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Journal of Scientific & Industrial Research Vol. 68, June 2009, pp.437-464 Developments of polyhedral oligomeric silsesquioxanes (PaSS), pass nanocomposites and their applications: A review D Gnanasekaran, K Madhavan and B S R Reddy* Industrial Chemistry Laboratory, Central Leather Research Institute, Chennai-600 020, India Received 01 September 2008; revised 24 February 2009; accepted 13 March 2009 This review presents a brief account of recent developments in synthesis and properties of organic-inorganic hybrids using polyhedral oligomeric silsesquioxanes (POSS) nanoparticIe, and applications of POSS monomers and nanocomposites. Therma], rheology and mechanical properties of polyimides, epoxy polymers, polymethymethacrylate, polyurethanes, and various other polymer nanocomposites are discussed. Keywords: Nanocomposites, Polyhedral oligomeric silsesquioxanes, Therma] and mechanical properties Introduction A variety of physical property enhancements (processability, toughness, thermal and oxidative stability) are expected to result from incorporation of an inorganic component into an organic polymer matrix. Many organic-inorganic hybrid materiels have shown dramatic improvement in macroscopic properties compared with their nonhybrid counterparts 1-6. Hybrid organic-inorganic materials based on incorporation of polyoctahedral oligomeric silsesquioxanes (PaSS) into polymeric matrices have received a considerable attention7-'3. Silsesquioxanes (Fig. I) 14,which consist of a rigid, crystalline silica-like core that is perfectly defined spatially (0.5-0.7 nm), have general formula (RSiOI)u(HP)o5b' where R is a hydrogen atom or an organic group and a and b are integer numbers (a = I, 2,3, ...; b = 0, 1,2,3, ), with a + b = 217, where 17 is an integer (n= I, 2, 3, ) and b = a + 2. Of several structures of silsesquioxanes (random, ladder and cage), cage structures contain 8 silicon atoms placed at cube vertices. Cubic structural compounds (completely and incompletely condensed silsesquioxanes) are commonly illustrated as T6, T7, Ts' T,o' and T12, based on the number of silicon atoms present in cubic structure (Fig. 2). In early 1900s, Kippingl5 found that polycondensation of siliconic acids invariably leads to * Author for correspondence Tel.: (044) 24404427; Fax: 91(44) 2491 ]589 E-mail: [email protected] extremely complex mixtures of silsesquioxanes. Later in 1946, pass were isolated along with other volatile compounds through thermolysis of polymeric products obtained from methyltrichlorosilane and dimethylchlorosilane cohydrolysisl6. A variety of pass nanostructured chemicals contain one or more covalently bonded reactive functionalities that are suitable for polymerization, grafting, surface bonding, or other transformationsI7-ls. Incorporation of nanosized pass macromers into polymers has produced significant improvement in thermal and oxidative resistance as well as reduction in flammability of several PaSS-based polymers; these systems then being suitable candidates in high-temperature and fire-resistant applications 19-21. pass compounds can be incorporated into polymers by blending, copolymerization or grafting22-29.PaSS-polymer blends create interesting materials, but occurring of microphase separation may decrease possible advantages connected with nanoscale incorporation30. Organic polymers can be reinforced with pass by attaching single or multiple polymerizable groups to pass cage. In catalysis chemistry, metallosilsesquioxanes are receiving considerable interest3I.32. Incompletely condensed silsesquioxanes [cyclopenty Isilanetriol CY7Si709(OH)3] share structural similarities with B-cristobalite and B-tridymite and are thus quite realistic models for silanol sites on silica surfaces33.34. Incompletely- condensed silsesquioxane frameworks have attracted attention as models for silica35.36, as ligands in

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Page 1: Developments of polyhedral oligomeric silsesquioxanes ... › 33d3 › bdca48f0a1f000683a2ac21… · Journal of Scientific & Industrial Research Vol. 68, June 2009, pp.437-464 Developments

Journal of Scientific & Industrial Research

Vol. 68, June 2009, pp.437-464

Developments of polyhedral oligomeric silsesquioxanes (PaSS), passnanocomposites and their applications: A review

D Gnanasekaran, K Madhavan and B S R Reddy*

Industrial Chemistry Laboratory, Central Leather Research Institute, Chennai-600 020, India

Received 01 September 2008; revised 24 February 2009; accepted 13 March 2009

This review presents a brief account of recent developments in synthesis and properties of organic-inorganic hybrids usingpolyhedral oligomeric silsesquioxanes (POSS) nanoparticIe, and applications of POSS monomers and nanocomposites. Therma],rheology and mechanical properties of polyimides, epoxy polymers, polymethymethacrylate, polyurethanes, and various otherpolymer nanocomposites are discussed.

Keywords: Nanocomposites, Polyhedral oligomeric silsesquioxanes, Therma] and mechanical properties

Introduction

A variety of physical property enhancements(processability, toughness, thermal and oxidative stability)are expected to result from incorporation of an inorganiccomponent into an organic polymer matrix. Manyorganic-inorganic hybrid materiels have shown dramaticimprovement in macroscopic properties compared withtheir nonhybrid counterparts 1-6.Hybrid organic-inorganicmaterials based on incorporation of polyoctahedraloligomeric silsesquioxanes (PaSS) into polymericmatrices have received a considerable attention7-'3.

Silsesquioxanes (Fig. I) 14,which consist of a rigid,crystalline silica-like core that is perfectly definedspatially (0.5-0.7 nm), have general formula

(RSiOI)u(HP)o5b' where R is a hydrogen atom or anorganic group and a and b are integer numbers (a = I,2,3, ...; b = 0, 1,2,3, ), with a + b = 217, where 17 is aninteger (n= I, 2, 3, ) and b = a + 2. Of severalstructures of silsesquioxanes (random, ladder and cage),cage structures contain 8 silicon atoms placed at cubevertices. Cubic structural compounds (completely andincompletely condensed silsesquioxanes) are commonly

illustrated as T6, T7, Ts' T,o' and T12, based on thenumber of silicon atoms present in cubic structure(Fig. 2). In early 1900s, Kippingl5 found thatpolycondensation of siliconic acids invariably leads to

*Author for correspondenceTel.: (044) 24404427; Fax: 91(44) 2491 ]589

E-mail: [email protected]

extremely complex mixtures of silsesquioxanes. Later in1946, pass were isolated along with other volatilecompounds through thermolysis of polymeric productsobtained from methyltrichlorosilane anddimethylchlorosilane cohydrolysisl6. A variety of passnanostructured chemicals contain one or more covalentlybonded reactive functionalities that are suitable for

polymerization, grafting, surface bonding, or othertransformationsI7-ls. Incorporation of nanosized passmacromers into polymers has produced significantimprovement in thermal and oxidative resistance as wellas reduction in flammability of several PaSS-basedpolymers; these systems then being suitable candidates inhigh-temperature and fire-resistant applications 19-21.passcompounds can be incorporated into polymers by blending,copolymerization or grafting22-29.PaSS-polymer blendscreate interesting materials, but occurring of microphaseseparation may decrease possible advantages connectedwith nanoscale incorporation30. Organic polymers can bereinforced with pass by attaching single or multiplepolymerizable groups to pass cage.

