Microscopy of polymers
Microscopy
• Experimental methods to obtain magnification of morphological
structures
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Optical microscopy (OM)
Scanning electron microscopy (SEM)
Transmission electron microscopy (TEM)
Scanning probe microscopy (SPM)
– Atomic force microscopy (AFM) is the most commonly used
Main features
Gedde, U., Polymer physics 1995
Optical microscopy
• The magnification is obtained via a two-lense system, referred to
as the objective and the eyepiece, respectively
• The maximum magnification obtained is about 2000x
• Surface topography is studied in reflected light mode
• Bulk structure is studied with the light transmitted through the
specimen
– More often used for polymers
– Sample thickness important, usually 5-40 mm
OM
• Phase contrast microscopy
• Differential interference-contrast microscopy
Electron microscopy
• Acceleration voltage in SEM is 1–30 kV, typically 15 kV
• A typical voltage in TEM is 100 kV
• The samples are inserted into a vacuum chamber; the vacuum
conditions mean that samples must not be liquid
• Samples must be conducting
– Coated with gold or platinum
• NOTE: Artificial structures
– Artefacts are not true features of the structure of a material, but
are created by the preparation method or during examination
(radiation damage), particularly TEM
Scanning electron microscopy
• In scanning electron microscopy (SEM), an electron beam is
focused into a small probe and scanned in a raster pattern across
the surface of a sample
• The electron beam interacts with the sample, generating different
signals. By detecting these signals and correlating signal intensity
with probe position, images of the sample surface are generated
• The nature of the image depends on the type of signal collected:
• secondary electrons for imaging surface morphology
• backscattered electrons for compositional imaging
• X-rays for compositional analysis
http://cime.epfl.ch/introduction-to-em
SEM
• The electron probe channels a current onto the sample, which must
be conducted away to prevent charge accumulation on the sample
surface. Samples must therefore be conductive
• Non-conductive samples can be made conductive by coating with
carbon or metallic films
• The coating is achieved with by vacuum evaporation or sputtering
of a heavy metal (Au or Pd) or carbon
Backscattering
image of surface
250 x
magnification
SEM, composite materials
Important to check the magnification when comparing data!
SEM, biological materials
Require chemical fixation and dehydration
TEM
• The transmission electron microscope uses a high energy electron
beam transmitted through a very thin sample to image and analyze
the microstructure of materials with atomic scale resolution
• The electrons are focused with electromagnetic lenses and the
image is observed on a fluorescent screen, or recorded on film or
digital camera
• The electrons are accelerated at several hundred kV, giving
wavelengths much smaller than that of light: 200kV electrons have
a wavelength of 0.025 Å
– The electron microscope is limited to about 1-2 Å
TEM
• Typical specimen examined with TEM consists of a series of thin
(50-100 nm) sections of stained polymer on a microscopy grid
• Thin sections are produced by ultra microtome
• Natural variation in density is seldom sufficient to achieve adequate
contrast
• Contrast is obtained by staining or by etching followed by replication
• Sample can be embedded in epoxy, polyesters or methacrylates
Electron microscopy
Gedde, U., Polymer physics 1995
AFM
• High resolution method
• Development from scanning tunnelling microscope
• Designed to measure the topography of a nonconductive sample
• A very sharp tip is dragged across a sample
surface and the change in the vertical position
(denoted the "z" axis) reflects the topography of
the surface
• By collecting the height data for a succession of
lines it is possible to form a three dimensional map
of the surface features
• Contact mode
– non-contact mode
– Tapping Mode (intermittent contact Mode)
AFM
• An Atomic Force Microscope can reach a lateral resolution of 0.1 to
10 nm
• Spherulites
• Poly(ferrocenyl-dibutylsilane)
• Contact Mode AFM images
of a reduced sample,
• Left height image, z range
1,5 mm; right deflection
• Isothermal crystallization
temperature 120 oC
AFM
Degradation and stability
General overview
• During polymerization
• During processing
• Product
– Shelf life
– Stability or controlled degradation
• Stability of polymers can be affected by;
– Chemical
• Water, oxygen, ozone, acids
– Physical
• Heat, mechanical action, radiation
– Biological environmental effects
Effect of environmental agents on polymers
Effects of degradation on polymers
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Changes in chemical structure
Changes on the surface
Loss in mechanical properties
Embrittlement
Reduction in molecular weight due to chain scission or increase due
to crosslinking
Generation of free radicals
Toxicity of products formed due to thermal degradation, pyrolysis or
combustion
Loss of additives and plasticizers (leaching)
Impairment of transparency (hazing)
Hamid et al. Handbook of polymer degradation (1992)
Thermal degradation
• Chain scission
– Random degradation (chain is broken at random sites)
– Depolymerization (monomer units are released at an active
chain end)
• Tc ceiling temperature; rates of propagation and depolymerization
are equal
– Weak-link degradation (the chain breaks at the lowest energy
bonds)
• Non-chain scission reactions
– One example is dehydrohalogenization which results from the
breakage of carbon-halogen bond and subsequent liberation of
hydrogen halide (PVC)
PVC
• Partial dehydrochlorination of a repeating unit of PVC resulting in
double bond formations and the liberation of hydrogen chloride
Picture: Fried
Polymers with high temperature stability
• For use at high T, the best polymers are those with highly aromatic
structures, especially heterocyclic rings
• Polymers having high temperature stability as well as high
performance properties are specialty polymers for limited use in
aerospace, electronics, etc.
