Nanophase separated amphiphilic microbeads

Article abstract of DOI:10.1021/ma0509715

The synthesis of nonporous nanophase separated amphiphilic microbeads was investigated. The synthesis was performed with a mixture of α,ω- methacrylate-terminated poly-dimethylsiloxane (MA-PDMS-MA), trimethylsilylated 2-hydroxyethylacrylate (TMSOEA) and t

Full text of DOI:10.1021/ma0509715

7536
Macromolecules 2005, 38, 7536-7539
phases, which can be swollen separately in a suited
solvent, being a typical behavior of amphiphilic conet-
works.15 Thereby, one selectively swollen polymer phase
is loaded with the catalyst dissolved, e.g., in water. After
drying and subsequent swelling of the conetworks in an
orthogonal solvent, the substrate, dissolved for instance
in n-heptane, diffuses into the other polymer phase of
the conetwork. The catalyzed reaction takes place at the
huge interface between the two polymer phases. Since
the demixing of the incompatible macromolecule seg-
ments during polymerization is very fast, only thin
coatings and membranes that are fully nanostructured
could be obtained in water-free systems.16 Thus, the
synthesis of amphiphilic conetwork microbeads could
not be realized so far. Here, we describe the first
synthesis of such thoroughly nanostructured nonporous
amphiphilic polymeric microbeads and their evaluation
as catalyst support.
We chose to apply a precursor strategy for am-
phiphilic conetworks,17 whereby a hydrophobic poly-
meric cross-linker (R,ω-methacrylate-terminated poly-
(dimethylsiloxane), MA-PDMS-MA, degree of functional-
ization 0.92, MW ) 5200 g/mol, PD ) 1.3) is copolymer-
ized with trimethylsilylated 2-hydroxyethylacrylate
(TMSOEA) to yield an hydrophobic precursor network.
Upon subsequent cleavage of the TMS groups, an
amphiphilic, nanophase separated conetwork is formed
as we could show in previous work.16 The challenge in
transferring this synthesis to a microbead preparation
via suspension polymerization was the use of water as
polymerization medium because the TMS protecting
groups are easily cleaved in an aqueous environment,
which immediately leads to a poly(2-hydroxyethyl acry-
late) (PHEA) cover on the respective surface.
The synthesis was performed with a mixture of MA-
PDMS-MA, TMSOEA, and the photoinitiator Irgacure
651 (3 wt % regarding to monomer mixture) emulsified
in a surfactant solution in 100 mM phosphate buffer,
pH 7.0. The resulting emulsion (10 wt % monomer
mixture) was UV-radiated at 340 nm for 3 min. The
monomer mixture was adjusted to 50 vol % PDMS, and
four different surfactants were used. While the nega-
tively charged surfactants SDS and Disponil FES77
effectively stabilized the emulsion of the PDMS/
TMSOEA mixture in phosphate buffer at concentrations
as low as 0.2 wt %, the neutral surfactants Lutensol
AT25 and Pluronic F68 needed to be used at 2 wt % to
avoid aggregation during polymerization. In all cases,
a dispersion of spherical polymer particles was obtained.
The beads were filtered off, were washed with the
phosphate buffer to remove the respective surfactant,
and were then immersed into a water/methanol mixture
(1:1, v/v) to cleave the TMS groups. The obtained
polymer beads of the amphiphilic conetworks are per-
fectly round shaped and do not show any porosity as
seen in the ESEM (environmental scanning electron
microscope) image displayed in Figure 1a on the ex-
ample of beads prepared with the aid of Disponil. The
diameter of the particles was measured to be in a range
of 30-80 µm. Additional measurements with light
scattering confirmed the particle’s diameter with a
maximum of 53 µm and a standard deviation of 27 µm,
which also shows the relatively broad distribution of the
polymer bead size. Similar results regarding particle
Nanophase Separated Amphiphilic
Microbeads
Gabriela Savin, Nico Bruns, Yi Thomann, and
Joerg C. Tiller*
Freiburg Materials Research Center and Institute for
Macromolecular Chemistry, Department of Chemistry,
University of Freiburg, Stefan-Meier-Str. 21,
D-79104 Freiburg, Germany
Received May 10, 2005
Revised Manuscript Received July 13, 2005
Organo-chemical reactions, where the substrates and
the catalyst are not soluble or stable in the same solvent,
e.g., enzyme-catalyzed reactions in organic solvents and
organo-metal-catalyzed reactions in water, are often
performed with suspended or immobilized catalysts or
as phase transfer reactions. In the first case the catalyst
often exhibits lower activity compared to that of its
dissolved form. Phase transfer reactions, on the other
hand, are usually elaborate because two nonmiscible
solvents are used. An alternative that afforded high
activity and easy handling was the use of catalysts
immobilized on polymeric microbeads.1,2 Since then, the
bead design was mainly focused on large porosity and
functional density. Later, the adsorption of molecules
was influenced more selectively by molecular imprint-
ing.3,4 More recently, the benefits of amphiphilic struc-
tures to various catalytic applications moved into focus
of research.5 Various efforts to prepare amphiphilic
polymer beads have been made in order to achieve high-
affinity catalyst supports or high-performance chroma-
tography materials. A common way is the modification
of the surface of prestructured inorganic microspheres,
e.g., silicate,6,7 or porous polymeric beads, mostly poly-
styrene,8 via physically or covalently grafting polymers
or amphiphilic block copolymers. Alternatively, hydro-
phobic beads made of chemically different but miscible
monomers have been prepared and subsequently modi-
fied. For example, Yashuda et al. prepared amphiphilic
microbeads by amination of beads composed of a mix-
ture containing poly(glycidyl methacrylate) and subse-
quently statistically modifying them with fatty acids.9
Further, the cross-linking of amphiphilic block copoly-
mers or dendrimers led to amphiphilic microparti-
cles.10,11 Mostly, PEG telechelics have been used as
cross-linking agent of polystyrene or acrylates to achieve
polymer beads swellable in water and organic solvents.12
Another way to amphiphilic beads was described by
Shahidi et al., who prepared microbeads consisting of
two interpenetrating networks, being cross-linked rub-
ber and a poly(acrylic acid).13 Although some of these
particles were found to have greater affinity to catalysts
or substrates, they all function in the common way; i.e.,
the catalysts is immobilized on the surface of the mostly
porous beads.
In previous work, we could show that nonporous
amphiphilic conetworks exhibit a great potential as
carriers for catalysts in phase transfer reactions, par-
ticularly for biotransformations in organic solvents.14
The concept for such an application is based on the
nanophase separation of two incompatible polymer
* To whom correspondence should be addressed: Fax (49)761
203 4709; e-mail joerg.tiller@fmf.uni-freiburg.de.
10.1021/ma0509715 CCC: $30.25 © 2005 American Chemical Society
Published on Web 08/10/2005

