International Journal of Biological Macromolecules 70 (2014) 340–346
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International Journal of Biological Macromolecules journal homepage: www.elsevier.com/locate/ijbiomac
Physicochemical, mechanical and thermal properties of chitosan films with and without sorbitol Mei Liu, Yibin Zhou ∗ , Yang Zhang, Chen Yu, Shengnan Cao School of Tea and Food Technology, Anhui Agricultural University, 130 Chang Jiang West Road, Hefei 230036, China
a r t i c l e
i n f o
Article history: Received 10 February 2014 Received in revised form 11 June 2014 Accepted 14 June 2014 Available online 28 June 2014 Keywords: Chitosan film Sorbitol Physicochemical Mechanical Thermal properties
a b s t r a c t The effect of sorbitol on the physicochemical, mechanical and thermal properties of chitosan films with different degrees of deacetylation (DD; i.e., DD85% and DD95%) was investigated. The thickness, moisture content (MC), water solubility (WS) and water–vapor permeability (WVP) of the films were evaluated. Sorbitol addition reduced MC, increased WS and significantly (p < 0.01) reduced WVP of both film types. DD95% films had lower MC and WVP, and higher WS than DD85% films. Static (thermomechanical analysis) and dynamic (dynamic mechanical analysis) tests indicated that sorbitol increased the strain and decreased stress for both DD films, but DD95% could sustain higher strain and DD85% could sustain higher stress. Thermogravimetrics analysis and differential scanning calorimetry showed that sorbitol elicited a lower degradation temperature for both films, and that DD95% films exhibited higher thermal stability than DD85% films. © 2014 Elsevier B.V. All rights reserved.
1. Introduction In recent years, exploring the possibility of using biopolymerbased films as packaging materials has garnered interest because of their biocompatibility, biodegradation, non-toxic nature, antimicrobial activity and other environmentally friendly properties [1,2]. Chitosan, obtained by the partial N-deacetylation of chitin [3], is a candidate material for packaging films because of its filmforming property. However, films formed of pure chitosan are fragile and brittle, which limits their application [4]. The addition of polyols could improve the related properties of chitosan by reducing the frictional forces among polymer chains [5–7]. According to one report, the properties of chitosan films are dependent on the type and quantity of plasticizer [8] and vary with the nature of plasticizer used [9]. Sorbitol has better physicochemical and mechanical properties than other polyols [10,11]. In our previous study, the mechanism of film formation of chitosan films as well as the structure of different degrees of deacetylation of chitosan with and without sorbitol were investigated and characterized [12]. Differences in the molecular structure of a compound usually result in different properties. Thus,
∗ Corresponding author. Tel.: +86 551 5786342; fax: +86 551 5786342. E-mail addresses: [email protected] (M. Liu), [email protected] (Y. Zhou). http://dx.doi.org/10.1016/j.ijbiomac.2014.06.039 0141-8130/© 2014 Elsevier B.V. All rights reserved.
discussion of the properties of different chitosan films with different components is important and necessary to further illustrate the relationship between films various properties and their structure. The objective of the present work was to analyze the different physicochemical (thickness, moisture content [MC], water solubility [WS] and water vapor permeability [WVP]), mechanical (thermomechanical analysis [TMA] and dynamic mechanical analysis [DMA]) and thermal (thermogravimetric analysis [TGA] and differential scanning calorimetry [DSC]) properties of various films. This was carried out to (i) further clarify the influence of chitosan formation on the various properties of its films and (ii) to shed light on the effect of sorbitol and different degrees of deacetylation on the properties of chitosan films. Ultimately, these results will provide information for the better choice of suitable formation in eventually various applications of chitosan films as biodegradable packaging materials.
2. Materials and methods 2.1. Materials Chitosan with different degrees of deacetylation, i.e., 85%DD (molecular weight [MW] = 343.75 kDa) and 95%DD (MW = 312.5 kDa), were purchased from Zhejiang Golden-shell Biochemical Co. Ltd. (Zhejiang, China). The MC of the 85%DD and 95%DD chitosan were 11.92% ± 0.032% and 12.97% ± 0.042%, respectively. Sorbitol was purchased from Sigma–Aldrich (St Louis,
M. Liu et al. / International Journal of Biological Macromolecules 70 (2014) 340–346
MO, USA). All of the other chemicals used were of analytical grade and available commercially. 2.2. Preparation of film-forming solutions and film casting Samples of chitosan with different degrees of deacetylation (2%, w/v), with and without sorbitol (2%, w/v), were dissolved in 2% (v/v) acetic acid solution. The different solutions were used to prepare films via a previously described film-casting method with some modification [13]. The detailed processing methodology can be found in our previous publication [12]. Unspiked chitosan films and those spiked with sorbitol as a plasticizer were abbreviated to “CHF” and “CHFP”, respectively. 2.3. Measurement of physicochemical properties 2.3.1. Thickness Film thickness was measured using a CHY-C2 Thickness Gauge (Lab Think Co., Jinan, China). Samples were tested at 10 random points, and the mean value was calculated. 2.3.2. MC About 10 mg of each sample was balanced in a drying vessel until a constant weight M1 was achieved. Then, it was dried in an oven at 105 ◦ C for 24 h, and a constant weight, M2 , was reached. The MC was calculated as the percentage of water removed from the system using the following formula: MC (%) = M1 −
M2 × 100% M1
(1)
2.3.3. WS Films were cut into strips (4 cm × 2 cm). The initial dry weight was determined by drying the strips in an oven at 105 ◦ C to constant weight (Wi), and then immersed them in 50 mL distilled water with stirring at 100 rpm. After 24 h, the strips were removed and dried at 105 ◦ C until a constant weight (Wf) was achieved. WS was calculated using the following formula: WS (%) = Wi −
Wf × 100% Wi
(2)
2.3.4. WVP The WVP of the films was measured on a TSY-TI Water Permeability Tester (Lab Think Co. Jinan, China) according to GB1037-88 in a similar way to the ASTM E96 method [14]. Approximately the same thickness of each film with the diameter of 10 cm was tested. All samples were dried for 4 h before testing, and then the tested film was sealed on the top of a permeation cell containing distilled water, placed in the tester at 20 ◦ C with a relative humidity of 57% with silica gel. The preheating time was 4 h and then the loss of water weight over 24 h was measured. The instrument was adjusted with a 200-g weight before each examination. The WVP of films was determined using the following formula: WVP = WVTR ×
L p
(3)
