PPC 10, Smolenice 2011
Positron annihilation and free volume in polymer-solid
interfaces and in nanocomposites
Klaus Rätzke, Stephan Harms, Franz Faupel
Technische Fakultät der Universität Kiel,
Institut für Materiawissenschaft - Materialverbunde
Kaiserstr. 2, 24143 Kiel, Germany
e-mail: kr@tf.uni-kiel.de
1/17
Polymeric Materials, Free Volume
fuel cell driven submarine
barnacles
color map:
carbon
oxygen
nitrogen
hydrogen
fluorine
Yacht during kiel week
Free volume, important for diffusion, viscosity, adhesion 2/17
Outline
Motivation
Polymeric materials, free volume, interphases and interfaces
Free volume / Positron annihilation lifetime spectroscopy
Principle, conversion lifetime - volume size, experimental setup
positron beam (PLEPS @ NEPOMUC)
Thin Films (SPP Polymer-solid interfaces and Interphases)
evaporated and spin coated Teflon AF on Si-Wafer
change of free volume due to solid substrate, (coiling changed?)
Polymer Nanocomposites
free volume as function of filler concentration, mixing rule and side effects
Summary
3/17
Our toolbox
Schematic of
free volume distribution
Hole Radius (nm)
„our toolbox“
positronium lifetime 3

measure for average free volume
width of lifetime distribution 3

measure of width of free volume distribution
Orthopositronium intensity I3

positronium formation probability *
concentration of free volume
Moderated pulsed positron beam

depth resolution possible
4/17
Positron beam at FRM II in Munich
NEPOMUC: Neutron induced Positron source Munich (Christoph Hugenschmidt)
PLEPS:
Pulsed Low Energy Positron System
(Werner Egger, Peter Sperr)
1 1.71
median implantation depth: zm  E
p
FWHM:  z  0.5 zm
zm (nm) ~ E
0
53
172
344
562
1.71
824 1125 1464 1840 2250
P(z, E)
1.0
P_1KEV
P_5KEV
P_10KEV
z
0.5
0.0
zm
0
2
4
6
8
10
E (keV)
12
14
16
18
J. Algers, P. Sperr, W. Egger, G. Kögel, F. Maurer
Phys. Rev. B (2003)
5/17
Outline
Motivation
Polymeric materials, free volume, interphases and interfaces
Free volume / Positron annihilation lifetime spectroscopy
Principle, conversion lifetime - volume size, experimental setup
positron beam (PLEPS @ NEPOMUC)
Thin Films (SPP Polymer-solid interfaces and Interphases)
evaporated and spin coated Teflon AF on Si-Wafer
change of free volume due to solid substrate, (coiling changed?)
Polymer Nanocomposites
free volume as function of filler concentration, mixing rule and side effects
Summary
6/17
schematic of polymer-solid contact
nanocomposite
property
{
solid
bulk value
Interphase polymer z
increased contribution
from interphase
char. length scale nm - µm
key property: free volume
SPP 1369 interfaces and Interphases
• controlled preparation
• property profiling
• modeling and simulation
Here: Free volume of Teflon AF 2400 as a function of distance to interface
S. Harms, K. Rätzke, V. Zaporojtchenko, F. Faupel, W. Egger, L. Ravelli, Polymer, 52 (2011) 505
7/17
Teflon AF 2400 evaporated onto Si
zm (nm) ~ E
evaporated, short chain length, no
relaxation, no interphase expected
0
53
172
344
562
1.71
824 1125 1464 1840 2250
15
3
Si substrate
I - exp. data
3
P(E=1ekV)
P(E=5ekV)
P(E=10ekV)
z
I3 (%)
0.5
5
zm
• E < 1 keV: surface effects
0
0
0.0
0
• 1 keV < E < 4/6 keV: bulk Teflon
3 AF2400 = 4 ns
“bulk”: 3 = 7 ns (M. Rudel)
2
2
4
4
6
6
8
8
10
E (keV)
10
E (keV)
12
12
14
16
14
18
16
5
24
4
20
16
 3 (ns)
3
• E > 4/6 keV: implantation into Si
3 remains constant
I3 decreases
33 mm
360 nm
717 nm
12
2
8
1
4
0
Reference sample, no interphase!
18
0
0
4
8
E (keV)
12
8/17
I 3 (%)
P(z, E)
I - calculation
Teflon
10AF 2400
1.0
Teflon AF 2400, spin coated
Aim:
Comparison with evaporated Teflon
7
30
220 nm
160 nm
6
110 nm
25
65 nm
5
20
TeflonAF spin coated
6
10
2
d = 220 nm
24
1
5
28
0
0
0
4
2
4
6
8
20
10
 3 (ns)
12
E (keV)
16
Interpretation:
• rearrangement of chains possible
• dcoil < 20-100nm = thicknessinterphase < dfilm
5
3
12
2
8
1
4
0
0
Remarks:
0
2
4
6
8
10
12
14
16
18
Same polymer, thick oxide layer (not shown)
E (keV)
 Clear influence of thick oxide layer on PALS, but not on interface width
Similar results for other polymers, temperature dependence (Tg profiling) planned
PALS suited to detect interphase width in thin films
9/17
14
I 3 (%)
15
3
I 3 (%)
Observations:
• Bulk value of 3 @ 2 keV
• voltage ↑  3 ↓
 3 (ns)
4
7
Outline
Motivation
Polymeric materials, free volume, interphases and interfaces
Free volume / Positron annihilation lifetime spectroscopy
Principle, conversion lifetime - volume size, experimental setup
positron beam (PLEPS @ NEPOMUC)
Thin Films (SPP Polymer-solid interfaces and Interphases)
evaporated and spin coated Teflon AF on Si-Wafer
change of free volume due to solid substrate, (coiling changed?)
Polymer Nanocomposites
free volume as function of filler concentration, mixing rule and side effects
Summary
10/17
Polymer-nanocomposites
Polymer:
polyethylenpropylen PEP, (deuterated)
Mw = 3000 g/mol (no entanglement)
Nanoparticles: SiOx
Filler:
Nanoparticles +
functionalized shell
Ø 18 – 20 nm
Our task: free volume as a function of temperature for various filler concentrations
We know:
system shows no change in dynamics at interphase (n-scattering)
We expect: - free volume: ↑
- dynamics: either ↑ (more free volume) or ↓ (less mobility)
S. Harms, K. Rätzke, F. Faupel, G. Schneider, L. Wöllner, D. Richter, Macromolecules, 43 (2010) 1050511/17
DSC, thermal analysis
Pyris DSC
Heating rate = 20 K/min
Tg from onset
Delta Cp from fit
1.0
-60
0.8
0.6
 cp
Results show clearly simple mixing
T g DSC (°C)
-65
0.4
-70
T
0.2
g DSC

