10254
Langmuir 2003, 19, 10254-10259
Surface Interactions between Propylene Glycol and Sodium Dodecyl Sulfate during Coalescence of MMA/nBA/ AA Colloidal Dispersions: A Spectroscopic Study W. R. Dreher and M. W. Urban* School of Polymers and High Performance Materials, Shelby F. Thames Polymer Science Research Center, Department of Polymer Science, The University of Southern Mississippi, Hattiesburg, Mississippi 39401
R. S. Porzio and C. L. Zhao BASF Charlotte Technical Center, 11501 Steele Creek Road, Charlotte, North Carolina 28273 Received June 18, 2003. In Final Form: August 18, 2003 These studies focus on the behavior of methalmethacrylate/n-butyl acrylate (MMA/n-BA) and methyl methacrylate/n-butyl acrylace/acrylic acid (MMA/nBA/AA) colloidal dispersions stabilized by sodium dodecyl sulfate (SDS). In MMA/n-BA, migration of SDS is apparent, but the presence of AA suppresses SDS mobility even at elevated temperatures. However, the addition of propylene glycol (PG) promotes SDS mobility. Internal reflection infrared imaging (IRIRI) data indicate that PG and SDS occupy the same areas, suggesting that both components reach the surface at the same time through the same transport mechanisms. As PG evaporates, SDS remains on the surface and likely forms crystalline phase. Using IRIRI imaging, with a spatial resolution of 1 µm, coalescence of MMA/nBA/AA was followed as a function of time. These studies show that the presence of AA in colloidal particles significantly alters not only film formation processes, but also SDS-particle interactions, thus minimizing mobility of SDS to the film-air (F-A) interfacial regions during coalescence. This behavior is attributed to the presence of localized ionomeric clusters (LIC) at the particle interstices that exhibit higher thermal stability.
Introduction While previous studies1-4 indicated that mobility of low molecular weight species, in particular surfactant molecules, may be affected by such intrinsic polymer properties as glass transition temperature, chemical makeup of colloidal particles, or particle morphologies, external coalescence conditions may also play an essential role. For example, external stimuli such as temperature and relative humidity (RH) are well-documented parameters, but less recognizable factors are surface tension effects of substrates as well as interactions of individual components. In essence, attempts to understand competing processes, especially on molecular levels, are challenging but important because minute variations of external and internal stimuli may lead to desirable and undesirable chemical/physical changes.5,6 In view of these considerations and the results of the previous studies, the importance of interactions among individual components in complex environments plays an important role during coalescence. For example, it is documented that by introducing acid functionalities onto colloidal particle surfaces the mobility of surface-stabilizing surfactants will be altered during coalescence.7 It has also been shown that COOH‚‚‚SO3-Na+ interactions will be altered in Sty/nBA/MAA systems, as was manifested * To whom all correspondence should be addressed. (1) Zhao, Y.; Urban, M. W. Macromolecules 2000, 33, 2184. (2) Niu, B. J.; Urban, M. W. J. Appl. Polym. Sci. 1996, 60, 371. (3) Niu, B. J.; Urban, M. W. Film Formation in Waterborne Coatings; American Chemical Society: Washington, DC, 1996. (4) Zhao, Y.; Urban, M. W. Langmuir 2001, 17, 6961. (5) Dreher, W. R.; Zhang, P.; Urban, M. W. Macromolecules 2003, 36, 1228. (6) Zhao, Y.; Urban, M. W. Macromolecules 2000, 33, 8426. (7) Zhao, Y.; Urban, M. W. Langmuir 2000, 16, 9439.
by sodium dioctylsulfosuccinate (SDOSS) migration to the F-A interface. Furthermore, mobility of low molecular weight species may be redirected if significantly higher or lower surface tension substrates are utilized as substrates for film formation.8 Thus, there is a spectrum of possibilities which result from internal or external stimuli during latex coalescence. With this in mind and considering inherent complexity of colloidal dispersions, we will expand the scope of the previous studies by investigating the effect of propylene glycol (PG) cosolvent on film formation as well as its interactions with surface active species such as SDS. Specifically, we will examine the behavior of SDS when utilized in MMA/nBA/AA colloidal dispersions on the presence of PG. While one objective of these studies is to elucidate the nature of SDS-particle interactions on the presence of polar cosolvents, our ultimate goal is to advance limited knowledge concerning the effects of cosolvents on surface characteristics as well as their affect on mechanistic aspects of coalescence. Experimental Section Methyl methacrylate (MMA), n-butyl acrylate (nBA), acrylic acid (AA), potassium persulfate (KPS), and sodium dodecyl sulfate (SDS) were purchased from Aldrich Chemical Co. All colloidal dispersions were synthesized under monomer-starved conditions using a semicontinuous polymerization process,9 as described elsewhere.10 Such prepared colloidal dispersions were cast on a poly(vinyl chloride) (PVC) substrate and allowed to coalescence (8) Evanson, K. W.; Urban, M. W. J. Appl. Polym. Sci. 1991, 42, 2287. (9) Lovell, P. A.; El-Aasser, M. S. Emulsion Polymerization and Emulsion Polymers; John Wiley & Sons: New York, 1997. (10) Zhao, C.; Wistuba, E.; Roser, J.; Fitzgerald, P.; Spitzer, J. BASF Aktiengesellschaft: USA, 2001.
