Chemical Processes - Isoconversional Kinetics of Thermally Stimulated Processes - page 193

k
k

dp

∆∆ ∆ GHTS RT =−=− ln

(4.41)

p

becomes zero. Therefore, the ceiling temperature can be defined as the ratio:

c = ∆
∆

H
S

(4.42)

T .

The concept of the ceiling temperature has limited use in estimating the thermal
stability of polymers. It assumes that polymer degradation occurs via simple de-
polymerization; i.e., polymer is converted into its monomer. However, there are
a few polymers (e.g., poly(α-methylstyrene); poly(methyl methacrylate), PMMA;
polytetrafluoroethylene; polymethacrylonitrile) that degrade predominately to the
monomer when heated. In addition to depolymerization, there are two other ma-
jor mechanisms of degradation: side-group scission and random chain scission
[ 62 ]. Side-group scission involves splitting off pendant groups without breaking
the main chain. Examples of side-group scission include elimination of hydrogen
chloride from poly(vinyl chloride), acid from poly(vinyl ester), and alkene from
poly(alkyl acrylate). Random chain scission refers to breaking the polymer chain
in random places that yields a variety of low molecular weight products, including
the monomer. Thermal degradation of polyethylene (PE) and polypropylene (PP)
are examples of this mechanism. Most of the time, the thermal degradation occurs
via a combination of depolymerization and random chain scission. Obviously, if
a polymer degrades by a mechanism other than depolymerization, the aforemen-
tioned equilibrium (4.40) becomes impossible and the ceiling temperature loses its
meaning.
The kinetics of degradation by depolymerization can be described in the simplest
case by using the same approach as the one used earlier for polymerization. Depo-
lymerization is the process reverse to polymerization:

k

dp
⋅→ +⋅

(4.43)

MM MM .

Its rate is:

(4.44)

r = k

dp MM

[ ⋅ ].

dp

The process starts by initiation:

k

i
→ ⋅

(4.45)

MM MM ,

whose rate is:

(4.46)

rk
i

= [

i MM

].