Post on 29-Apr-2023
Access to nanoscale blends between immiscible
polymers via simultaneous non-interfering
polymerisation
Philippe Zinck*1, Claire-Hélène Brachais*
1, Eric Finot
2 and Denise Barbier-Baudry
1
1 Laboratoire de Synthèse et d’Electrosynthèse Organométalliques, LSEO, UMR 5632,
Université de Bourgogne, Bâtiment Mirande, 9 Avenue Alain Savary, BP 47870 Dijon Cedex
France
Fax : 33 (3) 80 39 60 84 ; E-mail : Claire-Helene.Brachais@u-bourgogne.fr,
Philippe.Zinck@u-bourgogne.fr 2 Laboratoire de Physique, LPUB, UMR 5207, Université de Bourgogne, Bâtiment Mirande,
9 Avenue Alain Savary, BP 47870, 21078 Dijon Cedex France
Keywords : Blends, interpenetrating networks (IPN), NMR, polystyrene, ring opening
polymerisation
Summary
An important topic in polymer science seeks to improve the performances of polymer blends
using nanoscale phase segregation. Here, blends between polystyrene and polycaprolactone
are realised by a chemical route. The non-interfering character of the radical polymerisation
of styrene and the lanthanide halide initiated ring-opening polymerisation of caprolactone is
assessed. The molecular weights range from 2000 to 3500 for polycaprolactone and up to
140 000 for polystyrene, with reasonnable polydispersity indexes. From microcalorimetry
measurements, it is shown that polystyrene and low molecular weight polycaprolactone are
immiscible. The morphology of the blends between the two immiscible polymers studied by
atomic force microscopy is consistent with nanometer-scale phase segregation.
Introduction
Access to new high performance polymeric materials can be achieved either via
polymerisation of new monomers, use of new catalysts or by blending polymers together, the
latter being far less expensive and frequently investigated in the past decades. An important
topic in polymer science seeks to improve the performances of polymer blends using
nanoscale phase segregation. Several strategies are proposed including reactive blending[1]
,
starting from polymer nanoparticles[2]
and the use of copolymers, either as a raw material or
as compatibilisers. We reported the controlled diblock copolymerisation of isoprene or
isoprene/hex-1-ene copolymer with -caprolactone and the successive compatibilisation of
polyisoprene/polycaprolactone blends[3]
. Blends between polar and non-polar polymers are of
particular interest, as this enables tailoring of important properties such as surface energy and
adhesion. The increasing awareness toward sustainable growth and the related economical
constraints stimulate the development of recyclable functional nanomaterials based on linear
polymers. According to the targeted use, it can be convenient to deal with a component at the
glassy state at room temperature for the mechanical behaviour. Moreover, a glass transition
temperature far above the room temperature offers the opportunity to quench the system in a
thermodynamical state far from the equilibrium. In this frame, our goal is to obtain nanoscale
phase segregation in blends between immiscible polymers via simultaneous non-interfering
polymerisation, starting from an homogeneous mixture of the monomers.
The catalytic system described in this paper allows the simultaneous chain
polymerisation of two monomers by two non-interfering processes. Styrene and -
caprolactone were selected as monomers which are respectively polymerised by a classical
radical process and by ring opening polymerisation by a lanthanide halide[4,5]
. Blends between
polystyrene and polycaprolactone have been investigated mainly as a tool for fundamental
blend studies,[6-11]
and especially competition between liquid-liquid phase separation
(demixing) and liquid-solid phase separation (crystallisation). A wide variety of morphologies
was obtained according to the phase separation and crystallisation conditions. Most of the
reported studies are restricted to polystyrene oligomers, leading to blends showing a critical
upper solution temperature[12]
. Blends issued from polystyrene of high molecular weight are
immiscible, and their rheological[13]
and mechanical properties[14]
have been studied recently,
as they are considered as potential materials for improved biocompatibility and
biodegradability. In this paper, we report the formation of nanoscale blends between
polystyrene and polycaprolactone by a chemical route.
