Lecture 8 Flashcards
How do we synthesise linear Polydimethylsiloxane (PDMS)? How can we branch the polymer or terminate it?
One way to make PDMS is by the hydrolysis of its monomer dimethylsiloxane, as shown in digital notes. This reaction would lead to linear PDMS since each molecule can only react twice. By adding monomers like [Si(CH3)Cl3] or [SiCl4] one can promote branching as the additional chlorines can lead to points where more than two monomers can attach.
By contrast, adding [Si(CH3)3Cl] would serve to limit the growth of the polymer chains since this additional monomer can only react with one monomer thus functioning as a chain termination agent
What are PDMS useful properties?
PDMS has several useful properties: It is transparent, it is chemically inert, it can be transformed into silica by calcination, it is heat resistant, and it has widely tunable mechanical properties.
In short, we could say that PDMS displays some of the great properties of silica, with the added advantage of widely controllable mechanical characteristics
How are the mechanical properties of PDMS controlled?
The mechanical properties can be controlled by the length of the polymer chains (their molecular weight, MW), their branching, their crosslinking density, and the use of fillers.
What is the main difference between the surface of PDMS and the surface of silica?
The main difference between the surface of PDMS and the surface of Silica is the extremely low silanol density on the PDMS surface and the presence of organic methyl (-CH3) groups. Both factors imply a larger hydrophobicity, a lower surface charge, and a lower surface energy.
Given its low surface energy. PDMS generally has a low adhesion to most surfaces; this adhesion can be easily modified by exposure to plasma treatment which, just as for silica, temporarily increases the number of silanols on the surface.
What is soft lithography and why how does PDMS relate to it?
Soft lithography is a way of creating a surface pattern on a micro- or nanoscale which relies on self-assembly and template replication.
The tunable and adhesive properties of PDMS have been used to develop this method! The objective was to create a platform for surface patterning through chemistry, which could be cheap and reusable!
How do we prepare a PDMS stamp? IMP!
NOTE: the first step to creating the surface patterning is to prepare a PDMS stamp
Procedure:
The pattern is first designed into a master, which is usually made in silicon by the conventional lithography methods; this master will function as a template for the formation of our stamp. The silicon is first exposed to oxygen plasma and then made hydrophobic with molecules like trimethylsilyl chloride (CH3)3SiCl or perfluorooctyltrichlorosilane CF3(CF2)7SiCl3; A PROCESS CALLED SILANIZATION. the plasma will introduce silanol on the passivating silica layer on the surface of silicon. The silanization reacts most of the surface silanols, thereby reducing the surface reactivity and adhesion with PDMS, and increasing the accuracy of the template replication.
The PDMS pre-polymer is then poured over the master. This step is fairly delicate as you have to make sure not to form bubbles which might remain trapped in the viscous pre-polymer.
The PDMS then undergoes curing during which the pre-polymer molecules crosslink with each other forming the extended moleculear network which will imbue the final stamp with rubber-like properties.
The stamp can be successfully peeled off the master, providing a very accurate free-standing replica of the master and can be used in a multitude of ways to generate surface patterns.
What is the formal definition of silanization?
Silanization is the process of reacting surface silanols
What are the different procedures that use the PDMS stamp to generate surface patterns?
- Microcontact printing
- Micromolding
- Microebossing
- Microlithography
(All of the above are examples of soft lithography
What is microcontact printing and its procedure?
Microcontact printing is where a stamp transfers a specific ‘ink’ from one substrate to another employing the different adhesion characteristics of the substrates involved (see figure 4.1 in digital notes); the inks can be molecules, polymers, nanocrystals, nanowires, colloids, and whole thin films.
Procedure:
You initially deposit the ink on a substrate for which it has very low adhesion, for example, a silanized piece of silica, silicon or even PDMS itself. The stamp is then put in conformal contact with the ink, and if the adhesion of the ink towards the stamp is larger, the ink will transfer to the stamp in those protruding regions of the stamp designed in the master.
