Synthesis and characterization of methacrylate phospho-silicate hybrid for thin film applications

doi:10.1016/j.polymer.2007.09.050

Polymer

Volume 48, Issue 24, 16 November 2007, Pages 7078-7086

https://doi.org/10.1016/j.polymer.2007.09.050 Get rights and content

Abstract

Phosphorus containing methacrylate hybrids were synthesized from 2-(methacryloyloxy)ethyl phosphate (EGMP) and 3-[(methacryloyloxy)propyl] trimethoxysilane (MEMO) via dual-cure process involving sol–gel reaction and addition polymerization. The kinetics of the reactions was established using spectroscopic techniques. Photoacoustic Fourier transform infrared spectroscopy (PA-FTIR) and X-ray photoelectron spectroscopy (XPS) confirm the formation of Si–O–Si, P–O–P and Si–O–P linkages and simultaneous polymerization of methacrylate groups leading to a dense networked structure. The presence of silicate/phospho-silicate network in the hybrid enhances its thermal stability. Nanoindentation measurements on thin films show enhanced hardness and modulus with increasing silicate network. Topographic and conductivity images obtained using micro-thermal analysis (μTA) reveal a dense, homogenous and defect-free thin film formed on metallic substrate with a T_g of 93 °C.

Introduction

In recent years, phosphorus containing polymers have gained significant interest due to their unique characteristics in different applications such as reduced flammability, increased adhesion to metals, corrosion protection, membranes, biomedical areas etc. Phosphorus containing compounds when remain as additives in a physical mixture, can be depleted by evaporation or leaching by solvent or water. Such disadvantage can be overcome if they form a part of polymer network structure [1]. To achieve this goal, several approaches [2], [3], [4] of controlled homo or copolymerization have been investigated using phosphate precursors. However, phosphorus containing monomers can also be hybridized with other inorganic and organic precursor to form a new class of hybrid, which offers beneficial synergism between the properties of organic and inorganic materials. The ability to tailor the properties of such hybrids, at a molecular level, signifies their enormous potential in a wide variety of technologically advanced and conventional application fields [5], [6], [7].

There are several routes to form hybrid materials [8] on a nanoscale, but the most commonly employed method is the sol–gel method [9], [10], a simple process that allows the synthesis of hybrid at a relatively lower temperature at which organic compounds are stable. This can be achieved either through the sol–gel process in presence of a preformed polymer or polymerization in sol–gel networks or simultaneous formation of interpenetrating networks or using dual-network precursors. Among these types, the dual-network precursor method leads to strong covalent bonding between organic and inorganic components. Since the strength of interaction in this hybrid is much higher than the hybrid formed through other methods, it allows a better degree of homogeneity, transparency, thermal resistance and mechanical properties in the final hybrid [11], [12]. Hence, this class of hybrid with strong interaction is most commonly used for thin film applications.

[3-(Methacryloyloxy)propyl]trimethoxysilane (MEMO) is one of the most important precursors with dual-network forming capability. It has a polymerizable methacryloxy group at one end and the alkoxy silane groups capable of forming inorganic networks via sol–gel route at the other end. These hybrid materials can form a transparent, dense, uniform thin film on various substrates. It can combine the flexibility, density, toughness and easy processability of the organic component with the hardness, chemical and weather resistance of the inorganic component, thereby exhibiting multifunctional behavior. MEMO has been either homo or copolymerized with methyl methacrylate or acrylonitrile resulting in a uniformly distributed, transparent hybrid material [13], [14]. MEMO has also been used as a coupling agent to compatibilize organic and inorganic phases [15]. Till now, no attempt has been made to utilize the beneficial properties of phosphorus containing precursors with MEMO for thin film applications.

In this work, we have chosen to copolymerize 2-(methacryloyloxy)ethyl phosphate (EGMP) containing a polymerizable methacrylate group and functional phosphate group with MEMO. The selected materials thus have unique combination of three different components namely methacryloxy, phosphate functionality and silicon–alkoxy groups. The hydrolysable silicon alkoxide at one end can be condensed to form inorganic Si–O–Si, Si–O–P networks and simultaneous polymerization of the methacrylate group at the other end can lead to highly cross-linked dense networked structure [13]. Also, incorporation of the phosphate can improve thin film adhesion through acid–base type interaction of dissociated P–O⁻ from EGMP with Mⁿ⁺ of the metallic substrate rather than forming P 6-point double bond O/metal bond through induced dipole [16], [17]. The possible structure of the proposed hybrid network on a metallic substrate is given in Scheme 1.

Section snippets

Materials

MEMO, EGMP, acetone and hexane were purchased from Aldrich, Australia and used without further purification. Ethanol was used as a solvent. Commercial alkaline cleaner was purchased for the final cleaning of mild steel substrates.

Synthesis of methacrylate phospho-silicate hybrid

The hybrid material was synthesized in a two-step reaction process. Since a pre-hydrolysis of the sol–gel reactants promotes molecular scale mixing [18], 5 mmol of MEMO was first hydrolyzed in 10 mmol/15 mmol of ethanol/water solution mixture at room temperature. The

Structural evolution of the hybrid

PA-FTIR was used to evaluate the final structure of the hybrid. The FTIR spectra of MEMO, EGMP and the hybrid material (1:1 ratio) are shown in Fig. 1. The bands at 1635 cm⁻¹, 1405 cm⁻¹ and 817 cm⁻¹, that are present in the unreacted MEMO and EGMP monomers correspond to C 6-point double bond C, CH₂ wag, CH₂ twist, respectively [13], [21], [22]. These bands are weak and present in the cured hybrid indicating the reduction of the double bond (CC) in two monomers and subsequent network formation through the addition

Conclusions

In summary, we have demonstrated that a highly cross-linked dense hybrid material with phospho-silicate groups can be synthesized via a combination of sol–gel reaction and free radical polymerization. PA-FTIR and XPS results demonstrate the formation of highly cross-linked hybrid material through the formation of Si–O–Si, Si–O–P and P–O–P linkages and organic network through the polymerization of the methacrylate group. The cured hybrid material forms a highly networked, dense, uniform,

Acknowledgements

The authors gratefully acknowledge the financial support of the Australian Research Council's Special Research Centre for carrying out this work.

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Citation Excerpt :
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Synthesis and characterization of methacrylate phospho-silicate hybrid for thin film applications

Abstract

Introduction

Section snippets

Materials

Synthesis of methacrylate phospho-silicate hybrid

Structural evolution of the hybrid

Conclusions

Acknowledgements

Polym Degrad Stab

Prog Polym Sci

J Non-Cryst Solids

Polymer

Catal Today

Polym Degrad Stab

Polym Degrad Stab

Polym Degrad Stab

J Colloid Interf Sci

J Non-Cryst Solids

Spectrochim Acta A

Polym Degrad Stab

J Non-Cryst Solids

Polymer

Polym Degrad Stab

Polym Degrad Stab

Polymer

J Polym Sci Part A Polym Chem