Investigation of the Hydration Kinetics of Novel Poly (ethylene oxide

a Topometrix Explorer (Topometrix Corporation, Saffron Walden, Essex, U.K.). ..... Haschke, E.; Sendijarevic, V.; Wong, S.; Frisch, K. C.; Hill, G...
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Langmuir 2000, 16, 2744-2750

Investigation of the Hydration Kinetics of Novel Poly(ethylene oxide) Containing Polyurethanes R. J. Green,† S. Corneillie,‡ J. Davies,§ M. C. Davies,*,† C. J. Roberts,† E. Schacht,‡ S. J. B. Tendler,† and P. M. Williams† Laboratory of Biophysics and Surface Analysis, Department of Pharmaceutical Sciences, The University of Nottingham, University Park, Nottingham, NG7 2DX, United Kingdom, Polymer Materials Research Group, Institute of Biomedical Technologies, University of Gent, Krijgslaan 281, S-4 bis, B-9000 Gent, Belgium, and Johnson & Johnson Clinical Diagnostics Ltd., Pollards Wood, Chalfont St. Giles, Buckinghamshire, United Kingdom Received November 30, 1998. In Final Form: December 1, 1999 We report on the study of the hydration of a series of poly(ether urethanes) of varying poly(ethylene oxide) content within their soft segments using surface plasmon resonance (SPR). The study of the extent and dynamics of polymer film swelling upon hydration by SPR reveals that the technique is sensitive to the influence of polymer composition (soft segment and chain extender) of the polyurethane which are known to control polymer hydration. SPR analysis has been able to monitor the kinetics of these structural changes in situ, showing rapid initial hydration which appears to slow in an exponential fashion with time. The observed changes in surface elasticity of the polyurethane films on hydration were also confirmed by force-distance atomic force microscopy data. These data revealed a change in surface compliance which mirrored the measured modification in the surface refractive index as measured by SPR. These findings are discussed in the context of the polyurethane surface structure and the potential future use for screening hydration of such polymers is highlighted.

Introduction Polyurethanes are commonly used as biomaterials because of their superior physical and mechanical properties1 with applications such as contact lens materials,2 catheters, and drug delivery systems.3,4 Therefore, novel polyurethanes are continually being developed to increase their biocompatibility, often by incorporating hydrophilic materials into the polymer, such as poly(ethylene oxide)s (PEOs),5 or producing polymers that bind heparin or albumin.6,7 The properties of polyurethane block copolymers may be modified by changing the composition of the alternating hard and soft segments of the polymer. The soft segment determines the elasticity and hydrophilicity of the polymer and thus affects the polymer’s biocompatibility. In contrast, the hard segment and polymer chain extender determine the hardness of the polymer.1 Thermodynamic incompatibility between the hard and soft segments results in phase separation of the blocks within the bulk and, of relevance to this work, at the surface. As a result, surface properties differ from bulk polymer properties because of the ability of the polymer to reorientate at the interface, depending on its environment, to minimize interfacial free energy. For example, in an * To whom correspondence should be addressed. † The University of Nottingham. ‡ University of Gent. § Johnson & Johnson Clinical Diagnostics Ltd.. (1) Pinchuk, L. J. Biomater. Sci., Polym. Ed. 1994, 6, 225. (2) Haschke, E.; Sendijarevic, V.; Wong, S.; Frisch, K. C.; Hill, G. Clear Nonionic Polyurethane Hydrogels for Biomedical Applications. J. Elastomers Plast. 1994, 26, 41-57. (3) McNeill, M. E.; Graham, N. B. J. Biomater. Sci., Polym. Ed. 1993, 4, 305. (4) Bae, Y. H.; Kim, S. W. Adv. Drug Delivery Rev. 1993, 11, 109. (5) Lin, H. B.; Lewis K. B.; Leach-Scampavia, D.; Ratner, B. D.; Cooper, S. L. J. Biomater. Sci. Polym. Ed. 1993, 4, 183. (6) Marconi, W.; Galloppa, A.; Martinelli, A.; Piozzi, A. Biomaterials 1996, 17, 440. (7) Magnini, A.; Busi, E.; Barbucci, R. J. Mater. Sci. Mater. Med. 1994, 5, 839.

