Refractive index tuning of highly transparent bismuth containing polymer composites

Article abstract of DOI:10.1016/j.polymer.2011.05.046

A new method for the preparation of transparent bismuth containing composites is reported. With the organic ligands 2,3-Dihydroxypropylmethacrylate (HMA) and 2-(Methacryloyloxy)-ethylacetoacetate (AcMA), bifunctional monomers with a functional group for metal complexation on the one hand and a reactive group for polymerization on the other hand were found. By integrating up to 20 wt-% bismuth in the monomer mixtures, an increase of the refractive index of 4.0% (n = 0.058) is determined. The metal complexation was observed via infrared spectroscopy (AcMA) and NMR spectroscopy (HMA). Subsequent addition of trimethylolpropane triacrylate (TMPTA) as cross-linking agent and 2,4,6-trimethyl-benzoyldiphenylphosphine oxide (TPO) as photoinitiator to the bismuth containing monomers Bi-HMA and Bi-AcMA lead to a UV-curable liquid. Photoinduced polymerization was used to generate transparent bismuth containing hybrid materials with increased refractive index. The polymers were characterized by UV/VIS spectroscopy, ellipsometry and DTA/TG. The materials show a high transmission (>80% in 2 mm thick plates) in the visible spectral range, and an increased refractive index with increasing bismuth content.

Full text of DOI:10.1016/j.polymer.2011.05.046

Polymer 52 (2011) 3263e3268
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Refractive index tuning of highly transparent bismuth containing polymer
composites
J. Fritsch , D. Mansfeld , M. Mehring , R. Wursche , J. Grothe ,*, S. Kaskel a
a
a
,
b
b
c
a
a
Department of Inorganic Chemistry, Dresden University of Technology, Bergstraße 66, D-01069 Dresden, Germany
Department of Coordination Chemistry, Chemnitz University of Technology, Straße der Nationen 62, D-09111 Chemnitz, Germany
Evonik Degussa GmbH, Paul-Baumann-Straße 1, D-45764 Marl, Germany
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 11 February 2011
Received in revised form
A new method for the preparation of transparent bismuth containing composites is reported. With the
organic ligands 2,3-Dihydroxypropylmethacrylate (HMA) and 2-(Methacryloyloxy)-ethylacetoacetate
(
AcMA), bifunctional monomers with a functional group for metal complexation on the one hand and
17 May 2011
a reactive group for polymerization on the other hand were found. By integrating up to 20 wt-% bismuth
in the monomer mixtures, an increase of the refractive index of 4.0% (Δn ¼ 0.058) is determined. The
metal complexation was observed via infrared spectroscopy (AcMA) and NMR spectroscopy (HMA).
Subsequent addition of trimethylolpropane triacrylate (TMPTA) as cross-linking agent and 2,4,6-
trimethyl-benzoyldiphenylphosphine oxide (TPO) as photoinitiator to the bismuth containing mono-
mers Bi-HMA and Bi-AcMA lead to a UV-curable liquid. Photoinduced polymerization was used to
generate transparent bismuth containing hybrid materials with increased refractive index. The polymers
were characterized by UV/VIS spectroscopy, ellipsometry and DTA/TG. The materials show a high
transmission (>80% in 2 mm thick plates) in the visible spectral range, and an increased refractive index
with increasing bismuth content.
Accepted 23 May 2011
Available online 30 May 2011
Keywords:
Bifunctional monomers
Refractive index
Photo polymerization
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
prominent strategy is the incorporation of high refractive inorganic
2
nanoparticles (such as TiO , PbS or ZnS) into organic matrices
In recent years transparent hybrid materials have attracted
interests, particularly in optical applications [1]. Integrating inor-
ganic fillers into polymer matrices provides new materials
combining properties like luminescence [2e4], UV absorption
[10,17,18]. Due to the nanosized dimension of inorganic particles
and homogeneous dispersion in the polymer matrix, no loss of
transparency due to light scattering is observed. However, the
concept of nanocomposites is restricted by the amount of inte-
grated nanoparticles into a polymeric matrix. A high content of
particles often results in agglomeration of the particles and there-
fore in loss of transparency due to scattering. Though, a huge
amount of inorganic filler is essential to synthesize high refractive
hybrid materials. Therefore, most reports of transparent hybrid
materials with high refractive index focus on thin films. So far, very
few studies are based on the fabrication of transparent bulk poly-
mers. Yang et al. [19] synthesized highly transparent polymers by
integrating up to 30 wt-% mercaptoethanol-capped ZnS nano-
particles with a corresponding increase of the refractive index from
n ¼ 1.54 to n ¼ 1.585 (Δn ¼ 0.045) and Nakayama et al. [18] achieved
enhancements of refractive indices of Δn ¼ 0.11 (from n ¼ 1.54 to
[
5e8] and high refractive index [9e13] with the transparency of the
organic polymer matrices.
