Mixing of a self-assembled supramolecular polymer and a covalent polymer in organic solutions

Abstract

We report on a neutron scattering study of the molecular structure of a self-assembled supramolecular polymer composed of a bicopper complex within a solution of a covalent polymer (atactic polystyrene/trans-decalin). The study is achieved by putting two binary phases in contact and allowing for diffusion of both components in either phase. It is shown that the one-dimensional bicopper filaments are compatible to a high extent with the atactic polymer. These results are discussed in the light of a recently devised encapsulation process of the bicopper filaments within the fibrils of a physical network of isotactic polystyrene.

Introduction

In recent papers, it has been shown that the one-dimensional filaments produced from the piling of bicopper complex molecules of the type shown in Fig. 1 can be encapsulated within a polymer sheath [1], [2]. This is achieved through a heterogeneous nucleation process in which the filaments act as heterogeneous nuclei for the growth of fibrils of an isotactic polystyrene physical network in trans-decalin [1]. The heterogeneous nucleation phenomenon has been particularly evidenced through an increase of the gelation temperature up to a certain fraction of bicopper complex. To be sure, at higher bicopper fraction the gelation temperature levels off, an effect which has been assigned to an additional liquid–liquid phase separation. The occurrence of such a type of phase separation is not surprising as the system consists of a mixture of two polymers [3], [4]: a covalent polymer (iPS) of infinite lifetime, and a self-assembled supramolecular polymer of finite lifetime [5], [6] (the bicopper complex filaments). Note that, in addition to the scission-recombination characterizing the self-assembled supramolecular polymer, namely filaments are ceaselessly broken while new ones form, the 1D filament state is metastable as it transforms gradually into 3D crystals.

With the aim of grasping the details of the encapsulation process Lopez and Guenet [7] have attempted to differentiate the phase separation effect from the gelation process. They have studied the bicopper structure by small-angle neutron scattering in solutions of atactic polystyrene (aPS), a polymer which does not produce physical gel in the solvent used, and accordingly does not encapsulate the bicopper complex filaments. In particular, they expected to find out to which extent the covalent polymer and the self-assembled supramolecular polymer could be compatible. By starting from ternary systems at high temperature, from which homogeneous solutions can be obtained, these authors faced insuperable difficulty. Due to the fact that the kinetics of total demixing into two macroscopic phases occurs too slowly the growth of bicopper complex three-dimensional crystals took over and so hampered a study under near-equilibrium conditions. Only experiments carried out after minimum ageing on microscopically phase separated systems were worth interpreting. From these it was inferred that compatibility was probably quite important at least in one phase but it could not be told which phase was involved.

The purpose of this paper is to report on neutron scattering investigation of the molecular structure of the bicopper complex mixed with a covalent polymer, namely aPS, by using another experimental approach. Instead of starting from one homogeneous ternary system, two binary solutions are put into contact, and the diffusion process of each component in either phase is allowed to take place. After some time, a near-equilibrium is reached while two macroscopic phases are still observable. No 3D crystals are seen to grow so under these conditions so that a measure of the amount and the determination of the molecular structure of the state of the complex in each phase can be achieved.