Hypochromic Effects in UV Spectra of Polymers in Solution

Abstract

ABSTRACT:The hypochromic effect in the UV spectra of polystyrene and styrene copolymers in solution was investigated in detail. For this purpose, isotactic and syndiotactic-rich polystyrenes as well as their dimer model compounds, 2,4-diphenylpentanes, and styrene-methyl methacrylate copolymers were prepared. UV absorption measurements were carried out in dioxane and tetrahydrofuran at various temperatures. The extinction coefficients of phenyl rings depended not only on the solvent and the temperature but also on the microstructure of the polymers. A discontinuous temperature dependence of the extinction coefficients was observed between 40 and 50°C for homopolystyrenes as well as their dimer model compounds. It was suggested that this might be explained in terms of the rotation of phenyl rings. On the other hand, the hypochromicity in binary copolymers appeared to depend strongly on the sequential arrangement of monomers (the chain architecture and the chemical composition of copolymers). The measured average extinction coefficients were well interpreted by a simple treatment by using triad probability proposed by Harwood. The results were discussed in relation to the UV spectrometer which is being used as an auxiliary detector in gel-permeation chromatography and microcolumn chromatography.KEY WORDS Hypochromic Effect / UV Spectrum / Extinction Coefficient / 2,4-Diphenylpentane / Polystyrene / Styrene-Methyl Methacrylate Copolymer / Sequential Arrangement / The quantities of eluted homopolymers in gel-permeation chromatography (GPC) and microcolumn-solubility chromatography (MCC) can be determined through the detection of a single physical value, such as refractive index increment, which is proportional to the concentration of the polymer in the eluent. However, the determination of the chemical composition of a binary copolymer at a certain elution count (i.e., the point-by-point composition) requires another quantity to be specified. This can be performed by using an ultraviolet (UV) detector auxiliarily, so far as the extent of absorption at a certain wavelength is different for the two components of the copolymer. 1 • 2 If the UV extinction coefficient, e, depends solely on the population of the units able to absorb in the copolymer chains, the point-bypoint composition can be determined easily with such a dual detector system as mentioned above.However, it has been observed for binary copolymers that the UV extinction coefficient depends not only on the population of the units able to absorb but also on the sequence length of copolymer chains. 3 ' 4 Moreover, it has been reported that temperature and steric isomerism also affect the extinction coefficient of UV absorption. 5 ' 6 In this report detailed studies are made on the various factors which may influence the UV absorption of phenyl rings; i.e., (I) the steric isomerism of polystyrene and its dimer model compound, (2) the temperature, (3) the chain architecture of styrene (ST)-methyl methacrylate (MMA) copolymers (i.e., alternating, random, and ...

Detailed copolymer characterization using ultraviolet spectroscopy

“…These deviations have been attributed to the microstructure of the polymer chains (3,4,8,10, I I) and in some cases to the conformation of the molecules in solution (2, 6, 9). Vala and Rice (2) observed that the extinction coefficient of isotactic polystyrene at certain wavelengths is smaller than the corresponding extinction coefficients for atactic polystyrene.…”

“…They also concluded that the response of the uv detector appears to be a function of the length of the styrene sequences. Stutzel et al (8) investigated SMMA copolymers in THF and dioxane solutions. From their analysis using styrene dimers they concluded that the hypochromic effects are probably not due to differences in conformation.…”

“…Band shifts, solvent effects, and hypochromic effects have been observed as functions of the copolymer composition and the microstructure of the polymer chains (2)(3)(4)(5)(6)(7)(8). The band shifts, solvent effects, and other anomalies observed in the absorption behaviour of polymers have precluded the development of analytical techniques which can yield composition estimates directly from the uv spectra (4).…”

New fluorescent derivatives of oligopropylamines

“…Self-association of the dye molecules is often accompanied by a hypochromic effect in UVVis spectra. [10][11][12][13][14][15] Spectra of mono-NBD derivatives of OPAs are similar to the known NBDamines 5 and contain two main bands near 350 and 500 nm (Table 1, Figure 1) irrespective of pH values. A decrease of absorbance at 500 nm was observed for NBD-N2H at pH 10 only.…”

Influence of Composition on Ultraviolet Absorption Spectra of Styrene-Alkyl Methacrylate and Styrene-Alkyl Acrylate Random Copolymers

“…5 The UV spectra of the random copolymers might become similar to those for the alternating copolymers when the styrene content in the random copolymers is lower than 10%. The ratio of the molar absorption coefficients for a random copolymer having 20 mol% styrene and PS at 269.6 nm5 was 0.690; in this work it was 0.842.…”