Investigations of a New Reversible Strain Induced Chromic Transition in Polydiacetylene Elastomers
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INVESTIGATIONS OF A NEW REVERSIBLE STRAIN INDUCED CHROMIC TRANSITION IN POLYDIACETYLENE ELASTOMERS R. A. Nallicheri and M. F. Rubner* Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
ABSTRACT In-situ visible dichroism/tensile elongation studies of segmented polyurethanes containing conjugated polydiacetylene chains within their hard segment domains have revealed a new reversible mechanochromic transition that manifests itself as a color change from blue to yellow during tensile elongation. This unusual strain induced transition, found in an elastomer whose hard segment structure is derived from hexamethylene diisocyanate and 5,7-dodecadiyne-1,12 diol, closely resembles the thermochromic transition also exhibited by this same material. The color change and the associated shift in the position of the polydiacetylene absorption band to higher energies is not simply an order-disorder transition but reflects a strain induced reorganization of the hard domain structure. It has been found that this blue to yellow transition is reversible as long as the material is not strained beyond about 300%. This represents the first reversible, mechanically induced chromic transition in a cross-polymerized diacetylene containing polymer.
INTRODUCTION The optical properties of polydiacetylenes are known to be very sensitive to the molecular environment of their conjugated polymer backbones. Variations in backbone planarity and/or the level of stress experienced by the backbone can result in significant changes in the energy, shape and breadth of the absorption bands characteristic of these materials [1]. Thus, the electronic states of the conjugated backbone are strongly coupled to its conformation. As a result of this strong coupling, the polydiacetylenes exhibit a wide variety of chromic transitions that can be induced by temperature, stress or solvent changes. In this paper, we discuss a new reversible strain induced chromic transition observed during the tensile elongation of a polyurethane-diacetylene segmented copolymer. In such materials, the fully extended polydiacetylene chains reside exclusively in phase separated hard domains dispersed throughout a soft elastomeric matrix. Consequently, any changes in the electronic states of the conjugated backbone are a direct result of changes in the molecular organization of the hard domains. Previous work has demonstrated that these materials exhibit thermochromic behavior [2] and that their mechanical properties can be controlled systematically by varying the level of diacetylene cross-polymerization within the hard domains [3]. These new elastomers therefore exhibit an unusual collection of mechanical and optical properties that can be directly traced to their two phase microstructures.
EXPERIMENTAL Details concerning the synthesis, structure and characterization of the polyurethane diacetylene segmented copolymers can be found in our previous publications [2-4]. The elastomers are designated by the monomer
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