Multilayering of a pseudo polyelectrolyte (PVPh) with a strong polyelectrolyte (PDMAC) from aqueous media

Surface Science 604 (2010) 59–62

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Multilayering of a pseudo polyelectrolyte (PVPh) with a strong polyelectrolyte (PDMAC) from aqueous media Ronny Priefer a,*, Paolo N. Grenga a, Kathryn E. Leach b, Todd D. Krauss b, Ashley N. Mandrino a, Danielle M. Raymond a a b

Department of Chemistry, Biochemistry, and Physics, Niagara University, NY 14109, USA Department of Chemistry, University of Rochester, Rochester, NY 14627, USA

a r t i c l e

i n f o

Article history: Received 12 August 2009 Accepted for publication 26 October 2009 Available online 28 October 2009 Keywords: Atomic force microscopy Surface structure, morphology, roughness and topography Self-assembly Pseudo polyelectrolyte

a b s t r a c t The introduction of pseudo polyelectrolytes (pPE) into the field of multilayer thin films has recently been achieved with the successful combination of poly(4-vinylphenol) (PVPh) with the weak polyelectrolyte (WPE), polyallylamine hydrochloride (PAH). This paper examines the stretching of this limit by exploring the extremes of using the pPE with the strong polyelectrolyte (SPE), poly(diallyldimethylammonium chloride) (PDMAC). UV–Vis absorbance and atomic force microscopy (AFM) topography data reveal a linear growth trend in film thickness that depends critically upon the assembly pH. At an assembly pH of 11.0 the multilayer was five times thicker compared to that assembled at pH 12.0. AFM topography images also show that the surface roughness of the films increases as the assembly pH decreases. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction In the field of materials science there is a push towards progressively smaller, ordered, multifunctional, cheap, and easily tailored systems [1]. For 20 years multilayering of polyelectrolyte systems have been of interest in many technological and scientific areas [2–6]. Initially, polyelectrolyte multilayered systems were fabricated by mixing dilute solutions of a polycation and polyanion together. The resultant films exhibited very distinctive physical properties based upon their ionic nature [2]. However, this technique made producing thin, uniform coatings for multilayering purposes difficult [2]. Due to this inconsistency, a novel layer-bylayer deposition process, introduced by Decher, allowed polyelectrolyte systems to be created one layer at a time by repeatedly dipping surfaces into a polycation, and then a polyanion solution [7]. This allowed for thin, uniform polyelectrolyte systems of many different materials to be created. Numerous studies have been reported on the effects of a strong polyacid and polybase system, adding a salt to the polyelectrolyte aqueous solutions to help control the thickness of the adsorbed layers [8]. The layer-by-layer deposition process was extended from strong polyelectrolytes (SPE) to weak polyelectrolyte (WPE) systems. Multilayer films have been fabricated with WPE, such as poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) * Corresponding author. Tel.: +1 716 286 8261; fax: +1 716 286 8254. E-mail address: [email protected] (R. Priefer). 0039-6028/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2009.10.021

(PAH) and the effects of changing the pH of the system on the thickness of the layers has been measured [9–15]. It was revealed that by changing the assembly pH of the multilayering system, it was possible to create very thick or very thin bilayers based on the pKas of the polyelectrolytes used [9–15]. It was also shown that polyaniline can be assembled in multilayers with several different polymers including poly(vinyl pyrrolidone), poly(vinyl alcohol), poly(acrylamide), and poly(ethylene oxide) which all contain functional groups that are capable of hydrogen bonding with polyaniline [16]. This result demonstrated that along with molecular weight, solution pH, and polymer type, hydrogen bonding also plays a role in influencing the thickness of the self-assembled weak polyelectrolyte layers [16]. Recently, the creation of thin films of pseudo polyelectrolytes (pPE) assembled in a layer-by-layer fashion was accomplished [17] by incorporating the pPE, poly(4-vinylphenol) (PVPh) into a multilayer system with the WPE, poly(allylamine hydrochloride) (PAH). The basis for the assembly was that phenol is considered a very weak acid (pKa  10), therefore polymers of 4-vinylphenol would be pPE. In addition, phenol is capable of hydrogen bonding through its hydroxyl group and the electron cloud of the phenyl ring [18,19] which can be exploited when creating multilayer systems. The introduction of very weak polyelectrolytes from aqueous media has been accomplished not only with the PVPh/PAH system [17], but with a few other cases; such as with carbohydrates [20], poly(ethylene oxide) [21], and polyamides [22]. The success of multilayering PVPh with PAH [17] encouraged us to explore the

