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S ol - Gel Der ived Urea Cross-Linked Organic a l lyModif ied Si l ica tes . 1 . Room Tempe ra ture Mid-Inf rared

Spec t ra

V. de Zea Bermudez,*, L. D. Car los, and L. Alcacer

Secc a o de Qu mica, Universidade de Tra s-os-Montes e Alto Douro, Quinta de Prados,Apartado 202, 5001 Vila R eal Codex, Portugal, Departam ento de F

sica, Un iversidade d e

Aveiro, 3810 Aveiro, Portugal, Departamento de Engenharia Qu m ica, Instituto S uperior T ecnico, Rua Rovisco Pais, 1092 Lisboa Codex, Portugal

Received May 21, 1998. Revised Man uscript Received October 22, 1998

Organic- inorgan ic hybrids prepa red by th e sol- gel process ar e investigated by infraredspectroscopy at room tempera tu re. The ma tr ix of th e so-calledureasils is a silica network t owhich oligopolyoxyethylene cha ins a re grafted by mean s of ur ea cross-links. Th e u reasilsprepareds U(2000), U(900), an d U(600)s are obtained by reacting three diamines (containingabout 40.5, 15.5, and 8.5 oxyethylene units, respectively) with 3-isocyanatepropyltriethox-ysilane. The mid-infrared spectra of the diamines are examined. The FTIR spectrum of U(2000) shows that the polyether chains of the parent diamine become less ordered uponincorporation into the inorganic backbone. The assignment of the absorption bands

originating from the urea moieties in the ureasils is proposed. Spectroscopic data revealth at t he nu mber of oxyeth ylene un its present a ffects dra mat ically the a mide I and am ide IIbands and indicate tha t th e N- H groups of th e urea link age are involved in hydr ogen bondsof different strength. The existence of non-hydrogen-bonded urea groups and hydrogen-bonded urea- urea and urea- polyether associations is suggested. The format ion of ur ea-urea st ructu res is appa rent ly favored in U(600), whereas the n umber of freecarbonyl groupsis greatest in U (2000).

In t roduc t ion

Following Wrigh ts1 observations of ionically conduc-tive complexes formed with poly(oxyethylene) (POE,(OCH2CH2)n ) and sa lts, and t he r ecognition by Arman d

et al.2

that these polymer- salt complexes might workas suitable polymer electrolytes in all solid-state elec-trochemical devices, such a s ba tter ies, smart windowsand sensors, these unique ma teria ls have been the focusof an intense investigation in the past two decades.3

The motion of ions in polymer mat rices is a relat ivelynew concept; th e ma cromolecule itself acts a s a solventfor a salt that becomes partially dissociated in thematrix, leading to electrolyte behavior. The developmentof polymer- salt complexes has paralleled that of host-guest chemistry, exemplified by crown ethers whosepolymer ana logues ar e open ma cromolecular host str uc-

tures acting as polydentate l igands and containinpreferably oxyethylene r epeat u nits of the type OCH2-CH2, as i t is the case in POE for which the spacbetween t wo consecutive eth er oxygens is optimal coordination of metal cations. This hard polybasmolecule is able to solvat e not only most a lkali, alkaland rare earth cations, but also transition metalespecially in their+ 2 state.

The interest for lanthanide cation-containing polether s is quite recent.4- 19 New polymeric mater ials have

Univer sidade de Tr as-os-Mont es e Alto Douro.

Universidade de Aveiro. Inst itu to S uper ior Te cnico.(1) Wr ight, P. V.Br. Polym. J. 1975 , 7 , 319.(2) Arma nd, M.; Duclot, M. T.; Chaba gno, J. M. Proceedings of th e

Second International Meeting on Solid Electrolytes, St. Andrews,Scotland, 1978.

(3) (a)Polymer E lectrolyte Reviews s 1 ; MacCallum, J. R., Vincent,C. A., Eds.; Elsevier: London, 1987. (b)Electrochemical S cience an d Technology of Polymers 1 ; Linford, R. G., Ed.; Elsevier: London, 1987.(c) Polymer E lectrolyte Reviews s 2 ; MacCallum, J. R., Vincent, C. A.,Eds.; Elsevier: London, 1989. (d)Proceedings of the 2nd InternationalSym posium on Polymer Electrolytes ; Scrosati, B., Ed.; Elsevier AppliedScience: London, 1990; (e) Gray, F. M.S olid Polym er Electrolytes:fundamentals and technological applications ; VCH: New York, 1991.(f) Electrochemical S cience and Technology of Polymers 2 ; Linford, R.G., Ed.; Elsevier: London, 1991.

(4) H uq, R.; Farrington, G. C. InProceedings of the 2nd Interna-tional S ymp osium on Polym er Electrolytes ; Scrosati, B. Ed.; ElsevierApplied Science: London, 1990; p 281.

(5) Reis Machad o, A. S.; Alcacer, L. InProceedings of the 2nd International S ymp osium on Polymer Electrolytes ; Scrosati, B., Ed.;Elsevier Applied Science: London, 1990; p 283.

(6) Twomey, C. J.; Chen, S. H.J. Polym. Sci. B 1991 , 29 , 859.(7) Silva, C. J. R.; Smith, M. J .Solid S tate Ionics 1992 , 58 , 269.(8) Smith , M. J. R.; Silva, C. J.; Silva, M. M.Solid S tate Ionics 1993 ,

60 , 73.(9) Pet ersen, G.; Torell, L. M.; Pan ero, S.; Scrosati, B.; Silva, CR.; Smith, M. J .Solid S tate Ionics 1993 , 60 , 55.

(10) Silva, C. J. R.; Smith, M. J.Electrochim. Acta 1993 , 40 , 526.(11) (a) Bernson, A.; Lindgren, J .Solid S tate Ionics 1993 , 60 , 31.

(b) Bernson, A.; Lindgren, J.Solid S tate Ionics 1993 , 60 , 37.(12) (a) Carlos, L. D.; Assuncao, M.; Abr an tes , T. M.; Alcacer , L.

Solid State Ionics III , Materials Research Society Proceedings; NazriG.-A., Tara scon, J .-M., Arma nd, M. B., Eds.; MRS: Pittsbur gh, P1993. (b) Carlos, L. D.; Videira, A. L. L.Phys. Rev. 1994 , B49 , 11721.(c) Car los, L. D.; Videira, A. L. L.J. Chem. Phys. 1994 , 101 , 8827.

(13) Brodin, A.; Mattss on, B.; Torell, L.J. Chem. Phys. 1994 , 101 ,4621.

(14) (a) Carlos, L. D.; Videira, A. L. L.; Assunca o, M.; Alca cer , L.Electrochim. Acta 1995 , 40 , 2143. (b) Carlos, L. D.; Assunca o, M.;Alca cer , L.J. Mater. R es. 1995 , 10 , 202.

569Chem. Mater. 1999, 11, 569- 580

10.1021/cm980372v CCC: $18.00 1999 American Chemical SocietyPublished on Web 02/25/1999

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emerged in the past few years, and several groups havebecome involved worldwide in the research and develop-ment of luminescent polymeric systems for which el-egant technological applications including optical de-vices, such a s phosphors a nd solid-sta te lasers , may beforeseen.4,12- 15,17,19The desirable cha racteristics of plas-tic ma ter ials, in par ticular th eir easy processability intofilms, are thus combined with the optical propertiesprovided by th e luminescent center s (mostly Eu3+ and

Nd3+

cations). It was immediately recognized thoughtha t these mat erials, obtained by incorpora ting trivalentlanth anide salts int o POE, are a ssociated with two verysevere drawbacks: (1) they tend to crystallize, a phe-nomenon t hat considerably r educes optical qu ality; (2)their h ighly hygroscopic nature makes han dling in aglovebox compu lsory. Anoth er nu isan ce connected withthe POE-based systems is th e salting-out effect thatprevails at high salt concentrations.

