- Email: [email protected]
Influence of molecular weight on structure and rheological properties of microcrystalline chitosan mixtures
International Journal of Biological Macromolecules 79 (2015) 583–586
Contents lists available at ScienceDirect
International Journal of Biological Macromolecules journal homepage: www.elsevier.com/locate/ijbiomac
Influence of molecular weight on structure and rheological properties of microcrystalline chitosan mixtures Katarzyna Lewandowska ∗ Nicolaus Copernicus University in Toru´ n, Faculty of Chemistry, 7 Gagarin Street, 87-100 Toru´ n, Poland
a r t i c l e
i n f o
Article history: Received 11 March 2015 Received in revised form 27 April 2015 Accepted 10 May 2015 Available online 23 May 2015 Keywords: Chitosan Polyacrylamide Polymer mixture Rheological properties AFM
a b s t r a c t In the present work, the atomic force microscopy (AFM) studies and rheological properties of aqueous solutions of microcrystalline chitosan (MCCh), polyacrylamide (PAM) and their mixtures at different weight ratios have been investigated. Flow measurements were carried out using on solutions of native polymers and their mixtures with various weight fractions of components. It has been observed that the polymer solutions and their mixtures exhibited the non-Newtonian behavior with shear-thinning and/or shear-thickening areas. Rheological parameters from power law and activation energy of viscous flow are determined and discussed. The AFM images showed difference in surface properties films for the native polymers and their mixtures. The roughness of the mixtures increases with the increase of MCCh content. This may indicate a strong interaction between the polymeric components. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Polymer–polymer miscibility continues to attract the interest of many researchers. Blending of polymers with good miscibility is considered to be a very convenient method to meet new requirements in material properties [1–8]. It is known that the surfaces of mixtures prepared from natural and synthetic polymers are important for their use in biomedical applications [9–12]. The growth in practical applications for the natural polymer is related to the modification of its properties through suitable changes of molecular, supermolecular and chemical structure. New forms are manufactured by physico-chemical and chemical modifications. Microcrystalline chitosan (MCCh) is a special form of chitosan, which is prepared via its physicochemical modification using aqueous hydroxides and their salts [5]. The present work is a continuation of our previous studies on the physico-chemical properties of chitosan with partially hydrolyzed polyacrylamide or polyacrylamide [13,14]. In our earlier measurements, the surface properties of thin films based on the mixture of chitosan and partially hydrolyzed polyacrylamide have shown that the wettability and the surface roughness of chitosan and its blends films have been altered by mixing. These results have shown that chitosan/PAM is miscible at any composition.
The purpose of this study was to evaluate the physico-chemical properties of microcrystalline chitosan differing in molecular weight with partially hydrolyzed polyacrylamide on the basis of rheological measurements and atomic force microscopy. The rheological properties are important for various applications such as cosmetics, food and textile. Almost all concentrated polymer solutions show non-Newtonian behavior. The shear viscosity will be not constant but rather will depend upon the time, shear rate, temperature, etc. Flow properties of the used polymers and their mixtures solutions obey the power law relationship of the Ostwald de Waele model [15–22]: = k˙ n
where is the shear stress, ˙ is the shear rate, n and k are constants, known as the non-Newtonian index and the consistency index, respectively. From Eq. (1) it follows that if n is unity, then k is identical to and Eq. (1) appears as Newton’s law. The rheological parameters n and k are derived from the curve of logarithm shear stress versus logarithm shear rate. Activation energy of viscous flow (Ea ) was calculated with Arrhenius equation: a = A0 exp
∗ Tel.: +48 566114551. E-mail address: [email protected] http://dx.doi.org/10.1016/j.ijbiomac.2015.05.026 0141-8130/© 2015 Elsevier B.V. All rights reserved.
(1)
E a
RT
(2)
where A0 is preexponential parameter, Ea is the activation energy of viscous flow, R is the gas constant and T is the temperature.
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K. Lewandowska / International Journal of Biological Macromolecules 79 (2015) 583–586
Fig. 2. Logarithm of apparent shear viscosity of MCCh and PAM and their mixtures versus weight fraction of MCCh (wMCCh ) in the mixtures; T = 298 K, solvent: 0.1 M CH3 COOH0.2 mol/dm3 NaCl, dotted line – the values calculated according to the additivity rule. (A) MCCh I/PAM and (B) MCCh II/PAM.
