Polymer Film and Processes
The history of photopolymers may actually go back as far as the early 19th century when French photo pioneer Nicephore Niepce (http://www.niepce.com/pages/inv1.html#) carried out experiments with light-sensitive bitumen of Judea. Previously applied by the printing industry, photopolymer materials have meanwhile become an essential part of our daily life (from adhesives to optical memories).
In the late 1960s companies like Hughes, Bell, RCA, DuPont, Canon, Polaroid started introducing photosensitive polymers for holography. In the meantime a great many photopolymer systems have been developed. Generally these light-sensitive compositions were formed of a photopolymerizable monomer, an initiator system and a polymer binder. Photopolymerization may be defined as the reaction of monomers or macromers to produce polymeric structures upon light-induced initiation.
In photopolymer systems designed for holography index modulation usually is caused by refractive index changes. Upon actinic radiation polymerization is being initiated within the recording layer creating thus areas of increased molecular weight, converting a monomer into a polymer. Colburn/Haines (Volume hologram formation in photopolymer materials, Applied Optics, Vol. 10, 1971, pp.1636-1637) point out: In general, hologram formation in photopolymers is a three-step process. First, a normal exposure is made to the interference pattern to be recorded; this initial exposure polymerizes part of the monomer, with the amount of polymerization being a function of the intensity of the illumination. Monomer concentration gradients, caused by variations in the amount of polymerization, then give rise to the diffusion of the relatively small monomer molecules from regions of higher concentration to regions of lower concentration. With the completion of the diffusion step, the photopolymer is exposed to light of uniform intensity until the remaining monomer is polymerized.
In a holographic setup the variations of light intensity will translate into refractive index changes. So far most photopolymer materials represent almost ideal phase media. In order to achieve maximum index modulation each component needs to be adjusted to each other with regards to optimizing refractive index differences. Such criteria might equally apply to components like solvents, binders, plasticizers, surfactants etc. Photopolymers can be spectrally sensitized over a wide range of wavelengths from UV to IR.
Carré/Lougnot (Photopolymers for holographic recording: from standard to self-processing materials, J. Phys.III France 3, 1993, p.1445, www.edpsciences.org/articles/jp3/ref/1993/07/jp3v3p1445/jp3v3p1445.html) distinguish four categories:
"dry formulation containing a polymeric film-forming binder (substrate), a dissolved monomer and a photoinitiating system;
liquid or highly viscous coating containing monomers with several reactive functions and an initiating system;
dry film composition containing crosslinkable, dispersed or grafted structures or unreacted double bonds and an initiating system;
polymer film composition containing sensitive groups capable of undergoing photomodification (isomerization) or photodegradation (cleavage)."
In order to provide the photosensitive material with sufficient mechanical strength, many systems contain a binder material. However, the binder tends actually to lower the diffusion of the monomers within the recording layer, preventing thus full polymerization. Such systems may hence suffer from insufficient efficiency and reduced index modulation. One way to get around that issue is to subject the recording layer to a thermal treatment following the laser exposure and the subsequent UV post-exposure. The temperature rise will increase diffusion speed to enhance the degree of polymerization. Liquid post-treatment of the fully polymerized layers may be carried out to influence paramters like playback wavelength and bandwidth.
While most systems used to rely on radical polymerization, there has been increasing interest in cationic photoinitiators. The latter are based on the generation of an acid, which forms upon actinic radiation and promotes cationic polymerization.
Dupont makes a line of photopolymer films available for large applications. It is reported to nolonger be available for art holography markets. Dupont
Polaroid makes a line of photopolymers as well. DMP128.