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Thermoset Polymer Matrix–Based Natural Fiber Composites
Published in Shishir Sinha, G. L. Devnani, Natural Fiber Composites, 2022
The higher cross-linking density causes brittleness in thermoset polymers. One process of toughening can be lowering of cross-linking density. Decreasing the cross-linking density can also result in the reduction of desirable properties like resin modulus. There are two popular methods to reduce the cross-linking density. The first method is to change the main backbone chain by reducing the cross-linking density with a long-chain monomer. The decreasing glass transition temperature can be set off with the help of fabricating a long-chain monomer that has rigidity. The reduced mobility of the polymeric chain will remunerate the loss in transition temperature. The second method is to reduce the functionality of the monomer. Thermosets that exhibit high cross-linking for reactive end groups show functionality of four end groups that react and form the cross-link. If the part of a polymer has a monomer of functionality two, then during the curing there will be fewer active sites for cross-linking, which will result in improvement in toughness as the cross-linking density will reduce but the thermal resistance will be affected due to lower transition temperature of the monomer of functionality.
Telechelics by Free Radical Polymerization Reactions
Published in Eric J. Goethals, Telechelic Polymers: Synthesis and Applications, 2018
The characterization of telechelics is still one of the central problems. The determination of functionality by a combination of any end-group analysis and number-average molecular weight will give an average functionality. Average values are also obtained if the ratio of end-groups to monomer units is determined by means of spectroscopic methods, e.g., nuclear magnetic resonance (NMR). A functionality of two may be caused by a telechelic, but a compensation between molecules of different functionalities may give the same value as well. The limits of error of Mn-values by vapor phase osmometry is about ±3% up to molecular weights of 2000. Only in rare cases, can a statement about a functionality of better than 2.0 ± 0.1 be given. At higher molecular weights, the uncertainty is larger. The characterization of telechelics needs the analytical proof of a functionality of two and a chromatographic check that the product is made up of one polymer homologous series. High performance liquid chromatography (HPLC) and high resolution gel phase chromatography (GPC) are tools to do that. As Inagaki et al.106,107 have shown, the effect of molecular weight distribution can be eliminated in thin-layer chromatography (TLC) by proper choice of the solvent. Under these conditions, the separation occurs according to the microstructure and the chemical composition. Entelis et al. explored this principle for column chromatography.17 Major achievements can be expected from this method.
Step-Growth Polymerization
Published in Anil Kumar, Rakesh K. Gupta, Fundamentals of Polymer Engineering, 2018
We have already observed that the polymerization of trifunctional (and higher functionality) monomers leads to branched polymers, which ultimately form network molecules. The main commercial interest in the polymers of phenol and melamine has been in producing molded objects that exhibit high chemical and environmental resistance. These are network polymers and are formed in two stages. In the first step, a prepolymer is prepared that is, in the second step, cross-linked to the desired shape in a mold in the presence of a suitable cross-linking agent. The urea formaldehyde polymer has found extensive use in plywood industries; in its first stage, a syrupy prepolymer is prepared that is cross-linked between the laminates of the plywood.
Insights on self-assembly of carbon in the processes of thermal transformations under high pressures
Published in Functional Diamond, 2023
V. A. Davydov, V. N. Agafonov, T. Plakhotnik, V. N. Khabashesku
From the point of view of the theory of polymerization, an important characteristic of a monomer is its functionality (f), which means the maximum number of chemical bonds in which a particular monomer is able to participate. For C60, the number of covalent intermolecular bonds may be different in cases of 1D, 2D, or 3D polymerization. According to some model representations, the functionality of C60 in 3D polymers can reach values of 52, 54, and even 60 units per molecule [26]. In this case, polymerization can be both ordered and disordered.