Chap 6b Nano Composites

8/3/2019 Chap 6b Nano Composites

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Nanocomposites: mixing

CNTs into polymers


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Outline

1.Introduction

2. Composites of multiwalled carbon nanotubes (MWNT) withpolycarbonate (PC) produced by masterbatch dilution technique

Electrical resistivity

Dispersion and alignment

Influence of processing parameters on electrical resistivity3. Composites of MWNT and SWNT with PC produced by direct

incorporation

Percolation of different commercial MWNT in PC

Percolation of SWNT in PC

Stress-strain behaviour

4. Summary


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Electrical conductivity

Improvement of mechanical properties, especiallystrength

Enhancement of thermal stability Enhancement of thermal conductivity

Improvement of fire retardancy

Enhancement of oxidation stability

Effects at low CNT contents because of the very highaspect ratio

Benefits of CNTs to polymers


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How to introduce CNTs into

polymers


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Melt mixing of CNT with

thermoplastic polymers


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Preparation of the PC-MWNT

composites Masterbatch technology: polycarbonate(PC) +PC based masterbatch (15 wt% MWNT) masterbatch (Hyperion Catalysis International, Inc,

Cambridge, USA) diluted with PC Iupilon E2000(PC1), PC Lexan 121 (PC2) or PC as used for themasterbatch (PC3)

Haakeco-rotating, intermeshing twin screw extruderwith one kilogramm mixtures

DACA Micro Compounder, conical twin screwextruder (4.5 cm3capacity)

Brabender PL-19 single screw extruder


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Characterization of the

masterbatch (PC + 15 wt% MWNT)


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Dispersion in PC-MWNT

composites


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Alignment in PC-MWNT

composites


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Comparison for different set of PC

masterbatch dilution


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Detection of percolation and influence of processing

conditions investigated by dielectric spectroscopy


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Direct incorporation of different

kinds of commercial MWNT into PC


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Comparison of direct incorporation of CNT,

masterbatch dilution, and CB addition


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Direct incorporationof SWNT1 into PC


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Direct incorporation of SWNT1 into PC


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Direct incorporationof SWNT2 into PC


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Summary

Melt mixing is a powerful method to disperse CNT into polymers

Masterbatch dilution technique (based on a PC masterbatch)

percolation in the range of 1.0 wt% MWNT

suitable processing conditions can shift percolation to lower values(0.5wt%)

effects of mixing eq

uipment and PC viscosity on percolation are small Direct incorporation method

percolation strongly depends on the kind of CNT, production method(resulting in different sizes, purity and defect levels), and thepurifying/modification steps

for commercial MWNT percolation occurs between 1.0 and 3.0 wt% andis lower at lower MWNT diameters and higher purity

HipCO-SWNT (CNI) percolation between 0.30 and 0.35 wt% stress-strain behavior of the composites: modulus and stress are

enhanced, elongation at break reduced especially above percolationconcentration


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Graphenepolymer composite

Graphite oxide was prepared by the Hummers method from SP-1 graphite(Bay Carbon), and dried for a week over phosphorus pentoxide in a vacuumdesiccator. Dried graphite oxide (50 mg) was suspended in anhydrous DMF(5 ml, Dow-Grubbs solvent system), treated with phenyl isocyanate (2 mmol,Sigma-Aldrich) for 24 h, and recovered by filtration through a sintered glassfunnel (50 ml, medium porosity). Stable dispersions of the resulting phenyl

isocyanate-treated graphite oxide materials were prepared by ultrasonicexfoliation (Fisher Scientific FS60, 150 W, 1 h) in DMF (1 mg ml-1).Polystyrene (Scientific Polymer Products, approximate Mw = 280 kD, PDI =3.0) was added to these dispersions and dissolved with stirring (Fig. 1d, left).Reduction of the dispersed material (Fig. 1d, right) was carried out withdimethylhydrazine (0.1 ml in 10 ml of DMF, Sigma-Aldrich) at 80C for24 h. Upon completion, the coagulation of the polymer composites wasaccomplished by adding the DMF solutions dropwise into a large volume of

vigorously stirred methanol (10:1 with respect to the volume of DMF used).The coagulated composite powder (Fig. 1e) was isolated via filtration;washed with methanol (200 ml); dried at 130C under vacuum for 10 h toremove residual solvent, anti-solvent, and moisture; crushed into a finepowder with a mortar and pestle, and then pressed (Fig. 1f) in a hydraulichot press (Model 0230C-X1, PHI-Tulip) at 18 kN with a temperature of210C.


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Process flow of graphene

polymer composite fabrication a, SEM and digital image (inset) of natural graphite. b, A typical

AFM non-contact-mode image of graphite oxide sheets depositedonto a mica substrate from an aqueous dispersion (inset) withsuperimposed cross-section measurements taken along the red lineindicating a sheet thickness of 1 nm. c, AFM image of phenylisocyanate-treated graphite oxide sheets on mica and profile plotshowing the 1 nm thickness. d, Suspension of phenyl isocyanate-treated graphite oxide (1 mg ml-1) and dissolved polystyrene in DMFbefore (left) and after (right) reduction by N,N-dimethylhydrazine. e,Composite powder as obtained after coagulation in methanol. f, Hot-pressed composite (0.12 vol.% of graphene) and pure polystyrene ofthe same 0.4-mm thickness and processed in the same way. g, Low(top row) and high (bottom row) magnification SEM images obtainedfrom a fracture surface of composite samples of 0.48 vol.% (left) and2.4 vol.% (right) graphene in polystyrene.