aqueous environment the polymer will orientate such that the more hydrophilic soft segment dominates the surface.6 Polyurethanes based on PEOs are promising hydrogels for biomedical applications, such as contact lenses.2 These polymers absorb large amounts of water, depending on the proportion of PEO within the polymer and on the temperature.8,9 Many techniques have been used to monitor the hydration of such polymers. These include gravitational weighing techniques2,10 and differential scanning calorimetry (DSC)9,11 More recently, the potential of SPR12,13 to probe such dynamic processes at polymer interfaces has been realized. SPR has been used to investigate the adsorption of acrylic diblock copolymers,14 investigating the effect of block size, molecular weight, and the quality of the solvent system. SPR has also been employed to investigate the incorporation of organic vapors into polymer films within a gaseous environment.15 Within this laboratory SPR has been used to probe the degradation of polymer blends in situ, allowing the determination of the rate and extent of degradation.16 The group has further utilized SPR to investigate the adsorption of amphilic triblock copolymers to hydrophobic surfaces.17 Similar (8) Zulfiqar, M.; Quddos, A.; Zulfiqar, S. J. Appl. Polym. Sci. 1993, 49, 2055. (9) Schneider, N. S.; Illinger, J. L.; Karasz, F. E. J. Appl. Polym. Sci. 1993, 47, 1419. (10) Nathan, A.; Bolikal, D.; Vyaratiare, N.; Zalipski, S.; Kahn, J. Macromolecules 1992, 25, 4476. (11) Schneider, N. S.; Illinger, J. L.; Karasz, F. E. J. Appl. Polym. Sci. 1993, 48, 1723. (12) Davies, J. Nanobiology 1994, 3, 5. (13) Liedberg, B.; Lundstrom, I.; Stenberg, E. Sens. Actuators B 1993, 11, 63. (14) Rudolp, J.; Patzsch, J.; Mayer, W. H.; Wegner, G. Acta Polym. 1993, 44, 230. (15) Drake, P. A.; Bohn, P. W. Anal. Chem. 1995, 67, 1766. (16) Shakesheff, K. M.; Chen, X.; Davies, M. C.; Domb, A.; Roberts, C. J.; Tendler, S. J. B.; Williams, P. M. Langmuir 1995, 11, 3921. (17) Green, R. J.; Davies, J.; Davies, M. C.; Roberts, C. J.; Tendler, S. J. B. Biomaterials 1997, 18, 405.

10.1021/la981656x CCC: $19.00 © 2000 American Chemical Society Published on Web 02/04/2000

Hydration Kinetics of PEO Containing Polyurethanes

Langmuir, Vol. 16, No. 6, 2000 2745 Table 1. Composition of the Poly(ether urethanes) Investigateda.

Figure 1. Synthesis of segmented polyurethanes.

studies have been carried out using other optically based techniques. For example, Gauglitz et al.18 have used spectral elipsometry to investigate the interaction of polymer films with organic vapors. Knoll et al.19 used surface plasmon microscopy and other more novel evanescent wave techniques to investigate interactions with polymer films, including photoablation studies of polysilane films. In this paper the hydration kinetics of a series of novel segmented poly(ether urethanes) with varying PEO content is investigated by SPR. The polymer series has been selected to allow us to highlight the ability of SPR to probe dynamic changes at polymer/solution interfaces and determine the effect each segment of the polyurethane structure has on the rate and extent of hydration. Prior to SPR hydration experimentation, the surface chemistry and topography of the polymer films were characterized by X-ray photoelectron spectrocopy (XPS) and atomic force microscopy (AFM). Finally, to complement the SPR data, the hydration of a polyurethane film was probed using force-distance measurements by AFM, investigating the change in elasticity of the polymer film during hydration. Materials and Methods Materials. The polyurethane series was synthesized within the Biomaterial and Polymer Research Group at the University of Gent. The polyurethanes used in this study are synthesized by a two-step reaction process as shown in Figure 1. Initially a rigid isocyanate (DIC) is reacted with a macroglycol (soft segment, SS), a polyester or polyether, to produce a low molecular weight prepolymer of the structure DIC-SS-DIC. This prepolymer is extended by the addition of a chain extender (CE), a small hydroxyl- or amine-terminated molecule, to form a high molecular weight polyurethane structure with repeat units of -DIC-SSDIC-CE-.1 The soft segments of the poly(ether urethanes) used are made up of mixtures of two macroglycols, PEO and poly(propylene oxide) (PPO) or poly(tetramethylene oxide) (PTMO). The hard segment of the polymer contains hydrogenated methylene diisocyanate, also known as 4,4-methylene bis(cyclohexyl isocyanate) or Desmodur W. (Desm. W.), and a chain extender of either ethylenediamine (ED) or butanediol (BD). A catalyst of dibutyltin(IV) diacetate was used in their production. A control polymer is used, which contains no PEO, with a PTMO soft segment and a hexamethylene diisocyanate (HMDI) and BD hard segment. The compositions of all these polymers are detailed in Table 1. (18) Spaeth, K.; Kraus, G.; Gauglitz, G. Fresenius’ J. Anal. Chem. 1997, 357, 2292. (19) Knoll, W.; Hickel, W.; Sawodny, M.; Stumpe, J. Makromol. Chem.Macromol. Symp. 1991, 48-9, 363.