In catalysis chemistry, metallosilsesquioxanes arereceiving considerable interest3I.32. Incompletelycondensed silsesquioxanes [cyclopenty Isilanetriol

CY7Si709(OH)3] share structural similarities withB-cristobalite and B-tridymite and are thus quite realistic

models for silanol sites on silica surfaces33.34.Incompletely­condensed silsesquioxane frameworks have attractedattention as models for silica35.36, as ligands in

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438

-~--~---···"'"._'.''''·'''tll'111'' 1IlII'III"jl," III '~III•.'III"._h'~lt"I*"'I"II'

J SCI IND RES VOL 68 JU:\E 2009

a b

S.O,.Oct.m~ri<:Core

V-O.065nm'

v = 0.9 nm'

c

lr

Fig. I-Silsesquioxanes: (a) Q8 (Q = Siam)' R =H, vinyl, epoxy, methacrylate, etc.; (b) RxTx(T = R-SiOJ/2),

R = alkyl, alkene, acetylene, acrylate; (c) Typical sizes/volumesl4

homogeneous models for aluminosilicates37.3Sand silicasupported catalysts39.40,and as building blocks for networksolids41,42.

Methodologies adopted for Functionalization ofPOSS Monomer

New chemical reagents and nanocomposites (NCs)have been developed using POSS molecule. Polyhedral

silsesquioxanes are prepared from monomers of XSiY,type, where X is a chemically stable substituent

(CH3, phenyl, or vinyl), and Y is a highly reactivesubstituent (CI, OH, or OR). Thus, using XSiY3-type,POSS can form linear, cyclic, or polycyclic siloxanes as

nXSiY3 + l.SnHzO - (XSi015)n + 3nHY ... (1)

Octahydridosilsesquioxane43'45 (TsH), results fromhydrolysis condensation ofHSiCl3 with very low yield ofproducts [TsH (15-20%) and T10H (5-10%) cubes].Kudo ct af46 explored on synthesis of POSS and entirereaction scheme (Fig. 3). Reaction rate, degree ofoligomerization, and yield of polyhedral compounds formeddepend strongly on following factors47: i) Concentrationof initial monomer in solution is an important variable: ii)Nature of solvent: iii) Identity of substituent X in initialmonomer; iv) Nature of functional groups Y in monomer;v) Type of catalyst: vi) Reaction temperature; vii) Rateof water addition; and viii) Solubility of polyhedral oligomersformed. Using these conditions, octameric silicate anionsystems were developed from hydrolysis andpolycondensation of tetraalkoxysilane in presence of

tetramethylammonium (TMA) ions [N~(CH,)4] underappropriate conditions, which yield double four membered

ring silicate anion (Sis020s.) selectively. Hasegawa ct a14S.49

extended concept of Hoebbel et apo and identified ahigh-yield route to obtain POSS cubes with hydride andvinyl functional groups by silylation of silicate anion

(Sis020S)solution (Scheme 1).Cubes with vinyl and Si-H groups permit selective

attachment of numerous moieties to cubes throughPt-catalysed hydrosilylation (PCH).Octa

(dimethylsilyloxy) silsesquioxane (QsMsH) and octa(hydrido) silsesquioxane (TsH) cubes can be modifiedinto various octafunctionalized macromonomers byhydrosilylation reaction between terminal Si-H groupson POSS cube with an unsaturated carbon double bond

in presence of a Pt catalyst (Scheme 2). A series ofpolyethYlene glycol (PEG) substituted octasilsesquioxanes51 prepared by PCH of unsaturated PEG

with QsMJI, TsH and thermal effect of organicperipherals attached on vertices ofPOSS were studied.Another example of hydroxyi terminated POSS is

reporled52 through PCH of QsMll with allyl alcohol.Controlling catalyst concentration and choosingappropriate solvent undergo 100% C-silylation ratherthan O-silylation of -OH group of allyl alcohol.Matisons et af53 reported preparation of octaisocyanato

subslltuteo silsesquioxane by hydrosilylation ofQsMsHwith m-isopropenyl-cx-cx' -dimethylbenzyl isocyanate inTHF medium using Karstedt's catalyst, and proved thatIsocvanate functionalised POSS macromer was suitable

nanocrosslinkcr for preparation of organic-inorganic1:ybrid polyurethane. A simple two-step syntheticmethodology was developed for octaphenol-POSS

,hrough PCE of 4-acetoxystyrene with QsMsH andsubsequent hydrolysis of acetoxy units 54.OgunniYI cf al55 reported synthesis of etheric chlorine­terminated silsesquioxane using 2-chloroethyl vinylether

I 'I'I I '1"11 I I' ·1· , ,

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GNANASEKARAN et al: POLYHEDRAL OLIGOMERIC SILSESQUIOXANES (POSS) NANOPARTICLES 439

0--J

R O-Si-R\/ /-0 O-Si 0__\/ \ /R-Si O-Si-R\ \

° R/0 °R-S( -~i O~ L,R

/ \ \ / Si° O-SiI / \R O-Si-R/\

Random

R R R R\ ....--0, / O--S!"'--O~ (

;5' '5;- / ' 51-° \ ° °\ ° \. / S' ,/

--f'~Si_O-; 1"0.--8{-R \ R R

ladder

Fig. 2-ChemicaI structures of different types of silsesquioxanes

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440 J SCI IND RES VOL 68 JUNE 2009

A HSi(OH)3

~ B" · No of Hydrogen bond

b) T2: H(OHl2SiOSi(OH)2H

G' ~

~J) \ /cp\

(0)

T8 : (HSi01·5) 8

Fig. 3-Schematic representation of all processes from silanol (A, HSi(OH))) to TiU)

I 'II I I 'I I I ~I l'

" 'I"h'.jlj~.,.,~II." 0"". " .. , •. ,

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GNANASEKARAN et al: POLYHEDRAL OLIGOMERIC SILSESQUIOXANES (POSS) NANOPARTICLES 441

Scheme I-Synthesis of vinyl and hydride functionalised silsesquioxanes

with TsR through PCR. Various epoxy functionalizedPOSS [Octakis(dimethylsiloxypropyl glycidylether)silsesquioxane, Octa( ethy1cyclohexylepoxide dimethyl­siloxy)silsesquioxane and octakis( dimethylsiloxyhexafluoropropy I glycidyl) silsesq uioxane] weresuccessfully synthesized through PCR by reacting

QsMsR with allyl glycidyl ether (AGE), 4-vinyl-1,2­cyclohexene epoxide and allyl-1,1,2,3,3,3-hexa­fluoropropyl ether respectively56,57.

Octakis( dimethylsiloxypropyl aminobenzoate)silsesquioxane have been successfully prepared by

reacting QsMsR with allyl-4-aminobenzoate through PCRreaction5s. Stradiotto et a/59 reported synthesis ofoctacyanosilsesquioxane by reacting acetonitrile with

QsMsR in presence of toluene medium. Various otherfunctional groups such as silanes and a-olefins substitutedPOSS through PCR reactions also reported60. Ifstoichiometry is correctly controlled, PCR also offers agood method for synthesis of mono-, di- and poly­substituted silsesquioxanes61 ..

