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Viscoelastic Behavior and Sol-Gel Transition of Cellulose/Silk Fibroin/1-Butyl-3-Methylimidazolium Chloride Extended from Dilute to Concentrated Solutions.

INTRODUCTION

Ionic liquids are particular salts whose melting temperatures are below 100[degrees]C [1], and certain ionic liquids can dissolve natural polymers such as cellulose, chitosan, silk fibroin, etc. [2]. That's because the intermolecular hydrogens can be broken through the interaction between polymer chains and ionic liquids [3]. So far, ionic liquids are powerful tools in the application of natural polymers [4, 5]. For example, regenerated cellulose fibers with ionic liquids as solvent have been prepared by dry-jet wet spinning. It is found that the tensile strength of the fiber can be up to 5.34 cN/dtex, which is higher than that of commercial Lyocell fiber prepared with N-methylmorpholine-N-oxide solution (NMMO.[H.sub.2]O) as solvent [6].

As we know, the properties of regenerated cellulose materials can be traced back to the state of cellulose solutions. Rheology, an effective method to study the viscoelastic of polymers, has been widely used for cellulose/ionic liquid solutions [7, 8]. According to the study of Chen et al. [9], the typical shearing thinning behavior of cellulose/ionic liquids solutions under shear was found and l-butyl-3-methylimidazolium chloride ([BMIM]C1) is [theta] solvent for cellulose. More than that, Xia et al. [10] found different rheological behaviors of concentrated cellulose/[BMIM]C1 (4 wt%) solutions with diverse dissolving processes, the aggregation of cellulose in the solution can inherent from the raw cellulose pulp. In addition, Song et al. [11] proved that the phase transition occurred at a high concentration of cellulose/ionic liquid solution. From the research above, it can be understood that the viscoelastic behaviors with varying concentrations of cellulose provide comprehensive information about the microstructure of cellulose in ionic liquids.

Apart from cellulose, the rheological properties of protein dissolved in ionic liquids have also been investigated. Wang et al. [12] studied the conformation of silk in l-allyl-3-methylimidazolium chloride ([AMIM]C1), and [AMIM]C1 is good solvent for silk fibroin. In our previous research, [BMIM]C1 was used as co-solvent for cellulose and silk fibroin, the effect of cellulose/silk fibroin blend ratio on the rheology properties of the blend solutions were studied, and a typical continuous-discrete type of phase morphology can be imaged while comparing the experimental values with the data calculated from log-additivity rule [13]. Furthermore, the temperature induced sol-gel transition of the blend solution was proved [14], which is mainly caused by the intra- and intermolecular interaction between cellulose chains. Zhang et al. [15] used ionic liquid to co-dissolve cellulose with collagen, which is another important protein material. The phase separation occurred with increasing of collagen content, and the interaction of cellulose and collagen was found.

In the study of polymer blends, the effect of blend ratios on the structure and properties is always principal consideration. But for polymer blend solutions, the influences of polymer concentration are also important, which are rarely noticed. The scope of this work accounts for the cellulose/silk fibroin/[BMIM]Cl in the regimes from dilute to entangled concentration. The roles of cellulose and silk fibroin in the viscoelastic property of the solutions were proposed. The concentration dependence of specific viscosity of the blend solutions was also studied. The changes of microstructure in the blends with increasing cellulose/silk fibroin content and temperature were discussed.

EXPERIMENT

Materials

Cellulose (degree of polymerization (DP) = 500) was provided from Shandong Silver Eagle Chemical Fiber, Co., Ltd., China. Silk fibroin was degummed from silk cocoon in boiling 0.5% (w/w) [Na.sub.2]C[O.sub.3] solution with a bath ratio of 1:50 and then washed with deionized water to remove silk sericin [16].

1 -Butyl-3-methylimidazolium chloride ([BMIM]C1) was synthesized and purified in our laboratory as previously described [17].

Solution Preparation

Cellulose and silk fibroin were dried under vacuum at 80[degrees]C for 5 h. The cellulose/silk fibroin/[BMIM]Cl solutions with the concentration varying from 0.1 to 8 wt% were prepared by kneading at 90[degrees]C and the weight ratio of cellulose/silk fibroin is 8/2. After that, the blend solution was kept under vacuum at 90[degrees]C to remove air bubble.

Measurements

The rheology of all solutions was measured on a rotational rheometer (Physica MCR 301, Anton Paar). The 25 mm diameter concentric parallel plate geometry was utilized for solutions with viscosity above 100 Pa.s and 50 mm diameter concentric parallel plate geometry for solutions with viscosity below 100 Pa.s. The chosen gap was 1 mm for all solutions.

