Rheology and viscosity scaling of gelatin/1-allyl-3-methylimidazolium chloride solution

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1 Korea-Australia Rheology Journal, Vol.26, No.2, pp (May 2014) DOI: /s Rheology and viscosity scaling of gelatin/1-allyl-3-methylimidazolium chloride solution Congde Qiao, Tianduo Li*, Ling Zhang, Xiaodeng Yang and Jing Xu Shandong Provincial Key Laboratory of Fine Chemicals, Shandong Polytechnic University, Jinan , P.R. China (Received October 9, 2013; final revision March 14, 2014; accepted March 17, 2014) Gelatin/1-allyl-3-methylimidazolium chloride solutions are prepared by using the ionic liquid 1-allyl-3- methylimidazolium chloride as solvent. The rheological properties of the gelatin solutions have been investigated by steady shear and oscillatory shear measurements. In the steady shear measurements, the gelatin solutions with high concentration show a shear-thinning flow behavior at high shear rates, while another shear thinning region can be found in the dilute gelatin solutions at low shear rates. The overlap concentration of gelatin in [amim]cl is 1.0 wt% and the entanglement concentration is a factor of 4 larger (4.0 wt%). The high intrinsic viscosity (295 ml/g) indicates that the gelatin chains dispersed freely in the ionic liquid and no aggregation phenomenon occurs in dilute gelatin solution. The frequency dependences of modulus changed obviously with an increase in gelatin concentration. The empirical time-temperature superposition principle holds true at the experimental temperatures. Keywords: gelatin, 1-allyl-3-methylimidazolium chloride, rheology 1. Introduction Gelatin is a natural biopolymer derived from skin or bone collagen, by acid or alkali treatment to give type A and type B gelatins, respectively. It is known that gelatin is one of the most extensively applied potential biopolymers due to its low toxicity, availability, and biodegradability. It is widely used in pharmaceutical, food, cosmetic and photograghic industries (Segtnan and Isaksson, 2004; Kim et al., 2007; Wang et al., 2009). Because gelatin is soluble in water, water is perhaps the most frequently used solvent for gelatin. Generally, there is always a certain amount of water in the bulk gelatin, named structural water, which contribute to the helix structure of gelatin (Hüttenrauch and Fricke, 1984). However, the presence of such water complicated the relationships between the macromolecular structure and its solution properties. Generally, the conformational changes of macromolecules in solution can be reflected by its bulk rheological properties. Moreover, the gelatin molecules always aggregate in aqueous solution (Bohidar and Jena, 1994; Bohidar, 1998; Mohanty and Bohidar, 2005; Olivares et al., 2006), which limited the study of the single chain structure in polymer solution. Although gelatin can be dissolved in some organic solvents, such as dimethylformamide and acetic acid, the application of these solvent systems is still limited due to the disadvantages of toxicity, instability or difficulty in solvent recovery. Recently, much attention has been paid to the ionic liquids (ILs), which are a class of promising solvents, due to their remarkable properties such as strong polarity, low volatility, high thermal stability and ease of recycling *Corresponding author: litianduo@163.com, Tel./Fax: (Welton, 1999; Kubisa, 2004; Rogers and Seddon, 2005). Besides, they are excellent solvents for inorganic, organic and polymer materials, and they are widely used in organic synthesis, electrochemistry, chemical separation and material preparation. Among these materials, there is a novel ionic liquid, 1-allyl-3-methylimidazolium chloride ([Amim]Cl), with a low melting temperature of about 17 C and a prominent capability of dissolving biopolymers, such as cellulose (Kuang et al., 2008), starch (Wang et al., 2006), collagen (Zhou et al., 2010) and silk fibroin (Wang et al., 2012). In addition, the rheological behaviors of gelatin in ionic liquid