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limits of this novel system. We decided to explore how the PVPh would behave when assembled with the SPE poly(diallyldimethylammonium chloride) (PDMAC) (Fig. 1). It was anticipated that due to the pH independent charge of the PDMAC, the layers would be thinner compared to their counterpart, PAH/PVPh system. What was discovered was this was not always the case. At certain pH values the layer thickness was virtually identical, suggesting that PVPh dominates the assembly. 2. Experimental 2.1. Multilayer film assembly Using freshly piranha-washed quartz slides, polyelectrolyte multilayer films were assembled using a 2 mM poly(diallyldimethylammonium chloride) (PDMAC, Aldrich, Mw < 100,000) aqueous solution containing 100 mM NaCl (VWR) at the desired assembly pH, and a 2 mM poly(4-vinylphenol) (PVPh, Aldrich, MW  8000) with 100 mM NaCl solution of the same pH, via the layer-by-layer deposition technique. The first layer was created by dipping a quartz slide in the PDMAC solution for 10 min, which was followed by three consecutive washing with purified water (Millipure, MilliQ) for 1, 2, and 2 min, respectively. The second layer was adsorbed by dipping the slide into the PVPh solution for 10 min and washing again in the same manner. This alternating dipping pattern was continued until the desired number of layers was reached. For multilayer systems comprised of three polymers, a 2 mM polyallylamine hydrochloride (PAH, Aldrich, MW  15,000) with 100 mM NaCl was used.

[17], we initially monitored film growth using UV–Vis spectroscopy. The PDMAC/PVPh system clearly revealed three distinct absorption peaks centered at 200, 225, and 280 nm which had been previously observed [17,23]. Fig. 2 illustrates the multilayer growth for PDMAC/PVPh assembled at pH 12.0 for 10, 20, 30, 40, and 50 layers, whereby the absorbance increased with each subsequent layer. A linear increase at all three adsorption peaks with each PVPh additional layer was observed for assembly pH values 11.0, 11.5, and 12.0 systems. Fig. 3 illustrates the linear trend at 200 nm. As with the PAH/PVPh system [17], the absorbance of the multilayer system decreases as the assembly pH increases (Fig. 3). The lower absorbance at higher pH values suggests that the charge on the PVPh increases, hence less PVPh is required to interact with the highly charged PDMAC surface. At the lower pH values there would invariably be larger PVPh Gaussian electrostatic blobs [24], hence more of the phenol to absorb the UV–Vis lightwaves. These Gaussian electrostatic blobs are due to the chains not being fully stretched and thus are described as elongated chains divided into subunits [25]. The trend with pH also implies that there should exist a similar trend in film thickness, which was confirmed using atomic force microscopy (AFM). The surface morphology of the multilayer system was much rougher at lower pH values than at higher suggesting that indeed larger Gaussian electrostatic blobs are present. (Fig. 4).

3 10 Layer

2.5

2.2. UV–Vis monitoring

20 Layer

2.3. AFM – topography and thickness AFM topography images of polymer thin film samples were obtained using a Digital Instruments Nanoscope IIIa operated in tapping mode using a Si tip (300 kHz, 40 N/m). Polymer thickness was determined by making a scratch (1 cm by 100 lm) through the polymer film to the underlying glass substrate with a razor blade. Six cross sections were made across the scratch in the acquired AFM image. This provided six height profiles each with three data points. The 18 height measurements were averaged to yield the film thickness.

