Microscopy of polymers
Microscopy
• Experimental methods to obtain magnification of morphological structures
• 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 materialsImportant 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 non-
conductive 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-di-
butylsilane) • 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
• 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 photolysisHandbook 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
Time (h)
Oxi
datio
n
Oxidation of saturated polyolefins
• 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:
Effect of ozone on polymers
• Formation of ozonide
C CO3
+ O C CO O
CO
CO3
C
CO
O-
+
Amfoteerinenioni
Varsinainen otsonidiAmphoteric ionAmphoteric ion Actual ozonide
Radiation effects
Radiation• 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 radiation-
oxidative 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
C
CH3
CH CH2 CH2 C
CH3
CH CH2 C
CH3
CHC
CH3
CH CH2 +
Hydrolytic degradation
Hydrolytic degradation
• 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 partly-
crystalline; 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:
– 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:
– 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 contentwt.-%
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
CH
OH
N
Cu
OH
NCH
O O
sivuvalenssi
heteropolaarinen sidos
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 celluloseKuisma LittunenMaster’s thesis
Microfibrillated cellulose
• 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 variedO
O
O O
O
O
O
O
O
O
O
OH
GMA EA MMA
BuA HEMA
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
• 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 = conversionwG = graft polymer weight fractionGE = graft efficiency
0
50
100
150
200
250
300
350
400
0 5 10 15 20 25 30 35 40 45
n(M)/m(MFC) (mmol/g)
G (
%) GMA
EA
MMA
BuA
HEMA
Results
0
50
100
150
200
250
300
350
400
450
500
0 10 20 30 40 50 60 70
t (min)
G (
%)
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
0
500000
1000000
1500000
2000000
2500000
3000000
15 20 25 30 35 40 45
n(M)/m(MFC) (mmol/g)
Mn
(g
/mo
l)
EA
MMA
BuA
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
Temp Cel500.0400.0300.0200.0100.00.0
TG
mg
3.000
2.500
2.000
1.500
1.000
0.500
DTG
ug/m
in
400.0
350.0
300.0
250.0
200.0
150.0
100.0
50.0
0.0
265.0Cel3.022mg
318.3Cel438.2ug/min
395.7Cel408.9ug/min
351.1Cel1.956mg
Conclusions
• 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
Top Related