Impact of Silicone-Based Block Copolymer Surfactants on the Surface

Biomacromolecules 2003, 4, 696-703

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Impact of Silicone-Based Block Copolymer Surfactants on the Surface and Bulk Microscopic Organization of a Biodegradable Polymer, Poly(E-caprolactone) Pascal Viville* and Roberto Lazzaroni Service de Chimie des Mate´ riaux Nouveaux (SCMN), Centre de Recherche en Sciences des Mate´ riaux Polyme` res, Universite´ de Mons-Hainaut, 20 Place du Parc, 7000 Mons, Belgium

Philippe Dubois Service des Mate´ riaux Polyme` res et Composites (SMPC), Centre de Recherche en Sciences des Mate´ riaux Polyme` res, Universite´ de Mons-Hainaut, 20 Place du Parc, 7000 Mons, Belgium

Anton Kotzev and Yves Geerts Laboratoire de Chimie des Polyme` res, Universite´ Libre de Bruxelles, CP 206/1 Boulevard du Triomphe, B-1050 Bruxelles, Belgium

Gabriela Borcia and Jean-Jacques Pireaux Laboratoire Interdisciplinaire de Spectroscopie Electronique, Faculte´ s Universitaires Notre-Dame de la Paix, Rue de Bruxelles 61, B-5000 Namur, Belgium Received November 20, 2002; Revised Manuscript Received February 18, 2003

Two amphiphilic AB block copolymers, containing a highly compatible poly(-caprolactone) (PCL) block connected to a poly(dimethylsiloxane) (PDMS) block having a low surface energy, are synthesized and characterized in terms of their dispersion in a presynthesized PCL matrix. X-ray photoelectron spectroscopy, contact angle measurements, atomic force microscopy, and optical microscopy are used to describe the evolution of the surface chemical composition, as well as the surface and bulk morphology of the PCL/ copolymer blends as a function of the nature and weight surface free energy and the dispersion of the copolymers in the blends, leading to important modifications of the bulk and the surface morphology. These differences are interpreted in terms of the impact of the block copolymers on the semicrystalline polymer structure and related properties in the prospect of using the surfactants to improve the synthesis of PCL in supercritical CO2. Introduction Adjusting the properties of polymer materials is an important challenge from the fundamental, as well as from the applied, point of view. In particular, the importance of low environmental impact and biocompatible materials and technologies has been constantly increasing during the past two decades. Quite often, this means either a surface modification of the material or making blends of materials of different compatibility.1 In both cases, this would require the use of surfactants to achieve the desired surface properties or to stabilize the blend. To achieve both roles, a very promising molecular architecture for such a surfactant is a block or graft copolymer containing a low-surface-energy poly(dimethylsiloxane) (PDMS) sequence combined with polyester sequences such as poly(-caprolactone) (PCL) or polylactide, which have good miscibility with most polymeric materials. All of these polymer blocks are known to be highly biocompatible.2,3 Moreover, this type of polymer surfactant containing PDMS fragments is shown to promote dispersion polymerization in liquid and supercritical CO2.4-8 The latter

is considered as a promising and environment-friendly alternative for the replacement of the organic solvents traditionally used in polymerization procedures, in addition to other practical advantages such as low-cost, nontoxicity, nonflammability, and ease of removal, which could make CO2 the future reaction medium for a number of polymerization processes.4-8 However, one problem derived from the use of CO2 remains the control of the monomer and polymer dispersion in the polymerization medium. It is known that the solvency power of CO2 is quite good for small molecules (such as monomers) and quite poor for most polymers (with the important exception of noncrystalline fluorinated polymers and silicones). Particle stabilization is thus needed to get attractive polymerization rates and morphology control. To reach that goal, emulsifying agents are classically used to solubilize the CO2-phobic material. Along the same line, the use of surfactants can favor the recovery of the formed polymer from the reactor under a handable form, such as a fluff or a powder. Such surfactants are generally amphiphilic, that is, the different components of the molecule have quite

