Expansion of the polymer

Since the degree of expansion of the polymer coils is directly dependent on the solvating power of the solvent, under otherwise comparable conditions, both a and [q] provide a measure of the goodness of a solvent high values of a and [q] (at constant molecular weight and temperature) indicate remarkable coil expansion and therefore a good solvent. Low values of a and [q] indicate a bad solvent. For example, the values a for poly(vinyl acetate) in methanol and acetone are 0.60 and 0.72, respectively. 

The results of the delayed stress on radiation studies presented above (Figure 7) are also consistent with the mechanism of gas buildup within the polymer specimens as the cause of the accelerated creep. An additional interesting conclusion is that applied stress should increase the rate at which gases diffuse out of a polymer specimen. This is not unreasonable in view of the fact that this conclusion is reached for stress application during irradiation, when expansion of the polymer matrix by the internally generated gas would be expected to facilitate gas diffusion. (Actually, one would expect increased gas diffusion in stressed glassy polymers, even in the absence of radiation, owing to the low Poisson ratio in such materials.)... 

The LDPE production with tubular reactors (see Section 5.1) requires some sophisticated control valves [45]. The let-down valve (Fig. 4.2-6 B) controls the polymerization reaction via the pressure and temperature by a high-speed hydraulic actuator (9) together with an electronic hydraulic transducer. The position of the valve relative to the stem is determined by a high-resolution electronic positioner (7). The cone-shaped end of the valve stem (2), as well as the shrunk valve seat (3) are made from wear-resistant materials (e.g., sintered tungsten carbide) in order to tolerate the high differential pressure of around 3000 bar during the expansion of the polymer at that location. 

May and Wetton (72) obtained another indication that benzene leads to an expansion of the polymer coils. They concluded from viscosity measurements that the unperturbed end-to-end distance in benzene was 1.34 times as great as that in the mixed theta solvent consisting of 39.2% heptane and 60.8% cyclohexane at 26° C. 

The exponent a in the intrinsic viscosity-molecular weight relationship ([rj] = K.M ) of a polymer is associated with the expansion of the polymer in solution, and hence with the conformation and stiffness of the polymer (Table 24). The a values of tobacco mosaic virus, Kevlar and helical poly(a-amino acids) are close to 2, which means that they take rigid-rod structures. The a values of vinyl polymers are usually 0.5-0.8, indicating randomly coiled structures. In contrast, the a values of substituted polyacetylenes are all about unity. This result indicates that these polymers are taking more expanded conformations than do vinyl polymers. This is atrributed to their polymer-chain stiffness stemming from both the alternating double bonds and the presence of bulky substituents. 

West et al. 4, 5) have suggested a number of possible reaction intermediates, which include anions, radical anions, radicals, and diradicals. Zeig-ler (6, 7) has proposed, on the basis of some radical-trapping experiments, that the intermediate is, at least at some point, a radical. He showed that the diradical silylene was not an intermediate, and he also stressed the importance of bulk solvent composition on the course of the reaction. The bulk solvent composition determines the expansion of the polymer coil as it interacts with the sodium surface. Miller et al. (8) initially suggested that the reaction, which is promoted by the addition of diethylene glycol dimethyl ether, proceeds by an anionic process, although they later (9) accepted Zeigler s bulk-solvent model. 

In the absence of added electrolytes the reduced viscosity, Jjsp/C, of a polyelectrolyte rises upon dilution in a striking manner as a result of the expansion of the polymer chains (Fuoss and Strauss, 1948 Hermans and Overbeek, 1948 Kuhn et al., 1948). Empirically, Fuoss and Strauss have found that the viscosity data can be represented by the equations... 

The triphenylmethane leucohydroxide residues were incorporated into the pendant groups by copolymerizing thd vinyl derivative (72, R=CH=CH2, X=OH) with 7V,Af-dimethylacrylamide [20]. Upon ultraviolet irradiation (X > 270 nm), the solution became deep green, and at the same time its reduced viscosity increased from 0.55 to 1.6 dl/g, as depicted in Fig. 6. After the light was shut, the viscosity returned to the initial value with a half-life of 3.1 min. The close correlation between the viscosity change and the absorption intensity at 620 nm implies that the electrostatic repulsion was responsible for the expansion of the polymer conformation. 

