Adsorption Behavior and Adhesive Properties of Biopolyelectrolyte

1768

Biomacromolecules 2009, 10, 1768–1776

Adsorption Behavior and Adhesive Properties of Biopolyelectrolyte Multilayers Formed from Cationic and Anionic Starch Erik Johansson,*,† Lisa Lundstro¨m,†,‡ Magnus Norgren,§ and Lars Wågberg† Division of Fibre Technology, School of Chemical Science and Engineering, Royal Institute of Technology, ¨ rnsko¨ldsvik, Stockholm, SE-100 44 Stockholm, Sweden, Processum, Biorefinery Initiative AB, SE-891 90 O Sweden, and Department of Natural Sciences, Fibre Science and Communication Network, Mid Sweden University, SE-851 70 Sundsvall, Sweden Received February 13, 2009; Revised Manuscript Received April 22, 2009

Cationic starch (D.S. 0.065) and anionic starch (D.S. 0.037) were used to form biopolyelectrolyte multilayers. The influence of the solution concentration of NaCl on the adsorption of starch onto silicon oxide substrates and on the formation of multilayers was investigated using stagnation point adsorption reflectometry (SPAR) and quartz crystal microbalance with dissipation (QCM-D). The wet adhesive properties of the starch multilayers were examined by measuring pull-off forces with the AFM colloidal probe technique. It was shown that polyelectrolyte multilayers (PEM) can be successfully constructed from cationic starch and anionic starch at electrolyte concentrations of 1 mM NaCl and 10 mM NaCl. The water content of the PEMs was approximately 80% at both electrolyte concentrations. However, the thickness of the PEMs formed at 10 mM NaCl was approximately twice the thickness formed at 1 mM NaCl. The viscoelastic properties of the starch PEMs, modeled as Voigt elements, were dependent on the polyelectrolyte that was adsorbed in the outermost layer. The PEMs appeared to be more rigid when capped by anionic starch than when capped by cationic starch. The wet adhesive pull-off forces increased with layer number and were also dependent on the polyelectrolyte adsorbed in the outermost layer. Thus, starch PEM treatment has a large potential for increasing the adhesive interaction between solid substrates to levels higher than can be reached by a single layer of cationic starch.

Introduction Starch is a biopolymer which is the major component in many food plants and is, thus, of major importance for the food industry. Due to its abundance and relatively low cost starch is also widely used in other technical areas such as the chemical and pharmaceutical, cosmetics, and papermaking industries.1 When used for industrial purposes, starch is typically extracted from corn, wheat, tapioca, or potato but is also available from other plants. Native starch is a natural blend of amylose and amylopectin, in a proportion of approximately 20:80% in potato starch. However, the proportions varies widely with the plant source. The two polysaccharides both have a (1f4) linked polyR-D-glucan backbone. Amylose is a linear polymer (though with a helical structure in solution), while amylopectin is highly branched at the C6 positions.1 Starch is one of the most common paper chemicals and is used in different applications, such as a strength-increasing agent, surface sizing agent, retention agent and as a binder in paper coatings.1 Numerous studies have shown the paper strength-enhancing effect of cationic starch,2-4 which is added in the wet-end of the paper-machine. Due to its cationic charge, it adsorbs to the negatively charged cellulose fibers. The adsorbed cationic starch improves the adhesive interaction between the fibers, and though the molecular mechanisms are not fully understood, it has been suggested that the starch increases the specific joint strength4 and the molecular contact * To whom correspondence should be addressed. Phone: +46-(0)87908311. E-mail: [email protected]. † Royal Institute of Technology. ‡ Processum. § Mid Sweden University.

area. The number of efficient fiber-fiber joints2 are possibly also increased. Regardless of which mechanism is responsible for the increase in strength, it has been shown that the strength enhancement is dependent on the amount of starch that can be adsorbed onto the fibers.3 Lindstro¨m et al. used anionic polyacrylamide to precipitate cationic starch on the fibers; recent investigations have shown that it is possible to adsorb much higher amounts of starch to the fibers by consecutively treating the fibers with cationic and anionic starch in several layers using a technology called polyelectrolyte multilayering, PEM.5-7 The technique of constructing PEMs was first discussed by Iler8 and was later reintroduced by Decher9 in the early 1990s. The method has since then developed rapidly as a very efficient yet simple substrate treatment method for achieving various desirable properties.10 The PEM technique is already used in applications such as sensor technology11 and contact lens coating.10 PEMs are formed by consecutively treating a charged substrate with oppositely charged polyelectrolytes. By choosing different combinations of polyelectrolytes, and charged nanoparticles, a large variation in properties can be achieved.10 Once the polyelectrolyte combination has been chosen there is still a large toolbox for altering the properties of the layers by varying parameters such as salt concentration, type of salt, temperature, and molecular weight and charge of the polyelectrolytes.10,12 PEMs change the surface properties of substrates, thus, the method has the potential to be used to improve the adhesive interaction between different substrates. The interaction between substrates treated by individual polyelectrolyte layers has been discussed in several studies.13-16 To our knowledge, however, only a few studies of the interaction

10.1021/bm900191s CCC: $40.75  2009 American Chemical Society Published on Web 05/21/2009

