how morphology changes bonding in soft materials: a revelation through synchrotron studies
TRANSCRIPT
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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
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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
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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?
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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
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The Three ‘Old’ States
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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
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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
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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
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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
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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
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Layering in Complex Fluids: Polystyrene
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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
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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).
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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)
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The Layering Transition in Polystyrene
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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
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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)
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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)
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Schematic Model for Nanoconfined Polystyrene
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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
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Orientational Ordering of Benzene Rings on Confinement
The benzene ring ‘sandwich dimers’ are oriented 63° with the sample surface
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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
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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
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