Nylon surface modification. Part 1. Targeting the amide groups for selective introduction of reactive functionalities
Introduction
Nylon was one of the first commercialized polymers and more than 3.25 billion pounds of nylon are produced annually in the United States, most of which is nylon 6/6 [1]. Nylon 6/6 is a semicrystalline material that has a combination of strength, flexibility, toughness, and abrasion resistance. It is also known for its dye-ability, low coefficient of friction (self-lubricating), low creep, and resistance to solvents, oils, bases, fungi, and body fluids [1], [2]. The applications for nylon 6/6 include textile fibers, membranes, tapes, food packaging, electronics, and automotive parts. To further improve nylon's performance, it is necessary to introduce specific functional groups on its surface in pre-determined locations, densities, and patterns. For example, selective introduction of amine groups to nylon surfaces is likely to open up new possibilities for nylon because amine-enriched surfaces play an important role in processes such as the removal of heavy metal ions from aqueous solutions [3], for biofouling prevention [4], and for the covalent immobilization of biomolecules such as DNA and polysaccharides [5], [6]. On the other hand, when nylon films are used for food packaging, low gas permeability and a high barrier to water vapor are desirable properties. Since nylon readily absorbs water from air, we speculate that deposition of a dense silicon dioxide layer that is made hydrophobic by further silane chemistry may reduce the amount of water absorbed.
Nylon surfaces have been modified by physical and chemical methods. The first category includes activation of the surfaces through treatment with UV radiation [6], plasma activation [7] or plasma deposition [8]. While these methods are inherently clean, the disadvantages are that chemically well-defined surfaces cannot be designed and prepared and often, functionalization is accompanied by surface destruction [7c]. On the other hand, chemical surface modification of nylon surfaces has been accomplished by reaction at the terminal amine or carboxylic acid groups [9], or at the repeating amide group [10]. Since the concentration of terminal amine groups on the surface is low (0.011 amine ends/nm2) [10a], high coverage is achieved only when the molecular size and the number of reactive sites of the grafted species are high. Problems associated with this modification method include inhomogeneous distribution of functional groups, uncertainty of chain orientation at the surface, and instability of the grafted layers. A more efficient modification approach is by reaction at the amide groups through hydrolysis, O-alkylation and N-alkylation. Partial hydrolysis of the amide results in an increased concentration of amine and carboxylic acid groups, which have been used for the immobilization of ligands and enzymes [10c]. Examples of N-alkylation include cross-linking of nylon 6/6 yarns by reaction with diisocyanates and diacid chlorides [10a], and activation of the amide with formaldehyde to generate hydroxyl groups that can further be used to immobilize polysaccharides [9b] or covalently bind biocidal moieties [10b].
In this report, we describe the chemical surface modification of nylon films. The reactions employed were targeted at the naturally abundant amide functionality rather than the more reactive amine end groups. Several modification methods were evaluated using nylon 6/6, with the goal of obtaining surfaces with enriched reactive functionalities. N-alkylation with 2-bromoethylamine hydrobromide (BEA–HBr) gave rise to surfaces with a mixture of primary/secondary/tertiary amine groups. Ethoxylation of the activated amides with (3-glycidoxypropyl)triethoxysilane (GPTES) were utilized to prepare surfaces with silica-like reactivity. Reduction with BH3-THF complex proves to be a highly efficient method to introduce surface amine groups, and in-depth kinetics studies were carried out using different types of nylon films including nylon 6/6, nylon 4/6 and nylon 6/12. The surface modifications were monitored by contact angle, X-ray photoelectron spectroscopy (XPS), attenuated total reflectance infrared spectroscopy (ATR-IR) and atomic force microscopy (AFM).
Section snippets
Materials
Nylon 6/6 films, Dartek® C-101 (100 μm), were obtained from DuPont Canada. Nylon 4/6 and nylon 6/12 were purchased from Aldrich as pellets. Sodium hydroxide and solvents used for washing (HPLC grade) were obtained from VWR Scientific. (3-glycidoxypropyl)triethoxysilane (GPTES) was obtained from Gelest. Borane–tetrahydrofuran complex solution (BH3-THF, 1.0 M in THF), concentrated HCl, triethylamine (TEA, anhydrous), tetrahydrofuran (THF, anhydrous), potassium tert-butoxide (t-BuOK),
Results and discussion
Dartek® C-101 is a cast film made from nylon 6/6 for a wide range of industrial applications. One side of the film contains a slip agent (glass beads), and XPS analysis of this side indicates silicon atomic concentration of 3.79 and 1.13% at 15 and 75° take-off angles, respectively. The other side of the film, however, exhibits no contamination from the slip agent, and therefore, was used for XPS and contact angle analysis. Subsequent handling and modification was done ensuring that the slip
Summary
Several techniques were developed to introduce reactive functional groups to nylon surfaces using reactions that target at the naturally abundant amide groups. Activation of amides with potassium tert-butoxide facilitates the N-alkylation of surface amides. When 2-bromoethylamine was employed as the alkylation reagent, surfaces with branched oligmeric ethyleneimine were obtained. Alkylation with (3-glycidoxypropyl) triethoxysilane generated surfaces that contain poly (ethylene glycol) with
Acknowledgements
We thank the Office of Naval Research and the NSF-sponsored Materials Research Science and Engineering Center at University of Massachusetts at Amherst for financial support.
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Present address: Department of Chemical Engineering, Engineering Quadrangle Princeton university, Princeton, NJ 08544, USA.