Alokmay DattaSaha Institute of Nuclear Physics,

1/AF Bidhannagar, Kolkata 700 064, India

How Morphology Changes Bonding in Soft Materials:

A Revelation through Synchrotron Studies

Co-workers

Saha Institute of Nuclear PhysicsSudeshna Chattopadhyay (Northwestern University, USA)Smita MukherjeeNupur Biswas

TASC-INFM National Laboratory and UniVersita` di Modena e Reggio EmiliaStefano NannaroneAngelo GigliaNicola MahneBryan Doyle

A Question about Soft Materials

Soft Materials show drastic change in morphology when confined to nanometer scales in all or any dimensionsFormation of monomolecular layers at air-water interface and their restructuring in presence of metal ionsFormation of molecular layers parallel to the surface in films of simple and complex fluids including polymers, below a certain film thicknessBonding or electron distribution in materials depend on the molecular conformation

Does change in morphology cause change in molecular conformation?

Experimental Techniques Used

Studies at Saha Institute•X-ray Reflectivity – Density Profile across the sample•Atomic Force Microscopy – Surface Topography and Surface Energy Distribution•Infrared Spectroscopy – Bonding and Conformation

Studies at Elettra•Vacuum Ultraviolet Spectroscopy – Conformation•Near Edge X-ray Absorption Fine Structure Spectroscopy – Bonding

The Three ‘Old’ States

Fluids: Simple and Complex

Simple FluidIntermolecular potential

1. Spherically symmetric2. Short range

Isotropic and Viscous Complex Fluid

Intermolecular potential1. Anisotropic

2. Long/short rangeAnisotropic and Visco-elastic

X-ray Reflectivity: Principles Reflectivity: Principles

•In x-ray region, refractive index n < 1, i.e., phase velocity of x-rays in material > phase velocity in vacuum.

total external reflection (specular reflection)Incident and scattered wave-vectors in same plane normal to

surfaceIncident angle () = scattered angle ()10-6, electron density, r0 classical electron radius ~

2.810-5 Å•n = (1-) =1-(r0 2/2 ) •qz = normal momentum transfer = kf - ki= 4/(sin)c = critical angle for sample film = (2 )½

z

x

At > c, x-rays penetrate into sample, are scattered for each change in , and these scattered x-rays interfere interference (Kiessig) fringes in reflectivity profile with periodicity 2/d, d = thickness of a layer with a constant , while amplitude of fringes change in

kt

ki kf

n = 1-

n=1

qz = 2/d

dAir

Film

Substrate

Interference (Kiessig) fringes with periodicity 2Interference (Kiessig) fringes with periodicity 2/d, d = /d, d = thickness of a layer with a constant thickness of a layer with a constant , while amplitude of , while amplitude of fringes fringes change in change in M. K Sanyal, A. Datta, S. Hazra, Pure Appl. Chem. 74, 1553 (2002).

The Reflectivity Profile

Mirror

Laser Diode

Focusing Lens

Piezo Scanner

Sample Holder

Integrator

Divider / Multiplier

Differential amplifier

4-quadrant PSPD

X-Y Translator

X Y

TipSampleCantilever

Forc

e

attractive force

distance(tip-to-sample )

repulsive force

non-contact

contact

Intermittent-contact

Multimode Nanoscope IV (Digital

Instruments)Intermittent-Contact (tapping) mode; Etched Si tip; Phosphorus-doped Si cantilever; Force constant 40N/m; Characteristic frequency 344kHz

Atomic Force Microscope

Layering in Simple Fluids: TEHOS

C.-J. Yu, A. G. Richter, A. Datta, M. K. Durbin, and P. Dutta, Phys. Rev.Lett. 82 , 2326 (1999). This work used the National Synchrotron Light Source, USA as the X-ray source

Layering in Complex Fluids: Polystyrene

Sample preparation: Spin CoatingSample preparation: Spin Coating

Spin Coating Unit, EC101, Headway Research

Thin films are prepared by putting a drop of solution in toluene on acid-washed quartz mounted on rotating vacuum chuck.

