Rheology of Ordered and Disordered Symmetric Poly(Ethylenepropylene)-Poly(ethylethylene) Diblock Copolymers

A series of poly(ethylenepropylene)-poly(ethylethylene) (PEP-PEE) diblock copolymers, containing 55% by volume PEP, and the corresponding PEP and PEE homopolymers have been investigated by dynamic mechanical spectroscopy. The ordered and disordered states are characterized by qualitatively different low-frequency rheological properties in close agreement with previous reports on this subject. This paper addresses three aspects of such behavior in the vicinity of the order-disorder transition: identification of the microphase-separation transition temperature, T_MST, evaluation of fluctuation effects, and characterization of the ordering kinetics. We identify T_MST based on the distinct discontinuity in G’ (ω « ω_c’) and G” (ω « ω_c”) that accompanies the order-disorder transition; ω_c’ ≅ 3ω_c” « τ_d^-1 delineates the beginning of the phase-state-dependent low-frequency regime where rd is the conventional single-chain longest relaxation time. Above and below T_MST the rheological properties are complex for ω < ω_c (e.g., failure of time-temperature superposition), owing to the presence of composition fluctuations. In the disordered state these effects are evident 50 °C beyond TMST. The magnitude of these fluctuation effects is underpredicted by recent theory, although the relative fluctuation contributions to the dynamic elastic and loss moduli are in quantitative agreement with theory. Quenching experiments from slightly above to just below T_MST reveal a ca. 10 °C region within which the ordering process can be followed rheologically. On the basis of the temporal evolution of G’ (ω « ω_c’) following such temperature quenches, we conclude that the ordering process is governed by heterogeneous (secondary) nucleation.