Article pubs.acs.org/IECR
Design Rules for Microgel-Supported Adhesives Quan Wen and Robert Pelton* Department of Chemical Engineering, McMaster University, Hamilton, Ontario, Canada L8S 4L7 S Supporting Information *
ABSTRACT: The ability of carboxylated PNIPAM microgels bearing adsorbed PVAm to increase the strength of wet paper and wet cellulose film laminates was measured as a function of the microgel diameter, cross-linking degree, PVAm molecular weight, and roughness of the cellulose substrates. The experimental results and simulations from a model led to the following design rules: (1) For low microgel dosages, small microgels are better because they cover more surface; however, very small gels can get buried in pores. (2) For high microgel dosages, larger gels are better because a saturated monolayer of adsorbed large gels puts more adhesive in the joints. (3) Adhesion increases with decreasing microgel modulus (cross-linking). (4) Adhesion is not sensitive to the molecular weight of the reactive PVAm microgel coating polymer.
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INTRODUCTION Recently, we reported a new type of wet cellulose adhesive consisting of composite nanoparticles formed by sorbing adhesion-active polyvinylamine (PVAm) onto/into carboxylated poly(N-isopropylacrylamide) microgels (PNIPAM MGs).1 The PVAm-modified microgels are colloidally stable and can be freeze-dried and easily redispersed in water.2 Microgel-supported adhesives potentially offer an important new technology for papermaking that circumvents a situation we call the “adsorbed monolayer limit”, which is now explained. The strength of paper is a combination of the strengths of individual fibers and the adhesive strength in fiber−fiber bonds.3,4 In many cases, particularly when the paper is wet, the fiber−fiber bond strength is the controlling factor. Therefore, for applications such as filters and packaging, it is common industry practice to treat the fiber surfaces with a wet-strength resin when the fibers are in dilute aqueous suspension.5 However, most wet-strength resins are soluble polymers, and their adsorption is limited to the formation of a saturated monolayer, which limits the total amount of polymer that can be adsorbed onto the pulp, that is, the monolayer limit. Current industrial approaches to high polymer loading of paper include (1) impregnation of already made paper with polymer solution,6 (2) formation of polyelectrolyte complexes based on mixing oppositely charged polymers,7 and (3) layer-by-layer adsorption of oppositely charged polymers with intermediate washing steps.8 Microgel adsorption also tends to be limited by the formation of a monolayer. However, the saturation coverage (mass of polymer per square meter) of microgels scales with microgel diameter, giving a much higher coverage than than can be achieved with a linear polymer. In our preliminary study, employing cellulose film delamination experiments, we found that the adhesion of wet cellulose films laminated with an adsorbed layer of PVAm-coated microgels is 5−10 times greater than those of films laminated with PVAm alone.1 Furthermore, there is no adhesion advantage in covalently coupling PVAm to microgels: physical adsorption is sufficient. The goal of the work summarized in this report was to develop design rules for the microgel support particles with a © 2012 American Chemical Society
view toward providing guidance for the design of commercial microgel-based wet cellulose adhesive systems. The new results reported herein include the measurement of wet-paper strength with microgels; the determination of the effects of microgel diameter, cross-link density, and cellulose substrate roughness on the wet cellulose film delamination forces; and the quartz crystal microbalance characterization of microgel adsorption and swelling. The resulting design rules encapsulate all of our experimental observations, supplemented with predictions from our previously developed model.1 In particular, we identify those situations where microgel-supported adhesives might offer an advantage.
