Nanoparticle-sulphur "inverse vulcanisation" polymer composites

Article abstract of DOI:10.1039/c5cc03419a

Composites of sulphur polymers with nanoparticles such as PbS, with tunable optical properties are reported. A hydrothermal route incorporating pre-formed nanoparticles was used, and their physical and chemical properties evaluated by transmission and scanning electron microscopy, thermogravimetric and elemental analyses. These polymers are easily synthesised from an industrial waste material, elemental sulphur, can be cast into virtually any form and as such represent a new class of materials designed for a responsible energy future.

Full text of DOI:10.1039/c5cc03419a

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DOI: 10.1039/C5CC03419A
Nanoparticle-sulphur “inverse vulcanisation” polymer composites†
Joseph C Bear,a William J Peveler,b Paul D McNaughter,c Ivan P Parkin,b Paul O’Brien,c,dand
Charles W Dunnill∗a
Received Xth XXXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
First published on the web Xth XXXXXXXXXX 200X
DOI: 10.1039/b000000x
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Composites of sulphur polymers with nanoparticles such
as PbS, with tunable optical properties are reported. A hy-
drothermal route incorporating pre-formed nanoparticles
was used, and their physical and chemical properties eval-
uated by transmission and scanning electron microscopy,
thermogravimetric and elemental analyses. These poly-
mers are easily synthesised from an industrial waste mate-
rial, elemental sulphur, can be cast into virtually any form
and as such represent a new class of materials designed for
a responsible energy future.
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Sustainable chemical processes and those using waste materi-
als provide alternate routes to a more environmentally benign
economy of chemical utilisation. Sulphur is a promising al-
ternative feedstock to carbon for polymeric materials and is a
by-product from hydrodesulphurisation; a crucial step in the
petroleum refining process. Large slag heaps of elemental sul-
phur residues can be found at petroleum refineries, especially
when oil from sour fields is used, such as the sulphur rich oil
sands of Athabasca, Canada (Fig. 1a). To date the utilisation
of this waste is limited to sulphuric acid.1
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Fig. 1 (a) Sulphur wastes at Athabasca, Canada. Image used with
permission: David Dodge, The Pembina Institute -
www.pembina.org, (b) Scheme for formation of crosslinked
polymer. (c) Moulded polymer containing Fe3O4 nanoparticles, (d)
Raw polymer (no nanoparticles) in film and moulded forms.
In 2013, Pyun et al. showed that utilising elemen-
tal sulphur cross-linked with a divinylic species, 1,3-
diisopropenylbenzene (DIB), a rubber industry feedstock, can
create a thermosetting polymer, a process they dubbed inverse
vulcanisation.2,3 A deep red colour was observed on the addi-
tion of the DIB to molten sulphur at 185 ◦C. On further heating
the materials set due to the divinylic groups of the DIB react-
ing with the radical polymeric sulphur chain ends, forming the
final polymer.4,5
The DIB-sulphur polymer may be useful for the incorpora-
tion of nanomaterials, and subsequent processing and mould-
ing techniques multiply the potential uses of such compos-
ites.6 The combination of new polymers with nanomaterials
has potential to produce multi-functional optical materials for
lenses and filters, or durable composites for structural appli-
cations.7 In this paper, we describe a facile method for the
incorporation of different types of nanoparticles into a sulphur
polymer matrix, and demonstrate its malleability and useful
optical properties.
† Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See DOI:
10.1039/b000000x/
a
Energy Safety Research Institute, Swansea University, Bay Cam-
pus, Fabian Way SA1 8EN, UK. Tel: +44 (0)1792 606244; E-mail:
c.dunnill@swansea.ac.uk
b
Department of Chemistry, University College London, 20 Gordon Street,
London, WC1H 0AJ, UK.
Elemental sulphur has many allotropes, but the common
yellow powder mainly comprises eight-membered S8 rings.
On heating, yellow sulphur melts at 120-124 ◦C,8,9 before the
critical temperature (the so-called λ-transition) of ∼159 ◦C at
which the specific heat capacity, density, thermal expansion
c
School of Chemistry, University of Manchester, Oxford Road, Manchester,
M13 9PL, UK.
d
School of Materials, University of Manchester, Oxford Road, Manchester
M13 9PL,UK
1–4 | 1