To overcome these inconveniences, we decided tochange polymer architecture and to synthesize analo-gous luminescent polymers by using the sol- gel process.This chemical synthesis route is widely known aspotentially attractive for the preparation of novel ma-terials in many scientific fields. It relies on the greatversatility of the silicon chemistry resulting from therema rka ble stability of Si- O and Si- C bonds. Sol- geloffers subst an tial advant ages over other techniques: (1)the precursors used are highly reactive, pure, andhomogeneous; (2) the micro- and m acrostructur e of thehost matrix may be controlled through the optimizationof several parameters (e.g., water and/or alcohol content,tempera tu re, pressu re, type of catalyst, solvent ); (3) th epresence of the silica network provides simultaneouslygood mechan ical r esistance, extraordinary t herma l sta-bility, and a morph ous char acter ; (4) it is possible to graftorganic groups into the inorganic backbone at lowprocessing temperatures and to produce organic-inorgan ic hybrid mat erials designated as ormosils (or-ganically modified silicat es), ormolytes (organ icallymodified silicate electr olytes), or orm ocers (orga nicallymodified ceram ics), depending on th e fina l application;(5) elastomeric transpa ren t monolith s of variable thick-ness, as well as powders, may be prepared. The sol-gel route proceeds th rough th e hydrolysis a nd conden-sat ion of alkoxysilanes . After r emotion of the byproducts(water and alcohol), a rigid tridimensiona l network isformed. High-quality products may be obtained if specialcare is taken to prevent cracking during the dryingprocedure.

In t he second h alf of 1997, several europium organic-inorgan ic silicate hybrids were first introduced and th eirspectral char acteristics investigated.20,21 Our groupreported t he synthesis an d physical properties of a novelclass of inten sely luminescent Eu (III)-doped ormosils.20The sol- gel derived ormolytes proposed contain short,

highly solvating oxyethylene units grafted onto tinorganic network by means of urea bridges (- NHC-(d O)NH- ) and have been thus classed as ureasi l(ureasil icates).22

The practical relevance of the europium-based urasils is considerable. Apar t from th e fact t hat they aobta ined as amorph ous, slightly hygroscopic, and tr apar ent m onoliths, are th erma lly stable up to 250 C, aexhibit an ionic conductivity of about 10- 5 - 1 cm- 1 at

30 C, we ha ve shown th at these mat erials ar e mulwavelength phosphors (i.e., white-light emitters), for silica backbone itself is str ongly luminescent.20 Obvi-ously, th is optical feature is of the interest from tstandpoint of application in technological luminescedevices. In past years the light emission of highly porsilicon layers has been extensively explored.23- 29 A greatamount of effort has also been devoted t o the st udylight-emitting silicon-based materials with propertisimilar to those of porous silicon.29- 31 It is hoped thatthe luminescent pr operties of porous silicon an d relamaterials will contribute in the near future to thgrowth of optical communication devices directly silicon computer chips.28

The work present ed her e is the firs t of a ser ies of tpapers focused on the spectroscopic characterization

(15) (a) Ohno, H.; Yoshihara, H .Solid S tate Ionics 1995 , 80 , 251.(b) Brodin, A.; Matt sson, B.; Torell, A.; Torell, L. M.Electrochim Acta1995 , 40 (13- 14), 2393.

(16) Goncalves, A. R.; Silva, C. J. R.; Silva, M. M.; Smith, M. J.Ionics 1995 , 1 , 342.

(17) (a) Carlos, L. D.; Videira, A. L. L.J . Chem. Phys. 1996 , 105 ,8878. (b) Carlos, L. D.Solid S tate Ionics 1996 , 85 , 181.

(18) Bernson, A.; Lindgren, J.Solid State Ionics 1996 , 86 - 88 , 369.(19) (a) Carlos, L. D.; Videira, A. L. L.Chem. Phys. L ett. 1997 , 57 ,

264. (b) Ferr y, A.; Fu rlani, M.; Fran ke, A.; Jacobsson, P.; Mellander,B-E.J. Chem. Phys. 1998 , 109(7) , 2921.

(20) (a) de Zea Bermu dez, V.; Carlos, L. D.; Duart e, M. C.; SilM. M.; Silva, C. J .; Smith, M. J .; Assunca o, M. ; Alca cer , L.J. AlloysCompd . 1998 , 275 - 277 , 21. (b) Carlos, L. D.; de Zea Bermudez, V.;Duart e, M. C.; Silva, M. M.; Silva, C. J .; Smith, M. J .; Assunca o, M.;Alca cer , L.Physics and Chemistry of Lum inescent Materials VI ; Ronda,C., Welker, T., Eds.; The Electrochemical Society Proceedings 97-San Francisco, 1997; p 352.

(21) (a) Ribeiro, S. J. L.; Dahm ouche, K.; Ribeiro, C. A.; Sant illiV.; Pulcinelli, S. H.J . S ol - Gel Sci. Technol. , in pr ess. (b) Fra nville,A. C.; Zam bon, D.; Mahiou, R.; Chou, S.; Tr oin, Y.; Cousseins, J . CJ .Alloys Compd . 1998 , 275 - 277 , 831.

(22) (a) Armand, M.; Poinsignon, C.; Sanchez, J.-Y.; de Zea Bmudez, V.; Frenc Patent 91 11349, 1991. (b) De Zea Bermudez, V. PhThesis, University of Gren oble, Fra nce, 1992. (c) De Zea BermudV.; Baril, D.; Sanchez, J.-Y.; Armand, M.; Poinsignon, C.OpticalMaterials Technology for Energy Efficiency an d S olar Energy Conver-s ion XI: Chromogenics for Sm art Windows ; Hugot-Le Goff, A.,Granqvist, C.-G., Lampert, C. M., Eds.; P roceedings SPIE 17SPIE: Bellingham, Washington, 1992; p 180.

(23) Canham, L. T.Appl. Phys. L ett. 1990 , 57 , 1046.(24) (a) Chen, X.; Utt amchan dani, D.; Trager-Cowan, C.; ODonn

K. P. Semicond. Sci. T echnol. 1993 , 8 , 92. (b) Pifferi, A.; Taroni, P.;Torricelli, A.; Valentini, G.; Mut ti, P .; Guislotti, G.; Zanghieri, L.Appl.Phys. Lett. 1997 , 70 , 348.

(25) Fau chet, P. M.; Peng, C.; Tsybeskov, L.; Vandyshev, J.; DuboA.; Raisanen, A.; Orlowski, T. E.; Brillson, L. J .; Fouquet, J. Dexheimer, S. L.; Rehm, J . M.; McLendon, G. L.; Ett edgui, E.; GY.; Seifert, F.; Kurinec, S. K.Semiconductor Silicon/ 1994, 7thInternational Symnposium on Silicon Materials Science and Technol-ogy ; Huff, H. R., Bergholz, W., Sumino, K. Eds.; Electr ochem ical SociProceedings 94-10; San Francisco, 1994; p 499.

(26) (a) Cam eron, A.; Chen, X.; Trager -Cowan, C.; Utt am chandaD.; ODonnell, K. P.Optical Properties of Low Dim ensional S iliconStructures ; Bensah el, D. C., Can ha m, L. T., Ossicini, S., Eds.; Academ

Press: Amsterdam, 1993. (b) Ventur a, P . J .; Carmo, M. C.; ODonK. P. J.J. Appl. Phys. 1995 , 77 , 323.(27) (a) Brandt, M. S.; Fuchs, H. D.; Stut zmann, M.; Weber,

Cardona, M.Solid State Comm un. 1992 , 81 , 307. (b) Stut zmann, M.;Brandt , M. S.; Rosenbauer, M.; Weber, J.; Fuchs, H. D.Phys. Rev. B1993 , 47 , 4806.

(28) Brus, L. J.J. Phys. Chem. 1994 , 98 , 3575.(29) (a) Matsumoto, N.Semiconductor Silicon/ 1994, 7th Interna-

tional Symposium on Silicon Materials Science and Technology ; Hu ff,H. R., Bergholtz, W., Sumino, K., Eds.; E lectrochemical SocieProceedings 94-10: San Fra ncisco, 1994; p 461. (b) Matsu moto, Takeda, K.; Teramae, H .; Fujino, M.International Topical Workshopon Advan ces in Silicon-Based Polymer Science , Ha wai, 1987; Advancesin Chem istry S eries 224; American Chemical Society: Washington, D1990. (c) Miller, R. D.; Michl, J. J.J . Chem. Rev. 1989 , 89 , 1359.

(30) Fu rukawa, K.; Fujino, M.; Matsumoto, N.Macrom olecules1990 , 22 , 1697.

(31) (a) Furuka wa, K.; Fujino, M.; Matsumoto, N.Appl. Phys. Lett.1992 , 60 , 2744. (b) Heath, J . R.S cience 1992 , 258 , 1131.

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(1) the h ybrid n atur e of the u reasils, whose stru ctureinvolves flexible polyether chains trapped between rigidblocks of silica by mea ns of hard ur ea cross-links; (2)the well-kn own exceptiona lly strong self-as sociation of urea compounds originating from the presence in theurea moiety of three active centers capable of part ici-pating in h ighly directional hydrogen bonds.

