MAPR 2010 Presentation - Parrott Zhang

7/31/2019 MAPR 2010 Presentation - Parrott Zhang

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The number of publications with the phrase nanocomposites (NC) has risen

dramatically since 1990. Correspondingly, the same is true for polymer

nanocomposites (PNC) and polymer nanocomposites with clay and nanotubes.


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The three fundamental shapes analyzed here are spheres, rods, and plates. Theaspect ratio is an important parameter influencing the properties of the composite.Spheres maximize the interface volume fraction. Rods have the ability to impacttransport phenomenon, such as conductivity. Plates are important for barrier

properties, and can impact dimensional stability.The variable is a ratio relating particle size and interface thickness. The interface

thickness is typically on the order of 3-30 nm. As the filler size decreases, theinterface becomes thicker relative to the particle size. Each line on the graphrepresents a different size scale for filler particles. As the fillers shrink in size, thevolume fraction of the interface increases by orders of magnitude.

The key difference for nanoparticles is that at very small sizes (10 nm spheres), thevolume of the interface affected can be up to 1000 times larger than the volume ofthe particle. Interface properties will be shown to be the important factor forthermomechanical properties, among others.

The concept of a percolation threshold can be attributed to these size effects. Forsuch small particles, the interparticle distance becomes small. There is also a largenumber density. This means that a network of NFs can form when there is gooddispersion. Also, at this scale the particle size, distance between particles and therelaxation volume of the polymer chains are all of similar magnitude.

WINEY, K. and R. VAIA (2007). "Polymer nanocomposites." MRS bulletin 32(4): 314-322.


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Adding valuable properties can come with trade-offs. Desired functionality must be

gained while still providing precessibility, mechanical properties and weight. As a,

example, adding nanotubes to a polymer for enhanced mechanical properties can

have the trade off of increased viscosity, which can be very important for processing.

For a new material to be successful as an engineering material, it must meet

requirements across its entire lifetime, from processing and production to its

eventual disposal and recycling. Understanding how a nanofiller changes this lifecycle

will be important in decided its use as an engineering material.

In many coming examples, nanofillers will prove themselves to be comporable to,

or even improve beyond traditional fillers and composites.


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Nanofillers are materials with at least 1 dimension in the nano scale. Their

structure can be linear, planar or spherical (1-D, 2-D or 3-D).

Nanowires are examples of the 1-D structure. The two main families of nanowires

are carbon nanowires and clay nanowires. Both of them have a tube-like structure as

well as fiber-like structure. At the top the structure of a carbon nanotube is shown. A

TEM image at the top right shows the dispersed clay fiber in PS/MHABS matrix.

The 2-D structure forms nanosheets. One of the most widely studied nanosheet

materials is graphite . Clay mineral sheets also are a very common nanofiller.

The 3-D structured nanofillers are nanoparticles. They can be metal particles (such

as gold rods), metal oxide (such as TiO2 shown on the bottom left), nonmetal oxide or

others like SiC and Polyhedral oligomeric silsesquioxane (POSS (used as surfactant),

RSiO1.5), or even nanocellulose.


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There are two main challenge to be considered when choosing a polymer matrix.

First , the interfacial interaction and/or the compatibility with the nanofillers is

important. Second, processing which achieves a good dispersion of the nanofiller is

also very important.

Generally, we have three main families of polymers: thermoplastic, thermoset, and

elastomer. All the three types of polymers could be used as matrix in different

applications. The table gives the comparison of thermoplastic and thermosetting

resins.

SBR-styrene butadine rubber


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Good dispersion of NF into matrix is the key step to make the enhancement of nanoscale

become macro-scale. It is also a significant concern for all NCP processing.

Kinetically, we could add some mechanical stress to disrupt the aggregation of the NF, or

lower the viscosity to get a good dispersion. However, considering the time dependence

relaxation, we have to come to the thermodynamic point of view. Therefore, the interfacial

energy between NF and matrix is a key factor. We could enhance the interfacial properties

through functionalization of the NF

The examples are shown to illustrate functionalizaition methods.

1. Covalent funcionalization. One can take advantage of the dangling bond of the CNTs to

have chemical modifications. (Fig. 1 shows the chemical modification using aniline). However,

the disadvantage of covalent functionalization is it disrupts the conjugation.

2. Non-covalent functionalization. One can also wrap the CNTs by polymers with - stacking

(aromatic systems shown in Fig. 2 by triptycene orthoquinone), cation- stacking, etc.

