Rotating clamps

It is much easier to program a constant extension rate with the rotating clamp than with the translating clamp. Since we are always testing a constant length sample L from eq. 7.2.1, the velocity of 

The area still decreases exponentially, eq. 7.2.8 thus eq. 7.2.9 applies for the stress. 

Another problem with rotating clamp devices is slip at the clamp surface. Meissner (1969,1981,1985) reports that the sample end velocity was 8-10% less than QRc. 

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Figure 1.12 Rotating clamp device used by Meissner to impose a uniaxial extensional strain on a cylindrical filament of polymer of length Lq. Leaf springs in one of the sets of rotating clamps allow the extensional stress to be measured. (From Meissner 1971, reprinted with permission from Steinkopff Publishers.)...

Meissner and Hostettler [32] have described an improvement on an earlier double rotating clamp method in which metal conveyor belts were used at both ends. The test sample, a rectangular bar 60 x 7-8 x 2 mm, was supported on a cushion of inert gas. 

Moving clampSHiecking at bonded clamp rotating clamp-slip Sample inhomogeneity... 

Highest extensional strain e — 3-4 typical with careful sample preparation and 6-7 possible with rotating clamps Buoyancy bath required forik < 1 (eq. 7.2.13)... 

Rotating clamp with horizontal bath easiest to build and use must cut and measure length to get recovery... 

Schematic drawing of the double rotating clamp apparatus (adapted from Meissner, 1971). (a) Top view. The cylindrical sample P of initial length L is stretched by clamps Z and Zi rotating at f2 and S2 . Drive motor M is shown. Spring B and displacement sensor W measure force, (b) A detail of the rotating clamps. Gear teeth are used to prevent slip, (c) End view showing oil buoyancy bath with top surface O. Fourteen pairs of scissors T cut the sample into small lengths for recovery measurement La-...

Thus for lower rate testing some sort of buoyancy bath is required. Figure 7.2.5 indicates a horizontal buoyancy bath of the type most often used with rotating clamp instruments. Both horizontal (Vinogradov et al., 1970, Franck and Meissner, 1984) and vertical baths (Munstedt, 1975,1979) like the one shown in Figure... 

Nominal tensile stress versus strain for a low density polyethylene at 150 C rotating clamps, = 0.035 . The highest curve shows the effect of small temperature gradients (> 0.1°C710 cm) at high strain. iMth excellent control of temperature, data are reproducible to within the shaded region. From Meissner et al. (1981). 

Comparison of translating (o,—) and rotating clamp ( ) data for a low density polyethylene (lUPAC A) at 150°C. From Au Yueng and Macosko (1980). 

Figure 7.4.4 shows data from the rotating clamp device for the transient equibiaxial viscosity at three different extension rates. For comparison, the linear viscoelastic viscosity and the uniaxial viscosity are shown. Results for the biaxial viscosity compare well to those measured in lubricated compression on the same polyisobutylene sample as in Figure 7.4.4 (Chatraei et al., 1981). So far, only results with the rotating clamp method have been reported for this sample. Maximum strains were 2.5 in the biaxial and multiax-ial tests and k < 0.1 s. Friction on the talcum powder may limit the total strain and the detectable stress values. Much larger, more homogeneous samples are required than were used in the lubricated squeezing experiments. However, because the rotating clamps can...

Various arrangements of rotating clamps (/fC, ) for multiaxial elongation, (a) Equi-biaxial S = sample C,- = one of eight pairs of scissors transducers 7) (T-L of Figure 7.4.2) record the forces, (b) Planar clamps A-F rotate with constant speed, while G and H remain stationary, recording only force. L = 158 mm. If only G and H rotate and A-F are removed, we have uniaxial extension of a strip, (c) Multiaxial with m = 0.5 (eq. 7.1.4) ellipse axes a = 268 mm, b = 380 mm. From Meissner et al. (1982) and Demarmels and Meissner (1985). 

For high viscosity systems like polymer melts, where a solid sample can be prepared, grabbed, melted, and then tested, the first choice is tension of a cylindrical or rectangular sample. This is a homogeneous deformation that provides true data. The rotating clamp or simple windup is the easiest gripping method. High extension rates are still difficult with tension devices. 

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