LAYER-BY-LAYER DIRECTLY-ASSEMBLY OF POLYELECTROLYTE

MULTILAYERS WITH FOAMING STRUCTURES

A Thesis Presented to The Graduate Faculty of The University of Akron

In Partial Fulfillment of the Requirements for the Degree Master of Science

Lihua Xu August, 2015

LAYER-BY-LAYER DIRECTLY-ASSEMBLY OF POLYELECTROLYTE

MULTILAYERS WITH FOAMING STRUCTURES

Lihua Xu

Thesis

Approved: Accepted:

______Advisor Department Chair Dr. Nicole Zacharia Dr. Sadhan C. Jana

______Co-Advisor Dean of the College Dr. Kevin Cavicchi Dr. Eric J. Amis

______Committee Member Interim Dean of the Graduate School Dr. Sadhan C. Jana Dr. Rex D. Ramsier

______Committee Member Date Dr. Mark D. Soucek

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ABSTRACT

In recent decades, dynamic Layer-by-Layer (LbL) assembly of multilayer thin films has been widely recognized for many reasons, especially for its versatile functionality and ease of fabrication.1-3 Films can be fabricated towards huge amounts of materials, such as polyelectrolytes, organic components, polymeric microgels, etc. 4-7

Through the long history of LbL assembly research, few papers proposed ideas of making porous films or foamed films directly or indirectly, as foamed material contained plenty of pores, which could be used in insulation materials and fast food-packages.8-9 In several papers, the step of acid treatment aimed for irreversible structure transformation applied into films fabricated by the LbL assembly method.10 This indirect method makes procedures more complex, so direct fabrication with LbL is necessary. However, time- consuming problems and challenges from external environmental factors still limits the development of this technique.

In this thesis, a facile fabrication of porous films through the LbL assembly method was introduced. Strong polyelectrolyte, polycation poly(diallyldimethylammoniu

-m chloride) (PolyDADMAC) and polyanion poly(styrene sulfonate acid) (PSSA) are utilized as the original material. And PolyDADMAC was slightly modified by sodium hydroxide (NaOH) and carbon dioxide (CO2) in order to get the bicarbonate structure at the side groups along the chain of the polyelectrolyte. In order to deeply study the films, some other particles, such as the surfactant brij-76 and RDS laponite were added into certain solutions for comparison.

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Films were achieved by alternately putting clean glass slides into the polycation and polyanion solutions with the rinsing step in between. 20-bilayer film can be achieved after repeating the procedures above 20 times. The principle is that the bicarbonate functional group will react with the hydrogen ions for the sake of letting the gas carbon dioxide out and then make the porous structure throughout the film.

The morphology of the films measured by an atomic force microscope (AFM) and a scanning electron microscope (SEM) shows that porous structures are obvious through the whole film when no sodium bicarbonate (NaHCO3) is added into the anion solution.

The results help to solve the problem of directly making the foaming films as well as simplifying the procedures.

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ACKNOWLEDGEMENTS

I would like to express my great appreciations to my advisors Prof. Nicole

S. Zacharia and Prof. Kevin A. Cavicchi for their invaluable suggestions and guidance, both in scientific research and in my daily life. Thanks to their encouragement and support, I became confident in myself and eventually finished this Master's thesis. Also

I would like to thank all my committee members.

In addition, I appreciate the collaboration and advice from group members.

Especially for Mr. Guodong Deng who helped me a lot on the measurements of using an

Atomic Force Microscope (AFM).

Finally, I would like to express my gratitude towards my beloved parents and sister for their patience as well as their financial support to make this thesis complete.

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TABLE OF CONTENTS Page LIST OF TABLES ...... viii LIST OF FIGURES...... ix CHAPTER...... 1 I. INTRODUCTION...... 1 1.1 Layer-by-Layer(LbL) assembly...... 1 1.1.1 Basic concepts about LbL assembly...... 1 1.1.1.1 History of development of LbL assembly...... 1 1.1.1.2 Research fields using LbL assembly technique...... 2 1.1.1.3 Materials used for LbL assembly...... 3 1.1.1.4 Factors influence films made by LbL assembly...... 3 1.1.1.5 The interactions existing in the LbL film...... 4 1.1.2 Substrates for constructing LbL film...... 4 1.1.3 Methods operated in the LbL assembly...... 6 1.1.3.1 Dipping LbL assembly...... 8 1.1.3.2 Spin-assisted LbL assembly...... 9 1.1.3.3 Spray-assisted LbL assembly...... 12 1.1.3.4 Other film-making methods...... 14 1.2 Polyelectrolyte...... 17 1.2.1 Basic concepts about polyelectrolyte...... 17 1.2.2 Commonly used polyelectrolytes...... 19 II. MOTIVATION AND PROBLEM STATEMENT...... 21 2.1 Motivation...... 21 2.2 Problem statement...... 25 III. EXPERIMENT SECTIONS...... 26

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3.1 Material...... 26 3.2 Treatment of the substrates...... 28 3.3 Solution preparation...... 28 3.4 Layer-by-Layer assembly...... 31 3.5 Machine used in the experiment and the sample preparations...... 33 IV. RESULTS AND DICUSSIONS...... 36 4.1 Compare the structure of the different films...... 36 4.2 Analyzing SEM images of these different films...... 38 4.3 AFM images of 1-6 bilayers of 4 typical films...... 44 4.4 Thickness and roughness measurements...... 50 4.5 TGA data illustrates the content of the film...... 52 V. CONCLUSION...... 54 REFERENCES...... 55

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LIST OF TABLES Table Page 1. The components of different solutions to make different films...... 33

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LIST OF FIGURES Figure Page 1. Classification of substrates used in fabrication of layer-by-layer self-assembly. Copyright 2014 Nano-biomedicine.45...... 5 2. Schematic representation of the processes used to fabricate polyelectrolyte multilayer films by LbL assembly. (a) Dip coating: glass slides and beakers are used in this method. Steps 1) and 3) represent the exposure of a polyanion and polycation respectively, and steps 2) and 4) are rinsing steps. The four steps are in the basic build-up sequence and contribute for only one-bilayer film architecture. If 10-bilayer film is needed, the four procedures would be repeated 10 times. Construction of more complex film requires additional beakers and an extended deposition sequence. (b) Spin-assisted LbL assembly: High spinning speed would be operated after the droplet of the material is applied onto the center of the substrate surface. And the rinsing step would take place between steps 1) and 3), all the four steps are in a circulation system. (c) Spray-assisted LbL assembly: Instead of bringing the substrates surface into contact with the liquid of the adsorbing species, the liquid is sprayed against the receiving surface of the substrates. Multilayer films would be formed by repeating steps 1) to 4) in a cyclical fashion. Copyright 2012 Chemical Society Reviews.3...... 7 3. Schematics show the fabrication process of the drop casting including dropping of solution and spontaneous solvent evaporation...... 15 4. Procedures for describing the doctor blade method to create films. The first step: the solution is dropped onto glass substrate; the second step: the blade spreads the solution over the substrate; the third step: the obtained film is dried in the air. Copyright 2012 Advances in natural science: Nanoscience and Nanotechnology.70...... 15 5. Depiction of various solution deposition methods in order to make films. Copyright 2011 The Royal Society of Chemistry.69...... 16 6. A) The structure of a single polymer and B) a single polyelectrolyte (the whole chain is positively charged)...... 18 7. Polymer chain of polyelectrolyte in solution after addition of the salt NaCl...... 19 8. A small list of polyions already used for creating multilayer films. The words below each polyelectrolyte structures are the abbreviations...... 20 9. The AFM images of the morphology of a 21-bilayer PAH/PAA with PH 3.5/7.5 polyelectrolyte multilayer film before (A) and after (B) exposure in a transition bath within pH 2.50. Copyright 2000, Langmuir.10...... 22

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10. A: The porous structure films were made through a two-step procedure. In the first place, a certain amount of LbL films with the material LPEI and PAA were fabricated at the surface of the silica substrates. In the second place, the film was immersed into the acidic solution. B: The SEM images of both the morphology and cross-sectional area of the films which were exposed into different pH acidic solutions. Copyright 2008 Macromolecules.9...... 23 11. A shows cross-sectional (left two) and surface (right one) SEM images of multilayer films of (PDAC/LAP) with different number of bilayer films. B is a series of images illustrating surface topography and roughness of multilayer films of (PDAC/LAP) with 100 bilayers. Copyright 2013 US pattern number 20130341277 A1, porous films.91...... 24 12. The chemical structure of different polyelectrolytes. A:Poly(diallyldimethylammonium chloride)(PolyDADMAC). B:Poly(diallyldimethylammonium hydroxide). - C:Poly(diallyldimethylammonium bicarbonate)( PolyDADMA-HCO3 )...... 27 13. D:Poly(4-styrene sulfonate acid) (PSSA). E:Poly(sodium styrene sulfonate)(PSS)...... 27 14. The set-up of the experiment. The name of each letter is as follows. A: The rubber hose connected to the liquid CO2 cylinder. B: The long needle which let the CO2 into the solution. C: The syringe needles. D: The rubber plug. E: The round-bottom glass flask. F: The solution system. G: The stir bar. H: The CO2 bubbles...... 29 15. (a) Empirical formula for a Laponite particle. (b) Crystallographic structural formula of Laponite depicted in part. (c) Characteristic shape and dimensions of a single Laponite platelet.92...... 30 16. The chemical structure of Brij-76...... 31 17. Schematic representation of the procedures used to fabricate polyelectrolyte multilayer films by the mode of dipping LbL assembly...... 32 18. Different kinds of films made. The numbers are from 1 to 11 from left to right in relation to the number in table 1...... 36 19. The SEM images of the morphology and cross-sectional area of the control films number 12 and 13 in relation to the film number in table 1...... 39 20. The SEM images of the morphology and cross-sectional area of the film 1 and 2 in relation to the film number in table 1...... 40 21. The SEM images of the morphology and cross-sectional area of the film 3-6 in relation to the film number in table 1...... 41 22. The SEM images of the morphology and cross-sectional area of the film 7 to 9 in relation to the film number in table 1...... 42 23. Gather the former four typical different films' SEM images (morphology and cross- sectional area) together for the convenience of observation. In this figure, "+" means

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cation solution and "-" means anion solution. From the top to the bottom, the number of the film is in the following order 1, 2, 3, 7, 8...... 43 24. AFM images of film 3 in table 1...... 46 25. AFM images of film 4 in table 1...... 47 26. AFM images of film 7 in table 1...... 48 27. AFM images of film 8 in table 1...... 49 28. The thickness and roughness measured by profilometer of the different 11 films. "The number of the film" is in relation to the number in table 1...... 50 29. TGA data of film 7 in table 1 under the ambience of nitrogen gas. Region 1 represented the loss of the moisture in the sample. Region 2 and 3 illustrated two steps of decomposition, and region 4 is the remaining material which was non- degradable...... 53

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CHAPTER I

INTRODUCTION

1.1 Layer-by-Layer(LbL) assembly

Layer-by-Layer assembly (LbL) has been widely used in our daily life, here I would like to intruduce some basic concepts and assembly methods as well as the materials utilized in LbL field.

1.1.1 Basic concepts about LbL assembly

The long history of the development of LbL the significance of the LbL field. And during this long period of time, more and more approaches have been operated to further advance the technique of LbL assembly.

1.1.1.1 History of development of LbL assembly

Investigations on polymeric multilayer films, also called layer-by-layer (LbL) films can be traced back to the 1960s by the pioneering work of R. K. Iler and his coworkers on

"Multilayers of colloidal particles" which was published in J.Colloid and Interface

Science. It was the first time that researchers used the LbL approach to fabricate multilayer thin films with controlled architecture and functions.11-13

However, the significance of this LbL method was taken into account and recognized due to the re-discovery and re-establishment of Gero Decher and his co- workers on consecutive adsorption of polyanions and polycations to fabricate multilayer films (details introduced in the following paragraphs).14-16 The popularity among scientists was demonstrated by the increasing tendency of the citation curve of the review

1 articles of Gero Decher in 1997 in Science.14 Compared with the chemisorption, which can only be used with certain classes of molecules,11,17 the LbL assembly method has been extended to other materials such as proteins and colloids.14 Furthermore, by adjusting the conditions in the experiments, films with precisely controlled physical properties could be obtained.18-20 For example, variation of thickness could be limited in the range of hundreds of nanometers. Besides these merits, the paper also referred to some shortcomings, like the swelling behavior of the film during process would ruin the mechanical properties of the material.14 In summation, the LbL technique has been proven by the academic field that great attention was operated over the last two decades.

Moreover, the first commercially available product contact lens equipped with

LbL coating was claimed by CIBA-Vision in 2002.21 And the development of the LbL assembly resulted in solid and consistent growth. Specifically, in 2010, more than 1000 articles were published in this field.3

Though there was a long history about the development of LbL assembly, lots of limitations and obstacles were still existing. For instance, the principle of the polymeric multilayer films fabricated by this method remains elusive. And the theory why dynamic light scattering (DLS) techniques always show two peaks when measuring the particle size of the polyelectrolyte (two different peaks of particle size, but actually one polyelectrolyte should have one particle size) is still unclear. So, more effort is needed for exploring more deeply LbL assembly aspects.

1.1.1.2 Research fields using LbL assembly technique

Layer-by-layer (LbL) assembly has been widely used in an extremely large amount of aspects from the energy or physical fields to aspects of medical delivery.22-23 Specifically,

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solar-energy conversion,22,24 anti-reflection coatings,25-26 biosensors,27 solid-state ion- conducting materials,28 controlled drug-releasing coatings,29-30 and separation membranes.31-32 The LbL assembly approach offers the possibility to fabricate ultrathin films as well as controlling the film thickness.33 It offers functional coatings on solid substrates supported by alternate exposure to positive or negative kinds with spontaneous deposition of the oppositely charged polyelectrolytes, biological species, even the metallic or inorganic nano-particles.34-36 Thanks to the controlled thickness and the functional groups achieved in films fabricated from the LbL assembly method, more research areas are utilizing this method for the aim of advancing their current products.

1.1.1.3 Materials used for LbL assembly

Polyelectrolyte and proteins are the two most prominent materials for LbL assembly.7,17,37

In the next section, more details about polyelectrolyte will be introduced. Materials processed by LbL assembly should possess interactions in and between the bilayers, which will be specifically introduced in 1.1.1.5 "The interactions existing in the LbL film".

The material processed by LbL assembly, to some extent, will help bring in some functional groups in order to help improve the quality of the structures. For example, polymeric building blocks38 which possess versatile structures in solution are helpful to receive polymeric films with well-tailored structures as well as functionalities. Now, numerous researchers reported the successful LbL assembly fabrication on synthetic linear polymers,39 block copolymers,38 dentritic molecules, organic components, polymeric microgels, and polyelectrolytes, stabilized micelles, as well as complexes of these species.19

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1.1.1.4 Factors influence films made by LbL assembly

The very advantage of the prominent LbL assembly technique is that it is a convenient method for constructing composite films with precise control of film structures and subsequent properties (the growth of thickness and roughness.etc).3 What's more, structure and properties can be modulated through simple adjustment of parameters including the assembly pH,8,40 the salt concentration and type,40-41 the molecular weight of the material, the solvent qualities42 and the temperature or the humidity43 too.

1.1.1.5 The interactions existing in the LbL film

Diversity of weak interactions can be the driving force for the fabrication of LbL film, such as electrostatic interactions, coordination bonds, hydrogen-bonds, halogen-bonds, charge-transfer interactions, biospecific interactions (sugar–lectin interactions), cation– dipole interactions, and the combined interaction of the above forces, and so on.44-45 The sensitivity of a LbL film due to the assembly conditions as well as external stimuli can be ascribed to the weak nature of the non-covalent interactions referred to above, which exists between the two adsorbing species.

1.1.2 Substrates for constructing LbL film

The LbL assembly technique could be utilized for surfaces of almost any kind of any shape, including silicon wafer, golden wafer, micro glass, paper, quartz, polydimethylsiloxane (PDMS)34 or even biological cells (see Figure 1).46 All these substrates are either negatively or positively charged on their surface.

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Figure 1. Classification of substrates used in fabrication of layer-by-layer self-assembly.

Copyright 2014 Nanobiomedicine.45

The metal substrates are widely used for making fuel cells, biosensors or gas sensors, and stent-assisted gene transfer.46 While the substrates, glass slides, are usually implemented into perspectives, such as the synthesis of ultrathin organic-multilayer films or the synthesis of anti-reflection thin films. What's more, polymer substrates are often used in order to increase the solubility of CNTs in water. Other substrates like the platinum electrodes could be used in amphoteric biosensors. Due to the rapid development of LbL assembly research, we can even use particles and fibers as the

5 substrates, such as cotton fibers, calcium carbonate (CaCO3), even melamine formaldehyde microspheres.31,47

Generally, the most widely and commonly used substrates in the laboratory are micro glass slides and silica wafers.

1.1.3 Methods operated in the LbL assembly

According to the LbL technique, the operating principle could be performed in several different approaches including spin-assisted LbL assembly, dipping LbL assembly, spray-assisted LbL assembly and the least used method flow based techniques.3,47

Characterizations of LbL films are typically done by an optical polarizing microscope, scanning electron microscope (SEM), atomic force microscope (AFM) or the ellipsometry, and the mechanical properties are often measured by techniques such as quartz crystal microbalance (QCM) or quartz crystal microbalance with dissipation monitoring (QCM-D) .36,48 Recently, some groups have explored flowing deformation properties on the films with rheometers.49

6 Figure 2. Schematic representation of the processes used to fabricate polyelectrolyte

multilayer films by LbL assembly. (a) Dip coating: glass slides and beakers are used in

this method. Steps 1) and 3) represent the exposure of a polyanion and polycation respectively, and steps 2) and 4) are rinsing steps. The four steps are in the basic build-up

sequence and contribute for only one-bilayer film architecture. If 10-bilayer film is needed, the four procedures would be repeated 10 times. Construction of more complex

film requires additional beakers and an extended deposition sequence. (b) Spin-assisted

LbL assembly: High spinning speed would be operated after the droplet of the material is

applied onto the center of the substrate surface. And the rinsing step would take place

between steps 1) and 3), all the four steps are in a circulation system. (c) Spray-assisted

LbL assembly: Instead of bringing the substrates surface into contact with the liquid of the adsorbing species, the liquid is sprayed against the receiving surface of the substrates.

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Multilayer films would be formed by repeating steps 1) to 4) in a cyclical fashion.

Copyright 2012 Chemical Society Reviews.3

In addition, I would like to introduce the commonly used methods in doing LbL assembly. Simultaneously, the procedures, both the merits and the demerits, as well as limitations will all be concluded in the sections below.

1.1.3.1 Dipping LbL assembly

The dipping mode has already been widely generalized because of its convenience and cost, as the use of several beakers could help to finish experiments. The process of the dipping LbL assembly has been illustrated in the figure 2.a.3,50

Initially, the substrates were vertically exposed into the charged (positively charged or negatively charged) solution for several minutes, followed by the rinsing step.

After that, the substrates were vertically immersed into the oppositely charged solution for another several minutes. Finally, the rinsing step was performed, same as the second manner. The combination of all the above four procedures contributed to only one-bilayer film, thus the four steps can be seen as a circulation.3 20-bilayer films would be achieved after 20 circulations of the four procedures.

As a manner to make LbL films, dippinga LbL assembly has many merits. Firstly, it is a novel route to get a different number of bilayer films in order to achieve different film thicknesses. Secondly, it can help to control the film structures and mechanical properties. For instance, the thickness and roughness, and the Young modulus.51 Of the most importance, multilayer films of large scale could be obtained through dipping a LbL assembly. However, there are disadvantages as well, as a few severe limits still exist in

8 all these series of procedures,52-53 given that the dipping mode would be extremely time- consuming for each bilayer.

Considering the most common period of time for the above four steps in a cycle respectively, 10 min in solution, 1 min for rinsing each time three times, 10 min in another solution, and 1 min rinse each time three times. That is a lengthy process and film formation.13,53 Typically, researchers need more than 20-bilayer films or even several hundred-bilayer films, meaning more than 8 hours and sometimes even several days to finish. In addition, due to the gravity of the earth, we cannot rule out the possibility that thicker film would be obtained approaching the bottom of the substrates, and the upper areas of the substrate would have thinner film. Thus, some problems could take place when using equipment to measure the thickness of the film and, normally, we would measure the thickness of the middle place of the film, though there would be some fluctuations for the results under this circumstance. Besides the above limits and demerits, some other important factors would also influence the final surface and structure of the film, such as the air flow rate, and the humidity of the environment, which would affect the drying rate for the films. The lab temperature would affect the conditions of the film made by the temperature-sensitive solution. Furthermore, films would be more sensitive to light if materials are susceptible to change due to changes in light.

1.1.3.2 Spin-assisted LbL assembly

The construction of dip coating to make films concludes the following procedures, the diffusion, adsorption and rearrangement of polymer chains, and may occupy plenty of time, which depends on the dimensions of the macromolecule, the density of charge in the solution together with the mobility of chains.3 In order to accelerate the LbL assembly

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process, a spin-assisted LbL assembly technique was introduced by Char, Wang and co- workers at nearly the same time through the combination of both the spin-coating and

LbL assembly techniques.54-55

Spin-assisted coating is a technique utilized to create uniform thin films with nanoscale thicknesses on flat substrates for hundreds of years.56 A machine used for spin- assisted coating is called a spin coater, or simply spinner.

The description of the procedures for the spin-assisted coating involves four main parts. Firstly, a small puddle of fluid which is negatively or positively charged is applied on the center of the substrate, then a high speed spinning rate is performed on the substrate for tens of seconds (see Figure 2.b) (the amount of the fluid and the spinning speeds often depends on the properties of the fluid as well as the substrates. And uniquely, spinning speed is over 1000 rpm, the time needed for the spinning step could take from

10 seconds to several minutes).57 After that a rinsing step takes place, the high speed of spinning would be sustained for another several seconds, followed by the same volume of the oppositely charged fluid for another equal period of time. At the end, the rinsing step is performed again for the sake of removing the excess solution from the resulting film.58

Here, one-bilayer film would be done.

The spinning platform would be rotated under a high spinning speed in order to get centripetal force which could lead to the solution or solvent to spread to, and eventually off.3,59 The margins of the substrates surface would retain a thin film due to the surface intention. Undoubtedly, the combination of the spin speed, the period of time selected for each step, and the nature of the material used (concentration, viscosity, percent solids, surface tension, etc) would generally determine the final film properties.

