Electron Microscopy of Polymers - Stellenbosch...

Electron Microscopy of Polymers

Techniques and Examples

S. Henning, G.H. Michler*

Fraunhofer Institute for Mechanics of Materials IWM, Halle (Saale), Germany*Institute of Physics, Martin Luther University Halle-Wittenberg, Halle (Saale), Germany

Short course on polymer characterizationStellenbosch, 7 April 2014

Outline

1. Introduction: Morphology and Micromechanics of Polymeric Materials

2. TEM: Principles of Image Formation

2.1 Mass Thickness Contrast; 2.2 Electron Diffraction and Diffraction Contrast; 2.3 Analytical TEM:

3. TEM Preparation

3.1 Ultrathin Samples from Solution; 3.2 Ultramicrotomy and Cryo-Ultramicrotomy; 3.3 FIB;

3.4 Fixation and Staining

4. SEM: Principles and Imaging Modes

4.1 SE and BSE Signals, EDX; 4.2 ESEM

5. SEM Preparation

5.1 Preparation of Surfaces and Powders; 5.2 Fracture Surfaces; 5.3 Etching Techniques

6. in situ Deformation Techniques

6.1 Instrumentation; 6.2 Example

3. TEM Preparation

5. SEM Preparation

1. Introduction: Morphology and Micromechanics of Polymers

� Micromechanics:

� Micro- and nanoscopic processes during deformation and fracture

� TEM: History and State-of-the-Art

1931: first TEM Ernst Ruska and Max Knoll 2010: Cs-corrected HRTEMFEI Titan3 G2 60-300 *

Point resolution: 80 pm(Image corrector)

Cs-corrected HR-TEM image on SWCNTfilled with fullerenes acquired at 80 kV *

*[http://www.fei.com/uploadedFiles/DocumentsPrivate/Content/titan_cubed_g2_ds.pdf]

� Microscope resolution and structural details

� Comparison of probe design and bulk sample preparation strategies for different micsroscopic techniques

� Morphology

� Comparison of sample preparation strategies for different micsroscopic techniques

� Micromechanics

3. TEM Preparation

5. SEM Preparation

� Limitation of the resolution of light optical microscopy

� Rayleigh-Criterion: Two point sources cannot be resolved if their separation is less than the radius of the Airy disk.

NA … numerical aperture

[images: Swinburne University of Technology at: astronomy.swin.edu.au/ cosmos/R/Rayleigh+Criterion]

Accelerating voltage V [kV]

Velocity of electrons v [ms-1]

Ratio of electron velocity to light velocity vc-1

Wavelength of electrons λλλλ [pm]

10 5.83 x 107 0.194 12.20

100 1.64 x 108 0.548 3.70

200 2.08 x 108 0.695 2.51

500 2.59 x 108 0.863 1.42

1.000 2.83 x 108 0.941 0.87

3.000 2.97 x 108 0.989 0.36

� Imaging with electrons

� De Broglie

U …accelerating voltage, v … velocity of the electronseo = elementary electric charge = 1.6*10-19 C, m0 = rest mass of the electron = 9.11*10-28 g,c = velocity of light in vacuo = 3*108 ms-1, h = Planck´s constant = 6.625*10-34Js

� TEM principle: comparison of the path of rays with light optical microscope

2.1 Mass Thickness Contrast

ρ1 < ρ2

ρρρρ1

ρρρρ2d2

e-e- e- e-100 100 100 100

95 80 10 50

side view of the sample

PS particle

gold particles carbon film

100 nm

2.2 Electron Diffraction and Diffraction Contrast

Images of a sheaf-like lamellar structures in LDPE obtained by different operating modes of the TEM:

a) bright field image,b) dark field image,c) electron diffraction diagram

(cryosection, HVTEM)

2.3 Analytical TEM: EELS Investigations

[Werner: Lecture Electron Microscopy; MPI Halle]

2.3 Analytical TEM: EELS Investigations

Initial parts of electron energy-loss spectra of 50 nm thick ultra-thin sections of a PFS-PS block copolymer (a) and a PS homopolymer (b)

EFTEM investigation of the PFS-PS block copolymer: zero-loss image (c), electron spectroscopic image at 230 eV

energy-loss (d) and elemental mapping (f) by means of the iron L2,3-edge shown in the corresponding part of the

energy-loss spectrum (e)[PFS … pentafluorstyrol]

2.3 The Use of EFTEM: Electron Spectroscopic Imaging

Comparison of TEM images of ABS: a) „conventional“ image, b) image with energy filter in zero loss mode

[Product information, LEO Electron Microscopy]