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Fig. 2 a) Au nanoparticle-sulphur polymer composite, b) zero-loss TEM micrograph of Fe3O4 nanoparticle-sulphur polymer composite, c)
EELS iron map of the Fe3O4 nanoparticle-sulphur polymer composite, showing iron glowing white, d) EELS sulphur map with sulphur
glowing white, e) CoO nanoparticle-sulphur polymer composite, f) HAADF-STEM image of the PbS nanoparticle-sulphur polymer
composite, g) Pb EDS mapping of f) and h) S EDS mapping of f) showing high concentrations of sulphur in the PbS nanoparticles and diffuse
sulphur throughout.
and viscosity all alter dramatically. The changes are due to the
ring opening of the S8 species, forming long-chain oligomeric
components with radical end groups.2 A darkening in colour
is also observed on heating, held to be due to organic impuri-
ties within the sulphur.8
tal analysis and XPS. Elemental analysis on a 50 wt.% DIB-
sulphur polymer sample fitted with the postulated structure
(Fig. 1c),2 but any discrepancies were attributed to the ten-
dency of sulphur chains to oligomerise in their molten state as
well and oxidation.10 Calc. for S16C20H24 (MW 777.44): C,
30.90; H, 3.11; S, 65.99. Anal. Found: C, 35.85; H, 3.55;
S, 56.17. XPS analysis gave S2p [binding energy] peaks at
163.58 eV and 164.68 eV, consistent with that of elemental
sulphur (see S5.5†).
Full details of the sulphur polymer synthesis and composite
synthesis are given in the ESI(†), but briefly, 2 g (62.4 mmol)
of elemental sulphur was heated with vigorous stirring to
185 ◦C in an oil bath at which point 50 wt. % of DIB
(2.16 ml, 12.6 mmol) was injected rapidly. In order to incorpo-
rate nanoparticles ∼0.1 g of hydrophobically coated nanopar-
ticles were dispersed in the required amount of DIB before
addition. The mixture was then heated for 2-3 minutes before
pouring into a mould. This process is summarised in Fig. 1b.
Thin films were synthesised by drop-casting a small amount
(∼1 mL) of the molten mixture onto a cold microscope slide,
adding a second slide as a cover-slip before curing at 200 ◦C
for 30 minutes. Thin films were then analysed using UV/vis
absorption spectroscopy (Fig. 3).
Several types of nanoparticle were incorporated into the
DIB cross-linker, including: Au,11–13 InP/ZnS quantum
dots,14,15 Fe3O4,16 CoO,13,16 and PbS nanocrystals17 (sec-
tion S2 for syntheses†). All were capped with long-chain hy-
drocarbon ligands such as oleic acid, 1-dodecanethiol or tri-
octylphosphonic acid, making them readily dispersible in hy-
drophobic solvents and DIB. On addition to the molten sul-
phur, nanoparticle-DIB dispersions tended to cause efferves-
cence, postulated to be due to the presence of excess nanopar-
ticle surfactants in a near boiling solution, and the overall
colour of the mixture changed to reflect the type of added
nanoparticles.
The sulphur polymer itself is a ruby-red glassy material
with rheological properties that depend on the amount of DIB
in the synthesis, raising the glass transition temperature (Tg).2
The composition of the polymer was evaluated using elemen-
The sulphur-polymer nanoparticle composites were anal-
ysed using a variety of techniques including: Transmission
2 |
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DOI: 10.1039/C5CC03419A
electron microscopy (TEM), scanning electron microscopy
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(SEM), energy dispersive X-ray spectroscopy (EDS), electron
energy loss spectroscopy (EELS) mapping, thermogravimetric
analysis (TGA), differential scanning calorimetry (DSC) and
UV/vis spectroscopy. Extra compositional analysis on the sul-
phur polymer was carried out using elemental analysis and X-
Ray photoelectron spectroscopy (XPS). Solution phase char-
acterisation was severely impaired by the insolubility of the