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Taking the film thickness as an example, in short, the higher the angular speed of spinning, the thinner the film. When it comes to the concentration, the higher the concentration the larger the thickness.56

The most significant advantage for the spin-assisted LbL assembly is that it can help save a huge amount of time for each step in contrast to the dipping mode since it is unnecessary to drop as much material as possible to wet the entire surface of the substrate, and all the steps can be accomplished within a few seconds or minutes.3,60 Another important aspect is that film formed by the spin-assisted technique usually possesses a more progressively smooth surface, and a more ordered internal structure for the reason that interpenetration between adjacent layers is suppressed. Additionally, well-controlled film thickness can be easily achieved by changing the spinning speed of the spin-assisted mode.60 Furthermore, when it comes to the drying condition, less time is needed as the evaporation of the solution would be faster under a faster air flowing rate due to the high speed spinning.61 In many materials, authors pointed out that spin-assisted coating is a particularly beneficial approach when the materials or substrates themselves possess poor wetting abilities and can exclude voids that may otherwise form.59

Despite the advantages of the spin-assisted LbL technique, limitations still restrict it. There is no doubt that one of the most important factors when the spin-assisted technique is applied into the LbL assembly realm is repeatability. Slight variations in the parameters (concentration, viscosity of the materials, the angular speed of spinning and the time period of each spinning step and so on) could result in a tremendous variation on the final film properties.59 In addition, a bigger substrate cannot be spun at an adequate high rate for allowing the film to thin and dry in a timely manner, which leads to the

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descending wholly-put and unsmooth surface. Although in the former paragraph, I have mentioned that spin-assisted coating can help decrease the amount of material required for the LbL assembly, it still has a disadvantage due to waste of materials.3 For a typical spin-assisted coating process, merely 2-5 % of the material would be implemented onto the substrate, while the other remaining 95-98 % is flung off into the coating bowl and disposed of.57,62-63 No matter what the cost of the material is, the utilization ratio is so low that the improvement of material utilization is just around the corner.

1.1.3.3 Spray-assisted LbL assembly

Following the advances of the dip coating and the spin-assisted LbL assembly, spray- assisted LbL assembly has gradually caught our attention after the year 2000 as Schlenoff and co-workers reported on the spray-assisted coating instead of dipping mode.3,64 What's more, five years later, the Strasbourg group (the groups of Decher, Voegel and Schaaf) further affirmed the observation from Schlenoff's and explored further research.65 The manner of spray-assisted for both the planar films or the fabrication of polymeric particles has forwarded the development of the LbL field. Over recent years, papers about using the spray-assisted method have witnessed a strong growth, thus this technique can be considered a meaningful approach towards the wide application of LbL self-assembly technology.3

The schematic diagram of the spray-assisted LbL assembly to fabricate multilayer films is illustrated in Figure 2.c. Solutions are contained in the air pump spray cans which are pressurized for insuring the spraying rate in a successive condition during the entire

LbL assembly progress.3 As illuminated by the graph, the spraying solution is in a perpendicular situation in relation to the receiving substrates, which were vertically fixed

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by an instrument. So, under this circumstance, after the solution droplets merged on the substrates into a fully-wet film due to the gravity, the excessive solution is draining on the surface, which would be helpful to remove the excessive material as to form a smooth and homogeneous layer.50,66 So the spray-assisted LbL assembly contains certain steps

(use polyelecrolyte as an example): In the initial step, the substrates would be sprayed with the polyelectrolyte solution, then draining of the polyelectrolyte solution as well as surface adsorption of the polyelectrolyte molecules by the substrates would take place.

After the drainage, DI water is sprayed onto the substrate in order to remove the physically adsorbed polyelectrolyte molecules. Then the other oppositely charged polyelectrolyte solution is sprayed onto the surface again. Finally, another draining of DI water is operated.13 Alternate spraying of polycation and polyanion creates the polyelectrolyte multilayer films.

The film morphology can be controlled by certain parameters, for instance, solution viscosity, surface tension, air pressure (gas flow amount and rate), solvent properties (evaporation rate), fluid density as well as the design of the nozzle. Of which, a slight change would result in different consequences.67 Moreover, the film qualities are decided by the surface properties, wetting behavior, spraying speed, distance between the nozzle and the substrates, the geometry of the gun tip, and the number of sprayed layers.

Surface temperature as well as the kinetic impact of the droplets both play an important role in influencing the spreading state of the droplets.13,50 By the way, the first automatically, computer controlled, spray-assisted LbL assembly of the above approach has been developed by the Hammond group.3,13

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Advantages for the spray-assisted assembly are various. In contrast to the time- consuming dip coating process, it would be a popular application to be applied into the industrial fields as only several seconds applying in each step could reach the requirements.68-69 Moreover, spray-assisted assembly could avoid cross-contamination, which could always happen during the process of the dip coating when the substrates transfer from one recipient to another. Also, the spray-assisted assembly could be carried out on large substrates in a continuous manufacturing mode, during which the dipping mode could not be implemented.13

Limitations are also obvious for this method as large amounts of prepared solutions would be wasted, so more material-saving LbL assembly techniques are still under development.

1.1.3.4 Other film-making methods

Many other methods can be used to construct films too, such as drop casting and the doctor blade.69-70 Figure 3 demonstrates the simple steps for how the drop casting obtains the film. In the first place, a drop of solution is applied onto the substrate surface and the spontaneous solvent continues to evaporate. Here, no waste of materials would be a great benefit for using this method, but limitations still exist as the droplets are too small to cover large areas and the thickness would be difficult to control.71-72 What's more, the properties of the solvent would seriously influence the surface of the substrate since the evaporation rate would be different due to different solvents. To solve the problem, heating the substrate could adjust the evaporation rate as well as improve the film morphology, more or less.73

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Figure 3. Schematics show the fabrication process of the drop casting including dropping

of solution and spontaneous solvent evaporation.

Figure 4. Procedures for describing the doctor blade method to create films. The first step:

the solution is dropped onto glass substrate; the second step: the blade spreads the solution over the substrate; the third step: the obtained film is dried in the air. Copyright

2012 Advances in natural science: Nanoscience and Nanotechnology.70

The doctor blade method, derived from screen printing, is another method commonly used in the LbL assembled solar cells.74 Figure 4 shows an overall process of fabricating one-bilayer film on the substrate.70 Through using this method, more uniform

15 films could be achieved for the reason that the spreading of the solution is obtained through a moving blade onto a stationary substrate.74 And other techniques such as metering rod, chemical bath, slot-casting, and so on are all in different research or fields to fabricate films (see Figure 5).69

Figure 5. Depiction of various solution deposition methods in order to make films.

Copyright 2011 The Royal Society of Chemistry.69

A new method, spin-spray-assisted LbL assembly, combines the methods of spin- assisted and spray-assisted LbL assembly. Multilayer films could be achieved by directly spraying the two oppositely charged solutions in turn on high speed, rotating substrates, and in between the rinsing step is performed. The spin-spray-assisted LbL assembly tremendously accelerates the rate of creating films as well as reduces the demand of the materials used.3,66

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1.2 Polyelectrolyte

Polyelectrolytes are one of the most common materials used in the research in the LbL assembly field.75 Polyelectrolyte is defined as following:

"[...] a linear macromolecular chains bearing a large number (of the order of the degree of the polymerization) of charged or chargeable groups when dissolved in a suitable polar solvent, generally water."76

1.2.1 Basic concepts about polyelectrolyte

Here only a polyelectrolyte bearing the charges of the same sign (either positive or negative) is taken into account, thus the solution would contain only a single piece of polyelectrolyte with unknown polydispersity and one species of counter-ion. The counter-ion is defined as small ions with the charge of the sign opposite to that of the macromolecular charge, which should be in balance as being exposed to the electrolyte solution in order to reach the condition of electroneutrality.76-77 Consequently, the solution system may contain one species of polyelectrolyte and a counter-ion, unless otherwise specified, the counter-ion is assumed not to interact chemically with the polyelectrolyte.76

Usually, long chain polymers are always in a tangled state as shown in Figure

6A.78 But for the polyelectrolyte which bears plenty of identical signs of ions at the side group along the long chain would repel each other due to the theory "like charges repel each other, unlike charges attract each other", so that the chain stretches out, shown in

Figure 6.B.76

17

A B

Figure 6. A) The structure of a single polymer and B) a single polyelectrolyte (the whole

chain is positively charged).

