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Non-aqueous emulsions stabilized by block copolymers: application to liquid disinfectant-filled elastomeric films
Advances in Colloid and Interface Science 108 – 109 (2004) 43–48
Non-aqueous emulsions stabilized by block copolymers: application to liquid disinfectant-filled elastomeric films ´ Gerard Riessa,*, Andre´ Cheymolb, Pierre Hoernerb, Raffi Krikorianb a
´ Ecole Nationale Superieure de Chimie de Mulhouse, Institut de Chimie des Surfaces et Interfaces, 3 rue Alfred Werner, 68093 Mulhouse Cedex, France b Hutchinson, 2 rue Balzac, 75008 Paris, France
Abstract The emulsifying and stabilization efficiency of polybutadiene-b-poly(ethylene oxide) and poly(ter butylstyrene)–poly(ethylene oxide) diblock copolymers is examined in non-aqueous emulsions. These emulsions are formed by a dispersion of polyethylene glycol mixed with a cationic surfactant acting as a biocide, in a continuous phase of a thermoplastic elastomer (SEBS) dissolved in methylcyclohexane. Emulsions with controlled droplet size and excellent stability could be obtained, which by solvent evaporation lead to elastomeric films containing droplets of confined disinfecting liquids. 䊚 2004 Elsevier B.V. All rights reserved. Keywords: Non-aqueous emulsions; Block copolymers; Disinfectant; Surgical gloves; Thermoplastic elastomers
1. Introduction The increasing importance of block copolymers arises mainly from their unique colloidal and emulsifying properties which is a consequence of their molecular structure. It is now well established that block copolymers act as efficient emulsifiers for stabilizing emulsions composed of two immiscible liquids when each of them is a selective solvent of one of the blocks of the copolymer. Oil-in-water (OyW) and water-in-oil (Wy O) emulsions are thus typically obtained with hydrophobic–hydrophilic block copolymers, the hydrophilic block being either anionic, cationic or non-ionic w1x. Oil-in-oil (OyO) emulsions, so-called non-aqueous emulsions or water-free emulsions, have far less been studied. The possibility to form OyO emulsions is a very specific property of block copolymers and such systems are difficult, if not impossible, to obtain using classical low molecular weight surfactants. First examples reported for OyO emulsions were dimethylformamide–hexane and cyclohexane–acetonitrile emulsions stabilized, respectively, with polystyrene–polyisoprene and polystyrene–poly(methylmethacrylate) diblock copolymers w2,3x. Non-aqueous emulsions based on fluorocarbons, silicone oils, lipids and polyols have since *Corresponding author. Tel.yfax: q33-3-89-33-68-54. E-mail address: [email protected] (G. Riess).
been developed as described mainly in the patent literature for biomedical and cosmetic formulations w4,5x. As an extension of these concepts we became interested in non-aqueous emulsions where the dispersed phase consists of poly(ethylene glycol) (PEG) droplets, the continuous phase being formed by a polymer solution of a film forming polymer dissolved in a volatile organic solvent. Liquid-filled polymers, thermoplastics or elastomers, can thus be obtained by solvent evaporation as we have shown previously w6,7x. This kind of confined liquidfilled elastomeric film is for instance obtained by solvent evaporation from a polymeric OyO emulsion with a continuous phase of a styrene-hydrogenated diene thermoplastic elastomer SEBS dissolved in methylcyclohexane (MeCH) and having a dispersed phase of PEG, which is non-miscible neither with methylcyclohexane, nor with the styrenic and the ethylene–butylene blocks of SEBS. The stability of these emulsions is achieved with either polybutadiene-poly(ethylene oxide) (PB-PEO) or poly(ter butylstyrene)-b-poly(ethylene oxide) (PtBuSPEO) block copolymers, PEO being miscible selectively with PEG and PB, PtBuS, with the SEBS solution in MeCH. In the present study we were interested in the stabilization efficiency of PB-PEO and PtBuS-PEO diblock
0001-8686/04/$ - see front matter 䊚 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cis.2003.10.019
G. Riess et al. / Advances in Colloid and Interface Science 108 – 109 (2004) 43–48
44
Table 1 Characteristics of PB-PEO diblock copolymers Reference
PEO content wt.%
Mn PB
IPa PB
Mn PEO
Mn PB-PEO
BE BE BE BE BE BE BE BE
21 26 30 39 42 50 56 60
8300 58 000 21 100 11 300 9200 7700 6000 4000
1.06 1.06 1.06 1.05 1.06 1.04 1.05 1.07
2300 20 000 9300 6900 6700 7600 7500 6000
10 600 78 000 30 400 18 200 15 900 15 300 13 500 10 000
a
153-52 1074-455 391-211 209-157 170-153 143-173 111-170 74-136
29 wt.% polystyrene, was used as thermoplastic elastomer solubilized in MeCH at a concentration of 10–18 wt.%, unless otherwise stated. PEG, molecular weight 400, was purchased from Fluka. Its characteristics as determined at 22 8C are the following: volumic masss1130 kgym3; viscositys110 mPa s; surface tensions40 mNym. The disinfecting agent, didecyl-dimethyl ammonium chloride (BARDAC – Lonza) is well known for its quasi-instantaneous inactivation effect on virus. This cationic surfactant DDAC is of the following structure:
Polydispersity index (IP)sMw yMn.
