Miriam M. Unterlass

Geomimetics for green polymer synthesis: highly ordered polyimides via hydrothermal techniques

Inspired by geological ore formation processes, we apply one-step hydrothermal (HT) polymerization to the toughest existing high-performance polymer, poly(p-phenyl pyromellitimide) (PPPI). We obtain highly-ordered and fully imidized PPPI as crystalline flakes and flowers on the micrometer scale. In contrast to classical 2-step procedures that require long reaction times and toxic solvents and catalysts, HT polymerization allows for full conversion in only 1 h at 200 °C, in nothing but hot water. Investigation of the crystal growth mechanism via scanning electron microscopy (SEM) suggests that PPPI aggregates form via a dissolution–polymerization–crystallization process, which is uniquely facilitated by the reaction conditions in the HT regime. A conventionally prefabricated polyimide did not recrystallize hydrothermally, indicating that the HT polymerization and crystallization occur simultaneously. The obtained material shows excellent crystallinity and remarkable thermal stability (600 °C under N2) that stem from a combination of a strong, covalent polymer backbone and interchain hydrogen bonding.

Polyethylenes bearing a terminal porphyrin group

An α-[Cu(II)-porphyrin]-polyethylene was synthesized for the first time using copper catalyzed 1,3-dipolar azide-alkyne Huisgen cycloaddition yielding highly colored moiety-substituted polyethylene.

From dense monomer salt crystals to CO2 selective microporous polyimides via solid-state polymerization

Fully aromatic polyimides are synthesized via solid-state polymerization of the corresponding monomer salts. The crystal structure of salts shows strong hydrogen bonding of the reactive groups and thereby paves the way for solid-state transformations. The polycondensation yields copies of the initial salt crystallite habits, accompanied by the development of a porosity especially suited for CO2.

Towards a general understanding of hydrothermal polymerization of polyimides

Hydrothermal polymerization (HTP) has been recently established as a novel route to synthesize polyimides of outstanding crystallinity. In this contribution, we lay out the basic theoretical and experimental framework for understanding the mechanistic underpinnings of this process. For this purpose, we hydrothermally synthesize two representative polyimides that are known to form amorphous polymers when synthesized classically. Hydrothermal polymerization, in contrast, yields unprecedented crystallinity after only two hours. The co-monomers diamine and dianhydride form monomer salts via acid–base reaction when brought in contact in water. We show that the physicochemical properties of the crystalline monomer salts (i.e. solubility, solid-state polymerization temperature) are important factors for the crystallinity and the morphology of the corresponding hydrothermally synthesized polyimide. We develop a mechanistic model of hydrothermal polymerization processes allowing us to relate the polymerization parameters (concentration, reaction temperature, reaction time) to the obtained polyimide crystallinity and morphology. By adjusting the parameters, the achieved crystallinity can be further increased and high morphological homogeneity can be obtained. We believe that the developed mechanistic picture is applicable for the hydrothermal polymerization of any polyimide.

Mechanistic study of hydrothermal synthesis of aromatic polyimides

A mechanistic study regarding the synthesis of aromatic polyimides (PIs) under hydrothermal conditions is presented. We demonstrate evidence that the first step of the reaction is the formation of a nylon type salt, which can be isolated. Upon heating the salt in the presence of water at 180 °C, polyimides are formed from the salts. Additionally performed DFT calculations substantiate the experimental findings and give evidence that the formation of imides is energetically favorable in hot water. The intermediate salts as well as the resulting PIs are characterized by diverse analytical techniques (SEC, FT-IR, WAXS, SEM). By selective fractionation, it is possible to obtain polyimides of moderate to high molecular weights that form tough films. The polymerization mechanism is discussed, and it is shown that different loci of reaction are possible, namely interfacial polycondensation, heterophase polymerization and topochemical transformations, which seem to occur in parallel. Finally, poloxamers can be used as additives for the fine-tuning of the polymer morphology.