Due to the state of the polyelectrolyte in solution, the solution would be very viscous. Considering when the polyelectrolyte chain stretches out, it would occupy more space as to be more spatial in resisting the flow of the solvent molecules surrounding it.

That is the reason why the solution will be thick and syrupy. And also there is a way to stop this phenomenon from taking place, supposing that solution is made by mixing a certain amount of single polyelectrolyte in water and then pouring a certain concentration

41,79-80 of the salt (NaCl, AgCl, CuCl2, ZnCl2, FeCl2, FeCl3) into the system. Salt NaCl is used as an example, it could be divided into two parts, ions Na+ and Cl-. Under this circumstance, the negatively charged polyelectrolyte, such as Poly(acrylic acid) (PAA) or

Poly(allylamine hydrochloride) (PAH), the cation charges Na+ will get in between the anion charged polyelectrolyte chains to attract more negative charges. The chain wouldn't be extended but instead collapse back into the random coil condition.81-82 That is also the reason why adding salt into the polyelectrolyte solutions could make the chains more

18 coiled and attract more oppositely charged polyelectrolytes or particles, etc.79,83

(schematic diagram is represented in Figure 7)

Figure 7. Polymer chain of polyelectrolyte in solution after addition of the salt NaCl.

1.2.2 Commonly used polyelectrolytes

Polyelectrolytes have been used in the LbL assembly field for a long time, several examples of polyelectrolytes found in nature are proteins and lipids, DNA, and RNA.

RNA and DNA are negatively charged anions when dissolved in solution.84 Proteins would dissociate in solution because of the polar groups, which can help form an anion or cation solution.5 Charges along the chain of the above natural materials make them suitable for making LbL films. Several kinds of synthetic polyelectrolytes used in the

LbL field to create multilayer films were shown in Figure 8.5 What's more, polyelectrolytes can be divided into two kinds, one is the strong polyelectrolyte which is wholly charged along the chain in the solution, and typical examples are PolyDADMAC and PSSA. Another is the weak polyelectrolyte for which chains are partly charged in the

19

solution like PAA and PEI and more sensitive to pH, which means that changing pH would change the properties of the polyelectrolyte at the same time.5,86-87

Figure 8. A small list of polyions already used for creating multilayer films. The words

below each polyelectrolyte structures are the abbreviations.

(PolyDADMAC: Poly(diallyldimethylammonium chloride), PAH:

Poly(allylamine hydrochloride), PEI: Polyethyleneimine, PAAm: Poly(acrylamide),

NaPSS: Poly(styrene sulfonate acid), PSSA: Poly(sodium styrene sulfonate), PAA:

Poly(acrylic acid), PVS: Poly(vinyl sulfonate), PMA: Poly(methacrylic acid) )

20

CHAPTER II

MOTIVATION AND PROBLEM STATEMENT

2.1 Motivation

The foamed materials are found virtually everywhere in our modern world and are used in a wide variety of applications such as the disposable fast-food packaging, the cushioning of furniture and insulation materials.87

Due to the superiority of the LbL assembly technique in film fabrication, I am working on applying this method into directly making foamed films. According to the ideas from both my advisors, Dr. Nicole Zachair and Dr. Kevin Cavicchi, the porous structure should be all through the film from the edge to the center, or from the bottom to the top, and the foamed films were assembled directly.

In recent decades, few papers told about the production of the film with porous structures. In 2000, the Rubner's group at MIT successfully made nanoporous and microporous multilayer films by an indirect route by immersing PAH/PAA films with pH

3.5/7.5 into the acidic solution (pH 2.4).10 The final porous films may undergo another reorganization in the neutral, leading to an irreversible transformation of the film morphology (see Figure 9.A and B) as plenty of pores would appear.10,88

Zhang and his coworkers worked on the hydrogen-bonding-directed LbL films,

PAA/poly(4-vinylpiridine) (PAA/P4VP) multilayers. The microporous films were fabricated after the treatment in basic aqueous solution at different temperatures.89-90

21 Figure 9. The AFM images of the morphology of a 21-bilayer PAH/PAA with pH 3.5/7.5

polyelectrolyte multilayer film before (A) and after (B) exposure in a transition bath

within pH 2.50. Copyright 2000, Langmuir.10

Similarly, in 2008, the Hammond group published a paper "Nano and

Microporous Layer-by-Layer Assemblies Containing Linear Poly(ethylenimine) and

Poly(acrylic acid)", which provided more detailed information on making porous films with LPEI and PAA.9 The LbL assembled films were exposed to the acid treatment as to get porous structures (see Figure 10.A). However, SEM images showed that the nano- or even micro-porous structures not only appear on the surface of the film but also through the cross-sectional area (see Figure 10.B).

In 2013, our group used BPEI and PAA as the original materials to get LbL films, and porous structures could be obtained after the acid treatment.8 What's more, in a patent published in 2012 by the Hammond group, porous films from LbL assembly could be obtained when PolyDADMAC and clay were utilized. However, the porosity of the film

22

on the surface, which was highlighted by AFM, was blurred, while pores in the cross- sectional area of the film could be clearly observed.91

A B

Figure 10. A: The porous structure films were made through a two-step procedure. In the

first place, a certain amount of LbL films with the material LPEI and PAA were

fabricated at the surface of the silica substrates. In the second place, the film was

immersed into the acidic solution. B: The SEM images of both the morphology and

cross-sectional area of the films which were exposed into different pH acidic solutions.

Copyright 2008 Macromolecules.9

Looking back on the results and experimental processes given in those pioneering works, several of these works cannot rule out the utilization of the acid treatment which would make the procedures complicated. Moreover, most of them are indirect ways to get porous films, and some of the research could not prove that the porous structure is all through the films (some of them can only prove the porous structure either on the

23

morphology or through the cross-sectional area). All of the above situations gave us the idea of making films with fully-pore structure in a direct way.

A

B

Figure 11. A shows cross-sectional (left two) and surface (right one) SEM images of multilayer films of (PDAC/LAP) with different number of bilayer films. B is a series of images illustrating surface topography and roughness of multilayer films of (PDAC/LAP) with 100 bilayers. Copyright 2013 US pattern number 20130341277 A1, porous films.91

By mixing acid solutions with a solution which bears the bicarbonate functional groups, will lead to the reaction where carbon dioxide gas will be generated. With the help of this carbon dioxide (CO2) gas, porous structured films have the potential to be fabricated directly by LbL method. Therefore, in my research, two strong polyelectrolytes

24

Poly(diallyldimethylammonium chloride) (PolyDADMAC) and Poly(styrene sulfonate acid) (PSSA) were used. A slight modification of the cation solution, operated as CO2, would be taken through the cation solution system (PolyDADMAC solution). The reason why the weak electrolyte is not used is that they would be more sensitive to the slight change of pH due to the carbonic acid formed after CO2 was bubbled into the solution. In some cases, laponite91 (a kind of clay and slightly negatively charged, detailed information will be introduced in Charpter III) and brij-76 (a kind of surfactant, detailed information will be introduced in Charpter III) would be added into the solution due to their property as supposed to change the property of the film.

