Glasses, crosslinked

Hazardous Decomp. Prods. Heated to decomp., emits toxic fumes of CO, CO2, NOx emits toxic fumes under fire conditions Storage Hygroscopic protect from moisture store in cool, dry place keep container closed Uses Imparts water tolerance and vapor permeability to copolymer films latex and water-based coating compositions adhesion improver for cellulosics, concrete, glass crosslinking agent in polyester resins color photography... 

An ideal adhesive film system can be made by the method originally used by Roberts, shown in Fig. 14.4(a). He moulded gum rubber against an optically smooth surface such as glass, crosslinked the polymer by heating with sulfur or peroxide, peeled the rubber from the surface to reveal an optically smooth polymer surface, then adhered this material to another smooth surface to form the joint. Alternatively, low temperature reacting materials such as silastomers or... 

This is also known as Bulk Moulding Compound (BMC). It is blended through a mix of unsaturated polyester resin, crosslinking monomer, catalyst, mineral fillers and short-length fibrous reinforcement materials such as chopped glass fibre, usually in lengths of 6-25 mm. They are all mixed in different proportions to obtain the required electromechanical properties. The mix is processed and cured for a specific time, under a prescribed pressure and temperature, to obtain the DMC. 

The finite size effects in the contact between a spherical lens of polyurethane and a soft flat sheet of crosslinked polyfdimethyl siloxane) (PDMS) has been addressed by Falsafi et al. [37]. They showed that for deformations corresponding to contact diameters larger than the sheet thickness, the compliance of the system was affected by the glass substrate supporting the soft sheet. In order to minimize the finite size effects in the adhesion measurement of small elastomeric lenses, Falsafi et al. [38] and Deruelle et al. [39] used relatively thick elastic sheets to support their samples. 

In an attempt to determine the applicability of JKR and DMT theories, Lee [91] measured the no-load contact radius of crosslinked silicone rubber spheres in contact with a glass slide as a function of their radii of curvature (R) and elastic moduli (K). In these experiments, Lee found that a thin layer of silicone gel transferred onto the glass slide. From a plot of versus R, using Eq. 13 of the JKR theory, Lee determined that the work of adhesion was about 70 7 mJ/m". a value in clo.se agreement with that determined by Johnson and coworkers 6 using Eqs. 11 and 16. 

An example of a Maugis-Pollock system is polystyrene particles having radii between about 1 and 6 p.m on a polished silicon substrate, as studied by Rimai et al. [64]. As shown in Fig. 4, the contact radius was found to vary as the square root of the particle radius. Similar results were reported for crosslinked polystyrene spheres on Si02/silicon substrates [65] and micrometer-size glass particles on silicon substrates [66]. 

Increasing the amount of crosslinker extends the plateau modulus to higher and higher temperatures, eventually eliminating the flow of the polymer. The effect on the glass transition is minimal. 

Small deformations of the polymers will not cause undue stretching of the randomly coiled chains between crosslinks. Therefore, the established theory of rubber elasticity [8, 23, 24, 25] is applicable if the strands are freely fluctuating. At temperatures well above their glass transition, the molecular strands are usually quite mobile. Under these premises the Young s modulus of the rubberlike polymer in thermal equilibrium is given by ... 

At temperatures well above the glass transition of the polymers, the molecular segments are highly flexible and slip past each other almost without restriction. They behave like the molecules of a liquid except for the fact that their ends are linked with each other. Just the existence of crosslinks distinguishes rubberlike materials from ordinary liquids. The bulk moduli K of liquids and of rubberlike materials are of similar magnitude, e.g. K = 1 to 2 GPa [26]. 

The argument assumes that the forces act only at the crosslinked ends of the strands. No interactions between the strands exist in this simplistic picture. The interaction of molecular segments, well above the glass transition temperature is usually rather small. 

Upon cooling, molten and rubberlike polymers pass the glass transition and solidify as glassy materials. The temperature TB of the glass transition depends on the chemical nature of the polymer as well as on the number of crosslinks between the molecular chains. Two different test methods were used for the determination of the glass transition range ... 

Fig. 4.1. Glass transition temperatures of the polymers are plotted against l/IVlc, that is the inverse molecular mass between crosslinks.

Tgoa glass transition temperature of the uncrosslinked polymer Mc average molecular mass between crosslinks tj> empirical factor... 

Apparently, annealing was not impeded by crosslinks (Fig. 5.1). The density effects observed agree with the results of the glass transition temperature measurements (Sect. 4.2). There, the Tg of the annealed (and therefore denser) sample was consistently higher by about 2 K than the Tg of the quenched polymer. 

The free volume model seems to be more adequate to describe the plasticization behaviour of the systems of lower amine content. According to Eq. (5), the higher is the change of the expansion coefficient the lower is the influence of the diluent volume fraction. The three TGDDM-DDS mixtures cured with 20, 30 and 50 PHR of hardener were characterized l2) by changes of the expansion coefficient at the glass transition, respectively, of 0.63, 1.08 and 2.94x 10 3 °C l. The more dense and stiffer resin crosslinked with 50 PHR of DDS should be, in principle, the less... 

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