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Inorganic–organic nanocomposites based on sol–gel derived magnesiumfluoride†

Johannes Noack,ab Larissa Schmidt,ab Hans-J€urgen Gl€asel,ab Monika Bauer*ab and Erhard Kemnitz*ab

Received 15th July 2011, Accepted 5th September 2011

DOI: 10.1039/c1nr10843c

Monodispersed magnesium fluoride nanoparticles are utilized for the first time to prepare transparent

inorganic–organic nanocomposite materials with improved mechanical properties. The fluorolytic sol–

gel synthesis route has been modified for the preparation of monodispersed magnesium fluoride

nanoparticles with a size of 2–3 nm. MgF2 particles are effectively stabilised against agglomeration by

phosphonic acids, which strongly bind to the particles and lead to an increased compatibility of the

inorganic particles with the organic polymers. This way, highly transparent nanocomposite materials

with up to 20 wt% magnesium fluoride in different acrylates are obtained, featuring high dispersion of

MgF2 particles in the polymer matrix and an increased hardness by the factor of 2. The nature of

interaction between phosphonic acids and magnesium fluoride is thoroughly investigated by IR and

NMR showing a monodentate coordination of phosphonates to the particle’s surface.

1. Introduction

Composites of inorganic nanoparticles embedded in an organic

polymer matrix are of high interest for materials science. Inor-

ganic fillers are widely used for enhanced performance of organic

polymers in terms of thermal or mechanical behaviour and add

new functionality to the polymer material, such as electrical or

thermal conductivity or improved flame retardancy.1–3 Most

frequently used inorganic fillers like soot, SiO2, TiO2, ZnO or

clays are characterised by high mechanical strength and chemical

stability while most organic polymers are flexible and well

processible. Usually, composites are manufactured by mixing the

particles with monomers or polymer melts followed by further

processing, e.g. polymerization or extrusion. The particle sizes in

these composites typically range from a few hundred nanometres

up to some micrometres. The advances in sol–gel technology

offering an easy access to nanoparticle sols marked the starting

point for the development of nanocomposites, predominantly

based on metal oxides, and to elucidate the effects of those

nanoparticles on the materials properties.4–7 Due to large surface

energies of inorganic nanoparticles in contrast to the organic

matrix, agglomeration is one of the major problems in the

preparation of homogeneous composites. Stability of a particle

dispersion can be greatly improved by surface-modification of

aHumboldt-Universit€at zu Berlin, Department of Chemistry,Brook-Taylor-Straße 2, 12489 Berlin, Germany. E-mail: erhard.kemnitz@chemie.hu-berlin.de; Fax: +49 30 2093 7277bFraunhofer Research Institution Polymeric Materials and CompositesPYCO, Kantstrasse 55, 14513 Teltow, Germany. E-mail: monika.bauer@pyco.fraunhofer.de

† Electronic supplementary information (ESI) available: See DOI:10.1039/c1nr10843c

4774 | Nanoscale, 2011, 3, 4774–4779

the inorganic moieties, e.g. by silylation of the particle surface8,9

or by complexation with carboxylic or phosphonic acids.10–13 As

the surface of the nanoparticles increases, the particles become

smaller and the interactions at the interface of the particle and

polymer become increasingly dominant, determining the mate-

rial’s properties. Thus, it is apparent that the properties of

a composite directly depend on the size of filler particles and are

strongly affected by the reduction of particle sizes.

Optical properties are characterised by absorption and lumi-

nescence of all constituents and by scattering of the incident light

from the particles. Magnesium fluoride is one of the compounds

with the lowest refractive index of all inorganic substances and it

exhibits transparency over a wide range of wavelengths from the

vacuum UV (120 nm) up to the IR region (8.0 mm). Due to these

favourable optical properties, nanoscopic magnesium fluoride

films are investigated for different applications, such as anti-

refractive coatings and interference filters.14,15 Light scattering is

described by Rayleigh’s law (eqn (1)) giving the relative intensity

of transmitted light of spherical particles homogeneously

dispersed in a matrix—in the case of composites a polymer—in

relation to the volume fraction Fp, sample thickness x, radius of

particles r, incident wavelength l and refractive indexes of the

particles np and matrix nm.16

I

I0¼ e�

�3Fpxr

3

4l4

�np

nm� 1

��(1)

Since light scattering is strongly decreased with diminishing

particle size, it is necessary to homogeneously disperse nano-

particles with diameters not exceeding 1/20th of the wavelength of

incident light in the matrix and to prevent agglomeration in order

to obtain transparent composites.

