Effect of Cationic Polyacrylamides on the Interactions between

Article pubs.acs.org/Langmuir

Effect of Cationic Polyacrylamides on the Interactions between Cellulose Fibers Wade K. J. Mosse,*,† David V. Boger,‡ George P. Simon,§ and Gil Garnier*,† †

BioPRIA, Australian Pulp & Paper Institute, Department of Chemical Engineering, ‡Department of Chemical Engineering, Department of Materials Engineering, Monash University, Clayton, Victoria 3800, Australia

§

ABSTRACT: The interaction between cellulose fibers in the presence of cationic polyacrylamide (CPAM) was analyzed by rheology as a function of polyelectrolyte concentration, charge density, and molecular weight. CPAM was found to strongly influence the yield stress of cellulose suspensions; low doses of CPAM increased the yield stress, but at higher concentrations the yield stress declined. The charge density of the CPAM was the most significant factor in how yield stress responded to CPAM concentration; this effect was able to be normalized to a master curve by considering only the charged fraction of the polymer. The molecular weight of CPAM samples had some effect at high concentrations, but for lower CPAM doses the yield stress was independent of molecular weight over the range studied. The data suggest that CPAM modifies the interaction between cellulose surfaces via several mechanisms, with electrostatic interactions in the form of charge neutralization and charged patch formation dominating; polymer bridging and steric repulsion also influence the overall balance of forces between interacting cellulose fibers.

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INTRODUCTION Adsorbed polymers at the solid−liquid interface are important in a broad range of industrial applications, ranging from mining to pharmaceuticals. In many of these applications, polymers act by modifying the forces acting between two surfaces. The stability and rheology of a colloidal dispersion may thus be modified by adsorbing polymer to the surface of the particles. Where the polymer introduces an attraction between particles, the system tends toward aggregation with resulting increases in viscosity and yield stress; repulsion between particles typically results in a more stable suspension, with lower yield stress and viscosity.1,2 The interactions between cellulose fibers are of great importance in the papermaking industry, with suspension concentration and stability influencing the processing of cellulose pulp slurries as well as the properties of the finished paper.3 Cellulose fiber suspensions exhibit more complex behavior than most particulate suspensions; the flexible fibers possess a high aspect ratio and are easily entangled to form flocs up to 1 cm in diameter.4,5 The suspensions of cellulose fibers used in papermaking often include a range of polymers as additives: strength additives are used to increase the strength of finished paper, while retention aids are employed to flocculate fine particles, such as clays, used as filler in the paper.3 Cationic polyacrylamide (CPAM) is commonly used as a retention aid, due to its ability to flocculate fine anionic particles;3 some reports also show that CPAM may be added to improve the dry strength of paper.6 These polymer additives may reinforce the natural tendency of pulp fibers to form flocs, which can be a problem in papermaking, causing uneven distribution of fibers © 2012 American Chemical Society

in the paper and resulting in poor optical and strength properties.7 The effect of polymer on interactions between cellulose fibers may be studied using a range of methods. Industrial investigations often produce test samples of paper and measure properties such as dry strength, wet strength, and formation (the homogeneity of fiber distribution in the paper).8−10 However, such research usually leads to empirical observations, with little insight into mechanisms by which the polymer modifies interactions between cellulose. More detailed studies often make use of model cellulose surfaces formed by spin coating11 or Langmuir−Blodgett deposition,12 allowing the use of techniques such as AFM13 and neutron reflectometry14 to probe the interactions between polymer and cellulose. Unfortunately, it is not clear how closely these model systems mimic naturally occurring cellulose surfaces. A more balanced approach is to study the rheology of cellulose suspensions, enabling sensitive measurements of interactions in the system while retaining the use of natural cellulose fibers as a test substrate. Several recent studies have shown a close correlation between rheological properties, such as the yield stress and viscosity of suspensions, and the surface forces measured by techniques such as AFM.2,15 Most previous studies of cellulose rheology typically focused on simple suspensions of pulp in water, and discussion of the results is usually limited to rather empirical observations.16−18 Particularly relevant to this study is the work of Swerin19 that Received: December 16, 2011 Revised: January 17, 2012 Published: February 8, 2012 3641

