Ionic Self-Assembled Monolayer (ISAM) Deposition

Transcription

1 Chapter Two: Ionic SelfAssembled Monolayer (ISAM) Deposition 2.1 ISAM Film Deposition Process Ionic SelfAssembled Monolayer films, or ISAM films, are formed by depositing alternately charged layers of polyelectrolyte onto a substrate. These polymers incorporate charge groups within the backbone or as side chains, as shown in Figure Layers are formed by immersion in aqueous roomtemperature polyelectrolyte solutions for a short period of time. Coulombic attraction between the substrate surface and the polymer in solution result in a bonding of polymer to the surface. The new polymer layer forms a surface that has the same surface charge sign as the polymer in solution. This reversal of surface charge effectively limits film deposition, as the polymer in solution is now repelled from the substrate. The substrate is then removed and carefully rinsed in ultrapure water. After rinsing, the substrate is placed in a second, oppositely charged polyelectrolyte solution. Again, the polymer in solution is attracted to the oppositely charged surface ions, bonding to many of the available charge sites. Again the surface charge is reversed, halting film deposition when the available sites are either all occupied by polymer, counterions, or are screened by interposed polymer. The substrate is again removed and rinsed of excess solution. This process may be repeated indefinitely as a simple twostep process, or may be modified by the use of different polymers to form layers with different functionality. For the purposes of this investigation, the simple twostep process was used. The water source for all solution preparation and rinsing procedures was a Barnstead NANOpure II filtration/deionization system, with resistivity at 17.5MΩ or better. 25

2 ( ) n ( ) n NH NH SO 2 SO 2 N N N N HO 3 SO Na CO Na 2 OH (a) (b) Figure 2.1.1: Structure of polymer anions (a) PS119, commercially available from Sigma, and (b) Poly{1[4(3carboxy4hydroxyphenylazo)benzenesulfonamido]1,2ethanediyl, sodium salt}(pcbs), commercially available from Aldrich. 26

3 27 Figure 2.1.2: ISAM film deposition process. A charged substrate is immersed in an aqueous polyelectrolyte solution, where an oppositely charged layer of polymer is formed. Film adsorption is limited by reversal of surface charge, which repels any additional polymer. Film is removed from solution, rinsed of excess (unbonded) polymer, and immersed in a second, oppositely charged aqueous polyelectrolyte solution. A second layer is formed in a similar manner. After rinsing, this process may be repeated until the desired number of film layers is obtained.

4 Glass microscope slides obtained from Fischer Scientific were used as substrates for all films. All substrates were prepared for deposition using the RCA cleaning method 1. Slides are immersed for 10 minutes in a (5:1:1) solution of (H 2 O/H 2 O 2 (30%)/NH 4 OH (29 w/w % as NH 3 )) at a temperature of 70 C. After rinsing with filtered water, slides are immersed in a room temperature (6:1:1) solution of (H 2 O/H 2 O 2 /HCl (37 w/w%)) for 15 minutes. After a final rinse with filtered water, slides are baked in a drying oven for 60 minutes at 130 C. Absorbance measurements of ISAM films were made on a JASCO V530 UVVIS spectrophotometer. Film thickness was measured on an SCI FilmTek2000 unit, utilizing thin film reflectance and transmittance analysis to determine thickness, refractive index, and absorption coefficients. 2.2 Initial Deposition Experiments Initial ISAM films were produced by Y. Liu at the Fiber and ElectroOptic Research Center (FEORC) at Virginia Tech from commercially available polyelectrolytes. These films were generated from three different nonlinear optical polyanions and two different nonlinear optical inactive polycations. The most extensive studies were performed on the polyanions PS119 (obtained from Sigma) and Poly{1[4(3 carboxy4hydroxyphenylazo)benzenesulfonamido]1,2ethanediyl, sodium salt} (PCBS, obtained from Aldrich) (Figure 2.1.1). A third polycation, PR478 (Figure 2.2.1), also from Sigma, was also briefly investigated. Most first generation films were produced using poly(allylamine hydrochloride) (PAH) with molecular weight M w =70,000 from Aldrich as the nonlinear optical inactive polycation, though poly(diallyldimethylammonium chloride) (PDDA), also from Aldrich, was also used (Figure 2.2.2). PS119 and PCBS both consist of a poly(vinyl amine) backbone with an azo dye as a side chain on each repeat unit. The sulfonate group at the end of the PS119 chromophore sidechain provides an ionic bond site with pka between 1 and 0. PCBS contains a carboxylic acid for bonding which has a pka value of ~45. The azo dyes are conjugated over their full length, providing a reasonably large hyperpolarizability β 28