composites in most solvents (only sparingly soluble in 1,2-
dichlorobenzene).
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The composites themselves exhibited the colour of the em-
bedded nanoparticles and retained the shape of any mould in
which they were placed before curing, realising the potential
for complex engineering components to be cast (see Fig. 1c
and d). The incorporation of nanoparticles did little to alter
the physical properties of the sulphur polymer as evidenced
by the melting behaviour recorded by TGA/DSC (see S5.6†),
and all samples exhibited thermoset polymer characteristics.
SEM micrographs supported this observation with the poly-
mer shown to have sharp, glass-like features, with no observ-
able difference in external structure or integrity with incorpo-
rated nanomaterials (see section S5.2†).
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TEM was used to image the nanomaterials in the polymer,
although this was challenging due to the insolubility of the
composites for sample preparation and the high electron den-
sity of the sulphur polymer (relative to a conventional, or-
ganic polymer). This was overcome by partially dissolving
the composite in 1,2-dichlorobenzene overnight, effectively
giving cross-sections of the composite. TEM micrographs are
shown in Fig. 2.
Fig. 3 Top: UV/visible spectra of various nanoparticle containing
The PbS nanoparticles grew in size when incorporated into
the composite. The diameter increased from 3.6 0.5 nm in
the as synthesised17 sample to 36 9 nm in the composite.
The shape of the nanoparticles also changed from spherical
to cubic, the latter consistent with the cubic halite structure
of PbS (galena - see Fig. S5.3.4†). EDS mapping (Fig. 2f-
h) clearly indicated the presence of sulphur and lead with the
lead localised to the nanoparticles and the sulphur distributed
throughout, indicating a full sulphur-polymer coat.
S-polymer films. Bottom: Image of films produced.
sorption spectroscopy of composite thin films, with the pre-
viously stated amount of nanoparticles (∼ 0.1 g) causing an
observable difference in absorption.
The band-edge of the PbS parent nanocrystals was not
observable in the PbS-sulphur polymer composite, presum-
ably due to temperature induced agglomeration destroying the
quantum dot properties. Absorption spectra of PbS nanocrys-
tals and composite in the infrared range are given in Figure
S5.7.3†. The gold nanoparticle-sulphur polymer composite
does show the surface plasmon resonance band of the gold, al-
beit blue-shifted compared to the parent material dispersed in
toluene (Figure S5.7.1†). This is again consistent with ripen-
ing as confirmed by TEM images, with a size increase from
4.7 nm 1.2 nm to 22.3 nm 19.2 nm, although smaller par-
ticles of near original size do remain (Figure S5.3.2†).18 The
band-edge of the InP/ZnS quantum dots (455 nm in n-hexane)
used was not detectable in the UV/vis spectrum due to high
absorption of light by the sulphur polymer matrix in that re-
The EDS spectra of the other nanoparticle composites can
be found in section S5.4† where the elemental compositions
are confirmed. The most interesting sample is that containing
the InP/ZnS as the quantum dots are visible by TEM. EELS
mapping of sulphur and iron (Fig. 2b-d) in the Fe3O4-sulphur
polymer composite demonstrated the presence of unaltered
spherical iron oxide nanoparticles within the polymer, with
the iron and sulphur glowing white in respective micrographs.
TEM images of other nanoparticle-sulphur polymer compos-
ites can be found in section S5.3†.
On visible inspection, it was clear that the incorporated
nanoparticles had altered the optical absorption properties of
the material (Fig. 3). This was evaluated by the UV/vis ab-
1–4 | 3