Surface Morphology of Poly(caprolactone) - American Chemical Society

Biomacromolecules 2001, 2, 526-537

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Surface Morphology of Poly(caprolactone)-b-poly(dimethylsiloxane)-b-poly(caprolactone) Copolymers: Effects on Protein Adsorption M. A. Childs, D. D. Matlock, and J. R. Dorgan* Chemical Engineering Department, Colorado School of Mines, Golden, Colorado 80401

T. R. Ohno Physics Department, Colorado School of Mines, Golden, Colorado 80401 USA Received January 2, 2001; Revised Manuscript Received March 5, 2001

Certain triblock copolymer surfaces have beneficial blood contacting properties that remain unexplained from a mechanistic perspective. In this study, poly(caprolactone-block-dimethylsiloxane-block-caprolactone) (PCL-b-PDMS-b-PCL) surfaces are characterized by dynamic contact angle analysis, angle-resolved X-ray photoelectron spectroscopy (XPS), and phase detection imaging atomic force microscopy (AFM). Surface morphology of films cast from 10 wt % MEK solutions are found to be semicrystalline possessing spherulites on the micron scale and alternating semicrystalline PCL-rich and amorphous PDMS-rich lamellae on the nanometer scale. Surface enrichment of the lower surface free energy block, PDMS, is observed using angle-resolved XPS but the surface composition still consists of both copolymer blocks. Films cast from 1 wt % solutions showed similar morphologies but incomplete surface coverage. Different textural features of adsorbed fibrinogen layers on coated and uncoated polypropylene are observed. The hypothesis that patterned block copolymer surfaces can affect protein adsorption and thus influence compatibility is partially supported by the findings of this study. Introduction Health complications arise when blood flows external to its natural environment, the endothelium. Physiological defense mechanisms include activation of platelets, complement, intrinsic and extrinsic coagulation systems. Thrombus formation and positive feedback pathways within these systems further antagonize the immune system and lead to what is known as whole body inflammatory response. This is true in particular, for external cardiopulmonary bypass (CPB) patients whose blood circulates through vast, hostile areas there exists the need for surfaces compatible with blood. The majority of blood-contacting devices utilize polymeric materials because of their inertness, good mechanical properties, low cost, and relative biocompatibility. Modification of these surfaces is perhaps the simplest and most economical method to further enhance biocompatibility. A novel approach to inhibit initial activation steps at the bloodsynthetic material interface involves using patterned block copolymer surfaces.1 This approach exploits the heterogeneous structure of a cell membrane whose mobile glycoproteins and phospholipids can cluster to form microdomains that serve to aid in transmembrane communication.2 It also takes advantage of macromolecular domains of uniform physicochemical properties, such as wetability and charge, on the protein. It is hypothesized that microdomain-patterned synthetic surfaces interacting with these specified areas of * To whom correspondence should be addressed. e-mail: jdorgan@ mines.edu.

proteins profoundly alter adsorbed protein conformations, thus disrupting signaling pathways.3 Block copolymers are unique in their ability to demonstrate a myriad of complex morphologies that are characterized by variously shaped and sized microdomains. Complex bulk and surface morphologies arise when block copolymers are synthesized from thermodynamically incompatible blocks. Phase separation due to unfavorable monomer contacts or block crystallization competes with minimization of chain stretching associated with configurational entropy loss. The result is a frustrated system from which microdomains arise, with at least one domain dimension on the nanometer scale.4 Studies of block copolymers have demonstrated improved biocompatibility through reduced platelet activation, decreased surface thrombogenicity, and other factors. Okano and co-workers studied the copolymer synthesized from hydroxyethyl methacrylate (HEMA) and dimethylsiloxane (DMS). They concluded the most effective decrease in platelet activation occurs when the domain size is roughly 10 nm. Poly(propyleneoxide) (PPO)-b-polyamide copolymers have also been studied, and it was found, through X-ray analysis, that domains between 6 and 12 nm were most effective in decreasing thrombogenicity.5,6 Importantly, it has even been found that, by optimizing the crystalline state of pure polypropylene, interlamellar spacings of 11.3 and 10.8 nm minimize adsorption of rabbit serum albumin and rabbit plasma fibrinogen, respectively.7 In contrast to earlier studies, this later finding clearly points to the importance of patterning and domain size as both crystalline and amorphous domains of polypropylene are hydrophobic.

10.1021/bm0100054 CCC: $20.00 © 2001 American Chemical Society Published on Web 04/17/2001

Effects on Protein Adsorption

Figure 1. Structure of Tegomer and SMA triblock copolymers.

Siloxane-containing polymers have emerged in biomedical applications owing to their reputed biocompatibility, low glass transition temperature (Tg ) -127 °C), and oxidative, thermal, and ultraviolet stability. An example of a triblock copolymer in which a central string of amorphous PDMS is linked to semicrystalline poly(caprolactone) (PCL, Tg ) -70 °C, Tm ) 73 °C) chains is shown in Figure 1; two structural variations are shown. These are referred to according to their commercial names of SMA and Tegomer. A previous study by Lovinger et al. determined the morphology of bulk PCLb-PDMS-b-PCL triblock copolymers; crystalline PCL-rich and amorphous PDMS-rich regions were found using X-ray diffractometry, small-angle X-ray scattering, polarized optical micrography, and transmission electron microscopy.8 However, these techniques reflect bulk properties and lack the sensitivity required to resolve near-surface characteristics important in biomedical applications. In vitro studies indicate that compounding SMA into poly(vinyl chloride) (PVC) and coating polypropylene microporous membranes with SMA enhance the biocompatibility of these base polymers via suspending contact activation and diminishing coagulation activity in human blood.9 A separate study reports that this SMA lessens thrombin and fibrinogen adsorption and platelet binding when blended into PVC. 10 Clinical evaluation involving examination of PVC tubing and blood after CPB reveals markedly less prothrombin activation and platelet deposition on the SMA-treated CPB circuit.11 A separate clinical study of CPB circuits prepared with SMA shows patients’ blood sustaining platelets and resisting both fibrinolysis and thrombin generation more than patients undergoing CPB with untreated circuits.12 Results from these clinical studies demonstrate a significant improvement in the biocompatibility of materials modified with PCL-b-PDMS-b-PCL triblock copolymers. Accordingly, there exists a strong motivation to study these surfaces for the ultimate goal of achieving a deeper understanding of their biocompatibilizing mechanism. The present study investigates the surface microstructure of SMA and Tegomer films solution coated onto polypropylene substrates. Several analytical techniques are employed including dynamic contact angle (DCA) analysis, angle-resolved X-ray photoelectron spectroscopy (AR-XPS), and phase detection imaging atomic force microscopy (PDI-AFM). All of the techniques indicate that while the surface composition is enriched in PDMS, the surface morphology is heterogeneous and characterized by domains of both blocks. Materials and Methods Polypropylene plaques 8 × 3 cm2 in size were injection molded and used for DCA analysis. Plaques were also cut

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into approximately 1 × 1 cm2 pieces, rinsed with deionized water, and used as substrates. Polycaprolactone (Aldrich, catalog no. 440752) having an average molecular weight of 10 000 and poly(dimethylsiloxane) homopolymers were dissolved in reagent grade methyl ethyl ketone (MEK) obtained from Fisher Scientific (catalog no. M209-20). Two PCL-b-PDMS-b-PCL triblock copolymers differing in molecular weight and linker groups were studied. SMA (Thoratec Laboratories, Berkeley, CA) contains a 3200 PDMS midblock and two 2000 PCL end blocks. Tegomer (Goldschmidt Chemical Co., Hopewell, VA) has a 2700 PDMS midblock with 2000 PCL end blocks. Molecular weights were determined by nuclear magnetic resonance. Homopolymer and triblock copolymer surfaces were made by dip-coating polypropylene plaques into 1 and 10 wt % solutions of copolymer in MEK. The plaques were dipped in the appropriate solution for approximately 30 s, then placed in a vacuum oven to dry for 24 h. Annealed samples were achieved by placing samples in a vacuum oven at approximately 55 °C for 24 h (i.e., above the block Tg’s but below the Tm of the PCL block). Dynamic contact angle analysis performed in this study utilized a Cahn model DCA 312 analyzer, equipped with a Wilhelmy balance. Immersion and withdrawals rates in the present study were 40 µm/s, with top and bottom dwell times of 0 s. Contact angles using HPLC grade water were calculated from the second advancing (immersion) cycle at room temperature. Five samples of each surface were measuredseach reported contact angle has error bars spanning two standard deviations. Angle-resolved X-ray photoelectron spectroscopy experiments were performed with a Kratos AXIS HSi imaging spectrometer. A monochromatic Al KR1,2 source was employed for all experiments and was operated at 15.0 mA and 14.0 kV. Base pressure was maintained at 5.0 × 10-8 Torr. Identification of elements present was done by first performing wide energy range survey scans at high scan rates to rapidly determine elements present, and then slower scans were performed over the specific energy ranges for quantification of individual elements. Survey spectra with a binding energy ranging from 0 to 1200 eV were collected at a takeoff angle (measured from the substrate) of 90° and at a pass energy of 160 eV. High-resolution acquisitions of carbon 1s, silicon 2p, and oxygen 1s regions were conducted at a pass energy of 40 eV. All high-resolution measurements were conducted with takeoff angles ranging from 90 to 10° in 10° increments, which led to sampling depths ranging from 70 to 10 Å.13 Surfaces were imaged using a Nanoscope IIIa Extended MultiMode AFM (Digital Instruments, Inc., Santa Barbara, CA) and an AS-130V (“J” vertical) scanner. PDI-AFM was performed in tapping mode using TESP-10 Nanoprobe SPM tips, 125 µm in length, with a resonant frequency range of 292-322 kHz. All images were reproducible, acquired at room temperature, and rastered at 512 × 512 pixels.

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Figure 2. DCA results for variously treated polypropylene substrates coated with 1 wt % polymer solutions.

Results and Discussion The extent of surface enrichment of the PDMS block due to its low surface free energy is easily investigated by DCA analysis. If blooming of the PDMS block completely covers the surface, the contact angles of SMA and Tegomer surfaces should be the same as that of a PDMS homopolymer surface. Alternatively, the copolymer values will be intermediate between those characteristic of PDMS and PCL homopolymer surfaces if blooming does not fill the surface. DCA analysis performed on annealed samples detects changes in near surface composition. Figure 2 shows the contact angles for surfaces made by solution casting 1 wt % homopolymer and copolymer solutions; results for both as cast and annealed surfaces are shown. In addition, measured contact angles for polypropylene control plaques are presented. As expected, PDMS is more hydrophobic than PCL. The data reveal that contact angles for copolymer solution-coated substrates lie between those coated with homopolymer solutions. It is important to remark, however, that 1 wt % solution coatings do not always fully cover the polypropylene substrate, as shown in the PDI-AFM images and by the AR-XPS data presented below. Interpretation of the DCA data is further complicated since polypropylene control plaques have contact angles that also fall intermediate between those coated with homopolymer solutions; contact angle values for 1 wt % copolymer solution-coated surfaces may be intermediate between the homopolymer values simply because polypropylene is exposed at the surface. Annealing of the 1 wt % solution-coated surfaces results in contact angles within the standard deviations of the respective untreated samples. This indicates that all surfaces are stable and remain unaltered throughout the annealing process. It should be noted that the large standard deviations observed for untreated substrates coated with PCL solutions are likely due to surface roughness. Nonhomogeneous coatings of this semicrystalline polymer produce a large amount of visible surface roughness, a factor that strongly

reduces the reproducibility of DCA data. Annealing may serve to decrease this variability. A more convincing understanding of surface morphology is obtained from the 10 wt % polymer solution-coated surfaces. AFM shows that full coverage is achieved at this concentration. Figure 3 shows results for untreated and annealed 10 wt % homopolymer and copolymer solutioncoated surfaces. Values for the polypropylene control plaques are also shown. Data for the annealed 10 wt % PCL solutioncoated surfaces are not available, because upon heating these coatings dewet and bead up, making DCA analysis impossible. These results imply that it is PDMS, and not PCL, that wets the polypropylene substrate. Figure 3 shows that contact angles for the Tegomer surfaces are lower than those for the SMA surfaces indicating a greater amount of PDMS at the SMA solution-coated surface. This result agrees with stoichiometric calculations of mass percentages of PDMS in the copolymer based on AR-XPS that give 43 and 38 wt % PDMS for SMA and Tegomer, respectively. XPS demonstrates that PDMS is enhanced at the air-copolymer interface and this enhancement is apparent in the simple DCA experiments. A crucial finding from the DCA data is that both blocks appear to be present at the surface in both the 1 and 10 wt % solution coatings. This is consistent with the existence of a microdomain structure at the copolymer interface. Certainly conditions exist that permit, for example, the PDMS block to cover the entire surface while still producing a microdomained structure in the bulk. However, crystallization of one of the blocks in block copolymers is known to lead to the rapid formation of robust microstructure (one in the socalled strong segregation regime).14 The block copolymer samples did not change upon annealing significantly above the Tg’s of both blocks thus indicating considerable stability as a result of the PCL crystallinity. AR-XPS is an effective method to measure depth profiles of atomic composition. This type of analysis is germane for block copolymer systems since they can produce a dissimilar



