Dynamics of a Probe Molecule Dissolved in Several Polymer Matrices with Different Side-Chain Structures: Determination of Correlation Length Relevant to Glass Transition

Abstract

Dynamics of 4-pentyl-4′-cyanobiphenyl (5CB) dissolved in several polymers at concentrations of 3−7 wt % was examined by dielectric relaxation measurement. Glassy (segmental) mode of the matrix polymer was also investigated by viscoelastic measurements for the same samples. Polystyrene (PS), poly(4-methylstyrene) (P4MS), and poly(4-tertbutylstyrene) (PtBS) were used as host polymers, considering that they have the same backbone structure but different sidechain bulkiness. Two dielectric relaxation modes (slow and fast modes) of 5CB component appeared in all the mixtures, and the relative intensity of the fast mode increased in the order of PS < P4MS < PtBS, corresponding to the order of the side-chain bulkiness and main-chain stiffness. The effects of such chemical structure differences on the two relaxation modes, particularly their temperature dependence, were examined in detail. Comparison of relaxation times for the fast mode and the segmental dynamics of the matrix polymer suggests that the fast mode was attributed to the restricted orientational fluctuation, which includes precession motion around the long axis, of 5CB molecule in a confined space formed by slow polymer chains. The dielectric intensity of the fast mode increased with increasing side-chain bulkiness of polymers. This means that the larger side chain decreases the spatial restriction for the movement of guest 5CB molecules in the glassy state. The slow mode was attributed to the rotational motion of 5CB molecule nearly cooperative with the segmental motion of matrix polymer. The relaxation times of dielectric slow mode and viscoelastic glass mode were compared. Temperature dependence of the slow mode was slightly different from that of the segmental motion of polymer corresponding to the glass mode. From the difference between these relaxation times, the correlation length relevant to the glass mode was determined as functions of temperature for the three polymers.

Dynamic Heterogeneity and Cooperative Length Scale at Dynamic Glass Transition in Glass Forming Liquids

“…51,59 These segment sizes could be associated with Kuhn's segment length. 124 This assumption is in agreement with the work of Shogo et al, 119 showing that the cooperative motion in stiff backbone polymers may be correlated to Kuhn's segment length.…”

“…In fact, it can be found that for more rigid backbone polymers, cooperative motions involve some statistical segments of the macromolecule rather than the whole repeating units. , These segment sizes could be associated with Kuhn’s segment length . This assumption is in agreement with the work of Shogo et al, showing that the cooperative motion in stiff backbone polymers may be correlated to Kuhn’s segment length.…”

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Viscoelastic Relaxation of Polymerized Ionic Liquid and Lithium Salt Mixtures: Effect of Salt Concentration

“…According to the MD simulation [ 19 ], the increase of the LiTFSI concentration leads to the formation of some Li–TFSI coordinated species free from the polymer backbone, such as Li(TFSI) 3 , in addition to free TFSI anions. These species may work as plasticizers of the polymer component, i.e., behave as dynamically fast components, resulting in the dynamically heterogeneous behaviors, such as the broadening of the G * and tan δ spectral shapes and the appearance of prominent fast component dynamics in the high ω region [ 43 , 44 , 45 , 46 ]. Specifically, by looking at the high-frequency region of ω > 10 6 s −1 for the = 0.25 and 0.30 data, the slopes of tan δ (and G ″) are weaker than those at smaller , suggesting the existence of the fast relaxation component in the viscoelastic spectra.…”

Reduced stress‐optical coefficient of polycarbonate by antiplasticization

“…With addition of p ‐tPh, the α peaks in E ″ and O ″ shift to lower temperature but the peak strength in O ″ slightly increases. We previously reported that a small molecule exhibits a cooperative glass transition with a matrix polymer when the molecular size is consistent with the chain segment . Because p ‐tPh has large polarizability anisotropy originating from the three phenyl groups, the increase of the O ″ peak intensity suggests that p ‐tPh also contributes to the increase of the α peak with increasing p ‐tPh content.…”