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Calorimetric study of chitosan-graft-poly(2-ethylhexyl acrylate) copolymer
Thermochimica Acta 670 (2018) 136–141
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Calorimetric study of chitosan-graft-poly(2-ethylhexyl acrylate) copolymer Polina E. Goryunova, Semen S. Sologubov, Alexey V. Markin , Natalia N. Smirnova, Alla E. Mochalova, Sergey D. Zaitsev, Larisa A. Smirnova ⁎
T
National Research Lobachevsky State University of Nizhni Novgorod, 23/5 Gagarin Av., 603950 Nizhni Novgorod, Russia
ARTICLE INFO
ABSTRACT
Keywords: Chitosan Poly(2-ethylhexyl acrylate) Adiabatic calorimetry DSC Heat capacity Thermodynamic functions
In the present work, the temperature dependence of the molar heat capacity of chitosan-graft-poly(2-ethylhexyl acrylate) copolymer was measured for the first time in the temperature range from 6 to 350 K by precise adiabatic calorimetry, and the thermophysical properties of copolymer were studied over the range from 270 to 570 K by differential scanning calorimetry. In the above temperature intervals, the glass transition was detected for the investigated compound, and the standard thermodynamic characteristics were determined and analyzed. The standard thermodynamic functions: heat capacity Cpo (T ) , enthalpy [H°(T)−H°(0)], entropy [S°(T)−S°(0)], and the Gibbs energy [G°(T)−H°(0)] for the range from T → 0 to 350 K, as well as the standard entropy of formation ΔfS° and the Gibbs energy of formation ΔfG° of copolymer at T = 298.15 K in the glassy state were calculated based on the obtained experimental data. The standard thermodynamic properties of the investigated copolymer were discussed and compared with the literature data for the studied earlier copolymers of chitosan.
1. Introduction Over the past few decades, biodegradable materials have been extensively studied for a wide variety of biomedical and environmental applications, and thus they have significant commercial potential. According to the origin, two classes of biodegradable polymers can be distinguished: synthetic polymers or natural polymers. There are polymers produced from feedstocks derived either from petroleum resources (non-renewable resources) or from biological resources (renewable resources) [1,2]. Among the most widely distributed natural polymers, a special place is occupied by chitosan due to its non-toxicity, biocompatibility, biodegradability as well as antifungal and antimicrobial characteristics [3]. Structurally, chitosan is a linear polysaccharide composed of β-(1→ 4)-linked D-glucosamine residues with a variable number of randomly located N-acetyl-D-glucosamine groups. Chitosan has wide applications in biomedical fields such as blood anticoagulant, drug delivery systems, and tissue engineering [4–6], in addition to other fields such as wastewater treatment [7], food packaging [8,9], cosmetic preparations [10], and textile, paper and film technologies [11,12]. The unique structure and properties of chitosan make it valuable material for its intended use, and many applications of chitosan can be widely extended and improved following different chemical modifications. The investigation of acrylic polymers attracts increasing attention because these polymers are being widely applied in modern
⁎
technological processes [13]. Poly(2-ethylhexyl acrylates) (PEHA) with different molecular weights are characterized by low values of the glass transition temperature and extensively used for manufacturing of durable paints, mounting tapes, protective films, and as plasticizers for modification of pressure-sensitive adhesives [14–18]. The high chemical reactivity of amine and hydroxyl groups and the high complexing ability of chitosan enable its combination with different biologically active substances and synthetic polymers. The graft copolymerization of synthetic polymers into these groups allows preparing derivatives with tailored properties, the degree of branching, and the content of the modified functional groups of chitosan [19]. Therefore, it is promising to synthesize graft copolymers for improving different characteristics of each component [20–22]. The combination of unique properties of chitosan and PEHA as well as the synthesis of chitosan-graft-PEHA copolymers can provide the development of new biocomposite materials with the desired properties. This work continues previous calorimetric studies of copolymers of chitosan with poly(D,L-lactide) (PDLLA) [23,24]. The results of thermal analysis of some chitosan- and PEHA-based derivatives were presented in [15,16,18,25–31]. The temperature dependences of heat capacities of chitosan and PEHA were studied earlier [32–35]. The results of thermochemical study of chitosan-graft-PEHA copolymer were published in [36]. The purpose of the current study was to determine the temperature dependence of the molar heat capacity of chitosan-graft-PEHA
Corresponding author. E-mail address: [email protected] (A.V. Markin).
https://doi.org/10.1016/j.tca.2018.10.023 Received 22 August 2018; Received in revised form 22 October 2018; Accepted 25 October 2018 Available online 28 October 2018 0040-6031/ © 2018 Elsevier B.V. All rights reserved.
Thermochimica Acta 670 (2018) 136–141 IR spectroscopy, Elemental analysis, X-ray diffraction analysis, High performance liquid chromatography, Thermogravimetric analysis
copolymer in the temperature range from 6 to 350 K by precise adiabatic calorimetry (AC), and to investigate the thermophysical properties of copolymer in the interval from 270 to 570 K by differential scanning calorimetry (DSC). This included the detection of possible physical transformations and the evaluation of their thermodynamic characteristics. The obtained experimental data were used to calculate the standard thermodynamic functions for the range from T → 0 to 350 K and thermodynamic characteristics of formation of chitosan-graft-PEHA copolymer at T = 298.15 K. The comparative analysis of the standard thermodynamic properties of the investigated copolymer was made with those of the studied earlier copolymers of chitosan. 