Chemistry Reference
In-Depth Information
100 nm and the polymer chain in the crystalline state is oriented along
the thickness of the lamella, only a few tens of the repeating units can
be accommodated by the thickness. Thus in principle, a perfect lamella
of 100 nm thick can be formed by a polymer with a degree of polymeriza-
tion of only a few tens. If no chain ends are included inside the lamella,
only thinner lamellae are formed by polymers of lower molecular weights.
If chain end inclusion is allowed in the lamella, lamella with more defects
will result from samples of lower molecular weight. On the other hand,
for samples of high degrees of polymerization, extensive chain folding may
occur, otherwise the polymer chain has to travel through two or more
lamellae. Nevertheless, the limited low value of lamella thickness gives rea-
son for the fast attainment of the limited melting point of the polymer
crystals.
In order to study the general applicability of the above principle for
the transformation of the monotropic liquid crystals to the enantiotropic
ones by increasing the molecular weight of the monotropic liquid crystal
polymers, the polymer,
3
.
16
, reported by Ober
et al
. (1983), was restudied
(Duan
et al
. 1987).
O
O
O
3
.
16
C
O
C
C
O
CH
2
O
10
According to Ober
et al
., the polymer with an intrinsic viscosity of 0.17 dl/g
is monotropic. In this restudy, a sample with a viscosity of 0.30 dl/g was
obtained. The sample melts at 148
◦
C and forms a nematic phase. The
clearing point of the liquid crystal is 170
◦
C. The sample was fractionated to
obtain three fractions with viscosity of 0.14, 0.39 and 0.61 dl/g respectively.
In agreement with Ober
et al
., the sample of the lowest intrinsic viscosity
(0.14 dl/g) is monotropic, melts at 140
◦
C and forms the liquid crystal phase
only in the circle of cooling. However, the fractions with higher viscosity are
all enantiotropic liquid crystals. The one of 0.39 dl/g melts at 149
◦
C, the
clearing temperature of the liquid crystal phase is 159
◦
C. The fraction of
the highest viscosity also melts at 149
◦
C, but the clearing point is somewhat
higher (164
◦
C).
At about the same time, Percec and Nave (1987) also found that
the transformation of the monotropic liquid crystalline polymers to the
enantiotropic ones could be achieved by increasing molecular weight. The
polymer studied by these authors has the following structural formula
