Elsevier

Polymer

Volume 50, Issue 25, 27 November 2009, Pages 5933-5939
Polymer

Single-molecular hybrid nano-cylinders: Attaching polyhedral oligomeric silsesquioxane covalently to poly(glycidyl methacrylate) cylindrical brushes

https://doi.org/10.1016/j.polymer.2009.10.029Get rights and content

Abstract

We present the preparation of novel single-molecular hybrid nano-cylinders by covalently attaching a monothiol-functionalized polyhedral silsesquioxane (POSS-SH) to poly(glycidyl methacrylate) (PGMA) cylindrical brushes. Grafting of GMA from a long poly-initiator poly(2-(2-bromoisobutyryloxy)ethyl methacrylate) (PBIEM) via ATRP was first carried out. Gel permeation chromatography (GPC), 1H NMR, dynamic light scattering (DLS), static light scattering (SLS) and atomic force microscopy (AFM) measurements confirmed the well-defined worm-like structures of the PGMA brushes. Then POSS-SH was covalently linked to PGMA brushes by reaction with about 19% of the epoxy groups. The successful preparation of the PGMA–POSS hybrid brush was demonstrated by Fourier-transform infrared spectroscopy (FTIR), DLS, SLS, energy dispersive X-ray spectroscopy (EDX) and thermogravimetric analysis (TGA) measurements. An increase of the length and diameter of the brushes was shown by AFM and non-stained transmission microscopy (TEM) measurements. Residual SiO2 after pyrolysis of the PGMA–POSS hybrid brush in air displayed interesting cylindrical network structures.

Graphical abstract

Introduction

With the rapid development of nanotechnologies, a large number of materials with different functionalities on the nano-scale have been fabricated [1]. Combining inorganic materials’ outstanding mechanical, optical, electric and magnetic properties, with the soft organic materials’ excellent processabilities, functionalities and biocompatibilities, nano-composite materials have demonstrated their advanced performance. Their properties exceed the simple addition of each components characteristics, due to the special surface and quantum effects on the nano-scale. Polymer-based nano-composites have been widely investigated and a large category of inorganic materials, such as carbon materials, ceramics, clays, metals, metal oxides and silica, have been incorporated into different polymeric materials [2]. One advantage of these composites is that a very small amount of the added inorganic nano-materials can remarkably improve the mechanical, thermal and other properties of the polymer matrix [3]. Two general strategies have been applied to prepare the polymer-based nano-composites with inorganic materials. The easiest way is to directly disperse the nanometer-sized inorganic materials into the polymer by mechanical blending, melting or solution treatment. Surface functionalization is sometimes necessary to improve the compatibility of the inorganic materials with the polymer matrix. Recent research revealed the size of the blending materials is a key factor for the dispersing process [4]. However, the forces between the additive and the matrix are weak due to the non-covalent blending, which leads to poor property improvements and stabilities. The other method is to prepare the nano-composites via covalent linking, either by direct polymerization of functional monomers attached with inorganic groups, or by post-polymerization reactions. Thanks to the strong covalent linking of the inorganic part to the organic polymer chains, better performance of these materials is expected.

Recent progress has been achieved in the preparation of nano-composite materials on a molecular level, which find promising micro-electronic, optic and magnetic applications. Suitable nano-structured polymer matrices are in great demand. Spherically shaped polymers like star polymers [5], [6], dendrimers [7] and hyperbranched polymers [8] can meet the requirements to some extent. Recently, the interest in anisotropic polymer structures is increasing, owing to their interesting properties and potential applications. One-dimensional dendronized polymers [9] and cylindrical polymer brushes (CPBs) [10], [11] are the good examples. For instance, core-shell cylindrical brushes have been used as the templates for the preparation of inorganic nanowires [12], [13], [14]. Tedious synthetic procedures for dendronized polymers have limited their use. The relatively easier preparations of CPBs make them more attractive.

CPBs, comprised of very long backbones and densely grafted side-chains, adopt worm-like morphologies in good solvents. Their anisotropic nature causes special solution and bulk properties [15], [16]. So far, grafting-to [17], grafting-through [18] and grafting-from [19], [20] strategies have been explored to prepare CPBs. Generally, well-defined CPBs can be obtained by using the grafting-from method, since both the backbone and the side-chains can be grown by well-established living/controlled polymerization techniques.

In the past years, much attention has been paid to polyhedral oligomeric silsesquioxanes (POSS), which have special cage structures [21]. They are made up of silicon/oxygen (SiO1.5)n cages with organic substituents. Sized between 1 and 3 nm, they can be viewed as the smallest silica nanoparticles. Unlike the pure inorganic silica materials, the outside organic substituents provide them many possibilities for further functionalization, organo-compatibility, and even bio-compatibility. Incorporating POSS into polymers can dramatically improve their thermal and mechanical properties [21], [22], [23], [24]. Different methods have been developed to prepare POSS/polymers composites. When the shell of POSS is designed to be reactive, polymers can be connected covalently to them, either by grafting-from [25] or grafting-to [26] procedures. Another convenient way is to prepare monomers carrying POSS units followed by polymerization. Controlled radical polymerizations, such as atom transfer radical polymerization (ATRP), have been successfully used to prepare homopolymers or block copolymers of POSS-containing methacrylates [27], [28], [29].

Our group has achieved single-molecular nano-composites with semi-conducting [12], [13] and magnetic nanoparticles [14] non-covalently incorporated in core-shell CPBs. Very recently, we have reported the preparation of CPBs with silsesquioxanes as the core and poly[oligo(ethylene glycol) methacrylate] (POEGMA) as the water soluble shell, by consecutive ATRP of (3-acryloylpropyl)trimethoxysilane and OEGMA and further sol–gel process [30]. They were transformed into silica nanowires by pyrolysis on a substrate.

In this work, we employed another strategy to prepare silsequioxane-containg single-molecular cylindrical nano-hybrids. By attaching thiol-functionalized POSS to the preformed poly(glycidyl methacrylate) (PGMA) cylindrical brushes, we successfully obtained nano-cylinders (termed as PGMA–POSS) which contain the cage-like inorganic materials in an organic cylindrical matrix.

Section snippets

Materials

CuCl (97%, Aldrich) was purified by stirring with acetic acid overnight. After filtration, it was washed with ethanol and ether and then dried in vacuum oven. CuCl2 (99%, Acros) was used without purification. N,N,N′,N′′,N‴,N‴-hexamethyltriethylenetetraamine (HMTETA, Aldrich) was distilled before use. 4,4′-Dinonyl-2,2′-dipyridyl (dNbpy) was purchased from Aldrich and used without further purification. Glycidyl methacrylate (GMA) (97%, Aldrich) was purified by passing through basic alumina

Synthesis of the PGMA brush

To prepare the hybrid PGMA–POSS brushes, precursor CPBs with poly(glycidyl methacrylate) (PGMA) side-chains were first synthesized by grafting from a narrowly distributed poly-initiator PBIEM (Mn = 4.18 × 105, DPn = 1500, PDI = 1.08) [31] using ATRP. Scheme 1 depicts the synthetic process.

It has been known that the initiating efficiency for the grafting of bulky monomers, e.g. methacrylates, by ATRP is well below 100% [32], [33]. Some measures were taken to improve it. CuCl/CuCl2 catalysts were used

Conclusions

We have demonstrated the successful fabrication of single-molecular nano-hybrid cylinders by attaching thiol-functionalized POSS to CPBs with poly(glycidyl methacrylate) side-chains. A PGMA brush was first prepared by grafting of GMA from a long poly-initiator PBIEM via ATRP. 1H NMR, GPC, DLS, SLS and AFM measurements confirmed the well-defined worm-like structures of PGMA brushes. Then monothiol-functional POSS was covalently attached to the PGMA brushes by reaction with the epoxy groups.

Acknowledgements

The authors want to express their gratitude to Mr. Benjamin Gossler (BIMF, Universität Bayreuth) for the SEM and EDX measurements. Help from Christina Löffler and Sandra Ganzleben (MC I, Universität Bayreuth) with the TGA measurements is appreciated. Constructive discussions with Dr. Weian Zhang are gratefully acknowledged. We thank Deutsche Forschungsgemeinschaft (DFG) for the financial support within SPP 1165 (Nanowires and Nanotubes).

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