10.1021/la035078i CCC: $25.00 © 2003 American Chemical Society Published on Web 10/23/2003
Surface Interactions
Langmuir, Vol. 19, No. 24, 2003 10255
Scheme 1. Schematic Diagram of IRIRI Setup: A, The Light Path from the IR Source to a High Refractive Index Crystal and Reflection Back to the Detector; B, Hemispherical Shaped Ge Lens Enlarges the Image Signal Coming Out of the Surfacea
a Upon absorption at the point of the contact with the surface, the reflected IR radiation will carry molecular information, in this case vibrational energies of the surface species. Without the lens, the surface object “illuminates” an array detector without magnification, thus the spatial resolution is on the order of the array detector’s pixels (about 6 µm) (adopted from ref 10).
at 80% relative humidity (RH) for 3 days at 24 °C to form approximately 20-µm-thick films. Polarized attenuated total reflectance Fourier transform infrared (ATR FT-IR) spectra were collected using a Bio-Rad FTS-6000 FT-IR single-beam spectrometer set at a 4 cm-1 resolution which was equipped with a ZnSe polarizer. A 45° face angle Ge crystal with 50 × 20 × 3 mm dimensions was used. This configuration allows the analysis of the film-air (F-A) interface at approximately 0.2 µm from the surface. The use of a ZnSe polarizer facilitates orientation studies by utilizing TE (transverse electric) and TM (transverse magnetic) modes of polarized IR light. Each spectrum represents 100 coadded scans ratioed against the same number of reference scans collected using an empty ATR cell. All spectra were corrected for spectral distortions and optical effects using Q-ATR software.11 Internal reflection infrared (IRIR) images were obtained using a Bio-Rad FTS 6000 Stingray system with a Ge internal reflection element (IRE). This system consists of a Bio-Rad FTS 6000 spectrometer, a UMA 500 microscope, an ImagIR focal plane array (FPA) image detector, and a semispherical germanium IRE. IRIR images were collected using the following spectral acquisition parameters: undersampling ratio ) 4, step-scan speed ) 2.5 Hz, number of spectrometer steps ) 1777, number of images per step ) 64, and spectral resolution ) 8 cm-1. As recent literature12 indicates, the use of a Ge crystal in contact with the analyzed surface allows spatial resolution in the range of 1 µm, thus overcoming spatial detection limits in mid-IR. A schematic diagram of an experimental setup is shown in Scheme 1. In a typical experiment, a spectral data set acquisition time was approximately 15 min, thus allowing us to follow coalescence which in this case was significantly slower. Image processing was performed using ENVI (The Environment for Visualizing Images, Research Systems, Inc.) version 3.5. When necessary, baseline correction algorithims were used to compensate for a baseline drift.12
Results and Discussion As stated above, stimuli-responsive interactions between individual components of complex mixtures are one of the challenges associated with multicomponent systems. For example, one can envision that solubility, miscibility, and dispersibility of surfactants will significantly alter not only colloidal dispersions but also film formation from (11) Urban, M. W. Attenuated Total Reflectance Spectroscopy of Polymers Theory and Practice; American Chemical Society, Washington, DC, 1996. (12) Otts, D. B.; Zhang, P.; Urban, M. W. Langmuir 2002, 18, 6473.