Figure 3. DCA results for variously treated polypropylene substrates coated with 10 wt % polymer solutions.

elemental makeup as a function of depth from the interface. For example, Jalbert et al. utilized AR-XPS to study alkylamine terminal groups attached to a PDMS backbone.13 As a result of the lower surface energy of the PDMS backbone, the experimental atomic percentages of nitrogen from the higher surface energy alkylamine groups showed a depletion in the surface region compared with stoichiometric predictions if no enhancement occurred. Gorelova et al. employed AR-XPS to demonstrate phase segregation of PDMS to the air-surface interface from blends of poly(methyl methacrylate) (PMMA)/PDMS graft copolymers in poly(vinyl chloride).15 Similarly, Chen et al. employed the technique to study surface composition and migration phenomenon of different molecular weights, concentrations, and architectures of AB block copolymers blended into A-type homopolymers.16 These previous studies conclusively demonstrate that AR-XPS provides surface composition data as a function of depth, from the top few angstroms of a polymer surface to approximately 100 Å below. XPS is a particularly beneficial technique for the present SMA and Tegomer copolymers because it permits quantification of the surface composition. Figure 4 shows a representative wide energy range survey scan for a triblock copolymer surface. As expected, only carbon, oxygen, and silicon are detected. Figure 5 compares the mass % of PDMS for 1 and 10 wt % SMA solution-coated surfaces between depths of 10 and 70 Å. Mass % PDMS calculations are based upon silicon mass % detected by the XPS measurements and the stoichiometry of the polymer structure. Because silicon is present only in the PDMS block, concentrations can be calculated directly from measured silicon concentrations and a knowledge of the structure and molecular weight of the PDMS block in the triblock copolymer. For substrates coated with a 10 wt % SMA solution, PDMS concentrations at depths of 70 Å essentially agree with stoichiometric calculations (an experimental value of 40 wt % compared to 43 wt % calculated), indicating that bulk phase composition is nearly achieved at such depths. In addition, average mass concentrations of PDMS only change significantly with depth

Figure 4. Representative wide energy XPS scan for polypropylene substrates coated with triblock copolymer solutions.

starting at around 44 Å, further indicating that compositions below this depth correspond to the bulk phase. Enhancement of PDMS is evident as concentrations rise to 55 wt % at the near-surface. These results agree with DCA analysis, showing the surfaces are not completely covered by the lower surface energy PDMS block. Interpretation of the 1 wt % SMA solution-coated surfaces is not straightforward since detection of carbon from the exposed polypropylene substrates influences the measurements. Achievement of stoichiometric values of mass % PDMS at depths less than approximately 10 Å from the surface for the 1 wt % SMA solution-coated surfaces may be compared with the 10 wt % samples that achieve stoichiometric PDMS concentrations at a depth of 70 Å. The 1 wt % SMA solution coatings only partially cover the polypropylene substrate (see AFM results below), so when only the very near surface of the sample is probed, no polypropylene substrate is detected.

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Figure 5. AR-XPS results for 1 and 10 wt % SMA solution-coated substrates.

Figure 6. AR-XPS results for 1 and 10 wt % Tegomer solution-coated surfaces.

