Polymer International Polym Int 48 :277–282 (1999)

Acrylate intercalation and in situpolymerization in iron substituted nickelhydroxidesSte� phanie Rey, Jos efa Me� rida-Robles , Kyoo-Seung Han, LilianeGuerlou-Demourgues , Claude Delmas and Etienne Duguet*Ins titut de Chimie de la de Bordeaux (UPR 9048 CNRS) &Ecole Nationale de Chimie et de Phys ique deMatiere Condens e� e Supe� rieureBordeaux, avenue du Docteur Albert Schweitzer , F-33 608 Pes s ac Cedex, France

Abstract : Acrylic monomers and polymers have been intercalated in the lamellar structure of an iron

substituted nickel layered double hydroxide (LDH) (Ni:Fe = 7:3). The synthesis consists of an original

step-by-step soft chemistry preparation route in which the formation of the slabs is dis-Ni0.7

Fe0.3

O2

sociated from intercalation stage of anionic species. Acrylate, methacrylate and 4-pentenoate anions

have been intercalated and the interslab distance of the corresponding nanocomposite materials

reaches up to with 4-pentenoate. In the case of acrylate intercalated materials, the use of free-13.6 A�radical initiated polymerization conditions led to layered materials with interslab distances close to

that obtained through direct intercalation of a preformed poly(acrylic acid-co-sodium acrylate)

Some sulphate anions originating from the potassium persulphate initiator are simulta-(M1w

= 5000).

neously intercalated by anionic exchange.

1999 Society of Chemical Industry(

Keywords: acrylate monomer ; soft chemistry ; hybrid organic–inorganic materials ; intercalation; layereddouble hydroxides ; nanocomposites ; polymerization

INTRODUCTION

The preparation of nanocomposite materials basedupon the alternation of inorganic and organic layersis currently receiving considerable attention, due totheir potential use as catalysts, non-linear opticalmaterials, electronic devices and high strength com-posite materials.1 In particular, the intercalationof organic polymers is of great interest from theviewpoint of both intercalation chemistry andpolymer science. The layered inorganic hosts mainlyinvestigated are phyllosilicates,2h6 metal dichal-cogenides (with M \ Mo, Ti),7h9 ironMS2oxychloride FeOCl,10 vanadium pentoxidegels and molybdenum oxideV2O5 · nH2O,11h13

The most important methods of inser-MoO3 .14,15tion of polymers between inorganic sheets are pre-cipitation of mineral slabs, or restacking afterexfoliation in a solution of polymer,7,8 direct inter-calation of preformed macromolecules,2,3,6,11,12,14intragallery polymerization of intercalated mono-mer4,9,15h17 and redox intercalative polymerization(RIP),5,10,13,18 during which the intercalationprocess is accompanied by spontaneous mineralphase reduction and oxidative polymerization of themonomers.

In layered double hydroxides (M-L-LDHs)each slab has a[M1~y

II LyIII(OH)2]y`X

y@nn~(H2O)z,

brucite-type structure, made of edge-sharingand octahedra to form inüniteM(OH)6 L(OH)6

sheets, which are stacked on top of each other. Thepartial substitution of MII by L III generates an excesspositive charge in the slabs and allows thesematerials to be regarded as good host compounds foranionic interlayer guests Xn~. The amount of inter-calated anions is theoretically directly related to itsown negative charge (n) and to the concentration ofL trivalent cations within the slabs (y).

Organic polymer/LDH nanocomposites have beenmore rarely reported19h25 and their preparationroutes are based upon the LDH anionic exchangeproperties, or their ability to be precipitated frommetal nitrate–salt precursors in the presence of dis-solved anionic species.

The anionic exchange route has been explored byKato and co-workers, who have studied the acrylateintercalation in nitrate pre-intercalated Mg-Al-LDHand its subsequent polymerization by thermal treat-ment in the presence of potassium persulphate.19Schwarz and co-workers have used the sameapproach with 4-styrenesulphonate anions andreconstructed hydroxide Mg-Al-LDH.20

The co-precipitation method has been studied byseveral authors. Messersmith and Stupp21h23 pre-pared poly(vinyl alcohol)/Ca-Al-LDH nanocompo-

* Corres pondence to : Ins titut de Chimie de laEtienne Duguet,

Matiere Condens e� e de Bordeaux (UPR 9048 CNRS), avenue duDocteur Albert Schweitzer, F-33608 Pes s ac Cedex, France.

