Neutrons and Soft Matter Aurel RADULESCU Jülich Centre for Neutron Science JCNS, Outstation at MLZ,...
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Transcript of Neutrons and Soft Matter Aurel RADULESCU Jülich Centre for Neutron Science JCNS, Outstation at MLZ,...
Neutrons and Soft Matter
Aurel RADULESCUJülich Centre for Neutron Science JCNS, Outstation at MLZ, 85747 Garching, Germany
7 July 2014
2
Outline
• Soft Matter – definition, examples, applications
• Soft Materials – structural and dynamical properties
• Relevance of Neutron Scattering
• Small-Angle Neutron Scattering (SANS)
• Neutron Spin-Echo (NSE)
• SANS and NSE at JCNS and FZJ
• Conclusions
Soft Matter – Definition
Soft Materials
“molecular systems giving a strong response to very weak command signal” PG deGennes (1991)
- easily deformed by small external fields, including thermal stresses and thermal fluctuations- relevant energy scale comparable with RT thermal energy - subtle balance between energy and entropy rich phase behavior and spontaneous complexity
Soft Mattercrystalline state liquid state
structure: short range to long range orderdynamic response: elastic and viscous properties
Soft Materials
Soft Matter materials: common features
- structural units: much larger than atoms- large molecules, assemblies of molecules that move together
- large, nonlinear response to weak forces
- slow, non-equilibrium response
response time liquid ~ 10-9 spolymer or colloidal solution ~ 1 … 10-4 s
mechanical response rubbers elongated several hundred % of initial lenghtno linear relation between stress and strain
bulk modulus
shear modulus
Soft Matter – qualitative and quantitative
“Soft” – qualitative propertyshear modulus G – quantitative parameter
restoring force of a deformed material which
tends to recover its own shape (elastic materials)
“softness” – smallness of Gbulk modulus K of soft mater same order as for metals
Shear modulus Gmetals: some 10 GPasoft matter: < 0.1 GPaliquids: 0 Gpa
Bulk modulus Kmetals and soft matter: >1 GPa
Example: molecular vs macromolecular crystals
macromolecular (colloidal) crystals: molecule size ~1mmmolecular crystals (NaCl): unit size ~ 1Å unit size molecular crystal << unit size colloidal crystal
L
LG
L
F
2F – shearing forceDL – crystal deformationG ~ energy/(length)3
typical interaction energy ~ kBTGcolloidal crystal is 12 orders of magn. smaller than Gusual crystal
S. Kaufmann et al.J Mater Sci (2012) 47:4530–4539
Examples of soft matter systemsComplex fluids including colloids, polymers, surfactants, foams, gels, liquid crystals, granular and biological materials.
Y. Roiter and S. MinkoAFM
biological membrane
Soft-Matter Triangle
Applications – everyday life
Soft Matter – high-tech applications
understanding formation of nanoparticles: key for new products from detergents to cosmetics
tyres containing nanostructured aggregates: less energy to roll → save fuel
environmentally friendly cleaners
polymeric and soft composite materials as additives for oil industry
statistical „random walk“ effectsegment length: anumber of segments: Ncontour length: Na
Radius of gyration (average extension from the center of mass)
Full length contour:length of the stretched polymerL=((bond length)*(cos(109.47°-90°)/2))*(#C-1)
End-to-end length
N
RRR i
CMi
g
2
2
NaRee
6
1eeg RR
Static properties – statistical parameters
Polymer architecture
homopolymer
heteropolymer (diblock)
distance distribution function for different shapes
Polymer aggregates – shape
long-range repulsionR L aN
good solventR aN3/5
q-solventR aN1/2
poor solventR aN1/3
Polymer conformation
Monomer size a~0.1nmNumber of monomers N~102 – 1010 Contour length L~10nm – 1m
star-like block copolymer: n and m – number of repetitive units for the blue-solvophilic and the red solvophobic blocks
homopolymer
Polymer morphology
Morphologycal behavior of PEP-PEO in solution
polymer chains in the melt
each chain can be considered to be constrained within a tube –
topological constraintsRouse dynamics
local reptation
center-of-mass diffusion
3D Fickian diffusion
Dynamical properties
A. Wischnewski & D. Richter, Soft Matter vol. 1, 2006 Ed. G. Gompper & M. Schick
Dynamical properties – tube concept
Lateral confinement
Rouse model – dynamics of Gaussian chain at intermediate scale
Local reptation – random walk
Diffusion along the tube - reptation
Neutron Scattering – key in Soft-Matter
Length scale – Time scale
• Organic and biological compounds consist of primarily C, H, N, O
• Hydrogen (H) and Deuterium (D) scatter very differently
• Simple H/D substitution allows highlighting / masking structures
Ideal for Soft Matter
Neutrons exhibit very special properties
Scattering Theory
i
iA
A bV
1
Small-angle neutron scattering
Small-angle neutron scattering
intraparticle correlations
The form factor
hPS-dPB micelles (Fpol=0.25%) in different solvents for different contrasts
Contrast Variation
R. Lund et al., 2013
Experimental aspects – resolution and polydispersity
effect of asymmetry in MW
structure factor effect
PEP-PEO
J. Stellbrink et al., 2005
L. Willner et al., 2010
SANS - Examples
decoupling detectability of tiny velocity changes caused by the scattering process from the width of the incoming velocity distribution
the key is the neutron spin
/Dl l=10-20%
Neutron Spin-Echo
relaxation-type scattering, function of time
J – integral of the magnetic inductiong – gyromagnetic ratio
Neutron Spin-Echo
meaning of the scattering function
- deuterated polymer matrix containing a few % protonated chains → coherent single chain dynamics in the SANS regime
- sample containing only protonated chains → incoherent scattering function – self-correlation of protons of chain segments → segmental mean-square displacement <r2(t)>
Q=1nm-1
D. Richter et al., 1994
fit – Rouse model
Neutron Spin-Echo
Tube concept – pair correlation function of a single chain in the melt
A. Wischnewski et al., 2003
PEP melt, 492K
plateau – topological constraints
the only free parameter – the tube diameter: d=6nm
SANS and NSE at JCNS@MLZ
KWS-2 SANS diffractometer l=4.5 .. 20Å; /Dl l=2%..20%max. flux 2x108 ncm-2 s-1
Q-range: 1x10-4 .. 0.5Å-1 (with lenses)
J-NSE spectrometer l=4.5 .. 16Å; /Dl l=10%Fourier time range t=2ps.. 350ns
Phase behavior of C28H57-PEO
f=15%
fcc
f=30%
expected change in aggregation number Nagg → exploring the phase diagram
using chopper at KWS-2: solid-solid
phase transition
fcc → bcc observed
M. Amann et al., 2014
Conclusions
• Soft Matter Systems – great richness of properties, complex systems
• SANS – unique method for structural investigation
• NSE – unique method for dynamical investigation
• KWS-2 & J-NSE – dedicated neutron scattering instruments to soft-matter systems