Intimate mixing of two polymers at the molecularlevel is att ained if the intera ction tak es place between

a strongly self-associated polymer (e.g., polyurethanesand polyamides) and a weakly self-associated onecontaining donor sites, su ch as oxygen or nitrogenatoms, capable of forming strong hydrogen bonds.Polyethers belong to the last category. We believe thatthere is a great resemblance between the ureasils andthe extensively hydrogen-bonded polyurethane blockpolymers. The latter materials ar e kn own t o be com-posed of alterna ting low glass t ra nsition soft segments(generally polyethers or polyesters) and more rigid,polar uretha ne (- HNC(d O)O- ) hard segments formedfrom the extension of a diisocyanate (often aromatic)with low molecular weight diols. The donor s ites ar e th eN- H groups of the urethan e linka ges, and the hydr ogenbond acceptors may be either the hard urethane seg-ments (the carbonyl of the u reth an e groups) or t he softsegments (ester carbonyls or eth er oxygens).

Our analysis is mainly devoted t o the study of theinfluence of the oxyethylene chain length on the fre-quency and intensity of the ban ds ar ising from th e ureacross-links. The procedure adopted has been to closelyinspect these bands in terms of the absorption bandschara cteristic of the a mide stru cture (- HNC(d O)- ), inparticular the amide I, II, III, and V bands,68 and tocompare our experimental results with those widelyreferred for related materials.61- 67

E x p e r i m e n t a l S e c t i o n

Syn thes i s . In the preliminary stage of the preparation of the u reasils a covalent bond between th e alkoxysilane precur-sor (3-isocyanatepropyltriethoxysilane, ICPTES) and the oli-gopolyoxyethylene segment was formed by reacting the iso-cyanate group of the former compound with the terminalamine groups of doubly functional amines (R, -diaminepoly-(oxyethylene-co -oxypropylene)) in tetrahydrofuran, THF(Scheme 1). A urea cross-linked organic- inorganic hybridprecursors so-called ureapropyltriethoxysilane, UPTESs wa sthus obtained. The three diamines used in this study arecommer cially designat ed as J effamin e ED-2001 (witha + c )2.5 andb ) 40.5), J effamin e E D-900 (witha + c ) 2.5 andb) 15.5), and J effamin e ED-600 (witha + c ) 2.5 andb ) 8.5).In the second stage of the synthesis, a mixture of ethanol and

waters the solvents that start the hydrolysis and condensatireactions tha t lead t o the forma tion of the xerogelss was addedto the UPTES solution (Scheme 1).

Step 1 . S ynthes is of the Ureasi l Precursor UPTES. Anamount of 2.5 g of Jeffamine ED-2001 (or Jeffamine ED-9or Jeffamine ED-600) was dissolved with st irrin g in 10 mLTHF. A volume of 0.625 mL (or 1.388 or 2.081 mL, respectivof ICPTES was a dded to this solution in a fume cupboard. Tflask was sealed and the solution stirred overnight. Tgrafting process was infrared-monitored: as the reactiproceeds, the intens ity of the very str ong and sha rp absorptband located at about 2274 cm- 1, attributed to the vibrationof the t Si(CH2)3NCO group, progressively decreases, whiltha t of bands produced by urea groups increases.

Step 2. Synthesis of the Ureasils. A mixture of 0.582 mL (or1.293 or 1.941 mL, respectively) of eth an ol and 0.067 mL0.149 or 0.225 mL, respectively) of water (molar proportioICPES: 4 CH3CH2OH: 1.5 H2O) was added to th e solutionprepared in step 1. The mixture was st irred in a sealed flfor 30 min an d then cast int o a Teflon mold and left in a fucupboard for 24 h. The mold was covered with Parafiperforat ed with a syringe needle in order to ensur e the slevaporation of the solvents. After a few hours, gelatioccurred a nd t he mold was tr an sferred to an oven at 40 Ca period of 7 days. The sample was th en aged at about 80for 3 weeks to form a transparent, fairly rigid and britelastomeric monolithic film with a yellowish coloration. Tfinal mat erials, i.e., the ureasils, have been identified by designation U(Y ), where U originates from the word ureaand Y ) 2000, 900, or 600, indirectly indicatin g th e length th e oligopolyoxyethylene chains.

Mater ia ls and Methods . Jeffamine ED-2001 (Fluka) wasdried under vacuum for several days prior to being analyzJeffamine ED-600 (Fluka), CH3CH2OH (Merck), and THF(Merck) were kept in contact with dried molecular sieves prto being used. Jeffamin e ED-900 (Fluka) and ICP TES (Flu

(61) (a) Skrovanek, D. J.; Howe, S. E.; Painter, P. C.; Coleman, M.M. Macrom olecules 1985 , 18 , 1676. (b) Skrovanek, D. J.; Paint er, P.C.; Coleman, M. M.Macrom olecules 1986 , 19 , 699.

(62) Coleman, M. M.; Lee, K. H.; Skrovanek, D. J.; Paint er, P. C.Macrom olecules 1986 , 19 , 2149.

(63) Lee, H. S.; Wang, Y. K.; Hsu, S. L.Macrom olecules 1987 , 20 ,2089.

(64) Lee, H. S.; Wang, Y. K.; Macknight, W. J .; Hsu, S. LMacro-m olecules 1988 , 21 , 270.

(65) Coleman, M. M.; Skrovanek, D. J.; Hu, J.; Painter, P. C.Macrom olecules 1988 , 21 , 59.

(66) Zharkov, V. V.; Strikovsky, A. G.; Vertelet skaya, T. E .Polymer 1993 , 34 (5), 938.

(67) (a) Bradley, R. H.; Mat hieson, I.; Byrne, K. M.J. Mater. Chem.1997 , 7 (12), 2477. (b) Chur ch, J . S.; Corino, G. L.; Woodhead , A. L.J .Mol. Struct. 1998 , 440 , 15.

(68) Miyazawa, T.; Shimanouchi, T.; Mizushima, S.-I.J . Chem.Phys. 1956 , 24 (2), 408.

Scheme 1 . Syn thes i s o f the Ureas i l s


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were used as received. Distilled water was used in all experi-ments.

Mid-infrared spectra were acquired at room temperatureusing a Unicam FT-IR system. The spectra were collected overthe range 4000- 400 cm- 1 by averaging 60 scans at a maxi-mum resolution of 4 cm- 1. Solid samples (2 mg) were finelyground and a nalyzed by dispersing them in a pproximately 175mg of dried spectroscopic grade potassium bromide (KBr,Merck) by the pr essed-disk technique. The disks were vacuum-dried at 90 C for a long period of time in order t o reduce thelevels of solvent and adsorbed water in the samples prior torecording the spectra under ambient conditions. Consecutivespectra were recorded until reproducible results were obtained.The evaporation was infrared-monitored in the OH region.Liquid samples were spread between two NaCl windows th atwere immediately sealed with Teflon tape to exclude a tmo-spheric moisture. To evaluate complex band envelopes and toidentify underlying component bands of the mid-infrared

spectra, the curve-fitting procedure in the ORIGIN computersoftware for IBM PC compatible computers has been used.Band sh apes ha ve been described by Gaus sian-type functions.Una mbiguous result s are difficult to obta in, since they dependstrongly on the sta rting para meters. Unless stated otherwise,the curve-fitting procedure has been used mostly to confirmbands a lready observed as shoulders.

R e s u l t s a n d D i s c u s s i o n

The r oom-temperat ur e high- an d low-frequency mid-infrared spectra of Jeffamine ED-2001 are shown inFigures 1a and 2a, respectively. The wavenumbers of the mid-infrared absorption bands obtained for this

diamine an d th ose reported in t he literature for tetra-(ethyleneglycol) dimethyl ether (TEGDME, CH3O(CH2-CH2O)4CH3), hepta(ethyleneglycol) dimethyl ether(HEGDME, CH3O(CH2CH2O)7CH3), and poly(ethyleneglycol) (PE G, HO(CH2CH2O)nH) in the crystalline sta tear e listed in Table 1. The as signment s of the vibra tionalbands of the tetra mer, heptamer, an d high molecularweight PEG quoted in Table 1 are based on the attribu-tion proposed by Matsuura et al.39 The room-temper a-ture high- and low-frequency mid-infrared spectra of Jeffamine ED-600 ar e represented in Figures 1b an d2b, respectively. In Ta ble 2, the frequen cies of th e mid-infrar ed absorption bands of TEGDME, HEGDME, andPEG in th e liquid state r eported by severa l auth ors41,42

are compared with those obtained for J effamine E600. The room-temperature high- and low-frequenmid-infrared spectra of the ureasils are shown Figures 3 and 4, respectively. Parts a- c of Figure 5report the curve-fitting results of the room-temperatuam ide I r egion of U(600), U(900), an d U(2000), resptively. The att ribut ions pr oposed for t he ban ds observin the m id-infrared spectra of the u reasils ar e givenTable 3.

