Poly(ε-caprolactone)-block-poly(ethyleneoxide) -block-poly(ε-caprolactone): Biodegradable triblock copolymer spread at the air–water interface

European Polymer Journal 44 (2008) 2589–2598

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Poly(e-caprolactone)-block-poly(ethyleneoxide)-blockpoly(e-caprolactone): Biodegradable triblock copolymer spread at the air–water interface Angel Leiva *, Alejandro Farias, Ligia Gargallo, Deodato Radic´ Departamento de Química Física, Facultad de Química, Pontificia Universidad Católica de Chile, Casilla 302, Correo 22, Santiago, Chile

a r t i c l e

i n f o

Article history: Received 29 April 2008 Received in revised form 27 May 2008 Accepted 3 June 2008 Available online 11 June 2008

Keywords: Biodegradable copolymer Air–water interface Monolayer Block copolymer Static elasticity

a b s t r a c t Synthesis, characterization and behavior at the air–water interface of A–B–A triblock copolymers are reported. The copolymers consist of a poly(ethylene oxide) central block and poly(e-caprolactone) lateral blocks. The synthesis was controlled in order to obtain central and lateral blocks of variable length. Copolymer characterization was performed by FTIR and 1H NMR spectroscopy, size exclusion chromatography (SEC), and thermal analysis. Monolayers of the copolymers at the air–water interface were obtained by the Langmuir technique and the respective isotherms were obtained by monolayer compression. The limiting area per repeat unit (Ao) and the critical exponent of the excluded volume (m) for spread monolayers were obtained. The static elasticity (e0) of the monolayers was also determined. The obtained results allow proposing a schematic model of the orientation of the different blocks during the compression of the respective monolayers. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction Biodegradable polymers have been studied extensively in recent years due to their use in new applications and to their contribution towards diminishing the environmental problems caused by non-biodegradable materials, through the replacement of the latter by environmentally friendly materials. The evolution of synthetic biodegradable polymers has been achieved through modulating their chemical compositions using several polymerization techniques and also in some cases by using chemical modification of presynthesized polymers [1]. Since some properties of biodegradable polymers restrict their potential applications, copolymerization and mixture process of these materials have been used to reinforce properties such as biodegradation rates, permeability and mechanical properties [2,3].

* Corresponding author. Tel.: +56 2 6864743; fax: +56 2 6864744. E-mail address: [email protected] (A. Leiva). 0014-3057/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2008.06.001

Numerous studies about the properties and potential applications of biodegradable copolymers have been reported, highlighting, among other things, the capacity of adsorption, self assembly and micellization in water due to the amphiphilic character that biodegradable copolymers generally present. The adsorption phenomena of amphiphilic block copolymers on different interfaces are very important, due to their direct participation in different applications such as dispersion, stabilization, foaming and emulsification. Amphiphilic block copolymers have better stabilizing properties than ordinary surfactants [4] and for this reason they are extensively used in pharmaceutical formulations, personal care products and detergents. In this work, we have studied the behavior of a series of biodegradable tri-block copolymers spread at the air– water interface. The objective is to contribute to the knowledge of the properties and behavior that these copolymers present when forming a monolayer at the air–water interface. The study of this specific area in the wide field of materials science is very interesting due to the importance of these properties in possible applications

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in different fields where the properties of interfaces and surfaces of biodegradable materials are decisive. The studied copolymers are constituted of a poly(ethylene oxide) chain of variable length as central block and two poly(e-caprolactone) chains, also of variable length, as lateral blocks. The properties and behavior presented by the copolymers are compared with those of the respective homopolymers PCL and PEO at the same interface. Poly(ethylene oxide) (PEO) is an important biodegradable non-ionic synthetic water-soluble polymer. This polymer is widely used in different fields, such as the production of pharmaceuticals, cosmetics, lubricants and dispersion stabilizers. Poly(ethylene oxide)s hydrophobically end-capped are known as associative polymers [5–7] and show interesting rheological behavior when they are dispersed in aqueous solutions. These kinds of polymers have a great number of applications, for example as new building blocks for biomaterials [8]. PEOs form spread monolayers at the air– water interface and these monolayers are stable despite PEO being water-soluble at moderate temperature [9,10]. The adsorption behavior of the poly(ethylene oxide) at the air–water interface has been systematically characterized with respect to adsorption kinetics [11], segment density profiles [12] and thermodynamic characterization [13,14]. Adsorption of PEO capped at one end with fluoro carbon groups has been reported by Richards et al. [15], and they have concluded that the adsorption of PEO from aqueous solutions at the air–water interface is considerably enhanced by a hydrophobic fluorocarbon group inserted at one end of the macromolecule. PEOs spread at the air–water interface have been studied by Wave Damping [9], and surface dynamic elasticity [16], and thermodynamic and collapse parameters have been obtained [10]. Copolymers containing PEO blocks have been also studied at the air–water interface [17–20]. On the other hand, in the family of the polyesters, poly(e-caprolactone) is very important because it is a biodegradable polymer and has been the subject of several studies, such as: blend miscibilities of synthetic [21] and natural [22] polymers, hydrolytic degradation in blended monolayers [23], blend crystallization [24] and as a drug delivery system [25]. Copolymers of PEO/PCL have been studied in different aspects, such as preparation of nanoparticles in water [26], crystallization behavior [27], compatibilization effect [28], self assembly and micellization [29,30]. However, few works about these copolymers at the air–water interface have been reported. Duran and co-workers have studied Langmuir and Langmuir Blodgett films of star PEO/PCL block copolymers [31] and self assembly of PEO/PCL diblock copolymers at the air–water interface [4]. The results

of these works are considered and discussed later in the development of this manuscript. 2. Experimental 2.1. Reagents Reagents were purchased from Aldrich Chem. Co. eCaprolactone was dried twice over CaH2 and distilled under reduced pressure before use, poly(ethylene oxide)s of 1000 and 10,000 g/mol number average molecular weight M n were dried by azeotropic distillation with dry toluene, and tin(II) 2-ethylhexanoate was used as received. 2.2. Copolymerization Copolymer synthesis was performed by ring opening of

e-caprolactone, with tin(II) 2-ethylhexanoate as catalyst and polymerization of this over poly(ethyleneoxide) of 1000 and 10,000 g/mol according to the following procedure: 10 g e-caprolactone, poly(ethyleneoxide) and tin(II) 2-ethylhexanoate at 10% and 2% w/w proportion, respectively, were mixed. The reaction mixture was heated at 413 K with continuous stirring. The reaction time was con-

Table 1 Number average molecular weight Mn , polydispersity index and PCL content of the synthesized copolymers Copolymer

M n a (g/mol)

Mw Mn

copA1 copA2 copB1 copB2

3700 11,300 14,000 17,600

1.24 1.36 1.29 1.27

a b c

(a)

Mn b (g/mol)

% PCL weightc

6800 14,100 17,300 21,900

73 91 28 43

By SEC in THF. By 1 H-NMR. From SEC and PEOs M n .

Table 2 Intrinsic viscosity [g] values of copolymers and precursor PEOs Polymer

[g]a (dl/g)

[g]b (dl/g)

copA1 copA2 PEOc cop B1 cop B2 PEOd

0.11 0.15 0.02 0.23 0.26 –

0.12 0.26 0.04 0.32 0.41 0.29

a b c d

In toluene. In chloroform. 1000 g/mol. 10,000 g/mol.

O O O H

H

O O

O

x

y Scheme 1.

z

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trolled to obtain variable lengths of the lateral blocks of the e-caprolactone. Purification was achieved by repeated precipitation from CHCl3, with cold diethyl ether as non-solvent and the product was washed with cold methanol. Products were dried under vacuum until constant weight. Copolymers were labeled as copA1 and copA2 for those obtained from PEO 1000 g/mol at 7 and 24 h of polymerization, and copB1 and copB2 for those obtained from PEO 10,000 g/ mol at 7 and 16 h of polymerization, respectively.

