Photosensitized crosslinking of polymers

Photosensitized crosslinking of polymers has been the subject of numerous publications [l - 30], concerned mainly with poly(ethylene), poly(vinyl alcohol), various vinyl copolymers, copolymers of maleic anhydride and/or phtalic anhydride with styrene and some polymers derivated from cinnamic acid. The following compounds were used as sensitizers benzophenone, 4-chloro- and 4,4-dimethylbenzophenone [l, 3-6, 8, 9l, oC -and -derivatives of anthraquinone [3, 23] acetophenone, hydroquinone, triphenylmethane and pyridine li.] chlorobenzene and no less than trichlorinated n-paraffins [6], a complex of zink chloride with o-dia-nizidine fill potassium bichromate [l2j, anthracene fl3, 14] 2,5-methoxy-4-amino-trans-stilbene [l5], benzyl ideneacetophenone fl6-l8] -thiophenylacetophenone,... 

Results and discussion. In the course of our studies on light induced crosslinking of poly (vinyl buty-ral), new class of effective photosensitizers, neimely organic sulphoxides, was discovered. f4l] It should be noted, that four years after our invention, the use diphenyl sulphoxide simultaneously with o-ethylbenzo-ine and H2O2 as effective sensitizer for crosslinking of polymers, was reported 3 ]. 

Polystyrene alone or with photosensitizer was unaffected under photolysis conditions, apart from some yellowing, and polystyrene-blsmaleimlde solutions appear to be stable Indefinitely in the dark. Blsmalelmldes have beat used previously in peroxide-Induced crosslinking of polymers l however, polystyrene was reported to be unreactlve under these conditions. Thus, the crosslinking reactions described here are not free radical in nature and are best formulated according to Reaction 3. Blsmalelmldes have also been reported to accelerate the radlolysls-induced crosslinking of elastomers.—... 

The photochemical and photophysical properties of BPHTs and other phenothiazine derivatives have been investigated by means of a combination of time-resolved laser techniques, including laser flash photolysis and steady-state fluorescence spectroscopy [30,96]. The main goal of these studies was to develop novel photosensitizers for photoresist technology in the photoin-duced crosslinking of polymers. 

In the dye-photosensitized oxidation of polymers, the formation of polymeric hydroperoxides is paralleled by a rapid change in the viscosity numbers of polymer solutions (Fig. 6.5) and in their molecular weight distribution (Fig. 6.6). Both chain scission and/or crosslinking reactions occur [1785,1787,... 

If (P ) is terminated by a chain transfer to a solvent or a monomer, a graft copolymer is formed, or, if the termination is from a combination, a crosslinked network polymer is formed. If the pre-existing polymer (B) contains an end group that itself is photosensitive (or can produce a radical by interacting with photoinitiator) and in the presence of a vinyl monomer (A), block copolymer of type AB can be produced if the photosensitive group is on one end of the polymeric chain. Type ABA block copolymer can be produced if the polymer chain (B) contains a photosensitive group on both ends. 

These polymers need to be made photosensitive for use as photoresists and this is achieved by the incorporation of bisazide sensitisers. On exposure to light the photochemical reaction induced by the bisazide results in rapid crosslinking of the polymer rendering it insoluble in the developing solvent. 

The crosslinking process is specially extensive in the solid state in the presence of external initiators, photo-initiators and sensitizers [Refs. 40, 362, 393, 489, 503, 505, 632]. For example the decomposition of di-tert-butylperoxalate in polypropylene in the absence of oxygen gives a tightly crosslinked polymer. When the decomposition is carried out in the presence of oxygen, the polymer instead softens [405]. Benzophenone and its derivatives induce the crosslinking of solid polyethylene on exposure to UV light [Refs. 138, 469, 470, 665]. Table 4 shows the influence of various photosensitizers on the amounts of the gel fraction produced in solid polyethylene exposed to UV radiation. 

On this substructure a thin dense layer (in the range of 0.5 to 10 pm thick) is coated that has a very high separation capability. Different coating techniques are in use, most commonly a solution of the respective polymer in an appropriate solvent is spread onto the porous substructure. The solvent is evaporated, followed by further treatment to effect crosslinking of the polymer. Photosensitive, solvent-free prepolymers may be used for coatings that are later crosslinked by irradiation, e.g. with UV-light or electrons. 

Photoreactions of MA with 1,2-polybutadiene, 1,4-polybutadiene, poly(styrene-co-butadiene), poly(styrene-co-isoprene), polystyrene, and poly(styrene-co-methyl methacrylate) have been studied in air. " In homogeneous solutions, MA addition to the polymers proceeds efficiently by a chain mechanism, where the quantum yield of the photoaddition was greater than unity under irradiation at A >310 nm. From the effects of solvent and photosensitizers and spectroscopic data, a radical chain mechanism was proposed to account for addition and crosslinking of the polymers by MA molecules. The photoaddition reaction was applied to the surface of polymer films. The photoreactions were conducted at the interphase between solid polymer and acetone solution of anhydride and also at the interphase between solid polymer and gaseous anhydride. Irradiation with a 300-W high-pressure lamp brought about considerable surface modification, as shown by wettability and dyeability properties. 

---