Rheo-optical investigation of the dynamics of miscible polymers: blends and diblock copolymers
The dynamics of each component in miscible blends of polyisoprene / polyvinylethylene (PIP/PVE) are studied using dynamic stress-optical measurements. We first examine blends in which the two components are nearly equally entangled in order to avoid the effects of polydispersity on the dynamic modul...
| Summary: | The dynamics of each component in miscible blends of polyisoprene / polyvinylethylene (PIP/PVE) are studied using dynamic stress-optical measurements. We first examine blends in which the two components are nearly equally entangled in order to avoid the effects of polydispersity on the dynamic moduli of each species. While the homopolymers are thermorheologically simple and obey the stress-optic rule, the blends show failure of time-temperature superposition and complex stress-optic behavior. The way in which the stress-optic rule fails reveals the relaxation dynamics of each species. The blend dynamic modulus and complex birefringence coefficient are analyzed to infer the dynamics of each component. Orientational coupling, which affects the birefringence but not the stress, is incorporated based on the value of the coupling coefficient determined from rheo-optical studies of unequally entangled blends. While we cannot rule out contributions of compositional heterogeneity to thermorheological complexity in the PIP/PVE system, it is intrinsic differences between the temperature dependencies of the two species' dynamics which dominate the observed failure of time-temperature superposition.
The entanglement molecular weight, M[subscript e,i](φ), and monomeric friction coefficient,ζ[subscript o,i](φ,T), of each species are determined from the component dynamic moduli, as functions of blend composition, φ and temperature, T. The effect of blending on M[subscript e,i] of each component is small, with the values varying by only ~ 30% from those of the pure components over the range of composition. Blending has a much more dramatic effect on ζ[subscript o,i] of each species: it strongly speeds the rate of relaxation of the component with the higher glass transition temperature, T[subscript g], (PVE), while more modestly slowing the relaxation of the low-T[subscript g] component (PIP). The dynamics of each species have different temperature dependencies in the blend, which leads to the failure of the superposition principle. Furthermore, both the difference between the friction coefficients of the two species and the difference in their temperature dependencies is greater in blends rich in the high-T[subscript g], material (PVE).
We then look at blends in which the two species are unequally entangled and the effect of orientational coupling can be characterized. Since orientational coupling contributes to the birefringence but not the stress, simultaneous analysis of these observables allows us to extract the coupling coefficient, ε. For blends in which PIP is the faster-relaxing species and couples to a slower-relaxing PVE matrix, ε[subscript PIP,PVE] ≈ 0.35 ± 0.04. Analogous blends are also studied, where PVE is the faster-relaxing species coupling to a PIP matrix, for which ε[subscript PVE,PIP] ≈ 0.27 ± 0.08. Thus, within experimental uncertainty, it appears appropriate to use a single, average value of ε. No dependence of ε on composition or temperature were detected. Once ε was determined we extracted the component dynamic moduli in each blend over a range of temperatures and blend compositions. The trends observed for the equally entangled blends remain valid and the values determined for M[subscript e,i] and ζ[subscript o,i](φ,T) are reproduced. Although failure of time-temperature superposition in these blends is again due to the distinct temperature dependencies of the species' monomeric friction coefficients, both the contrast in friction coefficient and the relative molecular weights of the components determine the degree to which complex thermorheological behavior is observed.
Finally, the dynamics of nearly ideal, disordered PIP-PVE diblock copolymer melts are characterized using the same rheo-optical techniques, for two block compositions (φ[subscript PIP] = 0.25 and 0.75). Unlike miscible blends of PIP and PVE, for which time-temperature superposition fails, the dynamic moduli of the block copolymers suggest that they are thermorheologically simple. Here we show that this apparent paradox can be resolved by examining the relaxation of the constituent blocks, which can be determined from stress-optic measurements. In block copolymers rich in the low-T[subscript g] component (high φ[subscript PIP]), thermorheologically simple behavior is observed because both blocks have similar friction coefficients, in accord with our results on PIP/PVE blends. In copolymers rich in the high-T[subscript g] component (low φ[supscript PIP]), the individual blocks show distinct temperature dependencies, again in accord with the blend results. The reason that departure from time-temperature superposition was not previously observed in these diblocks is that the change in ζ[subscript o],PVE/ζ[subscript o],PIP produces subtle changes in the overall relaxation spectrum relative to a linear chain of uniform friction.
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