Piezochromic Polymer Materials Displaying Pressure Changes in Bar-Ranges

A piezochromic material exhibiting a pressure dependent reversible shift of the selective reflection wavelength via the entire visible range is developed. The material consists of a cholesteric mixture embedded in a cholesteric elastomer matrix bearing mesogenic side chains with similar molecular structure. The change of the selective reflection has its origin in a compressible helix structure. Thus, the pitch length is switched by pressure changes. To trigger the reversible piezochromic effect already pressure changes in bar-range are sufficient. The pressure controlled colour changes are excellent detectable by the human eye and still appear even after 100 cycles. Piezochromic effects based on modification changes of inorganic crystalline materials, such as LiF or NaCl monocrystals, are known phenomena since several decades. For example, the transition from the green α- into the red γ-modification of CuMoO4 requires a pressure of 2.5 kbar and transitions in palladium complexes are reported to re- quire pressures ranging from 1.4 to 6.5 GPa (=14 to 65 kbar)(1,2). However, the high pressure which is necessary for the modification changes makes these materials unsuit- able as pressure sensors in the ordinary area of life. Colour changes by pressure in organic polymer materials are fre- quently but not systematically described in literature so far. A bathochromic shift of the absorption band of poly (3-dodecylthiophene) was observed by increasing the pres- sure from atmospheric pressure to 8 kbar(3). In reference(4) a bathochromic shift from 605 nm to 672 nm (∆λ = 67 nm) is reported for poly(3-(1-dodecyl) thiophene-2,5-diyl) when the pressure is increased from normal pressure to 10.71 GPa (=107.1 kbar). The piezochromic effect of these conjugated polymers is based on pressure dependent shifts of the ab- sorption band. The aim of the present work was to develop piezochromic polymer materials exhibiting, for the first time, colour changes detectable with the human eye in dependence of pressure differences as small as a few bar. Such material could be used as an optical pressure sensor in the ordinary area of life. Our strategy is based on the knowledge about the preparation of cholesteric polymer materials with a selective