ARTICLE pubs.acs.org/Biomac
Dynamic Viscoelastic Properties of Cellulose Carbamate Dissolved in NaOH Aqueous Solution Yi Guo, Jinping Zhou,* and Lina Zhang Department of Chemistry, Wuhan University, Wuhan 430072, China ABSTRACT: Dynamic viscoelastic properties of cellulose carbamate (CC) dissolved in NaOH aqueous solution were systematically studied for the first time. CC was microwave-assisted synthesized from the mixture of cellulose and urea and then dissolved in 7 wt % NaOH aqueous solution precooled to 7 °C. The obtained CC solution is transparent and has good liquidity. To clarify the rheological behavior of the solution, the CC solutions were investigated by dynamic viscoelastic measurements. The shear storage modulus (G0 ) and loss modulus (G00 ) as a function of the angular frequency (ω), concentration (c), nitrogen content (N %), viscosity-average molecular weight (Mη), temperature (T), and time (t) were analyzed and discussed in detail. The solgel transition temperature of CC (Mη = 7.78 104) solution decreased from 36.5 to 31.3 °C with an increase of the concentration from 3.0 to 4.3 wt % and decreased from 35.7 to 27.5 °C with an increase of the nitrogen content from 1.718 to 5.878%. The gelation temperature of a 3.8 wt % CC solution dropped from 38.2 to 34.4 °C with the Mη of CC increased from 6.35 104 to 9.56 104. The gelation time of the CC solution was relatively short at 30 °C, but the solution was stable for a long time at about 15 °C. Moreover, the gels already formed at elevated temperature were irreversible; that is, after cooling to a lower temperature including the dissolution temperature (7 °C), they could not be dissolved to become liquid.
’ INTRODUCTION Cellulose carbamate (CC), an ester of cellulose and carbamic acid, can be obtained from cellulose and urea. The compound is soluble and shapeable in NaOH solution and can be used as a spinning solution that called the CarbaCell process.1 The CarbaCell process is considered as an environmentally friendly alternative method for the traditional viscose process with hazardous byproduct.2 Similarly to viscose, the substance can be processed to form fibers, foils, sponges, and other products.3 The methods for the synthesis of CC have been reported in many publications.48 For example, Mormann and Michel reported the synthesis of CC in dimethylacetamide with dibutyltin dilaurate as a catalyst.9 Iller et al. reported that the use of electron radiation as an activating method to provide the conversion of cellulose into CC. The synthesis was conducted throughout a period of 90 min using nonirradiated cellulose and urea exposed to 10 and 15 kGy doses.10 The WO/2003/064476 patent reported a method for manufacturing CC: an auxiliary agent and urea in solution form are absorbed into cellulose, and a reaction between cellulose and urea is carried out in a mixture containing cellulose, a liquid, the auxiliary agent, and urea.11 Yin et al. reported the synthesis of CC by supercritical CO2-assisted impregnation. Urea was first impregnated into the cellulose pulp by supercritical CO2, followed by the esterification of cellulose at the temperature above the melting point of urea (132.7 °C) for above 3 h.12 Vo et al. reported a facile method for the introduction of carbamate groups in cellulosic substrates. Woven cellulosic fabrics were padded with nonalkali treatment solutions containing poly(ethylene glycol) r 2011 American Chemical Society
(PEG) 2000, urea, and LiCl and heated at elevated temperatures to obtain CC fabrics.13 In the above conventional synthetic methods, there are many rigorous conditions, such as alkalized and preripened cellulose, a catalyst, organic solvents (pyridine, DMF, DMSO, toluene, and xylene, etc.), high temperature, and long reaction time, which lead to major limitations of the conventional synthetic methods for potential technical applications. For the first time, we presented a microwave-assisted synthesis of CC from the native cellulose under catalyst-free and solventfree conditions.14,15 The shortening of reaction time from hours to minutes is of great importance in the method. The obtained CC displayed good solubility in NaOH solution. The properties of spinning solution, a precondition of spinning good quality fibers, affect not only the processability, but also the choice of spinning process and the quality index of fibers. Therefore, studying on the rheological properties of CC solution is important in theory and has guiding significance for the choice of spinning process. However, the rheological properties of CC solution have been scarcely reported.16 In present work, the dynamic viscoelastic behavior of the CC solutions as a function of concentration, nitrogen content, and molecular weight of CC; temperature and time are studied by the oscillatory rheology. We hope to gain some meaningful information for the purpose of
Received: March 10, 2011 Revised: April 7, 2011 Published: April 08, 2011 1927
dx.doi.org/10.1021/bm200331g | Biomacromolecules 2011, 12, 1927–1934
Biomacromolecules
ARTICLE
their fundamental research and industrial application on the CarbaCell process.