Most transparent
commercial
polymers
like
poly-
methylmethacrylate (PMMA) have refractive indices in a narrow
range of 1.3e1.7 [14]. Only a few high refractive polymers such as
polythiophene (n ¼ 2.12) [14] are known. In contrast, inorganic
materials usually have a high refractive index between 2.0 and 5.0
[15,16], but they have disadvantages such as a high density. Thus,
the development of high refractive index transparent materials is
a challenging topic. Some strategies to integrate inorganic materials
into polymer matrices were reported in the last years. The most
*
Corresponding author. Tel.: þ49 351 46332029; fax: þ49 351 46337287.
E-mail addresses: Julia.Fritsch@chemie.tu-dresden.de (J. Fritsch), Dirk.
n ¼ 1.65 by integrating 30 wt-% ZrO
2 2
-TiO nanoparticles) but they
did not document the transparency quantitatively in the bulk
polymer blocks.
To avoid light scattering due to agglomeration of particles we
decided to generate high refractive hybrid materials integrating
Mansfeld@chemie.tu-dresden.de
(D.
Mansfeld),
michael.mehring@chemie.
tu-chemnitz.de (M. Mehring), roland.wursche@evonik.com (R. Wursche), Julia.
Grothe@chemie.tu-dresden.de (J. Grothe), Stefan.Kaskel@chemie.tu-dresden.de
(
S. Kaskel).
0
032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2011.05.046

3264
J. Fritsch et al. / Polymer 52 (2011) 3263e3268
molecular, high refractive inorganic compounds by complexation
into polymeric matrices. The incorporation of high molar refraction
elements for refractive index tuning has been used before. Paquet
et al. successfully introduced organometallic components gaining
high refractive index metallopolymers such as polyferrocenes and
polyferrocenylsilanes [20,21].
UV-1650PC with a scan range from 800 to 200 nm and a step width
of 0.5 nm. Ellipsometric determinations were made on a WOOLLAM
ellipsometer VASE with WVASE 32 evaluation program and a scan
ꢀ
ꢀ
ꢀ
range from 350 to 1050 nm (Dl ¼ 10 nm) at
F
¼ 60 , 65 , 70 at
Fraunhofer IWS, Dresden.
2þ
Lin et al. [22] prepared Pb containing methacrylates (MA),
obtaining transparent copolymers of Pb(MA) /styrene/MA with
0 wt-% Pb(MA) and a corresponding increase of the refractive
2.2. Synthesis of 2,3-dihydroxypropylmethacrylate (HMA) [23]
2
3
2
1
4.2 g (0.1 mol)
u
-Epoxypropylmethacrylate were dissolved in
index from n ¼ 1.554 to n ¼ 1.563 (Δn ¼ 0.009). In consideration of
the non-toxic behaviour of bismuth based materials we used
a similar strategy to prepare bismuth containing polymers.
Therefore it is necessary to create bifunctional monomers with
a functional group for metal complexation on the one hand and
a reactive group for polymerization on the other hand (Scheme 1
left).
ꢀ
3
0 ml THF. At 20 C, 100 ml of a 2 N sulphuric acid solution were
added and the mixture was stirred for 2 h. After finishing, the
reaction mixture was extracted four times with dichloromethane
4
and the combined phases were dried over MgSO . After removing
ꢁ
3
ꢀ
the solvent under vacuum (1 ꢂ10 mbar) at 20 C the product was
obtained as a pale yellow oil (yield: 60%).
