Design, synthesis and characterization of side chain liquid crystal segmented polyurethanes
Author(s)Nair, Bindu R., 1974-
Massachusetts Institute of Technology. Dept. of Materials Science and Engineering.
Paula T. Hammond.
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Side chain liquid crystal polyurethanes are liquid crystal elastomers with the potential to couple mechanical stress fields with optical changes brought on by liquid crystal alignment. These materials excite interest because they are the first thermoplastic liquid crystal elastomers (LCTPEs) that utilize the physics of microphase segregation rather than chemical crosslinking to achieve elastomeric properties: LCTPEs offer the potential to use traditional plastic processing technologies to effect the liquid crystal monodomain ordering required for mechanooptic applications. This thesis describes the design. synthesis and characterization of two series of side chain liquid crystal po I yurethanes. The first series of polyurethanes are made with traditional hard segments and cyano-biphenyl mesogens pendant on the soft segment. The synthesis of the soft segment macrodiol. a carbinol endcapped hydromethylsiloxane oligomer that can be functionalized with mesogen via a hydrosilylation technique, is effected by a polycondensation approach. These side chain liquid crystalline (LC) siloxane macromonomers are then converted to segmented polyurethanes using traditional urethane chemistry. Structural analysis and characterization of the resulting polymers is provided. along with a comparison study of the thermal and optical behavior of these urethanes as the spacer length is increased from 3 to 8 methylene units. Discussions of the effects of the hard segment mobility on the phase behavior of these LC polyurethanes is offered. Fourier-transform infrared (FT-IR) dichroism experiments performed on the cyano-biphenyl polyurethanes provide an opportunity to study the interplay between polyurethane morphology and liquid crystal ordering as the material is exposed to mechanical deformation. This complex material follows the trend established in the literature for both side chain liquid crystal siloxane homopolymers and segmented polyurethanes. At low strains. the soft segments align with the strain inducing an orientation in the "lone" horn segments. Up to strains of 40%, the LC mesogens align with the strain field and the hard segments in hydrogen bonded domains align perpendicular to the field. At strains above 40%, a rearrangement of the ordering that results in symmetric layers and hard segments aligning parallel to the field is found. A model is proposed to represent these findings. and reflections on the cooperative movement of the different macromolecular components of the polyurethane are offered. Phase modulated dynamic Ff-IR experiments are performed to study the response of these materials to sinusoidal mechanical perturbations in an attempt to further understand the response of the polyurethanes to mechanical fields. In dynamic Ff-IR. the viscoelastic reorientation of various segments of the macromolecule can be monitored as a function of applied strain. Evidence is presented for the two types of hard segments: those involved in hydrogen bonding within hard domains, and those found in "lone" hard segments in the soft matrix. Evidence is also presented for two types of mesogens: those found in smectic layers. and those not involved in smectic ordering postulated to be located at the hard domain interface. The hard domains and the smectic layers have strong viscous components to their mechanical response. The "free" mesogens and the "lone" hard segments, on the other hand, exhibit a more elastic response. Once again. evidence for the mechanical coupling of the hard segments and the LC mesogens is of~·ered. In an attempt to improve the mechanical properties of the LC segmented polyurethanes. and to explore piezoelectric and electro-mechanical behaviors. new LC segmented polyurethanes are designed with higher overall molecular weight. longer soft segments. higher degrees of microphase segregation. and smectic C* mesogens. Vinylmethylsiloxane macrodiols are anionically synthesized using a diifunctional intitiator. and then polymerized to form segmented polyurethanes by condensation reactions with diisocyanates and chain extenders. The degree of functionalization of the soft segment. and the length of the hard segment are varied in attempts to decrease the degree of mechanical coupling between the hard segment and the mesogens. It is found that polyurethanes with 50% functionalized soft segments provide phase segregated morphologies. form cohesive polyurethane films. and show decoupling in the thermal phase behavior of the LC soft segment and the hard segment. The study of side chain liquid crystal polyurethanes described in this thesis provides a fundamental understanding of the properties of this new class of materials. The interdependance of the hard segment and the mesogen on the thermal and mechanical responses of these materials is a key finding. The development of LC polyurethanes in which this cooperative interaction is muted leads to materials with great potential for mechano-optic applications. The findings reported here should be helpful in testing the piezoelectric and mechano-responsive behaviors of these promising materials.
Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2000.Includes bibliographical references.
DepartmentMassachusetts Institute of Technology. Dept. of Materials Science and Engineering.
Massachusetts Institute of Technology
Materials Science and Engineering.