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Recently, our group has established a fluorolytic sol–gel

process for the synthesis of nanoscopic metal fluorides17,18 which

are examined for their catalytic properties and applicability in

optics, such as anti-reflective coatings or wavelength filters. In

a previous work,13 we studied the agglomeration behaviour of

a magnesium fluoride sol derived from the fluorolytic sol–gel

route. It was found that addition of phosphonic acids which

coordinate to the particle surface apparently stabilises primary

particles with a size of/or below 5 nm. This paved the way for the

preparation of nanocomposite materials made of magnesium

fluoride homogeneously dispersed in organic polymers. Thus, the

present work focuses on the identification of the nature of

interaction between phosphonic acid and MgF2 surfaces by IR,

Raman and NMR and the preparation and the characterisation

of composites with up to 20 wt% magnesium fluoride in acrylate

polymers.

2. Experimental

Commercially available magnesium turnings (Aldrich, 99.98%),

phenylphosphonic acid (Aldrich, 98%), 2-hydroxyethyl methac-

rylate (Aldrich, 99%), 1,6 hexanediol dimethacrylate (Aldrich)

and benzoyl peroxide (Aldrich, 75%) were used as supplied.

Methanol was dried over Mg(OMe)2 and stored over molecular

sieves.

Magnesium methoxide was prepared by dissolving Mg turn-

ings (3.0 g, 123 mmol) in 250 ml dried methanol to give a 0.25

molar solution. A stoichiometric amount of HF which has been

dissolved in methanol (9.3 M) was added to the alkoxide solution

within 5 minutes under vigorous stirring, yielding a turbid

magnesium fluoride sol. For surface modification of the

magnesium fluoride particles 10 mol% of phenylphosphonic acid

were added to the sol and refluxed for 10 h.

Composites from magnesium fluoride and acrylates were

prepared by mixing the sol with the corresponding amount of

monomer and polymerisation initiator, evaporation of the

solvent and polymerisation at temperatures from 60 to 90 �C. Toproduce 10 g of the composite, 9 g of monomer were mixed with

65 ml ofMgF2-sol (0.25M) and 20 mg of benzoyl peroxide (BPO,

0.2 wt%).

FT-IR spectra of KBr pellets were recorded on a Perkin-Elmer

2000 spectrometer in transmission mode. About 250 mg of KBr

(Fluka, Germany) were pressed with 1.0–1.5 mg of the

sample and then the samples were measured in the regions

4000–400 cm�1.

NMR spectra were recorded on a Bruker AVANCE 400

spectrometer at spinning speeds of 30 and 10 kHz and resonance

frequencies of 376.4 MHz and 161.9 MHz for 19F and 31P,

respectively. The methanolic sol was measured in a concentration

of 0.25 mol l�1 with 5600 accumulations.

Dynamic light scattering (DLS) experiments were performed

using a Zetasizer Nano ZS (Malvern Instruments, Worcester-

shire, UK). Hydrodynamic diameter was calculated by decon-

volution of the correlation functions into exponential functions

using the non-negatively constrained least squares (NNLS)

fitting algorithm as implemented in the Malvern Nanosizer

software. Small-angle X-ray scattering (SAXS) patterns were

recorded from ground samples at the mSpot beamline at BESSY

II (Helmholtz Zentrum, Berlin, Germany).19 The focusing

This journal is ª The Royal Society of Chemistry 2011

scheme of the beamline is designed to provide a divergence <1

mrad (horizontally and vertically) and a beam diameter of

roughly 100 mm at a photon flux of 5� 109 s�1 at a ring current of

100 mA. All experiments were carried out employing a wave-

length of 1.00257 �A. The scattering curves were fitted with the

program scatter20 developed by S. F€orster and L. Apostol using

a model describing the scattering intensities of hard spheres using

a Schultz-Zimm distribution with 25% polydispersity. Micro-

indenter measurements were performed on a Fischerscope

H100C (Helmut Fischer GmbH, Germany) using a diamond tip

with a load of 100 mN for 20 s. Values are given as the mean of 10

single measurements.

3. Results and discussion

Homogeneous dispersion of nanoparticles in the polymer matrix

and suppression of particle agglomeration are crucial for the

material’s optical and mechanical properties. Unfortunately,

inorganic solids and organic polymers are not properly mixable.

Due to the high surface energy of the nanoparticles as compared

to the matrix, formation of agglomerates of sol particles is

favored. By grafting alkylsilanes, carboxylic acids, or phos-

phonic acids onto particle surfaces, thus acting as tensides,

a stabilization of nanoparticles can be achieved. Furthermore, by

introducing functional groups into the organic residue, inorganic

moieties may be covalently bonded to the polymer (class-II

composites).21

Phosphonates are known to strongly complex metal ions in

a mono-, di- or tridental coordination mode. Reaction of

magnesium fluoride surfaces with phosphonic acids yielding

phosphonate complexes can involve alcoholysis or condensation

such as given in eqn (2). The sol–gel synthesis of magnesium

fluoride as used in this study starts withMg(OCH3)2 as precursor

dissolved in dry methanol. Although nanocrystalline magnesium

fluoride is formed in a stoichiometric reaction by fluorination

with anhydrous HF, some methoxide groups might still exist at

the surface of the sol particle and react with phosphonic acid.

Mg–OCH3 + HO–P(O)(OH)R / Mg–O–P(O)(OH)R

+ CH3OH (2)

R: phenyl, vinyl,.Assuming a stoichiometry of MgF2 and non-charged particles,

due to energetic reasons, magnesium fluoride particles are mainly

terminated by (110), (001) and (100) surfaces, in which each

magnesium ion is surrounded by fluorine atoms. In the sol, the

free coordination site of magnesium in the MgF2 surface might

be occupied by adsorbed methanol that might be substituted by

stronger ligands, such as phosphonates. Coordination of nega-

tively charged ions is observed by measurements of the zetapo-

tential, which shows a decrease in the potential from 37 mV of

the pure sol to 25 mV when phenylphosphonic acid is added.13

The nature of interaction between magnesium fluoride and

phosphonic acids was studied by IR and NMR. In the IR spectra

(Fig. 1), phenylphosphonic acid (PPA) coordinated to magne-

sium fluoride is shown. Magnesium fluoride itself shows only one

broad band below 700 cm�1 assigned to Mg–F vibrations. All

other bands observed originate from adsorbed phosphonic acid

species. When compared to the spectrum of pure

Nanoscale, 2011, 3, 4774–4779 | 4775

Fig. 1 FT-IR spectra of phenylphosphonic acid coordinated to

magnesium fluoride nanoparticles (a) in comparison to the pure acid (b).

Fig. 2 31P MAS NMR spectra of pure phenylphosphonic acid (a), PPA

adsorbed to magnesium fluoride sol particles in MAS NMR at 10 kHz,

na: 1184 (b), and as a methanolic sol (c) at 0.25 molar concentration, na:

5600.

Fig. 3 19F MAS NMR spectra of crystalline magnesium fluoride (a) and

phenylphosphonic acid reacted with magnesium fluoride (b), measured at

30 kHz.

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phenylphosphonic acid, changes within the fingerprint are

noticed. Absorption bands from the phosphonic acid are much

broader and indistinct, implying strong coordination to the

magnesium fluoride particles. The frequency range from 900 to

1200 cm�1 is distinctive for the identification of the coordination

mode of the phosphonic acid to the magnesium fluoride

surface.22 Bands at 1220 cm�1 and 1019 cm�1 associated to P]O

stretch and ns(P–O) respectively, which are observed in the

phosphonic acid, disappear as PPA is adsorbed to MgF2. At the

same time, P–O at 1146 cm�1 and ns(P–O) at 1119 cm�1 remain in

the spectra as is characteristic of single deprotonated phosphonic

acids. The bands at 757, 717 and 697 cm�1, which are assigned to

phenyl ring vibrations and the P–C stretch at 1489 cm�1 can also

be identified in the PPA modified MgF2 sample. The Raman

spectrum shows no emission at 959 cm�1, which would be char-

acteristic for C3v symmetry of the tridentate coordination of the

phosphonate.23 These spectroscopic data point to an adsorption

of RP(O)(OH)O�-species to under-coordinated magnesium sites

at the MgF2 surface. However, additional coordination of the

phosphoryl oxygen to Lewis acid sites at the surface of the nano-

particles cannot be ruled out completely. Similar results are

found when other phosphonic acids (butyl, octyl or vinyl) were

used for particle stabilization instead of PPA.

