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 Low monomer concentration, bulky R groups High monomer concentration, most R groups High monomer concentration, small or reactive 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 Kyoto University 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 Low monomer concentration, bulky R groups High monomer concentration, most R groups High monomer concentration, small or reactive 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) 8 membered rings (as in T 8 ) are commonly formed Silica like-core with organic groups on surface Called smallest silica particle Low monomer concentration & reversible reactions

Some examples: Octamethyl- Polyhedraloligosilsesquioxanes: POSS No melting point Insoluble in organic solvents Sublimes above 240 °C 1,3,5,7,9,11,13,15- octamethylpentacyclo[ ,9.1 5,15.1 7,13 ]octasiloxan e

Polyhedral Silsesquioxanes J. Brown; M. Voronkov; R. Laine, A. R Esker, F. Fehr, T. Haddad & many others. T 8 : 1 nm cluster Cyclization & crystallization

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

Synthesis of T 12 POSS Dropwise add of 15.8 g (80 mmol) 14 days White crystalline precipitate Dalton Trans., 2012, 41,

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

Octaallyl- Polyhedraloligosilsesquioxanes: POSS Melts at 71 °C Soluble in organic solvents Sublimes above 140 °C 1,3,5,7,9,11,13,15- octapropenylpentacyclo[ ,9.1 5,15.1 7,13 ]octasiloxane Polymer 2005, 46, 2163

Networks based on POSS as polyfunctional monomers

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

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

Monomer functionality and phase separation 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 T 7 (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 T 7 (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 R = cyclohexyl: T g 396 °C, T d 445 °C T g 116 °C, T d 383 °C R = cyclohexyl = soluble in THF R = cyclopentyl = insoluble

TEM images of PS–POSS copolymers: (a) 6 and (b) 30 wt% i-Bu POSS. The samples were stained by RuO 4 vapor at ambient condition. P-methyl styrene-POSS Styrene Random copolymer

Methacrylate-POSS Dental Resin

Phase segregated block copolymer 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 Polymer 2003, 44, 2739–2750

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

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

Strength of POSS hybrids Energy Environ. Sci., 2013, 6,

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.