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