The 31P chemical shift in NMR can be correlated with the

coordination of the phosphonic acid to the magnesium fluoride

particle.24 While the pure phenylphosphonic acid shows a peak at

21.3 in MAS NMR (Fig. 2a), a shift of the peak maximum

towards the high field is observed. Previous studies investigating

the coordination of phosphonic acids to metal oxide particles

attributed the shifts in 31P NMR to the deprotonation and

coordination of the acid in a mono- or bidentate mode. Phe-

nylphosphonic acid reacted with magnesium fluoride particles

shows a very broad peak with a maximum at 16.4 ppm (Fig. 2b),

which is assigned to RP(O)(OH)O�. In addition to solid state

NMR, also measurements of the sols were conducted (Fig. 2c).

As for dissolved compounds, phosphonate species in solution

are expected to show a sharp signal with a width of well below

4776 | Nanoscale, 2011, 3, 4774–4779

0.1 ppm. In the case of PPA reacted with magnesium fluoride, the

NMR spectra of the sol and dried powders show almost the same

position and shape with a signal width of several ppm. The peak

broadening in liquid and solid-state NMR is a result of homo-

and heteronuclear dipolar coupling, which is not averaged by

Brownian motion or magic-angle spinning of the sample. Dis-

solved phosphonic acid or phosphonates are not observed as

superimposition of the spectra. Thus, comparison of the NMR

spectra leads to the following conclusions: (i) PPA is strongly

adsorbed to the MgF2 nanoparticle surface as single deproto-

nated phosphonate and (ii) no dissolved phosphonic acid or

phosphonate species due to equilibrium between coordination

and solvation are observed.

The 19F MAS NMR spectrum of crystalline magnesium fluo-

ride exhibits only one symmetrical peak at �198 ppm which

stands for the [MgF6]-octahedra in the rutile structure (Fig. 3,

bottom). Scholz et al. reported the correlation of the 19F NMR

chemical shift of magnesium oxide fluorides with the mean

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chemical composition of the first and second coordination

spheres of fluorine as probed based on the superposition model.25

When phosphonic acids coordinate to the surface of the

magnesium fluoride particle, oxygen is introduced in the prox-

imity of fluorine. As a consequence, a second peak at �180 ppm

appears when PPA is reacted with MgF2 (Fig. 3, top). Since the

ratio of surface to bulk atoms increases with decreasing particle

sizes, these species give a noticeable contribution to the NMR

spectrum. The signal of MgF2 remains unchanged, meaning that

no homogeneous product is formed by dissolution of magnesium

fluoride and crystallisation of fluorophosphonates but that PPA

is tightly bound to the particle’s surface.

Fig. 4 Photographs of composites from stabilised MgF2 (10 wt%) in

polyHEMA (left: phenylphosphonic acid, right: vinylphosphonic acid);

prepared by thermal polymerisation.

Preparation of composite materials by introducing MgF2

nanoparticles into acrylate polymers

The application of non-stabilised magnesium fluoride sols for the

preparation of nanocomposite films was not successful at all due

to agglomeration of the inorganic particles and thus loss of

optical transparency and gelation of the monomer/magnesium

fluoride mixture limiting its processability. When phosphonic

acids are used to stabilise the nanoparticles from agglomeration,

transparent dispersions of nanoparticles in the monomer were

obtained, which can be applied for the preparation of thin films

on glass or silicon substrates by spin- or dip-coating techniques.

Even bulk composites can be obtained by this procedure yielding

transparent composites. The size distributions of the sol particles

in methanol and in HEMA (2-hydroxyethyl methacrylate) are

listed in Table 1.

In DLS, an increase of hydrodynamic diameters is observed

when the solvent is exchanged by HEMA. For the case of pure

magnesium fluoride, agglomeration as seen by turbidity of the

dispersion is identified by shift of the correlation function

towards longer decay times. Determination of the particle size is

not advisable, due to large polydispersity of this sample. When

particles are stabilised by phosphonic acids, agglomeration of the

particles is effectively suppressed. Only a slight increase of the

particle size is noticed.

Fig. 4 shows two samples, each with a magnesium fluoride

content of 10 wt% in a polyHEMA matrix but using different

phosphonic acids for stabilisation of the nanoparticles. Although

the samples are several mm thick, both samples are obviously

highly transparent. If non-stabilized nanoparticles were used for

the preparation of the composites in the same way, agglomera-

tion takes place, which inevitably leads to intransparent and

brittle samples. Because magnesium fluoride and the phosphonic

acids are transparent in the UV-vis and light scattering is negli-

gible, spectra of both composites show no significant changes by

introduction of the magnesium fluoride nanoparticles when

compared to the pure polymer.

