Photocrosslinkable polymeric system

J.L.R. Williams, Fortschritte der chemischen Forschung, Springer Verlag, Berlin, Bd 13(2), pp. 227 250 (1969) G.A. Delzenne, S3mthesis and photocrosslinking of light sensitive polymers, Fur. Polym. J., Suppl., pp. 55 91 (1969) G.A. Delzenne, Photocrosslinkable polymeric systems and their technical applications, Makromol. Chem., Suppl. 2, 169 188 (1979). 

A novel photocrosslinkable polymeric system has been developed for processing into films having stable second-order nonlinear optical properties. In the present system, polymers bearing photocrosslinkable chromophores, such as polyvinylcinnamate, are reacted with appropriately designed nonlinear optical molecules with the cinnamate or other photocrosslinkable functionalities for photocrosslinking at one, two or more points. The system can be poled and photocrosslinked in the poled state to yield a material with stable optical nonlinearity and large electro-optic coefficients. 

Polymeric systems containing side-chain benzoin moieties, recently appearing [100,102] in the patent literature, have been applied as photoinitiators to UV curing of prepolymers of a different nature. In particular, siloxane-compatible photocrosslinking initiators (PSBME) are manufactured [100] by reacting silox-anes with benzoin ether derivatives (Scheme 23) and used in UV curing of acryloxypropyl-terminated dimethylsiloxane prepolymers. 

With a different ferroelectric LC-polymer it was even possible to obtain very homogenous FS-films of thicknesses of only 6 to 16 layers as determined by both optical and X-ray reflectometry (5polymeric films contained a small fraction of polymerizable mesogenic side-groups, it was even possible to photocrosslink these systems. It should be mentioned that in this first case the crosslinking caused some changes in film morphology, but the resulting films are very stable and do not even break when small holes are formed by mechanical stress (JS),... 

Rp Ra agrees well with Eq. 1, which describes the rate of the photocrosslinking process (Rp) under stationary irradiation conditions [1, 2]. Only this general expression reflects the real situation in the polymeric system investigated in all detail, because the conditions (viscosity, concentration of... 

A number of new resist materials which provide very high sensitivities have been developed in recent years [1-3]. In general, these systems owe their high sensitivity to the achievement of chemical amplification, a process which ensures that each photoevent is used in a multiplicative fashion to generate a cascade of successive reactions. Examples of such systems include the electron-beam induced [4] ringopening polymerization of oxacyclobutanes, the acid-catalyzed thermolysis of polymer side-chains [5-6] or the acid-catalyzed thermolytic fragmentation of polymer main-chains [7], Other important examples of the chemical amplification process are found in resist systems based on the free-radical photocrosslinking of acrylated polyols [8]. 

We have successfully demonstrated the concept of simultaneous polymerization and poling of photocrosslinkable systems to produce high quality films with large second-order nonlinear optical susceptibilities and low scattering losses. The ability to process these materials under mild conditions may help avoid potential problems encountered in... 

It has been tempting for researchers to assume that the crosslinking takes place when an excited double bond adds to one of its neighbors to form cyclobutane-type structures. Parallels from the poly(vinyl cinnamate) system have been drawn to the well-known dimerization of cinnamic acid to truxillic and truxinic acids. A study of a number of substituted cinnamic acids revealed that in order for efficient dimerization to take place the double bonds of two neighboring cinnamate structures must lie within 3.6—4.1 A of each other 25>. The regularity of structure in the crystalline state is not preserved in solution or in the polymeric medium. It has been difficult, therefore, to conclude that cyclobutane-type structures are formed predominantly during the photocrosslinking of poly (vinyl cinnamate). 

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