Experimental part
Materials
-caprolactone (99%), styrene (99%) and di-tert-butyl peroxide (98%) were supplied by
Aldrich. Benzoyl peroxide (75%, remainder water) was supplied by Janssen Chimica and
samarium chloride hexahydrate (99,9%) by Strem Chemicals.
Synthesis procedures
Monomers were stored on magnesium sulfate prior to synthesis. A 52/48 molar ratio of -
caprolactone vs. styrene was selected corresponding to a quantity of 2 ml for each monomer.
All constituents were mixed together and stirred 5 minutes at 120 °C to dissolve the
lanthanide salt. The magnetic stirrer was removed and the mixture was allowed to react at the
desired temperature. Once the synthesis completed, the blend was quenched to room
temperature.
Spin-coating procedure
The physical mixing is based on the spin coating procedure. Samples were first dissolved in
chlorobenzene. A droplet of the solution was then deposited at 400 rpm during 10s time
window. The spinning rate for a 300 nm film thickness was then maintained to 4,000 rpm for
60 s. Samples were left in air one day prior analysis.
Measurements
Nuclear magnetic resonance (NMR) 1H spectra were recorded on a Brücker Avance 300
spectrometer using CDCl3 as the solvent. Size exclusion chromatography (SEC) analyses
were carried out in THF solutions (20 mg.ml-1
– 20°C – flow rate 1ml.min-1
) on a Spectra
System P1000 apparatus, equipped with two PLgel 5 µm mixed C-columns and a IOTA 2
refractive index detector. Polystyrene standards were used for column calibration and Mark-
Houwink corrections were performed for the determination of the absolute values of the
molecular weights of polycaprolactone. Differential scanning calorimetry (DSC) analyses
were performed using a TA Instruments 2920 apparatus, at a heating rate of 20 K.min-1
under
nitrogen atmosphere. Surface observations were performed using a D3100 AFM microscope
equipped with a Nanoscope IIIa controller and a J type scanner (xy-scan range ~ 150 µm).
Images were acquired in both tapping and contact mode at room temperature. Unsharpened
`D' type silicon nitride cantilevers (Microlevers; nominal end-radius ~50 nm;
Thermomicroscopes (Sunnyvale, CA, USA) with a nominal force constant of 0.1 N/m) were
used.
Results and Discussion
Synthesis and catalytic system
The assessment of the non-interfering character of the ring-opening polymerisation and the
radical processes was the purpose of runs 1-4 (table 1). The molar ratio between polymers and
monomers were estimated from the 1H NMR integral values of the (O)CH2 signal at 4.06 and
4.23 ppm for polycaprolactone and -caprolactone, respectively, from the =CH signal at 5.74
ppm for the styrene and from the difference between the aromatic CH signal (6.30 - 7.24
ppm) and the latter for the polystyrene (figure 1). Runs 1 and 2 performed without the
samarium chloride indicate that the radical process does not open the ester ring in our
experimental conditions. The catalytic activity of SmCl3 toward styrene is also shown to be
negligible from run 3. The small amount in polystyrene formed can result from the thermal
activation of the radical polymerisation according to a Diels-Alder process. From run 1 and 3,
the ratio of polystyrene obtained from thermal activation is greater in the absence of SmCl3,
indicating that the polymerisation of styrene is not initiated by the lanthanide chloride. The
non-interfering character of the lanthanide halide initiated ring opening polymerisation of -
caprolactone and the radical polymerisation of styrene is therefore assessed.
Average molecular weights of the macromolecular chains are presented in table 2.
