Surface characterization data for tethered polyacrylic acid layers synthesized on polysulfone surfaces

The data presented are supplementary to an article [Kim et al., 2019] on synthesis and surface characterization of tethered polyacrylic acid (PAA) layers on polysulfone (PSf) film/membrane surfaces via atmospheric pressure plasma-induced graft polymerization (APPIGP). Data on surface characterization of the synthesized tethered PAA layers includes: AFM topographic surface images and height distributions of surface features, dry layer thickness, chain rupture length distributions determined via AFM based force spectroscopy (AFM-FS), in addition to measurements of water contact angles. Fouling propensity data for ultrafiltration of alginic acid as a model foulant are also provided for native and PAA grafted PSf ultrafiltration (UF) membranes.


Data
Tethered polyacrylic acid layers were synthesized via free-radical graft polymerization at initial monomer concentration ([M] 0 ) of 1e20 vol%. The resulting surfaces were characterized with respect to elemental surface composition determined via XPS (Table 1) and surface topography evaluated via AFM analysis (Fig. 1). Contact angles for the different surfaces ( Fig. 9, Fig. 10, Table 2, Table 3) were evaluated via the sessile drop method in air and via the captive bubble method in DI water and saline water (35 g/ L NaCl)), and the polar and dispersive components of the surface free energies are provided in Table 2. Typical AFM-FS retraction force curves for the native PSf film and PAA layer grafted PSf film surfaces are shown in Fig. 2 and Fig. 4, respectively. The AFM-FS determined chain rupture length and rupture force distribution data for the tethered PAA chains are presented in Figs. 5 and 6, respectively, and the rupture force distribution for the native PSf film in water is shown in Fig. 3

Materials
Helium (He), hydrogen (H 2 ) and oxygen (O 2 ) gases were used for plasma generation, and nitrogen (N 2 ) was utilized for drying and for degassing solvents. Poly (ethyleneimine) (PEI, M w~7 50,000) solution (50 wt% in water), PSf pellets (M w~3 5,000), chloroform and acrylic acid were all utilized in the synthesis of PAA tethered surfaces. Piranha solutions were prepared using sulfuric acid (H 2 SO 4 , 96%) and hydrogen peroxide (H 2 O 2 , 30% in water). Sodium chloride and deionized (DI) water (pH~6) were used for preparation of the saline water (35 g/L NaCl). The model foulant [2] for fouling tests was alginic acid sodium salt, and the prepared alginic acid solution pH was adjusted with 50 wt% aqueous sodium hydroxide solution. Finally, membranes with tethered chains were prepared using PSf base membrane of 100 kDa molecular weight cutoff (MWCO).

PSf film preparation
Smooth PSf substrate films were prepared on prime grade silicon wafers by spin-coating a PSf solution using a spin coater. A piranha solution (a 7:3 mixture of concentrated H 2 SO 4 and 30% aqueous H 2 O 2 ) was used to clean the silicon wafers at 90e100 C for 10 minutes, followed by rinsing with DI   water. Silicon wafer samples (~1 cm Â 1 cm) were prepared and cleaned with isopropanol and then DI water, followed by blow drying with N 2 . The wafer surface was first coated with PEI (using~0.1 mL of a 0.3 wt% aqueous PEI solution at a spin rate of 2500 rpm for 30 seconds). Subsequently, PSf was coated onto the silicon wafer with PEI coating using~0.1 mL of a 1 wt% PSf solution in chloroform that was spin-coated at 2500 rpm for 30 s. The produced PSf-Si substrates were then vacuum oven dried at~75 C prior to their subsequent use.

Plasma surface activation and graft polymerization
An impinging atmospheric pressure plasma (APP) jet [3] was used to activate the PSf substrates using He, He/H 2 or He/O 2 plasmas generated at the same He flow rate. All plasmas were generated at  60 W. Graft polymerization of acrylic acid onto the activated surfaces was carried out at monomer concentration of 1e20 vol% over a period of up to 2 h at 70 C. The DI water rinsed PAA-PSf-Si samples were vacuum dried at 40 C prior to surface characterization. Tethered PAA layers were synthesized onto a plasma activated base PSf membrane having 100 kDa MW cutoff. Following graft polymerization, the PAA-PSf-Membrane samples were rinsed and stored in DI water prior to ultrafiltration fouling tests.

Surface characterization
Confirmation of the presence of tethered PAA chains on the PSf surface was obtained by X-ray photo electron spectroscopy (XPS). Survey spectra were obtained for both the native PSf-Si and PAA-PSf-Si surfaces at a pass energy of 160 eV. Contact angles for the different surfaces were measured by both the sessile drop (SD) and captive bubble (CB) methods. CB contact angle measurements were for sample immersed in water or saline water at 20e22 C [4e6] allowing for~30 min equilibration prior to taking measurements [7]. All reported contact angles are averages based on measurements at five different locations on each sample. The surface free energy (g s , expressed as the sum of dispersive (g d s ) and polar (g p s ) components, g s ¼ g d s þ g p s ) was determined from the average air and n-octane CB contact angles (q) following the method described in Refs. [8,9], which required the dispersive, polar, and total surface tension of liquids (i.e., water and n-octane) reported in Ref. [6].  Atomic force microscopy (AFM) (Bruker Dimension Icon Scanning Probe Microscope with a NanoScope V Controller, Bruker, Santa Barbara, CA) was utilized to obtain surface feature height data in PeakForce Tapping mode, under air, DI water and saline water (35 g/L NaCl), using ScanAsyst-Air and ScanAsyst-Fluid þ probes (Bruker AFM Probes, Camarillo, CA). Cantilever deflection sensitivity and spring constant were determined following the approaches specified in Refs. [10,11], respectively. All AFM scans were obtained for areas of 1 mm Â 1 mm at 512 Â 512-pixel resolution and 0.5e0.8 Hz scan rates using a relatively small loading force (i.e.,~500 pN).
AFM force spectroscopy (AFM-FS; [12e14]) was accomplished using a Bruker MultiMode 8-HR Scanning Probe Microscope with a PicoForce Spectroscopy Control Module (Bruker, Santa Barbara, CA) under DI water and 35 g/L NaCl solution. A silicon nitride probe of a nominal tip radius of 20 nm (MLCT-D, Bruker AFM Probes, Camarillo, CA) was used, and force measurements were taken in contact mode at 1 mm ramp size and 500 nm/s tip velocity. The maximum surface applied force prior to tip retraction (i.e., trigger force) was 1 nN, and the approach and retraction force-distance profiles were obtained from 200 randomly selected locations for each sample. The rupture force and rupture length distributions as well as the equilibrium thickness [1] were determined from the retraction force profiles.
Sectioned substrate images were obtained via Focused Ion Beam (FIB) e Scanning Electron Microscopy (SEM) (Nova 600 NanoLab DualBeam TM -SEM/FIB; FEI company, Hilsboro, OR) [15]. The dry PAA layer thickness was calculated as the difference between the film thickness above the silicon wafer substrate before and after graft polymerization (an example of the thickness analysis is illustrated in Fig. 13).

Ultrafiltration fouling tests
Membrane fouling tests for the PSf (10 kDa MWCO) and PAA-PSf membranes were performed in a dead-end filtration mode (at 20 C) using a small (50 mL) stirred cell system (MilliporeSigma, Burlington, MA) of 13.4 cm 2 membrane filtration area. Fouling challenge tests were conducted with a 1 g/L alginic acid solution at pH 8 and at pH 6 in high salinity water (35 g/L NaCl). All filtration tests were carried out at 22 L m À2 h À1 initial permeate flux which is well within range of permeate flux for UF seawater pretreatment [16].

Transparency document
Transparency document associated with this article can be found in the online version at https:// doi.org/10.1016/j.dib.2019.103728.