Solid-State 13C and 15N NMR Analyses of Structure and Dynamics of Spacer Methylene Sequences and Mesogen Groups for Liquid Crystalline Polyurethanes with Different Spacer Lengths

Abstract

Solid-state 13C NMR analyses of the structure and dynamics have been performed for liquid crystalline polyurethanes with different spacer CH2 lengths, which were polymerized from 3,3‘-dimethyl-4,4‘-biphenyldiyl diisocyanate, α,ω-alkanediols having various even numbers m of carbons, and 1-hexanol. Each sample was crystallized by cooling from the melt through the liquid crystalline phase at an appropriate cooling rate depending on its phase transition behavior detected by DSC. 13C spin−lattice relaxation analyses have revealed that each CH2 carbon resonance line contains three components with different T1C values which are assigned to the crystalline, medium, and noncrystalline (supercooled liquid crystalline) components. For the polyurethane samples with m = 8 and 12, it has been found that the spacer CH2 sequences for the noncrystalline component will adopt the alternate trans (t) and trans−gauche exchange (x) conformation in the central part whereas those of the crystalline and medium components may be in the all-trans conformation. Accordingly, the conformation of the spacer CH2 sequences can be generally expressed as ttx(tx)𝓁 tt (𝓁 = m/2 − 3) for 8 ≤ m ≤ 12, including the result for m = 10 reported previously. Furthermore, to examine molecular motion of the mesogen units in detail, 13C CSA spectra have been measured at room temperature for the mesogen carbons by 2D SASS 13C NMR spectroscopy and compared with those simulated by using the two-site exchange model for the flip motions of the phenylene group. As a result, it has been suggested that flip angles of fluctuations around the phenylene axis may be limited to less than about 20° without any 180° flip motion in all components, including the supercooled liquid crystalline component. Natural abundant CP/MAS 15N NMR measurements also suggest that such rigidity of the mesogen groups may be due to the formation of intermolecular hydrogen bonding between NH and CO groups, and resulting segmental assemblies will induce the characteristic spacer conformation in the supercooled liquid crystalline component.