polyurethanes

soft segment (SS)

S-11 (control) L75 (2/1) L75 (1/2) L79 (1/2) L70 (1/1) L71 (1/1) L78 (2/1) L77 (1/2)

PTMO PEO/PTMO (2/1) PEO/PTMO (1/2) PEO/PTMO (1/2) PEO/PPO (1/1) PEO/PPO (1/1) PEO/PPO (2/1) PEO/PPO (1/2)

chain extender (CE) BD ED ED BD ED BD BD BD

diisocyanate (DIC)

Mn

Mw

HMDI Desm. W. Desm. W. Desm. W. Desm. W. Desm. W. Desm. W. Desm. W.

79 600 50 000 57 000 53 200 35 000 47 500 51 000 40 600

99 100 80 500 80 300 84 700 84 000 70 000 93 000 72 000

a DIC: Desm. W., OdCdN-(C H )-(CH )-(C H )-NdCdO; 6 10 2 6 10 HMDI, OdCdN-(CH2)6-NdCdO. CE: BD, HO-(CH2)4-OH; ED, NH2-(CH2)2-NH2. SS: PEO(2000 MW), HO-[-CH2-CH2-O-]nH; PPO(2000 MW) HO-[-CH2-CH(CH3)O-]m-H; PTMO (2000 MW), HO-[-O-(CH2)4-]x-H (2000 MW is the molecular weight before polymerization).

Sample Preparation. The polyurethanes were dissolved in chloroform (Fisons Scientific Equipment, Loughborough, U.K.) (0.5% (w/v)) and spin-coated onto silver-coated glass slides (Johnson & Johnson Clinical Diagnostics, Chalfont St. Giles, U.K.) at 1000 rpm immediately prior to analysis. After coating, all polymer-coated slides exhibited a reproducible and sizable shift to the value of the initial SPR angle when compared to an uncoated silver surface. SPR angle shifts observed upon polymer coating were considered in detail in earlier work and compared to ellipsometry data.20 That work suggested that for polymer films to completely cover the underlying rough silver surface, a 20-30-nm film was required. Thus, the substantial SPR shift observed for the polyurethane coatings studied in this paper could be assumed to correspond to, at least, film thicknesses in the range of 20-30 nm. SPR Analysis. The SPR instrumentation (Johnson & Johnson Clinical Diagnostics, Chalfont St. Giles, U.K.) was described in detail elsewhere.12,20 Briefly, SPR uses a monochromatic light source (780 nm) which is focused, through a glass prism, onto the underside of a silver-coated glass slide (50-nm-thick silver coating). The laser light which is totally internally reflected at the silver film is detected by a two-dimensional array of chargecoupled detectors (CCD). The SPR angle is observed as a minimum in the intensity of reflected light whose position is dependent upon the refractive index of material above the silver layer. The data are processed by a IBM ps/2 computer. Two types of liquid cell were used during SPR analysis. For the 10-min hydration experiments, the SPR instrument’s temperaturecontrolled liquid cell was used, where three channels are formed across the polymer surface by contacting a thermo-regulated block (34 °C) against the SPR silver-coated slide creating a watertight seal. For the longer term hydration experiments (>10 min) and the repeated hydration experiment, a single large circular open sample cell was constructed and the hydration was performed at room temperature. The use of this simple open cell enabled easy emptying of the cell contents and thus rapid removal of solution above the polymer films when needed. AFM Analysis. AFM imaging was performed in contact mode using a Rasterscope 3000 AFM (Danish Microengineering, Herlev DK2730, Denmark). Silicon nitride probes mounted onto cantilevers with spring constants ranging from 0.1 to 0.01 N m-1 were used at nominal imaging forces around 1 nN with a scan frequency of approximately 10 Hz. The sample was prepared by cutting out the center of the SPR slides and fixing it to a sample stub with double-sided sticky tape. Force-distance measurements monitoring polymer film hydration were carried out using a Topometrix Explorer (Topometrix Corporation, Saffron Walden, Essex, U.K.). The hydration of the films was observed by a change in the slope of the force-distance curves, measured as the change in deflection of the cantilever as the probe comes into contact with the sample. The AFM produces force-distance curves where the force is measured as (20) Green, R. J.; Davies, M. C.; Roberts, C. J.; Tendler, S. J. B. Biomaterials 1997, 18, 405.

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Table 2. Carbon, Oxygen, and Nitrogen Peak Areas Calculated from Narrow Scan XPS Spectra, and O/C, N/C Peak Area Ratios for the Polyurethane Series.

sensitivity factor L75 (1/2)a L75 (1/2)b L75 (2/1)a L75 (2/1)b L79 (1/2)b L77 (1/2)b L71 (1/1)b L78 (2/1)b L70 (1/1)b S-11b a

% carbon

% oxygen

% nitrogen

0.25

0.66

0.42

77.7% 77.0% 75.0% 76.2% 77.4% 75.3% 75.0% 74.6% 73.6% 79.0%

21.8% 20.4% 24.2% 21.0% 20.7% 22.6% 22.9% 25.0% 23.9% 19.0%

0.5% 2.7% 0.6% 2.7% 1.9% 2.1% 2.1% 4.2% 2.5% 1.9%

O/C peak area ratio

N/C peak area ratio

0.28 0.26 0.32 0.27 0.26 0.29 0.30 0.33 0.32 0.24

0.007 0.04 0.008 0.04 0.02 0.04 0.03 0.06 0.03 0.02

theoretical O/C ratio of soft segment

0.36 0.37 0.36 0.40 0.43 0.45 0.43 0.25

25° electron take-off angle. b 75° electron take-off angle.

the photodiode response (nA). This may be converted into force (nN) by the force conversion factor, i.e., (nN/nA) ) AFM probe spring constant (N/m)/sensor response (nA/nm). However, for the purposes of this paper the photodiode response is converted into units for cantilever deflection (nm) by dividing the photodiode signal (nA) by the sensor response (nA/nm). The sensor response is the change in photodiode signal with respect to a vertical movement of the piezo in the contact region of the force curve. XPS Analysis. Samples were mounted on a sample stub using double-sided sticky tape. XPS spectra were acquired employing MgKR X-rays (h ) 1253.6 eV) and an electron take-off angle of 25° or 75°, providing approximately 20- and 50-Å sampling depths, respectively. The X-ray gun was operated at 12 kV and 20 mA. A wide scan (0-1000 eV) was recorded for each sample followed by six narrow scans of the C 1s, O 1s, N 1s, Sn 3d, Si 2p, and Ag 3d regions. The Sn and Si regions were scanned to monitor any impurities in the films and the Ag region scanned to detect any discontinuities in the polymer films where the underlying silver substrate could be detected. The analyzer was operated in a fixed analyzer transmission (FAT) mode with a pass energy of 50 eV (wide scan) and 20 eV (narrow scans).

Results and Discussion Surface Characterization. Surface characterization of the spin-coated polyurethane films was performed by XPS and AFM. Initially, elemental analysis of the surfaces was carried out by XPS, determining the surface chemistry of the polymer film in its dry state. The polymer films were then visualized by AFM to check the films continuity and thus their suitability for SPR analysis. Each of the polyurethane films were analyzed by XPS, employing a 75° electron take-off angle, providing a wide scan dominated by strong peaks corresponding to the C 1s (292 eV) and O 1s (540 eV) regions and a small peak detecting lower surface levels of N 1s (400 eV). No silver was detected in any of the samples, suggesting that the polyurethane films were continuous. Trace amounts of Sn contamination were observed in three of the samples, L78(PEO/PPO, 2/1), L77 (1/2), and L71 (1/1) originating from the catalyst used during the synthesis of these polymers. The C 1s narrow scans were deconvoluted into three peaks, two of similar intensities corresponding to C-C (292 eV) and C-O (293.5 eV) carbon environments and a small shoulder at 295 eV corresponding to a O-CONH carbon environment from the urethane linkages. Table 2 details the peak areas observed for the carbon, oxygen, and nitrogen regions of the spectra and contains the calculations of the O/C and N/C atomic ratios. A comparison of the peak area ratios calculated from the 25° and 75° electron take-off angle spectra from polyurethanes L75 (PEO/PTMO, 2/1) and L75 (1/2) highlights