where WVTR is the measured rate of transmission of water vapor (g/(m2 s)) through the film, L is the mean thickness of the film (mm), and p is the difference in the partial pressure of water vapor (kPa) across the two sides of the film [15]. 2.4. Static and dynamic mechanical analyses Static and dynamic mechanical properties of the films were evaluated on a Dynamic Thermal Mechanical Analyzer (DMTA Q800; TA Instruments, New Castle, DE, USA). Each sample of identical
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thickness was cut to the same width; the length was measured automatically by the instrument. Conditions for static mechanical test were an applied force from 0 N to 18 N with a speed of 0.25 N/min under a constant temperature of 25 ◦ C. For the dynamic mechanical test, samples were tested at a frequency of 10 Hz, temperature was in the range −100 ◦ C to 200 ◦ C with a heating rate of 5 ◦ C/min and displacement amplitude of 30 m. Values of storage modulus, loss modulus and loss tangent (tan ) were obtained as a function of temperature. 2.5. Thermal analyses 2.5.1. TGA A Thermogravimetric Analyzer (TGAQ5000, TA Instruments) was utilized to determine the variations in quality and thermal degradation during heating. Temperature scans were carried out from room temperature to 800 ◦ C at a heating rate of 10 ◦ C/min under a nitrogen atmosphere. Approximately 5 mg of each sample was placed in an alumina pan (A1203) with an empty pan used as a reference. The starting, conclusion and peak temperature (Tp ) and percentage weight loss at each stage were obtained from the derivative (DrTGA) curves. 2.5.2. DSC DSC was undertaken with a Perkin Elmer DSC-8000 system (Perkin Elmer, Waltham, MA, USA) calibrated with indium as the standard. Approximately 6 mg of each sample was placed in an aluminum pan (A1203) and sealed hermetically with an empty pan of the same type (used as a reference). Measurements between 30 ◦ C and 350 ◦ C were made at a heating rate of 10 ◦ C/min under a nitrogen atmosphere. Data were analyzed using Pyris Manager version 8.0 (Perkin Elmer). The onset, peak and conclusion of melting temperatures in the endothermic and exothermic phases were utilized as the onset temperature (To ), peak melting temperature (Tm ) and conclusion temperature (Tc ). Melting enthalpy (H) was employed as a criterion for comparison of the thermal stability of the film during the two-phase transition. 2.6. Statistical analyses Differences between factors and levels were evaluated by oneway analysis of variance (ANOVA) using SPSS version 20 (SPSS, Chicago, IL, USA). Tukey’s test was employed to compare the means in order to identify groups that were significantly different from other groups at a confidence level of 95%. The results were expressed as the mean and SD. 3. Results and discussion 3.1. Physicochemical properties of different chitosan films 3.1.1. Thickness The thickness of the different films is presented in Table 1. The thickness of the sorbitol-spiked and sorbitol-unspiked films was similar (p > 0.05). This observation suggested that sorbitol did not influence the thickness of films, a finding that was in accordance with that of Piermaria et al. [16]. 3.1.2. MC The MC of different films is summarized in Table 1. DD95% films (spiked and unspiked with sorbitol) did not display significantly (p > 0.05) lower MC than those of the corresponding DD85% films. This result suggested that deacetylation degree of 85%, 95% presented little difference on chitosan films moisture content. The little variation might be ascribed to the higher crystallinity of the DD95% films elaborated in our previous work [12] which decreased
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Table 1 Physicochemical properties of chitosan films with different degrees of deacetylation (DD) with (CHFP) and without (CHF) sorbitol. Samples DD85% CHFP DD85% CHF DD95% CHFP DD95% CHF
Thickness (m) 28.3 28.1 28.7 28.5
± ± ± ±
a
0.8 1.1a 1.1a 1.4a
Moisture content (%) 16.98 17.15 16.72 17.02
± ± ± ±
WVP × 10−11 (g/m s Pa)
Water solubility (%)
ab
0.38 0.05b 0.15a 0.27ab
2.84 2.58 3.22 2.98
± ± ± ±
ab
0.27 0.14a 0.41b 0.17ab
2.82 2.96 2.78 2.94
± ± ± ±
0.034B 0.043C 0.032A 0.038C
Values are expressed as the mean and SD. Different letters in the same column indicate a significant difference (p < 0.05) on Tukey’s test. A capital letter denotes p < 0.01; a lowercase letter denotes p < 0.05.
3.1.3. WS The WS of the films is presented in Table 1. WS of DD95% chitosan films with and without sorbitol were not significantly (p > 0.05) higher than those of the corresponding DD85% chitosan films. This result indicated that deacetylation degree did not generate much difference on WS. Feng et al. [21] and Rivero et al. [22] found that highly deacetylated chitosan films possessed more amino groups, and this increase in the number of hydrophilic groups enhanced the WS of the chitosan films. Therefore, DD95% chitosan films with and without sorbitol had little higher WS than those of DD85% films. Addition of sorbitol to the DD85% and DD95% films did not significantly (p > 0.05) increased the WS of films (Table 1). The little WS increase for spiked films might be attributed to the higher hydrophilicity of sorbitol [23–25]. 3.1.4. WVP WVP is the most extensively studied property of edible films, mainly because of the importance of water in the deterioration of foods [26]. The WVP of the four samples is listed in Table 1. The WVP of DD85% chitosan films (spiked and unspiked with sorbitol) was slightly higher than that of DD95% films, which could be attributed to the higher MW of DD85%. Report had illustrated that the WVP of chitosan films increased with increase in MW [27]. As to the degree of deacetylation, it generated virtually no differences in the WVP [28], therefore, the WVP of DD85% unspiked film were not significantly (p > 0.05) higher than that of DD95% film. The WVP of sorbitol-spiked films was significantly (p < 0.01) lower than that of unspiked films for both DD95% and DD85% films, a finding that was in accordance with the results of the addition of glycerol and sorbitol to chitosan and cassava flour films [25,29,30]. This result corroborated the notion that sorbitol-spiked films showed increased bond reactions among the blend system, thereby contributing to the development of a more compact, dense, regular structure that led to the water vapor following a tortuous path through films [15,19,31]. This result verified the observations regarding the MC of spiked films. 3.2. Static and dynamic mechanical properties 3.2.1. Static mechanical properties The stress–strain properties of different films are presented in Fig. 1. Few differences were observed because they all displayed three types of deformation regions: elastic, yield and rupture. However, the mechanical behavior of unplasticized chitosan films for both degrees of deacetylation showed a typical pattern of brittle