Fig. 1. Apparent shear viscosity a versus shear rate of 1% of MCCh and PAM and their mixtures. (A) MCCh I/PAM, (B) MCCh II/PAM, wMCCh – the weight fraction of MCCh.
The value of activation energy of viscous flow calculated from Arrhenius equation reflects the influence of temperature on the interaction of polymer molecules with the used solvent. Analysis of polymer films was performed by Tapping Mode, which is one of the most dynamic techniques. Tapping Mode allows the examination of the sample surface topography and the surface roughness at the high resolution without damaging the surface of the sample. 2. Materials and methods Flow measurements were carried out on solution of native polymers: microcrystalline chitosan (MCCh) (with a degree of ¯ v = 5 × 105 for MCCh I and M ¯ v = 9 × 105 deacetylation of 86% and M for MCCh II, our laboratory) and polyacrylamide (PAM) (with a ¯ v = 1 × 106 , Aldich) and their mixdegree of hydrolysis of 1% and M tures with various weight fractions of components. Aqueous acetic acid/NaCl was used as solvents for MCCh, PAM and MCCh/PAM solution mixtures. The mixtures of different component ratios were obtained by mixing calculated volumes of both solutions. Polymer films were obtained by casting solutions onto glass plates. Rheological measurements were conducted on a Bohlin Visco 88 rotary viscometer with concentric cylinder at different temperatures (25–40 ◦ C) and shear rates (20–1230 s−1 ). The surface morphology of films were analyzed by using a commercial AFM a MultiMode Scanning Probe Microscope Nanoscope IIIa (Digital Instruments/Veeco Metrology Group, Santa Barbara, CA) operating in the tapping mode in air. 3. Results and discussion Based on the dependence of the shear viscosity on the shear rate, the type of solution was determinated. The viscosity curves for solutions of the investigated systems are presented in Fig. 1. It was found that studied polymer solutions exhibited the nonNewtonian behavior with shear-thinning and/or shear-thickening areas. As it can be observed, the apparent shear viscosity decreases
with the increase in the shear rate, indicating the pseudoplastic nature (the shear thinning region) for the solution PAM and MCCh II/PAM mixtures with wMCCh ≤ 0.5 (the weight fraction of MCCh). It is in accordance with previously reported data for the solutions of PAM [19,20]. The decrease of apparent shear viscosity with the shear rate is mainly related to the orientation of macromolecules along the streamline of flow and to the disentanglement of macromolecules with the increasing shear force [19–22]. However aqueous solutions of mixtures at wMCCh ≤ 0.5 exhibit the higher apparent viscosity than the apparent viscosity of native polymers. This synergy could be explained by the electrostatic interactions between the cationic chitosan and the partially anionic polyacrylamide molecules. For the solutions of MCCh I and MCCh I/PAM mixtures, the shear-thickening behavior is observed. The shear-thickening behavior may be related to the change of macromolecular conformation induced by flow [19–22]. In the case of the mixture solutions, the intermolecular interaction participates in the shear thickening phenomenon. The attractive force and/or electrostatic interactions between the polymeric components in the mixture solutions may cause the increase of apparent viscosity with the increase of shear rate. The comparison of flow behavior between the two different microcrystalline chitosan samples used in the blends is also shown in Fig. 1. For the solution of MCCh II, the apparent viscosity almost remains constant, indicating limiting Newtonian behavior. The reason is that these samples differ in their molecular weight which as could be expected influenced their rheological properties. Fig. 2 shows the logarithm of apparent shear viscosity (log a ) of the investigated mixtures versus weight fraction of MCCh (wMCCh ). The experimental log a values (solid line) have been drawn together with the log a values (dotted line), obtained according to the additivity rule. The positive deviations from the calculated straight-line illustrating the additivity rule have been observed in all investigated mixtures, with the exception of MCCh I/PAM mixtures at low values of shear rate. The deviations of log a decrease with the decrease of shear rate. Such behavior suggests that MCCh with PAM is miscible. It is also clearly seen that the differences of flow behavior in the two respective blends are visible. It is well known that the polymers with relatively the same molecular weight tend to be more miscible [23]. This may indicate the influence of the molecular weight of microcrystalline chitosan on the miscibility of polymeric components. The shear dependence of the viscosity was analyzed by using the well-know power law relationship [19–22]. The obtained values of rheological parameters are listed in Table 1. It was found the n values were less than 1 indicating shearthinning behavior for the solutions of PAM and MCCh II/PAM. For the MCCh I solution and MCCh I/PAM mixtures, where the shear
K. Lewandowska / International Journal of Biological Macromolecules 79 (2015) 583–586 Table 1 Values of rheological parameters of MCCh, PAM and their mixtures, 298 K, solvent: 0.1 mol/dm3 CH3 COOH/0.2 mol/dm3 NaCl. MCCh I/PAM wMCCh 0.0 0.2 0.5 0.8 1.0
n 0.88 1.27 1.53 1.51 1.50
Table 2 Activation energy of viscous flow of MCCh, PAM and their mixtures at = 400 s−1 solvent: 0.1 mol/dm3 CH3 COOH/0.2 mol/dm3 NaCl.
