Page 173 - Carrahers_Polymer_Chemistry,_Eighth_Edition

136 Carraher’s Polymer Chemistry

under considerably reduced temperatures, compared to the melt process, thermally induced side
reactions are minimized. Side reactions with the solvent may be a problem. Because the reactants
must be energetic, many condensations are not suitable for the solution technique.
The interfacial technique (IF), while old, gained popularity with the work of Morgan and
Carraher in the 1960s and 1970s. Many of the reactions can be carried out under essentially non-
equilibrium conditions. The technique is heterophasic, with two fast-reacting reactants dissolved in
a pair of immiscible liquids, one of which is usually water. The aqueous phase typically contains
the Lewis base such as diol, diamine, or dithiol. The organic phase contains the Lewis acid, gen-
erally an acid halide, dissolved in a suitable organic solvent such as hexane. Reaction occurs near
the interface, hence the name. With all the potential that the interfacial system offers, it has not
attracted wide industrial use because of the high cost of the necessarily reactive monomers and cost
of solvent removal. One commercial use for the IF system is the production of PC. Another involves
the synthesis of aramids. Morgan and others noted that some polymers formed with rapid stirring
would remain in solution for awhile before they precipitated. The problem with aramids was the
need to form fibers from their solutions. Thus, the aromatic nylons had to be redissolved after for-

mation. Today, aramids are synthesized using rapidly stirred systems where the polymer solution
is sent through a small hole into a nonsolvent. This allows fibers to be produced without needing to

redissolve the polymer.
Table 4.12 contains a comparison of these three major polycondensation processes.

4.18 LIQUID CRYSTALS

Everyday of our lives we “run across” LCs. They are commonly found in computer monitors, digital
clocks, TV screens, and other “read-out” devices, and so on.

Reintzer, in 1888, first reported “liquid crystal” behavior. In working with chloesteryl esters, he
observed that the esters formed opaque liquids, which on heating turned clear. We now know, that
as a general rule, that many materials are clear if they are anisotropic, random or if the materials
are composed of ordered molecules or segments of molecules, whereas they are opaque if there
exists a mixture of ordered and disordered regions. Lehmann interpreted this behavior as evidence
of a “third” phase that exists “between” the solid and isotropic liquid states. This new phase was
named by Lehmann as the liquid crystal phase. Friedel called this phase the mesophase after the

TABLE 4.12
Comparison of Requirements for Different Polycondensation Techniques

Requirement Melt Solution Interfacial
Temperature High Limited only by the MP and BP of the solvent used generally
about room temperature
Stability to heat Necessary Unnecessary Unnecessary
Kinetics Equilibrium, stepwise Equilibrium, stepwise Generally nonequilibrium,
chain-wise
Reaction time 1hour to several days Several minutes to 1 hour Several seconds to 1 hour
Yield Necessarily high Less necessary high Low to high
Stoichiometric Necessarily high Less necessary high Less necessary
equivalence
Purity of reactants Necessary Less necessary Less necessary
Equipment Specialized, often sealed Simple, open Simple to complex, can be
open
Pressure High, low Atmospheric Atmospheric

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