The next step is the printing step; the stamp, wet with ink, is put in contact with the desired substrate which, different from the first substrate, must have a strong adhesion for the ink. In this way, when the stamp is peeled off the desired substrate, the ink will remain on the substrate and not on the stamp.
(EXAMPLE) Sometimes it is necessary to use a trick to facilitate ink transfer, such as using a functional group that can react with the final substrate but not with the initial one; for example is the final substrate is gold you can choose to ‘dress’ your ink with a thiol group and to use as a first substrate a silica substrate: the thiol will not react with the silica or the PDMS stamp but will react promptly with the gold surface. In this way it is very easy to produce pattered self-assembled monolayers (SAMs) on metallic surfaces.
What is micromolding and its procedure?
It is a technique that uses a PDMS stamp as a template to create a negative replica of a chosen material.
Procedure:
The stamp is pressed against the desired substrate and the ink flows into channels, drawn by capillary forces; if the ink has a reasonable affinity for the substrate, placing a drop of the ink on one end of the channels will cause the ink to diffuse into the channels. Another requirement is of course that the ink must be fluid enough for it to diffuse into the channels of the stamp.
The ink is then consolidated, which can mean different things for different inks: Monomers can be polymerized, nanocrystals can be dried, and molecules, used to make a SAM on the substrate, can just flow through reacting as they pass.
The stamp is then peeled off leaving a patterned material behind.
What is microembossing?
A similar approach to micromolding (see Figure 4.1) in which the ink is first deposited on the desired substrate and then the stamp is strongly pressed against the substrate leading to an embossed pattern of ink on the substrate.
What is microlithography and its procedure?
Microlithography is one of the most sophisticated forms of patterning that can be performed with soft lithography. This technique is a combination of bottom-up and top-down techniques. This method is able to fashion material below 10 microns!!
Procedure:
The first step is to contact print an ink on a sacrificial layer which has been deposited onto the desired substrate. The stamp is peeled off leaving behind a sacrificial layer coated with a pattern of ink; the ink has here the tole of protecting the underlying sacrificial layer from etaching.
The whole structure is dipped into a wet etchant that will selectively etch the sacrificial layer only in the exposed regions in the case the sacrificial layer is SiO2 and the substrate is silicon you can use HF solution. This process leaves behind a pattern of posts from the sacrificial layer that will work as a template for the subsequent deposition of the desired material on the chosen substrate. Such depositions can be made via the Vapour phase (Like chemical vapour deposition, CVD) or liquid phase, depending on which is the most convenient and compatible with the survival of the template.
After the desired material is deposited the template is removed by first removing the ink with an appropriate solvent, and then removing the sacrificial layer with the appropriate etchant.
This process mimics standard photolithography in which the sacrificial layer is a photoresist, a material which can permanently change properties, like solubility, upon irradaition with light.
A final note about Soft lithography.
With soft lithography, you can pattern any substrate, even a curved one, since PDMS is a rubber and it can be made conformal with surfaces other than planar ones.
An important point before talking about Size
All the techniques that will be mentioned in this subchapter are ones that exploit the polymeric nature of PDMS (with it being fairly compressible and their porosity being small); meaning that they could not be applied on rigid stamps!
What are the soft reduction techniques in soft lithography?
- Reduction by compression
- Reduction by swelling
- Reducion by filler extraction
- Double printing
- Reactive spreading
- Overpressure contact printing
- V-shaped features
- Nanoskiving
What is the procedure for reduction by compression?
In the first step you wet your PDMS stamp with your chosen ink; you then apply the stamp to the surface and, in doing so, you strongly compress the stamp, leading to a lateral expansion of the protruding features and, as a consequence, of the printed pattern. After peeling off the stamp you can see that the distance between the printed regions is smaller than the distance between the protruding regions in the uncompressed stamp. The reduction in feature size is tracked in Figure 4.2 with the thin black lines
What is solvent-induced swelling? (procedure in the next flash card)
Solvent-induced swelling is a more elegant and controllable technique that is used to decrease the separation between the protruding features.
This is based on the phenomenon between the PDMS and certain solvents. Where PDMS is principally a single molecule: no part of it is chemically disconnected to any other. The main consequence of this is that polymer networks are not soluble in any solvent because the constituent atoms cannot disassociate from one another because every atom is covalently bound to the network. What the solvent can instead do is to penetrate and swell the polymer network! For the same reason you can compress a PDMS stamp - the vast amount of free volume in the network and the elasticity of the PDMS individual changes - the opposite can happen: The PDMS stamp can well in certain solvents. This happens for energetic reasons.
What is the procedure for solvent-induced swelling?
The technique is based on the following steps (Figure 4.2). The stamp is swollen in toluene (or any other good solvent for PDMS); the swollen sample is put in contact with the ink, which is transferred into the stamp; the stamp is then used to print the pattern on the desired substrate. Also, in this case, the lateral swelling of the protruding features leads to a reduction of the feature sizes in the printed pattern.
What is the reduction by filler extraction and its procedure?
A polymer network can be crosslinked in the presence of fillers (or additives), which are often used to fine-tune the properties of the final network; such fillers are 3D defects in the network as they are not covalently bound to the network.
Now that you have prepared a stamp filled with filler, you can always remove it by using a proper solvent like toluene, for example. Toluene will sell the PDMS stamp, as we just saw, and will remove the soluble components from it, including the filler. After having removed the toluene from the stamp, the resulting stamp will be smaller in size by a volume equal to the volume of filler you included. As shown in the scanning electron microscopy (SEM) micrographs in Figure 4.2, this allows one to strongly reduce the feature size of the printed pattern.
Note: In the example we show in Figure 4.2 the filler was PDMS oligomers. The reason why they used PDMS oligomers and not oligomers of another polymer is that they wanted to ensure thorough mixing of the filler into the network without PHASE SEPARATION. always consider that when solving problems!
What is double printing and its procedure?
Double printing is another way to get size reduction, even though in a hardly controllable way, is to print the ink twice. As shown in Figure 4.2, you can print twice, reducing the feature size. In the SEM micrograph is shown an example of the kind of pattern, called Moire patterns, you can make by printing twice a set of stripes at slightly different angles!
What is reactive spreading and its procedure?
It is a method based on ink diffusion. In the case in which your ink has low viscosity, you can let the stamp sit on the substrate long enough for the ink to diffuse laterally, thus reducing the feature size, as shown in Figure 4.2. The higher the ink viscosity the more slowly the ink diffuses and spreads on the surface. As you can see in the SEM micrographs below, by tracking the time of diffusion you can reliably control the feature size.
What is overpressure contact printing and its procedure?
It is an adaptation of the “Reduction of compression” printing. In this case, you completely wet the PDMS stamp with ink so that also the recessed regions are coated with ink. When the stamp is then used for printing, you can apply enough pressure to bring the recessed regions of the stamp in contact with the substrate (Figure 4.2). By this method, you can halve the feature size of your original stamp
What is ‘V-shaped features’ and its procedure?
This technique relies on the different ways to prepare the master. Instead of using flat-bottomed features, you can etch the silicon anisotropically to yield V-groovers.
Note that the anisotropic etching of silicon is founded upon the distinct etch rates of different crystallographic planes of single-crystal silicon.
If you use such a V-grover master to prepare a PDMS stamp, the protruding region of PDMS corresponding to the sharp apices of the V-grooves can be made as small as a few nanometers.
A shortcoming of this method is in the design of the master, as the specific etching method to produce grooves relies on the crystallographic orientation of silicon and thus you are limited in the angle you can have between features. The achievable resolution of the method also depends on the extent of crosslinking of the PDMS because the deformability of the tips will control the fidelity of the replication process.
What is nanoskiving and its procedure?
It is a method where the scientist transfers the fine control currently available for producing thin films to the production of 2D features.
Procedure:
The stamp is silanized and epoxy pre-polymer is poured on it. The epoxy is then cured and the PDMS stamp is peeled off. After this, you can deposit the material of interest on the epoxy mold in any way you like. In this case, we show gold as it can be easily sputtered or thermally evaporated on surfaces with nanometer control of the thickness.
