These bond disruptions can sever the molecular ties of pendant functional groups or the carbon-to-carbon backbone of the polymer chain The unbalanced electron states can be satisfied by any one of three modes: recombination (bonding at the original bond "break" site), crosslinking (bonding with alternate bond sites on the same or other polymer chains), or chain scission
(bonding of main chain breaks with a free radical.
For example, HDPE can suffer from either crosslinking or polymer chain scission
during reprocessing, both altering the molecular-weight distribution.
A possible mechanism of crosslinking and chain scission
of the EOC is presented in Fig.
The main chain scission
rate has been reported (ref.
At lower viscosity, chain scission
is less and equilibrium between it and crosslinking is achieved.
If used in a high enough concentration, they can be used to stabilize thermal and oxidative chain scission
while still offering the processing effects of fatty acids.
Colin and Verdu mentioned that a processing machine can be considered as a chemical reactor displaying nonuniform temperature and shear fields which can induce mechanochemical chain scission
of polymers, molecular weight, and rheological property changes .
This minimizes the potential for thermal degradation reactions such as chain scission
or crosslinking which often occur at residual double bonds.
They have considered that such increase can be used as an indicator of PE chain scission
. They showed that chain scissions
reduce the density of the entanglements in the amorphous phase of PE, which allows shorter molecules to crystallize because of their higher mobility.
In polyols, the terminal unsaturation should be an active participant in the propagation of the UV-initiated free radical reactions which lead to crosslinking and chain scission
. Evidence for increased crosslinking after UV exposure will be discussed.
Reprocessing recycled polyolefins is always accompanied with degradation, crystallization, and consequent processability problems, which result from molecular chain scission
, branching, and crosslinking.
It is this cycloperoxy intermediate which leads to chain scission
and softening of the polyisoprene on oxidation (refs.