Liquid crystalline behavior was first discovered in 1888 when Reinitzer observed that upon heating, cholesteryl benzoate melted to form a turbid fluid then appeared to melt again into a transparent phase at higher temperatures [25]. The first polymeric liquid crystal† was reported in 1937 when it was observed that above a critical concentration the tobacco mosaic virus formed two phases, of which one was birefringent [3]. The first synthetic polymeric system was a poly (γ-benzyl-L-glutamate) solution, which was reported in 1950 [8]. The first thermotropic LCPs were reported in the mid 70’s [9, 12, 28]. Since then a tremendous amount of research has been done from the quantumchemical to the macroscopic level on a great number of systems [22]. The result is that liquid crystalline polymer science has developed into its own discipline.
Liquid crystal (LC) describes a class of materials with long-range molecular order somewhere between the crystalline state, which exhibits three-dimensional order, and a disordered isotropic fluid [15]. Four classes have been identified to describe LC order:
nematic, cholesteric, smectic, and discotic [11]. Nematic describes a LC phase with only one-dimensional long range ordering; it possesses long-range orientational order (molecular alignment) but only short-range positional order (spatial ordering) [8]. A cholesteric is very similar to a nematic, but it is periodically twisted along the axis perpendicular to the long-range order axis. A smectic phase characterizes two-
† Liquid crystal was later renamed mesomorphic phase or mesophase. Mesomorphic is defined: of intermediate form [7]. Friedel found this more appropriate because these materials are not crystalline and may not even be liquids, it is a stable intermediate phase existing between the liquid and crystalline states.
dimensional order. A discotic mesophase can occur when disc-like liquid crystalline molecules align in columns. An illustration of these structures can be found in Figure 1.1.
Figure 1.1. Friedelian Classes: a. Nematic; b. Cholesteric; c. Smectic A; d. Discotic.
Liquid crystalline order originates, in polymeric systems, from nonflexible repeat units, called mesogenic units, with an axial ratio greater than three [8, 21]. In dilute solution the rigid molecules are capable of random arrangement. As concentration increases, the molecules are forced to adopt an oriented conformation because of
a. b.
c. d.
intermolecular repulsions or excluded volume interactions. Mesogenic units can be either rod, disc, or lathe-like and may appear within the molecule backbone either randomly or in a recurring rigid/ flexible structure. These are called main chain LCPs. The other common type of LCP structure, side chain LCPs, occur as a rigid pendent group to a flexible polymer backbone with orientation that can range from parallel to perpendicular to the backbone [21]. Of course another less common possibility would be for the LCP to contain both main chain and side chain units. This leads to a near infinite number of possibilities ranging in structure of mesogenic group and arrangement. Most of the property differences noted between side and main chain LCPs have been related to the greater mobility of side chain mesogenic units as a result of increased backbone flexibility [21].
LC order can exist in either solutions or melts. LC transition in solution is a function of concentration and temperature and is referred to as lyotropic systems [5].
Melts, since concentration is fixed, are only temperature sensitive and are referred to as thermotropics. The LC phase exists between the crystalline melting point, Tm (or in the case where no crystalline state exists, the glass transition temperature, Tg), and the upper transition temperature where the fluid reverts to an isotropic liquid, Tlc→i [20].
Unlike isotropic fluids, orientation is quantified to describe the state and dynamics of liquid crystals. Molecules are preferentially oriented about an axis, an apolar unit vector, n, called the director. The direction of the director is typically arbitrary but can be uniformly aligned through imposing boundaries, applying external magnetic or
electric fields, or inducing viscous flow [20]. Preferential orientation implies that molecules actually possess a distribution of orientation about the director and therefore a parameter is needed to describe that distribution. Assuming rigid rod molecules, the order parameter tensor is defined as [7]:
∫ ( ) −
= i i j ij i
ij f u t uu du
S δ
3
, 1 (1.1)
where ui is a unit vector which describes the orientation of a rigid rod molecule, δij is the Kronecker delta, and f(ui,t) is the orientation distribution function.
The order parameter tensor has a few properties worth noting. It is deviatoric, therefore its trace is equal to zero and it is symmetric. Its eigenvalues or principal values (S1, S2, S3) which define the principal axes of orientation must also add to zero. If all three are equal then S = 0 and the material is isotropic. If two of the eigenvalues are equal then the system is axially symmetric (as a nematic) and Sij can be represented by:
−
= i j ij
ij S n n
S δ
3
1 (1.2)
where S is a certain scalar equivalent to the Hermans orientation function,
2 1 cos
3 2 −
= θ
S (1.3)
ni the projection of the director in the ith direction, while the brackets denote the system average. Values of S range between, 1 > S > -1/2, with the values 1, 0, and –1/2
representing uniaxial orientation, random orientation, and biaxial orientation.
Valid application of a single order parameter to describe orientation requires that the material is uniform throughout or a “monodomain” with a single director. It is possible for elastic distortions to induce slight continuous variation in the director of monodomain systems [7]. This director variation is typically observed in low molecular weight materials and becomes less likely as molecular weight increases because of steric effects [8]. As elastic distortions become more difficult, free energy increases, and director variation becomes discontinuous, resulting in the formation of defects and polydomain textures.
Two types of defects have been identified for nematic liquid crystalline polymers [7]. In thick samples it is possible to observe a system of dark flexible filaments (defined as disclinations) that correspond to lines of singularity in molecular alignment and result in the formation of multiple domain texture. The other defect occurs when the imposed boundary conditions are continuously degenerate (no preferred axis in the plane of the walls). A system of singular nodes (noyaux) form on the surface and the resulting general texture is called a Schlieren texture.
Polydomain texture and defects are important to this particular project because of their influence on rheology. As previously eluded to, the formation of defects is associated with an increase in free energy. Under quiescent conditions the system attempts to minimize excess free energy by combining neighboring disclinations with opposite signs thus eliminating the pair and increasing domain size [13]. However, mechanical energy can be stored during deformation by an increase in the number of
defects. The development of texture and orientation during deformation can change the rheological response to imposed stresses and strains.
Liquid crystals have a number of interesting and potentially useful properties.
Optically, the material can be birefringent, although this may be limited to a local scale [8]. There are also a few polymeric liquid crystals that show nonlinear optical behavior [29]. Many systems have anisotropic diamagnetic and dielectric properties [20]. The modulus is often anisotropic, dependant upon the quality of alignment [19, 26]. An interesting rheological feature is that the nematic phase has a lower viscosity (parallel to the director) than the isotropic phase [8]. Negative normal stresses have also been reported for some systems when subjected to steady shear [15]. LCPs also tend to have low partial entropy of dissolution and therefore have a relatively high resistance to solvents [8]. Gas transport studies have revealed that they have excellent barrier properties because of their low gas solubility in their solid state [14]. Another notable property is that structures can be molded with extremely accurate dimensions because of their low or negligible coefficient of thermal expansion relative to flexible chain polymers [5]. They also exhibit high modulus, strength, and impact properties [18].
These properties can be exploited to apply LCPs in applications where flexible chain polymers perform inadequately.