Spectroscopic Studies of Surfactant Mobility and Stratification in Films

Chapter 12 Spectroscopic Studies of Surfactant Mobility and Stratification in Films from Homopolymer Latex Blends 1

Downloaded by STANFORD UNIV GREEN LIBR on August 18, 2012 | http://pubs.acs.org Publication Date: April 1, 1997 | doi: 10.1021/bk-1997-0663.ch012

Amy P. Chu, Lara Κ. Tebelius, and Marek W. Urban

Department of Polymers and Coatings, North Dakota State University, Fargo, ND 58105

Behavior of sodium dioctylsulfosuccinate (SDOSS) surfactant and stratification processes in poly(butyl acrylate)/polystyrene (p-BA/p-Sty) latex blends were investigated using attenuated total reflectance (ATR) Fourier transform infrared (FT-IR) spectroscopy. These studies indicate that at the early stages of coalescence, SDOSS surfactant exudes to the film-substrate (F -S) interface and its SO -Na hydrophilic heads are oriented preferentially parallel to the film surface. Although the p-Sty phase shows no stratification at either interface, p-Sty rings assume preferentially perpendicular orientations at both interfaces. However, at extended coalescence times, surfactant molecules migrate to the film-air (F-A) interface, and maintain their parallel orientation. At the same time, the p-Sty phase forms a stratified layer at approximately 1.4 μm from the F - A interface. At this depth, the p-Sty rings change their orientation to become preferentially parallel to the surface. +

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Interactions between latex components and surfactants, along with other factors that influence surfactant mobility, represent critical issues for latex film formation because their magnitude may ultimately influence macroscopic film characteristics. For that reason, our recent studies " focused on the behavior of sodium dioctylsulfosuccinate (SDOSS) surfactant molecules in latex matrices. The primary focus of these studies was the examination of surface and interfacial structures formed in ethyl acrylate/methacrylic acid (EA/MAA, 4 % w/w) and styrene/n-butyl acrylate (Sty/n-BA) latex copolymer interfaces. It became apparent that the mobility and structural changes of SDOSS after coalescence near the film-air (F-A) and film-substrate (F-S) interfaces may alter numerous physical and chemical film properties. 1

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Corresponding author

© 1997 American Chemical Society

In Technology for Waterborne Coatings; Glass, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.

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Films from Homopolymer Latex Blends

We extended these studies to examine the behavior of a 50/50 mixture of separately homopolymerized polystyrene (p-Sty) and poly(butyl acrylate) (p-BA) blended latex dispersions, mixed in equal volume ratios prior to coalescence. Although the primary interest was to understand SDOSS distribution across the film thickness, these studies opened up another avenue concerned with the stratification processes of individual latex components that may occur during or after coalescence. It appears that the presence of hard and soft latex particles does influence the distribution and mobility of individual components within the films, and the phase separation is not a uniform phenomenon across the film thickness, but it occurs in the direction normal to the film surface. Furthermore, the presence of core/shell type of particles may influence the direction of surfactant migration. 14


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In this study, we will expand the scope of previous findings and alter the latex composition by using homopolymerized polystyrene (p-Sty) and poly(ethyl acrylate) (pEA) blended latex suspensions which, upon mixing, will be allowed to coalesce. This approach will allow us to investigate surfactant-polymer interactions, and how latex composition will affect the distribution, mobility, and orientation of SDOSS. The 1046 and 1056 cm" bands due to the splitting of the 1050 cm' band, resulting from the S-0 stretching of S0 "Na hydrophilic ends on SDOSS will be used to trace these groups near the F - A and F-S interfaces. Similar to the previous studies, we will utilize attenuated total reflectance Fourier transform infrared (ATR FT-IR) spectroscopy along with the Q-ATR algorithm to perform surface depth profiling experiments. Monitoring the intensity changes of the 700 cm" band will allow us to follow the behavior of p-Sty within the p-EA/p-Sty blended latex films. 1

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EXPERIMENTAL L A T E X PREPARATION Ethyl acrylate (EA) and styrene (Sty) monomers were individually polymerized using a previously reported semi-continuous emulsion polymerization process. After synthesis of separate batches of poly(ethyl acrylate) (p-EA) and polystyrene (p-Sty), the homopolymers were mixed in a 50/50 w/w % ratio, stirred, and allowed to store for 3 days prior to coalescence. Such latex blends were deposited on a polytetrafluoroethylene (PTFE) substrate to achieve a film thickness ranging from 100 to 150 μηι, and allowed to coalesce for 24 to 72 hours under 40% relative humidity. 3,13