2. Experimental 2.1. Synthesis and characterization of sample Chitosan (CJSC Bioprogress, Russia) with Mr = 105000 g mol−1 was used without further purification. The molecular mass of chitosan was determined on the Ubbelohde-type viscometer at a room temperature. Chitosan was dissolved in aqueous solution of acetic acid, and the deacetylation degree was 82% [37]. The content of the main substance was no less than 99%. Poly(2-ethylhexyl acrylate) with Mw = 1560000 g mol−1, Mn = 875000 g mol−1, PDI = 1.78 was synthesized by polymerization of 2-ethylhexyl acrylate in sealed glass ampoules at 60 °C. Polymer was extracted with methanol dissolved in benzene, precipitated three times with methanol, and dried in vacuum at 50 °C to constant mass. The molecular mass characteristics of PEHA were determined by gel permeation chromatography (GPC) at 40 °C. Tetrahydrofuran (THF) was used as an eluent at a flow rate of 1 ml min−1. The graft copolymerization of PEHA onto chitosan was performed at 60–80 °C for 4 h under constant stirring. Azobisisobutyronitrile (AIBN) was used as a polymerization initiator. AIBN (0.01 mol l−1) was dissolved in a small amount of dioxane (2.0 mol l−1), and then mixed with the aqueous acetic acid solution of chitosan. The conversion of PEHA during copolymerization with chitosan was determined by gas chromatographic (GC) analysis of the residual monomer using the chromatographic system GCMS-QP2010 (Shimadzu, Japan). The synthesized copolymer was purified by extraction to remove possible residual homopolymers for evaluating the efficiency and the degree of grafting of PEHA onto chitosan. For this purpose, the sample of graft copolymer was dried to constant mass in vacuum (p ∼ 1 kPa). After that, PEHA was extracted in a Soxhlet extractor by use of THF for 48 h. Chitosan was extracted by use of aqueous acetic acid solution on a shaker at 40 °C for the same time. The yield of pure graft copolymer was 95% [22]. The molecular masses of the grafted PEHA chains (Mw = 2400000 g mol−1, Mn = 1900000 g mol−1) were determined by high performance liquid chromatography (HPLC) using the chromatographic system LC20 Prominence (Shimadzu, Japan) at 40 °C. The differential refractometer was used as a detector, and THF was used as an eluent. The formation of chitosan-graft-PEHA copolymer was confirmed by infrared (IR) spectrophotometer Infralum FT-801 (Simex, Russia). The IR spectrum of the studied copolymer showed the only characteristic peak at 1738 cm−1 corresponding to the carbonyl C = O group of PEHA. This may be caused by the formation of chitosan-graft-PEHA copolymer. The content of PEHA and composition of the synthesized copolymer were determined using the CHN elemental analyzer Vario EL Cube (Elementar, Germany). The results of the elemental analysis are: C, 50.28%; H, 7.62%; N, 6.73%; C:N = 7.47. The uncertainties associated to the percentages of carbon ( ± 0.05), hydrogen ( ± 0.08), and nitrogen ( ± 0.07) were estimated. The X-ray diffraction (XRD) analysis of chitosan-graft-PEHA copolymer was performed on a Bruker D8 Discover diffractometer using CuKα radiation. For this purpose, the copolymer films were ground into
Synthesis C7.14H12.9O3.77N0.82 Chitosan-graft-PEHA copolymer
0.99
Source Gross formula Sample
Table 1 Sample information.
Mole fraction purity
Analysis methods
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small particles, and then placed in a fused quartz cuvette. The X-ray patterns were recorded in the range of diffraction angles 2Θ = 10°–60° in the symmetric geometry, and no significant impurities were detected. The thermal stability of the synthesized copolymer was studied using the thermogravimetric (TG) analyzer TG 209 F1 Iris (NETZSCH, Germany). The TG analysis was carried out in the range from 300 to 570 K at a heating rate of 5 K min−1 in a high-purity argon atmosphere with a gas flow rate of 25 ml min−1. It was established that the investigated sample was thermally stable up to 490 K. The information for the studied graft copolymer is listed in Table 1. 2.2. Apparatus and measurement procedure 2.2.1. Adiabatic calorimetry The precise automatic adiabatic calorimeter BCT-3 with discrete heating was applied to measure the heat capacity of graft copolymer under study in the temperature range from 6 to 350 K. The sample mass loaded in a thin-walled cylindrical titanium ampoule of calorimeter was 0.2425 g. Liquid helium and nitrogen were used as cooling reagents. The calorimeter design and experimental procedures were described in detail elsewhere [38]. The calorimeter was calibrated by measuring the heat capacities of the standard reference samples of benzoic acid, corundum (α-Al2O3), and high-purity copper [39]. The expanded uncertainty of the heat capacity measurements was 0.02 Cpo between 6 and 15 K, decreased down to 0.005 Cpo in the interval from 15 to 40 K, and was equal to 0.002 Cpo over the range from 40 to 350 K. The phase transformation temperatures were determined with the standard uncertainty u(T) = 0.02 K. It should be noted that the studied copolymer existed in a totally amorphous state, and the loss of the sample mass was not observed after the Cpo measurements.
Fig. 1. Temperature dependence of the molar heat capacity of chitosan-graftpoly(2-ethylhexyl acrylate) copolymer: Tgo – the glass transition temperature.
from 6 to 290 K. Then, the glass transition of the studied sample was observed in the interval from 298 to 345 K. The thermophysical properties of chitosan-graft-PEHA copolymer were investigated in the temperature range from 270 to 570 K by DSC. The corresponding DSC curve is illustrated in Fig. 2. As shown in Fig. 2, two anomalies caused by the glass transition of two components of copolymer were detected. The onset of thermal decomposition of the studied compound is occurred at T > 490 K and accompanied by a strong exothermic effect. 3.2. Standard thermodynamic characteristics of glass transition and glassy state
2.2.2. Differential scanning calorimetry The differential scanning calorimeter DSC 204 F1 Phoenix with μsensor (NETZSCH, Germany) was used to study the thermal behavior of graft copolymer in the temperature range from 270 to 570 K. The sample mass loaded in the aluminium ampoule of calorimeter was 0.0174 g. The calorimeter design and the technique for determination of the thermophysical characteristics by the DSC measurements were described in detail in [40,41] and NETZSCH Proteus software. The calorimeter was calibrated by determining the temperatures and enthalpies of melting of the standard calibration samples (indium, zinc, tin, bismuth, mercury, potassium nitrate, caesium chloride, and biphenyl) according to the recommendation reported in the IUPAC Technical Report [42]. The DSC measurements of copolymer were performed at a heating rate of 5 K min−1 in a high-purity argon atmosphere with a gas flow rate of 25 ml min−1. The apparatus enables to determine the phase transformation temperatures and enthalpies with the standard uncertainty u(T) = 0.5 K and the relative standard uncertainty ur(ΔtrH°) = 0.01, respectively.
