Synthesis and Thermal Properties of Ferroelectric Side-Chain Liquid-Crystalline Polysiloxanes Based on Naphthyl Biphenylcarboxylate Mesogenic Groups and Oligooxyethylene Spacers

In this work, three series of ferroelectric side-chain polysiloxanes containing oligooxyethylene spacers and ( S )-2-methyl-1-butyl (2 S )-2-[6-(4-hydroxybiphenyl-4‘-carbonyloxy)-2‘-naphthyl]propionates, ( R )-1-methylheptyl (2S)-2-[6-(4-hydroxybiphenyl-4‘-carbonyloxy)-2‘-naphthyl]propionates, and (2 S,3 S )-2-chloro-3-methylpentyl (2 S )-2-[6-(4-hydroxybiphenyl-4‘-carbonyloxy)-2‘-naphthyl]propionates mesogenic groups were synthesized. The mesomorphic behaviors of three series of ferroelectric side-chain liquid-crystalline polysiloxanes were also studied using differential scanning calorimetry, optical polarizing microscopy, and high-resolution X-ray diffraction measurements. Some of these side-chain polysiloxanes containing four phenyl rings of ester cores (i.e., −Ph−Ph−COO−naphthyl−) and chiral heptyl tail present a smectic A (S A ), and a chiral smectic C (Sc*) phase (around 100 °C). Another two series containing four phenyl rings of ester cores and chiral butyl and pentyl chain tail reveal a cholesteric (N*), an S A phase and an Sc* phase. The polymer PS12NC derivative with one unit of oxyethylene spacer ( n = 1) of PSn2NC series reveals a N* phase, a twist grain boundary A (TGB A ) phase, and a Sc* phase. X-ray investigations reveal that the packing of mesogenic groups for PS02NA, PS02NB, PS02NC with the shortest spacers ( n = 0) exhibit a bilayer (two-layer) packing structures. Moreover, polymer PS32NA, PS32NB, PS32NC with the longest spacer chain ( n = 3) reveal a monolayer packing structure. However, these two types of packing structure appear simultaneously in PS12NA ( n = 1), PS12NC ( n = 1), PS12NB ( n = 1), and PS22NB ( n = 2), depending on the chiral tail length. Results obtained in this study again demonstrate that the tendency toward chiral smectic C mesomorphism increases as the rigidity of phenyl ester mesogens increases via the flexible oligooxyethylene spacers system. Furthermore, the thermal stability of the chiral smectic C mesophase is determined by the flexibility of the chiral tail.