Network chains molecular weight

With regard to elastomers of controlled structure, those having unusual distributions of network chain lengths have been of particular interest [88,89]. The most novel elastomer of this type consists of a binary combination of unusually short network chains (molecular weights of a few hundred) and the much longer chains typically associated with elastomeric behavior (molecular weights of ten or twenty thousand). Such a network is sketched in Figure 6. 

It should be pointed out that there are three requirements for obtaining these improvements. The first is that the ratio Ms/Ml of molecular weights of the short (Ms) and long chains (Ml) be very small (i.e., that their molecular weights be very different). The second is that the short chains be as short as possible for example, a network having network chain molecular weights of 200 and 20,000 g/mol... 

The elastic moduli can be calculated from the network chain molecular weight Me, and vice versa. Equation (2.13) can be substituted into Eq. (3.19) to give... 

Dependence of Mooney-Rivlin ratio, 2C2/2C1, on the molecular weight between cross links. The factor 2C measures the departure from affineness as the elongation increases, and 2Cj approximates the high-deformation modulus. The ratio decreases with decrease in network chain molecular weight, and with increase in junction functionality, as predicted by theory. ... 

Figure 15.1 a 2H NMR spectrum of perdeuterated crosslinked chains in a PDMS network (average molecular weight between junctions Mn= 10500 g.mol 1, i.e.,... 

Figure 15.8 2H NMR spectra obtained in a randomly crosslinked, deuterated PB network. Precursor chain molecular weight 115700 g.mol"1, 1.2% crosslink agent, the average molecular weight between junctions is 27400 g.mol 1 (as determined by swelling experiments) or 11600 g.mol"1 (elastic measurements). The smooth curves are fits with a most probable distribution of chain lengths with number average molecular weight Mc = 11600 and with non-Gaussian chain statistics...

Figure 1.8 Dependence of volume swelling in benzene of model networks vs. molecular weight M of chain precursors for three sets of networks each set prepared at a constant concentration and constant portion of DVB per living end. (After [126]).

All the quantities so far defined relate to ideal networks, i.e., continuous branched structures without free chain ends. In reality, the number of such free chain ends increases with decreasing primary chain molecular weight. The molar concentration MX niol/g of effective network chains can, according to P. J. Flory, be calculated from the molar concentration [MJ of all the chains present for M )q > (M ) ,... 

The next step in the development of a model is to postulate a perfect network. By definition, a perfect network has no free chain ends. An actual network will contain dangling ends, but it is easier to begin with the perfect case and subsequently correct it to a more realistic picture. We define v as the number of subchains contained in this perfect network, a subchain being the portion of chain between the crosslink points. The molecular weight and degree of polymerization of the chain between crosslinks are defined to be Mj, and n, respectively. Note that these same symbols were used in the last chapter with different definitions. 

Chains of polybutadiene were trapped in the network formed by cooling a butadiene-styrene copolymer until phase separation occurred for the styrene, effectively crosslinking the copolymer. At 25°C the loss modulus shows a maximum which is associated with the free chains. This maximum occurst at the following frequencies for the indicated molecular weights of polybutadiene ... 

As the length and frequency of branches increase, they may ultimately reach from chain to chain. If all the chains are coimected together, a cross-linked or network polymer is formed. Cross-links may be built in during the polymerisation reaction by incorporation of sufficient tri- or higher functional monomers, or may be created chemically or by radiation between previously formed linear or branched molecules (curing or vulcanisation). Eor example, a Hquid epoxy (Table 1) oligomer (low molecular weight polymer) with a 6-8 is cured to a cross-linked soHd by reaction of the hydroxyl and... 

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