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EXPERIMENTAL SECTION Materials. The recipes and methods for preparing the microgels are summarized in the Supporting Information, and the relevant microgel properties are listed in Table 1. Carboxylated polystyrene latex (4% w/v, 200 nm) was purchased from Invitrogen (C3748600) and used as received. Three PVAm types were provided by BASF (Ludwigshafen, Germany), and they are designated herein by the molecular weight given by the supplier: 10 kDa (Lupamin 1095), 45 kDa (Lupamin 5095), and 340 kDa (Lupamin 9095). The PVAm samples were further purified by dialysis, after which they were freeze-dried. PVAm is manufactured by the hydrolysis of poly(N-vinylformamide). The degree of hydrolysis was determined by 1H NMR spectroscopy (10 kDa, 73%; 45 kDa, 75%; 340 kDa, 91%) and the equivalent weight of stock solutions prepared from freeze-dried polymer was measured by conductometric titration (10 kDa/100 g/mol, 45 kDa/76.9 g/ mol, 340 kDa/83.3 g/mol). Sorption of PVAm onto Carboxylated PNIPAM MGs. We recently reported the detailed properties obtained by PVAm sorption onto carboxylated microgels.2 Briefly, vinylacetic acid (VAA)−N-isopropylacrylamide (NIPAM) MGs Received: Revised: Accepted: Published: 9564
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Table 1. Properties of the Carboxylated PNIPAM Microgels before and after PVAm Bindinga−c without PVAm
MG1 MG2 MG3 MG4 MG5 MG6 MG7 MG8 MG9
Table 2. Properties of Cellulose Films with Varying Surface Roughness Valuesa
with PVAm
crosslinker feed (wt %)
diameter (nm)
electrophoretic mobility (m2 V−1 s−1)
diameter (nm)
3 5 10 15 5 5 5 5 5
650 471 314 318 447 707 1332 2342 4479
−1.9 × 10−8 −1.9 × 10−8 −2 × 10−8 −1.9 × 10−8 −2 × 10−8 −1.9 × 10−8 −1.9 × 10−8 −1.8 × 10−8 −1.8 × 10−8
265 197 305 205 160 350 770 1117 2020
film
wafer
precipitation conditions
Ra (nm)
1 2 3 4
polished polished polished unpolished
water vapor 50% ethanol in water water water
27.08 327 634 1010
electrophoretic mobility (m2 V−1 s−1) 1.9 1.9 1.8 1.9 1.8 1.9 1.9 1.9 1.9
× × × × × × × × ×
Roughness defined as Ra = (1/MN)∑Nj=1∑M i=1|Zij|, where M = 736 and N = 480 are the numbers of pixels in the XY plane and Z is the surface height relative to the mean XY plane. a
10−8 10−8 10−8 10−8 10−8 10−8 10−8 10−8 10−8
(VSI) mode at a magnification of 20.6 (20× objective) over an area of 227 × 299 μm2, for all but the smoothest membranes. The images were processed using Vision32 software (Veeco Instruments). The smoothest membrane was characterized in phase shifting interferometry (PSI) mode, using a magnification of 5.1 (5× objective), and the roughness was determined over an area of 0.92 × 1.2 mm2. Laboratory Papermaking and Testing. Handsheets (60 g/m2) were prepared on a semiautomatic sheet machine (Labtech Instruments Inc., model 300-1) following TAPPI method T205 sp-95. Bleached softwood kraft dried pulp was dispersed in a disintegrator (Labtech Instruments Inc., model 500-1) and then diluted to a concentration of 0.5 wt %. The desired amount of polymer was added to the 0.5% (w/w) pulp with pH adjusted to 7 with 1 M NaOH for 30 min under constant stirring. The excess polymer was removed by filtration with a Buchner funnel fitted with a polycarbonate membrane (Sigma-Aldrich, pore size = 10 μm). All handsheets were prepared by wet pressing under 50 psi for 5 min. The wet handsheets were dried at room temperature or on a speed dryer (Labtech Instruments Inc.) for 10 min at 120 °C. After preparation, all handsheets were conditioned at 23 °C and 50% relative humidity (TAPPI standard T402 sp-98) overnight before measurements. For each test, at least five handsheets were prepared. The tensile strength was measured using an Instron 4411 universal testing system fitted with a 50-N load cell (Instron Corporation, Canton, MA) following TAPPI method T494 om-96. The paper specimen was soaked in 1 mM NaCl, pH 7, for 5 min, and the excess water was removed by pressing between blotting paper. Two strips were cut from each handsheet, and at least 10 measurements were performed for each set of experimental conditions.