Adsorption and Adhesion Properties of Starch Multilayers

between substrates treated by PEMs have been performed. Lowack and Helm and Blomberg et al. used the surface force apparatus (SFA) to measure the interaction between substrates coated by a maximum of two17 and four18 layers of polyallylamine hydrochloride (PAH) and polystyrene sulfonate (PSS), that is, PEMs constructed from one weak and one strong polyelectrolyte. In two recent studies,19,20 atomic force microscopy (AFM) and the colloidal probe technique21 was used to study the wet adhesive pull-off forces between silicon oxide substrates consecutively treated with PAH and poly(acrylic acid) (PAA), two weak polyelectrolytes, using different pH strategies during the PEM formation. These investigations showed that the adhesive forces between the PEM covered substrates increased with increasing layer number and that the pull-off forces were highly dependent on which polyelectrolyte capped the PEM. A higher pull-off force was achieved when PAH was adsorbed in the outermost layer.19,20 It has also been shown that there is a relationship between the adhesive properties of the PEMs and the viscoelastic properties of the PEMs as studied by quartz crystal microgravimetry.19 In another recent investigation, the interactions in asymmetric systems between a bare glass sphere and substrates coated by PEM formed from PAH/PSS and PAH/DNA were studied using AFM.22 The pull-off forces were higher when cationic polymer was adsorbed in the outermost layer than when anionic polymer capped the PEM. This is to be expected because a bare glass sphere in liquid is negatively charged. It was also suggested that the previously mentioned improvements in paper strength prepared from PEM-treated fibers were due to the ability of the PEMs to improve the adhesive interactions between the cellulosic fibers.23-25 These studies showed that paper strength increased as a function of the number of layers in the PEM by treating the fibers consecutively with PAH/PAA23,24 or polydiallyldimethylammonium chloride (PDADMAC)/PSS25 before sheet forming. However, the latter polyelectrolytes are all petroleum-based. In the light of global warming and decreasing oil resources, it is desirable to find alternative polymers from renewable resources that can be used to form PEMs. Starch, already extensively used in papermaking, would be an ideal candidate as a base for biopolyelectrolyte multilayers, since it is not only a biodegradable chemical but also very cost-efficient due to its abundance. A few studies have recently been performed where PEMs with up to three layers were formed from cationic starch/anionic starch5 and from cationic starch/carboxymethylated cellulose (CMC)6,7 onto cellulosic fibers. It was shown that starch PEMs could improve paper strength properties more than a single layer of cationic starch, that is, the adhesive interactions between the fibers were improved by using starch PEMs. However, to the knowledge of the authors, a more fundamental study of the adsorption behavior and adhesive properties of PEMs from cationic and anionic starch is still lacking. The purpose of the present study was to explore the potential of constructing well-defined biopolyelectrolyte multilayers using biodegradable polyelectrolytes from renewable resources, specifically cationic starch and TEMPO-oxidized anionic starch. The influence of the solution concentration of NaCl on the adsorption of starch onto silicon oxide substrates and on the formation of multilayers was investigated using stagnation point adsorption reflectometry and quartz crystal microbalance with dissipation. The wet adhesive properties of the starch multilayers were examined by measuring pull-off forces with the AFM colloidal probe technique. A further aim of this experimental approach

Biomacromolecules, Vol. 10, No. 7, 2009

1769

was to establish a relationship between the structural and viscoelastic properties of the starch multilayers and their adhesive properties. Because this work could be of interest for the paper industry, relevant experimental conditions were chosen. The ionic strengths were in the range of 1-100 mM NaCl, where typically the ionic strength in papermaking is in the order of 10 mM, and a pH of 6.3 was chosen which is also typically found in papermaking.

Materials and Methods Substrates. The silica used as model substrates was delivered as silicon wafers with a natural silicon oxide layer. It was supplied by MEMC, Electronic Materials SpA (Novara, Italy). The wafers were rinsed consecutively with ethanol and milli-Q water and were then blown dry with nitrogen gas. Finally the substrates were cleaned and rendered hydrophilic with the aid of a plasma cleaner (PDC-002, Harrick Plasma, Inc., Ithaca, NY) for 3 min at 30 W. The silica wafers used for stagnation point adsorption reflectometry (SPAR) measurements were oxidized to an oxide layer thickness of 80-100 nm in an oven at 1000 °C for 3 h before the cleaning procedure. The oxide layer thickness was measured using ellipsometry. Silica-coated quartz crystals for use in the quartz crystal microbalance with dissipation (QCM-D) were purchased from Q-sense AB (Va¨stra Fro¨lunda, Sweden). The crystals were cleaned as described above before use. Starches. Cationic and anionic potato starch was supplied by Lyckeby Industrial AB. The cationic starch had been modified with a quaternized ammonium group to a degree of substitution (D.S.) of 0.065 corresponding to a charge density of 378 µequiv/g, calculated from the D.S. and the molecular structure given by Solarek.26 The anionic starch had been modified by 2,2,6,6-tetramethylpiperidine-1-oxyl(TEMPO)-mediated specific oxidation of the C6 primary alcohols into aldehydes and subsequently into carboxylic acids. The D.S. of the anionic starch was 0.037 corresponding to a charge density of 226 µequiv/g and the aldehyde group content was 6.7%. The D.S. of the cationic starch was determined by analyzing the nitrogen content of the starch and the D.S. of the anionic starch was determined by polyelectrolyte titration. The refractive index increment (dn/dc) of the starches was assumed to be 0.146 based on earlier work and the relatively low D.S. of the molecules.27 The starch was cooked at 120 °C and elevated pressure in 5 mL vials using a ReactiTherm 18971 from Pierce (Rockford, IL). The cationic starch was cooked for 18 min at a concentration of 1.67 g L-1 and the anionic starch was cooked for 4 min at a concentration of 4.0 g L-1. Microscopy, with phase contrast and differential interference contrast, was used to check that the degree of gelatinization after cooking was sufficient. The cooking time should be long enough to solubilize all starch granules but not too long so that the starch molecules start to degrade. The cooked starch was diluted with milli-Q water and was filtered using 1.2 µm polyethersulfone membrane syringe filters to remove any small granular remnants. Solutions with a starch concentration of 50 mg L-1, pH 6.3, and different NaCl concentrations were prepared. Modig et al.28 have shown that the molecular weight of cationic potato starch amylopectin is typically in the order of 3.8 × 107 g/mol and the z-average rms radius in 10 mM NaCl was 140 nm. The molecular weight of TEMPO-oxidized anionic starch should be somewhat lower due to a slight degradation during the oxidation process. Milli-Q grade water was used to prepare the solutions. Water with a resistivity of 18.2 MΩ cm and total organic carbon content less than 10 ppb was obtained from a Millipore system comprising RiOs-10 and Milli-Q+ 185 units. The solutions were filtered through a 0.2 µm filter. Stagnation Point Adsorption Reflectometry (SPAR). A stagnation point adsorption reflectometer (SPAR) from the Laboratory of Physical Chemistry and Colloidal Science, Wageningen University, The Netherlands, was used to study the adsorption of cationic and anionic starches

1770


Johansson et al.