Macromolecules, Vol. 38, No. 18, 2005
Communications to the Editor 7537
Figure 2. AFM phase mode images of the surfaces of
amphiphilic PHEA-l-PDMS (60 wt % PDMS) microbeads
prepared with (a) Disponil, (b) Lutensol, (c) Pluronic, and (d)
SDS as emulsifier.
molecular weight compound SDS cannot provide this.
However, subsequent transformation of the obtained
precursor networks into amphiphilic ones by cleavage
of the TMS groups in methanol/water is fully achieved.
All ATR-FTIR spectra reveal the complete disappear-
ance of the TMS-typical signal at 836 cm-1 as shown in
the example of the PHEA-l-PDMS microbeads prepared
with Disponil (Figure 1b).
It was now investigated with atomic force microscopy
(AFM) if both polymer phases are present on the surface
of the particles, which is crucial for the application of
the microbeads in the phase transfer catalysis. Figure
2 displays the AFM images in phase mode (discrimi-
nates soft (PDMS, dark) and hard (PHEA, bright)
material) of the surfaces of PHEA-l-PDMS microbeads.
While the particles prepared with polymeric surfactants
(Figure 2a-c) show indeed both polymer segments on
their surface, the SDS-prepared microbeads do not. The
latter is most likely due to a surface demixing of the
hydrophilic PHEA segments during the polymerization,
which occurs according to the IR data shown in Figure
1b. This apparently leads to a surface covered with
PHEA. However, it seems that every surfactant affords
a specific surface morphoplogy. Thereby, Pluronic re-
sults in beads with the most distinguishable morphol-
ogy.
To investigate the phase separated bulk morphology
of the PHEA-l-PDMS microspheres, samples prepared
with Pluronic as emulsifier were embedded in a silicone
matrix and cut under liquid nitrogen, and the cross
sections were again explored with AFM. As seen in
Figure 3a, the amphiphilic polymer microbead shows a
continuous nanostructure from the inside to the surface.
Thereby, the PHEA segments form a spongelike con-
tinuous phase with domains sizes of about 3-6 nm and
the PDMS domains (18-30 nm) are embedded into the
PHEA structure. Even the surface morphology, which
looks similar, is evident in this picture (cf. Figure 2c).
Images with comparable morphologies were observed
for all preparations with polymeric emulsifiers. Com-
paring surface (Figure 2) with bulk morphology (Figure
Figure 1. (a) ESEM image of PHEA-l-PDMS microparticles
prepared with Disponil and subsequent immersion in water/
methanol for 24 h. (b) ATR-FTIR spectra of microparticles
synthesized from MA-PDMS-MA (50 vol %) and TMSOEA
prepared with Disponil (black s), with SDS (gray s), and with
Disponil and subsequent immersion in water/methanol for
24 h (- - -).
shape and size were found for polymer beads prepared
with SDS, Lutensol, and Pluronic.
ATR-FTIR was used to investigate the chemical
composition of the particles near their surface. A signal
at 836 cm-1 corresponds to the O-Si bond of the TMS-
protected PHEA chain segments and disappears upon
cleavage of the TMS groups. As seen in Figure
1b, when preparing the microbeads with Disponil,
the spectrum is that of a water-free fully protected
PTMSOEA-l-PDMS conetwork.18 Samples prepared with
Pluronic and Lutensol (spectra not shown) look similar
to that displayed in Figure 1b, i.e., no significant
cleavage of the TMS group occurred. However, when
using SDS as surfactant, the signal of the TMS-group
is significantly lower compared to that of the pure
PTMSOEA-l-PDMS network (cf. Figure 1b). Altogether,
according to the IR data, only polymeric surfactants
regardless their charge are suitable to protect the TMS-
modified HEA during polymerization, while the low

