Polymers of the crosslinking type

The rate of hydrogen evolution was found to decrease slightly during irradiation for Marlex-50 polyethylene but not for low-density polymers [274]. Savoley and Pascale [274] also reported that the hydrogen yield could be lowered by increasing the hydrogen pressure on the sample. This has, however, been the subject of much controversy in the literature [277]. 

Hydrogen evolution from single crystals has been studied. Salovey et al. [271] found a G value of 4.6 for bulk Marlex-50 polyethylene and thin films obtained from solutions of crystallized polyethylene. Values of 2.48 and 2.17 were obtained by Kawai et al. [278] for, respectively, bulk Marlex-50 and single crystals (annealed or not). 

The effect of temperature on the hydrogen yield has been investigated by many workers (Table 14) and the available data have recently been 

VOLATILE PRODUCTS ANALYSIS AFTER IRRADIATION OF POLYETHYLENE [272] 

The hydrogen yield has been measured recently by Dole and coworkers [272] as a function of temperature and dose. It increased slightly with temperature and was almost constant with dose (Table 16) for the solid polymer. A large increase (G = 5.6) was observed on passing to the liquid state at 140°C. 

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The mechanical properties of polymers of the crosslinking type are improved upon irradiation with low absorbed doses (up to 0.1 MGy) for example, the... 

Most synthetic polymers of the crosslinking type are crosslinked more effectively in solution than in the solid state. This is shown schematically in Figure 5.11, where Dgei, the dose for incipient gelation, is plotted versus the... 

Polyisobutylene. The polymers which we have dealt with until now are of the type which mainly crosslink under the influence of radiation in vacuo. Polyisobutylene with one tetrasubstituted carbon in each repeat unit can be considered as the simplest hydrocarbon polymer of the other type—i.e., the polymers degraded by radiation. 

Graft or block copolymers are expected to be formed via process (28) (i) and (ii), respectively, depending on whether the polymer Ap is of a crosslinking type or a degrading type. No homopolymerization occurs in... 

The porosity and permeability of CP are the most important factors determining their ability to sorb and immobilize BAS. For solving these problems, it was necessary to synthesize various types of porous and permeable CP differing in the mobility of elements of the crosslinked structure and in the rigidity of the polymer backbone. For biological problems related to the application of CP as biosorbents, it has been found necessary to use CP with a marked structural inhomogeneity. 

If such fillers are to be used, they should have a neutral or slightly alkaline pH, otherwise additives such as ethylene glycol and triethanolamine, which are preferentially adsorbed on the surface of the filler, should be used, preventing any undesirable interference reactions between the filler and the crosslinking peroxide. These additives must, however, always be added to the mix before the peroxide. With some mineral fillers, such as some types of clay, the polymer may be bound to the filler by means of silane treatment, and the surface of the filler becomes completely non-polar. Consequently, the interaction with the polymer matrix increases, while the adsorption of the crosslinking peroxide by the filler is severely suppressed. 

Recently, a novel hydrophylic polymer membrane based on poly(allylamine hydrochloride) (PAA.HC1)/PVA, crosslinked with GA, has been also tested for methanol dehydration by pervaporation technique [33], Even if the reported results show a small selectivity of the last type of membrane, the blend s composition, the curing degree and the process conditions (temperature, feed concentration, etc.) could be used to obtain a better separation of methanol. 

Macromolecules having identical constitutional repeating units can nevertheless differ as a result of isomerism. For example, linear, branched, and crosslinked polymers of the same monomer are considered as structural isomers. Another type of structural isomerism occurs in the chain polymerization of vinyl or vinylidene monomers. Here, there are two possible orientations of the monomers when they add to the growing chain end. Therefore, two possible arrangements of the constitutional repeating units may occur ... 

Condensation polymerizations (polycondensations) are stepwise reactions between bifunctional or polyfunctional components, with elimination of small molecules such as water, alcohol, or hydrogen and the formation of macromo-lecular substances. For the preparation of linear condensation polymers from bifunctional compounds (the same considerations apply to polyfunctional compounds which then lead to branched, hyperbranched, or crosslinked condensation polymers) there are basically two possibilities. One either starts from a monomer which has two unlike groups suitable for polycondensation (AB type), or one starts from two different monomers, each possessing a pair of identical reactive groups that can react with each other (AABB type). An example of the AB type is the polycondensation of hydroxycarboxylic acids ... 

A hydrocarbon prepolymer containing terminal carboxyl groups (28) is available to the propellant chemist. These polymers were synthesized to eliminate some of the variables found in the copolymers. The carboxyl groups can be made of the same types with like reactivity. These linear non-branched polymers impart greater extensibility to elastomeric formulations. The chemistry in propellants is similar to the random functionality polymer. As 20 years of the chemistry of crosslinked propellant binders is reviewed, one familiar with the art cannot fail to predict solid propellant formulations using these polymers tailored to the specific requirements of the solid rocket design with the confidence that any discipline of science can be practiced. 

The rate of this reaction is about ten times smaller than that of Eq. (I) and under typical cure conditions it becomes noticeable only after the end of the main process. Scheme (II) changes the structure of networks by the formation of additional crosslinks of the ether type. This makes the total connectivity of the network higher. This structural change influences some properties of polymers in the glassy and rubbery state (see Sects. 4 and 5), but it is really pronounced in nonstoichiometric systems with an excess (P < 0.8) of epoxy components 19,22). Crosslinks of the ether type may principally appear in polymers due to a condensation reaction between OH groups. Under our conditions this process normally does not take place. 

The results presented here demonstrate that static SIMS has unique capabilities for the characterization of the surfaces of polymers that have been modified by metal deposition or by plasma or corona techniques. Especially, the introduction of unsaturation and crosslinking are aspects that in some polymers can be observed directly. The formation of low-molecular oxidized material that can be inferred from XPS studies, can also be observed directly. A limitation of the quadrupole-type instrument, which is still the most widely used, is its limited mass range and mass resolution. It can be expected that a considerably more detailed description of modified polymer surfaces can be obtained by application of the more powerful reflectron-type Time-of-Flight SIMS spectrometers, but such studies have, to date, not yet been published. 

Recently, core-shell type microgels, which contain a hydrophobic core and a hydrophilic thermosensitive shell, have become attractive for scientists because such systems can combine the properties characteristic of both the core and the shell [53], We have prepared core-shell microgel particles consisting of a poly(styrene) core onto which a shell of polyCA-isopropylacrylamide) (PS-PNIPA) has been affixed in a seeded emulsion polymerization [54-56], In this case, the ends of the crosslinked PNIPA chains are fixed to a solid core, which defines a solid boundary of the network. In this respect, these core-shell latex particles present crosslinked polymer brushes on defined spherical surfaces. The solvent quality can be changed from good solvent conditions at room temperature to poor solvent conditions at a temperature... 

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