Under these conditions, only SMA that rests above the substrate is examined. This explains why at grazing takeoff angles the amount of silicon detected corresponds to the stoichiometric mass concentration of PDMS. As larger and larger depths are probed, more of the polypropylene substrate is examined, thus detecting additional carbon. Enhancement of PDMS at the interface is also observed in polypropylene substrates coated with Tegomer solutions; this effect is shown in Figure 6. The 10 wt % Tegomer solution-coated surfaces show that PDMS mass concentrations are somewhat higher (44 wt %) at the deepest depths probed than the calculated stoichiometric values (38 wt %). However, it should be remembered that the XPS technique does sample all of the material within the sampling depth. Accordingly, the PDMS enriched surface does influence the reported values at a “depth” of 70 Å. This effect is further indicated in Figure 6 where 10 wt % Tegomer solutioncoated surfaces are not shown to reach a plateau value of mass % PDMS at any depth. Enhancement of PDMS appears to occur from more than 70 Å below the surface, reaching mass PDMS concentrations of 65 wt % at the copolymer-

air interface. The greater surface enrichment found with this copolymer produces a thicker surface region of nonhomogeneous composition. The 1 wt % Tegomer solution-coated surfaces show similar enhancement phenomena. Certainly observation of PDMS enhancement by AR-XPS provides strong evidence of a block copolymer system in which the lower surface free energy blocks are preferentially driven to the interface. However, the XPS data do not support the hypothesis that only PDMS is present on the surface (the surface composition never reaches 100% PDMS even at the shallowest depths). A related XPS study of PDMS containing triblock copolymers also finds that the near surface is not saturated with PDMS, even upon annealing.17 For the present investigation, it is easy to rationalize the lack of a PDMS overlayer based on the crystalline block anchoring the morphology and preventing complete blooming. Atomic force microscopy (AFM) is a powerful and versatile scanning probe technique that revolutionized surface characterization for an extremely wide variety of materials. Application of AFM techniques has been shown to be especially appropriate for studying various polymer sys-



Figure 7. Series of low magnification height (left) and phase (right) PDI-AFM images of a 10 wt % SMA solution-coated surface: (a) 50.0 µm; (b) 13.0 µm; (c) 5.00 µm.

tems.18 Regarding copolymers, Magonov and Heaton employed PDI-AFM to verify TEM and XPS studies of poly(styrene)-b-poly(butadiene)-b-poly(styrene) (PS-b-PBDb-PS) triblock copolymers that showed an enhancement of lower surface energy PBD blocks at the surface.19 AFM not only was able to verify the previous studies, but also was able to generate more detailed information about this copolymer system, revealing a wormlike pattern of stiff PS within an amorphous sea of PBD. Many others have taken

advantage of such a powerful mode of analysis to reveal the wide variety of surface morphologies and properties demonstrated by block copolymer systems.16,20-22 The existence of phase-separated triblock copolymer surfaces is directly revealed with PDI-AFM. Height and phase images of a 10 wt % SMA solution-coated surface are shown for one sample at low magnification in Figure 7 and another sample at high magnification in Figure 8; the morphologies shown are typical of 10 wt % SMA solution-

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Figure 8. Series of high magnification height (left) and phase (right) PDI-AFM images of a 10 wt % SMA solution-coated surface: (a) 1.88 µm; (b) 1.00 µm.

coated surfaces. Bright areas in the height images presumably indicate elevated regions and dark regions represent depressed material. However, as discussed in ref 19, height images of block copolymer morphology in tapping mode are not reliable because the relative contrast of the blocks depends sensitively on the driving frequency in height images. For this reason, no scale is provided. Such sensitivity does not exist in phase images, and the contrast between rubbery PDMS-rich and crystalline PCL-rich areas is excellent. However assignment of each phase to a particular contrast cannot be definitively stated. The low magnification scans reveal semicrystalline spherulites on the order of 5-10 µm. Such spherulitic morphology is consistent with the studies by Lovinger et al. of PCL-b-PDMS-b-PCL triblock copolymers with similar molecular weights and linker groups.8 High magnification shows a lamellar morphology is present in which fibrous, PCL-rich semicrystalline spherulites are seen to be completely space filling with amorphous PDMS located in the interlamellar regions. Exact evaluation of interlayer thicknesses are made difficult by the lack of distinct, sharp boundaries that separate such layers; however, thicknesses measured using a ruler overlaid on the 1.00 µm phase image are estimated to be between 5 and 10 nm for bright regions and between 5 and 15 nm for dark regions. Assigning a value to the fully extended PCL chain length of 15 nm and to the fully extended PDMS of 13 nm leads to

Figure 9. Schematic models of possible molecular arrangements in the lamellar crystals of SMA and Tegomer copolymers (after Lovinger): (a) extended-type PCL blocks and (b) once-folded PCL blocks.

the conclusion that PCL chains are either folded once within the semicrystalline core or not folded at all, as shown in Figure 9. A once-folded conformation corresponds to a semicrystalline thickness of approximately 5 nm, whereas extended PCL chains are plausible for a 10 nm lamellar thickness. This is again in agreement with Lovinger et al. who found, by estimating crystallinity from X-ray diffractometry and assuming the density of amorphous phase to be 0.85 times that of the crystalline phase, a semicrystalline thickness of 6 nm corresponding to a once-folded PCL



Figure 10. Series of low magnification height (left) and phase (right) PDI-AFM images of a 10 wt % Tegomer solution-coated surface: (a) 50.0 µm; (b) 15.0 µm; (c) 7.73 µm.

morphology. It is again worth emphasizing that the rapid crystallization of the PCL block must lead to a pinning of the PDMS thus preventing the formation of an overlayer. A series of height and phase PDI-AFM images of a 10 wt % Tegomer solution-coated surface, shown in Figures 10 (low magnification) and 11 (high magnification), demonstrate spherulitic structures similar to those revealed in 10 wt % SMA solution-coated surfaces. However Tegomer spherulites range from approximately 5 to 25 µm. No other significant differences are observed in images of 10 wt % Tegomer solution-coated surfaces compared to those of

SMA. Measurement by hand of lamellar thicknesses result in approximately 10 nm for bright regions and range from 5 to 20 nm for the dark regions. The only significant compositional difference between the copolymers is that the middle block has a molecular weight of 2700 in Tegomer as opposed to 3200 in SMA. Lamellar thickness measurements of 10 wt % Tegomer solution-coated surfaces do not elucidate the conformation of the PCL chains since it is not known if the crystalline regions are bright or dark in the phase images. If the dark regions represented PCL-rich material, they would allow for both extended and

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Figure 11. Series of high magnification height (left) and phase (right) PDI-AFM images of a 10 wt % Tegomer solution-coated surface: (a) 3.78 µm; (b) 1.42 µm. The lamellar spacings of the domains are plainly visible in the high magnification PDI-AFM scans.

once-folded arrangements since they measure 5-20 nm wide. If, however, the bright regions corresponded to PCL-rich material, the measured thickness of 10 nm would indicate extended PCL chains with chain ends incorporated into the amorphous PDMS-rich region. Image analysis tools may be used to find the lamellar spacing from the AFM results. Power spectrum density analysis calculates an average spacing, or a length defined as one-half the length of the repeating unit (a repeating unit in the present case corresponds to the lamellar spacing). If one considers the present system as a clumpy mixture in which the repeating structures are statistically similar in shape, yet have no regular arrangement, then the linear scale of segregation, SL, can be calculated as the average spacing.23 The 10 wt % SMA solution-coated surfaces are not analyzed because the high magnification images do not have sharp enough boundaries between bright and dark regions. However, a 10 wt % Tegomer solution-coated surface is easily analyzed by this method, and the average spacing is calculated to be approximately 8 nm corresponding to a lamellar spacing of 16 nm. Utilizing a Fourier transform method, the average lamellar spacing (distance from one bright domain to the next) is calculated to be approximately 18 nm. Lovinger et al. determined average lamellar spacings of 15 and 18 nm by analysis of TEM micrographs and by

SAXS, respectively.8 Clearly the present results are in excellent agreement with these previous findings for bulk spacings. AFM demonstrates that the 1 wt % SMA and Tegomer solution-coated surfaces exhibit a variety of morphologies as a result of different surface coverages. A small fraction of 1 wt % copolymer solution-coated surfaces analyzed have morphologies identical to those coated with 10 wt % copolymer solutions, demonstrating the same characteristic space-filling crystalline spherulites at low magnification and fibrous lamellar phase separation at higher magnification. However, most 1 wt % copolymer solution-coated surfaces analyzed exhibit some exposed polypropylene regions. Figure 12 is a series of height and phase images of a 1 wt % SMA solution-coated surface in which semicrystalline spherulitic structures are evident in low magnification scans, yet randomly located copolymer-deficient areas are observed at higher magnification. These areas, indicated in the height images as dark spots, are consistent with low copolymer solution concentrations forming films that are too thin. In light of the AR-XPS data and the additional AFM results not presented, it is evident that full surface coverage is not guaranteed using a 1 wt % copolymer solution dip coating. Similar observations were made for the Tegomer triblock.