dx.doi.org/10.1021/la2049579 | Langmuir 2012, 28, 3641−3649

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as they were used, and stirred for at least 2 h to ensure proper dissolution of the polymer. Sodium chloride and branched polyethylenimine (molecular weight 25 kDa) were both purchased from Sigma-Aldrich (Australia). Deionized water was used in preparing all solutions. Procedure for Yield Stress Measurements. Rheological measurements were performed using a Haake Rotovisco RV20 system, with M5 measuring head, operating in controlled rate mode. The analogue signal output from the RV20 was passed through an analogue-digital converter and recorded on a PC at intervals of 0.1 s. The pulp was contained in a beaker 104 mm in diameter and tested with a 4-bladed steel vane 38 mm in diameter and 76 mm in length. This configuration was previously optimized and shown to be free from slip when measuring pulp suspensions.4 The experimental procedure involved filling the beaker with a sample from the stock pulp, adding a suitable amount of polymer solution and then diluting with deionized water to a final concentration of 2.0% w/w (for measurements of straight pulp, only water was added). The pulp was then stirred thoroughly using the vane rotating at 500 rpm (the highest speed setting). This stirring procedure was repeated 2−3 times over a 10 min period after adding the polymer, ensuring thorough mixing of the polymer throughout the pulp, and allowing time for polymer molecules to interact with the cellulose fibers. For measurements of suspensions containing both polymer and salt, the salt-containing suspensions were made from a saltfree pulp/polymer suspension, by adding a NaCl solution after the yield stress of the salt-free suspension had been measured. A pipet was used to carefully withdraw a small amount of clear liquid from the suspension (typically 18 mL from 900 mL of suspension), and then an equivalent amount of 5 M NaCl solution was added to the sample to return to the original volume. After this, the sample was thoroughly mixed over a 10 min period before measurements were made. Apparent yield stress measurements were performed using a procedure described previously.4 Initially, the suspension was mixed for approximately 10 s using the vane at maximum rotation speed. After this, the vane was stopped for 30 s to allow the suspension to settle. Finally a measurement was taken by initiating shear flow at a rotation rate of 0.65 rpm, and recording the torque measured as a function of time. The peak torque in each start-up of shear flow experiment was converted to yield stress using eq 121

used oscillatory rheometry to study the effect of CPAM on pulp suspension rheology. This research showed that CPAM addition causes a rise in elastic shear modulus followed by a decline above some optimal concentration, which is in broad agreement with the results reported in this study. However the analysis of the results tended toward a mechanical model.19 Although the long fibers of pulp do become mechanically entangled, any changes introduced by the addition of a polyelectrolyte can be attributed to changes in the interaction forces acting between the cellulose surfaces. Identifying and understanding these interactions is important to optimize the development of improved cellulose-based products. This study analyses the interaction between cellulose fibers in the presence of cationic polyacrylamide (CPAM) by measuring the yield stress as a function of polyelectrolyte concentration, charge density, and molecular weight. In conjunction with measurements of gel point and observations of floc strength, this is used to assess overall changes in strength of the pulp network. These data are used to determine the contribution of different surface forces to the overall interaction, and develop an overarching mechanism for how the addition of CPAM changes interactions between cellulose fibers. The mode of yielding of the pulp suspensions is also discussed, to develop an understanding of the relationship between colloidal forces and the rheology of fiber suspensions.

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MATERIALS AND METHODS Materials. The pulp suspensions used were prepared from the National Institute of Standards and Technology (NIST) reference material 8496 eucalyptus hardwood bleached kraft pulp (hardwood pulp) which has a mean length weighted fiber length of 0.65 mm20 and forms flocs averaging 1.8 mm in diameter.4 Pulp suspensions were prepared by soaking the dried pulp overnight in 2 L of deionized water, and then subjected to 75 000 revolutions (25 min at 3000 rpm) in a Messmer pulp disintegrator. Pulps were prepared at a stock solids concentration of approximately 2.5% w/w. Where a less concentrated pulp was used, this stock was diluted with deionized water. After preparation, the actual solids concentration of a sample of the pulp was determined by filtration and drying at 105 °C; the pulp was then allowed to rest for at least 2 h prior to any measurements to minimize any effects of pulp aging during the measurement process. In all experiments the pulp was used without any modification of pH; a fresh sample of pulp at 2.0% w/w solids had a pH of 5.9. The cationic polyacrylamide (CPAM) polymers used were supplied by AQUA+TECH (Switzerland) from their SnowFlake Cationics product range, and used as received. The specific polymer formulations were copolymers of uncharged acrylamide with cationic dimethylaminoethylacrylate methyl chloride as below: • E1 (50 wt % charge density, molar mass 13 MDa) • FHMW (40 wt % charge density, molar mass 15 MDa) • F1 (40 wt % charge density, molar mass 13 MDa) • F2 (40 wt % charge density, molar mass 8 MDa) • F3 (40 wt % charge density, molar mass 6 MDa) • GF1 (30 wt % charge density, molar mass 13 MDa) • GHMW (20 wt % charge density, molar mass 13 MDa) • H1 (10 wt % charge density, molar mass 13 MDa) Stock CPAM solutions were prepared at 1 mg/mL in deionized water (unless otherwise specified), on the same day

Tm =

1⎞ πD3 ⎛ H ⎜ + ⎟τy ⎝ 2 D 3⎠

(1)

where Tm is the maximum torque measured, D is the diameter of the vane, H is the height of the vane, and τy is the yield stress. Gel Point Determination. Gel point was determined according to the method of Tiller and Khatib.22 This method measures the height of the sediment formed from the settling of a series of dilute suspensions, according eq 222,23 ϕg = ϕ(h∞) =

d(ϕ0h0) dh∞

(2)

where φg is the gel point (the minimum concentration at which a self-supporting network is formed), h∞ is the equilibrium height of the sediment bed, φ0 is the initial volume fraction of solids in the original dilute suspension, and h0 is the height of the dilute suspension. By measuring a series of different 3642


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suspensions with different values of φ0h0 (in this study, the initial height was varied) a plot of φ0h0 vs h∞ may be constructed, with the gradient of this plot equal to the gel point. Figure 1 shows two sample data sets, demonstrating how the

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RESULTS Effect of Polymer Charge and Charge Screening on Pulp Yield Stress. The yield stress of hardwood pulp, at 2.0% w/w solids, was determined as a function of concentration for a series of CPAM samples with varying charge density; the polymer molecular weight was constant at 13MDa. Figure 2a

Figure 1. Data used in determining the gel point of a pure hardwood pulp suspension (diamonds), and a suspension with 8 mg/g CPAM H1 added (squares). Lines are fitted by least-squares regression, with the constraint that they pass through the origin, and the gradient is equal to the gel point for the suspension.

addition of 8 mg/g CPAM H1 changes the settling behavior of a pulp suspension. These experiments used of a set of 250 mL measuring cylinders, which were initially filled to different heights with pulp suspension. The initial pulp suspension was approximately 0.2% w/w solids prepared by dilution with water and/or polymer solution from 0.65% w/w pulp (except the study with CPAM E1, which required a more dilute 0.1% w/w pulp due to the very low gel point). After diluting the pulp and adding polymer, the sample was mixed for approximately 10 min, then poured into the cylinders and allowed to settle until equilibrium was reached. Floc Strength Observations. The strength of individual flocs was observed at the end of some rheology experiments, using the samples that remained in the original beaker. Most of the pulp was removed, leaving a small amount of pulp in the bottom of beaker. The beaker, with pulp flocs formed at the higher concentration, was then refilled with water. The robustness of the remnant flocs was then observed in these new conditions: flocs that disintegrated under the action of the pouring of the water were deemed “very weak”, while those that remained as flocs but were broken when mixed with the vane were considered “weak”. Finally, some systems formed “strong” flocs, which were unable to be broken even when the vane was reintroduced into the beaker and operated at maximum rotation speed (500 rpm). Streaming Potential Measurements. Streaming potential measurements used a hardwood pulp suspension diluted to 0.104% w/w solids and polymer solutions prepared at 0.5 mg/ mL. Suitable amounts of polymer solution (less than 2 mL in total) were added to pulp samples of approximately 100 mL and stirred for 2 min. Finally, a 10 mL sample was withdrawn and the streaming potential measured with a Mütek PCD-03 pH particle charge detector (BTG Instruments, Germany).

Figure 2. Effect of CPAM addition on the yield stress of a 2% w/w hardwood pulp suspension. Figure 1a shows data for CPAM with varying fractions of cationic monomer: 10% (closed circles), 20% (triangles), 30% (squares), 40% (open circles), and 50% (diamonds). All CPAM samples had the same molecular weight of 13 MDa. In Figure 1b, the yield stress data are replotted against the mass of cationic monomer added (ignoring the uncharged fraction of the various polymers). Error bars represent the 95% confidence interval for each measurement. Lines shown are to guide the eye.

shows that all CPAM samples significantly affected the yield stress measured, and differences in charge density strongly influenced the results obtained. From an initial yield stress of approximately 20 N/m2, the pulp suspension yield stress rose to a maximum of approximately 30 N/m2 and then began to decrease once this optimum dose was exceeded. More highly charged CPAM samples reached this maximum at a lower level of addition, and also showed a greater decline in yield stress at higher dosage; the most highly charged (50 wt % cationic) sample eventually declined to a yield stress of only 10 N/m2 when measured at 8 mg CPAM/g cellulose. In Figure 2b, these yield stress measurements are replotted against the effective mass of cationic monomer units added: 2 mg/g of 40% charge was treated as equivalent to 8 mg/g of 10% charged CPAM. With this analysis, the data sets take on a 3643


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very similar shape and a “master curve” may be fitted to all of the data. The addition of salt was found to strongly influence the ability of CPAM to modify the rheology of the pulp. Figure 3

% cationic). For both polymers, increasing levels of polymer addition were able to neutralize and eventually reverse the net charge of the suspension. 50% charged CPAM reaches neutral charge between 1 and 2 mg/g addition level, while 20% charged CPAM reaches neutral charge at an addition level of approximately 5 mg/g. Influence of Polymer Size on Suspension Yield Stress. The molecular weight of a polymer affects the size of the polymer coil in solution.24 In Figure 5, results are shown from

Figure 3. Effect of CPAM (13 MDa, 40% cationic) on the yield stress of hardwood pulp in deionized water (circles) and with the addition of 100 mM NaCl (squares). Error bars represent the 95% confidence interval for each measurement. Lines shown are to guide the eye.

shows how the addition of 100 mM NaCl changed the rheology observed with a 40 wt % charge density CPAM sample. Addition of salt to the blank pulp (without polymer) caused a significant rise in yield stress, as seen for the points at zero CPAM concentration in Figure 3. When salt was added to CPAM-containing samples a change in behavior resulted; the previously observed “peak and decline” in yield stress was replaced with a slow decrease in yield stress as CPAM concentration increased. Bleached cellulose pulp typically possesses a negative surface charge, so the overall charge of the suspension is expected to change with the addition of a cationic polymer. Figure 4 shows

Figure 5. Effect of CPAM molecular weight on the yield stress of a 2% w/w hardwood pulp suspension. A series of 40% charged CPAM samples were used: 6 (diamonds), 8 (triangles), 13 (squares), and 15 MDa (circles). Error bars represent the 95% confidence interval for each measurement. Lines shown are to guide the eye - the variegated line is shown for the 13 MDa and 15 MDa data sets as there was no significant difference.

testing with four different CPAM samples ranging from 6 to 15 MDa, all with constant 40 wt % charge density. For addition levels below 2 mg CPAM/g cellulose, the yield stress measured is unaffected by the molecular weight of the polymer used and similar peak yield stresses were recorded. However, at higher polymer doses, increasing molecular weight causes a more rapid decline in yield stress and lower yield stresses are measured. To further assess the role of polymer size, the effect of a different cationic polymer, branched polyethylenimine (molecular weight 25 kDa), on yield stress was also measured. Although still a cationic polymer, the PEI molecules are less than 1% of the weight of even the smallest CPAM sample studied, and branching of the polymer chain further reduces their size in solution. The results of these measurements are shown in Figure 6, and compared to the lowest charge density CPAM sample. The behavior of PEI is significantly different to CPAM. Although the yield stress rises with PEI addition, the increase is less rapid and the peak yield stress is significantly lower than was recorded for any CPAM sample. The yield stress also shows no indication of declining at high levels of PEI. Even when tested at a high dose of 20 mg PEI/g cellulose, the data show a plateau in yield stress at approximately 25 N/ m2 (compared to the peak yield stresses of around 30 N/m2 recorded with CPAM). Gel Point and Floc Strength. Observation during experiments suggested that high doses of highly charged CPAM led to the formation of fairly robust flocs. This result was somewhat surprising, in light of the low yield stresses reported, so additional experiments were conducted to quantify

Figure 4. Streaming potential measured for hardwood pulp with increasing CPAM addition. Two polymers of 13 MDa molecular weight were tested: 50 wt % charged CPAM (circles) and 20 wt % charged CPAM (squares). Lines shown are to guide the eye.

streaming potential measurements of pulp samples, measured with varying amounts CPAM added to the pulp. Two CPAM samples were investigated: highly charged CPAM E1 (50 wt % cationic) and more moderately charged CPAM GHMW (20 wt 3644