1 . S p e c t r a l C o n s e q u e n c e s R e s u l t i n g f r o m t h eI n c o rp o r a t io n o f t h e O x ye t h y l e n e C h a i n s o f J e f -f a m i n e E D - 2 0 0 1 i n t o t h e S i l i c a B a c k b o n e . If thebands at 3370 and 1598 cm- 1, whose attribution willbe discussed below, are excluded from the spectrum

J effamine E D-2001 (Figures 1a and 2a and Table the resulting spectral feature is essentially that crysta lline h igh molecular weight PE G (Table 1). Thear e also rema rka ble similar ities between the spectruof this diamine and those of TEGDME and HEGDMin th e crystalline st ate (Table 1). This is not sur prisiconsidering that the average number of oxy(ethylenunits in t his diam ine is a pproximat ely 40. The coindence of the wavenum ber of man y bands is not casusince the tetramer and heptamer contain four and sev(OCH2CH2) units , which corr espond, respectively, to1/ 10and about1/ 5 of the number of repeat units of JeffaminED-2001.

The spectru m of Jeffamine ED-2001 is deeply mo

fied after incorporating this diamine into the silinetwork, as i t may be immediately inferred froFigures 3c and 4c. In the hybrid material, most of tbands are broad and many have become ill-defineFur ther more, although some of the ban ds of J effamED-2001 undergo minor sh ifts, others simply disappe

At h igh frequen cies the spectr a of the u rea sils (Figu3 and Table 3) closely resemble not only those of PETEGDME, and HEGDME in the liquid state (Tablebut a lso that of J effam ine ED-600 (Figure 1b and Ta2), which is a viscous liquid at room t empera tur e.

The inspection of the spectral region between 15an d 800 cm- 1 reveals that the most striking effect upomelting of PEG is the presence of four new we

Figu re 1 . Room-temperat ure high-frequency mid-infrar edspectra of Jeffamine ED-2001 (a) and Jeffamine ED-600 (b).

Figu re 2 . Room-temperat ure low-frequency mid-infrar espectra of Jeffamine ED-2001 (a) and Jeffamine ED-600 (

Mid-IR of Cross-L inked S ilicates Chem . Mater., Vol. 11, N o. 3, 1999 573

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absorption bands in th e infrared spectrum (Table 2).According to Matsuura et al.,41 these bands are char-acteristic of the m olten or am orphous stat e and a re dueto disorder ed regions. The first ban d is th e one situ at edat 1326 cm- 1 (Table 2). It is downshifted to 1322 cm- 1in th e spectra of J effamine ED-600 and U(600) and doesnot show up in the spectra of U(900) and U(2000). The

second band, a t 992 cm- 1 (Table 2), is also observed inthe liquid oligomers, at lower wavenumber, and Jeffamine ED-600, though slightly upshifted (997 cm- 1).It is not detected in the spectra of the ureasils. Tshoulder pr oduced by U(600) and U(900) at 921 and cm- 1, respectively, will be connected with the third ba

Table 1 . Room Tempe ra ture Mid-Inf ra red Spe c t ra o f Te t ra (e thy leneg lyco l ) Dime thy l E ther (TEGDME,

CH 3O(CH 2CH 2O) 4CH 3) , Hepta (e thy leneg lyco l ) Dimethy lEther (HEGDME, CH 3O(CH 2CH 2O) 7CH 3) , and

Poly(e thy lene g lyco l ) (PEG, HO(CH 2CH 2O) n H) in theCrys ta l l ine S ta te and of Je ffamine ED-2001 a

crystallineTEGDME39

crystallineHEGDME39

crystallinePEG39

JeffamineE D-2 00 1 a t tr ib ut ion3370 w aNH2

2973 S 2973 m aCH32958 m 2953 m 2950 m 2951 sh aCH21923 S 2922 sh 2924 sh sCH32893 vS 2890 vS 2890 vS } sCH22885 vS 2884 vS2876 sh

}2864 sh 2863 sh 2865 S 2859 sh2851 sh2825 S 2819 m 2825 sh2810 sh 2805 sh 2814 sh combination2790 w vibrat ions2769 w2743 w 2743 w 2740 w 2740 w2727 w2708 w 2710 w2701 w 2698 w 2695 w 2694 w

1963 wb1675 w1598 wb NH2

1477 sh

}1468 m 1468 m 1470 m 1468 m1458 w 1463 m CH2 scissoring and

1457 m CH3 deformation1449 w 1447 w 1453 w 1452 sh1441 w1431 w

}1420 w1414 w 1412 w 1415 w 1413 w1383 w 1385 w CH2 wagging1370 w 1366 w1358 w 1361 m 1364 m 1359 m1352 w 1350 w 1345 S 1344 S1282 S 1280 S 1283 m 1280 m}1241 w 1244 m 1241 m CH2 twisting1235 w 1235 w 1236 w1200 m 1198 w CH3 rocking1160 w 1160 s h

}1147 S 1147 S 1149 S 1147 SCO + rCH21127 m

1111 S 1111 S 1119 S 1116 vS} CO1100 S 1102 S1085 sh1073 sh } CO, CC, rCH21067 w 1062 m 1062 m 1060 m1028 m 1030 m O- CH3967 w 961 sh 963 S 962 S}954 sh CC, rCH2948 w 947 m 948 m939 w 938 m859 sh

}853 S 853 m845 m CO, rCH2836 sh 842 m 844 S 842 S816 sh586 w 586 w

}562 m 557 w

538 w COC and CCO532 w 528 w 529 w 530 vw bending521 w

512 w 508 w 509 vwa Frequencies in cm- 1; vS, very str ong; S, strong; m, medium ;

w, weak; vw, very weak; sh, sh oulder.

Table 2 . Room-Tempera ture Mid-Inf ra red Spec t ra o f Te t ra (e thy leneg lyco l ) Dime thy l E ther (TEGDME,

CH 3O(CH 2CH 2O) 4CH 3) , Hepta (e thy leneg lyco l ) Dimethy lEther (HEGDME, CH 3O(CH 2CH 2O) 7CH 3) , and

Poly(e thy lene g lyco l ) (PEG, HO(CH 2CH 2O) n H) in theLiqu id S ta te and of Je ffamine ED-600 a

liquidTEGDME42

liquidHEGDME42

liquidPEG41

JeffamineE D-600 a t tr ibu t ion

3579 vwb3370 vw aNH2

3307 vw

sNH22975 sh 2963 sh aCH32920 sh 2930 sh 2936 sh} aCH22906 sh2880 S 2880 S 2865 S 2869 S sCH22820 sh combinat ion

2730 sh vibra t ions1592 vw NH2

1485 sh 1482 sh}CH2 scissoring a ndCH3 deformation1470 sh1456 m 1457 m 1460 m 1454 m1373 m} CH2 wagging1353 m 1354 m 1352 m 1350 m

1328 w 1328 w 1326 w 1322 w liqu id s ta t e1299 m 1300 m 1296 m 1299 m}1248 m 1252 m 1249 m 1 249 m CH2 twisting1199 m 1200 w1140 sh 1140 sh 1140 sh 1144 sh

CO + rCH21109 S 1110 S 1107 S 1112 vS CO1042 s h 1035 w 1038 m 1043 m CO, CC, rCH21028 w985 w 986 w 992 w 997 w liquid st at e965 sh } CC, rCH2943 w 948 w 945 m 946 m

915 sh liquid sta te850 m 852 m 855 m 863 vwb CO, rCH2

810 sh liquid sta te540 m 525 m 528 vwb COC+ CCO

bendinga Frequencies in cm- 1; vS, very st rong; S, strong; m, medium

w, weak; vw, very weak; sh, sh oulder; b, broad.

Figure 3 . Room-temperat ure high-frequency spectra of thun doped ur easils: (a) U(600), (b) U(900), an d (c) U(2000)


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of PEG indicative of the amorphous state at 915 cm- 1(Table 2). The fourth band of liquid PEG associated withthe m olten state, a shoulder a t 810 cm- 1 (Table 2), isnot detected in any of the spectra of the compoundsunder ana lysis.

