Novel thermo-sensitive membranes prepared by rapid bulk photo-grafting polymerization of N,N-diethylacrylamide onto the microfiltration membranes Nylon
Introduction
Recently, smart polymeric membranes attracted much attention, various types of stimuli-responsive membranes were prepared, such as temperature [1], [2], [3], [4], [5], pH [6], [7], [8], [9], [10], ionic strength [11], [12], light [13], [14] and multi-response [15], [16], [17]. Photo-grafting polymerization is one of the most effective methods to modify the surface of polymeric materials [18], [19], which is also available to prepare environment-sensitive membranes by grafting functional monomers onto porous polymeric backbones [20], [21]. One of grafting methods is bulk surface photo-grafting polymerization, regarded as a convenient and simple method to prepare well-defined membranes. In this process, the composite of monomer and initiator is 100% reactive with no solvent added. Bulk surface photo-grafting has the following advantages. The rate of the photo-grafting polymerization can reach high values due to the high concentration of the monomer and initiator with no solvent, so it can greatly shorten the irradiating time [22].
Poly(N-isopropylacrylamide) (PNIPAAm) has been extensively used as a monomer to prepare thermo-responsive membranes [23], [24], [25]. PNIPAAm exhibits its LCST near 32 °C and has a remarkable hydration–dehydration change in aqueous solutions in response to a relatively small change in temperature [26]. However, a problem with N-isopropylacrylamide (NIPAAm) is its solid state at room temperature that could not be blended sufficiently with common solid initiators such as benzophenone (BP) without solvents. This flaw of NIPAAm limits its use of bulk photo-grafting method.
In order to enhance the grafting rate and predigest the photo-grafting polymerization procedure, a substituted monomer is N,N-diethylacrylamide (DEAAm). DEAAm is in liquid state at room temperature and most of the initiators could be dissolved in it. Recently, attention has been paid to poly(N,N-diethylacrylamide) (PDEAAm) as an alternative thermo-sensitive water-soluble polymer to PNIPAAm [27], [28], [29]. PDEAAm belongs to a kind of polymer with similar thermo-sensitivity mechanism, containing hydrophilic amide groups and hydrophobic vinyl backbones and diethyl side groups in the main chain structure. This character leads to the inverse temperature solubility behavior of aqueous solution of PDEAAm, and the LCST of PDEAAm reported is near 33 °C. At low temperatures, the strong hydrogen bonding between the hydrophilic groups and water outweighs the unfavorable free energy related to the exposure of hydrophobic groups to water, leading to good solubility of the polymer in water. When temperature increases, hydrogen bonding weakens, while hydrophobic interactions between hydrophobic side groups strengthen. Above the LCST, interactions between hydrophobic groups become dominant, leading to an entropy-driven polymer collapse and phase separation [27], [28], [30], [31].
However, recent studies of thermo-induced phase transitions of PDEAAm have been focused on the field of hydrogel or solution. Polymer hydrogels respond slowly to environmental stimuli and the lower mechanical strength of hydrogels limits much of its applications [32].
The microfiltration membranes of Nylon have advantages at the aspects of mechanical strength, hydrophilicity, durable to organic solvents, which has been extensively used. However, few of studies on N-substituted polyacrylamide grafted onto Nylon membranes have been reported. The reason may be that N-substituted polyacrylamide has similar functional groups as Nylon, which results in the difficulty in characterization. In this study, thermo-sensitive membranes were prepared by bulk photo-grafting polymerization of DEAAm onto the microfiltration membranes of Nylon. Different membranes with a wide range of grafting yield were prepared by varying the photo-grafting conditions. Meanwhile, the characterization and the mechanism of the thermo-sensitive membranes were discussed, respectively.
Section snippets
Materials
Nylon 66 microfiltration membranes were supplied by Jinzheng Corp. (China), with an average pore size of 1.0 μm, a thickness of 30 μm. The membrane was extracted with acetone for 24 h, and then dried in vacuum at room temperature until constant weight. DEAAm was a colorless liquid (b.p. = 79–81 °C/78.660216 × 102 Pa (79–81 °C/59 mmHg)), prepared as described in Ref. [33] previously and 1H NMR spectra was measured on the Varian Unity 500 NMR (500 MHz, CDCl3, δ ppm: 6.5 (1H, CH−), 6.3 (1H, CH2), 5.7 (1H, CH2
Photo-grafting polymerization
The chemistry of BP initiated grafting on the microfiltration membranes of Nylon is similar as shown in Ref. [1]. Under UV irradiation, BP undergoes a photoreduction by reacting with a hydrogen donor, substrate Nylon membranes. The surface free radicals and the semipinacol radicals were generated. The surface free radicals initiate the grafting polymerization of DEAAm, while the semipinacol radicals initiate homo-polymerization. The grafting dominates the polymerization process owing to the
Conclusion
Bulk surface photo-grafting polymerization is an available, rapid and simple method to prepare well-defined membranes. The grafting yield of the membranes could be controlled through varying the grafting conditions, including UV intensity, the irradiation time, the content of initiator. The grafted polymers exist mainly on the top surface and inside the pores of the membranes rather than the backside. The grafted PDEAAm chains could act as a sensor of the temperature regulating the
Acknowledgements
Financially supports from the National Nature Science Foundation of China (NNSFC project Nos.: 20274023 and 20474036) and Guangdong Province Nature Science Foundation (GPNSF project No.: 021241) are gratefully acknowledged.
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