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the effect of the polymers on gel point and qualitatively measure floc strength. These results are shown in Table 1. Table 1. Effect of Polymer Addition on the Gel Point of Hardwood Pulp Suspensionsa

pulp only CPAM H1 (10% Charge, 13 MDa) CPAM E1 (50% Charge, 13 MDa) polyethylenimine (branched, 25 kDa)

concentration

gel point

apparent floc strength

N/A 1 mg/g 8 mg/g 1 mg/g 8 mg/g 1 mg/g 8 mg/g

0.0047 0.0028 0.0019 0.0024 0.0010 0.0029 0.0023

very weak

DISCUSSION

Mechanism by which CPAM Affects Pulp Yield Stress. The pulp-CPAM system exhibits fairly complex behavior, which may be understood by considering the colloidal forces acting between adjacent cellulose fibers in the presence and absence of CPAM. At first glance, it may appear that frictional forces should be considered, as well as the colloidal forces acting normal to the fiber surface. The flexibility and aspect ratio of the pulp fibers makes them prone to mechanical entanglement, and the strength of this entanglement is a product of friction between surfaces. However, an extended form of Amontons’ Law shows two components to the frictional force: a direct proportionality between friction and applied load, and a second component associated with adhesion resulting from the surface chemistry of the materials.25 Thus the colloidal forces, acting normal to the surface, exert control over friction between surfaces by modifying the strength of any adhesion. Where no polymer is present, the DLVO model may be a suitable starting point: attractive van der Waals forces will be present, and in competition with the repulsive electrostatic forces between the negatively charged cellulose surfaces. Due to the rapid decay of van der Waals forces with distance, it is likely that the electrostatic forces will be dominant at all but the shortest separations; there will be little attraction between fibers, and any network formed will possess little strength. This is supported by the results shown in Figure 3, where the addition of salt caused a rise in yield stress; at high ionic strength the range of the electrostatic force is greatly reduced and the interaction between two fibers becomes more attractive (or less repulsive). This model also agrees with direct surfaces forces measurements of model cellulose surfaces that showed a long-range repulsion at neutral pH and low ionic concentration, but that the repulsion was reduced or replaced by a net attraction between the surfaces when the amount of salt was increased.13 The addition of a cationic polyelectrolyte, which is likely to adsorb to the surface of cellulose, complicates the model for interactions between the fibers. Although the van der Waals force will be relatively unaffected, adsorption of the polymer may significantly alter the electrostatic interactions. With the adsorption of small amounts of cationic polymer, the average charge on the (initially negative) fibers is reduced, and eventually the surface charge may be neutralized. However, it is possible that more polymer will adsorb to the cellulose fibers than is required to neutralize the surface charge, causing reversal of the overall fiber charge and reintroducing an electrostatic repulsion between the now positively charged fibers. Other potential attractive mechanisms which may be introduced by the polymer include polymer bridging,24 and the formation of oppositely charged “patches” on the fiber surfaces.26 Depletion flocculation is unlikely to be relevant, as the opposite charge of polymer and surface makes adsorption inevitable. Polymer bridging occurs when a single polymer chain has segments bound to both the interacting surfaces; this creates an attraction as separating the surfaces stretches the polymer chain and reduces the entropy of the polymer.24 The need to span across two surfaces requires that unoccupied sites are present on both surfaces for bridging to occur. This criterion is likely to be met when low polymer concentration dictates only partial surface coverage, or when larger amounts of polymer first encounter adjacent surfaces and are able to

Figure 6. Effect of polyethylenimine (squares) on the yield stress of 2% w/w hardwood pulp. Data for the lowest charge CPAM (10% charge) from Figure 1 are shown as gray circles for comparison. Error bars represent the 95% confidence interval for each measurement. Lines shown are to guide the eye.

polymer

Article

strong strong weak

a

Apparent floc strength, upon dilution and agitation in water, is also shown.

The addition of CPAM was found to decrease the gel point of the pulp, with increases in polymer concentration further decreasing the gel point observed. Figure 1 shows an example of this change, with raw pulp showing markedly different settling behavior to pulp with 8 mg/g of CPAM H1 added. The more highly charged CPAM E1 had the strongest effect, lowering the gel point to just 0.1% v/v when added at 8 mg/g cellulose. Unlike the yield stress measurements, which displayed a peak at intermediate polymer concentrations, the gel point declined steadily with increasing CPAM addition. PEI addition was also found to lower the gel point, although the effect was smaller than for the CPAM samples. Tests of floc strength showed that pulp flocs formed with CPAM present at 8 mg/g were much more durable than the flocs formed in hardwood pulp alone. Whereas simple dilution was sufficient to disrupt the flocs present in unmodified pulp, the addition of CPAM meant that the flocs were unable to be broken, even when mixed with the vane at 500 rpm. PEI addition marginally increased floc strength: although some flocs remained after the dilution process, they were easily destroyed by a few seconds mixing. 3645


