Anisotropic nanoparticles of precise microstructure polyolefins

Article abstract of DOI:10.1039/c2cc32834h

Nanoparticles of precisely branched polyethylenes possess a distinct oblate shape resulting from a crystalline lamella in the particle as revealed by SAXS and TEM, and display controllable and well-behaved thermal behaviour.

Full text of DOI:10.1039/c2cc32834h

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Anisotropic nanoparticles of precise microstructure polyolefinsw
Justyna Trzaskowski,a Christian Rabe,b Karsten Vogtt,b Marina Krumova,a
Guenter Goerigk,b Matthias Ballauff*b and Stefan Mecking*a
Received 20th April 2012, Accepted 19th July 2012
DOI: 10.1039/c2cc32834h
Nanoparticles of precisely branched polyethylenes possess a
distinct oblate shape resulting from a crystalline lamella in the
particle as revealed by SAXS and TEM, and display controllable
and well-behaved thermal behaviour.
Bulk materials with such microstructures have recently been
studied extensively by Wagener et al.10,11 Methyl-branched a,o-
diene monomers H2CQCH(CH2)nCHMe(CH2)nCHQCH2,
n = 6: 1 and n = 8: 2 were prepared by a previously reported
five step procedure (Fig. S1, ESIw).10 GC analysis revealed a
purity of 499%, which is crucial for the subsequent step-growth
polymerization (Fig. S2, ESIw). Acylic diene metathesis (ADMET)
polymerization in a mechanically stirred microreactor with
[(PCy3)2Cl2RuQCHPh] (Grubbs’ 1st generation catalyst) for
six days yielded main-chain unsaturated poly-1 and poly-2,
respectively. Molecular weights as determined by GPC
For inorganic nanoparticles it is well established that their shape
and physical properties are governed by their crystalline structure
and the crystallization process.1,2 Other than for inorganic materials,
crystalline order in polymers results from van der Waals interactions
between adjacent chain segments. Its impact on the shapes and
properties of nanoparticles has been little studied.
(in THF vs. PS standards) amount to Mn = 4.1 Â 104 g molÀ1
,
,
Polyethylene is of particular interest in this regard. Its
crystalline structure and crystallization in the bulk have been
studied more extensively than for any other polymer, and it is
of enormous practical importance.3
Mw/Mn = 1.7 for poly-1, and Mn = 2.9 Â 104 g molÀ1
1
Mw/Mn = 1.8 for poly-2. H-NMR spectroscopic end group
analysis (vinyl endgroups at d 4.95 and 5.80 ppm, and alkyl
endgroups at d 0.86 ppm from isomerization during polymer-
ization) revealed a molecular weight of Mn 104 g molÀ1 for
both unsaturated polymers. This is in reasonable agreement with
the apparent molecular weight from GPC of the unsaturated
polymers, indicating that no excessive formation of cycles
has occurred.
Insertion polymerization of ethylene in aqueous solutions
afforded surfactant-stabilized dispersions of single crystal
nanoparticles of ca. 6 nm lamella thickness and 25 nm lateral
extension composed of linear polyethylenes.4 Likewise,
crystallization of the hydrophobic core of micelles formed
from block copolymers with a polyethylene and a hydrophilic
block yielded platelet or disc-like particles.5–7 Self-assembly in
aqueous solution of linear ethylene–acrylic acid copolymers affords
crystalline particles colloidally stabilized by covalently attached
carboxylate moieties.8 Anisotropic particles of moderately
branched polyethylene resulted from free-radical emulsion
polymerization at intermediate pressures.9
The double bonds in the main chain of poly-1 and poly-2
impede crystallization and enhance solubility. This is beneficial
for a secondary dispersion approach to nanoparticles. The
unsaturated polymers, diluted with toluene to lower the viscosity,
were dispersed in an aqueous surfactant (DTAC) solution by
means of ultrasonication. Dynamic light scattering revealed an
average particle size of ca. 35 nm with a particle size distribution
in the range from 0.12–0.26 (Fig. 1).
The aforementioned nanocrystals of high molecular weight
linear polymers feature sharp thermal transitions (ca. Tm
130 1C, Tc 75 1C), and recrystallize with a high supercooling
due to the confined nature of the nanodroplets. A control of
these transitions was not possible, however. The introduction
of branches alters the melting and crystallization point, but
also results in very broad thermal transitions which indicate
ill-defined crystalline structures. To this end, nanoparticles
composed of precisely branched polymers were investigated.
For hydrogenation, the ruthenium alkylidene catalyst was
quenched with ethyl vinyl ether to the Fischer carbene, in
order to suppress any undesired secondary metathesis activity,
which could enhance for example formation of macrocycles
and alteration of molecular weights. Hydrogenation of the
dispersion at 60 bar H2 pressure at 65 1C proceeded virtually
completely (Scheme 1), as concluded from the absence of olefin
a Chair of Chemical Materials Science, Department of Chemistry,
University of Konstanz, Universitatsstrasse 10, D-78457 Konstanz,
¨
Germany. E-mail: Stefan.Mecking@uni-konstanz.de;
Fax: +49 7531 88-5152
b Helmholtz–Zentrum Berlin for Materials and Energy,
Hahn Meitner-Platz 1, D-14109 Berlin, Germany.
E-mail: matthias.ballauff@helmholtz-berlin.de
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c2cc32834h
Fig. 1 Particle size distribution by number of poly-2 secondary
dispersion before (left) and after (right) hydrogenation.
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Chem. Commun., 2012, 48, 9153–9155 9153

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nanocrystals, however the crystalline nanoparticles’ thermal
transitions can be tuned over a wide range down to nearly
100 1C below the melting point of linear polyethylene, with
Tm = 57 1C for C19-NP and Tm = 39 1C for C15-NP. By
comparison, nanoparticles of a polyethylene similar to C19 in its
branching structure but with random placement of branches are
essentially non-crystalline (which also reflects in a spherical shape,
cf. below).12 Crystallization of nanoparticles occurs with a high
supercooling of ca. 28 1C, due to crystallization of individual
droplets, vs. typical heterogeneous nucleation in the bulk.
Small angle X-ray scattering (SAXS) in combination with
contrast variation can reveal the dimensions of polymer
particles in this small size regime.4,13,14 The small angle scattering
intensities of C19-NP were studied in neat water and in glucose
solutions in a contrast variation series (Fig. S12, ESIw).15 Due
to a non-negligible scattering contribution of DTAC-micelles
the analysis was only suitable for q-values lower than 0.7 nmÀ1
(refer to ESIw).
From a Guinier analysis of the scattering intensities at low
q-values (Fig. S13, ESIw) the initial parameters of the poly-
ethylene nanoparticles were extracted (see ESIw; Table S2).15,16
Further analysis of the data yielded the average particle volume
VP = 36 000 Æ 3000 nm3 and the average particle scattering
length density rP = 8.71 Æ 0.04 1010 cmÀ2, respectively.
For a more detailed determination of the shape of the particles
and their internal structure, the scattering intensities I(q) were
decomposed as a function of the contrast Dr.15,17–20 This technique
allows for the determination of the particles’ outer shape by its
specific scattering contribution unaffected by the contribution
arising from the particles inner structure (cf. ESIw).
Scheme 1 Preparation of nanoparticles of precisely methyl-branched
polyethylene.
resonances at d 5.35 ppm in 1H-NMR spectra of isolated bulk
material C15 and C19, respectively (Fig. S6 and S8, ESIw). This
corresponds to a degree of hydrogenation 499.9%, which
fulfills a prerequisite for meaningful studies of physical proper-
ties (vide infra). Particle sizes remain unaltered within experi-
mental error during the hydrogenation procedure, indicating
that the reaction occurs in individual particles which preserve
their identity (Fig. 1). Also, apparent molecular weights from
GPC are not altered significantly (C15: Mn = 3.2 Â 104 g molÀ1
,
Mw/Mn = 1.5 and C19: Mn = 3.0 Â 104 g molÀ1, Mw/Mn =
1.7) and correspond to absolute molecular weights determined
by alkyl endgroup analysis (resonance at d 0.86 ppm) in
1H-NMR spectra (Mn 104 g molÀ1). The nanoparticles, C15-NP
and C19-NP, were investigated by differential scanning calorimetry
(DSC) in dispersion and for comparison in bulk, which reveals
their crystalline nature (Fig. 2).
Melting in the particles and recrystallization occur without
any advert effect on colloidal stability. After five melting and
recrystallization cycles, the particle size was unaltered (Fig. S10,
ESIw). Moreover, narrow melt and crystallization transitions occur.
In this respect the nanoparticles resemble linear polyethylene
Fig. 3 displays the scattering intensities normalized by the
volume fraction f and the contrast Dr. This analysis shows
that particles are not homogeneous but exhibit a variation of
electron density in their interior. This can be argued directly
