1996 NATO Workshop

MANIPULATION OF ORGANIZATION IN POLYMERS USING TANDEM
MOLECULAR INTERACTIONS

Rudolf Zentel

Polymere Materialien, Institut für Chemie und Materialwissenschaften, Gaußstraße 20, L 13 15, 42097 Wuppertal, Germany

INTRODUCTION

Webster New World Dictionary defines tandems as: bicycles with two seats and two sets of pedals placed tandem or as a relation between two persons or things involving cooperative actions and mutual dependence to achieve a goal. A NATO workshop concerned with the "Manipulation of Organization in Polymers using Tandem Molecular Interactions" was held at Il Ciocco, Tuscany, Italy from May 28 to June 2, 1996. The purpose of the workshop was to bring together chemists, physicists and theorists to examine the developing field of simultaneously using two or more molecular interactions (tandem interactions) to adjust polymer organization and thereby create sophisticated polymer systems. A new concept? Certainly not - at least for mother nature - where tandem processes and tandem interactions are the basis of e.g. basically all cell and membrane interactions.

RESULTS AND DISCUSSION

The motivation for this workshop developed from disussions, between G. Galli (Pisa, Italy), C.K. Ober (Ithaca, USA) and R. Zentel (Mainz, Germany), who served later as the codirectors/ director of the workshop. These discussions showed an increasing interest in developing sophisticated polymer systems, by simultaneously using different molecular interactions. This led to the study of polymers, which organize on a variety of length scales. Synergistic effects occur in a series of tandem systems like LC-networks, block copolymers of complex structure and others. These systems were - so far however - discussed as isolated topics. The goal of this workshop was therefore twofold: 1) to review successful systems, which owe their special properties to tandem interactions and 2) to establish a more general concept for the use of tandem (competing or synergistic) interactions to organize polymers on different length scales.

The interactions to be combined included covalent networks to introduce rubber elasticity, liquid crystalline (LC) phases, phase separation in block copolymers and amphiphiles, ionic interactions, H-bonding and others. The systems considered were discussed in one session devoted to the "theoretical aspects of systems with competing interactions" (E.J. Kramer, J. Prost, E.M. Terentjev, A.R. Khokhlov, A. Halperin), one session devoted to the "characterization on different length scales" (G. Wegner, W. deJeu, G. Marrucci, E.L. Thomas, H.W. Spiess) and 4 sessions devoted to different combinations of tandem molecular interactions, like: "polymer networks and liquid crystalline phases" (S.I. Stupp, H. Finkelmann, G. Mitchell, D.J. Broer, A. Sirigu), "block copolymers and phase separated systems" (G. Galli, F.S. Bates, H.W. Schmidt, C.K. Ober, R. Stadler, N. Hadjichristidis), "effects of ionic interactions on ordering" (H. Ringsdorf, A. Eisenberg, A. Laschewsky, M. Möller, R.V. Talroze), and "effects of H-bonding, chirality and other interactions" (V. Percec, M. Laus, A.C. Griffin, R.J.M. Nolte).

In the following, some aspects discussed at the workshop will be summarized. These topics have been - somewhat arbitrarily - selected because they describe especially well the concepts of "tandem interaction" and the similiarities between the different fields, in which this concept is used.

Block copolymers owe their morphology to microphase separation and the successive tandem interaction of interfacial tension and entropic forces based on chain stretching. The first tries to minimize the interface between the microdomains, the second tries to increase it. Papers presented at the workshop showed how this interplay between two forces can be manipulated e. g. changing the connectivity between the blocks. Triblock copolymers (R. Stadler) give rise to new triphasic morphologies and miktoam block copolymers (N. Hadjichristidis) allow the preparation of bicontinuous phases. In this concept the similiarity to lyotropic liquid crystalline phases became evident (F. Bates). In addition the use of LC-main chain polymers, which favour chain stretching of one block, gives rise to new tilted structures (C. K. Ober, E. L. Thomas). Blockcopolymers from LC-main chain and LC-side group polymers allow the mutual orientation of two types of LC-phases and the lamellar block structure (G.Galli). Crystallization of one block can be used to change the shape of the phase separated structure e. g. from cylindrical to spherical (F. Bates, R. Stadler). Copolymers with ionic interactions tend to microphase separate because of the incompatibility of ionic and nonpolar (hydrophobic) segments. They show therefore many of the properties discussed for blockcopolymers with the speciality that their ionic block can be very short. In addition their incompatibility can be easily varied by the addition of salts or by changing the pH. For these systems especially the importance of non equilibrium structures were discussed, which result from a freezing of equilibrium structures like micelles or spinodal decomposition stages below Tg. This concept can be used for the defined preparation of micellar particles of different shape (A. Eisenberg) or for the lateral structure formation in thin films (M. Möller). Both systems can be used afterwards as minireactors for the preparation of CdS or gold nanoparticles. Alternatively systems were discussed that close the bridge between copolymers with ionic interactions and liquid crystalline networks. The combination of polyelectrolyte networks with oppositely charged surfactants leads to structure formation due to a combination of ionic and hydrophobic interactions with the elasticity of the polymer network (A. R. Khoklov). This system is although of a completely different chemical structure - very close to the LC-elastomers discussed below. In fact it is very close to a lyotropic LC-elastomer. The tandem interaction of LC-order and polymer networks adds rubber elasticity (slightly crosslinked systems) or hardness (densely crosslinked thermosets) to LC-properties.The anisotropic rubber possesses therefore strongly anisotropic mechanical properties. In smectic A phases these networks are entropically elastic parallel to the smectic layers and enthalpically elastic along the layer normal (H. Finkelmann). Another interesting aspect of network formation is the preparation of non-equilibrium structures, with a memory of the phase and director pattern during crosslinking (G. Mitchel). In addition the crosslinking freezes order fluctuations (E. Terentjev) and may lead to liquid crystals with improved internal order. In combination with ferroelectric liquid crystals, ferroelectric LC-elastomers are possible, which are switchable in electric fields, but relax - in the field free state - always to the same pola structure (R. Zentel).   Alternatively the combination of a network formation with the diffusion of monomers with different reactivities can be used to create a cholesteric material with a pitch gradient of the cholesteric helix (D. Broer). This material is useful as a broad band reflective polarizer.

At least two contributions tried to bridge the gap to nature. One talk (R.J.M. Nolte) discussed systems, which owe their superstructure to a molecular recognition process and another (J. Prost) discussed the question: "Can tandem interactions give rise to motion?" The answer seems simple. Systems, where one type of interaction strongly dominates, are close to equilibrium and one type of structure or morphology dominates strongly. Competing interactions open the possibility to have many morphologies or structures, with nearly the same stability. The stimulated transition between different polar structures or states is a precondition for motion.

Often when a concept is coined its importance becomes visible and the "new" view opens aspects otherwise overlooked or discussed in a "phase separated " manner. In addition to adding interesting new aspects to old fields, the concept of Tandem Molecular Interactions will help to bridge the gap between Materials Science and Life Science, a field where this concept is ubiquitous.

Last revised: 11/19/02