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5.2. POLYSACCHARIDE DEGRADATION: ACTIVATION
ENERGY AND ROLE OF pH
5.2.1.
Introduction
In the previous section (part I) the basic chemistry (mechanisms) were
described along with some of the kinetics of random depolymerization (how to
describe the changes in M
w
and M
n
and how to determine the rate constant
(k)).
In this part some practical aspects will be covered. In everyday work in the
laboratory we sometimes ask ourselves whether the degradation can be
speeded up by changing the conditions, for example elevating the
temperature, or by adding more reagents. How much faster will the reaction
go if the temperature is increased 10
°
C? What happens if we perform the
degradation in 0.1 M acid instead of 0.01 M? What about the case of a
polymer mixture being degraded simultaneously, but where the two polymers
are degraded at different rates? These are important questions in industrial
processing of polymers to obtain the desired product at minimum cost.
5.2.2. Role of
temperature: Activation energies and Arrhenius
plots
Simple reactions such as acid hydrolysis or alkaline
β
-elimination of glycosidic
linkages of a dissolved polysaccharide follow simple rules. These matters are
often covered in the first year chemistry course (did you keep you textbook?).
In general, the rate of a chemical reaction (k
40
) depends on the temperature
(T) according to the famous Arrhenius equation:
k
=
Ae
−
E
A
/
RT
A is a pre-exponential factor which is specific for each system. It is considered
being independent of the temperature. E
A
is termed the activation energy.
Again, it is for practical purposes also considered to be independent of the
temperature. R is the gas constant (1.986 cal K
-1
mol
-1
= 8.314 J K
-1
mol
-1
)
and T is the
absolute temperature
(in Kelvin).
If the rate constant has been determined for a range of temperatures:
40
Note rate constants are written as k, not capital K (which usually means equilibrium
constant)