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IUPAC Prize for Young
Chemists - 2001
Honorable Mention
Current address (at the time of application)
University of California at Santa Barbara
Department of Materials E3-106
Santa Barbara CA 93106 USA
Tel: +1 805 893 7551
Fax: +1 805 893 8971
E-mail: [email protected]
Academic degrees
Ph.D. Thesis
Title Supramolecular Concepts for Self-Organized
Polymeric Nanostructures
Adviser Prof. Olli Ikkala, Helsinki University of Technology
Thesis Committee Christopher K. Ober, Dept. of Materials Science
& Engineering, Cornell University, USA; Paula Hammond, Dept. of Chemical
Engineering, Massachusetts Institute of Technology, USA; Kari Rissanen,
Department of Organic Chemistry, University of Jyv�skyl�, Finland.
Essay
During the past decade, methods to prepare nanosized
structures have progressed greatly, stimulated by the continuing demand
for miniaturization of devices and electronic components. This imposes
new challenges for chemists to design materials suitable for submicron
size applications. For example, the design of suitable photoresist
materials for higher density chips has become difficult because feature
sizes are approaching the dimensions of only a few polymer chains.
In order to overcome these limitations more precise control in structure
size and orientation is needed. Polymeric materials designed to include
several competing molecular interactions, offer a useful method to
construct nanoscale domains, due their intrinsic ability to form self-organized
structures. Controlled microphase separation on a nanometer length
scale may prove to be a useful tool to provide material properties
suitable for future applications.
Polymer-amphiphile complexes
In this Thesis work we have been developing a supramolecular concept
to obtain polymeric nanostructures by using polymer-amphiphile complexation.
Traditionally, synthetic polymers consist of atoms and molecules in
a polymer chain held together by covalent interactions. In a supramolecular
approach, physical interactions play a major role in addition to covalent
bonds. Such interactions include all noncovalent intermolecular forces
like electrostatic interactions, coordination complexation, hydrogen
bonding, and van der Waals forces.
Many ordinary thermoplastic polymers form disordered
or semicrystalline structures in a solid state. In this work it was
shown that by the addition of simple amphiphile molecules, these polymers
can form highly organized nanostructures. Three different types of
polymer-amphiphile interactions were used to bind amphiphile molecules
to the polymer backbone: ionic interaction, coordination complexation,
and hydrogen bonding interaction. In all cases, microphase-separated
lamellar morphologies were formed, with domain spacings of 3 - 4 nm.
Two important factors that strongly affect the structure
formation were discussed:
-
the strength of the attractive interaction between
the polymer backbone and the amphiphile head group, and
-
the repulsion between the polar polymer backbone
and the nonpolar amphiphile tails.
In order to obtain self-organized microphase separated
structures, the attraction and the repulsion have to be sufficient
but balanced. The amount of repulsion can be tailored either by increasing
the polarity in the amphiphile head group or by varying the alkyl
tail length. Guidelines for the design of stable self-organized polymer-amphiphile
complexes were given.
The phase behavior of these polymer-amphiphile systems
was thoroughly studied, particularly in the case of mixtures of poly(4-vinylpyridine)
with alkylphenols. The most versatile phase behavior was observed
in the case of combined ionic interaction and hydrogen bonding interaction.
Microphase separation, order-disorder transitions, re-entrant closed-loop
macrophase separation, and high temperature macrophase separation
were discovered. Careful morphological characterization was also performed
and for the first time the lamellar polymer-amphiphile morphology
was directly imaged using transmission electron microscopy.
This work has many similarities to the fields of liquid
crystals and the block copolymers. The smectic-type lamellar morphologies
found in polymer-amphiphile systems resemble those found in liquid
crystals. However, in this case the structures are formed due to microphase
separation between the nonpolar alkyl tails and polar polymer backbone.
This self-organization due to microphase separation is well known
in the field of block copolymers.
Hierarchical structures
Inspired by our earlier results with the homopolymer-amphiphile complexes
we showed that interesting hierarchical nanostructured materials with
two length scales can be constructed by using the supramolecular route
where a functional block copolymer is complexed with suitable low
molecular weight amphiphilic molecules. This procedure provides a
straightforward method to construct structure-within-structure
materials. The block copolymers are known to spontaneously form self-organized
structures in a variety of morphologies: spherical, cylindrical, lamellar,
and bicontinuous gyroid phases. The characteristic sizes of these
structures mainly depend on the molecular weight of the block copolymer
and are typically in the range of 10 - 100 nm.
We used block copolymer consisting of a polystyrene
and a poly(4-vinyl pyridine). The alkylphenols were selectively hydrogen
bonded to the poly(4-vinyl pyridine) block . These systems resemble,
in some respect, side chain liquid crystalline block copolymers ("LC-coils"),
but are obtained simply by using hydrogen bonding of amphiphilic additives.
It was demonstrated that lamellar-within-lamellar, lamellar-within-cylindrical,
cylindrical-within-lamellar, spherical-within-lamellar,
and lamellar-within-spherical morphologies can be obtained
using supramolecular block copolymer-amphiphile complexation. All
of these structures were, for the first time, directly imaged using
transmission electron microscopy. This work gives some new insight
for phase behavior and structure formation of complex polymer systems.
It was also shown that polymeric supramolecular nanostuctures
with several length scales allow straightforward tailoring of hierarchical
order-order and order-disorder transitions and the concurrent switching
of functional properties. It was demonstrated that thermal switching
of electrical conductivity was achieved by using poly(4-vinyl pyridine)
containing block copolymer. The pyridine rings were acid doped to
form a protonically conductive polyelectrolyte/amphiphile complex.
This results in an interesting morphology where the conducting layers
alternate with insulating layers.
Applications and recent work done after Thesis
The supramolecular polymer-amphiphile complexation method is not only
restricted to the flexible thermoplastic polymers, but can be applied
to rigid rod-like polymers as demonstrated by using conjugated electroactive
polymer poly(2,5-pyridine diyl). By using suitable amphiphile molecules
a lamellar morphology with layer thicknesses of a few nanometers was
achieved. Conducting cylinders with a diameter of one nanometer are
also demonstrated by using conducting polyaniline-amphiphile complexes.
These concepts have many implications for new processing
routes and controlled nanoscale structures of rigid rod-like polymers
and clearly offer exciting possibilities for molecular engineering
of self-organized structures. .
Page last modified 13 April 2001.
Copyright ©2001 International Union of Pure and Applied Chemistry.
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