Transcript Physics and Chemistry of Hybrid Organic

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, small or
reactive 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
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All hits:
Frank Fehr (68)
Richard Laine (52)
F. C. Chang (51)
D. A. Loy (50)
K. J. Shea (50)
T. S. Haddad (48)
S.W. Kuo (48)
Y. Chujo (45)
Just Papers:
F. C. Chang (51)
S.W. Kuo (48)
Y. Chujo (45)
Richard Laine (42)
Frank Fehr (40)
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D.A. Loy (24)
Silsesquioxane Institutions
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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
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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, small or
reactive 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)
Low
monomer
concentration
& reversible
reactions
• Silica like-core with
organic groups on
surface
• Called smallest silica
particle
8 membered rings (as in T8) are commonly formed
Some examples: OctamethylPolyhedraloligosilsesquioxanes: POSS
1,3,5,7,9,11,13,15octamethylpentacyclo[9.5.
1.13,9.15,15.17,13]octasiloxan
e
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
POSS in polypropylene
Question: Are the POSS dissolved
or a separate phase?
OctaallylPolyhedraloligosilsesquioxanes: POSS
1,3,5,7,9,11,13,15octapropenylpentacyclo[9.5.1.13,9.15,15.1
7,13]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
• 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, 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.