Primary molecular weight

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... 

Morton and Salatiello have deduced the ratio kpp/kp for radical polymerization of butadiene by applying the above described procedure, appropriately modified for the emulsion system they used. The primary molecular weight was controlled by a mercaptan acting as chain transfer agent, as in the experiments of Bardwell and Winkler cited above. Measurement of the mercaptan concentration over the course of the reaction provided the necessary information for calculating % at any stage of the process, and in particular at the critical conversion 6c for the initial appearance of gel. The velocity constant ratios which they obtained from their results through the use of Eq. 

A further point which remains to be examined more carefully is the dependence of the tension on the proportion of flaws, or ends of primary molecules, in the network. Bardwell and Winkler s results cover a wide range in primary molecular weight as well as in degree of cross-... 

Fig. 103.—Tension r at a =4 vs. the reciprocal of the primary molecular weight for three butyl rubber series differing in degree of cross-linking (or in Me). (Flory. )...

Fig. 105.—Tensile strengths of butyl rubber fractions each vulcanized to the same cross-linking density p, corresponding to Me = 37,000, plotted against the primary molecular weight M. ...

The active strand concentration v is obtained from measurements of the initial modulus using Eq.(7.2) with g = 1. Values of v for samples at the same crosslink density but with different primary molecular weights are extrapolated to 1/M=0, giving vc + ve. Values of vc + ve obtained at two or more crosslink densities provide ve, and hence Me, by extrapolation. An advantage of this method is that only relative values of crosslink density must be known absolute values of vc are not required. If absolute values of vc are known, the g factor can be evaluated as well. 

Vander Hoff and Buchler (277) found Me= 13000 for natural rubber by applying the Bueche-Mullins analysis to the data of Bristow (288). Examination of the natural rubber results published some years ago by Flory et al. (289) yields Me% 15000. The data of Mullins (290) provides a value of 10000 for 1,4 polyisoprene if the total initial modulus is used in the analysis (Mullins reported Me = 16000 by examining the effects of primary molecular weight and extent of cure on C, alone the initial modulus G is C1 + C2 and C2 0.6 Cx in the samples examined). 

If the primary molecular weight distributions changes in the course of reaction qP, has to be replaced by (qP) (3). 

I. It is seen that the aggregated molecular weights can be as high as several hundred times the primary molecular weight. 

The authors collected the test data shown in Table 9.10. From these data it can be noted that the number-average molecular weight, or Af , of a commercial emulsion SBR such as IISRP 1500 or 1712 is typically 90,000-175,000. The primary molecular weight of a solution-polymerized polymer produced with an anionic lithium catalyst can, in contrast, be increased toward 250,000 without gelation. In addition, emulsion-polymerized SBR contains only about 92% rubber hydrocarbon as a result of the presence of residues from the production process solution polymers tend to be near 100% hydrocarbon. As a consequence, the authors concluded that the number-average molecular weight... 

Here, the shear modulus of the swollen hair G = (pRT/Mc) (v2 — j)/ (1 < ) 1/3(i 2M,/M) y n is the number of segments of the network chain L x) is the anti-Langevin fimction p is the dry density of fiie sample is the number average molecular weight between crosslinks of the rubbery phase M is the primary molecular weight R is the gas constant T is absolute temperature V2 is the volume fiaction of the polymer within the gel and y is the filler effect of the HS domain that exists in the rubbery phase. Equation (3) provides y. 

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