Unlike the situation found in the spectrum of Jef-famine ED-2001 for which the (OCH2CH2)-based cha insare expected to take regularly ordered conformations,

the effects just described and discussed regarding thespectrum of U(2000) provide conclusive evidence thatthe oligopoly(oxyethylene) chains of t his organic-inorgan ic hybrid ma terial ha ve chan ged from a helicalstru cture t o a new, less ordered, th ough not completelydisordered, st ructur e. In fact, the spectru m of U(2000)is substan tially identical n ot only to th e spectrum of liquid PEG but also to the spectra of Jeffamine ED-600or U(600), which contain fewer repea t un its (about 8.5)tha n the ones introduced in U(2000) (about 40.5). Theban ds indicat ive of the liquid st at e, i.e., 1326, 992, 915,and 810 cm- 1, are neverth eless missing in t he spectrumof U(2000). Although the considerable broadening andconsequent reduction of the a ppar ent intensity of most

of the bands produced by the oxyethylene moietiesindicate the partial r elaxation of the crystal stru ctureof Jeffamine ED-2001 upon synthesis of the hybridcompound, the absence of the liquid bands referredabove suggests that although the molecules as units aremore or less free to rotat e about their sk eletal axis, theyare semirigid about the individual skeletal bonds. Thisis logical considering that the grafting procedure mark-edly reduces their mobility. The semirigid moleculesprobably tend to form ordered r egions in th e ur easil togive the ma terial some short-ran ge order. Consequently,the changes in the structure of the backbone of thepolyether chains of Jeffamine ED-2001 upon inclusionin the sil ica network are not as complete as those

undergone by PEG upon melting. In other words, tpolymer cha ins in U(2000) have more segment al moity than in the diamine, but the individual moleculhave not a chieved total mobility. Fur thermore, thabsence of the liquid bands in the spectrum of tureasil, particularly the band a t 992 cm- 1, eliminat esthe possibility of the tran s structure chara cteristicthe molten state; i.e., the conformation of the oligpolyether chains of J effamine ED-2001 after beiincorporated into the silica matrix retains to a lardegree the TGT sequence cha ra cteristic of the crystline sta te of the d iamin e. Another p iece of spectroscoevidence supports our claim. The m ost intense ba nd

the spectru m of U(2000), at tribut ed to the C- O stretch-ing vibration, is found at 1122 cm- 1, closer to theposition observed in the spectrum of crystalline PE(1119 cm- 1) than in t ha t of liquid PEG (1107 cm- 1). TheX-ray powder diffraction pattern of U(2000)20,32 cor-roborat es t hese spectroscopic resu lts.

Let us now analyze the lower weight analogues U(2000). Our spectroscopic data clearly pr ove t hat both Jeffamine ED-600 and U(600) the oxyethylechains do at tain complete disorder. A situa tion simito that encountered in liquid PEG is present , indicated by the presence of bands attributed to tmolten state (at 1322 cm- 1 in both compounds and alsoat 921 cm- 1 in U(600)), thus implying the contributio

Figure 4 . Room-temperat ure low-frequency spectra of theun doped ur easils: (a) U(600), (b) U(900), an d (c) U(2000).

Table 3 . Room-Tempe ra ture Mid-Inf ra red Spec t ra o f theUndope d U reas i l s U(600) , U(900) , and U(2000) a

U(600) U(900) U(2000) a t t ribut ion3351 m 3340 m 3342 m b NH hydrogen-bonded

amide II 23116 w 3120 w2965 sh aCH32931 S 2933 S 2951 sh aCH22873 S 2877 S 2874 S sCH22748 sh 2751 sh 2733 sh combina t ion vibra t ions

1963 wb1751 s h 1750 S}1715 sh 1721 S 1716 sh amide I1671 sh 1674 S 1677 sh1642 S 1641 s h1565 S 1565 S 1548 w amide II1476 sh } CH2 scissoring a ndCH3 deformation1455 m 1457 m 1454 m1427 m1373 m 1373 sh 1380 sh} CH2 wagging1351 m 1351 m 1353 m1322 w liquid sta te1301 m 1303 s h1276 m 1284 m 1284 m amide III1253 m 1253 m 1249 m} CH2 twisting1193 m 1197 m 1205 S1151 sh 1145 sh 1148 sh CO + rCH2

1110 vS 1110 vS 1122 vS

CO1037 m 1035 S 1041 S CO, CC, rCH2950 w 950 m 950 w CC, rCH2921 sh 922 sh liquid sta te879 vw 883 vw 879 sh} CO, rCH2850 m 848 vw 850 w790 vw 798 vw 808 vw775 vw 771 vw 761 vw amide VI692 vw 692 w 698 vw amide V669 vwb

638 vw (N-C-N)582 wb 580 vw}528 vwb COC+ CCObending472 vwb 484 vw458 wb 449 wb

a Frequencies in cm- 1; vS, very st rong; S, strong; m, mediumw, weak; vw, very weak; sh, sh oulder; b, broad.


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of t he tran s conformat ions. The amorphous, disorga-nized natu re of U(600) is illustrated in its diffracto-gram.32 Finally, we may state that the general physi-cochemical beha vior of U(900) is very similar to th at of U(600). Although only one ba nd indicative of the liquidstat e is detected in its infrar ed spectru m (at 922 cm- 1),the compound gives rise to a spectral signature thatbasically resembles that displayed by liquid PEG or byU(600). This result is supported by X-ray powder

diffraction.32

2. Spec t ra l Analys i s o f Bands Produc ed by P ar-t icular Groups in J effamine ED-2001, in J effamineED-600, and in the Ure asi ls . Bands Originating fromAm ine Groups. Owing to the high number of (OCH2CH2)repeat units in t he str ucture of Jeffamine E D-2001, theterminal NH2 groups of this diamine give r ise to onlytwo absorption bands (Figure 1a and Table 1): a veryweak ill-defined one at 3370 cm- 1 and a weak broad oneat 1598 cm- 1, which are assigned to the asymmetricstr etching vibra tion and deformation of the NH2 groups,respectively.69- 71 The symm etric component of the NHstretching vibration is not detected. The other mostcharacteristic absorption bands of aliphatic primaryamines with secondaryR-carbons are also missing inthe spectrum of J effamine ED-2001:71 (1) the strongband pr oduced by the asymm etric stretching vibrat ionof th e C- N group, expected at 1040( 3 cm- 1, overlapsthe very str ong ban d at 1116 cm- 1; (2) the pa ir of broadand st rong bands originating from the as ymmetr ic an dsymmetric NH2 bending modes found at 794( 16 and840 ( 11 cm- 1, respectively, is masked by the strongband at 842 cm- 1.

In t he h igh-frequency spectra l region J effamine ED-600 displays two weak ban ds a t 3370 an d 3307 cm- 1that are attributed to the asymmetric and symmetricstret ching vibrations of the t ermina l NH2 groups of th ediamine69- 71 (Figure 1b and Table 2). The well-knownrelationship proposed by Stewart71, i.e.,j sym ) 0.98j asymholds here. Jeffamine ED-600 also gives rise to a weakabsorption band at 1594 cm- 1, assigned to the NH2deformation vibrat ion.69- 71 Similar to what happenedin the case of Jeffamine ED-2001, the strong infraredband assigned to the asymmetric stretching vibrationof the C- N group expected at 1040( 3 cm- 1 71 is againmask ed by the very strong band pr oduced by the C- Ostretching vibration situated in the spectrum of Jeffam-ine ED-600 at 1112 cm- 1. In the 850- 750 cm- 1 spectralregion of the latter compound, a very weak single bandis observed at 863 cm- 1. It might originate either fromthe coupled vibration of the C- O stretching and CH2rocking modes or from the symmet ric bending vibrationof the NH2 groups. However, considering the minorpercenta ge of amine moieties in the compound, we ha vepreferred to as sign it to the former vibrat ion m odes, asmentioned previously.

Bands Involving S ilicon Atoms. There is a minorpercent age of silicon atoms in th e ur easils (e.g., the silicaprecursor prepared from Jeffamine ED-2001 contains

92 car bon a toms between the u rea br idges versus o2 silicon atoms). The a bsorption bands resulting frthe vibrat ion of silicon-based bonds a re thu s expecto be less intense than those arising from the vibratioof the organic cha ins included in the hybrid ma teria

To avoid water quenching, an extremely low molratio of water versus silica precursor was chosen in sol- gel step of the synt hesis procedure of the urea s(i.e., 1.5 H2O:1ICPTES). It is thus highly probable tha

some unr eacted ethoxysilane groups, Si- OCH2CH3, of the precursor remain in the mater ia ls because incomplete h ydrolysis. Taking into a ccount the reveibility of th e h ydrolysis an d condensation r eactionsthe sol- gel process, two situations are equally posible: (1) the original eth oxy groups of ICPTES couhave r emained or h ave been h ydrolyzed to eth anol;the original etha nol groups could ha ve rema ined or bexcha nged with silanol groups to form ethoxy ones. Tethoxy group gives rise to a strong doublet at 110-1075 cm- 1 and two medium-intensity bands at 1175-1160 and 970- 940 cm- 1.70,72 In the spectra of U(600),U(900), and U(2000) the doublet and the band betwe1175 and 1160 cm- 1 are obscured by the very intense

C- O stretching absorption and the strong couplevibrations of the C- C str etching, C- O stretching, andCH2 rocking modes that fall in the same range ofrequencies (Figure 4 and Table 3). As to the lowfrequency band, it is masked by the medium-to-weintensity band a t 950 cm- 1, att ributed to the stretchingand rocking vibrations of the C- C and CH2 groups,respectively.