The standard thermodynamic characteristics of the glass transition and glassy state of copolymer under study are presented in Table 2. The glass transition temperature Tgo was determined using the method of Alford and Dole from the inflection of the S°(T) curve [43]. The interval of glass transition ΔT and the increase in the molar heat capacity ascribed to the glass transition Cpo (Tgo) were determined graphically o from the Cpo (T ) curve. The configuration entropy Sconf was calculated using the following equation [44]: o Sconf = Cpo (Tgo ) ln(Tgo/TK ),
where TK is the Kauzmann temperature [45], and the ratio (Tgo /TK ) = 1.29 [46]. We suggested that this ratio is valid for the investigated o copolymer. The assumption Sconf ≈ S°(0) was used for determining the absolute entropy S°(T) of compound. It was established that the heat capacity of chitosan gradually increases with rising of temperature in the range from 7 to 300 K [32,33]. The sharp Cpo (T ) changes were observed for chitosan in the intervals from 300 to 330 K, between 340 and 360 K, and from 400 to 420 K. The
3. Results and discussion 3.1. Heat capacity The temperature dependence of the molar heat capacity of chitosangraft-PEHA copolymer in the range from 6 to 350 K is presented in Fig. 1. The experimental Cp,m points of copolymer in the above temperature interval are listed in Table S1 (Supplementary material). The smoothing of the experimental values of the heat capacity was made using exponential and semilogarithmic polynomial equations in the regions where any transformations were absent, and the corresponding coefficients were found with special computer software. The tested sample of chitosan-graft-PEHA copolymer was cooled from the room temperature to the onset temperature (T ∼ 6 K) of measurements at a flow rate of 0.01 K s−1. The molar heat capacity of copolymer gradually increases with rising of temperature in the range
Fig. 2. DSC curve of chitosan-graft-poly(2-ethylhexyl acrylate) copolymer. 138
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Table 2 The standard thermodynamic characteristics of the glass transition and glassy state of chitosan, poly(2-ethylhexyl acrylate) and chitosan-graft-PEHA copolymer at p = 0.1 MPaa. Sample
ΔТ1/K
o (Tg,1 ± 1) /K
Chitosan-graft-PEHA copolymer Chitosan [32] PEHA [35]
322–339 298–345 — 162–212
329 330 320 196
o Cpo (Tg,1 ) /J K−1 mol−1
162 ± 3 153 ± 2 — 88 ± 2
ΔТ2/K
o (Tg,2 ± 1) /K
456–478 — — —
467 — 347; 413 —
o Cpo (Tg,2 ) /J K−1 mol−1
222 ± 3 — — —
o /J K−1 mol−1 Sconf
Method of determination
95
DSC AC DSC AC
— 22
o Conventional designations: ΔT – the interval of glass transition; Tgo – the glass transition temperature; Cpo (Tgo) – the heat capacity ascribed to the glass transition; Sconf – the configuration entropy. a The standard uncertainty for pressure u(p) = 10 kPa. The reported uncertainties correspond to the expanded uncertainty with 0.95 level of confidence (k ≈ 2).
Table 3 The standard thermodynamic functions of chitosan-graft-PEHA copolymer (M(C7.14H12.9O3.77N0.82) = 170.56 g mol−1) at p = 0.1 MPaa. T/K
Cpo (T ) /J K−1 mol−1
[Н°(Т)−Н°(0)]/kJ mol−1
[S°(T)−S°(0)]/J K−1 mol−1
−[G°(T)−H°(0)]/kJ mol−1
Glassy state 5 10 15 20 25 30 35 40 45 50 60 70 80 90 100 150 200 250 298.15 300 330
0.267 2.06 5.44 9.871 14.81 19.96 25.15 30.32 35.42 40.43 50.15 59.44 68.30 76.74 84.81 121.0 154.9 192.8 238.9 241.0 278.0
0.000324 0.0053 0.0237 0.06161 0.1232 0.2101 0.3228 0.4615 0.6259 0.816 1.269 1.817 2.456 3.182 3.990 9.159 16.05 24.72 35.07 35.51 43.28
0.0891 0.713 2.15 4.306 7.031 10.18 13.65 17.35 21.22 22.23 33.45 41.88 50.40 58.93 67.44 108.9 148.3 186.8 224.6 226.0 250.7
0.000111 0.00179 0.00859 0.02449 0.05254 0.09542 0.1549 0.2325 0.3288 0.4026 0.7379 1.114 1.576 2.122 2.754 7.171 13.61 21.99 31.88 32.30 39.45
High elastic state 330 350
431.2 448.5
43.28 52.07
250.7 276.6
39.45 44.72
Conventional designations: Cpo (T ) – the molar heat capacity; [H°(T)−H°(0)] – the enthalpy; [S°(T)−S°(0)] – the entropy; [G°(T)−H°(0)] – the Gibbs energy. a The standard uncertainty for pressure u(p) = 10 kPa. The standard uncertainty for temperature u(T) = 0.02 K. The combined expanded relative uncertainties Uc,r(Cpo (T ) ) = 0.02, 0.005, 0.002; Uc,r([H°(T)−H°(0)]) = 0.022, 0.007, 0.005; Uc,r([S°(T)−S°(0)]) = 0.023, 0.008, 0.006; Uc,r([G°(T)−H°(0)]) = 0.03, 0.01, 0.009 in the interval from 6 to 15 K, between 15 and 40 K, in the range from 40 to 350 K, respectively. The reported uncertainties correspond to the 0.95 confidence level (k ≈ 2).
first transformation of chitosan can be referred to as β-transition caused by the release of pyranose rings around glucoside bond. Two other transformations are due to the glass transition of the two parts of chitosan [47–49]. The glass transition of PEHA was revealed on the Cpo (T ) curve in the interval from 162 to 212 K [35]. In accordance with the literature data and the obtained experimental results, it is logical to assume that the first change in the heat capacity of chitosan-graft-PEHA copolymer in the temperature range from 298 to 345 K is caused by the glass transition of PEHA and the βtransition of chitosan. The second change is related to the glass transition of chitosan. The data of Table 2 are in good agreement with the above statements. The observed changes in the temperature and the Cpo (Tgo) values can be explained in terms of difference in the chain length of PEHA and chitosan, as well as their mutual arrangement in the investigated chitosan-graft-PEHA copolymer. For the same reason, chitosan-graft-PEHA copolymer has higher Tgo values than block copolymers of chitosan with PDLLA [24].
3.3. Standard thermodynamic functions The molar heat capacity of chitosan-graft-PEHA copolymer was extrapolated from the onset temperature of measurements to T → 0 to calculate the standard thermodynamic functions of the investigated compound. For this purpose, the Debye model of heat capacity was used [50]:
Cpo = nD(
D / T ),
where D is the symbol of the Debye function, n and ΘD are specially selected parameters. For the investigated sample, n = 3 and ΘD = 86.7 K in the temperature interval from 6 to 13 K. The experimental Cpo values of copolymer were described by the above equation with the relative standard uncertainty ur(Cpo ) = 0.014. The standard thermodynamic functions of chitosan-graft-PEHA copolymer are presented in Table 3. The calculations of the enthalpy [H°(T)−H°(0)] and the entropy [S°(T)−S°(0)] were made by the
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Table 4 The standard thermodynamic characteristics of formation of chitosan, poly(2-ethylhexyl acrylate) and chitosan-graft-PEHA copolymer at T = 298.15 K, p = 0.1 MPaa. Sample
−ΔfH°/kJ mol−1
−ΔfS°/J K−1 mol−1
−ΔfG°/kJ mol−1
Chitosan-graft-PEHA copolymer Chitosan [23,24] PEHA [35]
800.3 ± 2.4b 801.7 ± 2.5 704.7 ± 5.4
1025.8 ± 0.6 1099.5 ± 0.6 1198.2 ± 2.4
494.5 ± 3.2 473.9 ± 3.1 347.5 ± 6.1
Conventional designations: ΔfH° – the enthalpy of formation; ΔfS° – the entropy of formation; ΔfG° – the Gibbs energy of formation. a The standard uncertainty for pressure u(p) = 10 kPa. The reported uncertainties correspond to the expanded uncertainty with 0.95 level of confidence (k ≈ 2). b The standard enthalpy of formation ΔfH° of chitosan-graft-PEHA copolymer was published in [36].
