Developments of Polyhedral Oligomeric Silsesquioxanes (Pass), Pass Nanocomposites and Their Applications: a Review

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 extremely complex mixtures of silsesquioxanes. Later in A variety of physical property enhancements 1946, pass were isolated along with other volatile (processability, toughness, thermal and oxidative stability) compounds through thermolysis of polymeric products are expected to result from incorporation of an inorganic obtained from methyltrichlorosilane and component into an organic polymer matrix. Many dimethylchlorosilane cohydrolysisl6. A variety of pass organic-inorganic hybrid materiels have shown dramatic nanostructured chemicals contain one or more covalently improvement in macroscopic properties compared with bonded reactive functionalities that are suitable for their nonhybrid counterparts 1-6.Hybrid organic-inorganic polymerization, grafting, surface bonding, or other materials based on incorporation of polyoctahedral transformationsI7-ls. Incorporation of nanosized pass oligomeric silsesquioxanes (PaSS) into polymeric macromers into polymers has produced significant matrices have received a considerable attention7-'3. improvement in thermal and oxidative resistance as well Silsesquioxanes (Fig. I) 14,which consist of a rigid, as reduction in flammability of several PaSS-based crystalline silica-like core that is perfectly defined polymers; these systems then being suitable candidates in spatially (0.5-0.7 nm), have general formula high-temperature and fire-resistant applications 19-21.pass (RSiOI)u(HP)o5b' where R is a hydrogen atom or an compounds can be incorporated into polymers by blending, organic group and a and b are integer numbers (a = I, copolymerization or grafting22-29.PaSS-polymer blends 2,3, ...; b = 0, 1,2,3, ), with a + b = 217, where 17 is an create interesting materials, but occurring of microphase integer (n= I, 2, 3, ) and b = a + 2. Of several separation may decrease possible advantages connected structures of silsesquioxanes (random, ladder and cage), with nanoscale incorporation30. Organic polymers can be cage structures contain 8 silicon atoms placed at cube reinforced with pass by attaching single or multiple vertices. Cubic structural compounds (completely and polymerizable groups to pass cage. incompletely condensed silsesquioxanes) are commonly In catalysis chemistry, metallosilsesquioxanes are illustrated as T6, T7, Ts' T,o' and T12, based on the receiving considerable interest3I.32. Incompletely number of silicon atoms present in cubic structure condensed silsesquioxanes [cyclopenty Isilanetriol (Fig. 2). In early 1900s, Kippingl5 found that CY7Si709(OH)3] share structural similarities with polycondensation of siliconic acids invariably leads to B-cristobalite and B-tridymite and are thus quite realistic *Author for correspondence models for silanol sites on silica surfaces33.34.Incompletely• Tel.: (044) 24404427; Fax: 91(44) 2491 ]589 condensed silsesquioxane frameworks have attracted E-mail: [email protected] attention as models for silica35.36, as ligands in -~--~---···"'"._'.''''·'''tll'111'' 1IlII'III"jl," III '~III•.'III"._h'~lt"I*"'I"II'

438 J SCI IND RES VOL 68 JU:\E 2009

S.O,. Oct.m~ri<:Core l V-O.065nm'

v = 0.9 nm' r

a b c

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 silica extended concept of Hoebbel et apo and identified a supported catalysts39.40,and as building blocks for network high-yield route to obtain POSS cubes with hydride and solids41,42. vinyl functional groups by silylation of silicate anion (Sis020S)solution (Scheme 1). Methodologies adopted for Functionalization of Cubes with vinyl and Si-H groups permit selective POSS Monomer attachment of numerous moieties to cubes through New chemical reagents and nanocomposites (NCs) Pt-catalysed hydrosilylation (PCH).Octa have been developed using POSS molecule. Polyhedral (dimethylsilyloxy) silsesquioxane (QsMsH) and octa silsesquioxanes are prepared from monomers of XSiY, (hydrido) silsesquioxane (TsH) cubes can be modified type, where X is a chemically stable substituent into various octafunctionalized macromonomers by (CH3, phenyl, or vinyl), and Y is a highly reactive hydrosilylation reaction between terminal Si-H groups substituent (CI, OH, or OR). Thus, using XSiY3-type, on POSS cube with an unsaturated carbon double bond POSS can form linear, cyclic, or polycyclic siloxanes as in presence of a Pt catalyst (Scheme 2). A series of polyethYlene glycol (PEG) substituted octa

nXSiY3 + l.SnHzO - (XSi015)n + 3nHY ... (1) silsesquioxanes51 prepared by PCH of unsaturated PEG with QsMJI, TsH and thermal effect of organic peripherals attached on vertices ofPOSS were studied. Octahydridosilsesquioxane43'45 (TsH), results from Another example of hydroxyi terminated POSS is hydrolysis condensation ofHSiCl3 with very low yield of products [TsH (15-20%) and T10H (5-10%) cubes]. reporled52 through PCH of QsMll with allyl alcohol. Controlling catalyst concentration and choosing Kudo ct af46 explored on synthesis of POSS and entire reaction scheme (Fig. 3). Reaction rate, degree of appropriate solvent undergo 100% C-silylation rather oligomerization, and yield of polyhedral compounds formed than O-silylation of -OH group of allyl alcohol. depend strongly on following factors47: i) Concentration Matisons et af53 reported preparation of octaisocyanato of initial monomer in solution is an important variable: ii) subslltuteo silsesquioxane by hydrosilylation ofQsMsH Nature of solvent: iii) Identity of substituent X in initial with m-isopropenyl-cx-cx' -dimethylbenzyl isocyanate in monomer; iv) Nature of functional groups Y in monomer; THF medium using Karstedt's catalyst, and proved that Isocvanate functionalised POSS macromer was suitable v) Type of catalyst: vi) Reaction temperature; vii) Rate of water addition; and viii) Solubility of polyhedral oligomers nanocrosslinkcr for preparation of organic-inorganic formed. Using these conditions, octameric silicate anion 1:ybrid polyurethane. A simple two-step synthetic systems were developed from hydrolysis and methodology was developed for octaphenol-POSS polycondensation of tetraalkoxysilane in presence of ,hrough PCE of 4-acetoxystyrene with QsMsH and tetramethylammonium (TMA) ions [N~(CH,)4] under subsequent hydrolysis of acetoxy units 54. appropriate conditions, which yield double four membered OgunniYI cf al55 reported synthesis of etheric chlorine• terminated silsesquioxane using 2-chloroethyl vinylether ring silicate anion (Sis020s.) selectively. Hasegawa ct a14S.49

