Molecular weight between variation

Figure 11. Variation of molecular weight between cross-links as a function of molecular weight...

These data are consistent with the observation O) that increased crosslinking (i.e., epoxy resins with lower EEW s) will result in a decreased propensity for the cured polymer to absorb solvents. Thus, the solvent resistence of an organic coating can be controlled by the formulator via variations in the molecular weight between crosslinks. 

Some interesting results have already been obtained (JO, 11) on these polymers, where the effect of the above molecular parameters on the mechanical properties has been studied. Thus, Figure 11 shows the effect of variations in block length and styrene content on the stress-strain behavior of styrene-butadiene-styrene (SBS) polymers. As expected, the stress levels increase with increasing styrene ( filler ) content but are independent of the block lengths. In other words, the center block size does not exert the same influence as the molecular weight between cross-... 

Calculate the variation in entropy corresponding to a polydimethylsiloxane network of mass 6.89 g, with an average molecular weight between crosslinks Me = 8.3 X 10, subjected to a reversible uniaxial extension at 25°C until the length is double its initial length. 

Whatever the composition of the component molecules, association into larger species is certain. Variations in molecular weight between ca 3000 [15] and 150,000 [16] have been reported, as have asphaltene sizes of60-90A [16]-again considered to be an over-estimate [7,8]. In the context of hydroprocessing it is obviously the size of the entity than actually reacts on the catalyst rather than the absolute size of the molecule or conglomerate that is important. 

Fig. 6. Variation of interface toughne.ss with area den.sity of copolymer for a range of different molecular weight PS-PMMA copoly mens between PMMA and PPO (or PPE) [39J.

The formaldehyde-to-phenol molar ratios of most novolacs lie somewhere between 0.30 and 0.99. This is a very broad range in the eyes of a novolac chemist. Novolacs are extremely sensitive to molar ratio variation and they are usually specified to the nearest 0.001 molar ratio unit or less. Also unlike resoles, one does not have the option of selecting the development of molecular weight through viscosity control. The molecular weight and viscosity are largely determined by... 

Free phenol is a major concern in the manufacture of novolac resins. This is true for several reasons. The strongest drivers are probably EPA classification of phenol as a Hazardous Air Pollutant and worker safety concerns. However, free phenol also has significant technical effects on such parameters as melt flow characteristics. In this role, free phenol may undermine the desired effects of a molecular weight design by increasing flow beyond the desired point. Since free phenol is often variable, the effects on flow may also cause variation in product performance from batch to batch. Fig. 18 shows the effects of free phenol on the flow across a series of molecular weights. Free phenol contents between 1 and 10% are commonly seen. In recent years, much work has been aimed at reducing the free phenol. 

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