2. 2 Problem statement

This project presents the procedures of making foaming films and some necessary measurements of the foaming structures. In case of the conditions of the laboratory and the equipment, some inevitable factors were contained such as the humidity and the air flow, which would influence the drying rate and make it hard to control. In addition, because of the limits of the experiment conditions, the deposition during the dipping mode process is the only method used in this research, and other experiments establishment are still under exploration.

25

CHAPTER III

EXPERIMENT SECTIONS

3.1 Material

Three polyelectrolytes, poly(diallyldimethylammonium chloride) (PolyDADMAC)

(average molecular weight 100,000-200,000, 20 wt. % in water), poly(4-styrenesulfonate acid) (PSSA) (average molecular weight ~75,000, 18 wt. % in water), and poly(sodium

4-styrenesufonate) (PSS) (average molecular weight ~70,000) were purchased from the company Aldrich Chemistry. The salts sodium bicarbonate (NaHCO3), sodium chloride

(NaCl) and the base sodium hydroxide (NaOH) were all purchased from the Sigma-

Aldrich. Hydrogen peroxide 35% W/W(H2O2) and sulfuric acid (H2SO4) were obtained from BDH Chemicals and Sigma-Aldrich, respectively. Laponite RDS and the surfactant brij-76 were purchased from Southern Clay Products, INC and Aldrich, respectively. All the materials were used as received without any further purification. Deionized water (DI) which has 18.2 MΩ resistivity was obtained from a Milli-Q filtration system and used in all the experiments. The substrate, glass micro slides (25*75*1 mm) were received from

VWR North American Cat No. and the silica wafer (one side polished, 0-100 ohm-cm resistivity, 525+/- um thickness, 100mm diameter) was purchased from the University wafer. Single Edge Industrial Razor Blades (Surgical Carbon Steel) were purchased from

VWR.

26

A B C

Figure 12. The chemical structure of different polyelectrolytes.

A :Poly(diallyldimethylammonium chloride) (PolyDADMAC).

B:Poly(diallyldimethylammonium hydroxide).

- C:Poly(diallyldimethylammonium bicarbonate) ( PolyDADMA-HCO3 ).

D E

Figure 13. D: Poly(4-styrene sulfonate acid) (PSSA).

E:Poly(sodium styrene sulfonate) (PSS)

27

3.2 Treatment of the substrates

The substrates, glass micro slides and the silica wafers, were firstly immersed into the freshly prepared and slightly boiled piranha solution (the mixture of Sulfuric acid and

Hydrogen Peroxide 35% W/W with the volume ratio 7:3) for about one and a half hours.

After that, all the substrates were rinsed with an excessive amount of DI water until neutral. After the cleaning, the surface of the substrates would be hydrophilic. The substrates were then submerged into the DI water in a glass bottle which would be put into the Ultrasonics Cleaner (De-Gas, 117v, Model: B2500A-DTH, Input: 110-120V

50/60Hz 210W 1.8A, Output: 85W 42KHz+/-6%, from VWR North American Cat No.) for about 40 min. The substrates were firstly dried with the flowing Nitrogen gas (N2) before using, and then were all placed inside the equipment Plasma cleaner PDC-32G from the company Harrick Plasma to oxidize for 5 min in order to consign the surface with a better adhesion property.

(Caution: The Piranha solution should be handled under careful conditions as it will release a huge amount of heat and will be extremely caustic, and it also can react violently with organic materials like the clothing and the skin.)

3.3 Solution preparation

PolyDADMAC (20mmol/L, 30mmol/L, 50mmol/L with respect to its repeat unit), PSSA

(20mmol/L, 40mmol/L, 60mmol/L, 100mmol/L with respect to its repeat unit) and PSS

(20mmol/L with respect to its repeat unit) were prepared by directly adding

PolyDADMAC, PSSA and PSS into DI water, respectively.

28

Figure 14. The set-up of the experiment. The name of each letter is as follows. A: The rubber hose connected to the liquid CO2 cylinder. B: The long needle which let the CO2 into the solution. C: The syringe needles. D: The rubber plug. E: The round-bottom glass

flask. F: The solution system. G: The stir bar. H: The CO2 bubbles.

- The polyelectrolyte PolyDADMA-HCO3 was obtained by slight modification of the solution PolyDADMAC by the following procedures. First, the PolyDADMAC solution was transferred into the round-bottom flask and stirred for about an hour, then

1.5 times with respect to the molar of PolyDADMAC of NaOH was added into the solution system and stirred for another hour. After that, NaHCO3 with the aim concentration 0.1mol/L was added in the solution and the solution was then stirred until all NaHCO3 particles were dissolved. Finally, the gas CO2 was introduced into the round- bottom glass flask by a syringe needle, which was embedded in the rubber plug, the syringe needle connected the outside environment and the pressure system inside the

29

flask for the sake of keeping the pressure balance inside and outside the round-bottom glass flask. Specifically, the gas CO2 went into the solution system for at least 40 min through a long needle. The tip of this needle should be immersed below the solution level

(see Figure 14.).

Figure 15. (a) Empirical formula for a Laponite particle. (b) Crystallographic structural formula of Laponite depicted in part. (c) Characteristic shape and dimensions of a single

Laponite platelet.92

Laponite RDS was firstly dissolved into the DI water and stirred for about four hours and then centrifuged for about an hour. At the end, the laponite solution was mixed with the anion solution (PSSA) for the sake of making 0.05wt. %, 0.1wt. %, 0.2wt. %,

0.5wt. % Laponite in PSSA anion solution. The reason why only Laponite RDS was added into the anion solution is that it is negatively charged in nature as the structure and chemical formula are in Figure 15.

30

The surfactant brij-76 (C18H37(OCH2CH2)10OH (see Figure 16.) was dissolved in both the cation and the anion solution and stirred for about six hours until all the solid disappeared. As the critical micelle concentration (CMC) of the brij-76 is 0.02mmol/L, here I use 0.01mmol/L brij-76 in my work.93

Figure 16. The chemical structure of Brij-76.

3.4 Layer-by-Layer assembly

The assembly of the PEMs was operated orderly at room temperature through using a

StratoSequence VI dipper (NanoStrata Inc., U.S.A.). As the clean substrate is negatively charged, generally, the substrates were firstly exposed into the polycation solution for about 5 min, followed by rinsing in a DI water bath only one time for 1 min.

Subsequently, these substrates were immersed into the polyanion solution for another 5 min and washed with DI water. Heretofore, one-bilayer film was assembled on the substrate. These series of procedures will be implemented in cycles 20 times in order to make 20-bilayer films.(see Figure 17)

31

Figure 17. Schematic representation of the procedures used to fabricate polyelectrolyte

multilayer films by the mode of dipping LbL assembly.

For different kinds of films with different components of solution are utilized, the details are all shown in Table 1. For the convenience to present each film, different numbers are used. The number 12 and 13 films are the control samples. All of the films have identical 20 bilayers, and all of them are made on the surface of the glass micro slides. And different amounts number of bilayer film, 1,2,3,4,5,6 bilayers, are carried out on the clean silica wafer surface.

32

Table 1. The components of different solutions to make different films.

