Photografting-immersion

From another point of view, surface photografting techniques can be categorized as liquid-phase (immersion) photografting and vapor-phase photografting. The applicability of these approaches can be extended to modification of unable-to-be-irradiated surfaces, lamination of films, patterning, membrane modification, and so on. 

Immersion Photografting As the name implies, the surface to be grafted is immersed in a solution containing sensitizer and monomer or alternatively solutions of each sequentially. The examples given thus far also fall into this category, regardless of the number of steps grafting has been conducted. 

Fig. 11.6 Schematic depiction of surface photografting processes (a) continuous grafting [91], (b) immersion grafting [97], (c) vapor-phase grafting. Adapted from Ogiwara et al. [98] with permission from John Wiley Sons, Inc.

The photoinitiator may be loaded on the membrane surface by adsorption or may be dissolved in the monomer solution. For example, PET nucleopore membranes were modified using BP dissolved in the monomer solution following UV irradiation of the solution and the immersed membrane (Yang and Yang 2003). It was shown that photografting occurred mainly on the top membrane surface rather than in the membrane pores. This approach is relatively simple however, its main drawback is a low local concentration of BP on the membrane surface because BP moves to the membrane surface only by diffusion. This results in a low grafting efficiency. High bulk BP concentration may cause a side reaction, such as homopolymerization. In addition, the use of monomers that do not have a common solvent with BP is limited BP is almost insoluble in water. 

Micropatterning is a robust tool to surface-modify bioplastics like poly(lactic acid) (PLA) for biomedical applications. We used a sequential two-step photografting and photomask approach to micropattern poly(acrylamide) (PAAm) on PLA film. In step one, a PLA specimen, dip coated in benzophenone solution in ethanol covered with the photomask, was sandwiched between two glass plates and exposed to UV in an inert atmosphere. In step two, benzophenone-micropatterned film was immersed in 10% v/v monomer solution in water and exposed to UV for 3 h to grow poly(acrylamide) (PAAm) from the film surface. The resultant film surfaces were examined by AFM and optical microscopy, which revealed the resolution and acuity of the micropatterns. 

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