• Factors contributing to high temperature stability also contribute to
high Tg, high melt viscosity and insolubility in common organic
solvents
– Polymers are difficult or impossible to process by usual methods
such as extrusion or injection molding
Examples of thermally stable polymers
Radical reactions and stabilization
Oxidation of hydrocarbons in liquid phase:
These primary processes are followed by secondary branching reactions
initiated by hydroperoxide (ROOH) thermolysis and/or photolysis
Handbook of degradation
Effect of processing on PP and PE
Oxidative and UV stability
Oxidation
• Most polymers are susceptible to oxidation particularly at elevated
temperature or during exposure to UV light
• Oxidation leads to increasing brittleness and deterioration of
strength
• Mechanism of oxidative degradation is free radical and is initiated
by thermal or photolytic cleavage of bonds
• Free radicals react with oxygen to yield peroxides and
hydroperoxides
• Photolysis: combined effect of light and oxygen
• Ozonolysis: effect of ozone
Oxidation and UV
• Unsaturated polyolefins are susceptible to attack by oxygen and by
ozone
• Absorbed energy can break bonds and initiate free radical chain
reactions that can lead to discoloration, embrittlement and eventual
degradation
Oxidation
• Rate of oxidation is different for polymers
• Oxidation in saturated polymers is slow even at 100 °C, and is
enhanced by UV or metal ions. Activation energy of oxidation for
saturated polymers are 149 - 230 kJ/mol
• For unsaturated polymers, such as isoprene or polybutadiene,
physical properties are quickly affected even at room temperature
by oxidation. Activation energies are 100 - 110 kJ/mol
Oxidation of saturated polyolefins
Oxidation
• Heat and UV affect PE. The influence of heat on the oxidation of
LDPE has an induction period depending on the T; effect of UV has
no induction period:
Time (h)
Effect of ozone on polymers
• Formation of ozonide
O3
O3
C
C C
O
C+
O
O O
-
+
Amfoteerinen
Amphoteric
Amphotericion
ion
ioni
C
O C
C
O
C
otsonidi
Varsinainen
Actual ozonide
Radiation effects
Radiation
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High energy ionization radiation (radiolysis)
– Gamma radiation
– Electron beams
– X-rays
Causes degradation and crosslinking in polymers
– Whether it causes chain scission or crosslinking depends on the
chemical structure
Can be used on purpose
– Sterilization of medical devices and equipment can be done using g or
electron radiation
– Can be used to prepare graft polymers
Polystyrene and polysulfone very resistant to radiation, PP susceptible to
degradation
Antioxidants (radical scavenges) are effective stabilizers for radiationoxidative degradation
Mechanically induced degradation
of polymers
Mechanodegradation
• Degradation can result from stress, such as high shear deformation
of polymer solutions and melts
• Solids can be affected by machining, stretching, fatigue, tearing,
abrasion or wear
• Particularly severe for high molecular weight polymers in a highly
entangled state
• Stress induced degradation causes the generation of macro-radicals
originating from random chain rupture
Mastication
• Mastication is the process where natural rubber is softened by
passing between spiked rollers
• In this process, fillers and other additives (accelerators, vulcanizers,
and antioxidants) are dispersed
CH3
CH3
C CH CH2 CH2 C CH
CH3
CH3
C CH CH2
+
CH2 C CH
Hydrolytic degradation
Hydrolytic degradation
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Some polymers are susceptible to degradation due to water
Acidic conditions can enhance degradation
Requires labile chemical bonds such as ester-, ether, amide
Naturally occurring polymers such polysaccharides and proteins
• Chemical structure and physical properties influence hydrolysis rate
greatly
– Glass transition temperature
– Amorphous polymers are more easily hydrolyzed than partlycrystalline; water penetrates to the amorphous regions first
Effect of microorganisms
Effect of micro-organisms
• Biodegradation is a process by which bacteria, fungi, yeasts and
enzymes consume a substance
• Most synthetic polymers are not attacked by microorganisms but
some stabilizers or plasticizers may act as hosts
0 days
effect of enzymes after 5 days
Microbial degradation
Examples
Stabilizers
Stabilizers
• Applications:
– To prevent premature polymerization of the monomer
– To prevent degradation caused by heating during processing
– To reduce the environmental (weathering) effects such as
radiation (UV, visible), moisture, temperature cycling and wind
• Short term
– To protect against T and oxygen during processing
– Typically low molecular weight such as hindered phenols and
aromatic amines; serve as free-radical scavengers
Stabilizers
Handbook of degradation (1992)
Stabilizers
When adding stabilizers, the following properties and effects must be
considered:
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Miscibility with the polymer
Toxicity
Volatility
Effect on the colour and odour of the product