’ EXPERIMENTAL SECTION Materials. Four kinds of cotton linter pulps supplied by Hubei Chemical Fiber Co. Ltd. (Xiangfan, China) were selected as cellulose materials. Their viscosity-average molecular weights (Mη) were determined in cadoxen at 25 °C by viscometry and calculated by:17 ½η ¼ 3:85 102 Mw 0:76 ðmL=gÞ
ð1Þ
The obtained Mη values were 6.35 104, 7.78 104, 9.56 104, and 12.2 104 g/mol, which were coded as M6, M7, M9, and M12, respectively. NaOH and urea of analytical grade (Shanghai Chemical Reagent Co. Ltd., China) were used without further purification. Preparation of Cellulose Carbamate. CC samples were prepared according the previous work.14,15 Cellulose was immersed into urea aqueous solution; the mixture was stirred at ambient temperature for 24 h, followed by filtration, and was dried under vacuum. Then, the obtained mixture of cellulose/urea was heated in a microwave oven (Whirlpool, VIP 273F, 850 W, 10 levels) at 255 W for 25 min. Finally, the sample was washed with water and vacuum-dried at 55 °C for 24 h before use. Dissolution of Cellulose Carbamate. In a typical procedure, 7 wt % NaOH aqueous solution was used as a solvent for CC, which was stored in a refrigerator before use. After the solvent was precooled to 7 °C, CC was added immediately into it with stirring vigorously for 10 min at room temperature to obtain the transparent dope containing 4.0 wt % CC. The dope was subjected to centrifugation at 7200 rpm for 20 min at 05 °C to carry out the degasification. Characterization. The nitrogen content of CC was determined with an elemental analyzer (CHN-O-RAPID Hereaus Co., Germany). The experimental error caused by the instrument is (0.3%. The liquid 13 C NMR spectra of CC (M7) with nitrogen contents of 1.718%, 2.371%, and 5.878% in 7 wt % NaOH/D2O were recorded on a Mercury 600 MHz NMR spectrometer (Varian, USA) at 20 °C. The sample concentration was about 3.5 wt %. The dynamic rheology measurement was carried out on an ARES RFSIII rheometer (TA Instruments, USA). A parallel plate was used to measure dynamic viscoelastic parameters such as the shear storage modulus (G0 ) and loss modulus (G00 ) as functions of angular frequency (ω), temperature (T), or time (t). The rheometer was equipped with two force transducers allowing the torque measurement in the range from 0.004 to 1000 g 3 cm. The values of the strain amplitude were checked to ensure that all measurements were set as 10%, which was within a linear viscoelastic regime. For each measurement, a fresh CC solution was prepared, and then degassed CC solution was poured into the parallel plate instrument, which had been kept at each measurement temperature without preshearing or oscillating. Temperature control was established by connection with a Julabo FS18 cooling/heating bath kept within (0.5 °C over an extended time. To prevent dehydration during rheological measurements, a thin layer of low viscosity paraffin oil was spread on the exposed surface of the measured solution. For the frequency and time sweep measurements, time t = 0 min was defined when the temperature reached the desired value. The sweep of the frequency was from 0.1 to 100 rad/s. The gelation kinetics was studied at constant temperature as a function of time at a constant frequency of 1 rad/s. The dynamic temperature sweep measurements were conducted from 0 to 60 °C at an angular frequency of 1 rad/s and with heating or cooling rates of 2 °C/min.
’ RESULTS AND DISCUSSION Structure Analysis. Figure 1 shows the chemical structure of CC and the liquid 13C NMR spectra of CC (M7) with the nitrogen content of 1.718%, 2.371%, and 5.878% in 7 wt % NaOH/D2O
Figure 1. Liquid 13C NMR spectra of CC (M7) with different nitrogen content (a, 1.718%; b, 2.371%; c, 5.878%) in 7 wt % NaOH/D2O solutions at 20 °C.