and rigid materials because they exhibited high values of stress and low values of strain at break [25,32]. The deformation mode was affected by sorbitol addition, with the stress decreased and strain increased of films for both degrees of deacetylation. Garcia et al. [25] reported that the strain of plasticized films at break increased and stress decreased compared with those of unplasticized films. Other authors also observed similar results [9,32–34]. This phenomenon could have occurred because sorbitol addition interfered with the chitosan chains, which facilitated the mobility among different molecules and thereby enhanced flexibility [25,35]. For both spiked and unspiked films, the stress was slightly higher for DD85% films than for DD95% films, whereas the strain was slightly higher for DD95% films than for DD85% films (especially for unspiked films). Unspiked films of DD85% ruptured at a strain of 40%, whereas unspiked DD95% films ruptured at a strain of 50% (Fig. 1). Martínez-Camacho et al. [34] thought that the higher strain and stress were related to the MW and the deacetylation degree of chitosan. A higher MW would produce higher stress and lower strain [36,37]. 3.2.2. DMA patterns Sorbitol addition decreased the storage modulus and loss modulus of chitosan films for both degrees of deacetylation (Fig. 2A and B), thereby confirming the results of the work by Matet et al. [11]. Thakhiew et al. [38] reported that a lower storage modulus usually meant higher elongation at break, lower loss modulus meant lower tensile strength. Therefore, the presence of sorbitol increased the elongation at break and reduced the tensile strength of chitosan films. Thakhiew et al. [38] also described that higher galangal extract concentrations induced larger number of crosslinking interactions and then led to increase in storage modulus. Meanwhile, Fundo et al. [39] using NMR to observe the molecular mobility in chitosan films plasticized by glycerol and results showed that plasticizer reduces the intermolecular forces and increases the 60 DD95%CHF DD85%CHF
50
DD85%CHFP DD95%CHFP
40
Stress (MPa)
the MC [17]. Chen et al. also described that higher molecular chitosan films prepared by acetic acid had lower MC than those of low molecular chitosan films [18]. Spiked films did not show significantly (p > 0.05) lower MC than unspiked films (Table 1). The little difference for spiked and unspiked films might because that the hydroxyl groups of sorbitol reacted with the hydroxyl and amino groups of chitosan, which in turn decreased the extent of combination between chitosan and water through hydrogen bonding [19,20].
30 20 10 0
0
10
20
30
Strain (%)
40
50
60
Fig. 1. Stress–strain curves of degree of deacetylation (DD) 85% and 95% chitosan films with (CHFP) and without (CHF) sorbitol.
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According to the tan curves (Fig. 2C), two peaks appeared for each sample, which represented -relaxation and ␣-relaxation, respectively. -relaxation corresponds to the motions of the side chains, and ␣-relaxation reflects the characteristic glass transition temperature (Tg ) [13]. Values of -relaxation temperatures of the four samples varied slightly (−56 ◦ C to −58 ◦ C), while the ␣-relaxation temperatures differed a lot (35–63 ◦ C). This observation was similar to that of Epure et al. [13]. The -relaxation temperatures and ␣-relaxation temperatures of spiked films for both degrees of deacetylation were slightly higher than those of unspiked films, and the relaxation temperature of DD95% chitosan films was higher than that of DD85% films. These results suggested that sorbitol as a plasticizer increased the Tg , decreased the mechanical stiffness and increased the elasticity of chitosan films [42]. It also indicated that spiked films and DD95% chitosan films were more mobile, and exhibited less solid-like behavior and stiffness; these results were in accordance with the changes in storage modulus and loss modulus.
3.3. Thermal properties of different chitosan powders and films
Fig. 2. Storage modulus (A), loss modulus (B) and tan (C) of different deacetylation degrees (DD) chitosan films with (CHFP) and without (CHF) sorbitol.
overall mobility in the matrix. Thus, DMA test results suggested that the crosslinkage interactions among spiked films molecular were weaker than those in unspiked films and confirmed that the blend of sorbitol led to an increase in mobility within the blend system [38,40]. The storage modulus and loss modulus of DD95% films were higher than those of DD85% films, indicating that with the increase in deacetylation, the elongation percent of chitosan films decreased whereas the tensile strength increased [27,41].
3.3.1. TGA Thermal analysis is conducted to better understand the behavior and barrier properties of dry films [43]. Thermal analysis is considered to be the most important method for studying the thermal stability of polymers [44]. The thermal degradation behavior of both chitosan powders (CHs) and films with (CHFP) and without (CHF) sorbitol at both degrees of deacetylation is displayed in Fig. 3. The partial weight loss (Pw ) for each step, total weight loss (Tw ) and Tp values are summarized in Table 2. All types of CHs, spiked and unspiked films appeared to have two decomposition stages. The first degradation step was recorded from approximately 37–150 ◦ C, with a small weight loss from 11% to 15.5%. The second stage of degradation ranged from 190 ◦ C to 605 ◦ C, and corresponded to a weight loss of 50–56%. Similar results had been reported by Martínez-Camacho et al. [34] and Casariego et al. [15]. The first stage of weight loss resulted from the evaporation of water molecules or volatilization of small molecules; the second stage was attributed to a complex process involving the dehydration of saccharide rings as well as decomposition of the acetylated and deacetylated units of the polymers [43,45,46]. The Tp at each stage signified the fastest degradation rate at a particular temperature, and was obtained from the DrTGA curves. For both types of CHs, the first-event Tp was slightly lower than that for the films, whereas the second-event Tp was slightly higher than that for the films (Table 2). For the formed films, the first-event Tp of spiked films was lower than that of unspiked films, and for the second event, the Tp was almost identical. These data suggested that the thermal stability of spiked films was slightly lower than that of unspiked films for both degrees of deacetylation. Ramos et al. [35] also reported that plasticizers decreased the initial and peak degradation temperatures of edible films. For the two stages, the Tp of the DD95% samples was higher than that of the DD85% samples, which suggested that the thermal stability of DD95% CHs and films was slightly higher than that of DD85% CHs and films [45]. Table 2 suggested that all spiked films had slightly higher total weight loss than the unspiked films. These results could be ascribed to the evaporation of sorbitol. Similar results were observed by Kurek et al. [43] and Ramos et al. [35]. Besides, the total weight loss of the various CHs was much higher than that of the corresponding films, which could confirm that after formation into films with and without sorbitol, thermal stability was improved to some extent.