at high temperatures were investigated and the results showed that the storage modulus of gelatin solutions was essentially independent of gelatin concentration and temperature at all frequencies, while the loss modulus had a strong concentration and temperaturedependence (Zhang et al., 2012). Polyelectrolyte solution rheology is complicated due to the sensitivity of the polymer to the presence of ions in solution. In the literature, several theories have been put forward to explain the observed differences in rheological properties between polyelectrolytes and neutral polymers (de Gennes et al,. 1976; Rubinstein et al., 1994; Dobrynin et al., 1995; Barrat and Joanny, 1996; Colby, 2010; Chun and Ko, 2012). The scaling theory proposed by Dobrynin et al. has been successfully applied in both the flexible polyelectrolytes solutions (Dobrynin et al., 1995) and some rigid polyelectrolyte systems (Zirnsak et al., 1999; Wyatt and Liberatore, 2009). However, few work has been done on the rheological properties of polyelectrolytes in ionic liquid (Wang et al., 2012), and the application of scaling theory needs to be validated in further. Moreover, the molecular models, based on the linear viscoelastic behaviors of dilute solutions, have been proved to be very useful in explaining 2014 The Korean Society of Rheology and Springer 169

2 Congde Qiao, Tianduo Li, Ling Zhang, Xiaodeng Yang and Jing Xu the rheological behaviors of polymer solutions (Larson, 1999). However, the experimental data for dilute polyelectrolytes solutions are very deficient, particularly speaking of the dynamic data for testing various molecular models. It is widely accepted that understanding of the rheological properties of gelatin solution is crucial for the processing of high performance gelatin-based materials. For example, the applications of gelatin in the food, pharmaceutical, and photographic industries are directly attributed to its conformation transition. This conformational change of macromolecules in solution can be reflected by a rheological response. In addition, gelatin is commonly used in conjunction with surfactant to promote emulsification and control surface tension, while many of the room temperature ionic liquids behavior as surfactants. Hence, understanding of the interactions between gelatin and ionic liquid are of great importance in various industrial processes. The rheological behaviors of gelatin aqueous solution have been investigated extensively. However, few work has been done on the rheological properties of gelatin/[amim]cl solutions. In this paper, the rheological properties of gelatin/ils solution were investigated by steady and oscillatory shear measurements, and the effect of concentration on the rheological behaviors was discussed in details. Moreover, further knowledge on the structure of the biopolymer chains was obtained through further analysis of its rheological properties. 2. Experiments 2.1. Materials The ionic liquid, used in this study was 1-allyl-3-methylimidazolium chloride ([amim]cl), was supplied kindly by Lanzhou Institute of Chemical Physics, China. The water content in the [amim]cl sample with the Karl Fischer Titration was 0.25 wt%. The [amim]cl sample was used as received, and without further purification. The gelatin (G2500, Type A) was purchased from Sigma-Aldrich Company. The gelatin sample was dried at 50 o C in vacuum prior to use. cone/plate geometry (0.5 cone angle, 60 mm cone diameter) with the gap setting at mm. A thin layer of light silicon was placed around the borders of the measuring cell to prevent contact of the test fluid with air. The influence of this oil film on the measured rheological properties was found to be negligible by testing pure ionic liquid with and without the applied oil film. Steady shear and dynamic oscillatory shear experiments were carried out in the shear rate range of s -1, and frequency range of rad/s, respectively. The linear viscoelastic region was determined with a stress sweep at constant frequency