Yumi Matsumiya

Primitive chain network simulations for asymmetric star polymers

For branched polymers, the curvilinear motion of the branch point along the backbone is a significant relaxation source but details of this motion have not been well understood. This study conducts multi-chain sliplink simulations to examine effects of the spatial fluctuation and curvilinear hopping of the branch point on the viscoelastic relaxation. The simulation is based on the primitive chain network model that allows the spatial fluctuations of sliplink and branch point and the chain sliding along the backbone according to the subchain tension, chemical potential gradients, drag force against medium, and random force. The sliplinks are created and∕or disrupted through the motion of chain ends. The curvilinear hopping of the branch point along the backbone is allowed to occur when all sliplinks on a branched arm are lost. The simulations considering the fluctuation and the hopping of the branch point described well the viscoelastic data for symmetric and asymmetric star polymers with a parameter set common to the linear polymer. On the other hand, the simulations without the branch point motion predicted unreasonably slow relaxation for asymmetric star polymers. For asymmetric star polymers, further tests with and without the branch point hopping revealed that the hopping is much less important compared to the branch point fluctuation when the lengths of the short and long backbone arms are not very different and the waiting time for the branch point hopping (time for removal of all sliplinks on the short arm) is larger than the backbone relaxation time. Although this waiting time changes with the hopping condition, the above results suggest a significance of the branch point fluctuation in the actual relaxation of branch polymers.

Rheo-dielectrics in oligomeric and polymeric fluids: a review of recent findings

This paper gives a brief review of recent results of rheo-dielectric studies for oligo-styrene (OS) and cis-polyisoprene (PI). OS has type-B dipoles perpendicular to the backbone, and its dielectric α relaxation (glassy mode relaxation) is accelerated under the shear flow even at rates much smaller than the equilibrium relaxation frequency. This acceleration, resulting in the decrease of the viscosity (thinning) observed at those rates, is related to flow-induced reduction of the cooperativity of the segmental motion. For PI chains, having type-A dipoles parallel to the backbone, the dielectric relaxation detects the global chain motion. For well entangled PI chains, this relaxation is only moderately accelerated and its intensity is only mildly reduced even under fast flow in the non-Newtonian thinning regime. This result is related to a flow-induced orientational cross-correlation of entanglement segments. Within the context of the tube model for entangled chains, this cross-correlation can be related to the dynamic tube dilation induced by the convective constraint release.

2.28 – Rheological Characterization of Polymeric Liquids

Rheological properties of polymeric materials, which determine mechanical responses under externally applied deformation and/or force, reflect the static/dynamic behavior of polymer molecules and higher order structures in the materials. It is desired to determine the molecular parameters of polymers (such as the molecular weight and degree of branching) and the parameters of the higher order structures (such as the shape and interfacial tension of the droplet phases in blends) from rheological data. This rheological characterization of the polymeric materials can be achieved accurately if the molecular/structural origin of the mechanical stress is fully understood. This chapter summarizes the molecular/structural aspect of the rheological behavior of polymeric liquids and describes the method(s) for rheological characterization of the flexible polymer chains and higher order structures therein.

Entanglement relaxation of branched polymers: Constraint release and dynamic tube dilation

分岐鎖長および分岐点間分子量に分布のないpom-pom型および櫛型ポリブタジエン (PB) 試料について, 枝どうしは十分絡み合っているが幹どうしは絡み合わない濃度の溶液の粘弾性を検討した. その結果観察されたRouse型の終端緩和は, 枝運動で誘起される幹の束縛解放緩和によるものと推論された. 拡張管模型の枠内で, 束縛解放の結果として起こると考えられている管膨張過程が上記のpom-pomおよび櫛型PB試料の幹緩和を支配していないことを確認するため, より基本的な直鎖および星型鎖に対して管膨張過程の検証を行った. この目的のため, A型双極子を有する絡み合い鎖の規格化粘弾性緩和関数と誘電緩和関数に対して管が膨張する場合に成立する一般的関係を導出した. 単分散直鎖についてはこの関係がかなりよく成立し, 管膨張過程が実証された. 一方, 単分散星型鎖と希薄高分子量プローブについては, この関係が成立せず, 単純な管膨張は起こっていないことが見いだされた. この結果は, 隣接絡み合いセグメントが平衡化して管が膨張する際の素過程となる束縛解放運動の重要性を示す. また, この結果は, 上記のpom-pom型および櫛型PBの緩和に対する推察を支持する.