5 ( )( )( 2 4 ) 4 O NH NHAc SO Na 3 CH 3 H C 3 NH O O Figure 2.2.1: Structure of polymer anion PR478, commercially available from Sigma. 29

6 ( ) n CH 2 NH 2 Cl (a) ( ) n H C N Cl CH 3 3 (b) Figure 2.2.2: Structure of polymer cations (a) Poly(allylamine Hydrochloride) (PAH), from Aldrich, and (b) Poly(diallylimethylammonium chloride), (PDDA), from Aldrich 30

7 for secondorder nonlinear optical phenomena. The individual chromophore hyperpolarizabilities have not been measured at this point. Hyperpolarizabilities are generally measured through second harmonic generation of a solution of dye with an electric field present. Since these polymers are ionic, the poling field results in transport to the electrode rather than orientational alignment. Alternatively, hyperpolarizabilities may be determined from hyperrayleigh scattering. Such measurements are intended in the future. Four different types of ISAM films were fabricated by Y. Liu at FEORC. Absorption spectra for representative sample films are shown in Figure It can be noted from the curves that absorption spectra for PS119/PAH and PS119/PDDA are very similar in shape. This is expected as both PAH and PDDA, as inactive polycations, were chosen to be nonabsorbing in the region of interest (visible and nearinfrared spectrum). PAH and PDDA both show considerable absorbance in the UV region. Figure shows the absorbance at 480nm for a series of PS119/PAH ISAM films of varying numbers of bilayers. The linear fit indicates an increase in absorbance of 4.1*10 3 per bilayer. A similar linear increase of 8.2*10 3 per bilayer is shown in Figure for a series of PCBS/PAH ISAM films. The less satisfactory linearity for PCBS is believed to be due to insufficient carefulness in the fabrication of one of the films. These linear increases indicate that the amount of additional chromophore (and therefore polymer) being adsorbed with each bilayer is remaining constant. ISAM film thicknesses have been previously measured by ellipsometry, small angle xray scattering (SAXS) 2,3,4, and xray photoelectron spectroscopy (XPS) 5. ISAM film thicknesses for numerous polymers have been shown to be linear in the number of bilayers adsorbed. Ellipsometry measurements made by Y. Liu on PS119/PAH ISAM films deposited on single crystal silicon indicate that film thickness is linear in the number of bilayers adsorbed with a bilayer thickness of 1.2nm 6. This linearity suggests that polymer adsorbed in each additional bilayer is bonding in a similar fashion as the previous layers, with the same average conformation. This information, when taken with linearity of absorbance in the number of bilayers, suggests that film structure and density is remaining constant from layer to layer. 31

8 PCBS/PAH PS119/PDDA PS119/PAH PR478/PAH (*10) Absorbance Wavelength (nm) Figure 2.2.3: Absorbance spectra for representative ISAM films. Due to weak absorbance, PR 478/PAH spectrum has been scaled by a factor of 10 to show detail. Note that PS 119/PAH and PS119/PDDA show similar spectra. 32

9 Absorbance Number of Bilayers Figure 2.2.4: Absorbance at 480nm vs. number of bilayers for PS119/PAH ISAM films. Linear fit shows an increase in absorbance of 4.1*10 3 per bilayer. 33

10 Absorbance Number of Bilayers Figure 2.2.5: Absorbance at 480 nm vs. number of bilayers for PCBS/PAH ISAM films. Linear fit shows an increase in absorbance of 8.2*10 3 per bilayer. 34