(Received 28 May 1998; accepted 12 October 1998)

( 1999 Society of Chemical Industry. Polym Int 0959-8103/99/$17.50 277

S Rey et al

Figure 1. Scheme of the s ucces s ive reaction s teps involved in the

s oft chemis try preparation of LDHs .

sites by nucleation and growth of calcium aluminatelayers from homogeneous solutions containingmacromolecules as co-solutes. The layered structureof the nanocomposite was found to be stable up to400¡C, whereas pure LDH, lacking organic material,decomposed at 125¡C.24 Lastly, Lerner and co-workers25 have used this method for accommodatingpoly(acrylic acid), poly(vinylsulphonate) or poly-(styrenesulphonate) bilayers between diþerent LDHslabs.

Five years ago, a new preparation method ofLDHs was proposed in our laboratory which consistsof two main steps, during which formation of the

slabs (synthesis of throughM1~yL

yO2 NaM1~y

LyO2

a high temperature solid state reaction) is dissociatedfrom anion intercalation (soft chemistry redoxreactions) (Fig 1).26 The driving force for inter-calation is the compensation of the excess positivecharge brought by FeIII ions during the reduction ofmixed nickel–iron c-oxyhydroxide in the presence ofan outer anionic species Xn~. Such a method allowsthe synthesis of well-crystallized and chemicallywell-deüned LDHs, and has been successfully usedfor the intercalation of several mineral26 andvanadate27 anions in Ni-Co-LDHs.

The compositional and structural modiücations ofthe materials involved during the new soft chemistrypreparation (Fig 1) have been intensively discussedin previous papers.26,28,29 The aim of this study is todemonstrate that this original method may beapplied to the preparation of new organic–inorganichybrid materials, based upon Ni-Fe-LDHstructure (Ni : Fe \ 7 : 3). Organic monomers, suchas acrylic and methacrylic acids, have beenchosen because of their ability to be partiallyneutralized by strong bases and converted intoanionic species. The intercalation of the parentpolymer [poly(acrylic acid)] has also been studied.Moreover, the in situ free-radical polymerization ofacrylic monomers within interslab space has beencarried out.

EXPERIMENTAL

Materials

The inorganic starting materials were analyticalreagent grade. Acrylic acid, methacrylic acid(Prolabo), 4-penteno•�c acid and poly(acrylic acid)

(Aldrich Chemical Co) were usedM1 w \ 5000without further puriücation. Deionized and decarb-onated water was used in all experiments.

Synthesis by a soft chemistry route of intercalated

Ni-Fe-LDH materials

The preparation procedure consists of three suc-cessive steps as previously described28 (Fig 1): (i)building of slabs via preparation ofNi0.70Fe0.30O2

sodium nickelate by a high tem-NaNi0.70Fe0.30O2perature solid state reaction; (ii) oxidizing hydrolysis(by adding 5M KOH and 4M NaClO solutions) ofsodium nickelate, leading to layered c-oxyhydroxide

and (iii)(H0.20Na0.10K0.20(H2O)0.50Ni0.70Fe0.30O2);reduction by adding a hydrogen peroxide solution inthe presence of organic (poly)anions. This last step isprobably the most difficult one. In a typical synthe-sis, the organic (poly)anion solution was prepared bypartial neutralization with 2M NaOH solution of the0.3N parent acid solution to pH 4.3. This solutionwas reýuxed under a stream of argon for 2h in orderto avoid the presence of carbonate ions in the alkalinesolution. After cooling, 2g of c-oxyhydroxide phasewas dispersed in the solution and an excess of H2O2(3%) was added dropwise at room temperature understirring. The suspension was aged for 1h underargon and then centrifuged. Finally, the solid wasdried under vacuum at 50¡C for 15h. The resultingmaterials are named Ni-Fe-LDH(X), where X is theinserted anionic species.

Intragallery polymerization of acrylate monomers in

Ni-Fe-LDH(acrylate)

Ni-Fe-LDH(acrylate) phase (250mg) was dispersedin 100ml of 0.0037M potassium persulphate solu-tion. The reaction was carried out at 60¡C for 24hunder argon. The suspension was then centrifugedand the solid dried under vacuum at 50¡C for 15h.

