Measuring polymer melts

The measurement of rheological properties of polymers are most commonly done on dilute solutions or molten polymers, and are critical for determining molecular architecture (molecular weight, molecular weight distribution, and degree of branching), processing behavior and end product performance.

The most common device used for measuring rheological properties of melt polymers is a melt index, which is an empirical method that provides a single point measurement of viscosity. The melt index (MI) actually is the amount of material which flows through a capillary under a standard set of conditions (temperature, pressure and time). While it is most often used as a simple, quick way to grade the relative differences between polymers in a quality control environment, its limitations are that it is not very sensitive to differences in molecular architecture and provides little useful information to simulate how a material will behave in a process.

Polymers are complex rheological materials in that they exhibit both viscous and elastic (viscoelastic) properties under varying conditions of stress, strain and temperature. The best example of this is silly putty (or PDMS) – when left "at rest” in a lump, when time and natural forces of gravity are applied, it eventually flows like a fluid (albeit slowly) to form a puddle. Yet, when rolled into a ball and bounced, it behaves like an elastic solid. Lastly, when stretched rapidly, it snaps and breaks cleanly as if it was a solid piece of plastic. This demonstrates the importance of measuring the rheological properties of polymers under varying conditions of stress, strain and time if we want to understand how they will perform in a process or end use conditions.

Typically, stress and/or strain-controlled, rotational rheometers are used to measure properties such as:

• viscoelasticity (G’, G’’, tan delta) as a function of frequency (time) and temperature
  
• molecular architecture (molecular weight, molecular weight distribution,, branching) using frequency sweeps and creep/recovery tests (zero shear viscosity)
  
• influence of long chain branching on linear viscoelastic properties (zero shear viscosity, steady state recoverable compliance)

Also, capillary rheometers are used to measure:

• shear viscosity from low to high shear rates, directly simulating conditions experienced during processing
  
• melt fracture, die swell, and shark skin which are often due to elastic properties that manifest themselves at high shear rates during processing
  
• extensional viscosity and/or melt strength, which become important parameters to measure for many polymer processes

Available products:

	Bohlin Gemini II
	Bohlin CVO
	Rosand RH7/10-D
	Rosand RH2000