11 This uniformity is necessary for film design and application purposes. If the structure remains constant, then optical characteristics (absorbance and nonlinear effects) may be enhanced as necessary by depositing as many additional polyelectrolyte layers as appropriate. This information is not fully sufficient regarding nonlinear optical response, however, as second order nonlinear optical effects require chromophore orientational consistency. This will be examined further in Chapter Polyelectrolyte Adsorption Our interest in the secondorder nonlinear optical properties of ISAM films led us to more fully characterize the deposition process and the structure of the adsorbed polymer. Adsorption of Uncharged Polymers The conformation of uncharged polymer in dilute solution is determined by the polymersolvent interaction energy χ (also called the Flory χ) and the total number of polymer/solvent contacts 7 1 ε12 ( ε11 ε = 2 kt 22 ) Z χ (2.3.1) with coordination number Z and interaction energies ε between solvent molecules (1) and polymer segments (2). This parameter drives the conformation of the polymer in solution. For good solvents (χ < ½) the polymer stretches to increase the number of polymersolvent contacts. In poor solvents (χ> ½) the polymer coils and loops about itself and other polymer strands to reduce the number of polymersolvent contacts. Adsorption occurs when the net energy of adsorption s ε = 1 ( ε 2 kt 2s ε1s 11 ε 22 ) Z χ (2.3.2) 35

12 is greater than the critical enthalpy for adsorption. In general, three different types of polymer structures may occur at deposition trains, in which adjacent polymer segments lie along the interface, loops, in which a polymer chain (or portion thereof) is bonded to the surface at opposite ends leaving the portion of the chain between bonded sites unbound and arched into the bulk phase, and tails, in which a polymer chain is bonded to the interface at one end only, leaving the other to extend into the bulk phase (Figure 2.3.1a). The structure of the polymer at the interface is largely determined by the polymer conformation in solution (χ). However, if the polymer is weakly bound (small χ s ) then adsorbed chains may reorient over time to minimize their free energy. Adsorption of Polyelectrolytes 8 In ISAM deposition the coulombic interactions due to charged polyelectrolytes must be considered. The electrostatic interaction is primarily a function of three variables surface charge density σ, polymer charge, and salt concentration (ionic strength) c s. Since the surface layer is, in general, a previously deposited polyelectrolyte layer, surface charge density will be treated in a similar fashion as the polymer charge in the bulk phase. This polymer charge can be expressed as q m = ταe where τ represents the charge sign (±1), α the degree of dissociation, and e the elementary charge. For weak polyelectrolytes, the degree of dissociation is a function of the local electrostatic potential. This potential may be strongly affected by solution ph. In this case, the total ionic strength must include both salt terms and acid terms: c s = c = c salt salt c acid 10 ph (2.3.3) The contribution of ionic strength and solution ph may be more easily considered through the excluded volume, β. For uncharged polymers, this parameter may be written as 3 β = vl (2.3.4) 36

13 Loop Tail Train (a) (b) (c) Figure 2.3.1: Polymer film conformation. (a) Polymer conformation consists of three structures: trains, loops, and tails. At large ph and/or small or zero salt concentration, polyelectrolyte charge repulsion causes polymer chain to extend, predominantly forming trains (b). At smaller ph and/or larger salt concentrations, polyelectrolyte charges are screened, allowing polymer chains to loop and coil. This results in formation of thicker films than (b). 37

14 where l 3 is the hardcore volume of a polymer segment and v is the excluded volume parameter v = 1 2χ (2.3.5) If χ< ½, the excluded volume β>0 and segments repel each other, resulting in a stretching of the polymer chain noted earlier. If χ> ½, β<0 resulting in looping and coiling of the chain. For charged polyelectrolytes an electrostatic term must be introduced, β = κ 2 1 t e q, (2.3.6) where q t is the total persistence length, which represents the average rodlike length of the polymer. For an aqueous polyelectrolyte solution at 25 C with a 11 salt, the persistence length is inversely related to the total ionic strength: 9 q el = c s ( nm M) (2.3.7) In eq , κ 1 is the Debye length, which represents the effective range of the electrostatic interaction κ 1 = kt 2 2 2cs z e (2.3.8) where z is the counterion valency. The Debye length can also be calculated for a 25 C polyelectrolyte solution with a 11 salt 9 κ 1 = (nm M 2 c s ) (2.3.9) For weak polyelectrolytes the presence of added salt and the reduction of solution ph give nearly identical results: both serve to increase the concentration of small ions (c s ). In the limit of large salt concentration β e 0, and the exclusion volume is the same as that of the uncharged polymer. In this case polymer conformation is driven by χ and adsorption by χ s as high ionic strength screens the coulombic interaction. As the electrostatic screening decreases (whether due to decreased salt concentration or increased ph), the Debye length decreases as κ c and the electrostatic excluded volume increases. This causes the s electrostatic persistence length q e to increase, which reflects a stretching of the polymer chain 10. This is attributed to likecharge repulsion of the ionic sidegroups along the polymer strand. 38

