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Polymer Chemistry, Hypervelocity Physics and the CASSINI Space
Mission
Prof. Steve Armes
Department of Chemistry, School of Life Sciences,University of Sussex, Falmer, Brighton, BN1 9QJ.
First,
some Polymer Chemistry…
What is a Polymer?
“Polymer” = many units
A polymer is a long-chain molecule comprising many “links” (identical repeat units)
Long-chain structure was not recognised until the 1920’s
Many commercial/industrial uses:
Sealant, adhesives, paints, coatings, artificial hip joints, printed circuit boards, guttering, contact lenses, food
packaging, car parts, bullet-proof vests, double glazing, non-stick frying pans
Synthesis and Properties of Polypyrrole
NH
FeCl3
H2O, 20 °C NN
NH
H
H
+
nCl -
Conductivity is 1-10 S cm-1
Reasonable environmental stability
Poor processability: insoluble and infusible
Conductivity (S cm–1)105
104
103
102
101
10-1
10-2
10-3
10-4
10-5
10-6
10-7
Au, Ag, CuHg
Graphite, (SN)xTTF - TCNQ
100carbon black
Metallic Regime
Ge
Si
H2O10-8
Met
als
Sem
icon
duct
ors
Doped polyacetylene
Doped polypyrrole
Doped polyaniline
Insulators:Conductivities are off-scale:
10-8 → 10-18 S cm-1
•Diamond
•Quartz
•“Normal” polymers e.g., Teflon, polystyrene, polyethylene etc.
The Nobel Prize for Chemistry in 2000
Alan J. Heeger Hideki ShirakawaAlan G. MacDiarmidUSAUSA Japan
“ for the discovery and development of conductive polymers”
What is a Colloid?
A colloid comprises tiny particles within a solid, liquid or gas
such that: 1 nm < COLLOID < 1000 nm
1 nm = one-billionth of a metre
[or approximately 10 atoms]
1000 nm = one-millionth of a metre
[or one-thousandth of a millimetre]
Everyday examples of colloids :- milk, latex paint, mayonnaise, shampoo, ice cream, bitumen, cassette tape, toothpaste
At Sussex University we make polymer colloids
i.e. microscopic particles of long-chain molecules
Polystyrene Latex Synthesis
Polystyrene [PS] latex particles prepared by free radical polymerisation in alcohol
R•
60 oC, PNVP, methanol
1 - 2 µm
PS Latex Particle
PS Latex Particle
PS Latex Particle
PNVPStyrene
PNVP
How big are these 1.5 µm polystyrene latex particles?Typical human hair width is 75 µm
A chain of 50 latex particles would be required to span the width of a human hair
Everyday Uses of Latex Particles
1. Biomedical diagnostic assays
2. Latex paints
3. Rubber gloves
4. Surface modification of textiles
5. High quality glossy paper
6. Additive for concrete
7. Pressure-sensitive adhesives
8. Cosmetic formulations
Synthesis of Polypyrrole-Coated Polystyrene Latex Particles
PS Latex PS Latex
S. F. Lascelles and S. P. Armes, Adv. Mater. (1995) and J. Mater. Chem. (1997)
pyrrole
FeCl3, water
1-2 µm
Ultrathin polypyrrole coating 1-2 µm
Can easily control the polypyrrole overlayer thickness from
1 to 20 nm simply by varying the latex particle concentration
Polypyrrole-coated polystyrene latex (6 wt. % polypyrrole)
Polyaniline-coated polystyrene latex (10 wt. % polyaniline)
Polypyrrole-coated poly(methyl methacrylate) latex
Verification of Core-Shell Particle Morphology
Polypyrrole overlayer
PS Latex particle
S. F. Lascelles et al., J. Mater. Chem. 1997, 7, 1349.
solvent extraction at 20 oC
???remove PS
• Polystyrene latex core is soluble in THF, polypyrrole coating is insoluble.
• Complete removal of the underlying latex core (confirmed by mass balance and IR spectroscopy)
• What is the morphology of the remaining PPY residues?
Polypyrrole-coated polystyrene latex before solvent extraction
Polypyrrole ‘broken egg-shell’ residues obtained after solvent extraction
Now,
the Hypervelocity Physics…
Hypervelocity regime is > 1 km sec-1 i.e. > 2,000 mph (or Mach 3 !)
Use of Conducting Polymer-Coated Latex Particles asModel Projectiles in Hypervelocity Impact Physics
In collaboration with Dr. M. J. Burchell’s group @ University of Kent, UK
D d +
+ +
+
+
+ +
+
+ +
+
+
+ +
charge-up
PPy-coated PS latexparticle of diameter ‘d’
Mass spectroscopyanalysis of impact plasma
acceleration
1.5 million volt field
ionicplasma Target
crater formedby hypervelocityimpact (D >> d)
charged latex particlesmoving at 1-35 km sec -1
Schematic representation of a hypervelocity impact experiment:
Uncoated polystyrene latex cannot be accelerated (since q = 0 in q.V = ½mv2)the conducting polymer coating is essential !
What happens during a Hypervelocity Impact?
