544
|
143
|
231
|
237
|
|
6
|
1.301
|
1.564
|
1.561
|
0.301
|
0.564
|
0.561
|
137
|
241
|
244
|
|
9
|
1.292
|
1.621
|
1.634
|
0.292
|
0.621
|
0.634
|
133
|
262
|
267
|
|
12
|
1.284
|
1.613
|
1.636
|
0.284
|
0.613
|
0.636
|
130
|
259
|
268
|
Fig. 8
Intrinsic viscosity of CS, CS-DHBA, and CS-THBA as a function of UV irradiation time at 25°C.
Mechanical properties
The tensile strength (TS) and percentage elongation at break (E%) of CS, CS-DHBA, and CS-THBA before and after 12 h UV irradiation are shown in Fig. 9. The tensile strength of CS before and after 12 h UV irradiation was 28.83 ± 1.64 and 17.50 ± 2.7 MPa, respectively. The TS values of the degraded chitosan films were lower than those of the original chitosan films. This result is attributed to UV-induced chitosan degradation, leading to a decrease in its molecular weight. The effect of chitosan molecular weight on tensile strength and percentage elongation at break has been investigated in the literature, and Muzzarelli and others reported that the TS of chitosan films increased with increasing molecular weight of chitosan [70, 71].
Fig. 9
Percentage elongation at break (a) and tensile strength (b) CS, CS-DHBA and CS-THBA before and after 12 h UV irradiation.
The elongation at break shows a slight increase as a function of UV irradiation time by increasing UV irradiation time. The increase of elongation at break also could be attributed to the decrease of molecular weight. In contrast to CS, the TS of CS-DHBA and CS-THBA increase from 38.45 ± 2.41 to 45.55 ± 3.25 and from 44.52 ± 1.86 to 54.64 ± 2.52 MPa, respectively, after 12 h. The present study provides compelling evidence that UV irradiation leads to divergent behaviors in native versus functionalized chitosan derivatives. Specifically, while native chitosan experienced a significant decrease in molecular weight and mechanical strength upon prolonged UV exposure a trend consistent with prior studies [72], the benzoylated derivatives (CS-DHBA and CS-THBA) exhibited unexpected molecular weight growth and enhanced mechanical performance.The increase of tensile strength and decrease of elongation at break could be attributed to di- and trihydroxy groups in CS-DHBA and CS-THBA. Because, the radical formation pattern is possible for CS-DHBA and CS-THBA to make a denser cross-linking with hydroxyl groups and therefore, form macromolecules with polymerization of macroradicals [73].
Conclusions
The physicochemical and mechanical properties of the chitosan and functionalized chitosan’s CS-DHBA and CS-THBA, such as chemical structures, molecular weights, polydispersity index, intrinsic viscosity and mechanical properties before and after UV-irradiation were investigated and compared. The results showed that the molecular weight of chitosan decreased from 782,401 to 522,012 g/mol with increasing UV irradiation time. In contrast to chitosan, the molecular weight of CS-DHBA and CS-THBA increased from 824,152 to 826,187 g/mol and from 810,134 to 845,310 g/mol, respectively, with increasing UV irradiation time. In addition, the polydispersity index (Mw/Mn) of chitosan decreased from 7.41 to 5.62 under 12 h UV irradiation time but, the polydispersity index of CS-THBA increased from 9.84 to 9.94. The surface morphology and structure analysis of UV irradiated chitosan, CS-DHBA and CS-THBA by SEM, FT-IR and XRD patterns confirmed showed that the chemical structure of irradiated chitosan was not obviously changed but, chemical structures of irradiated CS-DHBA and CS-THBA were changed. The tensile strength (TS) of chitosan films decreased after 12 h radiation but, the UV irradiated CS-DHBA and CS-THBA showed a significant adhesive capacity and enhanced tensile strength. This work significantly advances the understanding of how chitosan and its derivatives change their response to UV radiation, moving it beyond a degradable biopolymer to a structurally reinforced material with functional groups under UV irradiation. Also, these findings introduce benzoylfunctionalized chitosans as promising UVresponsive materials with potential applications in adhesives, coatings, photocurable systems, and structural biopolymer composites.
Acknowledgments
The authors are thankful from Kimia Gostaran Novin Azmay Tabriz Co.
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Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
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Competing InterestsThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Author Contributions
Ali Bahadori - Conceptualization, Methodology, Investigation, Data curation, Formal analysis, Writing – original draft.
Mohammad Taghi Taghizadeh - Supervision, Validation, Resources, Writing – review & editing.
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Data Availability
The data that support the findings of this study are available from the corresponding author upon reasonable request.
Electronic Supplementary Material
Below is the link to the electronic supplementary material
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