Figure 2. Storage modulus G0 as a function of angular frequency ω at various temperatures for the 3.8 wt % CC (M7: N%, 1.718%) solutions.
solutions, respectively. Six major peaks at around 60 ppm to 105 ppm in the spectra are readily identified: C-1 (104.0 ppm), C-4 (79.2 ppm), C-3 (75.7 ppm), C-5 (75.4 ppm), C-2 (74.2 ppm), and C-6 (60.7 ppm). Except for the chemical shifts of the native cellulose, CC with different nitrogen content displays a remarkable signal at 168.5 ppm, which is typical for the carbonyl carbon of carbamate.18 It has been reported that the carbon chemical shifts of CdO for urea in NaOH/urea solution is at 162.7 ppm.19 Therefore, the 13C NMR results prove the successful introduction of carbamate groups in the cellulose backbone, and the chemical structure of CC did not change with the different nitrogen content. Because the degree of substitution (DS) is relative low, it is hard to determine the substitution distribution at the individual position (C-2, C-3, and C-6) of the anhydroglucose unit of cellulose. Dynamic Viscoelastic Properties. Figure 2 shows the shear storage modulus (G0 ) as a function of angular frequency (ω) for a 3.8 wt % solution of CC (M7) with the nitrogen content of 1.718% at various temperatures. The typical characteristics of the G0 curves can be divided into three segments: the G0 curves with the slope of 1 in the range of 1 to 30 °C; a flat site with G0 curves increasing as the temperature increased from 30 to 40 °C; a plateau of the G0 curves as the temperature at 50 °C. At the range of 1928
dx.doi.org/10.1021/bm200331g |Biomacromolecules 2011, 12, 1927–1934
Biomacromolecules
Figure 3. Storage modulus G0 (solid symbol) and loss modulus G00 (open symbol) as a function of angular frequency ω for the CC (M7: N %, 2.371%) solutions with indicated concentration at 25 °C. The data are shifted along the vertical axis by 10a with the given a value to avoid overlapping.
130 °C, the solutions exhibit liquidlike behavior with G0 scaling approximately with ω by G0 ∼ ω in the range of low frequency, but the terminal behavior (G0 ∼ ω2) for a Newtonian fluid was not observed.20 It is probably caused by a supermolecular structure maintained by intermolecular hydrogen bonds among CC. The phenomenon has also presented in the cellulose or cellulose derivation solutions.21,22 Above 30 °C, the G0 curves present a plateaulike behavior, and its height decreases sharply and the width expands with the raised temperature. Increasing to 50 °C, the G0 value increases and shows significantly frequency-independent plateaus at 0.1100 rad/s, which indicates the existence of the stable structure of the gel network. Within this temperature range, the G0 plateau is shown as a result of the formation of a gel network with the junctions formed by self-associated CC chains. It suggests that the elastic behavior of the CC solution has occurred at elevated temperature, because of the molecular entanglements and interchain interaction caused by self-association junctions on the CC backbone.23 Therefore, the dynamic viscoelastic properties of CC solution are studied below 30 °C. An explanation for thermally induced gelation is that gel formation involves the association of chain segments, which results in a three-dimensional network that contains solvent in the interstices. The associated regions are known as junction zones and formed with two or more chains.24 Different types of association responsible for the junction zones have been discussed.25,26 Cabane et al. suggested that raising the temperature causes some macromolecules or some sections of macromolecules to dump into lumps of a polymer-rich phase, which leads to a lower critical solution temperature behavior of the system where there are always interactions of hydrophobic groups.27 However, because there is no hydrophobic group in our system, the selfassociation tendency of CC plays a key role in the formation of gel. Figure 3 shows the angular frequency dependence of G0 and 00 G for CC (M7; N%, 2.371%) solution with different concentrations at 25 °C. At the concentration below 3.5 wt %, the elastic modulus G0 is smaller than the loss modulus G00 at all frequencies; both G0 and G00 are strongly dependent on the frequency, and the solution exhibits liquid-like behavior. As the CC concentration increases from 3.5 to 4.3 wt %, the difference between the G0 and G00 values becomes small. That is a sign of chain aggregation and entanglement taking place. The G0 values for the CC solutions at
ARTICLE
Figure 4. Storage modulus G0 and loss modulus G00 as a function of angular frequency ω for 4.0 wt % solutions of CC (M7) with different nitrogen content (N1, 1.718%; N2, 2.371%; N3, 5.878%) at 25 °C. The data are shifted along the vertical axis by 10a with the given a value to avoid overlapping.
