Incorporation of poly (acrylic acid), poly (vinylsulfonate) and poly (styrenesulfonate) within layered double hydroxides Christopher 0.Oriakhi, Isaac V. Farr and Michael M. Lerner* Department of Chemistry and Center for Advanced Materials Research, Oregon State University, Corvallis, Oregon 97331, USA Poly(acry1ic acid), poly(vinylsu1fonate) and poly(styrenesu1fonate) have been incorporated between the positively charged sheets of layered double hydroxides (LDHs) M1 -xA1,(OH)2f (M =Mg, Ca, Co) and Zn,-,M',(OH),+ (M'= Al, Cr) to form layered nanocomposites. The resulting nanocomposites contained the LDH sheet structures separated by 7.6-16.0 A,which is sufficient to accommodate polymer bilayers between the LDH sheets. Preparations were carried out in deaerated aqueous base by a template reaction, involving the formation and precipitation of nanocomposites from metal nitrate-salt precursors in the presence of the dissolved polymer.Structural and compositional details were provided by X-ray diffraction (XRD), FTIR spectroscopy, elemental analysis, differential scanning calorimetry (DSC) and thermogravimetry (TG). Scanning electron microscopy (SEM) indicates that the nanocomposition of LDHs with ionomers significantly alters the particle microstructure from that of the LDH carbonates derived from aqueous precipitation. The layered double hydroxides (LDHs), otherwise called 'anionic clays', and their intercalation compounds have received considerable attention in recent years in view of their potential technological importance as catalysts, ion exchangers, optical hosts, ceramic precursors and antacids.'-'' The LDH structure consists of brucite-like M(OH), sheets, where partial substitution of trivalent for divalent cations results in a positive sheet charge compensated by anions within interlayer galleries. LDHs are represented by the general formula [M111-xM111'x(OH)2]Xt*[(An"-),,,,, -nH,O] with representative examples M =Mg, Zn or Ca; M' =Al, Cr or Fe; An"-=C032-, C1-, OH-, NO3-or SO4'-, and x taking values between 0.16 and 0.45.'' Thus, the mineral hydrotalcite can be abbreviated as Mg-A1-C03-LDH with x=O.25, and has the approximate formula unit M&jA12(OH)16CO3 4H2O.The two most important methods of incorporation (or nanocomposition) of polymers within the smectite group of clay minerals and other layered inorganic hosts, such as metal dichalcogenides, are in situ polymerization, i.e.the intragallery polymerization of intercalated monomer,12-16 and exfoliation/ adsorption, i.e. the interaction of the separated sheets of an exfoliated host with a solution containing the desired poly- mer.17-20 In the resulting nanocomposites, the polymers are included as monolayers or bilayers between anionic sheets, and are either themselves positively charged or associated with other cations within the nanodimensional polymeric layers. Unlike the smectite clays and some layered inorganic solids, the LDHs do not exfoliate into single-sheet colloids in aqueous or other polar solutions, consequently, and there have been no reports of preformed polymers incorporated uia the exfoli- ationladsorption method.The incorporation of organic anions into LDHs has been accomplished by the following methods: (1) an ion-exchange reaction with an LDH containing monovalent anions such as itra rate;^'-^' (2) the reconstitution of calcined, amorphous LDH in the presence of an organic and (3) a 'template' synthesis where the LDH sheets are grown in the presence of by the slow diffusion associated with macromolecules. Attempts to produce nanocomposites by the second (reconstitution) method have thus far afforded only disordered or nearly amorphous prod~cts.