(252d) Integrating Plasma Surface Initiation with "Living" Nitroxide-Mediated Polymerization: Novel Approach to Graft Polymerization

Recent advances made in polymer surface engineering have demonstrated the ability to direct and control nano-scale polymer brush growth from substrate surfaces. Such polymer surfaces can be designed with unique chemical functionalities and surface topology. Synthetic routes towards films such as these are necessary for device miniaturization in microelectronics as well as the development of advanced materials for membrane separations, biotechnology, and tribology. Creating these films on surfaces have, in the past, relied on traditional adsorption and spin-coating techniques. Such films, however, can desorb (or delaminate) upon chemical and thermal stresses. In contrast, single molecule polymer chains can be grown directly from the surface to create robust, grafted or tethered chains in a process known as Graft Polymerization. There are two main challenges in this process: 1) creation of a high density of surface initiation sites, and 2) controlled growth of polymer chains from the surface. The classical approach, Free-Radical Graft Polymerization, relies solely on an initiator for polymerization but requires surface immobilized macroinitiators and results in polydisperse polymer growth. Recently, high energy plasma has been employed (under low pressure) to directly alter surface chemistry, creating surface peroxide groups on both organic and inorganic substrates that effectively function as surface initiators. However, following initiation by plasma treatment, polymer growth may be subject to early termination or chain transfer leading to a high polydispersity of polymer chain lengths.

In the present work, the objective is to integrate plasma surface treatment at atmospheric pressure with Controlled "Living" Radical Graft Polymerization to create a high density of uniform length polymer chains tethered to a stable inorganic substrate surface. The use of atmospheric plasma is particularly attractive since it does not require an ultra-high vacuum chamber for plasma processing. The substrate surface is first treated with atmospheric pressure plasma to create surface peroxide groups. Controlled polymer grafting via Nitroxide-mediated polymerization is then conducted by addition of Styrene monomer under specified reaction conditions and stoichiometric amounts of TEMPO (2,2,6,6-tetramethyl-1-piperidinoxyl). TEMPO is a free-radical scavenger that reversibly binds to the growing Polystyrene chain and both controls the growth of the chain and significantly reduces chain termination. While there are many approaches towards controlled "living" polymerization, the current approach was chosen because it does not require the presence of surface macroinitiators or catalysts and results in a relatively low polydispersity of polymer chain lengths. A series of graft polymerization studies were carried out to elucidate the kinetics of Polystyrene grafting under both classical and Nitroxide-mediated mechanisms. A strategy for growing long polymer chains from the surface was employed using low initial monomer concentration and relatively mild reaction temperature (< 100oC). Linear polymer chain growth was demonstrated through spectroscopic ellipsometry. Chemical functionality of the modified surface was confirmed by IR spectroscopy. Surface topology and surface feature uniformity was characterized by Atomic Force Microscopy (AFM). Post-chemical modification of the polymer surfaces produced by the present approach for chemical sensor development will be discussed and demonstrated.