A Mechanical Strain Sensor for Polymeric Materials and Photophusical Investigations of Large Molecules

The degree of deformation (strain) observed when a given force (stress) is applied to a polymeric material, and the relationship between deformation and force are a fundamental property when the mechanical state of a polymeric material is described. A number of methods exist to investigate this relationship on a macroscopic level (rheology measurements etc.). However, the molecular processes that parallels macroscopic deformation is not well understood. A tool that allows measurement of the molecular changes when the material is strained or stressed may produce new insight into the structure-property relationship of polymeric materials, and lay ground for new advanced polymer based materials. In addition, the current theory of rubber elasticity and molecular deformation might be experimentally probed. To investigate the molecular responds to deformation, a fluorescent sensor unit was placed in the middle of a polymer backbone, as depicted below. It was envisaged that straining the polymer chains would produce affect the fluorescent properties of the single site sensor unit. Four fluorescent strain probes with one or two carbazole units placed in the middle of an elastomeric tri-block of polystyrene-polyisoprene-polystyrene (SIS) have been prepared (see below) and their fluorescence behaviour investigated under strain. The idea with the probes 11-13 was that the carbazole-carbazole interaction could be changed with strain (see below). In the conformation shown left the carbazole units interact, and upon excitation and excimer might form. To right the carbazole moieties are isolated and excimer formation is not likely. Emission form the carbazole excimer can be distinguished from the fluorescence from the isolated carbazole. Thus the conformation change shown below should be detectable using fluorescence spectroscopy. The synthesised probes were dissolved in commercial SIS type rubbers in low levels of 0.1 weight-%. Films cast from this blend were subjected to tensile elongation while the fluorescence spectra were obtained. Below are the fluorescence spectra of probe 12 shown at varying strain levels. Thus the fluorescence of the probe changes considerably with macroscopic strain. Shown below is a plot of the relative intensity of the low wavelength peak (I₃₅₄/I₃₇₀) as function of tensile strain for the four probes. A rather large change from ca. 0.7 to 1.1 in the relative intensity at two wavelengths (I₃₅₄/I₃₇₀) was observed on elongations up to 400%. The spectroscopic change is strongest for small strain levels around 0-200% and levelled out above 300-400%. All four strain probes gave similar results on uniaxial elongation, even probe 10 with only one carbazole unit. Thus, there seems to be no significant effect of the close proximity of the two carbazole units in probes 11-13. This rules out any major contribution from excimer to monomer type shifts in the fluorescence of the probes. Since probe 10 show the same emission change as probes 11-13 it was concluded that changes in the emission vibronic bands are the major effect involved in the emission change measured.