I 'I 'I I '1"11 I I' ·1· , , GNANASEKARAN et al: POLYHEDRAL OLIGOMERIC SILSESQUIOXANES (POSS) NANOPARTICLES 439

0-- R O-Si-RJ / O-Si\/ -0 0__ R R R R R-Si\/ \O-Si-R/ \ ....--0, / O--S!"'--O~ ( \ \ ;5' '5;- / ' 51- ° R/0 ° °\ \° °\ ° R-S( -~i O~ L,R . / S' ,/ / \ \ / Si 1"0.--8{- ° O-Si --f'~Si_O-;R \ R R I R/ \ O-Si-R /\ ladder Random

Fig. 2-ChemicaI structures of different types of silsesquioxanes 440 J SCI IND RES VOL 68 JUNE 2009

A HSi(OH)3

b) T2: H(OHl2SiOSi(OH)2H G' ~ ~ B" · No of Hydrogen bond

~J) \ /cp\

(0)

T8 : (HSi01·5) 8

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

" 'I"h'.jlj~.,.,~II." 0"". " .. , •. , I I I 'I I I ~I l' I 'I GNANASEKARAN et al: POLYHEDRAL OLIGOMERIC SILSESQUIOXANES (POSS) NANOPARTICLES 441

Scheme I-Synthesis of vinyl and hydride functionalised silsesquioxanes

with TsR through PCR. Various epoxy functionalized and polycondensation of phenyl trichlorosilane in acetone POSS [Octakis(dimethylsiloxypropyl glycidylether) medium. Laine et a/63-65 successfully explored various silsesquioxane, Octa( ethy1cyclohexylepoxide dimethyl• reactive functional groups (amine, iodo- and bromo- ) on siloxy)silsesquioxane and octakis( dimethylsiloxy phenyl ring ofOPS (Scheme 3). Laine et a/63 successfully hexafluoropropy I glycidyl) silsesq uioxane] were . nitrated OPS using fuming nitric acid. successfully synthesized through PCR by reacting Octa(aminophenyl)silsesquioxane (OAPS) was prepared QsMsR with allyl glycidyl ether (AGE), 4-vinyl-1,2• in two steps by nitration of OPS in fuming nitric acid to cyclohexene epoxide and allyl-1,1,2,3,3,3-hexa• form octa- (nitrophenyl)silsesquioxane (ONPS) followed fluoropropyl ether respectively56,57. by mild reduction of ONPS using formic acid and Octakis( dimethylsiloxypropyl aminobenzoate) triethylamine in presence of PdlC catalyst, resulting in silsesquioxane have been successfully prepared by quantitative conversion with equal quantities of m- and reacting QsMsR with allyl-4-aminobenzoate through PCR p- nitro substituted in each phenyl ring. One step ahead reaction5s. Stradiotto et a/59 reported synthesis of of nitration of OPS, poly(bromooctaphenyl octacyanosilsesquioxane by reacting acetonitrile with silsesquioxane)s (BrxOPS) was synthesised via QsMsR in presence of toluene medium. Various other bromination with Br/Fe in dichloromethane64. Iodo• functional groups such as silanes and a-olefins substituted substituted OPS by iodination using iodinemonochloride POSS through PCR reactions also reported60. If (ICI) at -40°C gave highly crystalline, octaiodinated OPS, stoichiometry is correctly controlled, PCR also offers a which shows 93% selectivity for p- substitution, in good method for synthesis of mono-, di- and poly• 30-40% overall yield65.Feher et a/66 established a direct substituted silsesquioxanes61 .. synthetic strategy for amine functionalized POSS through Brown et a/62 extensively studied synthesis of another hydrolytic condensation of aminopropyltriethoxysilane in type of POSS, octaphenyl silsesquioxane (OPS) and MeOR and conc. RCI, but product obtained under these various other side products formation during hydrolysis conditions is aminehydrochloride salt. Octa 442 J seI IND RES VOL 68 JUl'JE 2009

0--CN pt Catalyst

8» pt Catalyst

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

I' II I I l' 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, bromine ilnd iodine groups

(aminehydrochloride) silsesquioxane IS highly soluble in as nanocunstruction sites for nanohybrids. water and poorly soluble or insoluble in most of organic Sllsesquioxane bearing various other functional solvents and is somewhat hygroscopic. To make stable groups67.70 (esters, norborenyl, choloro- and POSS amine, aliphatic amine ofPOSS using methacrylol chloroammoniumpropyl) have been developed. chloride gave methacrylic acid groups that could be used Concentration and type of initial monomer groups alter