SPECTROSCOPIC ANALYSIS A T R FT-IR spectra were recorded on a Mattson Sirus 100 spectrometer equipped with a variable angle rectangular ATR attachment (Spectra Tech) with a KRS-5 crystal. Typically, 200 coadded sample scans were acquired at a resolution of 4 cm' , and ratioed against the same number of scans of a single beam spectrum of an empty A T R cell. Polarization experiments were accomplished using a Specac 12000 IR polarizer. All spectra were corrected for optical distortions using recently developed algorithms 1


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TECHNOLOGY FOR WATERBORNE COATINGS

allowing simultaneous corrections for optical effects of strong and weak bonds. ' A l l spectra were normalized to the band at 853 cm* (not shown) which is due to the C-C skeletal modes of copolymer main chain. 1

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RESULTS AND DISCUSSION 1

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Previous studies " have established that the presence of the 1046 and 1056 cm" bands in the A T R FT-IR spectra recorded from the film-air (F-A) and film-substrate (F-S) interfaces is attributed to the splitting of the S-0 stretching band at 1050 cm" of the S0 "Na hydrophilic groups of sodium dioctylsulfosuccinate (SDOSS). The splitting results from the formation of the following molecular entities: the association of S0 " N a entities with H 0 and acid groups. The hydrophilic end of SDOSS and its associations are illustrated below: 1

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.0, ,0—-H'"

^Ή

H-O —

I ο We also have determined that, among other factors, the simultaneous presence of hard and soft particles in a latex suspension produces stratification processes in latex films which may occur during and after coalescence. In this study, we will analyze the exudation of SDOSS surfactant to the interfaces of p-EA/p-Sty blended latex films, with the focus on mobility and orientation of the surfactant molecules as a function of coalescence times, and how stratification processes of individual latex particles may influence surfactant concentration at the F-A and F-S interfaces. 14

To set the stage, let us examine a series of ATR FT-IR spectra obtained from the interfaces of a 50/50 p-EA/p-Sty blended latex films, which were allowed to coalesce for 24 hours under 40% relative humidity. Figure 1, traces A and C, show the transverse magnetic (TM) polarized spectra recorded from the F - A and F-S interfaces, respectively. The most pronounced spectral changes are detected at the F-S interface (traces C and D), where the presence of the 1046 cm" surfactant band in T M and T E polarizations is detected. A comparison of the T E (Trace D) and T M (Trace C) polarized spectra indicates that the band enhancement in the T E polarized spectrum results from orientation of the surfactant molecules. In this experimental setup, the TE wave is defined as having its electric vector parallel to the crystal plane (or perpendicular to the plane of incidence), and the T M wave has its electric vector perpendicular to the crystal plane. A pictorial definition of the T E and T M polarizations, and the orientation at the electric vector of the incident beam with respect to the film surface at the ATR crystal, are shown in Figure 2. * ' ' * The 1

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enhanced intensity of the 1046 cm" band in the TE polarization indicates that the SO3' N a groups of SDOSS are preferentially oriented parallel to the F-S interface. These 1


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Ifavenumbera (cm-1) Figure 1. A T R FT-IR spectra of a 50/50 p-EA/p-Sty blended latex in the 1350-950 cm' region after 24 hours of coalescence. A) F-A, T M polarization; B) F-A, TE polarization; C) F-S, T M polarization; D) F-S, TE polarization. 1

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results support our previous findings which indicated that the initial surfactant migration to the F-S interface is driven to alleviate interfacial tension between a substrate and latex suspension. These results also confirm preferentially parallel orientation of the sulfonate groups at the F-S interface. 13

For comparison purposes, let us now examine A T R FT-ER spectra recorded at the film interfaces after 72 hours of coalescence under 40% relative humidity. In


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contrast to the data presented in Figure 1, Figure 3 shows that the appearance of the 1046 and 1056 cm" bands is detected at the F-A interface (traces A and B). Along with the surfactant bands at 1046 and 1056 cm' , several other bands are enhanced in the spectra recorded from the F-A interface using T M and TE polarizations. 1

