Bulk Properties of Model Branched Polymers

Wyman and co-workers (120) studied the viscosity of narrow MWD linear and 4-armed star polystyrenes as a function of shear rate. At low shear rates, the star polymers (of MW 204000 and 430000) had low-shear viscosities much lower than linear ones of the same MW it may be remarked that the branch length 

Pannell (38) has studied a range of polystyrenes with comb-like branching, but with relatively long branches. He has correlated the low-shear melt viscosities with calculated values of Sq , finding i/o°c(so)4 8, whereas the exponent for linear polymers is about 3.4. Fujimoto s results can be correlated in a similar way, but with a rather higher exponent, 5.1, though rather better correlations would be obtained if separate lines were used for each branching frequency. 

The greater melt viscosities observed for some branched polymers, as compared with linear ones of the same MW, are not accounted for by current theories, as indicated in Section 5. The greater values of the steady state compliance mentioned above is also unexpected theory (128) would suggest a difference in the opposite sense. 

This covers methods that depend upon the fact that branching introduces groupings with different chemical structure from that of the repeat units of linear chain, namely branch-points and end-groups. These can sometimes be detected and estimated by physical or chemical methods. However, short branches as well as long ones introduce these groups, and it may not be justifiable to attribute them, or all of them, to long branches. Methyl groups in polyethylene are a case in point. 

These methods give a number-average estimate of the end-groups or branchpoints, and correspondingly the number-average DP must be known before the 

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