Physikalisches Institut, Fakultät für Mathematik und Physik
Hermann-Herder-Str. 3, 79104 Freiburg, Germany
Tel.: +49 761 203 5856 Fax: +49 761 203 5855
- Strukturanalyse mittels Röntgenstreuung (SAXS, WAXS, SWAXS)
- Cellulose nanocrystals’ production in near theoretical yields by 1-butyl-3-methylimidazolium hydrogen sulfate ([Bmim]HSO4) – mediated hydrolysis. Jia Mao, Barbara Heck, Günter Reiter, Marie-Pierre Laborie. Carbohydrate Polymers, 117, 443–451 (2015) Abstract
We report on near theoretical yield production of cellulose I nanocrystals (CNCs) using a two-step hydrolysis with the mildly acidic ionic liquid (IL) 1-butyl-3-methylimidazolium hydrogen sulfate ([Bmim]HSO4) in aqueous solution from common cellulosic sources. Two successive Taguchi experimental plans were performed to evaluate the impact of selected reaction parameters (T, t, H2O:IL ratio) and their interactions on the CNCs’ yield from bleached softwood kraft pulp (SWP), bleached hardwood kraft pulp (HWP) and microcrystalline cellulose (MCC). With these experimental plans, the molar yield for extraction of nanocrystals was optimized to near theoretical levels, reaching 57.7 ± 3.0%, 57.0 ± 2.0%, and 75.6 ± 3.0%, for SWP, HWP and MCC, respectively. The reaction yields corresponded to a relative crystalline region recovery of 84.1 ± 5.3%, 71.7 ± 1.3%, 76.0 ± 2.0% from SWP, HWP and MCC, respectively. The collected nanocrystals exhibited high aspect ratios (36–43), negligible sulfur content (0.02–0.21%) and high solvent dispersibility in comparison to those obtained with the traditional sulfuric acid method. Additionally these near theoretical yields were achieved for mild reaction conditions with the combined severity factor of 2 and 3 for MCC and pulp, respectively. Overall this two-stage IL-mediated preparation of nanocrystals combines the advantages of achieving high product quality, high reaction yields and mild conditions.
- Anisotropic Photophysical Properties of Highly Aligned Crystalline Structures of a Bulky Substituted Poly(thiophene). Yingying Wang, Barbara Heck, Daniel Schiefer, John O. Agumba, Michael Sommer, Tao Wen, and Günter Reiter. ACS Macro Letters, 3, 881-885 (2014) Abstract
The photophysical properties of a phenyl substituted poly(thiophene), poly(3-(2,5-dioctylphenyl)thiophene) (PDOPT), were studied as a function of polarization and degree of orientation of the crystalline structure. Under well-chosen controlled conditions, large-sized spherulitic crystals of PDOPT were successfully prepared from the melt. From polarized optical microscopy and X-ray diffraction, the molecular orientation of PDOPT within the spherulite was determined, indicating that the fastest growth direction of spherulite was the a-axis. This implied that crystallization of PDOPT was directed by the packing of the side chains rather than the backbones, which are significantly separated. As the crystalline lamellae were all radially oriented, the local absorbance strongly depended on the polarization of the incoming light. Compared to randomly oriented crystals in a quenched and thus rapidly crystallized sample, PDOPT spherulites displayed red-shifted absorption and emission spectra, combined with a reduced photoluminescence quantum yield. Even for these markedly separated polymer backbones (1.47 nm), the reduced photoluminescence suggests an enhancement of interchain interactions of highly ordered bulky substituted polythiophene induced by crystallization.
- Crystallization of Poly(γ-benzyl L-glutamate) in Thin Film Solutions: Structure and Pattern Formation. Kaiwan Jahanshahi, Ioan Botiz, Renate Reiter, Ralf Thomann, Barbara Heck, Roozbeh Shokri, Werner Stille and Günter Reiter. Macromolecules, 46, 1470-1476 (2013) Abstract
We have investigated the formation of poly(γ-benzyl L-glutamate) (PBLG) crystals from semi-dilute thin film solutions. Crystallization was initiated by adding methanol, a non-solvent. After drying, each PBLG crystal possessed an internal domain structure exhibiting a zig-zag pattern consisting of parallel stripes of alternating orientations between domains. X-ray scattering and electron diffraction revealed within these crystals a pseudo-hexagonal packing of the PBLG α-helices with their axis oriented parallel to the plane of the substrate. Based on optical anisotropy studies, it could be shown that the orientation of the helix axis was parallel to the stripes. While forming in solution, the objects are assumed to consist of a hexagonal columnar liquid crystalline phase. Upon drying, lateral packing density of the helices increased and resulted in a net dilative strain perpendicular to the columns, which is supposed to cause the formation of zig-zag patterns.