3.5 Machine used in the experiment and sample preparations

Scanning Electron Microscope (SEM) (Model JEOL-7401 Japan Electron Optics

Laboratory (JEOL), Accelerating voltage of 0.5-35 kV, Magnification range of 15-

600,000) was used to identify the morphology of the surface and the cross-sectional area of the 20-bilayer films, all the samples were sputter coated with gold (SPI) to increase the resolution under SEM. The morphology of different number of bilayer films (1,2,3,4,5,6 bilayer films) were examined by atomic force microscopy (AFM) utilizing a Dimension

33

V (Veeco). Film roughness and thickness were measured by a Veeco Dektak 150 profilmeter with a 0.18 mm/s scan rate. Thermogravimetric Analyzer (TGA) (TA

Instruments Q50) was used to determine the components of each film together with the degradable temperature of each components.

For SEM, there are some differences between making samples for the morphology and the cross-sectional areas. As for observing the morphology, glazier's diamond was used to cut the glass slides into several small pieces and then the double conductive carbon tape was used to adhere the small glass piece into the stub surface. For observing the cross-sectional area, as a slight force would change the structure of the cross-sectional area, the cold liquid nitrogen was used to break the glass slides in order to protect the structure as the film was freezing under so low a temperature. The broken small piece of glass was adhered to the half-cut stub for the convenience of observing the cross-sectional areas.

Compared with the sample preparation for SEM, it would be easy to prepare samples for the profilometer, a blade was used to scratch three lines on the film within 7-

9 mm in the middle of the films. For each film, I measured 8 times. The thickness of the films could be calculated as the average of these 8 results.

Generally, preparing samples for AFM would to some extent be the same as that for SEM, except the two following aspects, the substrate was the silica wafer instead of the glass slides for the reason that a silica wafer is easier to cut, and double sided adhesive tape was used instead of the double conductive carbon tape.

For the TGA measurement, blades were used to scratch the film from the substrates as to measure the degradable temperature of the different composition of these

34

films in order to get the content together with the proportion and degradable temperature of my production.

35

CHAPTER IV

RESULTS AND DISSCUSSIONS

4.1 Compare the structure of the different films

Figure 18. Different kinds of films made. The numbers are from 1 to 11 from left to right

in relation to the number in table 1.

Figure 18. shows the freshly prepared films with number 1 to 11 from the left to right and all of these films have 20 bilayers (Given the number to the films in the figure, as what I did). It is obvious that the macroscopic morphology of these films are different. It is clear that the white accumulation stemmed from the gravity that made the solution go down and due to the function of the surface tension, they hold near the bottom of each film.

In Figure 18, the remaining films 2 to 11 are much whiter than film 1. From film

3 to film 6, the differences were concentration and proportion (film 3 and 4) of both the anion and cation solutions as to study the influences of proportion and the concentration on the films morphology as well as on thickness and roughness. As we hope the film

36 would be in a foamed state in case of the reaction between bicarbonate groups and the hydrogen ion. In film 1, NaHCO3 was added into both the cation and the anion solutions as this salt would make the polyelectrolyte chains more colloidal to attract more oppositely charged materials.

But here, we could not rule out the possibility the acid in the anion solution might have been fully reacted with the salt NaHCO3 while the concentration of NaHCO3 was much higher than that of the PSSA anion solution (the concentration of NaHCO3 was

0.1M and that for the PSSA ranged from 20mM to 100mM according to different film requirements). In short, there is a potential that the acid (hydrogen ion) has been totally reacted before doing LbL assembly, and no more acid existed in the solution. Another point worth mentioning is that it is widely known that the H2CO3 is not stable, a slight shake of the flask would make bubbles come out. Here, adding NaHCO3 into the polycation solution not only can make chains more colloidal but also can make a solution system bearing more bicarbonate groups. Considering both of the above two reasons, films (films 2 to 6) without adding NaHCO3 into the PSSA solution were fabricated.

As in the chapter ahead, certain weight percentage of laponite into the anion solution may increase the properties of the film and a patent has shown that laponite could also be used as a material to get porous films through the utilization of the LbL assembly method. So, compared with the film 1, film 7 and 8 were fabricated, where laponite was added into the anion solution with or without NaHCO3, and the weight percentage for laponite usually ranges between 0.01% and 0.5%. And the variation factor for film 7, 9-11 is the weight percentage for laponite as to investigate the influence from different proportions of laponite (0.05%, 0.1%, 0.2%, 0.5%) on porous films.

37

Two control films were made during these experiments as they have no bicarbonate groups along both the polycation (PolyDADMAC) and polyanion (SPS). The first control film was made by the common strong polyelectrolytes PolyDADMAC and

SPS by an alternate dipping process, and the salt used in this experiment was NaCl instead of NaHCO3. Another control sample has the same procedures except for adding

0.05% laponite into its anion solution. We suppose that there should be no porous structure on the control samples' surface, and the two control films were compared with the above 11 films in order to make sure of the pores.

4.2 Analyzing SEM images of these different films

SEM is a useful method to observe the surface and inward structure of the films to further insure the film structures. And as the 12 different films were divided into 4 different categories above in 4.1 (category 1: film 1,2; category 2: film 3-6; catrgory 3: film 7-11; category 4: film 12-13). SEM images of the films were discussed first to prove whether these film had porous structures or not.

The SEM images of two control samples were shown in figure 19. For film 12, towards the left two morphology images, a small amount of accumulated particles were identified on the surface, which is more distinct in the second image (top center). The

SEM images imply that the cross-sectional area is in a compact condition, which is evidence that no porous structure is formed at the intersecting surface. The images of

Film 13 also indicate no pores at the intersecting surface.

38

Figure 19. The SEM images of the morphology and cross-sectional area of the control

films number 12 and 13 in relation to the film number in table 1.

When it comes to category 1 series of films shown in figure 20, SEM images of film 1 illuminates some hundreds of nanometer particles at the surface. According from the cross-sectional image, the film is too thin to be observed as the magnification is up to

*10,000. From the SEM images of film 2, no porous structures but some complex, which are connected to each other, is shown on the surface. Furthermore, the image for the cross-sectional area is also blurry under high SEM magnification mode (too thin to be caught by the SEM equipment).

39

Figure 20. The SEM images of the morphology and cross-sectional area of the film 1 and

2 in relation to the film number in table 1.

SEM images for film 3 to film 6 changes with an increasing tendency of concentrations of the polyelectrolyte in both the cation and anion solutions (Figure 21).

From the morphology images of the first two columns for the four films, it could be easily determined that they all have porous structures, and the film surface looks very shaggy. As they all have porous structures at the surface, I will analyze images of film 3 and 4 in detail. The SEM images for film 3 and 4 with the magnification *1000 and *500, respectively, show more obvious pores and the average size of the pores is around 15µm.

By increasing the magnification, many small pores between the big pores could be observed, too. Let us turn to the cross-sectional area, no doubt that films 3-6 are thicker and at the same time no more compact in contrast to film 1 and 2. Porous structures are apparent through the cross-sectional area for film 4, 5 and 6, distinctively. In general, category 2 series of films have porous structures all through the film.

40

Figure 21. The SEM images of the morphology and cross-sectional area of the film 3-6 in

relation to the film number in table 1.

SEM images of the third series of films were shown in Figure 22. Except for film

8, other films' anion solutions all have 0.1mol/L NaHCO3 inside. When the weight percentage of the laponite reaches 0.1%, the film surface became more nonuniform due to the laponite though there are many tiny pores at the film surface. According to the images, we notice a lot of cracks, which result from the force when preparing samples.

Furthermore, increasing the content of the laponite would change the macroscopic morphology, as shown in Figure 18. For the last 4 films, the film color becomes more transparent and more compact owing to the growth of the laponite content (other factors

41

are the same). Here, SEM images for film 10 and 11 are not given, as the increasing weight percentage of laponite would make films nonuniform. Therefore, it would be meaningless to get more SEM images for these films. Let's turn back to the point. From images of cross-sectional areas for film 9, some porous structures are obvious, but in relation to the former series of films (film 3-6), the cross-sectional area is more compact.