the probable surface segregation of the soft segments. The N/C ratio determines the amount of hard segment near the surface, and the results show that there is a lower N/C ratio at the surface (25°) compared to that found when probing deeper into the sample (75°). The O/C ratio will be dominated by the long polyether chains of the soft segment. The calculated O/C peak area ratio is higher from the 25° electron take-off angle spectra, particularly for L75 (2/1). These results suggest that the polymer surfaces are ordered such that the soft segment dominates the surface region. It is not possible to calculate the theoretical peak area ratios for these polymers because the molecular weights of the soft segment block copolymers are random, and thus the relative sizes of each block cannot be determined. However, the theoretical O/C ratios of the soft segment alone can be calculated and compared against the polyurethanes surface analysis data and are listed in Table 2. There is a correlation between the experimental O/C peak area ratios and the theoretical soft segment O/C ratios, suggesting that the O/C ratios for these polyurethanes are dominated by the soft segment and the amount of PEO in the polymer. S-11 contains no PEO and thus has the lowest O/C atomic ratio. However, for each polymer the experimental O/C ratio is lower than the expected theoretical soft segment O/C ratio, suggesting that while the soft segment may dominate the surface, it is not a complete overlayer. There are two possible explanations, first that the surface contains a hard/soft segment mix of which the soft segment dominates. This fits well with previous work carried out by Lin et al.,5 who also observed surface segregation by XPS for a similar polyurethane series. Second, there could also be a preferential orientation of the PPO or PTMO at the surface competing with the PEO within the soft segment, thus reducing the O/C ratio. When considering the O/C ratio for S-11, containing no PEO, the theoretical and experimental values are in close agreement. XPS analysis of these polyurethanes has determined that the soft segment dominates the polymer surface and that partial phase separation of the polymers does occur. No significant contamination was found, with only trace amounts of tin in three of the polymers and no silicon contamination. The polymer film was shown to be at least 20-nm thick and a continuous film with no detection of the silver substrate in any of the XPS spectra. AFM analysis confirmed the continuity of the polymer films by revealing smooth featureless films for each of the polymers. As an example, Figure 2 shows the surfaces of the polyurethanes L70 (1/1) and L75 (1/2), each sample exhibiting a continuous polymer surface with no hole defects through to the rough underlying silver substrate.16 Hydration of Poly(ether urethanes). The exposure of poly(ether urethanes) to an aqueous environment is likely to induce significant surface structural rearrangement during the hydration of the polymer interface. The absorption of water into the polymer surface and subsequently into the bulk will result in a reduction in the refractive index of the polymer film. Preliminary experiments with SPR revealed that it was sensitive to these changes in the polymer surface structure due to hydration. Once in contact with an aqueous solution, it is likely that the polymers will reorientate and hydrate, and the extent of swelling will be dependent upon the composition of the hard and soft segments of each polymer. An SPR experiment was devised during which the polymer films were incubated in a phosphate buffer for 10 min at 34 °C. Hydration of all the PEO-containing polymers resulted in a characteristic negative shift in the

Hydration Kinetics of PEO Containing Polyurethanes

Langmuir, Vol. 16, No. 6, 2000 2747

Figure 2. AFM images of the polyurethane films of (a) L70 (1/1) and (b) L75 (1/2).

Figure 3. Comparison of the SPR hydration profiles of three polyurethanes: (a) L75 (2/1), (b) L75 (1/2), and (c) S-11. (The units for the SPR angle shift are millidegree angles (mDA).)

SPR angle between 50 and 250 mDA (millidegree angles); Figure 3 shows the change in the SPR angle with time for polyurethanes L75 (2/1) and L75 (1/2) (PEO/PTMO soft segments) compared to the control, polyurethane S-11 (PTMO soft segment), a polymer that does not swell in water. From Figure 3 it can be seen that upon hydration of polymers L75 (1/2) and L75 (2/1) a rapid initial reduction in the SPR angle is observed which tails off with time as the polymer hydrates. Because hydration causes the polymer films to swell, an increase in the SPR angle may be expected with increasing polymer thickness. However, the SPR monitors the dielectric properties of the film. Therefore, although hydration of the film causes an increase in the film thickness, it also results in a decrease in the refractive index of the film due to the incorporation of water into the polymer. The SPR angle shift is a combination of these two effects and, thus, the net effect results in a reduction in the SPR angle upon polymer hydration. The negative shift observed in the SPR angle upon hydration of these polymers has been explained in terms of a corresponding reduction in their refractive index. One alternative explanation could lie in the loss of polymer from the surface. To confirm this, a film of L78 was hydrated, dehydrated, and rehydrated six times in cycle while being monitored by SPR. L78 was chosen because of its high PEO content, giving rise to the maximum SPR shift upon hydration. If a significant reduction in the initial measured SPR angle was observed, or any significant difference in the rate and extent of swelling of the film between each rehydration step, then the conclusion would be that material is leaving the surface. That would, thus, also explain a reduction in the SPR angle upon hydration. To minimize the observed inconsistencies in the SPR angle

