Storage stable photopolymerizable composition and element for refractive index imaging

DETAILED DESCRIPTION OF THE INVENTION The improved photopolymerizable compositions of this invention are substantially solid and are typically used as a layer applied to a permanent substrate.
The composition may be directly coated onto the substrate by any conventional method or may be laminated thereto as a storage stable, preformed element comprising the photopolymerizable layer releasably adhered to a temporary support film, e.
g.
, of polyethylene terephthalate.
The photopolymerizable layer is a thermoplastic composition which, upon exposure to actinic radiation, forms crosslinks or polymers of higher molecular weight to change the refractive index and rheological character of the composition.
Preferred photopolymerizable compositions are compositions wherein free radical addition polymerization and crosslinking of a compound containing one or more ethylenically unsaturated groups, usually in a terminal position, hardens and insolubilizes the composition.
The sensitivity of the photopolymerizable composition is enhanced by the photoinitiating system which may contain a component which sensitizes the composition to practical radiation sources, e.
g.
, visible light.
Conventionally a binder is the most significant component of a substantially dry photopolymerizable film or layer, in determining what physical properties the film or laminate will have while being used.
The binder serves as a containing medium for the monomer and photoinitiator prior to exposure, provide the base line refractive index, and after exposure contributes to the physical and refractive index characteristics needed for the refractive index image formed.
Cohesion, adhesion, flexibility, miscibility, tensile strength, in addition to index of refraction are some of the many properties which determine whether a binder is suitable for use in a refractive index media.
Dry film photopolymerizable elements of various types may be used.
Suitable elements are prepared by conventionally coating the photopolymerizable composition on a wide variety of transparent substrates