Brown et a/62 extensively studied synthesis of anothertype of POSS, octaphenyl silsesquioxane (OPS) andvarious other side products formation during hydrolysis

and polycondensation of phenyl trichlorosilane in acetonemedium. Laine et a/63-65 successfully explored variousreactive functional groups (amine, iodo- and bromo- ) onphenyl ring ofOPS (Scheme 3). Laine et a/63 successfully

. nitrated OPS using fuming nitric acid.Octa(aminophenyl)silsesquioxane (OAPS) was preparedin two steps by nitration of OPS in fuming nitric acid toform octa- (nitrophenyl)silsesquioxane (ONPS) followedby mild reduction of ONPS using formic acid andtriethylamine in presence of PdlC catalyst, resulting inquantitative conversion with equal quantities of m- andp- nitro substituted in each phenyl ring. One step aheadof nitration of OPS, poly(bromooctaphenylsilsesquioxane)s (BrxOPS) was synthesised via

bromination with Br/Fe in dichloromethane64. Iodo­substituted OPS by iodination using iodinemonochloride(ICI) at -40°C gave highly crystalline, octaiodinated OPS,which shows 93% selectivity for p- substitution, in30-40% overall yield65.Feher et a/66 established a directsynthetic strategy for amine functionalized POSS throughhydrolytic condensation of aminopropyltriethoxysilane inMeOR and conc. RCI, but product obtained under theseconditions is aminehydrochloride salt. Octa

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442 J seI IND RES VOL 68 JUl'JE 2009

0--CNpt Catalyst

8»pt Catalyst

Scheme 2: - Synthesis and structures of octafunctional POSS macromonomers through hydrosilylation reaction

I' II I I l'

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GNANASEKARAN et al: POLYHEDRAL OLIGOMERIC SILSESQUIOXANES (POSS) NANOPARTICLES 443

4 HC02H / NEt3

Pd/C THF

Scheme 3-Synthesis and structures of octafunctionai pass macrCJl1cnomcrs containing nitro, amine, bromineilnd iodine groups

(aminehydrochloride) silsesquioxane IS highly soluble inwater and poorly soluble or insoluble in most of organicsolvents and is somewhat hygroscopic. To make stablePOSS amine, aliphatic amine ofPOSS using methacrylolchloride gave methacrylic acid groups that could be used

as nanocunstruction sites for nanohybrids.Sllsesquioxane bearing various other functional

groups67.70 (esters, norborenyl, choloro- andchloroammoniumpropyl) have been developed.Concentration and type of initial monomer groups alter

11'1 I 1111 I

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444 J SCI IND RES VOL 68 JUNE 2009

Scheme 4--Structures of mixed octafunctionaI POSS macromonomers

cubic structure by changing feed composition7! ofphenyltrimethoxysilane(PTMS) and 3- (methacryloxy)propyltrimethoxysilane(MPTMS). Four different molarratios of monomers PTMS/MPTMS were co-condensed

in presence of formic acid to yield four differenttypes of pass with various functional group ratios(Scheme 4).

Among pass monomers, mono- functionalisedsilsesquioxane is a suitable monomer for synthesis oflinearthermoplastic NCs. Comer capping reaction yields aclosed cube with 7 comers of inert groups (cyclopentylor cyclohexyl) and remaining one vertices possesses

highly reactive functional groups (hydride, chloride,hydroxide, nitrile, amine, isocyanate, styryl, olefm, acrylic,epoxide, norbomyl, bisphenol, acid chloride, alcohol, and

acid) (Scheme 5)72.Lichtenhan et a[13 synthesized mononorborenyl substituted cyclopentyl siloxane through comer

capping reaction of cylopentylsilanetriol CY7SiP9(OH)3with trichloronorborenyl silane. Haddad et a[14 derivedmonostyryl functionalised pass through comer capingreaction using trichlorosilane styrene in presence ofTHFmedium. Aminopropyl substituted pass was synthesisedby reacting aminopropyl triethoxysiloxane with

CY7SiP9(OH)/s. Zheng et af76 synthesisedhydroxylpropyl corner capped phenylsilanetriol.2-(Trichlorosilyl)ethyl acetate was end capped on iso­butyl-paSS and acetate was hydrolysed to yield hydroxylfunctionalised POSS77. A pass framework with two

anilino pendant groups was prepared in good yield using

diexo-(c-CSHg)gSigO/OH)2 followed by adoptingvarious synthetic procedures (Scheme 6)7g.

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GNANASEKARAN et at: POLYHEDRAL OLIGOMERIC SILSESQUIOXANES (POSS) NANOPARTICLES 445

R-SiCI3

R = Cydopentyt, Cyclohexyl

Isobutyl, Phenyl

R' = hydride

chloride

hydroxide

nitriles

amines

isocynates

styryls

olefins

acrylics

epoxides

norbornyls

bisphenolsacid chlorides

alcohols

acids

Scheme 5-Synthesis and structures of monofunctional POSS macromonomers

Hybrid Nanocomposites from CompletelyCondensed Silsesquioxane Monomers

POSSfPolyimide Hybrid Nanocomposites (NCs)

Laine et al63 synthesized OAPS as a robust nanosizedbuilding block for construction of materials with nanometercontrol between organic and inorganic components. Athree-dimensional polyimide63 was synthesized by using

OAPS with pyromellitic dianhydride (PMDA) inNMPand DMF solutions (Scheme 7). Polyimide NCsshowthermal stabilities >500°C (5% mass loss temperature)and char yields >75% even at 600°C. Resulting materialsare completely amorphous, and extreme rigidity of organictethers linking cube vertices prevents long-range orderoccurring during curing reaction79,so.

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446 J SCI IND RES VOL 68 JUNE 2009

76 %

+H IMeOH

59%

two steps

'\./\/

/9\Li

Scheme 6-Synthesis of diamine functionalised POSS macromonomer

Po1yimide-POSS NCS81,82 were prepared by reactingOAPS with po1yamic acid (PAA) solution for lowdielectric properties and improved thermo-mechanicalproperties. Zheng et al83 synthesizedoctama1eimidophenyl/POSS (OmipPOSS) NCs viaimidization reaction between OAPS and maleic anhydride.Thermosetting hybrids containing OAPS (10 wt%) wereobtained via in situ polymerization of dig1ycidy1ether­bispheno1 A (DGEBA) and 4,4' -diaminodiphenylmethane(DDM) in presence of OAPS. OmipPOSS NCscontaining 2.5 and 5 wt% of OmipPOSS displayedenhanced glass transition temperatures (Tg's) (178 and174°C) compared with control, epoxy resin (172°C). Onthe other hand, hybrids containing 7.5 and 10 wt% ofOmipPOSS have lower Tg's than control epoxy.Enhancement in glass transition temperatures is due tonanoreinforcement effect of POSS on polymer matrix.

At very high percentage (7.5 and 10 wt%), restriction ofrigid POSS was lost and bulky groups behaved likep1ascticizer. A1agar et al84 showed improvedthermomechanica1 properties of OmipPOSS NCs.

