Polymer scission process

Molecular weight decreasing photoresists (a) polymer-scissioning processes (b) polymer ablative or erosion processes. 

This result should be vahd for sufficiently high density 0 where correlations, brought about by the mutual avoidance of the chains, are negligible. Due to the recombination-scission process a polydisperse solution of living polymers should absorb or release energy as the temperature is varied. This is reflected by the specific heat Cy, which can be readily obtained from Eq. (9) as a derivative of the internal energy U... 

To predict a volatilization rate, it is necessary to quantify how rapidly the sufficiently light species in the distribution evaporate. Some authors20 have employed relatively involved mass transfer processes in order to model evaporation in some detail. However, in the present case we shall take the view that as soon as sufficiently light species form, they immediately volatilize.27-30 For scission processes that do not involve recombination, this is tantamount to assuming that there is a characteristic number of repeat units mv below which polymer molecules are classified as volatile. So, to define the remaining mass in the distribution, consisting of nonvolatile species, the concept of a partial moment is used. The mth partial moment of the distribution is defined as... 

Extensive studies on different rubber compounds (see, for example, Table 1 in [105]) yield Ec 0.05 to 0.15 eV per filler-filler bond [105,106], i.e., typical values for physical (van der Waals like) bonds. Similar values were obtained within an approach which assumes a hypothetical analogy between the structure of a statistical carbon black network and that of a Gaussian elastomeric (unfilled) polymer network [107]. As in the Kraus approach, the carbon black network scission process is assumed to be thermally activated. 

Although the concepts outlined in this chapter are particularly appropriate for the interpretation of hydrolytic deterioration of cellulose, they show promise as an aid in the interpretation of thermal, photochemical, photolytic, and enzymatic degradation as well. Equations 3 and 4 are generally applicable to the scissioning process in linear polymers (33, 34). 

The quantum yields for scissioning ( >g) and crosslinking ( ) were determined for some representative polysilane derivatives both in solution and in the solid state (8). In all cases polymer scission is the predominant process and the values ranged from 0.2 to 1.0. For two cases, poly (methyl phenylsilane) and poly(cyclohexyl methylsilane) which were also examined in the solid state, the quantum yields were reduced by at least an order of magnitude from the solution values. 

One can consider that the kinetics carbonyl build-up is representative of the overall oxidation kinetics, at least when considered at the molecular scale (or monomer unit). It remains to establish a relationship between structural changes at this scale and molar mass changes. For the PE polymer understudy, random chain scission is predominant. It will be assumed that the main scission process is the rearrangement of alkoxyl radical (p scission). Then, every elementary reaction generating alkoxyl radicals will induce chain scission. In the chosen mechanistic scheme, both hydroperoxide decomposition processes and the nonterminating bimolecular peroxyl combination are alkoxyl sources. Thus, the number of moles of chain scissions per mass unit (s) is given by ... 

The acid cracking catalysts produce carbonium ions by the addition of protons to polyolefin chains or by abstraction of hydride ions from hydrocarbon molecules. This is followed by chain scission, which yields C30-C50 oligomeric hydrocarbons. Secondary cracking by P-scission of the C30-C50 hydrocarbons yields liquid (C10-C25) hydrocarbon fuel. Specific advantages of the Polymer-Engineering Process include ... 

The discussion of the essential features in the experimental and theoretical approaches to the free radical degradation of polymers is thus completed. We introduce the next section with a summary table which is subdivided according to the two extremes of degradation unzipping and random scission. The first part of Table III describes the influence of basic structure and the second deals with secondary factors for a given structure. When a polymer is processed at elevated temperatures, volatilization and deterioration of physical properties during short time intervals are a matter of concern hence initial rates are important. 

Quantum yields for chain scission, 0°, were determined from the Initial slopes of plots of the number of scissions, s, per average chain against the quanta absorbed by the average chain or from plots of s against the quanta, q, absorbed per g of polymer and calculated from the relation = (A/M ) (s/q). The course of the scission process was followed vlscometrlcally and s was estimated from ([tj]°/[t)]) -1 an approximation of a was taken as 0.65 for all solutions In DMM. 

Photocncldatlon of styrene-based polymers and copolymers In solution Involves a complex group of related and unrelated reactions. In part, the main chain scission process Is subject to the competition for migrating absorbed energy by various labile moieties that can be termed weak links and by energytrapping species that may themselves be weak links or the source of subsequent degradation reactions. The Intentional introduction of suitable trapping species can also serve as a means for the reduction of the rate of the scission process. 

Two types of stabilizers inhibit crack growth in rubbers microcrystalline waxes and alkylated phenylene diamines. A small quantity of the wax milled into a rubber will gradually diffuse to the surface where it will serve as a barrier impervious to ozone. A combination of wax and alkylated phenylene dicunine antiozonant is generally used for optimum protection. The exact function of antiozonant is still obscure but it is possible that it accelerates scission processes on the polymer surface producing a protective film of viscous products. 

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