~' Recently, Messersmith and Stupp rep~rted~'~~~ the prep- aration of well ordered layered nanocomposites derived from Ca-A1-LDH and poly(viny1 alcohol) by a template reaction similar to method (3) where the polymer is included within the host during the original sol-gel processing to form the LDH sheets.It appears likely that this method will prove to be generally applicable towards the introduction of preformed anionic polymers within LDHs. There are relatively few other reports describing the incorporation of polymers between LDH A poly(ani1ine)-LDH nanocomposite has been prepared by in situ oxidative polymerization of aniline in an organic-pillared hydr~talcite,~~and a similar method is described in the prep- aration of poly(acrilonitrile)-hydrotalcite.25 The in situ poly-merization routes employed thus far have some disadvantages, namely that the neutral monomers are incorporated into an LDH containing organic anions, limiting the polymer content of the prod~ct,~~,~~ and the factors controlling polymerization and the nature of a polymer formed within the interlayer space are not easily understood.In this study, we describe the synthesis and characterization of several new nanocomposites derived using the template method from Mg-Al-LDH, Ca-Al-LDH, Zn-Al-LDH, Zn-Cr- LDH, and Co-A1-LDH sheet structures and three vinylic polymers; poly(acry1ic acid) (PAA), poly(vinylsu1fonate) (PVS) and poly(styrenesu1fonate) (PSS). Experimental Materials The inorganic starting materials used for all preparations described in this work were analytical reagent grade and used without further purification.Poly(sodium styrene-6sulfonate) (M, =70 x lo3); poly(sodium acrylate) (M, =30 x lo3, 40% the desired organic anion with exclusion of carb~nate.~~?~' w/w in water); and poly(sodium vinylsulfonate) [M, = Since the ion-exchange reaction proceeds via a topotactic (40-60) x lo3, 25% w/w in water], were used as obtained from mechanism within the LDH layers, this method of incorpor- Aldrich Chemical Co. Distilled and deionized water was used ation of preformed anionic polymers will be kinetically limited in all preparations. J. Muter. Chem., 1996,6(l), 103-107 103 Preparation of LDHs M-A1-C0,-LDHs (M =Mg, Ca or Zn) were prepared following a standard aqueous precipitation and thermal crystallization method described by Reichle., In a typical synthesis, a solu- tion containing Mg(N03)2 *6H,O (2.88 g, 0.011 mol) and A1(N03), *9H20 (1.41 g, 0.0038 mol) in 70 ml water was added over 1 h to a vigorously stirred 100ml solution containing NaOH (3.00 g, 0.075 mol) and Na2C0, (2.34 g, 0.022 mol).The gel obtained was aged at 65 "C for 24 h and, on cooling, was filtered and washed with a copious amount of water. The resulting white solid was dried in air for 24 h. Mg-Al-CO,- LDH was prepared with starting Mg:A1 mole ratios of 2: 1 (x=0.33) and 3: 1 (x=0.25), and XRD powder data refined on a hexagonal cell provided lattice parameters of a =3.04 and 3.06 A, respectively. A correlation between the a parameter and x in the formula unit has been e~tablished,,~ and provides values of xxO.33 for the former and ~~0.25 for the latter.The formula units were therefore similar to the starting metal ratios, and were taken to be Mg4A1,(OH),,CO3 *nH20 and Mg,A12(OH)16C03 nH,O. Both samples gave similar diffraction results (Table l), and the water content for Mg4A12(OH)12C03-nH20 is estimated at nz3 from TG data. The Ca-A1-C0,-LDH and Zn-A1-C0,-LDHs were prepared from the corresponding nitrate precursors in a similar manner. Preparation of polymer-LDH nanocomposites Polymer-containing LDHs were synthesized by reacting mixed aqueous salt solutions with a basic solution containing dis- solved polymer. Nanocomposites were prepared with the fol- lowing metal ratios: x =0.33 for MI -xA1,(OH),+ (M =Mg, Ca), and x=0.25 for Zn,-,M',(OH),+ (M'=Al, Cr) and Col-,A1,(OH),+.It was necessary to carry out the prep- arations under an N2 atmosphere (using Schlenk procedures) in order to exclude air, as the carbonate form of these LDHs form preferentially in the presence of CO,. A typical synthesis, of the PAA-Mg-Al-LDH nanocomposite, is described. The polymer (2.50 g) was dissolved in 100 ml of deaerated water in a three-necked flask and solutions of Mg(NO,), *6H,O (3.16 g, 0.012 mol), Al(NO3), -9H20 (2.34 g, 0.0062 mol) and NaOH (3.00 g, 0.075 mol) were added simultaneously under vigorous stirring. PAA is generally prepared under alkaline conditions, and is stable in aqueous base.35 The resulting precipitate was aged at 65 "C for 24 h and then filtered, washed several times with hot water to remove excess polymer, and dried under vacuum for 24 h.The nanocomposite is stable in air once formed. Other nanocomposites were prepared by a similar route from