Page 39 - Engineering Plastics Handbook

Chemistry of Polymerization 13

E = activation energy for segmental diffusion
p
T = crystallization temperature, °C (°F)
c
D = constant for a given polymer
T = melt temperature, °C (°F)
m
The crystallization rate at constant temperature is calculated with the
Avrami equation [10]

ln (1 −θ) = Kt n

where θ= degree of crystallization
K = Avrami constant
t = crystalline time interval
n = Avrami exponent related to crystallization process—
nucleation type and spherulite growth form (see table)

Relationship of Avrami Exponent n to
Crystalization Growth Form

n Nucleation type Growth form
1 Predetermined Fibrillar
2 Sporadic Fibrillar
3 Predetermined Discoid
4 Sporadic Discoid
5 Predetermined Spherulite
6 Sporadic Spherulite
7 Predetermined Sheathlike
8 Sporadic Sheathlike

Beyond polycondensation and chain-growth polymerizations, further
selections must be made: whether to use bulk (mass), solution, emulsion
or interfacial, suspension, graft and solid-state polymerizations; ring
opening, free radical, anionic or cationic.
For example, SAN is an emulsion or suspension polymerized from
styrene plus acrylonitrile, and SAN is graft-polymerized to polybutadi-
ene to form ABS. Antioxidants can be introduced during polymerization
as well as during compounding, to protect the double bonds in polybu-
tadiene and acrylonitrile. Block and graft in situ polycondensation poly-
merization can be used to produce ABS composites and other engineering
thermoplastic composites reinforced with poly-p–phenylene terephthal-
amide (PPTA) liquid crystal fibers at the molecular level [4].
The choices are not always optimal. Solution and interfacial poly-
condensation polymerizations are used when bulk polymerization is
too exothermic, as noted earlier. Polymerization combines processes
such as polycondensation, bulk, graft, and solid-state polymerization
or copolymerization. Three solid-state polymerization methods include