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X-ray study of the comb-like polysiloxane at the different phase states
A. Alexandrov, T. Pashkova, B. Krücke, S. Kostromin, V. Shibaev
To cite this version:
A. Alexandrov, T. Pashkova, B. Krücke, S. Kostromin, V. Shibaev. X-ray study of the comb-like polysiloxane at the different phase states. Journal de Physique II, EDP Sciences, 1991, 1 (8), pp.939- 948. �10.1051/jp2:1991118�. �jpa-00247566�
J Phys II France 1 (1991) 939-948 Ao0T 1991, PAGE 939
Classification Physics Abstracts
61 30E 6140K
Proofs not corrected by the authors
X-ray study of the comb-like polysiloxane at the different phase
states
A. I. Alexandrov (I), T V. Pashkova (~), B Krbcke ~), S. G. Kostromin (3) and V. P.
Shibaev (3.*
(I) Ivanovo State University, Department of Physics, Ivanovo, 153377 US-S R (2) Martin-Luther-Universitat, Halle-Wittenberg, Sektion Chemle, Halle, DDR-4050
(3) M V Lomonosov Moscow State University, Department of Chemistry, Moscow 119899,
US S R
(Received11 July 1990, revised 2 May 1991, accepted15 May 1991)
Abstract. Structure of comb~shaped liquid crystalline polysiloxane with phenyl benzoate mesogeluc side group has been studied The analysis of the scattenng intensity and calculation of the Patterson function of polymer 8amples onented in the magnetic field over a wide temperature
range have been performed On the base of these data the structural model of mesogenic groups packing has been suggested
Introducfion-
The structural studies of the comb~like liquid crystalline (LC) polymers cawed out m recent years [1-5] have proved the s1rnllarity between the packing modes of the mesogenic fragments
and mesogenlc low molar mass liquid crystals. It has been shown that the vast majority of the comb-like LC polymers are charactenzed by the same types of mesophases as the low molar
mass liquid crystals now there are about ten different polymorphic modifications of the
smectic phases including the smectics A, B, C, E, F, etc, and there were obtained also the
nematic and cholestenc polymers
It is worth to mention that a polymenc backbone chain which connects all mesogemc groups into a structurally organized ensemble provJdes a number of the structural peculianties
of LC polymers. It has been clearly demonstrated m the reports on neutron study of some
magnetically oriented methacryhc [6, 7] and siloxane [8, 9] polymers containing deuterated backbone or spacer, respectively Although important information concerning the degree of the polymer coil asymmetry has been obtained by means of the above mentioned technique,
nevertheless one needs additional data in order to develop a universal structural model of LC
polymer taking into account the backbone conformation It will be noted that, from the structural vJewpoint, the comb-like polysiloxanes attract a special attention since the presence
~f yhc~n atoms m the backbone which are charactenzed by a greater scattering ability in (*) To whom all correspondence should be sent.
companson with the carbon ones can give us an additional opportunity to observe the backbone by means of the X-ray scattenng technique. In the early studies of the onented comb~hke polysiloxanes [3] only the geometry of the X~ray patterns has been analyzed, therefore no explicit information concerning the backbone conformation has been obtained.
In this work the structure of one of the comb~hke polysiloxanes PS~3,3 (1) has been studied
-(Si(CH~)-O)~g- (PS-3,3)
(CH~)~R-CjH~-OOC-~H~©-CjH~
and the analysis of the scattenng intensity and calculation of Patterson's function of PS~3,3 samples onented in the magnetic field over a wide temperature range have been performed.
Experhnental.
Poly methyL(3-~p~~p~propoxyphenylcarboxy)phenyloxy]propyl) siloxane (PS~3,3) (degree of
polyrnemation is equal to 36) was synthesized according to the technique reported in [[[j.
X~ray investigations were carried out in the special cell designed on the basis of X-ray apparatus URS~2 equipped with an electromagnet EM~I using the flat and cyhndncal films
(Ni~filtered CUK~-irradiation was used). The diffraction effects due to the presence of the white irradiation in the beam were revealed with the help of the set of X-ray diffraction patterns obtained at different voltages supplied to the X-ray tube Thus we reject the diffuse equatonal reflection with spacings 15 and 8.6 A.
PS-3,3 sample was onented in the magnetic field (15 000 Gs) under the cooling from the isotropic phase. Within the temperature range from 166 °C to room temperature we observed two types of X~ray pattems of PS-3,3 (Fig. la, b) and then the identification of the
polyrnorphlc states was camed out. The polymorphism scheme can be represented as follows
74'C 166 'C
K # S,, -
59 "C
The phase transition temperatures were determined through the change of the X~ray diffraction pattems within the accuracy of ± 0.5 °C
Results and discussion.
