Surface morphology and dewettability of self-organized thermosets involving epoxy and POSS-capped poly(ethylene oxide) telechelics

Abstract

A heptaphenyl polyhedral oligomeric silsesquioxane-capped poly(ethylene oxide) (POSS-capped PEO) telechelics was synthesized via the Huisgen 1,3-dipolar cycloaddition between 3-azidopropylheptaphenyl POSS and α,ω-dialkynyl-terminated poly(ethylene oxide). The organic–inorganic amphiphile was incorporated into epoxy to obtain the organic–inorganic nanocomposites. The morphology of the nanocomposites was investigated by means of atomic force microscopy (AFM) and dynamic mechanical thermal analysis (DMTA). It was found that the epoxy thermosets containing POSS-capped PEO telechelics were microphase-separated. The formation of the nanophases in the thermosets followed a self-assembly mechanism. The static contact angle measurements show that the nanocomposites displayed a significant enhancement in surface hydrophobicity as well as reduction in surface free energy. The improvement in surface dewettability was ascribed to the enrichment of POSS cages at the surface of the nanocomposites and the formation of the specific surface morphology as evidenced by X-ray photoelectron spectroscopy (XPS) and surface atomic force microscopy (AFM).

Introduction

Organic–inorganic composites have attracted considerable interest since they can possess the comprehensive properties from organic and inorganic components [1], [2], [3], [4], [5]. The properties of organic–inorganic composites are greatly dependent on the morphology of the materials. It is recognized that the nanoscaled dispersion of inorganics in organic polymers (i.e., the formation of organic–inorganic nanocomposites) can optimize the inter-component interactions and thus achieve the favorable combination of component properties. Generally, macroscopic phase separation would take place in the mixtures of inorganics and organic polymers owing to their inherent immiscibility unless the specific measurements were taken to suppress the unfavorable tendency. The following two strategies may be adopted to suppress the occurrence of macroscopic phase separation in the composite systems: i) the formation of inter-component chemical linkage between inorganic and organic components and ii) the utilization of organic–inorganic amphiphiles instead of inorganic components. Moderate inter-component reaction is effective to achieve the fine dispersion of inorganic components in organic polymers. In the past years, there has been ample literature to report the preparation of organic–inorganic nanocomposites via a variety of chemical approaches [1], [2], [3], [4], [5]. Relatively, the preparation of organic–inorganic nanocomposites via physical blending is less reported. For many applications, nonetheless, the preparation of organic–inorganic nanocomposites via physical blending is desirable owing to the simplicity in materials processing. The utilization of organic–inorganic amphiphiles can be taken as a physical blending approach, which employs their self-assembly behavior in organic matrices to access the nanostructures in the materials. The key to this approach is to design and prepare suitable organic–inorganic amphiphiles and to understand their self-assembly behavior in organic polymers.

Polyhedral oligomeric silsesquioxanes (POSS) are a class of important building blocks for the organic–inorganic nanocomposites. A typical POSS molecule possesses the structure of cube-octameric frameworks represented by the formula (R₈Si₈O₁₂) with an inorganic silica-like core (Si₈O₁₂) (∼0.53 nm in diameter) surrounded by eight organic corner groups, one or more of which is reactive. The structural and elemental features combining a nanosized inorganic core and reactive organic periphery allow a “bottom-up” approach to the creation of organic–inorganic nanocomposites. During the past years, POSS-containing hybrid composites have become the focus of many studies due to the excellent comprehensive properties of this class of materials [6], [7], [8], [9], [10], [11], [12], [13]. Generally, POSS cages are incorporated into polymers via copolymerization or inter-component reaction. The formation of chemical linkage between POSS and polymers is critical for suppression of macroscopic phase separation of POSS-containing composites. In the past years, there has been ample literature to report the preparation of the organic–inorganic nanocomposites containing POSS via chemical approaches [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]. Recently, there have been a few reports on the preparation of POSS-containing polymer nanocomposites via physical approaches. For instance, Zeng et al. [24], [25], [26] have ever reported the synthesis of several mono-hepta(3,3,3-fluoropropyl) POSS-capped polymer amphiphiles and these POSS-capped polymer semi-telechelics were then incorporated into epoxy thermosets to access the organic–inorganic nanocomposites via self-assembly approach. More recently, Zeng et al. [27] reported the preparation of the organic–inorganic nanocomposites involving epoxy and hepta(3,3,3-fluoropropyl) POSS-capped poly(hydroxyether of bisphenol A) telechelics. However, such studies still remain largely unexplored.

In this contribution, we report a new preparation of POSS-containing nanocomposites via the self-assembly of an organic–inorganic amphiphile in epoxy resin. The organic–inorganic amphiphile used in this work is a heptaphenyl POSS-capped poly(ethylene oxide) (PEO) telechelics and it is composed of a water-soluble midblock (i.e., PEO) and two hydrophobic end groups of POSS. The POSS-capped PEO telechelics was synthesized via the Huisgen 1,3-dipolar cycloaddition between 3-azidopropylheptaphenyl POSS and α,ω-dialkynyl-terminated poly(ethylene oxide). It should be pointed out that Mather et al. [28], [29], [30] first reported the synthesis of heptacyclohexyl POSS-capped poly(ethylene oxide) (PEO) telechelics via the direct reaction of isocyanatopropyldimethylsilylheptacyclohexyl POSS with poly(ethylene glycol) (PEG) and the behavior of crystallization and rheology was investigated. To the best of our knowledge, the investigations on self-assembly behavior of the organic–inorganic amphiphiles in other polymers have not been previously reported. The purpose of this work is twofold: i) to investigate the self-assembly behavior of POSS-capped PEO telechelics in epoxy thermosets and ii) to examine the surface morphology and properties of the nanostructured thermosets. In this work, atomic force microscopy (AFM) and dynamic mechanical thermal analysis (DMTA) were used to investigate morphology of the thermosets. The surface properties of the organic–inorganic nanocomposites were addressed on the basis of static contact angle measurements, surface AFM and X-ray photoelectronic spectroscopy (XPS).