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Polarizer

Figure 2. Schematic diagram of an ATR FT-IR experimental setup. 1

These bands are detected at 1207, 1233, 1261, and 1288 cm" . As was previously determined, the 1207 and 1261 cm' bands enhanced in the T E polarization result from the splitting of the 1216 cm" band which is attributed to the asymmetric S-0 stretching modes of the sulfonate groups of SDOSS. The 1233 and 1288 cm' bands, enhanced in the T M polarization, are due to the splitting of the 1241 cm' band, and are attributed to the C - 0 stretching of the surfactant backbone. 13,19

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This information, along with the band intensity changes under different polarization conditions, indicates that when water is present in the system, hydrophilic S0 "Na groups are oriented preferentially parallel near the F - A interface, whereas hydrophobic (CH ) surfactant backbone exhibits preferentially perpendicular orientation to the F-A interface which minimizes the "cross section" of the SDOSS molecules to allow water molecules to more freely migrate to the surface and diffuse out of the film. +

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Even though these conclusions agree with the previously reported results on pBA/p-Sty latex films, there are several differences between two latex systems which will inherently influence dynamics of the surfactant migration. Besides composition, the primary difference between p-BA/p-Sty and p-EA/p-Sty latexes is the time required for S0 ~Na heads to orient preferentially parallel to the F - A interface, and for the hydrocarbon tails to orient perpendicular to this interface. While in p-EA/p-Sty the hydrocarbon tails follow orientation of the sulfonate heads during early stages of coalescence, it takes about 15 days for the hydrocarbon tails to follow sulfonate orientation in p-BA/p-Sty. This transient effect of SDOSS tails results from the fact that the p-BA/p-Sty latex system was composed of a 5:1 ratio of p-BA/p-Sty, whereas the p-EA/p-Sty latex is a 50/50 mixture. Furthermore, having excessive 13

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amounts of p-BA, which has a lower glass transition temperature than the p-EA component, provides a significantly higher degree of free volume, thus permitting a random orientation of the hydrocarbon tails until water is removed from the latex. Therefore, hydrocarbon tails take longer time to position themselves against the water flux.

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Figure 3. A T R FT-IR spectra of a 50/50 p-EA/p-Sty blended latex in the 1350-950 cm' region after 72 hours of coalescence. A) F-A, T M polarization; B) F-A, TE polarization; C) F-S, T M polarization; D) F-S, TE polarization. 1

Let us focus on molecular level changes as a function of depth from the F-A and F-S interfaces. Figure 4, Traces A through E , illustrate spectra recorded after 72 hours of coalescence using T E polarized light, and obtained at angles of incidence between 60° and 40°. By using this range of incidence angles, we are able to vary the depth of penetration of light into the film surface from 1.3 um to 2.3 μπι. Thus, 19,20


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molecular level information from different depths within the film can be obtained; specifically, the distribution of surfactant molecules near the F - A interface after 72 hours of coalescence. Following the results shown in Figure 4, Trace A, which exhibits a spectrum recorded from the shallowest depths, the surfactant bands of 1046 and 1056 cm" , resulting from the association of S03*Na entities with H 0 and acid groups, respectively, exhibit the highest intensities. As the depth of penetration increases (Figure 4, traces Β through E), the band intensities decrease. These observations indicate that the highest concentration of SDOSS molecules is detected at the F - A interface, and decreases at greater depths. As water-soluble SDOSS surfactant molecules migrate with the water flux toward the F-A interface, the surfactant stays at this interface, while water diffuses out of the film. In contrast, the studies on poly(ethyl acrylate)/methacrylic acid (EA/MAA) latexes determined that when coalescing particles came into contact, surfactants are displaced into the aqueous phase, where they become free to migrate with the water front moving toward the F - A interface. Once at the interface, surfactants become trapped within partially coalesced particles, while water continues to diffuse out of the film. In the case of p-BA/p-Sty, the presence of the surfactant bands at 1046 and 1056 cm" at various depths from the F-A interface was detected, with the highest band intensities at about 1.3 μηι. At greater depths, a significant decrease of intensities was observed, indicating that the largest distribution of surfactants in the p-BA/p-Sty suspension exists at the F - A interface. As shown above, the same trends are detected for p-EA/p-Sty latex suspensions. 1