11'1 I 1111 I 444 J SCI IND RES VOL 68 JUNE 2009

Scheme 4--Structures of mixed octafunctionaI POSS macromonomers

cubic structure by changing feed composition7! of acid) (Scheme 5)72.Lichtenhan et a[13 synthesized mono phenyltrimethoxysilane(PTMS) and 3- (methacryloxy) norborenyl substituted cyclopentyl siloxane through comer propyltrimethoxysilane(MPTMS). Four different molar capping reaction of cylopentylsilanetriol CY7SiP9(OH)3 ratios of monomers PTMS/MPTMS were co-condensed with trichloronorborenyl silane. Haddad et a[14 derived in presence of formic acid to yield four different monostyryl functionalised pass through comer caping types of pass with various functional group ratios reaction using trichlorosilane styrene in presence ofTHF (Scheme 4). medium. Aminopropyl substituted pass was synthesised by reacting aminopropyl triethoxysiloxane with Among pass monomers, mono- functionalised CY7SiP9(OH)/s. Zheng et af76 synthesised silsesquioxane is a suitable monomer for synthesis oflinear hydroxylpropyl corner capped phenylsilanetriol. thermoplastic NCs. Comer capping reaction yields a 2-(Trichlorosilyl)ethyl acetate was end capped on iso• closed cube with 7 comers of inert groups (cyclopentyl butyl-paSS and acetate was hydrolysed to yield hydroxyl or cyclohexyl) and remaining one vertices possesses functionalised POSS77. A pass framework with two highly reactive functional groups (hydride, chloride, anilino pendant groups was prepared in good yield using hydroxide, nitrile, amine, isocyanate, styryl, olefm, acrylic, diexo-(c-CSHg)gSigO/OH)2 followed by adopting epoxide, norbomyl, bisphenol, acid chloride, alcohol, and various synthetic procedures (Scheme 6)7g. 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 bisphenols acid chlorides alcohols acids

Scheme 5-Synthesis and structures of monofunctional POSS macromonomers

Hybrid Nanocomposites from Completely OAPS with pyromellitic dianhydride (PMDA) inNMP Condensed Silsesquioxane Monomers and DMF solutions (Scheme 7). Polyimide NCsshow POSSfPolyimide Hybrid Nanocomposites (NCs) thermal stabilities >500°C (5% mass loss temperature) Laine et al63 synthesized OAPS as a robust nanosized and char yields >75% even at 600°C. Resulting materials building block for construction of materials with nanometer are completely amorphous, and extreme rigidity of organic control between organic and inorganic components. A tethers linking cube vertices prevents long-range order three-dimensional polyimide63 was synthesized by using occurring during curing reaction79,so. 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 reacting At very high percentage (7.5 and 10 wt%), restriction of OAPS with po1yamic acid (PAA) solution for low rigid POSS was lost and bulky groups behaved like dielectric properties and improved thermo-mechanical p1ascticizer. A1agar et al84 showed improved properties. Zheng et al83 synthesized thermomechanica1 properties of OmipPOSS NCs. octama1eimidophenyl/POSS (OmipPOSS) NCs via Po1yimide NCs are also obtained by reacting amine imidization reaction between OAPS and maleic anhydride. terminated PAA, with POSS containing epoxy pendant Thermosetting hybrids containing OAPS (10 wt%) were groups (instead of OAPS). In NC, network linkages obtained via in situ polymerization of dig1ycidy1ether• between POSS cube and po1yimide molecules primarily bispheno1 A (DGEBA) and 4,4' -diaminodiphenylmethane arise from curing reaction between terminal amine groups (DDM) in presence of OAPS. OmipPOSS NCs ofPAAand epoxy groups ofPOSS cube85.Epoxy POSS containing 2.5 and 5 wt% of OmipPOSS displayed incorporated NCs show high rubbery state region and enhanced glass transition temperatures (Tg's) (178 and with high tensile modulus 1045.6 Mpa at 400°C, which is 174°C) compared with control, epoxy resin (172°C). On 23 times greater from conventional epoxy polyimide the other hand, hybrids containing 7.5 and 10 wt% of composites. Very low dielectric values (dielectric OmipPOSS have lower Tg's than control epoxy. constant=2.30) of polyimide-POSS was reported by Enhancement in glass transition temperatures is due to synthesizing mixed glycidyl epoxy and fluorinated epoxy nanoreinforcement effect of POSS on polymer matrix. functionalised POSS86. Polyimide's with low dielectric

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o~'~O o 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 by having high molecular weights (up to ?inh=0.6ldUg) and the use of aliphatic amine POSS, but, thermal stabilities particularly, fluorinated POSS- PI shows' good combined of NCs became poor due to weak aliphatic groups of characteristics, including low dielectric constant (K.=2.43) POSS molecules87• with good mechanical properties (tensile strengths, 72.3 Wei et aZS8 reported about linear polyimide by reacting MPa; initial modulus, 1.8 GPa; and elongation at breakage, diamine functionalised POSS with dianhydrides. But, 8.0-15.9%), excellent optical properties, and is NCs obtained using these diamine functionalized POSS organosoluble as a good candidate for microelectronic shows lower thermal properties than that of pure polyimide applications. due to degradation of cyclopentyl groups present in Poly(methyl methacrylate) (PMMA)-POSS grafted 7 corners of POSS molecules (at 400°C). In-plane polyimides were synthesized by thermally initiated free• coefficient of thermal expansion ofPOSS/polyimide NCs radical graft copolymerization of methacrylcyclopentyl• increases with POSS amount as a result of increase in POSS (MA-POSS) with ozone-pre activated PAA, free volume in polyimide due to POSS molecules. Self• followed by thermal imidization to achieve material with assembly architecture was observed for polymer chain low dielectric values. Dielectric constant of film can be at 10% loading of POSS molecule. As POSS amount tuned by varying molar ratio of grafted MA-POSS side increases to 10 mol %, decrease in Young's modulus, chains in copolymer90• Brunsvold et al91 grafted maximum stress and elongation become more apparent silsesquioxane through a two-step chemical process. (10%). But, dielectric constant ofNCs got lowered and (3-Aminopropyl)(hepta-iso-butyl) POSS was covalently this could be tuned by varying mole ratio ofPOSS content. attached to polyimide backbone containing reactive acid Oikawa et aZS9 synthesized polyimide NCs using double• chlorides. PolyimidelPOSS NCs could be developed by decker-shaped POSS molecule (Scheme 8). In first step, crosslinking amine functionalsied trialkoxysilane precursor double-decker-shaped silsesquioxane containing aromatic with dianhyrdides92•95• tetracarboxylic dianhydride (DDSQDA) end groups were synthesized. This dianhydride is derived from DDSQ• POSS/Epoxy Hybrid Nanocomposites (NCs) Reactivity of mono- and multifunctional POSS-epoxy diamine by functional group conversion method. POSS• monomers and their effect on the formation of epoxy• Polyimide (POSS-PI) exhibits good thermal stability up amine networks containing POSS as a pendant unit or as to 500°C and POSS molecule in main chain (POSS-PI) 448 J SCI IND RES VOL 68 JUNE 2009