numerical integration of the Cpo (T ) and Cpo (ln T ) curves, respectively. The Gibbs energy [G°(T)−H°(0)] was calculated using the Gibbs–Helmholtz equation from the [H°(T)−H°(0)] and [S°(T)−S°(0)] values at the corresponding temperatures. The calculation technique was described in [51].
[4] M.N.V. Ravi Kumar, R.A.A. Muzzarelli, C. Muzzarelli, H. Sashiwa, A.J. Domb, Chitosan chemistry and pharmaceutical perspectives, Chem. Rev. 104 (2004) 6017–6084, https://doi.org/10.1021/cr030441b. [5] S.A. Agnihotri, N.N. Mallikarjuna, T.M. Aminabhavi, Recent advances on chitosanbased micro- and nanoparticles in drug delivery, J. Control. Release 100 (2004) 5–28, https://doi.org/10.1016/j.jconrel.2004.08.010. [6] R. Riva, H. Ragelle, A. des Rieux, N. Duhem, C. Jérôme, V. Préat, Chitosan and chitosan derivatives in drug delivery and tissue engineering, Adv. Polym. Sci. 244 (2011) 19–44, https://doi.org/10.1007/12_2011_137. [7] R. Yang, H. Li, M. Huang, H. Yang, A. Li, A review on chitosan-based flocculants and their applications in water treatment, Water. Res. 95 (2016) 59–89, https://doi. org/10.1016/j.watres.2016.02.068. [8] H.K. No, S.P. Meyers, W. Prinyawiwatkul, Z. Xu, Applications of chitosan for improvement of quality and shelf life of foods: a review, J. Food Sci. 72 (2007) R87–R100, https://doi.org/10.1111/j.1750-3841.2007.00383.x. [9] P.K. Dutta, S. Tripathi, G.K. Mehrotra, J. Dutta, Perspectives for chitosan based antimicrobial films in food applications, Food Chem. 114 (2009) 1173–1182, https://doi.org/10.1016/j.foodchem.2008.11.047. [10] I. Aranaz, N. Acosta, C. Civera, B. Elorza, J. Mingo, C. Castro, M. de los Llanos Gandía, A. Heras Caballero, Cosmetics and cosmeceutical applications of chitin, chitosan and their derivatives, Polymers 10 (2018) 213, https://doi.org/10.3390/ polym10020213. [11] S.H. Lim, S.M. Hudson, Review of chitosan and its derivatives as antimicrobial agents and their uses as textile chemicals, J. Macromol. Sci. Part C: Polym. Rev. 43 (2003) 223–269, https://doi.org/10.1081/MC-120020161. [12] C.K.S. Pillai, W. Paul, C.P. Sharma, Chitin and chitosan polymers: chemistry, solubility and fiber formation, Prog. Polym. Sci. 34 (2009) 641–678, https://doi.org/ 10.1016/j.progpolymsci.2009.04.001. [13] V. Vangani, A.K. Rakshit, Synthesis and characterization of homopolymer of 2ethylhexyl acrylate and its copolymers with acrylamide, acrylonitrile, and methyl methacrylate, J. Appl. Polym. Sci. 60 (1996) 1005–1013, https://doi.org/10.1002/ %28SICI%291097-4628%2819960516%2960%3A7<1005%3A%3AAIDAPP11>3.0.CO%3B2-%23. [14] C. Plessis, G. Arzamendi, J.M. Alberdi, M. Agnely, J.R. Leiza, J.M. Asua, Intramolecular chain transfer to polymer in the emulsion polymerization of 2ethylhexyl acrylate, Macromolecules 34 (2001) 6138–6143, https://doi.org/10. 1021/ma0018190. [15] D.J. Haloi, B.P. Koiry, P. Mandal, N.K. Singha, Synthesis and characterization of poly(2-ethylhexyl acrylate) prepared via atom transfer radical polymerization, reverse atom transfer radical polymerization and radical polymerization, J. Chem. Sci. 125 (2013) 791–797, https://doi.org/10.1007/s12039-013-0438-2. [16] Z. Czech, A. Kowalczyk, J. Kabatc, J. Świderska, Thermal stability of poly(2ethylhexyl acrylates) used as plasticizers for medical application, Polym. Bull. 70 (2013) 1911–1918, https://doi.org/10.1007/s00289-012-0887-7. [17] O.J. Chaudhary, E.P. Calius, J.V. Kennedy, M. Dickinson, T. Loho, J. Travas-Sejdic, Bioinspired dry adhesive: poly(dimethylsiloxane) grafted with poly(2-ethylhexyl acrylate) brushes, Eur. Polym. J. 68 (2015) 432–440, https://doi.org/10.1016/j. eurpolymj.2015.05.016. [18] Z. Su, H. Wang, X. Ye, K. Tian, W. Huang, Y. Guo, J. He, X. Tian, Non-covalent poly (2-ethylhexyl acrylate) (P2EHA)/functionalized graphene/h-boron nitride flexible composites with enhanced adhesive and thermal conductivity by a facilitated latex approach, Compos. Part A: Appl. Sci. Manuf. 99 (2017) 176–185, https://doi.org/ 10.1016/j.compositesa.2017.01.020. [19] V.K. Thakur, M.K. Thakur, Recent advances in graft copolymerization and applications of chitosan: a review, ACS Sustain. Chem. Eng. 2 (2014) 2637–2652, https://doi.org/10.1021/sc500634p. [20] A.E. Mochalova, L.A. Smirnova, S.D. Zaitsev, Yu.D. Semchikov, I.I. Zaitseva, G.M. Pavlov, Hydrodynamic and molecular characteristics of graft copolymers of chitosan with acrylamide, Polym. Sci. Ser. B 49 (2007) 232–235, https://doi.org/ 10.1134/S1560090407090059. [21] I.A. Baranov, N.A. Andriyanova, A.E. Mochalova, A.A. Sibirkin, M.A. Baten’kin, L.A. Smirnova, Grafting polymerization of acrylonitrile and methyl acrylate on chitosan in the presence of cobalt (III) complexes, Polym. Sci. Ser. B 54 (2012) 167–174, https://doi.org/10.1134/S1560090412030013. [22] A.E. Mochalova, E.N. Kruglova, P.A. Yunin, K.V. Apryatina, O.N. Smirnova, L.A. Smirnova, Graft and block copolymers of chitosan with vinyl monomers: synthesis, structure, and properties, Polym. Sci. Ser. B 57 (2015) 93–105, https:// doi.org/10.1134/S1560090415020116. [23] P.E. Goryunova, V.N. Larina, N.N. Smirnova, N.E. Tsverova, L.A. Smirnova, Thermochemical characteristics of chitosan–polylactide copolymers, Russ. J. Phys.