The intention of our work was to create bismuth containing
monomers for generating high refractive transparent polymers.
Due to the position in the high atomic number bismuth is an
adequate inorganic filler for increasing refractive index of poly-
mers. Particularly, the non-toxic behaviour of bismuth based
materials is superior to e. g. lead based materials.
In the following, we present the complexation of bismuth to the
bifunctional monomers 2,3-Dihydroxypropylmethacrylate (HMA)
and 2-(Methacryloyloxy)-ethylacetoacetate (AcMA) (Scheme 1
right). The influence of bismuth content in the monomer solution
on the refractive index is explored. Subsequent radical polymeri-
zation allows the generation of a hybrid material with the desired
optical properties.
2
.3. Synthesis of the bismuth containing monomers
To prepare the bismuth containing monomers Bi-HMA, the
complexing ligand 2,3-Dihydroxypropylmethacrylate was added to
a solution of 0.5 g (1.6 mmol) BiCl in 5 ml THF. Supplementary,
butanediol monoacrylate (Laromer BDMA) was added as como-
nomer and stabilizer due to the brittleness of the resulting poly-
meric composites and sensitivity to hydrolysis of the solvent-free
monomer mixtures. In Table 1 the respective reactants are listed.
3
Ò
ꢀ
The reaction mixture was stirred for 3 h at 20 C. After eliminating
ꢁ3
ꢀ
the solvent under vacuum (1 ꢂ10 mbar) at 20 C the product was
obtained.
t
For preparing Bi-AcMA, 0.428 g (1 mmol) Bi(O Bu)
3
were dis-
2
. Experimental
solved in 5 ml THF and 2-(Methacryloyloxy)-ethylacetoacetate was
added. The respective reactants are listed in Table 1. The mixture
was stirred 3 h at 20 C and the solvent was removed at 20 C in
vacuum. Due to the brittleness of the resulting polymer composites,
butanediol diacrylate (Laromer BDDA) was added as comonomer
in some cases.
2.1. Chemicals and analytical investigations
ꢀ
ꢀ
Commercially available bismuth chloride (S2) was sublimed two
times. The synthesis of bismuth tert-butylate was carried out at the
Ò
Institute of Coordination Chemistry at Chemnitz University of
t
Technology (S1). The precursors BiCl
3
and Bi(O Bu)
3
were handled
under argon atmosphere. The tetrahydrofurane for each synthesis
was dried using the M. BRAUN MB SPS-800. Other chemicals were
2.4. Polymerization
1
13
1 wt-% LucirinÒ TPO (photoinitiator) and 10 wt% LaromerÒ
TMPTA (cross linking agent) were added to the bismuth containing
monomer/(comonomer)-compound. The mixture was placed
between two glass plates and separated by a 2 mm thick PVC seal.
The polymerization was carried out in a UV cube inert (HOENLE AG)
used as received without further purification (S2). H and C NMR
1
13
spectra were recorded on a BRUKER DRX 500 P ( H: 500 MHz, C:
26 MHz). The refractive indices of the pure and bismuth con-
1
ꢀ
taining monomers were measured on a KRÜSS DR 6100-T at 20
C
(
l
¼ 589.1 nm). Infrared (IR) spectra were recorded on a BIO
RAD
ꢁ
1
2
Excalibur FTS 3000 between 650 and 4000 cm at room temper-
ature (RT), measured as liquid film between two KBr discs. Ther-
mogravic analyses were carried out using a NETZSCH STA 409 PC Luxx
using UV-radiation (200 W/cm , LAMP FOZFR 100 D22 U150 E3S9)
for 20 min. Scheme 2 illustrates the synthesis strategy.
ꢀ
thermal analyser with heating in air 5 K/min up to 800 C. Due to
Table 1
the resulting mass of the residue Bi
contents of the composites were calculated. UV/VIS analysis of the
mm thick polymer samples were performed on a SHIMADZU
2 3
O the effective bismuth
Respective reactants for the synthesis of the monomers Bi-HMA and Bi-AcMA.