4.0 and 4.1 wt % are larger than G00 at lower frequencies, suggesting an elastic behavior. However, both G0 and G00 increase with increased frequency and show a crossover, and then G0 becomes smaller than G00 at higher frequencies. It shows that the ability of the temporary networks was enhanced with the increase of polymer concentrations and behaves more like an elastic solid.28 For 4.3 wt % CC solution, the G0 becomes flatter than the G00 and is higher than the G00 . It is noted that a clear plateau of G0 curve for 4.3 wt % CC solution appears at all frequency ranges, indicating the existence of a more aggregated nature. Moreover, the crossover point shifts to a much higher frequency with an increase of CC concentration and shows a transition from frequency-dependent behavior with G00 values larger than those of G0 . This can be explained that molecular chains of CC are more close to each other at higher concentration. The angular frequency dependence of G0 and G00 curves for 4.0 wt % solutions of CC (M7) with different nitrogen content (N1, 1.718%; N2, 2.371%; N3, 5.878%) at 25 °C are illustrated in Figure 4. With the nitrogen content increase from 1.718% to 2.371%, the G00 values are greater than G0 for the entire frequency range measured, which indicates a liquid-like behavior is occurring. When the nitrogen content increases further, the difference between the G0 and G00 becomes small at low frequencies. For the solution of CC with the nitrogen content of 5.878%, the rheological behavior of the solution considerably differs from that of the earlier traces; G0 increases more rapidly than the G00 , and G0 exceeds G00 at low frequency, in which the G0 curve becomes flatter than that of the G00 ; this suggests that gelation has taken place. The results indicate that the solution of CC with relatively high nitrogen content is more elastic than that with low nitrogen content at 25 °C. There are several possible interactions between CC and solvent: hydrogen bonding between hydroxyl and acylamino groups of CC; hydrogen bonding among the hydroxyl and acylamino groups of CC and solvent molecules (NaOH hydrates and water); hydrogen bonding and electrostatic interaction between the solvent molecules.21 It can be explained that at relatively high nitrogen content, there are more acylamino groups along the molecular chains, leading to aggregate with each other to form a three-dimensional network easily, which shows much more elasticity. Selin et al. also claimed that 1929
dx.doi.org/10.1021/bm200331g |Biomacromolecules 2011, 12, 1927–1934
Biomacromolecules
Figure 5. Storage modulus G0 and loss modulus G00 as a function of angular frequency ω for 4.0 wt % solutions of CC with different molecular weights (N %: M6, 1.739%; M7, 1.718%; M9, 1.791%; M12, 1.728%) at 25 °C. The data are shifted along the vertical axis by 10a with the given a value to avoid overlapping.