copolymers for the preparation of ‘non-aqueous emulsions’ having as dispersed phase a liquid disinfectant, e.g. a solution of a quaternary ammonium derivative solubilized in PEG and as continuous phase a solution of a thermoplastic elastomer (SEBS) in MeCH. Under controlled evaporation of the cycloaliphatic solvent from this emulsion, it will thus be possible to obtain an elastomer film containing the liquid disinfecting phase in the form of small droplets embedded in the rubber matrix. 2. Experimental part 2.1. Synthesis and characterization of the diblock copolymers PB-PEO and PtBuS-PEO were synthesized by sequential anionic polymerization following standard procedures as published previously w6x. The block copolymer sample TS 22-55 supplied by Goldschmidt was prepared by ethoxylation of v-hydroxy-functionalized PtBuS w8x. The molecular weight and the molecular weight distribution of the precursor PB and PtBuS sequences were determined by SEC. The eluent was THF and the calibration curve was obtained using polystyrene standards. The actual molecular weights Mn and Mw of the precursor blocks were obtained by the classical ‘universal calibration’ technique w9x. The copolymer composition and the microstructure of the PB sequence were determined by 1H-NMR (Bruker AC 250 F) using CDCl3 as solvent. The microstructure of the PB sequence is approximately 85% of 1–2 configuration. The characteristics of the copolymers used in this study are given in Tables 1 and 2. The copolymers are referenced BE, respectively TS, with a first number indicating the DPn of the PB or PtBuS block and a second number for the DPn of the PEO block. 2.2. Materials Polystyrene-b-poly(ethyleneybutylene)-b-polystyrene (SEBS, Kraton G 1652, Kraton Polymers), containing
2.3. Solubilization of the copolymer Stock solutions of the block copolymers (20–50 mgy ml) were prepared by direct dissolution in MeCH at 80 8C during 3 h. The solution was then cooled to room temperature (22 8C) overnight. The solutions are isotropic at 80 8C and become birefrigeant by cooling down to 22 8C, as also observed by different authors for PEOcontaining block copolymers on crystallization in organic solvents w10–12x. In agreement with these authors it could be shown with various technique, and especially atomic force microscopy (AFM), that these colloidal structures obtained by crystallization could be in the form of small ‘platelets’ having a core of crystallized PEO with a fringe of PB or PtBuS sequences. An example of these structures is given in Fig. 1. Their amount in the stock solution was estimated by centrifugational separation (30 min at 5000=g). 2.4. Preparation of the emulsions A general procedure for the preparation of the emulsions is outlined in this section with the characteristics Table 2 Characteristics of the PtBuS-PEO diblock copolymers Reference
PEO content wt.%
Mn PtBuS
IPa PtBuS
Mn PEO
Mn PtBuS-PEO
TS TS TS TS TS
30 34 38 41 43
9700 7900 3400 3500 6000
1.20 1.43 1.13 n.d. 1.15
4100 4150 2100 2400 4500
13 800 12 050 5500 5900 10 500
56-93 47-95 21-47 22-55b 38-103
n.d. not determined. a Polydispersity index (IP)sMw yMn. b Sample supplied by Goldschmidt reference BSE 35-24.
G. Riess et al. / Advances in Colloid and Interface Science 108 – 109 (2004) 43–48
45
Fig. 1. AFM image (tapping mode) showing a ‘Platelet’ structure of PB-PEO copolymer (BE 74-136) crystallized in MeCH.
given at room temperature (22 8C) whereas additional details will be indicated with the corresponding experimental results. The continuous phase of the emulsion is prepared by adding a known amount of the copolymer solution to the SEBS solubilized in MeCH at a concentration of 10–18 wt.%. In this concentration range the volumic mass is rc(790 kgym3 and the viscosity hc varies from 30 to 300 mPa s. The dispersed phase is a mixture of DDACyPEG 400 in a weight ratio 30:70 having a volumic mass rDs 1170 kgym3 and a viscosity hDs110 mPa s. The DDACyPEG 400 mixture is slowly added to the SEBS solution under stirring (109 at 150–1000 rpm) by using classical lab scale emulsification equipment (blade stirrer and triple helix propeller). The volume fraction of the dispersed phase fc is kept between 0.05 and 0.08, which will lead after MeCH evaporation from the emulsion to an elastomer film containing a volume fraction of dispersed phase ff (PEGqBARDAC) of approximately 0.30. The concentration of copolymer, typically in the range of 2.5–40 wt., is given with respect to the dispersed phase.
2.5. Emulsion characterization The stability of the emulsion is evaluated from their sedimentation velocity using a Turbiscan MA1000 analyser, by which the transmission and the backscattering signals of the emulsion are monitored as a function of time. The average particle size given as the median diameter D50 (50% of the particles having a diameter -D50) is determined with a Photosedimentation Particle Size Analyser Shimadzu SA-CP3. Details about these different techniques can be found in Ref. w9x. 3. Results and discussion In the systematic study of this emulsification process, our major aim was to demonstrate – that the particle size of the dispersed phase can be adjusted which is necessary to impart the required disinfecting efficiency to the films; – that the non-aqueous emulsions of PEG dispersed in a solution of SEBSyMeCH can be obtained with excellent stabilities.
G. Riess et al. / Advances in Colloid and Interface Science 108 – 109 (2004) 43–48
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Table 3 Average particle size and stability of the emulsions as a function of the PB-PEO block copolymer characteristics Reference
PEO content wt.%
‘Platelets’ wt.%a
D50 at 5% copolymer mm
Stability t25 (h) 5% copolymer
D50 at 15% copolymer mm
Stability t25 (h) 15% copolymer
BE BE BE BE BE BE BE BE
21 26 30 39 42 50 56 60
0 16 18 10 10 12 33 65
15.4 15.8 13.6 11.6 11.7 11.1 10.4 9.8
8 6 10 15 15 15 72 100
11.8 12.1 11.7 8.6 9.1 7.7 6.6 7.3
60 100 72 245 400 740 )1000 )1000
153-52 1074-455 391-211 201-157 170-153 143-173 111-170 74-136
Experimental conditions: viscosity of the continuous phase hcs180 mPa s; fes0.07; ffs0.34, agitation 109 at 300 rpm. a Wt.% of ‘platelets’ separated by centrifugation (309 at 5000=g) from the stock-solution of copolymer in MeCH. Table 4 Emulsion characteristics as a function of the ‘platelets’ concentration Copolymer stabilizer PEO content D50 Stability t25 platelets wt.% at 5% copolymer (h) wt.% mm 0 65a 100
60 60 60
6.7 9.8 10.4
55 100 110
Experimental conditions: viscosity of the continuous phase hcs 180 mPa s; fes0.07; ffs0.34, agitation 109 at 300 rpm. a Copolymer dispersion before separation by centrifugation.
Table 3 shows as an example the particle size and the stabilities of the emulsions prepared with PB-PEO copolymers as a function of the copolymer characteristics and at two concentration levels of 5 and 15%. The stability is indicated by the time, expressed in hours, which is necessary to reach under static conditions a decantation of 25% for the emulsion. From Table 3 it can be noticed that the particle size of the dispersed phase decreases with increasing copolymer concentrations and with the increase in their PEO content. A correlation seems also to exist between the weight fraction of ‘platelets’ in the copolymer stock-solution in MeCH and the stability of the resulting emulsions. This could be an indication that this type of non-aqueous
emulsions are stabilized by molecular dispersed copolymer becoming anchored at the droplet surface, as well as by the colloidal dispersion of ‘platelets’ acting as a Pickering stabilizer w13–15x. Evidence of this stabilization mechanism by finely divided solid particles can be given by preparing the emulsion with the same amount of copolymer, on the one side with the ‘platelet’ fraction isolated by centrifugation, on the other with the supernating copolymer fraction. For the copolymer BE 74-136 it could be checked by NMR analysis that both fractions, the ‘platelets’ and the copolymers remaining in solution, have the same PEO content, which indicates that no detectable fractionation in composition has occurred during the preparation of the copolymer stock-solution in MeCH. The characteristics of the emulsions prepared under these conditions are given in Table 4. From this table it appears that the best emulsifying effect, e.g. the lowest particle size, is achieved in the absence of ‘platelets’. Molecular dispersed copolymer andyor in its micellar form has, therefore, a greater tendency to form smaller droplets than the ‘platelets’. However, emulsions of higher stability and larger particle size can be obtained in the presence of these finely divided solid particles.
Table 5 Particle size and stability of the emulsions as a function of the PtBuS-PEO block copolymer characteristics Reference
PEO content wt.%
D50 at 5% copolymer mm
Stability t25 (h) 5% copolymer
D50 at 15% copolymer mm
Blank TS 56-93 TS 47-95 TS 21-47 TS 22-55 TS 38-103
– 30 34 38 41 43
– 10.8 10.8 4.8 5.4 9.4
– 6 96 24 350 6
– 10.4 5.6 4.5 – 6.0
Experimental conditions: viscosity of the continuous phase hcs180 mPa s; fes0.07; ffs0.34; agitation 109 at 300 rpm. a Emulsion without copolymer.
Stability t25 (h) 15% copolymer a
2 120 )1000 )1000 )1000 )1000
G. Riess et al. / Advances in Colloid and Interface Science 108 – 109 (2004) 43–48 Table 6 Particle size of the emulsions as a function of the PtBuS-PEO block copolymer concentration % Copolymer D50 mm Stability t25 (h)
2.5 12.4 6
5 11.4 29
7.5 9.6 70
10 9.9 69
15 8.5 290
20 9.1 350
40 8.2 1600
Experimental conditions: copolymer T22-55 concentration with respect to the dispersed phase from 2.5 to 40%; viscosity of the continuous phase hcs80 mPa s; viscosity of the dispersed phase hds 110 mPa s; fes0.07; ffs0.34; agitation 109 at 600 rpm.
Similar results were also obtained with PtBuS-PEO copolymers as shown in Table 5. As copolymer TS 22-55, supplied by Goldschmidt, was meeting the requirement of stability, we examined in more details the influence of the copolymer concentration on the emulsion characteristics. The corresponding results are given in Table 6. As expected the particle size decreases and the stability increases with increasing amounts of copolymer. By comparing these results with those of Table 5, it can be noticed that at given copolymer concentrations, e.g. 5 and 15%, the particle size increases significantly by decreasing the viscosity of the continuous phase from 180 to 80 mPa s, even if the agitation is increased from 300 to 600 rpm. It could be confirmed that the average diameter of the dispersed phase decreases with increasing shear rates and levels off at approximately 1000 rpm. It was further shown that the particle size could also be adjusted by varying the viscosity ratio of the phases. In fact, it is well known that at a given shear rate and at constant interfacial tension between the two phases the particle size of the dispersed phase will be inversely proportional to the viscosity of the continuous phase, which in our case can easily be adjusted by increasing the amount of SEBS solubilized in MeCH w9 x . As an extension of these concepts, it was also confirmed more recently that poly(hydrogenated butadiene)b-poly(ethylene oxide) diblock copolymers are quite efficient as emulsifiers and stabilizers of the above mentioned non-aqueous emulsions w16x. It could be demonstrated that the solid colloidal particles, such as ‘platelets’, obtained under well-defined crystallization conditions in a cycloaliphatic solvent, are perfectly adapted to stabilize non-aqueous emulsion by a Pickering stabilization mechanism w13–15x. Starting with these emulsions and by controlled evaporation of MeCH, elastomeric films were obtained with confined droplets of PEGycationic surfactant, the particle size of the droplets being adjustable in the range of 0.5–50 mm. The detailed structure of these films could be established by various techniques, mainly electron microscopy, thermo-mechanical analysis, pulsed field gradient NMR, etc. w6,7x. It appeared quite clearly that the SEBS rubbery matrix of the film, organized in the
47
well-known mesomorphic structures of thermoplastic elastomers, contains embedded crystalline PEO structures resulting from the block copolymer ‘platelets’. Moreover, pulsed field gradient NMR experiments demonstrated that the diffusion coefficient of the confined PEG phase decreases with the size of the droplets w6,7x. This concept of confined liquid droplets in a rubber matrix was the starting point for the development by Hutchinson, Mapa of disinfectant-filled surgical gloves w17–19x. These G-VIR gloves are typically a multimaterial of three layers: – an elastomer SEBS layer of approximately 150-mm thickness which confers the required mechanical properties to the glove; – an ‘active layer’ of approximately 200–250 mm containing small droplets of the biocide solution in a rubbery matrix; – a third rubber layer of approximately 150 mm, which has to be anallergic as it will be in contact with the surgeon’s skin. This type of gloves are excellent barriers against pathogen penetration and in case of an accidental puncture they are highly efficient as disinfectant. They offer, therefore, as recently demonstrated an optimum protection for the surgeon w20x. 4. Conclusion The results of the present work indicate that nonaqueous emulsions are formed by dispersion of a liquid, such as PEG containing a cationic surfactant, in a continuous phase of SEBS in MeCH. Emulsions with controlled droplet size and excellent stability could be obtained in the presence of PB-PEO and PtBuS-PEO diblock copolymers acting as emulsifiers and stabilizers. The droplet size of the emulsion could be adjusted in a wide range as a function of shear rate and of relative viscosities of the two phases. The block copolymer concentration and its characteristics, such as molecular weight and PEO content, are, however, the major parameters in order to control the droplet size and the stability of the emulsions. Thus, PB-PEO and PtBuS-PEO block copolymers with PEO contents between 40 and 60 wt.% and molecular weights in the range of 5000–15 000 seem to be at the present stage of our study the most efficient emulsifiers and stabilizers for this type of non-aqueous emulsions. The stability of the emulsion is mainly obtained by a Pickering type stabilization mechanism, e.g. by the adsorption of colloidal copolymer particles, such as ‘platelets’ formed by controlled crystallization, at the interface of the two phases and to some extent due to the structure formation in the continuous phase.
G. Riess et al. / Advances in Colloid and Interface Science 108 – 109 (2004) 43–48
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Controlled solvent evaporation from these emulsions leads to thermoplastic elastomer SEBS films containing droplets of confined disinfecting liquids which have promising application possibilities as barrier materials in the biomedical area. Acknowledgments The authors would like to thank Hutchinson, Mapa and the European Community for supporting this project. They are grateful to Mr G. Argy for his interest taken in this study, to Goldschmidt for the generous supply of copolymers, and to Drs G. Reiter and Ph. Sonntag for helpful advices. References w1x G. Riess, G. Hurtrez, P. Bahadur, Block Copolymers, Encyclopedia Polymer Science and Technology, vol. 2, second ed., Wiley, 1985, pp. 324–434. w2x J. Periard, A. Banderet, G. Riess, Polym. Lett. 8 (1970) 109. w3x J. Periard, G. Riess, M.J. Neyer-Gomez, Eur. Polym. J. 9 (1973) 687. w4x A. Imhof, D.J. Pine, J. Coll. Interf. Sci. 192 (1997) 368.
w5x T. Hiroshi, K. Mitsuo, Kanebo Ltd, Japan Patent 622 25 240 (1987). w6x P. Hoerner, G. Riess, F. Rittig, G. Fleischer, Macromol. Chem. Phys. 199 (1998) 343. w7x G. Fleischer, J. Karger, ¨ F. Rittig, et al., Polym. Adv. Technol. 9 (1998) 700. w8x E. Esselborn, J. Fock, A. Knebelkamp, Surface active block copolymers, EPF Symposium, Basel, Switzerland, 10–12y10y 1994. w9x P. Hoerner, Ph.D. Thesis, University Mulhouse, 17y3y1998. w10x B. Lotz, A.J. Kovacs, Kolloid Z. Z. Polym. 209 (1966) 97. w11x K.A. Cogan, A.P. Gast, Macromolecules 23 (1990) 745. w12x G. Wu, Z. Zhou, B. Chu, Macromolecules 26 (1993) 2117. w13x T. Tadros, B. Vincent, in: P. Becher, M. Dekker (Eds.), Emulsion Stability, Encyclopedia of Emulsion Technology, vol. 1, New York, 1983. w14x S.U. Pickering, J. Chem. Soc. 91 (1907) 91. w15x B.P. Binks, J.H. Clint, Langmuir 18 (2002) 1270. w16x R. Krikorian, Ph.D. Thesis, University Mulhouse, 26y11y2001. w17x G. Riess, A. Cheymol, Hutchinson, French Patent 93 15561 (23y12y1993). w18x P. Hoerner, G. Riess, R.G. Busnel, A. Cheymol, Hutchinson, Europ. Patent EP 0771 837 A1 (29y10y1996). w19x R.G. Busnel, A. Cheymol, G. Riess, Hutchinson, US Patent 5 804 628 (8y9y1998). w20x F. Bricout, A. Moraillon, Ph. Sonntag, P. Hoerner, W. Blackwelder, S. Plotkin, J. Med. Vir. 69 (2003) 538.
Non-aqueous emulsions stabilized by block copolymers: application to liquid disinfectant-filled elastomeric films ´ Gerard Riessa,*, Andre´ Cheymolb, Pierre Hoernerb, Raffi Krikorianb a
´ Ecole Nationale Superieure de Chimie de Mulhouse, Institut de Chimie des Surfaces et Interfaces, 3 rue Alfred Werner, 68093 Mulhouse Cedex, France b Hutchinson, 2 rue Balzac, 75008 Paris, France
Abstract The emulsifying and stabilization efficiency of polybutadiene-b-poly(ethylene oxide) and poly(ter butylstyrene)–poly(ethylene oxide) diblock copolymers is examined in non-aqueous emulsions. These emulsions are formed by a dispersion of polyethylene glycol mixed with a cationic surfactant acting as a biocide, in a continuous phase of a thermoplastic elastomer (SEBS) dissolved in methylcyclohexane. Emulsions with controlled droplet size and excellent stability could be obtained, which by solvent evaporation lead to elastomeric films containing droplets of confined disinfecting liquids. 䊚 2004 Elsevier B.V. All rights reserved. Keywords: Non-aqueous emulsions; Block copolymers; Disinfectant; Surgical gloves; Thermoplastic elastomers
1. Introduction The increasing importance of block copolymers arises mainly from their unique colloidal and emulsifying properties which is a consequence of their molecular structure. It is now well established that block copolymers act as efficient emulsifiers for stabilizing emulsions composed of two immiscible liquids when each of them is a selective solvent of one of the blocks of the copolymer. Oil-in-water (OyW) and water-in-oil (Wy O) emulsions are thus typically obtained with hydrophobic–hydrophilic block copolymers, the hydrophilic block being either anionic, cationic or non-ionic w1x. Oil-in-oil (OyO) emulsions, so-called non-aqueous emulsions or water-free emulsions, have far less been studied. The possibility to form OyO emulsions is a very specific property of block copolymers and such systems are difficult, if not impossible, to obtain using classical low molecular weight surfactants. First examples reported for OyO emulsions were dimethylformamide–hexane and cyclohexane–acetonitrile emulsions stabilized, respectively, with polystyrene–polyisoprene and polystyrene–poly(methylmethacrylate) diblock copolymers w2,3x. Non-aqueous emulsions based on fluorocarbons, silicone oils, lipids and polyols have since *Corresponding author. Tel.yfax: q33-3-89-33-68-54. E-mail address: [email protected] (G. Riess).
been developed as described mainly in the patent literature for biomedical and cosmetic formulations w4,5x. As an extension of these concepts we became interested in non-aqueous emulsions where the dispersed phase consists of poly(ethylene glycol) (PEG) droplets, the continuous phase being formed by a polymer solution of a film forming polymer dissolved in a volatile organic solvent. Liquid-filled polymers, thermoplastics or elastomers, can thus be obtained by solvent evaporation as we have shown previously w6,7x. This kind of confined liquidfilled elastomeric film is for instance obtained by solvent evaporation from a polymeric OyO emulsion with a continuous phase of a styrene-hydrogenated diene thermoplastic elastomer SEBS dissolved in methylcyclohexane (MeCH) and having a dispersed phase of PEG, which is non-miscible neither with methylcyclohexane, nor with the styrenic and the ethylene–butylene blocks of SEBS. The stability of these emulsions is achieved with either polybutadiene-poly(ethylene oxide) (PB-PEO) or poly(ter butylstyrene)-b-poly(ethylene oxide) (PtBuSPEO) block copolymers, PEO being miscible selectively with PEG and PB, PtBuS, with the SEBS solution in MeCH. In the present study we were interested in the stabilization efficiency of PB-PEO and PtBuS-PEO diblock
0001-8686/04/$ - see front matter 䊚 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cis.2003.10.019
G. Riess et al. / Advances in Colloid and Interface Science 108 – 109 (2004) 43–48
44
Table 1 Characteristics of PB-PEO diblock copolymers Reference
PEO content wt.%
Mn PB
IPa PB
Mn PEO
Mn PB-PEO
BE BE BE BE BE BE BE BE
21 26 30 39 42 50 56 60
8300 58 000 21 100 11 300 9200 7700 6000 4000
1.06 1.06 1.06 1.05 1.06 1.04 1.05 1.07
2300 20 000 9300 6900 6700 7600 7500 6000
10 600 78 000 30 400 18 200 15 900 15 300 13 500 10 000
a
153-52 1074-455 391-211 209-157 170-153 143-173 111-170 74-136
29 wt.% polystyrene, was used as thermoplastic elastomer solubilized in MeCH at a concentration of 10–18 wt.%, unless otherwise stated. PEG, molecular weight 400, was purchased from Fluka. Its characteristics as determined at 22 8C are the following: volumic masss1130 kgym3; viscositys110 mPa s; surface tensions40 mNym. The disinfecting agent, didecyl-dimethyl ammonium chloride (BARDAC – Lonza) is well known for its quasi-instantaneous inactivation effect on virus. This cationic surfactant DDAC is of the following structure:
Polydispersity index (IP)sMw yMn.
copolymers for the preparation of ‘non-aqueous emulsions’ having as dispersed phase a liquid disinfectant, e.g. a solution of a quaternary ammonium derivative solubilized in PEG and as continuous phase a solution of a thermoplastic elastomer (SEBS) in MeCH. Under controlled evaporation of the cycloaliphatic solvent from this emulsion, it will thus be possible to obtain an elastomer film containing the liquid disinfecting phase in the form of small droplets embedded in the rubber matrix. 2. Experimental part 2.1. Synthesis and characterization of the diblock copolymers PB-PEO and PtBuS-PEO were synthesized by sequential anionic polymerization following standard procedures as published previously w6x. The block copolymer sample TS 22-55 supplied by Goldschmidt was prepared by ethoxylation of v-hydroxy-functionalized PtBuS w8x. The molecular weight and the molecular weight distribution of the precursor PB and PtBuS sequences were determined by SEC. The eluent was THF and the calibration curve was obtained using polystyrene standards. The actual molecular weights Mn and Mw of the precursor blocks were obtained by the classical ‘universal calibration’ technique w9x. The copolymer composition and the microstructure of the PB sequence were determined by 1H-NMR (Bruker AC 250 F) using CDCl3 as solvent. The microstructure of the PB sequence is approximately 85% of 1–2 configuration. The characteristics of the copolymers used in this study are given in Tables 1 and 2. The copolymers are referenced BE, respectively TS, with a first number indicating the DPn of the PB or PtBuS block and a second number for the DPn of the PEO block. 2.2. Materials Polystyrene-b-poly(ethyleneybutylene)-b-polystyrene (SEBS, Kraton G 1652, Kraton Polymers), containing
2.3. Solubilization of the copolymer Stock solutions of the block copolymers (20–50 mgy ml) were prepared by direct dissolution in MeCH at 80 8C during 3 h. The solution was then cooled to room temperature (22 8C) overnight. The solutions are isotropic at 80 8C and become birefrigeant by cooling down to 22 8C, as also observed by different authors for PEOcontaining block copolymers on crystallization in organic solvents w10–12x. In agreement with these authors it could be shown with various technique, and especially atomic force microscopy (AFM), that these colloidal structures obtained by crystallization could be in the form of small ‘platelets’ having a core of crystallized PEO with a fringe of PB or PtBuS sequences. An example of these structures is given in Fig. 1. Their amount in the stock solution was estimated by centrifugational separation (30 min at 5000=g). 2.4. Preparation of the emulsions A general procedure for the preparation of the emulsions is outlined in this section with the characteristics Table 2 Characteristics of the PtBuS-PEO diblock copolymers Reference
PEO content wt.%
Mn PtBuS
IPa PtBuS
Mn PEO
Mn PtBuS-PEO
TS TS TS TS TS
30 34 38 41 43
9700 7900 3400 3500 6000
1.20 1.43 1.13 n.d. 1.15
4100 4150 2100 2400 4500
13 800 12 050 5500 5900 10 500
56-93 47-95 21-47 22-55b 38-103
n.d. not determined. a Polydispersity index (IP)sMw yMn. b Sample supplied by Goldschmidt reference BSE 35-24.
G. Riess et al. / Advances in Colloid and Interface Science 108 – 109 (2004) 43–48
45
Fig. 1. AFM image (tapping mode) showing a ‘Platelet’ structure of PB-PEO copolymer (BE 74-136) crystallized in MeCH.
given at room temperature (22 8C) whereas additional details will be indicated with the corresponding experimental results. The continuous phase of the emulsion is prepared by adding a known amount of the copolymer solution to the SEBS solubilized in MeCH at a concentration of 10–18 wt.%. In this concentration range the volumic mass is rc(790 kgym3 and the viscosity hc varies from 30 to 300 mPa s. The dispersed phase is a mixture of DDACyPEG 400 in a weight ratio 30:70 having a volumic mass rDs 1170 kgym3 and a viscosity hDs110 mPa s. The DDACyPEG 400 mixture is slowly added to the SEBS solution under stirring (109 at 150–1000 rpm) by using classical lab scale emulsification equipment (blade stirrer and triple helix propeller). The volume fraction of the dispersed phase fc is kept between 0.05 and 0.08, which will lead after MeCH evaporation from the emulsion to an elastomer film containing a volume fraction of dispersed phase ff (PEGqBARDAC) of approximately 0.30. The concentration of copolymer, typically in the range of 2.5–40 wt., is given with respect to the dispersed phase.
2.5. Emulsion characterization The stability of the emulsion is evaluated from their sedimentation velocity using a Turbiscan MA1000 analyser, by which the transmission and the backscattering signals of the emulsion are monitored as a function of time. The average particle size given as the median diameter D50 (50% of the particles having a diameter -D50) is determined with a Photosedimentation Particle Size Analyser Shimadzu SA-CP3. Details about these different techniques can be found in Ref. w9x. 3. Results and discussion In the systematic study of this emulsification process, our major aim was to demonstrate – that the particle size of the dispersed phase can be adjusted which is necessary to impart the required disinfecting efficiency to the films; – that the non-aqueous emulsions of PEG dispersed in a solution of SEBSyMeCH can be obtained with excellent stabilities.