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Fig. 3. TGA and DrTGA curves of chitosan powder (CH) and films without (CHF) and with (CHFP) sorbitol, and different degrees of deacetylation (DD). (A) DD85% CH, (B) DD95% CH, (C) DD85% CHF, (D) DD95% CHF, (E) DD85% CHFP, (F) DD95% CHFP.
3.3.2. DSC DSC thermograms of CHs as well as unspiked (CHF) and spiked chitosan (CHFP) films are displayed in Fig. 4. The endothermic phase (40 ◦ C to 140 ◦ C) and exothermic phase (250 ◦ C to 320 ◦ C) of different samples are presented in Table 3. Feng et al. [21]
and Martínez-Camacho et al. [34] stated that the endothermic events between 40 ◦ C and 120 ◦ C could be attributed to the evaporation of absorbed and constituted water, volatile components and hydrophilic groups in CHs and films. Whereas the exothermic events between 250 ◦ C and 350 ◦ C could be ascribed to the cracking
Table 2 TGA and DrTGA data of chitosan powders (CH), unspiked films (CHF) and spiked films (CHFP) with different degrees of deacetylation (DD). Samples
DD85%
DD95%
CH
CHF
CHFP
CH
CHF
Tp (◦ C)
54.81 297.58
60.06 273.29
53.77 273.29
54.92 297.74
66.15 276.16
59.53 275.21
Pw (%)
11.65 55.90
15.15 50.05
13.85 51.90
11.25 55.47
14.52 50.21
14.62 50.87
Tw (%)
67.55
65.2
65.75
66.72
64.73
65.49
Tp , peak temperature; Pw , partial weight loss; Tw , total weight loss.
CHFP
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Table 3 Onset temperature (To ), peak melting temperature (Tm ), conclusion temperature (To ) and melting enthalpy (H) of chitosan powders (CH), unspiked films (CHF) and spiked films (CHFP) with different degrees of deacetylation (DD). Endothermic phase
Exothermic phase
Samples
To (◦ C)
Tm (◦ C)
Tc (◦ C)
H (J/g)
To (◦ C)
Tm (◦ C)
Tc (◦ C)
H (J/g)
DD85% CH DD85% CHF DD85% CHFP DD95% CH DD95% CHF DD95% CHFP
42.6 76.8 74 55.38 78.09 74
76.91 104.9 92.5 92.23 111.5 109.55
111.12 136.25 140.97 119.52 116.87 129.72
188.92 273.22 169.37 282.61 110.04 187.35
276.84 267.9 252.4 277.22 272.55 258.4
302.46 286.52 282.64 302.45 288.77 284.63
321.76 311.34 306.91 319.03 302.55 311.98
−231.18 −173.69 −168.68 −148.58 −178.35 −162.48
and degradation of CHs and films. In addition, the temperatures of the endothermic and exothermic phases verified the different temperatures of decomposition events in the above described TGA. The thermodynamic properties of the CHs changed to some extent after their formation into films (Table 3). Compared with the powders, the To and Tm of the spiked and unspiked films in the endothermic phase exhibited a right-shift, whereas they displayed a left-shift in the exothermic phase. This result might suggest that after CH formed into films, its Tm increased and thermal stability improved. This hypothesis was in accordance with our TGA data. The decrease in To and Tm in the exothermic phase could have been because when CH was dissolved in acetic acid solution with and without sorbitol, the polymer molecules in the chain unfolded gradually. To and Tm in the endothermic and exothermic phases of the DD95% CHs and films were higher than those of the DD85% CHs and films. It elucidated that the thermal stability of the DD95% samples was stronger than that of the DD85% samples. This was probably related to the degree of deacetylation of chitosan. Greater deacetylation was associated with more amide ions, which facilitated the formation of more hydrogen bonds and a more stable molecular structure [21]. To and Tm in the endothermic and exothermic phases were lower for the spiked films than for unspiked films. These results confirmed previous work showing that sorbitol as a plasticizer decreased the To and Tm of films [35,43]. These phenomena could also be associated with a decrease in crystallinity [26,31]. Sorbitol-spiked films exhibited crystal transformation during film formation [12], which may also explain the changes in the To and Tm of different films. Thermodynamic properties demonstrated that the melting range from To to Tc of a material reflects its crystallinity [47]. A
wide melting range of a material signifies that it has lower crystallinity. Table 3 showed that the melting range of the DD95% CHs and films was narrower than that of DD85% CHs and films. These results confirmed that the crystallinity of the former was higher than that of the latter [12]. Moreover, the melting range of CHFP was the widest, and that of CHF was the narrowest, for both DD85% and DD95%. These data implied that the crystallinity of CHFP and CHF was the lowest and highest, respectively, which was in accordance with our previous work [12]. 4. Conclusion The physicochemical properties of different chitosan films suggested that sorbitol addition reduced the MC and increased WS of chitosan films. WVP decreased significantly (p < 0.01) with the presence of sorbitol in DD85% and DD95% chitosan films. For spiked and unspiked films, DD95% displayed lower MC, higher WS and lower WVP than those of DD85% films. Static (TMA) and dynamic (DMA) mechanical analyses demonstrated that sorbitol increased the strain, decreased stress and increased mobility among the molecules in the blend, which resulted in spiked films being less stiff and more flexible. Spiked and unspiked DD95% films had higher tensile strength, lower elongation percent as well as higher -relaxation temperatures and ␣-relaxation temperatures than those of DD85% films. TGA and DSC revealed that the thermal stability of formed films for both degrees of deacetylation increased compared with their powders, and that spiked films showed lower thermal stability than unspiked films. DD95% films had higher thermal stability than DD85% films. DSC showed that the crystallinity of DD95% powder and film was higher than that of DD85% powder and film. This work analyzed the various properties of the four types of chitosan films and the reasons that caused the differences. However, there are still some properties such as WVP and mechanical properties needed to be improved to meet the demand of chitosan films various applications. Acknowledgments We gratefully acknowledge the staff of the University of Science and Technology of China for their support and comments during the preparation of this manuscript. This work was supported by a grant from the National Science Foundation of China (No. 31271960). References [1] [2] [3] [4] [5] [6]
Fig. 4. DSC thermograms of degree of deacetylation (DD) 85% and 95% chitosan powder (CH) as well as films without (CHF) and with (CHFP) sorbitol. (a) DD85% CH, (b) DD85% CHF, (c) DD85% CHFP, (d) DD95% CH, (e) DD95% CHF, (f) DD95% CHFP.