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

1050 C. 25h IToluene

Scheme 8-Synthesis ofpolyimide/POSS nanocomposites containing double-decker shaped silsesquioxanes junctions was evaluated96• pass epoxy monomers are processing techniques and thermomechanical stabilities less reactive towards amines than DGEBA, may be due are better than that of standard Ciba epoxy resins100, 101. to sterical crowding around epoxy groups caused by inert Chen et aP02 investigated epoxy NCs using octa(2,3• pass substituents and also due to reduced epoxy group's epoxypropyl)silsesquioxane (OE) with diamines of 4,4'• mobility. Epoxy pass could be chemically or methylenedianiline (DDM) and 5-trifluoromethyl-l,3• photochemically cured to give hard, scratch- and solvent• phenylenediamine (FPA). Resultant fluorinated epoxy resistant materials containing up to 65% masked silica97• NCs show Tg of 170°C, which was higher than diglycidyl Epoxy NCs with completely defined organic/inorganic ether of bisphenyl A (DGEBA)/DDM at the same phases were prepared by reacting stoichiometric ratio. It also possesses excellent thermal, octakis(gl ycid yldimeth ylsiloxy )octasilsesquioxane( OG) mechanical, and dielectric characteristics with high with DDM at various compositions to alter storage modulus of 1.8 GPa (30°C) and 0.3 GPa (250°C), thermomechanical properties98 (Scheme 9). From low coefficient of thermal expansion of86 /lm/moC, and resulting material, moduli and fracture toughness ofNCs dielectric constant of 2.19. Polybenzoxazine(PBA)• were found less than those of organic resins. Choice of epoxylPOSS nanocomposites (40 wt% of paSS) was short-chain, polyfunctional epoxies99 combined with obtained through intercomponent reaction between nontraditional resin stoichiometries is a possible way to phenolic hydroxyls of PBA and epoxide groups of control epoxy resin co-effiecient of thermal expansions epoxyPOSSI03. Introduction of pass, thermal stability over an order of magnitude. An additional advantage is of polymer matrix and oxidation resistance of materials that epoxy-PaSS resin has a low viscosity of about 350 were significantly enhanced. Nanoscale dispersion of CPs at room temperature, making it suitable for composite pass in NCs «30 wt% paSS) displayed higher storage

I 'I 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, storage ATRP methodl1O, PMMA-POSS hybrid NCs were moduli for NCs (> 30 wt% POSS) are lower than that of prepared and their thermal properties were investigated control PBA, and this could be responsible for porosity Hybrids were obtained by use of octafunctional octakis(3• of NCs. Enhanced thermal properties and good hydroxypropyldimethylsiloxy)octasilsesquioxane (ORPS) homogeneity of epoxy/POSS NCs was obtained by and OAPS nano-cages as ATRP initiators. No significant reaction of DGEBA with monoalkylamine functional improvement was observed in thermal stability ofNCs POSS and microphase separation of material could be as compared with linear PMMA. MandaI et allIl controlled by small molecule curing agentslO4• POSS developed an efficient and versatile method for synthesis monomers containing 8 aliphatic amine groups on vertices ofPOSS-polymethacrylate hybrids (Scheme 10), such were first incorportated into DGEBAI05. Improved as POSS-poly(methyl methacrylate) (POS,s .•.PMMA), thermal stability was achieved on increasing chain length POSS-poly(ethyl methacrylate) (POSS-PEMA), and of diamine with diepoxy hexavinyl POSSI06• Life time of POSS-poly(benzyl methacrylate) (POSS-PBzMA) of epoxy NCs at different temperatures was predicted using controllable molecular weights and low polydispersities kinetic parameter, based on thermal degradation of by thiol-mediated radical polymerization at elevated NCS107• Lu et al108 also investigated thermal properties temperature (l00°C). POSS content in these hybrid and curing kinetics of epoxylPOSS hybrid networks. materials could be varied, by tuning reactant concentrations and degree of polymerization of grafted Poly(methacrylate)/ POSS Hybrid Nanocomposites (NCs) polymethacrylate chains. Wu et al1I2 studied surface Matisons et a[l°9 reported various methyl methacrylate properties of poly(methyl methacrylate-co-n-butyl functionalised POSS macromonomers with different PEG acrylate-co-cyclopentylstyryl polyhedral oligomeric chain lengths and then polymerized with methyl silsesquioxane) [poly(MMA-co- BA-co-styryl-POSS)] methacrylate to produce a series of hybrid materials. terpolymers and found that incorporation of styryl-POSS Although POSS is a bulky and multifunctional crosslinker, into polymer resulted in increasing interactions between glass transition temperature increase with an increase in polymer and solvent, dispersive component of surface POSS concentration. In some instances, group bulkiness free energy of polymer and acidity of surfaces of created a free volume and chain separation, which led to polymers. Toughness propertyll3 ofPMMA using POSS a reduction in glass transition temperature. Incorporation nanocages at loadings between 0 and 15 wt% was ofPOSS always increases thermal stability. By applying investigated using Cyclohexyl-POSS, methacryl-POSS 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 and Gerard et all16 synthesized polymer networks either as PMMA were able to improve impact toughness of linear chains or as cross linked PMMA-POSS NCs. PMMA. In order to toughen PMMA with rigid fillers, pass-pass interaction was found to be main parameter weakly adhering particles (size, 100 nm) are required. Li governing network morphology. However, dynamic et al1I4 synthesized a soluble poly(MMA-co-octavinyl• mechanical properties remain nearly at the same level as paSS) hybrid by common free radical polymerization neat matrix. Multifunctional pass shows a higher and resulting hybrid NCs show higher Tg and better miscibility with dimethacrylate monomer and disperses thermal property than parent PMMA homopolymer. Tg very well in cured network. Molecular dynamic simulation improvement results from dual contributions of motion studies were carried out for PMMA-POSS NCslI7. hindrance ofPMMA chain imposed by nanometer pass cores and dipole-dipole interaction between PMMA chains Norborenyl/POSS Hybrid Nanocomposites (NCs) and pass cores. Thermal stability enhancement is mainly Mono- and tri- functionalized norbornenyl-POSS are attributed to incorporation of nanoscale inorganic pass copolymerized with dicyclopentadiene(DCPD). pass uniformly dispersed at molecular level. Silverstein et a[lls loading I 18 slightly decreases thermal oxidative resistance reported that degree of polymerization increased with of copolymer before primary decomposition temperature increase in pass content for linear PMMA-POSS NCs of PDCPD network due to lower thermal stability of and shows a relatively low PDI than conventional PMMA. pass. Modulus and yield stress, compression, and