10.1021/bm0257356 CCC: $25.00 © 2003 American Chemical Society Published on Web 03/25/2003

Impact of Si-Based Block Copolymer Surfactants

Biomacromolecules, Vol. 4, No. 3, 2003 697

Scheme 1

different solubility in the reaction medium. Block copolymers, consisting of two different polymer species that are covalently bound to each other, have been shown to be capable of solubilizing CO2-phobic materials and thus of promoting polymerization processes. Most copolymers used in CO2 are nonionic and, in most cases, contain poly(dimethylsiloxane) (PDMS) or fluorinated sequences because these two polymers are well-known for their solubility in CO2. Up to now, the dispersion polymerization has been widely studied, but there is very little research concerning the impact of the surfactant used in the process on the bulk and surface morphology and properties of the prepared polymer material. This is an important issue because surfactant removal is usually not considered.4-8 In this work, two particular diblock copolymers are studied the architecture of which is tailored to understand the impact of the surfactant on PCL prepared by dispersion polymerization in CO2.9 The first diblock copolymer is constituted of a “CO2-philic” PDMS sequence and an anchor made of PCL, PDMS70-PCL24; it is called hereafter type A surfactant. The second copolymer contains a short “CO2-phobic” segment instead of an entire PCL sequence, PDMS70-(CH2)3-O-(CH2)2-O-CO-C2H5 (∼PDMS70-PCL1); it is denoted type B surfactant (Scheme 1). In this case, the esterification of the OH group has the additional advantage of avoiding the chemical linking of the surfactant to the PCL during its formation. In this work, we aim at evaluating the impact of these two surfactant copolymers on the surface and bulk morphology and properties of PCL by characterizing, as model systems, blends of PCL and the copolymers. The effect of the addition of the surfactants is followed both at the surface of the films by means of X-ray photoelectron spectroscopy (XPS), contact angle measurements, and atomic force microscopy (AFM) and in the bulk by polarized optical microscopy (OM). The addition of few percent of surfactant agents in PCL is expected to modify the PCL crystalline structure and to have large effects on the surface morphology and chemical composition. The extent to which the micro-

structure of PCL is modified by the addition of the surfactants is related to the ability of the copolymer to interfere with the PCL solid-state organization and thus to the degree of interaction between the polymer and the copolymer. Experimental Section Materials and Methods. Chloroform (p.a., Aldrich), triethylamine (Aldrich), propionyl chloride (Aldrich), triethylaluminum (Aldrich), and PCL (Solvay, average Mn ) 50 000) were used as received. Toluene (Aldrich) and -caprolactone (Aldrich) were dried over CaH2 and distilled prior to use. Monohydroxy-terminated PDMS, CH3-(CH2)3[Si(CH3)2-O]70-Si(CH3)2-(CH2)3-O-(CH2)2-OH (Aldrich), was dried azeotropically into the reaction flask by distillation of anhydrous toluene. The average Mn of the PDMS precursor, as well as those of the derived block copolymers, was determined by 1H NMR taken on a Bruker Avance 300 spectrometer at 300 MHz in CDCl3; the polydispersity index (PDI) was estimated by size-exclusion chromatography (SEC) on Polymer Laboratories equipment in tetrahydrofuran (THF) using polystyrene standards. Thus, the average Mn of the monohydroxy-terminated PDMS (apparent Mn ) 6600 (SEC), PDI ) 1.09) was determined to be 5200 (i.e., 70 dimethylsiloxane units) by comparison of the integration of the Si-bound methyl signals between 0.0 and 0.2 ppm to that of the methyleneoxy signals of the functional end-group between 3.4 and 3.8 ppm. Synthesis of the Surfactants. The two block copolymers were synthesized from monohydroxy-terminated PDMS, CH3-(CH2)3-[Si(CH3)2-O]70-Si(CH3)2-(CH2)3 -O-(CH2)2-OH.10 Poly[(dimethylsiloxane)-block-(-caprolactone)] (Surfactant A). Ten grams (1.92 mmol) of monohydroxy-terminated PDMS dissolved in 200 mL of dry toluene and 4.41 mL of a 0.480 M solution (2.12 mmol) of AlEt3 in toluene were allowed to react for 30 min at 60 °C. Then, the solution was cooled to 0 °C, and 10.8 mL (11.6 g, 0.102 mol) of -caprolactone was slowly injected. The polymerization was