The concept to adopt the electrostatic repulsive force as a driving force for a photostimulated expansion of the polymer chain is useful and widely applicable to other polymer systems. Polystyrene and polyacrylamide having pendant leucohydroxide and leuco-cyanide groups were found to change their solution viscosity in methylene chloride and in water, respectively. 

A thermally good solvent, on the other hand, is one for which the heat of mixing (as measured by k) is low, while for a thermally poor solvent K is relatively large. In an athermal solvent (k = 0), the volume expansion of the polymer molecule will be independent of temperature, while in a thermally poor solvent it will increase with temperature. The change of viscosity with temperature will thus be low in a solvent with low K and relatively high in a solvent with relatively large k. 

However, even the pre-synthesized tetrazolides were still too reactive to be routinely used in the automated solid phase synthesis. The major advance that solved the problem was made in 1981, when Beaucage and Caruthers, who were experimenting with the nucleoside 3 -phosphoramidite derivatives [83] following some previous Russian work on the phosphorus(III)-amino compounds [84], discovered that these otherwise pretty stable compounds can be rapidly and very efficiently coupled to a solid-supported nucleoside in the presence of a mildly acidic nucleophilic catalyst, tetrazole [85]. This discovery combined with the already existing solid-phase assembly layout [68, 69, 82] paved the way for the very rapid expansion of the polymer-supported oligonucleotide synthesis, which has been summarized in the timely book edited by Gait [12a]. 

Ferry (23). This value of a is in reasonable agreement with the difference between the coefficients of thermal expansion of the polymer above and below Tg. 

Fig. 4.21. Schematic representation of the adsorption process of pyrene-labeled polyacrylic acid on alumina. (A) At low pH, polymer is coiled in solution which leads to (B) adsorption in coiled form. (C) Subsequent raising of the pH causes some expansion of the polymer. (D) Polymer at high pH in solution is extended and binds (E) strongly to the surface in this conformation. (F) Subsequent lowering of pH does not allow for sufficient intrastrand interactions for coiling to occur.

As can be seen from Fig. 2.23(a), above Tg there is an important contribution to Vf from the expansion of the polymer in the mbbery or molten state. The free volume above Tg can thus be expressed by... 

Since solvent power is a function of temperature, there is for any solvent a temperature, 0, at which ideal behavior should be observed—the solvent is then referred to as the theta-solvent. As the solvent power is increased, an isotropic expansion of the polymer molecule occurs, and the root-mean-square displacement length is then given by... 

Thus, macroscopic (e.g., viscosity) and molecular-level (e.g., D andA2) measurements are consistent with a model for NVP/SPE copolymers that is intramolecularly associated in water or low-salt solutions. These intramolecular aggregates are both of the intragroup and intrachain type. Such intramolecular aggregates are broken up by external electrolytes (e.g. NaCl), with a consequent modest expansion of the polymer coil (Figure 6). Thus, the solution properties for sulfo zwitterion copolymers with NVP confirm earlier trends reported for sulfo zwitterion homopolymers (3-6). 

Further research is needed in the area of liquid polymer solvent modification by (a) the use of end-caps or different monomers, (b) the addition of surfactants, or (c) expansion of the polymer with CO2. In each of these cases, measurement of solvent physical properties and evaluation of reaction performance are needed. Reaction performance in supported PEG phases with CO2 as the product-bearing phase, especially as a continuous-flow system, should also be investigated. One may anticipate greater ease of handling of the catalyst plus improved mass transfer between phases as a result of a greater surface area. 

Here and are the fractional free volumes at T and Tg, respectively. From Figure 12.5, it can be seen that Vf is assumed to remain constant during expansion of the polymer in the glassy state, but that above there is a steady increase with rising temperature. If ccj is the expansion eoeffieient of the free volume above Tg, then the temperature dependence of fj ean be written... 

Polymers can also be used to manufacture lenses and screens for projection television systems. These are most conveniently made from PMMA, or combinations of glass and PMMA, to counteract the high thermal expansion of the polymer. The use of ultraviolet curable coatings for lens replication and protective layers is widespread, and these systems are based on diacrylate or dimethacrylate monomers mixed with photoinitiators such as... 

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