title: Miscibility studies of two twist-bend nematic liquid crystal dimers with different average molecular curvatures. A comparison between experimental data and predictions of a Landau mean-field theory for the NTB-N phase transition.
abstract: We report a calorimetric study of a series of mixtures of two twist-bend liquid crystal dimers, the 1'',7''-bis(4-cyanobiphenyl)-4'-yl heptane (CB7CB) and 1''-(2',4-difluorobiphenyl-4'-yloxy)-9''-(4-cyanobiphenyl-4'-yloxy) nonane (FFO9OCB), the molecules of which have different effective molecular curvatures. High-resolution heat capacity measurements in the vicinity of the NTB-N phase transition for a selected number of binary mixtures clearly indicate a first order NTB-N phase transition for all the investigated mixtures, the strength of which decreases when the nematic range increases. Published theories predict a second order NTB-N phase transition, but we have developed a self-consistent mean field Landau model using two key order parameters: a symmetric and traceless tensor for the orientational order and a short-range vector field which is orthogonal to the helix axis and rotates around of the heliconical structure with an extremely short periodicity. The theory, in its simplified form, depends on two effective elastic constants and explains satisfactorily our heat capacity measurements and also predicts a first-order NTB-N phase transition. In addition, as a complementary source of experimental measurements, the splay (K1) and bend (K3) elastic constants in the conventional nematic phase for the pure compounds and some selected mixtures have been determined.
authors.affiliation: Grup de Propietats Físiques dels Materials (GRPFM-GCMM), Departament de Física, E.T.S.E.I.B, Universitat Politècnica de Catalunya, Diagonal, 647 08028 Barcelona, Spain. David.orencio.lopez@upc.edu.
authors.forename: D O
authors.initials: DO
authors.lastname: López
authors.affiliation: Grup de Propietats Físiques dels Materials (GRPFM-GCMM), Departament de Física, E.T.S.E.I.B, Universitat Politècnica de Catalunya, Diagonal, 647 08028 Barcelona, Spain. David.orencio.lopez@upc.edu and Departamento de Física Aplicada II, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Apartado 644, E-48080 Bilbao, Spain.
authors.forename: B
authors.initials: B
authors.lastname: Robles-Hernández
authors.affiliation: Grup de Propietats Físiques dels Materials (GRPFM-GCMM), Departament de Física, E.T.S.E.I.B, Universitat Politècnica de Catalunya, Diagonal, 647 08028 Barcelona, Spain. David.orencio.lopez@upc.edu.
authors.forename: J
authors.initials: J
authors.lastname: Salud
authors.affiliation: Departamento de Física Aplicada II, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Apartado 644, E-48080 Bilbao, Spain.
authors.forename: M R
authors.initials: MR
authors.lastname: de la Fuente
authors.affiliation: Institute for Experimental Physics, Otto-von-Guerike Universität Magdeburg, ANP, Universitätsplatz, 39106 Magdeburg, Germany.
authors.forename: N
authors.initials: N
authors.lastname: Sebastián
authors.affiliation: Grup de Propietats Físiques dels Materials (GRPFM-GCMM), Departament de Física, E.T.S.E.I.B, Universitat Politècnica de Catalunya, Diagonal, 647 08028 Barcelona, Spain. David.orencio.lopez@upc.edu.
authors.forename: S
authors.initials: S
authors.lastname: Diez-Berart
authors.affiliation: Grup de Propietats Físiques dels Materials (GRPFM-GCMM), Departament de Física, E.T.S.E.I.B, Universitat Politècnica de Catalunya, Diagonal, 647 08028 Barcelona, Spain. David.orencio.lopez@upc.edu.
authors.forename: X
authors.initials: X
authors.lastname: Jaen
authors.affiliation: School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK.
authors.forename: D A
authors.initials: DA
authors.lastname: Dunmur
authors.affiliation: Chemistry, University of Southampton, Highfield, Southampton, SO17 1BJ, UK.
authors.forename: G R
authors.initials: GR
authors.lastname: Luckhurst