Polymeric materials for piezoelectricity and second harmonic generation
Studies have been undertaken to utilise the possible potential of the rodlike, helical structures associated with polyglutamates and polyisocyanates for piezoelectric (PE) and nonlinear optical (second harmonic generation (SHG)) applications. Various techniques have been employed to form samples/films of these polymers containing oriented helices whose bulk structures are non-centrosymmetric, (an important criterium for the aforementioned applications). Owing to the poor yields obtained for certain intermediates in the synthetic stages of these polymers, only poly('S-benzyl-L-glutamate) (PBzLG) and poly(n- hexylisocyanate) (PHIC) were available for subsequent studies. Piezoelectric work was limited to hydrostatic measurements (d3h) made on electrically poled, composite (guest host), polymer samples. Phase separation was common in all the samples, but no piezoelectric response was observed. However, low concentration (not greater than 10% w/w) of active polymer (PBzLG or PHIC) and high conductivity during the poling stage, may have accounted for the lack of response. Electrically poled, homopolymer samples of PBzLG and PHIC were achieved using a solvent evaporation technique. Nonlinear optical studies of these poled samples revealed threshold fields above which detectable levels of SHG could be observed, (i.e. PBzLG > 40 V/mm, PHIC > 150 V/mm). The variation of SHG signal versus the angle of polarisation of the incident laser light (1064 nm) relative to the poling direction of the film, implied biaxial symmetry was present in the case of PBzLG, whereas the more conventional uniaxial symmetry was observed for PHIC. The variation in SHG signal observed for a fixed thickness was attributed to varying degrees of alignment, resulting from uneven poling. Although PHIC and PBzLG showed low SHG activity (less than value for urea), little ageing of this activity occurred over the 6 month testing period. Langmuir Blodgett studies carried out on chemically modified, low molecular weight PBzLG molecules showed poor transference of the monolayer to a quartz substrate once ten layers had been deposited. In addition, areas per molecule (or per residue) calculated from the pressure-area isotherms, proved inconclusive when deducing possible orientations for the PBzLG helices. No SHG signal was observed for the L.B. films.