Thermal Degradation

Report
Degradation of polymers
Polymers can degrade by
exposure to
high temperature
Shear action
Oxygen, ozone and chemicals
Electromagnetic (g, UV)
Ultrasonic radiation
Moisture
Thermal Degradation
Mechanical Degradation
Chemical Degradation
Light induced Degradation
Hydrolysis
Often, multiple exposures, such as a combination of
moisture heat or oxygen and light can result in accelerated
deterioration.
Shear
Ozone, oxygen
Heat
POLYMER
UV
moisture
Figure. Electron micrograph of the surface of a HDPE beer crate after nine years of use
and exposure to weathering.
Deterioration of plastics to normal environmental conditions is called WHEATHERING.
Thermal Degradation
Depending upon the presence of oxygen, temperature and structure of polymer,
degradation and/or oxidation reactions will occur.
Theoretical point of view most commercial polymer systems should be relatively
stable above their melting point in the absence of oxygen.
It is interesting to note that saturated hydrocarbons are much more stable then
polyethylene (PE) in the absence of oxygen as are chloroalkanes when compared with PVC.
In some cases this temperature difference may be as high as 200oC. There are mainly two
reasons for this difference in behavior;
- The first of which is simply that polymers by virtue of long chain nature are able to
breakdown into smaller molecular fragments i.e. monomer formation via unzipping
reactions
• The second is that commercial polymer structures are more
complex than their generic molecular formula indicates.
• They may contain various structural irregularities, branches,
unsaturated structures, carbonyl and hydroperoxide groups which
will act as initiation sites for degradation to occur. For example for
PVC,
Impurities; Generally, transition Metals (from catalyst residue or other sources) The sequence
of efficiency of metal ions to enhance degradation depends on its valence state and the type of
its ligand, but may be postulated as follows: Cu > Mn > Fe > Cr > Co > Ni
The average dissociation energy of bonds forming the structure of a macromolecule appears
thus to be a first criterion for estimating the thermal stability of a given polymer. The fraction
of bonds that reaches the energy equal to dissociation energy D is determined by the
Boltzman’s factor Exp(-D/RT) where T stands for absolute temperature and R for universal gas
constant. This may be exemplified as follows: the temperature at which in one mole of C–C
bonds at least one is dissociated into radicals is 486oC, while in one mole of O–O bonds it is
only 30oC.
Weak points/ links
It is possible to emphasize chain scission by working at low
temperature at which the evolution of volatile products is very slow.
If chain scission occurs in polymer molecule in the absence of
volatilization, then
Pt= Po(s+1)
(1)
in which Po and Pt are the chain lengths of original polymer and after
time at which s scissions have occurred on average per molecule.
Thus
S=(Po/Pt)-1
(2)
and the fraction of bonds broken, a, is given by equation (3)
a= s/Po = 1/Pt-1/Po
(3)
If chain scission is random, that is, every interunit bond in every
molecule is equally liable to break then
a= kt
In which k is the rate constant for chain scission. Thus for purely
random scission a plot of a against t should be linear and pass
through the origin.
On the other hands, if molecules contain some weak links in the
molecules which break more rapidly at the beginning of reaction
then
a= b+kt
(4)
In which b is the fraction of weak links in the molecules
b
Figure 2.6. shows that PS obey equation 4 and does indeed
incorporate weak links.
The differences can be accounted for in
terms of known mechanisms of degradation,
the lower temperature peak in radical
polymer being the result of degradation
through the unsaturated chain ends which
are absent in the ionic polymer.
Termination may occur by interaction of pair radicals in polymerization. Thus proportion
of molecules have unsaturated chain terminal structures. The bond indicated is
weakened by about 80 KJ which is resonance stabilization energy of the ally radical
which would be formed by its scission. Initiation by this pathway then allows a limited
amount of degradation at lower temperatures in radical-initiated polymers.
Depolymerization vs transfer reactions
Thermal analysis shows that polystyrene degrades thermally in single step that monomeric
styrene (approx. 40%) is the volatile product. A large cold ring fraction consists of decreasing
amount dimer, trimer ,tetramer and pentamer (oligomers).
Table 2.1. clearly show that it is a-hydrogen atom
which is involved in transfer process
a- methylstyrene has no a-hydrogen, therefore, depolimerize into its monomer completely.
