Surface Functionalization of IIR/PIB 20

The introduction of chemical functionalities through surface modification has been of great interest as it enables the desirable properties of a bulk material to be retained, while tuning the properties of the surface for a specific application. For example, the functionalization of surfaces with PEO can result in properties such as resistance to protein adsorption and the attachment and growth of cells that are critical for many biomedical applications.80

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Figure 1.2. Fluorescence confocal microscopy images (543 nm) of thin films (spin-cast at 20 mg/mL from CH2Cl2) following adsorption of a rhodamine-fibrinogen conjugate.

Images represent different wt% of PEO grafted on IIR backbone: a) 2%, b) 4%, c) 6%, d) 12%, e) 24%, and f) 34%. (Macromolecules, 2011, 44, 6405-6415, Reproduced with permission from the American Chemical Society).

Several approaches have been applied with the aim of functionalizing surfaces with PEO. Among all of them, a simple and effective one-step method was developed recently for the attachment of PEO onto non-functional polymer surfaces. This approach, developed by Lau and coworkers is known as hyperthermal hydrogen induced cross- linking (HHIC).81

As an attractive alternative to conventional plasma treatments,82 HHIC is an alternative way of cross-linking polymers to surfaces using the concept of collision kinematics. HHIC is a fast and efficient process for selectively cleaving the C-H bonds of organic compounds while preserving other chemical functionalities of the molecules on the surface. In this process, H2 is used as a light-mass projectile to collide with H of a

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C-H bond of an organic molecule on a surface; this collision very effectively knocks off hydrogen atoms from the molecule but leaves the molecule otherwise intact. The, carbon radicals produced can then recombine and the precursor organic molecules are cross- linked to each other and to any surface that contains C-H bonds. This C-H cleavage method has low energy-consumption and no reliance on any other chemical reagents. The development of HHIC has led to tailor-made surface modifications such as grafting molecules with specific functional groups onto different surfaces. A practical reactor providing a high flux of such hyperthermal hydrogen for complete surface grafting in a very short time has recently been developed (Figure 1.3).83

Figure 1.3. Photo of HHIC instrument.

Gillies and coworkers have demonstrated the utility of HHIC in the preparation of IIR-based surfaces exhibiting non-fouling properties.84 In this work, IIR and epoxidized IIR were spin cast and cross-linked via HHIC. Then, the cross-linked surfaces were spin coated with PEO followed by cross-linking by HHIC. The resulting surfaces were immersed in a fluorescently labeled fibrinogen. After washing off the nonadsorbed protein, the protein adsorption and cell growth were evaluated on the surfaces. As shown in Figure 1.4a, IIR was a good substrate for cell growth. On the other hand, upon coating IIR first with epoxidized IIR as an interfacial layer, followed by PEO and HHIC, a

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significant reduction in the number of cells was observed. This was attributed to the resistance of the PEO coated surfaces to protein adsorption, which is thought to be the first step in the adhesion of cells to a surface. Figure 1.4b shows the relative fluorescence values obtained by confocal microscopy. It can be concluded that HHIC is a promising technique for the preparation of protein- and cell-resistant properties from a diverse array of unreactive hydrophilic and hydrophobic surfaces containing C-H bonds.

Figure 1.4. a) Evaluation of cell growth on surfaces: 1a) IIR, 1b) epoxidized IIR coated with PEO, 1c) control surface of silane-functionalized PEO grafted on glass, 1d) PEO- coated silicon wafer following HHIC. b) Relative fluorescence obtained by confocal microscopy; 2a) IIR, 2b) epoxidized IIR, 2c) epoxidized IIR coated with PEO, 2d) PEO on clean silicon wafer, 2e) control surface of silane functionalized PEO grafted on glass (0.01 g/cm2). Error bars represent the standard deviation of 10 measurements on each of 3 samples.84

(Applied Materials & Interfaces, 2011, 3, 1740-1748, Reproduced with

permission from the American Chemical Society).

Functional polymer surfaces composed of polypropylene, IIR, and poly(vinyl acetate) (PVAc) were also prepared using HHIC.85 _{While many properties of IIR are}

advantageous, it would be useful to reduce its hydrophobicity for some applications.

Biaxially oriented polypropylene (BOPP) was used as a model substrate. Then, IIR was chosen as the next layer and PVAc was selected as the third layer as it can subsequently be hydrolyzed to poly(vinyl alcohol) (PVA). Furthermore, the choice of PVAc enables for the evaluation of the functional group tolerance of HHIC. Upon cross-linking each

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layer, the ester functionalities were hydrolyzed to convert the surface from hydrophobic to hydrophilic (Figure 1.5). This process demonstrated that multiple layers of cross- linked materials could be added, creating polymer laminates with each layer introducing new functionalities and properties.

Figure 1.5. a) Preparation of laminates using HHIC, b) chemical transformations of the PVAc layer.85 (Langmuir, 2011, 27, 14820-14827, Reproduced with permission from the American Chemical Society).