Transcript Smectic phases in polysilanes

Smectic phases in
polysilanes
Giorgio Cinacchi
Sabi Varga
Kike Velasco
polyethylene (organic polymer)
...-CH2-CH2-CH2-CH2-CH2-...
...
...
polysilane (inorganic polymer)
...-SiH2-SiH2-SiH2-SiH2-SiH2-...
...
...
1.96 x n A
PD2MPS = poly[n-decyl-2-methylpropylsilane]
16 A
L: length
m: mass
hard rods
+ vdW
s
persistence length l = 85 nm
correl. e  s / l
PDI = polydispersity index = Mw/Mn


 xm 
i i

mi

( m)
2
x
m
i

m
x
m

j
j



i
i
i i
mm 
Mw
j
  i
PDI 

 i (n)  
2
Mn
m n  i mi






  xi mi 
xi
i
i  x mi  i

number distribution
 j 
 j

mass distribution
number
distribution
m  m
2


m
mi
2
2

2
x
m
 i i
i


  xi mi 
 i

ml
2
 1  PDI  1
Chiral polysilanes (one-component)
Okoshi et al., Macromolecules 35, 4556 (2002)
• for small length polydispersity SmA phase
SAXS
• for large length polydispersity nematic*
• linear relation between polymer length and
smectic layer spacing
dL
SmA
Nem*
• Normal phase sequences as T is
varied:
isotropic-nematic*
isotropic-smectic A
• In intermediate polydispersity region:
isotropic-nematic*-smectic A
Non-chiral polysilanes (one - component)
Oka et al., Macromolecules 41, 7783 (2008)
DSC thermogram
dL
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 L1 / D = 1 x = 50%
A. Stroobants, Phys. Rev. Lett. 69, 2388 (1992)
MIXTURES
Hard 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
Cinacchi et al., J. Chem. Phys. 121, 3854 (2004)
• Parsons-Lee approximation
• Includes orientational entropy
x
x
Possible smectic structures for molar fraction x close to 50%
L2 / L1  2
Inspired by experimental work of Okoshi et al., Macromolecules 42, 3443 (2009)
Onsager theory for parallel cylinders
Varga et al., Mol. Phys. 107, 2481 (2009)
L2/L1=1.54
L2/L1=2.50
L2/L1=1.67
L2/L1=2.00
L2/L1=3.33
L2/L1=6.67
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., Macromolecules 42, 3443 (2009)
L1=1 (PDI=1.13), L2=2.09 (PDI=1.15)
L2 / L1 = 2.09
2
d
q
Macroscopic phase segregation?
NO
Two features:
• Peaks are shifted with x
• They are (001) and (002)
reflections of the same periodicity
L1=1 (PDI=1.13), L2=2.09 (PDI=1.15)
L2 / L1 = 2.84
x=75%
x = 75%
1.7 < r < 2.8
S1
S3
S1
S1
S3
S2
Onsager theory
Parallel hard cylinders (only excluded
volume interactions). Mixture of two
components with different lengths
Free energy functional:
F 1, 2   Fid 1, 2   Fex 1, 2 
Fid
Smectic phase:
d
1 2
   dzi ( z )log i ( z )  1
V
d i 1 0
Fex
V
d
2
1
2

D   dz  i ( z )  dz ' j ( z ' )
2d
i , j 1 0
 Li  L j

 
 z  z ' 
 2

i ( z ) N
  f ij cos jqz, i  1,2
Fourier expansion: fi ( z ) 
i
j 0
Fid
d
N
N
1 2
  i log i  1   i  f ij  dz cos jqz log  f ik cos kqz
V
d i 1 j 0 0
k 0
 Li  L j 

sin kq
2
2
N
2 
Fex 1
1
( ij )
  i  jVexc
 D 2  i  j  f ik f jk 
V
2 i , j 1
2
kq
i , j 1
k 0
excluded volume:
Minimisation conditions:
  F 
  F 

0
,



  0, j  1,...,N
f1 j  V 
f 2 j  V 
2
q
  F 
d

0
q  V 
smectic layer
spacing
(ij )
Vexc
 D2 Li  L j 
 ij 
f ij
2
smectic order
parameters
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
x=0.75
S3
S1
L1/L2
L1/L2
L1/L2
experimental range
where S3 phase exists
x
x
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

 
r  r'
  ˆ ˆ
V (r  r ' , , ' ; L, L' , ,  )
ˆ'

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: