|
|
|
Importance of polymer brushes and related
morphologies |
|
Why multiply-bound polymer chains? |
|
Approaches to synthesis of multiply-bound polymer
chains |
|
Diagram to help
understanding the telechelic attachment |
|
 |
|
Importance of polymer brushes and related
morphologies |
|
Manipulating properties of
multiphase systems by controlling the chemistry at the interface
is of paramount importance in a number of existing and emerging
technologies, including: paints, pharmaceuticals, dyestuffs,
adhesives, nano-filled polymers, biomedical implants, and tissue
engineering. One common and successful method for modifying an
interface of a material is to anchor polymer chains to the
surface to change the interfacial structure and physicochemical
properties of the interface. Polymer chains bound to an
interface at one end have been the subject of intense interest
in the past 25 years. This intriguing physical configuration
arises in a wide variety of polymer structures and systems,
including ordered phases of di- or triblock copolymer colloidal
stabilization, and polymer micelles. Beyond the need to
understand the properties of tethered chains for specific
applications, there is great interest in the fundamental
physical chemistry of these systems. Placing one end of the
polymer chain at an interface introduces another length scale,
i.e. the average distance between tethering points, which can
dramatically effect the conformation of the chain. In
particular, when the grafting density is high enough so that
neighboring chains overlap, the crowed chains stretch away from
the interface, forming a polymer brush. This stretched
conformation is the defining characteristic of brushes, and
results in unique aggregation, phase, and dynamic behavior. For
example, brushes limit the collapse of the polymer chains in
poor solvents. It has also been shown that frictional
interactions are severely reduced when two brush-covered
surfaces are brought into contact and sheared against each
other. These simple examples show that systems of tethered
chains can be used to dramatically change the structure and
properties of the interface. On the other hand, with a moderate
drafting density, the polymer chains can be viewed as more
structurally and dynamically independent. In the polymer brush
community, this regime is known as the "mushroom" regime. Bound
polymer chains in either regime can alter the wet-ability,
adhesion, chemical functionality, and structural stability of
the interface. They also play a key role in colloidal
stabilization, interfacial adhesion, and coatings formulations
due to the interaction and entanglement between grafted polymers
and their surrounding media. |
|
|
|
|
Why
multiply-bound polymer chains? |
|
The focus on tethered polymers in the
past two decades has been largely driven by their manifestation in a
broad range of polymer systems. These systems include polymer
micelles formed from diblock copolymers, the organization of diblock
copolymers in microphase separated melts, behavior of diblock
copolymers at fluid-fluid interfaces (i.e. as an interfacial
compatibilizer), the use of tethered polymers as colloidal
stabilizers, graft copolymers on surfaces, and the adsorption of
diblock copolymers at hard interfaces. However, in every application
of tethered polymers such as control of aggregation, adhesion,
friction, and microphase morphology, a convincing case can be made
that MBPCs can provide novel and often superior properties. For
instance, because the MBPCs will not have chain ends at the
periphery of the brush, MBPCs may further reduce interpenetration,
and because of their connectivity, may provide higher segment
densities at the edge of the layer. These factors may translate into
reduced interpenetration of penetrant molecules, reduced frictional
interactions, and reduced adsorption. For example, Irvine and
coworker demonstrated that homopolymer stars tethered by their ends
(thus producing loop-like structures) reduced protein adsorption more
efficiently than layers of tethered linear chains. We expect our
approach will allow further enhancement in properties by creating
layers of loops at a fluid-solid interface. Another example of how
our study could have significant technological impact is in
controlling morphologies and properties of microphase separated
multiblock (triblock, pentablock, etc...) copolymers, where their
morphologies often contain loops at the microphase separated
interfaces. The specific organization of these loops, controlling
their assembly, and their contribution to viscoelastic properties is
an unexplored, yet important, scientific and technological challenge
that a detailed examination of the organization and dynamic behavior
of MBPCs could significantly illuminate. Finally, because grafted
polymers straddle soft interfaces, they can be used to reinforce
interfaces. For example, in modifying a polymer-polymer interface, a
copolymer at the interface that entangles with the homopolymer,
strengthens that interface. The interaction of the extended chain
with its surroundings plays an important role in the performance of
this interfacial modifier. Recent results in our laboratory have
demonstrated that a surface modified with loops manifests superior
interfacial adhesion and steric stability than one modified with
dangling chains (i.e. tails). Loop formation provides a mechanism
for adhesion that is similar to a "molecular level Velcro", where
the doubly bound polymers behave as loops and the polymer chains in
the surrounding matrix act as the hooks. Thus, for applications such
as colloidal stability and improvement of interfacial adhesion, the
use of multiply-bound polymer chains should provide significant
performance enhancement an versatility in the surface modification
process.
Technologically, the grafting of a polymer chain is often associated
with modification of the interfacial properties of the system. For
instance in colloidal stabilization, a grafted polymer sterically
shields the colloidal particles, preventing aggregation. Grafting of
end-functionalized polymer chains to the surface is a common method
to modify interfacial properties of a bulk sample where the surface
coverage is an elementary parameter that defines the success of the
grafting procedure. Experimental results suggest that multiply-bound
polymer brushes offer a more efficient method to attain a
homogeneous surface coverage than with singly tethered polymer
brushes offering a more efficient and robust method for chemical
modification of surface sensitive properties. One primary goal of
this project is to optimize the MBPC as interfacial modifiers to
tune and control a material's surface sensitive properties,
including the stabilization of nanoparticles in a polymer matrix.
Furthermore, we expect that the guidelines provided will be
applicable to a wide range of materials. This is just one example of
the potential societal impact of an improved understanding of using
macromolecular chemistry to optimize the surface chemistry and
physical chemistry of these important materials.
Lastly, the reactivity, structure, and properties of singly
functional chains have also been examined at liquid-liquid
interfaces primarily related to the in-situ formation of polymeric
interfacial modifiers. These results indicate that the process
whereby two functional polymers migrate to the liquid-liquid
interface and react in-situ to form a diblock copolymer/brush
is feasible. However, the specific aspects of chain localization to
the interface, organization and reaction at the interface, and
resultant brush structures are not well known. As our current
understanding of interfacial modification by polymers suggests the
presence of loops at the interface is critical to efficient
interfacial strengthening, we will seek to expand this process to
the reaction of telechelic polymers at liquid-liquid interfaces to
form loop structures as interfacial modifiers in a reactive
processing scheme. This reactive processing of telechelic additives
in a phase separated multi-component polymer system can be viewed as
complex formation of multiply-bound polymer chains at an interface.
We will examine it as such, utilizing our knowledge of simpler MBPCs
as guidelines.
In summary, for many polymer systems where grafted polymer chains
are important, multiply-bound polymer chains will provide an
opportunity for developing novel surfaces with significant
performance improvement relative to singly tethered polymers.
Therefore, this project will provide a fundamental molecular level
understanding of the structure and properties of multiply-bound
polymer chains providing an exciting enabling technology for the
engineering of the next generation polymer interfaces and
polymer-modified surface. |
| |
|
|
|
Approaches to
synthesis of multiply-bound polymer chains |
|
This project will concentrate on three
methods for synthesizing multiply-bound polymer chains. Our
experiment indicates that each of these techniques has promise, but
involves challenges and limitations that make the choice of approach
dependent upon the specific application. We believe a thorough
understanding of the factors controlling formation of MBPCs for each
of these three processes will provide the versatility needed for the
chemical design of specific grafted polymer systems for a wide
variety of applications. These methods involve using I)
amphiphilic A-B-A triblock copolymers that tether through the A
blocks copolymers, II) end-functionalized star copolymers
and III) telechelic polymers. In this proposal we refer to
these methods as Processes I-III.
In process I, (the preferential adsorption of A-B-A triblock
copolymers) the loops are formed when a selective solvent good for
the B block but poor for the A blocks is used to tether the middle
block by the shorter end blocks. Experimental and theoretical
results suggest that these triblock 'telechelics' will adsorb on a
surface and form crowded doubly bound chains in a loop conformation
that are similar to that of a singly end-adsorbed polymer brushes of
half its length. However, there has been very little work regarding
the dynamic or mechanical properties of these brushes, or on the
kinetics of assembly. In this proposed work, we will examine this
process in detail to elucidate the fundamental properties that
govern the organization and arrangement of loops on surfaces and
their properties. process II leads to the formation of polymer loops
by adsorption of amphiphilic star diblock copolymers. These
materials will be assembled from selective solvents, where we
believe that the polymer adsorption process has a large probability
to produce loops at the interface than with a corresponding triblock.
With the possibility of forming "domes" for stars with more than two
bound ends. process III consists of designing a surface that
contains a controlled amount of a reactive functionality. This
surface is then brought into contact with a solution of telechelic
polymers/oligomers that contain reactive end groups capable of
reacting with the functional groups at the surface. Under the right
conditions, the telechelics will react with the surface to form
loops. This process is analogous to the 'grafting to' process for
the synthesis of polymer brushes. |
| |
|
|
|
Diagram to
help understanding the telechelic attachment |
 |
|
|
|
Last
Update:
10/13/2004 |
|