Table 1 Hydrodynamic diameters of MgF2 particles in methanolic soland dispersed in HEMA, determined by DLS

Methanolic sol MgF2 dispersed in HEMA

Non-stabilised 10–20 nm13 n.d.PA stabilised 2–3 nm 3–5 nm

This journal is ª The Royal Society of Chemistry 2011

When phenylphosphonic acid modified particles (Fig. 4, left)

are embedded in the polymer, hydroxyl groups of HEMA might

interact with the remaining P–OH from the phosphonic acid and/

or coordinate to the MgF2 surface. However, spectroscopic

investigations do not give any evidence for this hypothesis.

Introduction of polymerisable groups, such as vinyl in the case of

VPA-stabilised particles (Fig. 4, right), co-polymerisation with

HEMA can be expected giving composites of class II according

to the classification of Sanchez et al. Raman investigations prove

complete polymerisation of HEMA as the band at 1638 cm�1,

corresponding to the C]C double bond, can no longer be

identified (ESI†). Thus, introduction of magnesium fluoride

nanoparticles does not influence the degree of polymerization.

Unfortunately, the concentration of vinylphosphonic acid in the

sample is too small to record any Raman bands in order to

determine the degree of polymerization with the acrylate.

Dispersion of nanoparticles inside the polyHEMA matrix

A high degree of dispersion of nanoparticles in the polymer

matrix is relevant for (i) optical transparency and (ii) mechanical

properties. Considering the transparency of the composite

samples in Fig. 4, with no significant Rayleigh scattering, the

particle size should be below 20 nm, meaning that agglomeration

was effectively suppressed by surface modification using PPA or

VPA. Small-angle X-ray scattering experiments give integral

information on the size distribution of nanoparticles in the

polymer matrix. Fig. 5 shows the SAXS patterns of ground

nanocomposite from PPA stabilised magnesium fluoride (10 wt

%) in a PolyHEMAmatrix. The evaluation of the SAXS patterns

recorded is rather difficult because of the broad particle size

distribution and irregularly shaped magnesium fluoride particles.

Nevertheless, the scattering curves can be interpreted as super-

imposition of the respective scattering curves of hard spheres

with different sizes, the scattering intensities of which are given

by the Schultz-Zimm distribution. This approach seems reason-

able in this case due to high polydispersity of the nanocrystalline

MgF2 particles and spherical shape in TEM. As seen from the

comparison with the pattern expected for 4 nm sized magnesium

fluoride particles with 25% polydispersity, the measured SAXS

pattern is described quite well for the higher q values. Larger

particles with a size of approximately 10–20 nm are identified at

Nanoscale, 2011, 3, 4774–4779 | 4777

Fig. 5 SAXS pattern of PPA stabilized MgF2 nanoparticles in poly-

HEMA at 10 wt% content and approximation of the scattering curve

using Schultz–Zimm distribution of 4 nm sized spherical particles with

25% polydispersity.

Table 2 Martens hardness (MH), storage modulus (G0), penetrationdepth (PD) and initial surface hardness (ISH), determined by micro-indentation measurements, in dependence of filler content

MgF2

content (wt%) MH/N mm�2 G0/Gpa PD/mm ISH/N mm�2

0% 107 2.2 5.77 1495% 167 2.8 4.51 2177.5% 207 3.4 4.10 31610% 192 3.2 4.26 27115% 222 3.8 3.95 23520% 208 3.6 4.09 241

Fig. 6 Diagram of Martens hardness versus content of VPA- (black

circles) and PPA-(gray squares) MgF2 in polyHDDMA, determined by

microindentation.

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lower q. Agglomerates were not observed. TEM images of

ultramicrotome slices of these materials also show non-agglom-

erated magnesium fluoride particles in the polymer matrix, which

are identified by their crystal lattice plane distances (ESI†).

Turbid composites containing larger particles show an increasing

scattering curve at smaller q values, which indicates agglomera-

tion during preparation and processing. Vinylphosphonic acid

stabilized MgF2 particles in polyHEMA exhibit a scattering

pattern comparable to Fig. 5 (ESI†).

Homogeneous distribution of magnesium and fluorine over

the whole sample was proved by EDX measurements (not

shown). A consolidated view of all these factors indicates well

dispersed, isolated primary particles and small agglomerates

inside the polymer matrix when phosphonic acid stabilized

magnesium fluoride sols are used for the preparation of the

composites. In the case of non-stabilized nanoparticles partial

agglomeration is observed, which leads to a loss of transparency

of the composite material.