Figure 12. Series of height (left) and phase (right) PDI-AFM images of a 1 wt % SMA solution-coated surface: (a) 10.0 µm; (b) 4.11 µm; (c) 1.27 µm. Incomplete coverage is evident in the form of small elongated voids.

These findings may have important consequences for industrial coating processes utilizing such triblock copolymers. Knowing the intimate details of the surface morphology is a first step in understanding the mechanistic functioning of such surfaces in improving biocompatibility. Figure 13 shows an AFM result for a block copolymer surface that has been exposed to a 0.1 mg/mL human fibrinogen surface for 30 s. Again height and phase mode images are presented and it is seen that for this short exposure time, incomplete surface coverage is obtained. Particularly in the phase mode

image, it can be seen that the underlying lamellar morphology appears to template the adsorption of the fibrinogen. That is, the fibrinogen molecules or clusters lie with their long axis along the lamellae (see the upper right quadrant of the phase image). This finding supports the idea that protein adsorption can be mediated, in a conformational sense, through the interaction with block copolymers. However, this image represents scant experimental evidence for such an effect. Dewetting effects during sample drying could influence fibrinogen deposition. An in situ AFM experiment at

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Figure 13. Height (left) and phase (right) PDI-AFM images of a 1 µm2 area of a 10 wt % SMA solution-coated surface exposed to a 0.1 mg/mL human fibrinogen solution for 30 s. The fibrinogen adsorbs along the lamellae, showing a templating action of the surface on adsorption.

Figure 14. Comparison of the morphology of adsorbed fibrinogen layers over a 100 µm2 area. The sample on the left is untreated polypropylene, and the one to the right is polypropylene coated with block copolymer; both surfaces are covered with a layer of fibrinogen protein but exhibit different textures (see text).

the copolymer liquid interface is needed but facilities for such an experiment were not available to the authors. Further evidence of the effects of the block copolymer surface on protein adsorption is presented in Figure 14. Here, phase mode images of polypropylene surfaces exposed to a 0.1 mg/mL fibrinogen solution for 24 h at 37 °C are shown. The surface to the right is coated with a 10 wt % SMA solution. The morphologies of the two fully established fibrinogen layers are distinct; the copolymer treated surface exhibits a finer, grained texture. The untreated surface possesses coarser features (a morphology reminiscent of cauliflower blooms). These findings more directly support the premise that some type of templating action may exist. An alternative explanation is that the exposure of both copolymer blocks at the surface simply produces an intermediate contact angle. Given the combined findings of the

present study and those of other groups,7,17 this interpretation appears to be a rather simplistic view of the action of these block copolymer compatibilizers. Certainly, changes brought about by micropatterning lead to surfaces possessing fundamentally different blood contacting properties. Conclusions This study investigates the surface morphology of triblock copolymer solution coatings on polypropylene substrates and its affect on the adsorption of fibrinogen. DCA analysis shows that copolymer solution-coated surfaces have contact angles that lie within the range of the respective homopolymers and provides perhaps the most compelling indication of the presence of both PDMS and PCL at the surfaces. ARXPS studies show enhancement of the lower surface free


energy block, PDMS, at the interface. However, AR-XPS results also indicate the presence of both blocks at the surface. PDI-AFM images are also revealing, showing completely space-filling semicrystalline spherulites for all 10 wt % solution-coated surfaces. On the nanometer scale, such spherulites show alternating bright and dark lamellae corresponding to semicrystalline PCL-rich domains and amorphous PDMS-rich regions. The 1 wt % copolymer solutioncoated surfaces vary in morphology but commonly show semicrystalline weblike fibers. Utilization of the present powerful suite of analytical tools provides a detailed picture of the intimate nature of the surface morphology formed from solvent casting such films. Observation of the effects of the surface morphology on protein adsorption supports a novel approach to understanding the mechanism of improved blood compatibility on block copolymer surfaces. Namely, that microdomain patterned synthetic surfaces profoundly alter the nature of the adsorbed protein layer. This study reports some evidence for such effects by showing modified adsorbed protein morphology brought about by the micropatterned copolymer coating. Future design of biomedical materials should include a direct recognition of the surface as a two-dimensional object in which lateral compositional variations are of significant importance. Acknowledgment. This work was supported by the U.S. National Science Foundation, Division of Biological and Environmental Sciences, through Grant BES 9709959. The authors are grateful to the reviewers for a number of suggestions leading to the improvement of the presentation of the findings and for pointing out an error in the original treatment of the XPS data. References and Notes (1) Okano, T.; Nishiyama, S.; Shinohara, I.; Akaike, T.; Sakurai, Y. Polym. J. 1978, 10, 223-228.

Biomacromolecules, Vol. 2, No. 2, 2001 537 (2) Singer, S. J.; Nicolson, G. L. Science 1972, 175, 720-731. (3) Deppisch, R.; Gohl, H.; Smeby, L. Nephrol. Dial. Transplant. 1998, 13, 1354-1359. (4) Bates, F. S.; Fredrickson, G. H. Phys. Today 1999, 52, 32-38. (5) Okano, T.; Aoyagi, T.; Kataoka, K.; Abe, K.; Sakurai, Y.; Shimada, M.; Shinohara, I. J. Biomed. Mater. Res. 1986, 20, 919-927. (6) Yui, N.; Sanui, K.; Ogata, N.; Kataoka, K.; Okano, T.; Sakurai, Y. J. Biomed. Mater. Res. 1986, 20, 929-943. (7) Kawamoto, N.; Mori, H.; Terano, M.; Yui, N. J. Biomater. Sci. Polym. Ed. 1997, 8, 859-877. (8) Lovinger, A.; Han, B. J.; Padden, F. J., Jr.; Mirau, P. A. J. Polym. Sci., B: Polym. Phys. 1993, 31, 115-123. (9) Tsai, C.; Deppisch, R. M.; Lloyd, J. F.; Ritzau, G. H.; Orem, A. D.; Gohl, H. J.; Voorhees, M. E. ASAIO J. 1994, 40, 619-624. (10) Spijker, H. T.; Haan, J.; Graaff, R.; Boonstra, P. W.; van Oeveren, W. Cells Mater. 1997, 7. (11) Gu, Y. J.; Boonstra, P. W.; Rignsburger, A. A.; Haan, J.; van Oeveren, W. Thorac. Surg. 1998, 65. (12) Rubens, F. D.; Labow, R. S.; Lavalle´e, G. R.; Watson, M. I.; Robblee, J. A.; Voorhees, M. E.; Nathan, H. J. Ann. Thorac. Surg. 1999, 67, 689-698. (13) Jalbert, C. J.; Koberstein, J. T.; Balaji, R.; Bhatia, Q.; Salvati, L., Jr.; Yilgor, I. Macromolecules 1994, 27, 2409-2413. (14) Rangarajan, P.; Register, R. A.; Adamson, D. H.; Fetters, L. J.; Bras, W.; Naylor, S.; Ryan, A. J. Macromolecules 1995, 28, 1422-1428. (15) Gorelova, M. M.; Pertsin, A. J.; Muzafarov, A. M.; Gritsenko, O. T.; Vasilenko, N. G. J. Appl. Polym. Sci. 1995, 55, 1131-1135. (16) Chen, J. T.; Thomas, E. L. J. Mater. Sci. 1996, 31, 2531-2538. (17) Wen, J.; Somorjai, G.; Lim, F.; Ward, R. Macromolecules 1997, 30, 7206-7213. (18) Magonov, S.; Heaton, M. G. Am. Lab. 1998, 9-16. (19) Magonov, S. N.; Cleveland, J.; Elings, V.; Denley, D.; Whangbo, M. H. Surf. Sci. 1997, 389, 201-211. (20) Leclere, P.; Lazzaroni, R.; Bredas, J. L.; Yu, J. M.; Dubois, P.; Jerome, R. Langmuir 1996, 12, 4317-4320. (21) Motomatsu, M.; Mizutani, W.; Tokumoto, H. Polymer 1997, 38, 1779-1785. (22) Bar, G.; Thomann, Y.; Whangbo, M. H. Langmuir 1998, 14, 12191226. (23) Tucker, C. L. In Principles of Mixing Measurements; Rauwendaal, C., Ed.; Marcel Dekker: New York, 1991.

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Surface Morphology of Poly(caprolactone) - American Chemical Society

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