nitrate precursors. The Zn-Cr-PVS-LDH nanocomposite was prepared by dis- persing ZnO (3.03 g, 0.038 mol) in 100 ml of a deaerated aqueous solution containing 1.000 g of dissolved polymer. A solution of CrC1, (4.27 g, 0.019 mol in 50 ml water) was added slowly, and the precipitate was isolated and dried as described above. Characterization X-Ray powder diffraction (XRD) data were collected on a Siemens D5000 powder diffractometer, using Cu-Ka radiation, at 28 =0.02" s-'between 2 and 70". IR spectra were recorded on samples pressed into KBr disks using a Nicolet 510P FTIR spectrometer (resolution =2 ern- ', 100 scans averaged).A spectrum of pure KBr was collected for background correction. The morphology and microstructure of the samples were examined using an AMRAY lO00A scanning electron micro- scope. Thermal analyses of powdered samples (10-20 mg) were carried out using Shimadzu TGA-50 and DSC-50 instruments at 10°C min-' in flowing air or N, (50ml min-'). Carbon, hydrogen and nitrogen elemental analyses were carried out by Desert Analytics (Tuscon, AZ). Sulfur analyses were found to be unreliable due to the formation of incombustible sulfate salts. Results and Discussion Air must be carefully excluded (and solutions deaerated) prior to nanocomposite synthesis in order to avoid incorporation of carbonate ions into the product.The preferential accommo- dation of carbonate is readily explained as a result of the favourable lattice stabilization enthalpy associated with the small, highly charged C03,- anions, and is well known to cause difficulties in preparing LDHs with singly charged anions such as hydroxide and nitrate. Attempted syntheses of the polymer-LDH nanocomposites in air always resulted in the carbonate form with no evidence of polymer incorporation. Once prepared under N,, however, the polymer-containing nanocomposites are air stable. As a test, some nanocomposites were stirred in an aqueous solution of sodium carbonate for 1 week, with no evidence of exchange of polymeric anion for carbonate.The kinetic stability of the nanocomposites should derive from the slow diffusion of macromolecules within the galleries once formed. XRD patterns for Mg4A12(OH)12C03*nH,O and some nanocomposites are shown in Fig. 1 and 2, and the XRD data obtained are summarized in Table 1. The products described in Table 1 all exhibit only a single phase in the XRD patterns. The formation of nanocomposite products is demonstrated principally by the lack of peaks associated with the carbonate phase, and the appearance instead of a phase with an increased basal-plane repeat distance. The c repeat distance for Table 1 XRD data for LDH carbonates and nanocomposites including anionic polymers product M2' /M3' c repeat distance/A Ad/A" domain size/A Mg6A12(OH),&03 .nHzO 3 7.63 2.83 890 Mg4A12(OH),,C0, * nH20 2 7.63 2.83 940 Mg- A1-PSS-LDH 2 20.8 16.0 120 Mg- A1-PVS-LDH 2 13.1 8.3 120 Mg- A1-PAA-LDH 2 12.0 7.2 90 Ca-A1-C0,-LDH 2 7.62 2.82 - Ca- A1-PSS-LDH 2 19.6 14.8 160 Ca- A1-PVS-LDH 2 13.2 8.4 200 Ca- A1-PAA-LDH 2 12.4 7.6 290 Zn-A1-C03-LDH 3 7.65 2.85 3 70 Zn-Al-PSS-LDH 3 21.6 16.8 110 Zn-A1-PVS-LDH 3 13.3 8.5 100 Zn- A1-PAA-LDH 3 12.4 8.6 90 Zn- A1-PVS-LDH 3 13.0 8.2 130 CO- A1-PVS-LDH 3 13.3 8.5 130 a Ad is the gallery height taken as Ad =c repeat distance- brucite layer thickness (4.80 A).104 J. Muter. Chem., 1996, 6(l), 103-107 20.4 A Fig.1 Powder XRD patterns for Mg-Al-LDH containing (a) carbon-ate, (b) poly(acrylate), (c) poly(vinylsu1fonate) and (d) poly(styrene-sulfonate) ,7----I 4 6 8 io iz 14 16 is' '20 izT 28/degrees Fig. 2 Powder XRD patterns for (a) Mg-Al-PSS-LDH, (b) Ca-A1-PSS-LDH and (c) Zn-Al-PSS-LDH Mg,Al,(OH),,CO, *nH,O is 7.63 A,and the products in Fig. 1 have repeat distances of 12.0 (Mg-Al-PAA-LDH), 13.1 (Mg-Al-PVS-LDH) an! 20.8 A (Mg-Al-PSS-LDH). Allowing for a thickness of 4.8 A for the brucite-like LDH sheets, these distances correspond to galleries with polymer layer dimen-sions along the c axis of 7.2, 8.3 and 16.0A for the PAA-, PVS-and PSS-containing nanocomposite!, respectively. These can be compared with a distance of 2.8 A for the carbonate-containing phases.The dimensions obtained are consistent with those expected for the incorporation of