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stopped after 2 h at 0 °C by addition of 2.5 mL of aqueous HCl (1 M). The mixture was then precipitated in cold methanol. The copolymer was resolubilized in chloroform and washed successively with 2 × 30 mL of aqueous EDTA (0.1 M), 2 × 30 mL of aqueous NaOH (0.1 M), and 3 × 50 mL of distilled water to remove the catalyst. Finally, the block copolymer was reprecipitated in cold methanol from its chloroform solution to give 13.8 g of a colorless wax. 1H NMR (300 MHz in CDCl3, δ in ppm): 4.15-4.22 (m, 2H, -O-CH2-CH2-O-CO-), 3.98-4.13 (m, 48H, [-CO(CH2)4-CH2-O-]24), 3.57-3.63 (m, 2H, -O-CH2-CH2O-CO-), 3.37-3.44 (m, 2H, -CH2-O-CH2-CH2-OCO-), 2.25-2.35 (m, 48H, [-CO-CH2-(CH2)4-O-]24), 1.45-1.70 (m, 98H, [-CO-CH2-CH2-CH2-CH2-CH2O-]24, -Si-CH2-CH2-CH2-O-), 1.20-1.41 (m, 52H, [-CO-(CH2)2-CH2-(CH2)2-O-]24, CH3-(CH2)2-CH2Si-), 0.83-0.91 (m, 3H, CH3-(CH2)3-Si-), 0.47-0.56 (m, 4H, CH3-(CH2)2-CH2-Si-, -Si-CH2-CH2-CH2-O-), 0.00-0.20 (m, 426H, CH3-Si-). SEC (THF, PS standards): apparent Mn ) 13 500; PDI ) 1.23. The formation of the block copolymer was evidenced by the disappearance of the methylene peak (-CH2-OH) at 3.70-3.75 ppm of the PDMS precursor and the appearance of the typical ester methylene peak (-CH2-O-CO-) at 4.15-4.22 ppm. Moreover, SEC analysis shows a monomodal trace of apparent Mn of 13 500 and PDI of 1.23, which were clearly different from those of the PDMS precursor (apparent Mn ) 6600 and PDI ) 1.09). The length of the PCL block was calculated from the copolymer 1H NMR spectrum by comparison of the integration of the Si-bound methyl signals between 0.0 and 0.2 ppm to that of the PCL methylene signals at 3.98-4.13 ppm and 2.25-2.35 ppm. An average Mn of 8000 for the block copolymer was deduced. Poly[(dimethylsiloxane)-block-(trimethylenoxydimethylenyl propionate)] (Surfactant B). Twenty grams (3.85 mmol) of monohydroxy-terminated PDMS in 40 mL of chloroform and 1.70 mL (1.80 g, 19.5 mmol) of propionyl chloride were allowed to react for 6 h at room temperature. Then, 3 mL (2.16 g, 21.3 mmol) of triethylamine was added dropwise, and the reaction was continued overnight. Then, the chloroform solution was washed successively with 2 × 30 mL of distilled water, 2 × 30 mL of aqueous NaHSO3(0.5 M), and 3 × 30 mL of distilled water. Trace water is removed by drying over anhydrous MgSO4 to obtain 19.7 g of colorless oil. 1H NMR (300 MHz in CDCl3, δ in ppm): 4.16-4.22 (m, 2H, -O-CH2-CH2-O-CO-), 3.57-3.64 (m, 2H, -O-CH2-CH2-O-CO-), 3.36-3.46 (m, 2H, -CH2-O-CH2-CH2-O-CO-), 2.30-2.38 (m, 2H, -OCO-CH2-CH3), 1.54-1.66 (m, 2H, -Si-CH2-CH2CH2-O-), 1.20-1.34 (m, 4H, CH3-(CH2)2-CH2-Si-), 1.08-1.16 (m, 3H, -O-CO-CH2-CH3), 0.83-0.92 (m, 3H, CH3-(CH2)3-Si-), 0.46-0.57 (m, 4H, CH3-(CH2)2CH2-Si-, -Si-CH2-CH2-CH2-O-), 0.00-0.19 (m, 426H, CH3-Si-). The esterification was evidenced by the disappearance of the methylene peak (-CH2-OH) at 3.70-3.75 ppm of the PDMS precursor and the appearance of the typical ester methylene peak (-CH2-O-CO-) at 4.16-4.22 ppm. The

