Polymers can degrade by exposure to high temperature Shear
action Oxygen, ozone and chemicals Electromagnetic ( , UV)
Ultrasonic radiation Moisture Thermal Degradation Mechanical
Degradation Chemical Degradation Light induced Degradation
Hydrolysis Degradation of polymers
Slide 2
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. Often, multiple exposures, such as a combination of
moisture heat or oxygen and light can result in accelerated
deterioration. POLYMER moisture UV Heat Shear Ozone, oxygen
Slide 3
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
200 o C. 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
Slide 4
The second is that commercial polymer structures are more
complex than their generic molecular formula indicates.
Slide 5
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
Slide 6
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 Boltzmans 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 CC
bonds at least one is dissociated into radicals is 486 o C, while
in one mole of OO bonds it is only 30 o C.
Slide 7
Slide 8
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 P t = P o (s+1) (1) in which
P o and P t are the chain lengths of original polymer and after
time at which s scissions have occurred on average per molecule.
Thus S=(P o /P t )-1 (2) and the fraction of bonds broken, a, is
given by equation (3) = s/P o = 1/P t -1/P o (3) If chain scission
is random, that is, every interunit bond in every molecule is
equally liable to break then = 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 = +kt
(4) In which is the fraction of weak links in the molecules Figure
2.6. shows that PS obey equation 4 and does indeed incorporate weak
links.
Slide 9
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.
Slide 10
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).
Slide 11
- methylstyrene has no -hydrogen, therefore, depolimerize into
its monomer completely. Table 2.1. clearly show that it is
-hydrogen atom which is involved in transfer process
Slide 12
Depolymerization vs ester decomposition
Slide 13
Ester decompositon only becomes important when the monomer unit
incorporates at least five hydrogen atoms on the - carbon and
depolymerization is quantitative when there are at most one or two
-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.
Slide 14
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 - hydrogen 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.
Slide 15
Poly(ethylene terephthalate )
Slide 16
Slide 17
Slide 18
General Degradation Mechanism Chain scission can occur by one
of three mechanism. These include 1-Random degradation where the
chain broken at random sites. 2- Depolymerization where monomer
units are released at an active chain end Terminal initiation
Random initiation Depropagation Transfer Termination by
disproportionation or combination
Slide 19
Characterization techniques for polymer degradation &
stabilization
Slide 20
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.
Slide 21
Slide 22
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 > m- isopropylphenol > cardanol >
m-tert- butylphenol. The anomalous position of the m-
tert-butylphenol indicates that branching of the side chain has a
significant effect, particularly if branching occurs from the
-alkyl carbon atom which is attached to the phenolic nucleus.
Slide 23
Slide 24
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 non- volatile residue remaining
in the sample boat. PIPA= Polyisocyanate polyols
Slide 25
Slide 26
Slide 27
Pyrolysis- GC-MS
Slide 28
EGA-MS
Slide 29
Slide 30
Slide 31
Slide 32
Slide 33
Slide 34
Slide 35
Slide 36
ThermogravimetryMass 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
Slide 37
DSC
Slide 38
DTA (differential thermal analysis) and DSC (differential
scanning calorimetry) Measurement of oxidation induction times to
study a stabilizers effectiveness and its diffusion within the
solid
Slide 39
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. The oxidative-induction
time/oxidation induction time (OIT)
Slide 40
CL= chemiluminesans
Slide 41
Melt flow Index (MFI )
Slide 42
ThermogravimetryFourier 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.
Slide 43
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=CH 2 ) 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=CH 2 )
Slide 44
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).
Slide 45
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
Slide 46
Carbonyl Index= Abs at 1710 cm -1 /Abs at 2820 cm -1 Carbonyl
Index
Slide 47
Oxygen uptake The technique of oxygen uptake is an absolute
quantitative technique; it affords a direct measure of oxygen
consumption during polymer degradation
Slide 48
Slide 49
Slide 50
Slide 51
Thermo-oxidative Degradation of Polymers Eq.1. Initiation Eq.2.
Chain Branching) EQ.8 and 9. Termination
Slide 52
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=
10 8 dm 3.mol -1.s -1 )) In the presence of oxygen the temperature
of decomposition of most polymers decreases considerably and shift
from 300- 600 o C for inert atmosphere to 100- 200 o C. 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 PO 2. (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 o C. Diacyl peroxides and peracids are considerably less stable
than dialkyl peroxides and hydroperoxides.
Slide 53
Chain Branching Polymer oxy radicals (PO. ) undergo a number of
other reactions including (i) - scission reactions which results in
fragmentation of polymer chain together with formation of end
carbonyl (ketone or aldehyde) groups and radicals
Slide 54
(ii) Formation of in-chain ketone groups (iii) Induced
hydroperoxide decomposition
Slide 55
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. Metal catalyzed hydroperoxide
decomposition
Slide 56
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.
Slide 57
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.
Slide 58
Degradation of polyolefins during processing Processing of
polyolefins requires high temperatures (150- 300 o C) 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).
Slide 59
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 o C. 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.
Slide 60
Slide 61
Slide 62
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.
Slide 63
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.
Slide 64
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. 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.
Slide 65
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.
Slide 66
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 *, n * and *. In general, - *transition
is most difficult and the absorptions lie below the limit of 180
nm. These are associated with all organic molecules containing
bonds such as C-C or C-H.
Slide 67
- * transition is usually associated with multiple bonds of C,
N, O and S such as C=N, C=S or C=O. n- * transition occurs in all
covalently bonded compounds containing heteroatoms with non-
bonding electrons such as C-O-C, C-Cl or C-N. n- * 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.
Slide 68
Slide 69
Whats 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.
Slide 70
Excitation: the time required for singlet-singlet (S 0 -S 1 )
transition is only about 10 -15 s. S 0 -T 1 is forbidden transition
(10 -6 times less probable then (S 0 -S 1 ) transition). Lifetime
of an excited singlet is about 10 -9 - 10 -6 s. Internal Conversion
(IC): Molecule may lose energy in the S 1 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 (S 1 ) occurs within nanosecond to a microsecond (10 -6 - 10
-11 s) after excitation. Intersystem crossing (ISC) is
radiationless transition in which S 1 state in its lower
vibrational level is transformed to T 1 state. Phosphorescence is a
type of Luminescence in which there is a delay from 10 -4 10 2 s or
more before emission occurs (from T 1 ). Lifetime of an excited
triplet is about 10 -4 10 2 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.
Slide 71
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 -15 s) and triplet state (10 2
-10 -3 s) is an important factor deciding the dissociation
(cleavage) processes (5) of an excited state (S 1 and/or T 1 ) 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= E 2 - E 1 = h 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.
Slide 72
For example carbonyl groups exhibiting n- * 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.
Slide 73
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*).
Slide 74
Typical Energy transfer In solid state the rate or efficiency
of an energy transfer above glass transition temperature(T g ) 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.
Slide 75
Slide 76
Slide 77
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 o C and the transition associated with movement of the
phenyl ring polystyrene at -80 o K.
Slide 78
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 (T g ). 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.
Slide 79
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. Morphology of polymer In
the solid state amorphous polymers are more susceptible to
oxidation than crystalline polymers.
Slide 80
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 (T m ) 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.
Slide 81
Slide 82
Slide 83
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.
Slide 84
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.