a
Recipes given in the Supporting Information. bDynamic light scattering and electrophoresis measurements performed at pH 7.1 in 1 mM NaCl at 25 °C. c10-kDa PVAm content of 15 wt %.
were redispersed in 20 mL of 1 mM NaCl at a concentration of 4 g/L, and polyvinylamine (PVAm) was redispersed in 60 mL of 1 mM NaCl at a concentration of 0.5 g/L. The microgel dispersion was then added to the PVAm solution dropwise and the pH was maintained at 7 for 1 h. The unabsorbed PVAm was removed by several cycles of centrifugation. Adsorption of PVAm onto Carboxylated Polystyrene Latex. The latex dispersion was diluted to 0.5 g/L with 1 mM NaCl, and 5 mg of PVAm was dissolved in 10 mL of 1 mM NaCl. PVAm solution was then added to the carboxyl latex dispersion dropwise, and the pH was adjusted to 7 for 1 h. The unabsorbed PVAm was removed by several cycles of centrifugation. Cellulose Film Preparation. Our standard cellulose membranes for adhesion testing are smooth commercial membranes cut from dialysis tubing. Rougher cellulose surfaces were prepared by casting cellulose films from ionic liquids.9 Typically, a single-sided polished silicon wafer (diameter = 150 mm) was washed with ethanol and deionized water and dried with nitrogen. The clean wafer was fixed on a flat surface with masking tape. A 5.9-mm-diameter glass rod was then placed at one edge of the wafer, and 100 mL of ionic liquid containing 5 wt % dissolved cellulose (1-ethyl-3-methylimidazolium acetate, BASF) was spread on the wafer in front of the coating rod. The rod was then drawn across the wafer leaving a film of cellulose solution. The thickness of the film was controlled by winding 15 layers of tape (3M cloth adhesive tape, 1 in., catalog no. 2950-1) around each end of the rod, giving a cellulose film thickness of approximately 3.5 mm for the wet film and a precipitated cellulose membrane thickness of about 0.3 mm. After being coated with cellulose solution, the wafer was placed either in a bath or in a humidity chamber at 100% humidity to precipitate the cellulose. The precipitation conditions and the film roughness values are summarized in Table 2. The resulting films were exhaustively soaked in deionized water, peeled from the membranes, and dried under a vacuum at room temperature. Optical digital profilometry (Veeco WYKO NT1100 optical profiling system, DYMEK Company Ltd.) was employed to measure the topography of the cellulose membrane surfaces. Wet membranes were supported on glass slides, and measurements were performed in vertical scanning interferometry
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RESULTS A series of carboxylated PNIPAM microgels was prepared using Hoare and Pelton’s method.10 The microgel recipes are given in the Supporting Information, and the relevant microgel properties are summarized in Table 1. The microgel average diameters varied over an order of magnitude, whereas the electrophoretic mobilities were approximately equal. PVAm was adsorbed onto carboxylated microgels giving colloidally stable monodisperse microgels with PVAm-rich surfaces. Details of the PVAm treatment and the resulting composite microgel properties have been reported previously,2 and the swelling versus pH properties of the series of samples with varying cross-link density are shown in Figure SI-1 of the Supporting Information. The two rightmost columns in Table 3 show that, at pH 7, with PVAm sorption, the microgels shrunk by a factor of 2 and become positively charged, reflecting the presence of amine groups on the microgel surfaces. 9565
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Table 3. Influence on Wet Paper Strength of Molecular Weight of PVAm Adsorbed on MG2a PVAm molecular weight (kDa)
MG diameter at pH 7
MG content (wt % on fiber)
mass fraction of PVAm in MG
wet tensile index (N m g−1)
10 45 340
268 ± 5.4 305 ± 4.4 563 ± 11
0.92 0.91 0.92
0.11 0.12 0.14
16.7 ± 3.6 15.8 ± 2.8 17.2 ± 4.2
a
MG2 dosage in the pulp of 1 wt %.