Figure 1. Polarized ATR FT-IR spectra recorded from the F-A interface: A and B, TM and TE polarizations of MMA/ nMA/AA; C and D, TM and TE polarization of MMA/nBA/AA; D, SDS.
colloidal particles. Since particle surface morphology and its chemical makeup determine the nature of interactions present between particles and its surrounding environment, let us first consider MMA/nBA/AA and MMA/nBA films coalesced at 25 °C/80% RH, and utilizing ATR FTIR spectroscopy, determine the effect of AA on stratification of SDS after coalescence. This experimental setup allows us to obtain surface information from approximately 0.18 µm near the F-A interface. While Traces A and B of Figure 1 illustrate ATR FT-IR spectra of MMA/nBA/ AA copolymer film surfaces recorded using TM and TE polarizations, respectively, Traces C and D are the corresponding ATR FT-IR spectra of MMA/nBA copolymer latex films. For reference purposes, Trace E illustrates IR transmission spectrum of pure SDS in the 1120-950 cm-1 spectral region with the characteristic S-O stretching vibration at 1084 cm-1. As shown in Traces A and B of Figure 1, when AA is copolymerized into the particle surface, the 1084 cm-1 band is not detected, indicating that SDS does not migrate to the F-A interface. In contrast, without AA, SDS is mobilized and migrates to the F-A interface which is manifested by the presence of the band at 1079 cm-1. Two observations regarding the behavior of the S-O stretching vibrations are relevant: (1) vibrational energy changes from 1084 to 1079 cm-1 and (2) enhanced intensity for the spectrum recorded using TE polarization. While the intensity changes result from the preferential orientation of -SO3-Na+ groups near the surface,5 the S-O vibrational energy changes are attributed to local interactions of hydrophilic ends of SDS near the F-A interface. This is not surprising as recent studies showed that ionic environments may significantly alter the energy of S-O vibrations, ranging from 1068 cm-1 for SDS:CaCl2:NH4OH to 1105 cm-1 for SDS:Ca(OH)2:NH4OH.5 The shift to 1079 cm-1, as shown in Figure 1, Trace D, is likely attributed to the acidic environment of the colloidal dispersion (pH ) 2.2), and -SO3-Na+ groups are oriented preferentially parallel to the surface. Because of the presence of AA copolymerized particles inhibiting the ability of SDS to migrate to the F-A interface during film formation, let us now examine the effect of temperature. As previously shown, SO3-Na+‚‚‚ COOH interactions can be altered by temperature changes. MMA/nBA/AA films were annealed at 60, 90, and 120 °C for 2 h, and the results of these experiments (not shown)
10256
Langmuir, Vol. 19, No. 24, 2003
Scheme 2. Schematic Illustration Depicting the Interactions between -SO3-Na+ Groups of SDS in the Presence of MMA/nBA (A), MMA/nBA/AA (B), and Sty/nBA/MAA Particles (C)
indicated that, under these conditions, there is no surfactant mobility to the F-A interface. These results are in contrast to the recent studies5,6 which have shown that temperatures exceeding the glass transition temperature (Tg) of the coalesced film can induce surfactant migration to the F-A interface. Thus, the presence of AA demobilizes SDS. In an effort to elucidate the origin of interactions present on the surface of MMA/nBA/AA particles as well as determine the energies required to break SO3-Na+‚‚‚COOH entities, a schematic representation of interactions is illustrated in Scheme 2. As shown in Scheme 2A, interactions between AA of the polymer backbone and SO3-Na+ groups occur through the Na+ ion which can be easily mobilized in the presence of H2O. As shown in Scheme 2B, the presence of AA increases the strength of the SO3-Na+‚‚‚COOH ionic interactions, thus introducing more symmetry to the SO3-Na+ environment. These localized ionic clusters (LIC) facilitate the environment that exhibit significantly higher thermal stability, and the ionic content of the LICs will determine thermal stability of these entities.13 Previous studies also considered the behavior of surfactants in AA containing hard shell Sty/n-BA particles and related surfactant mobility to mechanical rupture of particles during film formation;14,15 one should not neglect that the free volume as well as other surface characteristics of the particles will influence mobility of surfactants. Also, the previous studies on Sty/nBA/MAA7 under similar conditions which utilized SDOSS have shown that SDOSS was capable of migrating to the surface. Although these data may appear to contradict the current results, MAA has an additional methyl group which serves as an electron donor, and the increased electron affinity present in MAA weakens the hydrogen bonding between SO3-Na+ and COOH entities. This is shown in Scheme 2C. Previous studies7 utilized concentration levels of MAA ranging from 2 to 10% (w/w%), and 10% of MAA was required to demobilize SDOSS, thus inhibiting its migration to the F-A interface. However, when AA is employed, only 1.5% (w/w%) is required to achieve the same effect. In view of these data, let us consider the effect of Tg. For Sty/nBA/MAA and MMA/ nBA/AA with a 49.25/49.25/1.5 copolymer composition ratio, both copolymers exhibit the Tg of 4-5 °C. For surfactant to be demobilized from the surface of Sty/ nBA/ MAA particles, 10% (w/w) of MAA is required, which for a random copolymer composition raises the Tg to 16 °C. As a result, surfactant molecules are trapped at the particle interstices during coalescence. The demobilization of surfactant molecules will be further amplified for mono(13) Tant, M. R.; Mauritz, K. A.; Wilkes, G. L. Ionomers; Synthesis, Structure, Properties, and Applications; Blackie Academic and Professional: New York, 1997. (14) Rharbi, Y.; Boue, F.; Joanicot, M.; Cabane, B. Macromolecules 1996, 29, 4346. (15) Joanicot, M.; Wong, K.; Cabane, B. Macromolecules 1996, 29, 4976.
