2.3. Grafting of PDMAEMA from the Nanochannels of PET TeMs (PDMAEMA-g-TeMs)

Benzophenone (BP) is frequently used as a photoactive tethering reagent to functionalize the surface of various materials, including commercial plastics and fabrics [46,47,48]. There is a straightforward relationship between the immobilized BP concentration on a surface and grafting yield. In order to increase the amount of BP immobilized on the PET TeMs, oxidation of the substrate, thus increasing the carboxyl group concentration on the surface, has been reported as an effective strategy [49,50,51]. Therefore, PET TeM films were first oxidized prior to UV-induced grafting experiments. In the oxidation step, pristine PET TeMs were treated with a 500 mM H₂O₂ solution at pH 3 for 180 min under UV irradiation (190 W at 254 nm) [52]. The samples were then washed twice with deionized water and air-dried at room temperature for 5 h. The amount of -COOH groups was determined via a titration method and was determined as 18.17 ± 3.2 nmol/cm² (average of five measurements). For the immobilization of BP, the oxidized PET TeMs were soaked in 5% BP in DMF (w/v) and kept in a shaking water bath at 150 rpm for 24 h at room temperature. The membranes on which BP was physically attached were then washed with water and ethanol, dried, and rapidly used for the grafting experiments.

Grafting of DMAEMA from the BP-immobilized PET TeMs was carried out in a controlled manner by RAFT polymerization using two molar ratios of DMAEMA and the xanthate-based RA1 RAFT agent ([DMAEMA]/[RA1] = 500, 1000). The concentration of DMAEMA was varied as 5%, 10%, 20%, 30%, and 40% (w/v) in a total solution volume of 10.0 mL. The solutions were prepared using four different solvents, i.e., water, acetone:water (1:1, v/v), ethanol:water (1:1, v/v), and ethanol. Reaction solutions were put into sealed flasks containing BP-immobilized TeMs and degassed with argon for 10 min prior to polymerization. Two different polymerizations, thermal and UV-initiated, were examined. For thermal polymerizations, a water-soluble thermal initiator, namely 4,4′-Azobis-4-cyanopentanoic acid (ACPA), was also added to reaction medium and the solutions were placed in a shaking water bath (150 rpm, IKA KS 3000 IS control, (IKA, Konigswinter, Germany)) at 70 °C for 60 to 1440 min. For UV-assisted graft copolymerization, samples were placed under a UV-lamp (15 W at 295 nm, Ultra-Vitalux 300 W, Osram, Augsburg, Germany) at a distance of 7 cm where the reaction time was varied from 30 to 1440 min. After each predetermined grafting period, the grafted films (PDMAEMA-g-TeMs) were taken from the reaction medium and washed with deionized water by periodically changing the solvent. Degree of grafting (DG) was determined from initial and final weights obtained using an analytical balance (Mettler Toledo, Columbus, OH, USA) and calculated with an accuracy of ±0.05 mg using the Equation (1):

where, w_f is the weight of the PDMAEMA-g-TeMs films and w_i is the weight of the BP-immobilized PET TeMs.