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Table 3 Average particle size and stability of the emulsions as a function of the PB-PEO block copolymer characteristics Reference
PEO content wt.%
‘Platelets’ wt.%a
D50 at 5% copolymer mm
Stability t25 (h) 5% copolymer
D50 at 15% copolymer mm
Stability t25 (h) 15% copolymer
BE BE BE BE BE BE BE BE
21 26 30 39 42 50 56 60
0 16 18 10 10 12 33 65
15.4 15.8 13.6 11.6 11.7 11.1 10.4 9.8
8 6 10 15 15 15 72 100
11.8 12.1 11.7 8.6 9.1 7.7 6.6 7.3
60 100 72 245 400 740 )1000 )1000
153-52 1074-455 391-211 201-157 170-153 143-173 111-170 74-136
Experimental conditions: viscosity of the continuous phase hcs180 mPa s; fes0.07; ffs0.34, agitation 109 at 300 rpm. a Wt.% of ‘platelets’ separated by centrifugation (309 at 5000=g) from the stock-solution of copolymer in MeCH. Table 4 Emulsion characteristics as a function of the ‘platelets’ concentration Copolymer stabilizer PEO content D50 Stability t25 platelets wt.% at 5% copolymer (h) wt.% mm 0 65a 100
60 60 60
6.7 9.8 10.4
55 100 110
Experimental conditions: viscosity of the continuous phase hcs 180 mPa s; fes0.07; ffs0.34, agitation 109 at 300 rpm. a Copolymer dispersion before separation by centrifugation.
Table 3 shows as an example the particle size and the stabilities of the emulsions prepared with PB-PEO copolymers as a function of the copolymer characteristics and at two concentration levels of 5 and 15%. The stability is indicated by the time, expressed in hours, which is necessary to reach under static conditions a decantation of 25% for the emulsion. From Table 3 it can be noticed that the particle size of the dispersed phase decreases with increasing copolymer concentrations and with the increase in their PEO content. A correlation seems also to exist between the weight fraction of ‘platelets’ in the copolymer stock-solution in MeCH and the stability of the resulting emulsions. This could be an indication that this type of non-aqueous
emulsions are stabilized by molecular dispersed copolymer becoming anchored at the droplet surface, as well as by the colloidal dispersion of ‘platelets’ acting as a Pickering stabilizer w13–15x. Evidence of this stabilization mechanism by finely divided solid particles can be given by preparing the emulsion with the same amount of copolymer, on the one side with the ‘platelet’ fraction isolated by centrifugation, on the other with the supernating copolymer fraction. For the copolymer BE 74-136 it could be checked by NMR analysis that both fractions, the ‘platelets’ and the copolymers remaining in solution, have the same PEO content, which indicates that no detectable fractionation in composition has occurred during the preparation of the copolymer stock-solution in MeCH. The characteristics of the emulsions prepared under these conditions are given in Table 4. From this table it appears that the best emulsifying effect, e.g. the lowest particle size, is achieved in the absence of ‘platelets’. Molecular dispersed copolymer andyor in its micellar form has, therefore, a greater tendency to form smaller droplets than the ‘platelets’. However, emulsions of higher stability and larger particle size can be obtained in the presence of these finely divided solid particles.
Table 5 Particle size and stability of the emulsions as a function of the PtBuS-PEO block copolymer characteristics Reference
PEO content wt.%
D50 at 5% copolymer mm
Stability t25 (h) 5% copolymer
D50 at 15% copolymer mm
Blank TS 56-93 TS 47-95 TS 21-47 TS 22-55 TS 38-103
– 30 34 38 41 43
– 10.8 10.8 4.8 5.4 9.4
– 6 96 24 350 6
– 10.4 5.6 4.5 – 6.0
Experimental conditions: viscosity of the continuous phase hcs180 mPa s; fes0.07; ffs0.34; agitation 109 at 300 rpm. a Emulsion without copolymer.
Stability t25 (h) 15% copolymer a
2 120 )1000 )1000 )1000 )1000
G. Riess et al. / Advances in Colloid and Interface Science 108 – 109 (2004) 43–48 Table 6 Particle size of the emulsions as a function of the PtBuS-PEO block copolymer concentration % Copolymer D50 mm Stability t25 (h)
2.5 12.4 6
5 11.4 29
7.5 9.6 70
10 9.9 69
15 8.5 290
20 9.1 350
40 8.2 1600
Experimental conditions: copolymer T22-55 concentration with respect to the dispersed phase from 2.5 to 40%; viscosity of the continuous phase hcs80 mPa s; viscosity of the dispersed phase hds 110 mPa s; fes0.07; ffs0.34; agitation 109 at 600 rpm.