MCCh II/PAM R2
k −2
8.08 × 10 3.78 × 10−3 5.81 × 10−4 5.04 × 10−4 4.24 × 10−4
0.999 0.999 0.998 0.992 0.990
n 0.88 0.79 0.86 0.94 0.95
MCCh I/PAM
k −2
8.08 × 10 1.23 × 10−1 8.82 × 10−2 4.50 × 10−2 3.78 × 10−2
585
MCCh II/PAM
R2
wMCCh
Ea (kJ/mol)
R2
Ea (kJ/mol)
R2
0.999 0.996 0.999 1.000 0.990
0.0 0.2 0.5 0.8 1.0
10.94 13.55 17.65 23.54 15.95
0.999 0.999 0.998 0.992 0.990
10.94 14.88 17.04 22.51 30.16
0.999 1.000 0.999 0.998 0.990
thickening behavior was observed, the parameter n was more than 1. The highest value of n is obtained for the MCCh I/PAM mixture with wMCCh = 0.5. It may be caused by the development of temporary network or the increase of number entanglements of unfolded macromolecules. Among various interactions, hydrogen bonding and electrostatic interactions play important roles in the structure and properties of the polymer mixtures. Such competitive interactions dependent on the solution composition and the shear stress can be a reason of variation of temporary structure and macromolecular conformation in rheological measurements. The values of parameter k decrease with the increase of MCCh content in the mixture solutions. The rise of molecular weight of MCCh sample causes the less pronounced non-Newtonian behavior. The value of n amounts to 0.95. Such behavior is caused by an increase in the chain length which influences the increase of entanglements of macromolecular chains, leading to the broadening of the shear thinning region. Hence the shift of shear-thickening phenomenon to higher values of shear rate is observed [20]. Table 2 presents the change of the activation energy of viscous flow of MCCh/PAM mixtures of various weight fractions of MCCh. The solution of MCCh II is characterized by the higher value of Ea than the solutions of PAM and MCCh I. In the case of MCCh I/PAM with wMCCh ≥ 0.5, the activation energy is higher than those for the individual polymers. This may suggest that the stronger
Table 3 The roughness parameters (Rq ) for films of MCCh/PAM mixtures of different composition. MCCh I/PAM
MCCh II/PAM
wMCCh
Rq (nm)
Rq (nm)
0.0 0.2 0.5 0.8 1.0
0.20 1.41 0.54 1.19 1.60
0.20 0.51 1.40 2.90 1.45
intermolecular interactions between polymer components occur and they are responsible for the increase of Ea with increase of MCCh I content in mixtures. For the MCCh II/PAM solutions, the values of Ea are lower than that of the MCCh II solution. This indicates that solution complexation of MCCh II with PAM can improve the temperature tolerance of MCCh II [18]. The surface properties of polymeric films were observed using Atomic Force Microscopy (AFM). The examples of AFM images of MCChI/PAM and MCCh II/PAM mixtures are shown in Figs. 3 and 4. Table 3 gives the values of the roughness parameters for the investigated films. The surface morphology characteristics of microcrystalline chitosan, PAM and their mixtures films are different. Pure PAM film exhibits the smoothest surfaces. This agrees
Fig. 3. AFM images of the surface of MCCh I/PAM. (A) MCCh I, (B) 80/20, (C) 50/50 mixture, (D) PAM.
