Properties of amorphous and crystallizable hydrocarbon polymers. IV. Melt rheology of linear and star‐branched hydrogenated polybutadiene

Abstract

SynopsisSome results on the melt rheology of hydrogenated polybutadiene (HPB) with narrow-molecular-weight distribution are reported and compared with the corresponding properties of the precursor polybutadienes (PBD) and fractions of linear polyethylene (PE). In linear samples the dynamic moduli obeyed frequency-temperature superposition. The relationship between melt viscosity and intrinsic viscosity at 190°C for HPB was indistinguishable from that for PE, but their flow activation energies were slightly different (E, = 7.2 kcal for HPB and 6.4 kcal for PE). Like PE, but unlike the PBD precursors, the dynamic storage modulus at low frequencies was anomalous. Otherwise, the dynamic moduli of HPB and its PBD precursor were essentially superposable. Plateau moduli from different samples were somewhat variable around an average of GR = 2.31 X lo7 dyn/cm2. The dynamic moduli for the HPB stars, unlike their PBD precursors, did not obey temperature-frequency superposition. At high frequencies the temperature coefficient approached that for linear HPB, but it increased with decreasing frequency, reaching limiting values which depended on the arm length. The flow activation energy ranged from 9 kcal to more than 15 kcal as arm length increased.

Modes of Molecular Motion in Low Molecular Weight Polystyrene

“…23 The same is true for (the structurally similar) hydrogenated polybutadiene. 24,25 These molecules, of course, are larger in size than PS2K, and so the putative averaging of spatial heterogeneities still occurs. However, the decoupling of η and D in high-M w polymers is observed at high temperatures, T > T g , where dynamic heterogeneity is not expected to be an issue.…”

Star Polymers and the Failure of Time−Temperature Superposition

“…We assume the thermorheological simplicity of the data at a high frequency (which has been successfully tested before 4 ) and use time-temperature superposition to collapse the modulus versus frequency data onto a single curve at the high-frequency ends where G attains the plateau modulus. We find that an…”

“…(In particular, this material appears to remain thermorheologically simple for frequencies near the rubber plateau with an activation energy equal to its value in a linear HPB melt. 4,5 ) Star melts of HPB present other problems to the theorist. While the present theory of stress relaxation in star polymer melts [7][8][9][10][11] has met with considerable success in elucidating both the exponential dependence of the melt viscosity upon arm molecular weight and the particular shape of star relaxational spectra, its application to melts of hydrogenated polybutadiene (HPB) reveals significant discrepancies between theory and experiment.…”

Rheology of Star-Branched Polyisobutylene

“…[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. If the gauche rotamers, which predominate in a more compact configuration, are higher in energy than the trans conformers, the model of Graessley 19,20 predicts the existence of an energy barrier, E A , whose magnitude is proportional to the molecular weight, M a , of the branch where M e is the entanglement molecular weight.…”