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IUPAC Prize for Young Chemists - 2001
Honorable Mention

 

  Janne Ruokolainen receives one of four Honorable Mention awards associated with the IUPAC Prize for Young Chemists, for his Ph.D. thesis work entitled "Supramolecular Concepts for Self-Organized Polymeric Nanostructures"

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. Helsinki University of Technology, June 6. 2000, Polymer Science.
  • M.Sc. Helsinki University of Technology, Nov.14. 1995, Semiconductor Technology .

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:

    1. the strength of the attractive interaction between the polymer backbone and the amphiphile head group, and
    2. 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. .


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