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K. Lewandowska / International Journal of Biological Macromolecules 79 (2015) 583–586
Fig. 4. AFM images of the surface of MCCh II/PAM. (A) MCCh II, (B) 80/20 mixture, (C) 50/50 mixture.
with the value of the roughness parameter which is about 0.2 nm (Table 3). For the MCCh films, the surface morphology is characterized by a more corrugated surface, with the roughness parameter being about 6 times higher than that for the PAM film. In the case of mixtures (Figs. 3 and 4), the surface topology began to alter after the addition of PAM. The surface of MCCh/PAM mixture film is similar to that of MCCh itself. It may suggest again that MCCh predominates on the mixture surface. The observed changes in morphology are related to the interactions between polymeric compounds. The repulsive forces and/or electrostatic interactions between components in the mixtures may lead to the increase of microdomains size what can be observed in Figs. 3 and 4, especially for MCCh II/PAM mixtures. The film surfaces become rougher than before the addition of PAM and the values of the roughness parameters increase (Table 3), especially for the mixtures containing MCCh II. This may indicate an increase in the heterogeneity of these mixtures in comparison to native polymer films. 4. Conclusions The solutions of microcrystalline chitosan and PAM samples as well as their solution mixtures behave as non-Newtonian fluids (Fig. 1). The rheological data has been well fitted to the Ostwald de Waele model described by the power law relationship. The n value is less than 1, indicating the pseudoplastic behavior of the PAM, MCCh II solutions and their mixtures (Table 1). In the case of the MCCh I solution and its mixtures, the n value is higher than 1, which indicates shear-thickening behavior. The highest Ea value is obtained for the MCCh II solution. Ea of the solution of MCCh I/PAM mixtures is higher than that of the individual component solutions in aqueous 0.1MCH3 COOH/0.2MNaCl (Table 2). This indicates on some interactions between the polymer components in the solution mixtures. AFM images show differences in surface properties between PAM films and films made of mixture of MCCh and PAM. The surface roughness of mixtures increases with the increase of MCCh content.
Summarizing, the structure and rheological properties of microcrystalline chitosan and polyacrylamide mixtures depend on the mixture composition and on the molecular weight of MCCh. The obtained results suggest that in the solution of MCCh/PAM mixtures, the components are miscible. References [1] J. Li, S. Zivanovic, P.M. Davidson, K. Kit, Carbohydr. Polym. 83 (2011) 375–382. [2] C. Castro, L. Gargallo, D. Radic, G. Kortaberria, I. Mondragon, Carbohydr. Polym. 83 (2011) 81–87. [3] S.J. Park, K.Y. Lee, W.S. Ha, S.Y. Park, J. Appl. Polym. Sci. 74 (1999) 2571– 2575. [4] K. Lewandowska, Appl. Surf. Sci. 263 (2012) 115–123. [5] K. Lewandowska, J. Solut. Chem. 42 (2013) 1654–1662. [6] K. Lewandowska, A. Sionkowska, B. Kaczmarek, G. Furtos, Int. J. Biol. Macromol. 65 (2014) 534–541. [7] A. Sionkowska, K. Lewandowska, A. Płanecka, J. Mol. Liq. 198 (2014) 354–357. [8] Y. Ren, R. Yang, X. Liu, F. Liu, Eur. Polym. J. 47 (2011) 2016–2021. [9] A. Sionkowska, A. Planecka, J. Kozlowska, J. Skopinska-Wisniewska, P. Los, Carbohydr. Polym. 84 (2011) 900–906. ˝ [10] T. Caykara, S. Demirci, M.S. Eroˇglu, O. Guven, J. Polym. Sci. B: Polym. Phys. 44 (2006) 426–430. [11] E. Karavas, E. Georgarakis, D. Bikiaris, J. Therm. Anal. Calorim. 84 (2006) 125–133. [12] S. Demirci, A. Alaslan, T. Caykara, Appl. Surf. Sci. 255 (2009) 5979–5983. [13] K. Lewandowska, Mol. Cryst. Liq. Cryst. 590 (2014) 186–192. [14] K. Lewandowska, Miscibility and physical properties of chitosan and polyacrylamide blends, J. Mol. Liq. (2015), http://dx.doi.org/10.1016/j.molliq.2015.05. 