Dreher et al.
mer-starved polymerization during which core-shell morphologies may be formed, which in the case of MAA and AA polymerizations would result in the soft-core hardshell morphologies. Let us go back to the main theme and focus on understanding SDS-colloidal particle interactions in the presence of cosolvents. Specifically, PG is an often used cosolvent for controlling film formation, rheological behavior, or freeze-thaw stability, and the question is how PG may, if at all, alter SDS behavior during coalescence and, ultimately, film properties thereafter. For that reason, PG was added to MMA/nBA/AA and coalescence was monitored at different time intervals. Although numerous studies utilized AFM as means for monitoring coalescence,16-18 these measurements do not provide molecular level information. While valuable surface morphological features may indeed be detected using AFM, in contrast, fluorescence measurements require a careful choice of fluorescence species which alters solubility, particle morphology, and ultimately coalescence, thus not adequately representing the chemistry of the entire system.19,20 We utilized in-situ IRIRI12 spectroscopy which allows chemical imaging with a 1000-nm spatial resolution during particle coalescence, and Figure 2 illustrates a series of images recorded at time intervals of 1.5, 3.5, 24, and 36 h after liquid MMA/nBA/AA films containing 20% (w/w) post-added PG were deposited. Because IRIRI allows tuning into a given IR band associated with a certain species, this approach facilitates chemical imaging of species in the xy directions as a function of time. The following vibrations are of particular interest in these studies: O-H stretching band of PG at 3330 cm-1, CdO stretching at 1724 cm-1 due to of MMA/ nBA/AA polymer matrix, and 1122 and 1084 cm-1 bands due to SDS. The images of these bands will be used to follow spatial distribution of PG and SDS with respect to the polymer matrix. Figure 2, A1-A4 illustrates IR images of these bands recorded from the same film location after 1.5 h of coalescence. While red color represents the highest concentration levels of the band which was “tuned in” to, black areas correspond to the lowest concentration levels. Analysis of the image shown in Figure 2, A1 shows that the presence of PG dominates a given area where none of the polymer matrix is detected. At the same time, the distribution of the 1084 cm-1 band due to SDS is similar to the distribution of PG (Figure 2, A4) indicating that its presence is associated with PG. High-intensity red areas in Figure 2, A2 are attributed to the presence of MMA/ nBA/AA polymer matrix. Although the origin of the 1128 cm-1 band is not exactly established, it is believed that this band results from a highly hydrated form of SDS.21,22 Figure 2, A3 and A4 also illustrates that the region dominated by PG are also SDS rich. While images shown in Figure 2, A1-A4 provide spatial distribution of different entities near the surface, Figure 2, A5 represents IR spectra obtained from the regions labeled in Figure 2, A2. As previously determined, region 1 is due mainly to the presence of the MMA/nBA/AA polymer matrix, but as shown in the 3400 cm-1 O-H (16) Cannon, L.; Pethrick, R. Polymer 2001, 43, 1223. (17) Linemann, R.; Malner, T.; Brandsch, R.; Bar, G.; Ritter, W.; Mulhaupt, R. Macromolecules 1999, 32, 1715. (18) Thomas, R.; Lloyd, K.; Stika, K.; Stephans, L.; Magallanes, V.; Sudol, E.; El-Aasser, M. Macromolecules 2000, 33, 8828. (19) Liu, Y.; Feng, J.; Winnik, M. A. J. Chem. Phys. 1994, 101, 9096. (20) Zhao, C. L.; Wang, Y.; Hruska, Z.; Winnik, M. A. Macromolecules 1990, 23, 4082. (21) Schreiber, K. C. Anal. Chem. 1949, 21, 1168. (22) Smith, L. A.; Hammond, R. B.; Roberts, K. J.; Machin, D.; McLeod, G. J. Mol. Struct. 2000, 554, 173.