X-ray diffraction pattem of PS~3,3 at room temperature onented in the magnetic field is
presented in figure la. It was found that this type of X-ray pattern does not change under the
heating of the sample up to 74 °C
The formation of the crystalline lattice is proved by the exJstence of a number of maxima on layer lines of X-ray diffraction patterns. At the same time this sample has an axJal texture if the magnetic field direction is parallel to X~ray beam, X~ray diffraction pattem are
charactenzed by the presence of the nngs similar to X-ray diffraction patterns of the powder samples (Fig. 2).
The diffractional and corresponding structural parameters of the sample at different temperatures are presented in table I. The indexing of reflections on X-ray diffraction patterns provides a possibility to estimate the parameters of the crystalline lattice Thls lattice
was found to be orthorombic one
,
a = 8.02 A, b
= 10.86 A,
c = 47 6 A. In this
case the axis
« C » and texture axis coincide. The analysis of the interference indexes gives the following (') These figures correspond to the number of the methylene fragments m the « spacer » and to the number of carbon atoms m the terminal oxyaliphatic group of the mesogeluc fragment, respecuvely
bt 8 X-RAY STUDY OF COMB-LIKE POLYSILOXANE 941
a)
b)
Fig. I.-X~ray di~racuon pattems of the magnet1caIIy onented samples PS-3,3 at 25 °C (a) and 80 °C ~b) X~ray beam is perpendicular to the magnetic field direcuon.
rule : K + L
=
2N for OKL and H
= 2N (or H + L
= 2N~ for HOL and there are no
restnctions for indexes HKL and HKO. According to reference [12] and table I it corresponds
to the following space groups D(=Pnam and ll~~=Pna2 (or D(=Pnnm and
ll~$ = Pnn2).
In order to determine the confornlations of the mesogenic groups and to establish the structural motive of the packing of the macromolecular fragnients the cylindrically symmetri~
Cal Patterson function Q(r, z) [13] was calculated as follows
im
Q (R, z m
= 2 I(Ji~ Z) Jo(2 arrR) cos (2 1rzZ) 2 1rR dR dZ
o o
where I(R, Z) is scattered intensity, Jo (21rRr) is the Bessel function of zero order r, z and Ji~ Z are the coor¬es of the direct and reciprocal spaces, respectively. The arrays of the intensities used in the calculations were obtained by densitometry of X-ray patterns by means
of the nficrodensitometer PDS1010A « Perkin Elmer ». Afterwards the above arrays of the
Fig. 2 X~ray diffraction pattem of the magneucally onented sample PS~3,3 at 25 °C; X~ray beam is
parallel to the magnetic direcuon.
intensifies were corrected with the account for the background due to the air scattering and for the geometry, polarization and absorption. The analysis of Q (r, z) was camed out via tile
_~
selGconvolution of the macromolecule fragment p~,(r, z) which contains one mesogenic
2
group with the spacer group and one monomer unit of the backbone. p (r, =) was obtained
according to the vector method [14] and then was fitted to Q (r, z varying the conformations of the macromolecule fragment. The various locations of the planes of the benzene rings
towards each other and towards the other pans of the side fragnient were also checked up.
The deviations of the side chain_ from the stretched conformation were taken into
2
consideration too. The superposition of p(r, =) and Q(r,z) substantially simplifies the
separation of the multiple peaks of the latter function and deternfinatlon of the structural motive.
The conformation of the macromolecule fragment, corresponding to it, self-convolution and the Patterson map are presented in figure 3a~c. One can easily observe the coincidence of
2
the mOSt pronoUnced maXbDa Of p(I,c) and Q(r,Z) associated With the Stretched conformation of the mesogenic group when the benzene nngs are located in the co-planar planes and the angle between these planes and the plane, where COO-group, spacer group
and « tail » he, is equal to 36°. Thus separated characteristic sets of the maxima in the
adjacent coordination zones of the Patterson map are found to be shifted by the length of the
mesogenic group (-211). A sinfilar situation is also observed for those maxJma that
correspond to the interatomic distances between the identical atoms of the different
mesogenic groups marked by tile crosses in figure 3c. As for the maxbna corresponding to the
bt 8 X-RAY STUDY OF COMB-LIKE POLYSILOXANE 943
Table I Dffractional andstructural characteristics ofthe magnetically oriented sample PS- 3,3.