bilayers of anionic polymers between LDH sheets. The c repeat distances for PAA and pOSS can be compared with those reportedofor acrylate (13.8 A),, and toluene-p-sulfonate (17.5-17.8 A)37q38 inter-calated into LDHs. Although details of the polymer confor-mation cannot be determined from the XRD data, a model for the arrangement of PVS within the intersheet galleries is shown in Fig. 3. The arrangement represented has anionic substituents oriented towards the LDH layers to maximize electrostatic LDH A+ + + + 13.1A + + + LDH Fig. 3 Schematic model for a bilayer packing of poly(vinylsu1fonate) in the LDH interlayer space attractions, and allows for the hydrophobic/hydrophilic bilayer separation favourable in many organic-pillared LDHs and other organoclays.This model may be generalized for the other polymer-LDH nanocomposites obtained. Reactions where the limiting reagent is PSS do not appear to form a related monolayer structure, instead mixtures of the bilayer phase and the LDH hydroxide are observed in XRD patterns of the products. The XRD pattern for Mg,A12(0H)12C03*nH,O exhibits a relatively sharp set of (001) reflections, indicative of a long-range ordering in the stacking dimension. The analogous peaks for the nanocomposites are broader, indicating a less organized stacking arrangement. The extent of peak broadening is similar 110 100 90 I T ' ' I " ' ' I " " 3000 2500 2000 1500 1000 50( wavenumberkm-' Fig.4 FTIR spectra for (a) M&A12(0H),,C03 *nH,O, (b) Mg-A1-PAA-LDH, (c) Mg-A1-PSS-LDH and (d) Mg-Al-PVS-LDH x (6) 3 100 10.1 5 0 E k --.o.oj0.080 .-0.1 4 60 -0.2 -0.3 40 Icci-0-3 J-0.4 0 500 1000 T/"C Fig. 5 TG-DrTG profiles for (a) Mg,Al,(OH),,CO, .nH,O and (b) Mg-A1-PSS-LDH J. Muter. Chem., 1996, 6(l), 103-107 105 with Mg, Ca and Zn-A1-LDH nanocomposites (Fig. 2). Domain sizes are obtained (Table 1) using the Scherrer relationship, d =0.9A/(D cos O), where d is the coherence length for crystal ordering, A is the X-ray wavelength, D is the peak width (in radians) at half height, and 8 is the diffraction angle.IR spectra of Mg4A12(OH),,C0, .nH,O and related nano- composites are provided in Fig.4. The strong peak at 1373 cm-l [Fig. 4(a)] is attributed to the v,(asym) stretching mode of the carbonate anion.39 The absence of both v3 peak splitting and a v1 peak near 1064cm-' also indicate that the C032- resides in a high symmetry (D3h)site. Peaks at 660 and 428 cm-l are associated with M-0 stretching modes in the LDH sheets. The nanocomposites [Fig. 4( b)-(d)] exhibit IR peaks characteristic of both the polymers and the LDH sheets. For example, the IR spectra of PVS-Mg-Al-LDH and PSS-Mg-Al-LDH both contain sharp peaks at 1040 cm-l and broader peaks at 1196 cm-' which are characteristic of S-0 vibrations in RSO, .40 The PAA-Mg-Al-LDH nanocomposite shows peaks characteristic of RC0,-at 1455 and 1566 cm-1.41 The absence or small size of carbonate-related peaks in the nanocomposite spectra provides evidence that the predominant anionic species incorporated between LDH sheets are the CHN elemental analyses and derived polymer contents for the Mg-Al-polyanion nanocomposites are provided in Table 2.The polymer content is established by assuming that carbon arises only from nanocomposited polymer, i.e. no residual carbonate content, which will tend to produce an overestimate of the polymer content. In all analyses, the N content was 0.03-0.06 mass(%), placing the maximum nitrate content at less than 1mol% of the anions incorporated. TG and derivative TG (DrTG) traces for Mg4A1,(OH),,C03 -nH,O and the Mg-Al-PSS-LDH nano-composite are shown in Fig.5. Mg4Al,(OH)12C03 *nH,O shows two mass loss events. The first, corresponding to 15% loss between 50 and 210 "C, has been attributed to elimination of both surface adsorbed water and interlayer water molecules. The second event, approximately 28% loss from 300-450 "C, derives from both C02 loss from the decomposition of carbon- ate and water loss by dehydroxylation of the brucite layers.' The total mass loss, 43.5%, provides a water content of n=2.8 based on complete conversion to the metal oxides, which agrees well with previous estimates of nw2-4 for air- dried samp1es.l In contrast, three events are observed in the