Kinetics of chain collapse in dilute polymer solutions: Molecular weight and solvent dependences

Abstract

The molecular weight and solvent dependences of the characteristic time of chain collapse were studied for poly͑methyl methacrylate͒ ͑PMMA͒ of the molecular weight M w = 6.4ϫ 10 6 and 1.14 ϫ 10 7 in pure acetonitrile ͑AcN͒ and in the mixed solvent of AcN + water ͑10 vol % ͒. The size of PMMA chains was measured as a function of the time after the quench by static light scattering and the chain collapse processes were expressed by the plot of the expansion factor ␣ 2 vs ln t. The chain collapse in the mixed solvent AcN + water ͑10 vol % ͒ was found to occur much faster than that in pure AcN, though the measurement of the former collapse process required several hours. In order to make a comparison between the rates of chain collapses, the fast chain collapse process was superposed on the slow one by scaling the time of the fast process as ␥t. The scale factor ␥ was determined by comparing the chain collapse processes of nearly the same equilibrium expansion factor with each other. Accordingly, the superposition of the collapse for M w = 6.4ϫ 10 6 on that for M w = 1.14ϫ 10 7 yielded ␥ m = 4.0± 0.6 for the process in AcN + water and 5.5± 0.6 in AcN. The superposition of the chain collapse process in AcN + water on that in AcN yielded ␥ s = 9.5± 1.4 for M w = 6.4ϫ 10 6 and 12.0± 1.8 for M w = 1.14ϫ 10 7 . This analysis suggests that ␥ m and ␥ s are constant independent of each other. Thus, by assuming the molecular weight dependence of ␥ m ϳ M z , the characteristic time exp of chain collapse was conjectured as exp ϳ M z , where reflects the nature of solvent species. The ratio of for PMMA in AcN to that in AcN + water is given by ␥ s . The exponent was estimated to be z = 2.4± 0.7 for AcN + water and 3.0± 0.7 for AcN. These values are compatible with the theoretical prediction z = 3 based on a phenomenological model, though the observed characteristic times are longer by several orders of magnitude than those of the theoretical prediction.

Kinetics of Swelling and Collapse of a Single Polyelectrolyte Chain

“…Light-scattering studies initially helped identify the role of temperature and solvent quality in collapse from Θ-temperatures of polystyrenes in cyclohexane. Further light-scattering experiments , identified the roles of the molecular weight and solvent quality in determining the characteristic time of chain collapse and found results closely in line with theoretical predictions and molecular simulations mentioned above. Both poly­(methyl methacrylate) (PMMA) in isoamyl acetate and PMMA in pure acetonitrile­(AcN) and in the mixed solvent of AcN + water­(10% of volume) found the coil–globule transition to be slower for higher molecular weight.…”

“…Further light-scattering experiments , identified the roles of the molecular weight and solvent quality in determining the characteristic time of chain collapse and found results closely in line with theoretical predictions and molecular simulations mentioned above. Both poly­(methyl methacrylate) (PMMA) in isoamyl acetate and PMMA in pure acetonitrile­(AcN) and in the mixed solvent of AcN + water­(10% of volume) found the coil–globule transition to be slower for higher molecular weight. In a later work, fluorescence experiments measured the ratio of excimer to monomer emissions intensities for single, synthetic poly­( N -isopropyl­acrylamide) (PNIPAM) chains, suggesting a two-stage collapse kinetics through a double-exponential decay (with two relaxation times).…”

“…The profiles plotted in Figure confirm the above. Experiments with PMMA (uncharged) in isoamyl acetate and in acetonitrile (AcN) and AcN + water identify slower kinetics with higher molecular weight as well. The effect of the dielectric mismatch parameter δ and monomer density ρ̃ on the deswelling kinetics is discussed in the Appendix.…”

“…The equilibrium size and configurational properties of isolated polymer molecules in solutions depend sensitively on the solvent quality and vary from expanded conformations in good solvents to random walk conformations (Gaussian) in theta (Θ) solvents and collapsed conformations in poor solvents. Since the first discovery of the coil–globule transition in polymers, a considerable number of theoretical − and simulation studies − and experiments − have been performed to investigate conformational properties, mainly collapse dynamics, of single polymer chains in solutions. This has been a major topic of interest owing to its intricate features as well as deep influences to many biological phenomena such as protein folding − or polymer translocations. − …”

Contraction and Collapsing Kinetics of Single Synthetic Polymer Chains at Small Quench Depths

“…[60] Previous experimentations in this field mainly employed the laser light scattering (LLS) technique. [61][62][63][64][65][66][67][68][69] The main limitation of the LLS facility in monitoring coil-toglobule transition kinetics is that quite a long (compared to chain collapsing time) thermal equilibration period is needed for temperature variation. Moreover, these experiments are typically conducted in the two phase region.…”