Chemistry Reference
In-Depth Information
these difunctional proton acceptors with bis-benzoic acids, which also bear
oligo(ethylene oxide) spacers, allowed the investigation of how flexibility in-
fluences the LC properties of such supramolecular polymers. As is seen in co-
valently bonded segmented LC polymers, a decrease in the isotropic-nematic
transition temperature (upon cooling) was observed for stoichiometric mix-
tures. Both nematic and smectic-A phases were observed for many composi-
tions, with isotropization temperatures in the range 170
◦
C
<
T
NI
<
190
◦
C.
In a similar vein, Weigel et al. [69] have also studied main-chain SLCPs ex-
ploiting the carboxylic acid/pyridine hydrogen-bonding motif, in their case
examining oligo(ethylene glyol)s symmetrically endcapped with 6-hydroxy-
2-naphthoic acid as the hydrogen bond donor. For the hydrogen bond ac-
ceptors, a variety of pyridine-terminated molecules were studied, including
4,4
-azopyridine, and 4,4
-(
p
-phenylenedi-1,2-ethylenediyl)bispyridyl, among
others. The researchers observed, using DSC and POM, that over narrow
temperature ranges a liquid crystal phase appeared and that, like the work
of He, increasing the spacer lengths afforded lower isotropization tempera-
tures. Further work by the same group [70] demonstrated how the nature
of the mesogenic supramolecular motif affects the LC properties. By com-
bining different aromatic bisacids (
5
,
6
,
7
) with a series of bisazopyridine
phenol derivatives (
8
,
9
) (Fig. 7) they showed that LC transition temperatures
are raised with increasing mesogen rigidity and aspect ratio. For example,
as in the case of covalently bonded LCPs, it was found that the 6-hydroxy-
2-naphthoic acid unit (
6
)-in this case serving as the hydrogen bond donor-
offers a clearing transition (e.g.,
6
·
8b
: I 142.7 N 40.4 K) higher than that of
the benzoic acid version (e.g.,
5
8b
: I 131.5 N 60.2 K), but lower than the high
transition temperatures witnessed for the biphenyl benzoic acid (e.g.,
7
·
·
8b
:
I 209.0 N 127.0 K) derivatives.
Liquid crystallinity has also been observed from supramolecular polymer-
ization of bipyridyl mesogens without a need for an aromatic mesogenic
component of the hydrogen donor. Bhowmik et al. [71] investigated the self-
assembly of 4,4
-bipyridyl (
BP
) with two aliphatic dicarboxylic acids: adipic
acid (
AA
; HOOC-(CH
2
)
4
-COOH) and sebacic acid (
SA
; HOOC-(CH
2
)
8
-
COOH). Despite the simplicity of using only commercially available building
blocks (monomers), rich LC behavior was observed. In particular, DSC and
POM studies revealed that the supramolecular homopolymers
BP
:
SA
and
BP
:
AA
exhibited smectic phases with phase sequences on heating of (K 127
Sm 172 I) and (K 143 Sm1 177 Sm2 181 I), respectively. Copolymeriza-
tion, namely mixing in different ratios of SA and AA while keeping the
acid/pyridine ratio at 1 : 1, was observed to broaden the temperature range for
liquid crystallinity quite substantially through melting point reduction (both
T
m
and heat of fusion). For example, the 50 : 50 copolymer (K 120 Sm 165 I),
though the melting transition was barely detectable by DSC.
An interesting example of how non-mesogenic rigid supramolecular main-
chain polymers can affect LC environments has recently been shown [72]
Search WWH ::
Custom Search