15 In the absence of surface affinity (χ s 0), adsorption is driven solely by coulombic interaction. Measurements of the electrostatic bridging force driving ISAM deposition have shown that the force exhibits the longest range when ionic strength (and therefore electrostatic screening) is lowest 14. As polymer is adsorbed, likecharges accumulate at the surface (reversal of sign of the surface charge), creating an electrostatic barrier (charge overcompensation), that results in limited deposition. The limited deposition is expected not through stochastic equilibrium (saturation of charge sites ion pair formation at each available charge site) but rather through kinetically hindered equilibrium as defined by Cohen Stuart 11. This process allows polyelectrolyte adsorption only until charges are electrostatically repelled from the surface by previously adsorbed polymer. The polymer charge groups responsible for the repulsion are not bonded to the surface but instead dangle into the bulk phase on polymer loops and tails. The actual ion pair formation stoichiometry is expected to depend on surface chemistry and ionic strength 5. Electrophoresis mobility 12 and surface forces apparatus (SFA) 14 experiments for several ISAM systems have shown that ion pairs are generally observed to form at less than half of available sites. This allows for a selfhealing feature during film production, as minor defects will generally not be propagated during adsorption steps. Due to this surface charge overcompensation, layer thickness will generally be limited by the size and conformation of polymer chains in solution. Thickness should not increase unchecked as immersion time increases: instead, polymer should increase surface density until charge overcompensation prohibits further deposition. At low salt concentrations (or high ph) the polymer film is expected to be primarily composed of trains as the electrostatic repulsion among charged sidegroups on the polymer extends the chain (Figure 2.3.1b). As salt concentration increases (or ph decreases) the deposited layer is expected to increase in thickness as polymer chains loop and coil in solution due to increased screening (Figure 2.3.1c). As salt concentration increases further, the screening should reduce the electrostatic interaction to zero, resulting in no adsorption for zero surface affinity. For very high screening, saltinduced depletion may occur as the small ions compete with the polymer for bond sites. 39

16 The formation of loops and tails at the film surface leads to an interface that is not constrained to the plane. Measurements with SFA on various ISAM films have determined that polymer chains may interpenetrate as much as two and three layers on either side. Initial layers on a bare substrate remain fairly thin, as polymer can not penetrate the substrate 13. After 35 bilayers, film thickness obtains a fairly uniform equilibrium value 5. This accompanies an increase in film density, as interpenetration allows additional charge sites, resulting in increased adsorption 14. In general, it has been found that increased ionic strength (small Debye length) result in greater interpenetration 12,14. Poorly screened polymer would be more rigid and rodlike and less able to snake around previously adsorbed polymer. Since secondorder nonlinearoptical susceptibility χ (2) is a function of film structure, it is then necessary to characterize the behavior of a representative NLOactive polyanion with varying solution ph and salt concentrations. A series of (AB) n PS119/PAH ISAM films were produced from 10mM solutions with salt concentrations of (0.0, 0.06, 0.10, 0.13)M and ph s of (3.5, 2.5, 1.5). Salt content was altered by adding NaCl (obtained from Fischer). Solution ph was altered by the addition of HCl (Fischer). Films at each combination of salt concentration and ph were fabricated except for the 0.13M salt concentration at 1.5 ph: at these conditions the polyanion PS119 precipitated out of solution. Absorbance was seen to increase linearly with the number of bilayers adsorbed for all phs and ionic strengths (Figures ). The data do not intercept the origin due to small differences in the transmittance of each substrate. As previously discussed, this indicates that the amount of material being deposited with each successive layer is constant. Since film thickness is also linear in the number of deposited bilayers, we know that absorbance is then linearly related to film thickness. Layer density is then constant from bilayer to bilayer, which allows us to consider each set of parameters as they affect a single bilayer. As expected, film thickness and absorbance both increase with increased salt concentration and acidity (Table 2.3.1). As the concentration of small ions (and therefore screening) increases, these variables become less dependent on the ionic strength/solution ph, as can be observed from Figures

17 1.0 ph = 1.5 ph = ph = 3.5 Absorbance Number of Bilayers Figure 2.3.2: Absorbance vs. number of bilayers of PS119/PAH films for varying solution ph values. Solutions contain 0.00M added salt. Lines represent linear fits to data. 41

18 Absorbance ph = 1.5 ph = ph = Number of Bilayers Figure 2.3.3: Absorbance vs. number of bilayers of PS119/PAH films for varying solution ph values. Solutions contain 0.10M added Salt. Lines represent linear fits to data. 42

19 M 0.06M 0.10M M Absorbance Number of Bilayers Figure 2.3.4: Absorbance vs. number of bilayers of PS119/PAH films for varying solution salt concentrations. Solutions held at ph=3.5. Lines represent linear fits to data. 43

20 M M 0.10M 0.8 Absorbance Number of Bilayers 30 Figure 2.3.5: Absorbance vs. number of bilayers of PS119/PAH films for varying solution salt concentrations. Solutions held at ph=1.5. Lines represent linear fits to data. 44

21 Table 2.3.1: Debye length, persistence length, absorbance, and film thickness for variations in salt content and solution ph. Small screening levels show large Debye and persistence lengths but small absorbance and film thickness. Increased screening (decreased Debye and persistence lengths) show increased absorbance and film thickness. Solution ph Salt Concentration (mm) Debye Length κ 1 (nm) Persistence Length q (nm) Absorbance A Film Thickness (nm)

22 Absorbance per Bilayer ph = 3.5 ph = 2.5 ph = Salt Concentration (M) Figure 2.3.6: Film absorbance per bilayer vs. salt concentration for varying acidities. Absorbance (and adsorbed mass of polymer) is shown to increase both with increased salt concentration and with increased solution acidity. 46

23 6 5 Film Thickness per Bilayer (nm) ph=3.5 ph=2.5 ph= Salt Concentration (M) Figure 2.3.7: Film thickness per bilayer vs. salt concentration. Thicknesses for films produced in three different solution ph s shown. In each case, film thickness increases with increased salt concentration. Dependence of thickness on salt concentration is seen to decrease with increasing acidity. 47

24 0.025 ph = ph = 2.5 ph = 1.5 Absorbance per Bilayer Debye Length (nm) Figure Absorbance per bilayer vs. Debye length for PS119/PAH ISAM films. Absorbance remains fairly constant when the electrostatic interaction range is large. As screening increases and Debye length decreases below 2 nm, small changes in Debye length show large increases in absorbance per bilayer. 48

25 6 Film Thickness per Bilayer (nm) ph = 3.5 ph = 2.5 ph = Debye Length (nm) Figure 2.3.9: Film thickness vs. Debye length for PS119/PAH ISAM Films. Film thickness remains fairly constant when the electrostatic interaction range is large. As screening increases and Debye length decreases below 2 nm, small changes in Debye length show large increases in film thickness. 49

26 Figures show absorbance and film thickness vs. Debye length. Most importantly, these figures show that the three separate curves representing absorbance and film thickness collapse to a single master curve when plotted against Debye length. Figures show that absorbance and film thickness remain fairly constant when the electrostatic range is large, but as the range decreases below 2 nm (due to increased screening), absorbance and film thickness increase dramatically. If deposition rates are a function of electrostatic interaction, it is expected that increased screening should diminish the coulombic attraction driving deposition, thereby decreasing deposition rates. It is therefore not expected that this increase in the amount of deposited material could be due to a change in deposition rates. Alternatively, the increase in film thickness and absorbance is thought to be due to a change in polymer conformation. Recall that at low ionic strengths and high phs the polymer chains are highly charged, and coulombic repulsion causes the polymer to stretch lengthwise. This would lead to the predominant formation of polymer trains in the adsorbed film. As the concentration of small ions increases (whether due to salt concentration or acidity) the electrostatic term in eq becomes less dominant, allowing the polymer strand to become more flexible. If the polymer is marginally soluble (χ s ½), then the polymer will loop about itself and other polymer strands to reduce the number of polymersolvent contacts. This conformation will be maintained to some degree as the polymer is adsorbed. Therefore, the increased ionic strengths and acidities lead to a film with more and larger loops, which results in an increased film thickness as observed. For perspective, the length of the PS119 chromophorebearing side group in Figure 2.1.1a is approximately 1.4 nm. In two of the low ionic strength, (relatively) high ph cases, bilayer film thickness is observed to be significantly less than this. This suggests two possibilities: first, the chromophore orientation is not normal to the surface, and secondly, layer boundaries are poorly defined. Average chromophore orientation is measurable through the use of second harmonic generation and will be discussed in the next chapter. It is generally accepted that since polymer conformation can consist of threedimensional structures such as loops and trains that layer boundaries will not be perfectly smooth, but will instead vary over the surface of the film. This leads to interpenetration by neighboring layers, which can decrease the overall average thickness of the region. This interpenetration is not generally thought to be a 50