cp
-75
0.0
0
10
20
30
40
50
60
filler concentration  (%)
12/17
Results: o-Ps lifetime = f(T, cfiller)
3.0
Lifetime3
Dispers.3
2.5
15%
18%
70

Intensity3
3.0
60
r
2.5
50
40
g
1.5
30
 3 (ns)
 3 (ns)

I 3 (%)
2.0
c
2.0
0
filler
6
1.0
18
20
1.5
0.5
35
10
60
100
0.0
0
-150
-100
T -50
g
0
T (°C)
50
1.0
-120
-80
-40
0
T (°C)
o-Ps lifetime ~ to hole size, mirrors macroscopic thermal expansion
Systematic behavior with concentration and temperature observable
=> Tg, glass, rubbery = f (cfiller)
13/17
Free volume and Intensity f (c) @ T = const.
3.0
t @ -120°C from fit T < T
3
2.5
I
3 mean
30
g PALS
@ 0°C  T  30°C
2.5
0
filler
6
I 3 (%)
c
2.0
 3 (ns)
 3 (ns)
20
2.0
18
1.5
10
35
60
1.5
100
1.0
-120
-80
-40
T (°C)
0
0
0
20
40
60
80
100
filler concentration (%)
Observation:
3 @ -120 °C   c filler   local disturbance of packing? No, see n-scattering
I3 = const  c filler   ?? Expected was decrease due to non-Ps forming nanoparticles
Ansatz: positrons, randomly implanted, do not annihilate in nanoparticles
diffuse out of nanoparticles and probe preferentially functionalized shell
14/17
Microscopic thermal expansion and glass transition
-50
T PEP
3.0
T
g
-60
-70
c
2.0
0
filler
6
18
1.5
T g PALS (°C)
 3 (ns)
2.5
g PALS
-80
-90
35
-100
60
100
1.0
-110
-120
-80
-40
T filler
g
0
0
T (°C)
40
60
80
100
filler concentration (%)
0.030

hg
0.025

hr
0.020
0.015
h
Observations:
Tg  cfiller 
glassy = const
rubbery   cfiller 
Ansatz for explanation:
Free volume is additive between functionalized shell
and polymer
No interphase needed for explanation
20
0.010
0.005
0.000
0
20
40
60
80
100
filler concentration (%)
15/17
Team, Cooperations & Sponsors
cooperations:
FRM II:
PD Dr. C. Hugenschmidt, Dr. W. Egger
nanocomposites: Dr. G. Schneider, Jülich and FRMII
funding:
DFG SPP 1369
BMBF Posimethod
Prof. Dr. Franz Faupel
Dipl.-Phys. Stephan Harms
MSc. Tönjes Koschine
MSc. Christian Ohrt
16/17
Summary
• PALS suitable method for investigation of free volume in polymers
average lifetime measure for average free volume

3

• TEFLON AF:
distribution of free volume at polymer-solid interface
no interphase for evaporated samples
clear interphase for spin-coated samples
• Polymer-nanocomposites
nanocomposites without interphase show deviation from mixing rule
interphase should be differentiated from functionalized shell
positrons probe preferentially functionalized shell
5
3.0
7
TeflonAF spin coated
24
6
d = 220 nm
4
28
24
20
2.5
5
20
12
2
8
4
16
3
12
 3 (ns)
717 nm
I 3 (%)
360 nm
 3 (ns)
 3 (ns)
33 mm
I 3 (%)
16
3
c
2.0
0
filler
6
18
2
8
4
1
4
0
0
0
1.5
35
1
60
100
0
0
4
8
E (keV)
12
0
2
4
6
8
10
E (keV)
12
14
16
18
1.0
-120
-80
-40
T (°C)
0
17/17