Figure 1. Schematic illustration of the in situ PEM formation in the AFM liquid cell. Solutions with cationic starch and anionic starch were consecutively injected into the liquid cell, and PEMs were formed on the negatively charged silica substrates. Both the flat silica substrate and the silica sphere were covered by PEMs. The substrates were never allowed to dry before the adhesion measurements.

onto silica substrates. The theory of the method is thoroughly described elsewhere.29,30 A brief description of the theory is available as Supporting Information. Quartz Crystal Microgravimetry with Dissipation (QCM-D). Adsorption was also studied using a quartz crystal microbalance with dissipation (QCM-D) supplied by Q-sense AB (Va¨stra Fro¨lunda, Sweden). A brief description of the theory is available as Supporting Information. Atomic Force Microscopy (AFM). Surface forces were measured using a Nanoscope IIIa AFM with a Picoforce scanner (Veeco, Ltd., Santa Barbara, CA). The general principle of force measurements using AFM31 is described in detail elsewhere and will not be further discussed here. The colloidal probe technique21 was used to measure the wet adhesive forces between two silica substrates covered with PEM. Borosilicate glass spheres (Duke Scientific, Inc., Fremont, CA) with a diameter of 10 µm were glued to standard V-shaped Si3N4 cantilevers (Veeco, Ltd., Santa Barbara, CA) with a nominal spring constant of 0.12 N m-1. For each individual measurement, the borosilicate probe diameter was measured using a light microscope (Nikon) and the normal spring constant was determined by the thermal noise method.32 PEMs were formed in situ in the AFM liquid cell: that is, both the flat silica substrate and the silica sphere were covered by PEMs, and the substrates were never allowed to dry (see Figure 1). Polyelectrolyte solution was injected into the liquid cell through a syringe filter with a 1.2 µm polyethersulfone membrane and was allowed to adsorb for 5 min, followed by rinsing with NaCl solution for 5 min. The forces were measured for each layer in the PEM at the end of the rinsing cycle when there was no free polyelectrolyte present in the solution. The maximum load was 20 nN, the scan size was 4 µm, and the scan rate was 0.268 Hz, resulting in a load/unload rate of 2.15 µm s-1. Ten force-distance curves were measured for each layer. Representative force curves are shown, and the adhesive pull-off forces, that is, the maximum force on separation of the substrates, are presented as average pull-off forces. To allow comparison between experiments, the pulloff forces were normalized by the probe radius.

Results SPAR. Stagnation point adsorption reflectometry (SPAR) was used to study the formation of polyelectrolyte multilayers (PEM) from cationic starch and anionic starch onto silica substrates at different ionic strengths. The PEMs were formed in situ in the SPAR equipment by consecutively adding solutions of cationic and anionic starch. Adsorption was conducted for 5 min for each layer followed by rinsing with NaCl solution for 5 min to wash away weakly attached polyelectrolytes and to prohibit the formation of aggregates. PEMs were formed from cationic starch, D.S. 0.065, and anionic starch, D.S. 0.037, at three different ionic strengths, 1, 10, and 100 mM NaCl. The ionic strengths of the rinsing solutions were the same as for the corresponding polyelectrolyte solutions, and the pH was 6.3 for all solutions. The results from the SPAR measurements are shown in Figure 2. ∆S/S0 is the normalized response in the reflectometer signal, which for thin adsorbed layers can be related to the adsorbed amount provided that the refractive index increment (dn/dc) of the polyelectrolyte solution is known.

Figure 2. Change in reflectometer signal upon consecutive adsorption of cationic starch (CS), D.S. 0.065, and anionic starch (AS), D.S. 0.037, onto silicon oxide substrates at different background electrolyte concentrations: 1 mM NaCl (9); 10 mM NaCl (0); and 100 mM NaCl (2). Each adsorption step was followed by a rinsing step with NaCl solution of corresponding electrolyte concentration. All solutions were adjusted to pH 6.3.

Figure 2 shows that at the lowest electrolyte concentration, 1 mM NaCl, the reflectometer signal increased at each addition of starch indicating that a multilayer was formed. The signal increase was larger when cationic starch was added than when anionic starch was added, indicating that a larger amount of cationic starch was adsorbed. At the anionic starch addition points the signal first increased rapidly and thereafter decayed. This could be interpreted as a desorption or a reconformation of the adsorbed polyelectrolytes. At 10 mM NaCl, the increase in reflectometer signal was much higher than at 1 mM NaCl, indicating higher adsorbed amounts and formation of a thicker PEM. At 10 mM NaCl, the stepwise adsorption was also more pronounced with a more distinctly increased signal. This is true also for the steps when anionic starch was adsorbed. It does not show the decay for the anionic layer as was seen at 1 mM NaCl. At the highest electrolyte concentration, 100 mM NaCl, the adsorption of the first layer of cationic starch reached the same level as reached at 10 mM NaCl. However, thereafter, the adsorption stagnated, showing only minor changes in reflectometer signal when more anionic and cationic was alternately added. This indicates that there was a nonionic contribution to the adsorption of the first layer of cationic starch onto silicon oxide, and that this interaction was too weak to contribute to further adsorption of starch on top of the first starch layer. Based on our experience the minor signal increase after the first layer of cationic starch was adsorbed indicates that there was no true PEM formation at 100 mM NaCl and pH 6.3 when using cationic starch, D.S. 0.065, and anionic starch, D.S. 0.037. QCM-D. Adsorption and PEM formation of cationic and anionic starch was also studied using the QCM-D technique for the above two electrolyte concentrations, 1 and 10 mM NaCl, at which PEMs were formed. The PEMs were formed in situ in the QCM-D equipment by consecutive adsorption of cationic



1771

Figure 3. QCM-D normalized frequency shift for the third overtone upon consecutive adsorption of cationic starch (CS), D.S. 0.065, and anionic starch (AS), D.S. 0.037, at different background electrolyte concentrations: 1 mM NaCl (9) and 10 mM NaCl (0). Each adsorption step was followed by a rinsing step with NaCl solution of corresponding electrolyte concentration. All solutions were adjusted to pH 6.3.

Figure 4. QCM-D energy dissipation shift upon consecutive adsorption of cationic starch (CS), D.S. 0.065, and anionic starch (AS), D.S. 0.037, at different background electrolyte concentrations: 1 mM NaCl (9) and 10 mM NaCl (0). Each adsorption step was followed by a rinsing step with NaCl solution of corresponding electrolyte concentration. All solutions were adjusted to pH 6.3.

