Smectic phases in polysilanes

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Smectic phases in polysilanes
Giorgio Cinacchi
Sabi Varga

• Kike Velasco

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polyethylene (organic polymer)...-CH2-CH2-CH2-CH2
-CH2-...
...
...
polysilane (inorganic polymer)
...-SiH2-SiH2-SiH2-SiH2-SiH2-...
...
...
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PD2MPS polyn-decyl-2-methylpropylsilane
L length m mass
1.96 x n A
16 A
hard rods vdW
s
persistence length l 85 nm
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PDI polydispersity index Mw/Mn
mass distribution
number distribution
number distribution
d
mi
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Chiral polysilanes (one-component)
Okoshi et al., Macromolecules 35, 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

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Non-chiral polysilanes (one - component)
Oka et al., Macromolecules 41, 7783 (2008)
DSC thermogram
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X rays
AFM
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NON-CHIRAL
9 7 16 15 34
32 39
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Freely-rotating spherocylinders
P. Bolhuis and D. Frenkel, J. Chem. Phys. 106,
666 (1997)
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Mixtures of parallel spherocylinders L1 / D 1
x 50
A. Stroobants, Phys. Rev. Lett. 69, 2388 (1992)
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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

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Previous results with more sophisticated model
Cinacchi et al., J. Chem. Phys. 121, 3854 (2004)

• Parsons-Lee approximation
• Includes orientational entropy

x
x
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Possible smectic structures for molar fraction x
close to 50
Inspired by experimental work of Okoshi et al.,
Macromolecules 42, 3443 (2009)
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Onsager theory for parallel cylinders
Varga et al., Mol. Phys. 107, 2481 (2009)
L2/L11.67
L2/L11.54
L2/L12.00
L2/L12.50
L2/L13.33
L2/L16.67
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Non-chiral polysilanes (two-component)
L11 (PDI1.11), L21.30 (PDI1.10) L2 / L1
1.30
S1 phase (standard smectic)
Okoshi et al., Macromolecules 42, 3443 (2009)
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L11 (PDI1.13), L22.09 (PDI1.15) 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
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L11 (PDI1.13), L22.09 (PDI1.15) L2 / L1
2.84
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x75
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x 75
1.7 lt r lt 2.8
S3
S1
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S1
S1
S2
S3
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Onsager theory
Parallel hard cylinders (only excluded volume
interactions). Mixture of two components with
different lengths
Free energy functional
Smectic phase
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Fourier expansion
excluded volume
Minimisation conditions
smectic order parameters
smectic layer spacing
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Conventional smectic S1
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Microsegregated smectic S2
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Two-in-one smectic S3
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Partially microsegregated smectic S4
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smectic period of S1 structure
L2/L11.54
L2/L11.32
L2/L11.11
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L2/L12.13
L2/L12.86
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x0.75
S3
S1
L1/L2
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experimental range where S3 phase exists
L1/L2
L1/L2
x
x
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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)

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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