Depolymerization vs ester decomposition
Ester decompositon only becomes important when the monomer unit incorporates at
least five hydrogen atoms on the b- carbon and depolymerization is quantitative when
there are at most one or two b-hydrogen atoms.
If a significant proportion of ester groups destroyed during early stage of heating then the
residual methacrylic acid units (or methacrylic anhydride units formed by elimination of
water) block unzipping process and thus inhibit formation of monomer.
If the radical depolymerization reaction can be initiated at a lower temperature than ester
decomposition even in poly(tert-butyl methacrylate) is replaced by quantitative production
of monomer.
Poly(vinyl acetate) - non radical processes
Ester decomposition also occurs in poly(vinylesters) but in this case carboxylic acids is
liberated and olefinic double bonds appear in the polymer chain backbone. The bhydrogen atom is effectively interacting as a proton with oxygen atom, so that the
reaction should be facilitated electron attracting group in vicinity.
.
The electron attracting properties of the carbon-carbon double bond causes the
reaction in PVAc to pass from unit to unit along the chain by reaction the ultimate
effect being to produce extended conjugation and colour.
Poly(ethylene terephthalate)
General Degradation Mechanism
Chain scission can occur by one of three mechanism. These include
1-Random degradation where the chain broken at random sites.
Random initiation
2- Depolymerization where monomer units are released at an active chain end
Terminal initiation
Depropagation
Transfer
Termination by disproportionation or
combination
Characterization techniques for polymer degradation & stabilization
Understand the thermal degradation mechanism
Frequently used techniques
1- Thermogravimetry (TG)
Thermogravimetry (TG) is a thermal analysis method in which the mass change of a sample
subjected to a controlled temperature programme is measured. The use of isothermal and
dynamic TG for the determination of kinetic parameters in polymeric materials has raised
broad interest during recent years Although TG cannot be used to elucidate a clear
mechanism of thermal degradation, dynamic TG has frequently been used to study the
overall thermal degradation kinetics of polymers because it gives reliable information on the
activation energy, the exponential factor and the overall reaction order.
To establish a criterion
for evaluating resin decomposition, the
temperatures at which 10% decomposition
[10% decomposition temperature (DT)] and
50% decomposition (50% DT) had
occurred were noted. Temperatures were also
recorded at which maximum rates of
decomposition occurred. From Table 1.1, it can
be seen that, based upon resin types 1, 2, 3, 4
and 7, the thermal stability of the resins
decreases with increasing molecular weight of
the meta-substituted phenol, i.e., stability
decreases in the order phenol > m-cresol > misopropylphenol > cardanol > m-tertbutylphenol. The anomalous position of the mtert-butylphenol indicates that branching of the
side chain has a significant effect, particularly if
branching occurs from the a-alkyl carbon atom
which is attached to the phenolic nucleus.
Evolved Gas Analysis (EGA)
Thermal Volatilisation Analysis (TVA)
In EGA, the sample is heated at a controlled rate under controlled conditions and
the weight changes monitored (i.e., TGA). Reaction products are simultaneously
led into a suitable instrument for identification and, in some cases, quantification.
Many variants of this approach have been developed based on three methods for
thermally breaking down samples: pyrolysis, linear-programmed thermal degradation
(i.e., without recording weight change), and the thermogravimetric approach (i.e.,
continuously recording of sample weight).
Using TVA experiment as a capstone, all
products of degradation can be isolated for
analysis by ancillary methods. At the end of
the experiment, three main product fractions
can be further examined: the volatile products
condensable in liquid nitrogen; the tar-wax
fraction that collected on the water-cooled
surface beyond the hot zone (referred to as
the cold ring fraction, CRF), and the nonvolatile residue remaining in the sample boat.
PIPA= Polyisocyanate polyols
Pyrolysis- GC-MS
EGA-MS
Thermogravimetry–Mass Spectroscopy (TG-MS)
TG-MS features are high sensitivity and high resolution, which allow
extremely low concentrations of evolved gases to be identified, together with overlapping
weight losses that can be interpreted qualitatively
This technique thus provides information about the qualitative aspects of the evolved
gases during polymer degradation that is otherwise unavailable for TG-only experiments.
This technique is therefore used for the structural characterisation of homopolymers,
copolymers, polymeric blends and composites and also fi nds application in the detection
of monomeric residuals, solvents, additives and toxic degradation products
DSC
DTA (differential thermal analysis)
and DSC (differential scanning
calorimetry)
Measurement of oxidation induction
times to study a stabilizer’s effectiveness
and its diffusion within the solid
The oxidative-induction time/oxidation induction time (OIT)
The oxidative-induction time/oxidation induction time (OIT) test is described in
standard test methods ISO 11357-6 [2] and ASTM D3895 [3]. OIT is expressed as
the time to onset of oxidation in a polymer test sample exposed to oxygen.
CL= chemiluminesans
Melt flow Index (MFI)
Thermogravimetry–Fourier Transform Infrared
Spectroscopy (TG-FTIR)
The combination of TG and FTIR provides a very useful tool for the determination of the
degradation pathways of a polymer, copolymer or the combination of one of these with an
Additive.
TG-FTIR makes it possible to assign the volatile components under investigation to the
decomposition stages detected by TG during an experiment.
Afterwards, a spectral range characteristic for a particular functional group can be selected and
the infrared (IR) absorption bands
in this range integrated and displayed as a function of time.
FT-IR
One of the most informative and
sensitive
techniques to observe functional
groups associated with oxygen is
infrared (IR)
spectroscopy, and many
researchers have used mid-infrared
spectroscopy to
study and investigate degradation
reactions and processes in
polymers.
For example, in low-density
branched polyethylene
photooxidation
tends to lead to an increase in the
level of the bands characteristic of
the
vinyl (–CH=CH2) end group, which is
characterised by a pair of bands
occurring at 990 and 910 cm1,
whereas thermal-oxidation tends to
lead to a reduction in relative
intensity of the band attributed to
vinylidene (>C=CH2)
Following a second compression moulding it was found that the hydroperoxide content,
determined by an iodometric test, decreased by rapidly transforming to additional carbonyl
groups (Figure 15).
Esters
1740 cm -1
Aldehydes
1730 cm -1
Ketones
1720 cm -1
Acids
1705 cm -1
Peracids
1785 cm -1
Peresters
1763 cm -1
The absorption bands due to
hydroxy species are observed in
the region 3600- 3200 cm -1
Carbonyl Index
Carbonyl Index= Abs at 1710 cm -1 /Abs at 2820 cm -1
Oxygen uptake
The technique of oxygen uptake is an absolute quantitative technique; it
affords a direct measure of oxygen consumption during polymer degradation
Thermo-oxidative Degradation of Polymers
Eq.1. Initiation
Eq.2. Chain Branching)
EQ.8 and 9. Termination
During the processing operation considerable shear and heat are applied to the
viscous polymer melt which causes some of polymer chains undergo homolytic scission at
the carbon-carbon bonds with the formation of macro-alkyl radicals. The macro-alkyl radicals
so produced are highly active species reacts with oxygen rapidly.( The rate constant of
reaction with alkyl radicals is extremely high (k= 108 dm3.mol-1.s-1))
In the presence of oxygen the temperature of decomposition of most polymers
decreases considerably and shift from 300- 600 oC for inert atmosphere to 100- 200 oC. This
is due to macroradicals with oxygen to form hydroperoxides which themselves are unstable
and breakdown rapidly forming more radicals, hence whole process becomes auotocatalytic.
Free radicals P. generated during the initiation process (reaction 1) are, in the
presence of oxygen, converted to peroxyl radicals PO2. (reaction 2), and subsequently to
hydroperoxides (reaction 3); intermediate hydroperoxides provoke further chain reaction
unless stabilizers (InH or D) are used to interrupt it (reactions 12 and 13). Respective reaction
of the scheme is completed by the method that monitors it.
Thermal decomposition of dialkyl peroxides, diacyl peroxides, hydroperoxides
and peracids depending on the structure of the peroxidic compound occurs in a measurable
rate usually above 60 oC. Diacyl peroxides and peracids are considerably less stable than
dialkyl peroxides and hydroperoxides.
Chain Branching
Polymer oxy radicals (PO.) undergo a number of other reactions including
(i) b- scission reactions which results in fragmentation of polymer chain together
with formation of end carbonyl (ketone or aldehyde) groups and radicals
(ii) Formation of in-chain ketone groups
(iii) Induced hydroperoxide decomposition
Metal catalyzed hydroperoxide decomposition
Some traces of metal and metal ions may initiate the decomposition of
hydroperoxides even at room temperature. Traces of metal ions are present in almost all
polymers and they may affect considerably the polymer oxidation and its subsequent
degradation. The sequence of efficiency of metal ions to enhance degradation depends on its
valence state and the type of its ligand, but may be postulated as follows:
Cu> Mn> Fe> Cr> Co> Ni
However, the mechanism for any particular ion may be more complex involving,
for example, the reaction of a lower oxidation state of metal ion with peroxyl radicals, etc.
