Polymer uncrossable

This material finds a limited application in films and coatings which require good abrasion and flexing resistance. Some typical properties of cross-linked and uncross-linked polymers are given in Table 18.9. 

As known, the free energy of an uncross-linked polymer melt can be calculated by the usual Gibbs formula,... 

Flocculation studies, considering the small-strain mechanical response of the uncross-hnked composites during heat treatment (annealing), demonstrate that a relative movement of the particles takes place that depends on particle size, molar mass of the polymer, as well as polymer-filler and filler-filler interactions (Figure 22.2). This provides strong experimental evidence for a kinetic cluster-cluster aggregation (CCA) mechanism of filler particles in the mbber matrix to form a filler network [24]. 

Simple physical entanglements can be sufficient to produce a structurally stable gel if the polymer has a sufficiently great molecular weight and if the polymer is of only modest hydrophilicity. In this case, the polymer will swell in water without dissolving, even in the absence of covalent cross-links. Poly(2-hydroxyethyl methacrylate) (PHEMA) is a prominent example of this type of hydrogel when uncross-linked, it will dissolve in 1,2-propanediol but only swell in water. 

The distribution of the thi monomer in molecular chains or in the whole polymer should affect the perfection of the vulcanizate network, free chain ends or the uncross-linked parts in the polymer making no contribution to the tensile strength but acting as a plasticizer of like structure as the polymer. 

Since the excellent work of Moore and Watson (6, who cross-linked natural rubber with t-butylperoxide, most workers have assumed that physical cross-links contribute to the equilibrium elastic properties of cross-linked elastomers. This idea seems to be fully confirmed in work by Graessley and co-workers who used the Langley method on radiation cross-linked polybutadiene (.7) and ethylene-propylene copolymer (8) to study trapped entanglements. Two-network results on 1,2-polybutadiene (9.10) also indicate that the equilibrium elastic contribution from chain entangling at high degrees of cross-linking is quantitatively equal to the pseudoequilibrium rubber plateau modulus (1 1.) of the uncross-linked polymer. 

A8. The Helmholtz elastic free energy relation of the composite network contains a separate term for each of the two networks as in eq. 5. However, the precise mathematical form of the strain dependence is not critical at small deformations. Although all the assumptions seem to be reasonably fulfilled, a simpler method, which would require fewer assumptions, would obviously be desirable. A simpler method can be used if we just want to compare the equilibrium contribution from chain engangling in the cross-linked polymer to the stress-relaxation modulus of the uncross-linked polymer. The new method is described in Part 3. 

A new stress-relaxation two-network method is used for a more direct measurement of the equilibrium elastic contribution of chain entangling in highly cross-linked 1,2-polybutadiene. The new method shows clearly, without the need of any theory, that the equilibrium contribution is equal to the non-equilibrium stress-relaxation modulus of the uncross-linked polymer immediately prior to cross-linking. The new method also directly confirms six of the eight assumptions required for the original two-network method. 

The mole fraction of the monomer units that are cross-linked in the polymer is X,., and nt is Ihe number-average number of atoms in the polymer backbone between cross-links. The temperature should be expressed in absolute degrees in this equation. The constant K is predicted to be between 1.0 and 1.2 it is a function of the ratio of segmental mobilities of cross-linked to uncross-linked polymer units and the relative cohesive energy densities of cross-linked and uncross-linked polymer (88). The theoretical equation is probably fairly good, but accurate tests of it are difficult because of the uncertainty in making the correction for the copolymer effect and because of errors in determining nf. 

The complete curve for the response of an uncross-linked polymer at a fixed temperature, depicted here, covers so many decades of time that it has only been measured at a single temperature on a very few low-molecular-weight polymers. The experimental results seen in the literature are actually a composite of data taken at several temperatures over a limited time scale. The effect of a temperature rise is to translate the main transition in the curve of Figure 5A to the left, toward shorter time, with essentially no change in shape. 

Give several possible ways of telling the difference between an uncross-linked polymer of very high molecular weight and one that has a very low degree of cross-linking. 

Experiments have been made in which uncross-linkable polymer rubbers have been added to a similar rubber that is subsequently cross-linked (199). As an example, polyisobutylene was added to butyl rubber before it was cross-linked. The polyisobutylene molecules were not attached to the network structure, so they could be extracted by a solvent. As expected, the polyisobutylene greatly increased the creep compliance over that of the pure butyl rubber. 

What will radiation during a test do to the stress relaxation of an elastomeric material if the radiation brings about chain scission Compare a cross-linked polymer with a high-molecular-weight uncross-linked one. 

A number of modifications of the structure of the allylstannane have been prepared with the aim of facilitating the removal of tin from the product. These include Curran s fluorous allylstannane (see Section 3.14.04.1), Pereyre s monoallylstannane AllylXSn[N(TMS)2]2 (Equation (92) above),258 allylstannanes with a polar (oligoethylene oxide) tail (e.g., 23 and 24), and the allylstannyl group bonded to a soluble or insoluble (cross-linked) polystyrene. The reaction using the allylstannane bonded to a soluble, uncross-linked, polystyrene resin occurs about 100 times faster than that on the cross-linked, insoluble resin, and the polymer can be recovered by recrystallization from methanol. 

The large scale molecular motions which take place in the rubber plateau and terminal zones of an uncross-linked linear polymer give rise to stress relaxation and thereby energy dissipation. For narrow molecular weight distribution elastomers non-catastrophic rupture of the material is caused by the disentanglement processes which occur in the terminal zone, e.g., by the reptation process. In practical terms it means that the green strength of the elastomer is poor. 

The simplified two-network experiment is performed in the following manner A thin strip of the uncross-linked polymer is stretched by about 60% and maintained with constant length throughout the remainder of the experiment. The force is monitored at all times. After a predetermined relaxation period, the temperature is decreased to below the glass transition temperature to quench all overall conformational changes. The sample is cross-linked in the glassy... 

Figure 5. Stretching force at constant length and a temperature of 283 K for the same polymer as in Figure 1. The sample is uncross-linked for the first 1000 s and then rapidly cooled to the glassy state and cross-linked with high energy electrons. The force f is obtained by subsequent heating to the stress relaxa-...

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