Exploitation of Polymer Degradation

In order to be successful as a photoresist the polymer must have three characteristics  

It must be sensitive to the desired wavelength of light, which in current manufacturing practice is 436 nm. Sensitivity must also be apparent in the speed of response shown to irradiation, since the reactions in the polymer must be complete within milliseconds. 

In addition to this high sensitivity the polymer must be capable of giving high-resolution images. 

The polymer must resist the etching solutions used to remove the unwanted surface layers of the silicon wafer as the image is transferred to the microchip. 

A number of polymers are capable of fulfilling these demanding requirements. Typically negative photoresists are based on cyclised poly(l,4-isoprene). These polymers are prepared by dissolving poly(l,4-isoprene) in an appropriate solvent and subjecting it to thermal degradation. This is followed by treatment with acid to produce the cyclised material (see Reaction 8.8). 

The earlier sections of this chapter have assumed that polymer degradation is undesirable, which is true to a large extent. 

However, there are two related areas of technology, lithography and microlithography, which make positive use of polymer degradation and for which polymers are designed to be as readily degraded by light as possible. 

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The processing of polymers should occur with dry materials and with control of the atmosphere so that oxidative reactions may be either avoided, to maintain the polymer s molar mass, or exploited to maximize scission events (in order to raise the melt-flow index). The previous sections have considered the oxidative degradation of polymers and its control in some detail. What has not been considered are reactions during processing that do not involve oxidation but may lead to scission of the polymer chain. Examples include the thermal scission of aliphatic esters by an intramolecular abstraction (Scheme 1.51) (Billingham et al., 1987) and acid- or base- catalysed hydrolysis of polymers such as polyesters and polyamides (Scheirs, 2000). If a polymer is not dry, the evolution of steam at the processing temperature can lead to physical defects such as voids. However, there can also be chemical changes such as hydrolysis that can occur under these conditions. 

Alkali-doped polyacetylene is extremely air-sensitive and deteriorates much more quickly than oxidatively doped polyacetylene. But, whereas the latter degrades rapidly upon heating as a result of polymer-dopant reactions that alter the chain, alkali-doped polyacetylene is surprisingly stable thermally, up to 200°C. This can be exploited to anneal cis-rich doped samples, which leads to a considerable decrease of disorder and evolution towards the trans lattice [88]. Especially in K-doped samples, this leads to a strong conductivity increase. 

Typically, rapid repair of main cracks in slabs is realized using bituminous masses covering only the damages. Unfortunately, this kind of protection is ineffective just after few months of exploitation, because the degraded mass (Fig. 2b, 2c) allows for infiltration of water under slabs [8]. The coming into being hydrodynamic pump effect (due to moving loads - Fig. 3a) destructs sealants in joints (Fig. 3b) and causes uneven settlement of concrete slabs (Fig. 3c). The solution of this problem is the use of special polymer flexible joints. 

Difference spectroscopy is often used to detect species produced by degradation when the absorption bands of the degradation product are masked by those of the base polymer. This technique was one of the earliest to be exploited by FTIR users (1) and has produced valuable results. Although simple in concept, experience has shown that there are a number of conditions that must be met, and that the results must be carefully interpreted. 

Because of this uncertainly, we try, first, to control the course of the synthesis as completely as possible during its processing. Second, we try to determine the quality of the synthetic end product on polymer by the aid of Edman degradation with quantitative exploitation of the phenylthiohydantoines obtained [161,164]. In this way contaminations of the product by false sequences can be detected in relative amounts of as small as 0.1% of the main chain. Generally we experienced purer peptides synthesized than liberated from polymer by any detachment reaction. This can be demonstrated qualitatively by the aid of thin layer chromatograms of the crude peptide products released from the support after the cleavage reaction and by end-group determinations before and after peptide detachment. 

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