of rad/s, for each solution at the temperature of 30 o C. The complex viscosity (η*) was constant for most of the tested stress range (from 0.1 to 100 Pa), and 10 Pa was chosen for all dynamic frequency tests to measure dynamic modulus, which fell well into the linear viscoelastic region during frequency sweep. The experiments were carried out at temperatures ranging from 20 to 50 o C. The steady shear viscosity of the gelatin/ [amim]cl solution was measured at 30 o C, All the samples showed appreciable shear thinning, and the apparent viscosity of the gelatin solution was determined as a function of shear rate. 3. Results and Discussions 3.1. Steady Shear on Gelatin/[amim]Cl Solutions Fig. 1 shows the steady shear viscosity as a function of the shear rate for gelatin solutions over a range of concentrations. It was found that the viscosity of gelatin/ [amim]cl solution increased by nearly three orders of magnitude with an increase in gelatin concentration. The gelatin solutions with low concentrations show a shear thinning behavior at low shear rates (from about 0.1 to 1 s -1 ). It is fol Samples Preparation To prepare gelatin/[amim]cl solution, a known amount of gelatin powder was mixed with [amim]cl in a threenecked flask by stirring under an inert atmosphere of N 2. The solutions were heated at 100 o C for 1h to obtain a homogeneous solution. The transparent gelatin/[amim]cl solutions with various concentration from 0.3 to 8.0 wt% were sealed and stored at room temperature and protected against moisture absorption Rheology The rheological behavior of gelatin solution was investigated by using a Haake RS75 rheometer with a Ti steel Fig. 1. Changes of steady shear viscosity, η, as functions of shear rate, γ, for gelatin/[amim]cl solutions with different concentrations at 30 o C. 170 Korea-Australia Rheology J., Vol. 26, No. 2 (2014)

3 Rheology and viscosity scaling of gelatin/1-allyl-3-methylimidazolium chloride solution lowed by Newtonian flow in the subsequent shear rate range, which was similar to that of pure [amim]cl solution as also shown in Fig. 1. As the concentration of gelatin increased, the shear thinning behavior at lower shear rates became less distinct or even disappeared, meanwhile a shear thinning behavior at higher rate arose. The similar rheological behaviors have been reported in cellulose/ [amim]cl (Kuang et al., 2008) and SF/[amim]Cl solutions (Wang et al., 2012). There is a dynamic network in the ionic liquid, which is resulted from hydrogen bonding between [amim]cl molecules confirmed by dynamic light scatting (Kuang et al., 2007) and oscillatory shear measurement (Kuang et al., 2008; Wang et al., 2012). In dilute gelatin/ [amim]cl solutions, there is a physical network formed by hydrogen bonding between [amim]cl molecules. Therefore, the shear thinning behavior of the dilute gelatin/[amim]cl solutions at low shear rates should be related to the gradual disruption of the physical networks formed by hydrogen bonds between [amim]cl molecules, as similar to the behavior of the pure [amim]cl ionic liquid. As the concentration of gelatin is further increased into the semidilute region, the physical network formed by [amim]cl molecules is replaced by a new physical network formed by the entangled gelatin chains in ionic liquid, which eventually led to the disappearance of the shear thinning region at low shear rates. At high shear rate, shear stress is strong enough to disrupt the entanglements of gelatin molecules in solutions. It was apparent that the shear thinning behavior can be observed easily for gelatin/ [amim]cl solutions with high concentrations (c>3.0 wt%), which was similar to that of typical polymer solution (Tam and Tiu, 1989). The mechanism of this shear thinning behavior can be explained as follows(fig. 2): at low shear rate, the gelatin chains exist as random coils, with the increase of shear rate, the polymer chains become extended. The changes of the polymer chain conformation lead to a decrease of viscosity of gelatin/[amim]cl