The mesophase behavior and structure of 1,1-bis(perfluoroalkyl)-2- vinylcyclopropane polymers

Article abstract of DOI:10.1080/15421400701205610

We introduced new poly(2-vinylcyclopropane)s carrying two perfluoroalkyl mesogens per repeat unit at the same main chain carbon atom. The relative lengths of the hydrocarbon-fluorocarbon segments were varied. Copolymers with a non-mesogenic 2-vinylcyclopropane were also prepared. X-ray diffraction studies proved the formation of smectic mesophases, the type of which depended on the chemical nature of the polymers: SmB2, SmI2 (or SmF2), and SmAd. We emphasize the role of intramolecular microphase segregation in determining the mesophase behavior and structure.

Full text of DOI:10.1080/15421400701205610

Mol. Cryst. Liq. Cryst., Vol. 465, pp. 81–92, 2007
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400701205610
The Mesophase Behavior and Structure of
1,1-Bis(Perfluoroalkyl)-2-Vinylcyclopropane Polymers
Elisa Martinelli
Elena Pucci
Giancarlo Galli
Dipartimento di Chimica e Chimica Industriale and INSTM,
`
Universita di Pisa, Pisa, Italy
Bernard Gallot
´
`
´
´
Laboratoire des Materiaux Organiques a Proprietes Specifiques,
Vernaison, France
We introduced new poly(2-vinylcyclopropane)s carrying two perfluoroalkyl meso-
gens per repeat unit at the same main chain carbon atom. The relative lengths
of the hydrocarbon-fluorocarbon segments were varied. Copolymers with a non-
mesogenic 2-vinylcyclopropane were also prepared. X-ray diffraction studies
proved the formation of smectic mesophases, the type of which depended on the
chemical nature of the polymers: SmB2, SmI2 (or SmF2), and SmAd. We emphasize
the role of intramolecular microphase segregation in determining the mesophase
behavior and structure.
Keywords: fluorinated polymer; microphase segregation; perfluorinated mesogen;
smectic mesophase; vinylcyclopropane
INTRODUCTION
Liquid crystalline polymers having perfluorinated chain segments as
side groups can form thin films in which the mesogens self-assemble
in an orientationally ordered array at the polymer-air interface [1].
This surface structure appears to be mediated by the bulk mesophase
The authors thank the Italian MIUR (PRIN 2005-03-5277) and the EU (G5RD-CT-
2001-00554 DENTALOPT) for financial support of the work.
Address correspondence to Giancarlo Galli, Department of Chemistry and Industrial
Chemistry, University of Pisa Via Risorgimento 35, Pisa, 56126, Italy. E-mail: gallig@
dcci.unipi.it
81

82
E. Martinelli et al.
order [2] which in contrast is not common for amorphous fluorinated
polymers [3]. In this context, poly(2-vinylcyclopropane)s carrying a
perfluorinated mesogen per repeat unit have proved to organize spon-
taneously in the bulk and at the surface, thereby creating a nanostruc-
tured low-energy surface [4].
Furthermore, the multifunctional character of the 2-vinylcyclopro-
pane monomers having two substituents in the 1,1-positions makes
them suitable for incorporation into a new type of polymers possessing
two mesogenic groups per repeat unit [5]. Examples of such polymer
structures are comparatively very few and are usually prepared by
more demanding polymerization procedures than the free radical
polymerization [6–8].
In this work we prepared a set of novel liquid crystalline poly(2-
vinylcyclopropane)s carrying two perfluoroalkyl mesogens per repeat
unit and investigated their thermotropic behavior by X-ray diffraction.
We emphasize the role of microphase segregation in affecting their
smectic behavior.
EXPERIMENTAL PART
Synthesis
2-(Perfluorohexyl)ethanol (m ¼ 2, n ¼ 6) and 2-(perfluorooctyl)ethanol
(m ¼ 2, n ¼ 8) were used as received from Fluorochem. 4-(Perfluoro-
octyl)butanol (m ¼ 4, n ¼ 8) was synthesized according to literature [9].
Monomer II, 1,1-di(ethyloxycarbonyl)-2-vinylcyclopropane, was
prepared following a literature procedure [10].
The syntheses of monomer I-2,8 and the corresponding polymer
p(I-2,8) are described here in detail as typical examples.
Monomer I-2,8. 19.30 g (91 mmol) of II was suspended with
18.92 g (330 mmol) of KOH in 20 ml of water under inert atmosphere.
The mixture was stirred at room temperature for 24 h and then acidi-
fied with 36% HCl to pH ¼ 2–3 and washed with diethyl ether. The
organic layer was dried over Na2SO4 and evaporated under vacuum
ꢀ
1
giving pure dicarboxylic acid 2(yield 80%): m.p. 105–107 C; H NMR
(DMSO-d6): d(ppm) ¼ 1.4 (2H, CH2), 2.4 (1H, CH), 5.1–5.5 (3H,
CH2¼CH), 10.6 (2H, COOH).
Then, 0.60 g (3.9 mmol) of 2, 0.10 g (0.7 mmol) of pyrrolidinopyridine
(PPy), 3.54 g (7.6 mmol) of 2-(perfluorooctyl)ethanol were dissolved in
25 ml of anhydrous CH2Cl2 and 5 ml of anhydrous THF at room tem-
perature. A solution of 1.57 g (7.6 mmol) of dicyclohexylcarbodiimide
(DCC) in 15 ml of anhydrous CH2Cl2 was then slowly added and the
mixture was let to react for an additional 3 days. The organic solution

