Radiation grafting of pH-sensitive acrylic acid and 4-vinyl pyridine onto nylon-6 using one- and two-step methods

Highlights

• A new binary graft of 4VP and AAc onto Ny₆ films was synthesized by γ-radiation.

• The binary grafted material has potential application for heavy ion retention.

• The two-step method shows better conditions in swelling and reversibility properties.

• Surface distribution of monomers was evaluate by XPS characterization.

Abstract

Acrylic acid (AAc) and 4-vinyl pyridine (4VP) were γ-ray grafted onto nylon-6 (Ny₆) films via pre-irradiation oxidative method. These monomers were grafted using a one-step method to render Ny₆-g–(AAc/4VP). A two-step or sequential method was used to render (Ny₆-g–AAc)-g–4VP. Random copolymer branches were obtained when the grafting was carried out via one-step method using the two monomers together. The two-step method was applied to graft chains of 4VP on both Ny₆ substrate and previously grafted AAc chains (Ny₆-g–AAc). The two types of binary copolymers synthesized were characterized to determine the amount of grafted polymers, the thermal behavior (DSC and TGA), the surface composition (XPS), and the pH responsiveness. In the two-step process, it is possible to achieve a higher graft yield, better control of the amount of each monomer, good reversibility in the swelling/deswelling process and shorter time to achieve equilibrium swelling.

Introduction

Nylon-6 (Ny₆) has attracted considerable interest due to its physicochemical and mechanical properties, especially its heat resistance. In addition, Ny₆ is a material with moderate water uptake capacity (9.5 wt%), which limits its application in the environmental decontamination and biomedical fields. Therefore, grafting reactions of Ny₆ with hydrophilic monomers have been reported to improve its water absorption and retention (Nayak, 1979, Takigami et al., 1984). Other reports have described grafting of polymer materials with stimulus-responsive monomers/polymers that exhibit relatively abrupt and large physical or chemical changes in response to low-intensity stimuli, such as temperature, pH, and ionic strength (Byung et al., 2003, Bucio et al., 2005, Liu and Urban, 2010). These species possess certain functional groups, such as amides, amines, carboxylic acids or epoxy groups, that can be grafted on the surface or in the bulk matrix by applying different chemical methods (Siow et al., 2006, Wang et al., 2006, Schmidt and Schmidt-Naake, 2007, Truica-Marasescu et al., 2007). Among the suitable techniques, ionizing radiation, ultraviolet light, plasma and chemical initiators are useful approaches for grafting polymers (Bucio and Burillo, 1996, Bucio et al., 2001, Burillo et al., 2006, Chen et al., 2007). Gamma radiation (γ) grafting is an attractive method for obtaining a modified material without chemical initiators or other additives that is applicable to nearly all polymer–monomer combinations (Clough, 2001). Taher et al. (2000) studied γ-radiation grafting of AAc and acrylamide (AAm) onto Ny₆ for separation of radioactive europium in wastewater. Other authors have grafted a combination of these monomers onto Ny₆ for use as cation-exchange membranes in different practical applications, especially for the separation of heavy metals in water. However, anionic/cationic membranes obtained by radiation grafting onto low-density polyethylene (LDPE) are more effective for the retention of heavy ions than the cationic ones (El-Sayed et al., 1997, El-Sayed et al., 2000). According to previous reports, binary grafting of AAc and 4-vinyl pyridine (4VP) onto Ny₆ is an alternative that is effective for heavy ion uptake.

The aim of this study was to synthesize pH-sensitive binary grafting copolymers containing AAc/4VP grafted onto Ny₆ with gamma radiation using two different pre-irradiation oxidative methods (i.e., one- and two-step methods). The one-step method is one of the more practical approaches for producing random copolymers. However, block copolymers are often prepared following a sequential method, such as the two-step approach. The effect of the swelling capacity, pH sensitivity and reversibility on the swelling/deswelling behavior was evaluated for all of the synthesized materials. The Ny₆ films modified with AAc/4VP have the potential for application as cation-exchange membranes for the separation of selected heavy metals in wastewater.