- Effect of Shear Stress on Crystallization of Isotactic Polypropylene from a Structured Melt, Bin Zhang, Jingbo Chen, Jing Cui, Fangfang Ji, Guoqiang Zhen, Hui Zhang, Barbara Heck, Günter Reiter, Changyu Shen. Macromolecules, 45, 8933–8937 (2012)
- Two Competing Crystallization Modes in a Smectogenic Polyester, Barbara Heck, Ernesto Perez and Gert Strobl, Macromolecules, 43, 4172-4183 (2010)
- Gradient Interfaces in SBS and SBS/PS Blends and Their Influence on Morphology Development and Material Properties, Yi Thomann, Ralf Thomann, Alfred Hasenhindl, Rolf Mülhaupt, Barbara Heck, Konrad Knoll, Helmut Steininger, Kay Saalwächter. Macromolecules, 42, 5684-5699 (2009)
- Stabilization, reformation and melting of poly(L-lactide) crystallites, Tai-Yon Cho, Barbara Heck, Gert Strobl, Chin. J. Polym. Science, 25, 83-94 (2007)
- A law controlling polymer recrystallization showing up in experiments on s-polypropylene, Barbara Heck, Silvia Siegenführ, Gert Strobl, Ralf Thomann, Polymer, 48, 1352-1359 (2007)
- Time dependent light attenuation measurements used in studies of the kinetics of polymer crystallization, Barbara Heck, Takahiko Kawai, Gert Strobl, Polymer, 47, 5538-5543 (2006)
- Crystallization of a poly(ethylene-co-octene): I - A precursor structure and two competing mechanisms, Andreas Häfele, Barbara Heck, Takahiko Kawai, Peter Kohn, Gert Strobl, Eur. Phys. J. E, 16, 207-216 (2005)
- Crystallization of poly(ethylene-co-octene): II - Melt memory effects on first order kinetics, Andreas Häfele, Barbara Heck, Thomas Hippler, Takahiko Kawai, Peter Kohn, Gert Strobl, Eur. Phys. J. E, 16, 217-224 (2005)
- Equations describing lamellar structure parameters and melting points of polyethylene-co-(butene/octene)s, Tai-Yon Cho, Barbara Heck, Gert Strobl, Coll. Polym. Science, 282, 825-832 (2004)
- Interplay between domain microstructure and nematic order in liquid crystalline/isotropic block copolymers, R. Ivanova, R. Staneva, S. Geppert, B. Heck, B. Walter, W. Gronski, B. Stühn. Coll. Polym. Science 282, 810-824 (2004)
- Crystallization of s-polypropylene: a qualitative change in the kinetics induced by the temperature of the melt, Barbara Heck, Gert Strobl, Coll. Polym. Science, 282, 511-513 (2004)
- Characteristic variations in the effect of diluents on polymer crystallization and melting observed for a sample of poly (ethylene-co-octene), Barbara Heck, Gert Strobl, Michael Grasruck, Eur. Phys. J. E, 11, 117-130 (2003)
- A temperature- and molar mass-dependent change in the crystallization mechanism of poly(1-butene): Transition from chain-folded to chain-extended crystallization? Qiang Fu, Barbara Heck, Gert Strobl, Yi Thomann, Macromolecules, 34, 2502-2511 (2001)
- Steps in the formation of the partially crystalline state, Barbara Heck, Thorsten Hugel, Masanori Iijima, Gert Strobl, Polymer, 41, 8839-8848 (2000)
- Steps in the transition of an entangled polymer melt to the partially crystalline state, Barbara Heck, Thorsten Hugel, Masanori Iijima, Emmanuel Sadiku, Gert Strobl, New Journal of Physics 1, 17 (1999) Abstract, PDF
For s-polypropylene, three different s-poly(propene-co-octene)s, two poly(ethylene-co-octene)s and poly(epsilon-caprolactone) we determined the relationships between the crystallization temperature Tc, the crystal thickness dc and the melting peak temperature Tf by carrying out time- and temperature-dependent small-angle x-ray scattering experiments. As a general law, dc is found to be inversely proportional to the supercooling below a characteristic temperature, Tc∞, which is always located above the equilibrium melting point following from an application of the Gibbs-Thomson equation. Tc∞ is not (for the polypropylene-based copolymers) or only weakly (for the polyethylene-based copolymers) dependent on the co-unit content. The 'crystallization line' Tc versus dc-1 and the Gibbs-Thomson melting line Tf versus dc-1 limit the range of the accessible partially crystalline states. The occurrence of two well defined independent boundary lines may be understood as indicating that the transformation from the melt into the partially crystalline state is generally, for homopolymers and copolymerized derivatives likewise, a two-step process, beginning with the building up of an initial form of lower order at the crystallization line which then becomes stabilized to end up in the state with layer-like morphology melting at Tf. Stabilization is achieved without a change in dc. AFM and TEM observations suggest that the initial structure could be composed of crystal blocks in planar assemblies. The stabilization then would result from their merging into a continuous lamella. It is proposed that the size of the blocks represents the minimum necessary to retain intrinsic stability. As a further result it was found that crystallinities reached at the end of isothermal crystallization processes remain invariant over larger ranges of Tc, in contrast to the changing length scales of the structure.
- Structural transition in a nematic LC block copolymer induced by the transition to the LC phase, J. Sänger, W. Gronski, S. Maas, B. Stühn, B. Heck, Macromolecules, 30, 6783-6787 (1997)
- SAXS and TEM studies on poly(styrene)-block-poly(ethene-co-but-1-ene)-block-poly(styrene) in bulk and at various interfaces, B. Heck, P. Arends, M. Ganter, J. Kressler, B. Stühn, Macromolecules, 30, 4559-4566 (1997)