3. TEM Preparation

5. SEM Preparation

3. TEM Preparation

3.1 Ultrathin Samples from Solution

� Preparation of electron transparent samples from polymer solutions by

� spin-coating

� dip-coating

3. TEM Preparation

3.1 Ultrathin Samples from Solution

� Examples: Deformation of semithin films formed by dip-coating on glass

Deformed film of Styrol-Butadiene-Blockcopolymer + PS + Alumina nanoparticles

3. TEM Preparation

3.2 Ultramicrotomy and Cryo-Ultramicrotomy

� Preparation of electron transparent samples from bulk materials

A - specimen is cut with controlled speed (downward stroke)B - retractionC - advance of specimen arm determines the specimen thickness

3. TEM Preparation

Dry sectioning Wet sectioning

3. TEM Preparation

� at room temperature for hard samples

� under cryo conditions for soft materials (Tcut < Tg)

� using glass or diamond knives

RMC PowerTome PT-PC with CRX cryo chamber

Diamond knives for dry, wet, and cry applications; different manufacturers

LEICA EM UC7 ultra-microtome

3. TEM Preparation

� Examples: Hydroxy apatite nanocrystal distribution in cortical bone

Ultrathin section 40 nm, 21 °C, diamond knife, RMC PT-PC; TEM LEO 912

Ultrathin section 40 nm, 21 °C, diamond knife, LEICA Ultracut; TEM LEO 912

3. TEM Preparation

� Examples: Layered silicate nanoplatelets in PA and PA/ASA blends

Ultrathin section 50 nm, - 80 °C, diamond knife, RMC PT-PC; TEM LEO 912

3. TEM Preparation

3.3 Focused Ion Beam Technology (FIB)

FEI Quanta 3D FEG DualBeam: e-, Ga+ Preparation of a lamella Rubber sample, BSE-SEM

3. TEM Preparation

� Very often, polymer samples are too soft for sectioning at RT, or they are sensitive to electron beam irradiation. Chemical or physical treatment can be used for hardening � „Fixation“

� Polymer samples very often do not show sufficient contrast (similar electron densities of the elements that are present). Heavy elements can be placed selectively into one or more phases of the material giving contrast � “Staining”

� Staining procedures can be applied prior to sectioning (staining of a trimmed block) or after ultramicrotomy (staining of ultrathin sections).

� Staining can be performed by immersion of the sample in the staining agent or in vapour.

� There are one-step procedures and more complex procedures with two or more steps.

3. TEM Preparation

� Examples

� Polyolefines: chlorosulfonic acid + osmium tetroxidechlorosulfonic acid + uranyl acetateruthenium tetroxide

� Polyamides: formalin + osmium tetroxidetungstophosphoric acid + osmium tetroxideruthenium tetroxide

� Styrol-Butadiene-Copolymers:osmium tetroxideruthenium tetroxide

� Polyurethanes: chlorosulfonic acid + osmium tetroxideruthenium tetroxide

� …

3. TEM Preparation

� Example: Semicrystalline morphology of HDPE

Two-step staining with chlorsulfonic acid and osmium

tetroxide, ultramicrotome

sections

3. TEM Preparation

� Example: Semicrystalline morphology of UHMWPE

One-step staining with ruthenium

sections Lamellar thickness distribution measured using the TEM image

3. TEM Preparation

� Example: α- and β- modifications of polypropylene

One-step staining with ruthenium

sections

3. TEM Preparation

� Example: Deformation structures in β- modification of polypropylene

Chevron formation, lamellar separation and nanovoid formation as

essential micromechanical mechanisms during tensile

deformation.

One-step staining with ruthenium tetroxide,

ultramicrotome sections

3. TEM Preparation

� Example: Deformation mechanisms in lamellar SBS block copolymers

The effect of thin layer yielding in an

asymmetric styrene-butadiene star block

copolymer

(74 % styrene)

One-step staining with osmium

sections

3. TEM Preparation

5. SEM Preparation

4.1 SE and BSE Signals, EDX

� SEM Principle: Instrumentation and signals

� SEM: Interaction volume

[Sketch modified after Röder, Uni Halle]

� Image formation using secondary electrons (SE)

� Inelastic scattering of primary electrons with weakly bonded electrons of the outer shells of atoms in the whole interaction volume

� Due to their low energy (3eV to 50 eV), only SE from volumina close to the surface are able to escape

� Collected and amplified by means of an apropriate detector, SE create a signal carrying information mainly on the surface topography

� The contrast is to a great extent a function of the tilting angle of the sample surface with respect to the incident beam and so-called edge effects

� Image formation using backscattered electrons (BSE)

� Elastic scattering of primary electrons when they interact with sample atoms

� Due a much higher energy than SE (60% to 80% of the energy of the original PE) they are able to escape from deeper regions of the interaction volume