Various Molecular
Fragments :-
PS Latex
n
Polypyrrole Overlayer
m/e
91
105
118
133
Impact at ν = 3 km sec-1
Before Impact After Impact
Molecular plasma suggests chemical bonds are broken?(and at least 90 % of the projectile mass is the polystyrene latex)
Kinetic Energy Calculation
Kinetic Energy = ½ mass.(velocity) 2
Mass of 1.8 µm PPy-coated PS latex ~ 5.5 x 10-15 kg or 5.5 picograms
ν ~ 3 km s-1 ~ 3000 m s-1 ~ 6,700 mph ~ Mach 9 !
For a typical hypervelocity impact:-
Thus Kinetic Energy = ½ x 5.5 x 10-15 x (3,000)2
= 2.5 x 10-8 Joules = 2.5 x 10-11 kJ (tiny!)
So Kinetic Energy per styrene residue ~ 470 kJ mol -1
But number of moles of styrene per latex particle ~
5.5 x 10-12
104~ 5.3 x 10-14
Hence we have sufficient energy to break bonds and do some chemistry!
Fragmentation of Polystyrene Chains
n
CH2 CH2CH2 CH CH CH
+ H+
C8H9+
m/e = 105
- e -
C9H10+
m/e = 118
+ 2H+ - e-
C10H13+
m/e = 133
+ H+
C7H7+
m/e = 91
Effect of Impact Velocity on Plasma Mass Spectra obtained from a Polypyrrole-coated Polystyrene Latex
~ 5 km sec-1
Molecular plasma
~ 9 km sec-1
Atomic plasma
Finally,
Some Space Science……
Various Types of Micro-meteorites:
Metallic (iron, nickel) Ice
Carbonaceous Silicates
Despite their broad size distributions, iron particles have been extensively used to mimic the behaviour of metallic micro-meteorites in lab-based experiments at U. Kent
There are no good synthetic mimics available for carbonaceous, silicate or ice-based micro-meteorites………
Electrical insulators cannot be easily charged up for acceleration in the van de Graaf instrument !
The competition: horribly polydisperse iron particles !
The CASSINI Space Mission (launched October 1997)
Cosmic Dust Analyser or ‘CDA’(duplicate detector at U. Kent)
CASSINI Flight Path
SUN
Launch Oct 1997
Arrival July 2004
VenusEarth
Mars
Jupiter
Saturn
CASSINI missionOBJECTIVE:Analyse micro-meteorites on its voyage through Solar System and
investigate the elemental composition of Saturn’s rings on arrival.
PROBLEM:
Cosmic Dust Detector is not yet calibrated……
SOLUTION:Calibrate duplicate detector @ University of Kent with a range of model
projectiles of known chemical composition during CASSINI’s voyage.
Conducting Polymer-Coated Latexes are ideal “model”
projectiles with variable sizes, compositions and densities!
Properties of Sussex ConductingPolymer-Based Latex Particles:
Variable Particle Diameter (100 to 5000 nm)
Narrow Particle Size Distributions (unlike iron !)
Low Particle Densities (1.06 to 1.50 g cm-3)
High carbon contents (up to 90 % carbon)
Can be readily accelerated up to hypervelocities (1 – 35 km sec-1)
Excellent mimics for carbonaceous micro-meteorites !
Recent Results from Sussex/Kent collaboration
M. J. Burchell et al. Planetary and Space Science (2002)
A wide range of model conducting polymer-based projectiles canbe accelerated. Elements include: C, Si, Br, Sn, …..(S ?).
Now have good synthetic mimics for both carbonaceousand silicate micro-meteorites.
Get higher hypervelocities with smaller projectiles.
M. J. Burchell et al. Astronomy and Astrophysics (2003)
Observed mass spectra are due to fragmentation of the latex core(90-95 % by mass), rather than the conducting polymer coating (5-10 %).
Get molecular ionic plasma at low hypervelocities (1-8 km s-1).
Get atomic ionic plasma at higher hypervelocities (> 10-12 km s-1).
Conclusions1. Conducting polymer-coated latexes are interesting new
“model” projectiles for Hypervelocity Impact experiments
2. Advantages over conventional iron projectiles include:
* Narrow size distributions
* Tunable size
* Variable chemical composition, density
3. Hypervelocity Impact experiments confirm molecular fragments at low hypervelocities. Can access high hypervelocities: (35 km s-1 or 70,000 mph!)
4. Implications for Space Science?
Proper calibration of Cosmic Dust Detector on CASSINI should lead to an improved understanding of the chemical composition of micro-meteorites and Saturn’s rings?
AcknowledgementsU. Sussex:Dr. Stuart LascellesDr. M. A. KhanDr. M. J. PercyDr. D. B. Cairns
Also: Dave Randall, our electron microscopy technician
U. Kent:Dr. J. Goldsworthy
andDr. M. J. Burchell
£££: EPSRC, DERA, DSM Research, PPARC, NASA/ESA
Finally: Dr. J. V. M. Weaver for helping to prepare this talk.
VUT, Australia:Dr. S. W. Bigger (interpretationof mass spectra)
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