2.3. Molecular characterization The copolymers were characterized by Fourier transform infrared spectroscopy (FTIR) and 1H NMR. Number average molecular weights M n were determined by size-exclusion chromatography (SEC). SEC measurements were performed with a modular SEC system composed of a Waters (USA) 515 HPLC pump, a Rheodyne (USA) 7010 injection valve, a differential refractometer ERC 7510 (Erma, Japan) and three PLgel

0.02

a

0.00

10

-0.02

dM/dT (mg/ºC)

-0.04

8

-0.06 -0.08 -0.10 -0.12 -0.14

6

-0.16

Mass (mg)

0

100

200

300

400

500

600

700

Temperatura (ºC)

4

2

0

300

400

500

600

700

800

900

1000

Temperature (K)

b

0

Heat Flow (mW)

-5

-10

-15

-20

-25

-30 150

200

250

300

350

400

450

500

Temperature (K) Fig. 1. (a) Thermal degradation profile and the respective derivate (dm/dT) (inset), for copA2. (b) DSC thermogram, for copA2.

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2.5. Thermogravimetric analysis

Table 3 Temperature of 5% weight loss TD,5%, glass transition temperature Tg and melting temperature Tm, of copolymers, PEOs and PCL Polymer

TD,5% (K)

Tg (K)

Tm (K)

PEOa PEOb PCL copA1 copA2 copB1 copB2

628 638 670 633 638 643 647

212 227 241 207 210 209 208

310 338 308 322 325 328 337

a b

Dynamic thermogravimetric measurements were performed using a Mettler calorimetric system TGA/SDTA 851e. The thermogravimetric results were processed using the STAR program. The samples were heated in Al2O3 pans. Measurements were carried out between 298.1 and 973.1 K at 20 K/min under nitrogen atmosphere at 60 ml/ min flow. 2.6. Differential scanning calorimetry

1000 g/mol. 10,000 g/mol.

Differential scanning calorimetry (DSC) measurements were performed using a Mettler Toledo DSC 821 (Switzerland) calorimetric system. The samples were heated until 538 K at 20 K/min, an isothermic step at 538 K was included for equalization of the thermal history of all samples, then the samples were cooled from 538 to 173 K at 20 K/min, the glass transition and melting temperatures (Tg and Tm) were determined in a second heating process until 538 K. The experiments were carried out under nitrogen atmosphere at 60 ml/min flow.

10 lm MIXED-B (300  7.5 mm) columns (Polymer Laboratories, UK). Tetrahydrofuran was used as eluent at a flow-rate of 1 ml/min. THF solutions were separated from solid impurities by filtration through 0.45 mm PTFE membrane filters. 2.4. Viscosity measurements Viscosities of copolymers in toluene and chloroform were determined at 298 K, using a capillary type Desreux-Bischoff [32] viscometer. Corrections for kinetic energy and shear rate were found to be negligible. Intrinsic viscosities [g] were calculated according to the classical empirical relations of Kraemer [33] and Huggins [34].

2.7. Monolayers at the air/water interface Monolayers were obtained by spreading of copolymer samples on aqueous subphase from chloroform. The concentrations of the spread solutions for different samples varied from 0.39 to 0.41 mg/ml and the temperature was

14

Surface Pressure (mN/m)

12 10

d

8

b 6

c

4

a 2

14

0 0

10

20

30

40

50

60

70

80

90

100

110

c

Area per repeat unit (A2/ru)

Surface pressure, π (mN/m)

12

b d

10 8 6 4

a 2 0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

Surface Concentration Γ (mg/m2) Fig. 2. Langmuir isotherms for, PEO 1000 g/mol (a), copA1 (b), copA2 (c) and PCL (d).

2.0

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-1

Surface Pressure (mNm )

12

10

d

8

6

4

c

14

a

b

2

0 0

10

20

30

40

50

60

70

80

90

100

110

2

c

Area per repeat unit (A /ur)

-1

Surface Pressure π (mNm )

12

b

10

d

8

a 6

4

2

0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2

Surface Concentration Γ (mg/m )) Fig. 3. Langmuir isotherms for, PEO 10,000 g/mol (a), copB1 (b), copB2 (c) and PCL (d).

