Hi Yasin. When Graphene is added to a thermoset, the curing reaction is afected by this addition. The 3 D network that is created by the thermoset will have a higher molecular weight between croslinkings (not entanglements, it is not a thermoplastic). This means that when a tensile stress experiment is performed, the young modulus will be lower and also the tensile strengh will be lower. You will need less force to achive the same strain. All this explanation takes into account only the matrix, not the interface that can drive the mechanical propeties (but this depends on the graphene concentration)

An old article of mine describe something similar with graphene, reinforcement usually comes from interaction with polymer. try to read the mechanical part of this article.

http://www.hindawi.com/journals/jnm/2012/938962/

Thank you...

Physico-mechanical properties will get better only if there is proper dispersion of the filler. otherwise, poor dispersion may give negative results. So try to get a proper mixing.

It is not because of poor dispersion, I had really good dispersion of graphene in polymer matrix. And also I got really good dielectric constant result. Most of the research prove that graphene increase tensile strength and decrease elongation.But my results are totally opposite. I just found some paper which have some similar result with me. But most of them could not explain reasons.

Also the paper, Dr, Bocchini mentioned, give some understanding about that.

What about storage & loss modulus data.. are you observing the same trend in those tests too?

Yes. I have also low tan loss data, which is desirable for it.

beside the dispersion of the filler within the matrix, the interface between the fillers and the matrix is also important. For the weak interface, it would have stress concentration, then the generation of cracks. So the weak interface would lead to a lower strength and also a lower strain at break.

If you are absolutely sure that you have a good dispersion, then the most likely reason is that you are changing the crosslink density of your thermoset (when you add the filler). A reduced cross linking density will explain lower strength, higher elongation and lower Tg. If the trend for these properties is as I mentioned, you need to go into some chemical details of your system. I am not familiar with what system and process you are using, but it is quite possible for nano particles to reduce the cross linking by physical/chemical hindrance.

As one adds a filler to a polymeric matrix, the initial consequence (0 to ~2% by volume) is to reduce Tg, tensile strength, and to increase elongation at break. The argument is that there is a slight dilation in density of polymer molecules adjacent to the filler particle since molecules with a gyration radius greater than the distance to the filler particle are simply missing (excluded volume). After that initial discrepancy, the typical relationships resume. It is rare to examine low concentrations of nanoparticles, so most researchers simply miss this behavior. In addition, the effect can be so subtle that one misses the numerical value in the noise of the experimental data. This behavior is independent of other effects.

Floyd, can you suggest a reference for this? because i would think that the excluded volume concept could explain the decrease in Tg, but not the strength and elongation characteristics. It could make an interesting reading. I would also imagine that dilation or the behavior of polymer in immediate vicinity of the nanoparticles will, to a large extent, depend on the interfacial chemistry.

Unfortunately, I have no acceptable reference for this. It was the result of a discussion with Uwe Bietan of BASF at a research conference about 30 years ago. I recently ran across it in my old notes while searching for something else.

In general, the bonding interactions between components of a composite makes them stiffen, while the absence of any interaction or the presence of a plasticizing component causes loss of strength or softening of materials. Possibly your thermoset polymer and graphene both doesn't have any suitable functional groups to interact with each other. Check what polymer and graphene functionalities others have used to get an increased tensile strength.

Other than that, you can find a number of papers talking about plasticization (loss of mechanical strength) and antiplasticization (increase in mechanical strength) of polymer films/membranes/composites.

I have papers on antiplasticization of nafion thin films in the presence of moisture at a low humidity level and gradual plasticization at higher humidity.

In case you are interested, here are the links:

1. http://pubs.acs.org/doi/abs/10.1021/mz200169a

2. http://pubs.acs.org/doi/abs/10.1021/ma3011137

Have you looked into Percolation Theory? Granted that humidity and processing conditions could create very dysfunctional results and yet, this may not be the case and found that PTT could offer a good rationalization when dealing with composites. Take a look at the excerpt below.