049 (in press). [15] A.H.P. Skellard, Non-Newtonian Flow and Heat Transfer, Wiley, New York, 1967. ´ [16] K. Lewandowska, D.U. Staszewska, S. Trzcinski, in: H. Struszczyk (Ed.), Progress on Chemistry and Application of Chitin and its Derivatives, Monograph, vol. IV, Polish Chitin Society, Łódz, 1998, pp. 17–36. [17] M. Mucha, React. Funct. Polym. 39 (1998) 19–25. [18] L.M. Zhang, Colloid Polym. Sci. 277 (1999) 886–890. [19] B. Briscoe, P. Luckham, S. Zu, Rheol. Acta 38 (1999) 224–234. [20] K. Lewandowska, J. Appl. Polym. Sci. 103 (2007) 2235–2241. [21] K. Lewandowska, in: M.M. Jaworska (Ed.), Progress on Chemistry and Application of Chitin and its Derivatives, Monograph, vol. XIV, Polish Chitin Society, Łódz, 2009, pp. 41–48. ˛ [22] K. Lewandowska, A. Dabrowska, H. Kaczmarek, e-Polymers no. 015, 2012, pp. 1–13. [23] A. Dondos, V. Christopoulou, D. Papanagopoulos, J. Polym. Sci. B: Polym. Phys. 37 (1999) 379–387.
Contents lists available at ScienceDirect
International Journal of Biological Macromolecules journal homepage: www.elsevier.com/locate/ijbiomac
Influence of molecular weight on structure and rheological properties of microcrystalline chitosan mixtures Katarzyna Lewandowska ∗ Nicolaus Copernicus University in Toru´ n, Faculty of Chemistry, 7 Gagarin Street, 87-100 Toru´ n, Poland
a r t i c l e
i n f o
Article history: Received 11 March 2015 Received in revised form 27 April 2015 Accepted 10 May 2015 Available online 23 May 2015 Keywords: Chitosan Polyacrylamide Polymer mixture Rheological properties AFM
a b s t r a c t In the present work, the atomic force microscopy (AFM) studies and rheological properties of aqueous solutions of microcrystalline chitosan (MCCh), polyacrylamide (PAM) and their mixtures at different weight ratios have been investigated. Flow measurements were carried out using on solutions of native polymers and their mixtures with various weight fractions of components. It has been observed that the polymer solutions and their mixtures exhibited the non-Newtonian behavior with shear-thinning and/or shear-thickening areas. Rheological parameters from power law and activation energy of viscous flow are determined and discussed. The AFM images showed difference in surface properties films for the native polymers and their mixtures. The roughness of the mixtures increases with the increase of MCCh content. This may indicate a strong interaction between the polymeric components. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Polymer–polymer miscibility continues to attract the interest of many researchers. Blending of polymers with good miscibility is considered to be a very convenient method to meet new requirements in material properties [1–8]. It is known that the surfaces of mixtures prepared from natural and synthetic polymers are important for their use in biomedical applications [9–12]. The growth in practical applications for the natural polymer is related to the modification of its properties through suitable changes of molecular, supermolecular and chemical structure. New forms are manufactured by physico-chemical and chemical modifications. Microcrystalline chitosan (MCCh) is a special form of chitosan, which is prepared via its physicochemical modification using aqueous hydroxides and their salts [5]. The present work is a continuation of our previous studies on the physico-chemical properties of chitosan with partially hydrolyzed polyacrylamide or polyacrylamide [13,14]. In our earlier measurements, the surface properties of thin films based on the mixture of chitosan and partially hydrolyzed polyacrylamide have shown that the wettability and the surface roughness of chitosan and its blends films have been altered by mixing. These results have shown that chitosan/PAM is miscible at any composition.
