2 Dependence of Thermodynamic Properties on Degree
of Polymerization
Blumstein et al. [10] has reported the molecular weight dependence of the latent
entropy ΔS NI for a mainchain LC polyester, poly(4,4
0 -dioxy-2,
2
0 -dimethylazoxybenzene dodecanedioyl) (DDA-9). A homologous series of polymer samples (M n ¼ 700–19,000) , together with the monomer and dimer model
compounds were employed in their studies. The value of ΔS NI, as expressed in terms
of a repeating unit, increases very rapidly with the degree of polymerization (DP),
reaching an asymptotic value in the oligomeric region. When the unit is converted to
the entropy change per spacer, the magnitude of ΔS NI becomes nearly invariant over
a wide range of DP from the dimer (9-DDA-9) to polymers: ΔS NI /R ¼ ~ 2.1 (where
R designates the gas constant) (Fig. 1). These observations immediately suggest
that the conformational contributions arising from the spacer intervening between
the two mesogens at both terminals are nearly identical for a given series of LCs
beyond the dimer.
It is thus advantageous for conformational studies to work with oligomeric
compounds having neat chemical structures. Shortcomings inherent to polymeric
LCs, such as polydispersity in DP, structural imperfections due to irregular arrangements such as kink-conformations or hairpins [11], and experimental difficulties
associated with the enhancement of NI transition temperatures with DP, can thus be
avoided. In the studies mentioned hereafter, detailed analyses were mostly carried
out for oligomeric LCs. The knowledge gained through studies on the low DP
analogs has been found useful in understanding the spatial arrangements and
thermodynamic properties of polymer LCs [12, 13].
3 Influence of Bond Angle Restrictions and Rotational
Characteristics on the Odd-Even Effect of
Thermodynamic Quantities
For the sake of comparison, thermodynamic properties were examined for a series
of dimer model compounds carrying the same chemical constitution except for
the linking group X (see Fig. 2). The observed values of ΔS NI /R are shown for
X ¼ carbonate (CBC-n), ether (CBA-n), and ester linkages (CB-n) in Fig. 3 [14].
In accordance with Roviello and Sirigu’s finding [5], the carbonate linkage is
undoubtedly the origin of the less pronounced oscillation of the dimer [7]. Also
included herewith are those obtained for the monomer analog with a carbonate(nOCCB) and ether-type tail (nOCB) (see Fig. 2). The odd-even alternation is very
weak for both monomer LCs [15–19]. These observations suggest that the chemical
structure of the linking group may be an important factor affecting the
order–disorder properties of the mainchain LC compounds beyond the dimer.
Nematic Conformation of Chain Molecules Predominating in the Ordered Mesophase
111
of Polymerization
Blumstein et al. [10] has reported the molecular weight dependence of the latent
entropy ΔS NI for a mainchain LC polyester, poly(4,4
0 -dioxy-2,
2
0 -dimethylazoxybenzene dodecanedioyl) (DDA-9). A homologous series of polymer samples (M n ¼ 700–19,000) , together with the monomer and dimer model
compounds were employed in their studies. The value of ΔS NI, as expressed in terms
of a repeating unit, increases very rapidly with the degree of polymerization (DP),
reaching an asymptotic value in the oligomeric region. When the unit is converted to
the entropy change per spacer, the magnitude of ΔS NI becomes nearly invariant over
a wide range of DP from the dimer (9-DDA-9) to polymers: ΔS NI /R ¼ ~ 2.1 (where
R designates the gas constant) (Fig. 1). These observations immediately suggest
that the conformational contributions arising from the spacer intervening between
the two mesogens at both terminals are nearly identical for a given series of LCs
beyond the dimer.
It is thus advantageous for conformational studies to work with oligomeric
compounds having neat chemical structures. Shortcomings inherent to polymeric
LCs, such as polydispersity in DP, structural imperfections due to irregular arrangements such as kink-conformations or hairpins [11], and experimental difficulties
associated with the enhancement of NI transition temperatures with DP, can thus be
avoided. In the studies mentioned hereafter, detailed analyses were mostly carried
out for oligomeric LCs. The knowledge gained through studies on the low DP
analogs has been found useful in understanding the spatial arrangements and
thermodynamic properties of polymer LCs [12, 13].
3 Influence of Bond Angle Restrictions and Rotational
Characteristics on the Odd-Even Effect of
Thermodynamic Quantities
For the sake of comparison, thermodynamic properties were examined for a series
of dimer model compounds carrying the same chemical constitution except for
the linking group X (see Fig. 2). The observed values of ΔS NI /R are shown for
X ¼ carbonate (CBC-n), ether (CBA-n), and ester linkages (CB-n) in Fig. 3 [14].
In accordance with Roviello and Sirigu’s finding [5], the carbonate linkage is
undoubtedly the origin of the less pronounced oscillation of the dimer [7]. Also
included herewith are those obtained for the monomer analog with a carbonate(nOCCB) and ether-type tail (nOCB) (see Fig. 2). The odd-even alternation is very
weak for both monomer LCs [15–19]. These observations suggest that the chemical
structure of the linking group may be an important factor affecting the
order–disorder properties of the mainchain LC compounds beyond the dimer.
Nematic Conformation of Chain Molecules Predominating in the Ordered Mesophase
111