Physical Properties of Composites Near Percolation

Annual Review of Materials Research

Vol. 40: 131-151 (Volume publication date August 2010)

First published online as a Review in Advance on January 13, 2010

DOI: 10.1146/annurev-matsci-070909-104529

C.-W. Nan,1 Y. Shen,2 and Jing Ma1

1Department of Materials Science and Engineering, State Key Lab of New Ceramics and Fine Processing, Tsinghua University, Beijing 100084, China; email: [email protected], [email protected]

2School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138; email: [email protected]

ABSTRACT

Dramatic changes in the physical properties of composites occur when filler particles form a percolating network through the composite, particularly when the difference between the properties of the constitutive phases is large. By use of electric conductivity and dielectric properties as examples, recent studies on the physical properties of composites near percolation are reviewed. The effects of geometric factors and intrinsic properties of the fillers and the matrix, and especially of the interface between fillers and matrix, on electric and dielectric properties near percolation are discussed. Contact resistivity at the interface is less desirable for enhancing electrical conductivity. By contrast, an interface with high resistivity suppresses tunneling between adjacent fillers and leads to percolative composites with higher dielectric constant but lower dielectric loss. This review concludes with an outlook on the future possibilities and scientific challenges in the field.

Robert,

I guess I am getting where you are going with the percolation theory to understand the change in mechanical properties. At low filler concentration or at situation where inter-filler interaction is not favorable over filler-polymer interaction, the mechanical stiffening can occur. While at higher filler concentration, filler molecules are more prone to interact with each other as compared to with the polymer molecules leading to percolated tunnel of interacting fillers. As a result, the onset of softening or reduction in modulus occurs. At least, this happens at low humidity (stiff) and high humidity (soft) with water molecules (as filler) in polymer films.

Graphen has no active groups and it is weakly bonded to thermoset. This suggests that Young's modulus shoud increase and strength should decrease. The key question is dispersion, If particles agglomerate, they are weak, and strength decreases. If agglomeration is not observed, surface treatment is needed.

I think it's either:

(a) poor dispersion of graphene within the matrix that you are using. so may be you have a polymer with graphene clusters rather than uniform distribution. In this case, it is not only that graphene is not strengthening the polymer but also it could be that these clusters are introducing stress concentrations within the part that make it more susceptible to failure.

(b) It could be that the interface between the matrix and the reinforcement is not a strong one, i.e. in-homogeneous with the rest of the material that it actually weakens your material. Other than the interface, it could be the interphase (some new phase that has been created between the matrix and the composite that has different properties) that is introducing problems within your material.

(c) the matrix-reinforcement ratio is not the optimal one, either lower than the critical limit that graphene's amount is not enough to reinforce your matrix or higher than the critical limit that it is causing agglomerates

(d) Maybe your matrix and reinforcement are not compatible

Dear Altin,

The typical reason could be that the filler is not well exfoliated. You have to use some compatibiliser or some more effective system for mixing.

And the other reason could be higher concentration when percolation effect takes place. Try with smaller concentration

Hi Yasin. When Graphene is added to a thermoset, the curing reaction is afected by this addition. The 3 D network that is created by the thermoset will have a higher molecular weight between croslinkings (not entanglements, it is not a thermoplastic). This means that when a tensile stress experiment is performed, the young modulus will be lower and also the tensile strengh will be lower. You will need less force to achive the same strain. All this explanation takes into account only the matrix, not the interface that can drive the mechanical propeties (but this depends on the graphene concentration)

Besides the fact that interfacial properties between the graphene and thermoset must be improved, through functionallization or etc., you might have had some micro pores within your structure after mixing the resin and the graphene. So, these micro-pores can be another reason of having this reduction in tensile strength. What I suggest is that after mixing put the solution in a vacuum oven before curing. So that any possible vapor that may generate bubbles within the solution or any misalignment that could have originally exist after mixing can be removed from your system. so at around 50-80oC you can apply the vacuum and try to remove these defects for a few hours.

I believe you will see a big difference in your results. Please let me know if this issue can be solved through this simple experiment.

It depends upon the stiffness of the filler particles. Stiffer particles normally increase Young's modulus, but reduces tensile strength of the composite. Such results are published and explained in various articles. For example, please search the contents of journals "Polymer composites' and 'Polymer Sc. & Engg."

Uniform dispersion of any filler in polymer matrix is required to get reinforcement in the properties. i think, u need to use processing method like ultrasonication, probe sonication to disperse the graphene in thermoset polymer. In my own reaserch, I used this technique to disperse the graphene in epoxy matrix and got improvement in the mechanical properties

Since you are doing tensile strength not shear strength, I still think the interphase bonding or dispersion are relevant but not so important for your case.

Whether the orientation of graphene in polymer is along the tensile test direction is important. Imaging that graphene is a 2-D plate, so if it is in the plane for the tensile stretch. it will give a higher strength, otherwise, the answer may be no.