and anionic starch with intermediate rinsing steps to wash away weakly attached polyelectrolytes. Figure 3 shows the normalized frequency shift for the third overtone for PEMs formed from cationic starch (D.S. 0.065) and anionic starch (D.S. 0.037) at 1 and 10 mM NaCl. The polyelectrolytes were allowed to adsorb for 10 min followed by 10 min rinsing. At each starch addition point the frequency decreased, indicating adsorption at both electrolyte concentrations, as was expected from the SPAR results. When anionic starch at 1 mM NaCl was added in the QCM-D experiments, there was a quick decrease in frequency followed by a gradual increase. This is a similar trend to the SPAR measurements. However, the relative regress in signal upon anionic starch adsorption was lower in the QCM-D experiments than it was in the SPAR measurements. The stepwise change of the QCM-D frequency curves clearly shows that PEM formation is taking place. The frequency shift at 10 mM NaCl was similar to the frequency change at the lower electrolyte concentration for the first two monolayers. Thereafter, the frequency decreases much more at 10 mM NaCl than at 1 mM NaCl, indicating a higher adsorbed amount at the higher electrolyte concentration. This also agrees with the SPAR results. The frequency shift after adsorption of four bilayers was around 90 Hz at 1 mM NaCl and around 180 Hz at 10 mM NaCl. Figure 4 shows the change in energy dissipation during starch PEM formation. The dissipation can be related to the viscoelastic properties of the layer. A low dissipation indicates a thin rigidly adsorbed film and a high dissipation suggest a thicker, more water-rich and mobile film. As can be seen in Figure 4, there was a large increase in dissipation when cationic starch was adsorbed at 1 mM NaCl and even more so at 10 mM NaCl. Upon adsorption of anionic starch, however, there was a smaller increase in dissipation. At 1 mM NaCl the dissipation even decreased for the three outer bilayers when anionic starch was adsorbed. This suggests that the properties of the PEMs depend on which polyelectrolyte is adsorbed in the outermost layer, with a more rigid film when anionic starch is in the outermost layer and a more mobile, flexible, and water-rich multilayer when cationic starch is adsorbed outermost. This is in accordance with other PEM systems, such as polyallylamine (PAH)/poly(acrylic acid) (PAA),19 in which similar trends have been seen. However, the absolute levels were much higher for the starch PEMs than for the PAH/PAA PEMs. The starch PEMs showed dissipation shifts of around 7-8 units after 3.5 bilayers at 1 mM NaCl and around 20 units after 3.5 bilayers at 10 mM

NaCl. This should be compared with only around 3 units after 3.5 bilayers for PAH/PAA PEMs adsorbed at pH 7.5/3.5.19 The large difference in structure, molecular weight, and charge density between starch and PAH/PAA probably affect both the adsorbed amounts and the mechanical properties of the PEMs. AFM. Force measurements using the AFM colloidal probe technique were made at the same electrolyte concentrations as for QCM-D, that is 1 mM NaCl and 10 mM NaCl. The multilayers were constructed in situ in the AFM liquid cell. Between each adsorption step, the cell was rinsed with electrolyte solution with the same NaCl concentration and pH in order to wash away weakly attached polyelectrolytes. The force measurements were made at the end of the rinsing cycle. Figure 5a,b shows representative force-distance curves upon approach for silica substrates covered by PEMs constructed from cationic starch and anionic starch at 1 mM NaCl (Figure 5a) and at 10 mM NaCl (Figure 5b). The curves shown correspond to anionic starch in the outermost layer. The forces on approach were purely repulsive at both NaCl concentrations. The forces increased and became longer range with an increased number of adsorbed layers. This trend was seen at both ionic strengths even though it was most distinct at the higher ionic strength. At 10 mM NaCl, the repulsion started at a longer range than at 1 mM NaCl. Figure 6 shows a representative force-distance curve upon retraction for cationic starch in the outermost layer for the fourth bilayer. The appearance of this curve was typical for all force curves upon retraction though of course the range and magnitude of the forces differed. Two aspects should be noted in Figure 6. First of all, the range of the adhesive forces on retraction was very high, reaching over several micrometers. Second, the curves were characterized by multiple adhesive events, that is, the joints between the PEM-covered silica substrates did not break in one single snap, but instead broke in steps. Figure 7 shows the average normalized pull-off forces as a function of layer number for the starch PEM-covered silica substrates. The silica substrates were negatively charged and cationic starch was adsorbed in the first layer. Thus odd layer numbers correspond to cationic starch adsorbed in the outermost layer, and even numbers correspond to anionic starch adsorbed in the outermost layer. The pull-off forces were of the same order of magnitude, and increase with layer number at both background electrolyte concentrations. At 1 mM NaCl a trend could be seen from the fifth layer onward that the pull-off forces were significantly higher when cationic starch was adsorbed in

1772


Johansson et al.

Figure 7. Normalized pull-off force as a function of layer number for PEM-covered silica substrates. Cationic starch, D.S. 0.065, and anionic starch, D.S. 0.037, were adsorbed at pH 6.3 in background electrolyte concentrations of 1 mM NaCl (9) and 10 mM NaCl (0).

Discussion

Figure 5. Normalized force vs apparent separation upon approach for PEM-covered silica substrates. Cationic starch, D.S. 0.065, and anionic starch, D.S. 0.037, was adsorbed at pH 6.3 in background electrolyte concentrations of 1 mM NaCl (a) and 10 mM NaCl (b). The force curves correspond to anionic starch in the outermost layer, that is, layers 2, 4, 6, 8, 10, and 12. Layer 2 (9), layer 4 (2), layer 6 (() layer 8 (0), layer 10 (4), and layer 12 ()).

Figure 6. Normalized force vs apparent separation upon retraction for PEM-covered silica substrates. Cationic starch, D.S. 0.065, and anionic starch, D.S. 0.037, was adsorbed at pH 6.3 in a background electrolyte concentration of 1 mM NaCl. The force curve corresponds to cationic starch in the outermost layer of the 4th bilayer, that is, the 7th monolayer.

the outermost layer than when anionic starch was adsorbed in the outermost layer. This trend has been seen in previous colloidal probe measurements on multilayers constructed from PAH/PAA.19,20 At 10 mM NaCl, however, the trend was the opposite from the sixth layer onward: higher pull-off forces when anionic starch was adsorbed in the outermost layer than when cationic starch was adsorbed outermost.