solution. It should be pointed out that the gelatin solutions with moderate concentrations (c<3.0 wt%) show another shear thinning behavior at higher shear rates (not shown). Fig. 2. (Color online) Illustration of the conformation of gelatin chains under different shear rates (solvent not shown) Concentration dependence of Specific viscosity The Dobrynin scaling theory has been successfully applied in polyelectrolytes solutions (Dobrynin et al., 1995; Dobrynin and Rubinstein, 2005; Wyatt and Liberatore, 2009; Zhulina and Rubinstein, 2012). By plotting the specific viscosity, the effect of the solvent viscosity is eliminated so that we can look for scaling behaviors in the low concentration range as well. The specific viscosity η η η sp = s (where η and η s are the viscosities of the η solution s and pure solvent, respectively) is plotted against concentration for the gelatin/[amim]cl solutions presented in Fig. 3. It was obvious that the specific viscosity showed a power-law dependence on concentration with the equation as follows: η sp c α. (1) There are two critical concentration c * (overlap concentration) and c e (entanglement concentration) identified as about 1.0 wt% and 4.0 wt%, respectively, which divided the gelatin solution into three regions. The power law scaling of η sp for our gelatin sample have an exponent( α ) of 1.0, 1.3 and 3.7, belonging to the dilute, semidilute unentangled, and semidilute entangled regions, respectively, according to the scaling theory (Colby and Rubinstein, 1990; Rubinstein and Colby, 2003; Colby, 2010). In the dilute solution region (c < 1.0 wt%), the gelatin existed mainly as single chains and the excluded volume between chains kept them away from each other and made them adopt a random coil conformation. The specific viscosity increased linearly with concentration ( η sp c ). In the semidilute unentangled region (1.0 wt%<c<4.0 wt%), the single gelatin chains start to contact each other, and the specific viscosity increased moderately with concentration ( η sp c 1.3 ). When the gelatin concentration was increased to a semidilute entangled region (c > 4.0 wt%), entanglement occurred among the crowded polymer chains, and the scaling of η sp is much stronger with concentration( η sp c 3.7 ). Very good agreements with the scaling predictions for a neutral polymer in a good solvent were observed in the dilute, semidilute unentangled, and semidilute entangled concentration regimes (de Gennes, 1979; Adam and Delsanti, 1983), respectively. It is concluded that the gelatin in ionic liquid behavior as neutral polymer in good solvent, and the Coulombic interactions which the polymer experiences in salt-free aqueous solution are fully screened Intrinsic Viscosity Generally, the intrinsic viscosity( [ η] ) was correlated with the hydrodynamic volume of polymer chain and can be described by [ η] = lim( η sp c). (2) c 0 Korea-Australia Rheology J., Vol. 26, No. 2 (2014) 171

4 Congde Qiao, Tianduo Li, Ling Zhang, Xiaodeng Yang and Jing Xu Fig. 3. (Color online) Concentration dependence of specific viscosity( η ) for gelatin/[amim]cl solutions at 30 o sp C with overlap concentration c * = 1.0 wt % and entanglement concentration c e = 4.0 wt % indicated by vertical dashed lines. Fig. 4 showed the reduced viscosity ( η sp c ) as a function of concentration. The inset of Fig. 4 presented data for dilute solutions, together with a linear regression. The intrinsic viscosity was obtained to be about 295 ml/g as the extrapolated value of η sp c as the gelatin concentration approached zero, according to Eq. (2). The specific viscosity η sp as a function of c[ η] was plotted in Fig. 5. The data can be fitted with a truncated version of the general Huggins equation, defined in Eq. (3) (Kulicke and Kniewske, 1984) η sp = c[ η] + K H ( c[ η] ) 2 + Acη ( [ ]) n (3) where K H is the Huggins coefficient and A and n are fitting parameters. In Fig. 5, the plot of the specific viscosity as a function of c[ η] resulted in a linear relationship in the