The work described here was performed with a single ternary blend of poly(ethyl ethylene) (PEE), poly(dimethyl siloxane) (PDMS), and a PEE–PDMS diblock copolymer. These polymers were synthesized by standard anionic polymerization and catalytic saturation techniques that are described elsewhere [Hillmyer et al. (1999)]. PEE was prepared by saturating 1,2-polybutadiene with deuterium, in both the homopolymer and the diblock copolymer samples. Number average molecular weights, determined by nuclear magnetic resonance (NMR) spectroscopy, were 1770, 2130, and 10,400 for PEE, PDMS, and PEE-PDMS, respectively. The densities were measured using a density gradient column and the volume fraction of PEE in the block copolymer was determined to be 52%. This block copolymer molecular weight places the order–disorder transition temperature (90 °C) close to the critical temperature of the corresponding binary homopolymer blend (150 °C); an additional benefit afforded by these relatively low molecular weights is rapid phase transition dynamics, which facilitate equilibration. A single symmetric composition containing 10% by volume block copolymer and 45% PEE and PDMS homopolymers was used throughout this study. This mixture lies in the bicontinuous microemulsion channel which was identified in earlier publications [Hillmyer et al. (1999); Morkved et al. (1999)]. Specimens were prepared by simple mixing at 80 °C (in the disordered state) followed by cooling to room temperature.

Small angle neutron scattering (SANS) experiments were conducted at the National Institute of Standards and Technology (NIST), Gaithersburg, Maryland, on the 30 m NSF/CHRNS SANS instrument using =12 Å wavelength neutrons (/=0.11). A quartz couette shear cell was used, with the neutron beam directed radially through two sections of the device (Fig. 2). Additional SANS experiments were conducted with the neutron beam impinging tangentially. The Couette cell gap was 0.5 mm, leading to a total scattering thickness of 1 mm. Steady state patterns were collected at various temperatures and shear rates. The data were corrected for background and cell scattering, sample thickness, transmission, and detector sensitivity.

Figure 2.

Small angle light scattering (SALS) experiments were carried out using a custom-built flow-light scattering apparatus, illustrated in Fig. 3. This instrument contains a shear stage (Linkam Scientific Instruments) that employs a rotating parallel plate geometry with transparent disks. The temperature can be controlled to a precision of ±0.2 °C using a silver heating block along with circulating cooling water in the body of the stage. A He–Ne laser (=632.8 nm) supplies the light beam, which is conditioned by a set of neutral density filters, an iris, and a pair of alignment mirrors before passing through the sample. Two-dimensional scattering patterns are projected onto a translucent frosted glass screen, imaged by a charge coupled device (CCD) camera connected to a VCR, and recorded using high quality S-VHS videotapes. Subsequently these images were digitized from the videotapes using a computer equipped with frame-grabbing capabilities. Real-space microscopic images were obtained using the same shear stage mounted on an optical microscope (Olympus), with a long working-distance objective.

Figure 3.

Rheological measurements were carried out with a strain-controlled Rheometric Scientific ARES rheometer. A cone and plate geometry with a 50 mm plate diameter and 0.02 rad cone angle was used. The sample was maintained under nitrogen atmosphere and the temperature was controlled to within ±0.1 °C. Dynamic frequency sweep experiments were performed in the linear regime with frequencies from 0.01 to 100 rad/s. Steady shear rate sweep experiments were executed over the range from 0.01 to 250 s^–1. Transient shear flow startup experiments were conducted prior to the rate sweep in order to determine the time required to reach steady state at several representative shear rates. The sample was viewed at high magnification using a video camera to verify the absence of edge fracture at high rates. Rate sweeps were carried out with both increasing and decreasing rates, and no hysteresis effects were evident in the viscometric properties. These experiments were repeated at various temperatures over the range of 0–35 °C, while maintaining a constant cone-to-plate gap by taking into account thermal expansion of the tools.