Layer Thickness and Adsorbed Amounts. The SPAR and QCM-D results of the present study clearly suggest that the combination of cationic starch (D.S. 0.065) and anionic starch (D.S. 0.037) can be used to construct PEMs from solutions with background electrolyte concentrations of 1 mM NaCl and 10 mM NaCl. At 100 mM NaCl, the SPAR results indicated that PEM formation did not take place since there was only a minor increase in SPAR signal after the adsorption of the first layer of cationic starch (Figure 2). This is also expected from earlier published33 results. Considering the D.S. of the cationic and anionic starch, it would be reasonable to expect the adsorbed amounts of anionic starch, with the lower D.S., to be higher than the adsorbed amounts of cationic starch. The SPAR results (Figure 2), however, indicate that the cationic starch adsorbed in larger amounts than the anionic starch both for 1 mM NaCl and for the first layers at 10 mM NaCl. These results could possibly be explained by the influence of the silica surface on the first layers in the multilayer. The charges on the silica surface will neutralize a substantial part of the adsorbing cationic starch and only a fraction of the charge of the cationic starch will be available for the anionic starch in the second layer. Another possible explanation for the low increase in SPAR signal upon anionic starch adsorption is that the dn/dc value, that was assumed to be 0.146 for both cationic and anionic starch, was not completely accurate for the anionic starch. When anionic starch was added at 1 mM NaCl there was a rapid increase in the SPAR signal followed by a slow decrease, which suggested a possible desorption of starch in the form of soluble complexes or a reconformation of the adsorbed layer. A similar decrease was also detected in the QCM frequency results (Figure 3), but this decrease was comparably smaller than the decrease in the SPAR signal. This indicated that there was a minor desorption of anionic starch after the initial adsorption and also that the decrease in the SPAR signal most probably is linked to a change in the polarizability of the adsorbed layer. The lack of significant desorption of the anionic starch is shown by the adsorption of cationic starch in the following adsorption steps, since the surface has to be recharged to allow for adsorption of the cationic starch. So far the results have only been discussed in terms of conditions for possible multilayer formation. The results can also be used to calculate layer thicknesses and adsorbed


amounts. For thin homogeneous adsorbed layers, eq 1 links the normalized SPAR signal, ∆S/S0, to the adsorbed amount, Γ.

Γ)

1 ∆S AS S0

(1)

where AS is a sensitivity factor. A more detailed description is available as Supporting Information. Although in this case of starch multilayers the adsorbed film can be considered neither thin nor homogeneous, eq 1 together with the SPAR software was used to estimate the adsorbed amounts of solid polyelectrolytes, see Table 1. For the first layer of cationic starch, the adsorbed amount was higher at 10 mM NaCl than at 1 mM NaCl and at 100 mM NaCl, where adsorption due to electrostatic interactions should be significantly reduced, the adsorbed amount was at the same level as for 10 mM NaCl. These results indicate that there is a nonionic contribution to the interaction between the starch and the silica surfaces. At 10 mM NaCl, the adsorbed amount of cationic starch (charge density 378 µeq/g) in the first layer was 1.7 mg/m2 corresponding to an adsorption of 0.64 µeq/m2. The charge density of the silica surface at 10 mM NaCl and ph 6.3, estimated from Bolt 1957,34 was 1.08 µC/cm2, corresponding to 0.112 µequiv/m2. Samoshina et al.35 recently showed that the surface charge density of silica increases significantly due to adsorption of polyelectrolytes with a similar charge density to the cationic starch used in the present study. Considering this, the charge density of the silica was quite well-matched by the amount of charges on the adsorbed starch. Still, the fact that the adsorbed amount of charges exceeded the amount of charges on the silica surface supports the suggestion that there was a nonionic interaction between the starch and the silica. Another factor that possibly limits the adsorption of cationic starch to the silica surface is crowding at the surface, where the dimensions of the starch molecules become important. The dominating species of potato starch is amylopectin and its dimensions can hence be used to estimate the importance of crowding. Modig et al.28 determined the radius of gyration, r, of amylopectin to be 140 nm in 10 mM NaCl and the weight average molecular mass, Mw, was 3.8 × 107 g/mol. The maximum possible adsorbed amount of cationic starch to 1 m2 silica surface at 10 mM NaCl can be calculated according to eq 2

Ads amount )

Mw π · r2 · NA

(2)

where NA is Avogadros number. The calculation yields a maximum adsorbed amount of 1.0 mg/m2. This is in fair agreement with the present experimental adsorption results at 10 mM NaCl, 1.7 mg/m2, especially considering the complexity of the starch system used with 80% amylopectin and 20% amylose and that close packing of spheres at a surface does not fully cover the surface. It is thus suggested that steric restrictions and crowding at the surface should be added to electrostatic interactions and nonionic interactions as a factor that might influence the adsorption of cationic starch to a silica surface. With the QCM-D results it is also possible to calculate the adsorbed amount of solid polyelectrolytes including the water immobilized by the adsorbed layer using the Sauerbrey relationship,36 eq 3, applied to the third overtone.


∆m ) C

∆f n

1773

(3)

The adsorbed mass, ∆m, is proportional to the change in resonance frequency upon adsorption, ∆f. C is a sensitivity constant and n is the overtone number. A more detailed description is available as Supporting Information. The adsorbed amounts of polyelectrolytes including immobilized water should be considered to be approximations, because the Sauerbrey equation is only valid for thin, rigidly attached films. Because the Sauerbrey relationship is known to underestimate the adsorbed amounts for viscous layers,37 a viscoelastic model developed by Voinova et al.38 was also used to calculate the adsorbed amounts and layer thicknesses. Based on the assumption of very water-rich layers, the density of the adsorbed layers was estimated to be 1000 kg m-3 in both the Sauerbrey and Voinova model calculations. The calculated thicknesses of the starch multilayers formed at 1 and 10 mM NaCl are shown in Figure 8 and the adsorbed amounts are shown in Table 1. As can be seen in Figure 8, the viscoelastic Voinova model calculations yielded higher thickness values than the Sauerbrey model did for both electrolyte concentrations. Considering the earlier discussion of missing mass37 in Sauerbrey calculations, this is as expected. The viscoelastic model gives between 15 and 30% higher values than the Sauerbrey model. Figure 8 also shows that thicker PEMs were estimated at 10 mM NaCl compared to 1 mM NaCl. For 10 mM NaCl, the estimates level off at about 40 nm; for 1 mM NaCl, the estimate was 18-19 nm. Both estimates use the viscoelastic model. Thus, the thickness was approximately twice as high at the higher NaCl concentration than it was at the lower electrolyte concentration. The AFM force curves on approach (Figure 5) also show that the adsorbed layers were thicker at 10 mM NaCl and the repulsive forces on approach were of longer range than at 1 mM NaCl. The difference seen in the AFM approach curves was not as large as could have been expected from the SPAR and QCM-results. However, this could possibly be due to the difficulty in determining the point of zero separation in the AFM experiments. In addition, the PEM may be too soft for the cantilever used (i.e., the spring constant of the cantilever is too high to “see” the most external soft layers of the PEM). However, these layers can be detected with the QCM-D since they significantly contribute to the viscoelastic response of the signal from this equipment. An interesting parallel can be drawn between the viscoelastic model thickness curve at 1 mM NaCl and the SPAR curve at the corresponding electrolyte solution. At the addition of anionic starch the thickness first increases but then decays back to thickness of the preceding cationic starch layer. This is similar to what was seen in the SPAR signal but unlike the pure QCM-D frequency response, and also the Sauerbrey curve. This indicates that when the anionic starch is adsorbed at this ionic strength the interdiffusion of the chains into the previous layers is relatively low compared to the interdiffusion at 10 mM NaCl. The anionic starch’s interactions with the cationic starch at 1 mM would thus mostly be directed to the outermost layer, giving a low adsorption after desorption/reconformation. Together with the adsorbed amounts, the water content of the PEMs could be calculated from the solid adsorbed amount of polyelectrolytes (SPAR) and the total adsorbed amounts of polyelectrolytes and water (QCM-D) of the PEMs. The results are shown in Table 1. As expected, the water content was very high, at approximately 80%. Slightly higher water content values