Characterization techniques

The X-ray diþraction (XRD) patterns were recordedon a PW 1820 Philips diþractometer equipped with acopper anticathode, in steps of 0.02¡ between 3 and70¡. Thermogravimetric experiments (Setaram MTB10–8) on about 20mg of material were performed instatic air at a ramp rate of 240¡Ch~1 to 600¡C. TheFTIR spectra were obtained using a Perkin–ElmerParagon 1000 spectrophotometer. The materialswere dispersed and gently ground in a few drops ofacetone, placed on a CsI plate and analysed by trans-mission after acetone evaporation.

278 Polym Int 48 :277–282 (1999)

Iron substituted nickel hydroxides

RESULTS AND DISCUSSION

Acrylate intercalation in Ni-Fe-LDH

A preliminary experiment consisted of verifying thathydrogen peroxide is not able to initiate the poly-merization of acrylic acid or sodium acrylate mono-mers under reduction conditions of the soft chemistrylast step. Thus, this reaction was carried out withouta c-oxyhydroxide phase and, after 2h at pH 4.3 andambient temperature, no polymer was formed, asveriüed by infrared spectroscopy and an attemptedprecipitation of the supposed polymer in acetone.

The X-ray diþraction pattern of Ni-Fe-LDHwith intercalated acrylate anions is displayed in Fig2, curve A. The presence of two large and asym-metric ‘bands’ in the 33–45¡ and 60–65¡ (2hCu)ranges is characteristic of some disorder in the stack-ing of the slabs. Such a phenomenon is very prob-ably related to local distortions within the slabs, asalready demonstrated in Ni-Co- or toLDH(VO3),28a turbostratic eþect induced by both the large inter-slab distance and the weak bonding interactionsbetween the interlayer species and the host lattice. Asa consequence, a complete indexation of the X-raydiþraction pattern is not possible. Nevertheless, theürst diþraction peak (001) corresponds to a basalspacing of which can be compared to the13.6A� ,value of reported by Kato and co-workers.1913.4A�This large interslab distance indicates the inter-calation of acrylate anions into the LDH interslabspace. Allowing for a thickness of for the4.6A�brucite-like LDH slabs, this distance corresponds togalleries with an acrylate layer dimension along the caxis of This value is larger than the length of9.0A� .acrylate anions, which is estimated to be about 6A� .

The infrared spectrum of the LDH(acrylate)materials conürms the presence of intercalated acry-late anions (Fig 3, curve A). The assignments areconsistent with those of sodium acrylate, as pre-viously described:30 (1634cm~1),lC/C lCO2

(as)(1538cm~1), and scissors (1423cm~1),lCO2

(s) CH2CH bending (1369 and 832cm~1), (1274cm~1),lC~C

Figure 2. X-ray diffraction patterns of : A, LDH(acrylate) ; B,

LDH(methacrylate) ; C, LDH(4-pentenoate) ; D, LDH(polyacrylate)

prepared by the s oft chemis try method peaks due to[L,aluminium carrier-s ample of the X-ray diffractometer ; peaks+,due to LDH(CO

3)].

Figure 3. IR s pectra of : A, LDH(acrylate) ; B, LDH(polyacrylate)

obtained by direct intercalation of a preformed polymerM1w

\C, LDH(polyacrylate) obtained by in s itu acrylate5000;

polymerization of LDH(acrylate).

rocking, twisting and wagging (1059, 990 andCH2962cm~1) and rocking and wagging (659 andCO2523cm~1). The slight shifts, observed for somebands, probably result from the constrained environ-ment of the interslab space and would deserve acareful spectroscopic study.

In order to verify the dependence of the basalspacing upon the size of the intercalated anions,larger similar species were intercalated: methacrylateand 4-pentenoate anions.