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average Mn of the copolymer is equal to 5300, as calculated from its 1H NMR spectrum using the same procedure as for the PDMS precursor (see above). Sample Preparation and Characterization. In this work, the changes occurring upon addition of an increasing amount of copolymer in PCL are characterized. For that purpose, two series of PCL (Mn ) 50 000) + surfactant samples were prepared. PCL and surfactant copolymers were first solubilized in CHCl3 and blended in varying ratios (from 1 to 10 wt % copolymer). Films were then prepared by spin-coating at ambient conditions a 5 mg/mL solution of the blends, yielding films with a thickness of a few micrometers. The spin-coating method was preferred to solvent casting because it yields more homogeneous films, avoiding local thickening. Typically, 50 µL of the solution was spin-coated on a 1 × 1 cm2 piece of silicon for AFM analysis and on gold-coated silicon for XPS analysis. Samples were analyzed after complete evaporation of CHCl3 at room temperature. XPS spectra were recorded on a HP 5950A spectrometer using monochromatic Al KR radiation (1486.6 eV) under a vacuum of about 10-9 Torr at electron takeoff angle of 51.5°. In these conditions, XPS probes the top 50 Å of the polymer films. An X-ray power of 600 W was used during analysis. The high-resolution spectra were taken in the constant analyzer energy mode with a 120 eV pass energy. The value of 285.0 eV of the hydrocarbon C1s core level was used as a calibration of the energy scale. The peak envelopes were fitted by mixed Gaussian-Lorentzian component profiles. Contact angle measurements were performed by the sessile drop technique (0.5 µl drops of bidistilled water) using an image processing system (VCA 2500 XE from AST corporation). The AFM microscope has been operated in tapping mode (TM), a procedure which is known to minimize the sample distortion due to mechanical interactions between the AFM tip and the surface. In tapping mode, AFM can provide threedimensional imaging of the surface morphology with very accurate lateral and vertical resolution without damaging the surface.11 In addition, we also present phase imaging of the sample surface. Combined with the topography, phase imaging is useful because it yields information concerning the local mechanical properties.12,13 All TM-AFM images were recorded in ambient atmosphere at room temperature with a Nanoscope IIIa (Veeco, Santa Barbara, CA). The probes were commercially available silicon tips with a spring constant of 24-52 N/m, a resonance frequency lying in the 264-339 kHz range, and a typical radius of curvature in the 10-15 nm range. In this work, we show topography and phase images that are recorded with the highest sampling resolution, that is, 512 × 512 data points. Several locations (at least five) of each sample were imaged to ensure that the data are representative. Optical microscopy observations were carried out with a Leitz Wetzlar Orthoplan microscope operating in the reflection mode. We worked in polarizing mode to follow the evolution of the birefringence in the bulk of the films upon addition of increasing amounts of surfactant.



Figure 1. XPS spectra for (a) PCL thin film (C1s and O1s) and (b) PDMS thin film (C1s, O1s, and Si2s). The experimental curves (full line) are compared to the results of spectral fitting (dotted lines).

Results and Discussion

Si2s spectra of PDMS, respectively, along with the spectral fitting.

