Transcript Controlled Synthesis of Polymer Brushes by Atom Transfer

Controlled Synthesis of Polymer Brushes
by Atom Transfer Radical Polymerization
Jinsheng Zhou
Membrane Research Group
Department of Chemistry
7/17/2015
Presentation, Advanced Polymer
Chemistry (Dr. H. D. Stover)
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Introduction
What is Polymer Brushes?
Polymer brushes are long-chain polymer molecules attached by one end to a surface
or interface with a density of attachment points high enough so that the chains are
forced to stretch away from the interface.
----Milner. S.T., Science 1991,251,905
Why are we interested in these systems?
Applications: Colloidal stabilization
Modification of bulk surfaces and interfaces
(to improve adhesion, wetting, and wear properties)
* The interface may be a solid substrate, interface between two solvents and
between solvent and air.
** But a solid substrate is the subject of today presentation
7/17/2015
Presentation, Advanced Polymer
Chemistry (Dr. H. D. Stover)
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Methods of Producing Polymer Brushes
• Physical process
The binding to surface is
noncovalent and desorption
of the brushes always occurs.
• Chemical process
“Grafting to”
Serious steric hindrance,
hard to achieve a high density
of grafting polymer
“Grafting from” (Pioneered by Sogah et al, Macromolecules 1990, 23, 1264)
Surface-initiated conventional radical, cation, anion or living radical polymn.
7/17/2015
Presentation, Advanced Polymer
Chemistry (Dr. H. D. Stover)
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Why choose ATRP?
*
Accurate control of the thickness of
polymer layer on the nanometer
scale has great significance in the
application for chemical separation,
sensor, composite materials.
•
ATRP permits control of MW of
polymers with low MWD and thus
precisely control the thickness of
polymer brushes.
A wide range of monomers available
and allow block polymers to be
formed, thus tailoring the properties
of polymer brushes over a wide
range
•
•
Relatively easy to perform, and can
be performed at the relatively low
temperature.
Shah et al. Macromolecules 2000, 33, 7617
7/17/2015
Presentation, Advanced Polymer
Chemistry (Dr. H. D. Stover)
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The Mechanism of Atom Transfer Radical
Polymerization
Matyjaszewski et al. J. Am. Chem. Soc. 1995, 117,5614
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Controlled synthesis of polymer brushes by
ATRP (Surface-initiated ATRP)
A Two-Step Process
1. Immobilization of initiator on the
substrate surface
* The current studies focus on two substrates,
silicon and gold .
2. Atom transfer radical polymerization
7/17/2015
Presentation, Advanced Polymer
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Silicon Substrate
2-(4-chlorosulfonyl phenyl)
ethyl trimethoxysilane
Ejaz et al. Macromolecules, 1998,31, 5934
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Chemistry (Dr. H. D. Stover)
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Gold Substrate
Me6TREN
Baker et al. J. Am. Chem. Soc. 2000,122,7616
7/17/2015
Presentation, Advanced Polymer
Chemistry (Dr. H. D. Stover)
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How control polymer chain growth at the
extremely low initiator concentration?
• Under typical conditions of LB self-assembly, the concentration of
initiator can be no larger than ~51014 molecules/cm2( i.e., 10-9
mol/cm2, or 10-7mol/L).
-----Wasserman et al. Langmuir, 1989, 5, 1074
Two initial attempts: ----Husseman et al. Macromolecules, 32, 1424
Standard living radical polymerization conditions
(Polymer brushes are formed while no control is observed, )
Lower monomer concentration
(polymerization rate reduces dramatically when monomer
concentration is less than 25wt%)
7/17/2015
Presentation, Advanced Polymer
Chemistry (Dr. H. D. Stover)
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Why is ATRP not well controlled here?
A typical ATRP process
P-X +
CuIX/2L
Activator
Kact
Kdeact
P*+ CuIIX2/2L
Deactivator
t=0
t
0
C(P*)
C(Cu )
The formation of P-P
C(P*)
C(Cu )
II
II
High C(CuII)
Reduction of P* termination
Keq 
0
kact
[ P*][Cu( II )]

Kdeact [ P  X ][Cu( I )]
The persistent radical effect
** In a typical ATRP process, and the concentration of Cu(II) detected by
EPR is the range 10-3 mol/L.
** Cu(II) concentration formed by surface-initiator is 10,000 times less than
that required for well-controlled ATRP.
Matyjaszeweski et al. Macromolecules 1999, 32,8716
7/17/2015
Presentation, Advanced Polymer
Chemistry (Dr. H. D. Stover)
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How to solve this problem?
P-X + CuIX/2L  P*+ CuIIX2/2L
Activator
Deactivator
Synthesis with Free Initiator
* Add free untethered initiator in the solution to increase the
Initiator (P-X) concentration.
Synthesis without Free Initiator
* Intentionally add Cu(II) at the beginning of reaction.
7/17/2015
Presentation, Advanced Polymer
Chemistry (Dr. H. D. Stover)
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The Results of two Approaches
TsCl=2.4mM
1%CuBr(PMDETA)
0.03%CuBr2(PMDETA)
4.8mM
Plots of graft-layer thickness vs polymerization
time.
Synthesis with Free Initiator
---Ejaz et al. Macromolecules, 1998,31, 5934
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Synthesis without Free Initiator
---Matyjaszeweski et al. Macromolecules 1999, 32,8716
Presentation, Advanced Polymer
Chemistry (Dr. H. D. Stover)
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Characterization of Grafted polymer chains
1. Direct measurement
• Polymer chains are detached from surface and measured by GPC
analysis to get MW and MWD. But this method is limited by the too
small amount of grafted polymers ( e.g. 0.01mg of polymer
corresponds to 100nm thick film grown from a flat surface 1 cm2 )
2. Indirect measurement
• This is the dominating method to study grafted polymer chains. It is
generally assumed that the MW of covalently bound polymer chains is
related to that of the “bulk” polymer.
7/17/2015
Presentation, Advanced Polymer
Chemistry (Dr. H. D. Stover)
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Tailoring of Polymer Brushes
---Shah et al. Macromolecules, 2000,33, 597
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---Matyjaszeweski et al. Macromolecules 1999, 32,8716
Presentation, Advanced Polymer
Chemistry (Dr. H. D. Stover)
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Grafting density and Surface Evenness
M
Ax 
t N A
Ax : a cross-sectional area per chain
 : is the mass density
NA : is Avogadro’s number
t: polymer brushes thickness
M: molecular weight of the chain
Comparison:
Ax:
180Å(ATRP) ~ 900Å(Adsorption)
Baker et al. J. Am. Chem. Soc. 2000,122,7616
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----Wirth et al. Anal. Chem. 1998, 70, 4023
Presentation, Advanced Polymer
Chemistry (Dr. H. D. Stover)
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Conclusion
• The persistent radical effect plays an important role in surfaceinitiated ATRP
• The thickness of polymer brushes can be effectively controlled by
surface-initiated ATRP.
• The properties of polymer brushes can be tailored by
copolymerization and the selection of monomers
• However, how to directly study the grafted polymers is still a
challenge (MW, MWD, Chain Conformation).
• Surface-initiated ATRP shows a great potential in application for
Biocompatible medical materials, nonlinear optical materials,
Chemical separation(capillary for electrophoresis),
microfabrication.
7/17/2015
Presentation, Advanced Polymer
Chemistry (Dr. H. D. Stover)
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