And I agree with Carlos, if graphene changes the crosslink density of thermoset, absolutely it will change the strength.

noorunnisa khanam Patan asked a similar question as follows:

How can I explain the decrease in tensile strength of graphene/thermoplastic composites?

I commented there as follows:

I have not studied graphene / polymer composites before, but from my experience, you can never achieve a real nano-scale dispersion of such nano-platelets in a polymer.

This is because the polymer matrix needs to wet the particles you want to disperse. Wetting means: a monomolecular polymer layer will adsorb on the particles. This can only be done with a certain bending angle.

For normal medium/high Mw of your polymer matrix, the best (lowest) particles size you can achieve by dispersion in a polymer is around 100 nm (between 50 ... 70 and 150 nm, depending on Mw).

Please look at these publications:

https://www.researchgate.net/publication/229816762_Electrical_conductivity_in_heterogeneous_polymer_systems._V_(1)_Further_experimental_evidence_for_a_phase_transition_at_the_critical_volume_concentration

https://www.researchgate.net/publication/253651172_Conductive_Polymers_as_Organic_Nanometals

Any further question is welcome, I can then explain in more detail.

Article Electrical conductivity in heterogeneous polymer systems. V ...

Chapter Conductive Polymers as Organic Nanometals

later I added:

The key question is whether or not the graphene (nano!) platelets can be dispersed. I assume they can't as I have explained above, and I explained why (you need to study the publications before further guessing).

If the graphene nanoplatelets are not dispersed well (which I assume is not possible this way), then the tensile strength which theoretically a nano-platelet has and could somehow transfer to the pokymer matrix is lost because the graphene nano platelets simply glide between each other, there is no adhesion, no strength: ZERO!

even later, I explained in a little more detail:

Again, I can only repeat what I explained above: for dispersion and for wetting of any material in a polymer matrix, the polymer must build a (mono)molecular layer adsorbed on the particles to be dispersed in that polymer. For this, the polymer layer must have a curvature to bend around the particle. This allows for only (at best) something like around 50 - 150 nm small particles being individually dispersed in a polymer matrix.

Graphene is MUCH thinner than that, no polymer matrix can fully disperse it, that is my assumption, and I would like to get solid arguments for supporting or disproving this assumption, which is based on my studies in this field:

https://www.researchgate.net/publication/229816762_Electrical_conductivity_in_heterogeneous_polymer_systems._V_(1)_Further_experimental_evidence_for_a_phase_transition_at_the_critical_volume_concentration

https://www.researchgate.net/publication/253651172_Conductive_Polymers_as_Organic_Nanometals

and many more (incl a non-equilibrium theory for dispersions in polymer systems) which can be found on my RG page.

Article Electrical conductivity in heterogeneous polymer systems. V ...

Chapter Conductive Polymers as Organic Nanometals

Hello everyone, Does there any solution to increase the tensile strength of composite materials made up of carbon fibre/epoxy/ nanoclays? 

To answer the question of the original poster and add to additional answers:

Bubbles are a huge problem for nanocomposites as sonication to disperse the nanoparticle or planetary/high shear mixing when adding the hardener can introduce voids. I recommend a high vacuum treatment. 

Another issue with epoxy nanocomposites is that really good dispersion essentially has a rheological definition of behaving like a solid so even with vacuum it can be hard to remove all the voids.

An alternative solution is to cure under really high pressure to make voids/air bubbles really small.

The other thing I would add is to consider the degree of cure. Sometimes nanoparticles can inhibit curing due to making it hard for the epoxides to find the curing agent due to not being able to diffuse/move around.

So do some modulated DSC or FTIR to look for exotherms or the epoxide peak at ~916 cm^-1

I'm actually about to publish a epoxy/graphene paper too.

PM me if you have more questions. Good luck

Upto 3% wt% to 4% wt% will give better result in yield strength as well as ultimate tensile strength (UTS) but further addition (wt%) will start clustering effect ( like intercalated) for which strength start to decrease. Intercalated graphenes have less effect on strengthening mechanism as well toughening mechanism in nanocomposite.

Dear Dr Yasin, what is the range of graphene content added into the polymer composite?

Maybe at a lower content, the tensile strength will increase.

It looks that it follows Percolation effects. In few words, the larger the amount of graphene, the closer the properties will be to graphene, and the lower amount of graphene, the closer the properties will be to the host polymer.