Surface Interactions Langmuir, Vol. 19, No. 24, 2003 10257
Figure 2. A. IRIRI analysis of the F-A interface after 1.5 h of coalescence: A1-A4, IRIRI images produced by tuning into the 3400, 1724, 1128, and 1084 cm-1 IR bands, respectively; A5, averaged IR spectra of regions 1-3 as indicated in A2, in the IR regions of 3400-2900 cm-1 and 1700-1000 cm-1. B. IRIRI analysis of the F-A interface after 3.5 h of coalescence: B1-B4, IRIRI images produced by tuning into the 3400, 1724, 1128, and 1084 cm-1 IR bands, respectively; B5, averaged IR spectra of regions 1 and 2 as indicated in B2, in the IR regions of 3400-2900 cm-1 and 1700-1000 cm-1. C. IRIRI analysis of the F-A interface after 24 h of coalescence: C1-C4, IRIRI images produced by tuning into the 3400, 1724, 1128, and 1084 cm-1 IR bands, respectively; C5, averaged IR spectra of regions 1 and 2 as indicated in C2, in the IR regions of 3400-2900 cm-1 and 1700-1000 cm-1.
10258
Langmuir, Vol. 19, No. 24, 2003
stretching region and the 1045 cm-1 PG band, low PG concentrations are also detected. Region 2 represents an island which is detected after tuning into the 1128 and 1084 cm-1 bands, thus being attributed to SDS. It is apparent that the polymer matrix can be also partially detected in this region, as demonstrated by the presence of the CdO stretching band. Furthermore, the C-H stretching band at 2924 cm-1 as well as OH stretching vibrations at 3330 cm-1 are also detected. Finally, region 3 is mainly attributed to the presence of a large island of PG, as manifested by the increased intensity of the 3330 and 1045 cm-1 bands. In summary, after 1.5 h of coalescence, PG is present on the surface of the film in the form of large islands and surfactant is located in the areas near PG. Since there are no significant concentration level changes of SDS present in the polymer matrix, this suggests that PG ultimately stimulates migration of SDS to the F-A interface, thus releasing it from the MMA/ nBA/AA particles. The same analysis was performed after 3.5 h of coalescence. Because at this stage the surface of the film was still a liquid, a different area on the film was analyzed. Similarly, Figure 2, B1-B4 illustrates spatial distribution of the bands due to PG, SDS, and MA/nBA/AA, with the same color scheme as above. The intense red regions in Figure 2, B1 result from the presence of PG, and due to migration to the F-A interface and subsequent evaporation, significantly smaller amounts are present. As one would expect, the image of the 1724 cm-1 band masks a majority of the F-A interface (Figure 2, B2). Figure 2, B3-B4 illustrates distribution of the 1128 and 1084 cm-1 bands due to SDS, and as seen, the presence of SDS directly coincides with the location of PG islands. Figure 2, B5 represents IR spectra obtained from regions 1 and 2 of Figure 2, B2, which confirm the results presented earlier: regions 1 and 2 represent the polymer matrix and PG/SDS, respectively. This is clearly manifested by the IR spectrum recorded from region 1 illustrating MMA/ nBA/AA matrix with a strong CdO band at 1724 cm-1 and low band intensities of the 3330 and 1045 cm-1 glycol bands. On the other hand, region 2 exhibits the simultaneous presence of PG and SDS species, as demonstrated by the intensity of the 3330 and 2927 cm-1 bands. The IR spectrum representing region 2 is dominated by the 1131 cm-1 band which is due to S-O stretching vibrations of the SDS-phase. To verify that this is indeed SDS, the islands detected spectroscopically and visually at the F-A interface were exposed to water and solubilized on the surface of the film. Such specimens were collected for further analysis, which upon heating for 24 h at 80 °C, a white powder was obtained and 1H NMR spectra (not shown) indicated identical spectral features characteristic of SDS. These experimental conditions and spectroscopic analysis suggest that the 1131 cm-1 band is due to a crystalline form of SDS in the presence of PG. Although the crystallographic phase of SDS produced under these conditions is not known and would require further investigation, it is quite apparent that this process occurs during coalescence. The results of the IRIRI analysis after 24 h of coalescence are illustrated in Figures 2, C1-C5, and indicate that PG migrated and evaporated from the surface of the film, as indicated by the absence of intense red regions in the image produced when tuning into the 3400 cm-1 band in Figure 2, C1. When tuning into 1724 cm-1 (Figure 2, C2), distinct islands are present and distributed on the surface, as manifested by the red regions representing the copolymer matrix and dark regions responsible for its deficiency and spectroscopic data illustrated in Figure 2, C5 confirms
