Flexibility of the Polymer Backbone

The problem of backbone flexibility has not yet been addressed for polysoaps. The vast majority of polymers reported have very flexible backbones. Clearly there must be gradual differences in the flexibility of the polysoaps prepared, depending on the main chain spacer and the reactive moieties employed. E.g., polysoaps based on polystyrene [51, 83, 104, 130, 238, 277, 278, 288, 292, 319-323] or poly(/V,Af-dialkyl,iV,jV-diallyl-ammonium salt)s [161, 168, 199, 200] should differ from polyvinyl-esters [224, 225, 232], polybutadienes [168, 313] or aliphatic polysulfones [164, 168], 

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None of the models address the question of how the main chains are packed, and details of crystallinity are neither factored into nor predicted by mathematical models of the structure and properties of Nafion. Chains packed in crystalline arrays are usually considered to be rigid within the context of certain properties for example, with regard to diffusion, crystallites are viewed as impenetrable obstacles. F NMR studies indicate otherwise. Molecular motions that do not significantly alter symmetry can in fact occur in polymer crystals. It would seem, for example, that the response of the Nafion structure to applied stress would depend on the flexibility of the polymer backbone, a certain fraction of which is incorporated in crystalline regions. On the other hand. Starkweather showed that the crystallinity and swelling of Nafion are not correlated. 

The loss in effectiveness in 0.1M Na2S04 is particularly drastic. The high salt concentrations must surely influence the solvation characteristics of the polyelectrolyte and change the thermodynamic quality of the solvent. The effects on the solution structure and the flexibility of the polymer backbone must also be discussed in this connection, as, for example, suggested by Brostow (1984). 

One of the methods for estimating experimental thermodynamic flexibility of the polymer backbone is determination of parameters, associated with the size of isolated... 

The study by Percec, Tomazos and Willingham (15) looked at the influence of polymer backbone flexibility on the phase transition temperatures of side chain liquid crystalline polymethacrylate, polyacrylate, polymethylsiloxane and polyphosphazene containing a stilbene side chain. Upon cooling from the isotropic state, golymer IV displays a monotropic nematic mesophase between 106 and 64 C. In this study, the polymers with the more rigid backbones displayed enantiotropic liquid crystalline behavior, whereas the polymers with the flexible backbones, including the siloxane and the polyphosphazene, displayed monotropic nematic mesophases. The examples in this study demonstrated how kinetically controlled side chain crystallization influences the thermodynamically controlled mesomorphic phase through the flexibility of the polymer backbone. 

Pn rather than by the HLB and the size of the surfactant fragments. E.g. above a critical value of P , spherical aggregates could no longer be realized [126] (cf. Fig. 8). Compared to the local micelle , the molecular micelle requires less flexibility of the polymer backbones, and thus the hydrophobic tails should be better shielded from contacts with water molecules. Therefore the HLB of polysoaps should be closer to that of analogous low molecular weight surfactants. 

We have discussed the influence of the flexibility of the polymer backbone on the mesophase formation by examples of polyacrylates and polymethacrylates. More flexible polymers should have a stronger tendency to form more stable mesophases. Nevertheless, smectic liquid crystalline phases are the most common mesophases formed, if they do indeed form, by polymers in which no flexible spacers are used to connect main chain and... 

Comparison of the phase diagrams plotted in Fig. 14 of poly(5- [ -[4 -4"-cyano-phenyl)phenoxy]alkyl]carbonyl]bicy-clo[2.2.1]hept-2-ene]s [189] and poly(n-[(4 -(4"-cyanophenyl)phenoxy)alkyl]vinyl ethers [122-127, 212, 213] which contain a single mesogen per repeat unit demonstrates that the glass transition temperature decreases as the flexibility of the polymer backbone increases from polynorbornene to poly(vinyl ether), whereas the isotropiza-tion temperature increases. In addition to revealing additional mesophases at lower temperatures, this increase in polymer flexibility enables the poly(vinyl ether)s to form more ordered mesophases. That is, poly(5- [ -[4 -(4"-cyanophenyl)phenoxy]al-kyl]carbonyl ]bicyclo[2.2.1 ]-hept-2-ene ]... 

The data in Table 10 demonstrates that for a constant spacer length and mesophase, both the change in enthalpy and entropy of isotropization decrease as the flexibility of the polymer backbone increases from poly-norbornene to poly (vinyl ether). However, the change in entropy decreases more rapidly than the change in enthalpy, and the isotropization temperature (T = AH /AS ) therefore increases with increasing flexibility. Since lower entropies of fusion are associated with more rigid structures, the lower entropy of isotropization of poly(vinyl ether)s is obviously not due to a lack of inherent flexibility of its polymer backbone, but rather to the more flexible backbone being more ordered and therefore more... 

Increasing the flexibility of the polymer backbone enhances the decoupling of the motions of the side and main chains and therefore tends to give rise to a higher thermal stability of the mesophases including the SmC phase. 

Due to the fact that, in SPEEK, the separation into hydrophilic and hydrophobic domains is less pronounced (Rikukawa and Sanui 2000), the hydrophobic/ hydrophilic domain difference is smaller, the backbone is less hydrophobic, and the sulfonic acid functional group is less acidic. Therefore, SPEEK is less polar, its conductivity is lower, and the flexibility of the polymer backbone is less. In order to improve the proton conductivity, the DS needs to be higher. However, this will also increase the methanol permeability and other problems of high swelling and loss of mechanical properties will arise. As a result, in order to overcome these problems. 

The smaller hydrophobic/hydrophilic separation and the lesser flexibility of the polymer backbone of SPEEK produce narrow proton channels and a highly branched structure, which baffles the transfer of methanol. 

Let us first consider very briefly the influence of various parameters (i.e., nature of flexible spacer and its length, nature and flexibility of the polymer backbone and its degree of polymerization) on the phase behavior of a side chain liquid crystalline polymer. According to some thermodynamic schemes which were described elsewhere, the increase of the degree of polymerization decreases the entropy of the system and therefore, if the monomeric structural unit exhibits a virtual or monotropic mesophase, the resulting polymer should most probably exhibit a monotropic or enantiotropic mesophase. Alternatively, if the monomeric structural unit displays an enantiotropic mesophase, the polymer should display an enantiotropic mesophase which is broader. It is also possible that the structural unit of the polymer exhibits more than one virtual mesophase and therefore, at high molecular weights the polymer will increase the number of its mesophases. All these effects were observed with various polymer systems. ... 

Our results to date suggest that the flexibility of the polymer backbone plays the most important role in polymer reactivity, particularly if there is a significant change in the polarity of the polymeric derivative. Rigid polymer chains can not contribute significantly to solvation. Retardation of substitution rates occurs... 

Membranes based on aromatic poly(ether ether ketone) (PEEK) seem very promising. In addition to their low methanol permeability, they have good mechanical properties, and high thermal stability, and the proton conductivity can be controlled through the degree of sulfonation. The main difference between Nafion and SPEEK membranes, which makes the latter less permeable to methanol, can be attributed to the difference in their microstructure. In SPEEK membranes, there is less pronounced hydrophiUc/hydrophobic separation as compared with Nafion membrane, and the flexibility of the polymer backbone of SPEEK produce narrow proton channels and a highly branched structure that baffles the transfer of methanol [18]. Thus, SPEEK membranes have lower electro-osmotic drag and methanol permeability values. 

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