Mesophase Identifications in a Series of Liquid Crystalline TPP Polyethers and Copolyethers Having Highly Ordered Mesophase Structures. 2. Phase Diagram of Even-Numbered Polyethers

Abstract

Liquid crystalline polyethers have been synthesized from 1-(4-hydroxy-4‘-biphenylyl)-2-(4-hydroxyphenyl)propane and α,ω-dibromoalkanes with even-numbers of methylene units [TPP(n = even)s]. Multiple phase transitions are found during cooling and heating via differential scanning calorimetry (DSC), and they show little undercooling dependence. Ordered structure identifications are based on experimental observations of wide angle X-ray powder and fiber diffraction experiments at different temperatures. Polarized light and transmission electron microscopy observations on mesophase morphology combined with DSC results on thermodynamic transition properties also provide additional evidence for these phase assignments. Moreover, the contributions of the mesogenic groups and the methylene units to each ordering process are obtained based on the changes of transition enthalpy and entropy. In TPP(n ≤ 8)s the highest temperature transition is from the isotropic melt to a nematic phase. This nematic phase is only stable in a narrow temperature range. For instance, it is 12 °C for TPP(n = 4) and 6 °C for TPP(n = 8). When the number of methylene units n ≥ 10, the isotropic melt directly enters a smectic F phase. The second transition in TPP(n ≤ 8)s is from the nematic to the smectic F phase. As a result, the smectic F phase exists for all TPP(n = even)s. Decreasing the temperature further leads to another transition in TPP(n = even)s to form a smectic crystal G phase which is followed by a transition to a smectic crystal H phase. This smectic crystal H phase remains for TPP(n ≤ 8)s down to their glass transition temperatures, while in TPP(n ≥ 10)s further ordering processes occur and crystal phases are observed. A phase diagram of TPP(n = even)s is constructed.

Phase structures, transition behavior and surface alignment in polymers containing rigid rod-like backbones with flexible side chains, 3. The even-odd effect on phase transitions in combined liquid crystal polyesters

“…2). This behavior is similar to previously reported N e I or S A e I transitions observed in several main-chain LC polymers [15][16][17][18][19] . Note that the diffuse scattering halo represents the average distance between the lateral packing of molecular chains.…”

Self-Assembly of Chemically Linked Rod−Disc Mesogenic Liquid Crystals

“…In most cases, this represents an isotropic (I) melt to a low-ordered LC phase transition. [48][49][50][51][52][53][54][55][56] For the lower temperature transition, the transition temperatures (both the peak and onset) and heat of transition are cooling rate dependent. This type of transition behavior is usually observed for a crystallization process.…”

Structures and phase transformations of odd-numbered asymmetric main-chain liquid crystalline polyesters

“…For the high-temperature transition, the heat of transition and the onset transition temperature at the high-temperature side do not exhibit cooling rate dependence. This indicates that the high-temperature transition takes place close to thermodynamic equilibrium, and it may be associated with the transitions between an isotropic melt (I) and a low ordered LC phase [28][29][30][31][32][33][34][35]. In contrast, the heat of transition (6.5 kJ/mol) at the lower transition temperature and its onset transition temperature are cooling rate dependent, which is often observed during polymer crystallization processes.…”

“…For example, a correlation length is estimated to be between 1.8 and 2.2 nm below 181 8C (see the inset of Fig. 3(b)), indicating a liquid-like, short-range order in the lateral molecular packing in the smectic layers [32][33][34][35][36][37]. The vertical dash-lines are indications of the thermal transitions based on the DSC cooling results.…”