Dreher et al. Scheme 3. Schematic Illustration of the Film Formation Process of MMA/nBA/AA Colloidal Particles without (A) Propylene Glycol and with (B) Propylene Glycol Added as a Coalescing Agent, and the Interactions Displaced in the Presence of PG (C)
these findings. The same analysis performed after 36 h of coalescence (not shown) indicated the same behavior. At this point, let us attempt to determine why the addition of PG to the MMA/nBA/AA colloidal dispersion not only promotes the migration of SDS to the F-A interface but also results in its crystallization. As we recall, Scheme 2 illustrated interactions between SDS/MMA/ nBA, SDS/AA, and SDS/MAA and indicated that stronger interactions present in MMA/nBA/AA demobilized SDS. This is schematically illustrated in Scheme 3, A2. When AA is not present on the surface of MMA/nBA particles, SDS is capable of migrating to the F-A or F-S interfaces, such as illustrated in Scheme 3, A1 and A3. Although the current study utilizes the F-A interface, surfactants may also migrate to the F-S interface in the presence of high and low surface tension substrates.8 In the presence of PG, however, SDS is not only mobilized and stratified at the F-A interface, but SDS also crystallizes (Scheme 3, B). This process requires the release of SDS molecules from the particles during coalescence by breaking the SDS-AA interactions shown in Scheme 2. As OH groups of PG compete with the OH functionalities of AA, they will displace both entities from colloidal particles, thus leading to the release of SDS and PG to the surface. This is schematically illustrated in Scheme 3, C, and also indicates that PG is in serum, not in colloidal particles. It should also be pointed out that apparent differences in δt of the polymer matrix (∼20) and H2O (48.0) give rise to H2O being a poor solvent for MMA/nBA/AA, and at the same time, the solubility of SDS in H2O is about 10-15%. Therefore, during coalescence, H2O is unable to displace the interactions of SDS with MMA/nBA/AA, thus leaving SDS distributed throughout the bulk of the film. On the other hand, when PG is introduced into the system, excess OH groups of PG are able to displace -SO3-Na+ groups from MMA/nBA/AA particles as illustrated in Scheme 3, B, and due to the difference between δt of PG (33.0) and
Surface Interactions
Langmuir, Vol. 19, No. 24, 2003 10259
the polymer matrix (∼20), which is smaller than that of H2O, MMA/nBA/AA becomes slightly soluble in PG.23,24 As a result, SDS can be mobilized by its displacement from colloidal particles, thus enabling it to migrate through the polymer matrix with PG serving as the vehicle, and upon evaporation of PG from the F-A interface, SDS islands crystallize and remain on the surface of the coalesced MMA/nBA/AA film. Conclusions Film formation processes resulting from coalescence of MMA/nBA colloidal particles stabilized by SDS are affected by the presence of AA on the surface of the particles. MMA/nBA colloidal dispersions are capable of coalescing under room-temperature conditions, with SDS stratifying at the F-A interface, but interactions with SDS through the OH groups of AA that demobilize SDS, (23) Barton, A. F. M. Handbook of Solubility Parameters and Other Cohesions Parameters; CRC Press: Boca Raton, FL, 1983. (24) Brandrup, J.; Immergut, E. H. Polymer Handbook, 3rd ed.; John Wiley & Sons: New York, 1989.
rendering a latex film with SDS distributed in the bulk. Furthermore, the addition of PG as a cosolvent in MMA/ nBA/AA colloidal dispersions displaces the interactions between the -SO3-Na+ groups of SDS and the COOH groups of AA and thus mobilizes SDS in such a way that PG/SDS islands formed near the F-A interface. In-situ IRIRI experiments show the presence of SDS at the surface of MMA/nBA/AA films which contained PG as a cosolvent. SDS was shown to stratify on the surface in areas that were rich in PG. This behavior is attributed to the ability of PG to displace SDS from the surface of MMA/nBA/AA particles and thus serve as a vehicle for carrying SDS to the F-A interface. While at the F-A interface, PG facilitates crystallization of SDS which results in the formation of SDS islands. Acknowledgment. This work was supported in part by the MRSEC Program of the National Science Foundation under the Award Number DMR 0213883. The authors are also thankful to BASF Corp. for partial support of these studies. LA035078I