I am sorry, Robert, "percolation" is not the appropriate term as the percolation theory requires the dispersed particles to be statistically evenly distributed. That's definitely not the case. In contrast, dispersion in any continuous medium - be it a polymer matrix or a liquid - is quite complex and can be understood as follows:

- the particles to be dispersed need to be separated from each other (overcome cohesion forces)

- during dispersion, the matrix needs to create inner surfaces (in polymers by melt fracture, in liquid media by cavitation) (need to overcome surface tension)

- these surfaces adsorb on the open surfaces of the particles to become dispersed (here graphene) - that results in a small gain of interfacial energy

- as the cohesion and the surface energy necessary for creating the surfaces (see above) are much bigger than the interfacial energy gain, these systems are far away from equilibrium.

Percolation theory is - whether people know it or not - a theory describing equilibrium systems with the particles statistically evenly distributed.

But now, we have a non-equilibrium systems, and for these systems, it is characteristic that they exhibit sponaneous self-organisation of highy complex structures. And exactly that is the case.

I did not analyse graphene containing composites, but carbon black and conductive polymer dispersions, and also other dispersions - they all show highly comple structures, so that percolation theory can not be applied.

See literature on my RG site which can be foundhere:

https://www.researchgate.net/project/Non-equilibrium-thermodynamics-of-dispersions-and-emulsions

for instance:

Article Electrical conductivity in heterogenous polymer systems (V) ...

Article Frozen dissipative structures in heterogeneous polymer syste...

Article Critical Shear Rate - the Instability Reason for the Creatio...

Disagree since percolation could explain factors such as these. Also, if the distribution is not homogeneous then could explain part of this. However, from experience, if the distribution is not homogeneous, the responses (tensile strength vs. concentration) will be erratic and not consistent. The other part is if particles (particle size and distribution) are not evenly and thus possible creating responses like the one observed. In experimental occasions, there is no one single answer but all options should be listed and then which is the closest that can explain the phenomena seen.

funny, you disagree without having experimental or other evidence to show. Percolation theory starts with the requirement that the dispersed particles are statistically evenly distributed (AND in addition: no interaction between the dispersed particles and the matrix). Both is not the case. So, percolation theory can not be applied.

Before trying to explain, one should carefully analyze the system and not speculate which possible explanation could be "closest".

Please read the literature which I have cited, make your analysis homework and then come back with some modified statement.

I would like to add the following:

a) while I have proven that the dispersed particles are not statistically evenly distributed, this does not mean they are irregularly distributed, in contrast (as I explained above): they form highly complex 3-dimensional structures

b) moreover, the properties of the whole system (matrix/dispersed phase) do not linearly change with the percentage of particles dispersed therein (nor with the particle size used);

- very often, properties show an S-shaped curve, i.e., at a critical concentration, a property suddenly increases or decreases (that's due to a phase transition happening at this critical concentration)

- sometimes, a property shows a U-shaped (going through a minimum) or inversed U-shaped (going through a maximum) form,

depending on the structures formed during dispersion.

c) in no case, you will (as Robert speculated: " the larger the amount of graphene, the closer the properties will be to graphene") make a system withj large amount of graphene exhibiting properties similar to graphene, already simply because

- the matrix has strongly adsorbed on the graphene surface, you will not be able to remove it (i.e., by pyrolysis - it won't get away, try it! read my papers!)

- the maximum concentration is far below 80% or 90%, the smaller the particle size, the lower the maximum concentration achievable (because after dispersion of - say - 10% of a very small particle size material, all of the available matrix polymer has already been adsorbed in form of a single molecular layer

- the graphene "sheets" will not be sheets any more, but rolls or something like that.

So, again, answering the title question requires careful deep analysis, not speculation; and the analysis should consider previously made and published experimental evidence and build on that (like the ones I have cited above), should not be built on speculation.

And whether the effect is statistically reliable? About what maintenance of a graphen there is a speech? And what at the same time happens to the elasticity module?

if dispersion is well done, the properties are reproducible; as I said before, I did not study graphene composites, but many other polymer / nanoparticle dispersions, and not only I have generated a whole science around it (the papers are accessible on my RG site), but real products which I have successfully sold, so your question, Petr, can be positively answered: yes, statistically reliable.

I can not predict what properties graphene composites would have, but can recommend to study them carefully,

- what polymer matrix to take

- what percentage to be incorporated

- under which conditions

- which additives to use

- and several questions more,

so one can see this can not be a work consisting of just one test and that's it.

I am already in the polymer compounding business using twin-screw co-rotating extruders. I shall be receiving my Farrel CP45 mixer & extruder in two months. I believe this does an excellent job of homogenising and compounding. I hope to manufacture graphene polymer compounds.