The PVAm-coated microgels were employed in two types of experiments: (1) as a wet-strength-enhancing additive in paper and (2) as an adhesive in model film delamination experiments. Before introducing the adhesion results, we consider the adsorption properties of the PVAm-coated microgels. PVAm-abs-MG Adsorption on Cellulose QCM-D Sensors. MG2 bearing 15 wt % 10-kDa PVAm dispersed in 1 mM NaCl at pH 7 was adsorbed onto oxidized cellulose surface on a quartz crystal microbalance sensor leading to a frequency change. The quartz crystal microbalance with dissipation monitoring (QCM-D) results in Figure SI-2 (Supporting Information) show that the frequency change with microgel adsorption was 330 Hz, corresponding to a Sauerbrey coverage of 58 mg/m2, including the bound water. This, in turn, suggests that the packing fraction of the 200-nmdiameter water-filled spheres was λm = 0.45. Similar values have been reported for cationic latex deposition onto silica.11 Randomly packed hard spheres should give a packing fraction of λm = 0.55; the lower experimental values are explained by electrostatic replusion between neighboring particles.11 After deposition of the cationic microgels onto the cellulose sensors, the pH was lowered from 7 to 4, giving an increase in swelling due to increased charging of the gel (see the diameter versus pH data for coated microgels in Figure SI-2 of the Supporting Information). The QCM sensors were then dried at room temperature and rehydrated, and the experiments were repeated. The frequency changes with pH, before and after drying, are summarized in Table 4. These results show that
Figure 1. Wet tensile strength of handsheets prepared from oxidized cellulose fibers with 1 wt % polymer. MG2 was treated with 10-kDa PVAm to give 15 wt % PVAm on the microgel.
fibers treated with PVAm-coated microgels (PVAm-abs-MG) was more than 4 times greater than for fibers treated with PVAm alone. The superior wet-strengthening characteristics of the microgels are further illustrated in Figure 2, which compares three
Figure 2. Comparison of microgel- (MG2 with 15 wt % 15-kDa PVAm) supported particles with 200-nm-diameter polystyrene particles (PVAm-abs-PS) and no support particles. The pulp fibers were slightly oxidized before polymer treatment.
Table 4. Change in MG2 Swelling with 15 wt % 10-kDa PVAm and Adsorbed onto a Silica QCM Sensor, Induced by Lowering the pH from 7 to 4 in 1 mM NaCl
ways of depositing high-molecular-weight PVAm onto the oxidized fibers: direct PVAm adsorption from solution, adsorption of PVAm-coated microgels, and adsorption of PVAm-coated 200-nm-diameter carboxylated polystyrene latex. Direct adsorption linear PVAm from solution gave modest, if any, improvement in paper strength. Supporting the PVAm on the hard polystyrene latex particles gave slightly improved strength, whereas the softer microgels gave substantially stronger wet paper. Finally, the influence of PVAm molecular weight on the paper strength was investigated. The results summarized in Table 3 suggest that the strength is not sensitive to PVAm molecular weight. This is in marked contrast to the behavior of PVAm not supported on microgels. As is well-known in the papermaking technology literature, it takes much more lowmolecular-weight polymer to strengthen paper because much of the low-molecular-weight polymer enters pores in the cellulose fiber wall, where the polymer does not contribute to fiber−fiber bonding.15 An example of this behavior with PVAm is given in Figures SI-4 and SI-5 (Supporting Information). The role of the PVAm was to couple the PNIPAM microgels to the cellulose fibers. PNIPAM is somewhat hydrophobic16
Δf pH 4 − Δf pH 7 (Hz) change in dissipation factor (×10−6) before drying after drying
−79 −62
4.57 23