3.4. Standard thermodynamic characteristics of formation The standard thermodynamic functions of formation of chitosangraft-PEHA copolymer are listed in Table 4. The standard entropy of formation ΔfS° of the studied compound was calculated using the [S°(T)−S°(0)] value (Table 3), as well as the residual entropy S°(0), and the absolute entropies of elemental substances C(gr), H2(g), O2(g), N2(g) at T = 298.15 K [52]. The standard enthalpy of formation ΔfH° = −(800.3 ± 2.4) kJ mol−1 of copolymer was published in [36]. According to the Gibbs–Helmholtz equation, the standard Gibbs energy of formation ΔfG° was calculated at T = 298.15 K. The obtained values correspond to the following reaction of formation of chitosan-graftPEHA copolymer in the glassy state: 7.14C(gr) + 6.45H2(g) C7.14H12.9O3.77N0.82(gl).
+
1.885O2(g)
+
0.41N2(g)
→
4. Conclusions The heat capacity of chitosan-graft-poly(2-ethylhexyl acrylate) copolymer was measured in the temperature range from 6 to 350 K by AC, and its thermophysical properties were studied in the interval from 270 to 570 K by DSC. The glass transition was detected for copolymer in the range from 298 to 345 K, Tgo = 330 K. The complex of the standard thermodynamic functions of the studied compound in various states for the range from T → 0 to 350 K, as well as the standard entropy and the Gibbs energy of its formation at T = 298.15 K were calculated based on the obtained experimental data, including the estimation of the residual entropy. It was established that the regularities of change in thermodynamic properties are the same for chitosan and its copolymers. In addition, the inclusion of PEHA into the chitosan macromolecules led to an increase in the glass transition temperatures of copolymer. Acknowledgments This work was performed with the financial support of the Ministry of Education and Science of the Russian Federation (Contracts Nos. 4.5706.2017/8.9 and 4.6138.2017/6.7). References [1] V.K. Thakur, M.K. Thakur, M.R. Kessler, Handbook of composites from renewable materials, Scrivener Publishing LLC, Beverly, 2017, https://doi.org/10.1002/ 9781119441632. [2] A. Dufresne, S. Thomas, L.A. Pothan, Biopolymer nanocomposites: processing, properties, and applications, John Wiley & Sons, Inc., Hoboken, 2013, https://doi. org/10.1002/9781118609958. [3] M. Rinaudo, Chitin and chitosan: properties and applications, Prog. Polym. Sci. 31 (2006) 603–632, https://doi.org/10.1016/j.progpolymsci.2006.06.001.
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Calorimetric study of chitosan-graft-poly(2-ethylhexyl acrylate) copolymer Polina E. Goryunova, Semen S. Sologubov, Alexey V. Markin , Natalia N. Smirnova, Alla E. Mochalova, Sergey D. Zaitsev, Larisa A. Smirnova ⁎
T
National Research Lobachevsky State University of Nizhni Novgorod, 23/5 Gagarin Av., 603950 Nizhni Novgorod, Russia
ARTICLE INFO
ABSTRACT
Keywords: Chitosan Poly(2-ethylhexyl acrylate) Adiabatic calorimetry DSC Heat capacity Thermodynamic functions
In the present work, the temperature dependence of the molar heat capacity of chitosan-graft-poly(2-ethylhexyl acrylate) copolymer was measured for the first time in the temperature range from 6 to 350 K by precise adiabatic calorimetry, and the thermophysical properties of copolymer were studied over the range from 270 to 570 K by differential scanning calorimetry. In the above temperature intervals, the glass transition was detected for the investigated compound, and the standard thermodynamic characteristics were determined and analyzed. The standard thermodynamic functions: heat capacity Cpo (T ) , enthalpy [H°(T)−H°(0)], entropy [S°(T)−S°(0)], and the Gibbs energy [G°(T)−H°(0)] for the range from T → 0 to 350 K, as well as the standard entropy of formation ΔfS° and the Gibbs energy of formation ΔfG° of copolymer at T = 298.15 K in the glassy state were calculated based on the obtained experimental data. The standard thermodynamic properties of the investigated copolymer were discussed and compared with the literature data for the studied earlier copolymers of chitosan.
1. Introduction Over the past few decades, biodegradable materials have been extensively studied for a wide variety of biomedical and environmental applications, and thus they have significant commercial potential. According to the origin, two classes of biodegradable polymers can be distinguished: synthetic polymers or natural polymers. There are polymers produced from feedstocks derived either from petroleum resources (non-renewable resources) or from biological resources (renewable resources) [1,2]. Among the most widely distributed natural polymers, a special place is occupied by chitosan due to its non-toxicity, biocompatibility, biodegradability as well as antifungal and antimicrobial characteristics [3]. Structurally, chitosan is a linear polysaccharide composed of β-(1→ 4)-linked D-glucosamine residues with a variable number of randomly located N-acetyl-D-glucosamine groups. Chitosan has wide applications in biomedical fields such as blood anticoagulant, drug delivery systems, and tissue engineering [4–6], in addition to other fields such as wastewater treatment [7], food packaging [8,9], cosmetic preparations [10], and textile, paper and film technologies [11,12]. The unique structure and properties of chitosan make it valuable material for its intended use, and many applications of chitosan can be widely extended and improved following different chemical modifications. The investigation of acrylic polymers attracts increasing attention because these polymers are being widely applied in modern
⁎
technological processes [13]. Poly(2-ethylhexyl acrylates) (PEHA) with different molecular weights are characterized by low values of the glass transition temperature and extensively used for manufacturing of durable paints, mounting tapes, protective films, and as plasticizers for modification of pressure-sensitive adhesives [14–18]. The high chemical reactivity of amine and hydroxyl groups and the high complexing ability of chitosan enable its combination with different biologically active substances and synthetic polymers. The graft copolymerization of synthetic polymers into these groups allows preparing derivatives with tailored properties, the degree of branching, and the content of the modified functional groups of chitosan [19]. Therefore, it is promising to synthesize graft copolymers for improving different characteristics of each component [20–22]. The combination of unique properties of chitosan and PEHA as well as the synthesis of chitosan-graft-PEHA copolymers can provide the development of new biocomposite materials with the desired properties. This work continues previous calorimetric studies of copolymers of chitosan with poly(D,L-lactide) (PDLLA) [23,24]. The results of thermal analysis of some chitosan- and PEHA-based derivatives were presented in [15,16,18,25–31]. The temperature dependences of heat capacities of chitosan and PEHA were studied earlier [32–35]. The results of thermochemical study of chitosan-graft-PEHA copolymer were published in [36]. The purpose of the current study was to determine the temperature dependence of the molar heat capacity of chitosan-graft-PEHA
Corresponding author. E-mail address: [email protected] (A.V. Markin).