1774


Johansson et al.

Table 1. Adsorbed Amounts and Water Content of the Starch PEMs as a Function of Layer Numbera electrolyte concentration 1 mM NaCl

10 mM NaCl

a

layer number

SPAR adsorbed mass [mg/m2]

QCM-D adsorbed mass Sauerbrey/Voinova [mg/m2]

estimated water content Sauerbrey/Voinova [%]

1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8

1.0 1.0 1.7 1.9 2.6 2.9 3.6 3.9 1.7 1.7 3.1 3.5 4.6 5.4 5.9 7.1

6.2/7.3 7.2/8.5 9.8/12.2 10.4/12.1 12.6/15.6 13.4/15.6 15.2/18.6 16.2/18.8 5.5/6.0 6.7/7.8 12.3/15.1 15.5/18.4 20.1/25.3 24.1/29.2 27.6/35.3 31.8/39.6

84/87 86/88 83/86 82/85 80/83 79/82 76/80 76/79 69/72 75/78 75/80 77/81 77/82 78/82 79/83 78/82

As calculated from the SPAR and QCM-D results. A layer density of 1000 kg m-3 was assumed.

Figure 8. Thickness of the adsorbed starch multilayers calculated from the QCM-D results assuming a constant adsorbed layer density of 1000 kg m-3. The frequency of the third overtone was used to calculate the Sauerbrey thickness of starch PEMs formed at 1 mM NaCl (9) and at 10 mM NaCl (0). The viscoelastic model included both frequency and dissipation from all three overtones, n ) 3, 5, and 7, to calculate the thickness of starch PEMs formed at 1 mM NaCl (2) and at 10 mM NaCl (4).

were found when the values from the viscoelastic model were used in the calculations instead of the Sauerbrey relationship. The high water content is reasonable considering that the cationic and anionic starches had a low charge density. Considering that the QCM-D energy dissipation data (Figure 4) show much higher values at 10 mM NaCl, it might have been logical to assume that the PEMs formed at this electrolyte concentration had higher water content than the PEMs formed at 1 mM NaCl. However, the calculated values in Table 1 show that the water content was very similar at both electrolyte concentrations except for the first three layers. Thus, the large difference in dissipation is probably due to larger amounts of adsorbed polyelectrolytes at 10 mM NaCl, not due to a difference in entrapped water. For the first three layers, the PEMs formed at 10 mM NaCl concentration showed a slightly lower water content, indicating a more compact structure of the adsorbed layer. Because the size of the starch molecules will decrease at the higher salt concentration, there will be a closer packing of the molecules in the first layer as seen from the higher adsorbed amounts and this will hence lead to a lower water content of the adsorbed layer. It is striking, however, that at higher layer numbers where the influence of the silica surface on the adsorption is lower and where adsorption is not limited by packing of molecules on a flat surface, the water contents

Figure 9. Viscosity (a) and shear modulus (b) of starch PEMs calculated by a Voight-type viscoelastic model developed by Voinova et al.,38 assuming a layer density of 1000 kg m-3. PEMs formed at different background electrolyte concentrations: 1 mM NaCl (9) and 10 mM NaCl (0).

levels out at very similar levels at both 1 and 10 mM NaCl concentration. Viscoelastic Properties of the Adsorbed PEMs. The viscoelastic model developed by Voinova et al.38 was also used to evaluate the mechanical properties of the starch multilayers. Being a model, it is sensitive to the values of the parameters used; thus, the absolute values from the model should be treated with care. However, some general trends can definitely be detected in the calculated values of viscosity, Figure 9a, and shear modulus, Figure 9b. First, the results in Figure 9 show that the viscosity and shear modulus were of the same order of magnitude for PEMs formed


at both electrolyte concentrations. This was somewhat unexpected considering the large difference in dissipation, but on the other hand, it was in perfect agreement with the similarity of the water contents of the PEMs (Table 1). While the water content was rather similar for the cationic starch and anionic starch layers in the PEMs, the mechanical properties seemed to be dependent on the polyelectrolyte that was adsorbed in the outermost layer. This agrees with earlier studies of PEMs formed from PAH/PAA.19,20 Apart from the first layer of cationic starch, the PEMs showed significantly higher viscosity and shear modulus values when anionic starch was adsorbed outermost than when cationic starch was adsorbed outermost (i.e., the PEMs were stiffer and more rigid when capped by anionic starch). This trend was most distinct for the PEMs formed at 1 mM NaCl where there was no decrease in mechanical properties with increasing layer numbers. At 10 mM NaCl the shear modulus and viscosity showed a slight decrease with increasing layer number. The difference in mechanical properties dependent on the polyelectrolyte in the outermost layer was not as large as it was at the lower electrolyte concentration. From the calculated values for the shear modulus (Figure 9b), it is also clear that there is a difference in the way the properties change, especially when the anionic polyelectrolyte is added. After the second addition of anionic starch there is a sharp increase in the shear modulus at the lower ionic strength. However, there is only a gradual increase in modulus for the higher ionic strength. The final level attained is also lower than for the lower ionic strength. This indicates a different structure of the adsorbed layer, that can be linked both to the electrostatic interactions between the polyelectrolytes and the crowding of the surface at the higher salt concentration. Adhesive Properties of the PEM. The initial objectives of this research were to determine the possibility of forming biopolyelectrolyte multilayers from cationic starch and anionic starch, and to determine their properties. An additional objective was to investigate the potential of starch PEMs for increasing the adhesive interaction between two solid substrates beyond the adhesion possible by the adsorption of a single cationic starch layer. The AFM colloidal probe technique was used to measure adhesive pull-off forces between PEM-covered silica substrates under wet conditions. Wet conditions are typical of those under which joint formation takes place in many biological systems and also during the formation of strong fiber-fiber joints in papermaking. A typical force curve on retraction for the starch PEM covered silica substrates was shown in Figure 6. The forces were very long-range and the curves were characterized by multiple adhesive events. Considering that the maximum thickness of two interacting starch PEMs, for example, at 10 mM NaCl and four bilayers, should be about 80 nm, the adhesive forces measured (ranging over several micrometers) are very surprising. Thus, the entire PEMs must have stretched out several times their own thickness, probably by the formation of multiple molecular bridging necks on retraction. The high molecular weight, highly branched amylopectin might facilitate a possible bridging. Another sign of molecule bridging is the multiple adhesive events. This shows that the adhesive contact was not broken in one single snap, but broke step by step and perhaps molecule by molecule. As can be seen in Figure 7, the pull-off forces measured by colloidal probe AFM increased with increasing layer number, probably due to larger molecular contact area at higher layer numbers, for starch PEMs formed both at 1 mM NaCl and at 10 mM NaCl. It could also be noted that the pull-off forces