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Contents lists available at ScienceDirect
International Journal of Biological Macromolecules journal homepage: www.elsevier.com/locate/ijbiomac
Physicochemical, mechanical and thermal properties of chitosan films with and without sorbitol Mei Liu, Yibin Zhou ∗ , Yang Zhang, Chen Yu, Shengnan Cao School of Tea and Food Technology, Anhui Agricultural University, 130 Chang Jiang West Road, Hefei 230036, China
a r t i c l e
i n f o
Article history: Received 10 February 2014 Received in revised form 11 June 2014 Accepted 14 June 2014 Available online 28 June 2014 Keywords: Chitosan film Sorbitol Physicochemical Mechanical Thermal properties
a b s t r a c t The effect of sorbitol on the physicochemical, mechanical and thermal properties of chitosan films with different degrees of deacetylation (DD; i.e., DD85% and DD95%) was investigated. The thickness, moisture content (MC), water solubility (WS) and water–vapor permeability (WVP) of the films were evaluated. Sorbitol addition reduced MC, increased WS and significantly (p < 0.01) reduced WVP of both film types. DD95% films had lower MC and WVP, and higher WS than DD85% films. Static (thermomechanical analysis) and dynamic (dynamic mechanical analysis) tests indicated that sorbitol increased the strain and decreased stress for both DD films, but DD95% could sustain higher strain and DD85% could sustain higher stress. Thermogravimetrics analysis and differential scanning calorimetry showed that sorbitol elicited a lower degradation temperature for both films, and that DD95% films exhibited higher thermal stability than DD85% films. © 2014 Elsevier B.V. All rights reserved.
1. Introduction In recent years, exploring the possibility of using biopolymerbased films as packaging materials has garnered interest because of their biocompatibility, biodegradation, non-toxic nature, antimicrobial activity and other environmentally friendly properties [1,2]. Chitosan, obtained by the partial N-deacetylation of chitin [3], is a candidate material for packaging films because of its filmforming property. However, films formed of pure chitosan are fragile and brittle, which limits their application [4]. The addition of polyols could improve the related properties of chitosan by reducing the frictional forces among polymer chains [5–7]. According to one report, the properties of chitosan films are dependent on the type and quantity of plasticizer [8] and vary with the nature of plasticizer used [9]. Sorbitol has better physicochemical and mechanical properties than other polyols [10,11]. In our previous study, the mechanism of film formation of chitosan films as well as the structure of different degrees of deacetylation of chitosan with and without sorbitol were investigated and characterized [12]. Differences in the molecular structure of a compound usually result in different properties. Thus,
∗ Corresponding author. Tel.: +86 551 5786342; fax: +86 551 5786342. E-mail addresses: [email protected] (M. Liu), [email protected] (Y. Zhou). http://dx.doi.org/10.1016/j.ijbiomac.2014.06.039 0141-8130/© 2014 Elsevier B.V. All rights reserved.
discussion of the properties of different chitosan films with different components is important and necessary to further illustrate the relationship between films various properties and their structure. The objective of the present work was to analyze the different physicochemical (thickness, moisture content [MC], water solubility [WS] and water vapor permeability [WVP]), mechanical (thermomechanical analysis [TMA] and dynamic mechanical analysis [DMA]) and thermal (thermogravimetric analysis [TGA] and differential scanning calorimetry [DSC]) properties of various films. This was carried out to (i) further clarify the influence of chitosan formation on the various properties of its films and (ii) to shed light on the effect of sorbitol and different degrees of deacetylation on the properties of chitosan films. Ultimately, these results will provide information for the better choice of suitable formation in eventually various applications of chitosan films as biodegradable packaging materials.
2. Materials and methods 2.1. Materials Chitosan with different degrees of deacetylation, i.e., 85%DD (molecular weight [MW] = 343.75 kDa) and 95%DD (MW = 312.5 kDa), were purchased from Zhejiang Golden-shell Biochemical Co. Ltd. (Zhejiang, China). The MC of the 85%DD and 95%DD chitosan were 11.92% ± 0.032% and 12.97% ± 0.042%, respectively. Sorbitol was purchased from Sigma–Aldrich (St Louis,
M. Liu et al. / International Journal of Biological Macromolecules 70 (2014) 340–346
MO, USA). All of the other chemicals used were of analytical grade and available commercially. 2.2. Preparation of film-forming solutions and film casting Samples of chitosan with different degrees of deacetylation (2%, w/v), with and without sorbitol (2%, w/v), were dissolved in 2% (v/v) acetic acid solution. The different solutions were used to prepare films via a previously described film-casting method with some modification [13]. The detailed processing methodology can be found in our previous publication [12]. Unspiked chitosan films and those spiked with sorbitol as a plasticizer were abbreviated to “CHF” and “CHFP”, respectively. 2.3. Measurement of physicochemical properties 2.3.1. Thickness Film thickness was measured using a CHY-C2 Thickness Gauge (Lab Think Co., Jinan, China). Samples were tested at 10 random points, and the mean value was calculated. 2.3.2. MC About 10 mg of each sample was balanced in a drying vessel until a constant weight M1 was achieved. Then, it was dried in an oven at 105 ◦ C for 24 h, and a constant weight, M2 , was reached. The MC was calculated as the percentage of water removed from the system using the following formula: MC (%) = M1 −
M2 × 100% M1
(1)
2.3.3. WS Films were cut into strips (4 cm × 2 cm). The initial dry weight was determined by drying the strips in an oven at 105 ◦ C to constant weight (Wi), and then immersed them in 50 mL distilled water with stirring at 100 rpm. After 24 h, the strips were removed and dried at 105 ◦ C until a constant weight (Wf) was achieved. WS was calculated using the following formula: WS (%) = Wi −
Wf × 100% Wi
(2)
2.3.4. WVP The WVP of the films was measured on a TSY-TI Water Permeability Tester (Lab Think Co. Jinan, China) according to GB1037-88 in a similar way to the ASTM E96 method [14]. Approximately the same thickness of each film with the diameter of 10 cm was tested. All samples were dried for 4 h before testing, and then the tested film was sealed on the top of a permeation cell containing distilled water, placed in the tester at 20 ◦ C with a relative humidity of 57% with silica gel. The preheating time was 4 h and then the loss of water weight over 24 h was measured. The instrument was adjusted with a 200-g weight before each examination. The WVP of films was determined using the following formula: WVP = WVTR ×
L p
(3)