https://doi.org/10.1016/j.tca.2018.10.023 Received 22 August 2018; Received in revised form 22 October 2018; Accepted 25 October 2018 Available online 28 October 2018 0040-6031/ © 2018 Elsevier B.V. All rights reserved.
Thermochimica Acta 670 (2018) 136–141 IR spectroscopy, Elemental analysis, X-ray diffraction analysis, High performance liquid chromatography, Thermogravimetric analysis
copolymer in the temperature range from 6 to 350 K by precise adiabatic calorimetry (AC), and to investigate the thermophysical properties of copolymer in the interval from 270 to 570 K by differential scanning calorimetry (DSC). This included the detection of possible physical transformations and the evaluation of their thermodynamic characteristics. The obtained experimental data were used to calculate the standard thermodynamic functions for the range from T → 0 to 350 K and thermodynamic characteristics of formation of chitosan-graft-PEHA copolymer at T = 298.15 K. The comparative analysis of the standard thermodynamic properties of the investigated copolymer was made with those of the studied earlier copolymers of chitosan. 2. Experimental 2.1. Synthesis and characterization of sample Chitosan (CJSC Bioprogress, Russia) with Mr = 105000 g mol−1 was used without further purification. The molecular mass of chitosan was determined on the Ubbelohde-type viscometer at a room temperature. Chitosan was dissolved in aqueous solution of acetic acid, and the deacetylation degree was 82% [37]. The content of the main substance was no less than 99%. Poly(2-ethylhexyl acrylate) with Mw = 1560000 g mol−1, Mn = 875000 g mol−1, PDI = 1.78 was synthesized by polymerization of 2-ethylhexyl acrylate in sealed glass ampoules at 60 °C. Polymer was extracted with methanol dissolved in benzene, precipitated three times with methanol, and dried in vacuum at 50 °C to constant mass. The molecular mass characteristics of PEHA were determined by gel permeation chromatography (GPC) at 40 °C. Tetrahydrofuran (THF) was used as an eluent at a flow rate of 1 ml min−1. The graft copolymerization of PEHA onto chitosan was performed at 60–80 °C for 4 h under constant stirring. Azobisisobutyronitrile (AIBN) was used as a polymerization initiator. AIBN (0.01 mol l−1) was dissolved in a small amount of dioxane (2.0 mol l−1), and then mixed with the aqueous acetic acid solution of chitosan. The conversion of PEHA during copolymerization with chitosan was determined by gas chromatographic (GC) analysis of the residual monomer using the chromatographic system GCMS-QP2010 (Shimadzu, Japan). The synthesized copolymer was purified by extraction to remove possible residual homopolymers for evaluating the efficiency and the degree of grafting of PEHA onto chitosan. For this purpose, the sample of graft copolymer was dried to constant mass in vacuum (p ∼ 1 kPa). After that, PEHA was extracted in a Soxhlet extractor by use of THF for 48 h. Chitosan was extracted by use of aqueous acetic acid solution on a shaker at 40 °C for the same time. The yield of pure graft copolymer was 95% [22]. The molecular masses of the grafted PEHA chains (Mw = 2400000 g mol−1, Mn = 1900000 g mol−1) were determined by high performance liquid chromatography (HPLC) using the chromatographic system LC20 Prominence (Shimadzu, Japan) at 40 °C. The differential refractometer was used as a detector, and THF was used as an eluent. The formation of chitosan-graft-PEHA copolymer was confirmed by infrared (IR) spectrophotometer Infralum FT-801 (Simex, Russia). The IR spectrum of the studied copolymer showed the only characteristic peak at 1738 cm−1 corresponding to the carbonyl C = O group of PEHA. This may be caused by the formation of chitosan-graft-PEHA copolymer. The content of PEHA and composition of the synthesized copolymer were determined using the CHN elemental analyzer Vario EL Cube (Elementar, Germany). The results of the elemental analysis are: C, 50.28%; H, 7.62%; N, 6.73%; C:N = 7.47. The uncertainties associated to the percentages of carbon ( ± 0.05), hydrogen ( ± 0.08), and nitrogen ( ± 0.07) were estimated. The X-ray diffraction (XRD) analysis of chitosan-graft-PEHA copolymer was performed on a Bruker D8 Discover diffractometer using CuKα radiation. For this purpose, the copolymer films were ground into
Synthesis C7.14H12.9O3.77N0.82 Chitosan-graft-PEHA copolymer
0.99
Source Gross formula Sample
Table 1 Sample information.
Mole fraction purity
Analysis methods
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small particles, and then placed in a fused quartz cuvette. The X-ray patterns were recorded in the range of diffraction angles 2Θ = 10°–60° in the symmetric geometry, and no significant impurities were detected. The thermal stability of the synthesized copolymer was studied using the thermogravimetric (TG) analyzer TG 209 F1 Iris (NETZSCH, Germany). The TG analysis was carried out in the range from 300 to 570 K at a heating rate of 5 K min−1 in a high-purity argon atmosphere with a gas flow rate of 25 ml min−1. It was established that the investigated sample was thermally stable up to 490 K. The information for the studied graft copolymer is listed in Table 1. 2.2. Apparatus and measurement procedure 2.2.1. Adiabatic calorimetry The precise automatic adiabatic calorimeter BCT-3 with discrete heating was applied to measure the heat capacity of graft copolymer under study in the temperature range from 6 to 350 K. The sample mass loaded in a thin-walled cylindrical titanium ampoule of calorimeter was 0.2425 g. Liquid helium and nitrogen were used as cooling reagents. The calorimeter design and experimental procedures were described in detail elsewhere [38]. The calorimeter was calibrated by measuring the heat capacities of the standard reference samples of benzoic acid, corundum (α-Al2O3), and high-purity copper [39]. The expanded uncertainty of the heat capacity measurements was 0.02 Cpo between 6 and 15 K, decreased down to 0.005 Cpo in the interval from 15 to 40 K, and was equal to 0.002 Cpo over the range from 40 to 350 K. The phase transformation temperatures were determined with the standard uncertainty u(T) = 0.02 K. It should be noted that the studied copolymer existed in a totally amorphous state, and the loss of the sample mass was not observed after the Cpo measurements.
