Structure and Phase Transitions of Poly(heptamethylene p,p′‐bibenzoate): Time‐Resolved Synchrotron WAXS and DSC Studies

Abstract

Time‐resolved wide‐angle X‐ray scattering (WAXS), as well as differential scanning calorimetry (DSC) and polarisation microscopy studies, were applied to investigate the structure and phase transitions of poly(heptamethylene p,p′‐bibenzoate). Temperature dependencies of several structural parameters were determined. Complete transformation from an isotropic melt to a smectic phase was suggested whereas the transition from a smectic to crystalline phase is only partial (around 30%), although it takes place from the ordered SCA phase. Crystals are formed within the SCA domains with nearly the same coherent length. On the basis of the analysis of the position and the profile of the diffuse wide‐angle X‐ray scattering and mesogenic layer spacing, it was assumed that either crystallisation modifies the smectic structure, or mesophase losses its positional order because of the lack of mobility of the spacers at low temperatures.

Tailoring the Phase Behavior in Thermotropic Copolyesters

“…A second interesting aspect deduced from Figure 9 is the fact that the smectic layer spacing displays a negative temperature coefficient for the SmA phase, as usual in other polybibenzoates 14, 17. This negative coefficient is interpreted by assuming that the normal temperature expansion in counterbalanced by a decreasing degree of orientational order on heating the SmA phase,45 and/or by the adoption of less extended chain conformers (higher gauche proportions) at higher temperatures.…”

“…This is the case of thermotropic polybibenzoates, an interesting family showing a wide range of transition temperatures, and different kinds of smectic mesophases, with varying degree of order 10–17…”

Synthesis of copolybibenzoates with thioether and ether groups in the flexible spacers

“…A rough extrapolation of transition temperatures to zero comonomer content, considering the samples with higher viscosity, gives a value of around 210 °C for the isotropisation temperature of the homopolymer PDESB, which is slightly higher than that of poly(diethylene glycol p , p ′‐bibenzoate),8, 15 the homologue with an oxygen atom, which is 200 °C. However, it is lower than that for the all‐methylene homologue,3 218 °C.…”

“…In previous work,3–7 the thermotropic properties of poly[oxybis(trimethylene) p , p ′‐bibenzoate] ([PhPhCO 2 (CH 2 ) 3 O(CH 2 ) 3 O 2 C] DP or PDTMB) were compared with those of its methylenic homologue poly(heptamethylene p , p ′‐bibenzoate) (P7MB) and it was found that the presence of an ether group in the spacer increased the range of thermal stability of the mesophase in such a way that only the mesophase was observed on the time‐scale of cooling inside the calorimeter (even though, during longer annealing times at the appropriate temperature, a crystalline phase starts to develop). On the other hand, the mesophase of P7MB is rapidly transformed into a crystalline phase upon cooling 2, 3, 8. No pure liquid crystal is obtained even at the highest cooling rates and a small amount of crystallinity (10–17 %) is found after quenching the sample from the melt 9, 10…”

Structure and Phase Transitions of Poly (Hexamethylene p,pʹ-Bibenzoate) as Studied by DSC and Real-Time SAXS/WAXS Employing Synchrotron Radiation

“…The main-chain liquid crystalline polymers (LCPs) have generated much interest because of understanding such systems and owing to their industrial applications [1][2][3][4]. Typical examples of LCPs are poly (p,p'-bibenzoate)s (PBBn) with different number n of methylene groups in the spacer.…”