Introduction to Mesomorphism and Thermotropic Liquid Crystals.ppt

1、Introduction to Mesomorphism and Thermotropic Liquid Crystals,Terminology,A Mesogen has mesomorphic properties, exhibits mesomorphism, and is mesomorph. Mesogenic is a material that is “mesogen-like” but to necessary mesomorph. A liquid crystal (LC) is a mesogen but not every mesogen is a liquid cry

2、stal. Thermotropic LCs show mesomorphism upon heating of the neat solid material (pure compounds or mixtures). Lyotropic LCs display mesophases when dissolved in a solvent (often water) or a solvent mixture and their mesomorphism depends on the concentration and temperature.,States of Matter,MESOPHA

3、SES,plasmas,liquid crystals 0-2D l.-r. positional order l.-r. orientational order macroscopic fluidity,solids,liquids,gases,soft & plastic crystal phases 3D l.-r. positional order reduced l.-r. orientational order macroscopic plasticity,low-dimensional solids (e.g. soft polymer glasses) no 3D l.-r.

4、positional order no l.-r. orientational order macroscopic plasticity,high temperature,low temperature,?,?,Order of N, Sm, and Col Mesophases,n,director,2D positional order,1D positional order,no positional order,N,SmA,Colh,orientational order,calamitic,discotic,Calamitic LCs,Discotic LCs,Molecules w

5、ith an elongated rigid core (one molecule axis is much longer than the other two; high aspect ratio),Disc-shaped molecules (one molecule axis is much shorter than the other two; low aspect ratio),Nematic (N) LCs (Gr. for thread-like) Smectic (Sm) LCs (Gr. for soap-like),Main- & side-chain calamitic

6、LC polymer,Rigid-rod and board-shaped LC polymers,Main- & side-chain discotic LC polymer,Discotic nematic (ND) mesophases Discotic columnar (Col) mesophases,The mesophase is marked with an Asterisks if it contains one enantiomer of a chiral molecule, which could be a dopant or the LC itself. However

7、, a mesophase containing chiral molecules does not necessary show chiral bulk properties (e.g. racemic mixtures). The chiral nematic phase (N*) is also called cholesteric phase.,Chiralety and Mesophases,Polymorphism of Smectic Mesophases,Collings PJ and Michael H, Introduction to LCs, Taylor & Franc

8、is, 1997.,Structure of Chiral Mesophases,N* or cholesteric mesophase,Chiral smectic C (SmC*) phases,A pitch can be as small as 100 nm, as large as 10 mm, and varies with temperature.,Collings PJ and Michael H, Introduction to LCs, Taylor & Francis, 1997.,Polymorphism of Discotic Mesophases,Discotic

9、nematic (ND),Discotic hexagonal columnar Colh or Dh Colho ordered; Colhd disordered,Discotic rectangular and tetragonal columnar Colr or Dr; Colt or Dt,Discotic oblique columnar Colob or Dob,An ordered molecular arrangement within a column requires a persistence length of at least 20-30 molecules,un

10、iaxial,uniaxial,biaxial,biaxial,uniaxial,(Colh plastic crystal),Thermotropic mesomorphism governed by molecular design,Shape anisotropy,Intermolecular interactions,Microphase separation,hard-rod potential by Onsager: Ann. New York Acad. Sci. 51 (1949), 627,rods,discs,not mesomorph,dipole-dipole inte

11、raction induced mesomorphism,hydro- and fluorocarbons do not display mesomorphism,semifluorinated n-alkanes display soft crystal phases,Liquid Crystals of Unusual Shape,Triptycene derivative (Epitaxygen) Norvez et al. Liq. Cryst. 14 (1993), 1389,SmA layers,Dendrimeric liquid crystal (SmA) Lorenz et

12、al., Adv. Mater. 8 (1996), 414,Discotic liquid crystal without side-chains (Colh) Barber at al., Angew. Chem. Int. Ed. 37 (1998), 296,Introduction to Lyotropic Liquid Crystals,Thermotropic LCs show mesomorphism upon heating of the neat solid material (pure compounds or mixtures). Lyotropic LCs displ

13、ay mesophases when dissolved in a solvent (often water) or a solvent mixture and their mesomorphism depends on the concentration and temperature. The term lyotropic stems from the Greek word for solve.,All pictures taken from H. Stegemeyer, “Liquid Crystals”, Topics in Physical Chemistry; Steinkopff

14、 Darmstadt, Springer New York, 1994,The Hydrophobic Effect,The entropy of water is exceptionally high and the tetrahedral network of water molecules, maintained by hydrogen bonds, can accommodate small guest molecules in its cavities. Large solute molecules brake the hydrogen bonds between adjacent

15、molecules which is a enthalpy-costly process. Most often, the entropy of mixing can not overcompensate the enthalpy-loss for solved molecules that do not form hydrogen bonds. This is why hydrocarbons show very low solubility in water. They are expelled from the water phase.,Molecular Structure of Su

16、rfactants,Examples for amphiphilic molecules, composed of a polar hydrophilic headgroup and a hydrophobic tail.,Aggregation of Surfactants,CMC = Critical Micelle Concentration,Surfactant molecules start aggregating in water at rather low concentration. Both of the two parts of the amphiphilic molecu

17、les profit by gathering into locally hydrophobic and hydrophilic micro-domains. Micelles, “finite” size dynamic aggregates of 50 up to several hundred molecules, can only exist at temperatures above the Krafft-point temperature (TK). The normal micelle turns into a reverse micelle at high surfactant

18、 concentrations confining the remaining water molecules to its now hydrophilic interior.,solid,micellar “solution”,solution,Structures of micellar aggregates and lyotropic mesophases of rod- shaped surfactants. Disc-shaped water soluble, but non- amphiphilic, ionic aromatic pharmaceuticals also self

19、-organize into p,p-stacks that form lyotropic mesophases (“chromonics”).,Three-component phases diagram of sodium octanoate/water/decanol (co- surfactant),Two-component phases diagram of C12EO6 in water,Phase Diagrams of Surfactants in Water,Isotropic normal/inverse micelles (L1/L2), Isotropic bicon

20、tinuous (sponge) (L3); Hexagonal normal/inverse (H1/H2); Nematic rod-like (NC), disc-like (ND, NL); Rectangular (R); Lamellar liquid chains (La), all-trans chains (Lb); Cubic normal/inverse spherical (I1/I2), normal/inverse bicontinuous (V1/V2);,Packing Considerations developed by Israelachvili,The

21、hydrophobic tail is geometrically characterized by its volume v and length lc. The area occupied by the head group at the interface is defined as ao and includes the first hydration layer. The ratio of v/lc to ao determines the shape of the aggregates (micelles).,SDS = sodium dodecyl sulfate,CTAB = Hexadecyltrimethyl- ammonium bromide,