Figure 2, curves B and C show the X-ray diþrac-tion patterns of LDH(methacrylate) and LDH(4-pentenoate). They are similar to the previousLDH(acrylate) curve, but show an increase of theinterslab distance with a respective basal spacing of

and The variation of the interslab dis-13.9A� 18.2A� .tance can be directly related to the size of the carbox-

Polym Int 48 :277–282 (1999) 279

S Rey et al

ylate anion present within the interslab space.Although details of the carboxylate anion conforma-tion between the layers cannot be determined fromX-ray diþraction data, a calculation based on com-puter simulation and the construction of molecularmodels have led to measurements of the steric hin-drance and to determination of the longer andshorter dimensions of the anions. As shown in Fig 4,the interslab distances are systematically larger thanthe longer dimension of the organic guest species.This phenomenon is not observed with anions suchas carbonate, chloride or sulphate, prepared undersimilar conditions from appropriate sodium-saltsolutions. In those cases, the thickness of the inter-slab space is directly related to the anion steric hin-drance, and the interslab distance is approximatelythe sum of the thickness of a slabNi0.7Fe0.3(OH)2and that of the inserted species. The particular caseof intercalated sulphate anions, the interslab thick-ness of which is smaller than the shorterSO42~dimension, has already been observed and dis-cussed.26

Thus, the organic anions are probably orientedperpendicularly to the LDH slab. The carboxylategroups are placed facing the inorganic slabs in thevicinity of FeIII ions. Therefore, the basal spacingsresult from the balance of the electrostatic attractionsbetween the slabs and the electrostatic repulsionbetween CxC double bonds.

Air was carefully excluded and water decarbonat-ed, before nanocomposite synthesis in order to avoidincorporation of carbonate ions into the LDHs.Nevertheless, in all cases, the coexistence of twoLDH phases is observed: the major one has beendiscussed above and is related to the various carbox-ylate intercalated anions ; the second one, with ashorter interlayer distance of corresponds to7.8A� ,the presence of carbonate anions in some inter-lamellar spaces.31 The total exclusion of carbonate

Figure 4. Evolution of LDH inters lab dis tances vers us gues t s teric

hindrance (the diagonal line corres ponds to inters lab dis tances

which would be exactly adapted to the s ize of the gues t s pecies ).

from the interslab space of LDHs is known to bedifficult, due to its preferential accommodation,which can be readily explained as a result of thefavourable lattice stabilization enthalpy associatedwith the small and highly charged anions.19,25CO32~

Polyacrylate intercalation in Ni-Fe-LDH

Figure 2, curve D represents the X-ray diþractionpattern of a similar LDH material intercalated bypolyacrylate with Under the conditionsM1 w \ 5000.of the reaction (pH 4.3), the precursor poly(acrylicacid) is partially neutralized and consists of(pKa 5.1)a statistical copolymer with the average formulaw(CH2wCHCOOH)0.7w(CH2wCHCOO~)0.3w.The basal spacing is and is thinner than in the12.6A�LDH(acrylate) intercalate This phenome-(13.6A� ).non may be explained by the absence of electrostaticrepulsions between CxC double bonds. In the caseof the incorporation of poly(acrylic acid) orpoly(vinylsulphonate) in LDH by Lerner and co-workers,25 similar interslab distances have beenreported. According to the authors, these dimensionsare consistent with the arrangement of bilayer poly-mers.

The infrared spectrum of the LDH(polyacrylate)phase conürms the presence of intercalated polymerchains (Fig 3, curve B). The assignments are consis-tent with those of sodium poly(acrylate) as quoted onthe spectrum.

The intercalation of such macromolecules provesfor the very ürst time that soft chemistry redox reac-tions are able to insert large guest species.

In situ polymerization of intercalated acrylate

monomers in Ni-Fe-LDH

The interlayer space of LDHs has been used as anoriginal nano-reactor for the in situ free-radical poly-merization of acrylate. The water-soluble initiatorwas potassium persulphate, the dissociation of whichis described in the following reaction scheme:

O O O> > > *`OwSwOwOwSwO` —— Õ 2`OwS wOÕ> > >O O O

Figure 5 compares the XRD patterns before andafter polymerization of the LDH(acrylate) inter-calate. After polymerization, the basal spacing of theLDH(acrylate) decreases from 13.6 to and is12.6A�consistent with that of LDH(polyacrylate) obtainedby direct intercalation of the polymer. However, nearthe main peak at another one appears at12.6A� 9.0A� ,which is not present for the initial LDH(acrylate)intercalate and seems to take the place of the mainpeak of parasitic The value of this inter-LDH(CO3).slab distance and the inventory of anions(9.0A� )likely to be present in the solution, allow the pres-ence of an phase to be suggested, mainlyLDH(SO4)obtained by anionic exchange of carbonate anionsduring free-radical polymerization.