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Advantages of Nanosized Additions

The Nanocomposites 2000 conference has revealed clearly the

property advantages that nanomaterial additives can provide in

comparison to both their conventional filler counterparts and base

polymer. Properties which have been shown to undergo substantial

improvements inclu

de: Mechanical properties e.g. strength, modulus and dimensional stability Decreased permeability to gases, water and hydrocarbons

Thermal stability and heat distortion temperature

Flame retardancy and reduced smoke emissions

Chemical resistance

Surface appearance

Electrical conductivity

Optical clarity in comparison to conventionally filled polymers


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Disadvantages of Nanosized

Additions To date one of the few disadvantages associated with

nanoparticle incorporation has concerned toughness andimpact performance. Some of the data presented hassuggested that nanoclay modification of polymers such

as polyamides, could reduce impact performance.Clearly this is an issue which would requireconsideration for applications where impact loadingevents are likely. In addition, further research will benecessary to, for example, develop a better

understanding of formulation/structure/propertyrelationships, better routes to platelet exfoliation anddispersion etc.


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Examples of Mechanical Property

gains due to Nanoparticle Additions Data provided by Hartmut Fischer of TNO in the Netherlands

relating to polyamide montmorillonite nanocomposites indicatestensile strength improvements of approximately 40 and 20% attemperatures of 23C and 120C respectively and modulusimprovements of 70% and a very impressive 220% at the sametemperatures. In addition Heat Distortion Temperature was shown toincrease from 65C for the unmodified polyamide to 152C for thenanoclay-modified material, all the above being achieved with just a5% loading of montmorillonite clay. Similar mechanical propertyimprovements were presented for polymethyl methacrylate clayhybrids.

Further data provided by Akkepeddi ofHoneywell relating to

polyamide-6 polymers confirms these property trends. In addition,the further benefits of short/long glass fibre incorporation, togetherwith nanoclay incorporation, are clearly revealed.


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Area of Applications

Such mechanical property improvements have resultedin major interest in nanocomposite materials innumerous automotive and general/industrial applications.These include potential forutilization as mirror housings

on various vehicle types, door handles, engine coversand intake manifolds and timing belt covers. Moregeneral applications currently being considered includeusage as impellers and blades for vacuum cleaners,power tool housings, mower hoods and covers for

portable electronic equipment such as mobile phones,pagers etc.


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Gas Barrier

The gaseous barrier property improvement that can result fromincorporation of relatively small quantities of nanoclay materials isshown to be substantial. Data provided from various sources indicatesoxygen transmission rates for polyamide-organoclay compositeswhich are usually less than half that of the unmodified polymer.Further data reveals the extent to which both the amount of clay

incorporated in the polymer, and the aspect ratio of the fillercontributes to overall barrier performance. In particular, aspect ratio isshown to have a major effect, with high ratios (and hence tendenciestowards filler incorporation at the nano-level) quite dramaticallyenhancing gaseous barrier properties. Such excellent barriercharacteristics have resulted in considerable interest in nanoclaycomposites in food packaging applications, both flexible and rigid.Specific examples include packaging for processed meats, cheese,

confectionery, cereals and boil-in-the-bag foods, also extrusion-coating applications in association with paperboard for fruit juice anddairy products, together with co-extrusion processes for themanufacture of beer and carbonated drinks bottles. The use ofnanocomposite formulations would be expected to enhanceconsiderably the shelf life of many types of food.


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Fuel Tanks

The ability of nanoclay incorporation to reduce solventtransmission through polymers such as polyamides hasbeen demonstrated. Data provided by De Bievre andNakamura of UBE Industries reveals significant

reductions in fuel transmission through polyamide6/66polymers by incorporation of a nanoclay filler. As a result,considerable interest is now being shown in thesematerials as both fuel tank and fuel line components forcars. Of further interest for this type of application, the

reduced fuel transmission characteristics areaccompanied by significant material cost reductions.


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Films

The presence of filler incorporation at nano-levels has also beenshown to have significant effects on the transparency and hazecharacteristics of films. In comparison to conventionally filledpolymers, nanoclay incorporation has been shown to significantlyenhance transparency and reduce haze. With polyamide basedcomposites, this effect has been shown to be due to modifications inthe crystallisation behaviour brought about by the nanoclay particles;spherilitic domain dimensions being considerably smaller. Similarly,nano-modified polymers have been shown, when employed to coatpolymeric transparency materials, to enhance both toughness andhardness of these materials without interfering with lighttransmission characteristics. An ability to resist high velocity impactcombined with substantially improved abrasion resistance wasdemonstrated by Haghighat of Triton Systems.


