Scission of polymers

Absorbance of Novolac Resists Synthesized from o-, m-, and p-Cresoh 

Since KrF excimer laser emits light at 248 nm, the resolution limit is expected to be more than twice the i-line exposure. Novolac-DQ polymer can be applied to KrF resist because DQ shows photosensitivity at 248 run. However, the depth of the coating layer is limited because of strong absorption of the benzene ring in novolac. 

The absorption peak of cresol, which is synthesized for novolac, depends on the position of substitution. Table 1.19 shows the absorbance of novolac resists synthesized from o-, m-, and p-cresol. Although p-cresol shows strong absorption at peak wavelength, 289 run, it has the weakest absorption among the three cresols. On the other hand, while o-cresol shows weak absorption at 278 nm, it has the strongest absorption, at 250 nm, among the three cresols. Absorbance can be controlled by the substituent position of the cresol. Since p-cresol has an absorption window near 250 nm, it is possible to apply it to the KrF resist. Since absorbance of p-cresol is 1.0 at 250 nm for a 1.0 p film thickness, 10% of irradiated light can be penetrated at the deepest layer of the film. 

Burnett, Mechanism of Polymer Reactions, Interscience, New York (1954). 

Bevington, Radical Polymerization, Academic Press, New York (1961). 

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Small particle size resins provide higher resolution, as demonstrated in Fig. 4.41. Low molecular weight polystyrene standards are better separated on a GIOOOHxl column packed with 5 /u,m resin than a GlOOOHg column packed with 10 /Ltm resin when compared in the same analysis time. Therefore, smaller particle size resins generally attain a better required resolution in a shorter time. In this context, SuperH columns are best, and Hhr and Hxl columns are second best. Most analyses have been carried out on these three series of H type columns. However, the performance of columns packed with smaller particle size resins is susceptible to some experimental conditions such as the sample concentration of solution, injection volume, and detector cell volume. They must be kept as low as possible to obtain the maximum resolution. Chain scissions of polymer molecules are also easier to occur in columns packed with smaller particle size resins. The flow rate should be kept low in order to prevent this problem, particularly in the analyses of high molecular weight polymers. 

Recently, we have shown that non-isothermal chemiluminescence measurements for oxidized cellulose provide the same rate constants of cellulose degradation as may be measured from experiments on the decay of polymerization degree determined by viscometry. This may be also taken as indirect evidence that the light emission is somehow linked with the scission of polymer chains [29]. 

The neutral polymer radicals which are produced also often undergo further reactions, which can result in chemical changes in the polymer. These reactions may include crosslinking or scission of polymer chains, formation of small molecule products, changes in the stereochemistry of the polymer chains, changes in the crystallinity of the polymer or a variety of other chemical and physical processes. 

Modified PTFE surfaces show a high degree of biocompatibility with good cell adhesion and proliferation [7-11], However, the UV-treatment results also in a loss of mechanical stability due to the scission of polymer chains, especially for light-sources with wavelengths below 193 nm [6], Similarly to the ion implantation or plasma modification, also the UV light-irradiation is performed on both sides of a polymer foils in order to avoid the material torsion. 

Note 1 Examples of reactive blending are (a) blending accompanied by the formation of a polymer-polymer complex, (b) the formation of block or graft copolymers by a combination of radicals formed by the mechanochemical scission of polymers during blending. 

If a mechanical degradation of a solution of two polymers is carried out by high speed stirring, the formation of a block copolymer is not probable as the scission of polymer molecules at low concentration is not caused mainly by intermolecular interaction, such as by collision of molecules and through entanglements, but by displacements due to hydrodynamic forces in velocity gradients. Nakamo and Minoura (98) did obtain reaction by stirring a benzene solution of polyethylene oxide and poly(methyl methacrylate). 

Scission -of polymer chain [INITIATORS - FREE-RADICAL INITIATORS] (Vol 14)... 

Typical data from such an experiment for PDN6S are seen in Figure 3. For multiphoton exposure, the birefringence initially rises until a saturation value is reached, then begins to diminish. This corresponds to the initial scission of the polymer chain segments aligned with the pulsed laser exposure and subsequent scission of polymer chain segments of other orientations. 