Temperature hkf or 8, Interplanal A(21J) x 102 gj L Intensity,
renecuon grad. distance, A rdd % A conditional
number units
25 °C 110 6.855 6 46 ± 0 05 16
120 9 843 4.51 ± 0.05 354 5 0 104 90
200 Ii.085 4.01 ± 0.05 38
210 11.742 3.77 ± 0.04 8
220 13 854 3.22 ± 0.04 7
040 16.211 2.76 ± 0.03 2
310 17.150 2.61 ± 0 03 2
320 18.685 2.41 ± 0 03 3
240 19.793 2 28 ± 0.03 2
011 4 171 10 60 ± 0 08 14
121 9.888 4.49 ± 0.05 153
002 1.856 23 8 ± 1.0 0.79 1.5 196 250
112 7.109 6.23 ± 0.05 7
202 11.227 3.96 ± 0 05 130
013 4.97 8 90 ± 0 08 7
213 12.127 3.67 ± 0.04 45
004 3 684 12.0 ± 0.3 0.92 90
024 8.979 4 94 ± 0.05 7
006 5 530 8.00 ± 0 08 102
008 7 489 5.90 ± 0 05 26
0010 9 361 4.74 ± 0 05 10
0012 II.285 3.94 ± 0.04 6
0014 13 186 3.38 ± 0.04 4
0016 15 043 2.97 ± 0.03 3
0018 16.784 2.67 ± 0.03 2
0020 19.235 2.34 ± 0 03 2
0022 20.715 2.18 ± 0 03
0024 22.670 2 00 ± 0 03
85 °C Ml j 793 24.6 ± 0 0 87 1.9 178 76
Ml 3 628 12 19 ± 0.30 10 84
M,* 5.460 8 10 ± 0 08 24
Ml 7 301 6 07 ± 0 05 3
E** 9 844 4.51 ± 0.05 2.818 10 80 63
M* mendional reflections, E** equatonal reflection
interatomic distances along the backbone, the contnbution from which is increased due to Si- Si-atoms existence, they can be easily identified on the Patterson map by means of the self- convolution of the backbone fragment corresponding to Si~si distances They are located within the region of the shaded maxima in figure 3c. The regular arrangement of the above maxima fitting the general structural motive allows one to suppose that the backbone is crystallized together with the mesogenic groups. Th1s conclusion seems to be quite essential
z
,
I
4s o
40
35
25
a 20
". 0
5 3 5 lo 15 p (
a b c
Fig 3 -a) The conformation of the side chain mesogemc group ; b) the self-convolution of the macromolecular fragment 2/p(r, z), c) cylindrically synJrnetnc Patterson function Q(r, z)
since the reflections which drop out from the indexing scheme based on the orthorhombic lattice are absent in X~ray pattems of polysiloxane.
The consideration of the possible packing types of the macromolecule fragments with the
account for symmetry elements of the spatial groups Pnam, Pna2, Pnnm and Pnn2 and the
results of Q(r, z) analysis reveal that the packing of the mesogemc groups together with the backbone fragment to form the orthorhombic cell corresponds to the spatial group Pnam
(Fig. 4). However one should mention that since the length of the backbone doesn't exceed
loo1(36
monomer units) being macroscopically in the state of the anisotropic coil (see
neutron scattenng data [7j) its crystallization may have only the local character. The chain
bendings m the plane perpendicular to the texture axis and the terminal groups of the backbone can result m the formation of the defects. It was found that the crystalline sizes are
dependent on these defects, and as a result the fine~grained crystalline polymer structure is
bt 8 X-RAY STUDY OF COMB-LIKE POLYSILOXANE 945
i
$
,
c
Fig. 4 -Scheme of the backbone and side chain packing of PS-3,3 macromolecule in elementary crystalline cell
formed. Actually, the evaluation of the crystallite sizes through the width of the reflection 120 (Tab. I) gives the value of ~100 A.
