Biomedical Engineering Reference
In-Depth Information
as the weight loss due to volatilization of low molar mass fragments after pro-
longed thermal and photodegradation.
Another powerful tool for the qualitative and quantitative evaluation of the oxida-
tion processes is the CO
i
as determined by FTIR spectroscopy. It has been reported
repeatedly that most of the degradation intermediates from PE peroxidation
contain carbonyl groups so that their concentration, as determined by CO
i
meas-
urements, can be used to monitor the progress of degradation [22]. These deter-
minations are usually carried out on test fi lms by recording the ratio of the optical
density of the carbonyl absorption bands in the range 1780-1700 cm
− 1
to the optical
density of the band at 1463cm
− 1
(CH
2
in - plane vibration - scissoring peak). In
addition, FTIR analyses provide information on the presence and formation over
time of oxidation products with absorption maxima at 1712 cm
− 1
(carboxylic acids),
1723 cm
− 1
(ketones), 1730 cm
− 1
(aldehydes), and 1780 cm
− 1
(lactones) [35] .
16.4.1
Biodegradation of Polyolefi n Oxidation Products
The determination of the wettability of fi lm surfaces by contact angle measure-
ments may provide useful information about the increasing polarity of fi lm sur-
faces as a consequence of oxidation and the formation of functional groups. This
information is also useful in order to predict the propensity for microbial attack
on PE fi lms, by recognizing that one of the reasons suggested to explain the
intrinsic recalcitrance of PE to biodegradation is the hydrophobic character that
hinders the adhesion and interaction of microbial cells. In terms of potential
ultimate biodegradation (i.e., conversion to CO
2
and H
2
O, mineralization), the
assessment of molecular weight changes is of fundamental importance as well.
Indeed, it has been suggested from a theoretical point of view that since PE is a
nominally straight-chain hydrocarbon, it should be metabolized according to the
biochemical pathway for linear alkanes. On the other hand, it has been established
that there is a molecular weight upper limit for the utilization of
n
- alkanes as a
carbon source by microorganisms. Haines and Alexander established that linear
hydrocarbons with more than 44 carbon atoms (tetratetracontane) cannot be
metabolized by soil microorganisms [38]. Recently, this dimensional limit has
been extended to 0.72kDa corresponding to 60 carbon atoms in a study using
single bacterial strains [39]. In any case, these limits are thought to be related to
the bacterial metabolism of
n
-alkanes that need the accessibility to methyl chain
ends by extracellular oxidizing enzymes to start the biodegradation process. Thus,
the rate and eventually the ultimate extent of biodegradation of solid
n
- alkanes is
strongly affected by the availability of -CH
3
chain ends susceptible to enzymatic
oxidation. It follows that the chain ends present at the surface of solid
n
- alkane
decrease with an increase in molecular weight with extremely low values in the
case of high - molecular - weight PE.
Finally, other information about the relationship between the levels of oxidation
reached during the abiotic stage of degradation of “ degradable ” PE - the molecular
weight reduction as well as the potential to be biodegraded in the environment - can
