We report the synthesis of azobenzene-containing coil-liquid crystal-coil triblock copolymers that can serve as mechano-optic actuators for applications that include non-invasively steering fiber optics. The coil (polystyrene) end-blocks phase segregate from the liquid crystal midblock forming of uniform and uniformly-spaced physical crosslinks, resulting in highly reproducible and thermoreversible networks by self-assembly. These polymers are elastic in the melt (at room temperature) and can be easily spun, coated or molded. Mechanical stretching results in a temporary monodomain alignment. Starting from identical triblock prepolymers (with polystyerene end blocks and 1,2-polybutadiene midblocks), a matched pair (azobenzene-containing, and non-azobenzene-containing) of liquid crystal triblock copolymers was synthesized. These triblocks were then be blended to prepare a series of elastomers with 0 to 5% azobenzene groups, while matching in nearly all other physical properties (cross-link density, modulus, birefringence, etc.), allowing the effect of concentration of photo-responsive groups to be unambiguously determined. Results will be presented that demonstrate this approach to independent control of optical density and photo-mechanical sensitivity.
We report the synthesis of azobenzene-containing coil-liquid crystal-coil triblock copolymers that form uniform and highly reproducible elastomers by self-assembly. To serve as actuators to (non-invasively) steer a fiber optic, for example in deep brain stimulation, the polymers are designed to become monodomain “single liquid crystal” elastomers during the fiber-draw process and to have a large stress/strain response to stimulation with either light or heat. A fundamental scientific question that we seek to answer is how the interplay between the concentration of photoresponsive mesogens and the proximity to the nematic-isotropic transition governs the sensitivity of the material to stimuli. Specifically, a matched pair of polymers, one with ~5% azobenzene-containing side groups (~95% cyanobiphenyl side groups) and the other with 100% cyanobiphenyl side groups were synthesized from identical triblock pre-polymers (with polystyerene end blocks and 1,2-polybutadiene midblocks). These can be blended in various ratios to prepare a series of elastomers that are precisely matched in terms of the backbone length between physical crosslinks (because each polymer is derived from the same pre-polymer), while differing in % azobenzene side groups, allowing the effect of concentration of photoresponsive groups to be unambiguously determined.
Soft-solids that retain the responsive optical anisotropy of liquid crystals (LC) can be used as mechano-optical, electro-optical and electro-mechanical elements. We use self-assembly of block copolymers to create reversible LC gels and elastomers that flow at elevated temperatures and physically cross link upon cooling. In the melt, they can be spun, coated or molded. Segregation of the end-blocks forms uniform and uniformly spaced crosslinks. Matched sets of block copolymers are synthesized from a single "prepolymer." Specifically, we begin with polymers having polystyrene (PS) end blocks and a poly(1,2-butadiene) midblock. The pendant vinyl groups along the backbone of the midblock are used to graft mesogens, converting it to a side-group LC polymer (SGLCP). In the present case, cyanobiphenyl groups are used as the nonphotoresponsive mesogens and azobenzene groups are used as photoresponsive mesogens. Here we show that matched pairs of block copolymers, with and without photo-responsive mesogens, provide model systems in which the optical density can be adjusted while holding other properties fixed (cross-link density, modulus, birefringence, isotropic-nematic transition temperature). For example, a triblock in which the SGLCP block has 95% cyanobiphenyl and 5% azo side groups is miscible with one having 100% cyanobiphenyl side groups. Simply blending the two gives a series of LC elastomers that have from 0 to 5% azo, while having all other physical properties matched. Results will be presented that show the outcomesof this approach to systematic and largely independent control of optical density and photo-mechanical sensitivity.
Self-assembly of coil-SGLCP-coil block copolymers provides a route to model liquid crystalline (LC) gels in concentrations ranging from bulk polymer to gels with as little as 5 wt % polymer in a small-molecule LC host. Triblock copolymers with LC-phobic endblocks associate in a nematic liquid crystal to create crosslinks. An SGLCP midblock allows for solubilization of the network. These materials have a precisely tailored network structure ideal for comparison to theory1 of liquid crystalline elastomers. In addition, the gel responds readily to external fields since the concentration of polymer can be relatively low. In this report we discuss rheological measurements that demonstrate gelation at low polymer concentrations. We show that the association of the PS blocks at low concentration is driven by the presence of the SGLCP, rather than incompatibility between PS and 5CB. We then discuss the alignment of the gels via shear, magnetic fields, electric fields, and alignment surfaces. Finally, we present results on a distinctive striped texture observed in aligned gels when subjected to an applied electric field or normal force. The exceptional electro-optic and mechano-optic responsiveness of these gels coupled with thermally reversible gelation suggests they would be ideal candidates for use in large-area printable display technology.
The Display and Beam Steering Thrust of the AFOSR Liquid Crystal MURI addressed key materials and device technology issues affecting performance of liquid crystal (LC) electro-optic (EO) devices, particularly device structures useful in information Displays and for Laser Beam Steering and Switching. Two basic themes were development of bulk LCs having high performance characteristics (nematic LCs, and chiral smectic LC devices having analog response), and development of novel LC electro-optic structures. Research on novel device structures led to advances in LC alignment and on photonic band-gap materials.
Recent exciting advances have been made using infrared (IR) diode lasers to obtain sensitive and rapid measurements of molecular-level dynamics that control polymer material properties. Infrared diodes provide a bright, nearly monochromatic, polarized source that is excellent for use in polarimetry. Optical polarimetry is the measurement of the state of polarization of light. It is a powerful technique for studying molecular and microstructural anisotropy in a sample by measuring how the sample alters the polarization of a transmitted beam. In polymers, it is precisely such molecular anisotropy that underlies their viscoelastic properties. As theories are developed to predict the relaxation dynamics of individual parts of a polymer molecule, and of different species in polymer blends, there is increasing interest in experimental methods to distinguish these dynamics. Infrared polarimetry is extremely valuable in this regard, not only because of the direct relationship between the IR dichroism and molecular orientation, but also because it is amenable to labeling. The present paper reviews the basis of this technique and its application to study the effects of polydispersity on melt rheology, and the relaxation dynamics at different positions along a polymer chain. It concludes with an overview of research in progress and future problems to be tackled.
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