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.

This article reviews the physicochemical aspects of surface‐initiated polymer films used to modify planar and non‐planar surfaces and to produce micro‐ and nanoscale patterned features. Particular emphasis is placed on the molecular composition of the polymer and its effect on surface and bulk properties of ultrathin films. Recent advances in the use of responsive polymer films that exhibit dramatically altered properties upon changes in solvent, temperature, or ionic strength are reviewed. The uses of surface‐initiated polymer films to modify materials' properties and impact applications in chromatography, nanoparticle‐templated synthesis, and carbon nanotube dispersion are highlighted.

Poly(2-hydroxyethyl methacrylate) (PHEMA) films were grown onto gold via water-accelerated, surfaceinitiated atom transfer radical polymerization (ATRP), and the resulting side chains were modified by reaction with alkanoyl chlorides (C m H 2m+1 COCl; m ) 1, 7, 11, 13, 15, and 17) to incorporate hydrocarbon side groups within the film. We have previously demonstrated the ability to react ∼70-80% of PHEMA hydroxyl side chains with fluorocarbon acid chlorides to prepare partially fluorinated films. Here we convert the side chains to hydrocarbon esters with diminishing conversion (80 to 40%) as m is increased and compare the resulting films with fluorocarbon-modified PHEMA. Based on IR spectra and wetting data, hydrocarbon side chains structure the film to a greater extent as m is increased. A critical chain length (m ) 15) was required to orient chains normal to the interface at the outer film surface and impart the wetting properties of a dense methyl surface. The resistances of the films against the transport of redox probes were greatly enhanced with increasing conversion of hydroxyl groups within the film and were modestly affected by film structuring to create a densely packed methyl surface. For example, capping the hydroxyl groups of PHEMA by reaction with acetyl chloride resulted in an unstructured film with >90% conversion that increased film resistance by almost four orders of magnitude over the base PHEMA film without a significant volumetric enhancement of the side chains. Also, the decreasing conversion as m is increased from 7 to 17 resulted in a decreasing film resistance even though the longer chains structured the film and surface to a greater extent. These results illustrate the important effect on barrier properties of unreacted hydroxyl groups, which possibly associate to create water and ion-diffusing pathways within the film. The barrier properties of these hydrocarbon-modified PHEMA films are comparable to those of fluorocarbon-modified PHEMA when both conversion and the molecular weight of the modifying group are similar.

Acylation of a surface-initiated poly(hydroxyethyl methacrylate) (PHEMA) film with a perfluoroalkyl acid chloride (C 7 F 15 COCl) results in a highly blocking film with fluorocarbon side groups that orient at the airfilm interface to produce an ultralow-energy surface. The kinetics of the acylation step was monitored with reflectance-absorption infrared spectroscopy to reveal that the rate of fluorination depends on the chain length and solution concentration of the perfluorinated acid chloride as well as the solvent used. By control of the time of exposure to C 7 F 15 COCl, films with fractional conversion of the hydroxyl groups to fluorinated esters between 0 and 0.8 were produced. Increasing fractional conversion from 0 to ∼0.2 has a dramatic effect on surface wettability and electrochemical barrier properties as a densely packed, oriented fluorocarbon surface region is formed that greatly reduces water and ion transport. Additional fluorination has no effect on wettability but gradually increases film resistance while lowering film capacitance. These results suggest that structuring of fluorocarbon groups at the outermost surface of the film has a major impact on film properties and establish the minimum content of fluorocarbon groups required to achieve the structured interface via this grafting process.

We report a new method to prepare ultrathin blocklike copolymer films on metal surfaces with molecularly optimized surface and barrier properties. Copolymer films containing various fluorocarbon and/or hydrocarbon side chains were created by a one-step surface-initiated polymerization of poly-(hydroxyethyl methacrylate) (PHEMA) followed by straightforward derivatization steps. Exposure of PHEMA to perfluorobenzoyl chloride results in a perfluoroaryl-modified PHEMA film that exhibits high conversion and outstanding barrier properties but does not present an oleophobic surface. We have previously demonstrated that fluorinated esters created in this manner may be hydrolyzed back to PHEMA by brief exposure to base. Controlled hydrolysis results in regeneration of PHEMA in the outer surface region that can be subsequently rederivatized with alkyl or fluoroalkyl acid chlorides to create copolymer films with tailored surface composition. Surface properties are solely affected by the species used during rederivatization while barrier properties result from the combined conversion, structuring, and surface properties of the copolymer film.

Size exclusion chromatography and matrix-assisted laser desorption ionization mass spectrometry (SEC/MALDI) coupled with selective degradation reactions have been applied for characterization of polyurethane soft and hard blocks. A series of model PUR's were prepared from 4,4‘-diphenylmethane diisocyanate (MDI) and poly(butylene adipate) (pBA)−polyols with molecular weights of 1000 and 4000 Da. The weight ratio of the pBA polyols was varied:  1:3, 1:1, and 3:1; the amount of MDI was adjusted accordingly. In these model PUR systems no additional chain extender was added in addition to that in the polyester soft segments (butanediol), as a consequence their Flory distribution was used. Therefore, the model systems only have a minimum of so-called hard segments (oligo urethanes consisting of MDI and butanediol). Molecular weights of soft blocks, liberated by isocyanatolysis using phenyl isocyanate and measured by SEC/MALDI, showed reasonable agreement with those estimated from tandem light scattering and SEC (MALS/SEC). The increase in molecular weights observed with increasing amounts of pBA4000 indicated that selective degradation combined with SEC/MALDI is sensitive to the polymer soft block composition. Polydispersity indices (PDs), determined for the soft blocks recovered from phenyl isocyanate degradation, were lower than those expected on the basis of reaction theory. Partial acid-catalyzed hydrolysis was applied to determine the hard block chain length distribution for polyester-based PUR samples having different amounts of MDI. MALDI spectra of the degraded products provided proof for a degradation mechanism proposed in the literature. The results presented here demonstrate that applying partial acid hydrolysis to polyester−polyurethane generates exclusively a series of hydroxy-terminated oligomers, which can be identified as former hard segments of the polyester−polyurethane elastomer. The methodology hydrolyzes selectively all ester bonds while leaving the urethane groups containing hard segments completely intact, thus providing an additional tool for the complete characterization of polyurethanes.