The above mentioned packJng model allows one to determine easily the signs of the structural amplitudes by calculating the projection of the electron density of PS~3,3 on the axis
z. The function p (z) is approximated as follows
p (z)
= 2 £ ± fi cos(2 1rLz/c)
where I(L) is the intensity normalized to the averaged atom of PS-3,3 L an interference index z current coordinate of the direct space c translation in the direct lattice The
coefficient signs of the Fourier's senes alternate in the following manner
-, +, +, +, -. The
corresponding function p (z) is represented in figure 5 (curve I). Its maxJma at the beginning
of the coordinates and at z = 23.8 A describe the backbone electron density. The maxJma at
z = 8 and 16 A correspond to the region where the contacts of the aromatic fragments of the
neighbour side groups take place, and the central maximum at z = 12 A corresponds to the
contacts between the aromatic fragments and Coo~group connecting the benzene nngs. The
minima of the function are associated with the regions where the spacer groups and « tolls » of
the neighbour mesogenic side groups are overlapped
~~~~ b
120
~
80
40
4 8 12 16 20 24
z
,
1
Fig 5 The electron density profiles along the texture axis for PS-3,3 at 25 °C (I) and 85 °C (2)
Under the heating of the magnetically onented sample from the crystalline phase dunng its phase trans1tlon one can observe the certain transformation of the X-ray diffraction picture
For example, the reflections m the layer lines disappear and only the maxJma charactenstic for the smectic structure of type A are kept m the equatonal and mendional directions
(Fig. lb). The equatonal reflections become diffuse ones and along the mendional direction
one can observe a certain intensity re-distnbution of the maxJma of different orders
accompanied by the transition of a part of the reflection intensity to the background scattenng (Fig. 6). This intensity change of the mendional reflections is connected with the change of the function p (z) (Fig. 5, curve 2). One can expect a transformation of three central maxJma (it proceeds in SA phase over the temperature range 74~85 °C) corresponding to the contact of
the different parts of the mesogenic groups, to a wide maximum. One can believe that the above phenomenon is due to the violation of the ngld fixation of these contacts accompanied by the formation of the irregular shifts and angular disarrangement of the mesogemc groups in
the smectic layers. As a consequence, the perturbation of the long range order in the
longitudinal ~Tab I) as well as in the lateral (Fig. 7b) packing of the mesogenic groups takes
place
It was found that the structural changes in the vJcimty of the crystal-smectic transition have
a hysteresis character The difference of the transition temperatures in the heating-cooling cycles is equal to 15 °C.
It is worth mentioning that, on the whole, within the smectic phase, the antiparallel single- layer packing of the side chains is kept The value of the paracrystalhne distortions in the side
packins of the mesogemc groups gi [14] and the character of temperature dependence of gi are quite comparable to those observed for the low molar mass smectics « A ». This is the
difference of polysiloxane from polyacrylates and polymethacrylates with the same side chain groups where the stabilizJng influence of the backbone under the thermal treatment is more
pronounced
K -W SA ' '
,
g ~-
2~°
j
160 °j
.j j,o
~
~°
~ 't~ll~l~~'~ '
.o .o
.
I
O 80 160
~ oc
Fig 6 Intensity of the first (I) and the second (2) mendional reflections on X-ray diffraction pattem of PS-3,3 as a funcuon of temperature
N 8 X-RAY STUDY OF COMB-LIKE POLYSILOXANE 947
~ f ~ $~ ' ~ ~
,
~
#
j ,
~
~'~
2~ j ZZ(~$l~ ~
,& 50
, , ~& '
20 )3 ~i~* t * ~ ~ ~ ~ ~'~
~ it'( '
&
4,0 16
0 40 80 120 160 200
a T,Oc
~~"~ ~ ~
~'
~~ '
~~ j~
J
, I t(
~~
, t ~
a a
, a ~
jn~
8 j..,
aa,I I ti
#J 4
0
0 40 80 120 160 200
b ,°C
Fig 7 Temperature dependence of some structural parameters of PS-3,3 a) Smectic penods, d ii),
and the penodicity of lateral packing of the mesogenic groups, D(2), b) distortion parameters gi charactenzJng the lateral mesogenic group packing
If one compares the above results with the data obtained by Zugenmaier on crystalline and
smectic phases of polysiloxanes [3] one can find them to be quite comparable conceming the
packing of the mesogenic groups. As for the backbone packing the results are difserent. The conclusion drawn in [3] concerning the regular conformation of the backbone the packing of which differs from the packing of the mesogenic groups was based on the presence of the
reflection l 5 I
on the X-ray diffraction pattern. As we have already shown th~s reflection is due to the presence of the continuous component in the irradiation, I-e the reflection 15 I
is an artefact. Just the absence of the reflections that do not fit the general scheme at rather h~gh regulanty of the sample allows one to suppose that the structunng of the backbone is accompanied by the structuring of the mesogenic groups. The results of the analysis of the
Patterson function do fit the above considerations