298 K. The water subphase was purified by a Millipore Milli-Q system (resistivity greater than 18.0 MX cm). 2.8. Surface pressure/area isotherms Polymer monolayers on the aqueous subphase were studied by the Langmuir Technique. Surface pressure/area (p/A) isotherms, were obtained using a Nima Model 611 surface film balance (NIMA Instruments, Coventry, UK). The entire system was covered with a box of poly(methyl methacrylate) in order to prevent environmental perturbations. A constant compression rate of 10 cm2/min (16.7 mm2/seg) was used in all experiments. The experiments were carried out in triplicate to ensure their reproducibility. 3. Results and discussion 3.1. Synthesis and characterization

tone carbonyl units at 1726 cm1, methylene groups from the PCL and PEO between 2951 and 2865 cm1 and the hydroxyl end groups signal between 3437 and 3439 cm1. The chemical structure of the synthesized copolymers is shown in Scheme 1. The molecular parameters of the synthesized copolymers are summarized in Table 1. The number average w molecular weights M n and polydispersity indexes M were Mn obtained from size exclusion chromatographic measurements in THF. As expected, the molecular weight increases when the incorporation of the PCL over central PEO block increases. The relative PCL and PEO contents of copolymers were determined directly by the difference between the copolymers’ molecular weights and the respective PEO

Table 4 Areas per repeat unit Ao and critical exponent m obtained from Langmuir isotherms of PEOs, PCL and copolymers Polymer

1

The main signals in the H MNR spectra of copolymers correspond to the expected structure, i.e. the characteristic signals of PEO–CH2-protons appear at 3.7 ppm; for the PCL blocks, –CH2–O– and –CH2–CO–O– protons appear at 4.0 and 2.3 ppm, respectively, and central CH2– between 1.4– 1.7 ppm. The FTIR spectrum agrees with the expected copolymer structure and shows characteristic signals of e-caprolac-

a

PEO copA1 copA2 PEOb copB1 copB2 PCL a b

1000 g/mol. 10,000 g/mol.

Ao (A2/r.u.) ± 2

m ± 0.05

23 37 54 32 20 28 65

0.75 0.58 0.61 0.70 0.56 0.62 0.73

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central blocks molecular weights. As an alternative the PCL contents were also determined from 1H NMR spectrum by means of the analysis of intensity corresponding to –CH2groups of the PEO moiety (3.7 ppm) and that corresponding to –CH2–O protons from PCL moiety (4.0 ppm). The molecular weight of the copolymer was estimated from its composition and the known molecular weight of the central PEO block. The results are also compiled in Table 1. The molecular weights obtained by 1H NMR are higher than those obtained by SEC, but the tendency is the same. Similar differences were obtained for Poly(e-caprolactone)-Grafted Dextran copolymers between molecular weights obtained from gravimetry and 1H NMR [35]. For this copolymer, the authors attribute the difference to the overlapping of e-CL signals in the 1H NMR spectra, which causes an overestimation of the content of e-CL in the copolymers. Considering the above-mentioned, the values obtained by SEC in our opinion are more reliable. To obtain information regarding the influence of copolymerization on the hydrodynamic volume of the copolymers a viscometric study was carried out. The intrinsic viscosity values of the precursor PEOs and the respective copolymers in chloroform and in toluene are shown in Table 2. An increment in the hydrodynamic volume of the copolymers relative to the precursor PEOs was obtained and this is in agreement with the incorporation of the e-caprolactone lateral chains. The highest values of intrinsic viscosity obtained in chloroform indicate that this solvent is thermodynamically a better solvent than toluene for the copolymers. Copolymers were also characterized by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). In Fig. 1, the thermal degradation profile obtained by TGA and the thermogram obtained by DSC for copA2 are shown as an example of the general behavior. Starting with the degradation profiles the thermal stability, usually presented as temperature of 5% weight loss under nitrogen TD [36–38] was obtained. Table 3 summarizes the obtained values. The degradation profiles of all copolymers show