280 Polym Int 48 :277–282 (1999)

Iron substituted nickel hydroxides

Figure 5. Typical evolution of the X-ray diffraction pattern of

LDH(acrylate) before and after polymerization peak due to[+,LDH(CO

3)].

Infrared spectroscopy investigation after poly-merization shows the disappearance of the bandsassociated with the CxC double bonds (Fig 3, curveC) and conürms the chain polymerization of co-intercalated acrylate monomers. Moreover, the pres-ence of a large band at approximately 1100cm~1attests to the presence of intercalated sulphate anions

mode).26(l3In order to verify the ability of carbonate anions to

be exchanged by sulphate anions, a LDH(CO3)intercalate was dispersed in a potassium persulphatesolution, and the mixture was aged at 60¡C for 24hunder argon (acrylate polymerization conditions).X-ray diþraction patterns show that carbonateanions are readily exchanged by sulphate anions, assuggested by the increase of the basal spacing to

which corresponds to that of8.50A� , LDH(SO4)(Fig 6).

Figure 7 compares the thermogravimetric curvesof LDH(acrylate) before and after polymerization.The shapes of the curves are similar but slightly dif-ferent. The ürst stage of weight loss has been attrib-uted to the removal of water adsorbed on both sur-faces and interlayer water molecules. The secondstage of weight loss derives from both water loss bydehydroxylation of the brucite layers, and lossCO2from the decomposition of acrylate or polyacrylate(and parasitic carbonate) inserted in LDH slabs.

Figure 6. Evolution of the X-ray diffraction pattern of inLDH(CO3)

the pres ence of potas s ium pers ulphate under free-radical

polymerization conditions (here diffractograms were recorded in

s teps of 0.1¡).

These last decompositions are completed at about300¡C for LDH(acrylate) and at about 375¡C forLDH(polyacrylate) obtained by in situ poly-merization. On further heating, a decomposition ofthe material occurs, resulting in the formation ofNiO and The weight losses for both inter-Fe2O3 .calated compounds before and after polymerizationare quite similar, indicating that there is no greatchange in their chemical formula before and afterpolymerization. The delay during decomposition ofLDH after polymerization is additional proof of thepresence of intercalated macromolecules.

Figure 7. Thermogravimetric profiles of LDH(acrylate) before and

after polymerization, s howing changes .

Polym Int 48 :277–282 (1999) 281

S Rey et al

All these results show that sulphate anion inter-calation is possible and lead to the supposition thatone step of the intragallery free-radical poly-merization of acrylate monomers consists of inter-calating the initiator species. Nevertheless, furtherexperiments will have to be undertaken to determinewhether the persulphate dissociation step occursbefore, after or simultaneously to its intercalation.

CONCLUSIONS

The feasibility of intercalation of diþerent car-boxylate anions in Ni-Fe-LDH using a new softchemistry preparation route has been demonstrated.The nanocomposite materials contain LDH slabsseparated by with 4-pentenoate. The inter-13.6A�calation of macromolecules was also successfully per-formed by such a redox route. Moreover, it wasshown that an additional step may consist of poly-merizing intercalated monomers by using a free-radical initiator. The results of X-ray diþraction,infrared spectroscopy and thermogravimetry experi-ments are consistent with the in situ polymerizationof intercalated monomers.

Attempts are currently underway to achieve eradi-cation of some parasitic carbonate intercalation,before studying the inýuence of the Ni :Fe ratio uponthe density of intercalated monomers and its conse-quences for the polymerization mechanism. Thedesintercalation of the macromolecules (by acidicdissolution of inorganic slabs) is also studied in orderto determine their molar mass and tacticity. Lastly,intercalation and in situ polymerization of bifunc-tional monomers would allow synthesis of originaltwo dimensional macromolecules.

ACKNOWLEDGEMENTS

The authors thank Bruno Delatouche, Cathy Denageand Ma•� te� Basterreix for the synthesis of the initialmaterials, and Joe� l Villot and Jean-Pierre Cazorla fortechnical help.

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