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highest molecular weight show the lowest yield stress. This is consistent with the longer polymer molecules forming a layer which extends further from the fiber surface, and thus introduces a larger steric repulsion. Polyethylenimine was tested as a contrast to the CPAM samples: the small size of the PEI molecules means that polymer bridging and steric repulsion are unlikely to be active mechanisms in this case, but PEI is capable of reversing the charge on cellulose pulp in a similar fashion to what was measured with CPAM.29 As seen in Figure 6, this produces a very different type of result to CPAM; there is a slower rise in yield stress to a lower peak value, and there is no decline at high polymer concentrations. Again, this shows that although electrostatic forces are a major contributor they are not the only mechanisms acting. Interestingly, the plateau value of the yield stress with PEI is similar to the yield stress seen for pulp in 100 mM NaCl (approximately 25 N/m2), suggesting that the main mechanism of PEI is to neutralize the surface charge and so eliminate the electrostatic repulsion between fibers. Based on this analysis, the following model is proposed (and discussed with reference to the example of CPAM with 50 wt % charged segments). At low doses of CPAM (less than 1 mg CPAM/g cellulose) a mixture of polymer bridging, charged patches, and reduction in electrostatic repulsion between the negatively charged fibers causes the interaction between hardwood fibers to become more attractive and the yield stress rapidly rises. As the amount of CPAM added increases (around 1 mg CPAM/g cellulose) the overall negative charge continues to decline, and the fiber surfaces become coated with polymer to the degree that bridging and patch formation become less effective attractive mechanisms. The balance between the decline in electrostatic repulsions and the loss of bridging and patch-based attractions means the yield stress reaches a maximum. Further CPAM addition (between 1 and 2 mg CPAM/g cellulose) shows that the yield stress begin to decline. Surface charge is neutralized, and the increasing CPAM layer begins to cause a steric repulsion between adjacent cellulose surfaces. As concentration increases further (from 2 to 4 mg CPAM/g cellulose), the excess CPAM adsorbed to the fibers causes the net charge to become positive, and electrostatic forces become increasingly repulsive. Combined with steric repulsion from the thicker polymer layer, the yield stress of the suspension declines rapidly. Finally, for very large doses of CPAM (above 4 mg CPAM/g cellulose) the adsorption of CPAM to the fibers approaches saturation, so although CPAM is added to the suspension there is less change in the amount adsorbed to the cellulose fibers. Increasing the electrosteric repulsion means that the yield stress continues to decline, but the rate of decline is reduced. It is important to note that this research was carried out with a near-neutral pH and that the proposed mechanism applies to these conditions. The charge on the pulp fibers primarily results from carboxyl groups introduced during bleaching, and these groups would become uncharged under strongly acidic conditions.3 Similarly under basic conditions the CPAM molecules will become discharged, which would be expected to cause changes in the behavior of the system. Floc Strength and the Mode of Yielding of Pulp Suspensions. At the concentration studied (2% w/w solids), hardwood pulp forms a continuous network, although within the network there are discernible flocs forming dense patches with less dense regions between adjacent flocs. At the point of yielding, this network must be disrupted for flow to commence;