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Environmental Protection

Water laden atmospheres have long been regarded as one of themost damaging environments which polymeric materials canencounter. Thus an ability to minimize the extent to which water isabsorbed can be a major advantage. Data provided by Beall fromMissouri Baptist College indicates the significant extent to whichnanoclay incorporation can reduce the extent of water absorption in apolymer. Similar effects have been observed by van Es of DSM withpolyamide based nanocomposites. In addition, van Es noted asignificant effect of nanoclay aspect ratio on water diffusioncharacteristics in a polyimide nanocomposite. Specifically, increasingaspect ratio was found to diminish substantially the amount of waterabsorbed, thus indicating the beneficial effects likely from nanoparticleincorporation in comparison to conventional microparticle loading.Hydrophobic enhancement would clearly promote both improvednanocomposite properties and diminish the extent to which waterwould be transmitted through to an underlying substrate. Thus,applications in which contact with water or moist environments is likelycould clearly benefit from materials incorporating nanoclay particles.


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Preparation and Characterization of

Novel Polymer/Silicate Nanocomposites

Five categories cover the majority of composites

synthesized with more recent techniques being

modifications or combinations from this list.

Type I: Organic polymer embedded in aninorganic matrix without covalent bonding

between the components.

Type II: Organic polymer embedded in an

inorganic matrix with sites of covalent bonding

between the components.


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Type III: Co-formed interpenetrating networks of

inorganic and organic polymers without covalent

bonds between phases.

Type IV: Co-formed interpenetrating networks ofinorganic and organic polymers with covalent

bonds between phases.

Type V: Non-shrinking simultaneous

polymerization of inorganic and organic

polymers.


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The great majority of nanocomposites

incorporate silica from tetraethoxysilane

(TEOS). The formation of the inorganic

component involves two steps, hydrolysis

and condensation as seen in Scheme 1.


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Polymers considered: PEO, PEO/PPO,

PVAc, PVA, PAN, MEEP

A general synthesis for a base, acid, or salt catalyzed polyphosphazene,polyethylene oxide (PEO), and polyethylene oxide/polypropylene oxide(PPO/PEO) block nanocomposite is as follows: 300 mg of polymer isdissolved into 10 mL of a 50/50 by volume tetrahydrofuran (THF)/ethanolmixed solvent in a capped vial. To this solution is added TEOS (336 mg). Acatalyst is then introduced as an aqueous solution (150 l) and the mixture

is capped and sonicated at 50o

C for 30 minutes. The sol

ution is aged fromhours to days depending upon the catalyst used in a sealed vial and poured

into a Teflon mold and loosely covered at room temperature. Thenanocomposite self assembles as the volatile solvent slowly escapes duringthe condensation process.

The synthesis of polyvinyl acetate (PVAc)/silicate nanocomposites requiresa different approach from the other nanocomposites. PVAc (300 mg) isdissolved into an 50/50 by volume acetic acid/methanol (10 mL) mixed

solvent in a capped vial. To this solution is added TEOS (373 mg). Thesolution is then sonicated for 5 minutes in a sealed vial at room temperatureand poured into a Teflon mould and loosely covered at room temperature.The nanocomposite self assembles during the curing process, whichtypically lasts up to 24 hours. Additional heating at 100C for 30 minutesaids in removing lingering acetic acid from the nanocomposite.


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Novel Rubber Nanocomposites with

Adaptable Mechanical Properties

Silica particles have become more important in tire applicationssince the introduction of the Green Tire by Michelin. As a filler,silica has greater reinforcing power, such as improving tear strength,abrasion resistance, age resistance and adhesion properties thancarbon black [6-8]. However, due to the strong inter-particlehydrogen bonds between hydroxyl groups, the agglomeration nature

of silica is generally believed to be responsible for the significantPayne effect which brings about considerable rolling resistance fortire applications. In order to reduce the filler-filler interaction and/orto enhance the mechanical properties of silica filled composites,researchers have been working for many years on differentstrategies to improve silica-rubber interaction and, in turn, to reducethe rolling resistance. Among these strategies, chemicalmodifications of rubbers by attaching functional groups interactingwith silica [9-22] and surface treatments of silica by reducing surfacepolarity with different silane coupling agents [22-36] are the mostpopular techniques.


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Novel Rubber Nanocomposites with

Adaptable Mechanical Properties

However, these techniques admittedly have quite a few drawbacks.For the former technique, the chemical modification reaction ofrubber was usually not applicable to commercial production and itsdegree of modification was usually very low [9,11,14,18,22].Additionally, the chemical modification was limited to rubber chainends [12,17,20], meaning that the final silica composite was

unsatisfactory in terms of reducing silica agglomeration. For thelatter, the used coupling agents are expensive and it could possiblylower the crosslinking density by reacting with the chemicalingredients for vulcanization. This technique would lead to loweroverall cure rates [34,35], and at the same time it degraded themechanical performance of such silica filled material for tireapplications. In summary, due to these flaws none of the methodsmentioned above could simultaneously ensure both the ability inreducing the silica agglomeration and improving the materialperformance.


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Chap 6b Nano Composites