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Smectic phases in polysilanes

Giorgio Cinacchi. Smectic phases in polysilanes. Sabi Varga. Kike Velasco. polyethylene (organic polymer) ...-CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -. polysilane (inorganic polymer) ...-SiH 2 -SiH 2 -SiH 2 -SiH 2 -SiH 2 -. PD2MPS = poly[n-decyl-2-methylpropylsilane]. L : length m : mass.

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Smectic phases in polysilanes

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  1. Giorgio Cinacchi Smectic phases in polysilanes Sabi Varga Kike Velasco

  2. polyethylene(organic polymer)...-CH2-CH2-CH2-CH2-CH2-... ... ... polysilane(inorganic polymer) ...-SiH2-SiH2-SiH2-SiH2-SiH2-... ... ...

  3. PD2MPS = poly[n-decyl-2-methylpropylsilane] L: length m: mass 1.96 x n A 16 A hard rods + vdW s persistence length l = 85 nm

  4. PDI= polydispersity index = Mw/Mn mass distribution number distribution number distribution d mi

  5. Chiral polysilanes (one-component) Okoshi et al., Macromolecules35, 4556 (2002) SAXS • for small length polydispersity SmA phase • for large length polydispersity nematic* • linear relation between polymer length and • smectic layer spacing SmA Nem* • Normal phase sequences as T is • varied: • isotropic-nematic* • isotropic-smectic A • In intermediate polydispersity region: • isotropic-nematic*-smectic A

  6. Non-chiral polysilanes (one - component) Oka et al., Macromolecules 41, 7783 (2008) DSC thermogram

  7. X rays AFM

  8. NON-CHIRAL 9% 7% 16% 15% 34% 32% 39%

  9. Freely-rotating spherocylinders P. Bolhuis and D. Frenkel, J. Chem. Phys.106, 666 (1997)

  10. Mixtures of parallel spherocylinders L1 / D = 1 x = 50% A. Stroobants, Phys. Rev. Lett.69, 2388 (1992)

  11. MIXTURES Hard rods of same diameter and different lengths L1, L2If L1,L2 very different,for molar fraction x close to 50% there is strong macroscopic segregation +

  12. Previous results with more sophisticated model Cinacchi et al., J. Chem. Phys.121, 3854 (2004) • Parsons-Lee approximation • Includes orientational entropy x x

  13. Possible smectic structures for molar fraction x close to 50% Inspired by experimental work of Okoshi et al., Macromolecules42, 3443 (2009)

  14. Onsager theory for parallel cylinders Varga et al., Mol. Phys.107, 2481 (2009) L2/L1=1.67 L2/L1=1.54 L2/L1=2.00 L2/L1=2.50 L2/L1=3.33 L2/L1=6.67

  15. Non-chiral polysilanes (two-component) L1=1 (PDI=1.11), L2=1.30 (PDI=1.10) L2 / L1 = 1.30 S1 phase (standard smectic) Okoshi et al., Macromolecules42, 3443 (2009)

  16. L1=1 (PDI=1.13), L2=2.09 (PDI=1.15) L2 / L1 = 2.09 Macroscopic phase segregation? NO • Peaks are shifted with x • They are (001) and (002) • reflections of the same periodicity Two features:

  17. L1=1 (PDI=1.13), L2=2.09 (PDI=1.15) L2 / L1 = 2.84

  18. x=75%

  19. x = 75% 1.7 < r < 2.8 S3 S1

  20. S1 S1 S3 S2

  21. Onsager theory Parallel hard cylinders (only excluded volume interactions). Mixture of two components with different lengths Free energy functional: Smectic phase:

  22. Fourier expansion: excluded volume: Minimisation conditions: smectic order parameters smectic layer spacing

  23. Conventional smectic S1

  24. Microsegregated smectic S2

  25. Two-in-one smectic S3

  26. Partially microsegregated smectic S4

  27. smectic period of S1 structure L2/L1=1.54 L2/L1=1.32 L2/L1=1.11

  28. L2/L1=2.13 L2/L1=2.86

  29. x=0.75 S3 S1 L1/L2

  30. experimental range where S3 phase exists L1/L2 L1/L2 x x

  31. Future work: • improve hard model (FMF) to better represent period • check rigidity by simulation • incorporate polydispersity into the model • incorporate attraction in the theory • (continuous square-well model)

  32. Let's take a look at the element silicon for a moment. You can see that it's right beneath carbon in the periodic chart. As you may remember, elements in the same column or group on the periodic chart often have very similar properties. So, if carbon can form long polymer chains, then silicon should be able to as well. Right? Right. It took a long time to make it happen, but silicon atoms have been made into long polymer chains. It was in the 1920's and 30's that chemists began to figure out that organic polymers were made of long carbon chains, but serious investigation of polysilanes wasn't carried out until the late seventies. Earlier, in 1949, about the same time that novelist Kurt Vonnegut was working for the public relations department at General Electric, C.A. Burkhard was working in G.E.'s research and development department. He invented a polysilane called polydimethylsilane, but it wasn't much good for anything. It looked like this:

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