Fig. 1. Temperature dependence of the molar heat capacity of chitosan-graftpoly(2-ethylhexyl acrylate) copolymer: Tgo – the glass transition temperature.
from 6 to 290 K. Then, the glass transition of the studied sample was observed in the interval from 298 to 345 K. The thermophysical properties of chitosan-graft-PEHA copolymer were investigated in the temperature range from 270 to 570 K by DSC. The corresponding DSC curve is illustrated in Fig. 2. As shown in Fig. 2, two anomalies caused by the glass transition of two components of copolymer were detected. The onset of thermal decomposition of the studied compound is occurred at T > 490 K and accompanied by a strong exothermic effect. 3.2. Standard thermodynamic characteristics of glass transition and glassy state
2.2.2. Differential scanning calorimetry The differential scanning calorimeter DSC 204 F1 Phoenix with μsensor (NETZSCH, Germany) was used to study the thermal behavior of graft copolymer in the temperature range from 270 to 570 K. The sample mass loaded in the aluminium ampoule of calorimeter was 0.0174 g. The calorimeter design and the technique for determination of the thermophysical characteristics by the DSC measurements were described in detail in [40,41] and NETZSCH Proteus software. The calorimeter was calibrated by determining the temperatures and enthalpies of melting of the standard calibration samples (indium, zinc, tin, bismuth, mercury, potassium nitrate, caesium chloride, and biphenyl) according to the recommendation reported in the IUPAC Technical Report [42]. The DSC measurements of copolymer were performed at a heating rate of 5 K min−1 in a high-purity argon atmosphere with a gas flow rate of 25 ml min−1. The apparatus enables to determine the phase transformation temperatures and enthalpies with the standard uncertainty u(T) = 0.5 K and the relative standard uncertainty ur(ΔtrH°) = 0.01, respectively.
The standard thermodynamic characteristics of the glass transition and glassy state of copolymer under study are presented in Table 2. The glass transition temperature Tgo was determined using the method of Alford and Dole from the inflection of the S°(T) curve [43]. The interval of glass transition ΔT and the increase in the molar heat capacity ascribed to the glass transition Cpo (Tgo) were determined graphically o from the Cpo (T ) curve. The configuration entropy Sconf was calculated using the following equation [44]: o Sconf = Cpo (Tgo ) ln(Tgo/TK ),
where TK is the Kauzmann temperature [45], and the ratio (Tgo /TK ) = 1.29 [46]. We suggested that this ratio is valid for the investigated o copolymer. The assumption Sconf ≈ S°(0) was used for determining the absolute entropy S°(T) of compound. It was established that the heat capacity of chitosan gradually increases with rising of temperature in the range from 7 to 300 K [32,33]. The sharp Cpo (T ) changes were observed for chitosan in the intervals from 300 to 330 K, between 340 and 360 K, and from 400 to 420 K. The
3. Results and discussion 3.1. Heat capacity The temperature dependence of the molar heat capacity of chitosangraft-PEHA copolymer in the range from 6 to 350 K is presented in Fig. 1. The experimental Cp,m points of copolymer in the above temperature interval are listed in Table S1 (Supplementary material). The smoothing of the experimental values of the heat capacity was made using exponential and semilogarithmic polynomial equations in the regions where any transformations were absent, and the corresponding coefficients were found with special computer software. The tested sample of chitosan-graft-PEHA copolymer was cooled from the room temperature to the onset temperature (T ∼ 6 K) of measurements at a flow rate of 0.01 K s−1. The molar heat capacity of copolymer gradually increases with rising of temperature in the range
Fig. 2. DSC curve of chitosan-graft-poly(2-ethylhexyl acrylate) copolymer. 138
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Table 2 The standard thermodynamic characteristics of the glass transition and glassy state of chitosan, poly(2-ethylhexyl acrylate) and chitosan-graft-PEHA copolymer at p = 0.1 MPaa. Sample
ΔТ1/K
o (Tg,1 ± 1) /K
Chitosan-graft-PEHA copolymer Chitosan [32] PEHA [35]
322–339 298–345 — 162–212
329 330 320 196
o Cpo (Tg,1 ) /J K−1 mol−1
162 ± 3 153 ± 2 — 88 ± 2
ΔТ2/K
o (Tg,2 ± 1) /K
456–478 — — —
467 — 347; 413 —
o Cpo (Tg,2 ) /J K−1 mol−1
222 ± 3 — — —
o /J K−1 mol−1 Sconf
Method of determination
95
DSC AC DSC AC
— 22
o Conventional designations: ΔT – the interval of glass transition; Tgo – the glass transition temperature; Cpo (Tgo) – the heat capacity ascribed to the glass transition; Sconf – the configuration entropy. a The standard uncertainty for pressure u(p) = 10 kPa. The reported uncertainties correspond to the expanded uncertainty with 0.95 level of confidence (k ≈ 2).
Table 3 The standard thermodynamic functions of chitosan-graft-PEHA copolymer (M(C7.14H12.9O3.77N0.82) = 170.56 g mol−1) at p = 0.1 MPaa. T/K
Cpo (T ) /J K−1 mol−1
[Н°(Т)−Н°(0)]/kJ mol−1
[S°(T)−S°(0)]/J K−1 mol−1
−[G°(T)−H°(0)]/kJ mol−1
Glassy state 5 10 15 20 25 30 35 40 45 50 60 70 80 90 100 150 200 250 298.15 300 330
0.267 2.06 5.44 9.871 14.81 19.96 25.15 30.32 35.42 40.43 50.15 59.44 68.30 76.74 84.81 121.0 154.9 192.8 238.9 241.0 278.0
0.000324 0.0053 0.0237 0.06161 0.1232 0.2101 0.3228 0.4615 0.6259 0.816 1.269 1.817 2.456 3.182 3.990 9.159 16.05 24.72 35.07 35.51 43.28
0.0891 0.713 2.15 4.306 7.031 10.18 13.65 17.35 21.22 22.23 33.45 41.88 50.40 58.93 67.44 108.9 148.3 186.8 224.6 226.0 250.7
0.000111 0.00179 0.00859 0.02449 0.05254 0.09542 0.1549 0.2325 0.3288 0.4026 0.7379 1.114 1.576 2.122 2.754 7.171 13.61 21.99 31.88 32.30 39.45
High elastic state 330 350
431.2 448.5
43.28 52.07
250.7 276.6
39.45 44.72
Conventional designations: Cpo (T ) – the molar heat capacity; [H°(T)−H°(0)] – the enthalpy; [S°(T)−S°(0)] – the entropy; [G°(T)−H°(0)] – the Gibbs energy. a The standard uncertainty for pressure u(p) = 10 kPa. The standard uncertainty for temperature u(T) = 0.02 K. The combined expanded relative uncertainties Uc,r(Cpo (T ) ) = 0.02, 0.005, 0.002; Uc,r([H°(T)−H°(0)]) = 0.022, 0.007, 0.005; Uc,r([S°(T)−S°(0)]) = 0.023, 0.008, 0.006; Uc,r([G°(T)−H°(0)]) = 0.03, 0.01, 0.009 in the interval from 6 to 15 K, between 15 and 40 K, in the range from 40 to 350 K, respectively. The reported uncertainties correspond to the 0.95 confidence level (k ≈ 2).