from the shallow maximum at around q B 0.4 nmÀ1 (Fig. 3).
We ascribe this finding to a crystalline core of the particles, which
is further corroborated by TEM (vide infra). Unfortunately, the
polydispersity of the particles is too high to analyse the internal
variation of electron density by SAXS in further detail. However,
the data were of sufficient quality to extrapolate the scattering
intensities in infinite contrast. The resulting partial intensity IS(q)
Fig. 3 Scattering intensities I(q) from a contrast variation series of
C19-NP normalized by volume fraction f and contrast Dr. The data
are background corrected for contributions of glucose and DTAC
(see ESIw for further details).
Fig. 2 DSC traces of nanoparticle dispersions C19-NP (top) and
C15-NP (bottom), and bulk material for comparison (10 K minÀ1
,
2nd heating cycle).
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understanding is that this might arise from the crystallization
process in the nanoscale droplets. A lamella forms by crystal-
lization with its thickness dimension defined by the crystallization
conditions, essentially by the temperature. Due to confinement in
the droplet and particle, respectively, material is not incorporated
into the lamella but is left as surrounding amorphous phase.
In conclusion, the crystallization of precisely branched
linear polyethylene in nanoscale droplets provides particles
with a distinct oblate shape. This is found independently by
X-ray scattering and electron microscopy. The strongly anisotropic
nature appears to arise from a crystalline lamella in the
particles, which also imparts shape upon the amorphous
phase, present in an approximately equal volume as the
crystalline portion. The strongly anisotropic nature appears
to arise from a crystalline lamella in the particles, which also
imparts shape upon the amorphous phase, present in an
approximately equal volume as the crystalline portion.
The authors would like to thank Dr Narayanan from the
ID02 beamline at the ESRF in Grenoble (France) for the kind
help with the SAXS measurements. J. T. gratefully acknowledges
a stipend provided by the DFG (IGK 667).
Fig. 4 Scattering contribution of the shape-term IS(q) derived from the
contrast decomposition (open symbols). The corresponding scattering
intensities of an oblate ellipsoid were obtained by modelling (red line).
For comparison, a fit for polydisperse spheres is shown (dashed line).
refers solely to the shape of the particles. Fig. 4 displays IS(q)
together with the corresponding fit. IS(q) could be modelled best
by a polydisperse oblate-like spheroid with an equatorial-axis
with req = 23.8 Æ 8.8 nm and a perpendicular semi-axis with rp =
4.9 Æ 1.8 nm (Fig. 4). This fit also affords the correct particle
volume in agreement with the Guinier-fit (vide supra). For
comparison, the best fit for polydisperse spheres (with d =
27 Æ 10 nm) shows significant deviations from the decomposed
shape function in the high-q region and can be ruled out.
Notes and references
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Further insight into the particle internal structure was
obtained from transmission electron microscopy (TEM) (Fig. 5).
The nanoparticles appear to consist of one ca. 10 nm thick lamella
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Fig. 5 TEM images of nanoparticle dispersion C19-NP. Left: over-
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c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 9153–9155 9155