3. Reversible functionalization. As Shown in Fig.3, Prato developed a reversible

functionalization method by treating the initally functionalized SWCNTs (with alkyl groups)

with sodium or lithium metal in liquid ammonia, and the CNTs will be defunctionalized. One

can also add other functional groups after to form different functionalization.


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Commonly used clays belong to the family of phyllosilicates. The clay structure consists of two

tetrahedral silicate layers sandwiching an octahedral layers that is either magnesium or aluminum

hydroxide. Each layer is roughly 1 nm. Van der Waals energy between adjacent layers forms the

interlayers or gallery layer between sheets. When isomorphic substitution occurs in the octahedral

layer, a negative charge occurs which is balanced by alkali cations in the gallery layer. Montmorilloniteis the most used clay. It is only compatible with hydrophillic polymers.

As dispersion is a critical property for our NC properties, we are very interested in separating layers

of clay into individual sheets. There are three states of separation which are seen in clay. The first is an

unmixed state, where platelets are in tight stacks and can be aggregated. When these plate stacks are

broken apart from one another, and interlayers are expanded slightly we have an intercalated state.

Finally when individual sheets of clay are separated we have an exfoliated state. Achieving exfoliation

is very important for creating clay nanocomposites.

Similarly to the case of CNT, both kinetic and thermodynamic modifications can be used to disperse

the clay. Kinetic energy in the form of mechanical shear is in a mixer for extended periods of time. This

process will separate aggregates. However, for extensive exfoliation to occur a chemical modification

must be made.

In 1987 two researchers demonstrated that replacing inorganic cations in interlayers with

alkylammoniumsurfracant resulted in separation of layers and exfoliation. This modified clay has since

been referred to as organoclay. The resulting surface energy of the clay is low enough to allow for

good wetting with the polymer matrix. The addition of compatibalizers also can encourage dispersion.

Finally, some polymers, such as nylon 6, exhibit a better affinity for platelet dispersion.


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Various techniques have been developed to process nanocomposites. Melt blending

by name is to disperse the nanfillers by heating to molten state and usually co-

extrusion blending. Solution blending is to dissovle polymers and disperse the

nanofillers in certain solvent. Roll milling is considered as low shear mixing, which is

to use cylinder rollers. Emulsion polymerization is carried out in solvent with the help

of surfactant. In-situ polymerization is to disperse the nanofillers within the

monomers, and then carry out the polymerization process.


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Pure nanofillers can exhibit remarkable properties. Nanotubes can have a Youngs modulus

exceeding 1 TPa, and a yield strength of 45 GPa. This extreme elasticity means CNT can

undergo large strains while staying in the elastic region. They also exhibit high electrical and

thermal conductivity. It should be noted that under traditional mixing rules

Composites traditionally can be described by simple mixing rules. In the case of a fiber

reinforced structure, with respective Youngs modulus and volume fraction values the

following mixing rule applies:

Ec = VmEm + VfEf

To compare this equation with the results shown in the top right, taking PDMS tensile

modulus as 1 GPa (an overestimate, but reasonable for these purposes), and 5% filling we should

expect a composite modulus in the range of 50 GPa. However we only achieve 9 GPa in our

real composite.

Despite the fact that they do behave below mixing rules, substantial changes can still bemade to a matrix with very low filler volumes (1-5%). Stiffness and strength typically increase

as filler size decreases. They also usually increase (sometimes nonlinearly) as filler volume

increases up until a critical value is reached where properties decline. The case of PDMS and

SWNT on the top right exhibits this property, where at 4 or 5% the maximum tensile modulus

is reached.

To understand fundamentally how a NF increases the bulk properties of a material, the

interface must be studied. Important parameters here are the dispersion quality, aspect

ratio, orientation, interface thickness and particle size.


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The material selection chart shown in the bottom left illustrates a common tradeoff

within engineering materials, where toughness and modulus are compromised. One

potential of composites is to create a synthesis of these two properties, pushing the

material selection chart into the new yellow are marked with a ?. Two examples are

given here.

First, we will describe a macroscopic example. The case of brittle polyamideimide

shows 3% fracture strain under 80 MPa stress. Addition of 2 wt% alumina

nanoparticles extends the stress strain cuve up to 18% elongation and 110 MPa

stress. This is an exceptional case, where the stiffness of the material is preserved

while the ductility is massively increased.

Second, a nanoscale mechanism is described which can increase toughness. In the

bottom right nanosized particles are shown to create nanocracks in the FPZ zone

during crack propagation. These nanocracks are smaller than the critical crack size,

such that they do not accelerate damage but instead relieve stress.


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While engineers and scientists work to develop synthetic materials in labs, nature has beendeveloping and optimizing materials for millions of years. The phylum Mollusca includes soft-bodiedinvertebrates wholly or partially enclosed in a calcium carbonate shell. It is one of the most diversegroups in the animal kingdom with over 100,000 species. A study of 19 species of mollusks revealedelastic modulus ranging from 40 to 70 GPa, and strength on the order of 20-120 MPa. The nacreous

inner layer reinforces the outer part of the shell in the case of a crack, preventing total failure. Thisstructure is able to achieve a work of fracture 3000 times greater than the single crystal of purearagonite[10]. No man-made composite can approach this level of toughness amplification (which hasmotivated over 30 years of research into nacre).

Nacre is made of aragonite CaCO3 (95% vol) together with soft organic biopolymers (5% vol).Aragonitic CaCO3 is arranged in a brick and mortar microstructure. The roughly 500 nm thick bricksare separated by 30nm thick layers of organic mortar. Under stress, the mortar serves to glue thestructure together, and the plates provide strength. A unique mechanism occurs where platelets slidepast each other allowing for ductile behavior. The sliding motion between plates, as well as the platesthemselves, contain many nanostructures such as nanobridges, nanoasperities, and nanograins .

Tang et al. in 2003 demonstrated sequential deposition of polyelectrolytes and clays, resulting in ananoscale version of nacre with alternating layers. Macromolecular folding effects were

demonstrated. Tensile strength approached that of nacre (100 10 MPa) while Youngs modulus wascomparable to bone (11 2 GPa). LBL deposition of montmorillonite clay platelets and polyelectrolytesformed the brick and mortar structure characteristic of nacre. These layers were shown to have a highaffinity and compatibility for each other. Sacrificial bonds and successive opening of intra-chain loopsin the polymer yielded step patterns in the stress strain curve.

Mollusk. Encyclopaedia Britannica 2010; Availablefrom:http://www.britannica.com/EBchecked/topic/388398/mollusk.

Currey, J.D. and J.D. Taylor, The mechanical behaviour of some molluscan hard tissues. Journal ofZoology, 1974. 173(3): p. 395-406.

Tang, Z., et al., Nanostructuredartificial nacre. Nature Materials, 2003. 2(6): p. 413-418.


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Carbon fiber reinforced composites can benefit from a range of nanofillers. First,

the matrix resin itself can be modified. Toughening by nanoclay platelets, conductivity

effects due to nanotubes, or even self-healing effects of resin filled clay nanotubes

can be introduced into the matrix. Strength improvements are especially important if

they allow for lighter parts to be made. Reducing weight improves energy usage,

which also reduces emissions. Although lifetime considerations may make

automotive applications illogical, the point can be made that simply lightening a car

by 25 kilograms can affect fuel economy by as much as one percent.

The carbon fibers themselves can also be interesting to modify. Making conductive

and strengthened fibers is an interesting way to modify the overall composite

properties, as illustrated by the simple composite mixing rule on slide 10. Nanofillers

have the potential to modify the processing of the CFRP product, and as a result the

construction of the plate must consider their effects. For example, adding nanofillers

to resin can result in a large viscosity, which would make resin transfer moldingimpossible. One way around a problem such as this would be by introducing the

nanoproducts into the layered assembly in the form of a sheet. In this case the resin

can still be injected as it is not yet modified. Coatings can provide a number of

functionalities, from thermal or electrical conductivity to toughness or permeability

barriers. Nanoclays are known for their ability to resist permeability, which will be

illustrated in more detail later in the presentation.


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Nanocomposites can modify a wide range of thermal properties including the glass

transition tempertature, flammability resistance, thermal conductivity and coefficient

of thermal expansion.

The glass transition temperature of polymers occurs when thermal energy allowschains to slide past one another. When a significant portion of the matrix is a part of

an interfacial region, the activation energy for polymer chains to slide will change.

Two possibilities can be distinguished on the graph on the right. Here SiO2 particles

are added up to 5%. By varying the molecular weight of the polymer matrix (shown

on the right scale), the wetting of the polymer chains around the nanofiller can be

adjusted. For a wetting matrix, the chains stick to the surface of the particle. The

energy needed to overcome this attraction raises the glass transition temperature of

our overall material. Similarly, if the nanofiller does not adhere to the matrix,

dewetting can occur which is similar to a free surface. It has been shown that for freesurfaces, the Tg of a polymer will drop. This is because there is added ability for

chains to move and slide.

The diagram on the lower left shows different functionalization possibilities for a

nanoparticle. By adding block copolymers, short functional groups, long functional

oligomers, or hyrdrogen bonding groups the particle can be made such that it wets or

dewets a matrix.


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Dimensional stability here refers to the ability of a material to retain its shape under

varying temperatures. The coefficient of thermal expansion (CTE) characterizes the

change in size according to temperature. Low CTE nanofillers can be added to

stabilize against higher CTE polymers. The large tensile modulus allows the filler to

withstand the stresses due to different thermal expansion. Nanofiller size, aspect

ratio and orientation are important for determining its CTE.

A common example comes from the nylon 6 nanoclay composite produced by

Toyota. Among other remarkable properties, this material had half the CTE of neat

resin. The graph on the right shows how montmorillonite clay can be much more

effective than talc as a filler. Large reductions are made with as low as 3% filler.


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The costs of fire damage are shown in the highlighted box. Every year 4000 people

are killed and 80000 are injured in the European Union alone. Worldwide the death

count reaches 166,000. The cost within the EU of this damage per year is as high as

80 billion. Current fillers for fire protection consisted of halogenated retardants,

which when burned release toxic and corrosive gases. These gases are harmful to the

ozone layer, and increasingly subject to government regulation.

Clays offer another mechanism to protect against fire. A qualitative observation in

many studies finds a protective char at the surface of a burned polymer. This layer

forms a barrier against the release of heat. The comparison on the top right makes

this clear. Pure polystyrene is shown burning, whereas 5% filler of nanoclay forms the

distinct black char.

The graph on the right shows a heat release rate experiment with platelet fillers.

The release of heat is cut by a factor of three with nanofiller. This can be measured by

cone calorimetry. Reducing the maximum heat release, flame propagation is retardedin the adjacent areas near the ignited material. These fillers promise to substitute for

the regulated halogenated retardants.


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To explain the electrical properties of nanocomposites, the concept of thepercolationthresholdmust be introduced. From a mathematical and physical point of view, thepercolation threshold is the formation of long-term connectivity in a random system. Forinsulating polymers containing conducting fillers there is a critical concentration for the fillersto form a conducting path, and thus a sharp jump of conductivity (4~10 order magnitude) atthis percolation threshold. The left figure gives the conductivity behavior of extendedgraphite (EG)/PVDF nanocomposite when increasing the volume fraction of EG. The schemesabove show the formation of conducting path. Near the percolation concentration, theconductivity follows:

cnp= f(- c)t, >c

where cnp is the total conductivity of nanocomposite, f is the conductivity of the filler, is the volume fraction of the filler, and the c is the critical volume fraction. This relationshipis shown in the inside figure log v.s. log (- c).

There are several factors can influence the percolation threshold value: the aspect ratio ofnanofillers, the dispersion, the alignment of fillers, etc. In addition, different nanofillers

provide different percolation thresholds. For single wall carbon nanotubes SWCNTs), thisvalue could be down to 0.002 vol%, while the general order of percolation for CNT is 1-4 vol%.For graphite, normally, the threshold is around 4-9 vol%.

According to the percolation theory, the dielectric constant obtains very large values nearthe percolation threshold. As the right figure shown, within the same system as in the leftfigure (EG/PVDF), the dielectric constant increases significantly near the percolationthreshold (6 vol%). Metal and ceramic fillers are usually used for obtaining high dielectricmaterials (~ 10-1000). The combination of processibility (polymer matrix) and the obtainedhigh dielectric constant made this kind of nanocomposite advantageous in electronicapplications.


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As mentioned previously, nanofillers can enhance the electrical properties of

composites in conductivity and dielectric constant. There are various applications

based on these properties.

With different orders of conductivity the nanocomposites can attain, they can beused for electrostatic dissipation, electrostatic painting , electromagnetic interference

(EMI), printable circuit wiring, etc. The top graph shows the working principle of the

electrostatic painting: the painting which is made of conductive nanocomposites is

positively charged, and the object (column made of steel) is negatively charged; thus,

the spray goes around the object and coats it automatically.

The high dielectric constant property of polymer nanocomposites is very

interesting for electronic applications. It can be applied in embedded capacitors,

integrated circuits, organic thin film transistors, etc. The major advantage is the ease

of processing. The second graph shows the polymer nanocomposite capacitor film (A)and the array of capacitors with different capacitances obtained by etching (B).


