Polymers photo-cross-linking

Trinh, X. A., Fukuda, J., Adachi, Y, Nakanishi, H., Norisuye, T. and Tran-Cong-Miyata, Q. (2007) Effects of elastic deformation on phase separation of a polymer blend driven by a reversible photo-cross-linking reaction. Macromolecules, 40, 5566-5574. 

The most commonly practiced means for fabricating micro-patterned hydrogels is based on in-situ photo-polymerization and photo cross-linking using UV light in a liquid phase. A drawback of this method is that the prepolymer or polymer solution has to be brought into the irradiation set-up. An alternative approach is to cross-link a prefabricated dry film of the sensitive polymer. 

Recently the photopolymerisation of polysiloxanes has gained interest, because photo cross-linking allows the synthesis of indicator-free polymers... 

The first permanent photography was obtained with this system. Very soon bitumen was replaced by chromium colloids which remained Irreplaceable for a long time. The real start of modern photosensitive polymer technology occurred in 1930 when Murray showed that the unsaturated ketones could be photo-cross-linked to form resists. First limited to the graphic field, these techniques became later an indispensable part of electronic microminiaturization by making printed and Integrated circuitry possible. 

PET reactions of benzylic silanes with polycyano-aromatic compounds have been applied to photoresist and electron-beam resist technologies. "" The photoreaction of poly(4-trimethylsilyl-methylstyrene) in benzene-acetonitrile with DCB affords insoluble polymer via photo-cross-linking, which contains 4-cyanophenyl-methyl groups. In the case of 1,2,4,5-tetracyanobenzene, soluble 2,4,5-tricyanophenylmethyl-substituted polystyrene is produced. But,... 

Elisseeff, J. et al.. Synthesis and characterization of photo-cross-linked polymers based on poly(D-lactic acidaspartic acid). Macromolecules, 30, 2182,1997. 

This mode of photo-cross-linking has attracted attention for applications in resist technology, since it became apparent that the photodecomposition of organic azides in polymeric systems leads to insolubility. Azide groups can be chemically attached to polymer chains, as demonstrated here by two examples ... 

Alternatively, bisazides, i.e. low molar mass compounds containing two azide groups, can be added to the polymer. Several commercially used bisazides are presented in Table 7.2. Many linear polymers can be photo-cross-linked with the aid of bisazides [17]. Of note in this context is poly(cis-isoprene), which contains some cyclized structures (Chart 7.7). It has been frequently applied as a resist material in photolithography applications. 

Table 7.2 Bisazides of practical importance for the photo-cross-linking of linear polymers [17].

Free radical mechanisms also serve to explain the photo-cross-linking of various polymers, such as that of polyethylene accomplished with the aid of lightabsorbing additives such as benzophenone, quinone, benzoin, acetophenone, or their derivatives. When electronically excited by light absorption, these additives either directly abstract hydrogen from the polymer or decompose into free radicals capable of abstracting hydrogen, as shown in Schemes 7.10 and 7.11. 

The occurrence of these reactions is restricted to the amorphous phase. Therefore, the photo-cross-linking process has to be performed at temperatures exceeding the crystalline melting point in the case of highly crystalline polymers such as polyethylene. The cross-linking efficiency can be strongly enhanced by the addition of small amounts of multifunctional compounds such as triallyl cyanurate, TAG (see Chart 7.9), or by the incorporation of special diene moieties into copolymers such as ethylene propylene diene copolymers (EPDM elastomers) [33]. 

The discoloration is due to a dehydrochlorination process restilting in the formation of long conjugated polyene sequences in the polymer chain [Eq. (7-10)]. Polyenes can give rise to photo-cross-linking reactions. 

The earhest photoresists used in integrated circuit manufacture consisted of polymers that were rendered insoluble by photo-cross-linking and thus operated in the negative tone mode. For instance, partially cycUzed poly(cis-isoprene) containing a bisazide as additive served for a long time as the workhorse resist material in photohthography applications [15]. This system has already been described in Section 7.2.3. Subsequently, Novolak-based positively functioning sys-... 

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