the increase in carbamate content in the solution resulted in an increase in the viscosity of the solution, which in turn restricted the fiber spinning capacity.29 The G0 and G00 curves in Figure 5 for the CC (Mη below 9.56 104) solution exhibit typically dominant viscous behavior in the frequency range from 0.1 to 100 rad/s, whereas the one with Mη of 12.2 104 displays a much more elastic behavior at low frequency. As the molecular weight increases, the CC solution with the same concentration exhibits relatively more elasticity on the whole. This phenomenon can be explained that, as the length of the CC chain increases, its relaxation capacity decreases, resulting in easy entanglement of the chains. Therefore, decreased molecular weight leads to the relatively stable liquid state of the CC solution. In view of the above results, the gelation of the CC solution is related to the temperature, the concentration, the nitrogen content, and molecular weight of CC. Gelation Temperature. Traditionally, many researchers use the crossover of G0 (ω) and G00 (ω) as an indicator of apparent gel point.20 This method is simple and convenient. Therefore, the traditional definition is chosen for the determination of gel point. However, the gel point determined by this method is frequencydependent, which is as shown in Figure 2. To be consistent on apparent gel point determination, the frequency of 1 rad/s is selected for all of the samples, and the value at the crossover of the G0 and G00 curves was taken as apparent gel point. Figure 6 shows the temperature dependence of the storage and loss modulus for the 3.0, 3.8, and 4.3 wt % CC (M7: N%, 1.718%) solutions with a heating rate of 2 °C/min. The G0 and the G00 values exhibit three regions. The first region is characterized by a slightly decreasing of G0 and G00 , where G0 is lower than G00 , showing the common viscoelastic behavior of a liquid. Subsequently, with the partial formation of aggregates or clusters through self-association of CC, both G0 and G00 increase greatly. Meanwhile, G0 exhibits a much higher increasing rate, suggesting that the evolution of the self-association mainly contributes to the increase in the elasticity of the system. The intersection temperature of the G0 and G00 curves, that is, the apparent gel point, decreases from 36.5 to 31.3 °C with an increase of the CC concentration from 3.0 to 4.3 wt %. Then, the increasing speed of G0 and G00 decreases but never equals 0; namely, no plateau was
ARTICLE
Figure 6. Temperature dependence of the storage modulus G0 and loss modulus G00 for CC (M7: N%, 1.718%) solutions with different concentrations at the elevated temperature with a heating rate of 2 °C/min at a frequency of 1 rad/s. The data are shifted along the vertical axis by 10a with the given a value to avoid overlapping.
Figure 7. Temperature dependence of the storage modulus G0 and loss modulus G00 for 3.8 wt % solutions of CC (M7) with different nitrogen contents (N1, 1.718%; N2, 2.371%; N3, 5.878%) at the elevated temperature with a heating rate of 2 °C/min at a frequency of 1 rad/s. The data are shifted along the vertical axis by 10a with the given a value to avoid overlapping.
observed, suggesting that the gelation process is much different from a chemically cross-linked gelation process, in which there is always a G0 plateau following the second rapid increase of the G0 region. This probably results from the gradual formation of the condensed gel network accompanied with microphase separation. Similar behaviors were observed in gelling methylcellulose solution and cellulose/NaOH solutions.22,30 The G0 value of each solution is temperature-dependent, which is attributed to the fact that the thermal disposal created the physical interaction junctions in the concentrated solutions. It is worth noting that the solgel transition temperature significantly shifts to lower temperatures with an increase in the polymer concentration. This implies that the association is caused not only by heating but also by elevating the concentration because elevating the concentration increases the chance of collision of CC chains. The temperature dependence of G0 and G00 for the 3.8 wt % solutions of CC (M7) with different nitrogen content (N1, 1930
dx.doi.org/10.1021/bm200331g |Biomacromolecules 2011, 12, 1927–1934
Biomacromolecules
ARTICLE
Figure 8. Temperature dependence of the storage modulus G0 and loss modulus G00 for 3.8 wt % solutions of CC with different molecular weight (N%: M6, 1.739%; M7, 1.718%; M9, 1.791%) at the elevated temperature with a heating rate of 2 °C/min at a frequency of 1 rad/s. The data are shifted along the vertical axis by 10a with the given a value to avoid overlapping.
1.718%; N2, 2.371%; N3, 5.878%) at ω = 1 rad/s are shown in Figure 7. With increasing nitrogen content, the apparent solgel transition temperature significantly shifts from 35.7 to 27.5 °C; this suggests that the aggregation and entanglements of the CC chain can be also caused by the nitrogen content of CC. The reason for this could be that the nitrogen content of CC increases, and acylamino groups increase, promoting the formation of the aggregates and cross-linking network structure. Figure 8 shows the temperature dependence of G0 and G00 analyzed in the 3.8 wt % solutions of CC with different molecular weight. The apparent gelation temperature decreases from 38.2 to 34.4 °C with the increase of Mη from 6.35 104 to 9.56 104. This can be explained by the observation that CC could create a chance of self-association of the CC chains, further leading to the network structure. Hence the elastic modulus of gels at the gel point increases with increased molecular weight. According to the WinterChambon criterion, the following power law behavior exists at the gel point: G0 ðωÞ ¼ G00 ðωÞ ∼ ωn
0
