1. THE NIST SANS USER RESEARCH

2. SANS RESEARCH TOPICS Boualem Hammouda National Institute of Standards and Technology Center for Neutron Research
1. SANS from Pluronics
2. Polymer Blend Thermodynamics
3. Helix-to-Coil Transition in DNA

3. 1. SANS FROM PLURONICS Pluronics are triblock copolymers: PEO-PPO-PEO
PEO: -CH2CH2O- is hydrophillic
PPO: -CH2CH2(CH3)O- is hydrophobic

4. P85 Pluronic forms micelles at high temperatures

5. GUINIER PLOT
I(Q) = I(0) exp(-Q2Rg2/3)
Guinier region
Guinier region

6. SANS from Pluronics Micelles
PEO
PPO
Polymer
Sphere
Sphere and polymer
low temperature
high temperature

7. SINGLE PARTICLE AND INTER-PARTICLE
STRUCTURE FACTORS
I(Q) = (NA/V)VA2 (bA/vA-bB/vB)2 P(Q) S(Q)
NA: number of particles, VA: particle volume, V: sample volume
(bA/vA-bB/vB)2 = contrast factor
P(Q): single-particle structure factor
S(Q): inter-particle structure factor
P(Q) = [F(QR)]2 = 3[sin(QR)/(QR)3-cos(QR)/(QR)2]2 for sphere of radius R.
P(Q) = 2[exp(-Q2Rg2)-1+Q2Rg2]/(Q2Rg2)2 for polymer of radius of gyration Rg.
S(Q) given by Percus Yevick model for solution of hard spheres.
S(Q) given by the Random Phase Approximation model for polymer mixtures.

8. Solution of Spheres Percus Yevick Model Solution of spheres
Single sphere
Solution of spheres

9. Solution of Spheres with Polymers
PPO
PEO

10. Concentrated Core-Shell Particles
Fit SANS Data to a Model of
Concentrated Core-Shell Particles
RA=42.6 Å
RB=71.4 Å
(b/v)A = 1*10-6 Å-2
(b/v)B = 5.9*10-6 Å-2
(b/v)C = 6.4*10-6 Å-2
core region A
shell region B
solvent region C
In the core:
2,795 PPO monomers
690 PEO monomers
490 D2O molecules
In the shell:
2,943 PEO monomers
34,167 D2O molecules
10% P85 Pluronic/D2O, 40 oC

11. 2. POLYMER BLENDS THERMODYNAMICS
SANS Intensity: I(Q) = dS(Q)/dW = (b1/v1-b2/v2)2 ST(Q)
Thermodynamics: ST-1(Q=0) = (1/kBT)(d2G/df12); G: Gibbs Free Energy.
The Random Phase Approximation:
ST-1(Q) = 1/(n1f1v1P(QRg1) + 1/(n2f2v2P(QRg2) -2 c12(T)/v0
Mixed polymer blend
Phase separated blend
1 nm
0.1 mm

12. Gibbs Free Energy

14. SANS FROM POLYMER BLEND MIXTURES
Polymers: Polyethylbutylene / Polymethylbutylene
hPEB -(C6H12)- / dPMB -(C5H5D5)-
Molecular Weights: Mw=44,100 g/mole Mw=88,400 g/mole
Volume Fractions: fhPEB=0.57 fdPMB=0.43

16. ZIMM PLOT

18. 1 nm
0.1 mm

19. 3. HELIX-TO-COIL TRANSITION IN DNA
DNA is the basic building block for life. It encodes for the synthesis of proteins. H P O C N HO
CH2 O O
Nucleotide
Phosphate group
Sugar
Base
Phosphate
group
Amine
bases OH
HOCH2
D-desoxyribose
Purines
Pyrimidines
Adenine (A)
Guanine (G)
Cytosine (C)
Thymine (T)
H3C
THE DNA MOLECULE

20. THE DNA HELIX A T G Major groove Pitch 30-40 Å Minor groove C
Repeat distance
per base pair=3.4 Å
Major groove
Minor groove

21. HELIX-TO-COIL TRANSITION IN DNAUV

22. SANS

23. POROD PLOT I(Q) ~ C/Qm

24. NONLINEAR LEAST-SQUARES FIT
Functional form: I(Q) = C/[1+(QL)m] + Background
C: solvation intensity
L: correlation length
m: Porod exponent

25. Solvation Intensity

26. Correlation Length
12.3 Å
8.5 Å

27. Porod Exponent
Reference: B. Hammouda and D. Worcester, “The DNA Denaturation Transition of DNA in Mixed Solvents”, Biophysical Journal (accepted 2006).

28. POROD EXPONENTS Porod region 1D object 2D object 3D object 1/Q1 1/Q2
MASS FRACTALS
SURFACE FRACTALS

29. CONCLUSIONS ACKNOWLEDGMENTS
-- The SANS technique is a valuable characterization method.
-- SANS has been effective in complex fluids, polymers, biology, etc.
-- SANS can determine structures, phase transitions, and morphology.
-- The NG3 SANS instrument at NIST gets over 150 users per year,
resulting in over 40 publications per year.
ACKNOWLEDGMENTS
NSF-DMR, Steve Kline, Nitash Balsara, David Worcester.
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