Rheological behavior of star-shaped polymers

Abstract

The rheological properties of star-shaped polyisoprenes having a wide range of arm numbers and arm molecular weights are reported. In contrast to linear polymers, stars have a broad relaxation spectrum and a viscosity that increases exponentially with arm molecular weight. A comparison of eight pairs of samples having 3 and 4 arms and identical arm molecular weights showed that the viscosity of 3-arm stars is approximately 20% lower. For higher degrees of functionality, 4 < f < 33, the effect of functionality saturates and the viscosity is determined by arm molecular weight only. The nonlinear properties of one sample were studied using step-strain tests and found to be essentially identical to those of linear polymers. The predictions of a molecular theory for star polymers based on an extension of the reptation model are reviewed and shown to be in good agreement with the experimental data.

Structure and Dynamics of Irregular Multiarm Star Polymers

“…These rearrangements are of the same character as these suggested for the cooperative rearrangements in low molecular liquids [21]. Note that because of the presence of the slow relaxation, the zero shear viscosity η 0 of multiarm stars is not independent of the functionality f , as in the case of low-f stars [22].…”

Polymeric Drift Control Adjuvants for Agricultural Spraying

“…[143] Increasing the span molecular weight increased the difference between branched and linear polymers, in line with theory which states that the viscosity of branched structures increases exponentially with molecular weight, while linear polymers follow a power law (Equation 3). [144] These results contrast with earlier work which suggested that the viscosity of branched polymers is always less than that of a linear polymer of equal total molecular weight. An explanation may be found in the large arm molecular weight of the polymers tested in the above study, which ranged from 3 x 10 3 to 5 x 10 4 gmol -1 .…”

Rheology of Star-Branched Polyisobutylene

“…The viscosity of polymers having sufficiently long branches will have a stronger dependence on molecular weight than the power law found for linear chains (eqs 2 and 3). An exponential relationship has been proposed [15][16][17] 37 vertically scaled to superpose on the data for L176.…”

“…The general consensus is that stress relaxation and diffusion entail arm retraction, which causes an exponential increase with branch length of both the zero-shear viscosity and the diffusion constant. [10][11][12][13][14][15][16][17][18] This arm retraction is believed 19 to be the cause of two other phenomena associated with branched polymers: more temperature-dependent rheological properties and thermorheological complexity in the terminal zone. [20][21][22][23][24][25][26][27] The transient, compact structure would alter the distribution of rotational states, in turn giving rise to a thermal barrier to terminal relaxation, which is absent for linear polymers.…”