Smectic phases in polysilanes Sabi Varga Kike Velasco Giorgio Cinacchi.

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Smectic phases in polysilanes Sabi Varga Kike Velasco Giorgio Cinacchi

Transcript of Smectic phases in polysilanes Sabi Varga Kike Velasco Giorgio Cinacchi.

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

Smectic phases in polysilanes

Sabi Varga

Kike Velasco

Giorgio Cinacchi

Page 2: Smectic phases in polysilanes Sabi Varga Kike Velasco Giorgio Cinacchi.

polyethylene (organic polymer)

...-CH2-CH2-CH2-CH2-CH2-...

polysilane (inorganic polymer)

...-SiH2-SiH2-SiH2-SiH2-SiH2-...

... ...

......

Page 3: Smectic phases in polysilanes Sabi Varga Kike Velasco Giorgio Cinacchi.
Page 4: Smectic phases in polysilanes Sabi Varga Kike Velasco Giorgio Cinacchi.

PD2MPS = poly[n-decyl-2-methylpropylsilane]

1.96

x n

A

persistence length = 85 nm/correl. se

s

hard rods + vdW

16 A

L: length

m: mass

Page 5: Smectic phases in polysilanes Sabi Varga Kike Velasco Giorgio Cinacchi.

PDI = polydispersity index = Mw/Mn

2

2

)(

)(

iii

iii

ii

jj

i

ii

jjj

ii

ii

ni

ii

mi

n

m

n

w

mx

mx

mxx

mmxmx

m

m

m

m

M

MPDI

112

2

2

22

PDI

mx

mx

m

mm

iii

iii

mass distribution

number distribution

number distribution

lm

mi

Page 6: Smectic phases in polysilanes Sabi Varga Kike Velasco Giorgio Cinacchi.

• 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*

Ld

Okoshi et al., Macromolecules 35, 4556 (2002)

Page 7: Smectic phases in polysilanes Sabi Varga Kike Velasco Giorgio Cinacchi.

Non-chiral polysilanes (one - component)

DSC thermogram

Oka et al., Macromolecules 41, 7783 (2008)

Page 8: Smectic phases in polysilanes Sabi Varga Kike Velasco Giorgio Cinacchi.

X rays

AFM

Ld

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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)

Page 12: Smectic phases in polysilanes Sabi Varga Kike Velasco Giorgio Cinacchi.

Mixtures of parallel spherocylinders L1 / D = 1 x = 50%

A. Stroobants, Phys. Rev. Lett. 69, 2388 (1992)

Page 13: Smectic phases in polysilanes Sabi Varga Kike Velasco Giorgio Cinacchi.

MIXTURESHard rods of same diameter and different lengths L1, L2

If L1,L2 very different, for molar fraction x close to 50% there is strong macroscopic segregation

+

Page 14: Smectic phases in polysilanes Sabi Varga Kike Velasco Giorgio Cinacchi.

Previous results with more sophisticated model

x x

• Parsons-Lee approximation

• Includes orientational entropy

Cinacchi et al., J. Chem. Phys. 121, 3854 (2004)

Page 15: Smectic phases in polysilanes Sabi Varga Kike Velasco Giorgio Cinacchi.

Possible smectic structures for molar fraction x close to 50%

Inspired by experimental work of Okoshi et al., Macromolecules 42, 3443 (2009)

2/ 12 LL

Page 16: Smectic phases in polysilanes Sabi Varga Kike Velasco Giorgio Cinacchi.

L2/L1=1.54 L2/L1=1.67 L2/L1=2.00

L2/L1=2.50 L2/L1=3.33

L2/L1=6.67

Onsager theory for parallel cylindersVarga et al., Mol. Phys. 107, 2481 (2009)

Page 17: Smectic phases in polysilanes Sabi Varga Kike Velasco Giorgio Cinacchi.

L1=1 (PDI=1.11), L2=1.30 (PDI=1.10) L2 / L1 = 1.30

S1 phase(standard smectic)

Non-chiral polysilanes (two-component)

Okoshi et al., Macromolecules 42, 3443 (2009)

Page 18: Smectic phases in polysilanes Sabi Varga Kike Velasco Giorgio Cinacchi.

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

qd

2

Macroscopic phase segregation?

NO

• Peaks are shifted with x

• They are (001) and (002) reflections of the same periodicity

Two features:

Page 19: Smectic phases in polysilanes Sabi Varga Kike Velasco Giorgio Cinacchi.

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

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x=75%

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1.7 < r < 2.8

S3S1

x = 75%

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S1 S1

S2S3

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2

1 0

1)(log)(1

iii

did zzdz

dV

F

2

1, 0

2 )'(')(2

1

jiji

dex zdzzdzD

dV

F

Onsager theory

212121 ,,, exid FFF

Parallel hard cylinders (only excluded volume interactions). Mixture of two components with different lengths

Free energy functional:

Smectic phase:

'

2zz

LL ji

Page 25: Smectic phases in polysilanes Sabi Varga Kike Velasco Giorgio Cinacchi.

2

1 000

coslogcos1

1logi

N

kik

dN

jijiii

id kqzfjqzdzfdV

F

kq

LLkq

ffDVV

Fji

ji

N

kjkikji

ji

ijexcji

ex

2sin

2

1

2

1 2

1, 0

22

1,

)(

jiijexc LLDV 2)(

NjV

F

fV

F

f jj

,...,1 ,0 ,021

0

V

F

q

2,1 ,cos)(

)(0

ijqzfz

zfN

jij

i

ii

dq

2 2

ijij

f

Fourier expansion:

excluded volume:

smectic order parameters

smectic layer spacing

Minimisation conditions:

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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/L1=1.54

L2/L1=1.32

L2/L1=1.11

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L2/L1=2.13 L2/L1=2.86

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L1/L2

x=0.75

S3 S1

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L 1/L

2

L 1/L

2

x x

experimental range where S3 phase exists

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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)

),,',;'ˆ,ˆ,'( LLrrV

''rr

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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: