Modulating Solubility and Enhancing Reactivity of Photo-Cross-Linkable Poly(styrene sulfonyl azide-alt-maleic anhydride) Thin Films

Abstract

To formalize our understanding of indiscriminate grafting chemistries as they pertain to cross-linkable polymers and emerging patterning technologies, we designed a new polymer, poly(styrene sulfonyl azide-alt-maleic anhydride) (PSSMA). By modulating its solubility, it can be deposited into smooth, ultrathin films atop polar and nonpolar polymers. Upon heating above 120 °C or exposure to UV light, highly reactive nitrene intermediates are generated from the azide groups which form covalent adducts and cross-link the PSSMA. Azide photolysis and polymer gelation were studied in the context of a statistical model to gain insight into the network outcomes of nitrenes in a polymer film. For every azide group converted to a nitrene in ambient atmosphere, it has an 11% likelihood of grafting to another chain and a 5% chance of causing a scission. These values can be increased over 3-fold by reducing the O 2 content by 85%. Alternatively, the effects of quenching by ground-state O 2 can be mitigated by adding Michler's ketone (MK) to the film. PSSMA/MK blend films possess a 39% (±13) likelihood for grafting and 29% (±10) for scission. The higher ratio of scission to grafting is a consequence of the sensitized azides producing triplet-state nitrenes, which favor hydrogen abstraction. These broadly generalizable considerations will be useful to others who wish to maximize light sensitivity in related polymer systems.

Orthogonally Spin-Coated Bilayer Films for Photochemical Immobilization and Patterning of Sub-10-Nanometer Polymer Monolayers

“…50 The 2.6 J/cm 2 exposure dose corresponds to a fractional conversion of azide, p = 0.70, based on previous examination of irradiated PSSMA films. 24 h remains constant at higher doses and p, suggesting that h is not influenced significantly by crosslinking or scission of the interfacial overlayer polymer. A wide process window exists that can accommodate deviations in light intensity or exposure time.…”

“…22,23 Upon exposure to ultraviolet light or thermal annealing, sulfonyl azides are converted to highly reactive nitrene intermediates that can form covalent bonds with C−H groups by insertion or recombination mechanisms. 24,25 Therefore, not only does PSSMA cross-link to itself, it will also covalently graft to polymers and other soft materials with which it is in contact. After grafting and rinsing away unbound overlayer material, an immobilized monolayer is obtained.…”

Photochemical Immobilization of Polymers on a Surface: Controlling Film Thickness and Wettability

“…Covalently attached polymer films can be prepared either by surface initiated polymerization or grafting a pre‐existing polymer to a reactive group on the surface . Among methods to enact the latter method, photochemical methods are highly desirable because, provided the right chromophores are selected, they can be used to covalently immobilize polymers containing aliphatic C‐H groups, which are present in a wide variety of commercially important macromolecules . A major advantage of the approach is that a polymer does not need to be designed with a specific functional group that reacts with a complementary surface‐bound functional group as long as abstractable hydrogen atoms are present.…”

Electroless Deposition of Nickel on Photografted Polymeric Microscale Patterns

“…Among aromatic ketones, benzophenone, phthalimide, and binaphthyl ketone have been used to construct surfaces that covalently bind a wide variety of preexisting polymers. Similarly, carbenes and nitrenes have been photogenerated on surfaces derivatized with azidobenzoyl, aryl trifluoromethyl and ethyl diazirine, azidoformate, aryl sulfonyl azide, and diazomethylcarbonyl . The photogenerated surfaces have been used to immobilize numerous polymers in a variety of applications including carbohydrate microarrays, protein patterning, lipid bilayers, low surface tension substrates, nanoparticle patterning, and coatings for biomedical devices, and have provided a general method for patterning surfaces to study a variety of phenomena on chemically structured surfaces .…”