where WVTR is the measured rate of transmission of water vapor (g/(m2 s)) through the film, L is the mean thickness of the film (mm), and p is the difference in the partial pressure of water vapor (kPa) across the two sides of the film [15]. 2.4. Static and dynamic mechanical analyses Static and dynamic mechanical properties of the films were evaluated on a Dynamic Thermal Mechanical Analyzer (DMTA Q800; TA Instruments, New Castle, DE, USA). Each sample of identical
341
thickness was cut to the same width; the length was measured automatically by the instrument. Conditions for static mechanical test were an applied force from 0 N to 18 N with a speed of 0.25 N/min under a constant temperature of 25 ◦ C. For the dynamic mechanical test, samples were tested at a frequency of 10 Hz, temperature was in the range −100 ◦ C to 200 ◦ C with a heating rate of 5 ◦ C/min and displacement amplitude of 30 m. Values of storage modulus, loss modulus and loss tangent (tan ) were obtained as a function of temperature. 2.5. Thermal analyses 2.5.1. TGA A Thermogravimetric Analyzer (TGAQ5000, TA Instruments) was utilized to determine the variations in quality and thermal degradation during heating. Temperature scans were carried out from room temperature to 800 ◦ C at a heating rate of 10 ◦ C/min under a nitrogen atmosphere. Approximately 5 mg of each sample was placed in an alumina pan (A1203) with an empty pan used as a reference. The starting, conclusion and peak temperature (Tp ) and percentage weight loss at each stage were obtained from the derivative (DrTGA) curves. 2.5.2. DSC DSC was undertaken with a Perkin Elmer DSC-8000 system (Perkin Elmer, Waltham, MA, USA) calibrated with indium as the standard. Approximately 6 mg of each sample was placed in an aluminum pan (A1203) and sealed hermetically with an empty pan of the same type (used as a reference). Measurements between 30 ◦ C and 350 ◦ C were made at a heating rate of 10 ◦ C/min under a nitrogen atmosphere. Data were analyzed using Pyris Manager version 8.0 (Perkin Elmer). The onset, peak and conclusion of melting temperatures in the endothermic and exothermic phases were utilized as the onset temperature (To ), peak melting temperature (Tm ) and conclusion temperature (Tc ). Melting enthalpy (H) was employed as a criterion for comparison of the thermal stability of the film during the two-phase transition. 2.6. Statistical analyses Differences between factors and levels were evaluated by oneway analysis of variance (ANOVA) using SPSS version 20 (SPSS, Chicago, IL, USA). Tukey’s test was employed to compare the means in order to identify groups that were significantly different from other groups at a confidence level of 95%. The results were expressed as the mean and SD. 3. Results and discussion 3.1. Physicochemical properties of different chitosan films 3.1.1. Thickness The thickness of the different films is presented in Table 1. The thickness of the sorbitol-spiked and sorbitol-unspiked films was similar (p > 0.05). This observation suggested that sorbitol did not influence the thickness of films, a finding that was in accordance with that of Piermaria et al. [16]. 3.1.2. MC The MC of different films is summarized in Table 1. DD95% films (spiked and unspiked with sorbitol) did not display significantly (p > 0.05) lower MC than those of the corresponding DD85% films. This result suggested that deacetylation degree of 85%, 95% presented little difference on chitosan films moisture content. The little variation might be ascribed to the higher crystallinity of the DD95% films elaborated in our previous work [12] which decreased
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Table 1 Physicochemical properties of chitosan films with different degrees of deacetylation (DD) with (CHFP) and without (CHF) sorbitol. Samples DD85% CHFP DD85% CHF DD95% CHFP DD95% CHF
Thickness (m) 28.3 28.1 28.7 28.5
± ± ± ±
a
0.8 1.1a 1.1a 1.4a
Moisture content (%) 16.98 17.15 16.72 17.02
± ± ± ±
WVP × 10−11 (g/m s Pa)
Water solubility (%)
ab
0.38 0.05b 0.15a 0.27ab
2.84 2.58 3.22 2.98
± ± ± ±
ab
0.27 0.14a 0.41b 0.17ab
2.82 2.96 2.78 2.94
± ± ± ±
0.034B 0.043C 0.032A 0.038C
Values are expressed as the mean and SD. Different letters in the same column indicate a significant difference (p < 0.05) on Tukey’s test. A capital letter denotes p < 0.01; a lowercase letter denotes p < 0.05.
3.1.3. WS The WS of the films is presented in Table 1. WS of DD95% chitosan films with and without sorbitol were not significantly (p > 0.05) higher than those of the corresponding DD85% chitosan films. This result indicated that deacetylation degree did not generate much difference on WS. Feng et al. [21] and Rivero et al. [22] found that highly deacetylated chitosan films possessed more amino groups, and this increase in the number of hydrophilic groups enhanced the WS of the chitosan films. Therefore, DD95% chitosan films with and without sorbitol had little higher WS than those of DD85% films. Addition of sorbitol to the DD85% and DD95% films did not significantly (p > 0.05) increased the WS of films (Table 1). The little WS increase for spiked films might be attributed to the higher hydrophilicity of sorbitol [23–25]. 3.1.4. WVP WVP is the most extensively studied property of edible films, mainly because of the importance of water in the deterioration of foods [26]. The WVP of the four samples is listed in Table 1. The WVP of DD85% chitosan films (spiked and unspiked with sorbitol) was slightly higher than that of DD95% films, which could be attributed to the higher MW of DD85%. Report had illustrated that the WVP of chitosan films increased with increase in MW [27]. As to the degree of deacetylation, it generated virtually no differences in the WVP [28], therefore, the WVP of DD85% unspiked film were not significantly (p > 0.05) higher than that of DD95% film. The WVP of sorbitol-spiked films was significantly (p < 0.01) lower than that of unspiked films for both DD95% and DD85% films, a finding that was in accordance with the results of the addition of glycerol and sorbitol to chitosan and cassava flour films [25,29,30]. This result corroborated the notion that sorbitol-spiked films showed increased bond reactions among the blend system, thereby contributing to the development of a more compact, dense, regular structure that led to the water vapor following a tortuous path through films [15,19,31]. This result verified the observations regarding the MC of spiked films. 3.2. Static and dynamic mechanical properties 3.2.1. Static mechanical properties The stress–strain properties of different films are presented in Fig. 1. Few differences were observed because they all displayed three types of deformation regions: elastic, yield and rupture. However, the mechanical behavior of unplasticized chitosan films for both degrees of deacetylation showed a typical pattern of brittle
