Self-Assembled Liquid Crystal Polymer Gels

I would like to thank the various members of the Kornfield group with whom I have had the opportunity to work and have not already mentioned: Dr. I also want to thank the members of the Fuzzy Bunnies of Death, the Two Baggers, Ernie's. A.

Introduction

The applications listed are derived from a special class of material known as liquid crystal elastomers. The first liquid crystal elastomer was prepared in 1981 [9], and later the method was improved so that liquid crystal line elastomers could be produced [10].

Liquid crystals

The nematic mesophase

We begin by defining the nematic liquid crystal phase and describing the elastic and viscous forces present. Molecules in the nematic mesophase have a preferred direction of orientational order, known as the n-direction, but no long-range positional order.

Liquid crystal elasticity

Dynamic properties of the nematic director

These modes are a combination of bending and torsion, and the relaxation rates associated with each will be different due to the different associated viscosities and elastic constants. The amplitude and lifetime of a vibration depends on its orientation with respect to the nematic director, and different orientations will have different degrees of contribution from stretching, twisting, and bending, and different associated viscosities.

Figure 1.3: Splay, bend, and twist distortions of a nematic liquid crystal (Adapted from de Gennes and Prost, The Physics of Liquid Crystals, 1993, Oxford University Press, Inc., Oxford)

Liquid crystalline elastomers and gels

LC polymers: anisotropic chains
LC elastomers
LC gels
Block copolymer self-assembly for LC elastomers and gels

LC gels can also exhibit shape changes in response to temperature and external fields, similar to LC elastomers. LC gels can also be made by mixing small molecule gelators with a liquid crystal solvent [36, 28].

Figure 1.4: Schematic of (a) main-chain LCP, (b) side-on SGLCP, and (c) end-on SGLCP. (Adapted from Warner and Terentjev, Liquid Crystal Elastomers, 2003, Oxford University Press, Inc., Oxford.) As shown, main-chain LCPs and side-on SGLCPs typically have a

The molecular theory of nematic rubber elasticity

Soft elasticity

Soft elasticity refers to a phenomenon within the framework of the neoclassical theory of rubber elasticity (Eq. 1.6) where a nematic rubber can undergo a non-trivial deformation without cost to the free energy. Under some imposed loads, the network chains can rotate the distribution of chains at constant average shape of the network chains, and therefore the load is perfectly "soft".

Semi-softness

All soft deformations can be expressed in terms of the initial and final step length tensors, and an arbitrary rotationWα(Equation 1.7) [40]. Semi-softness arising from polydispersity in the network or fluctuations in the composition can be explained by including an additional term, weighted by the semi-softness parameter α, in the free energy expression (Eq.

Objectives

Electro-optical properties of liquid-crystalline physical gels: a new oligo(amino acid) gelator for light scattering materials. Synthesis and characterization of aba-triblock copolymers containing smectic c* liquid crystal side chains via ring-opening metathesis polymerization using a bimetallic molybdenum initiator.

Abstract

Introduction

LC gels have unique thermal, mechano-optical and electro-optical properties, and they offer several advantages for display applications such as thermo-reversible gelation. We find that LC gels have a fast electro-optical response and, because of their homogeneous structure, they are also useful for testing theoretical predictions for liquid crystalline networks.

Experimental

Gel permeation chromatography (GPC)
Nuclear magnetic resonance
Liquid crystal phase identification
Conoscopic imaging
Rheometry
Electro-optic measurements
Synthesis of SGLCPs: homopolymers and triblocks
Mixing of nematic gels

In this chapter, we present the synthesis of LC gels and investigate the phase behavior, alignment, and electro-optical properties of LC gels. The fraction of 1,4-polybutadiene groups in the prepolymer was determined by an NMR of the ABA prepolymer by comparing the peaks at 5.3 and 5.0 ppm.

Figure 2.1: (a) Chemical structure of LC 5CB and the end-associating triblock copolymer ABASiBB used in this study and (b) an NMR of ABASiBB.

Results

Polymer characteristics
Monodomain alignment of LC gels
Dynamic mechanical analysis
Reversible electro-optic response

The interference figure indicates that the nematic director is oriented slightly off-axis in the plane of the sample. The transmitted intensity is normalized by the transmitted intensity of the gel in the isotropic state.

Figure 2.5: (a) A sample of LC physical gel, and (b) a schematic of the LC triblock gel

Discussion

The dynamic electro-optical response of LC gels also depends on the polymer concentration and sample history (Appendix A). The time required to reach 90% of the maximum throughput is denoted by τ90. b) Switching off the electric field.