very similar patterns, with the main weight loss taking place between 528 and 723 K. TD values indicate that PCL is thermally more stable than PEOs and the copolymers present a stability intermediate to both homopolymers. In addition, the TD of the copolymers with higher content of PCL and/or PEO is closer to the corresponding homopolymers. The glass transition Tg and melting Tm temperatures obtained for copolymers, PEOs and PCL are also summarized in Table 3. The Tg values of the copolymers vary between 207 and 210 K, these values closer to the Tg of PEO than that of PCL. The Tg and Tm values of the copolymers are very similar to those reported for other copolymers containing PEO [38]. 3.2. Copolymer behavior at the air–water interface Copolymer monolayers spread at the air–water interface were obtained and studied by compression using the Langmuir technique. The spreading solvent used in all cases was chloroform. The Langmuir isotherms obtained are shown in Figs. 2 and 3 for copolymers copA1, copA2 and copB1, copB2, respectively. The isotherms of PEO and PCL are also included for comparison. Two representations of the isotherms are shown: the surface pressure as a function of the surface concentration and as a function of the area per repeat unit. In the case of copolymers this last one corresponds to an average repeat unit. The obtained isotherms are of the expanded type [39,40], i.e. surface pressure increases gradually upon monolayer compression and the compressibility of the copolymer monolayers is higher than the PCL monolayer and smaller than the PEO monolayer. The areas per repeat unit Ao, obtained by projection to zero surface pressure of the linear variation of p with the surface concentration in the semidilute region [40] are summarized in Table 4. Ao values in the case of the copolymers copA1 and copA2 have an expected behavior, showing values intermediate to those of the respective homopolymers. But in the case of

Scheme 2.

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the copolymers copB1 and copB2, the areas per repetitive units are oddly smaller than those of both homopolymers. The explanation which seems to be more appropriate for this particular behavior is an analogy to that reported by Duran and colleagues for PEO/PCL diblock copolymers, i.e. in the region where Ao for the copolymers is obtained, the blocks of e-caprolactone would crystallize at the air– water interface, while poly(ethyleneoxide) blocks would be oriented toward the water subphase (see Scheme 2a). In this way, a smaller area per repeat unit compared to the respective homopolymers is obtained (see Scheme 2c and d). In these last cases, Ao values are obtained in regions of surface concentration where most of the repeat units re-

main at the interface. For copolymers with smaller hydrophilic PEO blocks this process would be restricted by the smaller possibility of loop formation at the subphase (see Scheme 2b). In the semidilute region, the polymer chains partially interpenetrate each other and the surface pressure p varies with the surface concentration according to:

p / C2v=½2v1

ð1Þ

where C is the surface concentration expressed as mg/m2 and m is the critical exponent of the excluded volume [41,42]. The values of m, obtained from double logarithmic

16

PEO1K copA1 copA2 PCL

14 12

-1

ε0 (mNm )

10 8 6 4 2 0 0

2

4

6

8 -1 π (mNm )

10

12

14

16

PEO1K copA1 copA2 PCL

14 12

-1

ε0 (mN m )

10 8 6 4 2 0 0

0.2

0.4

0.6

0.8

1.0 1.2 2 Γ (mg/m )

1.4

1.6

1.8

2.0

Fig. 4. Static elasticity of monolayers, PEO 1000 g/mol (j), PCL (.), copA1 () y copA2 (N).

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graph of p as function of C, are summarized in Table 4. These values would indicate that the air–water interface is a better solvent for homopolymers PEO and PCL than for copolymers. These results agree well with the Ao values and with the representation suggested in Scheme 2, considering the interface as a pseudo-three-dimensional solvent, since the homopolymers would be ‘‘more expanded” at the interface than the copolymers. The static elasticity for the monolayers, eo was obtained from the surface pressure-surface area per repetitive unit isotherm by

  op eo ¼ A oA T

eo indicates the effect of an area change on the surface pressure, and can be better understood considering it as the inverse of its analogue in bulk, the modulus of the isothermal compressibility j,

j¼

  1 oV V oP T

ð3Þ

where P is the isotropic pressure and V is the volume [43,44]. In Figs. 4 and 5, the static elasticity of the copolymers copA1, copA2 and copB1, copB2, respectively are shown. Two representations are shown, eo vs p and eo vs C. For the copolymers and precursors the maximum static elas-