1775

were of the same order of magnitude at both background electrolyte concentrations. Entanglements of polymer chains across the interface have been shown to be crucial for the adhesive interaction between polymer coated substrates39-41 for different polymer systems. Upon separation of the substrates, the polymer chains must disentangle. Thus, an increasing degree of entanglements leads to increased adhesive forces. Polymer chain mobility is very important in creating extensive entanglements across the interface, especially if the time of interaction is limited.41 If the QCM-D viscoelastic modeling data is considered, and assuming that interdiffusion of polyelectrolytes across the interface is also very important for the development of joint strength between PEM-covered substrates, it is therefore reasonable that the pull-off forces were of the same order of magnitude at both electrolyte concentrations. The viscosity and shear modulus was of the same order of magnitude at 1 and 10 mM NaCl, suggesting that the mobility of the polyelectrolytes building up the PEMs was also similar. Thus, the interdiffusion rate of the polyelectrolytes should be similar at both NaCl concentrations, leading to about the same degree of entanglement. From the fifth layer onward at 1 mM NaCl the adhesive forces were higher when the PEMs were capped by cationic starch than when anionic starch was adsorbed in the outermost layer. Considering the data from the viscoelastic model again, the higher adhesive forces coincide with a lower rigidity (i.e., lower viscosity and shear modulus) when the PEMs were capped by cationic starch. When the PEMs were instead capped by anionic starch they were more rigid, the polymer chains were less mobile and thus less prone to create adhesion-enhancing entanglements across the interface. Similar correlations between rigidity, mobility, and adhesion have been seen in earlier studies of PAH/ PAA PEMs.19 At 10 mM NaCl, however, the opposite trend in pull-off forces were seen from the sixth layer, that is, the adhesive forces were higher when anionic starch was adsorbed outermost. Thus, in this particular case, the correlation between pull-off forces and modeled viscoelastic properties of the formed layers was broken because the PEMs seemed to be more rigid when capped by anionic starch than when capped by cationic starch. This is true even if the changes in shear modulus and viscosity were lower than at 1 mM NaCl. This suggests that it is not only the polymer mobility and rate of interdiffusion that determine the adhesion in this case at 10 mM NaCl; possibly other differences in the PEM structure are also important. There is also a clear difference in the amount of oppositely charged starches on the surfaces when comparing data from 1 and 10 mM NaCl. In addition, the differences between the properties of the formed layers have to be considered. It is reasonable to assume that the properties of the formed layers and the amount of polyelectrolyte on the surfaces will both be of large importance for wet adhesion. The influence of chain mobility might be different depending on the absolute amount of polyelectrolyte on the surface. For example, as proposed by Luengo et al.,40 the polymer mobility might in some cases be too high, leading not only to a high entanglement rate but also to a high disentanglement rate. Thus, the contribution to adhesion from polymer interdigitation across the interface will not be very high. Instead, an intermediate mobility of the polymer chains is optimal for creating strong adhesive joints.40 Further work is needed to determine the mechanisms responsible for starch PEM adhesion. In summary, it can be concluded that the adhesive properties of starch PEMs can be tailored by choosing different polyelec-

1776


trolytes in the outermost layer. The background electrolyte concentration can also be manipulated to alter the PEM properties. Earlier studies of PEMs formed from PAH/PAA have shown that the results of model experiments using the AFM colloidal probe technique19,20 can be linked to joint strength measurements of single fiber crosses42 and also to direct studies of paper strength.23,24 Thus, based on the results of the model experiments in the present work, there should be a real potential for cationic starch/anionic starch PEMs to improve the adhesive interactions in more applied systems, for example between cellulosic fibers in papermaking. The work also shows the potential of forming strong adhesive joints with oppositely charged biopolyelectrolytes. Furthermore, the TEMPO-oxidized anionic starches have another yet unexplored potential in the aldehyde groups, which could possibly be utilized for chemical reactions such as crosslinking of the PEMs, which might lead to further improved properties or entirely new properties.

Conclusions SPAR and QCM-D measurements showed that biopolyelectrolyte multilayers can be successfully constructed from cationic starch (D.S. 0.065) and anionic starch (D.S. 0.037) at electrolyte concentrations of 1 and 10 mM NaCl. At 100 mM NaCl there was no multilayer formation. The thickness of the PEMs formed at 10 mM NaCl was approximately twice as large as the thickness of the PEMs formed at 1 mM NaCl. The starch PEMs formed at both electrolyte concentrations were very water-rich with about 80% water content. Viscoelastic modeling suggested that the viscoelastic properties of the PEMs were dependent on the polyelectrolyte that was adsorbed in the outermost layer. Anionic starch-capped PEMs showed higher viscosity and shear modulus values than the PEMs capped by cationic starch. The wet adhesive properties of the starch PEMs were successfully measured using colloidal probe AFM. It was shown that adhesion increased with layer number. Thus the starch PEM treatment has a large potential of being used to increase the adhesive interaction between solid substrates beyond the levels of adhesion that can be obtained by a single layer of cationic starch. It was also shown that the adhesive forces were dependent on the polyelectrolyte that was adsorbed in the outermost layer. This creates the possibility of tailoring the surface properties of different substrates by treatment with PEMs formed from cationic starch and anionic starch. Acknowledgment. E.J. acknowledges the Swedish Center for Biomimetic Fiber Engineering (Biomime) and Lyckeby Research Foundation for financial support. L.L. thanks Holmen Paper AB and Kempestiftelserna for financial support. Therese Sennerfors, Johan Lindgren, Eva Larsson, and Ingela Thulin are acknowledged for valuable discussions. Supporting Information Available. Theory of stagnation point adsorption reflectometry and theory of quartz crystal microgravimetry with dissipation. This material is available free of charge via the Internet at http://pubs.acs.org.

