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)