So this experiment proved that laponite can help slightly increase pores in both the morphology and the cross-sectional area.

Figure 22. The SEM images of the morphology and cross-sectional area of the film 7 to 9

in relation to the film number in table 1.

For the convenience of comparing these different series of films, SEM images of five typical films were selected in figure 23. It is clear and obvious that film 3 has more

42

porous structures both on the surface and through the cross-sectional area. Film 7 and 8 which contain 0.05 wt% laponite in the anion solution also possess small pores though the morphology is nonuniform.

Figure 23. Gather the former four typical different films' SEM images (morphology and

cross-sectional area) together for the convenience of observation. In this figure, "+"

means cation solution and "-" means anion solution. From the top to the bottom, the

number of the film is in the following order 1, 2, 3, 7, 8.

43

4.3 AFM images of 1-6 bilayers of 4 typical films

SEM was used to study the morphology of the 20-bilayer films, here we use AFM to get

1-6 bilayer for different films as to do investigations on how the porous structures were formed. 4 typical kinds of films were studied and each film 1-6 bilayers on the silica wafer was characterized. Figures 24 to 27 represent the four different films with three different sizes 10µm, 2 µm, and 1 µm AFM images.

Noticing that, for each figure, from the top to the bottom, the number of the bilayer is in the following order 1, 2, 3, 4, 5, 6 bilayers. And from the left to the right image, the size of the area is in decreasing tendency 10 µm, 2 µm, 1 µm. For each image, the scale bar of each image is in its right place.

Figure 24 and 25 give a clear process on when the porous structure became clear.

From the 1-6 bilayer films, the morphology change is obvious especially from 2-bilayer film to 3-bilayer film. The image of 3-bilayer film demonstrates many pores at the surface and the pores are surrounded by successive complex. This phenomenon is attributed to the fact that pores on the surface could be obtained after the reactions between the acid and the bicarbonate groups. Furthermore, porous structures are more distinct when the number of bilayers reaches 5 and 6. In addition, large and small pores both exist in the film surface which is consistent with the results from the SEM images.

Figure 26 shows the AFM images for film 8 of which 0.05wt. % laponite and

NaHCO3 were both added into the anion solution. No pores appeared and there may be some complex and particles at the surface, that may be due to the number of the bilayers is less. For the 10 µm of the 6-bilayer film, there are some crystals existing on the surface, which is due to the crystallization of NaHCO3.

44

AFM images for film 8 gave a different story. When only 0.05wt. % laponite was added into the anion solution, porous structures are distinct in 5-bilayer and 6-bilayer films. The results are also in agreement with that of SEM results.

45

Figure 24. AFM images of film 3 in table 1.

46

Figure 25. AFM images if film 4 in table 1.

47

Figure 26. AFM images of film 7 in table 1.

48

Figure 27. AFM images of film 8 in table 1.

49

4.4 Thickness and roughness measurements

Figure 28. The thickness and roughness measured by profilometer of the different 11

films. "The number of the film" is in relation to the number in table 1.

To some extent, the AFM and SEM images qualitatively inform us the structures of both the morphology and the cross-sectional area of the films. Other measurements for quantitatively analyzing properties of the film are also significant, such as the approximate content of each component in the films, and the roughness and thickness of different films.

50

In the case of typical film structures, the thickness and roughness was measured by profilometer (Figure 28). In addition, each film was measured 8 times in the substrates' middle place to obtain the average results of the thickness and roughness .

In contrast with the non-porous film with a smooth and uniform surface, the surface condition for foamed films would be extremely rough. Clearly from Figure 28, even when the amount of bilayer was up to 20, both the thickness and roughness for film

1 and 2 are below 1 µm, which is the least among the 11 different films. Without adding

NaHCO3 into the anion solution, it is found that the thickness and roughness for films 3-6 are larger than any other films. The increasing tendency for the thickness is not under the norm that increasing the concentration of the solution would increase the thickness. That is due to the different environmental conditions resulting from the lab such as the temperature, the humidity and rate of air flow together with some inevitable factors.

Let's transfer our attention on the data on film 7-11, also the influence of whether adding NaHCO3 to the anion solution or not was distinct between film 7 and 8. However, this time, film 8 with only laponite in the anion solution has a greater thickness than film

7 whose anion solution contains both laponite and NaHCO3. For films 9-11, the average thickness and roughness is nearly half of that compared with films 3-6.

Above all, Figure 28 shows that adding NaHCO3 into the anion solution would to some degree influence not only the final porous structure through the film but also the thickness and roughness of the film.

51

4.5 TGA data illustrates the content of the film

As proven by the AFM and SEM images, films 3-6 contain more foaming structures than other films, and films with laponite came to the second place. However, it would be more reliable to prove the existence of laponite in the composition of the films. Therefore, the

TGA of film 7 was measured.

TGA is a typical method to measure the percentage of weight loss due to the increasing temperature. During this period, the degradable temperature of different materials as well as the proportion for each component could be obtained. The experiment would be surrounded in a typical ambience such as in nitrogen, air, and so on.

For the test, the speed for increasing the temperature is 10 oC/ min in the nitrogen ambience, and the temperature range is between the room temperature to 800 oC. TGA data is shown in Figure 29.

Figure 29 demonstrated the TGA data of a 20-bilayer film under a flow of dry nitrogen gas. The sample was stored at room temperature for about 3 hours. According to

TGA, the beginning drop within the range 0oC to 150oC is due to the evaporation of the moisture in the sample as the boiling temperature for water is 100oC. After that, there are two clear drops in weight, the first one occurs between the temperature 330oC and 380oC and another occurs in the range 460oC to 530oC. When the temperature is up to 550oC, 8% weight of residue was still existing. This phenomenon may be attributed to the adding of the laponite which is stable in high temperatures.94-95

52

Figure 29. TGA data of film 7 in table 1 under the ambience of nitrogen gas. Region 1 represented the loss of the moisture in the sample. Region 2 and 3 illustrated two steps of

decomposition, and region 4 is the remaining material which was non-degradable.

We could notice that the TGA data reveals other interesting aspects, the 20- bilayer film is in a thermally stable state up to 300 °C and the decomposition would occur in a two-step of approximately equal weight loss.94 Consequently, all the possible components (water, polyelectrolyte and laponite) in this 20-bilayer film have now been proven and quantitatively treated.

53

CHAPTER V

CONCLUSION

From all the above data from this experiment, we could come to the following conclusions: when no salt NaHCO3 and no laponite are added into the anion solution, this will lead to the more porous structure through all the films wherever, from the centrer to the edge or from the top to the bottom. The morphology and cross-sectional area have been demonstrated by AFM and SEM.

There is no doubt that the foaming films' surface would be extremely rough and the thickness of the film would be larger due to the fluffy structures inside. Shown by the thickness and roughness data of different films measured by the profilometer, we also found that films number 3-6 have the largest thickness and roughness.

TGA data for film 7 showed degradation steps indicating the multiple components including approximately 10% moisture, 8% laponite and 82% polyeleclytes.

Above all, the foaming films could be made by this direct LbL assembly method by the mixture of a modified polycation solution, which contains polyelectrolyte

- PolyDADMA-HCO3 and the salt NaHCO3 and the polyanion solution including only

PSSA, which could lead to pores appearing after reactions between a hydrogen ion and the bicarbonate groups. This research is still under way as to further advance and develop both the physical and the chemical properties of the foamed films. Also, works on reducing the influence from the environment are still underway.

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