ð2Þ

16

PEO 10K copB1 copB2 PCL

14 12

-1

ε0 (mNm )

10 8 6 4 2 0 0

4

2

6

8

10

12

14

-1

π (mNm ) 16

PEO 10K copB1 copB2 PCL

14 12

-1

ε0 (mNm )

10 8 6 4 2 0

0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2

Γ (mg/m ) Fig. 5. Static elasticity of monolayers of, PEO 10,000 g/mol (j), PCL (.), copB1 () y copB2 (N).

A. Leiva et al. / European Polymer Journal 44 (2008) 2589–2598

ticity, where an area change produces a larger effect on the surface pressure, is around 5 m Nm1 surface pressure. In the surface concentration scale, the maximum static elasticity for copolymers occur at slightly higher surface concentrations than for PCL and PEOs, 0.55–0.78 mg/m2 for copA1 and copA2 and 0.60 mg/m2 for copB1 and copB2. According to these results, the static elasticity e0 shows higher values in the semidilute region, and this behavior seems to be general for polymeric systems [45,46]. In this interval of concentrations, the surface pressure is more sensitive to changes in area or surface concentration. In this same portion of the isotherms the extrapolation to p = 0 was performed to obtain Ao. Therefore, this region would be where the preferential orientation of the PEOs blocks to aqueous subphase takes place, giving rise to the loop formation shown in Scheme 2. As is observed in this Scheme, this phenomenon of loop formation would present a larger structural hindrance in the case of the copolymers with the shorter PEO block (copA1 and copA2) compared to those with the larger PEO block (copB1 and copB2). Due to this, the values of Ao for copA1 and copA2 are bigger than those of copB1 and copB2. In summary, we report the synthesis of a series of biodegradable triblock copolymers containing PEO and PCL as central and lateral blocks, respectively. The series includes copolymers of different composition. These copolymers present interesting behavior when they are spread at the air–water interface to generate a monolayer. A schematic model that describes this behavior is proposed, different orientations for hydrophilic PEO blocks and hydrophobic PCL blocks are also proposed. The model agrees with experimental values obtained for characteristic parameters of the monolayer, such as area per repeat unit Ao, critical exponent of the excluded volume m, and the static elasticity eo. Acknowledgment The authors thank FONDECYT Project 1040550 for the partial financial support for this work. References [1] Shalaby SW, Burg KJL. Absorbable and biodegradable polymers. New York: CRC Press; 2004. [2] Rusa C, Toneli A. Polymer/polymer inclusion compounds as a novel approach to obtaining a PLLA/PCL intimately compatible blend. Macromolecules 2000;33:5321–4. [3] Leiva A, Gargallo L, González A, Araneda E, Radic´ D. Biodegradable poly(dl-lactide)/poly(e-caprolactone) mixtures: miscibility at the air/water interface. Euro Polym J 2006;42:316–21. [4] Joncheray TJ, Denoncourt KM, Meier MAR, Schubert US, Duran RS. Two-dimensional self-assembly of linear poly(ethylene oxide)-bpoly(e-caprolactone) copolymers at the air–water interface. Langmuir 2007;23:2423–9. [5] Chassenieux C, Nicolai T, Durand D, Francois J. 1H NMR study of the association of hydrophobically end-capped poly(ethylene oxide). Macromolecules 1998;31:4035–7. [6] Beaudoin E, Hiorns RC, Borisov O, Francois J. Association of hydrophobically end-capped poly(ethylene oxide). 1. Preparation of polymers and characterization of critical association concentrations. Langmuir 2003;19:2058–66. [7] Barentin C, Muller P, Joanny JF. Polymer brushes formed by endcapped poly(ethylene oxide) (PEO) at the air–water interface. Macromolecules 1998;31:2198–211.