Figure 4. The hydration and rehydration of a spin-cast film of polyurethane L78 (2/1). The initial hydration is represented by plot (a). Subsequent rehydrations were performed at (b) 1 h, (c) 2 h, (d) 3 h, (e) 4 h, and (f) 19 h after the initial hydration in (a).

shifts during polymer hydration, the amount of time between the addition of buffer/water and the start of SPR monitoring has been optimized during this and subsequent experiments, such that SPR monitoring begins less than 1 s after the addition of the aqueous solution to the polymer surface. The SPR setup used in this experiment was composed of a single liquid cell with no cover to allow the complete removal of water between hydration stages. Therefore, the SPR temperature-controlled liquid cell could not be used and the hydration was performed at room temperature. Approximately 8 min of hydration was monitored by SPR after which the water was removed from the surface and the surface left for at least 50 min to dehydrate, before the cycle was repeated at random intervals over a 24-h time period. Figure 4 shows six SPR profiles corresponding to repeated hydration cycles of one film. It is clearly evident from the data that the rate and extent of hydration was within error reasonably consistent each time the polymer was hydrated. Therefore, one can conclude that there was minimal loss of material upon hydration of the film and suggest that the observed reduction in the SPR angle is indeed primarily due to changes in the refractive index of the polymer film as it swells. A histogram of the monitored SPR angle shifts observed after 10 min of hydration of all the polymer films investigated can be seen in Figure 5. While the standard deviations observed for the swelling of these polymers

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Figure 6. Long-term SPR hydration profile of polyurethane L78 (2/1). Figure 5. A histogram comparing the SPR angle shifts for each of the polyurethanes studied after 10 min of hydration. (For each polymer, n ) 9 for the number of curves used in statistical analysis.)

have an error margin of around 20%, significant differences in SPR angle shifts of the polymers allow for the critical analysis of these data and the observation of some significant trends. A possible reason for these large deviations may be due to the inhomogenicity of the polymer films, where despite careful attention to a standard protocol, each spin-coated film may have resulted in polymer films of different thicknesses. From Figures 3 and 5 it can be clearly observed that the SPR analysis is able to distinguish the differences in hydration of the polymers examined which may be readily interpreted in terms of the polymer composition. Figure 5 shows data for two specific series which contain PEO with either PTMO (L75 (2/1), L75 (1/2), and L79 (1/2)) or PPO (L78 (2/1), L77 (1/2), L71 (1/1), and (L70 (1/1)) within their soft segment. In both series, it is clear that PEO has the anticipated profound effect on hydration. In the case of PEO/PPO, changing the relative ratios of the polymers from 1:2, 1:1, and 2:1, as in L77, L71, and L78, respectively, produced a gradual increase in the hydration kinetics and a negative SPR angle shift observed after 10 min of hydration. Similar observations could be made for the PEO/PTMO series as illustrated in Figure 3 for L75 (1/2) and L75 (2/1). Another notable trend is the difference in the hydration of the polymers which have identical chemical structures but differing chain extenders, i.e., L71 (1/1) and L70 (1/1) and also L79 (1/2) and L75 (1/2) where BD and ED, respectively, are the chain extenders in each group. For both these groups, the swelling of the polymers appears to increase by the use of BD. Further observations can be made when the hydration curves in Figure 4 are compared with the SPR angle shifts quoted in Figure 5, where higher negative shifts are observed in the latter experiment. The difference between these two experiments is the temperature at which the hydration was performed, suggesting that the polymer swells to a lower extent upon hydration at increased temperatures. This observation again agrees with other experiments found in the literature. Zulfiqar et al.8 showed that the water adsorption of similar polyurethanes is dependent upon polymer composition and temperature. Schneider et al.9 observed temperature dependence for the water uptake of a series of polyurethanes containing PEO/PPO/PEO triblock copolymer soft segments. They suggested that the temperature dependence was due to the phase compatibility of the PEO and PPO blocks.

Figure 7. SPR hydration profiles for the 1 h of hydration of polyurethanes L78 (2/1), L71 (1/1), and L77 (1/2).