The purpose of this study was to evaluate the physico-chemical properties of microcrystalline chitosan differing in molecular weight with partially hydrolyzed polyacrylamide on the basis of rheological measurements and atomic force microscopy. The rheological properties are important for various applications such as cosmetics, food and textile. Almost all concentrated polymer solutions show non-Newtonian behavior. The shear viscosity will be not constant but rather will depend upon the time, shear rate, temperature, etc. Flow properties of the used polymers and their mixtures solutions obey the power law relationship of the Ostwald de Waele model [15–22]: = k˙ n
where is the shear stress, ˙ is the shear rate, n and k are constants, known as the non-Newtonian index and the consistency index, respectively. From Eq. (1) it follows that if n is unity, then k is identical to and Eq. (1) appears as Newton’s law. The rheological parameters n and k are derived from the curve of logarithm shear stress versus logarithm shear rate. Activation energy of viscous flow (Ea ) was calculated with Arrhenius equation: a = A0 exp
∗ Tel.: +48 566114551. E-mail address: [email protected] http://dx.doi.org/10.1016/j.ijbiomac.2015.05.026 0141-8130/© 2015 Elsevier B.V. All rights reserved.
(1)
E a
RT
(2)
where A0 is preexponential parameter, Ea is the activation energy of viscous flow, R is the gas constant and T is the temperature.
584
K. Lewandowska / International Journal of Biological Macromolecules 79 (2015) 583–586
Fig. 2. Logarithm of apparent shear viscosity of MCCh and PAM and their mixtures versus weight fraction of MCCh (wMCCh ) in the mixtures; T = 298 K, solvent: 0.1 M CH3 COOH0.2 mol/dm3 NaCl, dotted line – the values calculated according to the additivity rule. (A) MCCh I/PAM and (B) MCCh II/PAM.
Fig. 1. Apparent shear viscosity a versus shear rate of 1% of MCCh and PAM and their mixtures. (A) MCCh I/PAM, (B) MCCh II/PAM, wMCCh – the weight fraction of MCCh.
The value of activation energy of viscous flow calculated from Arrhenius equation reflects the influence of temperature on the interaction of polymer molecules with the used solvent. Analysis of polymer films was performed by Tapping Mode, which is one of the most dynamic techniques. Tapping Mode allows the examination of the sample surface topography and the surface roughness at the high resolution without damaging the surface of the sample. 2. Materials and methods Flow measurements were carried out on solution of native polymers: microcrystalline chitosan (MCCh) (with a degree of ¯ v = 5 × 105 for MCCh I and M ¯ v = 9 × 105 deacetylation of 86% and M for MCCh II, our laboratory) and polyacrylamide (PAM) (with a ¯ v = 1 × 106 , Aldich) and their mixdegree of hydrolysis of 1% and M tures with various weight fractions of components. Aqueous acetic acid/NaCl was used as solvents for MCCh, PAM and MCCh/PAM solution mixtures. The mixtures of different component ratios were obtained by mixing calculated volumes of both solutions. Polymer films were obtained by casting solutions onto glass plates. Rheological measurements were conducted on a Bohlin Visco 88 rotary viscometer with concentric cylinder at different temperatures (25–40 ◦ C) and shear rates (20–1230 s−1 ). The surface morphology of films were analyzed by using a commercial AFM a MultiMode Scanning Probe Microscope Nanoscope IIIa (Digital Instruments/Veeco Metrology Group, Santa Barbara, CA) operating in the tapping mode in air. 3. Results and discussion Based on the dependence of the shear viscosity on the shear rate, the type of solution was determinated. The viscosity curves for solutions of the investigated systems are presented in Fig. 1. It was found that studied polymer solutions exhibited the nonNewtonian behavior with shear-thinning and/or shear-thickening areas. As it can be observed, the apparent shear viscosity decreases