range of larger c[ η] and the parameter, n, was determined by a linear regression with a value of about 3.7, Then, through a polynomial fitting of the specific viscosity η sp against c[ η] and the Huggins coefficient K H was obtained as about 0.1. It is known that K H is sensitive to chain aggregation and the association of the polymer chains always results in high K H (Scho, 1999). Generally, a solvent with K H less than 0.5 could dissolve the solute well. It can be included that [amim]cl was a good solvent for gelatin, and the gelatin chains were expected to be dispersed freely in [amim]cl solution rather than aggregate as they do in aqueous solution (Olivares et al., 2006). In addition, our previous work validated the aggregation behavior of gelatin in aqueous solution(xu et al., 2012). It was also found that the morphology of the gelatin aggregate displayed a series of changes with the concentration variation, and this structural transitions was strongly influenced by hydrogen bonding, hydrophobic interactions and electrostatic repulsion. Fig. 4. (Color online) Concentration dependence of reduced viscosity ( η ) for gelatin/[amim]cl solutions at 30 o sp c C. The intrinsic viscosity of the gelatin in [amim]cl is around 295 ml/g. However, the intrinsic viscosity of the gelatin with similar molecular weight in aqueous solution is about 50 ml/g (Bohidar and Jena, 1994; Li and Cheng, 2006; Olivares et al., 2006). Generally, the intrinsic viscosity is a molecular parameter which can be interpreted in terms of molecular conformation. Obviously, the conformation of the gelatin molecules in [amim]cl is very different from that in aqueous solution. It is well known that the gelatin molecules tend to aggregate in dilute aqueous solution (Olivares et al., 2006). In these aggregates the increase of the hydrodynamic volume is smaller than the increase of their molecular mass, which results in decrease in reduced viscosity (Dondos et al., 1989). The aggregation behavior, which may be induced by intramolecular or intermolecular interactions in dilute gelatin aqueous solution, leads to the reduction in the intrinsic viscosity. In addition, the Flory-Huggins interaction parameter χ was determined to be around 0.5 for gelatin/h 2 O system at 35 o C, which indicated the polymer was nearly in θ state (Bohidar and Jena, 1994). When the temperature decreased to 30 o C, water probably became a poor solvent for gelatin. The gelatin chains aggregate in aqueous solution and this needs further study. The situation is different for gelatin in [amim]cl solution. It is known that the ionic liquid has a unique structure, especially in its cationic part. The cationic structure is composed of two parts, which are a large imidazolium ring with positive charge and a nonpolar alkane chain. The nonpolar alkane chain of imidazolium cation was believed to interact with the hydrophobic blocks of gelatin molecules through solvophobic interaction and the polar imidazolium ring was exposed on the outside to decrease the possibility of interaction among hydrophobic blocks of 172 Korea-Australia Rheology J., Vol. 26, No. 2 (2014)

5 Rheology and viscosity scaling of gelatin/1-allyl-3-methylimidazolium chloride solution gelatin molecules, aroused by either electrostatic repulsion or spatial hindrance of adjacent imidazolium rings. The tendency to aggregation of gelatin molecules was highly inhibited, and the expanded gelatin chains exist freely in ionic liquid rather than aggregate as they do in aqueous solution. Therefore, the ionic liquid seemed to be a good solvent for gelatin, which can be confirmed by the low Huggins coefficient Gelatin chain size in Ionic Liquid Solution viscosity is an extremely precise and sensitive measure of the polyelectrolyte coil size in solution. The relationship between the intrinsic viscosity and chain size can be well described by the known Flory-Fox equation (Flory and Fox, 1951) [ η] Φ h2 32 = , (4) M Fig. 5. Specific viscosity, as a function of [amim]cl solution at 30 