adsorb to both surfaces simultaneously. The charged patch mechanism is similar to bridging, in that it requires partial surface coverage, but under this theory the polymer need not span across two surfaces. Instead, one or more molecules of cationic polymer creates a positively charged “patch” which interacts with a negatively charged “patch” of bare cellulose creating an electrostatic attraction between the fibers.26 Polymers may also cause the appearance of one more, repulsive, component to the overall interaction: steric forces.24 Steric repulsion is caused when two surfaces are covered with a layer of polymer; as these surfaces approach the layers are forced to adopt a denser configuration resulting in a loss of entropy for the polymer molecules. This creates a restoring force which tends to drive the surfaces apart.24 This requires a thicker and denser layer of polymer, meaning that steric forces tend to occur at higher concentrations than bridging or the patch-based attraction. Also important is the role of layer thickness: if other parameters are constant, thicker layers cause the repulsion to extend over a longer range, and mean the size of the repulsion will be greater for a given separation. When the steric layer is formed by a polyelectrolyte, the properties of the layer become intertwined with the charge-based properties of the polymer, the charge causing the polymer to swell, which expands the layer and increases the steric force. This combined electrostatic and steric effect is commonly referred to as an electrosteric interaction.27 Considering Figure 2b, it becomes clear that the charged fraction of the polymer plays a dominant role in any interactions, with the uncharged acrylamide fraction having little effect on the result. The dramatic change in yield stress with the addition of salt further shows that charge-based interactions are important in the action of CPAM, and that screening these interactions changes the properties of the pulp. Taken together, this suggests that an electrostatic mechanism should be a major component of any model. However, Figure 3 also shows a decline in yield stress with increasing CPAM addition − even though the high salt concentration severely curtails the ability of the charge on the CPAM to have any effect. Analysis of these results is complicated by the polyelectrolyte effect, which means that the presence of salt will directly change the properties of CPAM as well as screening charges between surfaces. For a polyelectrolyte in aqueous solution, the addition of salt will screen intrachain repulsions between charged segments and the polymer coils in solution will shrink.28 However, the steady decrease in yield stress shown in Figure 3 cannot be explained by electrostatic forces alone, suggesting that other nonelectrostatic interactions are involved. Figure 4 shows that the charge on the pulp is neutralized at around 5 mg CPAM/g cellulose for the 20% charged polymer, and between 1 and 2 mg CPAM/g cellulose for the 50% charged polymer. Comparing this with the data in Figure 2a is instructive: the 20% charged CPAM shows peak yield stress at around 3 mg CPAM/g cellulose, while the yield stress peaks with 50% charged polymer at around 1 mg/g cellulose. Thus, the peak yield stress appears at a point where the overall suspension charge is not yet neutralized, but the most rapid declines in yield stress occur once the overall suspension charge has been reversed. This is clear evidence that although electrostatic forces are important, they cannot exclusively explain the interaction of CPAM and cellulose. Figure 5 suggests that there is a steric contribution to the overall interaction: at high polymer doses, the polymers with the 3646


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true then flocs should not remain in the suspension. It seems likely that the differences may be explained by the history of the pulp suspensions and the kinetics of polymer-pulp interaction. The CPAM is added to an already concentrated and flocculated pulp suspension, and immediately begins to adsorb to the cellulose fibers. With the swollen fibers in close contact, the polymer is unable to penetrate all the way into fiber−fiber junctions; instead the polymer adsorbs across both neighboring fibers and forms strong polymer bridges. This polymer bridging acts to strengthen and stabilize the flocs, and as the suspension is mixed to allow for proper dispersion of the polymer throughout the suspension, these reinforced flocs are not disturbed internally. Rather, the flocs are mixed like large, solid particles, with polymer coating only the accessible regions of fibers outside the flocs. Thus at high CPAM concentration, the interstices between the flocs are weak as they are filled with fibers which are completely coated with polymer. The flocs themselves are made up of incompletely covered fibers, reinforced by strong polymer bridges. Once these polymer bridges are formed, the large number of polymer segments means that spontaneous desorption of the bridging polymer is statistically unlikely and thus breaking these bridges is kinetically slow. As a result, the flocs behave as unbreakable “superparticles”, with the suspension properties driven by the interactions between the flocs. General Implications. This study shows that CPAM strongly influences the rheological properties of a cellulose fiber suspension in water. Yield stress was found to peak at around 50% above the value for a pure cellulose suspension, while high gel point decreased to less than a quarter of the gel point for cellulose alone. Given these results, it seems obvious that CPAM is able to change the bonding between cellulose fibers in the presence of water. However in the papermaking industry, CPAM is not generally thought to influence the strength of wet paper; the most common uses of CPAM are as a retention aid and perhaps a dry strength agent.3,6 Thus there appears to be some conflict between industrial practice and the results of this study, making this a worthy area for further investigation. One possible factor impacting on the performance of CPAM (and similar polymers) in papermaking may be the balance between floc formation, floc stabilization, and bonding between cellulose fibers. Although additional bonding between cellulose fibers is theoretically desirable, as it improves the mechanical properties of the paper, it also tends to cause flocculation in the pulp stock when the paper is made. This flocculation may lead to poor formation (uneven mass distribution throughout the paper) which harms the optical and mechanical properties of the finished product.3 One possible method for improving paper quality is that a polymer such as CPAM may be added af ter the paper has been formed. That is, the paper is made from an unflocculated pulp resulting in an even sheet, and the CPAM may then adsorb across the existing joints between cellulose fibers and form polymer bridges which reinforce the sheets in the same way as CPAM was found to reinforce pulp flocs in the rheological measurements. The pronounced reduction in the gel point with CPAM addition may also have implications for the production of cellulose aerogels, which is a topic attracting recent research interest.30,31 Achieving a low final density is important in many of the proposed applications of aerogels.30 However, most recent research in this area reports the manufacture of aerogels from cellulose nanofibers without any attempt to use additives to improve the final product. If the reduction in the gel point of

this situation is shown as a diagram in Figure 7. This gives rise to two potential ways in which this yielding may occur. In the