2
Sample
name
Comonomer
Molar ratio ligand:
bismuth:comonomer
Theoretical
bismuth content
[wt-%]
Functional group
2-(Methacryloyloxy)-ethylacetoacetate (AcMA)
HMA-0
HMA-1
HMA-2
HMA-3
HMA-4
HMA-5
AcMA-0
AcMA-1
AcMA-2
AcMA-3
AcMA-4
AcMA-5
AcMA-6
e
1:0:0
e
BDMA
BDMA
BDMA
BDMA
e
4.7:1:5.3
4.7:1:4
4.7:1:2.6
3:1:1.7
4.2:1:0
1:0:0.25
4.5:1:0
4:1:0
11.5
13
14.5
20
21
e
O
O
O
OH
O
O
OH
O
OR
O
O
2,3-Dihydroxypropylmethacrylate (HMA)
Monomeric group
OH
BDDA
e
O
O
O
OH
15
16
20
12
15
17
O
e
e
3:1:0
BDDA
BDDA
BDDA
3:1:3.2
3:1:1.6
3:1:0.8
Scheme 1. left: Examples of bifunctional monomers with functional groups for metal
complexation on the one hand and monomer groups for polymerization on the other
hand; right: used monomers HMA and AcMA.

J. Fritsch et al. / Polymer 52 (2011) 3263e3268
3265
Scheme 2. Scheme of the composite preparation.
Table 2
with increasing atom mass of the coordinated atom is characteristic
(1639 cm is the characteristic band for the enol-form without
metal complexation).
ꢁ1
Change of the refractive index (
monomer mixtures of Bi-AcMA.
l
¼ 589.1 nm) with different bismuth contents in the
Molar ratio AcMA:
Bi:BDDA
Theoretical content
of bismuth
Refractive index
Due to the brittleness of the resulting polymer composites,
a comonomer was added to the Bi-AcMA monomer mixtures in
some cases. Using the comonomer BDDA, an increase of the
refractive index with increasing content of bismuth is observed
without losing transparency. The strategy affords an integration of
up to 17 wt-% bismuth (AcMA-6), the refractive index increases
from 1.457 to 1.495 according to an enhancement of 2.6% (Table 3).
AcMA-0
AcMA-1
AcMA-2
AcMA-3
1:0:0.25
4.5:1:0
4:1:0
e
1.457
1.496
1.502
1.515
15 wt-%
16 wt-%
20 wt-%
3:1:0
3
. Results and discussion
3.2. Characterization of Bi-HMA monomers
3
.1. Characterization of Bi-AcMA monomers
By complexing bismuth to the bifunctional monomer HMA, also
an increase of the refractive index is observed. As for AcMA,
different contents of bismuth were integrated into the monomer
matrix. Due to the hydrolysis sensitivity of the complexes, BDMA
was added as stabilizer. The change of refractive index was deter-
mined in the comonomer mixture Bi-HMA/BDMA. As shown in
Table 4 it is possible to integrate up to 21 wt-% (HMA-5) bismuth
into the monomer matrix Bi-HMA and up to 20 wt-% (HMA-4) into
the system Bi-HMA/BDMA. By integrating 21 wt-% Bi to the Bi-HMA
monomer, an increase of the refractive index from 1.472 to 1.508,
according to an enhancement of 2.5%, is observed. The comonomer
strategy leads to an increase of the refractive index from 1.465 to
1.507, according to an enhancement of 2.8%
With the complexation of bismuth to the bifunctional monomer
-(Methacryloyloxy)-ethylacetoacetate (AcMA), an increase of the
2
refractive index is observed. To study the correlation between
bismuth content and refractive index, different contents of bismuth
were integrated into the monomer matrix of AcMA. As shown in
Table 2 it is possible to integrate up to 20 wt-% bismuth. The
refractive index increases from 1.457 to 1.515, corresponding to an
enhancement of 4.0%.
To prove the complexation of bismuth in the monomer, IR
ꢁ
1
spectroscopy was used (Fig. 1). The broad band at 3440e3560 cm
A) indicates an increased number of OH-groups in the composite.
(
By complexing bismuth,tert-butanol is generated in the reaction
mixture and a preferred shift to the enol-form of the monomer is
built (Scheme 3). A second indication for the enol-form is shown
The bismuth containing complexes, exemplified by the sample
1
13
of HMA-5, were characterized by H and C NMR and ESI-MS. The
content of chloride ions was analysed by elemental analysis. Due to
the mentioned hydrolysis sensitivity it was not possible to examine
the Bi-complexation via IR spectroscopy.