and rigid materials because they exhibited high values of stress and low values of strain at break [25,32]. The deformation mode was affected by sorbitol addition, with the stress decreased and strain increased of films for both degrees of deacetylation. Garcia et al. [25] reported that the strain of plasticized films at break increased and stress decreased compared with those of unplasticized films. Other authors also observed similar results [9,32–34]. This phenomenon could have occurred because sorbitol addition interfered with the chitosan chains, which facilitated the mobility among different molecules and thereby enhanced flexibility [25,35]. For both spiked and unspiked films, the stress was slightly higher for DD85% films than for DD95% films, whereas the strain was slightly higher for DD95% films than for DD85% films (especially for unspiked films). Unspiked films of DD85% ruptured at a strain of 40%, whereas unspiked DD95% films ruptured at a strain of 50% (Fig. 1). Martínez-Camacho et al. [34] thought that the higher strain and stress were related to the MW and the deacetylation degree of chitosan. A higher MW would produce higher stress and lower strain [36,37]. 3.2.2. DMA patterns Sorbitol addition decreased the storage modulus and loss modulus of chitosan films for both degrees of deacetylation (Fig. 2A and B), thereby confirming the results of the work by Matet et al. [11]. Thakhiew et al. [38] reported that a lower storage modulus usually meant higher elongation at break, lower loss modulus meant lower tensile strength. Therefore, the presence of sorbitol increased the elongation at break and reduced the tensile strength of chitosan films. Thakhiew et al. [38] also described that higher galangal extract concentrations induced larger number of crosslinking interactions and then led to increase in storage modulus. Meanwhile, Fundo et al. [39] using NMR to observe the molecular mobility in chitosan films plasticized by glycerol and results showed that plasticizer reduces the intermolecular forces and increases the 60 DD95%CHF DD85%CHF
50
DD85%CHFP DD95%CHFP
40
Stress (MPa)
the MC [17]. Chen et al. also described that higher molecular chitosan films prepared by acetic acid had lower MC than those of low molecular chitosan films [18]. Spiked films did not show significantly (p > 0.05) lower MC than unspiked films (Table 1). The little difference for spiked and unspiked films might because that the hydroxyl groups of sorbitol reacted with the hydroxyl and amino groups of chitosan, which in turn decreased the extent of combination between chitosan and water through hydrogen bonding [19,20].
30 20 10 0
0
10
20
30
Strain (%)
40
50
60
Fig. 1. Stress–strain curves of degree of deacetylation (DD) 85% and 95% chitosan films with (CHFP) and without (CHF) sorbitol.
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According to the tan curves (Fig. 2C), two peaks appeared for each sample, which represented -relaxation and ␣-relaxation, respectively. -relaxation corresponds to the motions of the side chains, and ␣-relaxation reflects the characteristic glass transition temperature (Tg ) [13]. Values of -relaxation temperatures of the four samples varied slightly (−56 ◦ C to −58 ◦ C), while the ␣-relaxation temperatures differed a lot (35–63 ◦ C). This observation was similar to that of Epure et al. [13]. The -relaxation temperatures and ␣-relaxation temperatures of spiked films for both degrees of deacetylation were slightly higher than those of unspiked films, and the relaxation temperature of DD95% chitosan films was higher than that of DD85% films. These results suggested that sorbitol as a plasticizer increased the Tg , decreased the mechanical stiffness and increased the elasticity of chitosan films [42]. It also indicated that spiked films and DD95% chitosan films were more mobile, and exhibited less solid-like behavior and stiffness; these results were in accordance with the changes in storage modulus and loss modulus.
3.3. Thermal properties of different chitosan powders and films
Fig. 2. Storage modulus (A), loss modulus (B) and tan (C) of different deacetylation degrees (DD) chitosan films with (CHFP) and without (CHF) sorbitol.
overall mobility in the matrix. Thus, DMA test results suggested that the crosslinkage interactions among spiked films molecular were weaker than those in unspiked films and confirmed that the blend of sorbitol led to an increase in mobility within the blend system [38,40]. The storage modulus and loss modulus of DD95% films were higher than those of DD85% films, indicating that with the increase in deacetylation, the elongation percent of chitosan films decreased whereas the tensile strength increased [27,41].
3.3.1. TGA Thermal analysis is conducted to better understand the behavior and barrier properties of dry films [43]. Thermal analysis is considered to be the most important method for studying the thermal stability of polymers [44]. The thermal degradation behavior of both chitosan powders (CHs) and films with (CHFP) and without (CHF) sorbitol at both degrees of deacetylation is displayed in Fig. 3. The partial weight loss (Pw ) for each step, total weight loss (Tw ) and Tp values are summarized in Table 2. All types of CHs, spiked and unspiked films appeared to have two decomposition stages. The first degradation step was recorded from approximately 37–150 ◦ C, with a small weight loss from 11% to 15.5%. The second stage of degradation ranged from 190 ◦ C to 605 ◦ C, and corresponded to a weight loss of 50–56%. Similar results had been reported by Martínez-Camacho et al. [34] and Casariego et al. [15]. The first stage of weight loss resulted from the evaporation of water molecules or volatilization of small molecules; the second stage was attributed to a complex process involving the dehydration of saccharide rings as well as decomposition of the acetylated and deacetylated units of the polymers [43,45,46]. The Tp at each stage signified the fastest degradation rate at a particular temperature, and was obtained from the DrTGA curves. For both types of CHs, the first-event Tp was slightly lower than that for the films, whereas the second-event Tp was slightly higher than that for the films (Table 2). For the formed films, the first-event Tp of spiked films was lower than that of unspiked films, and for the second event, the Tp was almost identical. These data suggested that the thermal stability of spiked films was slightly lower than that of unspiked films for both degrees of deacetylation. Ramos et al. [35] also reported that plasticizers decreased the initial and peak degradation temperatures of edible films. For the two stages, the Tp of the DD95% samples was higher than that of the DD85% samples, which suggested that the thermal stability of DD95% CHs and films was slightly higher than that of DD85% CHs and films [45]. Table 2 suggested that all spiked films had slightly higher total weight loss than the unspiked films. These results could be ascribed to the evaporation of sorbitol. Similar results were observed by Kurek et al. [43] and Ramos et al. [35]. Besides, the total weight loss of the various CHs was much higher than that of the corresponding films, which could confirm that after formation into films with and without sorbitol, thermal stability was improved to some extent.