Similar results were also obtained with PtBuS-PEO copolymers as shown in Table 5. As copolymer TS 22-55, supplied by Goldschmidt, was meeting the requirement of stability, we examined in more details the influence of the copolymer concentration on the emulsion characteristics. The corresponding results are given in Table 6. As expected the particle size decreases and the stability increases with increasing amounts of copolymer. By comparing these results with those of Table 5, it can be noticed that at given copolymer concentrations, e.g. 5 and 15%, the particle size increases significantly by decreasing the viscosity of the continuous phase from 180 to 80 mPa s, even if the agitation is increased from 300 to 600 rpm. It could be confirmed that the average diameter of the dispersed phase decreases with increasing shear rates and levels off at approximately 1000 rpm. It was further shown that the particle size could also be adjusted by varying the viscosity ratio of the phases. In fact, it is well known that at a given shear rate and at constant interfacial tension between the two phases the particle size of the dispersed phase will be inversely proportional to the viscosity of the continuous phase, which in our case can easily be adjusted by increasing the amount of SEBS solubilized in MeCH w9 x . As an extension of these concepts, it was also confirmed more recently that poly(hydrogenated butadiene)b-poly(ethylene oxide) diblock copolymers are quite efficient as emulsifiers and stabilizers of the above mentioned non-aqueous emulsions w16x. It could be demonstrated that the solid colloidal particles, such as ‘platelets’, obtained under well-defined crystallization conditions in a cycloaliphatic solvent, are perfectly adapted to stabilize non-aqueous emulsion by a Pickering stabilization mechanism w13–15x. Starting with these emulsions and by controlled evaporation of MeCH, elastomeric films were obtained with confined droplets of PEGycationic surfactant, the particle size of the droplets being adjustable in the range of 0.5–50 mm. The detailed structure of these films could be established by various techniques, mainly electron microscopy, thermo-mechanical analysis, pulsed field gradient NMR, etc. w6,7x. It appeared quite clearly that the SEBS rubbery matrix of the film, organized in the
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well-known mesomorphic structures of thermoplastic elastomers, contains embedded crystalline PEO structures resulting from the block copolymer ‘platelets’. Moreover, pulsed field gradient NMR experiments demonstrated that the diffusion coefficient of the confined PEG phase decreases with the size of the droplets w6,7x. This concept of confined liquid droplets in a rubber matrix was the starting point for the development by Hutchinson, Mapa of disinfectant-filled surgical gloves w17–19x. These G-VIR gloves are typically a multimaterial of three layers: – an elastomer SEBS layer of approximately 150-mm thickness which confers the required mechanical properties to the glove; – an ‘active layer’ of approximately 200–250 mm containing small droplets of the biocide solution in a rubbery matrix; – a third rubber layer of approximately 150 mm, which has to be anallergic as it will be in contact with the surgeon’s skin. This type of gloves are excellent barriers against pathogen penetration and in case of an accidental puncture they are highly efficient as disinfectant. They offer, therefore, as recently demonstrated an optimum protection for the surgeon w20x. 4. Conclusion The results of the present work indicate that nonaqueous emulsions are formed by dispersion of a liquid, such as PEG containing a cationic surfactant, in a continuous phase of SEBS in MeCH. Emulsions with controlled droplet size and excellent stability could be obtained in the presence of PB-PEO and PtBuS-PEO diblock copolymers acting as emulsifiers and stabilizers. The droplet size of the emulsion could be adjusted in a wide range as a function of shear rate and of relative viscosities of the two phases. The block copolymer concentration and its characteristics, such as molecular weight and PEO content, are, however, the major parameters in order to control the droplet size and the stability of the emulsions. Thus, PB-PEO and PtBuS-PEO block copolymers with PEO contents between 40 and 60 wt.% and molecular weights in the range of 5000–15 000 seem to be at the present stage of our study the most efficient emulsifiers and stabilizers for this type of non-aqueous emulsions. The stability of the emulsion is mainly obtained by a Pickering type stabilization mechanism, e.g. by the adsorption of colloidal copolymer particles, such as ‘platelets’ formed by controlled crystallization, at the interface of the two phases and to some extent due to the structure formation in the continuous phase.
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Controlled solvent evaporation from these emulsions leads to thermoplastic elastomer SEBS films containing droplets of confined disinfecting liquids which have promising application possibilities as barrier materials in the biomedical area. Acknowledgments The authors would like to thank Hutchinson, Mapa and the European Community for supporting this project. They are grateful to Mr G. Argy for his interest taken in this study, to Goldschmidt for the generous supply of copolymers, and to Drs G. Reiter and Ph. Sonntag for helpful advices. References w1x G. Riess, G. Hurtrez, P. Bahadur, Block Copolymers, Encyclopedia Polymer Science and Technology, vol. 2, second ed., Wiley, 1985, pp. 324–434. w2x J. Periard, A. Banderet, G. Riess, Polym. Lett. 8 (1970) 109. w3x J. Periard, G. Riess, M.J. Neyer-Gomez, Eur. Polym. J. 9 (1973) 687. w4x A. Imhof, D.J. Pine, J. Coll. Interf. Sci. 192 (1997) 368.
w5x T. Hiroshi, K. Mitsuo, Kanebo Ltd, Japan Patent 622 25 240 (1987). w6x P. Hoerner, G. Riess, F. Rittig, G. Fleischer, Macromol. Chem. Phys. 199 (1998) 343. w7x G. Fleischer, J. Karger, ¨ F. Rittig, et al., Polym. Adv. Technol. 9 (1998) 700. w8x E. Esselborn, J. Fock, A. Knebelkamp, Surface active block copolymers, EPF Symposium, Basel, Switzerland, 10–12y10y 1994. w9x P. Hoerner, Ph.D. Thesis, University Mulhouse, 17y3y1998. w10x B. Lotz, A.J. Kovacs, Kolloid Z. Z. Polym. 209 (1966) 97. w11x K.A. Cogan, A.P. Gast, Macromolecules 23 (1990) 745. w12x G. Wu, Z. Zhou, B. Chu, Macromolecules 26 (1993) 2117. w13x T. Tadros, B. Vincent, in: P. Becher, M. Dekker (Eds.), Emulsion Stability, Encyclopedia of Emulsion Technology, vol. 1, New York, 1983. w14x S.U. Pickering, J. Chem. Soc. 91 (1907) 91. w15x B.P. Binks, J.H. Clint, Langmuir 18 (2002) 1270. w16x R. Krikorian, Ph.D. Thesis, University Mulhouse, 26y11y2001. w17x G. Riess, A. Cheymol, Hutchinson, French Patent 93 15561 (23y12y1993). w18x P. Hoerner, G. Riess, R.G. Busnel, A. Cheymol, Hutchinson, Europ. Patent EP 0771 837 A1 (29y10y1996). w19x R.G. Busnel, A. Cheymol, G. Riess, Hutchinson, US Patent 5 804 628 (8y9y1998). w20x F. Bricout, A. Moraillon, Ph. Sonntag, P. Hoerner, W. Blackwelder, S. Plotkin, J. Med. Vir. 69 (2003) 538.