Prepolymer molecular weight

Molecular Weight Distribution. The determination of prepolymer molecular weight distribution requires fractionation followed by the determination of the molecular weights of the individual fractions. The most satisfactory molecular weight fractionations have been achieved through either solvent precipitation or gel permeation chromatography. 

Table 2.8 Characteristics of commercial polyurethane prepolymers. Molecular weight information is withheld by the manufacturers... 

Random Prepolymers. These prepolymers are built up from polyfunctional monomers reacting statistically according to the theories of Flory. Reaction is stopped at a desired prepolymer molecular weight, usually by cooling. Final polymerization is achieved by heating therefore the term thermoset is used for them. 

Epoxy Prepolymers. These prepolymers are commonly formed from Bisphenol A and epichlorhydrin (Figure 4). A variety of other materials are also employed, but to a lesser extent. The lowest member of the series might well be considered structoterminal, while all the others must be considered as structopendant prepolymers. Molecular weights range from a few hundred to about 4000 for commonly used industrial epoxy prepolymers. Most common cross-linking agents are amines and anhydrides however, epoxies may also be combined with a variety of other prepolymer systems (25). 

Parallel study with changed by changing prepolymer molecular weight (33). 

In conclusion, the temperature dependence of shift factors for the networks studied here do not follow the WLF equation, but rather an Arrhenius-type relationship. The apparent activation energies are independent of stoichiometric variation [as they are when is varied by changing prepolymer molecular weight (13)]. ... 

Figure 7. Dependence of the critical gel concentration (Cgep upon the prepolymer molecular weight in a,to -Mg dicarboxylato polymers (toluene, 25 0 ( ) PMS, ( ) PTBS, (x) PBD, (A)PIP and ( ) PS.

Anderson and Culbertson have pointed out that ortho-ortho resoles are very useful as laminating and impregnating resins, because they have a good prepolymer molecular weight (PMW) efficiency, i.e., a large ratio of PMW to starting phenol content. 

The first report of ADMET polymerization in the solid state [171] was in 2003 by Wagener and coworkers, who polymerized a variety of dienes with both [Ru] 1 and [Ru]2. Their data proved that solid-state polymerization did occur, even at relatively low temperatures (30 °C), and produced polymers with number average molecular weight as high as 3.1 X lO gmol , a 20-fold increase over the prepolymer molecular weight. A series of rigid-rod polymers were also produced [172]. 

Effect of Prepolymer Molecular Weight on Ultimate Properties. ... 

The influence of the prepolymer molecular weight has already been indicated and must have an obvious effect on the fracture energy. Presence of a solvent will aggravate the formation of crosslink density inhomogeneities. This explains the poor ultimate mechanical properties of networks crosslinked in the presence of relatively small amounts of diluents. A broad prepolymer molecular weight distribution will have a similar effect i.e. considerable... 

It has been shown that various factors such as prepolymer molecular weight, functionality of prepolymer and of crosslinking agent, chemical conversion of reactive groups, and stoichiometry of reactive groups influence the ultimate properties of the networks. It can be predicted, however, that these factors also affect the morphology and the coherence of the network structure. 

Table 5. Thermomechanical properties of TPUs with trifuctional polyols. Mj, is the prepolymer molecular weight...

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