Fluorinated Polymer Films from Acylation of ATRP Surface-Initiated Poly(hydroxyethyl methacrylate)

Abstract

We report a new strategy to prepare partially fluorinated polymer films by utilizing surface-initiated films of poly(2-hydroxyethyl methacrylate) (PHEMA) grown onto gold surfaces via atom transfer radical polymerization (ATRP). Hydroxyl side chains of PHEMA were reacted with perfluoroalkyl (C3F7 and C7F15) and perfluoroaryl (C6F5) acid chlorides to yield partially fluorinated surface-initiated polymer films with conversions ranging from ∼70 to 85%. The resulting fluorinated PHEMA films dramatically altered surface and barrier properties, particularly lowering the critical surface tension to as low as 9 mN/m and boosting film resistance by as much as 5 orders of magnitude compared to that of PHEMA. Film properties depend on the chemical composition and length of the fluorinated side chains. The longer C7F15 fluoroalkyl chain structured the film to a greater extent than the shorter C3F7 chain and yielded improved barrier properties and lower critical surface tension. The C7F15 groups are oriented nearly normal to the surface at the air−film interface and predominantly parallel to the surface in the bulk. Modification of PHEMA with the fluoroaryl side chain produced the best barrier properties of the three side chains investigated.

Physicochemical Properties of Surface‐Initiated Polymer Films in the Modification and Processing of Materials

“…The wettability of these films, also shown in Scheme 1 via advancing contact angles of water and hexadecane, depended on the composition and chain length of the side group. For m = 7, [69] the partially fluorinated film exhibited an advancing hexadecane contact angle of 79, similar to that observed for SAMs containing densely packed, homogeneous CF 3 surfaces. [75] The critical surface energy of this partially fluorinated film is 9 mN m ±1 , which is remarkably lower than that of polytetrafluoroethylene (18 mN m ±1 ), a pure fluorocarbon polymer.…”

“…[64] Low-energy surfaces are commonly generated by preparing thin films in which relatively non-polar groups such as ±CF 3 or ±CH 3 dominate the surface composition. We have recently used surface-initiated ATRP to grow poly(hydroxyethyl methacrylate) (PHEMA) films on gold that were subsequently modified by acylation with acid chlorides (RCOCl) of hydrocarbon (R = C p H 2p+1 ) [71] or fluorocarbon (R = C m F 2m+1 ; R = C 6 F 5 ) [69,70] composition to produce ester-linked side groups (Scheme 1). The wettability of these films, also shown in Scheme 1 via advancing contact angles of water and hexadecane, depended on the composition and chain length of the side group.…”

“…The low critical surface tension and high hexadecane contact angle for this film are consistent with a normal orientation of the fluorocarbon chains at the outer surface, such that the terminal CF 3 group dominates surface properties. [69] With hydrocarbon modification of PHEMA, [71] the advancing hexadecane contact angle increases with chain length and approaches the value for a densely packed methyl-terminated SAM (~50). [76] The critical surface tension of these fluorocarbon-and hydrocarbon-modified films can be tuned from 9±27 mN m ±1 , depending on the selection of R. These results demonstrate the importance of molecular engineering of polymer films to tailor surface composition, and show that surface-initiated polymer films can present densely packed surfaces that approach the wettability behavior of monolayer films.…”

“…Of particular importance is the penetration of water and ions into coatings, for which electrochemical impedance spectroscopy is an invaluable method of characterization. As shown in Scheme 1, we have modified ATRP, surface-initiated PHEMA films with fluorocarbon [69] and hydrocarbon [71] acid chlorides to investigate the effect of film composition on surface wettabilities (described above) and impedance properties, such as film resistance and capacitance (Table 2). In general, modification of a 250 nm thick PHEMA film leads to dramatic increases in film resistance by four to five orders of magnitude and reductions in film capacitance by two orders of magnitude.…”

Formation of surface-grafted polymeric amphiphilic coatings comprising ethylene glycol and fluorinated groups and their response to protein adsorption

“…[21][22][23][24][25] Three different acylchlorides were employed ͑all obtained from Sigma-Aldrich͒: heptafluorobutyryl chloride ͑C 3 F 7 COCl, F3͒, pentadecafluoro-octanoyl chloride ͑C 7 F 15 COCl, F7͒, and pentafluorobenzoyl chloride ͑C 6 F 5 COCl, PFA͒. Substrates with PHEMA brushes were exposed to 80 mM solutions of a given acylchloride with 100 mM pyridine in dichloromethane for 24 h at room temperature to modify PHEMA films with fluorinated side chains ͑see Fig.…”