Applicability to processing
Compatibility with possible fillers
Economical effects, price
Elimination of active sites
• Active sites are where primary reactions occur. The site is either
inherently part of the structure or it is developed during processing
or aging of the product. Examples of active sites:
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Carbon double bonds
Labile Cl atoms (PVC)
Catalyst residues
OH-end groups (polyacetal)
Hydrogen atoms at branching sites
Groups formed by oxidation
Prevention of oxidations
• Too high a dose of inhibitor will have a negative effect
Oxidation
Inhibitor content
wt.-%
Metal complexes
• Metal catalyst residues have been found to enhance the effects of
oxidation, for example on PVC
• Metal traces are bound with substances forming complexes with
them. Most common are chelates due to their great stability
OH
CH N
OH
N CH
heteropolaarinen sidos
Cu
O
sivuvalenssi
O
Against the effects of UV
• Pigments can be added to protect the polymers from the effects of
UV light
• Carbon black is added (2-3 wt.%) to the formulation
• Titanium oxide, Zn white and other lighter pigments prevent the
penetration of radiation but will not protect the effects on the surface
i.e. yellowing and brittleness
• UV-light absorbents are colorless, organic compounds which
change the UV energy, making it less harmful to the polymer by:
– Fluorescence
– Heat production
– Disproportionation
Free radical graft copolymerization
of microfibrillated cellulose
Kuisma Littunen
Master’s thesis
Microfibrillated cellulose
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Can be produced by combining
mechanical,
chemical
and
enzymatic treatments of cellulose
Microfibrils (MFC) have a very
large specific surface area and
impressive mechanical properties
Raw material is cheap and
abundant in nature
MFC can only be stored as a dilute
water
suspension
to
avoid
aggregation of fibrils
Incompatible with many common
polymers
MFC graft copolymerization
• Redox initiated free radical polymerization with Ce(IV) initiator
– Reaction can be done in an aqueous medium and low
temperature
– Radicals are formed selectively on cellulose chains
– Method is well known and tested
MFC graft copolymerization
• Experiments were done with several acrylic monomers
• Goals:
– To achieve hydrophobization and/or functionalization of MFC
– To find out differences in grafting tendency between different
acrylates and methacrylates
• Monomer/MFC ratio, initiator concentration and polymerization time
were varied
Characterization
• Starting material and some products were studied by AFM imaging
to compare fine structures
• Monomer conversion, polymer weight fraction and the amount of
homopolymer were determined gravimetrically
• Graft copolymerization was verified by FTIR, XPS and solid state
NMR
• Grafted polymer was cleaved by acid hydrolysis and analyzed with
GPC
• Thermal behavior was studied by DSC and TGA
Results
• High graft yield and low
homopolymer formation
(depending on monomer)
• Yield could be adjusted with
monomer/MFC ratio. Initiator
concentration and reaction time
had only a minor effect
C = conversion
wG = graft polymer weight fraction
GE = graft efficiency
400
500
350
450
400
300
350
GMA
200
EA
MMA
150
BuA
100
HEMA
50
300
G (%)
G (%)
250
250
200
150
100
50
0
0
0
5
10
15
20
25
30
n(M)/m(MFC) (mmol/g)
35
40
45
0
10
20
30
40
t (min)
50
60
70
Visual observation of dried products
AFM images
MFC
MFC-g-PGMA
FTIR spectrometry
FTIR spectrometry
Solid state 13C NMR
• Polymeric carbon signals
detected and assigned in
all samples
• Signals were integrated to
calculate molar and
weight fraction of polymer
XPS (X-ray photoelectron spectroscopy)
• MFC that was used as
raw material contains
some nonpolar impurities
• Possible sources are
lignin residues or surface
contamination
XPS spectrometry
• MFC-g-PGMA spectrum matched
pure PGMA, indicating a very
dense polymer coating
• MFC-g-PMMA sample showed
also cellulosic O-C-O bonds
GPC analysis
• Hydrolysis was successful with
products grafted with PEA, PMMA
and PBuA
3000000
2500000
Mn (g/mol)
2000000
EA
MMA
1500000
BuA
1000000
500000
0
15
20
25
30
35
n(M)/m(MFC) (mmol/g)
40
45
DSC results
DSC results
TGA results
• Initial decomposition temperature
was increased by 10 oC and 20 oC
in products grafted with PEA and
PBuA
• Other grafted polymers had only
minor effects
265.0Cel
3.022mg
400.0
318.3Cel
438.2ug/min
3.000
395.7Cel
408.9ug/min
350.0
2.500
300.0
351.1Cel
1.956mg
TG mg
DTG ug/min
2.000
250.0
200.0
1.500
150.0
1.000
100.0
50.0
0.500
0.0
0.0
100.0
200.0
300.0
Temp Cel
400.0
500.0
Conclusions
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MFC was successfully grafted in aqueous solution
Selected polymerization method was efficient and quite selective
Results varied greatly between different monomers
Reaction seemed suitable for scale-up
MFC was hydrophobized to some degree by all tested modifications
Nanostructure was at least partially preserved
PBuA grafting improved the heat resistance of MFC