Bis(Perfluoroalkyl)-Vinylcyclopropane Polymers
83
was washed with 5% HCl, 5% NaHCO3, water and dried over Na2SO4.
After evaporation of the solvent, the crude product was purified by
liquid chromatography on a silica gel column with ethyl acetate=
hexane (1=4 v=v) as the eluent. Pure monomer I-2,8 was obtained as
a white wax (yield 66%).
FT-IR (liquid film): n (cmꢀ1) ¼ 3094 (n C–H vinyl), 2920–2860 (n C–H
ꢀ
aliphatic), 1732 (n C¼O), 1640 (n C¼C), 1250–1117 (n C(O)–O, n C–F),
658 (xCF2).
1H_NMR (CDCl3): d ppm ¼ 1.6–1.8 (2H, CH2 cyclopropane), 2.5 (5H,
_
€
CH2CF2 þ CH cyclopropane), 2.6 (1H, CH cyclopropane), 4.4 (4H,
COOCH2), 5.1–5.5 (3H, CH2 ¼ CH).
Polymer p(I-2,8).
0.53 g (0.5 mmol) of monomer I-2,8 and 6 mg
(0.04 mmol) of azobis(isobutyronitrile) (AIBN) were introduced into a
pyrex vial under nitrogen atmosphere. After 3 freeze-thaw pump
cycles the vial was sealed under vacuum and the polymerization was
let to proceed for 19 h at 65ꢁC. The polymer was purified by repeated
precipitations from Cl2FCCF2Cl solutions into methanol, and dried
under vacuum. The final polymer p(I-2,8) was obtained as a white
powder (yield 78%).
FT-IR (polymer film): n (cmꢀ1) ¼ 2959–2850 (n C–H aliphatic), 1737
ꢀ
=
(n C O), 1203–1082 (nC(O)–O, n C–F), 659 (xCF2).
_
1H NMR (trifluorotoluene):
=
d
ppm ¼ 2.0–3.0 ppm (8.5H,
_
CH2CH CHCH2 þ cyclobutane þ CH2CF2), 4.4 (4H, COOCH2), 5.5
=
(1.5H, CH CH).
19F NMR (trifluorotoluene=CDCl3): d ppm from CF3CO2H ¼ –12
(3 F, CF3), –47 (2F, CF2CH2), –54 to –57_(5CF2), –59 (2 F, CF2€CF3).
Characterization
The 1H, 13C, and 19F NMR spectra were recorded with a Varian Gemini
VXR 300 spectrometer (operating at 299.9, 75.4, and 282.2 MHz,
respectively). Chemical shifts were referenced to external TMS, unless
specified otherwise.
DSC measurements were carried out with a Mettler DSC-30 instru-
ment (10ꢁC=min scanning rate) that had been calibrated using tin,
indium and zinc standard samples. The clearing temperature (Ti)
was taken as the maximum temperature in the DSC enthalpic peak
of the second heating cycle. The glass transition temperature (Tg)
was set at the half-devitrification temperature.
X-ray diffraction experiments were performed on powder samples
with a pinhole camera under vacuum at various temperatures
ꢁ
˚
(ꢂ1 C) up to Ti. Ni-filtered CuK a beam (k ¼ 1.54 A) was used. Several