Frequency sweep measurements were tested from the range of 628 to 0.1 rad/s at given temperature (10-50[degrees]C, with an interval of 10[degrees]C). Before this test, dynamic strain sweep at the frequency of 6.3 rad/s was performed to get the linear viscoelastic regime, and 10% (within a linear viscoelastic regime) was chosen as set strain amplitude for the frequency sweep measurement.

Steady shear experiments were examined at a shear-rate range from 0.1 to 100 [s.sup.-1] at the temperature of 40[degrees]C.

Temperature sweeps measurements were performed from 70 to 0[degrees]C with the cooling rate of 2[degrees]C to obtain the dynamic storage modulus (G') and loss modulus (G").

Before every test, the sample were kept at specific temperature in the rheometer for 5 min to stable the solution property. Rheological data processing was achieved using the software of Rheoplus. Each sample was made and tested three times in order to avoid errors, such as incorrect blend ratio or experimental procedure.

RESULTS AND DISCUSSION

Dynamic Oscillation Behaviors of Cellulose/Silk Fibroin/[BMIM]Cl Solutions

The frequency dependence of complex viscosity for cellulose/silk fibroin/[BMIM]Cl solution at 20[degrees]C is shown in Fig. 1. At very low concentration (c = 0.1 wt%), the curve of complex viscosity is almost linear within the frequency of 0.1-10 rad/s. With the increase of cellulose/silk fibroin concentration, shear thinning behavior is obvious, and the onset of shear thinning shifts to lower frequency.

The dynamic oscillation curves were fitted with three-parameter Carreau model [18, 19], shown in Eq. 1, which can be used to fit the transition from Newtonian to non-Newtonian region of cellulose/silk fibroin/[BMIM]Cl solution.

[eta]([??])= [[eta].sub.0][(1 + [([tau] x [??]).sup.2]).sup.(n-1)2] (1)

where [[eta].sub.0] is the zero shear-rate viscosity, [tau] is the relaxation time, and n is the power law exponent.

It is obvious that the complex viscosity can be fitted well with three-parameter Carreau model. The zero shear-rate viscosity ([[eta].sub.0]), relaxation time ([tau]), and power law exponent (n) of the blend solutions are shown in Table 1, and the [[eta].sub.0] and [tau] increased with increasing of cellulose/silk fibroin concentration. The n values for the blend solutions are nearly 1 in the concentration range from 0.1 to 0.3 wt%, showing a Newtonian fluid character.

The effect of temperature on the viscosity of the blend solutions can be calculated by Arrhenius equation [20], as shown in Eq. 2.

[mathematical expression not reproducible] (2)

where [[eta].sub.0] is the zero viscosity of the solution at the temperature T, [DELTA][E.sub.[eta]] is the flow activation energy, A is the pre-exponential factor, R is the gas constant.

The [DELTA][E.sub.[eta]] of the blend solutions is shown in Fig. 2, when the concentration of cellulose/silk fibroin is above 2 wt%, the [DELTA][E.sub.[eta]] decreased with increasing of cellulose/silk fibroin concentration. That's maybe caused by the change of phase morphology in the blends, the solutions are less sensitive to temperature at high concentrations.

The specific viscosity of the blend solution was calculated as follows, indicates the viscosity of the solution without the effect of solvent.

[[eta].sub.sp] = [[eta].sub.o] - [[eta].sub.s]/[[eta].sub.s] (3)

where [[eta].sub.sp] is the specific viscosity, [[eta].sub.0] is the zero viscosity and [[eta].sub.s] is the viscosity of [BM1M]C1.

Figure 3 shows the dependence of specific viscosity with varying concentration of cellulose/silk fibroin. The blend solutions can be divided as three regimes: the dilute regime, semidilute unentangled regime and entangled regime [21]. The overlap concentration (c*) is 0.5 wt% and entangled concentration ([c.sub.e]) is 2 wt% for cellulose/silk fibroin/[BMIM]Cl solutions can be found, and the slopes of these three regions are 1.01, 1.86, and 3.66, respectively. According to scaling prediction, the slopes should be 1, 2, 14/3 for neutral polymers in [theta] solvent and 1, 1.3, 3.9 for neutral polymers in good solvent [22, 23]. In the study of Chen et al, the slopes are 1, 2, and 14/3 for cellulose in [BMIM]Cl at 25[degrees]C has been found [9]. Obviously, the slopes of blend solution cannot be used to classify [BMIM]C1 as [theta] or good solvent for cellulose/silk fibroin, as the blend solution is cellulose, silk fibroin, [BMIM]C1 ternary system.