After this step is completed epoxy pre-polymer is again poured and cured on the gold-coated epoxy mold. At the end of this step, you have a solid piece of epoxy with a film of gold having controlled thickness on a nanometer scale and morphology that is defined by the first PDMS stamp. What you can do now is slice this epoxy block with a microtome.
The slices are transfered to the desired substrate and the epoxy is selectively removed from each slice by plasma etching. What is left behind is a 2D pattern in the material of choice with nanometer thickness and lateral size.
What is microfluidics? (Important)
Microfluidics is the science and technology of the manipulation of fluids at the micrometre scale. One reason why it is relevant is that it allows one to miniaturize fluid-based processes. Another reason is that the fluids behave differently at micrometre length scales than in macroscopic equivalents; the flow has no turbulence (laminar) and this means that only mixing which occurs has to happen through diffusion.
Microfluidics allows you to minimize the use of expensive reagents by downscaling the reaction size, while also allowing you to perform combinational chemistry, where hundreds or thousands of different reactions can be performed in parallel thus drastically reducing the time invested into screening processes, where hundreds or thousands of compounds need to be evaluated for some property like, for example, their toxicity.
It also raises the idea that instead of producing chemicals in conventional macro reactors, it would maybe be better to make the same chemicals in similar quantities, but in plants built from millions of integrated microreactors.
PDMS is the most useful platform material for microfluidics.
(This is depicted in digital notes from the slides)
How can we fabricate a microfluidic device?
A microfluidic device is often fabricated by rapid prototyping:
This process was developed after the realization that the feature size needed for microfluidics could be achieved by using an inkjet printer. The circuit is thus designed on a computer and printed in actual size on a transparency. such transparency will work as a mask for the fabrication of the master, against which the PDMS is going to be cast.
In order to do so, a glass substrate is coated with a layer of SU-8 (a commercial photoresist widely used in laboratory settings). The printed transparency is then put on top of the SU-8 layer and the whole film is irradiated with light; the areas that are exposed to the light will polymerize and will resist further washing, which will instead remove material from all unexposed areas.
Now that your glass substrate is patterned with SU-8 features resembling the channels you need to add glass capillaries where you want to have inlets for fluids; you can then fabricate the PDMS mold out of this construct, peel off the master, and remove the capillaries the PDMS mold can now be welded against another PDMS flat surface to yield the microfluidic device; this delicate step is performed by treating the two opposite faces of PDMS with oxygen plasma. this will create silanols on each face which will then react upon contact of the two surfaces, leading to the cold-welding of the two PDMS pieces.
(This is depicted in digital notes from the slides)
What is the formal definition of rapid prototyping and its advantages?
Rapid prototyping - building solid forms by delivery of material and/or energy to predefined regions in space.
The enormous practical advantage of rapid prototyping is that it does not require the use of sophisticated facilities and that the whole process can be carried out in a few hours instead of a few days and for a minute fraction of the cost. This has obvious positive repercussions when it comes to the often trail-and-error protocols that sometimes need to be used in nanochemistry research.
How is the shape of a PDMS stamp used to affect microfluidic devices?
The case we will consider is about the flow-focussing device that is shown in the central diagram in Figure 4.3.
three different fluids are focused towards a narrow opening, Liquid A is a polymerizable monomer, while Liquid B is a carrier liquid, The flow-focussing device produces exceedingly monodisperse droplets from liquid A; such droplets are carried away by the carrier liquid and their reciprocal distance ensures they will not coalesce. In this specific case, the droplet of liquid A can be polymerized within the channels by two diffrent mechanisms: i) IUv irradiation or by temp. ii) initiator.
If the droplet is small enough to fit into the channel it would form a spherical colloid, if the channel is shallower than the diameter of the droplet then you would obtain disks. If the channel is instead both shallower and thinner than the diameter of the droplet, you can form rod-like colloids.
(If confused about the job that PDMS plays I think it is the nuzzle itself, look at the description written for Figure 4.3)
What are finial notes about the flow-focussing device?