84
E. Martinelli et al.
exposures were recorded so as to measure the strongest and weakest x
orders of reflections on the smectic layers. Experimental amplitudes ax
were corrected for the Lorentz-polarization factor and normalized to
the strongest one.
RESULTS AND DISCUSSION
A set of novel poly(2-vinylcyclopropane)s was prepared that contained
two fluorinated mesogens as lateral substituents at the same chain pos-
ition of the repeat unit (Fig. 1). In designing these polymer structures
we introduced side groups where the relative lengths of the fluorocar-
bon rodlike segment (n ¼ 6 or 8) and the hydrocarbon spacer segment
(m ¼ 2 or 4) were varied. Accordingly, the fluorinated monomers will
be denoted as I-m,n, and the relevant polymers as p(I-m,n) after the
numbers of CH2 and CF2 moieties in their side groups, respectively.
It is known [11] that perfluorinated chain segments of greater
aspect ratio favor the onset of smectic mesophases in various classes
of polymers; on the other hand, more flexible hydrocarbon chain
spacers increase conformational freedom and segregation. Thus, we
aimed at finely tuning the formation of mesophases over a wide tem-
perature range on a chemical ground.
The reaction scheme followed for the synthesis of the fluorinated
monomers I-m,n starting from diethyl malonate (1) is depicted in
Figure 2. This method was relatively facile and versatile, in that it
additionally yielded intermediate II that could then be used as a
non-mesogenic monomer in subsequent copolymerization experiments
for one further chemical modification of p(I-2,8) to give copolymers
p(I-2,8-co-II)a–c (Fig. 1). An alternative, more straightforward pro-
cedure (also shown in Fig. 2) involved the synthesis of bisfluorinated
malonates 4 in the first step. However, the eventual monomers were
difficult to purify accurately and were obtained in lower reaction
yields, and this synthetic pathway was not optimized.
FIGURE 1 Structures of the homopolymers p(I-m,n) and copolymers p(I-2,
8-co-II)a–c (for clarity the linear repeat units are shown only).

Bis(Perfluoroalkyl)-Vinylcyclopropane Polymers
85
FIGURE 2 Reaction schemes for the synthesis of the 2-vinylcyclopropanes
I-m,n and II.
The polymerization experiments were performed by free radical
polymerization (AIBN, 60–65ꢀC) in either bulk or trifluorotoluene sol-
ution (yields 70–85%). The copolymers were extracted with acetone
that is a selective solvent for p(II). No sizeable amounts of polymeric
products were isolated in the acetone extracts, consistent with the
formation of copolymers comprised of both reacted monomers in a
random fashion. The polymers were soluble in fluorinated solvents,
with the exceptions of the copolymers with higher contents of
non-fluorinated units that could be dissolved in common solvents for
characterization. Their SEC analysis provided values of Mn ¼ 38000–
45000 g=mol and Mw=Mn ¼ 2.0–2.2.
The free radical homopolymerization of 2-vinylcyclopropanes carry-
ing electron-withdrawing substituents in the 1,1-positions (X and Y in
Fig. 3) may involve the introduction of both linear ring-opened units
and cyclic ring-closed units into an actual copolymer structure
FIGURE 3 Structure of a poly(2-vinylcyclopropane) with both linear and
cyclic repeat units (X, Y: electron-withdrawing groups).