thermal trace obtained on the Mg-A1-PSS-LDH nanocomposites.The first polymers. nancomposite Mg- A1-PSS-LDH Mg-Al-PVS-LDH Mg-A1-PAA-LDH Table 2 Elemental analyses and derived stoichiometries for the Mg-AI-LDH series mass% C H N polymer 24.63 4.64 0.03 47.0 7.43 4.24 0.06 33.1 14.12 4.61 0.05 27.9 empirical formula Mg2A1(OH),[CH2CH(C6H4S03)] * 1.6 H20 Mg,Al(OH), [CH2CHS03] * 2.2 H20 Mg,AI(OH),[CH,CHCO,] -0.4H20 Fig. 6 SEM images (3000x magnification) for (a) M&A12(OH),,C03 -nH,O, (b) Mg-Al-PAA-LDH, (c) Mg-A1-PVS-LDH and (d) Mg-Al-PSS-LDH 106 J. Muter. Chem., 1996, 6(l), 103-107 event, a 15% loss completed by ca. 100 "C, should correspond to the elimination of adsorbed water at the surface and between LDH layers. A second loss from 300 to 500°C (23%) is ascribed to the dehydroxylation of the LDH layers and partial decomposition of the polymer.After heating to 500"C, the sample colour changes from white to black, indicating that the polymer has begun to carbonize. The material obtained by heating to 500°C is also amorphous in the XRD analysis, which suggests a disruption of the brucite-like sheet structure due to intrasheet dehydroxylation. The final mass loss is observed at approx. 800 "C (22%) and is ascribed to complete oxidative elimination of the carbonaceous residue derived from the initial polymer degradation. The mixed oxide recovered after heating above 900 "C is white, confirming the low carbon content in the final product, and exhibits XRD reflections due to both MgA120, and MgO.Quantitative conversion of the starting material to metal oxides can be written as: where n is taken as ca. 4 to provide an H20 content near 15 mass% for the starting composition, and 'Mg3A103.5' reflects the overall stoichiometry of a mixture of spinel (MgAl,O,) and MgO. The calculated total mass loss of 68% is greater than that observed (6O%), which allows that a small quantity of sulfate is formed during thermolysis. A previous study on poly(viny1 al~ohol)-Ca-Al-LDH~~ pro-vides interesting observations on the morphology of products obtained by thermolysis and indicates an enhanced thermal stability associated with the nanocomposite over the parent phases. The morphology of catalysts derived from pyrolysis of LDH structures is also of considerable interest.' We have examined in some detail the thermal characteristics of the nanocomposites described above and evaluated the microstruc- tures of the thermolysis products, and will report these results separately.,, SEM images indicating the microstructures of Mg,Al,(OH),,C03 *nH20 and the three Mg-Al-LDH nano- composite powders are shown in Fig.6. The more crystalline carbonate phase [Fig. 6(u)] shows an aggregate of submicro- metre-sized particles that are estimated to be 0.1-0.3 pm in diameter under higher magnification. Some of these particles have aggregated into a larger platey mass, and a similar fine microstructure is evident on the surface of these aggregates. In contrast, all the nanocomposites [Fig.6(b)-(d)] show only large (>5 pm) platey aggregates with no observable structural details of submicrometre dimension. Conclusions We have described the synthesis and characterization of several new nanostructural materials based on Mg,A1,(OH)12C03 nHzO and related LDH sheet structures and the vinyl ionomers poly(acry1ic acid), poly(vinylsu1fonate) and poly(styrenesu1fonate). The products are obtained under air-free conditions by a template reaction where the LDH sheets are grown in a solution containing the desired polymer. The nanocomposi!es contain the LDH sheet structures separ- ated by 7.2-16.0 A, which is consistent with polymer bilayers between the sheets. These results broaden the application of the template method, indicating that the method is generally suitable for the preparation of nanocomposites between LDH and anionic polymers.The authors gratefully acknowledge supporting grant DMR- 9322071 from the National Science Foundation. References 1 F. Cavani, F. Trifiro and A. Vaccari, Catal. Today, 1992, 11, 173 and references therein. 2 W. T. Reichle, Chemtech, 1986,16, 58. 3 W. T. 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Paper 5/04853B;Received 24th July, 1995 J. Muter. Chem., 1996, 6(l), 103-107 107