If homolytic scission of polymer main-chain is induced by mechanical fracture, primary mecbano-radical must be formed in a pair. However, for homogeneous polymers, like PE, PTFE and PB, it is hard to demonstrate the pair formation of mechano-radical. Simplest heterogeneous polymer, polypropylene, is probably the best sample for the purpose of experimental verification of pair formation of the mechano-radical. 

A new macroscopic degradation mechanism of polymers studied by Murata et al. [6] was suggested with two distinct mechanism in the thermal degradation of PE, PP and PS. One is a random scission of polymer links that causes a decomposition of macromolecnles into the intermediate reactants in liquid phase, and the other is a chain-end scission that caused a conversion of the intermediate reactants into volatile prodncts at the gas-liqnid interface. There are parallel reactions via two mechanisms. The random scission of polymer links causes a reduction in molecular weight of macromolecules and an increase of the number of oligomer molecules. The chain-end scission causes a dissipation of oligomer molecules and a generation of volatile products. 

Comparison of Crosslinking Vs Scission of Polymers Polymer Radiation Heat of Monomer... 

C. Average Molecular Weight as a Function of Depth in UV-Irradiated Films. Random scission of polymer chains within a... 

It has more recently been shown that there are advantages in separating the chemical reaction of antioxidants with solid polymers from the vulcanization process. It has long been recognized (32) that mechano-scission of polymer chains during processing or polymers gives rise to free radicals which can initiate radical chain reactions (32). Block copolymers can be synthesized in this way, reaction 5 ... 

The mechanism in Scheme 1 makes polypropylene an attractive substrate for study, because it implies a proportionality between rates of light emission and scission of polymer chains. 

High sensitivity resists Table II summarizes the properties of representative positive electron resists. The polymers are classified into four groups according to the chemical structure. Almost all positive electron resists operate by main chain scission of polymer, resulting in a molecular weight decrease in exposed areas. Resist patterns are produced by development in a suitable solvent in which degraded polymer dissolve much faster than unexposed polymer. The sensitivity is determined by the scission probability and the solubility rate ratio for the degraded polymers. 

Now let me try to get some insists into molecular mechanisms of the mechanic fracture in polymers 73). The l teilin model was originally proposed in ex]danation of the mechano-radical formatirm in the hi y stretched fibre. However, one can apply the Peterlin model to the fracture ptenomena in crystalline polymers, because large deformations proceed always in advance of a mechanical fracture. Thus, the tie molecules are assumed to be only parts vtdiich are broken in the case of destmction of bulk polymers. The fact that no mechano-radical is formed from the polymer having no tie mdecules even after the milling supports the interpretation mentioned ove. However, for amorjdious pd[ymers such as PMMA and PB, formation of the mechano-radkals is not attributed to the ruptures of the tie molecules, becau% neither the crystalline parts nor frie tie molecule exist in an amorphous polymer and no particular part of the polymer, on which the applied stress is concentrated, can be assumed in the amorphous polymer. It was found that the polymer chains are ruptured even in the case of an amorphous polymer, like PMMA, PB, and other elastomers, as mentioned in the Section III. The medianism other than the Peterlin model is needed to explain the bond scissions of polymer chains in the amorphous pdymers. 

Molecular Design for Position-Selective Scission of Polymer Skeleton... 

Nakano, A.7 Minoura, Y. Effect of solvent and concentration on scission of polymers with high-speed stirring. J. Appl. Polym. Sci. 19 (1975) 211-2130... 

Aging of polymers in solution under the influence of light, microorganism, temperature and mechanical energy are often to be observed and in such cases the viscosity decreases with storage time, because chain scission of polymers occur. 

Degradation process can be described in terms of the changes of molecular weight due to the scission of polymer chain and its efficiency can be estimated by a radiation yield of scission Gs (mol/J) according to Equation 31.9 (Janik et al. 2003) ... 

Guven O. Cross-Linking and Scission of Polymers. Dordrecht Kluwer, 1990. 

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