Q. I've seen a device similar to the SER being marketed by another instrument manufacturer. What's the difference between the SER and this other fixture?
A. This answer requires a brief background... Development of the SER technology first began in 1997 with the pioneering work of Dr. Martin Sentmanat at the Goodyear Tire & Rubber Company in Akron, OH. This revolutionary dual wind-up drum rheometer technology was patented by Goodyear and later licensed exclusively to Xpansion Instruments, LLC. Since its introduction back in 2003, the SER has generated a tremendous amount of interest not only in the global rheological community but in the physical material characterization community as well. In 2004, another instrument manufacturer looking to capitalize on the breakthrough success of the SER launched an imitation technology similar in appearance to the original model SER-HV and began marketing the device as an extensional viscosity fixture for an ARES rotational rheometer. Although this imitation technology has the identical drum dimensions, securing means, and sample geometry of the original model SER-HV, it has serious shortcomings and is far less capable. The SER's significant performance advantages are summarized on a dedicated page (click here to learn more).

Q. How do you control the strain rate and is it possible to measure it during the test?
A. For many homogeneous and soft materials, the applied strain rate is EQUAL to the true strain rate. This has been verified for Hencky strain rates up to 20+ s^-1 using stop frame videography (laser micrometers have also been used in the past). Unlike the stretching means incorporated by other melt extensional rheometers, SER sample deformation remains in a fixed plane and is clearly visible AT ALL TIMES, and with each incremental drum rotation the sample becomes less and less likely to slip during stretching. For very high modulus materials (solids and cured elastomers), the use of a thin double-sided adhesive (such as carpet tape) or sandpaper strips have been found to be very effective in preventing slip during a stretching experiment, and thus you can get the true strain rate to EQUAL the applied strain rate even for high modulus solids.

Q. Is the SER sensitive/reproducible enough to distinguish between good and bad polymer samples for quality control purposes?
A. As described on our Principle of Operation page, uniaxial extensional is the "strongest" flow field one can generate. Because of the high level of reproducibility and the degree of molecular orientation and chain stretch that can be achieved with the SER, extensional rheology measurements with the SER are ideally suited for distinguishing subtle differences in molecular architecture, examples of which are provided on our Extensional Rheology mode of operation page.

Q. How do you compensate for “sag” and gravity effects in the case of low viscosity samples?
A. The minimum recommended sample viscosity for extensional characterization with the SER is a zero-shear viscosity of 10,000 Pa-s when operated under a gas environment. For molten polymers with such a viscosity, the operator has to work rather quickly (approx. 60-90 seconds after sample loading in the pre-heated oven) to prevent any large effects due to sag. Unlike other extensional melt rheometers, the SER has a small distance between points of contact (1.27 cm) and holds the sample on its side instead of flat such that the width dimension of the sample actually deters sagging to a large extent due to the increased moment of inertia with regard to sample/beam flexure. As witnessed by the excellent superposition between the low strain portions of the tensile stress growth curves and the LVE shear stress growth curves presented in the Rheology Results section, sag is typically not an issue for materials with a zero-shear viscosity above this recommended value of 10,000 Pa-s.

Because both of the detachable drums are cantilevered and suspended from the SER2 base chassis, the SER2 models that are configured for use on controlled stress/strain rotational rheometers such as the SER2-P, SER2-G, SER2-M, and SER2-T are capable of fluid immersion testing. The drums of said SER2 models can be raised from and lowered into a controlled temperature fluid environment contained within a jacketed beaker or other such fluid containment vessel. Applications include biomaterials testing as well as high-temperature silicon oil bath testing to eliminate any effects associated with molten sample sag at low viscosities.

Q. Does the air flow in a forced convection oven disturb the measurements at low rates?
A. Hencky strain rates as low as 0.00001 s-1 have been run with no problems due to air flow in most forced convection environmental chambers.

Q. What size of sample is needed?
A. The following is the recommended range of sample sizes for use on the SER:

Minimum Length = 15 mm
Width = 1 - 12.7 mm
Thickness = 0.005 - 1 mm
Mass = 5 - 200 mg

Q. What is the maximum reachable Hencky strain with the SER?
A. One complete drum rotation corresponds to a Hencky strain of 5, however, beyond a Hencky strain of about 4 the sample begins to overwrap the securing clamps and the previously wound sample. With proper care, the SER can be operated well beyond a single drum rotation with little to no problem for molten polymers with large extensability. If the polymer does survive to such strains, sample overwrapping typically does not affect the torque signal by an appreciable degree and is usually observed as a brief perturbation in the torque signal as the sample overwraps the securing clamps. Usually by this point either the polymer has reached its plateau viscosity or the sample has already initiated its failure mode. At high rates (>1 s^-1), most melts will rupture prior to sample overwrap.