there was no measurable change in the swelling potential of the microgels bound to oxidized cellulose before and after drying. Thus, any cross-links that might have formed during the drying process did not influence microgel swelling. Paper Wet Strength. A series of handsheets (i.e., laboratory handmade paper) was prepared with unbeaten bleached kraft pulp fibers filtered from dilute aqueous suspension. Figure 1 shows the wet tensile strength of the resulting paper for various types of fiber treatments before the paper was made. In these experiments, the polymer or microgel dosage was 1% based on dry pulp fibers. It has long been established that PVAm is a far superior wet adhesive when the cellulose substrate is slightly oxidized to give aldehyde groups.12−14 The results for both linear PVAm and the PVAm-treated microgel in Figure 1 demonstrate the advantage of oxidation. The wet strength of paper made with oxidized 9566
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prepare the supporting microgels for MG1−MG4 with adsorbed PVAm (see Table 1). The delamination force decreased with increasing cross-linking. Note that the starting microgels were not all the same size (see Table 1) and that the microgels with the lowest cross-linker concentration underwent the greatest shrinkage when treated with PVAm, presumably reflecting the relatively high chain mobility. The solid lines in Figure 4 were calculated with a model that is presented in the Discussion session. Diameter is a standard microgel design parameter. Figure 5 shows the cellulose film delamination force as a function of the
and has no ability to increase either wet or dry paper strength because of low cellulose−PNIPAM adhesion.15,17 From a practical perspective, PVAm is relatively expensive, giving an economic incentive to minimize its use. Figure 3 shows the paper wet tensile strength as a function of PVAm content in the microgels. As little as 10% PVAm in the PVAm/microgel complex gave significant adhesion improvements.
Figure 3. Influence of the amine content in MG2 on the wet paper strength.
Cellulose Film Delamination. Paper is a complex material with broad distributions of porosity, roughness, and structure. Presented in this section are results from model cellulose film delamination studies, providing the opportunity to probe the effects of substrate roughness, microgel diameter, and microgel elasticity. Specifically, laminates consisting of two regenerated cellulose films, bonded together with an adhesive layer of PVAm-coated microgels, were soaked in water, and the wet delamination forces were measured with 90° peeling experiments. We view these laminates as physical models for fiber− fiber bonds in paper. However, unlike paper, the surface topology and adhesive coverage (Γ in units of milligrams of dry polymer per square meter of cellulose−adhesive−cellulose contact area) is well-defined. The stiffness or elastic modulus of microgels varies with the degree of cross-linking. Figure 4 shows the wet delamination force as a function of the percentage of cross-linker used to
Figure 5. Influence of microgel size on the wet adhesion to rough cellulose surfaces. MG5−MG9 with 15% 10-kDa PVAm with Γ = 30 mg/m2.
microgel diameter and the roughness of the regenerated cellulose substrate films. For these measurements, cellulose films of varying roughness were prepared by casting from ionic liquid solution. The film properties are summarized in Table 2, and images from the optical profilometer are given in the Supporting Information. The results in Figure 5 show thath adhesion increased with the diameter of the microgel PVAm composite particles when the microgels were smaller than the roughness scale of the cellulose. By contrast, adhesion was independent of microgel diameter when the microgels were larger than the cellulose roughness. The entire adhesion data set in Figure 5 varies by only a factor of 2 when the microgel diameter and surface roughness values spanned 1 and 2 orders of magnitude, respectively. Thus, neither roughness nor microgel diameter is an extraordinarily sensitive parameter.
