Branching Polydispersity effects

Table XII shows how polybutadiene microstructure and macrostructure, i.e., molecular weight, Mw, and Mn, polydispersity, and branching can effect the processability of a polymer [14]. A study with both cobalt- and neodenium-catalyzed polybutadiene showed the relationship between polydispersity or molecular weight distribution and increases in stress relaxation. Increases in stress relaxation, as measured by the Mooney viscometer, will infer greater...

In this experiment, the LLDPEs were simulated by hydrogenated (or deuterated) polybutadienes, because they can be prepared as monodisperse molecules (with a ratio of weight- and number-averaged molecular weights, Mw/M < 1.1) and a homogenous branch distribution within the chains. Such studies are therefore unaffected by polydispersity effects, either in the branch content or molecular... 

A further area for which CSF turns out to be helpful lies in the field of synthetic polymers that cannot be produced with low polydispersities. Among these, non linear homopolymers or block copolymers play an interesting roll. In order to investigate the influences of the molecular architecture on certain properties, it is compulsory to fractionate the polymers to exclude polydispersity effects. Examples where the CSF has been applied to solve such problems are branched polyiso-prene ° and butadiene-styrene block copolymers of different molecular architectures. In both cases, the branched material still contains non-negligible amounts of the linear precursors. Samadi and co-workers ° showed that small remainders of the linear polymer can have significant... 

The molecular weight distribution obtained from SEC analysis was also shown in Fig. 8. In order to check the effect of the estimated exponent a(-0.55) on molecular weight distribution for Ei branched PVAc, we used another a(-0.58) value to compute a new calibration curve as shown in Fig. 9. The two calibration curves almost overlapped with each other. The results are listed in Table 2. In both cases, we obtained the same weight-average molecular weight and the polydispersity index (M /M ). Thus, we could confirm that in using a two-point (Bq and %l) estimate for a, we have not introduced an appreciable error in the determination of molecular weight distribution of branched PVAc. 

Bueche s results apply to monodisperse polymers. Ajroldi and co-workers (62) have calculated the melt viscosity of polydisperse polymers such as would be produced by random trifunctional and tetrafunctional branching, as a function of Mw, assuming Eq. (5.1) to hold and making specific assumptions about the additivity of melt viscosity in polydisperse polymers. Because of these assumptions their results must be regarded as illustrative only but they show that large effects may be expected, the calculated melt viscosity of the branched polymer being lowered by more than two orders of magnitude in some cases. 

The facts just mentioned indicate that the influence of polydispersity is by far the most important effect when compared with those discussed in the previous sections (branching, excluded volume, theoretical refinements). 

This work examines the effect of long-chain branching on the low-shear concentrated solution viscosity of polybutadienes over a broad range of molecular weights and polydispersity. It will show that the reduction in molecular coil dimension arising from long-chain branching is more sensitively measured in concentrated than in dilute solutions for the polymers examined. 

The foregoing discussion assumed that using a good solvent in place of a solvent has no significant effect on the ratio of viscosities. With some exception (32, 33), the evidence in the case of g shows the validity to be unimpaired for polydisperse as well as monodisperse systems (29, 34-36). It is expected that polymer expansion in good solvents would be in the same direction for branched and linear polymers in dilute and concentrated media so that any errors would be compensating. 

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