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Nanofillers can be used to modify the optical properties of composites. Themodification of optical constant is not only dependent on the optical nature of thefillers and polymer matrices, but also significantly affected by the size and sizedistribution of the fillers. The equation of intensity loss for spherical particles is:

where r is the radius of nanoparticles, is the wavelength of light, x is the opticallength, np and nm are the refractive index for fillers and matrix respectively.

The absorption spectra will also shift with varying particle size as shown in thefigure. Therefore the color of the nanocomposite is dependent on the filler size.

The spectra shift mentioned above is due to the dielectric confinement effect,which results from the refractive index difference between the filler and matrix. Thelocal field intensity near or at the interface of the filler and matrix is enhanced when

illuminated by light.

When the particle size is small enough (less than of light wavelength), aninteresting effect appears: the non-scattering of light. This property providesnanocomposites optically homogenous media, and in favor of transparency of thematerials. We can see from the picture that, because of the small size, the right one(epoxy resin with nanoclay) is still transparent and has no big differency in clarity thanthe left one (neat epoxy).

The other very important optical property of nanocomposite is their non-linearlity,which is the dielectric polarization response nonlinearly to the electrical field of light.


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Based on the specific optical properties of nanocomposites, there are lots of

applications, such as:

Tunable LED devices: one can varify the color by changing the size of NF.

Antireflextion coatings: Using high refractive index nanoparticles (metal, metal oxide,etc.), the nanocomposites with high RI properties can be obtained. They can be used

for solar cells coating or lenses.

Hard Transparent coatings: Hard Nanoparticles with small enough size which is non-

scattering. (Polyimide/silica)

Decorative coatings: Related to their absorption behavior.


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Permeability of nanocomposite can be expressed as:

where Pp is the permeability of polymer matrix, p is polymer phase fraction, and is

tortuosity.

Where nf is the nanofiller phase fraction, D and L are the diameter and thickness of

NF, respectively.


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Nanocomposites are widely used as gas and other barriers. They are made for food

packaging (barrier of oxygen and water), fuel tanks (reduction of the permeability of

fuel), water & gas treatment (polymer nanocomposite membranes), etc.

The figures show Baseball inner coating, nanocomposite pouch for sports shoes, andthe Mitsubishi super gas barrier Nylon are made of nano-clay composites. The pictre

of the tire shows that it contains silica to reduce oxygen permeability.


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Bionanocomposites are impacting diverse areas. Hydroxyapatite (HAP)polymer

nanocomposites have been used as a biocompatible and osteoconductive substitute

for bone repair and implantation. HAP is the hard inorganic component which has

long been used in bone repair surgeries. However, HAP is brittle and hard to shape,

moreover, their powders cannot disperse well easily. Incorporating HAP into polymer

matrix enables us better processing and dispersion.

MMT (clay) can be incorporated into polypeptides matrices (biodegradable) to be

make use of in tissue engineering.

Silver or silver oxide nanoparticles are employed in antimircobial applications.

Nanocomposites can also provide improvements in drug release/ delivery.

Nanoparticles can act to allow slower and more controlled release. Iron oxide

nanoparticles have been widely applied in drug delivery, enhancing the contrast in

magnetic imaging, reducing swelling and improving mechanical integrity.


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In conclusion, nanofillers offer the ability to not only improve upon existing

properties, but also to add further multifunctionality to composites. Optical, electrical,

mechanical, and thermal properties can be specifically tailored for performance

specifications. An interesting area for the future includes nanocomposites made with

a hierarchical structure, controlled on size scales from the nanoscale up to the bulk.

Biological materials such as nacre serve as natural models for composite

development. For NCs to be successful, spatial and orientational control will need to

be achieved in a cost effective manner. This will help transform some polymers from

commodity items to critical components of active devices.


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Polypropylene was first polymerized by Giulio Nata in 1954, whom the famous

Ziegler-Nata polymerization process is named after. Isotactic-PP is the most

commericial form of PP. The properties of it are shown on this slide.


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Operational efficiencies at plants require a minimum of 85% of capacity, which can

strain the supply chain.

PP as well as Polyethylene is polymerized using Zeigler-Natta catalyst.

Zeigler-Natta Catalyst:1. Titanium chlorides /MgCl2 and organoallumina (e.g. Al(C2H5)3)

2. Metallocene catalysts

3. Non-metallocene metal complexes


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Some applications are shown here, but PP can be found absolutely everywhere

around us. A more appropriate question may actually be where can you not find PP.

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MAPR 2010 Presentation - Parrott Zhang