Rich Dynamics in Diblock Copolymers
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Rich Dynamics in Diblock Copolymers G.Fytas1,2 1 Department of Materials Science and Technology University of Crete and FO.R.T.H, P.O. Box 1527, 71110 Heralkion Greece 2 Max Planck Institute for Polymer Research, P.O. Box, 3148, 55128 Mainz ,Germany
ABSTRACT The dynamic structure factor S(q,t) of ultra-high molecular mass diblock copolymers BC in a common solvent was systematically investigated by photon correlation spectroscopy in different regimes of the phase diagram. Through proper probing of the composition fluctuations in symmetric and asymmetric BC the different relaxation mechanisms were identified and the influence of the disorder to order (ODT) transition was assessed. In the disordered regime, S(q,t) sensitively depends on the composition polydispersity of BC and the proximity to ODT complementing the real phase morphology of BC. The dynamic response of BC is therefore predictable.
INTRODUCTION Covalently linking of two chemically distinct polymer chains creates a diblock copolymer BC which is an intra-molecular mixture as opposed to a polymer blend. Unfavorable enthalpic interactions between dissimilar monomers summed up over the chains lead to a microphase separation in contrast to the macrophase separation of a polymer blend. The real phase morphology of BC manifested in the static structure factor S(q) has been thoroughly investigated and is nowadays well understood[1]. The evolution of the maximum S(q*) and the characteristic spacing 2π/q* with either temperature or concentration (in neutral solvents) towards the disorderto-order transition (ODT) is well established. Alternatively, the dynamics of BC have received less attention partly due to paucity of experimental techniques with broad spatiotemporal resolution. The understanding of the dynamic response that influences the rheological properties of BC requires sensitive measurements of the dynamic structure factor S(q,t) over a broad time range in the relevant wave vector q range ,i.e. q about q*.Neutron spin echo and x-ray photon correlation spectroscopy in the high q-range either cover narrow time range or possess low sensitivity[2]. Employment of the photon correlation spectroscopy (PCS) over the time range (10-7-103)s but in the low q-range requires very high molar mass(Mw) BC in order to match q*. To reach, however, the ODT the experimental studies are restricted in solutions of BC in neutral solvents [3,4]. Violation of the solvent neutrality can have affect the equilibrium phase behavior of BC [5,6] in particular for very high Mw. For nearly monodisperse BC, fluctuations of the equilibrium structure relax via two channels: S(q,t)=S1exp(-Γ1(q)t)+S2(q) exp(-Γ2t)
(1)
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where the mode (2) is inherent to BC, has an intensity S2(q) exhibiting a maximum at q* and a pure relaxational rate Γ2≈τ1-1 controlled by the longest chain overall relaxation time, τ1 .Mode (1) is the analogue of the interdiffusion in compatible polymer mixtures and is visible due to the composition polydispersity S1~ which is virtually q-independent in BC.
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