Po1yimide NCs are also obtained by reacting amineterminated PAA, with POSS containing epoxy pendantgroups (instead of OAPS). In NC, network linkagesbetween POSS cube and po1yimide molecules primarilyarise from curing reaction between terminal amine groupsofPAAand epoxy groups ofPOSS cube85.Epoxy POSSincorporated NCs show high rubbery state region andwith high tensile modulus 1045.6 Mpa at 400°C, which is23 times greater from conventional epoxy polyimidecomposites. Very low dielectric values (dielectricconstant=2.30) of polyimide-POSS was reported bysynthesizing mixed glycidyl epoxy and fluorinated epoxyfunctionalised POSS86. Polyimide's with low dielectric

I II '_._1

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GNANASEKARAN et at: POLYHEDRAL OLIGOMERIC SILSESQUIOXANES (POSS) NANOPARTICLES 447

o~'~Oo 0

Heat 1- ~O

o 0

- = -O-N~N-O-o 0

Scheme 7-Synthesis ofpolyimideIPOSS nanocomposites

constants (K=2.65) (pure PI, K=3.22), are obtained bythe use of aliphatic amine POSS, but, thermal stabilitiesof NCs became poor due to weak aliphatic groups ofPOSS molecules87•

Wei et aZS8 reported about linear polyimide by reactingdiamine functionalised POSS with dianhydrides. But,NCs obtained using these diamine functionalized POSSshows lower thermal properties than that of pure polyimidedue to degradation of cyclopentyl groups present in7 corners of POSS molecules (at 400°C). In-planecoefficient of thermal expansion ofPOSS/polyimide NCsincreases with POSS amount as a result of increase in

free volume in polyimide due to POSS molecules. Self­assembly architecture was observed for polymer chainat 10% loading of POSS molecule. As POSS amountincreases to 10 mol %, decrease in Young's modulus,maximum stress and elongation become more apparent(10%). But, dielectric constant ofNCs got lowered andthis could be tuned by varying mole ratio ofPOSS content.Oikawa et aZS9 synthesized polyimide NCs using double­decker-shaped POSS molecule (Scheme 8). In first step,double-decker-shaped silsesquioxane containing aromatictetracarboxylic dianhydride (DDSQDA) end groups weresynthesized. This dianhydride is derived from DDSQ­diamine by functional group conversion method. POSS­Polyimide (POSS-PI) exhibits good thermal stability upto 500°C and POSS molecule in main chain (POSS-PI)

having high molecular weights (up to ?inh=0.6ldUg) andparticularly, fluorinated POSS- PI shows' good combinedcharacteristics, including low dielectric constant (K.=2.43)with good mechanical properties (tensile strengths, 72.3MPa; initial modulus, 1.8 GPa; and elongation at breakage,8.0-15.9%), excellent optical properties, and isorganosoluble as a good candidate for microelectronicapplications.

Poly(methyl methacrylate) (PMMA)-POSS graftedpolyimides were synthesized by thermally initiated free­radical graft copolymerization of methacrylcyclopentyl­POSS (MA-POSS) with ozone-pre activated PAA,followed by thermal imidization to achieve material withlow dielectric values. Dielectric constant of film can be

tuned by varying molar ratio of grafted MA-POSS sidechains in copolymer90• Brunsvold et al91 graftedsilsesquioxane through a two-step chemical process.(3-Aminopropyl)(hepta-iso-butyl) POSS was covalentlyattached to polyimide backbone containing reactive acidchlorides. PolyimidelPOSS NCs could be developed bycrosslinking amine functionalsied trialkoxysilane precursorwith dianhyrdides92•95•

POSS/Epoxy Hybrid Nanocomposites (NCs)

Reactivity of mono- and multi- functional POSS-epoxymonomers and their effect on the formation of epoxy­amine networks containing POSS as a pendant unit or as

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448 J SCI IND RES VOL 68 JUNE 2009

o

O~"<:::::, 0# 0 +

o o

IToluene1050 C. 25h

Scheme 8-Synthesis ofpolyimide/POSS nanocomposites containing double-decker shapedsilsesquioxanes

junctions was evaluated96• pass epoxy monomers areless reactive towards amines than DGEBA, may be dueto sterical crowding around epoxy groups caused by inertpass substituents and also due to reduced epoxy group'smobility. Epoxy pass could be chemically orphotochemically cured to give hard, scratch- and solvent­resistant materials containing up to 65% masked silica97•

Epoxy NCs with completely defined organic/inorganicphases were prepared by reactingoctakis(gl ycid yldimeth ylsiloxy )octasilsesquioxane( OG)with DDM at various compositions to alterthermomechanical properties98 (Scheme 9). Fromresulting material, moduli and fracture toughness ofNCswere found less than those of organic resins. Choice ofshort-chain, polyfunctional epoxies99 combined withnontraditional resin stoichiometries is a possible way tocontrol epoxy resin co-effiecient of thermal expansionsover an order of magnitude. An additional advantage isthat epoxy-PaSS resin has a low viscosity of about 350CPs at room temperature, making it suitable for composite

processing techniques and thermomechanical stabilitiesare better than that of standard Ciba epoxy resins100, 101.

Chen et aP02 investigated epoxy NCs using octa(2,3­epoxypropyl)silsesquioxane (OE) with diamines of 4,4'­methylenedianiline (DDM) and 5-trifluoromethyl-l,3­phenylenediamine (FPA). Resultant fluorinated epoxyNCs show Tg of 170°C, which was higher than diglycidylether of bisphenyl A (DGEBA)/DDM at the samestoichiometric ratio. It also possesses excellent thermal,mechanical, and dielectric characteristics with highstorage modulus of 1.8 GPa (30°C) and 0.3 GPa (250°C),low coefficient of thermal expansion of86 /lm/moC, anddielectric constant of 2.19. Polybenzoxazine(PBA)­epoxylPOSS nanocomposites (40 wt% of paSS) wasobtained through intercomponent reaction betweenphenolic hydroxyls of PBA and epoxide groups ofepoxyPOSSI03. Introduction of pass, thermal stabilityof polymer matrix and oxidation resistance of materials

were significantly enhanced. Nanoscale dispersion ofpass in NCs «30 wt% paSS) displayed higher storage

I 'I

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GNANASEKARAN et al: POLYHEDRAL OLIGOMERIC SILSESQUIOXANES (POSS) NANOPARTICLES 449

Epoxy-polymer

--- =

Scheme 9-Synthesis of epoxylPOSS nanocomposites

moduli in glassy state than control PBA. But, storagemoduli for NCs (> 30 wt% POSS) are lower than that ofcontrol PBA, and this could be responsible for porosityof NCs. Enhanced thermal properties and goodhomogeneity of epoxy/POSS NCs was obtained byreaction of DGEBA with monoalkylamine functionalPOSS and microphase separation of material could becontrolled by small molecule curing agentslO4• POSSmonomers containing 8 aliphatic amine groups on verticeswere first incorportated into DGEBAI05. Improvedthermal stability was achieved on increasing chain lengthof diamine with diepoxy hexavinyl POSSI06• Life time ofepoxy NCs at different temperatures was predicted usingkinetic parameter, based on thermal degradation ofNCS107• Lu et al108 also investigated thermal propertiesand curing kinetics of epoxylPOSS hybrid networks.