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[8] Rieger J, Bernaerts KV, Du Prez FE, Jérome R, Jérome C. Lactone end-capped poly(ethylene oxide) as a new building block for biomaterials. Macromolecules 2004;37:9738–45. [9] Shuler RL, Zisman WA. Study of the behavior of polyoxyethylene at the air–water interface by wave damping and other methods. J Phys Chem 1970;74:1523–34. [10] Kuzmenka DJ, Granick S. Collapse of poly(ethylene oxide) monolayers. Macromolecules 1988;21:779–82. [11] Sauer B, Yu H. Adsorption kinetics of poly(ethylene oxide) at the air/ water interface. Macromolecules 1989;22:786–91. [12] Lu JR, Su TJ, Thomas RK, Penfold J, Richards RW. The determination of segment density profiles of polyethylene oxide layers adsorbed at the air–water interface. Polymer 1996;37:109–14. [13] Ross S, Chen ES. Adsorption and thermodynamics at the liquid– liquid interface. Ind Eng Chem 1965;57(7):40–52. [14] Glass JE. Adsorption characteristics of water-soluble polymers. II. Poly(ethylene oxide) at the aqueous–air interface. J Phys Chem 1968;72:4459–67. [15] Richards R, Sarica J, Webster JRP, Holt SA. Direct determination by neutron reflectometry of the distribution of hydrophobically end capped polyethylene oxide at the air/water interface. Langmuir 2003;19:7768–77. [16] Akentiev AV, Noskov BA. Surface dynamic elasticity of poly(ethylene oxide) monolayers on a water surface. Colloid J 2002;64: 129–34. [17] Blomqvist BR, Warnheim T, Claesson PM. Surface rheology of PEOPPO-PEO triblock copolymers at the air–water interface: comparison of spread and adsorbed layers. Langmuir 2005;21:6373–84. [18] Fauré MC, Bassereau P, Desbat B. Orientation of grafted polymer chains at the air–water interface studied by PM-IRRAS. Eur Phys JE 2000;2:141–51. [19] Rivillon S, Muñoz MG, Monroy F, Ortega F, Rubio RG. Experimental study of the dynamic properties of monolayers of PS-PEO block copolymers: the attractive monomer surface case. Macromolecules 2003;36:4068–77. [20] Baker JA, Berg JC. Investigation of the adsorption configuration of polyethylene oxide and its copolymers with polypropylene oxide on model polystyrene latex dispersions. Langmuir 1988;4:1055–64. [21] Kuo SW, Huang CF, Chang FC. Study of hydrogen-bonding strength in poly(epsilon-caprolactone) blends by DSC and FTIR. J Polym Sci Part B: Poly Phys 2001;39:1348–59. [22] Path T, Has Y, Inoue Y. Biodegradable blends of poly(e-caprolactone) with a-chitin and chitosan: specific interactions, thermal properties and crystallization behavior. Polym Int 2002;51:33–9. [23] Lee WK, Nowak RW, Gardella Jr JA. Hydrolytic degradation of polyester blend monolayers at the air/water interface: effects of a slowly degrading component. Langmuir 2002;18:2309–12. [24] Nojima S, Sato K, Ashida T. Morphology formation by combined effect of crystallization and phase separation in a binary blend of poly(e-caprolactone) and polystyrene oligomer. Macromolecules 1991;24:942–7. [25] Bei J, Wang W, Wang Z, Wang S. Surface properties and drug release behavior of polycaprolactone polyether blend and copolymer. Polym Adv Technol 1996;7:104–7. [26] Vangeyte P, Gautier S, Jerome R. About the methods of preparation of poly(ethylene oxide)-b-poly(e-caprolactone) nanoparticles in water Analysis by dynamic light scattering. Colloid Surf A 2004;242: 203–11. [27] He C, Sun J, Ma J, Chen X, Jing X. Composition dependence of the crystallization behavior and morphology of the poly(ethylene oxide)-poly(e-caprolactone) diblock copolymer. Biomacromolecules 2006;7:3482–9. [28] Na YH, He Y, Shuai X, Kikkawa Y, Doi Y, Inoue Y. Compatibilization effect of poly(e -caprolactone)-b-poly(ethylene glycol) block copolymers and phase morphology analysis in immiscible poly(lactide)/poly(e-caprolactone) blends. Biomacromolecules 2002;3:1179–86. [29] Xiong XB, Mahmud A, Uludag H, Lavasinafar A. Conjugation of arginine-glycine-aspartic acid peptides to poly(ethylene oxide)-bpoly(e-caprolactone) micelles for enhanced intracellular drug delivery to metastatic tumor cells. Biomacromolecules 2007;8:874–84. [30] Sachl R, Uchman M, Matejieek P, Prochazka K, Stepánek M, Spirkova M. Preparation and characterization of self-assembled nanoparticles formed by poly(ethylene oxide)-block-poly(e-caprolactone) copolymers with long poly(e-caprolactone) blocks in aqueous solutions. Langmuir 2007;23:3395–400. [31] Joncheray TJ, Denoncourt KM, Mathieu C, Meier MAR, Schubert US, Duran RS. Langmuir and Langmuir-Blodgett films of poly(ethylene

