Polymer Basics - Fundamentals of Industrial Chemistry: Pharmaceuticals, Polymers, and Business - page 103

upon their affinity for a polar column material such as silica. In reversed-phase
HPLC, non-polar packing material is used as the stationary phase. In contrast,
GPC separates materials based upon molecular size. The stationary phase
has pores. Smaller molecules can enter the pores and therefore take longer to
elute from the column. Larger molecules are too big to enter the pores; they
are excluded from the pores. Therefore, larger molecules flow faster through
the column. Because of this principle, GPC is sometimes called size exclu-
sion chromatography. The shorter the retention time, the larger the molecule.
Commonly more than one column is used with the different columns using
stationary phases of different pore sizes. By doing this, distinctions can be
made between polymer chains of various molecular weight. Standards of
known molecular weight can be analyzed by GPC and a calibration curve
developed that correlates retention time to molecular weight. Polystyrene
standards are readily available and are often used, even when analyzing other
types of polymers. The GPC can give M n ,M w and polydispersity data.
Sometimes other techniques are used to gain information about molecular
weight. Because molecular weight has such a profound influence on rheolog-
ical properties, sometimes the viscosity is analyzed and then inferences are
made about molecular weight. The tests can be very simple. For example, a
polymer can be placed in a heated cylinder and, after allowing time for melt-
ing, forced through a die by placing a weighted rod on top of the cylinder. The
weight pushes the rod which in turn extrudes the polymer melt through the
die. By weighing the amount of extrudate per minute, you can have a measure
of viscosity. The greater the weight of extrudate, the less viscous the material.
After multiplying by ten, you have the melt index which is the grams extru-
date per ten minutes. The melt index is also called the melt flow index and is
an easy quality control test.
Another technique which gives rheological information is Dynamic
Mechanical Analysis (DMA), also known as Dynamic Mechanical Thermal
Analysis (DMTA). Small deformations are applied to a polymer melt in a
cyclic manner. The polymer melt can be held between two parallel plates
which oscillate. This technique is much more sophisticated than a melt index
and gives much more information. The force on the melt can be varied and
the effect of force on flow (called shear sensitivity) can be measured. Some
polymers, especially those with high molecular weight or some branching,
have high shear sensitivity. To understand shear sensitivity, think about water
which flows easily and then toothpaste. Toothpaste doesn't flow unless a
force (shear) is applied. Then, when you squeeze the tube, toothpaste flows
readily. Toothpaste has high shear sensitivity.
For some polymers, portions of their chains can assemble with portions
of other polymer chains and form highly ordered crystalline regions. This is
the same as when a small molecule forms crystals. Like small molecules,