References and Notes (1) Whistler, R. L.; BeMiller, J. N.; Paschall, E. F. Starch: Chemistry and Technology, 2nd ed.; Academic Press: Orlando, FL, 1984. (2) Moeller, H. W. Tappi J. 1966, 49 (5), 211–214.

Johansson et al. (3) Lindstro¨m, T.; Flore´n, T. SVensk Papperstidn. 1984, 87 (12), R99– R104. (4) Howard, R. C.; Jowsey, C. J. J. Pulp Pap. Sci. 1989, 15 (6), J225– J229. (5) Eriksson, M.; Pettersson, G.; Wågberg, L. Nord. Pulp Pap. Res. J. 2005, 20 (3), 270–276. (6) Pettersson, G.; Ho¨glund, H.; Wågberg, L. Nord. Pulp Pap. Res. J. 2006, 21 (1), 115–121. (7) Pettersson, G.; Ho¨glund, H.; Wågberg, L. Nord. Pulp Pap. Res. J. 2006, 21 (1), 122–128. (8) Iler, R. K. J. Colloid Interface Sci. 1966, 21 (6), 569–594. (9) Decher, G.; Hong, J. D.; Schmitt, J. Thin Solid Films 1992, 210-211 (Part 2), 831–835. (10) Decher, G.; Schlenoff, J. B. Multilayer Thin Films: Sequential Assembley of Nanocomposite Materials; Wiley-VCH: Weinheim, Germany, 2003. (11) Sun, Y.; Zhang, X.; Sun, C.; Wang, B.; Shen, J. Macromol. Chem. Phys. 1996, 197 (1), 147–153. (12) Klitzing, R. v. Phys. Chem. Chem. Phys. 2006, 8 (43), 5012–5033. (13) Marra, J.; Hair, M. L. J. Phys. Chem. 1988, 92 (21), 6044–6051. (14) Claesson, P. M.; Dahlgren, M. A. G.; Eriksson, L. Colloids Surf., A 1994, 93, 293–303. (15) Biggs, S.; Proud, A. D. Langmuir 1997, 13 (26), 7202–7210. (16) Notley, S. M.; Biggs, S.; Craig, V. S. J. Macromolecules 2003, 36 (8), 2903–2906. (17) Lowack, K.; Helm, C. A. Macromolecules 1998, 31 (3), 823–833. (18) Blomberg, E.; Poptoshev, E.; Claesson, P. M.; Caruso, F. Langmuir 2004, 20 (13), 5432–5438. (19) Notley, S. M.; Eriksson, M.; Wågberg, L. J. Colloid Interface Sci. 2005, 292 (1), 29–37. (20) Lingstro¨m, R.; Notley, S. M.; Wågberg, L. J. Colloid Interface Sci. 2007, 314 (1), 1–9. (21) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Nature 1991, 353 (6341), 239–241. (22) Gong, H. F.; Garcia-Turiel, J.; Vasilev, K.; Vinogradova, O. I. Langmuir 2005, 21 (16), 7545–7550. (23) Wågberg, L.; Forsberg, S.; Johansson, A.; Juntti, P. J. Pulp Pap. Sci. 2002, 28 (7), 222–228. (24) Eriksson, M.; Notley, S. M.; Wågberg, L. J. Colloid Interface Sci. 2005, 292 (1), 38–45. (25) Lingstro¨m, R.; Wågberg, L.; Larsson, P. T. J. Colloid Interface Sci. 2006, 296 (2), 396–408. (26) Solarek, D. B. Cationic Starches. In Modified Starches: Properties and Uses; Wurzburg, O. B., Ed.; CRC Press, Inc.: Boca Raton, FL, 1986; pp 113-130. (27) Bello-Pe´rez, L. A.; Roger, P.; Baud, B.; Colonna, P. J. Cereal Sci. 1998, 27 (3), 267–278. (28) Modig, G.; Nilsson, P.-O.; Wahlund, K.-G. Starch/Sta¨rke 2006, 58 (2), 55–65. (29) Dijt, J. C.; Stuart, M. A. C.; Hofman, J. E.; Fleer, G. J. Colloids Surf. 1990, 51, 141–158. (30) Dijt, J. C.; Stuart, M. A. C.; Fleer, G. J. AdV. Colloid Interface Sci. 1994, 50, 79–101. (31) Senden, T. J. Curr. Opin. Colloid Interface Sci. 2001, 6 (2), 95–101. (32) Hutter, J. L.; Bechhoefer, J. ReV. Sci. Instrum. 1993, 64 (7), 1868– 1873. (33) Glinel, K.; Moussa, A.; Jonas, A. M.; Laschewsky, A. Langmuir 2002, 18 (4), 1408–1412. (34) Bolt, G. H. J. Phys. Chem. 1957, 61 (9), 1166–1169. (35) Samoshina, Y.; Nylander, T.; Shubin, V.; Bauer, R.; Eskilsson, K. Langmuir 2005, 21 (13), 5872–5881. (36) Sauerbrey, G. Z. Phys. 1959, 155 (2), 206–222. (37) Voinova, M. V.; Jonson, M.; Kasemo, B. Biosens. Bioelectron. 2002, 17 (10), 835–841. (38) Voinova, M. V.; Rodahl, M.; Jonson, M.; Kasemo, B. Phys. Scr. 1999, 59 (5), 391–396. (39) Creton, C.; Kramer, E. J.; Hui, C. Y.; Brown, H. R. Macromolecules 1992, 25 (12), 3075–3088. (40) Luengo, G.; Pan, J.; Heuberger, M.; Israelachvili, J. N. Langmuir 1998, 14 (14), 3873–3881. (41) Chen, N.; Maeda, N.; Tirrell, M.; Israelachvili, J. Macromolecules 2005, 38 (8), 3491–3503. (42) Eriksson, M.; Torgnysdotter, A.; Wågberg, L. Ind. Eng. Chem. Res. 2006, 45 (15), 5279–5286.

BM900191S

Adsorption Behavior and Adhesive Properties of Biopolyelectrolyte

Recommend Documents