o C. c[ η] for gelatin/ where [ η] is the intrinsic viscosity, M is the gelatin molecular weight ( M 100 kda), h 2 is the meansquared end-to-end length of the macromolecular coil at infinite dilution and Φ = mol -1 is the Flory constant. Then we can obtain the root-mean-squared end-toend length h nm, which, in turn, allows an estimate of the radius of gyration R g = h nm. In addition, the overlap concentration of the gelatin solution can be obtained from c = ( MN A )( h ) 3 (Ying and Chu, 1987), which gives a value c* 1.1 wt%. This value for c* agrees very well with the concentration (c* 1.0wt%) at which the viscosity dependence changes from linear to a power law, as observed in Fig Viscoelasticity of Gelatin/[amim]Cl Solution Fig. 6 exhibits the changes of storage modulus ( G ) and loss modulus ( G ) of [amim]cl as functions of frequency at 30 o C. It has been shown that [amim]cl is a viscoelastic fluid in the experimental frequency range (Kuang et al., 2008; Wang et al., 2012), hence both the storage and loss modulus of [amim]cl as backgrounds should be subtracted when only the gelatin contribution is concerned. The conventional method was used to subtract the contribution of solvent (Ferry, 1986). The storage modulus, G and loss modulus, G ωη s ( η s is the solvent viscosity and ω is the angular frequency), as functions of frequency for various gelatin concentrations in the range of wt% were plotted in Fig. 7. In the Fig. 7, the values of G and G ωη s at respective concentrations are shifted along the vertical axis by multiplying 10 m with the m value indicated in the figure caption to prevent data overlap. For lower concentrations (below 1.0 wt%), it presented a typical plastic flow behavior, which was characterized by the increase of G and G ωη s in line with ω on logarithmic scales. When the gelatin concentration is below 1.0 wt%, the solution can be attributed to the dilute regime. It shows Fig. 6. Changes of storage modulus, G, and loss modulus, G, as functions of frequency for [amim]cl at 30 o C. a more liquid like behavior due to the single chain characteristic in dilute solution. For the semidilute solution(1.0 wt% < c < 4.0 wt%), though the plots of G and G ωη s tend to converge at high frequencies, G ωη s is always larger than G over the whole frequency range. With further continuously increasing concentration(over 4.0 wt%), a crossover in G and G ωη s in the intermediary-frequency region can be observed more clearly (Fig. 7). The presence of the crossover in G ωη s and G signifies the crossover from more liquid like to elastic behaviors, and it is a signature of entanglement of gelatin molecules. The master curves of storage and loss modulus at various temperatures (with reference temperature T r = 30 o C) for 3.0 wt% gelatin/[amim]cl solution are shown in Fig. 8. The data can be successfully superposed, which means that the empirical time-temperature superposition principle holds true and the solutions are homogeneous at the experimental temperatures. In addition, in the low-fre- Korea-Australia Rheology J., Vol. 26, No. 2 (2014) 173

6 Congde Qiao, Tianduo Li, Ling Zhang, Xiaodeng Yang and Jing Xu N ωτ G = ( ρrt M) p ω τ p p = 1 (6) τ p = 6η 0 M ( π 2 p 2 RT). (7) 4. Conclusions Fig. 7. Changes of storage modulus (filled symbols), G, and loss modulus (open symbols), G ωη s, as functions of frequency for gelatin/[amim]cl solutions with different concentrations. The concentrations are 0.6, 1.0, 3.0, and 5.0 wt % from the bottom up. The data are shifted along the vertical axis by multiplying 10 m with the corresponding m values of -1, 0, 1, and 2, respectively, to prevent data overlap. The rheological properties of the gelatin/[amim]cl solutions were investigated by using steady shear and oscillatory shear measurements. The concentration dependence of specific viscosity could be used to define the three concentration regimes: dilute(c < 1.0 wt%), semidilute unentangled (1.0 wt% < c<4.0 wt%), and entangled solutions (c>4.0 wt%). The observed