ꢁ1
with the decreased signal at 1757 cm (B) which is representative
ꢁ
1
for carbonyl-group vibrations. Additionally, the signal at 1549 cm
C) in the composite material is based on the complexation of
1
(
The H NMR spectra of the pure HMA and Bi-HMA show
bismuth. Due to chelating coordination of atoms to carbonyl
a change of the broad signal at 2.5e2.8 ppm which is attributed to
groups, especially in ß-ketoesters, a shift to lower wave numbers
the OH-groups (S3). The integration of the OH-signal decreases
Fig. 1. FT-IR spectrum of the bismuth containing sample AcMA-1 (black) in comparison with the pure monomer AcMA-0 (grey, AcMA-0 was measured without BDDA as
comonomer).

3266
J. Fritsch et al. / Polymer 52 (2011) 3263e3268
R2
Bi
Table 4
H
t
Bi(O Bu)3
Change of the refractive index (
l
¼ 589.1 nm) with different bismuth contents in the
O
O
O
O
(co)monomer mixture of Bi-HMA(/BDMA).
R
O
O
O
R
R
O
O
Sample
name
Molar ratio HMA:
Bi :BDMA
Theoretical content
of bismuth [wt-%]
Refractive index
(T ¼ 20 ꢀC)
O
t
with R = O Bu /
O
with R =
HMA-0
HMA-0*
HMA-1
HMA-2
HMA-3
HMA-4
HMA-5
1:0:0
0
0
11.5
13
14.5
20
1.472
1.465
1.481
1.489
1.495
1.507
1.508
O
O
3:0:1.7
4.7:1:5.3
4.7:1:4
4.7:1:2.6
3:1:1.7
4.2:1:0
Scheme 3. Proposed mechanism of the complexation of bismuth to AcMA.
from 2 to 0.5 which confirms the decrease of pure monomer
compared to the complex. This result confirms the proposed
mechanism of complexation shown in Scheme 4. The enlargement
and splitting of the multiplets as well as a slight shift of the
hydrogen atoms on C-1, C-2 and C-3 also evidences the presence of
21
to still bonded t-butylate and chloride ions due to incomplete
conversion of the bismuth precursors with the ligand, which was
not considered in the calculation of the theoretical bismuth
contents. Additionally, the formation of volatile bismuth
compounds at elevated temperatures as described in the literature
13
two compounds, Bi-HMA and the pure HMA. By comparing the
C
NMR spectra of HMA and Bi-HMA no further changes were
observed. The exchange of the hydrogen of the hydroxyl groups by
bismuth seems to have no influence on the proximate carbon atoms
because the oxygen atoms have a stronger substituent influence on
these carbon shifts [24]. With the twofold determination of the
content of chloride ions the schematically mechanism is also
underlined. The determined content of 2.77 and 2.97 wt-% chloride
in comparison with the calculated maximal amount of 9.92 wt-%
confirms the successful complexation of bismuth and the discharge
of chloride by forming of hydrogen chloride gas (HCl). Both analysis
methods, NMR and the determination of the chloride content, show
[
25,26] is possible.
To verify the transparency of the polymers, transmittance UV/
VIS spectra were recorded. The spectra of the materials of Bi-AcMA
and Bi-AcMA/BDDA with different contents of bismuth are shown
in Fig. 3.
By the integration of up to 20 wt-% bismuth into the polymer
matrix of AcMA (Fig. 3 left) insignificant loss in transparency is
detected. All materials with a sample thickness of 2 mm show
a transmittance higher than 80% over the whole range of visible
light. Furthermore, slight decreases of transparency are due to
absorption, not to light scattering. The sharp absorption edges of
the blank polymer at about 330 nm and of the bismuth containing
polymers at 400 nm are caused by excess of the photoinitiator
3
that the reaction of BiCl and HMA is incomplete but the equilib-
rium of the reaction is on the product side. ESI-MS analysis (S4)
only locates the presence of the pure ligand HMA and a signal at m/
z ¼ 303 indicates a complex of Bi(OMe)
3
resulting from the carrier
Ò
Lucirin TPO in the material. The differences between the
solvent methanol. The latter indicates a higher interaction between
bismuth and methanol than between bismuth and HMA. There is
a complete exchange of the ligands during the measurement.
Concluding, the structural analysis of the complexes of Bi-HMA
is difficult due to the hydrolysis sensitivity but the studies indicate
the expected reaction shown in Scheme 4.
composite samples are caused by the different colours of the
polymer materials which change from colourless to yellow with
3þ
increasing content of Bi . The increasing Bi-content results in
a shift of absorption to higher wavelengths.
The polymer materials based on AcMA (independent on the
amount of bismuth) reveal a kind of brittleness (shown in the
picture of Fig. 3). Due to this fact, BDDA was added as a comonomer
(
internal softener) to the Bi-AcMA mixtures before photo
3.3. Characterization of the bismuth containing polymers
polymerization.
With the copolymerization of Bi-AcMA/BDDA, highly transparent
less brittle composite materials are received. Fig. 3 (right) presents
the transmittance spectra of a pure polymer AcMA-0 and Bi-AcMA/
BDDA copolymers with different contents of bismuth. All samples
have a high transmittance in the visible spectral range. Some loss of
transparency for the bismuth containing materials is attributed to
absorption effects due to the yellow colour of the samples. The steep
absorption edges at about 330 nm and 380e400 nm are caused again
After in situ polymerization of the bismuth containing mono-
mers, highly transparent composite materials were obtained. To
identify the effective content of bismuth in the polymer, ther-
mogravimetric analyses (TGA) were performed. Due to the result-
2 3
ing mass of the residue Bi O the effective bismuth content was
calculated (Fig. 2).
As shown in the table at the left of Fig. 2 the calculated bismuth
contents (column 3) due to the resulting mass of the residue Bi
2 3
O
Ò
by excess of photoinitiator Lucirin TPO in the material. The differ-
are lower than the theoretical values used in the manuscript
Ò
ences between are due to the different compositions. The reduced
brittleness of the copolymeric composites leads to defined polymer
plates (shown in the picture of Fig. 3 right).
(
column 4). As a result of the addition of Lucirin TPO (photo-
Ò
initiator) and Laromer TMPTA (cross linking agent) in the poly-
merization process, the bismuth contents in the polymers must be
lower than the bismuth contents in the pure monomer mixtures.
Therefore the bismuth contents were calculated taking into account
the addition of these additives (column 5). Comparing column 3
and 5 the values are similar. Minor differences could be attributed
The composite materials of Bi-HMA also show a high trans-
parency over the whole range of visible light (Fig. 4). The sharp
absorption edge for the blank polymer at 400 nm is caused again by
Ò
excess of photoinitiator Lucirin TPO in the material. Slight
1
-
Table 3
Change of the refractive index (
comonomer mixtures of Bi-AcMA/BDDA.
l
¼ 589.1 nm) with different bismuth contents in the
R
O
O
O
O
O
+
6
HMA + 3 BiCl3
3
+
3 H
Bi
Molar ratio AcMA:Bi:
BDDA
Theoretical content
of bismuth
Refractive index
- 9 HCl
O
with R =
R
AcMA-4
AcMA-5
AcMA-6
3:1:3.2
3:1:1.6
3:1:0.8
12 wt-%
15 wt-%
17 wt-%
1.473
1.487
1.495
O
Scheme 4. Proposed mechanism of the complexation of bismuth to HMA.

J. Fritsch et al. / Polymer 52 (2011) 3263e3268
3267
Fig. 2. Calculated bismuth contents from DTA/TG-analysis in comparison with the theoretical bismuth contents (left) and the PXRD pattern of the combustion residue Bi
2
O
3
(right).
Ò
Ò
*
: The theoretical bismuth contents in the polymer samples were calculated in relation to the addition of Lucirin TPO and Laromer TMPTA in the polymerization process.
