Deformation-induced color changes in melt-processed photoluminescent polymer blends

Binary blends of linear low-density polyethylene (LLDPE) and a series of highly photoluminescent (PL) 1,4-bis(alpha-cyano-4-alkoxystyryl)benzenes were prepared using melt-processing techniques. It was shown that the phase behavior of these blends, which comprised 0.01-3% w/w of the PL guest, could be controlled via the molecular structure of the dye, its concentration, and the processing conditions. Because of the dyes' strong tendencies to form excimers, the emission characteristics of the blends strongly depended on the extent of molecular aggregation of the PL guest molecules. Bathochromic shifts of up to 147 nm were observed when comparing the PL emission spectra of molecularly mixed blends with samples that comprise aggregates of the dyes. Phase-separated blends with apparently very small dye aggregates could be produced by rapidly quenching the samples after melt processing and subsequent plasticization with a hydrocarbon solvent. The mechanical deformation of the blends thus produced led to a pronounced change of the materials PL characteristics. When the films were stretched to a draw ratio lambda = (l - l(0))/l(0) = 500%, the monomer to excimer emission ratios I-M/I-E were increased by a factor of up to 10. This effect appears to bear significant potential for the use of such dyes as internal strain sensors in polymer objects.