Besides the control of shape, you are also fairly free to choose composition. You are not limited to polymers in the formation of monodisperse colloids by microfluidics. you can use any material that can be consolidated in conditions compatible with a microfluidic device.
Another twist to this methodology was obtained by changing the liquid A for a gas (See Figure 4.3). In this case, you can obtain the same monodisperse colloids but, instead of being made of a liquid monomer, they are made of gas.
A quick note about PDMS self-assembly
When we talk about controlling the properties of PDMS we refer to the study of millimeter-scale self assembly. Not the nanometer scale.
Now we ask are self this is nanochemi why are we talking in terms in of nanoscale???
This is becasue what self-assembly can teach us at the millimeter scale will be useful aslo at the nanoscale and vice versa!
What are capillary forces?
They are forces that are caused by a meniscus, for example at the interface between a particle and another particle or a surface
How do we use capillary forces in order to self-assemble PDMS? (part 1 background)
Background (just read and try to understand):
The study involves fabricating hexagonal PDMS disks with a hydrophobic bottom surface and a hydrophilic top surface (achieved by oxygen plasma treatment). These disks are placed at the interface between a denser, hydrophobic liquid (perfluorodecalin, PFD) and water. The disks stay at the interface due to the contrasting surface properties—hydrophilic tops contact water, and hydrophobic bottoms avoid it.
The hexagonal shape was chosen to introduce directional interactions between disks. If all sides were hydrophobic, PFD would bend the interface to wet the sides, while fully hydrophilic sides would cause water to bend the interface downward. When opposite sides have opposite wettability, the disk tilts, as water and PFD pull on the hydrophilic and hydrophobic edges, respectively.
How do we use capillary forces in order to self-assemble PDMS? (part 2 self-assembly imp)
Now the actual self-assembly:
If two disks with all-hydrophobic or all-hydrophilic sides would get close to each other, the best way for the system to minimize surface energy is to get the building blocks attached. In this way, from four menisci you are down to two and the sides of the disks have similar metallicities which make them compatible with each other. The force exerted on the building blocks in such cases will thus be attractive allowing for self-assembly. In the case in which oppositely wettable disks are approached, a repulsive force will ensure an opposite energetic consideration would be valid.
So, here you have a controllable system subject to two opposing forces of similar kind, which govern the interaction between the building blocks. PDMS hexagonal disks with different combinations of hydrophilic/hydrophobic edges were placed at the interface and then gently stirred in order to allow the system to find the equilibrium conditions (to self-assemble) faster (Figure 4.4). In the first case, only one edge of the disk was hydrophobic so that the disks autonomously formed stable couples, in the case in which two neighbouring edges were hydrophobic, triplets were observed. You can see in Figure 4.4 how other edge functionalizations gave rise to different self-assembled architectures.
The important message of this example is that we can control self-assembly if we can control the characteristics and directionality of the interaction between building blocks.
The background information about surface tensions and Young’s equation (reminder)
The interface between three phases (Liquid, solid and vapor) is defined by an angle, called the contact angle, which is determined by the interface energies γ between each couple of phases. Each surface-energy component will act on the line of contact between the three phases trying to find the best compromise in terms of minimum free energy.
The surface energy can be expressed as a vector representing the force that derives from it and it measures the amount of energy necessary to increase the surface area by a surface unit. Such surface energy will generate a force called surface tension, which will have a direction tangential to the surface and be directed in a way to reduce the interface.
If you see Figure 4.5 the γsv vector has the magnitude of the solid-vapour surface energy and its direction indicates how the force will try to increase the spreading of the droplet on the surface, thus decreasing the area of the solid surface exposed to the vapour phase. Oppositely, the γsl vector representing the surface energy of the solid-liquid interface is pointing towards the bulk of the droplet trying to decrease the contact between the solid substrate and the liquid. All these opposing forces will have to equilibrate and this can be expressed mathematically with the century-old Young’s equation:
γsv = γslcos(a)
How can we obtain superhydrophilicity?