concentration dependence of specific viscosity agrees well with the scaling predictions for neutral polymers in a good solvent. In addition, the Huggins constant of gelatin in [amim]cl was estimated to be less than 0.5, showing that [amim]cl is a good solvent for gelatin. The intrinsic viscosity of gelatin in ionic liquid is much higher than that in aqueous solution at similar situation, which indicates that the expanded gelatin chains exist freely in ionic liquid rather than aggregate as they do in aqueous solution. The frequency dependences of modulus changed obviously with an increase in gelatin concentration. When the polymer concentration c < 4.0 wt%, the loss modulus is always larger than storage modulus over the whole frequency range. A crossover in loss modulus and storage modulus can be observed for the concentration c > 4.0 wt%, due to the presence of entanglement among gelatin chains. The empirical time-temperature superposition principle holds true at the experimental temperatures. Acknowledgment Fig. 8. Master curves for storage modulus( G G s ) (filled symbols) and loss modulus ( G G s ) (open symbols) for the 3.0 wt% gelatin/[amim]cl solution at different temperatures (the reference temperature T r = 30 o C). The parameter a T is the shift factor. quency region, the slopes of the storage modulus curve and the loss modulus curve are 2 and 1, respectively, and then merged into one curve with the slope of 0.5 at high frequencies, behaving like the Rouse model. According to Eqs. (5)-(7) for the Rouse model, molecular weight of a given solution can be obtained by fitting experimental viscoelastic data to Rouse model, and M w of gelatin in our work was calculated to be around 80 kda N ω 2 2 τ G = ( ρrt M) p ω τ p p = 1 (5) The work was financially supported by the Project of Shandong Province Higher Educational Science and Technology Program (No. J11LB13). References Adam, M. and M. Delsanti, 1983, Viscosity and longest relaxation time of semidilute polymer solutions. 1. good solvent, J. Phys.(Paris) 44, Barrat, J. L. and J.F. Joanny, 1996, Theory of polyelectrolyte solutions, Adv. Chem. Phys 94, Bohidar, H.B. and S.S. Jena, 1994, Study of sol-state properties of aqueous gelatin solutions, J. Chem. Phys 100, Bohidar, H.B., 1998, Hydrodynamic properties of gelatin in dilute solutions, Int. J. Biol. Macromol 23, 1-6. Chun, M.-S. and M.J. Ko, 2012, Rheological Correlations of Relaxation Time for Finite Concentrated Semiflexible Polyelectrolytes in Solvents, J. Korean Phys. Soc, 61, Korea-Australia Rheology J., Vol. 26, No. 2 (2014)

7 Rheology and viscosity scaling of gelatin/1-allyl-3-methylimidazolium chloride solution Colby, R.H., and M. Rubinstein, 1990, Two-parameter scaling for polymers in θ solvents, Macromolecules 23, Colby, R.H., 2010, Structure and linear viscoelasticity of exible polymer solutions: comparison of polyelectrolyte and neutral polymer solutions, Rheol. Acta 49, de Gennes, P.G., P. Pincus, R. Velasco, and F. Brochard, 1976, Remarks on polyelectrolyte conformation, J. Phys. Fr 37, de Gennes, P.G., 1979, Scaling Concepts in Polymer Physics, Cornell University Press, New York. Dobrynin, A.V., R.H. Colby, and M. Rubinstein, 1995, Scaling theory of polyelectrolyte solutions, Macromolecules 28, Dobrynin, A.V. and M. Rubinstein, 2005, Theory of polyelectrolytes in solutions and at surfaces, Progr. Polym. Sci 30, Doi, M., and S.F. Edward, 1978, Dynamics of concentrated polymer systems. Part 3. The constitutive equation, J. Chem. Soc., Faraday Trans II 74, Dondos. A., C. Tsitsilianis, and G. Staikos, 1989, Viscometric study of aggregation phenomena in polymer dilute solutions and determination of the critical concentration c**, Polymer 30, Ferry, J. D., 1990, Viscoelastic Properties of Polymers, 3rd ed., Wiley, New York. Flory, P.J. and T.G. Fox, 1951, Treatment of Intrinsic Viscosities, J. Am. Chem. Soc. 73, Haward, S.J., V. Sharma, C.P. Butts, G.H. McKinley, and S.S. Rahatekar, 2012, Shear and Extensional Rheology of Cellulose/Ionic Liquid