7538 Communications to the Editor
Macromolecules, Vol. 38, No. 18, 2005
Figure 3. (a) AFM phase mode image of a cross section of a silicone embedded PHEA-l-PDMS microbead (60 wt % PDMS, 500
× 500 nm2). (b) Proposed structure according to the AFM images measured on and within the PHEA-l-PDMS microbeads.
Table 1. Initial Activities of Free and PHEA-l-PDMS
Embedded r-Chymotrypsin (CT) in the
Transesterification of APEE with Propanol in Dry
n-Octane (Water Content < 0.02 wt % According to Karl
Fischer Titration) at 30 °C, 30 mM APEE, and 1 M
Propanol
which lowered the water content below the detection
limit of some 0.1 wt %. The initial rates of the trans-
esterification reaction shown in Table 1 reveal the great
activation potential of the amphiphilic network beads
with the best activity being 165 times greater than that
of the merely suspended native enzyme. The enzyme
does not leach in the medium during the reaction as
proven by a complete halt of the product formation after
removing the particles.
The activation of the immobilized CT increases with
greater PDMS content. This is probably due to smaller
hydrophilic phases at greater PDMS content, affording
the loaded enzyme molecules to closer proximity to the
interface between the hydrophilic and the hydrophobic
domains of the network. Exploring this phenomenon
will be part of future investigations, applying networks
loaded with labeled enzymes to high-resolution micros-
copy such as AFM and TEM. Comparing the results to
literature experiments, where CT was activated upon
entrapment, shows the good performance of the am-
phiphilic microbeads. The best results so far were
obtained by Novick and Dordick,20 who covalently
immobilized CT on a polysaccharide matrix and afforded
a 34-fold activation of the enzyme, and by van Unen et
al.,21 who entrapped the enzyme into a siloxane matrix
via a sol-gel technique, which led to a 45-fold activation
of CT.
PDMS
content of
carrier
CT
activity of CT
TOFb (min-1 U/mgc
loadinga
(wt %)
activity
)
enhancementd
free enzyme
40 wt %
60 wt %
0.08
2.0
2 × 10-6
50 × 10-6
355 × 10-6
2.4
1.5
25
165
13.4
a mg of CT/mg of carrier. b TOF ) turnover frequency [mol of
APPE/(mol of CT min)]. c µmol of APPE/(min mg of CT). d v(em-
bedded enzyme)/v(free enzyme).
3a) leads to the conclusion that the prepared PHEA-l-
PDMS are truly thoroughly nanophase separated mi-
crobeads, as illustrated in Figure 3b.
The suitability of the amphiphilic PHEA-l-PDMS
microbeads as phase transfer matrix for catalysis was
explored in the example of the R-chymotrypsin(CT)-
catalyzed transesterification of N-acetyl-L-phenylalanine
ethyl ester (APEE) with propanol in n-octane to N-
acetyl-L-phenylalanine propyl ester (APPE), a common
reaction to explore the performance of CT in organic
solvents.19 To this end, PHEA-l-PDMS microbeads of
two different compositions (40 and 60 wt % PDMS)
prepared with Pluronic as surfactant were loaded with
the enzyme by immersing them into a buffered aqueous
solution of the enzyme (10 mg CT/mL 10 mM phosphate
buffer, pH 7.8) overnight. The loaded amount was
determined by measuring the decrease in absorbance
of the enzyme at 280 nm in the aqueous solution. As
expected, greater PHEA content resulted in increased
CT loading (see Table 1). However, taking into account
the volume fraction of PHEA, the enzyme loading into
the PHEA phase is nearly constant. Further, the
enzyme is greatly concentrated (5-10-fold) into the
network compared to the surrounding enzyme solution.
The particles were then simply air-dried overnight,
In closing, we could demonstrate the first successful
synthesis of nonporous nanophase separated am-
phiphilic conetwork microbeads. The beads show the
phase separation in bulk as well as on the surface and
could be shown to be effective as solid-phase transfer
medium on the example of a R-chymotrypsin-catalyzed
biotransformation in octane.
Acknowledgment. This work was supported by the
Deutsche Forschungsgemeinschaft (Emmy-Noether-
Programm and SFB 428) and by the Fonds der Che-
mischen Industrie. The authors are grateful to Dr. Jonas
Scherble for synthesizing the MA-PDMS-MA macro-
monomer and to CIBA AG for kindly providing the
photoinitiator Igracure 651.
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MA0509715