Polycaprolactone molecular weigths determined from the CH2OH signal at 3.65 ppm on the
1H NMR spectrum are in good agreement with experimental values measured by size
exclusion chromatography using Mark-Houwing corrections. Molecular weights distributions
of polycaprolactone and polystyrene are mainly superimposed, the separation between the two
peaks being observed for run 7 only. Average number molecular weights range from some ten
thousands up to 140 000 for polystyrene and from 2000 to 3500 for polycaprolactone. As
discussed in a previous work[5]
, the low range obtained for polycaprolactone results from
transfer reactions induced by the small amount of water in the commercial monomers and the
coordinated water in the hydrated catalyst. Water is able to break the lanthanide-oxygen bond
of the growing macromolecular chain, leading to oligomers of low molecular weights as
represented Equation 1. Anhydrous catalysts, in the form of lanthanide THF adducts, as well
as appropriate chain extenders such as diethylene glycol[5]
, can be used in order to obtain
higher molecular weights. Reasonable polydispersities are obtained for the ring opening
polymerisation of the cyclic ester at 120°C ( wM / nM = 1,3 – run 7) and for the radical
polymerisation of styrene ( wM / nM = 1,7 – run 7).
Ln-O-(CH2)5COO-(CL)n-COOH Ln-OH + HO-(CL)n+1-COOH (1)H2O
The final composition of the blends derives from the initial molar ratio of -caprolactone vs.
styrene (52/48). It may result from styrene loss through evaporation (Teb = 152°C under
atmospheric pressure). Optimisation was achieved for run 5 by considering the complete
conversion of the monomers, the lowest loss of styrene via evaporation and the very early
stage of demixtion as discussed hereafter.
Morphology
The first steps of the formation of macromolecular chains were followed by 1H NMR.
1H
NMR spectra for run 5 after 40 minutes of reaction at 140°C (figure 1a) corresponds well to
the superposition of polycaprolactone, polystyrene, -caprolactone and styrene spectra. Figure
2 shows that the styrene reacts very rapidly, in contrast to the formation of polycaprolactone
chains. Since both polymers are soluble in both styrene and -caprolactone, the final
morphology may result from a phase separation driven by the growth of the polycaprolactone
chains, i.e the ring opening polymerisation process.
Chemical composition of the surface and the bulk were measured using 1H RMN spectra with
polymerisation time at 140°C for run 5 (table 3). If the compositions remained the same after
2h30, the surface and bulk compositions differ clearly after 4h, indicating a macroscopic
demixtion at high temperature.
In immiscible polymer blends, with one of the component being semi-crystalline, the non-
crystallisable component is segregated as a dispersed phase. The dispersed domains may be
present in the blend before crystallisation, or may develop after solidification of the
crystallisable component[15]
. Since the polymerisation temperature is far above the melting
temperature of the crystalline polycaprolactone, crystallisation occurs during the quench of
the blend from polymerisation temperature (120°C-150°C) to room temperature (20°C). The
resulting crystalline ratio is given in table 4 from the DSC measurements of the heat of fusion,
assuming (i) a heat of fusion of 139.5 J/g for fully crystalline PCL[16]
and (ii) that the
surrounding polystyrene does not influence the melt or in a negligible way. With the latter
assumptions, the crystalline ratio obtained ranges from 15% (runs 4,5) up to a quasi fully
crystalline polycaprolactone (runs 6,7). The melting temperature varies from 51 to 58 °C, and
increases with increasing the crystalline ratio. Changes in the melting temperature are not
considered as representative of compatibilisation, since Tm is dependant on the nature and the
size of crystalline entities which may change from one sample to another.
Glass transition temperatures of the amorphous phases are around those of the bulk polymers,
i.e -62°C and 110 °C for polycaprolactone and polystyrene respectively. The changes in glass
transition temperature of polystyrene are attributed to residual amounts of caprolactone in the
blends (runs 3,6 and 7) which may cause some plasticisation of the polymer, and the use of a
new batch of styrene (runs 5,6 and 7). Discrepancies observed for the glass transition
temperature of polycaprolactone (runs 3 and 7) may also be ascribed to a plasticisation by the
residual monomer. From run 5, where the conversion is complete and the glass transition
temperatures correspond to those of the bulk polymers, it is assessed that polystyrene and low
molecular weight polycaprolactone are immiscible.