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The next question is if surfactant distribution in 50/50 p-EA/p-Sty blended latex films is affected by stratification processes which may occur during or after coalescence. As we recall, stratification processes have been detected for hard p-Sty and soft p-BA particles. In this case, monitoring intensity changes of the 700 cm" band resulting from the aromatic out-of-plane C - H normal deformation modes of styrene, distribution of the p-Sty latex component can be monitored. For the pEA/p-Sty latex system, we will take advantage of these findings and follow the p-Sty phase by monitoring the same band. Figure 5 shows A T R FT-IR spectra obtained from the interfaces recorded in both polarizations after 24 hours of coalescence. It appears that the 700 cm' band intensity is stronger in the spectra recorded from the F - A interface (traces Β and D). This observation indicates that, initially, there is a greater concentration of p-Sty component at the F-A interface. Again, this observation agrees with the previous findings on p-BA/p-Sty latex films, where hydrophobic p-Sty phase stratified near the F - A interface allowing the p-EA phase exposure to humidity. Furthermore, the 700 cm' band is enhanced in the T M polarization at the F-A and F-S interfaces (traces C and D), indicating that the styrene rings of p-Sty are oriented preferentially perpendicular to the F - A interface. Their perpendicular orientation results from the fact that when residual water molecules diffuse out of the film, hydrophobic styrene units facilitate this process. 14

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Figure 4. A T R FT-IR spectra of a 50/50 p-EA/p-Sty blended latex in the 1350-950 cm" region at the F-A interface with TE polarization at various depths after 72 hours of coalescence. A) 1.3 μπι; Β) 1.4 μηι; C) 1.6 μιη; D) 1.9 μπι; Ε) 2.3 μπι. 1

Based on these data, the following scenario can be proposed for 50/50 p-EA/p-Sty blended latex films after 24 hours of coalescence. Surfactant molecules occupy predominantly the F-S interface, with S03~Na hydrophilic groups and hydrophobic tails taking preferentially parallel orientation. The presence of p-Sty is detected at both interfaces, however, higher concentration levels are present at the F-A interface and pSty rings of the p-Sty phase are oriented preferentially perpendicular. This is schematically depicted in Figure 6. +



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Figure 5. A T R FT-IR spectra of a 50/50 p-EA/p-Sty blended latex in the 720-680 o n region after 24 hours of coalescence. A) F-A, T M polarization; B) F-S, T E polarization; C) F-A, T M polarization; D) F-S, T M polarization.

Figure 6. Schematic representation and location of SDOSS surfactant molecules and styrene in a 50/50 p-EA/p-Sty blended latex after 24 hours coalescence.


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With this in mind, let us extend coalescence times to 72 hours. Examination of the 700 cm" band of the same films after 72 hours of coalescence, which is shown in Figure 7, indicates that the concentration of p-Sty at the F-A interface (traces C and D) is higher as compared to the 24 hour coalescence data (Figure 5), and the orientation of styrene rings changes. Furthermore, the intensity changes of the 700 cm" band in the TE and T M polarizations shown in traces Β and D of Figure 7 indicate that the styrene rings assume a preferentially parallel orientation to the film surfaces. As compared to p-EA phase, p-Sty rings are hydrophobic, and at the early stages of coalescence, will assume perpendicular orientation, in order to facilitate water flux from the film. After water is removed, the styrene rings became parallel. 1


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Figure 7. A T R FT-IR spectra of a 50/50 p-EA/p-Sty blended latex in the 720-680 cm" region after 72 hours of coalescence. A) F-S, T M polarization; B) F-S, T E polarization; C) F-A, T M polarization; D) F-A, TE polarization.


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Let us further examine the p-Sty distribution at the F - A interface at various depths. By changing the angle of incidence, thus changing the depth of penetration of light into the specimen, the concentration of p-Sty also changes. Figure 8 shows that the 700 cm" band reaches its maximum intensity at approximately 1.4 μιη from the F - A interface, and decreases at greater depths. Although these results agree with the previous studies for the 50/50 p-BA/p-Sty blended latex films, where p-Sty layers were observed at 1.6 μπι, for 50/50 p-EA/p-Sty blended latex films, phase separation of a p-Sty layer is detected at approximately 1.4 μπι from the F-A interface. This change in distance from the surface is attributed to the length of the acrylate chains; which exhibit higher compatibility with SDOSS. 1


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Figure 8. A T R FT-IR spectra of a 50/50 p-EA/p-Sty blended latex in the 720-680 cm" region at the F - A interface with TE polarization at various depths after 72 hours of coalescence. A) 1.3 μιη; Β) 1.4 μπι; C) 1.6 μπι; D) 1.9 μπι; Ε) 2.3 μπι.


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Extensive research efforts have been made to understand compatibility of components in latex films " and numerous studies have shown that the compatibility between p-BA latex and SDOSS is higher than that for p-EA latex and SDOSS. This, in turn, reduces the ability of SDOSS to exude in p-BA copolymer latex films. Furthermore, neutralization of the acid groups in p-EA copolymers causes swelling of the latex particles, and extension of the hydrophobic segments into the aqueous phase, which facilitates a greater degree of interactions between copolymer and surfactant hydrophobic ends. Ultimately, this process results in greater overall compatibility. Since p-BA has longer pendant hydrophobic segments (butyl versus ethyl), their presence will enhance hydrophobic interactions between the copolymer and surfactant, and thus will enhance compatibility. For that reason, p-Sty layers in p-EA/p-Sty stratify at 1.4 μπι from the F - A interface, whereas for the p-BA/p-Sty blends, stratification was detected at 1.6 μπι. Because p-BA has longer hydrophobic segments, it is more compatible with surfactant hydrophobic ends detected at this interface. This reduces exudation of surfactant molecules to the F - A interface, and increases their concentration below the surface. 1


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In summary, these as well as previous studies on p-BA/p-Sty blended latexes demonstrated again that there are significant differences in the latex surface and interfacial properties, depending upon the latex composition. In latex films composed of individual homopolymer particles, hard p-Sty and the soft p-EA particles exhibit a significant degree of phase separation, which occurs near the F - A interface, and stratification of the p-Sty phase is detected. The phase separation during film formation not only influences the mobility and orientation of SDOSS surfactant molecules and p-Sty rings within the latex film, but occurs in the direction normal to the film surface. A schematic representation of stratification processes for 50/50 pEA/p-Sty blended latex is depicted in Figure 9. After 72 hours of coalescence, the surfactant migrates with the water flux to the F-A interface, and maintains its preferentially parallel orientation. A stratified layer of p-Sty occurs at 1.4 μπι from the F-A interface, and the styrene rings change their orientation, and become preferentially parallel to the F-A surface. Finally, it is appropriate to compare the behavior of the latex systems composed of a 50/50 homopolymer mixture with that of a 50/50 latex copolymer. In both cases, the surfactant presence is initially detected to be the highest at F-S interfaces and its presence has been attributed to the interfacial surface tension between the latex and the substrate. Furthermore, hydrophilic S03~Na heads of the surfactant are preferentially parallel to the F-S surface. However, as the coalescence process continues, surfactant molecules in both latex systems migrate with the water front toward the F - A interface, and the surfactant distribution is highest near the F-A interface. Once at the F - A interface, the surfactant sulfonate heads maintain their preferentially parallel orientation to the surface. Whereas for the homopolymer mixture, the hydrophobic (CH ) tails exhibit preferentially perpendicular orientation to the surface, in the copolymer latexes, the tails have a random orientation. 12,13

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Compatibility of latex copolymers and homopolymerized components further affects latex properties. While homopolymerized latex mixtures coalesce into a non-


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Figure 9. Schematic representation and location of SDOSS surfactant molecules and styrene in a 50/50 p-EA/p-Sty blended latex after 72 hours coalescence. uniform composite of hard and soft particles near the F-A interface, with two separate T s, each representing individual components; the copolymer latex exhibits only a single T . Thus, phase separation in the homopolymer mixture is evident. However, it occurs in such a way that the layers with excessive amounts of p-Sty are present at depths of 1.4-1.6 μπι from the F - A interface. For the copolymer latexes, the distribution of p-Sty was uniform across the film. As a result, homopolymer mixtures and latex copolymers exhibit different mobility and orientation of surfactants during coalescence. g

