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Ind. Eng. Chem. Res. 2006, 45, 6665-6671

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MATERIALS AND INTERFACES Mechanical Properties of Polyelectrolyte Complex Films Based on Polyvinylamine and Carboxymethyl Cellulose Xianhua Feng, Robert Pelton,* and Marc Leduc† Department of Chemical Engineering, McMaster UniVersity, 1280 Main Street West, Hamilton, Ontario, Canada L8S 4L7

Homogeneous polyelectrolyte complex films were cast from mixtures of poly(vinylamine-co-vinylformamide) and carboxymethyl cellulose (CMC) in 50% formic acid aqueous solution with compositions varying between CMC blended with pure polyvinylamine (PVAm) and CMC blended with pure poly(N-vinylformamide) (PNVF). The tensile strength and tensile modulus of the complex films were measured as functions of polymer ratio, molecular weight, function group content, and water content. PVAm addition lowered the strength and modulus of dry CMC whereas PNVF did not; intermediate PVAm-PNVF copolymers gave intermediate results. Mechanical strength decreased with increasing molecular weight of CMC and carboxyl content, but was not affected by molecular weight of PVAm. The mechanical properties of all the films decreased with increasing water content. CMC:PVAm films were swollen gels in water, whereas CMC:PNVF films dissolved in water. Heating did not improve the mechanical strength, whereas cross-linking with 5% glutaraldehyde doubled the strength. It was proposed that hydrogen bonding was the predominant intermolecular force responsible for the strength of dry film blends whereas ionic bonds between CMC-carboxyl and PVAmammonium ions were responsible for the integrity of water swollen polyelectrolyte films. Introduction

Table 1. Published Mechanical Properties of Polyelectrolyte Complexesa

Water-insoluble polyelectrolyte complexes (PECs) often form when oppositely charged aqueous polyelectrolyte solutions are mixed. Since the pioneering work of Michaels1 and Rembaum2 in the 1960s and 1970s and the more recent work of Dautzenberg3, Kokufuta,4 Decher,5 and many others, a wide range of PEC applications has been reported, including dialysis and ultrafiltration membranes,6 surgical adhesives,7 and microcapsules for drug delivery.8 One of the highest volume uses of PECs is the papermaking process, where colloidal-sized PECs are generated in situ with the goal of depositing them onto aqueous wood pulp fibers to ultimately locate in dry paper sheets. This is done as a method to either remove unwanted soluble polyelectrolytes from the papermaking processes9 or increase the mechanical strength of the paper.10,11 Our interests involve the application of colloidal PEC for paper strengthening,12 where deposited PEC particles act as an adhesive, enhancing the strength of cellulose fiberfiber bonds in paper. In this report we address the mechanical properties of macroscopic, homogeneous films of carboxymethyl cellulose (CMC) complexes with polyvinylamine (PVAm) and with poly(N-vinylformamide) (PNVF). PEC mechanical properties are important because, to be a useful adhesive, the PEC must have adequate cohesive strength. We have found little data in the literature describing the mechanical properties of PECs, and in particular, there is little information linking polyelectrolyte structures to complex strength. * To whom correspondence should be addressed. Tel.: (905) 5297070, ext 27045. Fax: (905) 528-5114. E-mail [email protected]. † BASF Aktiengesellschaft, 67056 Ludwigshafen, Germany.

dry (MPa)

wet (MPa)

tensile tensile strength modulus strength PAA chitosan PAA/chitosan gelatin chitosan CMC/gelatin, cross-linked PAA/PEPP PSS/PVBTAC PSS/PDADMAC PVA PC/PA PTC/PSA

5 6 13 29 64 40

0 3.5 3.4 16 1b

34c 20c 32c

214c 83c 560c

0.14 0.27

modulus

ref

19 19 19 32 32 16 140 21 1000 13 10b 7 15 0.03 15 0.007 15

a Polymers: PAA, poly(acrylic acid); CMC, carboxylmethyl cellulose; PEPP, poly(ethylenepiperazine); PSS, sodium poly(styrenesulfonate); PVBTAC, poly(vinylbenzyltrimethylammonium chloride); PDADMAC, poly(diallyldimethylammonium chloride); PVA, poly(vinyl alcohol); PC, carboxymethylated poly(vinyl alcohol); PA, aminoacetalized poly(vinyl alcohol); PTC, poly(vinyl alcohol) acetalized with diethoxyethyltrimethylammonium; PSA, sulfated poly(vinyl alcohol). b Water content 80%. c 65% RH. d 50% RH.