The presence of r esidual silanol groups, Si- OH, acommon situation in many sol- gel derived materials,reflects incomplete polycondensation. In the higfrequency region, free Si- OH groups u sua lly give riseto a weak band at 3690 cm- 1, which downshifts to3400- 3200 cm- 1 with hydrogen bonding producing a

medium-intensity band.70,72 The tetramethoxyortosi-lane, TMOS, derived xerogels pr epar ed by Orcel et a73absorb at 3750, 3660, and 3540 cm- 1 because of th estr etching vibrat ion of, respectively, free sur face, intnal, an d hydrogen-bonded Si- OH gr oups. Accordin g toOu et al.,74 the shoulder found in the spectrum of thphenyl-modified silicate at 3606 cm- 1 should be at-tribut ed t o hydrogen-bonded sur face silanols. The sanol stretching bands in th e spectra of the u reasils anot easily detected (Figure 3), since the complex brofeature observed at this range of frequencies moriginate not only from t he s tret ching vibrat ion of S-OH groups but also from the stretching vibration of NH groups of the urea cross-links and from the streting vibration of adsorbed water. In the case of U(20the ill-defined contour observed makes the identificatespecially difficult (Figure 3c). Silanols also absostrongly at 950- 830 cm- 1 because of the Si- OH stretch-ing vibration.70,72 In the spectra of the vinyl sampleobtained by Ou et al.74 th is vibra tion mode gives rise toa strong band at 964 cm- 1 and in those of the phenyl-derived materials to a medium intensity one at 9cm- 1. In TMOS this ban d is seen a t 950 cm- 1.73 In thespectra of the u reasils this band is ma sked by the bro(69) Batist a de Carva lho, L. A. E.; Amorim da Costa, A. M.; Duar te,M. L.; Teixeira-Dias, J . J . C.Spectrochim. Acta 1988 , 44A (7), 723.

(70) (a) Nakanishi, K.Infrared Absorption Spectroscopy: Practical ;Holden-Day, Inc.: San Fra ncisco, 1962. (b) Colthup, N. B.; Daly, L.H.; Wiberley, S. E.Introduction to Infrared and Raman Spectroscopy ,3rd ed.; Academic Press: London, 1990.

(71) Stewart, J. E.J. Chem. Phys. 1959 , 30 (5), 1259.

(72) Smith, A. L.Spectrochim. Acta 1960 , 16 , 87.(73) Orcel, G.; Phalippou, J .; Hench, L. L.J. N on-Cryst. Solids 1986 ,

88 , 114.(74) Ou, D. L.; Seddon, A. BJ. Non-Cryst. Solids 1997 , 210 , 187.


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band at 950 cm- 1 assigned t o the coupled vibration of the C- C stretching and CH2 rocking modes.

In short, infrared spectroscopy is not conclusive forthe existence or absence of Si- OCH2CH3 or Si- OHgroups in the ur easils. The latter s hould be much moreeasily distinguishable in the near-infrared overtone andcombination region.

Bands Originating from Urea Groups. NH StretchingRegion. The broadness of the hydrogen-bonded N- Hband reflects a distribution of hydrogen-bonded N- Hgroups of varying strength dictated by distance andgeometry. However, owing to the bad resolution andcomplexity of the spectra in the high-frequency region,absorption ban ds due to N- H group vibrat ions can notbe used for quantitative assessment of the hydrogen-bonded urea groups. Although this spectral region willnot be discussed in detail in this work, several bandswill be tentatively assigned. This attribution will bebased mainly on the extensive infrared spectroscopicstudies carr ied out on polyamides61 and poly-urethanes.62- 66 The word free in quotations will alsobe used here t o describe urea N- H or Cd O groups tha tare not directly hydr ogen-bonded.

Amorph ous polyamides ar e kn own t o exhibit a weakshoulder at 3444 cm- 1 and a very broad band a t 3310cm- 1 that have been assigned to free and hydrogen-bonded N- H st ret ching modes, respectively.61 Remark-ably similar frequ encies (3440 and 3320 cm- 1) have beenreported for simple linear semicrystalline aliphaticpolyuretha nes in this spectra l r egion.62 In the modelpolyurethanes investigated by Lee et al.63 the samemodes give rise to bands at slightly different frequencies(3450 cm- 1 an d from 3330 to as low as 3200 cm- 1). Inpolyurethane- ether blends the free N- H groupsproduce a ban d a t 3440 cm- 1, whereas t he h ydrogen-bonded one is found at approximately 3340 cm- 1.65 Jadahas r eported t he presence of a very strong band a t 3346cm- 1 in t he s pectru m of 1,3-dimeth ylur ea, (CH3)HNC-(d O)NH(CH3).59 In polyurea a single band is seen at3330 cm- 1.60

Although dialkylur eas of the t ype R HNC(d O)NHRare expected to produce only one band in the N- Hstret ching region, RHN C(d O)NHR diureas give rise totwo distinct bands in this range of frequencies. In thespectra of the dialkylur eas investigated by Mido thesebands appear between 3320 and 3380 cm- 1 and are asa rule separat ed by approximately 20- 60 cm- 1.55 Forinstance, 1-methyl-3-tert -butylurea displays two bandsat 3376 an d 3318 cm- 1.55 According to the same author,this experimenta l evidence is attr ibutable to the exist-

ence in the lattt er t ype of compounds of substitu ents of different sizes whose steric effect on the molecularstru cture creates an imbalance between both h ydrogenbondings through thetrans -amide hydr ogens.

Since the ureasils belong to the second group of disubstituted ureas, two bands originating from theN- H stretching vibration should in principle be foundin th e high-frequen cy region of their spectra. A medium-intensity single broad band and a weak one are ob-served, respectively, at about 3351 and 3116 cm- 1 inthe spectrum of U(600) (Figure 3a) and at 3340 and3120 cm- 1 in t he spectru m of U(900) (Figure 3b). Thestronger band is undoubtedly associated with the stretch-ing vibration of the secondar y hydr ogen-bonded N- H

groups of the urea bridges. Given its bad r esolutionis impossible t o resolve it into t wo components. Mencountered the same problem in the spectra of sodialkylureas.55 We will assign th e weaker ban d to anovertone of the 1565 cm- 1 band.69,70 As for U(2000), itdisplays only a br oad, ill-defined ba nd center ed at ab3342 cm- 1 in this spectral region (Figure 3c). Obviouslit is extremely difficult to distinguish the N- H groupsfrom hydrogen bonds in the spectra of the ureasils.

Bands in the Am ide I Region. The am ide I mode is ahighly complex vibra tion tha t involves the contr ibutiof the Cd O stretching, the C- N stretching, and theC- C- N deformat ion vibrations.68 Unlike the N- Hstretching vibration, t he Cd O stretching vibration issensitive to th e specificity an d m agnitude of hydrogbonding. Thus, th e amide I vibrat ion consist s of sevecomponents reflecting Cd O groups in different en viron-ments.