27 major detriment to the nonlinear optical effects considered, though the presence of verticallyoriented polymer backbone at or near the interface could lead to an increased chromophore orientation angle, as the chromophore is expected to be oriented normal to the polymer backbone. If film density is assumed to be constant throughout the film layer, then we can use Beer s law to determine the absorptivity α. Given absorbance A, bilayer thickness l b, and transmittance T we can write T = e αlb = 10 b A A* ln(10) α = l (2.3.10) Figure shows the absorbance per bilayer vs. bilayer thickness for the PS119/PAH ISAM films made in varying acidities and ionic strengths. Best fits to each ph value are shown as dashed lines. The slopes of these lines, from eq , represent the average absorptivities of the films. As can be seen, these absorptivities are very similar. Average absorptivity from individual films is calculated to be 8.9*10 3 *nm 1 with an error of 12%. If chromophore orientation is taken as uniform, absorbance may be used as a measure of the mass of adsorbed polymer. It follows that in these cases the absorptivity may in many cases be used as a measure of the film density. It can be seen from Figure that film density for PS119/PAH ISAM films is reasonably constant and invariant under changes in solution acidity or ionic strength. The 12% variation in the absorptivity coefficients for individual ISAM films does suggest a possible small systematic variation in film density. As can be seen from Figure , these variations seem to show a slight increasing trend in absorptivity with increased screening. This factor might be expected, as the increased screening would allow closer packing of otherwise mutually repulsive chromophores, resulting in an increased film density. This also agrees with the expectation of increased interpenetration and therefore higher film densities associated with a decreased Debye length. 51

28 Absorbance per Bilayer ph = 3.5, α = 8.522*10 ph = 2.5, α = 9.225*10 3 ph = 1.5, α = 9.214* α = 8.540* Bilayer Thickness (nm) Figure : Absorbance per bilayer vs. bilayer thickness for varying solution acidities. Slopes of bestfits represent film absorptivity coefficient, α which in turn may be used as a measure of film density. Data suggest a reasonably constant film density that is invariant under varying solution acidities and salt concentrations. Solid line indicates best fit for all samples taken together. 52

29 Absorptivity, α (nm ) ph=3.5 ph=2.5 ph= Salt Concentration (M) Figure : Absorptivity α vs. salt concentration for varying solution acidities. In general, low ionic strength and decreased acidity produces large absorptivity values. With the exception of two zerosalt/high ph films, α seems to show an increase with increased ionic strength and acidity. 53

30 The interpretation of absorptivity strictly as a measure of film density is a simplification, however, since chromophore alignment at the layer boundaries will alter the absorbance measurements. Absorbance measurements made with transmitted light normal to the surface would show a reduced interaction with chromophores oriented normal to the interface. This would result in a lower absorbance measurement. In this respect we can no longer safely treat absorbance as a sufficient measure for adsorbed mass of polymer. This will be discussed in greater detail in Chapter 3 when we use second harmonic generation to make measurements of the orientation angle. 2.4 ISAM Film Formation as a Function of Immersion Time In order to determine adsorption times for ISAM deposition, a series of PS119/PAH ISAM films were made with varying immersion times in the polyelectrolyte solutions. In order to reduce experimental parameters, both anionic and cationic polyelectrolyte solutions were maintained at equal ph and salt concentration. Based on the previous salt and ph studies, representative values of ph=1.5 and salt concentration of M=0.0mM were chosen. Absorbance of each film was measured every five bilayers. Each film showed a linear increase in absorbance with successive bilayers (Figure 2.4.1). As seen in Table and Figure 2.4.2, films exhibit rapid growth in absorbance and film thickness over the first 45 seconds of immersion. After 45 seconds the deposition rate decreases, showing only a 10% increase in absorbance per bilayer over the next 255 seconds. The increase in absorbance indicates that as exposure time to the polyelectrolyte solution increases, the amount of adsorbed material also increases. The deposition rate slows as the surface is covered due to charge reversal of the surface. Initially, it was suspected that this increase in absorption must be due to increased polymer surface coverage, as the surface charge reversal would eliminate the alternative description of incremental layer growth. However, it is shown in Figure that film thickness grows nearly consistently with the absorbance. We suspect that 54

