Phase Behavior and Rheology
Block copolymer mesophase morphology and microdomain dimensions are readily characterized in-situ by small-angle x-ray scattering (SAXS). Over the past several years, we have examined the phase behavior of these materials in response to temperature; in response to pressure; in the presence of solvents of varying selectivity; as functions of the chemical constitution of the blocks; and as functions of how the blocks are topologically connected. Time-resolved SAXS is especially useful in tracking the dynamics of such a transition in response to temperature or pressure changes. An example is shown below, where we observed a hexagonally-perforated lamellar (HPL) structure in a polystyrene-poly(ethylene-alt-propylene) diblock copolymer, S/EP. The figure below shows an exceptionally well-ordered HPL structure; the SAXS pattern can be fully indexed according to a regular ABCABC… stacking of the perforations. In this particular polymer, the HPL structure can be rapidly (seconds) and reversibly accessed from the lamellar phase; other S/EP diblocks of slightly different composition show reversible HPL-to-cylinder transitions.
The L-to-HPL transition in an S/EP diblock copolymer. Lower left shows the structure of the the HPL phase; in this particular diblock, the S domains form the perforated layers, while EP forms the perforations. Right shows the SAXS patterns for the diblock at the temperatures indicated; at 245oC and above, the structure is HPL, 210oC and below is L, and from 210-245oC an apparently stable (reversible and reproducible) coexistence of the two structures pertains. Upper left shows the 7% contraction in layer spacing which occurs in the L-to-HPL transformation.
SAXS can also reveal the temperature at which microphase separation becomes unstable (the boundary between "DIS" and the various ordered phases in the phase diagram), and thus quantify the strength of the thermodynamic interactions between the blocks. With this thermodynamic information as a backdrop, we also investigated the flow behavior of sphere-forming block copolymers. Despite the widespread application of block copolymers, their rheology in the two-phase state, characterized by large viscosities and elasticities, has only recently yielded to understanding. In the ordered state, at low shear stresses, the microdomains are ordered, giving the molten rubber a large viscosity. However, as shown in the figure below, at a critical shear stress the viscosity drops precipitously as the microdomains are "shear-disordered". We have shown that this stress is uniquely related to the spacing between ordered microdomains, allowing us to control the value of this stress through molecular design or the addition of a selective solvent.
Steady-shear viscosity of a styrene-isoprene diblock forming polystyrene spheres, with TODT = 197°C. Note the factor-of-104 drop in viscosity at the critical stress, where the microdomain lattice disorders.
Supported by the National Science Foundation through the Princeton Center for Complex Materials and the Polymers Program
Recent/Current People and Projects:
Daniel Vega – Phase Behavior and Rheology of Block Copolymer Gels
Selected Recent Publications:
Y.-L. Loo, R.A. Register, D.H. Adamson, and A.J. Ryan, “A Highly Regular Hexagonally-Perforated Lamellar Structure in a Quiescent Diblock Copolymer”, Macromolecules, 38, 4947-4949 (2005).
J.M. Sebastian, W.W. Graessley, and R.A. Register, “Steady-Shear Rheology of Block Copolymer Melts and Concentrated Solutions: Defect-Mediated Flow at Low Stresses in Body-Centered-Cubic Systems”, J. Rheol., 46, 863-879 (2002).
C. Lai, W.B. Russel, and R.A. Register, “Scaling of Domain Spacing in Concentrated Solutions of Block Copolymers in Selective Solvents”, Macromolecules, 35, 4044-4049 (2002).