II I 'Iii I II 1III 11'llil;III' III GNANASEKARAN et a/: POLYHEDRAL OLIGOMERIC SILSESQUIOXANES (POSS) NANOPARTICLES 451

X~enes.Base Sid' D---

RellUX4h~\\o I \/O/~ RIS~O_--~\

R=C)"lopentyl

Scheme I I-Synthesis ofnorbornene block polyethylene-POSS random copolymers fracture toughness of copolymers decrease with POSS Polyurethane networksl22 with POSS were obtained by loading, irrespective of cross link density. For mono• reacting OAPS with isocyanate terminated polyurethane functionalized POSS, decrease in toughness is in prepolymer. Thermal stability of hybrid polyurethanes is agreement with loss of irreversible damage occurring more than conventional PU elastomer due to nanoscale during fracture. Microstructural and mechanical reinforcement effect ofPOSS on polyurethane networks. relaxation investigation was carried out for norbomene/ In glassy state (-75 to -25°C), all POSS-containing hybrids POSS-norbornene hybrid polymersl19• POSS are significantly higher than that of control PU even at copolymerization enhances a-relaxation temperature, T. 2 wt% ofPOSS incorporation. PU-POSS networks have in proportion to the weight fraction ofPOSS-norbomenyl been synthesized by taking epoxy-POSS monomerl23• comonomer. However, magnitude of this dependence is Thermal and mechanical properties show similar trend larger for POSS-norbomenyl comonomer possessing to that of PU-POSS networks prepared using OAPS cyclohexyl comer groups (CyPOSS) than for copolymer macromonomer. Contact angle measurements showed with cyclopentyl comer groups (CpPOSS). Norbomene that organic-inorganic NCs displayed a significant block polyethylene-POSS random copolymers73 enhancement in surface hydrophobicity, as well as (Scheme 11) were obtained by ring-opening metathesis reduction in surface free energy. Improvement in surface copolymerizations of cyclooctene and POSS monomer properties was ascribed to the presence ofPOSS moiety 1-[2-(5-norbornen-2-yl)ethyl]-3,5,7 ,9,11,13,15• in place of polar component of polyurethane. Wu et a[l24 heptacyclopentylpentacyclo [9.5.1.13,9.15,15.17,13] studied thermal and rheology properties of hybrid octasiloxane (POSS- norbomylene) using Grubbs's polyester-POSS resins and PU-POSS using catalyst, followed by reduction of double bonds in the 1-(2,3-propanediol)propoxy-3,5, 7,9,11,13,15• copolymers. Polyethylene-POSS copolymers showed a isobutylpentacyclo-POSS diol cured with isophorone 70°C improvement, relative to a polyethylene control diisocyanate (IPDD trimer. Pure polyester resin displayed sample of similar molecular weight. Kwon et a[l20 Newtonian flow behaviour, but all resins with POSS reported synthesis ofPOSS-containing block copolymers presented a first shear-thinning branch. for the first time via living ring-opening metathesis polymerization. POSS/Polystyrene Hybrid Nanocomposites (NCs) Polystyrene-POSS NCs (Scheme 13) were obtained

Polyurethane(PU)/POSS Hybrid Nanocomposites (NCs) by free radical polymerization using AIBN-initiator74• Linear PU/POSS NCS121 were obtained through Both homopolymers and copolymers with varying bisphenol functionalised POSS (Scheme 12). proportions ofPOSS content modifies thermal properties 452 J seI IND RES VOL 68 JUNE 2009

Scheme 12-Synthesis ofpolyurethane/POSS linear copolymer drastically and allows for tailoring polymer softening Dramatic improvement in thermal properties ofNCs were temperature (> 200°C range). Thermal stability, compared observed for poly(styrene-co-octavinyl-polyhedral with polystyrene modified with pendent C60 fullerenes oligomeric silsesquioxane) (PS-POSS) organic-inorganic (diam 7.07 A), shows a Tg of 166°C for an approx. 11 hybrid NCs containing various percent ofPOSS prepared mol% C60 incorporation. This is comparable to Tg's seen via one-step free radical polymerization127• Ferrocene in 9 mol% polystyrene-POSS copolymers. A series of metal bonded polystyrene-POSS NCS128 were hybrid organic/inorganic triblock copolymers125,126 of synthesized and exhibit paramagnetic properties with a polystyrene-butadiene-polystyrene (SBS) grafted with remnant magnetization of 0.035 emulg and indicated that POSS molecules with different chemical constituents polystyrene-POSS shows ideal soft magnetic properties. groups (phenyl, cyclopentyl, cyclohexyl and cyclohexenyl) in POSS cube have been investigated and observed that Nanocomposites (NCs) based on Incomplete POSS with phenyl moiety, when grafted to polybutadiene Silsesquioxanes (PB) phase, appears to show favourable interaction with Heptameric siloxanes with partially formed cages polystyrene (PS) phase and Ph-POSS plasticizes SBS. containing 2 or 3 residual silicon hydroxyl functional