The segregation state was studied by atomic force microscopy on native surfaces, in order to
get information on the size of the segregated domains. From figure 3, an homogeneous blend
at the micrometer scale can be observed. Crystalline parts of polycaprolactone could not be
resolved at this scale. It can be explained by the low crystalline ratio of polycaprolactone for
run 5, and is in agreement with the presence of lamellaes of polycaprolactone. The formation
of higher dimension crystalline entities such as spherulites for instance is indeed restricted by
the quench of the blends after polymerisation. Those observations are consistent with a
segregation state at scale down to a few dozen of nanometers, explaining the optical
properties of the samples which are translucent to visible light. The analysis conducted in
tapping mode leads to similar results. Blends with the same composition were prepared by
physical mixing, i.e dissolution of the quenched samples in chlorobenzene and spin-coating.
From Figure 4, inhomogeneities can be observed at the micrometer scale (4 µm zones) as well
as at the nanometer scale (200 nm). Segregation obtained via simultaneous non-interfering
polymerisation occurs therefore at scales well above those observed after spin-coating, and
AFM experiments on native surface are consistent with a nanometer phase segregation.
Further investigation is needed to precise the exact morphology and the thermodynamical
stability of blends realised by the chemical route.
Conclusion
The non-interfering character of the radical polymerisation of styrene and the lanthanide
halide initiated ring-opening polymerisation of caprolactone has been assessed. The molecular
weights range from 2000 to 3500 for polycaprolactone and up to 140 000 for polystyrene,
with reasonnable polydispersity indexes. From microcalorimetry measurements, it is shown
that polystyrene and low molecular weight polycaprolactone are immiscible. The morphology
of the blends between the two immiscible polymers studied by atomic force microscopy is
consistent with nanometer-scale phase segregation. Further work is now under progress with
other ROP sensitive monomers.
Table 1.
Simultaneous non-interfering polymerisation of -caprolactone and styrene – Molar ratio of
polycaprolactone (PCL), polystyrene (PS), -caprolactone (CL) and styrene
Runa)
Polymerisation time
and temperature
[S]/[A]b)
[CL]/[Sm]c) PCL
d)
(%) CL
d)
(%)
PSd)
(%)
Styrened)
(%)
1 3h 150°C - - -e)
62 27 1
2 3h 150°C 16 - -e)
60 40 -e)
3 3h 150°C - 67 87 6 7 -e)
4 3h 150°C 50 67 81 -e)
16 3
5 2h30 140°C 100 200 62 -e)
38 -e)
6 4h 120°C 100 f
200 54 7 38 -e)
7 4h 120°C 100 200 70 12 18 -e)
a)
Polymerisation conditions :V(-caprolactone) = 2 ml and V(styrene) = 2 ml corresponding
to a molar ratio 52/48 b)
Styrene vs. radical initiator ratio c)
-caprolactone vs. samarium chloride ratio d)
Determined by 1H NMR
e) There was no evidence of the corresponding compound on the
1H NMR spectra or less than
1% f) Benzoyl peroxide as radical initiator
Table 2.
Simultaneous non-interfering polymerisation of -caprolactone and styrene – Polymer number
average molecular weights and polydispersity indexes
Runa)
Polymerisation time and
temperature
[S]/[A]b)
[CL]/[Sm]c)
nM
SEC
PDI nM (PCL)
1H NMR
1 3h 150°C - - 32800 2.4 -
2 3h 150°C 16 - d)
-
3 3h 150°C - 67 d)
2800
4 3h 150°C 50 67 7000 3.0 2900
5 2h30 140°C 100 200 8600 4.0 3300
6 4h 120°C 100e)
200 d)
1900
7 4h 120°C 100 200 137000
1900
1.7
1.3
2100
a)
Polymerisation conditions :V(-caprolactone) = 2 ml and V(styrene) = 2 ml corresponding
to a molar ratio 52/48 b)
Styrene vs. radical initiator ratio c)
-caprolactone vs. samarium chloride ratio d)
Bimodal distribution e)
Benzoyl peroxide as radical initiator
Table 3.
