An XPS investigation of thermal degradation and charring
on PMMA clay nanocomposites
Jianxin Du
a
, Jin Zhu
b
, Charles A.Wilkie
b
, Jianqi Wang
a,
*
a
School of Chemical Engineering and Materials Science, Beijing Institute of Technology, 100081 Beijing, China
b
Department of Chemistry, Marquette University, PO Box 1881, Milwaukee, WI 53201, USA
Received 2 January 2002; received in revised form 3 March 2002; accepted 9 March 2002
Abstract
Poly(methyl methacrylate)–clay nanocomposites have been studied using X-ray photoelectron spectroscopy.It is clear that as the
polymer undergoes thermal degradation, the clay accumulates at the surface and the barrier properties which result from this clay
accumulation have been described as the reason for the decreased heat release rate for nanocomposites.The surface composition of
the clay changes as the nanocomposite is heated and the changes are affected by the organic-modification that were applied to the
clay in order to prepare the nanocomposite. # 2002 Elsevier Science Ltd.All rights reserved.
Keywords: XPS; PMMA–clay nanocomposites; Thermal degradation; Charring
1. Introduction
There has been a great deal of interest in nanocompo-
sites recently, because the combination of a polymer with
3–5% of an organically modified clay shows enhanced
mechanical properties.For instance, a nylon–clay nano-
composite, containing 5% clay, shows a 40% increase in
tensile strength, 68% in tensile modulus, 60% in flexural
strength and 126% in flexural modulus, while the heat
distortion temperature increases from 65 to 152
C and
the impact strength is lowered by only 10% [1].It also
exhibited lower water sensitivity and permeability to
gases, along with no loss of clarity in the polymer.
Polymer–clay nanocomposites have been prepared by
polymerization techniques, including bulk, suspension,
emulsion and solution polymerization, and by melt
blending and solution blending [2,3].Since the polymer
must insert into the clay, it is essential that the sodium
cation, which is normally the counter-ion in the clay, be
replaced by a more organophilic cation so that a poly-
mer is able to penetrate between the clay layers.The
combination of a clay with a polymer may give either a
microcomposite, also known as an immiscible nano-
composite, in which the clay is present mostly as a filler,
or a nanocomposite.If the registry between the clay
layers is maintained, the material is referred to as an
intercalated nanocomposite.If this registry is lost, it is
described as an exfoliated, also known as delaminated,
nanocomposite.
In addition to mechanical properties, fire properties,
notably heat release rates from cone calorimetric experi-
ments, are also reduced, typically by 50% or more for
both intercalated and exfoliated materials [4–10].Among
the many polymers that have been examined are poly-
amide 6, polyamide 12, polystyrene, poly(methyl metha-
crylate) and polypropylene-graft-maleic anhydride.Two
mechanisms have been suggested to account for the
reduction in heat release rate, a barrier mechanism, in
which the clay functions as a barrier to insulate the
polymer from the fire and a barrier to mass transfer of
the polymer [11], and a radical trapping mechanism,
which occurs due to the presence of iron, or other para-
magnetic impurities, as a structural component in the
clay [12].
In studies from these laboratories, we have examined
the X-ray photoelectron spectra, XPS, of polystyrene–clay
nanocomposites [13].This work shows that carbon is lost
and oxygen accumulates at the surface of the degrading
polymer, thereby confirming the barrier properties as a
mechanism by which these materials function.Current
thinking is that the radical trapping mechanism is most
important at lower amounts of clay while the barrier
mechanism becomes more important as the fraction of
clay increases.
0141-3910/02/$ - see front matter # 2002 Elsevier Science Ltd.All rights reserved.
PII: S0141-3910(02)00074-5
Polymer Degradation and Stability 77 (2002) 377–381
www.elsevier.com/locate/polydegstab
* Corresponding author.Tel.: +86-10-6891-3075; fax: +86-10-
6891-2798.
E-mail address: bitjq@public.bta.net.cn (J. Wang).