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Encyclopedia of Physical Science and Technology EN012c-593 July 26, 2001 15:56

612 Polymer Processing

Rheology Science of the deformation and flow of rials typically have molar masses in the range of 20,000
materials. to 150,000 g/g-mol. When the molar masses are greater
Rheometry The equipment and process by which the rhe- than some critical value, M c , the large molecules physi-
ological properties of polymeric fluids are determined. cally entangle with each other as shown schematically in
Thermoforming A process by which a sheet of polymer Fig. 1. This entanglement network leads to not only highly
is heated, usually by radiation heating, to the point at viscous fluids but fluids with elastic properties. Under fi-
which it is pliable and then pushed or pulled into a cav- nite deformations, macromolecules stretch, disentangle,
ity by applied mechanical force or pressure or applied and orient (Fig. 1). On cessation of flow, thermal motion
vacuum. Used frequently for producing materials for causes the molecules to recover their initial conforma-
packaging applications, but large parts for recreational tion and re-entangle. As a result of this recovery process,
use can also be produced. polymeric fluids partially resemble purely elastic materi-
Viscoelasticity The response of a fluid or solid which is als such as cross-linked rubber and are therefore referred
a combination of viscous and elastic behavior as deter- to as being viscoelastic.
mined by the rate of deformation relative to the relax- The processing behavior of polymeric melts is highly
ation time of the material. dependent on several molecular features. The size of the
molecule or its molar mass is one of the key factors. Poly-
mers rarely consist of a single molar mass; they are instead
IN THE CONTEXT of this article, polymer processing a distribution of molar masses. Because of the distribution
refers to the operations by which polymer resin is con- of chain sizes it is common to use moments of the distri-
vertedtofinishedplasticpartsandobjects.Ofparticularin- bution to specify the molar mass of a resin. For example,
terest are those resins referred to as thermoplastics which the number average molar mass, M n , is the total mass of
can be softened by the application of heat, processed, so- polymer divided by the number of polymeric chains (i.e.,
lidified, and reheated and processed again. This is opposed the average molecular weight). The weight average molar
to thermosetting resins which, once solidified via the pro- mass, M w , is the number of chains of given weight times
cess of cross-linking, cannot be softened for reprocessing. the weight of material with that given molar mass divided
Virgin resin is rarely processed; instead, a wide variety of by the total weight of polymer. It is a higher moment of
additives are compounded into the resin to improve pro- the distribution. The ratio of M w /M n is referred to as the
cessing performance and properties. These resins plus ap- polydispersity of the polymer. For polymers synthesized
propriate additives are then heated and shaped by flow and by means of a step polymerization process (e.g., PET and
deformation using a number of processing operations (ex- nylon), this ratio is around 2.0 (the most probable distri-
trusion, injection molding, film blowing, fiber spinning, bution), whereas for polymers produced by means of ad-
blow molding, thermoforming, compression molding, dition polymerization (polyethylenes), the ratio can vary
etc.). from 4.0 to 30.0. With the advent of metallocene catalysts,
polyethylenes with polydispersity indices in the range of
2.0 are possible.
I. INTRODUCTION Although techniques such as gel permeation chro-
matography or size exclusion chromatography are typ-
The conversion of polymers to finished parts and arti- ically used to provide details about the molar mass
cles differs significantly from that used to process low distribution, several other techniques are used by man-
molar mass (sometimes molecular weight is used) fluids ufactures to specify the molar mass of their polymers. For
because of their unique molecular features that lead to polymers such as those produced via means of step poly-
high viscosity and viscoelastic behavior. Polymeric mate- merization the inherent viscosity (IV) is commonly used.

FIGURE 1 Entangled macromolecules in the undeformed and deformed states.