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CONCLUSIONS In this study, stratification processes that occur during latex film formation and their effect on the distribution of SDOSS surfactant molecules within the latex film were examined. After 24 hours of coalescence, SDOSS surfactant molecules are detected at the F-S interface and S0 "Na hydrophilic groups are preferentially parallel, whereas the p-Sty phase is present at both interfaces, without stratification at either interface and a preferentially perpendicular orientation to the F-S interface. Upon extending coalescence times to 72 hours, SDOSS surfactant molecules migrate to the F - A interface with the water flux, where the hydrophilic S0 "Na groups maintain their preferentially parallel orientation to the film surface, and the hydrophobic tails have a preferentially perpendicular orientation. These processes are accompanied by a phase separation of the blended p-EA and p-Sty homopolymers which occurs at the F - A interface. The p-Sty phase separation occurs at a depth of approximately 1.4 μπι from the F - A interface and p-Sty rings are oriented parallel to the surface. +

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These studies also show that ATR FT-IR spectroscopy can be effectively used in the analysis of polymeric surface and interfacial systems when the penetration depths do not exceed 3 - 4 μπι. When molecular level information from greater depths is sought, step-scan photoacoustic FT-IR provides a better, although at this stage not a quantitative tool. 20


ACKNOWLEDGEMENTS The authors (APC) are thankful to the 1995 North Dakota Governor's School for Science and Mathematics directed by Dr. Allan G. Fischer, Dean of the College of Science and Mathematics, and to numerous industrial sponsors for their financial support (LKT and MWU).

REFERENCES 1. M.W. Urban and K.W. Evanson, Polym. Comm., 31, 279 (1990). 2. K.W. Evanson and M.W. Urban, in Surface Phenomena and Fine Particles in WaterBased Coatings and Printing Technology, M.K. Sharma and F.J. Micale, Eds., Plenum: New York, 1991, P. 197. 3. K.W. Evanson and M.W. Urban, J. Appl. Polym. Sci., 42, 2287 (1991). 4. K.W. Evanson, T.A. Thorstenson, and M.W. Urban, J. Appl. Polym. Sci., 42, 2297 (1991). 5. K.W. Evanson and M.W. Urban, J. Appl. Polym. Sci., 42, 2309 (1991). 6. T.A. Thorstenson and M.W. Urban, J. Appl. Polym. Sci., 47, 1381 (1993). 7. T.A. Thorstenson and M.W. Urban, J. Appl. Polym. Sci., 47, 1387 (1993). 8. T.A. Thorstenson, L.K. Tebelius, and M.W. Urban, J. Appl. Polym. Sci., 49, 103 (1993). 9. T.A. Thorstenson, L.K. Tebelius, and M.W. Urban, J. Appl. Polym. Sci., 50, 1207 (1993). 10. J.P. Kunkel and M.W. Urban, J. Appl. Polym. Sci., 50, 1217, (1993). 11. T.A. Thorstenson, K.W. Evanson, and M.W. Urban, in Advances in Chemistry Series #236, M . W . Urban and C.D. Craver, Eds., American Chemical Society, Washington, DC, 1993. 12. B.-J. Niu and M.W. Urban, J. Appl. Polym. Sci., 56, 377, (1995). 13. L . K . Tebelius and M.W. Urban, J. Appl. Polym. Sci., 56, 387 (1995). 14. L . K . Tebelius, E . M . Stetz, and M.W. Urban, J. Appl. Polym. Sci., 62, 1887, (1996) . 15. L.R. Martin and M.W. Urban, J. Appl. Polym.Sci., 62, 1893, (1996). 16. M . W . Urban, A T R Spectroscopy of Polymers - Theory and Practice, American Chemical Society, Washington, DC, 1996. 17. M . W . Urban, Vibrational Spectroscopy of Molecules and Macromolecules on Surfaces, Wiley-Interscience, New York, 1993. 18. G. Socrates, Infrared Characteristic Group Frequencies, Wiley-Interscience: New York, 1980, pp. 83-85. 19. B.J.Niu and M.W. Urban, J. Appl. Polym.Sci., 62, 1893, (1996). 20. B.-J. Niu, L.R. Martin, L.K.Tebelius, and M.W. Urban, in Film Formation in Waterborne Coaitngs, Eds. T.Provder, M.A.Winnik, and M.W.Urban, A C S Symp. Series # 648, American Chemical Society, Washington DC, 1996.


Spectroscopic Studies of Surfactant Mobility and Stratification in Films

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