Dynamic Light Scattering from Block Copolymer Solutions under the Zero Average Contrast Condition

Abstract

Dynamic light scattering measurements on a symmetric polystyrene-polyisoprene diblock copolymer, Mw = 3.4 x 106, dissolved in neutral good solvents are reported. The solvents comprise mixtures of toluene and -chloronaphthalene, and the solvent compositions necessary to index-match the entire copolymer, the polystyrene block, and the polyisoprene block have been determined. The first of these, the zero average contrast (ZAC) condition, is particularly advantageous as it eliminates the cooperative diffusion mode, thus enhancing the relative amplitude of the elusive internal mode. However, the ZAC condition is sensitive to small changes in either solvent composition or temperature. In dilute and semidilute ZAC solutions, the correlation functions consistently show two modes. The slower mode is diffusive, with a diffusivity in close agreement with that measured by pulsed-field-gradient NMR; it is thus interpreted as the heterogeneity mode, which is visible due to chain-to-chain variations in composition.The faster mode is the internal mode, with a decay rate that is independent of scattering angle and decreasing with concentration; the concentration dependence is that observed for the viscoelastic longest relaxation time. For polymer concentrations within a factor of 2 of the order-disorder transition, the total scattered intensity increases markedly, which is attributed to the appearance of large-amplitude concentration fluctuations. It is also shown that in dilute solution, the heterogeneity and cooperative modes have comparable decay rates, which can complicate the interpretation of the diffusional second virial coefficient, k¿, for copolymers.

Dynamic light scattering from asymmetric block copolymers in neutral good solvents: Evidence of association in the disordered state

“…Two were É 0(dn/dc) B , the cooperative mode can be eliminated, rendering the internal mode more accessi- ble. 7 The associated decay rate, G I , is approximately independent of q, and follows the conceng/mol, M PI Å 9.0 1 10 3 g/mol, designated SI(36-9). A poly(styrene-b-isoprene-b-styrene) triblock tration dependence expected for the inverse longest viscoelastic relaxation time, 1/ t 1 .…”

“…A poly(styrene-b-isoprene-b-styrene) triblock tration dependence expected for the inverse longest viscoelastic relaxation time, 1/ t 1 . 7 The with block molecular weights of 1.0 1 10 4 , 1.0 1 10 5 , and 1.0 1 10 4 g/mol, designated SI(10-identity of the heterogeneity mode has been confirmed by comparison with pulsed-field gra-100-10), was also used. SI(10-50) and SIS(10-100-10) were provided by D. Handlin (Shell Dedient NMR measurements of self-diffusion, and through artificial enhancement of the composivelopment Company), and SI(9-36) was donated by F. Bates (University of Minnesota).…”

“…6,10 A fourth, slower mode has sometimes been obsion chromatography (SEC) confirmed that the polydispersities were less than 1.06 for all three served, but is not yet fully characterized or understood. 6,7,9 When visible, its decay rate tends to materials. Two solvents that are good for both components, toluene and THF, and two selective vary as q x , with 2°x°3, and the apparent diffusivities extracted are consistently an order solvents, cyclohexane (theta solvent for PS at 34.5ЊC) and dioxane (theta solvent for PI at of magnitude or move slower than translational diffusion of single chains.…”

“…Filtered solsity and erratic. 6,7 We were inclined to view it as representative of some nonequilibrium associavent was then added to achieve the desired concentration, and the cells sealed. mode that is weakly dependent on concentration, but the dominant feature is a slow mode that grows strongly with increasing concentration.…”

Simulation of the collective dynamics scattering of diblock copolymers with heterogeneous composition in the semidilute regime

“…A collective mode, describing faster local fluctuations in the polymer-solvent composition around the entanglement points is present in all types of semidilute solutions of homopolymers [6] and should also appear for copolymer solutions except in the case that the refractive index of solvent has been exactly set to the so called 'zero-average contrast conditions' [12]. This mode is independent of the polymer length and its frequency shows a diffusive (q 2 -dependent) behavior.…”