Fig. 3. Left: transmittance spectra of Bi-AcMA (dashed line: AcMA-0, black: AcMA-1, dark grey: AcMA-2, light grey: AcMA-3) and photo of AcMA-1; Right: transmittance spectra of
Bi-AcMA/BDDA (dashed line: AcMA-0, black: AcMA-4, dark grey: AcMA-5, light grey: AcMA-6) and photo of AcMA-5.
decreases of transparency are due to absorption and insignificant
small asperities.
In our work, the photochromic effect appears only by using BiCl
3
as
t
bismuth precursor. By Using Bi(O Bu)
could not be observed.
3
as bismuth source this effect
As illustrated in the picture on the right side of Fig. 4, photo
polymerization of the monomer mixture Bi-HMA shows another
interesting result e a photochromic effect. The colour of the
material changes from colourless to dark brown after UV irradia-
tion. This process is reversible by exposing the sample in air.
Further UV irradiation colours the samples brown again. This effect
is caused by redox processes between bismuth and halogen con-
taining structural units and the radical initiator in the material [27].
To determine the change of refractive index in the polymer
materials and compare those with the corresponding monomer
mixtures, it was possible to record ellipsometric spectra of some
polymers. Exemplified for AcMA-1, AcMA-4 and the pure AcMA-0,
the results are shown in Fig. 5. At lower wavelengths an increase
of the refractive index was detected, the pure and bismuth con-
taining materials show normal dispersion. For determination of the
Fig. 4. Transmittance spectra of Bi-HMA on the left (dashed line: HMA-0, black: HMA-1, dark grey: HMA-2, light grey: HMA-3) and demonstration of the photochromic effect on the right.

3268
J. Fritsch et al. / Polymer 52 (2011) 3263e3268
Subsequently photo polymerization of the bismuth containing
monomer mixtures yields in highly transparent polymer plates.
UV/VIS spectroscopic measurements show the visual transparency
with transmittance values >80% in the visible spectral range. Slight
losses are due to absorption effects of the yellow samples, not to
scattering. In case of the composite materials of Bi-HMA with BiCl
3
as starting material, a photochromic effect was observed during
photo polymerization.
Exemplified for AcMA-1 (15 wt-% bismuth) and AcMA-4 (12 wt-
%
bismuth), an increase of the refractive index was determined via
spectroscopic ellipsometry, confirming the results of the monomer
mixtures.
Appendix. Supplementary data
Fig. 5. Results of the spectroscopic ellipsometry (dashed line: AcMA-0, grey: AcMA-4,
black: AcMA-1).
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.polymer.2011.05.046.
increasing refractive index of the polymers with different bismuth
contents, the sodium D-line at 590 nm was investigated. The
refractive index increases from n ¼ 1.498 (AcMA-0) to n ¼ 1.530
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(
AcMA-4 with 12 wt-% bismuth) and n ¼ 1.538 (AcMA-1 with
15 wt-% bismuth). Consequently, the refractive index increases with
increased bismuth content. These results confirm the investigations
in the monomer mixtures.
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[
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4
. Conclusion
1
[
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Summarizing, we have developed a new method for the synthesis
Degrad Stabil 2004;85(3):927e46.
of bismuth containing composite materials with a thickness of 2 mm
and high transparency after polymerization. The concept using
bifunctional organic ligands such as 2-(Methacryloyloxy)-ethyl-
acetoacetate and 2,3-Dihydroxypropylmethacrylate with functional
groups for metal complexation on the one hand and monomer units
on the other hand allow the complexation of Bi , and subsequent
photo polymerization yields in composite materials with the desired
properties.
Through complexation it was possible to integrate bismuth
contents up to 20 wt-% into the monomer AcMA and up to 17 wt-%
into the AcMA/BDDA comonomer matrices. In case of the system of
HMA it was possible to integrate up to 21 wt-% bismuth. With the
variation of bismuth contents in the monomer mixtures an increase
of the refractive index was observed. With the integration of 20 wt-%
Bi (AcMA-3) it is possible to achieve an enhancement of 4.0%, in the
case of the Bi-AcMA/BDDA comonomer mixtures the integration of
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3þ
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17 wt-% Bi (AcMA-6) affords an increase of the refractive index of
[
2
.6%. In the system Bi-HMA it was possible to achieve an enhance-
ment of the refractive index of 2.5% by integration of 21 wt-%
bismuth (HMA-5).
[
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