Composition of the blend corresponding to run 5 as a function of time at 140°C and location
(surface and bulk) – Initial molar ratio -caprolactone / styrene = 52/48
Polymerisation time
and temperaturea)
[S]/[A]b)
[CL]/[Sm]c) PCL (%)
d) PS (%)
d)
Bulk Surface Bulk Surface
2h30 140°C 100 200 62 62 38 38
4h 140°C 100
200 86 30 14 70
a) Polymerisation conditions :V(-caprolactone) = 2 ml and V(styrene) = 2 ml corresponding
to a molar ratio 52/48 b)
Styrene vs. radical initiator ratio c)
-caprolactone vs. samarium chloride ratio d)
Determined by 1H NMR
Table 4.
Simultaneous non-interfering polymerisation of -caprolactone and styrene – Glass transition
temperature and melting point
Runa)
Polymerisation
time and
temperature
[S]/[A]b)
[CL]/[Sm]c) Tg
PCLd)
(°C)
Tg
PSd)
(°C)
Tmd)
(°C)
Hd)
(J/g)
Cristallinity
percentage e)
3 3h 150°C - 67 -70 95 55 58 48
4 3h 150°C 50 67 -62 98 54 16 14
5 2h30 140°C 100 200 -62 111 51 14 16
6 4h 120°C 100 200f
-63 103 58 70 93
7 4h 120°C 100 200 -75 107 57 95 96
a) Polymerisation conditions :V(-caprolactone) = 2 ml and V(styrene) = 2 ml corresponding
to a molar ratio 52/48 b)
Styrene vs. radical initiator ratio c)
-caprolactone vs. samarium chloride ratio d)
Determined by differential scanning calorimetry e)
Calculated assuming a heat of fusion of 139.5 J/g for fully crystalline PCL f) Benzoyl peroxide as radical initiator
Figure 1.
1H NMR spectra for a molar ratio styrene / di-tert-butyl peroxide of 100, -caprolactone /
samarium chloride of 200 and -caprolactone / styrene 52/48 corresponding to run 5 after 40
minutes (a) and 2h30 (b) reaction time at 140°C
(a)CDCl3
CH2OH
b’
d’
c’
c
bd
a
a‘
e
e’
ii’
i’
fgh
f’g’
(a)CDCl3
CH2OH
b’
d’
c’
c
bd
a
a‘
e
e’
ii’
i’
fgh
f’g’
H
H
CHCC
O
O
CH2
CH2
CH2
CH2
CH2
a
b
d
ce
a’ b’ c’ d’ e’O (CH2) C
O
(CH2)(CH2)(CH2)(CH2)
f
g
h
i
( CH CH2 )
f ’ g’
i’
H
H
CHCC
O
O
CH2
CH2
CH2
CH2
CH2
a
b
d
ce
C
O
O
CH2
CH2
CH2
CH2
CH2
a
b
d
ce
a’ b’ c’ d’ e’O (CH2) C
O
(CH2)(CH2)(CH2)(CH2)
a’ b’ c’ d’ e’O (CH2) C
O
(CH2)(CH2)(CH2)(CH2)
f
g
h
i
( CH CH2 )
f ’ g’
i’
( CH CH2 )
f ’ g’
i’
Figure 1. (continued)
(b)
CDCl3
CH2OHi’i’
b’
d’
a‘
e’
c’
g’
f ’
(b)
CDCl3
CH2OHi’i’
b’
d’
a‘
e’
c’
g’
f ’
Figure 2.
Conversion of styrene (▲) and -caprolactone (■) as a function of reaction time at 140°C for
a molar ratio styrene / di-tert-butyl peroxide of 100, -caprolactone / samarium chloride of
200 and -caprolactone / styrene of 52/48 corresponding to run 5
0
0,2
0,4
0,6
0,8
1
0 10 20 30 40 50
Time (min)
Co
nve
rsio
n
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Table of contents
The simultaneous lanthanide halide
initiated ring-opening polymerisation of ε-
caprolactone and radical polymerisation of
styrene afford the formation of blends by a
chemical route. Polystyrene and low
molecular weight polycaprolactone are
shown to be immiscible, and the
morphology studied by atomic force
microscopy is consistent with nanometer-
scale phase segregation.