Ions of Al, Ti, Zn and V usually reduce the rate of oxidation.
Structure of Polymers
The structure of polymers is again an important factor in controlling its relative
stability. The presence of a labile hydrogen atom is particularly important in this regard and
ease of oxidation to form peroxides. Thus in structure below the rate of oxidation of
polymers decreases from polyethylene to polyisoprene. The electron delocalizing effect of
attached group is primary important here which controls the stability of subsequent carbon
centered radical after hydrogen abstraction. In the solid state PS anomalous since it is more
stable than predicted here due to the shielding effect of bulky phenyl group.
Hydrogen abstraction
Any type of free radical may participate in the hydrogen abstraction from a
polymer macromolecule. Depending on the polymer chain structure,, the hydrogen
atoms can be abstracted in order of primary < secondary < tertiary C-H sites and this
process is independent of the nature of the attacking radical.
Hydrogen abstraction occurs principally the tertiary carbon atoms
It may also occur from the secondary carbon atoms in methylene groups.
Degradation of polyolefins during processing
Processing of polyolefins requires high temperatures (150- 300 oC) depending on
the type of polymer. At these temperatures thermal oxidative and mechanochemical
degradation occur. The main processes observed during the thermal oxidation of polymers
are the formation of hydroperoxy groups and carbonyl groups .
Figures 3.13 and 3.15 show that the maximum rate of initial carbonyl formation in
polyethylene (LDPE).
Figures 3.14 and 3.16 show that the hydroperoxide (as primary oxidation product)
concentration rises to a maximum and then decays with heating time both in melt and the
solid phase and that the maximum concentration achieved increases with decreasing
temperature. In the absence of oxygen, hydroperoxide concentration decayed to zero in less
than 20 h at 110 oC.
Polyethylene undergoes crosslinking reactions, leading to an decrease in MFI
where polypropylene under similar conditions undergoes chain scission, leading to an
increase in MFI. In both cases, the initial reaction occuring is chain scission due to shearing
forces acting on the polymer when its viscosity is high.
Photodegradation of Polymers
Photodegradation (chain scission and/or crosslinking) occurs by activation of
the polymer provided by absorption of a photon of light by the polymer.
In the case of photoinitiated degradation light is absorbed by photoinitiators (or
chromophore groups) which are photocleaved into free radicals, which further initiate
degradation (in non-photochemical processes) of the polymer.
In photo-thermal degradation both photodegradation and thermal degradation
processes occur simultaneously and one of these can accelerate another.
Photoageing is usually initiated by solar UV radiation, air, and pollutants, whereas
water, organic solvents, temperature and mechanical stress enhance these processes.
Molecular Orbital Theory and Electronic Transitions
Hints:
*The energy of the electrons is determined by their particular orbital. Each
element has its own particular orbitals so that the energy values of its electrons are
characteristic of it and different from those electrons of other elements.
* Normally electrons in an atom occupy those orbitals which are essentially
nearest to the nucleus to form most stable arrangement.
* If the most loosely bound electron is moved to an orbital which is farther
away from the nucleus then energy must be supplied i.e. the electron must absorb
energy which corresponds exactly to the difference in energy of starting orbital from
that of final orbital, therefore, electronic transitions will involve definite amount of
energy.
Covalent bonds in organic molecules are formed by the overlap of atomic
orbitals to form molecular orbitals in which, the electrons are, on average, closer to
the atomic nuclei than they were in the atomic orbitals and therefore have lower
energy.
Atomic orbitals can combine and overlap to give more complex standing waves. We
can add and subtract their wave functions to give the wave functions of new orbitals. This
process is called the linear combination of atomic orbitals (LCAO). The number of new orbitals
generated always equals the number of orbitals we started with.
Thus, when a chemical bond forms, the outer orbitals can be divided into three types:
1- Antibonding orbitals (of higher energy)
2- Bonding orbitals (of lower energy)
3- orbitals not involved in bonding and are therefore, referred to as non-bonding orbitals and
are at a different energy than the other two types. They occur in heteroatoms such as nitrogen
and oxygen.
