Microscopy of polymers Degradation. Application of analytical techniques

Transcription

1 Microscopy of polymers Degradation Application of analytical techniques

2 Key learning outcomes Microscopy methods (SEM, TEM, optical microscopy) Operating principles, uses, limitations Atomic force microscopy (AFM) Degradation phenomena Causes, mechanisms, what can be done to prevent them

3 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

4 Main features Gedde, U., Polymer physics 1995

5 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

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 SEM/TEM, so what is the difference? SEM: Electrons reflected from sample surface Surface imaging Fine, focused electron beam that scans over the sample surface in a raster pattern TEM: Electrons pass through the sample Internal imaging Broad, static electron beam transmitted through the specimen

14 Comparison of SEM/TEM: how electrons are collected

15 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)

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17 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 o C

18 AFM

19 Degradation and stability

20 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

21 Effect of environmental agents on polymers

22 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)

23 Thermal degradation Chain scission Random degradation (chain is broken at random sites) Depolymerization (monomer units are released at an active chain end) T c 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)

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

25 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 T g, 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

26 Examples of thermally stable polymers

27 Radical reactions and stabilization

28 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

29 Effect of processing on PP and PE

30 Oxidative and UV stability

31 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

32 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

33 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

34 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: Oxidation Time (h)

35 Effect of ozone on polymers Formation of ozonide C C O 3 CO 3C C O O O C C O O O C Amfoteerinen ioni Amphotericion ion Varsinainen otsonidi Actual ozonide

36 Radiation effects

37 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 radiationoxidative degradation

38 Mechanically induced degradation of polymers

39 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

40 Mastication: useful degradation! 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 CH 3 C CH CH 3 CH 2 CH 2 C CH CH 3 C CH CH 2 + CH 3 CH 2 C CH

41 Hydrolytic degradation

42 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 partlycrystalline; water penetrates to the amorphous regions first

43 Examples

44 Effect of micro-organisms

45 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

46 Microbial degradation

47 Stabilizers

48 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

49 Stabilizers Handbook of degradation (1992)

50 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

51 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

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

53 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 OH N CH sivuvalenssi Cu heteropolaarinen sidos O O

54 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

55 Free radical graft copolymerization of microfibrillated cellulose Kuisma Littunen Master s thesis

56 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

57 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

58 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

59 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

60 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 w G = graft polymer weight fraction G E = graft efficiency G (%) GMA EA MMA BuA HEMA G (%) n(m)/m(mfc) (mmol/g) t (min)

61 Visual observation of dried products

62 AFM images MFC MFC-g-PGMA

63 FTIR spectrometry

64 FTIR spectrometry

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

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

67 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

68 GPC analysis Hydrolysis was successful with products grafted with PEA, PMMA and PBuA Mn (g/mol) EA MMA BuA n(m)/m(mfc) (mmol/g)

69 DSC results

70 DSC results

71 TGA results Initial decomposition temperature was increased by 10 o C and 20 o C in products grafted with PEA and PBuA Other grafted polymers had only minor effects Cel 3.022mg 318.3Cel 438.2ug/min 395.7Cel 408.9ug/min DTG ug/min Cel 1.956mg TG mg Temp Cel

72 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