A05 Constrained side chain dynamics in semi-crystalline polymers with comb-like architecture

Beiner

Primary target of project A05 was to study and understand the influence of self-assembled constraints on the cooperative dynamics of CH₂ units in long-range ordered comb-like polymers with rigid backbone. Such polymers show in many cases a well-defined layered morphology with a periodicity of 20-40 Å where main chain layers and side chain layers alternate. While the rigid backbones are stacked in a crystal-like manner within the main chain layers, the methylene sequences are often in a disordered state in the side chain domains (alkyl nanodomains). In the relaxation spectra of such systems a polyethylene-like glass transition α_PE reflecting a cooperative relaxation dynamics of the CH₂ units in the alkyl nanodomains is commonly observed. Central aim was to understand and quantify the influence of (i) nanodomain size, (ii) average density and (iii) constraints at interfaces on the cooperative dynamics in small amorphous alkyl nanodomains. Shear, dielectric and wide angle X-ray diffraction experiments have been performed. A comparison of results for alkoxylated polyesters with 6-12 alkyl carbons per side chain with those for regio-regular poly(3-alkyl thiophenes) shows that geometrical confinement effects are of major importance while average density and constraints at interfaces have obviously only a moderate influence on the cooperative dynamics of CH₂ units in small alkyl nanodomains.[E1] This supports earlier interpretations explaining systematic changes in the α_PE process depending on side chain lengths by a ‘hindered glass transition’, i.e., as a conflict between nanodomain size and size of cooperative rearranging regions (CRRs) being an intrinsic feature of the glass forming materials with typical dimensions in the 10-30 Å range.
Second aim of project A05 was to study the physical aging behavior of self-assembled polymers.
Studies have been performed in particular on a microphase-separated P(S-block-isoMMA) copolymer
with cylindrical morphology where a polystyrene matrix (T_g,PS ~ 100 °C) is containing isotactic PMMA cylinders (T_g,PisoMMA ~ 60 °C). It could be shown that there are peculiarities in the physical aging behavior considering dynamic mechanical data for samples which are isothermally annealed under different conditions. In contrast to commonly reported trends for homopolymers the storage modulus G’ is increasing only in a certain temperature-frequency range slightly below T_g,PS while a decrease of G’ is observed in the glassy state at lower temperatures near T_g_,PisoMMA. In order to clarify the origin of these peculiarities structure, of the block copolymer and density contrast between PS and PisoMMA are investigated after different aging times based on detailed temperature-dependent scattering experiments. These studies have been complemented recently by pVT measurements providing information about of the overall density depending on physical aging. In the remaining time of the final funding period dilatometric measurements at ambient pressure were planned after different aging times in collaboration with partners. The completion of these interesting studies was expected by the end of 2015.
Based on the knowledge about temperature-dependent structure formation and mechanical properties of alkoxylated polyesters gathered in project A05 and methods to prepare and characterize shear oriented block copolymer samples with anisotropic mechanical properties developed in project B09, a new project B14 was proposed for the next funding period of SFB/TRR 102 focusing on structure formation processes under the influence of external constraints (shear fields, fiber interfaces). This new project proposal came from the Fraunhofer Institute for Mechanics of Materials IWM Halle being meanwhile the main affiliation of the PI.

Beiner

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