MICELLES OF POLYBUTADIENE-bPOLY(ETHYLENE OXIDE) IN A BINARY SOLVENT SYSTEM

Postgraduate

ABSTRACT

 

We studied the assembly behavior of a polybutadiene-b-poly(ethylene oxide) diblock copolymer in methanol, cyclohexane, and the corresponding partially miscible binary solvent system.  Dynamic light scattering indicates that the copolymer forms coexisting spherical and cylindrical micelles in both of the pure solvents.  In the binary solvent system, spherical micelles form in the methanol-rich phase for a wide range of temperatures.  Conversely, micelles are present in the cyclohexane-rich phase only near the critical temperature.  At the critical solvent composition, micelles form in the single phase region above the critical temperature.    Size exclusion chromatography results for the binary solvent system show that the copolymer generally prefers the methanol-rich phase.  The preference becomes more pronounced as temperature decreases.


Introduction

 

Aqueous solutions of polybutadiene-b-poly(ethylene oxide) (PB-b-PEO), like other amphiphilic block copolymers, have received much attention in the literature.

  Investigations have determined the configuration of the microphase (e.g. isolated assemblies, networks, etc.) as a function of copolymer concentration3, 4 as well as molecular mass and copolymer composition (i.e., relative length of the two blocks).  In “dilute” aqueous solutions (~18 wt% or less8), PB-b-PEO forms aggregates spanning the complete range of configurations from spherical micelles (for relatively short PB blocks), through cylindrical (worm-like) micelles, to bilayers (for relatively long PB blocks).

We have not found any studies of PB-b-PEO in solvents other than water.  Likewise, the behavior of any amphiphilic block copolymer in the presence of a binary solvent (i.e., two partially miscible solvents that form coexisting liquid phases) has received relatively little attention.  Chemical intuition, corroborated by limited

theoretical and experimental work, indicates that dilute solutions of amphiphilic block copolymers in binary solvents should yield micelles in the polar phase and

reverse micelles in the non-polar phase.  The only relevant experimental work developed a complete three-component (tri-block copolymer/water/“oil”) phase diagram that appears to indicate the presence of spherical micelles in both of the coexisting phases at low copolymer concentration.  However, this work focused on the copolymer-rich corner of the phase diagram and the multitude of microstructures formed there.  It is not clear how rigorously the copolymer-lean side of the phase diagram was investigated, nor if the authors meant to imply the presence of micelles in coexisting phases.

Therefore, we have investigated the behavior of a PB-b-PEO diblock copolymer of narrow polydispersity (1.04) and roughly equivalent block lengths (54.7 wt% PEO) in the methanol/cyclohexane binary solvent system, which is partially miscible at room temperature and exhibits an ambient pressure upper critical solution

temperature between 45 ºC22, 23 and 48 ºC24 at 29 wt% methanol (51.7 mol% methanol).22  This work focuses specifically on the copolymer-lean side of the threecomponent phase diagram in order to determine if micelles are formed in both of the coexisting liquid phases, and to determine if micelles are present above the critical temperature.  PB-b-PEO was selected for this study because the solubility properties of the two blocks are very different, which leads us to believe that microphase separation should occur for a wide variety of solvents even for small concentrations of copolymer.

Before investigating the binary solvent system, the micellization behavior of the copolymer in each of the pure solvents was characterized.  In methanol, the copolymer appears to form coexisting spherical and cylindrical regular micelles.  Changes in temperature and concentration have little effect.  In cyclohexane, spherical and cylindrical reverse micelles are observed at elevated temperatures, but only cylindrical micelles exist at lower temperatures.  There is no concentration effect.  We also attempted to find the critical micelle concentration in cyclohexane, but it is experimentally inaccessible for the method we used.

Results in the binary solvent system indicate that micelles do form in both coexisting liquid phases for a small range of temperatures near the critical temperature.  Micelles disappear from the cyclohexane-rich phase as the temperature is lowered, but remain in the methanol-rich phase.  Micelles were also found above the critical temperature at the critical solvent concentration.  

The disappearance of micelles in the cyclohexane-rich phase can be explained by the fact that the copolymer preferentially partitions into the methanol-rich phase and that the temperature effect for this phenomenon is strong (i.e. the ratio of the concentration in the methanol-rich phase to the concentration in the cyclohexane-rich phase increases as the temperature decreases).


Micelle Formation in Selective Solvents

 

 

It is well documented that block copolymers assemble to form micelles as well as other micro- and nanostructures in solvents that are poor for one block and good for the other. Like any phase transition, the solvation, i.e., complete dissolution of both blocks of the copolymer, or microphase separation of block copolymers in solution is governed by the Gibbs free energy change of mixing (ΔGmix), which is shown in equation (2.1) and must be positive for microphase separation to occur.