Silica/poly(N,N′-methylenebisacrylamide) composite materials by encapsulation based on a hydrogen-bonding interaction

Abstract

Monodisperse silica/poly(N,N′-methylenebisacrylamide) core–shell composite materials with silica as core and poly(N,N′-methylenebisacrylamide) (PMBAAm) as shell were prepared by a two-stage reaction, in which the silica core with diameter of 500 nm was synthesized in the first stage according to Stöber method. The PMBAAm shell was then encapsulated over the silica core by distillation–precipitation polymerization of N,N′-methylenebisacrylamide (MBAAm) in neat acetonitrile with 2,2′-azobisisobutyronitrile (AIBN) as initiator. The encapsulation of PMBAAm on silica particles was driven by the hydrogen-bonding interaction between the hydroxyl group on the surface of silica core and the amide unit of PMBAAm during the polymerization without modification of the silica surface in the absence of any stabilizer or surfactant. The shell thickness of the core–shell composite particle was controlled via altering the mass ratio of MBAAm monomer to silica core during the polymerization. Hollow PMBAAm microsphere was further developed after removal of silica core with hydrofluoric acid. The resultant core–shell composite and hollow microspheres were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier-transform infrared spectra (FT-IR) and elemental analysis (EA).

Introduction

In recent years, the combination of the properties of inorganic and organic building blocks within a single material has attracted rapidly expanding interest for material scientists because of the possibility to combine the various functional groups of organic components with the advantages of a thermally stable and robust inorganic substrate [1], [2]. These composite materials can exhibit novel and excellent properties, such as mechanical, chemical, electrical, rheological, magnetic, optical and catalytic, by varying the compositions, dimensions, and structures of the core and shell, which have promised diverse applications as drug delivery system, diagnostics, coatings, and catalysis [3], [4], [5], [6], [7], [8], [9].

The silica/polymer hybrid particles with various interesting morphologies, such as silica core/organic shell [10], organic core/silica shell [11], raspberry-like [12], snowman-like [13], daisy-shaped and multipod-like [14], and raisinbun-like [15], have been prepared by different methods. The synthesis of the silica/polymer hybrid particles can be generally classified as two categories: the self-assembly of the resultant silica and polymer particles via physical or physicochemical interaction, and the direct polymerization of monomer on the surface of silica particles. Kulbaba et al. [16], [17] assembled positively charged polyferrocenylsilane microspheres with negatively charged silica particles through the electrostatic forces. Layer-by-layer [18], [19], [20], [21], [22] deposition technique by sub-sequential adsorption of polyelectrolytes (PEs) with opposite charges onto silica particles via the electrostatic interaction has been reported for the preparation of a range of polymer-core/inorganic-shell particles. Fleming et al. [8] fabricated raspberry-like composites from silica microspheres and polystyrene nanospheres by either the reaction of amine and aldehyde groups or the biochemical interaction between avidin and biotin. Bourgeat-Lami et al. [23], [24], [25] synthesized silica/organic hybrid particles with silica as seeds by dispersion polymerization, in which the vinyl group was introduced by 3-(trimethoxysilyl)propyl methacrylate. The hedgehog-like or raspberry-like hybrid particles were prepared by miniemulsion polymerization [26], in which the silica nanoparticles acted as surfactants and fillers. Bourgeat-Lami et al. [27] synthesized silica/poly(methyl methacrylate) nanocomposite particles by emulsion polymerization with a cationic initiator 2,2′-azobis(isobutyraidine) dihydrochloride (AIBA·2HCl) in the presence of a nonionic polyoxyethelenic surfactant (NP30). However, it was difficult to control the morphology of the resultant silica/polymer hybrid particles and the encapsulation efficiencies of the polymer on the silica core were much low for both dispersion polymerization and emulsion polymerization. Surface-initiated atom transfer radical polymerization (ATRP) has been widely utilized to prepare well-defined silica/polymer hybrids with the initiator-modified silica particles as macroinitiators [28], [29], [30], [31], in which the synthesis was tedious with long reaction time and low conversion of monomer to polymer.

We have previously reported distillation–precipitation polymerization as a novel and powerful technique to prepare monodisperse poly(divinylbenzene) (polyDVB) [32], poly(ethyleneglycol dimethacrylate) (polyEGDMA) [33], poly(N,N′-methylenebisacrylamide) (polyMBAAm) [34] and other polymer microspheres with various functional groups [35], [36], [37]. In the present work, monodisperse silica/polyMBAAm core–shell composite materials were prepared by distillation–precipitation polymerization with silica particles as seeds in the presence of 2,2′-azobisisobutyronitrile (AIBN) as initiator in neat acetonitrile, in which polyMBAAm was encapsulated onto silica particles with the aid of the hydrogen-bonding interaction between the hydroxyl group on the surface of silica and the amide unit of polyMBAAm. Furthermore, hollow polyMBAAm microspheres were developed after removal of the silica cores by hydrofluoric acid.