In the first case (in ref with fig 4.5), of superhydrophilicity, the surface energy of the solid-liquid interface will be smaller than γsv - ylv; the contact angle will approach zero, representing the maximum spreading of the liquid on the solid surface. This can be achieved by decreasing the surface energy of the solid-vapor interface or by increasing the surface energy of the solid-vapor interface.
In the case of PDMS, increasing the hydrophilicity can be accomplished by exposing the surface to oxygen plasma this will produce some temporary silanols on the surface, increasing the surface charge, and decreasing the PDMS-water interface energy. The reason for such a decrease in energy is due to the good solvation that water will provide to the exposed silanol.
How can we obtain hydrophobic conditions?
when γsl is greater than γvl the contact angle will be higher than 90, thus leading to the condition defined as hydrophobic.
How can we obtain super hydrophobic conditions?
In order to further increase the contact angle increasing the γsl does not seem to be enough, at least in general. Even using perfluorinated molecules grafted to teh surface of PDMS hardly brings you to contact angles higher than 120. What is necessary is the introduction of specific nanoscale roughness in the surface. The droplet will transition from a so-called wentzel state, in which the droplet uniformly wets all the recesses of the surface, to a Cassie-Baxter state, in which the droplet sits on top of the protrusions without wetting the recesses in the surface, In such conditions, when high γsl is combined with the right roughness of the surface the contact angle approaching 180 can be approached.
What are the formal definitions of superhydrophilic and superhydrophobic?
Superhydrophilic - really water-loving; when a liquid spreads as much as possible on a surface, with a contact angle approaching 0.
Superhydrophobic - really water-fearing; where a droplet sits wholly on top of a surface, with a contact angle approaching 180
What is the lotus leaf?
The leaf of the lotus plant; it repels water almost completely (i.e., exhibits superhydrophobicity)
What are the applications of polymers in bionano?
- Antifouling
- drug delivery
- Anti-fogging
How do polymers play a role in anti-fouling
One of the most common problems when using materials in biology is that proteins tend to stick to almost any material via non-specific adsorption, thus complicating the design and the reliability of any device.
It was thus very important to discover that protein would not stick as much if the surface was covered with a specific polymer called poly(ethylene oxide) PEO.
How do polymers play a role in drug delivery?
Surfaces of PEO are called ‘stealth surfaces’ for their ability to go mostly unnoticed in vivo for prolonged periods of time. This is of fundamental importance in the design of the surfaces of drug delivery vehicles, since you want them to go unnoticed till they hit the region of interest. Without stealth surfaces, most drug delivery systems would be immediately attacked by proteins and sequestered.
What is the mechanism behind the protein reistance of PEO?
PEO grafted into PDMS surfaces in water solution will extend into the solvent in the attempt to maximize its interaction with it: The oxygen atoms within the backbone form hydrogen bonds with water, greatly increasing the affinity of the polymer for this solvent.
A PEO monolayer will be compressed by approaching proteins (Figure 4.6), the response to such compression can be compared to the response of a mattress. The PEO monolayer will resist compressions due to two parallel factors. On the one hand, the compression of the chain will reduce their conformational freedom thus reducing their entropy; a reduction of entropy, with no significant change in enthalpy leads to an increase in free energy, and thus to a non-spontanous process. This will lead the chains to spring back into their extended conformation. PEO is special in this regard in the sense that its ether groups are very flexible - particularly when swollen with water - and thus confer large entropy to the molecule.
The other force involved in this process is osmosis. When a part of a solution gets artificially concentrated in a certain solute, the outlying liquid will develop osmotic pressure to enter that part of the solution in order to equilibrate the concentration across the whole system. This is what happens to our PEO monolayer when it gets compressed by a protein. The local concentration of PEO will increase and this will generate an osmotic pressure from the rest of the solution. The resulting flow of water molecules in the spaces of the PEO monolayer will lead to its springing back and repelling the protein.
This process is dependent on the surface denisty of PEO, since the tighter the PEO is packed, the more unlikely it will be for the proteins to find an unprotected spot of surface to stick to
How do polymers play a role in anti-fogging?
Found in digital notes