Poly(methacrylate)/ POSS Hybrid Nanocomposites (NCs)

Matisons et a[l°9 reported various methyl methacrylatefunctionalised POSS macromonomers with different PEG

chain lengths and then polymerized with methylmethacrylate to produce a series of hybrid materials.Although POSS is a bulky and multifunctional crosslinker,glass transition temperature increase with an increase inPOSS concentration. In some instances, group bulkinesscreated a free volume and chain separation, which led toa reduction in glass transition temperature. IncorporationofPOSS always increases thermal stability. By applying

ATRP methodl1O, PMMA-POSS hybrid NCs wereprepared and their thermal properties were investigatedHybrids were obtained by use of octafunctional octakis(3­hydroxypropyldimethylsiloxy)octasilsesquioxane (ORPS)and OAPS nano-cages as ATRP initiators. No significantimprovement was observed in thermal stability ofNCsas compared with linear PMMA. MandaI et allIl

developed an efficient and versatile method for synthesisofPOSS-polymethacrylate hybrids (Scheme 10), suchas POSS-poly(methyl methacrylate) (POS,s .•.PMMA),POSS-poly(ethyl methacrylate) (POSS-PEMA), andPOSS-poly(benzyl methacrylate) (POSS-PBzMA) ofcontrollable molecular weights and low polydispersitiesby thiol-mediated radical polymerization at elevatedtemperature (l00°C). POSS content in these hybridmaterials could be varied, by tuning reactantconcentrations and degree of polymerization of graftedpolymethacrylate chains. Wu et al1I2 studied surfaceproperties of poly(methyl methacrylate-co-n-butylacrylate-co-cyclopentylstyryl polyhedral oligomericsilsesquioxane) [poly(MMA-co- BA-co-styryl-POSS)]terpolymers and found that incorporation of styryl-POSSinto polymer resulted in increasing interactions betweenpolymer and solvent, dispersive component of surfacefree energy of polymer and acidity of surfaces ofpolymers. Toughness propertyll3 ofPMMA using POSSnanocages at loadings between 0 and 15 wt% wasinvestigated using Cyclohexyl-POSS, methacryl-POSS

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450 J SCI IND RES VOL 68 JUNE 2009

liS

AIBN •

s· Methacrylate propagation••

Hexane Termination

Scheme 10--Synthesis ofthiol mediated polymethacrylate/POSS nanocomposites

and trisilanol-phenyl-POSS. Binary blends of pass andPMMA were able to improve impact toughness ofPMMA. In order to toughen PMMA with rigid fillers,weakly adhering particles (size, 100 nm) are required. Liet al1I4 synthesized a soluble poly(MMA-co-octavinyl­paSS) hybrid by common free radical polymerizationand resulting hybrid NCs show higher Tg and betterthermal property than parent PMMA homopolymer. Tgimprovement results from dual contributions of motionhindrance ofPMMA chain imposed by nanometer passcores and dipole-dipole interaction between PMMA chainsand pass cores. Thermal stability enhancement is mainlyattributed to incorporation of nanoscale inorganic passuniformly dispersed at molecular level. Silverstein et a[lls

reported that degree of polymerization increased withincrease in pass content for linear PMMA-POSS NCs

and shows a relatively low PDI than conventional PMMA.

Gerard et all16 synthesized polymer networks either aslinear chains or as cross linked PMMA-POSS NCs.

pass-pass interaction was found to be main parametergoverning network morphology. However, dynamicmechanical properties remain nearly at the same level asneat matrix. Multifunctional pass shows a highermiscibility with dimethacrylate monomer and dispersesvery well in cured network. Molecular dynamic simulationstudies were carried out for PMMA-POSS NCslI7.

Norborenyl/POSS Hybrid Nanocomposites (NCs)

Mono- and tri- functionalized norbornenyl-POSS arecopolymerized with dicyclopentadiene(DCPD). passloading I 18 slightly decreases thermal oxidative resistanceof copolymer before primary decomposition temperatureof PDCPD network due to lower thermal stability ofpass. Modulus and yield stress, compression, and

I II 1III 11'llil;III' IIIII I 'Iii

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X~enes.Base ' D---Sid

RellUX4h~\\o I

\/O/~IS~O_--~\R

R=C)"lopentyl

Scheme I I-Synthesis ofnorbornene block polyethylene-POSS random copolymers

fracture toughness of copolymers decrease with POSSloading, irrespective of cross link density. For mono­functionalized POSS, decrease in toughness is inagreement with loss of irreversible damage occurringduring fracture. Microstructural and mechanicalrelaxation investigation was carried out for norbomene/POSS-norbornene hybrid polymersl19• POSS

copolymerization enhances a-relaxation temperature, T.in proportion to the weight fraction ofPOSS-norbomenylcomonomer. However, magnitude of this dependence islarger for POSS-norbomenyl comonomer possessingcyclohexyl comer groups (CyPOSS) than for copolymerwith cyclopentyl comer groups (CpPOSS). Norbomeneblock polyethylene-POSS random copolymers73(Scheme 11) were obtained by ring-opening metathesiscopolymerizations of cyclooctene and POSS monomer1-[2-(5-norbornen-2-yl)ethyl]-3,5,7 ,9,11,13,15­heptacyclopentylpentacyclo [9.5.1.13,9.15,15.17,13]octasiloxane (POSS- norbomylene) using Grubbs'scatalyst, followed by reduction of double bonds in thecopolymers. Polyethylene-POSS copolymers showed a70°C improvement, relative to a polyethylene controlsample of similar molecular weight. Kwon et a[l20

reported synthesis ofPOSS-containing block copolymersfor the first time via living ring-opening metathesispolymerization.

Polyurethane(PU)/POSS Hybrid Nanocomposites (NCs)

Linear PU/POSS NCS121 were obtained throughbisphenol functionalised POSS (Scheme 12).

Polyurethane networksl22 with POSS were obtained byreacting OAPS with isocyanate terminated polyurethaneprepolymer. Thermal stability of hybrid polyurethanes ismore than conventional PU elastomer due to nanoscale

reinforcement effect ofPOSS on polyurethane networks.In glassy state (-75 to -25°C), all POSS-containing hybridsare significantly higher than that of control PU even at2 wt% ofPOSS incorporation. PU-POSS networks havebeen synthesized by taking epoxy-POSS monomerl23•

Thermal and mechanical properties show similar trendto that of PU-POSS networks prepared using OAPSmacromonomer. Contact angle measurements showedthat organic-inorganic NCs displayed a significantenhancement in surface hydrophobicity, as well asreduction in surface free energy. Improvement in surfaceproperties was ascribed to the presence ofPOSS moietyin place of polar component of polyurethane. Wu et a[l24

studied thermal and rheology properties of hybridpolyester-POSS resins and PU-POSS using1-(2,3-propanediol)propoxy-3,5, 7,9,11,13,15­isobutylpentacyclo-POSS diol cured with isophoronediisocyanate (IPDD trimer. Pure polyester resin displayedNewtonian flow behaviour, but all resins with POSSpresented a first shear-thinning branch.

