# pH-Responsive Hollow Polymeric Microspheres from Irradiated Cyclic Ether Aqueous Solution
###### Abstract  
Smart hollow polymeric microspheres have been widely applied in various fields such as controlled release, drug delivery, catalysis, and so on. Herein, a facile, green and one-step template-free method is introduced for preparing pH-responsive hollow polymeric microspheres via gamma irradiation of cyclic ether aqueous solution. The hollow polymeric microspheres are synthesized by radiation-induced polymerization and following the self-assembly and self-organization of amphiphilic polymer with cyclic ethers as monomers in water. SEM, TEM, micro-FTIR, and NMR confirmed the morphology and structures of the resultant microspheres. The confocal laser scanning microscope was used to investigate the stimuli-responsiveness and release behavior of hollow microspheres using 1-pyrene carboxaldehyde as a hydrophobic molecule model. The well-defined hollow polymeric microspheres with an average diameter of ca. 2.6 \(\upmu\)m or 1.6 \(\upmu\)m were prepared directly from dicyclohexal-18-crown-6 or tetrahydropyrane aqueous solution, respectively. The prepared hollow microspheres exhibit obvious pH stimuli-responsiveness and can release the encapsulated hydrophobic molecules when pH is higher than 5.0. Moreover, the reversible morphology transition between hollow microspheres and micelles makes the prepared hollow polymeric microspheres potentially suitable for a wide range of applications, including removal of dyes, oil field engineering, and biomedical fields. \(\gamma\)-irradiation; cyclic ethers; stimuli-responsiveness; hollow microspheres +
Footnote †: journal: Applied Sciences  
15 September 2021  
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**1. Introduction**  
Stimuli-responsive hollow particles, especially hollow micro-sized or nano-sized spheres, continue to attract major interest because they have wide potential applications in fields such as dye precipitation [1], catalysis [2, 3], sensors [4, 5, 6], and especially controlled delivery systems [7, 8, 9, 10]. Early studies on hollow polymeric microspheres were conducted by [PERSON] et al. [11]. Up to today, many'smart' hollow polymeric microspheres have been prepared via various methods such as the template method [11], the microencapsulation method [12], and the self-assembly layer-by-layer method [13], which are usually based on pre-prepared stimuli-responsive copolymers. Though having the advantage of controlling the supramolecular architectures and functions by constructing copolymer structures, these methods commonly require multi-step and complex procedures to synthesize the stimuli-responsive copolymers. Moreover, the complicated bottom-up processes and scrupulous core-removing techniques are also required [14, 15]. All of these would restrict the practical applications of those methods in fabricating'smart' hollow microspheres. As a facile and efficient route to generate novel polymeric materials, \(\gamma\)-irradiation has attracted increasing interest since the preparation can be carried out without any initiators or catalysts under mild conditions [16, 17, 18]. Compared with chemical-initiated polymerization, radiation-induced polymerization allows decoupling initiation kinetics from theeffects of temperature on the induced chemical reactions, particularly on chain propagation and transfer reactions for polymer synthesis. This makes it possible to follow polymerization kinetics under stationary and various experimental conditions, thus reducing the risks of chain branching and the products with a high purity could be obtained easily [18]. Gamma irradiation has enough energy to ionize atoms and molecules and could provide high penetration depth in matter. Therefore, gamma irradiation has been widely used in the synthesis of hydrogels [16], nanocomposites [17], and so on.