I 'I I II' j I I "" ..:t GNANASEKARAN et at: POLYHEDRAL OLIGOMERIC SILSESQUIOXANES (POSS) NANOPARTICLES 453

NEt3 in THF• Q CI-Si-CI I CI

R= Cyclopentyl, Cyclohexyl

AIBN BOoC

4-Methyl styrene! Tolvene

Scheme l3-Synthesis of polystyrene/POSS nanocomposites groups, obtained through hydrolysis/condensation of alkyl• temperature range of35-350°C. Continued curing leads or aryl trichlorosilanes, are called as incomplete to aggregation ofPOSS, which is bound to resin molecules silsesquioxane. Feher et al129,I30 synthesized partial cage and aggregation process depends on concentration of silsesquioxane from hydrolytic cleavage of completely POSS. There is a formation ofSi-O-Si bonds from sHanol condensed POSS. A variety of NCs were obtained groups ofPOSS also occurring, both intramolecularly and through reactive silicon hydroxyl functional groups(-SiOH) intermolecularly at 250-300°C. Latter could create dimers (Scheme 14). Pittman et af131 reported effect of partially or trimers of POSS when there is enough freedom of caged trisilanol phenyl POSS on thermal and mechanical motion or when molecules ofPOSS are in close proximity. properties of cyanate~ester NCs and suggested that three• Storage modulus for 1, 3, 5 and 10 wt% POSS are greater hydroxyl groups in POSS aid solubility into cyanate-ester than that of pure resin composites except when 15 wt% resin and also reacts with cyanate ester resins at high ofPOSS was present. Also, Tg of 1, 3, and 5 wt% POSS temperatures to aid dispersion. Storage moduli (E) is composites, flexural strength and flexural modulus are higher than those of pure cyanate ester over entire higher than that of neat cyanate ester composites. 454 J SCI IND RES VOL 68 JUNE 2009

o C

Nf IN;Toluene/ DBTL C a I OH H~O OH

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

Choudhury et alI32 reported synthesis of transparent better melt elasticity and a broader window of hybrid polyurethane using partially caged pass for thin processability of material for commercial applications. film coating. Partially caged pass components retain Synthesis of polyurethane NCs based on completely and their partial cage structure after curing and impart high incomplete condensed silsquioxanes have been reported glass transition temperature and high thermal stability to and effect of dual cage structure on thermal, mechanical product as compared to conventional polyurethanes, and morphological properties was evaluatedI34• which is essential in their application as weatherable Completely condensed silsesquioxane decreases thermal coatings. Application of hybrid as a coating resulted in and mechanical properties ofNCs with increase in pass formation of a homogeneous "ue-phase lamellar coating aggregation ofPU matrix. with excellent coverage of substrate. Similarly, prevention Influence of trisilanol phenyl-PaSS on thermo• of discoloration of poly(ethylene terephthalate) (PET) mechanical properties and curing of epoxy-amine ,during melt processing, by usage of trisilanolisobutyl• networks were investigated135• Phenyl-trisilanol polyhedral pass have been reported133• Thermal studies displayed silsesquioxane is highly soluble in DGEBA epoxy resin that partially caged pass increases thermooxidative and addition ofPOSS-triol improves cross-linking reaction stability with increase in shear storage modulus, indicating of epoxy-amine networks. Methyl silicone resinIPOSS