POSS/Polystyrene Hybrid Nanocomposites (NCs)

Polystyrene-POSS NCs (Scheme 13) were obtainedby free radical polymerization using AIBN-initiator74•Both homopolymers and copolymers with varyingproportions ofPOSS content modifies thermal properties

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452 J seI IND RES VOL 68 JUNE 2009

Scheme 12-Synthesis ofpolyurethane/POSS linear copolymer

drastically and allows for tailoring polymer softeningtemperature (> 200°C range). Thermal stability, compared

with polystyrene modified with pendent C60 fullerenes(diam 7.07 A), shows a Tg of 166°C for an approx. 11

mol% C60 incorporation. This is comparable to Tg's seenin 9 mol% polystyrene-POSS copolymers. A series ofhybrid organic/inorganic triblock copolymers125,126 ofpolystyrene-butadiene-polystyrene (SBS) grafted withPOSS molecules with different chemical constituents

groups (phenyl, cyclopentyl, cyclohexyl and cyclohexenyl)in POSS cube have been investigated and observed thatPOSS with phenyl moiety, when grafted to polybutadiene(PB) phase, appears to show favourable interaction withpolystyrene (PS) phase and Ph-POSS plasticizes SBS.

Dramatic improvement in thermal properties ofNCs wereobserved for poly(styrene-co-octavinyl-polyhedraloligomeric silsesquioxane) (PS-POSS) organic-inorganichybrid NCs containing various percent ofPOSS preparedvia one-step free radical polymerization127• Ferrocenemetal bonded polystyrene-POSS NCS128 weresynthesized and exhibit paramagnetic properties with aremnant magnetization of 0.035 emulg and indicated thatpolystyrene-POSS shows ideal soft magnetic properties.

Nanocomposites (NCs) based on IncompleteSilsesquioxanes

Heptameric siloxanes with partially formed cagescontaining 2 or 3 residual silicon hydroxyl functional

I 'II II' j I I "" ..:t

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R= Cyclopentyl, Cyclohexyl

QCI-Si-CI

ICI

NEt3 in THF•

AIBN BOoC

4-Methyl styrene! Tolvene

Scheme l3-Synthesis of polystyrene/POSS nanocomposites

groups, obtained through hydrolysis/condensation of alkyl­or aryl trichlorosilanes, are called as incompletesilsesquioxane. Feher et al129,I30 synthesized partial cagesilsesquioxane from hydrolytic cleavage of completelycondensed POSS. A variety of NCs were obtainedthrough reactive silicon hydroxyl functional groups(-SiOH)(Scheme 14). Pittman et af131 reported effect of partiallycaged trisilanol phenyl POSS on thermal and mechanicalproperties of cyanate~ester NCs and suggested that three­hydroxyl groups in POSS aid solubility into cyanate-esterresin and also reacts with cyanate ester resins at hightemperatures to aid dispersion. Storage moduli (E) ishigher than those of pure cyanate ester over entire

temperature range of35-350°C. Continued curing leadsto aggregation ofPOSS, which is bound to resin moleculesand aggregation process depends on concentration ofPOSS. There is a formation ofSi-O-Si bonds from sHanol

groups ofPOSS also occurring, both intramolecularly andintermolecularly at 250-300°C. Latter could create dimersor trimers of POSS when there is enough freedom ofmotion or when molecules ofPOSS are in close proximity.Storage modulus for 1, 3, 5 and 10 wt% POSS are greaterthan that of pure resin composites except when 15 wt%ofPOSS was present. Also, Tg of 1, 3, and 5 wt% POSScomposites, flexural strength and flexural modulus arehigher than that of neat cyanate ester composites.

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H~O

OHIOH

oC

IN;TolueneNf / DBTLC

a

Scheme l4--Synthesis and structures of polyester, polyurethane, polydimethylsiloxane and cyanate esternanocomposites using partially caged silsesquioxanes

Choudhury et alI32 reported synthesis of transparenthybrid polyurethane using partially caged pass for thinfilm coating. Partially caged pass components retaintheir partial cage structure after curing and impart highglass transition temperature and high thermal stability toproduct as compared to conventional polyurethanes,which is essential in their application as weatherablecoatings. Application of hybrid as a coating resulted information of a homogeneous "ue-phase lamellar coatingwith excellent coverage of substrate. Similarly, preventionof discoloration of poly(ethylene terephthalate) (PET),during melt processing, by usage of trisilanolisobutyl­pass have been reported133• Thermal studies displayedthat partially caged pass increases thermooxidativestability with increase in shear storage modulus, indicating

better melt elasticity and a broader window ofprocessability of material for commercial applications.Synthesis of polyurethane NCs based on completely andincomplete condensed silsquioxanes have been reportedand effect of dual cage structure on thermal, mechanicaland morphological properties was evaluatedI34•

Completely condensed silsesquioxane decreases thermaland mechanical properties ofNCs with increase in passaggregation ofPU matrix.

Influence of trisilanol phenyl-PaSS on thermo­mechanical properties and curing of epoxy-aminenetworks were investigated135• Phenyl-trisilanol polyhedralsilsesquioxane is highly soluble in DGEBA epoxy resinand addition ofPOSS-triol improves cross-linking reactionof epoxy-amine networks. Methyl silicone resinIPOSS

I 'II I

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composites with various proportions ofPOSS monomerusing trisilanolisobutyl-POSS were synthesizedI36,137,Thermal stability of methyl silicone resin greatly improvedby introduction of POSS cages. Segmental motion ofPDMS chain was retarded by large mass and sterichinderance of bulky POSS. Organic-inorganic hybridcomposites evolved via in situ polymerization ofDGEBAin presence of trisilanolphenyl- POSS 138.Phase separationof trisilanolphenyl-POSS induced by reaction occurred,and heterogeneous morphology was obtained for hybridcomposites. Spherical POSS-triol particle aggregates(diam, 0.3-0.5 JLm) were dispersed in continuous epoxymatrices. Phase separated NCs show very high Tg's andwith high storage modulus in glassy states. Dissolvingtrisilanolphenyl-POSS and phenolic resin into THF,followed by solvent removal and curing prepared phenolic­POSS NCs. Both nano- and micro-sized POSS filler

aggregates and particles were heterogeneously dispersedin cured matrix 139.POSS silanol/phenolic hydroxylhydrogen bonc~ing increases compatibility, which isbeneficial for POSS dispersion in matrix that does notprevent phase separation. Thermal stability and Tg ofNCs is only slightly affected by loading ofPOSS.

