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 methacrylate) (PMMA) and polytertbutylmethacrylate (PtBMA). For oligomer 1, the tetramer, decamer, and hexadecamer (4-mer, 10-mer, and 16-mer) were used. For oligomer 2, the chain lengths 10, 16, 20, and 24 were examined. In liquid solution larger oligomers can exist either in a folded (helical) or unfolded state (random coil) or a mixture thereof depending upon the compatibility with 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 the 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 function of pressure. The pressure derivative of this quantity gives the difference in partial molar volume ΔV̄′ = V_F -V_U. In all cases ΔV̄′ decreased with increasing pressure. ΔV̄′ 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 all pressures in both media, as is the 10-mer in PMMA.