Solutions, Biomacromolecules 13, Hüttenrauch, R. and S. Fricke, 1984, Importance of water structure to helical conformation and ageing of gelatin in aqueous solutions, Naturwissenschaften 71, Kim, Y., Y. Hong, R.M. Kimmel, J. Rho, and C. Lee, 2007, New approach for characterization of gelatin biopolymer films using proton behavior determined by low field 1 H NMR spectrometry, J. Agric. Food. Chem. 55, Kuang, Q.L., J. Zhang, and Z.G. Wang, 2007, Revealing longrange density fluctuations in dialkylimidazolium chloride ionic liquids by dynamic light scattering, J. Phys. Chem. B 111, Kuang, Q.L., J.C. Zhao, Y.H. Niu, J. Zhang, and Z.G. Wang, 2008, Celluloses in an ionic liquid: the rheological properties of the solutions spanning the dilute and semidilute regimes, J. Phys. Chem. B 112, Kubisa, P., 2004, Application of ionic liquids as solvents for polymerization processes, Prog. Polym. Sci. 29, Kulicke, W.M. and R. Kniewske, 1984, The shear viscosity dependence on concentration, molecular weight and shear rate of polystyrene solutions, Rheol. Acta 23, Larson, R.G., 1999, The Structure and Rheology of Complex Fluids, Oxford University Press, New York. Li, Y. and R.S. Cheng, 2006, Viscometric Study of Gelatin in Dilute Aqueous Solutions, J. Polym Sci, Part B: Polym Phys. 44, Mohanty, B. and H.B. Bohidar, 2005, Microscopic structure of gelatin coacervates, Int. J. Biol. Macromol. 36, Olivares, M.L., M.B. Peirotti, and J.A. Deibe, 2006, Analysis of gelatin chain aggregation in dilute aqueous solutions through viscosity data, Food Hydrocolloids 20, Rogers, R.D. and K.R. Seddon, 2005, Ionic Liquids IIIB: Fundamentals, Progress, Challenges and Opportunities, (ACS Symposium Series 902; American Chemical Society: Washington, DC). Rubinstein, M., R.H. Colby, and A.V. Dobrynin, 1994, Dynamics of semidilute polyelectrolyte solutions, Phys. Rev. Lett. 73, Rubinstein, M. and R.H. Colby, 2003, Polymer Physics, Oxford University Press, New York. Schoff, C.K., 1999, Polymer Handbook, 4th ed., John Wiley & Sons, New York. Segtnan, V.H. and T. Isaksson, 2004, Temperature, sample and time dependent structural characteristics of gelatine gels studied by near infrared spectroscopy, Food Hydrocolloids 18, Tam, K.C. and Tiu, C., 1989, Steady and dynamic shear properties of aqueous polymer solutions, J. Rheol, 33, Wang, L.Z., M.A.E. Auty, A. Rau, J.F. Kerry, and J.P. Kerry, 2009, Effect of ph and addition of corn oil on the properties of gelatin-based biopolymer films, J. Food. Eng, 90, Wang, N., X.X. Zhang, H.H. Liu, and B.Q. He, 2006, 1-Allyl-3- methylimidazolium chloride plasticized-corn starch as solid biopolymer electrolytes, Carbohydr. Polym. 76, Wang, Q., Y.H. Yang, X. Chen, and Z.Z. Shao, 2012, Investigation of rheological properties and conformation of silk fibroin in the solution of AmimCl, Biomacromolecules 13, Welton, T., 1999, Room-temperature ionic liquids. Solvents for synthesis and catalysis, Chem. Rev. 99, Wyatt, N.B. and M.W. Liberatore, 2009, Rheology and viscosity scaling of the polyelectrolyte xanthan gum, J. Appl. Polym. Sci. 114, Xu, J., T.D. Li, X.L. Tang, C.D. Qiao, and Q.W. Jiang, 2012, Effect of aggregation behavior of gelatin in aqueous solution on the grafting density of gelatin modified with glycidol, Colloids. Surf. B: Biointerfaces 95, Ying, Q.C. and B. Chu, 1987, Overlap concentration of macromolecules in solution, Macromolecules 20, Zhang, L., C.D. Qiao, Y.Q. Ding, J.Y. Cheng, and T.D. Li, 2012, Rheological Behavior of Gelatin/1-Allyl-3-Methylimidazolium Chloride Solutions, J Macro Sci, Part B: Physics, 51, Zhou, Y.W., B.Z. Cheng, Y. Deng, and H.P. Shang, 2010, Dissolution characteristics of collagen fiber in ionic liquids, China Leather 7, Zhulina, E.B and M. Rubinstein, 2012, Ionic strength dependence of polyelectrolyte brush thickness, Soft Matter 8, Zirnsak, M.A., D.V. Boger and V. Tirtaatmadja, 1999, Steady shear and dynamic rheological properties of xanthan gum solutions in viscous solvents, J. Rheol. 43, Korea-Australia Rheology J., Vol. 26, No. 2 (2014) 175