Investigation of surface hardness and E0 modulus by

microindentation

Based on the same procedure, composites with other acrylate

matrices, such as isobornylmethacrylate or 1,6-hexanediol

dimethacrylate (HDDMA), were also successfully prepared,

giving fully transparent films or bulk materials. The influence of

different amounts of VPA-stabilised MgF2 nanoparticles in the

HDDMA matrix was evaluated by microindentation measure-

ments. The response of the composite to the indentation of

a diamond tip pressed into the surface under defined load (100

mN) and time (20 s) is recorded in relation to the penetration

depth. Several specific parameters can be derived from the

hardness-depth curves, such as Martens hardness (MH), initial

surface hardness, penetration depth and storage modulus (G0)(Table 2). Fig. 6 illustrates the increase of Martens hardness as

a function of the MgF2 content in the composite. While as

prepared PolyHDDMA has a Martens hardness of 107 N mm�2,

only small amounts of nanoscopic magnesium fluoride already

4778 | Nanoscale, 2011, 3, 4774–4779

result in a strong increase of surface hardness. A maximum

Martens hardness is reached at a filler content of 15 wt%

exceeding the initial value by the factor of 2. Simultaneously the

initial surface hardness is increased as a consequence of MgF2

nanoparticle introduction into the acrylate matrix and penetra-

tion depth is reduced as hardness is increased. The storage

modulus of elastic deformation (G0), derived from the first part of

the hardness–depth curve, also shows the same increase from

initially 2.2 MPa with raising the MgF2 content up to 3.8 MPa

at 15 wt%.

4. Conclusion

The preparation of transparent nanocomposite materials with

a high dispersion of magnesium fluoride in the polymer matrix is

a quite challenging task. Without adequate stabilisation,

agglomeration of the MgF2 nano-particles and thus phase

separation and loss of desired properties are observed. In the

course of the previously studied agglomeration behaviour of sol–

gel derived MgF2 nanoparticles13 we found that phosphonic

acids are able to coordinate to the surface of the inorganic

particles and stabilise primary particles with a hydrodynamic

radius of 2–3 nm in methanolic sols. Based on the present IR,

Raman, and NMR investigations it is evident that single

deprotonated phosphonates strongly coordinate to

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undercoordinated magnesium sites in the surface of the nano-

scopic MgF2.particles. Thus, stabilisation of primary particles is

attributed to energetic relaxation of the magnesium fluoride

surfaces and steric effects of the phosphonic acids rather than

electrostatic repulsion of the sol particles. As a consequence, such

MgF2 nanoparticle dispersions are much more stable when

compared to non-stabilised particles. DLS experiments reveal

that the hydrodynamic diameter of these sol particles increases

only slightly when the solvent is exchanged by acrylate mono-

mers. Thus, transparent dispersions of MgF2 nanoparticles with

a content of 20 wt% and more are obtained, which can be used

for the preparation of optically transparent thin films or bulk

composite materials. SAXS and TEM measurements evidence

particles with a size of about 4 nm, which are finely dispersed in

the polymer matrix. Agglomerates are not identified.

The influence on mechanical properties of the composite

materials was evaluated by microindentation analysis and shows

a distinct increase of Martens hardness and the elastic storage

modulus G0 by introduction of nanoMgF2 (up to 20 wt%). Thus,

based on this procedure using phosphonic acid stabilised

magnesium fluoride nanoparticles, wide adaptability towards the

preparation of nanocomposites either as thin films or bulk

materials with high particle dispersion and transparency is

demonstrated. Mechanical properties are significantly altered by

introduction of little amounts of magnesium fluoride

nanoparticles.

Acknowledgements

Gudrun Scholz and Michael Teltewskoi (Humboldt-Universit€at

zu Berlin) are kindly acknowledged for conducting the NMR

measurements. We thank Franziska Emmerling (BAM Federal

Institute for Materials Research and Testing, Berlin) for SAXS

measurements of the composite. J. N. and E. K. are grateful for

financial support from ABCR GmbH (Karlsruhe, Germany). J.

N. is a member of the graduate school ‘‘Fluorine as key element’’

(GRK 1582) of Deutsche Forschungsgemeinschaft, DFG.

This journal is ª The Royal Society of Chemistry 2011

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