Table 1 summarizes the published mechanical properties of PEC films. Based on these papers, the following generalizations can be made about the wet and dry properties of PEC films: 1. Although they are water-insoluble, most PECs are hygroscopic, and in water they are hydrogels.13 2. Dry PECs are often brittle plastics because the polar groups which render the polymer hydrophilic usually give strong intermolecular interactions in the dry state.

10.1021/ie060511f CCC: $33.50 © 2006 American Chemical Society Published on Web 08/25/2006

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3. In many cases, dry films made solely of one the constituent polymers are stronger than the dry PECs. By contrast, in water the constituent polymers dissolve whereas the PEC swells but has measurable mechanical strength. 4. Water is a plasticizer that lowers the tensile strength and modulus, and the higher the degree of swelling, the lower the tensile strength of wet gels.7,14 5. The mechanical properties of wet PEC gels are at a maximum when the oppositely charged polymers are stoichiometrically balanced.7 6. Covalently cross-linking PEC increases tensile strength and modulus and decreases swelling.15,16 The paucity of PEC mechanical property data perhaps reflects the difficulties in preparing uniform, homogeneous films necessary for conventional physical testing. Simply mixing oppositely charged aqueous polymer solutions and casting a film is ineffective because the mixture is often a sticky, nonuniform suspension of polyelectrolyte complex species. For this work, we were unable to prepare suitable samples for mechanical testing by mixing the aqueous polymer solutions. Although there is some suggestion that pressing this mixture in the presence of alcohol will give films,17 it seems better to cast films from homogeneous solutions. Three types of procedures have been reported for the preparation of uniform PEC films from homogeneous solutions. The earliest method employs a ternary solvent mixture of salt, organic solvent, and water.13 The salt inhibits complex formation by screening electrostatic interactions, whereas the organic solvent prevents salting out of polyelectrolytes. Films cast from this mixture contain salt whose removal produces holes.18 This may be advantageous for membrane production; however, holes complicate mechanical testing. The second method for making uniform PEC films uses water, a polar organic solvent, and a volatile acid or base. This approach is effective for PEC in which one of the polyelectrolytes is a weak acid or a weak base whose charge can be suppressed by the corresponding volatile acid or base.19-21 This method is simple and eliminates the need to remove salt crystals from the dried films. Decher’s layer-by-layer assembly technique is (arguably) the third method to make polyelectrolyte complex films.5 In a typical process, a monolayer of polyelectrolyte is adsorbed onto an oppositely charged surface, the surface is washed, and then an adsorbed monolayer is formed from the oppositely charged polymer to generate a PEC.22 Although multiple replications of this process increases the PEC film thickness, this method seems best suited for ultrathin PEC films. In this paper we describe the mechanical properties of CMC: PVAm films and CMC:PNVF films (made by the second method) as functions of composition and water content. Surprisely, the dry CMC:poly(N-vinylformamide) blends are much stronger than the CMC:polyvinylamine films, although the latter are much stronger in the presence of water. Although our work is aimed at papermaking applications, the results are relevant to microcapsules, biomaterials, and other applications of polyelectrolyte complexes.

Table 2. Polyvinylamine Propertiesa PVAm

PVAm-co-PNVF

MW (kDa) 34 150 340 950 1500 500 500 500 degree of hydrolysis (%) 100 100 100 100 100 10 50 95 equiv wt (g/mol N) 98 72 92 93 80 1470 181 133 a The molecular weights are for the parent poly(N-vinylformamide) polymers. The degree of hydrolysis values were measured by 1H NMR, and the equivalent weights were measured after exhaustive dialysis and freeze-drying.

Polyvinylamine (PVAm) is obtained by completely hydrolyzing the corresponding poly(N-vinylformamide) (PNVF). Thus for a given PNVF there is a range of possible copolymer compositions which are specified by the parent PNVF molecular weight and the degree of hydrolysis. BASF provided PNVF samples with viscosity average molecular weights of 34, 150, 340, 950, and 1500 kDa. In addition, BASF supplied poly(vinylamine-co-vinylformamide) with parent PNVF molecular weight 340 kDa which was hydrolyzed to give 10 mol %, 50%, and 90% amine. We further hydrolyzed these polymers to give PVAm using 5% NaOH at 75 °C for 48 h, under a nitrogen purge.23 All samples, both with and without undergoing further hydrolysis, were exhaustively dialyzed against water and freezedried. The final degree of hydrolysis was determined by 1H NMR, and conductometric titration was used to measure the equivalent weight of the freeze-dried polymers. The polyvinylamine polymer properties are summarized in Table 2. We prepared one sample of PNVF by precipitation polymerization of N-vinylformamide in toluene medium using Vazo 52 (1,1′-azobis(cyclohexanecarbonitrile)) as initiator. The polymerization was carried out at 50 °C for 8 h, under a nitrogen purge. The product was washed four times with ethanol and was freeze-dried. The molecular weight was estimated to be 500 kDa by static light scattering. Film Preparation and Characterization. PVAm or PVAmco-PNVF and CMC were dissolved in water and stirred for 24 h. The solutions were filtered through 5 µm Millipore filters and were diluted with an equal mass of formic acid to give 1% polymer solutions. For casting, the polymers in 50% formic acid were mixed to give CMC:PVAm weight ratios ranging from 1 to 5. After mixing for 24 h at room temperature, the solutions were placed in 10 × 10 cm2 polystyrene Petri dishes and dried in a fume hood for 24 h. The resulting polyelectrolyte complex films were washed by soaking in ethanol. During 24 h of washing, the ethanol was changed three times. Finally, dry transparent films were obtained after drying under vacuum at room temperature overnight. The formic acid contents in the washed films were determined by the following procedure. First, the complex film was either dissolved in pH 12 NaOH solution or underwent Soxhlet extraction with water for 2 days. The extracts were mixed with nicotinamide adenine dinucleotide (NAD) which was dissolved in pH 7.5 potassium phosphate buffer. The UV absorbance of the solution was measured at 334 nm. Then formate dehydrogenase (FDH) aqueous solution was added into the mixture to initiate the following reaction: FDH

Experimental Section Materials. Carboxymethyl cellulose sodium salt (CMC) polymers (from Aldrich) were designated by the molecular weight (MW) and the degree of carboxymethylation (DS) values given by Aldrich. The CMC polymers employed in this work were MW/DS 90 kDa/0.7, 250 kDa/0.7, and 250 kDa/1.2.

formate + NAD+ + H2O 98 bicarbonate + NADH + H+ (1) in which formic acid or formate was stoichiometrically oxidized to bicarbonate.24 Finally, the absorbance of the solution was measured after 20 min of reaction and the absorbance difference before and after reaction was used to calculate formic acid

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content using an extinction coefficient of NADH of 6.181 mmol-1 cm-1 at 334 nm.24 The atomic compositions of the film surfaces were measured by X-ray photon spectroscopy (XPS) using a Leybold Max 200 XPS system equipped with a nonmonochromatized Al KR (15 kV, 20 mA) X-ray source. The energy scale was corrected by using the C 1s value of the main C-C component at 285 eV. Spectra were obtained at a takeoff angle of 90° corresponding to a depth of analysis of 10 nm, at 2 × 10-9 bar, using a 2 × 4 mm2 aperture. Film morphology was examined by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and a confocal laser scanning microscope (CLSM). In the TEM measurement, the film was first placed into the cryo-chamber and was sectioned at approximately -120 °C with a cold, dry cryo-diamond knife. Then sections were placed on Formvarcoated 200 mesh copper grids and viewed in the TEM. A second method for sample processing was also employed. For this, the film was embedded in nanoplast melamine resin (Nanoplast FB101, Rolf Bachhuber, Germany) and heated at 40 °C for 48 h, followed by heating at 60 °C for 48 h. The resulting blocks were sectioned with a diamond knife on an ultramicrotome (Leica Ultracut UCT) with a section thickness of approximately 60 nm. Finally, the sections were placed on Formvar-coated 200 mesh copper grids, stained with 1% aqueous uranyl acetate, and viewed in a JEOL JEM 1200 TEM. In the SEM measurement, the film was first fractured in liquid nitrogen. Then the cross section was sputter-coated with gold and viewed on a Philips SEM 515. For the CLSM measurement, a film was made from fluorescein-labeled PVAm and CMC. The top surface of the film was viewed with a Zeiss LSM 510. A stack of images was acquired in the z-direction with 2 µm per layer. The labeled PVAm was prepared by the following procedures.25 To 50 mL of 2 mg/mL PVAm in pH 9 borate buffer was added a mixture of 1.5 mg of fluorescein-5-isothiocyanate (FITC) and 1.5 mL of DMSO. The reaction was stirred with a magnetic stirring bar for 10 h at 4 °C, followed by quenching with 1.3 mL of 2 M NH4Cl for 2 h. A 5 mL volume of the solution was eluted through a Sephadex G-25 gel filtration column with pH 9 borate buffer. The polymer passed through the column before the free fluorescein molecules. Finally, the labeled PVAm was dialyzed against water for 3 days and then was freeze-dried. Note that all of these procedures were carried out in the dark. The attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) of the films was performed on a BioRad FTS-40 spectrometer (Hercules, CA). Mechanical Strength Measurements. The washed and freeze-dried polyelectrolyte complex films were conditioned at 23 ( 1 °C and 50 ( 2% relative humidity for 24 h and were then cut into 8 × 1.5 cm2 (length by width) strips. Typically, five strips were obtained from each film. The thickness of each strip was measured using a digital micrometer (Testing Machines Inc., Point Claire, Quebec) and was recorded as the average of 10 measurements. Typical thickness was about 40 ( 3 µm. Finally, the mechanical strength measurement was performed on an Instron 4411 universal testing system (Instron Corp., Norwood, MA), located in a humidity-controlled room and fitted with a 50 N load cell. The film was clamped on the instrument with a gauge length of 5 cm and was tested at a crosshead rate of 5 mm/min following the ASTM D882-02 procedure for thin plastic films. For the measurement of mechanical strength at other relative humidity values, the films were placed in an open plastic zipper

Table 3. Atomic Composition of Complex Films Determined by Elemental Analysis (EA) and by XPS Using a Takeoff Angle of 90° a XPS CMC/PVAm

C

N

O

1:1 2:1 3:1 4:1 5:1

63.5 61.3 60.4 59.9 61.4

8.9 8.6 6.4 4.6 4.4

22.7 26.8 28.5 30.6 29.0

EA Cl Na

Si

0.3 0.3 0.3 0.1 0.1

3.8 1.9 3.1 3.0 3.1

0.9 1.2 1.4 1.9 2.1

C

N

O

H

40.76 10.32 30.48 7.07 39.06 6.63 31.83 6.64 39.18 5.21 30.84 6.52 39.37 4.18 34.09 6.49 38.74 3.53 33.64 6.10

a The CMC MW was 90 kDa and the DS was 0.7. The PVAm MW was 150 kDa.

Table 4. Comparisons of O/N Atomic Ratios Measured by XPS and Elemental Analysis to the Calculated Values CMC:PVAm

O/N (XPS)

O/N (EA)

O/N (calcd)

1:1 2:1 3:1 4:1 5:1

2.6 3.1 4.5 6.7 6.6

2.6 4.2 5.2 7.1 8.3

3.2 5.0 6.8 8.6 10.4

bag and were conditioned at 23 °C and at desired humidity (6090%) for 48 h in an ESL-2CA CTH chamber (ESPEC North America, Inc., Hudsonville, MI). Then the bag was sealed and the tensile test was performed immediately with the sample protected inside the plastic bag. The water contents of the films were determined by weighing before and after conditioning. Cross-Linking with Glutaraldehyde. Mixtures containing 70 mL of methanol and 10-30 mL of 50% glutaraldehyde were adjusted to 100 mL with water, and the pH of the solutions was adjusted to 2 with hydrochloric acid. The films made from 3:1 of CMC 90 kDa, DS 0.7 to PVAm 150 kDa were then immersed in the mixture and kept in room temperature for 48 h. After cross-linking, the films were washed by immersion into 95% ethanol for 24 h to remove excess glutaraldehyde and hydrochloric acid.26 The ethanol was changed three times during this process. Results CMC:PVAm films were cast from polymer solutions in 50% formic acid. After air-drying, the films were washed with ethanol and the residual formate concentration, measured by an enzyme based method, was 1.5 ( 0.4 wt %. The atomic compositions of the films, as measured by elemental analysis and XPS, are summarized in Table 3. The corresponding O/N atomic ratios are compared with the calculated O/N ratio in Table 4. Presumably the sodium originated from the CMC, which was supplied as the sodium salt, whereas the silicon is an impurity. For the calculations it was assumed films contained no formate and that bicarbonate was the counterion in the freeze-dried, exhaustively dialyzed PVAm. The O/N ratios were systematically lower for the surface-sensitive XPS, suggesting a slight tendency for PVAm to concentrate at the surface of the cast films. The calculated O/N ratios were about 25% higher than those determined with elemental analysis. In the following sections the film compositions are expressed as mass ratios of CMC to PVAm. In most aqueous polyelectrolyte complex studies the compositions are expressed as charge ratios, which are functions of the mass, the equivalent weights, and the degree of protonation of the added polymers. Since both CMC and PVAm are weak polyelectrolytes, the degrees of protonation are functions of the pH. In this work we do not present charge ratio data since most of the films are dry, or nearly dry, making estimation of the degree of protonation difficult. In a following paper, we will present the swelling

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Figure 1. Morphology of films made from 3 parts CMC 90 kDa, DS 0.7 and 1 part PVAm 150 kDa: (A) TEM cryosectioned, scale bar 50 nm; (B) TEM embedded in resin and thin sectioned, scale bar 100 nm, where the darker part is complex; (C) SEM scale bar 15 µm; (D) LSCM, scale bar 20 µm.

Figure 3. Modulus and tensile strength of PVAm/CMC complex films as a function of polymer ratio at various PVAm molecular weights, and maximum modulus and maximum tensile strength as a function of PVAm MW. Error bars denote standard deviation of five measurements

Figure 2. Examples of stress versus strain curves for CMC:PVAm complex films. The ratios beside the curves represent weight ratios of CMC to PVAm. PVAm 1500 kDa; CMC 90 kDa, DS 0.7. CMC:PVAm ) 1:1 to 4:1.

characteristics of the films. In this situation we can estimate the pH and thus the charge contents of CMC and PVAm in the films. The mechanical properties of polymer blends can depend on the degree of polymer mixing. Polymers can be completely miscible if there are significant intermolecular attractions. In many cases, however, one polymer is present as a dispersed phase embedded in a continuous phase rich in the second polymer. Our CMC:PVAm films were examined by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and laser scanning confocal microscopy (LCSM). For the latter, the PVAm was labeled with a fluorescent dye. Figure 1 shows example photomicrographs from each technique. Even at the nanometer-scale resolution of the electron microscopy there was no evidence of phase separation within the polymer films. The PEC film tensile properties were measured, and example data are plotted in Figure 2 for a series of films with varying CMC:PVAm mass ratios. Before testing, most films were conditioned at 23 °C, 50% RH (relative humidity). Tensile strengths (stress at break) and moduli were extracted from these, and similar curves, and are presented in the following paragraphs where they illustrate the influences of film composition, polymer structure, water content, and cross-linking on the mechanical properties. Generally, the tensile strength of a polymer increases with molecular weight up to a limiting value.27 Figure 3 shows the influence of PVAm molecular weight on the mechanical properties of CMC:PVAm films. The error bars show the

standard deviation of the results based on five measurements. For most of the samples, the maximum strength occurred at CMC:PVAm ratios of 2:1 or 3:1. The modulus values were rather insensitive to PVAm molecular weight, although the 340 kDa PVAm gave lower strengths. The tensile strengths showed greater scatter and there was no clear trend with PVAm molecular weight. By contrast, PEC film strength and modulus increased with decreasing CMC molecular weight (see Figure 4). Similar effects have been reported for cellulose acetate films.28 The usual explanation is that because cellulosic chains are stiff, shorter ones are more likely to align during drying to maximize intermolecular interactions. The degree of CMC carboxymethylation also influenced film strength. DS 1.2 CMC gave much weaker PEC films than DS 0.7 CMC. Apparently the advantage of increased electrostatic interaction with PVAm afforded by increasing the DS was more than offset by the interference in CMC/CMC hydrogen bonding. Poly(N-vinylformamide) was hydrolyzed to different extents to give a range of PVAm-co-PNVF copolymers. The influence of copolymer composition on PEC film tensile strength and modulus is shown in Figure 5. PNVF gave the strongest dry films when mixed with CMC, whereas PVAm gave the weakest. The mechanical properties were not too sensitive to amine content for amine contents (equivalent to the degree of PNVF hydrolysis) less than 50%. By contrast, higher amine contents gave much weaker PEC films. Note that because the mechanical properties of PNVF were close to those of CMC, the tensile strength and modulus values were insensitive to the mixing ratio (see 0 amine data in Figure 5). Note that although PNVF forms rather strong dry films when mixed with CMC, the materials are of limited technological interest in many applications because the films completely dissolve in water, whereas CMC:PVAm PEC films persist in