After 10 min of hydration, the gradient of the SPR hydration profiles (not shown) suggest that some of the polymer films are completely hydrated. However, the SPR plots show that polyurethane films with the highest PEO content (L78 and L75 (2/1)) are still changing in composition at this time period. To investigate the time taken to completely hydrate these polymer films, a long-term hydration was performed as part of an SPR experiment on a film of L78 (2/1). The same conditions were employed as those for the cycled rehydration experiment described in the previous section, using an open liquid cell at ambient temperature. An SPR hydration profile of this film is shown in Figure 6. This graph reveals that for L78 the polymer surface takes ≈4 h before it is completely saturated with water under these conditions, and that only half of its total hydration has occurred after 10 min in water. It appears that the majority of the film swelling has occurred after ≈1 h of hydrationbecause the initial rate of hydration is rapid. Polymer film hydration over 1 h was monitored for three similar polyurethanes, L78 (2/1), L71 (1/1), and L77 (1/2), differing only in their PEO content. The SPR hydration profiles are shown in Figure 7, and as noted in Figures 3 and 5, the greater the presence of PEO within the polymer structure, the greater the degree of hydration of the polymer. The hydration profiles also highlight an initial rapid rate of hydration which tails off in an exponential-like fashion as the hydration proceeds. For each polymer the majority of the hydration occurs during the initial minutes of hydration. In Table 3 the SPR angle shifts for hydration of each polymer discussed in Figure 7 are compared with experimental swelling data, calculated by taking the difference

Hydration Kinetics of PEO Containing Polyurethanes

Langmuir, Vol. 16, No. 6, 2000 2749

Table 3. Comparison of SPR Angle Shifts with Calculated Hydration Values for Polyurethane Films. SPR angle shift (mDA)

% hydration

-752 ( 93 -474 ( 43 -377 ( 46

73% 58% 54%

L78 L71 L77

between the mass of the polymers dehydrated and after 48 h of hydration using the equation

% hydration )

mass at 48 h - mass at t ) 0 × 100 mass at 48 h

Table 3 highlights the agreement of the trends between the values for SPR angle shifts and the calculated percentage hydrations observed for the hydration of this polymer series. This provides evidence that SPR is able to successfully probe the hydration of these polymers, agreeing with data calculated using crude methods such as monitoring the hydration by mass, but with the added advantage of also monitoring the rates of hydration in situ. Polymer Hydration Investigated by AFM. SPR analysis of polyurethane hydration resulted in a decrease in the SPR angle which was suggested to be caused by swelling of the polymer films. AFM analysis was used as a complementary technique to provide supporting data by investigating the change in the elasticity of the polymer films as they hydrate. When the forces between the probe and the sample are monitored, as the tip comes into contact with the surface, a force-distance curve may be produced, which is able to provide information on specific forces of interaction21,22 and surface elasticity.23,24 A schematic of a typical force-distance plot showing the approach of the cantilever as it comes into contact with the surface is shown in Figure 8. The gradient of the force-distance curve once the tip is in contact with the surface depends on the hardness of the surface. If the polymer film is softer than the cantilever spring constant, deformation of the sample occurs and is manifested in the gradient of the forcedistance curve as the probe comes into contact with the surface. The gradient increases until the sample can no longer be compressed or the probe has penetrated the polymer film and reached the substrate. After this point, the force-distance slope becomes constant, corresponding to a linear increase in cantilever deflection, and its gradient equals the spring constant of the AFM probe.25 The force-distance curves corresponding to the hydration of a L71 film are shown in Figure 9. Here, forcedistance curves have been converted so that the y axis (force) is measured in terms of cantilever deflection (nm). The figure shows the force-distance curves of the polymer film in air and after 3, 15, and 25 min of hydration. The force-distance curve corresponding to the polymer sample in air revealed a sharp point of contact after a linear gradient corresponding to the deflection of the cantilever upon reaching a hard surface. As the polymer began to hydrate, the initial gradient of the force-distance curve decreased upon the cantilever coming into contact with a softer surface. The probe was able to penetrate into the (21) Lee, J. H.; Andrade, J. D. In Polymer Surface Dynamics; Andrade, J. D. Ed.; Plenum Press: New York, 1988; p 119. (22) Allen, S.; Davies, J.; Dawkes, A. C.; Davies, M. C.; Edwards, J. C.; Parker, M. C.; Roberts, C. J.; Sefton, J.; Tendler, S. J. B.; Williams, P. M. FEBS Lett. 1996, 390, 161. (23) Radmacher, M.; Fritz, M.; Hansma, P. K. Biophys. J. 1995, 69, 264. (24) Tao, N. J.; Lindsay, S. M.; Lees, S. Biophys. J. 1992, 63, 1165. (25) Lea, A. S.; Andrade, J. D.; Hlady, V. Colloids Surf. A 1994, 93, 349.