Various Other POSS Nanocomposites (NCs)Crosslinked polysiloxanes were directly synthesized

by anionic ring-opening copolymerization of octaisobutyl­POSS as a multifunctional monomer with

octamethy1cyclotetrasiloxane (D4) using base catalysts[potassium hydroxide (KOH) or tetramethylammonium

hydroxide (Me4NOH) siloxanolate], indicating thatcrosslinked polysiloxanes exhibit distinct Tg and excellentthermal stability 140.Synthesis ofliquid crystal POSS andspecific problems connected with the nature ofsilsesquioxane cage, and special properties that theirgeometry imparts to their mesogenic behaviour of liquid­crystal polyhedral silsesquioxane materials have beendescribed 141.Synthesis of these materials is based on asilsesquioxane cage modification process, starting with asuitably functionalized cage, to which mesogens areattachedl42. Many systems are obtained based onhydrosilylation reaction of hydrido-silsesquioxanes andmeso genic groups 143.Richardson et a[l44 reported ahexadecamer, first-generation, octasilsesquioxane liquid­crystalline dendrimers.

POSS-POSS nanohybrids were synthesized byhydrosilylatively copolymerized with stoichiometricamounts of octavinylsilsesquioxanes, with TgH and

QsMsH in toluene using Pt catalystl45. POSS cubes with

longer spacer groups are more reactive than those withno spacer groups. Pores within POSS cube interiors(diam, 0.3 nm), and pores between cubes (diam, 1-50nm) were determined according to nitrogen absorption,positron annihilation lifetime spectroscopy (PALS), andsmall angle X-ray scattering (SAXS) data. Benzoxazine/POSS hybrids were synthesized by using octafunctionalcubic silsesquioxane (MBZ-POSS) with 8 organiibenzoxazine tethers as a curing agentI46-14S. Inbenzoxazine/POSS hybrids, POSS aggregates occur inlarger scale at higher POSS contents and reason forheterogeneous phase separation may be from lesscompatibility of inorganic silsesquioxane core with organicbenzoxazine species. During formation ofpolybenzoxazine/POSS hybrids, POSS particles wereseparated from polybenzoxazine rich region, leading toPOSS rich domains (50-1000 nm). Zheng et azt°3 OGwas used to prepare polybenzoxazine(PBA)/POSS NCs.Crosslinking reactions involved during formation ofpolybenzoxazine(PBA)/POSS NCs can be divided intotwo types: 1)Ring-opening polymerization ofbenzoxazine;and 2) Subsequent reaction between in situ formedphenolic hydroxyls of PBA and epoxy groups of OG.Other polybenzoxazine/POSS NCs obtained by reactionof OAPS and 2,2' -(1,3-phenylene)-bis(4,5-dihydro­oxazoles) (PBO) are reported149. Dynamic mechanicalanalyses indicated that NCs exhibited higher Tg valuesthan pristine PBZ and PBZ-PBO resins. Storage modulusof NCs was maintained at higher temperatures even witha small amount of OAPS incorporated into compositesystems. Thermal stability of hybrid was also improvedby inclusion ofOAPS. Crosslinked poly(4-vinylpyridine)/POSS NCs were obtained by the reaction of epo~y groupofOG with pyridine ring of poly(4-vinylpyridine) 150.

Amphiphilic silsesquioxane derivative, 1-(1,0)­propylenemethoxy )oligo(ethyleneoxide)- 3,5,7,9,11,13,15­heptahydridopentacyclo [9.5.13,9.15,15.17,13]

octasiloxane has been prepared by reacting TsH and allylfunctional oligo(ethyleneoxide) (Mn=750glmol) throughPCHI5l. Associative behaviour of new' amphiphilictelechelics containing POSS as an end-group of PEG of

varying chain length welf investigated using capillaryviscometry. Viscosities were strongly affected by solventcomposition in THF/water mixturesl52.

POSS modification increases storage modulus and

Young's modulus of poly.amides, slightly decreases theirTg from 312° to 305°C, and signi(icantly lowered theirdielectric constants from 4.45 to 3.35153.Supramolecularinclusion complexation (ICs) of POSS-capped

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polycaprolatone (PCL) with a-and y-cyclodextrins (CL)were derived.76,77,154 Crystallization kinetics ofsilsesquioxane based hybrid star, poly(3-caprolactone)was investigated by synthesising a series of silsesquioxanebased hybrid star poly(3-caprolactone) having differentarm lengths through ring-opening polymerisation of3-caprolactone155.

Poly(alkyl silsesquioxane), PASSQ, copolymersconsisting of various chemically linked alkyl units overmethyl group as a pore forming moiety, and 1,2- bis­trimethoxysilylethane (BTMSE), were synthesized andtheir thermo-mechanical and optical properties wereinvestigatedl56. Higher alkyl groups in PASSQ werecomposed and thereby lower refractive indices wereachieved from 1.45 to 1.27 due to formation of nanoporesin film. Modulus for BTMSE based PASSQ films weresignificantly higher than 3.8 GPa of typical thin film ofpoly(methyl silsesquioxane). Smith et atl57 synthesizedand characterized PFCB aryl ether copolymers andmultiblock copolymers with pendant cyclobutyl and iso­butyl-functionalized POSS cages. Synthesis ofPOSS aryltrifluorovinyl ether (TFVE) monomers was accomplishedby condensation of commercial monosilanolalkyl-POSSwith a TFVE-functionalized chlorosilane. POSS/PFCB

aryl ether copolymers demonstrated excellent solutionprocess ability, producing optically transparent and flexiblefilms. Incorporation ofPOSS showed no change in thermalstability as compared to PFCB aryl ether homopolymer.

Blends of POSS Nanocomposites (NCs)Polysiloxane composites containing POSS were

prepared by melt blending. Crosslinking polysiloxanecaused changes in POSS solubility that enhanced phaseseparation. But crosslinks caused constraints, whichdecreased domain sizes of precipitated phases158.Octamethyl-POSS-HDPE NCs were prepared by meltmixing route. Joshi et atl59 observed that POSS does notinterfere with crystallization of HDPE. At lowconcentrations (0.25-0.5 wt%), POSS particles act aslubricants, lead to decrease in complex viscosity ascompared to neat HDPE, show significantly high storagemodulus and also enhanced thermomechanical propertiesthan HDPE.

Composites ofpoly(methyl vinylsiloxane) with POSSwere prepared by melt blending showing that highlycrystalline POSS macromers could undergo condensationreactions at 230°C in air, leading to partially amorphousstructures. Also, POSS crystals apparently dissolved inpolysiloxane at high temperatures and POSS crystals with

hexahedral or flake-like structures recrystallized out uponcooling. Both crystallites and POSS molecules co-existedin these blends, with the amount of dispersed molecularPOSS being increased at higher temperatures. POSSmolecules exhibited some physical interactions with freepolysiloxane chains that are not crosslinked. But,crosslinking induced phase separation. In curing process,POSS molecules could react with polysiloxane, resultingin decreases in crosslink density. Original POSS crystalscould also be dissolved in polysiloxane during initial curingstages, but recrystallization upon cooling gaveregenerated crystals that were roughly sphericaP60.Thermal properties and morphological development ofisothermally crystallized isotactic polypropylene (IPP)blended with nanostructured POSS molecules at verysmall loading ofPOSS were studiedl61.