Figure 7. Diagram of a 4-bladed vane in a network of closely packed flocs. Upon yielding, the vane will rotate and describe a circle (dashed), while the flocs outside the circle remain stationary. For this to occur, either the highlighted flocs must rupture, or these flocs must move and deform while breaking contacts with the flocs inside the circle.

first route, yielding would occur throughout the network, resulting in a near-perfect cylinder of fluid rotating with the vane. This would require that some of the flocculated fiber groups are ruptured, but mean that the minimum possible surface area of pulp needs to be disrupted. The other possible mode of yielding would leave the flocculated groups intact, and instead create a failure surface only through the less densely packed fibers between the flocs. This would mean that the overall area disrupted was greater, and may require some floc deformation or rearrangement to allow for rotation of the imperfect cylinder of pulp, but would mean that the (presumably stronger) flocculated regions were not disrupted. Based on the data presented in Table 1, and comparing this to the yield stress measurements in Figure 2, the evidence suggests that the second mechanism is active in the case of high CPAM doses. For CPAM E1 (50 wt % cationic) the yield stress at 8 mg CPAM/g cellulose is approximately 10 N/m2, half the yield stress of blank pulp. If the first mechanism were active, this would suggest that the overall network and the constituent flocs were significantly weaker with CPAM than they were in the blank sample. However, the gel point for pulp with 8 mg CPAM/g cellulose was the lowest of all samples measured, and less than a quarter the gel point measured for pulp alone; similarly the flocs with CPAM were far stronger than those present in pulp alone. These flocs are thus unlikely to rupture at such a low yield stress, leading to the conclusion that yielding must occur only in the less-dense region between neighboring flocs. This conclusion is only valid for CPAM at high concentration, where the yield stress and floc strength are moving in opposite directions. For lower CPAM concentrations, where floc strength and yield stress both increase, no such inferences may be drawn. This high floc strength seems to contradict the mechanism outlined earlier, which proposes a strong repulsion between fibers at high CPAM concentrations; if this were universally 3647


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hardwood fibers with CPAM extends to other cellulosic materials, the application of polymers like CPAM may be of great value in developing improved cellulose aerogels.

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CONCLUSION The addition of cationic polyacrylamide causes dramatic changes in the interactions between cellulose fibers. Small amounts of CPAM increase the suspension yield stress but, above an optimal amount of CPAM, further addition causes a reduction in yield stress to below that of unmodified pulp. This complex behavior is the result of a mechanism where simultaneous variations in polymer bridging, charged patches, electrostatic repulsion and (electro)steric repulsion all influence the overall fiber−fiber interaction. At low CPAM concentrations, the introduction of polymer bridging and the formation of charged patches reduces the repulsion between anionic cellulose surfaces, and the yield stress increases. The peak yield stress appears at a point where the overall suspension charge is not yet neutralized; although additional CPAM reduces electrostatic repulsion this is offset by increasing surface coverage reducing the effectiveness of polymer bridging and charge patches. Once sufficient polymer is added to neutralize and reverse the suspension charge, the yield stress declines rapidly with increasing CPAM concentration, as electrosteric forces create a strong repulsion between polymer coated fibers. Unlike the yield stress, the floc strength and network strength rise consistently with CPAM concentration, with no decline at high CPAM concentration. This demonstrates a difference in polymer-pulp interactions within individual flocs, as opposed to the properties of the entire suspension. We theorize that at high CPAM concentrations, fibers within the flocs are in close contact and stabilized by polymer bridging, while fibers in the periphery of the flocs become completely coated with polymer, which reduces the ability to bond with adjoining flocs. Interestingly, the strength of these flocs means the pulp begins to behave as a system of elastic “superparticles” instead of a suspension of discrete high aspect ratio fibers.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support was provided by ARC Linkage LP0989823, Visy, and Nopco Paper Technology. The authors gratefully acknowledge the samples donated by AQUA+TECH (Switzerland) which enabled this research.

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REFERENCES

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