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These are made by the reaction of phenols, formaldehyde and primary amines which at elevated temperatures (400 °F (200 °C)) undergo ring–opening polymerisation forming polybenzoxazine thermoset networks; when hybridised with epoxy and phenolic resins the resulting ternary systems have glass transition temperatures in excess of 490 °F (250 °C).
Left: individual linear polymer chains Right: Polymer chains which have been cross linked to give a rigid 3D thermoset polymer. In materials science, a thermosetting polymer, often called a thermoset, is a polymer that is obtained by irreversibly hardening ("curing") a soft solid or viscous liquid prepolymer (). [1]
Conventional PU foams are cross-linked materials or thermosets. PU foams can either be mechanically recycled (where PU foams are grinded and used as fillers), or chemically recycled (where PU foams are downcycled into polyols or other monomeric components via chemical degradation). [16] [17] However, most PU foams end up on landfills.
Carbon on Earth naturally occurs in two stable isotopes, with 98.9% in the form of 12 C and 1.1% in 13 C. [1] [8] The ratio between these isotopes varies in biological organisms due to metabolic processes that selectively use one carbon isotope over the other, or "fractionate" carbon through kinetic or thermodynamic effects. [1]
The research group led by Ludwik Leibler demonstrated the operating principle of vitrimers at the example of epoxy thermosets. Epoxy thermosets can be represented as vitrimers, when transesterification reactions can be introduced and controlled. In the studied system, carboxylic acids or carboxylic acid anhydrides must be used as hardeners. [13]
The underlying force driving these reactions is the Gibbs free energy of the reactants relative to the products. If donor and acceptor (the reactants) are of higher free energy than the reaction products, the electron transfer may occur spontaneously. The Gibbs free energy is the energy available ("free") to do work.
Because of this, P680 + is highly oxidizing and can take part in the splitting of water. [ 2 ] Photosystem II (of cyanobacteria and green plants) is composed of around 20 subunits (depending on the organism) as well as other accessory, light-harvesting proteins.
Electrons in pigment molecules can exist at specific energy levels. Under normal circumstances, they are at the lowest possible energy level, the ground state. However, absorption of light of the right photon energy can lift them to a higher energy level. Any light that has too little or too much energy cannot be absorbed and is reflected.