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Figure 5. Influence of amine content (degree of hydrolysis) of PVAmco-PNVF copolymers on the strength of PEC complex films. The parent PNVF molecular weight was 500 kDa, and the CMC MW was 90 kDa, DS 0.7. The films were conditioned at 50% RH.

Figure 4. Influence of CMC structure on mechanical properties of PECs. PVAm 150 kDa. CMC 90 kDa, DS 0.7; CMC 250 kDa, DS 0.7, 1.2. The films were equilibrated in 50% RH, 23 °C, before testing.

water because of the ionic cross-links formed between charged amines and dissociated carboxyl groups. The role of water content in the mechanical properties of the PEC films was investigated by equilibrating the films in different relative humidity environments. Figure 6 shows the tensile strength and moduli of PEC as functions of water content and film composition. As expected, the tensile strengths and moduli of all the materials dramatically decreased with increasing water content, with the major changes occurring around 10% (w/w) water. The single component polymer film results are interesting. Both CMC and PNVF gave strong, brittle films, whereas PVAm films were nearly an order of magnitude weaker. In general, the higher the mass fraction of PVAm in the film, the weaker was the blend at all water contents. There are subtle indications that ionic bonds between PVAm and CMC contributed to the mechanical properties. For example, the CMC:PVAm 3:1 film, corresponding to 25% PVAm, is slightly stronger and stiffer than the CMC:PVAm 5:1 film (17% PVAm). The introduction of covalent cross-links is a conventional approach to improving mechanical properties of materials. With the CMC:PVAm blends, cross-links could be formed either by inducing amide formation between amines and carboxyls or by introducing a cross-linking reagent that reacts with both CMC and PVAmswe evaluated both approaches. CMC (90 kDa, DS 0.7):PVAm (150 kDa) films of various compositions were heated at 90, 120, and 160 °C for 1 h, and the mechanical properties were measured. There was no increase in either tensile strength or modulus compared to the standard samples dried at 23 °C, nor was there any evidence of amide formation in the FTIR spectra. Indeed, there was some evidence of strength loss at high temperature. We concluded that amide

bonds did not form under these conditions. Note that recent work has shown that polyallylamine will form amides with poly(acrylic acid) when heated to 130 °C; therefore, our conclusion that amides do not form is system dependent.29 CMC (90 kDa, DS 0.7):PVAm (150 kDa) ) 3:1 films were treated with 5% glutaraldehyde, and the modulus increased from 2156 MPa (no glutaraldehyde) to 3259 MPa. The corresponding tensile strengths increased from 21 to 52 MPa. We measured the extent of PEC film swelling in water. Before cross-linking the equilibrium film water content was 381%, whereas the crosslinked film contained only 86% water. Discussion CMC:PVAm films cast from concentrated formic acid were transparent with no indication of microphase separation. The CMC:PVAm blend properties follow the six general trends of polyelectrolyte complexes given in the Introduction. However, there were some surprises in the detailed behaviors. For example, the higher strength of CMC blended films based on PNVF compared to those based on PVAm was unexpected. We were also surprised by the limited contribution of ionic bonds to the dry mechanical propertiessthese observations are now discussed. Poly(N-vinylformamide) (PNVF) is a nonionic water-soluble polymer, much like polyacrylamide, produced by the free radical polymerization of N-vinylformamide. Polyvinylamine (PVAm) is produced by the complete hydrolysis of PNVF, whereas partial hydrolysis gives copolymers. Approximately half of the primary amino groups are protonated (charged) in water at pH 7.30 The results in Figure 5 clearly show that nearly dry CMC: PNVF blends are much stronger than CMC:PVAm blends, whereas wet CMC:PVAm films form hydrogels when CMC: PNVF films dissolve. Since our PVAm and PNVF polymers had identical polymer backbones, the strength of individual polymer chains was not the critical factor, suggesting that the