To understand the evolution of the surface chemical composition for the blends, the XPS analysis was first performed on pure PCL and PDMS thin films, as reference compounds. In Figure 1a,b, we present the C1s and O1s corelevel photoemission spectra of PCL and the C1s, O1s, and

The C1s spectrum for PCL consists of four distinct components, and the O1s envelope decomposes into two distinct components (Figure 1a, Table 1). Spectra were fitted on the basis of standard measurements.14 The relative elemental composition for PCL is C/O ) 3.1:1.0, in very


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Table 1. Binding Energies (BE) and Relative Peak Areas of the PCL C1s and O1s Fitted Peaks

C1s BE (eV) area (%)

O1s

1

2

3

4

1

2

285.0 51

285.5 18

286.6 17

289.1 14

532.2 51

533.5 49

Table 2. Binding Energies (BE) of the PDMS peaks

Figure 2. Relative amount of Si at the surface of PCL + surfactant thin films as determined from the XPS spectra. BE (eV)

C1s

O1s

Si2p

285.0

532.6

152.0

Table 3. Elemental Surface Composition for PCL + Surfactant Samples A. A Surfactant C (%) O (%) Si (%)

PCL only

PCL+1%

PCL+2%

PCL+5%

PCL+10%

75.6 24.4 0

73.7 24.5 1.8

72.4 24.5 3.1

70.0 24.4 5.6

67.3 24.6 8.1

PCL only

PCL+1%

PCL+2%

PCL+5%

PCL+10%

75.6 24.4 0

74.3 24.8 0.9

73.4 24.7 1.9

71.4 24.7 3.9

68.7 25.1 6.2

B. B Surfactant C (%) O (%) Si (%)

good agreement with the stoechiometric formula, which predicts a 3:1 ratio. The C1s, O1s, and Si2s spectra for PDMS consist of single peaks (Figure 1b, Table 2) showing a ratio for the atomic percentages C/O/Si ) 2.0:1.0:1.0, identical to the chemical formula. For the “PCL + surfactant” thin films, the PCL and PDMS characteristic elements (C, O, and Si) were identified on the large-energy-scan spectra. Atomic concentration percentages for these elements were determined taking into account the corresponding area sensitivity factor for the different highresolution spectral regions (C1s, O1s, and Si2s). The fitting and quantification of the spectra were performed on the basis of the above results for pure polymer samples. Table 3,sections A and B, summarizes the elemental surface compositions for the two series of samples (A and B surfactants). These results clearly indicate an increase in the Si content at the surface of thin films with increasing surfactant percentage, this trend seeming larger for the A surfactant as compared to the B surfactant (Figure 2). The interpretation of these results was carried out using a simple model, in which we propose that the samples consist of mixtures of the type PCL + x% surfactant, expressed as mass units. Based on the stoichiometric formula for A surfactant, [DMS]70-b-[-CL]24, and for B surfactant, [DMS]70-(CH2)3-O-(CH2)2-O-CO-CH2CH3 (∼[DMS]70-

Table 4. Theoretical Bulk Values and Surface Measured Values for the Surfactant Repeat Units to [-CL] Monomer Units Ratio in PCL + Surfactant Thin Films PCL+1% PCL+2% PCL+5% PCL+10% A surfactant bulk surface B surfactant bulk surface

0.01 0.19 0.02 0.07

0.02 0.32 0.04 0.18

0.05 0.76 0.09 0.36

0.10 1.73 0.18 0.72

[-CL]1), calculations were performed to compare the global elemental composition of the thin films, as deduced from the relative amounts of PCL and surfactant incorporated in the blend, and the actual surface composition, as measured with XPS. The comparison between the “bulk theoretical” values and the XPS experimental results, which are probing only a few surface monolayers, permits us to monitor the evolution of the surface properties as a function of the nature and amount of surfactant added to PCL. The results, presented in Table 4, are expressed as ratios between the number of surfactant repeat units and [-CL] units. Let us note that XPS measurements on pure A and B surfactants (i.e., no PCL added) showed surface chemical compositions with 100% [DMS] moieties. The results indicate that both A and B surfactants migrate to the surface of the samples because the XPS measured values are higher than the “theoretical” values based on the bulk stoichiometric formula by at least 1 order of magnitude for the A surfactant and a factor of 4 for the B surfactant. Nevertheless, PCL is still present in the surface layer (∼50 Å) probed by XPS for the studied range of PCL + surfactant mixtures composition, that is, e10%. Clearly, it appears that [DMS] moieties and surfactant repeat units are much more present at the surface in the case of the A surfactant as compared to the B surfactant. Contact angle measurements are presented in Figure 3. The values are averaged over 10 measurements. The value at 0% surfactant corresponds to a pure PCL thin film. Contact angle measurements showed very uniform thin films on goldcoated silicon wafers having a reduced dispersion for θ values ((2°, which is within the bar of experimental errors). Results for the B surfactant show that the surface wettability, evaluated by θ, is practically constant and not significantly



Figure 3. Contact angle measurements for PCL + surfactant thin films.

different from that of pure PCL over the entire investigated range of PCL + surfactant mixtures. In contrast, thin films containing the A surfactant show a significant increase in contact angle as compared to the pure PCL surface; this trend stabilizes for samples containing more than 2% surfactant, θ remaining almost constant for samples containing 2-10% surfactant. XPS and contact angle data thus clearly demonstrate the more pronounced migration of the A surfactant to the surface of the thin films. It is to be noted that the surface of PCL + 5% surfactant A and PCL + 10% surfactant B have similar compositions (see Tables 3 and 4), while the contact angles are significantly different. One possible explanation could be the effective depth of the surface layer analyzed by the two techniques. The contact angle measurements refer practically only to the first monolayer at the surface, while XPS probes about 50 Å, the escape depth of the photoelectrons. The difference in the contact angles for comparable formulated surfaces of PCL + A surfactant and B surfactant could actually indicate a higher amount of surfactant repeat units at the topmost surface of PCL + A surfactant sample than the one indicated by XPS, which is “averaged” on 4-5 monolayers. Nevertheless, this behavior emphasizes the strong migration of surfactant moieties to the surface. Complementary to this chemical approach, AFM can provide information on the morphology of the films. It offers the possibility to characterize the incorporation of the copolymers in terms of the influence of the copolymer on the PCL microscopic morphology. On the basis of the XPS and contact angle measurements, we report here on blends containing 10 wt % of copolymer to characterize samples of which the surface contains the highest amount of surfactants. In these conditions, one could expect to observe clear differences between the surface morphology of pure PCL and that of the blends. The surfaces of pure PCL and “PCL + surfactants” samples were found to be homogeneous over large length scales. The AFM images shown in the present paper are thus completely representative of the whole sample surface. Figure 4 shows TM-AFM topographic and phase images of pure PCL, PCL containing 10 wt % of A surfactant, and PCL containing 10 wt % of B surfactant. Pure PCL (top

Figure 4. Topographic (left) and phase (right) TM-AFM (5 × 5 µm2) images of the surface morphology of thin films of (A) pure PCL, (B) a 90:10 PCL/A surfactant blend, and (C) a 90:10 PCL/B surfactant blend. The thin films are obtained from 5 mg/mL solutions in CHCl3.

images) exhibits a surface morphology made of spherulites, as expected regarding the semicrystalline character of the polymer. In the phase image (right), the internal fibrillar morphology of the spherulites is clearly distinguished. The relatively small size of the spherulites (in the 3-7 µm range) and the particular curvature of the frontier between neighboring spherulites indicate that the spherulites are not simultaneously nucleated.15 This morphology most probably originates from the fact that spin-coating was used for the film preparation; this method brings the system into nonequilibrium conditions and can significantly slow the spherulites growth rate. As a comparison, the casting method is less drastic because, under the same concentration conditions, spherulites of pure PCL are much larger (50-60 µm wide) (results not shown here). PCL containing 10 wt % of A surfactant exhibits a different surface morphology. The presence of spherulites is again identified, but less crystalline areas appear between the spherulitic entities. The surface morphology observed in this case is illustrated by the two images in the middle of Figure 4. In particular, the phase image on the right side clearly shows the disappearance of the crystalline organization over large portions of the surface (for instance, in the lower right quarter of the image). The change in morphology is attributed to the presence of a significant amount of A

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surfactant at the surface, coherent with the chemical composition mapping performed by XPS and contact angle measurements. Probably, in the surfactant-richer areas, PCL crystallization is impeded and the spherulitic morphology does not develop. By comparison, the addition of 10 wt % of B surfactant has a much more pronounced impact on the morphology (bottom images in Figure 4). In this case, the fibrillar morphology, which was observed for the two previous samples, has totally disappeared. Instead, the morphology becomes much smoother and less organized. We thus observe strong differences in the morphology of the blends depending on the nature of the copolymer. We believe that this evolution of the morphology from the PCL/A surfactant blend to the PCL/B surfactant blend originates in a different incorporation of surfactants A and B in PCL. On one hand, the morphology observed for the PCL/A surfactant, that is, modified amorphous regions sandwiched between PCL spherulites, is coherent with an interspherulitic incorporation of the A surfactant. On the other hand, the B surfactant seems to incorporate more homogeneously in PCL, inducing the complete disappearance of the spherulite morphology. These differences can be explained in terms of the different molecular masses between surfactants A and B. The lower molecular mass of the B surfactant (and thus its shorter length) could indeed favor its incorporation in PCL, while the A surfactant, because of its larger size, would incorporate between the spherulites. To further confirm the higher alteration of the PCL crystalline organization upon addition of the B surfactant, we carried out optical microscopy (OM) observations. In a cross-polarized configuration, OM can easily identify the bulk crystallinity of the blends by measuring the birefringence. Again, we focus here on blends containing the highest amount of surfactant, that is, 10 wt %. Figure 5 shows three micrographs illustrating the evolution of the bulk birefringence from pure PCL to PCL/A surfactant and to PCL/B surfactant thin films. For pure PCL, we observe a high level of birefringence coherent with a bulk spherulitic organization of PCL. Note that this result is also coherent with the surface spherulitic organization highlighted by the AFM experiments; in particular, the range of spherulite size is similar to that measured by AFM (3-7 µm). With 10 wt % of A surfactant added to the PCL, we clearly observe birefringence, but in contrast to pure PCL for which the birefringence is homogeneous over the whole sample, birefringence originates here from local areas of the sample, revealing a lower amount of crystalline entities embedded in amorphous regions. Consistent with the AFM images, these observations thus also indicate the presence of A surfactant in the blend (both at the surface and the bulk) and supports the hypothesis that the surfactant molecules are located between the spherulites. Finally, when 10 wt % of B surfactant is added to the blend, we clearly observe a quasi-total suppression of the birefringence, coherent with an intimate insertion of B surfactant molecules in PCL, which prevents crystallization.

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Figure 5. Polarized optical micrographs (175 × 115 µm2) of thin films of (A) pure PCL, (B) a 90:10 PCL/A surfactant blend, and (C) a 90: 10 PCL/B surfactant blend.

AFM and OM data thus converge to the same conclusions, demonstrating a much better incorporation of the B surfactant in PCL. Conclusions Both diblock copolymers studied in this work present an interest as surface modifiers, as well as potential surfactants for the polymerization of -caprolactone in supercritical CO2. The impact of these surfactants on the surface and bulk morphology of PCL has been evaluated by studying their dispersion in “PCL + surfactant” blends.