31 Absorbance t = 5s t = 10 s t = 30 s t = 300s Number of Bilayers Figure 2.4.1: Absorbance vs. number of bilayers for four PS119/PAH ISAM films produced under varying solution immersion times shown. All samples were produced with solution ph=1.5 and no added salt. Samples show increased absorbance with increased immersion times. 55

32 Table 2.4.1: Absorbance, film thickness, and absorptivity data for solution immersion time variations. All PS119/PAH ISAM films with 30 bilayers. Absorbance and bilayer thickness both increase with increasing immersion time. Immersion Time (s) Absorbance per Bilayer Thickness per Bilayer (nm)

33 Relative Value Thickness per Bilayer Absorbance per Bilayer Immersion Time (seconds) Figure 2.4.2: Relative absorbance per bilayer and thickness per bilayer vs. polyelectrolyte solution immersion time. Rate of deposition is seen to increase dramatically in first 45 seconds of immersion, after which time adsorption rate appears to reach equilibrium. 57

34 this might be explained as follows. Initial polymer strands are adsorbed onto a clean surface in that the entire surface is oppositely charged. As more polymer is deposited, charge sites are occupied, forcing additional polymer to bond in some places then arch over existing polymer, forming loops if the other end bonds to the surface or tails if it does not. This would result in an increase in film thickness with absorbance. The absorbance data for PS119/PAH ISAM films shown in Figure indicate that kinetically hindered equilibrium is reached within 50 seconds of immersion. After this time no additional polymer is adsorbed on the surface. It has been observed, however, that this layer is in a dynamic equilibrium state with the solution, with polymer chains continually depleting from and adsorbing to the surface 14. This suggests that it is possible that polymer may change its conformation (from train to loop, for example) in order to minimize its free energy. This could result in increased film thickness. Figure indicates that film thickness and absorbance reach equilibrium simultaneously. The reorientation might occur on a fast time scale, with the new conformation liberating unoccupied bond sights for further deposition. This would allow absorbance and thickness to progress lock step. This process would conceivably be a small effect, possibly accounted for by the slow rise observed in the equilibrium region after 50 seconds. This method might well be unlikely, though, as the depositionlimiting factor has been shown not to be a saturation of bond sites but rather the charge overcompensation at the surface. While the reorientation of polymer could conceivably increase the number of available bond sites, it would likely not decrease the charge overcompensation. Instead, the formation of loops and tails extending towards the bulk phase would tend to increase the charge overcompensation. Since film thickness and absorbance reach equilibrium simultaneously, we then expect that the dynamic equilibrium observed does not generally include a reorientation of adsorbed polymer. Figure shows relative absorbance per bilayer vs. bilayer thickness. The slope of this plot is the relative absorptivity as given in eq This quantity may be used as a measure of film density. It can be seen that the absorptivity increases over the first 20 seconds of film deposition, whereupon it reaches 58

35 Relative Absorbance per Bilayer Relative Bilayer Thickness Figure 2.4.3: Relative absorbance per bilayer vs. relative bilayer thickness for varying solution immersion times. Slope of plot shows relative absorptivity. After an initial increase, absorptivity obtains constant value by 20 second immersion time. Absorptivity remains constant after that time. 59

36 some constant value. This implies that film density remains constant after the first 20 seconds of exposure to solution. In the initial seconds of deposition, the adsorbed polymer sparsely covers the surface, resulting in lower densities. It can be noted from Figure that near the 20second immersion time the absorbance and bilayer thickness rates decrease dramatically. This should be due to this reduction in available charge sites on the surface. 60

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