The time-temperature superposition (tTS) principle is widely used to study the dynamics of homogeneous or heterogeneous polymers [22], and the failure of tTS is often believed as the phase separation of polymer blends [24, 25], Based on the tTS principle, the master curves of the blend solutions were obtained from the dynamic oscillation curves at the temperature of 10, 20, 30, 40, 50[degrees]C. As shown in Fig. 4, G" is larger than G' in the whole frequency range, indicating a liquid behavior for the blend solution in the semidilute unentangled regime (0.5 wt%). It is found that tTs does not work well, for the reason that the phase morphology of the blend solutions is continuous-discrete type, and the phase domains in the blend are temperature dependent. However, tTS works well for concentrated cellulose/silk fibroin/[BMIM]Cl solution (8 wt%), as the interaction of cellulose and silk fibroin is enhanced in the entangled region.

Figure 5 givers log G' versus log G" plots for cellulose/silk fibroin/[BMIM]Cl solutions at different temperatures, known as Han plot. According to the theoretical interpretation from Han et al, the plots of log G' versus log G" are temperature independent, and the slope of the curve in the terminal region should be 2 for homopolymers [26, 27]. If the slope is less than 2 for polymer blends, it may be caused by polydispersity or microheterogeneity [28]. As shown in Fig. 7, the plots show temperature dependence for 0.5 wt% blend solution, caused by the microheterogeneity structure of the blends. However, the curve is temperature independence for 8 wt% solution, even though the slope in the terminal region is 1.82, less than 2. Thus, we conclude from Han Plots, as the same result from Master Curves, that the miscibility of cellulose and silk fibroin increased with increasing of cellulose/silk fibroin concentration.

The comparison between complex viscosity and steady-shear of the blend solutions is shown in Fig. 6. It is found that Cox-Merz empirical rule is followed at low shear rates or frequencies [29]. But [eta]([??]) is larger than [absolute value of [eta]*([omega])] at high frequency for the blend solutions in dilute regime, semidilute unentangled regime. In contrast, [eta]([??]) is less than [absolute value of [eta]*([omega])] for concentrated cellulose/[BMIM]Cl or concentrated cellulose/silk fibroin/[BMlM]Cl solutions [14], It can be speculated that the deviations from cox-merz rule of dilute and semidilute blend solution are caused by two phase structures of the polymer microaggregates dispersed in the system. Then with the increase of cellulose/silk fibroin blend concentration, the intra- and intermolecular interactions between the polymers were improved [30, 31].

In view of the above results, the two-phase system of cellulose and silk fibroin exists in the dilute concentration regime of the blend solution. In the concentrated regime, the miscibility of cellulose and silk fibroin is improved due to the interphase interaction, and the phase size of silk fibroin becomes smaller in cellulose continuous phase, the possible phase morphology is shown in Scheme 1.

Sol-Gel Transition of Cellulose I Silk Fibroin/[BM1MJCI Solutions

The sol-gel transition of cellulose/silk fibroin/[BMIM]Cl has been proved in our previous study [14], It is found that the gel structure is mainly formed by cellulose molecules and it can be loosened with the existence of silk fibroin. The gel point can be determined from the scaling law of the temperature dependence of tan o at different frequency, it can be expressed as fellows [32].

G'[([omega]).sup.~] G"[([omega]).sup.~][[oemga].sup.n](0 < p < 1) (4)

tan [delta]= G"([omega])/G'([omega]))=tan(p[pi]/2)=const (5)

where p is the scaling exponent, gelation temperature ([T.sub.gel]) is determined from the intersection plot of multifrequency tan [delta] versus temperature [33].

The effect of cellulose/silk fibroin concentration on the sol-gel transition process of the blend solution is shown in Fig. 7. The intersection of tan [delta] at three frequencies cannot be found for dilute concentration of blend solutions. When the concentration of cellulose/silk fibroin reached 0.5 wt%, the gel structure was formed at the temperature of 7.2[degrees]C. The tan [delta] and p values at the gelation point can be obtained by Eq. 5, which are listed in Table 2. The p values can be used to describe the compactness degree of the gelation structure [34], and it increases with the increasing of cellulose/silk fibroin concentration, indicating a denser gel structure of cellulose/silk fibroin.

