The solvents, initiator, and butadiene were purified using standard techniques for anionic polymerization [Morton and Fetters (1975)].

Butadiene was polymerized in n-hexane using sec-BuLi as the initiator in an all-glass apparatus which had been flamed under a vacuum before use. Reagents were added from ampoules by the rupturing of glass breakseals. Reactions were carried out under a high vacuum at 30 °C for a minimum of 48 h to ensure completion. The reaction was then terminated with methanol.

The synthesis of PS was carried out on a scale of up to 200 g and, as such, involved some modifications to the usual methodology. Polymerizations were carried out in benzene, and purified by passing the solvent sequentially through a column of activated alumina to remove polar impurities and a column of supported copper catalyst to remove traces of oxygen [Pangborn et al. (1996)]. Styrene was dried over calcium hydride and degassed by several freeze-thaw cycles. The polymerizations were carried out in a 5 glass reactor, which was evacuated prior to use. After transferring up to 2 of solvent and the monomer to the reactor, any residual impurities were removed by drop-wise addition of the initiator, sec-BuLi. Upon the formation of a sustained pale yellow/orange color, attributable to "living" polystyryllithium, the required quantity of initiator was added. All polymerizations were carried out at room temperature and the reactions quenched with degassed methanol. Labels and descriptions of materials used in this study are shown in Table I.

Molecular weights were obtained by size exclusion chromatography using a Viscotek 200 with differential refractometer/viscometer/RALLS detectors. Three (300×7.5  mm) PLgel 5µ MIXED C columns were used with tetrahydrofuran as the eluent and a flow rate of 1.0 ml/min. The level of 1,4-enchainment of the butadiene was determined by ¹H nuclear magnetic resonance in CDCl₃ at 30 °C using a Bruker AC250MHz spectrometer. For each sample, 93% (±1%) 1,4-addition was found.

Polymer samples were vacuum dried, compacted using a piston and cylinder as necessary, and then premolded to a suitable thickness and geometry using a template and platen press. Typical molding press temperatures were 90 °C for PB and 180 °C to 200 °C for PS depending on molecular weight. Molding cells with a nitrogen atmosphere or vacuum were available.

All measurements were made using Rheometrics ARES or RDAII rotational rheometers with a nitrogen atmosphere.

G() was obtained using time-temperature superposition and a measurement geometry of either 10 mm diameter parallel plates or a 10 mm diameter cone and plate with 2° included angle. PB data were obtained from measurements between –80 °C and 60 °C and PS data between 120 °C and 210 °C.

The relaxation modulus G(t) as a function of strain () and the transient viscosity (t) as a function of shear rate () were obtained with the cone and plate geometry. The 10 mm diameter and 2° angle were used to reduce large normal forces, when installing the sample and during measurement, and to minimize edge fracture instability [Lee et al. (1992); Tanner and Keentok (1983)]. To reduce the tendency for slip at high strains and strain rates, the cone and plate surfaces were grit blasted to produce a surface roughness of approximately 600 grit. Transient viscosity data were corrected for the start time error (strain rate acceleration). At higher strain rates, the transient viscosity was limited to the minimum time and strain to indicate the stress overshoot. This avoided instability allowing repeat measurements on the same test specimen. PB was measured at 20 °C and 50 °C and PS at 170 °C to conform with the elongational data.

Extensional viscosity was measured in nitrogen using a Rheometrics RME elongational rheometer as described by Meissner and Hostettle (1994). Changes in the rectangular bar specimens, initially 60 mm long with a cross section between 3.5  mm² and 10.5  mm² depending on the strain rate and peak force, were monitored during extension by a camera and frame grabber. Corrections were made for true strain rate [Schulze et al. (2001)], force base line drift, and start time error. However, the strain rate was determined from change in the sample width due to the need to recover polymer and avoid contamination by particles or markers required for direct extension monitoring. Spacer pins were used between the gripping belts to prevent premature squeeze flow in the sample. There is a narrow temperature window for optimum measurement and the PS was measured at 170 °C.

The multipass rheometer (MPR) used for the flow visualization experiments was a two-piston capillary-type rheometer developed at the University of Cambridge [Mackley et al. (1995)] and is shown schematically in Fig. 1(a). The latest (Mk IV) design has a reduced volume and requires less than a 10 g sample, thereby enabling microprocessing studies to be made on the synthesized monodisperse polymers. The molten polymer was forced backward and forward at controlled piston velocities through an optical flow cell and the flow-induced birefringence technique was used to observe the stress field within the melt. Additional time dependent pressure difference data could also be followed. These techniques have previously been used in combination with an MPR for molten polyethylene [see for example, Lee and Mackley (2001); Lee et al. (2001)].

Figure 1.

After the system was loaded and sealed, the pistons were used to set a mean pressure and to carry out a series of experiments for a range of wall shear rates, if necessary, over long periods of time with the same small sample. As the system is enclosed, there was no noticeable degradation of the samples with time. For the experiments reported in this article, we used a "multipass steady" shear operation, which was moving the pistons in unison at a constant velocity. For each stroke, the pressure difference develops quickly across the test section, and for most of the stroke the flow is in a steady state. The general layout of the MPR and test section is shown in Fig. 1. The slit flow cell used was a contraction and expansion flow through a narrow slit, with a cross section as shown in Fig. 1(b); the depth of the cross section was 10 mm and so the channel upstream and downstream of the slit was square. Within the slit the aspect ratio was 11/1 where the flow can approximate to being two dimensional.

For comparison with the inherent relaxation times of the polymers (so determining the regime of non-Newtonian flow), the wall shear rates were estimated both by inverting the Pouseuille expression for the (measured) volume throughput, and by checking against the relevant numerical simulations. The use of flow birefringence is now a well-developed technique for the determination of stress distributions within flowing polymer systems [see for example Baaijens et al. (1997)]. The system used here followed that employed by Lee et al. (2001). Linearly polarized monochromatic light was passed through quarter-wave plates and received through an analyzer. The observed isochromate fringes then represented integer differences in principal refractive index, which in turn can be expressed as contours of principal stress differences using the stress optical law. In comparison to the cases in which large quantities of commercial material are employed, the quality of the birefringence photographs was not as high, although the geometry and number of fringes were generally clear. This was partly due to the presence of debris material from the synthesis and processing, and the inability to flush the apparatus through with quantities of melt.