Physics and Chemistry of Hybrid Organic-Inorganic Materials Lecture 9: Polyhedral Oligosilsesquioxanes (POSS)

Key concepts Polyhedral Oligosilsesquioxanes (POSS) are made from polymerization of organotrialkoxysilanes Used as inorganic particles in hybrid composites by mixing or covalently attaching to organic polymers POSS raises thermal stability, mechanical strength and modulus of organic polymers, particularly when attached through bonds. Common POSS are T8, T10 and T12. T6 is less stable. Larger POSS are a major challenge and goal for chemists studying hybrids POSS form with stable eight membered rings (cyclotetrasiloxanes). POSS will form crystals. POSS can melt and dissolve.

Hydrolysis & Condensation of Organotrialkoxysilanes:possible architectures High monomer concentration, small or reactive R groups Low monomer concentration, bulky R groups High monomer concentration, most R groups

Publications on silsesquioxanes Web of science 3446 papers

Citations of silsesquioxane papers Web of science 63,000 citations

Top four silsesquioxane papers (all reviews)

Major Authors in Silsesquioxane Field All hits: Just Papers: Frank Fehr (68) F. C. Chang (51) Richard Laine (52) S.W. Kuo (48) F. C. Chang (51) Y. Chujo (45) D. A. Loy (50) Richard Laine (42) K. J. Shea (50) Frank Fehr (40) T. S. Haddad (48) . S.W. Kuo (48) . Y. Chujo (45) D.A. Loy (24)

Silsesquioxane Institutions UC Irvine Kyoto University Chinese Academy of Sciences University of Michigan Shanghai Jiao Tong University Harbin Institute of Technology University of Montpellier Beijing University of Chemical Technology

Chinese Silsesquioxane Institutions Chinese Academy of Sciences Shanghai Jiao Tong University Harbin Institute of Technology Beijing University of Chemical Technology University of Science Technology of China Donghua University Suzhuo University

Hydrolysis & Condensation of Organotrialkoxysilanes:possible architectures High monomer concentration, small or reactive R groups Low monomer concentration, bulky R groups High monomer concentration, most R groups

Making silsesquioxanes: Sol-gel polymerizations

Sol-Gel Chemistry: details with lower monomer concentrations: cyclization dominates leading to Polyhedral Oligosilsesquioxanes or POSS

products of sol-polymerization: polyhedral oligosilsesquioxanes (POSS) • Silica like-core with organic groups on surface • Called smallest silica particle Low monomer concentration & reversible reactions 8 membered rings (as in T8) are commonly formed

Some examples: Octamethyl-Polyhedraloligosilsesquioxanes: POSS 1,3,5,7,9,11,13,15-octamethylpentacyclo[9.5.1.13,9.15,15.17,13]octasiloxane No melting point Insoluble in organic solvents Sublimes above 240 °C

Polyhedral Silsesquioxanes T8: 1 nm cluster Cyclization & crystallization J. Brown; M. Voronkov; R. Laine, A. R Esker, F. Fehr, T. Haddad & many others.

An Atomic Force Microscope (AFM) image of a single POSS molecule on a silicon surface Used to make dielectric layers in computer chips

What about POSS with 6–membered rings? Instead only T8 & POSS with 8 membered rings T6 forms under anhydrous conditions only 25% yield with R = octyl 2 six membered rings & 3 eight membered rings

Synthesis of T12 POSS Dropwise add of 15.8 g (80 mmol) 14 days White crystalline precipitate Dalton Trans., 2012, 41, 10585-10588

Class 1 Hybrids: Prefab POSS are dispersed in an organic polymer. * Each “black dot” represents a 1.5nm POSS cage Non-covalently mixed into solid plastic Question: Are the POSS dissolved or a separate phase? POSS in polypropylene

Octaallyl-Polyhedraloligosilsesquioxanes: POSS 1,3,5,7,9,11,13,15-octapropenylpentacyclo[9.5.1.13,9.15,15.17,13]octasiloxane Melts at 71 °C Soluble in organic solvents Sublimes above 140 °C Polymer 2005, 46, 2163

Networks based on POSS as polyfunctional monomers

Octa-functional epoxide versus commercial epoxide Impossible to react at all epoxide groups Comparable toughness and strength!! (Just 100X as expensive) Some Improvement in thermal stability

Chemists often believe network polymers are infinite and homogeneous in structure They are not. Particulate morphology suggests otherwise.

Monomer functionality and phase separation Gel point = 14% of groups reacted Degree of condensation at Gel point Gel point = 14% of groups reacted

What happens as polymer grows? Entropy cost for polymerization increases with extent of reaction Enthalpy dominates solubility thermodynamics

Chemistry and physics of gelation Sol-gel polymerizations create solid particles that eventually percolate and gel Kinetics lead to amorphous, high free energy structures in gels

Even this thermodynamically controlled polymerization gives kinetic structures

POSS attached covalently to linear polymers Dental resins, adhesives, oxidation resistant polymers

POSS: The T7(OH)3 can be used to make a new monomer. The polymer will form due to chemistry in the organic component only

Making POSS monomers from T7(OH)3

POSS as a Nanoscale Filler & Modifier for Polymers Higher Tg Improvements in Strength Oxidation Resistance Wear from abrasion Hybrid Plastics

Example of POSS modified polystyrene Tg 116 °C, Td 383 °C R = cyclohexyl = soluble in THF R = cyclopentyl = insoluble R = cyclohexyl: Tg 396 °C, Td 445 °C

P-methyl styrene-POSS Styrene Random copolymer TEM images of PS–POSS copolymers: (a) 6 and (b) 30 wt% i-Bu POSS. The samples were stained by RuO4 vapor at ambient condition.

Methacrylate-POSS Dental Resin

Phase segregated block copolymer Transmission electron microscopy (TEM) images of a series of poly(norbornene)-bpoly( norbornene-POSS) di-block copolymers varying with CyPOSS(upper) and CpPOSS(below) derivatives. The POSS loading is 10, 30, 60 wt% from left to right. • Phase segregates with > 10% POSS monomer •Dark phase in TEM is due to silicon in POSS MRS Symp. Proceedings 2000, 628, CC2.6.1–CC2.6.7

Phase segregated triblock copolymer The microstructures and rheological behavior of tri-block copolymer: p(MA-POSS)10- b-p(BA)210-b-p(MA-POSS)10: (left) Transmission electron microscopy (TEM) images of (a) low magnification images with a overall morphological feature, (b)–(c) high magnification micrographs showing well-defined bright pBA cylindrical phase regularly dispersed in the dark pMA-POSS continuous phase. The local hexagonal packing of the cylinderswas confirmed by (d) Fourier transform of selected area frommicrograph, (right)Master curves of dynamic oscillatory shear storage modulus(G) and loss modulus(G) for the tri-block copolymer p(MA-POSS)10-b-p(BA)210-b-p(MA-POSS)10. The reference temperature was 80◦C. Polymer 2003, 44, 2739–2750

Oxidative Stability from POSS in hybrids POSS-formaldehyde resorcinol Better barriers to fire or oxidation

POSS-Silicones Stronger and transparent to light Solar lens Solar Cell cover sheet Stronger and transparent to light

Strength of POSS hybrids Energy Environ. Sci., 2013, 6, 1929-1937

Summary POSS hybrids with organic polymers are made by attaching reactive organic group to POSS Hybrids are stronger higher modulus Higher glass transition temperature Less permeable to gases. POSS is cheaper than fullerenes and carbon nanotubes.