XPS measurements clearly indicate that both surfactants tend to migrate to the surface of the thin films. This observation, together with contact angle values, points to the lowering of the surface energy at the polymer-air interface (mainly for the surfactant A), which is a necessary but nonsufficient condition for their use as stabilizing agents in sc-CO2 polymerizations. Our results also demonstrate that the surface enrichment is dependent on the copolymer molecular architecture. Because of its longer PCL sequence, PDMS70-PCL24 (A surfactant) has a more pronounced trend to segregate to the surface than the copolymer containing the shorter “PCLphile” sequence, that is, PDMS70-(CH2)3-O-(CH2)2-OCO-C2H5 (B surfactant). The less-pronounced surface segregation of the B surfactant can be understood in terms of its better incorporation within the PCL bulk, consistent with the suppression of crystallinity observed with optical microscopy and with the very small change of contact angle in PCL + surfactant B blends compared to that of pure PCL. AFM images confirm that this bulk modification is also strongly reflected at the surface of the films because deep changes of the surface morphology are indeed observed when a few percent of B surfactant is added to the blend. Making the link with the XPS data, we believe that a lower amount of B surfactant at the surface is sufficient to deeply modify the surface morphology of the blends because of its finer incorporation. Because of that better incorporation and its easier synthesis, the PDMS70-(CH2)3-O-(CH2)2-O-COC2H5 copolymer thus appears as a better candidate to eventually promote the dispersion of the PCL forming in CO2-based processes. It is worth noting here that PCL biodegradability is preserved upon addition of the surfactants. Indeed, it is well-known that PDMS is a biocompatible


polymer and is employed in many biomedical applications. Therefore, the surface localization of PCL-PDMS copolymers is not expected to alter the PCL biocompatibility. Acknowledgment. This work was partly supported by the Re´gion Wallonne Program WDU-TECMAVER and by the Belgian Federal Government Office of Science Policy (SSTC) “Poˆle d’Attraction Interuniversitaire en Chimie Supramole´culaire et Catalyze Supramole´culaire” (PAI 5/3). R.L. is Directeur de Recherches du FNRS (Belgium). References and Notes (1) Arkles, B. CHEMTECH 1983, 13, 542 and references therein. (2) Vert, M., Feijen, J., Albertsson, A.-C., Scott, G., Chiellini, E., Eds. Biodegradable Polymers and Plastics; Royal Society of Chemistry: Cambridge, U.K., 1992. (3) Albertsson, A.-C., Chiellini, E., Feijen, J., Scott, G., Vert, M., Eds. Degradability, Renewability and Recycling-Key Functions for Future Materials; Macromolecular Symposia 144; Wiley-VCH: Weinheim, Germany, 1999. (4) Canelas, D. A.; DeSimone, J. M. AdV. Polym. Sci. 1997, 133, 103. (5) Ajzenberg, N.; Trabelsi, F.; Recasens, F. Chem. Eng. Technol. 2000, 23, 829. (6) Cooper, A. I. J. Mater. Chem. 2000, 10, 207. (7) Kendall, J. L.; Canelas, D. A.; Young, J. L.; DeSimone, J. M. Macromolecules 1999, 32, 543. (8) Canelas, D. A.; DeSimone, J. M. Macromolecules 1997, 30, 5673. (9) Stassin, F.; Halleux, O.; Je´roˆme, J. Macromolecules 2001, 34, 775. (10) Mecerreyes, D.; Je´roˆme, J.; Dubois, Ph. AdV. Polym. Sci. 1999, 147, 1 and references therein. (11) Zhong, Q.; Inniss, D.; Kjoller, K.; Elings, V. B. Surf. Sci. Lett. 1993, 290, L688. (12) Nony, L.; Boisgard, R.; Aime´, J. P. J. Chem. Phys. 1999, 111, 1615. (13) Kopp-Marsaudon, S.; Lecle`re, Ph.; Dubourg, F.; Lazzaroni, R.; Aime, J. P. Langmuir 2000, 16, 8432. (14) Beamson, G.; Briggs, D. High-resolution XPS of organic polymers, The Scienta ESCA300 Database; John Wiley & Sons: Chichester, U.K., 1992. (15) Villers, D.; Dosie`re, M.; Paternostre, L. Polymer 1994, 35, 1586.

BM0257356

Impact of Silicone-Based Block Copolymer Surfactants on the Surface

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