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Fig. 3. TGA and DrTGA curves of chitosan powder (CH) and films without (CHF) and with (CHFP) sorbitol, and different degrees of deacetylation (DD). (A) DD85% CH, (B) DD95% CH, (C) DD85% CHF, (D) DD95% CHF, (E) DD85% CHFP, (F) DD95% CHFP.
3.3.2. DSC DSC thermograms of CHs as well as unspiked (CHF) and spiked chitosan (CHFP) films are displayed in Fig. 4. The endothermic phase (40 ◦ C to 140 ◦ C) and exothermic phase (250 ◦ C to 320 ◦ C) of different samples are presented in Table 3. Feng et al. [21]
and Martínez-Camacho et al. [34] stated that the endothermic events between 40 ◦ C and 120 ◦ C could be attributed to the evaporation of absorbed and constituted water, volatile components and hydrophilic groups in CHs and films. Whereas the exothermic events between 250 ◦ C and 350 ◦ C could be ascribed to the cracking
Table 2 TGA and DrTGA data of chitosan powders (CH), unspiked films (CHF) and spiked films (CHFP) with different degrees of deacetylation (DD). Samples
DD85%
DD95%
CH
CHF
CHFP
CH
CHF
Tp (◦ C)
54.81 297.58
60.06 273.29
53.77 273.29
54.92 297.74
66.15 276.16
59.53 275.21
Pw (%)
11.65 55.90
15.15 50.05
13.85 51.90
11.25 55.47
14.52 50.21
14.62 50.87
Tw (%)
67.55
65.2
65.75
66.72
64.73
65.49
Tp , peak temperature; Pw , partial weight loss; Tw , total weight loss.
CHFP
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Table 3 Onset temperature (To ), peak melting temperature (Tm ), conclusion temperature (To ) and melting enthalpy (H) of chitosan powders (CH), unspiked films (CHF) and spiked films (CHFP) with different degrees of deacetylation (DD). Endothermic phase
Exothermic phase
Samples
To (◦ C)
Tm (◦ C)
Tc (◦ C)
H (J/g)
To (◦ C)
Tm (◦ C)
Tc (◦ C)
H (J/g)
DD85% CH DD85% CHF DD85% CHFP DD95% CH DD95% CHF DD95% CHFP
42.6 76.8 74 55.38 78.09 74
76.91 104.9 92.5 92.23 111.5 109.55
111.12 136.25 140.97 119.52 116.87 129.72
188.92 273.22 169.37 282.61 110.04 187.35
276.84 267.9 252.4 277.22 272.55 258.4
302.46 286.52 282.64 302.45 288.77 284.63
321.76 311.34 306.91 319.03 302.55 311.98
−231.18 −173.69 −168.68 −148.58 −178.35 −162.48
and degradation of CHs and films. In addition, the temperatures of the endothermic and exothermic phases verified the different temperatures of decomposition events in the above described TGA. The thermodynamic properties of the CHs changed to some extent after their formation into films (Table 3). Compared with the powders, the To and Tm of the spiked and unspiked films in the endothermic phase exhibited a right-shift, whereas they displayed a left-shift in the exothermic phase. This result might suggest that after CH formed into films, its Tm increased and thermal stability improved. This hypothesis was in accordance with our TGA data. The decrease in To and Tm in the exothermic phase could have been because when CH was dissolved in acetic acid solution with and without sorbitol, the polymer molecules in the chain unfolded gradually. To and Tm in the endothermic and exothermic phases of the DD95% CHs and films were higher than those of the DD85% CHs and films. It elucidated that the thermal stability of the DD95% samples was stronger than that of the DD85% samples. This was probably related to the degree of deacetylation of chitosan. Greater deacetylation was associated with more amide ions, which facilitated the formation of more hydrogen bonds and a more stable molecular structure [21]. To and Tm in the endothermic and exothermic phases were lower for the spiked films than for unspiked films. These results confirmed previous work showing that sorbitol as a plasticizer decreased the To and Tm of films [35,43]. These phenomena could also be associated with a decrease in crystallinity [26,31]. Sorbitol-spiked films exhibited crystal transformation during film formation [12], which may also explain the changes in the To and Tm of different films. Thermodynamic properties demonstrated that the melting range from To to Tc of a material reflects its crystallinity [47]. A
wide melting range of a material signifies that it has lower crystallinity. Table 3 showed that the melting range of the DD95% CHs and films was narrower than that of DD85% CHs and films. These results confirmed that the crystallinity of the former was higher than that of the latter [12]. Moreover, the melting range of CHFP was the widest, and that of CHF was the narrowest, for both DD85% and DD95%. These data implied that the crystallinity of CHFP and CHF was the lowest and highest, respectively, which was in accordance with our previous work [12]. 4. Conclusion The physicochemical properties of different chitosan films suggested that sorbitol addition reduced the MC and increased WS of chitosan films. WVP decreased significantly (p < 0.01) with the presence of sorbitol in DD85% and DD95% chitosan films. For spiked and unspiked films, DD95% displayed lower MC, higher WS and lower WVP than those of DD85% films. Static (TMA) and dynamic (DMA) mechanical analyses demonstrated that sorbitol increased the strain, decreased stress and increased mobility among the molecules in the blend, which resulted in spiked films being less stiff and more flexible. Spiked and unspiked DD95% films had higher tensile strength, lower elongation percent as well as higher -relaxation temperatures and ␣-relaxation temperatures than those of DD85% films. TGA and DSC revealed that the thermal stability of formed films for both degrees of deacetylation increased compared with their powders, and that spiked films showed lower thermal stability than unspiked films. DD95% films had higher thermal stability than DD85% films. DSC showed that the crystallinity of DD95% powder and film was higher than that of DD85% powder and film. This work analyzed the various properties of the four types of chitosan films and the reasons that caused the differences. However, there are still some properties such as WVP and mechanical properties needed to be improved to meet the demand of chitosan films various applications. Acknowledgments We gratefully acknowledge the staff of the University of Science and Technology of China for their support and comments during the preparation of this manuscript. This work was supported by a grant from the National Science Foundation of China (No. 31271960). References [1] [2] [3] [4] [5] [6]
Fig. 4. DSC thermograms of degree of deacetylation (DD) 85% and 95% chitosan powder (CH) as well as films without (CHF) and with (CHFP) sorbitol. (a) DD85% CH, (b) DD85% CHF, (c) DD85% CHFP, (d) DD95% CH, (e) DD95% CHF, (f) DD95% CHFP.
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