Thermal initial region

Peroxides are used most commonly either as thermal initiators or as a component in a redox system. While peroxides are photochemically labile, they seldom find use as photoinitiators other than in laboratory studies because of their poor light absorption characteristics. They generally have low extinction coefficients and absorb in the same region as monomer. Kinetic parameters for decomposition of some important peroxides are given in Table 3.5,... 

It is reported in Refs 42 67 that moderate quantities of waxy materials incorporated with such expl materials as HMX, RDX and PETN do not appreciably affect their initiation sensitivity, but do inhibit propagation to expin. Ref 67 further states that no relationship between the specific and latent heats of the desensitizers and its ability to desensitize could be found, and concludes that the desensitizer cannot be regarded merely as a thermal sink. This is somewhat in conflict with results of work in Ref 91, where it is reported that it has been established firmly that the sensitivity of the RDX compositions decreases with increasing specific heats of the additives, and a solid desensitizer functions primarily by absorbing heat from local regions of initiation including any hot spots which arise from adiabatic compression of occluded gas ... 

Normally, thermal initiation is used and the critical energy is acquired by collisions. In photochemical initiation the critical energy is accumulated by absorption of radiation. This can only be used if the reactant molecule has a sufficiently strong absorption in an experimentally accessible region, though modem laser techniques for photochemical initiation increase the scope considerably. 

Radicals can be produced by ultraviolet irradiation of a monomer like styrene which absorbs sufficiently strongly in this wavelength region. Photochemical initiation may also be provided by thermal initiators or by compounds like benzoin ethers ... 

We became interested in poly(l-trimethylsilyl-l-propyne) [poly(TMSP)] after the initial report of its synthesis (9). Poly(TMSP) possesses many of the qualities needed in a practical deep-UV resist material a high silicon content (—25%) thermal stability excellent film-forming properties and a chromo-phore, the conjugated backbone, that absorbs strongly in the deep-UV region. Our initial work with poly(TMSP) showed that it had one serious drawback its sensitivity toward degradation by deep-UV radiation was poor, and doses of >1 J/cm are required for the definition of a positive-tone pattern. 

The thermal initial region is the region just after the EPR leading edge, where the flow velocity decelerates by (3.37), and the flow temperature is changed due to only the sinks on obstructions,... 

The solution of the last equation gives the law for a temperature decrease on the thermal initial region ... 

In the case Ah = A, the length of the thermal initial region equals the hydrodynamic initial region. If AH > A, the temperature decreases faster that the velocity, so the length of the thermal initial region is shorter than that of the hydrodynamic one. The region ends, when T becomes very small, T = s. The estimation of its length can be made by the formula... 

It is conventional to define the length of the thermal initial region as the distance from the inlet cross-section to the point at which the Nusselt number differs from its limit value (3.5.27) by 1%. Calculations show that the dimensional length of the thermal initial region is l = 0.1 la Pe --... 

Isothermal frontal polymerization (IFP) is a self-sustaining, directional polymerization that can be used to produce gradient refractive index materials. Accurate detection of frontal properties has been difficult due to the concentration gradient that forms from the diffusion and subsequent polymerization of the monomer solution into the polymer seed. A laser technique that detects tiny differences in refractive indices has been modified to detect the various regions in propagating fronts. Propagation distances and gradient profiles have been determined both mathematically and experimentally at various initiator concentrations and cure temperatures for IFP systems of methyl methacrylate with poly(methyl methacrylate) seeds and wilh the thermal initiator 2,2 -azobisisobutryonitrile. 

FP systems must have certain conditions for the front to autocatalytically occur. The necessary and sufficient conditions for TFP include a monomer that will polymerize via free-radical polymerization and a thermal initiator (a photoinitiator may be used to start the reaction, but a thermal initiator is necessary to sustain the reaction) [9,10]. The necessary and sufficient conditions for IFP include a monomer that will dissolve the polymer seed, polymerize via free-radical polymerization, and exhibit the gel effect a thermal initiator and a viscous region in which the gel effect can occur (i.e., the seed dissolving) [6, 11]. Ideally, another necessary IFP condition is a monomer-polymer system that produces an optically clear product because most IFP products are used in optical applications. 

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