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238 Rheology of Polymeric Liquids

they can be melted by heating to recover the original liquid concerned here primarily with the rheological behavior of
state. Thermoset resins, in contrast, are thermally irre- thermoplastic polymeric liquids.
versible in that, once solidified, they cannot be melted. Thermoplastic polymeric materials may be classified in
Thermoplastic polymer molecules are large, with molec- the following way, on the basis of chemical constituents:
ular weights on the order of tens to hundreds of thousands. (1) bulk polymers (or polymer solutions), which are con-
Furthermore, many of the repeat units are linked in sidered to be homogeneous liquids; (2) polymer liquid
a one-dimensional direction. The size of thermoplastic crystals; (3) polymer blends consisting of two or more
polymer molecules does not change during the fabrication similar or dissimilar polymers, the latter of which gener-
of products. Thermoplastic polymers (e.g., polyethylene, ally form heterogeneous phases in which one component
polystyrene, poly(vinyl chloride), nylon, and polycarbon- is suspended in the other; (4) filled polymers in which
ate) are used for manufacturing films, fibers, furniture, particulates are suspended in the polymeric liquid; and
etc. Thermoset resins, however, have smaller molecules, (5) polymeric foams, in which gas bubbles are suspended
with molecular weights on the order of a few thousand. in the polymeric liquid.
They usually form three-dimensional networksin the pres-
ence of a catalyst (often referred to as curing agent), giv-
ing rise to strong structures. Therefore, parts made from II. UNUSUAL CHARACTERISTICS
thermoset resins are much stronger than those made from OF POLYMERIC LIQUID FLOW
thermoplastic polymers and are widely used for structural
purposes. Thermoset resins (e.g., unsaturated polyester, Before we quantitatively discuss the rheological behavior
epoxy, and urethane) are used for manufacturing boat of polymeric liquids, let us observe some of their unusual
hulls, parts for automobiles and airplanes, bath tubs, but- flow characteristics. Figure 1 demonstrates a dramatic
tons, etc. difference between two types of liquid—a low-mole-
Thus, in dealing with the flow properties of thermoplas- cular-weight polybutene and an aqueous solution of
tic polymeric liquids, one need not be concerned with a polyacrylamide—when a rotating rod is dipped into them.
change of molecular size, and in this sense, the measure- The polyacrylamide solution climbs up the rotating rod,
ment of the flow properties of thermoplastic polymeric while the low-molecular-weight polybutene does not.
liquids is much simpler than that of thermoset resins. In Note that low-molecular-weight polybutene has a molec-
dealing with the flow properties of thermoset resins, how- ular weight of hundreds or thousands, whereas the poly-
ever, one must understand that the molecules grow dur- acrylamide dissolved in water is believed to have a molec-
ing fabrication. Consequently, measurement of the flow ular weight of tens of thousands.
properties of thermoset resins is very difficult. An under- The direction in which the polyacrylamide solution
standing of the flow behavior of thermoset resins during climbs in Fig. 1 is perpendicular to the rotating direc-
fabrication requires an understanding of the nature of the tion of the solution, and therefore one can conjecture that
chemical reactions occurring during fabrication. Because there ought to be a force generated by the liquid that over-
of the complexity of the problems involved in dealing with comes the effect of centrifugal force and points in a direc-
the rheological behavior of thermoset resins, we shall be tion perpendicular to the direction of rotation. The liquid

FIGURE 1 Photograph describing the liquid climb-up effect: (1) low-molecular-weight polybutene; (b) 2 wt% aqueous
solution of polyacrylamide.