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Table 5. Compositions of the PVAm/CMC Filmsa film

1:1

2:1

3:1

4:1

5:1

HCOOH (wt %) free amine (mol/100 g film) protonated amine formate (mol/100 g film) protonated amine carboxylate (mol/100 g film)

2.4 ( 0.5 0.733 0.052 0.216

1.4 ( 0.2 0.488 0.030 0.256

1.3 ( 0.5 0.366 0.030 0.185

1.1 ( 0.5 0.293 0.024 0.148

1.4 ( 0.4 0.244 0.030 0.113

a The formic acid content was measured in triplicate by a formate dehydrogenase assay. It was assumed that protonated amines were only present as formate or carboxylate salts and that all of the CMC carboxyls were present as salts.

Figure 6. Modulus and tensile strength of PVAm:CMC complex films at various water contents. The numbers beside the CMC curve represent the relative humidity. The error bars were omitted for a better view. The ratios refer to weight ratios of CMC to PVAm.

PNVF vs PVAm results reflect differences in the types of intermolecular interactions in the films. It is generally accepted that hydrogen bonding between hydroxyl groups is the primary intermolecular interaction in dry carbohydrates, explaining the strong, brittle nature of CMC films.31 The interaction of water with hydroxyls is stronger than the hydroxyl-hydroxyl interactions. Thus water disrupts CMC: CMC intermolecular interactions, at first plasticizing and eventually dissolving CMC films. The amide groups in PNVF are also effective hydrogen bond formers, giving both strong PNVF and CMC:PNVF films. However, like the CMC films, the PNVF and its blends are plasticized by a small amount of water and are water soluble. The details of bonding in the PVAm blends are more complex. When polyvinylamine is dissolved in 50% formic acid, all the amine groups should be protonated with formate counterions. Under the same conditions the carboxyl groups on CMC should also be protonated. Upon drying, the volatile formic acid evaporates. Formate, present as counterions, can accept a proton back from the charged vinylamine ions and

allow formic acid to be released to the headspace. Finally, as the formic acid content of the drying film becomes low, the CMC carboxyl groups can donate a proton to vinylamine groups to give an ionic linkage. The analyses of the residual formate in the ethanol-washed films are summarized in Table 5. From this information we estimated the contents of free amines, protonated amine formate, and protonated amine CMC carboxylate, assuming all of the added carboxylate was present as saltssthe estimates are also given in Table 5. Based on these estimates, most of the amines are present in an unprotonated form, and the concentration of formate salts in the films is much less than the concentration of CMC carboxylate-PVAm salts. The cohesive strength of pure, dry PVAm films is much less than that of pure PNVF or pure CMC because dry PVAm cannot hydrogen bond to itself since there are no acidic protons. By contrast, both the amide groups in PNVF and the alcohol groups in CMC can serve as hydrogen bond donors and acceptors. For the blended films, the unprotonated amines should be able to hydrogen bond with the alcohols in CMC. However, the mechanical properties in Figure 5 suggest that the pendant amide groups in PNVF form more or stronger hydrogen bonds than do the amines in PVAm. In water PVAm gives much stronger bonds because of ionic bond formation. The contribution of ionic bonds to film strength is most obvious when the films are placed in water. The CMC-PVAm films swell, whereas the CMC-PNVF films dissolve. The swelling behavior will be described in more detail in a future publication. For the dry films, there is little evidence that ionic bonds contribute to mechanical properties. Presumably the contribution of ionic bonds is swamped by the overwhelming number of hydrogen bonds present in the nearly dry films. We employed homogeneous macroscopic PEC films as models for the colloidal polyelectrolyte complexes because the macroscopic films facilitated mechanical testing. Like most modeling exercises, this is an approximation. PEC made from high molecular weight polymers in aqueous solution are kinetically controlled structures whose properties are sensitive to details of the fabrication. Nevertheless, this work gives important hints about links between polymer structure and complex strength. In a future publication we will describe the influence of colloidal CMC:PVAm complexes on the mechanical properties of wet and dry paper. We will employ the results from this work to infer the effects of polyelectrolyte composition on the cohesive properties of a film of dried polyelectrolyte complex binding two cellulose surfaces. For example, the current work suggests that the higher vinylformamide content of poly(vinylamine-co-vinylformamide), the higher is the dry cohesive strength. On the other hand, the presence of amine groups is critical if wet strength is important. Conclusions The major conclusions from this work are as follows: 1. Macroscopic uniform, transparent CMC/PVAm films can be prepared by casting from 50% formic acid solutions. 2. Polyvinylamine (PVAm) and carboxymethyl cellulose (CMC) form completely miscible blends, and blended films with