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DISCUSSION This work describes a new way of assembling an adhesive layer onto wood pulp fibers, leading to stronger paper. A lowmolecular-weight, adhesion-active polymer is sorbed onto an adhesion-inert microgel, which is subsequently adsorbed onto cellulose fibers. The result is stronger paper with the microgelsupported adhesive compared to the paper obtained by directly treating the fibers with the adhesion-active polymer. Microgel-supported PVAm gives stronger paper compared to PVAm alone for two reasons. First, the large microgels ensure that the PVAm remains on the exterior of the fibers and thus participates in fiber−fiber bonding. It is well-known that cellulose fibers are porous; thus, low-molecular-weight polymers enter the cellulose pores, where they do not participate in fiber−fiber bonding. For example, Figure SI-5 (Supporting Information) shows that it takes twice as much 10-
Figure 4. Effect of microgel cross-link density on the wet adhesion to smooth cellulose surfaces (Ra = 27 nm). MG1−MG4 with sorbed PVAm (see Table 1) were applied between pairs of cellulose films giving a coverage of 30 mg/m2. Model parameters: D = 200 nm, Γsat = 30 mg/m2, α = 0.1, and εmE = 395 kPa. 9567
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2 Γsat = 2 Dρλm 3
kDa PVAm to give the same wet tensile strength as 340-kDa PVAm. By contrast, Table 3 shows that the adhesion performance of microgel-supported PVAm is independent of the PVAm molecular weight. The second reason microgel-supported PVAm is more effective is because of what we call the adsorption limit. In papermaking, strength-enhancing polymers are adsorbed onto wood pulp fibers from dilute aqueous suspension. The maximum polymer adsorption on a fiber surface corresponds to a monolayer, usually about 1 mg/m2.18,19 Therefore, when two polymer-coated fibers form a fiber−fiber bond, the maximum coverage adhesive in the joint is 2 mg/m2. By contrast, the adsorption limit for microgels scales with the microgel diameter. A general observation in adhesion science is that adhesion increases with the amount of adhesive in the joint.20 We have confirmed that adhesion increases with the coverage of PVAm-coated MGs using model wet cellulose delamination experiments.1 In our previous publication, we derived a model to predict the peel delamination forces as functions of the microgel properties.1 We assumed that each load-bearing microgel behaved as an ideal spring: see Figure 6. The role of PVAm
where λm, the maximum packing fraction, was assumed to be equal to 0.45 based on the QCM-D results described earlier. Predictions of the model are compared with the experimental data in the following sections, where design rules for the surface adhesion-active polymer (i.e., PVAm) and for the supporting microgel particles are proposed. Design Rules for the Surface Polymer. The role of the surface polymer is to couple the microgels to the cellulose substrate. Based on previous work, we believe that PVAm forms covalent bonds with the oxidized cellulose surfaces,13 whereas PVAm forms a polyelectrolyte complex with the carboxylated microgel. Previously, we showed that no increase in adhesion occurs when PVAm is chemically graftedd to the microgels.1 Our results also suggest that PVAm molecular weight is not an important parameter (Table 3), whereas there are adhesion advantages to increasing the quantity of PVAm sorbed onto the microgel (Figure 3). The role of the surface polymer is captured by one term in the model, εm, the elongation required to detach the microgels. The model predicts that the delamination force increases approximately linearly with εm (results not shown). Design Rules for Supporting Microgel Particles. PNIPAM microgels are remarkably uniform spherical particles that are usually described in terms of the average diameter as a function of the degree of swelling, the content of charged comonomer, and the content of cross-linking monomer. At a higher level of detail are the distributions of charged groups and cross-linker within individual microgel particles.21 The microgels employed in this work were prepared with vinylacetic acid as the source of charged groups. We have shown that, in these microgels, the carboxyl groups are mainly present near the exterior gel surface on the end of polymer chains, whereas the density of cross-links is highest at the microgel centers.10 PVAm binding has a significant influence on microgel properties. In the first article in this series,2 we showed that, at neutral pH, corresponding to the adhesion measurements in this work, the coated microgels are much less swollen compared to parent uncoated PNIPAM microgels. Examples of the swelling versus pH as a function of cross-linker content are shown in Figure SI-1 (Supporting Information). We employed our model to explore further the influence of microgel cross-linker content and diameter on adhesion. Classical rubber elasticity theory gives the following relationship between the elastic modulus, E, and the molecular weight per cross-link, MWc, where ρ is the density of polymer in the microgel and α is the fraction of the added cross-linker that is elastically active22
Figure 6. Schematic illustration of the peel delamination model.