Jada has attributed the band at 1635 cm- 1 in thespectrum of 1,3-dimethylurea to th e a mide I mode59Iijima et al.60 ha ve pointed out t he pr esence of a singlamide I band at 1650 cm- 1 in the spectrum of polyurea.Skrovanek et al.61 have associated the intense band a1640 cm- 1 and the shoulder at about 1670 cm- 1 ob-served in the spectra of am orphous polyamides whydrogen-bonded and free carbonyl groups, resptively. Zharkov et al.66 have recently investigated indetail the amide I absorption band contour of thinfrared spectra of polyether urethane elastomers ahave concluded th at it comprises five individual coponents at 1740, 1730, 1725, 1713, and 1702 cm- 1.Several associations for t he u reth an e groups have beproposed to account for the isolated bands observeColeman et al.62 have reported the presence of threebands in the carbonyl stretching region of a simppolyurethane, at about 1721, 1699, and 1684 cm- 1,which have been assigned to non-hydrogen-bonded adisordered an d ordered h ydrogen-bonded Cd O groups,respectively. In model polyurethanes two clearly dcernible components have been found in t he amidregion: one at 1732 cm- 1, assignable t o carbonyls freeof hydrogen bonding, and another at 1703 cm- 1, associ-ated with hydrogen-bonded carbonyls.63 In urethane-ether blends two components have also been foundthis r ange of frequencies: the free carbonyl band a t 1cm- 1 and the hydrogen-bonded carbonyl band locatbetween 1694 an d 1704 cm- 1.65

It m ay be immediately inferred from Figure 4 ththe amide I region in the u rea sils is extrem ely complThis is not sur prising considering tha t th e urea moi

conta ins t wo N- H groups, whereas in polyamides anpolyurethanes only one N- H group contributes tohydr ogen bonding. Moreover, in the u rea sils, as pointout a bove, the t wo N- H groups of the ur ea cross-linkar e expected to behave differen tly, since one of thembonded t o a propyl cha in, which is in tur n bonded trigid silicon-based n etwork, wherea s th e other is bondto soft organic oxy(ethylene) chains.

In t he spectru m of U(600) the amide I mode gives rto a very intense broad band whose maximum occuat 1643 cm- 1 (Figur e 4a). The maximum of the band upshifted to 1751 cm- 1 in the spectrum of U(2000)(Figur e 4c). The amide I region of U(900) (Figure spans a larger ra nge of frequencies t han the previo


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materials, since two strong broad envelopes centered atabout 1724 and 1650 cm- 1 are now observed. Twoimmediate conclusions may be drawn from the analysisof these results. (1) The spectra l featu re of the a mide Iband of these hybrid materials is drastically modified

as the length of the oxy(ethylene) chains present isincreased from 8.5 in U(600) to about 40.5 in U(2000).(2) The spectra of the u reasils in th e ran ge 1600- 1800cm- 1 provide conclusive evidence that several compo-nents contribute to the envelope of the a mide I band,meaning tha t var ious m ore or less ordered structuresinvolving hydr ogen bonding between the N- H groupsof urea and the oxygen atoms of ether or carbonylmoieties are possible.

To determine the frequency of these components, wehave performed curve-fitting of the bands with theORIGIN comput er softwar e for IBM P C-compat iblecomputers. Gaussian bands shapes were employed inthis procedure, since they produced the best fits. A

linear baseline was assumed in the three cases from2000 to 400 cm- 1 for the points where no significantabsorbances were detected. Curve-fitting was limited tothe 1760- 1610 and 1830- 1610 cm- 1 spectra l ran ges inthe case of U(600) and U(900), respectively; for U(2000)the limits were set between 1805 and 1640 cm- 1. Aminimum of three components was isolated for theamide I band of U(600) at 1715, 1671, and 1641 cm- 1(Figure 5a). The amide I spectral region of U(900) wasresolved into four component s at 1751, 1721, 1674, and1641 cm- 1 (Figure 5b). At la st, th ree individual compo-nents were detected for U(2000) at 1750, 1716, and 1677cm- 1 (Figure 5c). Thus, four componentss centeredaround 1751, 1721, 1674, and 1641 cm- 1s seem to

describe entirely the a mide I region of the th ree hybcompounds.

However, these results should be looked at wiextreme care. First of all, the second-derivative absban ce spectr a (smooth ed by a cubic function by avering 10 points) were not always consistent with tdeconvolution pr ocedur e. Only in t he case of U(900) the frequency of the bands ma tch exactly that ind icaby th e second-derivative plot. Second, the fitting p

cedur e is subject t o much a mbiguity, since it depenheavily on the baseline drawn, on the band frequenlimits set initially, and on the number and position the bands. Third, the breadth of the components wtotally unkn own and could not be estimat ed previousConsidering all these items, the results just describmight be physically meaningless. An alternative aproach within a stat istic fra mework will be considerin the near future to confirm them.

Bands in the Am ide II Region. The am ide II mode isa m ixed contr ibution of th e N- H in-plane bending, theC- N stretching, and t he C- C stretching vibrations.68It is sen sitive to both chain conforma tion a nd int ermlecular hydrogen bonding. In noncyclic secondary amid

where the N- H and Cd

O groups exist mainly in a tra nsconfigur at ion, th e in-plane N- H bending frequency andthe resonan ce-stiffened C- N bond stretching frequencyfall close together an d inter act consider ably, giving rto a strong absorption near 1550 cm- 1.68,70

The amide II band is seen at 1545 cm- 1 in amorphouspolyamides61 and at 1540 cm- 1 in simple polyure-thanes.62 In disubstituted ureas bands, bands in th1510- 156058 and 1565- 1590 cm- 1 59 spectra l r egionshave been ascribed to the am ide II mode. In th e specof theN ,N -disubstituted ureas studied by Mido,55 theamide II band is located between 1565 and 1590 cm- 1.Nevertheless, this auth or h as n oted tha t th e amideband of derivatives having branched alkyl groups apear at slight ly lower frequen cies. Therefore, we assoate the single intense band seen at 1565 cm- 1 in thespectra of U(600) (Figure 4a) and U(900) (Figure 4with this vibration mode. This assignment is coroborat ed by the fact th at the a mide II band of 1-eth3-tert -butylurea has been found at exactly this frquency.55 The band is downshifted to 1548 cm- 1 in thespectrum of U(2000), and its intensity is markedreduced (Figure 4c). The polyurea thin films invesgated by Iijima et al.60 display this band a t 1550 cm- 1.Person et al.75 have demonstrated that the reductionin frequency and intensity of this mode may be crelated with a decrease in h ydrogen-bonding str engThis would imply tha t t he h ydrogen bonds in U(200are less strong than in the other t wo ur easils considerIt is interesting to mention the inter preta tion given Myers to account for th e changes observed in th e 151-1560 cm- 1 spectr al r egion of urea- formaldehyde poly-mers during curing.58 He has reported the presence ofa s t rong amide II band in the spectra of uncurpolymers bet ween 1550 an d 1560 cm- 1 and the appear-ance and growth of a band at 1510- 1520 cm- 1 duringpolymer cure. According to this author, although in uncured solid form the material m ust exist primaras a linear chain in a highly hydrogen-bonded sta

(75) Pers on, W. B.; Zerbi, G.Vibrational Intensities in Infrared and Raman Spectroscopy ; Elsevier: Amsterdam, 1982.

Figu re 5 . Curve-fitting results of the room-temperatureamide I region of the undoped ureasils: (a) U(600), (b) U(900),and (c) U(2000).


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after curing hydrogen bonding is possibly repressedbecau se of steric constra ints.

Bands in the Am ide III Region. The amide III modeinvolves th e str etching vibrat ion of the C- N group. Itis highly mixed an d complicated by coupling with NHdeformation modes. The amide III band is expectedbetween 1250 an d 1350 cm- 1.68

Urea- formaldehyde r esins are known to absorbbroadly at 1260 and 1290 cm- 1 because of this vibrat ionmode prior to and after curing, respectively.58 A strongand broad amide III band has been observed between1260 and 1300 cm- 1 in the spectra of the r esins prepar edby Jada.59 The frequency of this band in the spectrumof polyurea ha s not been reported.60 In th e spectr um of dialkylur eas this band appea rs in the 1240- 1330 cm- 1region.55 We therefore ascribe the medium-intensityband observed at 1276 cm- 1 in th e spectrum of U(600)(Figure 4a) and at 1284 cm- 1 in both the spectra of U(900) (Figur e 4b) and U(2000) (Figur e 4c) to th is mode.This as signment is support ed by the following evidence.(1) In 1-n -butyl-3-sec -butylurea and in 1-sec -butyl-3-sec -butylurea the amide III mode gives r ise to an intenseabsorption band at 1275 cm- 1.55 The ban d at 1276 cm- 1

in th e spectru m of 1-ethyl-3-sec -butylurea has also beenatt ributed to this vibrat ion.55 (2) 1-Methyl-3-tert -butyl-urea and 1-n -butyl-3-butylurea display the amide IIIband at 1282 and 1286 cm- 1, res pectively.55

Band s in the Am ide IV, V, and VI R egions . The amideIV, V, and VI bands are produced by highly mixedmodes containing a significant contribution from the NHout-of-plane deformat ion m ode. They a re expected t o bein the 800- 400 cm- 1 region.55

Owing to the influence of the alkyl framework on itsstrength, the amide IV band is often absent in thespectra of urea derivatives. For instance, Mido55 hasbeen unable to detect it experimentally in any of thespectra of the 30 dialkylureas he analyzed.

