Entanglement Dynamics in Miscible Polyisoprene/Poly(p-tert-butylstyrene) Blends

Abstract

Viscoelastic, dielectric, and rheo-optical behavior was examined for miscible blends of high-M cis-polyisoprene (PI) and poly(p-tert-butylstyrene) (PtBS). The slow dielectric relaxation of the blends was exclusively attributed to the global motion of the PI chains having the type-A dipoles. The PI and PtBS chains behaved as the fast and slow (low- and high-friction) components and were well entangled with each other. The dynamics of these chains changed significantly with temperature T. At high T, the blend exhibited two-step entanglement plateau of the storage modulus G′(ω), and the plateaus at high and low angular frequencies (ω) were attributed, with the aid of the dielectric data, to the entanglement among all component chains and that between the PtBS chains, respectively. The entanglement length a characterizing the high-ω plateau was well described by a simple mixing rule based on the number fraction n of the Kuhn segments of the components, a = n PI a PI bulk + n PtBS a PtBS bulk. This result was consistent with the current molecular picture relating the entanglement density to the packing length p (≅a/20). The complex moduli G* of the blends in the high-ω plateau zone were well described by a simple blending law combined with this mixing rule of a, which was consistent with the rheo-optical data. At low T, the blend exhibited the Rouse-like power-law behavior of storage and loss moduli, G′ = G′′∝ ω1/2, in the range of ω where the high-ω plateau was supposed to emerge. This lack of the high-ω plateau was attributed to retardation of the Rouse equilibration of the PI chain over the entanglement length a due to the hindrance from the slow PtBS chains: The PI and PtBS chains appeared to be equilibrated cooperatively/simultaneously at a rate essentially determined by PtBS. The Rouse equilibration time, evaluated from the G* data of the blend, was just moderately shorter than the dielectrically determined relaxation time of PI. Thus, the high-ω plateau zone was too narrow to be resolved experimentally, and the PI chains relaxed almost immediately after their Rouse equilibration (retarded by PtBS). This PI relaxation activated the constraint release (CR) relaxation of PtBS to dilate the entanglement mesh for PtBS. A simple model considering the Rouse equilibration and CR/dilation processes described the G* data of the blend surprisingly well, lending support to the molecular picture of the cooperative/simultaneous Rouse equilibration of the PI and PtBS chains. The model calculation was consistent with the rheo-optical data, which lent further support to this molecular picture.

Dynamics in miscible blends of polyisoprene and poly(p-tert-butyl styrene): thermo–rheological behavior of components

“…[31][32][33][34] The analysis showed that the PI relaxation was slower in the blends, by a factor of 2-3, than in the iso-t s bulk, possibly because of a topological constraint from the slow PtBS chains. 26 In addition, the entanglement length a in the blends was found to be well described by a simple mixing rule based on the number fraction n of the Kuhn segments of PI and PtBS: 27 a ¼ n PI a This rule, differing from mixing rules assumed in literature, 20,23 is consistent with the current molecular picture that relates the entanglement density to the packing length. [35][36][37] The previous study also examined the global dynamics of the PtBS chains entangled with the PI chains.…”

“…[18][19][20][21][22][23] Thus, blends of PI and poly(p-tert-butyl styrene) (PtBS) were chosen as model systems to examine the global dynamics of the components therein. [24][25][26][27] PI has the type-A dipole parallel along the chain backbone and its global motion activates slow dielectric relaxation, 28,29 whereas PtBS has no type-A dipole and is dielectrically inert in long-time scales. 27 This difference between PI and PtBS enabled us to dielectrically examine, without any ambiguity, the global dynamics of PI in the blends.…”

“…[24][25][26][27] PI has the type-A dipole parallel along the chain backbone and its global motion activates slow dielectric relaxation, 28,29 whereas PtBS has no type-A dipole and is dielectrically inert in long-time scales. 27 This difference between PI and PtBS enabled us to dielectrically examine, without any ambiguity, the global dynamics of PI in the blends. Furthermore, PI and PtBS have a negative interaction parameter and are miscible in a surprisingly wide range of T, 24,30 enabling a thorough test of the thermorheological behavior of the PI and PtBS chains in the miscible blends.…”

“…In addition, the subtraction, based on the entanglement concept, is valid only in a range of frequencies where the PI and PtBS chains have been cooperatively Rouse-equilibrated within the entanglement length, as revealed in recent work. 27 Thus, for completeness, the PtBS dynamics was examined for model PI/PtBS blends having G PtBS *cG PI * (having the PI relaxation time t PI much shorter than t blend of the blend) in the entire range of temperature (T). For these blends, small numerical uncertainties in G PI * hardly affected the G PtBS * values.…”

“…An oligomeric PI sample, PI3, was anionically synthesized at 30 1C in vacuo, with benzene and sec-butyllithium being used as the solvent and initiator, respectively. The other PI and PtBS samples were anionically synthesized in the previous studies, 24,27,38 except the commercially supplied PI20 sample (from Kuraray Co, Tokyo, Japan). 24 The PI3 sample was characterized with GPC (Co-8020 and DP-8020, Tosoh Co, Tokyo, Japan) having a refractive index monitor (LS-8000, Tosoh Co) and also with 1 H-NMR (MERCURYplus AS400, Varian, Palo Alto, CA, USA) for the chain end/microstructure analysis.…”