Figure 2.10: Transient electro-optic properties of a 5 wt % ABASiBB, 25 µm thick layer, under application of a.c

Abstract

Introduction

The middle block is a random copolymer of LC side group and butadiene monomer, where the LC side group makes up approx. 70 mol% of the center block. In the scheme, the spotted background represents the LC solvent 5CB, the dark black lines represent the SGLCP midblock, and the red chains represent the polystyrene endblocks.

Experimental

Nematic gels thermoreversibly change to a viscous liquid when heated above the nematic-isotropic phase transition temperature (TN I) 5CB, 35.2oC, allowing easy loading of the gel into the cell gap. After removing the cell from the magnetic field, the cell was warmed to 34 °C and then cooled back to room temperature at 1.

Results

Details of the stripe texture

The orientation of the director imprinted during crosslinking and the orientation of the crossed polarizers is shown in the upper left corner of each image. The positions of the extinction bands in the images are shifted relative to each other, indicating a spatially varying orientation of the director in the sample.

Mathematical model

The free energy density of the gel consists of three terms: the ideal elasticity of the nematic network, the elasticity of the non-ideal nematic network, and the Frank elasticity (Eq. 3.1). We analyze the behavior near the transition to the striped state, where we assume small values for the driver rotation amplitudes and displacements.

Figure 3.3: Dependence of pitch on gap in 10 and 5 wt % polymer gels.

Discussion

The penalty arises from the boundary condition, which necessitates a bending distortion and displacement from the edges to the center of the specimen. The model also accounts for the disappearance of stripes at small gaps from a prediction of the threshold anisotropy (Fig. 3.5).

Figure 3.5: (a) Predicted value of the threshold anisotropy r th , as a function of gap thickness d and rubber modulus µ

Abstract

We thank Professor Sprunt's students, Sunil Sharma and Krishna Neupane, for their help with the light scattering experiments. The contents of this chapter were submitted for consideration for publication in the journal Soft Matter on January 18, 2007.

Introduction

Physical SGLCP-PS gels and, for comparison, in homopolymer solutions of the SGLCPs on which the gels are based. Thus, the dynamics of physical LC gels, which exhibit a broader spectrum combining hydrodynamic director modes with very slow relaxation associated with gel structure rearrangement, are quite different from previous studies of covalently bonded networks such as LC elastomers [5], where light scattering only reveals director modes that are non-hydrodynamic, due to a direct coupling of director rotation to network motion.

Experimental

The time autocorrelation of the scattered intensity (< I(q,0)I(q, t) >) collected at discrete angles was recorded in the homodyne regime. The intensity of the scattered, depolarized light is recorded at a discrete angle, θs, in the laboratory frame, corresponding to a specific final wave vector,kf and scattering vectorq = kf−ki.

Figure 4.1: Chemical structure of the side-group LC homopolymers (HSiCB4 and HSiBB) and triblock copolymers (ABASiCB4 and ABASiBB).

Results

5 wt% side-on homopolymer (HSiBB) in (a) H-orientation and (b) V-orientation; and 5 wt% side-on triblock copolymer gel (ABASiBB) in (c) H-orientation and (d) V-orientation. In contrast, the relaxation time for side-on homopolymer in the V geometry is accelerated by orders of magnitude (Fig. 4.10).

Figure 4.3: Normalized time correlation functions g 2 (t) at 25 o C and θ s = 30 o C for 5CB, 5 wt % homopolymer solutions, and 5 wt % triblock copolymer gels in the H and V orientations

Discussion

However, at long enough times, the physical junctions in the network reorganize and cause the nematic director to lose full correlation with its initial orientation. Another aspect of anisotropic gel structure is observed in the initial decay of the correlation data - specifically the faster initial relaxation observed in the edge-on gels relative to the corresponding homopolymer solution (Fig. 4.6d).

Figure 4.12: Schematic for network relaxation via a fast mode that retains the network structure and a slow mode that requires the reorganization of the physical network

Conclusions

Dynamic light scattering from a nematic monodomain containing a side-chain liquid crystal polymer in a nematic solvent. Effect of molecular architecture on the electrorheological behavior of liquid crystal polymers in nematic solvents.

Abstract

Much of the work in this chapter represents the combined efforts of both myself and Neal Scruggs. Finally, the work in this chapter could not be done without the materials received from Dr.

Introduction

Both SANS and rheometry show that the driving force for self-assembly increases with PS block size: a larger PS block size results in a higher TM ST and a higher sol–gel transition temperature. Also, the PS block length is an important parameter for the phase behavior of the gels, and samples with large (>100 kg/mol) PS blocks form gels even above TN I.