The amide V vibration mode absorbs strongly andbroadly in t he 630- 690 cm- 1 region.55,68 Although theN ,N -disubstitu ted u reas investigated by Mido exhibitthis band at frequencies lower than 672 cm- 1,55 wetent at ively assign the weak-to-very-weak band observedat 692 cm- 1 in the spectra of U(600) and U(900) (Figures4a and 4b, respectively) and at 698 cm- 1 in the spectrumof U(2000) (Figure 4c) to this mode.

Taking into account that in the spectra of disubsti-tuted urea derivatives the amide VI band has beenreported to appear constantly ca. 770 cm- 1,55 we at-tribute t he very weak bands observed at 775, 771, an d761 cm- 1 in t he s pectra of U(600), U(900), and U(2000),

respectively, to t he amide VI mode.At last, a medium-intensity band over the regionbetween 600 and 500 cm- 1 observed in the spectra of disubstituted ureas ha s been at tributed to (N- C- N)by analogy to the case of methylalkylureas.55 Consider-ing that t he very weak bands at 669 an d 638 cm- 1 foundrespectively in the spectra of U(600) and U(2000) arenot observed in th e spectra of the par ent diam ines (seeTables 1 and 2), we tentatively assign them to thismode.

Let us now go back to the inter preta tion of the room-temperatur e amide I an d II ban ds of the ur easils. Thediscussion tha t follows shows tha t t he pr esent spectro-scopic work has helped to clarify several points related

to the structure of the materials. I t is based on twimporta nt a ssumptions. (1) Because of the h ybrid natuof the ureasils, the N- H groups of the urea moietiesar e expected to beha ve different ly. (2) The amide I banof U(600), U(900), and U(2000) are definitely decoposed into three, four, and three components, respetively, a s suggested by the curve-fitting pr ocedudescribed ea rlier.

The mid-infrared spectrum of U(600), in particulthe location of the amide I band, indicates that tinteraction between the N- H groups and the oxygenatoms of the Cd O or OCH2CH2 groups is appar entlysterically favorable for the formation of strong aordered hydrogen-bonded structures. This particulbehavior ma y be associated with th e intrinsic natu rethe material. Since U(600) contains fewer oxyethyleuni ts than the other ureasi ls , the number of uremoieties presen t is extr emely high. Hence, self-assoction of th e latt er group is expected to occur extensivthr oughout t he ma terial. This situa tion is aided by tflexibility of the chaotically distributed oligopolyethsegments th at a re able to adopt easily adequa te spatconformations for highly efficient hydrogen bonding

In semicrystalline U(2000) the opposite situatita kes place. The rema rkable upshift of the amide I ba(108 cm- 1) of U(600) upon incorporation of 32 moroxyethylene un its is accompanied by a mar ked decreof the int ensity of the a mide II band. J udging from thresults, we may sta te t hat the associations formed U(2000) via hydrogen bonds must be considerabweaker than those in U(600). The steric constraincreated by the extr a r epeat u nits very possibly accoufor this behavior. Because of the considerable n umbof oxyethylene units present , the influence of the urlinka ges in t his ur easil is minimized and th e forma tiof hydrogen-bonded urea- polyether associations fa-vored. On the basis of the premise that whenevhydrogen bonding occurs between a urea moiety and ether oxygen of a soft polyether segment one carbogroup is liberated, a large number of free carbongroups is expected to exist in U(2000).

The proportion of hydrogen-bonded ur ea- polyetherand urea- ur ea associations in th e thr ee ureasils helpto explain th e num ber an d intensity of the componenobserved in the amide I spectral region of the corsponding spectra.

The contour of the amide I band in the spectrum U(2000) (Figure 5c) shows th at the most inten se coponent is situated at 1750 cm- 1. Considering its ex-trem ely high frequency, we tentat ively assign it to t

absorption of urea groups completely devoid of ahydrogen bonding (either by N- H or by Cd O groups).We att ribute th e remaining two component s at 1716 a1677 cm- 1 to the vibration of NHC(d O)NH groupsbelonging t o ur ea- polyether str uctures, wh ich will bedesignat ed as A and B, respectively. The am ide I regof semicrystalline nylon is also composed by three bantha t h ave been at tributed t o hydrogen-bonded car bogroups in ordered domains, h ydrogen-bonded carbogroups in disordered conforma tions, and free carbogroups.61 Since th e bands at approximately 1716 an1677 cm- 1 ar e present in the thr ee ureasils studied, wcannot associate them unequivocally with amorphoor crystalline domains.


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In U(600), ordered self-associated ur ea- urea struc-tur es C dominate, giving r ise to the strong componentat 1641 cm- 1 (Figur e 5a). Since the ba nd a t 1641 cm- 1is missing in the spectru m of U(2000), we a re led t oconclude that no urea- ur ea hydrogen-bonded associa-tions are present in the latter ureasil . On the otherhand, the absence of an individual band at 1750 cm- 1in the spectrum of U(600) is an indication that theextension of hydr ogen bonding throughout th is mater ial

is such that n either Cd

O nor N- H groups are left free.This claim is acceptable considering th e length of thepolyether chains in both materials; there are approxi-mately 5 times more (OCH2CH2) units in U (2000) tha nin U(600). The other components of the amide I absorp-tion band of U(600) at 1671 and 1715 cm- 1 are inherentto progressively less order ed ur ea- polyether st ructur esB and A, respectively. The prominence of hydrogenbonding in U(600), caused by the presence of ureagroups, is not unexpected at all. Quite r ecently, similarinter actions ha ve allowed De Loos et a l.76 and van E schet al .77 to synthesize new durable organic gels withpotential applications through self-assembly and po-lymerization. Highly specific int eractions between neigh-

boring urea groups of a bis-ureido derivative provideexcellent gelling properties to this compound. Whenadded to common solvent s, elonga ted fibers ar e form edvia hydrogen bonding.

The spectrum of U(900) exhibits a more complicatedam ide I band cont our (Figure 5b), suggest ing definitelya situation between that found in the spectra of thehigher and lower analogues. Probably short and longN- HO conta cts ar e established, for the length of thesoft segments and the number of urea moieties are suchtha t the N- H groups of the urea linkage may nowinteract indiscriminately with the carbonyl and the

eth er oxygen at oms of th e soft polyeth er segmen ts. Tcomponent of the amide I absorption band of U(90centered a t 1641 cm- 1 (Figure 5b) indicates tha t thoughthe formation of the urea- polyether st ructures B a ndA is preferred, th at of ur ea- urea associations (structureA) is also feasible. The presence of the band at 17cm- 1 proves that free Cd O groups exist too.

It is worth mentioning that the relative breadth the bands in t he t hr ee compounds gives some confide

to th e curve-fitting procedure. The infrar ed ban d tributed to urea- urea interactions at 1641 cm- 1 is, asexpected, considerably nar rower than t hose of the othcomponents. Its width at half-height has the samma gnitu de for U(600) and U(900) (W 1/2 ) 24 cm- 1). Thewidth at half-height of the band at 1750 cm- 1, associat edwith free carbonyl groups, is larger and has a simivalue for U(2000) and U(900) (W 1/2 ) 37 and 42 cm- 1,respectively). The width at half-height of t he basituated at approximately 1674 cm- 1, as sociated withstructure B, is 37, 43, and 31 cm- 1, respectively, forU(600), U(900), and U(2000). Fina lly, the ba nd a t a b1716 cm- 1, attr ibuted to the hydrogen-bonded str ucturA, has a width at half-height of 40, 28, and 30 cm- 1,respectively, for U(600), U(900), an d U(2000).The temperature-dependent spectral studies of tureasils that will be performed in the near future whopefully provide valuable and enlightening informatabout the rich spectral feature of these fascinatimaterials.

A c k n o w l e d g m e n t . The authors thank Dr. A. M.Amorim da Costa, of the Department of Chemistryth e Un iversity of Coimbr a, and Dr . Ivone Delgadillothe Department of Chemistry of the University Aveiro, for helpful discuss ions. Th e finan cial su pportFundaca o pa ra a Cien cia e Tecnologia (cont ract n um bPBIC/CTM/1965/95) is greatly appreciated.CM980372V

(76) De Loos, M.; van E sch, J.; St okroos, I.; Kellogg, R. M.; Feringa,B. L.J . Am . Chem. Soc. 1997 , 119 , 12675.

(77) van Esch, J.; Kellogg, R. M.; Feringa, B. L.Tetrahedron Lett.1997 , 38 (2) , 281.


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