Photomodulation of the Mechanical Properties and Photo-Actuation of Chitosan-Based Thin Films Modified with an Azobenzene-Derivative.

A sophisticated comprehension of the impacts of photoisomerization and photothermal phenomena on biogenic and responsive materials can provide a guiding framework for future applications. Herein, the procedure to manufacture homogeneous chitosan-based smart thin films are reported by incorporating the light-responsive azobenzene-derivative Sodium-4-[(4-(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)phenyl)diazen-yl]-benzenesulfonate (TEGABS) in the biopolymer through electrostatic interactions. When irradiated with UV-light the TEGABS/chitosan films show a biresponse, comprising the E→Z photoisomerization with a half-life of 13 - 20 h and the light-induced evaporation of residual moisture leading to an increase in the reduced indentation modulus (up to 49%) and hardness. Freestanding films of TEGABS/chitosan show actuation up to 13° while irradiated with UV-light. This work shows the potential of biogenic polysaccharides in the design of biresponsive materials with photomodulated mechanical properties and unveils the link between the humidity of the environment, residual moisture, and the photomodulation of the mechanical properties.

Dual Catalytic Ring-Opening Polymerization of Ethylene Carbonate for the Preparation of Degradable PEG.

A dual catalytic setup, consisting of a range of different Lewis bases (including N-heterocyclic olefins, phosphazenes and nitrogen bases) and simple Lewis acids (such as LiCl, MgF2, BEt3), was employed to prepare poly(ether carbonate)s from five-membered, cyclic ethylene carbonate (EC). Polymerizations were conducted under microwave irradiation at T = 160-200 °C with low catalyst loading (0.4-0.005 mol % regarding organobases) in the bulk, enabling access to molar masses of up to 10 000 g/mol. A combination of kinetic investigation, GPC, MALDI-ToF MS, and NMR analysis underlines that the polymerization can be highly effective (TON up to >17 000) but side reactions still occur, resulting in a moderately controlled process. In contrast to traditional procedures, where increased ether contents can typically only be realized by higher temperatures and longer reaction times (at the cost of much reduced molar masses), the dual catalytic approach reveals the choice of the Lewis acid as a more effective tuning parameter for this property. Thus, while carbonate contents of up to 30% are possible, application of LiCl as cocatalyst provides a polymer with high ether content (90-99%, Mn = 800-10 000 g/mol), a finding that also seems to apply for other lithium salts. Thus, using this operationally simple setup and EC as a cheap feedstock, a copolymer which is essentially a degradable poly(ethylene glycol) can be prepared in a one-pot, one-step approach. Notably, the obtained low-carbonate content allows for the preparation of semicrystalline poly(ether carbonate), further underlining the "PEG-likeness" of the material.