Maitra et ap62 grafted various oligomeric PEO with

different chain lengths (n=2,4,8,12) onto QgMgH, andreported that silica surface affect thermal behaviour ofoligomeric PEOs. Most dramatic effect was observedfor PEO(n=4), where originally crystalline materialbecame completely amorphous; 4 PEO repeat units wereinsufficient for crystallization to occur at surface and PEOoligomers crystallized with increasing side chain length.Maxima in ion conductivity qbserved for PEO chainlengths (n=4 and 8) have been attributed to the absenceof crystallinity as well as enhanced mobility of short sidechains in comblike polymers. Mya et a[l63 discussedcrystallization behaviour of star-shaped poly( ethyleneoxide) with cubic silsesquioxane (CSSQ).

Bridged PolysilsesquioxanesBridged polysilsesquioxanes (BSSs) represent class

of highly crosslinked hybrid organic-inorganic polymers,which are derived from hydrolytic polycondensation oforgano-bridged silsesquioxane precursors having general

molecular formula (RO)3-Si-R'-Si-(OR)3' where R andR' are organic groupsI64,165.BSSs, prepared by sol-gelpolymerization of organo-bridged trialkoxysilaneprecursor, are network polymers, in which basic buildingblock contains two silicons directly attached to ahydrocarbon bridging group. Remaining three bonds toeach silicon are siloxane linkages. By connecting two ormore silsesquioxane groups to R' , a material with as manyas six siloxane linkages (Si-O-Si) per monomer unit canbe prepared, as opposed to just four in tetraalkoxy silanes.By introducing hydrocarbon spacers into siloxanenetwork, properties (hydrophobicity, surface area, poresize, UV-visible absorption, and fluorescence) can be

I 'I1

I 'I ~" 1 ' I " II" ''I'

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significantly modifiedl66,167.These materials allow to haveproperties (porosity, permeability, permselectivity,chemical functionality, and chemical, mechanical, andthermal stability) to be fine-tuned because of vast varietyof synthetically available monomersI64,165,168,169.Honma et ai170,171 synthesized a protonic conductivepolysilsesquioxane membrane containing PEG,polypropylene oxide and polytetramethylene oxide bridginggroups functionalized with isocyanotopropyltriethoxysilaneand condensed in presence of phosphotungstic acid ormonododecyl phosphate. Khiterer et ail72 reported BSSscontaining covalently bound sulfonic acid group materialsthat were utilized to prepare mechanically stable gelmembranes for fuel cell application, BSS molecules areutilized as precursors for synthesis of periodic mesoporousorganosilicas (PMOs) or bifunctional PMOs. Newmethodology was adopted for preparation of PMOs filmsby adopting evaporation-induced self assembly procedureto obtain spherical NPs from initially developed basicaqueous medium precipitation procedure173.

Surfactant-directed self-assemblyI74,175 and self­directed assemblyI76,177methods have been successfullydeveloped for fabrication of BSSs with well-organizedstructure. Self-directed assembly that takes advantageof weak interactions, such as H bonding, p-p, and/orhydrophobic interactions between bridging groups (R'),provides a very easy method for fabricating hierarchicalstructure. By judicious choice of organic substructure(R') in precursor, new intrinsic nanomaterials includingboth the nature of molecules and their collective propertieswithin aggregate will be realized 178.All mesoporousorganosilicas contain aliphatic organic groups withrelatively short chains (ethane or ethylene) or aromatic(arylene, thiophene and biphenylene) moieties174.179-182.A very few reports are available for amines and thiolsincorporated PMOSI83.

Applications

Poly(carbonate-urea)urethane (PCU)/(POSS) NCswere potentially used in cardiovascular bypass grafts andmicrovascular component of artificial capillary beds.POSS NCs possess greater thromboresistance thanpolytetrafluoroethylene and poly( carbonate-urethane),making it an ideal material for construction of both bypassgrafts and microvessels 184,185.Kaneshiro et ai186 reportedthat L-Iysine dendrimers with an octa(3­aminopropyl)silsesquioxane core (OAS) with 4th

generation is a suitable candidate for controlled in vitrogene delivery and transfection in human breast carcinoma

cells (MDAMB- 231 cells). Preliminary toxicityevaluation predicted that L-Iysine dendrimers is anexcellent biocompatibile NC. POSS monomer (POSS­MA) was used as a novel dental restorative composites

to place commonly used dental base monomer 2,2' -bis­[4-(methacryloxypropoxy)- phenyl]-propane (Bis­GMA)187.Amino functionalized silsesquioxane providecurl retention for hairl88.

Polyfluorenes/POSS NC shows maximumluminescence intensity and quantum efficiency, which isalmost twice as good as those of PFO EL device, and isan excellent material for optoelectronic applications 189.Introduction of POSS moieties into PPY s improves ELproperties of PPY derivatives. Improvement in ELproperties of POSS incorporate PPY is due to formationof suitable insulation domains of POSS moieties in

conjugated polymer matrices, resulting in a balance ofcharge carriers of electrons and holes 190.Castaldo etail91 presented polymeric NC sensors, based on a POSSby selecting proper matrix such as poly [(propylmethacryl­heptaisobutyl- POSS)-co-(n-butylmethacrylate)] and asuitable choice of other external home-made fillers

(graphite, copper, silicon, zinc and their alloys). Hybridscould be used in sensing of both polar and apolar analytes.Sulfonic acid containing bridged POSS hybrid materialsis used as proton-exchange membranes for fuel cellapplications and electrolyte materials are suitablematerials for automotive industryl72.

Polyphenylsilsesquioxane is used as interlayerdielectrics and protective coatings films for semiconductor devices, liquid crystal display elements,magnetic recording media and optical fiber coatingl87.Polymethyl silsesquioxane is used as an additive materialin cosmetics, polypropylene films and methacrylic resins.Polymethyl silsesquioxane with epoxy containingsiloxanes adheres well to rubber and plastics and couldbe used to provide nonsticking, water repellant andabrasion resistant films on paper, rubber, plastics andmetalsl87. Poly(aminopropyl silsesquixane) and specificcarbonyl compounds of silsesquioxane acts as anantitumour agentl87. Silsesquioxane films, particularlyOAPS/imide and OAPS/ epoxide films, provide excellent

O2 barrier properties, and is an ideal candidate forpackaging applicationsl92. In PYC, POSS behaves as aplastizecer like dioctyl phthalate (DOP) and could be usedas a plastizicerl93.Metal containing POSS cages (gallium­containing cage silsesquioxanes andaluminosilsesquioxane) 194,195have been synthesized, forthe use of silicasupported metal catalysts.

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Conclusions

Each specific pass can behave differently in aspecific resin attributed to size of pass cage, nature oforganic periphery, number of reactive functionalities, andconcentration and solubility of pass in the resin. Thesefactors determine whether pass is incorporated asisolated and uniformly dispersed molecules, unreactedand phase separated particles, or matrix-boundaggregates. Different morophologies affect physical andmechanical properties of final material. There has beenenormous growing application for pass NCs. passmonomers and some copolymers of pass arecommercially available in Hybrid Plastics Company (http://www.hybridplastics.coml).FountainValley.CA.

AcknowledgementsD Gnanasekaran thanks DST, New Delhi for financial

support (No. SR/SlIPC-33/2006).

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