first transformation of chitosan can be referred to as β-transition caused by the release of pyranose rings around glucoside bond. Two other transformations are due to the glass transition of the two parts of chitosan [47–49]. The glass transition of PEHA was revealed on the Cpo (T ) curve in the interval from 162 to 212 K [35]. In accordance with the literature data and the obtained experimental results, it is logical to assume that the first change in the heat capacity of chitosan-graft-PEHA copolymer in the temperature range from 298 to 345 K is caused by the glass transition of PEHA and the βtransition of chitosan. The second change is related to the glass transition of chitosan. The data of Table 2 are in good agreement with the above statements. The observed changes in the temperature and the Cpo (Tgo) values can be explained in terms of difference in the chain length of PEHA and chitosan, as well as their mutual arrangement in the investigated chitosan-graft-PEHA copolymer. For the same reason, chitosan-graft-PEHA copolymer has higher Tgo values than block copolymers of chitosan with PDLLA [24].
3.3. Standard thermodynamic functions The molar heat capacity of chitosan-graft-PEHA copolymer was extrapolated from the onset temperature of measurements to T → 0 to calculate the standard thermodynamic functions of the investigated compound. For this purpose, the Debye model of heat capacity was used [50]:
Cpo = nD(
D / T ),
where D is the symbol of the Debye function, n and ΘD are specially selected parameters. For the investigated sample, n = 3 and ΘD = 86.7 K in the temperature interval from 6 to 13 K. The experimental Cpo values of copolymer were described by the above equation with the relative standard uncertainty ur(Cpo ) = 0.014. The standard thermodynamic functions of chitosan-graft-PEHA copolymer are presented in Table 3. The calculations of the enthalpy [H°(T)−H°(0)] and the entropy [S°(T)−S°(0)] were made by the
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Table 4 The standard thermodynamic characteristics of formation of chitosan, poly(2-ethylhexyl acrylate) and chitosan-graft-PEHA copolymer at T = 298.15 K, p = 0.1 MPaa. Sample
−ΔfH°/kJ mol−1
−ΔfS°/J K−1 mol−1
−ΔfG°/kJ mol−1
Chitosan-graft-PEHA copolymer Chitosan [23,24] PEHA [35]
800.3 ± 2.4b 801.7 ± 2.5 704.7 ± 5.4
1025.8 ± 0.6 1099.5 ± 0.6 1198.2 ± 2.4
494.5 ± 3.2 473.9 ± 3.1 347.5 ± 6.1
Conventional designations: ΔfH° – the enthalpy of formation; ΔfS° – the entropy of formation; ΔfG° – the Gibbs energy of formation. a The standard uncertainty for pressure u(p) = 10 kPa. The reported uncertainties correspond to the expanded uncertainty with 0.95 level of confidence (k ≈ 2). b The standard enthalpy of formation ΔfH° of chitosan-graft-PEHA copolymer was published in [36].
numerical integration of the Cpo (T ) and Cpo (ln T ) curves, respectively. The Gibbs energy [G°(T)−H°(0)] was calculated using the Gibbs–Helmholtz equation from the [H°(T)−H°(0)] and [S°(T)−S°(0)] values at the corresponding temperatures. The calculation technique was described in [51].
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3.4. Standard thermodynamic characteristics of formation The standard thermodynamic functions of formation of chitosangraft-PEHA copolymer are listed in Table 4. The standard entropy of formation ΔfS° of the studied compound was calculated using the [S°(T)−S°(0)] value (Table 3), as well as the residual entropy S°(0), and the absolute entropies of elemental substances C(gr), H2(g), O2(g), N2(g) at T = 298.15 K [52]. The standard enthalpy of formation ΔfH° = −(800.3 ± 2.4) kJ mol−1 of copolymer was published in [36]. According to the Gibbs–Helmholtz equation, the standard Gibbs energy of formation ΔfG° was calculated at T = 298.15 K. The obtained values correspond to the following reaction of formation of chitosan-graftPEHA copolymer in the glassy state: 7.14C(gr) + 6.45H2(g) C7.14H12.9O3.77N0.82(gl).
+
1.885O2(g)
+
0.41N2(g)
→
4. Conclusions The heat capacity of chitosan-graft-poly(2-ethylhexyl acrylate) copolymer was measured in the temperature range from 6 to 350 K by AC, and its thermophysical properties were studied in the interval from 270 to 570 K by DSC. The glass transition was detected for copolymer in the range from 298 to 345 K, Tgo = 330 K. The complex of the standard thermodynamic functions of the studied compound in various states for the range from T → 0 to 350 K, as well as the standard entropy and the Gibbs energy of its formation at T = 298.15 K were calculated based on the obtained experimental data, including the estimation of the residual entropy. It was established that the regularities of change in thermodynamic properties are the same for chitosan and its copolymers. In addition, the inclusion of PEHA into the chitosan macromolecules led to an increase in the glass transition temperatures of copolymer. Acknowledgments This work was performed with the financial support of the Ministry of Education and Science of the Russian Federation (Contracts Nos. 4.5706.2017/8.9 and 4.6138.2017/6.7). References [1] V.K. Thakur, M.K. Thakur, M.R. Kessler, Handbook of composites from renewable materials, Scrivener Publishing LLC, Beverly, 2017, https://doi.org/10.1002/ 9781119441632. [2] A. Dufresne, S. Thomas, L.A. Pothan, Biopolymer nanocomposites: processing, properties, and applications, John Wiley & Sons, Inc., Hoboken, 2013, https://doi. org/10.1002/9781118609958. [3] M. Rinaudo, Chitin and chitosan: properties and applications, Prog. Polym. Sci. 31 (2006) 603–632, https://doi.org/10.1016/j.progpolymsci.2006.06.001.
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