� The signal detected by a semiconductor device is strongly dependent on the average atomic number of the interacting atoms

� Singnal detection can be designed so that contrast of BSE images represents differences in the material composition of the sample (left hand image)

� Energy dispersive X-ray analysis (EDX)

� Ionization of inner shells, gaps are filled with electrons from outer shells

� The energy difference is emitted as characteristic X-ray emission

� Analysis of elemental composition of the sample

� Example: Morphology of flame retarded PP

� ESEM principle and advantages

� Variable pressure in the chamber due to pressure limiting aperture (PLA)

� „Wet“ samples (with cooling)

� No conductive coating (new detection principles)

4.2 ESEM

� Gaseous secondary electron detector

4.2 ESEM

� Example: Drying of dental composites, ESEM-GSED

4.2 ESEM

Left: ambient conditions 7 mbar and 277 K, Right: after drying at lower pressure (3 mbar, 277 K)

3. TEM Preparation

5. SEM Preparation

� Fixation on sample holder, conductive carbon or metal coating

5. SEM Preparation

5.1 Preparation of Surfaces and Powders

Urinary stent

Bone cement

� Fracture or cryofracture, mounting, conductive carbon or metal coating

5. SEM Preparation

5.2 Fracture Surfaces

Pores in acrylic bone cement; fracture surface, SEM

X-ray opacifier particles in acrylic bone cement, SEM

� Principle: Transformation of morphological features to topography

5. SEM Preparation

5.3 Etching Techniques

Example for permanganic etching:

Development of a topography at the

surface of a polypropylene sample;

SEM-SE images

Literature on peramganic etching:

e.g. Olley et al., J. Mat. Sci. 28 (1993),

1102-1112

� Example: α- and β- modifications of polypropylene

5. SEM Preparation

Comparison of lamellar structures after permanganic

etching;

SEM-SE images

� Examples: UHMWPE

5. SEM Preparation

Semicrystalline morphology of UHMWPE; permanganic etching, SEM

Semicrystalline morphology of UHMWPE with graphite; permanganic etching, SEM

5. SEM Preparation

� Example: Deformation of the α- and β- modifications of polypropylene

Micromechanical mechanisms in PP

observed after tensile deformation

Permanganic etching, SEM-SE

images

3. TEM Preparation

5. SEM Preparation

� Straining holder model 671, Gatan

� for TEM JEOL JEM 2010, JEM 4000 FX and others

� Sample thickness: 0,1 μm bis 0,5 μm

� Temperature range: -180 °C bis 120 °C

6.1 Instrumentation

� Tensile and bending devices for AFM and ESEM

� Kammrath & Weiss bending device for deformation at RT

� Kammrath & Weiss tensile device for deformation at RT

� Registration of load-displacement diagrams

6. in situ Techniques

6.1 Instrumentation

� Typical results for HDPE/Copolymer blends

6.2 Examples

SEM TEM AFM

� Semithin cryo-section (300 nm), straining holder, TEM

6.2 Example: Rubber Toughened Polystyrene (HIPS)

� Semithin cryo-section (300 nm), sandwich technique, TEM JEOL JEM 4010

6.2 Example: in situ straining of a rubber sample

Comparison to AFM Results

� TEM Micrograph (a) and AFM Phase Image (b) of Ethylene / 1-Hexene Copolymer Blend

Comparison to AFM results

Examples

� TEM and SEM – modern, developing, challenging techniques

� One of the original keys to nanotechnology

� Resolutions down to 80 pm

� Analytical methods (EDX elemental analyses, EELS chemical analyses, crystallographic analyses, …) – with nanospot resolution!

� Needs some expertise for problem solution, preparation, image interpretation

� Relatively expensive and time consuming (sample preparation, HRTEM image formation, in situ-techniques; TEM installation, maintenance, and operation)

Summary

References

• G.H. Michler: Electron Microscopy of Polymers; Springer-Verlag Berlin 2008• G.H. Michler: Kunststoff-Mikromechanik: Morphologie, Deformations- und Bruchmechanismen von

polymeren Werkstoffen; Hanser Verlag 1992• G.H. Michler, F.J. Baltá-Calleja: Nano- and Micromechanics of Polymers; Hanser 2012• L.C. Sawyer, D.T. Grubb, G.F. Meyers: Polymer Microscopy, Third ed.; Springer 2008• L. Reimer: Scanning Electron Microscopy; Springer-Verlag Berlin 1985• L. Reimer: Transmission Electron Microscopy; Springer-Verlag Berlin 1989

Thank you for your attention!

Electron Microscopy of Polymers - Stellenbosch...

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