Auxetic Polymeric Materials: Expanding Materials and Applications

Andy Alderson A.Alderson@shu.ac.uk

What are auxetics?

Poisson’s ratio:xy = -(y/x)

Auxetic Material: Material with a negative Poisson’s ratio

+ve xy

-ve xy

Am I in the right place?

The Poisson’s ratio scale

0 +0.5 -1

Cork

‘Typical’ materials

Rubber

a-cristobalite

Crystalline cellulose

Microporous polymers

Annulus fibrosus

Auxetic or ‘anti-rubber’ materials

Why are auxetics of interest?

• Usefulness of the novel property itself: – Counter-intuitive (‘opposite’) response

– High volume change

• A route to achieving unusual or extreme values of other material properties not easily achievable in conventional materials

Indentation resistance

Non-auxetic Auxetic

For isotropic materials:

and -1 < < +0.5

xE

H

21 • Auxetics: a route to enhancing other material properties, e.g.:

• Enhanced indentation resistance

• Enhanced fracture toughness

• Enhanced volumetric strain energy

dissipation (Uv)

• Enhanced shear modulus

• Ability to naturally adopt dome shape

when bent out of plane

E

U v6

21 2

212

r

ET

Curvature

Non-auxetic Auxetic

12

213KG

12

12

RR

Naturally-occurring auxetic biomaterials

• Increasing number of natural soft biological tissues reported to display auxetic behaviour o Cat skin

o Cow teat skin

o Bovine common carotid arteries

o Human achilles tendon

o Stem cells

o Early stage amphibian embryo tissue (gradient properties?)

o Human annulus fibrosis?

• Likely to have evolved for optimised specific functionality • Gradient structure and properties evident

(Chena & Brodland, Journal of the

Mechanical Behaviour of Biomedical

Materials 2 (2009) 494-501)

Auxetic cellular solids

Honeycombs

Re-entrant hexagon

LIGA auxetic honeycomb

structure - pore dimension ~

100mm

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C C

C C

C

C

C

C

C

C

C

C

C

C

C

Theoretical molecular structures -

pore dimension ~ 10Å

Femtosecond laser ablated

auxetic honeycomb structure -

pore dimension ~ 1mm

Star (connected Hoberman spheres analogy)

single ‘building block’

2 x 2 building block assembly

4 x 4 building block assembly

32 x 32 building block assembly

Rapid prototyped

auxetic honeycomb

structure - pore

dimension ~ 1cm

• A shape-changing (gradient) honeycomb – In-plane uniaxial loading – stress-induced shape change – Complex curvatures under out-of-plane bending

Auxetic cellular solids

Foams

• Cleanable/tuneable filters (BNFL)

• Anisotropic foams (QinetiQ)

• Large area flat and curved thin foams (Sara Lee Branded Apparel, Hanesbrands Inc & Auxetic Technologies Ltd)

• Detailed x-ray microtomography and modelling of structure-properties relationships (Univs of Manchester & Malta)

(A. Alderson et al, Phys. Stat. Sol. B 2007, 244 817.)

PCT Patent application, No. WO 99/22838

US Patent application, No. US 2003/0042176

PCT Pub. No. WO2007/052054,

US Patent Publication US2009119820

UK Patent Application No. 0821287.0

(S.A.McDonald et al, Phys. Stat. Sol. B, 248 (1) (2011) 45)

(S.McDonald et al, Script. Mater., 60 (2009) 232)

(Movie courtesy Sam McDonald, Univ. Manchester)

Impact testing: R60FR foam (Custom Foams),

2.27kg flat dropper, height = 0.1m

Unconverted (UC) Auxetic (0.7 LCR)

R60FR foam, 2.27kg flat dropper, height = 0.1m

Unconverted

Auxetic

R45 foam (Custom Foams), 2.09kg dome

dropper, 1mm PP shell, height = 0.1m

Unconverted Auxetic

Hemispherical dropper R45 foam

Unconverted

Auxetic

Auxetic microporous polymers

10 mm PTFE

PTFE Tape

• Auxetic effect observed in

expanded PTFE tape/ribbon

(Evans & Caddock, 1989)

• Auxetic effect arises due to

material microstructure

consisting of nodules and fibrils

• Batch processing route

(compaction, sintering &

extrusion) developed (K.

Alderson et al, 1992/6/8)

• Nodule-fibril microstructure

replicated in other microporous

polymers

• Auxetic cylinders of UHMWPE,

PP and nylon fabricated

A. Alderson & Evans, 1995/7

Veronda & Westmann, J. Biomechanics (1970)

Cat skin

In-plane

Thru’

thickness

Frohlich et al, J. Zool. Lond. (1994)

Veronda & Westmann, J. Biomechanics (1970)

Caddock & Evans, 1989

Ex-PTFE

Auxetic fibres and textiles

• Polypropylene fibres (Alderson

et al, 2002)

• Polyester fibres (Ravirala et al,

2005a)

• Polyamide fibres (Ravirala et

al, 2005a)

• Polypropylene films (Ravirala

et al, 2005b)