The possible of electronic transitions between the orbitals are presented by the
vertical arrows.
Electronic transitions occur upon absorption of light of energy equal to the energy
difference between the orbital from which the electron originates and the orbital into which
the electron is promoted.
These transitions are seen as absorption bands in the UV-Visible spectrum of an organic
compounds. For example, the UV absorption spectrum of formaldehyde exhibits three bands
has shown in figure below, due to n
p*, n
s* and p
p*.
In general, s-s*transition is most difficult and the absorptions lie below the limit of 180 nm.
These are associated with all organic molecules containing s bonds such as C-C or C-H.
p-p* transition is usually associated with multiple bonds of C, N, O and S such as
C=N, C=S or C=O.
n-s* transition occurs in all covalently bonded compounds containing heteroatoms with nonbonding electrons such as C-O-C, C-Cl or C-N.
n-p* transition is associated with multiple bonds containing heteroatoms such as C=N, C=O or
C=S.
When two molecular orbitals of two conjugated double bonds delocalise, the
energy of highest occupied orbital is rised and that of the lowest unoccupied antibonding
orbital is lowered.
What’s happen when molecule absorbs light
quanta ?
When a molecule absorbs UV
or visible light, an electron can
be promoted to a higher
energy orbital. The resulting
excited state is transient and
in many cases the excess
energy is lost as HEAT when
the molecules return to its
ground state. However,
excited states of some
substances return to the
ground state with emission of
radiation. LUMINESCENCE is a
general term for such
behavior.
Excitation: the time required for singlet-singlet (S0-S1) transition is only about 10-15s.
S0-T1 is forbidden transition (10-6 times less probable then (S0-S1) transition).
Lifetime of an excited singlet is about 10-9- 10-6 s.
Internal Conversion (IC): Molecule may lose energy in the S1 state by vibrational relaxation to
the lowest vibrational level of the excited singlet state ( for this reason, the wavelengths of the
fluorescence band are always longer than the wavelengths of the exciting photons) or the
ground state through the evolution of heat.
Fluorescence is a type of Luminescence in which the emission from a photoexcited state (S1)
occurs within nanosecond to a microsecond (10-6- 10-11 s) after excitation.
Intersystem crossing (ISC) is radiationless transition in which S1 state in its lower vibrational level
is transformed to T1 state.
Phosphorescence is a type of Luminescence in which there is a delay from 10-4– 102 s or more
before emission occurs (from T1).
Lifetime of an excited triplet is about 10-4– 102 s.
The triplet life time is so long that there is a good opportunity for the loss of excitation energy
by collision with oxygen and with solvent molecules. This process is referred as quenching. For
this reason phosphorescence is rarely observed at room temperature in solutions.
The excitation energy of a molecule in its excited state may be dissipated by the following
processes:
1- Radiative processes: LUMINESCENCE ( fluorescence and phophorescence)
2- Radiationless processes: Intersystem crossing(ISC) and Internal conversion (IC)
3- Bimolecular deactivation processes: Quenching
4- Energy transfer processes
5- Dissociation (cleavage) processes.
Lifetime of an excited singlet state (10-6- 10-15s) and triplet state (102-10-3s) is an
important
factor deciding the dissociation (cleavage) processes (5) of an excited state (S1 and/or T1) into
free radicals. If the lifetime is very short, the above reaction is less probable.
When a molecule (polymer molecule) absorbs electromagnetic radiation (light) its energy
increses by an amount egual energy of the absorbed photon (E):
E= E2- E1= hn
According to Grothus and Draper law “only the light which is absorbed by a
molecule can be effective in producing photophysical process (bond dissociation) or
photochemical process (e.g. photo-rearrangement) in that molecule” However absorbed light
has to have enough energy for example to cause a bond dissociation( see table)
Most pure polymers contain only C-C, C-H, C-O, C-Cl, C-N and C-P bonds and are
not, therefore, expected to absorb light at wavlengths longer than 200nm.
The fact that photodegradation occurs even with ligth > 300nm indicates some
kinds of chromophoric groups must be present in these polymers.
For example carbonyl groups exhibiting n-p* type absorption bands in the range
300- 360 nm can be responsible for the absorption of radiation from spectral region in which
many poymers themselves do not absorb (>300 nm).
The extended absorption of many polymers can result from formation of charge Transfer
(CT) complexes between polymer and molecular oxygen.
Bimolecular deactivation and Energy transfer processes
An electronic energy transfer is the one-step transfer of electronic excitation
energy from excited donor (D*) to an acceptor (A) molecule in separate molecules
(intermolecular energy transfer) or in a different part of the same molecule ( intramolecular
energy transfer).
Electronic energy transfer process may occur only if the absorption spectrum of an
acceptor (A) overlaos an emission spectrum of an excited donor (D*).
Typical Energy transfer
In solid state the rate or efficiency of an energy transfer above glass transition
temperature(Tg) is the same order of magnitude as in solution.
In polymer having aromatic groups (e.g. polystyrene, polyvinylnaphthalene etc)
energy transfer process may occur via formation of excimers (excited dimer).
Electronic energy transfer process may also occur via formation of exciplexes. An exciplex
(excited charge transfer complex) is a well defined complex which exits in electronically
excited states. An exciplex is formed between an excited donor (D*) and/or an excited
acceptor (A*) and donor (D) molecules.
Effect of Free volume
When polymer is cooled to 0 K no motion of the groups or constituents is possible. As
the temperature is increased from 0 K the specific volume of the polymers increases. Since
there is very little change in the bond lengths with temperature the observed increases in
specific volume must be due to the formation of small holes or voids in the system which
collectively increases in size and/or number as the temperature rises. As the free volume in the
polymer increases, various types of molecular motion can begin to occur and these are
identified by transitions (e.g. the crank shaft motion in polyethylene observable at -85 oC and
the transition associated with movement of the phenyl ring polystyrene at -80 oK.
Effect of The Glass Transition Temperature
In generally, dissociation of free radical pairs can be relatively efficient in the
solid-state if one component is a small free radical but will be significantly inhibited if
both are polymeric radicals.
Reaction which can be considered to be associated with caged radicals,
such as the photo-Fries, will require very little free volume and can be expected to
be quite efficient in solid polymers below transition, whereas photochemical process
such as Norrish Type II process are substantially reduced in glassy polymers below
the glass transition temperature (Tg).
Bimolecular reactions which require the diffusion of a small molecule
reagent to a species in a polymer matrix will depend on both the diffusion constant
and solubility of the material in the matrix.
Solid polymers generally have internal viscosities only two to three orders
of magnitude less than those for simple liquids such as benzene or hexane so that
under suitable conditions quite efficient bimolecular reactions can be induced to
occur by diffusional processes in polymer materials.
Morphology of polymer
In the solid state amorphous polymers are more susceptible to oxidation
than crystalline polymers.
This is because of the rate of diffusion of oxygen will be faster in the amorphous
material which will oxidize faster followed by moulded film and the single crystals.
Oxygen diffuses only into amorphous regions of a polymer making them
more susceptible to photo-oxidative degradation.
The presence of crystalline domains in a polymer matrix has two effects on oxygen
diffusion and solubility behavior.
1- At temperatures well below the melting point (Tm) crystalline regions are generally
inaccessible to oxygen and to most penetrants.
2- Crystalline domains require penetrant migration around them, which increases the
average pathlength relative to nominal dimensions of the sample.
Role of Mechanical stress
Mechanical stress causes changes in the
physical Properties of polymers. Macroscopic
extension of a polymer film causes anisotropic
orientation and extension of polymer chains.
Stress can cause chain breaks and introduce
radicals which can initiate degradative processes
Such as oxidation or microcraking.
In solid polymers in the glassy state, where
mobility of the macromolecules is limited, chain
end radicals formed by mechanical stress may
only abstract hydrogen atoms from adjacent
molecules. Following the chain reaction
neighbouring macromolecules may be degraded
in fast reactions, and local sites of disintegration
are formed in a stressed polymer.
Such local degraded sites may be considered
as submicrocracks.
Microcracks are generally formed at the
weakest links in polymeric materials.
The overall embrittlement of material is a
result of the formation of microcracks.
In this study, the data trend shown in Figure 5 correlated well with activation energies derived
from the thermal analysis, which showed that the thermal oxidative stability followed the order
LLDPE> mPE>HDPE, whereas the trend for photo-oxidative stability was mPE>HDPE>LLDPE. The
thermal-oxidative results were also in accord with CL measurements, although the CL data for
the photo-oxidative series demonstrated a higher light stability for the LLDPE compared to HDPE
and mPE, both of which exhibited CL emission below their melting points.

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