Study on the effect of hexamethylene diamine functionalized graphene oxide on the curing kinetics of epoxy nanocomposites

Highlights

• Non-isothermal DSC was used to study the curing kinetics of the epoxy nanocomposite.

• Peak temperature (T_P) of epoxy curing reaction increases on loading P-GO or A-GO.

• Kinetic analysis of these epoxy nanocomposites revealed the autocatalytic nature.

• Model equations for non-isothermal curing kinetics of epoxy system were derived.

Abstract

The curing kinetics of epoxy nanocomposites prepared by incorporating pristine graphene oxide (GO) and hexamethylene diamine functionalized graphene oxide (AGO) was studied using non-isothermal differential scanning calorimetry (DSC) experiments. Loading of AGO in epoxy matrix resulted in the decrease of peak exotherm temperature (T_P) at all heating rates corroborating the enhanced curing reactions, when compared to the pristine GO filled epoxy system. The kinetic parameters of the curing processes of the neat, pristine and functionalized GO filled epoxy were determined using isoconversional methods viz. Kissinger and Friedman methods. In comparison to pristine GO filled epoxy system, epoxy nanocomposites loaded with AGO showed lower activation energy (E_α) over the range of conversion (α) revealing the enhanced curing reactions in these system. The predicted curves determined using the kinetic parameters fit well with the non-isothermal DSC thermograms revealing the proposed kinetic equation clearly explain the curing kinetics of the prepared epoxy nanocomposites.

Introduction

Graphene, a single-atom-thick sheet of sp² bonded carbon atoms, has generated much interest due to its high specific area and novel mechanical, electrical, and thermal properties [1], [2], [3]. The discovery of graphene with its unique combination of extraordinary physical properties has motivated the researchers to use it as nanofillers in polymer system leading to the development of a new class of polymer nanocomposites. Reported methods for fabricating graphene, such as micromechanical exfoliation [4], epitaxial growth [5] and [6] and chemical vapor deposition [7] and [8] are difficult to scale up for the fabrication of polymer nanocomposites. Alternatively, chemical methodologies such as harsh oxidation of graphite leading to the formation of graphene oxide (GO) [9] and its modified derivatives [10] is an effective technique to produce graphene based nanofillers in bulk quantities, which can be subsequently used in the preparation of polymer nanocomposites. However, loading of GO in the polymer matrix resulted in the formation of composites materials with little or no conductivity and highly reduced strength due to the presence of irreversible defects and disorderness in the GO [11]. To overcome this critical issue, reduction techniques such as chemical [12] or thermal reduction [13] are often employed to reduce the surface defects in the GO. However, employing reduction techniques resulted in the formation of graphene agglomerates or restacking of graphene sheets due to the strong van der Waals interaction between the graphene sheets restricts the performance of nanocomposites, when loaded in polymer matrix [11], [14]. Alternatively, surface functionalization techniques such as reactions with alkyl amines [14], [15], [16], [17], [18], isocynate [19] and polyethylene glycol [20] were employed by various research groups to modify the GO surfaces. It is expected that these modified GO could significantly improve the physical properties, when loaded in the polymer matrix due to the improvement in polymer–filler interactions.

Epoxy as one of the most important engineered polymers has drawn attention due to its wide applications including structural materials, tissue substitutes [21], anti-corrosion coatings [22] and flame retardant additives [23]. Epoxy nanocomposites reinforced with nanofillers have attracted much interest because of their cost effective processability and tunable physical properties, such as mechanical, magnetic, optical, electrical and electronic properties [24], [25], [26]. Wide range of nanofillers including inorganic nanoparticles such as silica (SiO₂) [24], titania (TiO₂) [27], alumina (Al₂O₃) [28] and layered silicate nanoplatelets [29] and hard nanocarbon fillers such as carbon nanotubes [30], [31], [32], [33] and fullerenes [34] have been studied. Recently, graphene nanofiller loaded epoxy nanocomposites have received significant research importance due to the significant improvement in physical, mechanical, fatigue and fracture properties [35], [36], [37], [38], [39], [40], [41], [42].

Generally, improvement in the properties of the epoxy nanocomposites filled with pristine or modified graphene oxide depends on the formation of the crosslinked molecular network, which is often influenced by the mechanism and kinetics of the epoxy resin curing that involves various chemical reactions. Understanding the cure process in the epoxy system is the essential part in order to get better control of the cure reactions and in consequence to optimize the physical properties of the final product. One of the most widely used techniques for studying the kinetics of the cure reaction of epoxy system is thermal analysis by differential scanning calorimetry (DSC) in isothermal or dynamic modes followed by kinetic analysis using phenomenological models [43]. Among these models, Borchardt–Daniels [44], Ozawa and/or Flynn and Wall [45], Kissinger [46], isoconversional [43], [44], [45], [46], [47] and autocatalytic cure rate methods [48], [49], [50] are widely applied to understand the curing mechanism of thermosetting resins. Significant studies have been conducted on the curing reaction kinetics of the thermosetting epoxy resins and its nanocomposites by employing various techniques, experimental procedures and data analysis methods [51], [52], [53], [54], [55]. Recently, we have fabricated epoxy nanocomposites filled with hexamethylene diamine modified graphene oxide and observed significant improvement in the mechanical, electrical and flexural properties in comparison to its counterpart. However, to the best of our knowledge no research report has been published that depicts the curing mechanism and kinetics of these epoxy nanocomposites filled with hexamethylene diamine modified graphene oxide nanofillers.

The aim of the present work is to study the effect of hexamethylene diamine grafted graphene oxide on the cure mechanism and kinetics of the epoxy resin (diglycidyl ether of bis-phenol A, DGEBA) cured with polyamidoamine (G-A0533) so as to understand the structure–property relationship. For this purpose, non-isothermal DSC measurements has been carried out to reveal the cure behavior of these epoxy systems and the empirical approaches are used to model the kinetics of the curing reactions.