The effect of pressure on the conformation of two sets of m-phenylene ethynylene oligomers in PMMA and PtBMA

The effect of pressure has been studied on the emission characteristics of two sets of m-phenylene ethynylene oligomers in solid poly(methyl methacryl ate) (PMMA) and polytertbutylmethacrylate (PtBMA). For oligomer 1, the tetr amer, decamer, and hexadecamer (4-mer, 10-mer, and 16-mer) were used. For o ligomer 2, the chain lengths 10, 16, 20, and 24 were examined. In liquid so lution larger oligomers can exist either in a folded (helical) or unfolded state (random coil) or a mixture thereof depending upon the compatibility w ith the solvent. The 10-mer and 16-mer of oligomer 1 and 10-mer, 16-mer, 20 -mer and 24-mer of oligomer 2 exist primarily in a folded conformation in t he polymeric media at 1 atm. With increasing pressure, they unfold so that, between 30 and 60 kbar, one can establish the equilibrium constant as a fu nction of pressure. The pressure derivative of this quantity gives the diff erence in partial molar volume Delta(V) over bar ' = V-F - V-U. In all case s Delta(V) over bar ' decreased with increasing pressure. Delta(V) over bar ' is larger for oligomer 1 than for 2, it increases with oligomer size and is larger in PtBMA than in PMMA. The 4-mer of oligomer 1 is unfolded at al l pressures in both media, as is the 10-mer in PMMA.