Constant-force rheometer
The present invention is generally directed to a method for measuring interfacial stick-slip transitions (SST) and an improved constant-force shear capable of measuring interfacial SST. Some embodiments are capable of measuring SST under simple shear conditions and/or in highly entangled polymer melts. Some embodiments include the application of a constant shearing force to a polymer sample.
1. A rheometric device for determining shear rate in an entangled polymer sample that is subject to a stick-slip transition, the device comprising:
a tube having an open end, a closed end, an inside wall and an inside back-wall, wherein the tube comprises two members having semi-circular cross-sections divided along the longitudinal axis of the tube, wherein the tube is capable of receiving a piston at the tube's open end, while maintaining a gap between the inside wall of the tube and the piston, wherein the gap is capable of containing a molten polymer sample;
a means for maintaining the polymer sample in a molten state;
the piston having a first end and a second end, wherein the first end enters the tube before the second end, and wherein the position and/or speed of the piston is monitored;
a means for driving the piston with a known constant force and speed into the tube while the tube contains a molten polymer sample;
a means for recording piston speed and/or position as a function of time; and
a means for recording the force applied to the piston.
2. The device of claim 1 , wherein the gap is from 0.1 to 0.4 mm thick.
3. The device of claim 1 , wherein the means for maintaining the polymer in a molten state comprises a resistive heating element.
4. The device of claim 1 , wherein the means for monitoring the speed and/or position of the piston comprises a linear variable differential transformer.
5. The device of claim 1 , wherein the means for recording piston speed and/or position as a function of time comprises a computer, microprocessor and/or computer memory.
6. The device of claim 1 , wherein the means for recording the force applied to the piston comprises a computer, microprocessor and/or computer memory.
7. The device of claim 1 , wherein the piston comprises a material selected from one or more of iron, nickel, and aluminum.
8. The device of claim 1 , wherein the tube comprises a material selected from one or more of iron, nickel, and aluminum.
9. A process for making rheometric measurements of entangled polymer samples, comprising:
loading a polymer sample into the apparatus of claim 1 ;
maintaining the polymer sample in a molten state;
driving the piston into the tube at a known speed and with a known constant applied force, so that the polymer sample is forced to flow into the gap between the piston and the tube;
calculating the shear rate; and
recording the shear rate.
10. The process of claim 9 , further comprising the step of correcting for a stick slip transition when the applied stress is greater than the predetermined value of the critical stress of the sample, wherein the correction comprises subtracting the effect of the transition from the recorded shear data.