Andreas Geissler

Moisture-responsive films of cellulose stearoyl esters showing reversible shape transitions

Moisture-responsive materials are gaining greater interest for their potentially wide applications and the readily access to moisture. In this study, we show the fabrication of moisture-responsive, self-standing films using sustainable cellulose as starting material. Cellulose was modified by stearoyl moieties at first, leading to cellulose stearoyl esters (CSEs) with diverse degrees of substitution (DSs). The films of CSE with a low DS of 0.3 (CSE0.3) exhibited moisture-responsive properties, while CSEs with higher DSs of 1.3 or 3 (CSE1.3 and CSE3) not. The CSE0.3 films could reversibly fold and unfold as rhythmical bending motions within a local moisture gradient due to the ab- and desorption of water molecules at the film surface. By spray-coating CSE3 nanoparticles (NPs) onto CSE0.3 films, moisture-responsive films with non-wetting surface were obtained, which can perform quick reversible bending movements and continuous shape transition on water. Furthermore, bilayer films containing one layer of CSE0.3 at one side and one layer of CSE3 at the other side exhibited combined responsiveness to moisture and temperature. By varying the thickness of CSE0.3 films, the minimal bending extent can be adjusted due to altered mechanical resistances, which allows a bending movement preferentially beginning with the thinner side.

Formation of nanostructured cellulose stearoyl esters via nanoprecipitation

Nanoparticles (NPs) from derivatives of native polysaccharides have not been as intensively studied yet as those from synthetic polymers. In this report, NPs in aqueous suspensions were fabricated using cellulose stearoyl esters (CSEs) with different molecular weights via nanoprecipitation using dropping and dialysis techniques. The average diameters of NPs depended strongly on the concentrations of CSE solutions, molecular weights of CSE and also the nanoprecipitation technique. Both nanoprecipitation techniques are based on different mechanisms and NPs from dialysis are generally larger than NPs from dropping. The mechanism for dropping nanoprecipitation was further analyzed based on the properties of NPs which contain crystallized stearoyl groups in CSE chains. The average diameters of freshly-prepared CSE nanoparticles decreased with rising temperature, which is accompanied by the release of THF from the interior of NPs. The intensity of the size reduction of up to 35% depended on the one hand on the concentration of CSE solutions, and on the other hand on the molecular weights of CSEs. Finally, it was shown that these NPs can be used for the fabrication of temperature-responsive superhydrophobic surfaces.

Solid-state spectroscopic characterization of α-chitins deacetylated in homogeneous solutions.

Synthesis and spectroscopic characterization of α-chitins exhibiting degrees of acetylation (DA) between 0.5 and 1 were reported. Crab shell α-chitin consisting of microfibrils with a width of less than 1 μm was regenerated from or alkaline deacetylated in 30% aqueous NaOH solution. After the deacetylation in homogeneous solution, the DA of α-chitins decreased steadily with prolonged deacetylation times from 1 to 0.54. The native microfibrils of starting α-chitin were destroyed, while aggregates or sheets were formed depending on the DA of chitins. According to WAXD measurements, deacetylated chitins with DA lower than 0.76 have different crystalline structures than starting α-chitin. Furthermore, FT Raman spectroscopy demonstrated a novel rapid method to determine the DA of chitin with correlation coefficients greater than 0.99. On the basis of the FT Raman signals ascribed to the amide groups, the amount of hydrogen bonds linked to C═O groups decreased strongly with lower DA and the relationship between them was presented.

Synthesis of carboxyl cellulose sulfates with regioselective sulfation and regiospecific oxidation using cellulose trifluoroacetate as intermediates

Synthesis of cellulose sulfates (CSs) and carboxyl cellulose sulfates (COCSs) with regioselectively or regiospecifically distributed functional groups within anhydroglucose units was reported. CS with regioselectively distributed sulfate groups at 2,3-O- or 2,6-O-position were homogeneously synthesized and cellulose trifluoroacetate (CTFA) was used as intermediates. The trifluoroacetyl groups were detected primarily at 6-O-position and their distributions could be altered by changing the amount of trifluoroacetyl anhydride (TFAA). Various sulfating agents were used for further homogeneous sulfation of CTFA. The total degree of sulfation (DSS) and the distribution of sulfate groups within the repeating units were affected by the amount of TFAA, the type and amount of sulfating agents. Subsequent homogenous 4-acetamide-TEMPO or TEMPO-mediated oxidation of CS led to COCS with carboxyl groups regiospecifically distributed at C6 position, which may be interesting structural mimics for natural occurring heparin.