Research Description:

I.  Significance of Research

Mixtures of oil and water organize into a variety of structures, such as vesicles, bilayers, lamellae, and microemulsions, upon addition of a surfactant [1].  This rich phase behavior is due to the availability of different types of surface-active molecules such as ionic and non-ionic surfactants, cosurfactants, phospholipids, proteins, etc.  Our objective is to use surfactant design concepts established in aqueous systems to create a library of surfactants for organizing immiscible polymers.    

Numerous papers have been written on the interface between A and B homopolymers [2].  It is now recognized that A-B block and graft copolymers cannot serve as surfactants for highly immiscible homopolymers A and B.  Theoretical calculations show that as the incompatibility between the homopolymers increases, a large unorganized 3-phase window consisting of two homopolymer-rich phases and one copolymer rich phase is obtained [3].  Our surfactant design consists of an A-C diblock copolymer, in which the C-block has attractive interactions with the B homopolymer and repulsive interactions with the A homopolymer.  Therefore, we have independent control over all of the interactions that govern our multicomponent system.  Systems with both attractive and repulsive interactions have been a recent topic in literature. 

II.  Experimental Results

We have studied the phase behavior of an A/B/A-C system with the following components: component A is a saturated polybutadiene with 89% 1,2-addition (sPB89), component B is polyisobutylene (PIB), and the diblock copolymer consists of block A (also sPB89) and block C which is a saturated polybutadiene with 63% 1,2-addition (sPB63).  These components are amorphous and rubbery in the temperature range between –30 C and 250 C.  We synthesized the polybutadiene polymers through anionic polymerization and saturated the double bonds with either hydrogen or deuterium gas, thus giving our system a neutron contrast.  All of the binary chi parameters that govern the thermodynamics of this A/B/A-C system have been measured and are summarized in Figure 1.  The binary interactions (A/B, A/C, and B/C) are analogous to that in the oil/water/non-ionic surfactant system (in the aqueous system, component A is oil, component B is water, and component A-C is an alkyl polyglycol ether surfactant).  The A-C diblock copolymer will thus have "balanced" interactions with the A and B homopolymers due to the combination of attractive and repulsive interactions [4].

FIGURE 1: Binary Chi Parameters in A/B/A-C System

We have conducted a study varying the size of the homopolymers as well as the size of the diblock copolymer, in order to understand the key parameters for designing an effective surfactant.  One highlight of this study was a series of blends utilizing homopolymers that are weakly segregated.  Through neutron and light scattering studies we have observed the formation of lamellae and spherical or bicontinuous microemulsions.  A phase diagram summarizing these results is shown in Figure 2.  This phase diagram looks very similar to the upper half of the fish phase diagram that is observed in oil/water/non-ionic surfactant systems.  Furthermore, we observed a scattering peak indicating the presence of a microemulsion for a blend with only 5% of the A-C diblock copolymer.  If we increase the size of the diblock copolymer, only 1% of the A-C diblock copolymer is required to form a microemulsion phase.  To our knowledge, this is the lowest concentration of diblock copolymer that has been used to successfully form an organized phase in 50/50 mixtures of immiscible polymers, and the lowest concentration of a single surfactant to organize any two immiscible fluids.  Additionally, we have completed self-consistent field theory and random phase approximation calculations to predict the domain spacing as a function of temperature for each blend.  The results for the blend with 50% diblock copolymer are shown in Figure 3, and remarkably good agreement is observed between theory and experiment. 

FIGURE 2: Multicomponent Blend Phase Diagram

FIGURE 3: Domain Spacing from Theory and Experiment

1. Kahlweit, M.; Strey, R. Angew. Chem. Ed. Engl. 1985, 24, 654.

2. Bates, F. S. et. al. Phys. Rev. Lett. 1995, 75, 4429.

3. Janert, P. K.; Schick, M. Macromolecules 1997, 30, 3916.

Publications:

Ruegg, M. L; Reynolds, B. J.; Lin, M. Y.; Lohse, D. J.; Balsara, N. P. Macromolecules 2006, 39, 1125-1134..

Reynolds, B. J.; Ruegg M. L.; Mates T. E.; Radke C. J.; Balsara N. P. Macromolecules 2005, 38, 3872-3882.

Reynolds, B. J.; Ruegg, M. L.; Balsara; N. P., Radke; C. J.; Shaffer, T. D., Lin, M. Y.; Shull, K. R.; Lohse, D. J. Macromolecules 2004, 37, 7401-7417.

Ruegg, M. L.; Newstein, M. C.; Balsara, N. P.; Reynolds, B. J. Macromolecules 2004, 37, 1960-1968.

Lee, J. H.; Ruegg, M. L.; Balsara, N. P.; Zhu, Y. Q.; Gido, S. P.; Krishnamoorti, R.; Kim, M. H. Macromolecules 2003, 36, 6537-6548.