2598

[32] [33] [34]

[35]

[36]

[37]

[38]

A. Leiva et al. / European Polymer Journal 44 (2008) 2589–2598 oxide)-b-poly(e-caprolactone) star-shaped block copolymers. Langmuir 2006;22:9264–71. Yamakawa H. Modern theory of polymer solutions. New York: Harper and Row; 1971. Kraemer EO. Molecular weights of celluloses. Ind Eng Chem 1938;30:1200–3. Huggins ML. The viscosity of dilute solutions of long-chain molecules. IV. Dependence on concentration. J Am Chem Soc 1942;64:2716–8. Ydens I, Rutot D, Degée P, Six JL, Dellacherie E, Dubois P. Controlled synthesis of poly(e-caprolactone)-grafted dextran copolymers as potential environmentally friendly surfactants. Macromolecules 2000;33:6713–21. Li X, Hay AS. Synthesis of a high molecular weight fluorinated poly(phthalazinone ether) by self-condensation of an AB-type monomer. Macromolecules 2006;39:3714–6. Wu CW, Tsai CM, Lin HC. Synthesis and characterization of poly(fluorene)-based copolymers containing various 1,3,4oxadiazole dendritic pendants. Macromolecules 2006;39: 4298–305. Li J, Ni X, Zhou Z, Leong KW. Preparation and characterization of polypseudorotaxanes based on block-selected inclusion complexation between poly(propylene oxide)-poly(ethylene

[39] [40]

[41] [42]

[43]

[44]

[45]

[46]

oxide)-poly(propylene oxide) triblock copolymers and acyclodextrin. J Am Chem Soc 2003;125:1788–95. Gaines G. Insoluble monolayers at liquid-gas interfaces. New York: Interscience; 1966. Crisp DJ. In: Danielli JF, Pankhurst KGA, Riddiford AC, editors. Surface phenomena in chemistry and biology. Elmsford, NY: Pergamon; 1958. Vilanove R, Rondelez F. Scaling description of two-dimensional chain conformations in polymer monolayers. Phys Rev Lett 1980;45:1502–5. Leiva A, Gargallo L, González A, Radic´ D. Poly(itaconates) monolayers behavior at the air/water interface. Study at different surface concentration. Euro Polym J 2004;40:2349–55. Kim C, Ester AR, Runge FE, Yu H. Surface rheology of monolayers of poly(1-alkylene-co-maleic acid) at the air/water interface: surface light scattering studies. Macromolecules 2006;39:4889–93. Kim C, Yu H. Surface rheology of monolayers of triblock copolymers of PEO and PPO: surface light scattering studies at the air/water interface. Langmuir 2003;19:4460–4. Monroy F, Ortega F, Rubio RG. Dilatational rheology of insoluble polymer monolayers: poly(vinylacetate). Phys Rev E 1998;58:7629–41. Ferenczi TAM, Cicuta P. Shear and compression viscoelasticity in polymer monolayers. J Phys: Condens Matter 2005;17:s3345–453.