with the increase in the shear rate, indicating the pseudoplastic nature (the shear thinning region) for the solution PAM and MCCh II/PAM mixtures with wMCCh ≤ 0.5 (the weight fraction of MCCh). It is in accordance with previously reported data for the solutions of PAM [19,20]. The decrease of apparent shear viscosity with the shear rate is mainly related to the orientation of macromolecules along the streamline of flow and to the disentanglement of macromolecules with the increasing shear force [19–22]. However aqueous solutions of mixtures at wMCCh ≤ 0.5 exhibit the higher apparent viscosity than the apparent viscosity of native polymers. This synergy could be explained by the electrostatic interactions between the cationic chitosan and the partially anionic polyacrylamide molecules. For the solutions of MCCh I and MCCh I/PAM mixtures, the shear-thickening behavior is observed. The shear-thickening behavior may be related to the change of macromolecular conformation induced by flow [19–22]. In the case of the mixture solutions, the intermolecular interaction participates in the shear thickening phenomenon. The attractive force and/or electrostatic interactions between the polymeric components in the mixture solutions may cause the increase of apparent viscosity with the increase of shear rate. The comparison of flow behavior between the two different microcrystalline chitosan samples used in the blends is also shown in Fig. 1. For the solution of MCCh II, the apparent viscosity almost remains constant, indicating limiting Newtonian behavior. The reason is that these samples differ in their molecular weight which as could be expected influenced their rheological properties. Fig. 2 shows the logarithm of apparent shear viscosity (log a ) of the investigated mixtures versus weight fraction of MCCh (wMCCh ). The experimental log a values (solid line) have been drawn together with the log a values (dotted line), obtained according to the additivity rule. The positive deviations from the calculated straight-line illustrating the additivity rule have been observed in all investigated mixtures, with the exception of MCCh I/PAM mixtures at low values of shear rate. The deviations of log a decrease with the decrease of shear rate. Such behavior suggests that MCCh with PAM is miscible. It is also clearly seen that the differences of flow behavior in the two respective blends are visible. It is well known that the polymers with relatively the same molecular weight tend to be more miscible [23]. This may indicate the influence of the molecular weight of microcrystalline chitosan on the miscibility of polymeric components. The shear dependence of the viscosity was analyzed by using the well-know power law relationship [19–22]. The obtained values of rheological parameters are listed in Table 1. It was found the n values were less than 1 indicating shearthinning behavior for the solutions of PAM and MCCh II/PAM. For the MCCh I solution and MCCh I/PAM mixtures, where the shear
K. Lewandowska / International Journal of Biological Macromolecules 79 (2015) 583–586 Table 1 Values of rheological parameters of MCCh, PAM and their mixtures, 298 K, solvent: 0.1 mol/dm3 CH3 COOH/0.2 mol/dm3 NaCl. MCCh I/PAM wMCCh 0.0 0.2 0.5 0.8 1.0
n 0.88 1.27 1.53 1.51 1.50
Table 2 Activation energy of viscous flow of MCCh, PAM and their mixtures at = 400 s−1 solvent: 0.1 mol/dm3 CH3 COOH/0.2 mol/dm3 NaCl.
