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

Figure 3.

AFM surface image in contact mode for run 5

Figure 4.

AFM surface image in contact mode for run 5 after dissolution and spin-coating

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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.