Figure 8. Schematic representation of the approach cycle of an AFM force-distance curve monitoring the elasticity of a polymer film.

Figure 9. The cantilever deflection (nm) verses probe/sample separation (nm) for a L71 (1/10) polymer film in air and after 3, 15, and 25 min of hydration. The x axis, z height is expressed relative to the point of contact between the probe and sample (at 0 nm).

hydrated polymer, thus resulting in a force-distance curve with a curved gradient and no observed sharp point of contact. The changes in gradient between 0 and 3, 3 and 15, and 15 and 25 min of hydration highlight the nonlinearity of the hydration process. This suggests that the initial rate of hydration, between 0 and 3 min, is rapid and shows that the rate of hydration decreases with time. After 25 min of hydration little to no difference was observed between subsequent force-distance curves, suggesting that the hydration process was completed. This interpretation of the AFM data agrees well with the SPR hydration curve shown in Figure 7, both revealing initial rapid hydration which slows exponentially with time. A control hydration experiment was performed where force-distance curves were taken of the hard silver substrate, resulting in curves that appeared identical in air and in water, and showed no change in gradient as the contact time in water increased. This confirmed that the observed changes in the gradient of the force-distance curves for a L71 film were due to the hydration of the film and demonstrated that as the polymer hydrated and swelled it became softer and more elastic in nature. Conclusions The hydration of a series of polyurethane films has been studied by SPR and AFM analysis. The initial chemical characterization of the films showed that the surface is dominated by the soft segment as observed by Lin et al.,5 who investigated a similar polyurethane series by XPS. The surface morphology of the films was also shown to be smooth and continuous over the underlying silver substrate by AFM and XPS. The SPR analysis was sensitive to the hydration of the series of these polyurethane films. Real time in situ kinetic

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analysis of the water sorption and swelling of the films was dependent on both the soft segment composition and the chain extender employed. A clear dependence on the influence of PEO levels upon hydration was observed which is in line with known swelling behavior of these polymers.8 In addition, polymers prepared with the BD chain extender revealed greater hydration than those with ED. As in previous studies using other methods,2,9,10 the swelling of the polymers was found to be dependent on temperature as well as polymer composition where a reduction in hydration was observed with increased temperatures.8 SPR has been able to monitor the rate of hydration, showing that the process is initially very rapid and that the majority of the water absorption occurs within the first 10 min of hydration. A range of experiments has been performed to confirm that a negative SPR response during polymer hydration is a result of the incorporation of water into the polymer film as it swells. This work has shown that SPR has the sensitivity to monitor changes in the structure of polymer films in situ, providing valuable information on the kinetics of the process. Such an approach could potentially be used for the investigation of any hydrogel system prepared as a thin film. These SPR studies have been complimented by AFM analysis to monitor polymer hydration by observing the increase in the surface elasticity of the polymer as it swells. This is one of the first examples of its kind in the literature. Radmacher et al.23 demonstrated the use of AFM to investigate the hydration of a material by looking at thin films of gelatin in a range of water/propanol mixed solutions. His work showed that the elasticity of the gelatin

Green et al.

increased (decrease in elastic modulus) with increasing water content, and therefore swelling, of the solution. This work shows an investigation of a complex novel polymer system with monitoring of the polymers’ hydration by AFM measurement of surface elasticity, revealing similar hydration kinetics to those observed by SPR. This work has further demonstrated the value of SPR to probe dynamic processes, gaining information on the extent and rate of interactions at polymer surfaces,15,20 and has also demonstrated the effectiveness of the combined approach of techniques to provide complementary data to aid in the understanding of the complex processes involved at polymer interfaces. Here, AFM has proved itself as a valuable complementary tool to SPR used to provide synergistic data for the characterization of the polymer films during the investigation of polymer hydration. Such SPR studies may be extended to examine the hydration behavior of thin films of other classes of polymers or may possibly employ the SPR technique as a rapid screening approach to examine hydration of such systems. These measurements may be linked with further analyses of polymer surface interactions by SPR such as biomolecular adsorption or polymer degradation.26 Acknowledgment. The authors would like to thank the Brite/Euram Program for funding. S.J.B.T. is a Nuffield Foundation Science Research Fellow. LA981656X (26) Chen, X.; Shakesheff, K. M.; Davies, M. C.; Heller, J.; Roberts, C. J.; Tendler, S. J. B.; Williams, P. M. J. Phys. Chem. 1995, 99, 11537.