Molar mass effect upon

TABLE 3.5 Theoretical effects of low molar mass impurities upon the value of M as calculated using Equation (1.6)... 

Purity is of great importance when evaluating because low molar mass impurities can give rise to major errors in the measured value of A , as is shown by the data in Table 3.5. These errors are reduced in MO due to equilibration of the impurity on either side of the membrane and in VPO by evaporation of volatile impurities from the solution drop. Thus the effects of low molar mass impurities upon measurement of M can be negligible but should never be ignored. 

A review of the literature demonstrates some trends concerning the effect of the polymer backbone on the thermotropic behavior of side-chain liquid crystalline polymers. In comparison to low molar mass liquid crystals, the thermal stability of the mesophase increases upon polymerization (3,5,18). However, due to increasing viscosity as the degree of polymerization increases, structural rearrangements are slowed down. Perhaps this is why the isotropization temperature increases up to a critical value as the degree of polymerization increases (18). 

Fig. 15. Memory effect. Gradient elution of 500 pg phosphorylase kinase on a RP 18 column (250 x 4.6 mm dP = 5 pm). Mobile phase A 0.1% TFA in water B 0.08% TFA in acetonitrile gradient program 0 % B (0-8 min), 46 % (9 min) 68 % (24 min), 75% (33 min) flow rate 1 ml/min. The lower chromatogram was obtained upon sample injection the five blank gradients were performed immediately after the initial separation. Molar mass of the subunits a — 132,000 daltons P — 113,000 y - 43,000 5 - 16,680. (From Ref. 66> with permission)...

The molar mass (molar weight) is an example (the oldest additive molar quantity). This most simple system of additivity however, has a restricted value. Accurate comparison of molar properties of related compounds reveals that contributions from the same atoms may have different values according to the nature of their neighbour atoms. The extent to which this effect is observed depends upon the importance of outer valence electrons upon the property concerned. 

Effects of monomer-starved conditions upon molar mass and molar mass distribution... 

Thus, intermolecular chain transfer to polymer leads to premature termination of the growth of one propagating chain and the reactivation of a dead chain which then grows a long-chain branch. As a consequence, the molar mass distribution of the polymer broadens. The changes in skeletal structure and molar mass distribution inevitably have major effects upon bulk polymer properties. 

Since chain transfer to polymer does not change the numbo- of polymo molecules formed, it has no effect upon the number-average molar mass, M . Furthermore, although it results in the formation of branched polymer molecules, only in the case of intermolecular diain transfer to polymer is thrae an effect upon molar mass distribution (see Section 1.3.4) which broadens, leading to increases... 

Other strategies for controlling copolymer composition Although the use of monomer-starved conditions for control of copolymer composition is widespread, the low monomer feed rates which need to be used give rise to low rates of copolymerization and have significant effects upon the molar mass and molar mass distribution of the copolymers formed (see Section 7.4.4.4). Hence, alternative procedures have been developed which facilitate higher feed rates, but nevertheless allow for control of copolymer composition. These procedures are briefly described in this section. 

Table 15.5 shows the results of Mochel [22] for the effect of conversion upon the gel content of polychloroprene rubbers prepared by emulsion polymerization at 40 °C. In section (a) of this table are shown results for polymers produced in the absence of added sulfur section (b) shows results for polymers produced with the addition of 0.6 parts of sulfur per 100 parts by mass of chloroprene, before chemical peptization of the polymer. In both types of reaction system, polymer gel begins to form quite early in the reaction. However, these results indicate that sulfur has a slight tendency to act as a modifier during the polymerization, in that the onset of gel-formation is delayed when sulfur is present. Also delayed is the pdnt at which the polymer is virtually entirely gel. Mochel et al. [23] have reported results for the molar mass distribution of a thiuiam-modified polychloroprene rubber produced by emulsion polymerization at 40 °C,... 

The tendency for a polychloroprene rubber to crystallize increases as the polymerization temperature is reduced. This tendency is illustrated by results of Maynard and Mochel [26], given Figure 15.13, for the effect of polymerization temperature upon the melting temperature of the pol3mien as the polymetizadon temperature is reduced, so the temperature at which crystallization is able to occur increases progressively. This is believed to be primarily a consequence of die effect of temperature upon the stereoregularity of the polymer. The results shown in Table 15.6 indicate that it is probably the enhanc concentration of ds-1,4 units in polychloroprenes produced at higher temperatures which is responsible for the retardation of crystallization, rather than the presence of the 1,2 and 3,4 units. It appears that, for chloroprene homopolymers, other variables, such as molar mass and crosslink density, have little effect upon tendency to crystallize. 

The fracture resistance of a thermoplastic depends critical upon its relative molar mass, essential because Gjc and fall rapidfy when the chains are too short to form effective entanglements. This point is illustrated in Figure 5.19. Very short chains are held tt ther oi by van der Waals forces, and are unable to form stable crazes. 

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