Film thickness can be varied by adjusting the rotation speed and concentration of the solution

Surface Energy Variation from Phase Measurement

000

sinkAAQE

AA D

SiPSSic

D Az

rE 203

2

2/12

2/12/1 4

Si

SiPSPS

Si

SiPSH A

AA

AA

A

Tip Parameters: = phase, (0) = working (resonant) frequency, A (A0) = set-point (free) amplitude, k = spring constant, Q = quality factor, ED = energy dissipated per cycle, rc = radius of curvature, Si = Si atomic radius, ASi = Si Hamaker constant z0 = Tip-sample separation,

ASiPS = Si-PS Hamaker constant, APS = Bulk PS Hamaker constant, AH = PS Hamaker constant in film

J. Tamayo and R. Garcia, Appl. Phys. Lett. 73, 2926 (1998).

First Indication of Layering

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.3510-10

10-9

10-8

10-7

10-6

1x10-5

1x10-4

10-3

10-2

10-1

100

101

Elec

tron

den

sity

,

z (Å)

Ref

lect

ivity

qz (Å-1)

0 200 400 600 800

0.28

0.32

0.36

0.40

0.26 0.28 0.30 0.32 0.34 0.36

500

400

300

200

100

0

Dept

h fro

m s

urfa

ce

Electron Density (Å-3)

~212 Å ~Rg

Rg is the radius of gyration ofPolystyrene, i.e. the size of thePolystyrene molecule in its most Disordered state

M. K. Sanyal, J. K. Basu, A. Datta and S. BanerjeeEurophys. Lett. 36, 265 (1996)

The Layering Transition in Polystyrene

Nanoconfined State: An Ordered State with Low Cohesion (Out-of-plane)

S.Chattopadhyay and A.Datta, Phys. Rev. B 72, 155418 (2005)

Reduction in cohesive energy caused by the variation of density due to layering AH= PS (max

2 - min2), = (max -

min), AH = Hamaker Constant

Lowering of In-plane Cohesion in Nanoconfined Polystyrene

Polystyrene Thickness

~7Rg (150 nm) 4Rg (84 nm) ~2Rg (50 nm)

PS = the change in PS Surface EnergyGPS –PS = the change in in-plane PS cohesion

S. Chattopadhyay and A. Datta, Macromolecules 40, 3613 (2007)

Intermolecular Potential in Nanoconfined State

From X-ray Reflectivity (Out-of-plane)

G (in mJm−2) AH (in J)/(2.1×10−21)Spatial variation in G fits the Modified Pöschl-Teller PotentialGPS−PS() = V0 cosh-2(/), = generalized co-ordinate, = range of potential

Polystyrene film thickness shown beside each curve

From Atomic Force Microscopy (In-plane)

Schematic Model for Nanoconfined Polystyrene

Nanoconfinement and Molecular Conformation

The non-zero dihedral angle has non-zero dipole moment, whereas the dipole moment vanishes as the dihedral angle becomes zero

Orientational Ordering of Benzene Rings on Confinement

The benzene ring ‘sandwich dimers’ are oriented 63° with the sample surface

Confinement versus Entanglement

PS D1/D2 (eÅ-3)

PS-1C 0.69 0.131

PS-5C† 1.061 0.045

PS-9C 0 0

D1 = out-of-plane periodicity as obtained from GIXR dataD2 = in-plane diameter of gyration ‘spheres’ as obtained from TM-AFM images = average difference between electron density maxima and minima in the layered ‘spheres’ as obtained from EDP

†Phy. Rev. B, 72, 155418 (2005)

0 as MW increases

Conclusions

Confinement causes change in morphology through change in molecular conformationsThese changes may give rise to new intermolecular potentialsThe new conformations and consequent forces are seen to lower the entropy by orientational orderingIn polymers increase in chain length increases ‘entanglement’, possibly a force opposing confinement induced changes