Crosslinked glassy polymers that

If this interpretation is correct, it should also be possible to prepare crosslinked glassy polymers that proliferate in contact with a monovinyl compound. Such materials have been obtained by polymerizing styrene with a high concentration of divinylbenzene in the presence of a diluent... 

In this contribution we present results obtained with tetra-ethyleneglycol diacrylate (TEGDA). This compound was chosen since its polymer shows an easily discernible maximum in the mechanical losses as represented by tan 5 or loss modulus E" versus temperature when it is prepared as a thin film on a metallic substrate. When photopolymerized at room temperature it forms a densely crosslinked, glassy polymer, just as required in several applications. Isothermal vitrification implies that the ultimate conversion of the reactive double bonds is restricted by the diffusion-limited character of the polymerization in the final stage of the reaction. Therefore, the ultimate conversion depends strongly on the temperature of the reaction and so does the glass transition. 

To address these limitations, a new constitutive model was developed for conventional and highly crosslinked UHMWPEs (Bergstrom, Rimnac, and Kurtz 2003). This model, which is inspired by the physical micromechanisms governing the deformation resistance of polymeric materials, is an extension of specialized constitutive theories for glassy polymers that have been developed during the last 10 years, is discussed later. 

Pa, would deform appreciably under the action of loads comparable to the pull-off force given by Eq. 16. It is for this reason that the JKR type measurements are usually done on soft elastic materials such as crosslinked PI rubber [45,46] or crosslinked PDMS [42-44,47-50]. However glassy polymers such as polystyrene (PS) and PMMA are relatively hard, with bulk moduli of the order of 10 Pa. It can be seen from Eq. 11 that a varies as Thus, increasing K a factor of... 

In conclusion, the yield behavior of thermosets is similar to that found for other glassy polymers. The presence of crosslinks does not basically affect the yield behavior of polymer networks. 

Reaction 1 produces a linear polymer (a thermoplastic) that should be soluble in acetone, while reaction 2 produces crosslinked, insoluble polymer (a thermoset resin). The viscosity of the crosslinked polymer when hot should be noticeably higher than that of reaction 1. Although individual results will depend upon the purity of the starting materials and the heating rate, often the linear product is glassy and hard while the crosslinked one tends to be more brittle and porous. The latter results from the extremely high viscosity that develops as the crosslinked polymer increases in molecular weight. 

Thermosets are polymeric materials which when heated form permanent network structures via the formation of intermolecular crosslinks. Whether the final product has a glass transition temperature, Tg, above or below room temperature, and therefore normally exists as an elastomer or a glass, it is, strictly speaking, a thermo-set. In practice, however, thermosets are identified as highly crosslinked polymers that are glassy and brittle at room temperature. These materials typically exhibit high moduli, near linear elastic stress-strain behavior, and poor resistance to fracture. 

The stress-strain behavior of thermosets (glassy polymers crosslinked beyond the gel point) is not as well-understood as that of elastomers. Much data were analyzed, in preparing the previous edition of this book, for properties such as the density, coefficient of thermal expansion, and elastic moduli of thermosets [20,21,153-162]. However, most trends which may exist in these data were obscured by the manner in which the effects of crosslinking and of compositional variation were superimposed during network formation in different studies, by... 

As indicated in Fig. 23.1, a sample consists of a rigid glass indenter and an elastomer substrate of crosslinked poly(dimethyl siloxane) (PDMS), which are both coated with the polymer layers of interest. These layers include a semicrystalline layer of poly(ethylene oxide) (PEO) sandwiched between glassy polymer layers of poly(tetramethyl bisphenol A polycarbonate) (TMPC). These polymers will be described in more detail within this section along with the steps that were taken to select these polymers for the study of a glassy/semicrystalline interface. 

In order to facilitate chemical modification of crosslinked supports, and indeed to allow subsequent use of the immobilised species in reactions, it is vital that the crosslinked network is designed to be accessible. This can be achieved in two ways. Firstly, the proportion of crosslinker (divinyl monomer) can be kept low, say below 10 vol%, and often as low as 1-2 vol%. This yields a hard glassy polymer known as a gel-type resin. Figure 6.5. 

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