Changes in the Viscoelastic Mechanisms of Polyisobutylene by Plasticization

Our dynamic mechanical measurements in the glass-rubber transition zone of polyisobutylene (PIB), expressed in terms of the relaxation spectrum H(tau) and loss tangent (tan delta), have found an additional shoulder. Following the interpretation of previous works, the shoulder is attributed to the sub-Rouse modes, which account for motions intermediate in length scale to the Rouse and local segmental modes. The fact that the sub-Rouse modes are well resolved in PIB and not in other amorphous polymers was traced to its weak intermolecular coupling, which ultimately originates from the compact and symmetrical structure of its repeat unit. We test this interpretation further by studying the change on adding liquid paraffin (LP) to PIB, which should disrupt the effective chain packing of undiluted PIB. We found on adding LP to PIE that the softening dispersion becomes narrower, and eventually the disappearance of the shoulder. The effect is due to the mobility of entropic Rouse modes enhanced significantly more than that of the sub-Rouse modes and the enthalpic local segmental motion on increasing the weight fraction of LP. Thus, the Rouse modes get closer to the sub-Rouse modes and local segmental motion. Incorporation of LP does not reduce much the intermolecular coupling, as evidenced by the small change of glass transition temperature. On the other hand, the disruption of the effective chain packing of PIB by adding LP endows higher mobility to the Rouse modes with longer motional units, while it has little effect on the local segmental relaxation and the sub-Rouse modes. Consequently, there is the narrowing of the softening dispersion and eventually the disappearance of the shoulder in H(tau) and tan delta. The result supports the compact and symmetric chemical structure of the repeat unit of PIB, and the efficient chain packing is the root cause that engenders the broad softening dispersion, and the observation of the sub-Rouse modes in H(tau) and tan delta. Positron annihilation lifetime spectra are presented to show the average size and concentration of the free-volume holes increase with incorporation of LP and the disruption of the effective chain packing of PIB.