Molecular organization in nematic polymers. 1. Biphasic structures vs the nematic phase

Abstract

The objective of this work has been to investigate how molecules are organized in nematic polymers within the temperature range between melting of the solid phase and complete isotropization. The main experimental technique in this study has been broad-line proton NMR and the polymers used were a sequentially aperiodic aromatic-aliphatic copolymer and its isomeric periodic compound. The study included also calculations of conformational energy and molecular shape in seven-mer chemical sequences present in the polymers investigated. NMR measurements as a function of temperature on the aperiodic copolymer provided evidence for the segregation of chains by chemical composition in the nematic and isotropic regions of biphasic patterns observed at high temperatures. The more flexible chains of the system congregate in isotropic regions whereas nematic regions retain the more rigid ones. Experiments at lower temperatures where biphasic patterns are no longer observed by optical microscopy suggest that nematic polymers form orientationally ordered phases that tolerate isotropic motion in their molecular segments. We propose that these motions can occur through "nematic-isotropic fluctuations" with spatial and temporal periodicities which depend on temperature and molecular structure. The physical origin of these fluctuations is not known but molecular weight distribution and the aperiodicity of chemical sequences could be important factors.

Effect of Mesophase Order on the Dynamics of Side Group Liquid Crystalline Polymers

“…Any branches associated with sparse cross linking in the higher molecular weight samples (the presumed origin of the higher polydispersity) would also serve to disrupt molecular packing and suppress T s-i ; however, given the reduced degree of mesogen substitution found at high molecular weights (Table 1), "missing mesogen defects" should be much more prevalent than branch points and are thus likely the primary factor explaining the trend toward lower clearing points. The consequences of random and incomplete mesogen attachment can be considered in relation to concepts of Stupp et al, 32 who discuss the origins of a biphasic region in a melt of a main chain nematic random copolymer of monomers of different intrinsic stiffness. They invoke the concept of "polyflexibility," referring to a distribution of chain flexibility, reflecting in turn the statistical distribution of composition among individual molecules in a random copolymer.…”

Study of Odd−Even Effect of Flexible Spacer Length on the Chain Dynamics of Main-Chain Thermotropic Liquid-Crystalline Polymers Using High-Resolution Solid-State 13C Nuclear Magnetic Resonance Spectroscopy

“…As a matter of fact, during the past 2 decades, solid-state NMR spectroscopy has been used extensively to investigate the local structures and chain dynamics of polymeric materials. Many research groups have investigated local structures of TLCPs using solid-state NMR spectroscopy. − Specifically, the molecular order in TLCPs has been estimated from 1,2 H and 13 C NMR spectra; − the ratio of trans and gauche conformations in a methylene group has been determined using 13 C NMR chemical shifts; − the molecular motions in TLCPs have been investigated using 1,2 H and 13 C NMR spin−lattice relaxation time ( T 1 ); ,,, the motion of a methylene group in TLCPs has been analyzed using models, such as rotational diffusion about a bond and rotational jump among a few sites; the mobility at various local sites and the molecular motion in TLCPs with varying spacer lengths have been investigated using 13 C NMR T 1 ; the 13 C NMR spectra of the rigid and mobile components in TLCPs have been analyzed using the difference in T 1 of both components; − the slow director fluctuation in TLCPs has been investigated using 13 C NMR spin−lattice relaxation time in a rotating frame ( T 1 ρ ) 34 and the two-dimensional exchange NMR method. ,,, In the high-resolution solid-state 13 C NMR spectrum, when the frequency of the molecular motion is comparable to the proton decoupling...…”

Aperiodic Copolymers

“…Thus, different types of copolymers can be described as aperiodic (Figure ). It should be noted that the term aperiodic copolymer has already been employed in a few publications. − However, previous uses of this term are not always matching the above definition.…”