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-...

... ...

......

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

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

• 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)

Non-chiral polysilanes (one - component)

DSC thermogram

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

X rays

AFM

Ld

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 L1 / D = 1 x = 50%

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

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

+

Previous results with more sophisticated model

x 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)

2/ 12 LL

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)

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)

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:

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

x=75%

1.7 < r < 2.8

S3S1

x = 75%

S1 S1

S2S3

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

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:

Conventional smectic S1

Microsegregated smectic S2

Two-in-one smectic S3

Partially microsegregated smectic S4

smectic period of S1 structure

L2/L1=1.54

L2/L1=1.32

L2/L1=1.11

L2/L1=2.13 L2/L1=2.86

L1/L2

x=0.75

S3 S1

L 1/L

2

L 1/L

2

x x

experimental range where S3 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)

),,',;'ˆ,ˆ,'( LLrrV

''rr

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: