Polymer segmental motion

A crystalline or semicrystalline state in polymers can be induced by thermal changes from a melt or from a glass, by strain, by organic vapors, or by Hquid solvents (40). Polymer crystallization can also be induced by compressed (or supercritical) gases, such as CO2 (41). The plasticization of a polymer by CO2 can increase the polymer segmental motions so that crystallization is kinetically possible. Because the amount of gas (or fluid) sorbed into the polymer is a dkect function of the pressure, the rate and extent of crystallization may be controUed by controlling the supercritical fluid pressure. As a result of this abiHty to induce crystallization, a history effect may be introduced into polymers. This can be an important consideration for polymer processing and gas permeation membranes. 

The importance of polymer segmental motion in ion transport has already been referred to. Although classical Arrhenius... 

The structures and charge transport mechanisms for polymer electrolytes differ greatly from those of inorganic solid electrolytes, therefore the purpose of this chapter is to describe the general nature of polymer electrolytes. We shall see that most of the research on new polymer electrolytes has been guided by the principle that ion transport is strongly dependent on local motion of the polymer (segmental motion) in the vicinity of the ion. 

Many polymer-salt complexes based on PEO can be obtained as crystalline or amorphous phases depending on the composition, temperature and method of preparation. The crystalline polymer-salt complexes invariably exhibit inferior conductivity to the amorphous complexes above their glass transition temperatures, where segments of the polymer are in rapid motion. This indicates the importance of polymer segmental motion in ion transport. The high conductivity of the amorphous phase is vividly seen in the temperature-dependent conductivity of poly(ethylene oxide) complexes of metal salts. Fig. 5.3, for which a metastable amorphous phase can be prepared and compared with the corresponding crystalline material (Stainer, Hardy, Whitmore and Shriver, 1984). For systems where the amorphous and crystalline polymer-salt coexist, NMR also indicates that ion transport occurs predominantly in the amorphous phase. An early observation by Armand and later confirmed by others was that the... 

The indication that polymer segmental motion is necessary for ion transport has focused most of the current research and development on amorphous materials with low glass transition temperatures. 

At room temperature, these molecules occupy well-defined locations in their respective crystal lattices. However, they tumble freely and isotropically (equally in all directions) in place at their lattice positions. As a result, their solid phase NMR spectra show features highly reminiscent of liquids. We will see an illustration of this point shortly. Other molecules may reorient anisotropically (as in solid benzene). Polymer segmental motions in the melt may cause rapid reorientation about the chain axis but only relatively slow reorientation of the chain axes themselves. Large molecular aggregates in solution (such as surfactant micelles or protein complexes or nucleic acids) may appear to have solidlike spectra if their tumbling rates are sufficiently slow. There are numerous other instances in which our macroscopic motions of solid and liquid may be at odds with the molecular dynamics. Nuclear magnetic resonance is one of the foremost ways of investigating these situations. 

A third type of ionic conduction occurs in polymer electrolytes, such as polyethylene oxide. The mechanism of ionic conduction in polymer electrolytes is not entirely understood. However, it is believed to involve rapid polymer segmental motion which creates regions of an elastomeric nature. These elastomeric regions have relaxation times similar to liquids, and, thus, allow a higher ionic mobility than would be concluded from the polymer s macroscopic properties [2]. 

A high ambient temperature conductivity of 10 Scm was measured with LiTFSI dissolved in the P2S5-PEO matrix, i.e, in salt-in-polymer materials. The decoupling of Li+ motions from polymer segmental motions was demonstrated, suggesting that, in certain cases, the transport number for Li+ ions in the phosphorus sulfide-PEO matrix will be much higher than in a typical PEO-based salt-in-polymer system. 

Effect of aging on the permeability and molecular motion of the membranes of PMSP, poly(TMSP-co-PP) and blend of PMSP/PPP Glassy polymers, such as PMSP, are nonequilibiium materials and their permeation and sorption properties drift over time as thermally driven, small-scale polymer segmental motions cause a relaxation of nonequilibrium excess free volume. The microcavities of large size which are present in PMSP membrane have been considered to be responsible for the decay of C h and the gas permeability (4). Therefore, it is possible to stabilize the gas permeability by control the C by copolymerization or blending with the other acetylene derivatives such as PP and PPP, respectively. 

The characterization of the segmental motions of polymers is difficult, especially in concentrated solutions. One technique that is well suited to study polymer segmental motions is NMR relaxation measurement.(i, Studies have mostly focussed on the carbon or proton relaxation behavior at lower concentrations. Dipolar interactions among protons, or protons and tie the relaxation phenomena to local motions of polymer segments. Proton and techniques have been of limited use in more concentrated solutions, which to some extent is the most important regime for the development of many polymer properties. In more concentrated solutions, the overlap of spectral features and/or the complexity of the interactions make extracting motional information difficult, even if the relaxation measurements can be made. 

Elmahdy, M. M., Chrissopoulou, K., Afratis, A., Floudas, G., and Anastasiadis, S. H. 2006. Effect of confinement on polymer segmental motion and ion mobility in PEO/layered silicate nanocomposites. Macromolecules 39 5170-5173. 

Experimental results indicate that the ionic transport property for the PVA-based SPE is highly dependent on both the alkali salt and the solution concentration. All the values of anionic transport numbers for different PVA-based SPEs with KOH solution are much higher than those with alkaline NaOH and LiOH solutions. This trend is consistent with literature results [58] that ion movement in polymer is related to polymer segmental motion, and the order of ionic conductivity has been K >Na >Lr. 

PTMSP has a glass transition temperature of more than 250 C (2). In aU glassy polymers, small-scale polymer segmental motions lead to relaxation of non-equhibrium excess free volume and, as a result, the physical properties of glassy polymers drift over time. Because PTMSP has more free volume than other glassy polymers, a dramatic decline in gas permeability occurs when the non-equilibrium excess volume in PTMSP relaxes (3,4). Membrane contamination via absorption of organics (such as pump oil vapor) also decreases gas permeability of PTMSP membranes (4). In the absence of such contaminants, the decrease in gas permeability... 

The importance of polymer segmental motion in ion transport has already been referred to. Although classical Arrhenius theory remains the best approach for describing ion motion in solid electrolytes, in polymer electrolytes the typical curvature of the log a vs. 1/T plot is usually described in terms of Tg-based laws such as the Vogel-Tamman-Fulcher (VTF) [61] and Williams-Landel-Ferry (WLF) [62] equations. These approaches and other more sophisticated descriptions of ion motion in a polymer matrix have been extensively reviewed [6, 8, 63]. 

Theoretically, the free-volume model for ionic conduction suggests that the parameter B, unlike Tq, should have different values from those used for polymer segmental motion B is expected to reflect the relative sizes of the diffusant and the chain segments and should also vary with the type of ion. The VTF equation should therefore only apply to a partial conductivity due to a single carrier, and it is surprising that the total conductivity normally measured should exhibit a VTF form of temperature dependence. 

FIGURE 1.6 Cation motion in a PEO-Li-salt PEM. (a) Cation moves from one ionic cluster to another ionic cluster via positive-negative charge interaction, (b) Intrachain polymer segmental motion promotes cation mobility. The O represents the ether oxygen atom and the curve line represents the CHj-CHj in PEO. (Modified with permission from Gary, F.M., Polymer Electrolytes, The Royal Society of Chemistry, Cambridge, U.K., 1997.)... 

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