Identification of nanostructural development in epoxy polymer layered silicate nanocomposites from the interpretation of differential scanning calorimetry and dielectric spectroscopy

Abstract

a b s t r a c tThe effect of nanoclay on the non-isothermal cure kinetics of polymer layered silicate nanocomposites based upon epoxy resin is studied by calorimetric techniques (DSC and TGA) and by dielectric relaxation spectroscopy (DRS) in non-isothermal cure at constant heating rate. The cure process takes place by homopolymerisation, initiated anionically using 3 wt% dimethylaminopyridine (DMAP), and the influence of the nanoclay content has been analysed. Interesting differences are observed between the nanocomposites with 2 wt% and 5 wt% clay content. At low heating rates, these samples vitrify and then devitrify during the cure. For the sample with 2 wt% clay, the devitrification is accompanied by a thermally initiated homopolymerisation, which can be identified by DRS but not by DSC. The effect of this is to improve the exfoliation of the nanocomposite with 2 wt% clay, as verified by transmission electron microscopy, with a corresponding increase in the glass transition temperature. These observations are interpreted in respect of the nanocomposite preparation method and the cure kinetics.

Non-isothermal cure and exfoliation of tri-functional epoxy-clay nanocomposites

“…heat of reaction first increases, from 74.0 to 88.5 kJ/ee for a clay content of 2 wt%, and thereafter decreases with increasing clay content. Such a reduction in the heat of reaction with increasing clay content (here from 2 to 10 wt%) has been observed in much of our earlier work on epoxy/clay systems [16,17,20,37], and can be attributed either to the effect of the stoichiometry of the reaction or to the fact that the heat of reaction for homopolymerisation is smaller than that for the epoxy-amine reaction [17,38,39]. The initial increase in the heat of reaction for the clay content of 2 wt% may be explained on the basis of stoichiometry, as follows.…”

“…We found that, in the isothermal cure of TGAP-based nanocomposites, two distinct reactions occurred: the first of these reactions was very rapid and could be associated with the homopolymerisation reaction of the resin within the clay galleries, while the second reaction represented the cross-linking reaction of the TGAP with the curing agent, diaminodiphenyl sulphone (DDS) [22]. These TGAP systems resulted in significantly better exfoliation of the cured nanocomposite, as observed by both small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM), in comparison with earlier studies of DGEBA-based nanocomposites [16][17][18]20]. We attributed this to the beneficial occurrence, in the TGAP system, of the intra-gallery reaction before the bulk cross-linking reaction, whereas the homopolymerisation reaction in the DGEBA system does not occur until after the major part of the bulk crosslinking reaction has occurred, which inhibits any further separation of the clay layers from taking place.…”

“…The curing process, which may involve isothermal or non-isothermal (dynamic) cure schedules, or a combination or sequence of either of these, may be monitored and characterised by thermal analysis techniques, from which a number of kinetic parameters may be obtained. These parameters and other details of the cure process can be related to the nanostructure development, and hence thermal analysis is a useful tool in the study of these PLS nanocomposites [8][9][10][11][12][13][14][15][16][17][18][19][20]. Most of the studies of the cure reaction of epoxy PLS nanocomposites have concentrated on the iso -thermal or non-isothermal cure of bi-functional epoxy resin systems, typically diglycidyl ether of bisphenol-A (DGEBA); for example, only reference 13 of those cited immediately above (references 8 to 20) does not use DGEBA.…”

A novel comparative study of different layered silicate clay types on exfoliation process and final nanostructure of trifunctional epoxy nanocomposites

“…Indeed, in some cases, calorimetric studies of the reaction can obviate the necessity for nanostructural analysis of the cured nanocomposite by SAXS and TEM, since they can demonstrate that the relationship between the intra-and extra-gallery reaction rates is not favorable for exfoliation. Examples of this have been given in earlier publications [11,13,15,35,36].…”

Atomic Force Microscopy of Polymer/Layered Silicate Nanocomposites (PLSNs): A Brief Overview

“…The nanocomposites materials with polymers as matrix have invaded a wide area of applications of materials which demand light weight, controlled functionalities, and improved physico‐mechanical properties . If the size of the fillers used in the composite materials is decreased to nanometer range, a dramatic increase in the interfacial area takes place leading to novel and better properties of the composites on addition of a small amount of filler.…”