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water contents less than 10% are strong brittle plastics, whereas higher water contents weaken (plasticize) the films, giving rubbery behavior. 3. PVAm weakens nearly dry CMC films, whereas ionic bonds, effectively cross-linking PVAm amine groups to CMC carboxyls, maintain film integrity in water in contrast to both pure CMC and pure PVAm films, which are water soluble. 4. The mechanical strength of PVAm/CMC films decreases with increasing CMC molecular weight and DS, but is not significantly affected by the molecular weight of PVAm. 5. Poly(N-vinylformamide) (PNVF) forms blends with CMC, giving strong dry films. PNVF is much less detrimental to the strength of CMC films than PVAm. However, CMC:PNVF blends are water soluble and do not form hydrogels. 6. We propose that nearly dry CMC/PVAm film properties are dominated by hydrogen bonding between polymers with negligible contribution from ionic bonds, whereas in water the ionic bonds were the only intermolecular interactions controlling the hydrogel properties. 7. Heating CMC:PVAm films to 160 °C does not form amide cross-links between CMC and PVAm, whereas cross-linking with glutaraldehyde gives covalent cross-links, which increase mechanical strength. Acknowledgment This work was supported by BASF Canada and Natural Science and Engineering Research Canada. We thank Marcia Reid for the TEM measurements, Rana Sodhi, University of Toronto, for the XPS measurements, and Drs. Harald Sto¨ver and Raja Ghosh for helpful suggestions in this work. John-Lou DiFlavio is acknowledged for help with the labeling of PVAm. Literature Cited (1) Michaels, A. S. Polyelectrolyte Complexes. Ind. Eng. Chem. 1965, 57, 32. (2) Rembaum, A.; Yen, S. P. S.; Landel, R. F.; Shen, M. Synthesis and Properties of a New Class of Potential Biomedical Polymers. J. Macromol. Sci. Chem. 1970, A4, 715. (3) Dautzenberg, H. Polyelectrolyte Complex Formation in Highly Aggregating Systems: Methodical Aspects and General Tendencies. In Physical Chemistry of Polyelectrolytes; Radeva, T., Ed.; Surfactant Science Series 99; Marcel Dekker: New York, 2001; pp 743-792. (4) Kokufuta, E.; Matsumoto, W.; Nakamura, I. Use of Polyelectrolyte Complexes For Immobilization of Microorganisms. J. Appl. Polym. Sci. 1982, 27, 2503. (5) Decher, G. Fuzzy Nanoassemblies: Toward Layered Polymeric Multicomposites. Science 1997, 277, 1232. (6) Richau, K.; Schwarz, H. H.; Paul, D. Dehydration of Organics by Pervaporation with Polyelectrolyte Complex Membranes: Some Considerations Concerning the Separation Mechanism. J. Membr. Sci. 1996, 113, 31. (7) Vogel, M. K.; Cross, R. A.; Bixler, H. J. Medical Uses for Polyelectrolyte Complexes. J. Macromol. Sci. 1970, A4, 675. (8) Lee, K. Y.; Park, W. H.; Ha, W. S. Polyelectrolyte Complexes of Sodium Alginate With Chitosan or Its Derivatives for Microcapsules. J. Appl. Polym. Sci. 1997, 63, 425. (9) Sundberg, A.; Ekman, R.; Holmbom, B.; Sundberg, K.; Thornton, J. Interactions Between Dissolved and Colloidal Substances and A Cationic Fixing Agent in Mechanical Pulp Suspensions. Nord. Pulp Pap. Res. J. 1993, 1, 226. (10) Hubbe, M. A.; Moore, S. M.; Lee, S. Y. Effects of Charge Ratios and Cationic Polymer Nature on Polyelectrolyte Complex Deposition Onto Cellulose. Ind. Eng. Chem. Res. 2005, 44, 3068.

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ReceiVed for reView April 23, 2006 ReVised manuscript receiVed June 27, 2006 Accepted July 27, 2006 IE060511F