Experimental

Preparation of nematic gels
LC phase identification
Rheometry
SANS

SANS experiments were performed on the Small Angle Scattering Instrument (SASI) at the Intense Pulsed Neutron Source at Argonne National Laboratory. The scattering experiments were performed on polydomain samples at temperatures varying between 25oC and 60oC.

Results

Polymer characteristics and phase behavior

The phase behavior of PS [3] in 5CB and current SGLCP [5] in 5CB has been previously studied. All polymers were soluble in 5CB in both nematic and isotropic phases at concentrations up to 10 wt.

Neutron scattering

A proper fit could provide quantitative data on the size of the micelle core, the micelle corona and the intermicellar distance [12, 16]. The cores may be very swollen by solvent and the aggregation number of the micelles is unknown.

Rheometry

First, the data suggest that micelles can persist well above the sol–gel transition as measured by rheology. The data also show that the location of the sol–gel transition is correlated with micelle formation.

Table 5.3: Gel storage modulus (Pa) at 20 o C and 1 rad/s

Discussion

Neutron scattering and rheometry studies establish the relationship between micelle formation and gel mechanical properties. PS block size is an important variable that determines the driving force for self-assembly, with larger PS blocks being associated with gelation even above TN I .

SANS of diblock and triblock gels

The mixtures are single-phase nematic for all T ≤ 33.5 oC and single-phase isotropic for T >33.5oC.

Figure 5.5: Azimuthally averaged SANS pattern for 5 and 10 wt % 530(60)AB in perdeuterated 5CB

Rheometry of diblock and triblock mixtures

The mixture is single-phase nematic for allT ≤35.5oC (closed symbols) and single-phase isotropic for T ≥40.0oC (open symbols). The mixture is single-phase nematic for all T ≤32.0oC (closed symbols) and single-phase isotropic for T ≥35.7oC (open symbols).

Figure 5.13: Dynamic storage modulus(G 0 ) and loss modulus (G 00 ) of 5 wt % 530(60) AB diblock in 5CB

Introduction

Here we report the preparation of a covalent LC network by controlled "click" cross-linking of telechelic LC polymers. Recently, it has been used to fabricate well-defined hydrogels [20] and model networks [21] by controlled cross-linking of telechelic polymers.

Experimental

Synthesis of 5,6-disubstituted cyclooctene-based mesogen 2

The mixture was refluxed for 24 h and the product was purified by extraction with 1 N HCl (20 mL, repeated 3 times), followed by extraction with a saturated solution of aqueous NaHCO3 (50 mL) and with a saturated aqueous solution of KCl (50 ml). The product was dried over MgSO4 and purified by fractionation on a silica gel column with 30% ethyl acetate in hexanes (75% yield).

Polymerization via ROMP and end-group functionalization

HCl (from JT Baker) (50 mL) was added to precipitate the acidic product, which was collected by filtration, washed with H 2 O and dried in vacuo at 60 °C in 95% yield.

Crosslinking of telechelic polymers by “click” chemistry

Results

Synthesis of covalent LC network

Controlled "click" cross-linking of these telechelic polymers was achieved by reacting a tri-acetylene species, tripropargylamine, with the polymer azide end groups in the presence of CuBr as catalyst and PMDETA as a ligand in DMF at 50oC (Fig. 6.3 ). IR spectrometry shows complete disappearance of the azide absorption, indicating that most of the azide end groups have reacted in the crosslink.

Figure 6.2: Reaction scheme for ROMP of substituted cyclo-octene

Temperature-dependent swelling of covalent LC networks in liquid crystalcrystal

Electro-optical properties of LC gels

We are interested in the threshold for obtaining a response, the dynamics of the response, and the texture and shape changes associated with the electro-optical response. Unlimited samples were prepared by carefully cutting away a thin (~7 µm) piece of the LC elastomer and placing it in a 100 µm thick gap between ITO coated plates filled with 5CB.

Figure 6.4: LC gels at a) 34.5 o C, b) 35.5 o C, and c) 37.0 o C. Image a was taken with crossed polarizers, and images c and dwere taken with uncrossed polarizers

Discussion

The present system with a controlled network structure and appropriate choice of LC side group is a promising candidate to elucidate the dynamic behavior of LC elastomers. One of the unsolved challenges of these materials is achieving a uniformity of alignment and control of the sample size, and this represents the next major obstacle to the use of covalent LC networks for careful physical studies.

Conclusions

A novel liquid crystalline polymer of 5-substituted cis-cyclooctene side chain by ring-opening metathesis polymerization. The field of liquid crystal elastomers and gels has usually been dominated by polymer and liquid crystal scientists with a physics background.