Absorbance

30 Layer

The growth of the multilayered films was monitored using a Perkin-Elmer (Lambda 650) UV/Vis spectrophotometer.

40 Layer 50 Layer

1.5 1 0.5 0 190

240

290

340

390

Wavelength (nm) Fig. 2. Absorption spectra for PDMAC/PVPh multilayer system assembled at pH 12.0.

3. Results and discussion

3.5

n

n

NH .HCl 2

+ N Cl-

OH Fig. 1. From left to right, chemical structures of poly(4-vinylphenol) (PVPh), polyallylamine hydrochloride (PAH), and poly(diallyldimethylammonium chloride) (PDMAC).

pH 11.0 pH 11.5 pH 12.0

3 2.5

Absorbance

For this study we examined the assembly pH values of 11.0, 11.5, and 12.0. It was previously noted that at pH 10.5 and lower, the PVPh tends to precipitate out of solution as more ambient CO2 dissolves into the solution increasing the acidity and protonating this pPE [17]. As with the PAH/PVPh system previously reported

n

2

2 1.5 1 0.5 0

0

10

20

30

40

50

Layer # Fig. 3. Absorption at 200 nm of the PDMAC/PVPh multilayer system assembled at pH 11.0, 11.5, and 12.0.

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Fig. 4. AFM images showing film topography created at assembly pH (a) 11.0, (b) 11.5, and (c) 12.0. The z-limit (i.e., maximum height feature) for each scan was 1 lm, 200 nm, and 50 nm, respectively.

AFM was also used to determine the thicknesses of the multilayer systems. As observed with the absorbance data, there was a linear increase in film thickness as additional layers were deposited, as well as an increase in film thickness with decreasing assembly pH (Table 1 and Fig. 5). It is interesting and illustrative to compare this SPE/pPE (PDMAC/PVPh) system to the WPE/pPE (PAH/PVPh) film. For the PDMAC/PVPh system the layers assembled at pH 11.0 and 11.5 were comparable in thickness to that of PAH/PVPh [17]; however, at pH 12.0, the PDMAC/PVPh system is five times thicker (82.8 compared to 21.8 nm) than that of the PAH/PVPh system at 50 layers [17]. Ranges in multilayer thickness at with the use of strong versus weak polyelectrolytes have been previously reported [10,26]. Typically, the SPE should have thinner layers compared to that of a WPE, as each monomer unit is always charged and is independent of pH [27]. It was hypothesized that the remarkable thinness of the PAH/PVPh system at pH 12.0 was due to PAH beginning virtually completely uncharged at this pH and thus very little

Table 1 Average thickness (nm) of multilayer films with 10, 20, 30, 40, and 50 layers assembled at pH 11.0, 11.5, and 12.0.