86
E. Martinelli et al.
[12,13]. The relative proportion of the two isomer repeat units
depends, for given experimental polymerization conditions, on the
steric hindrance and electronic configuration of the substituents X
and Y, with bulkier groups being better accommodated in the 1,5-
ring-opened linear repeat unit [14].
In accord with previous results [13,14] we found that the linear unit
was incorporated principally, albeit not exclusively, in the homopoly-
merization experiments, e.g., up to 75% content in p(I-2,8). The pre-
dominance of the linear repeat units is illustrated in the simplified
structures of the polymers in Figure 1. The precise influence of the
configurational features of the main chain on the polymer properties
is difficult to investigate and remains to be ascertained.
Each of the polymers formed one smectic mesophase (Tables 1 and 2).
The clearing temperature increased with the length of the perfluor-
oalkyl segment (Ti from 98ꢀC to 129ꢀC), while it was depressed by the
longer hydrocarbon spacer (down to 96ꢀC). The glass transition
temperature was little affected by the structural variations (Tgꢁ40–
50ꢀC), and therefore the range of mesophase existence was maximized
for p(I-2,8) (Ti – Tg ¼ 86ꢀC). This mesogenic character was preserved in
the copolymers with non-fluorinated monomer II for which Ti was
located in the range of 150–180ꢀC (Table 2).
This essential insensitivity of Ti to chemical composition is in
marked contrast to what is observed in customary random copolymers
when the mesophase-isotropic transition temperature normally drops
in a substantial way with increasing content of non-mesogenic units.
However, it is not surprising for copolymers based on monomers carry-
ing long perfluorinated side groups [15]. It is assumed that strong intra-
molecular phase segregation confines the incompatible hydrocarbon
and fluorocarbon constituents into individual smectic sub-layers. This
ordered array would be less affected by chemical composition than in
a totally mixed distribution of mesogenic and non-mesogenic units
[16]. The details of the structure and packing of the smectic mesophase
depend on the chemical features in a more subtle way than is simply
anticipated by inspection of Ti.
TABLE 1 Thermal Properties of Polymers p(I-n,p)
Polymer
Tg (ꢀC)
Ti (ꢀC)
DHi (J=g)
p(I-2,6)
p(I-2,8)
p(I-4,8)
40
43
52
98
129
96
5.6
8.3
8.3

Bis(Perfluoroalkyl)-Vinylcyclopropane Polymers
87
TABLE 2 Thermal Properties of Copolymers p(I-2,8-co-II)a–c
Polymer
I-2,8 (mol%)
Tg (ꢀC)
Ti (ꢀC)
DHi (J=g)
p(I-2,8)
100
65
44
23
0
43
44
46
32
33
129
156
161
153
–
8.3
8.1
8.9
6.5
–
p(I-2,8-co-II)a
p(I-2,8-co-II)b
p(I-2,8-co-II)c
p(II)
We investigated the mesophase behavior by X-ray diffraction in
order to identify the nature of the smectic mesophases and their struc-
tural parameters and to confirm the phase segregation of incompatible
components. The detected physical parameters of the smectic phases
are collected in Table 3, where they are also compared with the maxi-
mal lengths calculated by the CPK models for smectic bilayer (L2) and
monolayer (L1) spacings.
The homopolymers gave rise to a pseudo-hexagonal mesophase
˚
(side of the hexagon bH ¼ 5.7 A) (Fig. 4) with a layer periodicity d typi-
cal of a double layer structure (d >> L1). While the mesophase of p(I-
2,6) was orthogonal (SmB2, d>L2), the mesophase of p(I-2,8) and
p(I-4,8) was tilted (SmI2 (or SmF2), d<L2) [17]. The respective tilt
angles b of 15ꢀ and 21ꢀ were unaffected by temperature variations over
the entire mesophase range. In these two last polymers, the longer
perfluorooctyl chain segments were inclined to the smectic layer nor-
mal in order to fill space effectively. In both cases the average surface
2
˚
area S per side group was in fact too small (Sꢁ28 A ) to allow interdi-
gitation of the bulky perfluorinated chains.
TABLE 3 Structural Parameters of the Smectic Mesophases (for symbols,
see text)
d
bH
S
b
L2
(ꢂ1 A)
L1
(ꢂ1 A)
Polymer
(ꢂ0.2 A)
(A)
(A )
(ꢂ2ꢀ)
Mesophase
2
˚
˚
˚
˚
˚
p(I-2,6)
p(I-2,8)
p(I-4,8)
p(I-2,8-co-II)a
p(I-2,8-co-II)b
p(I-2,8-co-II)c
31.9
32.3
36.1
33.3
24.9
24.1
5.7
5.7
5.7
28
28
28
0
15
21
0
0
0
28
34
39
34
34
34
16
19
22
19
19
19
SmB2
SmI2 (SmF2)
SmI2 (SmF2)
SmB2
SmAd
SmAd
5.8
29
5.5a
5.5a
30b
30b
aIntermolecular distance.
bBased on a square lattice.

