1. Microscopy of polymers

2. 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

3. Main features
Gedde, U., Polymer physics 1995

4. 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

5. OM Phase contrast microscopy
Differential interference-contrast microscopy

6. 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

7. 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

8. 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

9. SEM, composite materials
Important to check the magnification when comparing data!

10. SEM, biological materials
Require chemical fixation and dehydration

11. 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 Å
The electron microscope is limited to about 1-2 Å

12. TEM
Typical specimen examined with TEM consists of a series of thin ( 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

13. Electron microscopy
Gedde, U., Polymer physics 1995

14. 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)

16. 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

17. AFM

18. Degradation and stability

19. 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

20. Effect of environmental agents on polymers

21. 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)

22. 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)

23. PVC
Partial dehydrochlorination of a repeating unit of PVC resulting in double bond formations and the liberation of hydrogen chloride
Picture: Fried

24. 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

25. Examples of thermally stable polymers

26. Radical reactions and stabilization

27. 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

28. Effect of processing on PP and PE

29. Oxidative and UV stability

30. 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

31. 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

32. 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 kJ/mol
For unsaturated polymers, such as isoprene or polybutadiene, physical properties are quickly affected even at room temperature by oxidation. Activation energies are kJ/mol

33. 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:
Time (h)
Oxidation

34. Effect of ozone on polymers
Formation of ozonide
Amphoteric ion
Amphoteric ion
Actual ozonide

35. Radiation effects

36. 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

37. Mechanically induced degradation of polymers

38. 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

39. 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

40. Hydrolytic degradation

41. 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

42. Effect of microorganisms

43. 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

44. Microbial degradation

45. Examples

46. Stabilizers

47. 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

48. Stabilizers
Handbook of degradation (1992)

49. 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

50. 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

51. Prevention of oxidations
Too high a dose of inhibitor will have a negative effect
Oxidation
Inhibitor content
wt.-%

52. 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

53. 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

54. Free radical graft copolymerization of microfibrillated cellulose
Kuisma Littunen Master’s thesis

55. 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

56. 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

57. 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

58. 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

59. 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

60. Visual observation of dried products

61. AFM images
MFC
MFC-g-PGMA

62. FTIR spectrometry

63. FTIR spectrometry

64. Solid state 13C NMR
Polymeric carbon signals detected and assigned in all samples
Signals were integrated to calculate molar and weight fraction of polymer

65. XPS (X-ray photoelectron spectroscopy)
MFC that was used as raw material contains some nonpolar impurities
Possible sources are lignin residues or surface contamination

66. 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

67. GPC analysis
Hydrolysis was successful with products grafted with PEA, PMMA and PBuA

68. DSC results

69. DSC results

70. 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

71. 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