ABSTRACT:The apparent short time cutoff of the relaxation-time spectrum at surprisingly long times for polymers in solution is a well known but not yet understood observation. To elucidate its origins we revisit viscoelastic and oscillatory flow birefringence data for solutions and melts of two linear polymers (polystyrene and polyisoprene) and present new measurements of oscillatory flow birefringence of the latter. Previous measurements have suggested that the "flexibility" of both polymers in solution is smaller than in the melt on the basis of the breadth of the relaxation-time spectrum of the solution as compared with that of the melt. Our new measurements have explored a higher effective frequency range than was previously possible. This has allowed us to observe the effect of the rotational relaxation time of the solvent on the dynamics of the solution at high frequencies. To obtain the polymer global motion contribution, one now needs to subtract from the solution properties a frequencydependent complex solvating environment contribution. We show that the decrease in apparent "flexibility" for solutions arises from the presence of a solvent that exhibits a rotational relaxation time and thus simple viscoelastic behavior somewhat near the frequency window of the experiment. Although recent predictions of a model for a chain in a solvent with a single relaxation time are in qualitative agreement with our results, our data suggest that the solution results may reflect the influence of solvent on the development of the "entropic spring" forces at short times.

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Multidimensional chromatography using on‐line, coupled, micro‐column size exclusion chromatography and reversed‐phase high performance liquid chromatography, has been successfully implemented for the quantitative determination of photoinitiator concentrations in photosensitive, interlayer dielectric resin formulations. The actual formulation consists of the prepolymer, aromatic solvent, antioxidant, and a variety of bis‐azide photocrosslinkers. This paper describes the development of an automated system for Micro SEC‐LC, as well as accuracy and precision data for the photoinitiator analysis. The technology is very effective for the analysis of small molecules (e.g. additives) in polymer products, and as such can have wide ranging applicability to other polymer and complex products. The use of microcolumns in the first dimension allows the introduction of the complete fraction containing the components of interest into a reversed‐phase second dimension without deterioration of the separation when using a very strong mobile phase such as THF.