was modeled by assuming that the microgels detached from the surface at a critical elongation. Our model is summarized by the following two equations, where F is the delamination force (N/ m), n is the number of microgels per area in the joint (m−2), E is the elastic modulus of the microgels, L0 is the thickness of the microgel layer in the joint (nm; see Figure 6), D is the microgel diameter (nm), Γ is the mass of dry microgel in the joint, Γsat is the coverage corresponding to a single layer of microgels (mg/ m2) on each of the cellulose surfaces brought together to form a joint, xm is length of the load-bearing region at the delamination front (nm), and εm is the elongation at which the microgels detach F=
nE π 2 D L0 4
∫0
xm
r−
r 2 − x 2 dx
E=3
2 ⎛ Γ ⎞ r 2 − ⎜r − εm 2D ⎟ Γsat ⎠ ⎝
ρRT α MWc
(4)
Equation 4 predicted very large E values unless α was lowered to 10%. In batch microgel polymerizations, the cross-linker is consumed early, meaning that the cross-linker distribution is not uniform, with most of the cross-links in the particle core making little contribution to the elastic network.23 Equations 1−3 were used to simulate the influence of microgel cross-linking on adhesion. The calculated curves are compared to the experimental results in Figure 4. In these calculations, we assumed that the polymer density in the microgel, ρ, was independent of the cross-linker density
(1)
where xm =
(3)
(2)
Γsat is a function of the microgel diameter estimated by the equation 9568
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because PVAm binding caused significant shrinkage (see Table 1). Curves for two values of ρ are shown, and for both cases, the stiffer the microgels (i.e., the higher the cross-linking), the lower the adhesion. This effect arises because, with stiffer microgels, the load-bearing width (xm) in the peel front is lower, meaning fewer microgels are contributing to the peel force (see Figure 6). This might explain the experimental trend in Figure 4: the lower the cross-linking density, the higher the adhesion. The role of the microgel diameter is complex. For smooth cellulose substrates (roughness ≪ microgel diameter), our adhesion model provides a framework for understanding the experimental results and thus developing microgel design rules. Figure 7 shows the influence of the microgel coverage (Γ) on Figure 8. Influence of microgel diameter on the delamination force simulated for two cases: (1) coverage equal to two monolayers (Γ = Γsat) and (2) constant coverage (Γ = 30 mg/m2).
Our adhesion model does not account for substrate roughness. The experimental results in Figure 5 suggest that microgels smaller than the roughness scale are less effective, presumably because small gels sitting in crevices cannot participate in fiber−fiber bonding. Finally, in this work, we demonstrated the advantages of microgel-supported PVAm compared to PVAm without the supporting microgels. The conclusions section articulates our main findings as four design rules. Although these conclusions are based on our specific results, we believe that they might have widespread applicability and could be applied to other types of reactive polymers coating other types of support particles.
Figure 7. Influence of microgel coverage on adhesion: comparison of wet paper strength measurements with delamination model predictions. Experimental data from Figure 2 assuming that the specific surface area of pulp was 1 m2/g. Model parameters: E = 100 kPa, εm = 5, r = 2 mm, λm =0.45, and ρmg = 0.25.
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CONCLUSIONS Described herein is a novel adhesive for wet cellulose prepared by sorbing adhesion-active PVAm onto adhesion-inert carboxylated PNIPAM microgels. Experimental adhesion results from paper testing and model cellulose film delamination experiments, coupled with a simple model, have led to the following design rules for our adhesive: (1) If the microgel dosage is relatively low, smaller microgels will give greater adhesion because a larger fraction of the fiber surfaces is covered with adhesive. However, the gels must be larger than the surface pores in the fibers. (2) For high microgel dosages, leading to high strengths, larger microgels are superior because they give more adhesive mass in the fiber−fiber joint when the surfaces are saturated with adsorbed microgels. (3) Adhesion strength increases with decreasing microgel modulus. (4) Adhesion is not sensitive to the molecular weight of the surface polymer (PVAm).