MCCh II/PAM R2
k −2
8.08 × 10 3.78 × 10−3 5.81 × 10−4 5.04 × 10−4 4.24 × 10−4
0.999 0.999 0.998 0.992 0.990
n 0.88 0.79 0.86 0.94 0.95
MCCh I/PAM
k −2
8.08 × 10 1.23 × 10−1 8.82 × 10−2 4.50 × 10−2 3.78 × 10−2
585
MCCh II/PAM
R2
wMCCh
Ea (kJ/mol)
R2
Ea (kJ/mol)
R2
0.999 0.996 0.999 1.000 0.990
0.0 0.2 0.5 0.8 1.0
10.94 13.55 17.65 23.54 15.95
0.999 0.999 0.998 0.992 0.990
10.94 14.88 17.04 22.51 30.16
0.999 1.000 0.999 0.998 0.990
thickening behavior was observed, the parameter n was more than 1. The highest value of n is obtained for the MCCh I/PAM mixture with wMCCh = 0.5. It may be caused by the development of temporary network or the increase of number entanglements of unfolded macromolecules. Among various interactions, hydrogen bonding and electrostatic interactions play important roles in the structure and properties of the polymer mixtures. Such competitive interactions dependent on the solution composition and the shear stress can be a reason of variation of temporary structure and macromolecular conformation in rheological measurements. The values of parameter k decrease with the increase of MCCh content in the mixture solutions. The rise of molecular weight of MCCh sample causes the less pronounced non-Newtonian behavior. The value of n amounts to 0.95. Such behavior is caused by an increase in the chain length which influences the increase of entanglements of macromolecular chains, leading to the broadening of the shear thinning region. Hence the shift of shear-thickening phenomenon to higher values of shear rate is observed [20]. Table 2 presents the change of the activation energy of viscous flow of MCCh/PAM mixtures of various weight fractions of MCCh. The solution of MCCh II is characterized by the higher value of Ea than the solutions of PAM and MCCh I. In the case of MCCh I/PAM with wMCCh ≥ 0.5, the activation energy is higher than those for the individual polymers. This may suggest that the stronger
Table 3 The roughness parameters (Rq ) for films of MCCh/PAM mixtures of different composition. MCCh I/PAM
MCCh II/PAM
wMCCh
Rq (nm)
Rq (nm)
0.0 0.2 0.5 0.8 1.0
0.20 1.41 0.54 1.19 1.60
0.20 0.51 1.40 2.90 1.45
intermolecular interactions between polymer components occur and they are responsible for the increase of Ea with increase of MCCh I content in mixtures. For the MCCh II/PAM solutions, the values of Ea are lower than that of the MCCh II solution. This indicates that solution complexation of MCCh II with PAM can improve the temperature tolerance of MCCh II [18]. The surface properties of polymeric films were observed using Atomic Force Microscopy (AFM). The examples of AFM images of MCChI/PAM and MCCh II/PAM mixtures are shown in Figs. 3 and 4. Table 3 gives the values of the roughness parameters for the investigated films. The surface morphology characteristics of microcrystalline chitosan, PAM and their mixtures films are different. Pure PAM film exhibits the smoothest surfaces. This agrees
Fig. 3. AFM images of the surface of MCCh I/PAM. (A) MCCh I, (B) 80/20, (C) 50/50 mixture, (D) PAM.
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K. Lewandowska / International Journal of Biological Macromolecules 79 (2015) 583–586
Fig. 4. AFM images of the surface of MCCh II/PAM. (A) MCCh II, (B) 80/20 mixture, (C) 50/50 mixture.
with the value of the roughness parameter which is about 0.2 nm (Table 3). For the MCCh films, the surface morphology is characterized by a more corrugated surface, with the roughness parameter being about 6 times higher than that for the PAM film. In the case of mixtures (Figs. 3 and 4), the surface topology began to alter after the addition of PAM. The surface of MCCh/PAM mixture film is similar to that of MCCh itself. It may suggest again that MCCh predominates on the mixture surface. The observed changes in morphology are related to the interactions between polymeric compounds. The repulsive forces and/or electrostatic interactions between components in the mixtures may lead to the increase of microdomains size what can be observed in Figs. 3 and 4, especially for MCCh II/PAM mixtures. The film surfaces become rougher than before the addition of PAM and the values of the roughness parameters increase (Table 3), especially for the mixtures containing MCCh II. This may indicate an increase in the heterogeneity of these mixtures in comparison to native polymer films. 4. Conclusions The solutions of microcrystalline chitosan and PAM samples as well as their solution mixtures behave as non-Newtonian fluids (Fig. 1). The rheological data has been well fitted to the Ostwald de Waele model described by the power law relationship. The n value is less than 1, indicating the pseudoplastic behavior of the PAM, MCCh II solutions and their mixtures (Table 1). In the case of the MCCh I solution and its mixtures, the n value is higher than 1, which indicates shear-thickening behavior. The highest Ea value is obtained for the MCCh II solution. Ea of the solution of MCCh I/PAM mixtures is higher than that of the individual component solutions in aqueous 0.1MCH3 COOH/0.2MNaCl (Table 2). This indicates on some interactions between the polymer components in the solution mixtures. AFM images show differences in surface properties between PAM films and films made of mixture of MCCh and PAM. The surface roughness of mixtures increases with the increase of MCCh content.
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