88
E. Martinelli et al.
FIGURE 4 X-ray diffraction pattern of the SmI2 (or SmF2) phase of p(I-2,8) at
80ꢀC.
The structure of the mesophase in the copolymers p(I-2,8-co-II)a–c
depended critically on their chemical composition (Table 3). While it
was a bilayer and high order (SmB2, dꢁL2) for the copolymer p(I-2,8-
co-II)a containing a lower amount of non-mesogenic units, the meso-
phase was partly interdigitated and low order (SmAd, L1<d<L2) in
the two copolymers having larger contents of such units (Fig. 5). Dilution
of the perfluorooctyl side groups along the macromolecular chain gradu-
ally disrupted their in-plane order and enabled partial overlap within the
FIGURE 5 X-ray diffraction pattern of the SmAd phase of p(I-2,8-co-II)b at
100ꢀC.

Bis(Perfluoroalkyl)-Vinylcyclopropane Polymers
89
FIGURE 6 Schematic of the microphase segregated structure of the smectic
phase of copolymer p(I-2,8-co-II)a with fluorocarbon (df) and hydrocarbon
(dh) sub-layers.
2
˚
layers (S ꢁ 30 A ). Nonetheless, an effective microphase segregation
operated in the copolymer rich in mesogenic units (Fig. 6).
Further insight into the structure of the smectic mesophase was
gained by analysis of the small angle diffraction region. In fact the
diagrams of the homopolymers and copolymer p(I-2,8-co-II)a pre-
sented three orders of Bragg reflections on the smectic layer planes.
The normalized amplitudes ax of the layer reflections are collected in
Table 4. In the other two copolymers with lower contents of I-2,8 units
the higher harmonic reflections were either too weak to be measured
with reliability or not detected. Taking into account the symmetry of
TABLE 4 Normalized Amplitudes ax of the Smectic Layer
Reflections
Polymer
a1
a2
a3
p(I-2,6)
p(I-2,8)
p(I-4,8)
p(I-2,8-co-II)a
1
1
1
1
0.76
0.69
0.72
0.70
0.41
0.38
0.42
0.39

90
E. Martinelli et al.
the smectic structure, the electron density profile, or its projection,
q(z) along the layer normal is given [18] by:
qðzÞ ¼ R½ax cosðx2pz=dÞꢃ
ð1Þ
FIGURE 7 Electron density profiles for the smectic mesophase of p(I-2,8).

Bis(Perfluoroalkyl)-Vinylcyclopropane Polymers
91
For x reflections at small angles, there are 2x combinations of signs
or electron density profiles [19]. This is illustrated for p(I-2,8) in
Figure 7. The profile q
was more physically acceptable because
ꢄþþ
it exhibits an absolute maximum in correspondence of the fluorocar-
bon side groups (z=dꢁ0.3–0.7) and secondary comparatively weak
maxima for the hydrocarbon main chains (z=d ¼ 0 and 1). The central
maximum in q(z) does not seem to be sufficiently intense to account for
interdigitation of the fluorinated chains. On the other hand, the unu-
sually high electron density for the polymer chains reflects the large
number of chain carbon atoms per repeat unit. It also provides further
supporting evidence of microphase segregation within the smectic
layers.
Analogous considerations apply to the rest of the polymer samples
investigated here for which very similar ax values were evaluated.
Thus, only minor details of their q(z) were affected in the microphase
segregated smectic structures. One significant difference is that the
interlayer correlation was shorter in the copolymers with lower con-
tents of mesogenic units, probably because of a more blurred interface.
CONCLUSIONS
The poly(2-vinylcylopropane)s carrying fluorinated side groups (n ꢅ 6)
present smectic mesophases irrespective of essentially any other struc-
tural feature, including chemical composition of the copolymers with
non-mesogenic components. Such inherent smectogenic tendency
was enhanced owing to the concomitant presence of two fluorinated
chain segments per repeat unit, that are phase segregated from the
hydrocarbon backbone and spacer segments.
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