I 'I I I GNANASEKARAN et al: POLYHEDRAL OLIGOMERIC SILSESQUIOXANES (POSS) NANOPARTICLES 455 composites with various proportions ofPOSS monomer longer spacer groups are more reactive than those with using trisilanolisobutyl-POSS were synthesizedI36,137, no spacer groups. Pores within POSS cube interiors Thermal stability of methyl silicone resin greatly improved (diam, 0.3 nm), and pores between cubes (diam, 1-50 by introduction of POSS cages. Segmental motion of nm) were determined according to nitrogen absorption, PDMS chain was retarded by large mass and steric positron annihilation lifetime spectroscopy (PALS), and hinderance of bulky POSS. Organic-inorganic hybrid small angle X-ray scattering (SAXS) data. Benzoxazine/ composites evolved via in situ polymerization ofDGEBA POSS hybrids were synthesized by using octafunctional in presence of trisilanolphenyl- POSS 138.Phase separation cubic silsesquioxane (MBZ-POSS) with 8 organii of trisilanolphenyl-POSS induced by reaction occurred, benzoxazine tethers as a curing agentI46-14S. In and heterogeneous morphology was obtained for hybrid benzoxazine/POSS hybrids, POSS aggregates occur in composites. Spherical POSS-triol particle aggregates larger scale at higher POSS contents and reason for (diam, 0.3-0.5 JLm) were dispersed in continuous epoxy heterogeneous phase separation may be from less matrices. Phase separated NCs show very high Tg's and compatibility of inorganic silsesquioxane core with organic with high storage modulus in glassy states. Dissolving benzoxazine species. During formation of trisilanolphenyl-POSS and phenolic resin into THF, polybenzoxazine/POSS hybrids, POSS particles were followed by solvent removal and curing prepared phenolic• separated from polybenzoxazine rich region, leading to POSS NCs. Both nano- and micro-sized POSS filler POSS rich domains (50-1000 nm). Zheng et azt°3 OG aggregates and particles were heterogeneously dispersed was used to prepare polybenzoxazine(PBA)/POSS NCs. in cured matrix 139.POSS silanol/phenolic hydroxyl Crosslinking reactions involved during formation of hydrogen bonc~ing increases compatibility, which is polybenzoxazine(PBA)/POSS NCs can be divided into beneficial for POSS dispersion in matrix that does not two types: 1)Ring-opening polymerization ofbenzoxazine; prevent phase separation. Thermal stability and Tg of and 2) Subsequent reaction between in situ formed NCs is only slightly affected by loading ofPOSS. phenolic hydroxyls of PBA and epoxy groups of OG. Other polybenzoxazine/POSS NCs obtained by reaction Various Other POSS Nanocomposites (NCs) of OAPS and 2,2' -(1,3-phenylene)-bis(4,5-dihydro• Crosslinked polysiloxanes were directly synthesized oxazoles) (PBO) are reported149. Dynamic mechanical by anionic ring-opening copolymerization of octaisobutyl• analyses indicated that NCs exhibited higher Tg values POSS as a multifunctional monomer with than pristine PBZ and PBZ-PBO resins. Storage modulus octamethy1cyclotetrasiloxane (D4) using base catalysts of NCs was maintained at higher temperatures even with [potassium hydroxide (KOH) or tetramethylammonium a small amount of OAPS incorporated into composite hydroxide (Me4NOH) siloxanolate], indicating that systems. Thermal stability of hybrid was also improved crosslinked polysiloxanes exhibit distinct Tg and excellent by inclusion ofOAPS. Crosslinked poly(4-vinylpyridine)/ thermal stability 140.Synthesis ofliquid crystal POSS and POSS NCs were obtained by the reaction of epo~y group specific problems connected with the nature of ofOG with pyridine ring of poly(4-vinylpyridine) 150. silsesquioxane cage, and special properties that their Amphiphilic silsesquioxane derivative, 1-(1,0)• geometry imparts to their mesogenic behaviour of liquid• propylenemethoxy )oligo(ethyleneoxide)- 3,5,7,9,11,13,15• crystal polyhedral silsesquioxane materials have been heptahydridopentacyclo [9.5.13,9.15,15.17,13] described 141.Synthesis of these materials is based on a octasiloxane has been prepared by reacting TsH and allyl silsesquioxane cage modification process, starting with a functional oligo(ethyleneoxide) (Mn=750glmol) through suitably functionalized cage, to which mesogens are PCHI5l. Associative behaviour of new' amphiphilic attachedl42. Many systems are obtained based on telechelics containing POSS as an end-group of PEG of hydrosilylation reaction of hydrido-silsesquioxanes and varying chain length welf investigated using capillary meso genic groups 143.Richardson et a[l44 reported a viscometry. Viscosities were strongly affected by solvent hexadecamer, first-generation, octasilsesquioxane liquid• composition in THF/water mixturesl52. crystalline dendrimers. POSS modification increases storage modulus and POSS-POSS nanohybrids were synthesized by Young's modulus of poly.amides, slightly decreases their hydrosilylatively copolymerized with stoichiometric Tg from 312° to 305°C, and signi(icantly lowered their amounts of octavinylsilsesquioxanes, with TgH and dielectric constants from 4.45 to 3.35153.Supramolecular QsMsH in toluene using Pt catalystl45. POSS cubes with inclusion complexation (ICs) of POSS-capped 456 J SCI IND RES VOL 68 JUNE 2009 polycaprolatone (PCL) with a-and y-cyclodextrins (CL) hexahedral or flake-like structures recrystallized out upon were derived.76,77,154 Crystallization kinetics of cooling. Both crystallites and POSS molecules co-existed silsesquioxane based hybrid star, poly(3-caprolactone) in these blends, with the amount of dispersed molecular was investigated by synthesising a series of silsesquioxane POSS being increased at higher temperatures. POSS based hybrid star poly(3-caprolactone) having different molecules exhibited some physical interactions with free arm lengths through ring-opening polymerisation of polysiloxane chains that are not crosslinked. But, 3-caprolactone155. crosslinking induced phase separation. In curing process, Poly(alkyl silsesquioxane), PASSQ, copolymers POSS molecules could react with polysiloxane, resulting consisting of various chemically linked alkyl units over in decreases in crosslink density. Original POSS crystals methyl group as a pore forming moiety, and 1,2- bis• could also be dissolved in polysiloxane during initial curing trimethoxysilylethane (BTMSE), were synthesized and stages, but recrystallization upon cooling gave their thermo-mechanical and optical properties were regenerated crystals that were roughly sphericaP60. investigatedl56. Higher alkyl groups in PASSQ were Thermal properties and morphological development of composed and thereby lower refractive indices were isothermally crystallized isotactic polypropylene (IPP) achieved from 1.45 to 1.27 due to formation of nanopores blended with nanostructured POSS molecules at very in film. Modulus for BTMSE based PASSQ films were small loading ofPOSS were studiedl61. significantly higher than 3.8 GPa of typical thin film of Maitra et ap62 grafted various oligomeric PEO with poly(methyl silsesquioxane). Smith et atl57 synthesized different chain lengths (n=2,4,8,12) onto QgMgH, and and characterized PFCB aryl ether copolymers and reported that silica surface affect thermal behaviour of multiblock copolymers with pendant cyclobutyl and iso• oligomeric PEOs. Most dramatic effect was observed butyl-functionalized POSS cages. Synthesis ofPOSS aryl for PEO(n=4), where originally crystalline material trifluorovinyl ether (TFVE) monomers was accomplished became completely amorphous; 4 PEO repeat units were by condensation of commercial monosilanolalkyl-POSS insufficient for crystallization to occur at surface and PEO with a TFVE-functionalized chlorosilane. POSS/PFCB oligomers crystallized with increasing side chain length. aryl ether copolymers demonstrated excellent solution Maxima in ion conductivity qbserved for PEO chain process ability, producing optically transparent and flexible lengths (n=4 and 8) have been attributed to the absence films. Incorporation ofPOSS showed no change in thermal of crystallinity as well as enhanced mobility of short side stability as compared to PFCB aryl ether homopolymer. chains in comblike polymers. Mya et a[l63 discussed crystallization behaviour of star-shaped poly( ethylene Blends of POSS Nanocomposites (NCs) oxide) with cubic silsesquioxane (CSSQ). Polysiloxane composites containing POSS were prepared by melt blending. Crosslinking polysiloxane Bridged Polysilsesquioxanes caused changes in POSS solubility that enhanced phase Bridged polysilsesquioxanes (BSSs) represent class separation. But crosslinks caused constraints, which of highly crosslinked hybrid organic-inorganic polymers, decreased domain sizes of precipitated phases158. which are derived from hydrolytic polycondensation of Octamethyl-POSS-HDPE NCs were prepared by melt organo-bridged silsesquioxane precursors having general mixing route. Joshi et atl59 observed that POSS does not molecular formula (RO)3-Si-R'-Si-(OR)3' where R and interfere with crystallization of HDPE. At low R' are organic groupsI64,165.BSSs, prepared by sol-gel concentrations (0.25-0.5 wt%), POSS particles act as polymerization of organo-bridged trialkoxysilane lubricants, lead to decrease in complex viscosity as precursor, are network polymers, in which basic building compared to neat HDPE, show significantly high storage block contains two silicons directly attached to a modulus and also enhanced thermomechanical properties hydrocarbon bridging group. Remaining three bonds to than HDPE. each silicon are siloxane linkages. By connecting two or Composites ofpoly(methyl vinylsiloxane) with POSS more silsesquioxane groups to R' , a material with as many were prepared by melt blending showing that highly as six siloxane linkages (Si-O-Si) per monomer unit can crystalline POSS macromers could undergo condensation be prepared, as opposed to just four in tetraalkoxy silanes. reactions at 230°C in air, leading to partially amorphous By introducing hydrocarbon spacers into siloxane structures. Also, POSS crystals apparently dissolved in network, properties (hydrophobicity, surface area, pore polysiloxane at high temperatures and POSS crystals with size, UV-visible absorption, and fluorescence) can be

I 'I I 'I ~" 1 ' I " II" ''I' 1 GNANASEKARAN et al: POLYHEDRAL OLIGOMERIC SILSESQUlOXANES (POSS) NANOPARTICLES 457 significantly modifiedl66,167.These materials allow to have cells (MDAMB- 231 cells). Preliminary toxicity properties (porosity, permeability, permselectivity, evaluation predicted that L-Iysine dendrimers is an chemical functionality, and chemical, mechanical, and excellent biocompatibile NC. POSS monomer (POSS• thermal stability) to be fine-tuned because of vast variety MA) was used as a novel dental restorative composites of synthetically available monomersI64,165,168,169. to place commonly used dental base monomer 2,2' -bis• Honma et ai170,171 synthesized a protonic conductive [4-(methacryloxypropoxy)- phenyl]-propane (Bis• polysilsesquioxane membrane containing PEG, GMA)187.Amino functionalized silsesquioxane provide polypropylene oxide and polytetramethylene oxide bridging curl retention for hairl88. groups functionalized with isocyanotopropyltriethoxysilane Polyfluorenes/POSS NC shows maximum and condensed in presence of phosphotungstic acid or luminescence intensity and quantum efficiency, which is monododecyl phosphate. Khiterer et ail72 reported BSSs almost twice as good as those of PFO EL device, and is containing covalently bound sulfonic acid group materials an excellent material for optoelectronic applications 189. that were utilized to prepare mechanically stable gel Introduction of POSS moieties into PPY s improves EL membranes for fuel cell application, BSS molecules are properties of PPY derivatives. Improvement in EL utilized as precursors for synthesis of periodic mesoporous properties of POSS incorporate PPY is due to formation organosilicas (PMOs) or bifunctional PMOs. New of suitable insulation domains of POSS moieties in methodology was adopted for preparation of PMOs films conjugated polymer matrices, resulting in a balance of by adopting evaporation-induced self assembly procedure charge carriers of electrons and holes 190.Castaldo et to obtain spherical NPs from initially developed basic ail91 presented polymeric NC sensors, based on a POSS aqueous medium precipitation procedure173. by selecting proper matrix such as poly [(propylmethacryl• Surfactant-directed self-assemblyI74,175 and self• heptaisobutyl- POSS)-co-(n-butylmethacrylate)] and a directed assemblyI76,177methods have been successfully suitable choice of other external home-made fillers developed for fabrication of BSSs with well-organized (graphite, copper, silicon, zinc and their alloys). Hybrids structure. Self-directed assembly that takes advantage could be used in sensing of both polar and apolar analytes. of weak interactions, such as H bonding, p-p, and/or Sulfonic acid containing bridged POSS hybrid materials hydrophobic interactions between bridging groups (R'), is used as proton-exchange membranes for fuel cell provides a very easy method for fabricating hierarchical applications and electrolyte materials are suitable structure. By judicious choice of organic substructure materials for automotive industryl72. (R') in precursor, new intrinsic nanomaterials including Polyphenylsilsesquioxane is used as interlayer both the nature of molecules and their collective properties dielectrics and protective coatings films for semi within aggregate will be realized 178.All mesoporous conductor devices, liquid crystal display elements, organosilicas contain aliphatic organic groups with magnetic recording media and optical fiber coatingl87. relatively short chains (ethane or ethylene) or aromatic Polymethyl silsesquioxane is used as an additive material (arylene, thiophene and biphenylene) moieties174.179-182. in cosmetics, polypropylene films and methacrylic resins. A very few reports are available for amines and thiols Polymethyl silsesquioxane with epoxy containing incorporated PMOSI83. siloxanes adheres well to rubber and plastics and could be used to provide nonsticking, water repellant and Applications abrasion resistant films on paper, rubber, plastics and Poly(carbonate-urea)urethane (PCU)/(POSS) NCs metalsl87. Poly(aminopropyl silsesquixane) and specific were potentially used in cardiovascular bypass grafts and carbonyl compounds of silsesquioxane acts as an microvascular component of artificial capillary beds. antitumour agentl87. Silsesquioxane films, particularly POSS NCs possess greater thromboresistance than OAPS/imide and OAPS/ epoxide films, provide excellent polytetrafluoroethylene and poly( carbonate-urethane), O2 barrier properties, and is an ideal candidate for making it an ideal material for construction of both bypass packaging applicationsl92. In PYC, POSS behaves as a grafts and microvessels 184,185.Kaneshiro et ai186 reported plastizecer like dioctyl phthalate (DOP) and could be used that L-Iysine dendrimers with an octa(3• as a plastizicerl93.Metal containing POSS cages (gallium• aminopropyl)silsesquioxane core (OAS) with 4th containing cage silsesquioxanes and generation is a suitable candidate for controlled in vitro aluminosilsesquioxane) 194,195have been synthesized, for gene delivery and transfection in human breast carcinoma the use of silicasupported metal catalysts. 458 J SCI IND RES VOL 68 JUNE 2009

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