Polymers have a vast number of advantages and attractiveness as a material system. However, despite these plus factors, polymers have limitations. In general, special surface properties with regard to chemical composition, hydrophilicity, roughness, crystallinity, conductivity, lubricity, and cross-linking density are required for successful application of polymers in such wide fields as adhesion, membrane filtration, coatings, friction and wear, composites, microelectronic devices, thin-film technology and biomaterials, and so on. Unfortunately, polymers very often do not possess the surface properties needed for these applications. In fact, polymers that are mechanically strong, chemically stable, and easy to process usually will have inert surfaces both chemically and biologically. Vice versa, those polymers having active surfaces usually do not possess excellent mechanical properties which are critical for their successful application.
Due to this dilemma, surface modification of the polymers without changing the bulk properties has been a classical research topic for many years, and is still receiving extensive studies as new applications of polymeric materials emerge, especially in the fields of biotechnology, bioengineering, and most recently in nanotechnology.
The main purpose of surface modification is to alter surfaces by either chemically or physically altering the atoms/molecules in the existing surface (treatment, etching, chemical modification), changing the surface topography or coating over the existing surface with a material having a new composition (solvent coating or thin film deposition by chemical vapor deposition, radiation grafting, chemical grafting, or RF-plasmas) [19,20]. There are a few factors to consider when modifying a surface [21-24]:
1. Thickness of the surface is crucial. Thin surface modifications are desirable, otherwise mechanical and functional properties of the material will be altered. This is more so when dealing with nanofibers as there is less bulk material present.
2. Sufficient atomic or molecular mobility must exist for surface changes to occur in reasonable periods of time. The driving force for the surface changes is the minimization of the interfacial energy.
3. Stability of the altered surface is essential, achieved by preventing any reversible reaction. This can be done by cross-linking and/or incorporating bulky groups to prevent surface structures from moving.
4. In some cases a transparent scaffold is desired, especially in optical sensors or ophthalmology; after surface treatment they should remain transparent. Any cloudiness introduced is of real concern.
5. Uniformity, reproducibility, stability, process control, speed, and reasonable cost should be considered in the overall process of surface modification. The ability to achieve uniform surface treatment of complex shapes and geometries can be essential for sensor and biomedical applications.
6. Precise control over functional groups. This is a challenging yet difficult scope. Many functional groups might bond to the surface such as hydroxyl, ether, carbonyl, carboxyl, and carbonate groups, instead of one desired functional group.
Common surface modification techniques used on polymer substrates include treatments by blending, coating, surface segregation, layer by layer electrostatic interaction, radiation of electromagnetic waves, electron beam, ion beam [25,26] or atom beams , corona or plasma treatment [28-30], chemical vapor deposition (CVD), gas oxidation, metallization, chemical modifications using wet-treatment and surface grafting polymerization [31,32], and so on. In recent years, many advances have been made in developing surface treatments to alter the chemical and physical properties of polymer surfaces and progress in recent years has been summarized by many reviews [19,33,34].
To date, no specific method of surface modification has been developed for nanofibers. In principle, all the current techniques mentioned above may be considered for the surface modification of polymer nanofibers. Nevertheless, they have to be carried out under moderate conditions for nanofibers. Of the above-mentioned techniques, only blending, grafting, radiation, and plasma treatment have been utilized to date on nanofibers with success.
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