• Novel continuous melt extrusion process developed to

produce auxetic monfilaments and films

PP Film

PP Fibre

LAB-SCALE MELT EXTRUSION

University

PROCESS DEVELOPMENT & FIBRE DEMONSTRATORS

INCREASE RANGE OF POLYMERS

INDUSTRIAL-SCALE MELT EXTRUSION

APPLICATIONS

2 Ends x 2 Picks Woven

Fabric (Auxetic Monofilament)

1 x 1 Rib Knitted Fabric

(Auxetic Monofilament)

Fabrics incorporating Auxetic fibres

Non Woven Fabric (PP Non

Woven containing 40%

Auxetic Monofilament)

Naturally-occurring auxetic nanopolymers

Crystalline cellulose – Kraft cooked spruce

Experimental data

Tc [min]

Poisson’s ratios

zx

(before yield)

zx

(after yield)

120 -1.06±0.53 -0.98±0.46

150 -0.91±0.25 -1.00±0.25

180 -0.76±0.3 -0.86±0.25

210 -1.17±0.26 -1.05±0.26

240 -0.26±0.15 N/A

M. Peura, et al (2006),

Biomacromolecules, 7, 1521-1528

z x

z y

Molecular mechanics simulations on

cellulosic auxetic mechanism

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

-0.01 -0.005 0 0.005 0.01 0.015 0.02

z

ij

xy

yx

xz

zx

yz

zy

ij

Y. T Yao, PhD thesis (University of Bolton) 2010

0

5

10

15

20

25

-0.01 -0.005 0 0.005 0.01 0.015 0.02

z

|di| (o)

δ1

δ2

z (GPa)

C4-H...O2

d(D…A)

(Å)

2 2.491

1.5 2.491

1 2.491

0.5 2.491

0 2.491

-0.5 2.491

-1 2.491

-1.5 2.491

-2 2.491

• z (z) > 0

• x > 0 → zx < 0

• y < 0 → zy > 0

(a) Before rod rotation (b) After rod rotation

y

x

y

xx

b

a

d

b

a

b

a

d

b

a

Rigid spacers

Rigid rod

b

a

b

a

Rigid spacers

Rigid rod

Gradient auxetic

structures =

morphing wing

boxes

Auxetic composites = reduced

composite damage volume

for replacement

Auxetic

honeycombs =

curved nose cone

panels

Auxetic constituents in hybrid

adhesives = stiffer joints

‘Auxetic balancing’ = lightweight

composite structures through

reducing thermal distortion

1

MN

MX

XY

Z

-2.595

-2.062-1.529

-.996486-.463755

.068976.601707

1.1341.667

2.2

OCT 17 2007

15:29:59

NODAL SOLUTION

STEP=1

SUB =1

TIME=1

UZ (AVG)

RSYS=0

DMX =2.595

SMN =-2.595

SMX =2.2

1

MNMX

XY

Z

OCT 16 2007

13:56:32

NODAL SOLUTION

STEP=1

SUB =1

TIME=1

S1 (AVG)

Distorted (no auxetic)

No distortion (with auxetic)

Multifunctional auxetic

structures and systems =

vibration damping

Sector case study: Aerospace

US ARO

Sector case study: Healthcare

Auxetic foam pads will improve wearer

acceptance and compliance in hip protector

devices due to:

• improved comfort/fit (double curvature)

• enhanced energy absorption (impact

response)

• lower device weight and/or volume

Hip implants with auxetic mesh stems

provide

• improved match to bone mechanical

properties

• reduced ‘stress shielding’

• a compensation mechanism for

loosening of the stem over time -

extending device lifetime and time

between implant replacement operations

Deployable gradient

auxetic structures for

space creation and

organ retraction in

keyhole surgery

The 'smart bandage' concept delivers controlled drug release

from wound dressings in response to swelling of the wound

Gradient one-piece foams

mimic the concentric core-

sheath structure of the

natural intervertebral disc.

The auxetic sheath

reduces disc bulge under

compression to reduce

lower back pain

Acknowledgements • Prof Kim Alderson (The Open University)

• Dr Tom Allen (Sheffield Hallam University)

• Prof Subhash Anand (University of Bolton)

• Titus Augustine (CMFT)

• Dr Vicky Coenen (University of Bolton/Rolls-Royce)

• Dr James Corden (Trustech)

• Dr Phil Davies (University of Bolton)

• Prof Ken Evans (University of Exeter)

• Dr Leon Foster (Sheffield Hallam University)

• Trishan Hewage (Sheffield Hallam University)

• Dr Sam McDonald (University of Manchester)

• Dr Frank Nazare (University of Bolton)

• Dr Naveen Ravirala (University of Bolton)

• Dr Amit Rawal (Indian Institute of Technology)

• Mohammad Sanami (University of Bolton)

• Prof Fabrizio Scarpa (University of Bristol)

• Dignesh Shah (Sheffield Hallam University)

• Dr Ginny Simkins (University of Bolton)

• Jonathan Shepherd (Griffith University, Gold Coast, Australia)

• Dr Gill Smart (University of Bolton)

• Dr Muhammet Uzun (University of Bolton)

• Prof Phil Withers (University of Manchester)

• Dr Yong Tao Yao (Harbin Institute of Technology)