Molecular Weight Effect on Local Motion of Polystyrene Studied by the Fluorescence Depolarization Method

Abstract

ABSTRACT:The molecular weight effect on the local motion of polystyrene (PS) was examined in dilute solutions by the fluorescence depolarization method. Four PS samples with the fluorescent probe, anthryl group, in the middle of the main chain were synthesized by the living anionic polymerization. The molecular weight of samples varied from ca. 6.4 x 10 3 to 9.2 x 10 4 Solvents were benzene, a good solvent and ethyl acetate, a poor solvent. In both solvents, the relaxation time increased with the molecular weight up to MW = 10 4 at which it reached an asymptotic value. The activation energies were also estimated from the temperature dependence of the relaxation time, and its molecular weight dependence appeared to be similar to that of the relaxation time. It was suggested that the relaxation time of the local motion is determined by the potential for the conformational transition of main chain bonds, rather than by the segment density. Finally, the molecular weight effect on the relaxation time for PS was compared with that for poly(oxyethylene) (POE The flexible polymer chain in dilute solution has various motional scales with regard to time and space resulting from its high degree of intramolecular freedom. Because this chain dynamics governs a variety of properties of polymers, extensive experimental and theoretical efforts have been made to understand the polymer chain dynamics. 1 -21 For the local motion, which is fairly a fundamental process in chain dynamics, many experimental methods have been utilized, e.g., NMR, 5 -7 ESR, 8 dielectric relaxation, 9 -11 dynamic light scattering, 12 • 13 neutron scattering, 14 and fluorescence depolarization.15-21 The fluorescence depolarization method provides direct information about the local motion of polymer chains through a fluorescent probe that is covalently bonded to the polymer main chain. By using this method, we have examined the influence of molecular structure/ 6 · 19 stereoregularity, 17 and quality of solvent16·18 on the chain dynamics of a variety of polymers.The local chain dynamics in dilute solution is influenced by the molecular weight of the polymer in addition to those factors mentioned above. Concerning the fluorescence depolarization study, Waldow et a!. have examined the molecular weight effect on polyisoprene (PI) chain dynamics and concluded that the chain dynamics is governed by the segment density in the vicinity of the fluorescent probe labeled in the middle of the main chain. 21 Previously, we reported the molecular weight effect for poly( methyl methacrylate) samples and explained the behavior of the local chain dynamics by the segment density as well. 19 We also have reported the local chain dynamics of poly(oxyethylene) (POE) in good solvents and discussed the molecular weight effect. 20The static and dynamic properties of polystyrene (PS), a common polymer, have been widely studied. 2 · 6 -9 • 16 · 17 We have reported the effects of the solvent quality on the local dynamics of PS 17 and discussed the difference t To whom correspondenc...

Effect of Low-Concentration Polymers on Crystal Growth in Molecular Glasses: A Controlling Role for Polymer Segmental Mobility Relative to Host Dynamics

“…The same analysis above also suggests that the segmental mobility of polymer chains is a possible limiting factor for crystal growth. It is known that a polymer’s segmental mobility in a dilute solution decreases with increasing molecular weight but reaches a plateaus near DP = 100 . This dependence is consistent with standard models .…”

“…It is known that a polymer's segmental mobility in a dilute solution decreases with increasing molecular weight but reaches a plateaus near DP = 100. 32 This dependence is consistent with standard models. 31 This molecular weight dependence of segmental mobility is in excellent agreement with that of a polymer's effect on crystal growth (Figure 8).…”

Local Chain Mobility of Gellan in Aqueous Systems Studied by Fluorescence Depolarization

“…Figure 3 shows that the value of τ r increases slowly with decreasing temperature and then increases steeply below the transition temperature. The temperature dependence of τ r can basically be attributed to the variation in the activity of the thermal motion of the gellan chains, as was reported for nonelectrolyte polymer systems 10–14. The rotational relaxation of the fluorescent probe labeled in the polymer chain is achieved by the thermal motion of neighboring polymer segments, consisting of coupled librations and conformational transitions 22.…”

“…Therefore, if the fluorescent probe is labeled in a polymer chain, the local chain mobility can be estimated as the rotational relaxation time of the fluorescent probe 10–19. Fluorescence depolarization studies have focused on non‐electrolyte polymer systems; it has been reported that the chain mobility of nonelectrolyte polymers is determined by the steric hindrance in internal rotation and the chain entanglement 10–17. Regarding polysaccharides, amylose and dextran in dilute aqueous solutions were previously examined, and the effect of coil‐helix conformational transition on the chain mobility was discussed 15.…”

Influence of a Fluorescent Probe on the Local Relaxation Times for a Polystyrene Chain in the Fluorescence Depolarization Method

“…The relaxation time of the probe was estimated through the measurements of the decay of the fluorescence anisotropy ratio. The local motion of polymer chain has been extensively studied for the polymers with the fluorescent probe labeled in the main chain. − We examined the local motion of polymer chains labeled with anthracene as the fluorescent probe in dilute solutions and discussed the effects of several molecular factors such as solvent condition, , molecular structure of polymer, − molecular weight, − and the position along a chain on the local chain mobility . However, there still remain some questions to be solved.…”