Smectic phases in polysilanes Sabi Varga Kike Velasco Giorgio Cinacchi

polyethylene (organic polymer)...-CH 2 -CH 2 -CH 2 -CH 2 -CH polysilane (inorganic polymer)...-SiH 2 -SiH 2 -SiH 2 -SiH 2 -SiH

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

PDI = polydispersity index = M w /M n mass distribution number distribution  mimi

for small length polydispersity SmA phase for large length polydispersity nematic* linear relation between polymer length and smectic layer spacing Chiral polysilanes (one-component) SAXS Normal phase sequences as T is varied: isotropic-nematic* isotropic-smectic A In intermediate polydispersity region: isotropic-nematic*-smectic A SmA Nem* Okoshi et al., Macromolecules 35, 4556 (2002)

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

X rays AFM

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

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

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

MIXTURES Hard rods of same diameter and different lengths L 1, L 2 If L 1,L 2 very different, for molar fraction x close to 50% there is strong macroscopic segregation +

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

Possible smectic structures for molar fraction x close to 50% Inspired by experimental work of Okoshi et al., Macromolecules 42, 3443 (2009)

L 2 /L 1 =1.54 L 2 /L 1 =1.67 L 2 /L 1 =2.00 L 2 /L 1 =2.50L 2 /L 1 =3.33 L 2 /L 1 =6.67 Onsager theory for parallel cylinders Varga et al., Mol. Phys. 107, 2481 (2009)

L 1 =1 (PDI=1.11), L 2 =1.30 (PDI=1.10) L 2 / L 1 = 1.30 S 1 phase (standard smectic) Non-chiral polysilanes (two-component) Okoshi et al., Macromolecules 42, 3443 (2009)

L 1 =1 (PDI=1.13), L 2 =2.09 (PDI=1.15) L 2 / L 1 = 2.09 Macroscopic phase segregation? NO Peaks are shifted with x They are (001) and (002) reflections of the same periodicity Two features:

L 1 =1 (PDI=1.13), L 2 =2.09 (PDI=1.15) L 2 / L 1 = 2.84

x=75%

1.7 < r < 2.8 S3S3 S1S1 x = 75%

S1S1 S1S1 S2S2 S3S3

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

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

Conventional smectic S 1

Microsegregated smectic S 2

Two-in-one smectic S 3

Partially microsegregated smectic S 4

smectic period of S 1 structure L 2 /L 1 =1.54 L 2 /L 1 =1.32 L 2 /L 1 =1.11

L 2 /L 1 =2.13 L 2 /L 1 =2.86

L 1 /L 2 x=0.75 S3S3 S1S1

L 1 /L 2 xx experimental range where S 3 phase exists

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)

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: