Molecular weight per cross-link

The isocyanates used with rigid foam systems are either polymeric MDI or specialty types of TDI. Both contain various levels of polymerized isocyanate groups which contribute to molecular weight per cross-link and also may affect reactivity due to steric hindrance of some isocyanate positions. 

Molecular weight per cross-linked unit. Hence also the molecular weight per chain of a perfect network. 

Urethane foams can be classified into two principle types, i.e., flexible and rigid foams. In some cases, flexible foams can be further subdivided into flexible and semi-flexible (or semi-rigid) foams. The differences in physical properties of the two foams are mainly due to the differences in molecular weight per cross-link, the equivalent weight and functionality of the polyols, and the type and ctionality of the isocyanate. 

M, molecular weight per cross-linked unit MDA molecular density molding... 

There are two parameters used as a measure of cross-Unk density the number of network chains, v, usually expressed as v/ V, where V is the volume of the unstrained network and the number of cross-links (p) per unit volume, p/F. The relationship between p and v is established by knowing the number of chains starting from a particular cross-linking point, (functionality). The two most important types of network are the tetrafunctional (c ) =z 4) and the trifunctional ( = 3). Another characteristic parameter of a network is the number-average molecular weight between cross-links,... 

On the other hand if the network is produced by long chains linked at a T-shaped junction point (as may occur with the polyurethane rubbers for example) the molecular weight per junction point is 1-5 times the molecular weight between cross-links (Fig. 3A. 1(b)). For a tetrafunctional junction point the ratio of the two values will be 2 0/1 0 (Fig. 3A.l(c)). 

FIG. 14-2. Dependence of diffusion coefficient of n-hexadecane on the degree of cross-linking in three polymers in the rubberiike state at 25°C. (PB) 1,4-Polybutadiene, cis trans vinyl = 40 53 7 (NR) natural rubber (pips denote four different initiai molecular weights before cross-linking, from 2.3 to 7.7 X 10 ) (SBR) styrene-butadiene random copolymer, 23.5% styrene. Abscissa is moles effective network strands per cubic centimeter estimated from swelling measurements. (Chen. )... 

A typical resin has an initial molecular weight of 150 to perhaps 1500. For systems of unsubstituted phenols, the final cross-link density is 150—300 atomic mass units (amu) per cross-link. In other words, 25—75% of the ring-joining reactions occur during the cure phase. 

The relation (13) should hold regardless of the primary molecular weight distribution, provided only that the cross-linking proceeds between units at random. The number of cross-linked units per primary molecule, which has been called the cross-linking indexequals pyn-For a homogeneous primary polymer — 2/, and the critical value... 

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