the delamination force. The discontinuity in the adhesion versus coverage curves corresponds to a single layer of microgels in the cellulose−microgel−cellulose joint (i.e., Γ = Γsat/2). Above the discontinuity for the larger microgels, the model predicts that adhesion is independent of microgel diameter. The maximum microgel loading in conventional papermaking corresponds to Γsat, where the fibers are coated with a single layer of deposited microgels. The experimental data in Figure 7 are the wet paper strength data from Figure 2, with dosages converted to coverage values by assuming that the specific surface area of the pulp was 1 m2/g. Although the delamination model could never be quantitatively linked to paper strength data, the model and the data show similar dependencies on microgel coverage. Under conditions of saturation coverage (i.e., Γ = Γsat, eq 3), the model predicts that larger microgels give stronger adhesion because larger gels put more adhesive into the fiber−fiber joint. This effect is illustrated in Figure 8, which shows the delamination force as a function of microgel diameter for two scenarios. The curve labeled Γsat shows that adhesion increases with particle diameter because Γsat increases with D (see eq3). By contrast the curve with Γ = 30 mg/m2 shows more complex behavior. With smaller microgels, the adhesion is independent of microgel diameter because all of the joints have a coverage greater than Γsat/2. With the larger gels, the adhesion drops off at constant Γ because the bonding surfaces are not fully covered with microgels. The illustrations in Figure 8 show the interrelationships between microgel diameter and coverage for these two cases.
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ASSOCIATED CONTENT
S Supporting Information *
Details of microgel preparation and characterization, data on QCM-D microgel adsorption onto cellulose surfaces, profilometer images of wet cellulose surfaces, PVAm adsorption isotherms on pulp fibers, and paper wet tensile strength as a function of the concentrations of 10- and 340-kDa linear PVAm. This material is available free of charge via the Internet at http://pubs.acs.org. 9569
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(16) Pelton, R. Poly(N-isopropylacrylamide) (PNIPAM) Is Never Hydrophobic. J. Colloid Interface Sci. 2010, 348, 673−674. (17) Zhao, B. X.; Bursztyn, L.; Pelton, R. Simple Approach for Quantifying the Thermodynamic Potential of Polymer−Polymer Adhesion. J. Adhes. 2006, 82, 121−133. (18) Fleer, G. J.; Cohen Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at Interfaces; Chapman & Hall: London, 1993. (19) Geffroy, C.; Labeau, M. P.; Wong, K.; Cabane, B.; Cohen Stuart, M. A. Kinetics of Adsorption of Polyvinylamine onto Cellulose. Colloids Surf. A 2000, 172, 47−56. (20) Satas, D. Peel. In Handbook of Pressure Sensitive Adhesive Technology, 2nd ed.; Satas, D., Ed.; Van Nostrand Reinhold: New York, 1989; Chapter 5. (21) Hoare, T.; McLean, D. Multi-Component Kinetic Modeling for Controlling Local Compositions in Thermosensitive Polymers. Macromol. Theory Simul. 2006, 15, 619−632. (22) Tagit, O.; Tomczak, N.; Vancso, G. J. Probing the Morphology and Nanoscale Mechanics of Single Poly(N-isopropylacrylamide) Microgels across the Lower-Critical-Solution Temperature by Atomic Force Microscopy. Small 2008, 4, 119−126. (23) Wu, X.; Pelton, R. H.; Hamielec, A. E.; Woods, D. R.; Mcphee, W. The Kinetics of Poly(N-isopropylacrylamide) Microgel Latex Formation. Colloid Polym. Sci. 1994, 272, 467−477.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the Natural Sciences and Engineering Research Council of Canada for funding this work through a cooperative research grant with BASF Canada. Andrew M. Vincelli, Antonyos Fahmy, and Steven Zecchin are acknowledged for performing some of the experiments. The authors acknowledge many stimulating conversations with Drs. Esser, Kroener, Mijolovic, and Stährfeldt, all from BASF in Europe. The authors also thank the Canada Foundation for Innovation for support of this work. R.P. holds the Canada Research Chair in Interfacial Technologies
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REFERENCES
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