“Schizomorphic” Emulsion Copolymerization Particles

Cryo-electron microscopy, atomic force microscopy, and light microscopy investigations provide experimental evidence that amphiphilic emulsion copolymerization particles change their morphology in dependence on concentration. The shape of the particles is spherical at solids content above 1%, but it changes to rod-like, ring-like, and web-like structures at lower concentrations. In addition, the shape and morphology of these particles at low concentrations are not fixed but very flexible and vary with time between spheres, flexible pearl-necklace structures, and stretched rods.

Green one-pot synthesis and processing of polyimide–silica hybrid materials

Inorganic–organic hybrid materials allow for combining features typical of the inorganic component with those of the organic component in one material. Generally, the preparation of organic and inorganic compounds requires considerably different synthesis conditions. Hence, the development of one-pot routes to inorganic–organic hybrid materials is challenging. We herein report a fully green one-pot synthesis of polyimide/silica (PI/SiO2) hybrids. Specifically, we co-condense both components hydrothermally, using nothing but the respective precursors and water. Furthermore, we show that the PI and the SiO2 component can be covalently connected under hydrothermal conditions, using the compatibilizer (3-aminopropyl)-triethoxysilane. We thoroughly investigate the effect of different reaction conditions, including temperature, pH, precursor concentration and reaction time on the morphology and crystallinity of the final materials. The polyimide component, poly(hexamethylene pyromellitimide) was chosen for its thermoplasticity, which allows for processing both the PI and the PI/SiO2via sintering. For being a solvent-free method, sintering qualifies as a green processing technique. This work is the first report of the simultaneous hydrothermal condensation of an inorganic and an organic material.

Extending the Scope of a New Cyanation: Design and Synthesis of an Anthracene Derivative with an Exceptionally Low LUMO Level and Improved Solubility

The preparation of cyanated acenes from quinones has been improved for the conversion of electron-poor starting materials. The new procedure was used to prepare rationally designed 2,7-dinitro-9,10-dicyanoanthracene. Crystallographic, morphological, and electrochemical investigations have revealed most promising properties for applications in organic electronics.

Geomimetics and Extreme Biomimetics Inspired by Hydrothermal Systems—What Can We Learn from Nature for Materials Synthesis?

‘Extreme biomimetics’ and ‘geomimetics’ are relatively recent fields of materials chemistry. Both take inspiration from natural materials for generating novel synthetic materials or enhanced properties in known materials. In geomimetics, the source of inspiration is geological systems, while extreme biomimetics is motivated by organisms operating in—from an anthropocentric point of view—extreme conditions. This review article focuses on geomimetic and extreme biomimetic hydrothermal synthesis. Since hydrothermal preparative chemistry typically uses nothing but water and the required precursors, the field belongs to the research area of ‘green materials chemistry’. Geomimetics, on the one hand, takes inspiration from natural materials formation. Extreme Biomimetics, on the other hand, is inspired by materials found in extremophile organisms, instead of aiming to implement their actual biosynthesis. In this contribution, both extreme biomimetics and geomimetics are first defined, and further critically discussed on the basis of recent, selected examples. Moreover, the necessity for the two closely related fields as well their prospects are commented on.

Processing of Carbon Nanotubes and Carbon Nanofibers towards High Performance Carbon Fiber Reinforced Polymers

Carbon fiber reinforced polymers (CFRPs) are promising composite materials for high-performance and lightweight applications, gaining increasing interest in aerospace and automotive industries. Epoxy thermosets are frequently used as polymer matrices of CFRPs, which are usually responsible for failure of the composite. In this work different types of carbon nanotubes (CNTs) and carbon nanofibers (CNF) are added to the epoxy resin to improve mechanical properties of the whole CFRP composite. The dispersion of the fillers on a three-roll mill (TRM) is shown comparing their dispersion behavior in the resin. Results of increased modulus and strength of the hierarchical composite in four-point bending tests are presented.