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PO: Polymerphysik

PO 8: Poster: Polymermischungen und Copolymere

PO 8.6: Poster

Tuesday, March 2, 1999, 14:00–16:00, Foyer

Dynamic Structure Factor of Diblock Copolymers. Polydispersity Effects — •F. Rittig^1,2, K. Chrissopoulou¹, G. Fytas¹, G. Fleischer², S. Pispas³, and N. Hadjichristidis^3,1 — ¹FORTH, IESL,71110 Heraklion, Greece — ²Universität Leipzig, Fakultät f. Physik und Geowissenschaften, Linnéstraße 5, 04103 Leipzig (rittig@physik.uni-leipzig.de) — ³University of Athens, Department of Chemistry, Greece

The most probable composition (order parameter) fluctuations Ψ_q, in diblock copolymers are of mesoscopic length, O(R) (R being the polymer size) whereas long wavelength Ψ_q are suppressed. For incompressible monodisperse diblock copolymer the static structure factor S(q) exactly vanishes at the thermodynamic limit q=0 and attains its maximum value for x^* ≡ (q R)² ≃ 4 (for a symmetric diblock copolymer). It was soon recognized that unavoidable polydispersity for real diblock copolymer leads to a shift of S(q^*) to lower q’s and a fill-up of the correlation hole i.e. an excess S_exc = S(q → 0) ≠ 0 as well as a reduction of the (χN) of the transition. In the present report, we examine the upper limits for the polydispersity and reduced length x beyond which the low q relaxation behavior of S_c (q,t) is not valid with photocorrelation spectroscopy. To cover a broad x range, we have used styrene-isoprene (SI) block copolymers with low (170 K) and high (2M) molecular weights. Since the main cause of polydispersity in high molecular weight diblock copolymers is incomplete removal of homopolymer, we have included SI/S mixtures using the same parent SI and a different amounts on homopolymer.