a

Total layers

pH 11.0

pH 11.5

pH 12.0

10 20 30 40 50

37.4 ± 7.23 142.1 ± 18.79 224.8 ± 64.78 265.8 ± 56.63 481.1 ± 93.09

41.1 ± 7.79 76.0 ± 11.59 123.6 ± 23.11 177.1 ± 17.12 234.9 ± 41.88

NAa 13.4 ± 3.26 43.9 ± 6.05 51.4 ± 5.81 82.8 ± 6.67

was adsorbed onto the surface [17]. For PDMAC, however, the charge will remain the same at all pH values and thus the thickness would solely be due to the charge on PVPh. The stability of these surfaces was also examined in varying solvents. In dH2O, the surfaces did not seem to lose any polymer (as indicated by matching absorbance profiles), even when immersed in water for 1 week. However, in boiling dH2O, there was a decrease of 50% after 30 min. This however, did not lose any more material, even with a further 1 h of boiling. Upon dry in a dessicator, the absorbance profile matched that of after the initial 30 min of boiling. This behavior was also observed when slides assembled at different pH values and of differing layer numbers were immersed in MeOH. In hexanes, acetone, dichloromethane, ethyl acetate, and acetonitrile no material was lost after half hour of immersion. The final system that we explored consisted of an alternating film of 10 layers of PDMAC/PVPh then 10 layers of PAH/PVPh, repeating up to a maximum of 40 total layers. We then assembled these pairs, starting with the PAH/PVPh then changing to the PDMAC/PVPh system again up to a total of 40 layers. Using the change of absorbance at 200 nm as a gauge of relative film thickness, we observed a gradual growth of the films with alternating slopes in a step-ladder fashion (Fig. 6–8). Under assembly pH values of 11.0 and 11.5, the film thickness was greater after 10 layers with the PAH/PVPh system compared to PDMAC/PVPh. This is evident by the greater absorbance profile. This is presumably due to the fact that SPE’s, such as PDMAC, tend to form thinner layers than WPE [8]. After 20 and 40 layers the

Too thin to measure accurately.

5 600

4

pH 11.0 pH 11.5 pH 12.0

3.5

Absorbance

500

Thickness (nm)

PAH/PVPh start PDMAC/PVPh start

4.5

400 300

3 2.5 2 1.5

200

1 0.5

100 0

0 0

10

20

30

40

50

Layer # Fig. 5. Average film thickness of PDMAC/PVPh multilayer systems assembled at pH values 11.0, 11.5, and 12.0 as measured from AFM images (Table 1).

0

10

20

30

40

Layer # Fig. 6. Assembly pH 11.0. Alternating PAH/PVPh and PDMAC/PVPh every 10 layers either starting with either PAH/PVPh (blue) or with PDMAC/PVPh (red). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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R. Priefer et al. / Surface Science 604 (2010) 59–62

1.8

4. Conclusions

PAH/PVPh start PDMAC/PVPh start

1.6

Poly (4-vinylphenol) (PVPh), herein classified as a pseudo polyelectrolytes (pPE), can be adsorbed onto surfaces via the layer-bylayer assembly technique to create multilayered polymer coatings. By using this pPE with a SPE (PDMAC), we have pushed the limits of this polyelectrolyte. Although not classically considered a traditional polyelectrolyte, we have demonstrated that PVPh does behave as such, albeit only in a very narrow pH range. The thickness of the surfaces obtained were dependant upon assembly pH, with a five times greater thickness at pH 11.0 compared to 12.0.

Absorbance

1.4 1.2 1 0.8 0.6 0.4 0.2 0

0

10

20

30

40

Acknowledgements

Layer # Fig. 7. Assembly pH 11.5. Alternating PAH/PVPh and PDMAC/PVPh every 10 layers either starting with either PAH/PVPh (blue) or with PDMAC/PVPh (red). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

0.8

References

PAH/PVPh start PDMAC/PVPh start

0.7

Absorbance

0.6 0.5 0.4 0.3 0.2 0.1 0

0

10

20

30

The authors thank the Niagara University Academic Center for Integrated Science, the Rochester Academy of Science (RP) and the Department of Energy Office of Basic Energy Sciences for their financial support. PG would like to thank the Barbara S. Zimmer Memorial Research Award for financial aid.

40

Layer # Fig. 8. Assembly pH 12.0. Alternating PAH/PVPh and PDMAC/PVPh every 10 layers either starting with either PAH/PVPh (blue) or with PDMAC/PVPh (red). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

UV–Vis spectra are virtually indistinguishable regardless whether starting multilayering with the PAH/PVPh or PDMAC/PVPh system. With the systems assembled at pH 12.0, there was more bound PVPh after 10 and 30 layers for the PDMAC/PVPh compared to PAH/ PVPh system. This is what was also observed when these systems were assembled alone (vide infra) and was the only assembly pH where the PDMAC/PVPh demonstrated thicker layers compared to the PAH/PVPh system. As expected, the absorbance after assembly of 20 and 40 layers, were virtually identical whether beginning with PDMAC/PVPh or with PAH/PVPh.

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