CONCLUSIONS

The effect of cellulose/silk fibroin concentration on the viscoelastic behavior and sol-gel transition of cellulose/silk fibroin/ [BMIM]C1 solutions was studied by rheology. According to the concentration dependences of specific viscosity, the blend solution can be divided as three regimes: the dilute, semidilute unentangled and entangled regimes. A two-phase structure of the blend was speculated from the deviation of the tTS principle, Han plot and empirical Cox-Merz rule, the miscibility of cellulose and silk fibroin was improved with the increase of cellulose/silk fibroin concentration, which is caused by the enhanced interaction between cellulose and silk fibroin. The temperature induced solgel transition of the blend solution can be found when the concentration of cellulose/silk fibroin is above 0.5 wt%, then the [T.sub.gel] and gel compactness further increased with the increase of cellulose/ silk fibroin concentration.

ACKNOWLEDGMENTS

This work was financially supported by Zhejiang Provincial Natural Science Foundation of China (LQ17E030002) and National Natural Science Foundation of China (21704034, 51273041).

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Yongbo Yao (ID), (1) Zhiyong Yan, (1) Zhe Li, (1) Yumei Zhang (ID), (2) Huaping Wang (2)

(1) Jiaxing University, Zhejiang Jiaxing 314001, China

(2) State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University, Shanghai 201620, China

Correspondence to: Y. Yao; e-mail: yaoyongbo@foxmail.com Contract grant sponsor: Zhejiang Provincial Natural Science Foundation of China; contract grant number: LQ17E030002; contract grant sponsor: National Natural Science Foundation of China; Contract grant numbers: 21704034, 51273041.

DOI 10.1002/pen.24802

Published online in Wiley Online Library (wileyonlinelibrary.com).

Caption: FIG. 1. Dynamic oscillation rheological curves of cellulose/silk fibroin/[BMIM]Cl solution at 20[degrees]C. (Solid lines are calculated according to Eq. 1).

Caption: FIG. 2. Temperature dependence of zero shear-rate viscosity (a) and flow activation energy (b) of cellulose/silk fibroin/[BMIM]Cl.

Caption: FIG. 3. Double logarithmic plot of specific viscosity [[eta].sub.sp] against concentration of cellulose/silk fibroin/[BMIM]Cl solutions at 20[degrees]C.

Caption: FIG. 4. Master curves of cellulose/silk fibroin/[BMIM]Cl solutions obtained by time-temperature superposition with a reference temperature of 20[degrees]C.

Caption: FIG. 5. Double logarithmic plot of G' versus G" for cellulose/silk fibroin/[BMIM]Cl solutions at different temperatures.

Caption: FIG. 6. Cox-Merz plots of cellulose/silk fibroin/[BMIM]Cl solution at 40[degrees]C.

Caption: SCHEME 1. Possible phase morphology of cellulose and silk fibroin in the blend solution with the increase of cellulose/silk fibroin concentration. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 7. Loss tangent tan [delta] as function of temperature for cellulose/silk fibroin/[BMIM]Cl.
TABLE 1. Carreau model parameters of
cellulose/silk fibroin/[BMIM]Cl at the
temperature of 20[degrees]C.

Sample (wt%)   [[eta].sub.0]   [tau] (s)   n (-)
               (Pa.s)

0.1                     7.8        0.78    1.05
0.2                     9.8        0.75    1.10
0.3                    15.8        0.96    1.11
0.5                    19.9        0.82    1.19
0.6                    31.6        1.15    1.23
0.8                    33.4        1.14    1.19
1.0                    48.2        1.27    1.28
1.5                    86.3        1.94    1.31
2.0                   287.7        2.78    1.43
4.0                  2823.9        5.38    1.58
6.0                 18477.3       11.21    1.43
8.0                 40436.4       10.04    1.73

TABLE 2. The viscoelastic parameters of
cellulose/silk fibroin/[BMIM]Cl at gel point.

Sample (wt%)   [T.sub.gel]     tan    P (--)
               ([degrees]C)   [tan]

0.1                 --         --       --
0.2                 --         --       --
0.3                 --         --       --
0.5                7.2        3.16     0.80
0.6                7.8        2.89     0.79
0.8                9.3        2.61     0.77
1.0                10.4       2.07     0.71
1.5                10.9       1.34     0.59
2.0                11.3       0.89     0.46
4.0                12.6       0.71     0.39
6.0                14.5       0.57     0.33
8.0                18.3       0.44     0.26
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Author:Yao, Yongbo; Yan, Zhiyong; Li, Zhe; Zhang, Yumei; Wang, Huaping
Publication:Polymer Engineering and Science
Article Type:Report
Date:Nov 1, 2018
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