I think the melting transition appears when there is any crystalline contents in the sample. if the sample have no melting peak , in my view it has no exothermic peak on the in the DSC scan. If there is any exothermic peak (cold crystallization) then definitely, it should have a melting transition. But as the composites have the melting peak, means the filler you used have the crystalline contents and that melts when it gets enough heat flow. Or it is also possible that the fillers have acted as nucleating agent to accelerate it crystallization process. If you can do DSC again for pure polymer matrix, do the Heat-cool-Heat method to remove the previous thermal history. I think it might help you.

Could you please reproduce the DSC curves together with test conditions (exo-endo direction, manufactruing and storage conditions of the sample)? What do you mean by melting point in the case of an amorphous polymer? What kind of polymer and what kind of metal did you use? If the peak is in the exo direction it can alos be an enthalpy relaxation peak.

@ Gyorgy Banhegyi: After earning from different sources, it is true that for amorphous polymer melting point not make any sense and glass transition is generally observable for the same. But question is, what happened when particles were introduced in that polymer matrix? Nanocopper, CNTs, extended graphite (nanosized) and a ceramic filler (having average particle size of 1 micron, not a nanosized filler) were introduced in that polymer matrix separately . In all the cases glass transition was disappeared and melting peak was observed. Please help to get the answer.

The crystalllinity of the polymer is determined by the structure of the polymer chain. In the case of atactic vinyl polymers e.g. it is impossible to get crystallization and melting peak as the symmetry necessary for crystal formation is simply missing. There is only local order, as in liquids (described by the radial distribution funcition). In the case of isotactic or syndiotactic vinyl polymer chains there is a possibility of partial (!) crystallization. (Due to polymerization defects, molecular weight distribution and chain-end defects polymers are always partially crystalline even if the symmetry necessary for crystallization is present). Thus, if the pure polymer is amorphous (e.g. atactic vinly polymer, as aPS, aPP) - it will not crystallize in the presence of any filler. If the ppolymer may crystallize, the addition of fillers may change the degree of crystallinity, the spherulite size (related to nucleation density) or the so-called long-period (the width of the repeating crystalline and amorphous phases in the folded lamellae). In a very extreme case it may happen that a potentially crystallizing polymer does not crystallize becaues of over-cooling (too high viscosity, too low nucleus concentration) in pure state, while it does so in the presence of nano-fillers, which may act as nuclei. This is why I asked about the structure of the polymer and wanted to see the thermograms - if they are not confidential. It may also happen that there is a chemical rection between your polymer and the metal nano-particle. A new polymer or odified interphase may also be formed - so your question cannot be answered in abstract.

usually DSC information are coming to complete TGA data (Thermogravimetric analysis); what is TGA telling you about your polymer?

I am not sure whether TGA is really informative here - unless the presence of the nanoparticles causes decomposition of the matrix. DTA or DSC detect thermal transitions due to phase transitions (primary, as melting, crystallization of pseudo-secondary as glass transition) and chemical reactions (such as oxidation or thermal decomposition). Of these only decomposition (and sometimes oxidation) is coupled with significant mass change (maybe also desorption or evaporization - but these are less frequent in polymers), so instead of TGA I would recoemmend using DMA (dynamical mechanical analysis) or DEA (dielectric anaylsis) that detect relaxation transitions. If there is e.g. a significant difference in the tan delta peak of DMA, you could conclude that the mobility is changed significantly by adding nanoparticles. Changes in crystallinity would also be detected by WAXS (wide angle X-ray scattering). Here, of course, you would have to use the X-ray diffractogram of the nano-particles for comparison, in order to see the potential new peaks that can be assigned to the changing crystallinity of the matrix. If you have WAXS apparatus with temperature control possibilities you can follow also the melting process. (For this purpose the best tool is syncrotron radiation source, where the total angle range can be detected simultaneously during heating. Such facilities, are, however, rare).

Amorphous polymer doesn't melt. Even in extrusion or injection molding, the amorphous polymers like PS,PMMA are shear deformed and forced to move through the die orifice or gate opening in case of mold.

while in crystalline polymers, certainly Tg appears in DSC curve.

@ All contributors please provide your mail IDs

It's going to be dependent upon what the parent matrix is - which - I don't happen to see in the string above.

What is the polymer we are talking about ? If we know, what the polymer is, it will be easy to point out the reasoning.......should be straight-forward !!!

Dr. Yash Khanna

DSC is based on heat diffiential on tested materials and reference. If amorphous polymer does not have transition point at so called melting point, it should have no much info you can get. For composites, you may first need to know what components in your composites and if there are no interaction among each other, they should have all components behaviour (linear addition). If they have certain interaction, they will change the begaviour, and new info could come out.

Polymer is Polycarbonate

@ Selcuk The same result is also observed with ceramic filler having very high melting temperature. The endothermic (melting indication) peak appeared near the theoretical melting point of the polymer. There should be something related to polymer - filler interface.

For PolyCarbonate, there should be a clear Tg at about 150C with no Tm.

Maybe, you don't see the Tg @ 150C for the resin because of experimental issues. I suggest, you run the DSC upto 160C and then REHEAT the same. You will get a Tg of 150C without any doubt IF you are dealing with PolyCarbonate.

[email protected]

Melting is a phenomenon typical of crystalline domains. If your polymer is completely amorphous you do not have any of such domain. As a consequence there is not any melting but only glass transitions.

Yes you are right Tg appeared at 151 for pure Polycarbonate...but it disappeared when filler was added and endothermic peak observed near the melting point of Polycarbonate

Do the same REHEAT with higher sample size (15 mg) and higher heating rate (20C/min), you will DEFINITELY get the Tg

When you see "Endothermic Peak near Tg", it is due to ENTHALPIC RELAXATION and nothing to do with melting. In any case, upon REHEAT, you will get the Tg you are looking for.

You will have melting peaks in DSC only if there is a crystilline phase which is absent in amorphous polymers. Tha standard procedure to get Tg is heating at 20 K/min, cooling and re-heating. Then you will have stepwise glass transition.

An amorphous polymer has no melting point (by definition). When filled with metallic particles, it is very likely that the polymer chains orient themselves in the vicinity of the particles creating an ordered (crystalline) phase. The extent of this phenomenon should be dependent on the volume fraction of the metallic particles, their size and the molecular weight of the polymer.

Of course answering such a question would be greatly helped by sufficient information - we now know it is polycarbonate but not ist MW which crystallises under standard conditions very slowly ie > 30 days. We dont know the equipment this was observed or the nature of the metal filler, material or size. As a number of contributors have pointed out an amorphous polymer does not exhibit a melting point, it does exhibit a a softening point or glass transition which manifests itself in the DSC as a step in the heat capacity. Sometimes, a sharp melting like peak is observed due to enthalpic relaxation( common in PC) or due to trapped solvent loss at the glass transition. the addition of the metal filler (how much added?) will greatly alter the thermal conductivity and I doubt that your 2 samples are in the same thermal condition. The filler will also affect the transfer of heat in the DSC.

I suggest you perfect your experimental procedure so that you can observe the step in heat capacity of the PC alone before looking for complications. Observing the step in heat capacity is simple to acheive, a reasonable mass, an appropriate heating rate. Good luck

Your Polymer is possibly semicrystalline, but you didnt record a melting event, at the expected temperature region, possibly due to unvlear thermal history of your sample. You can do the folowwing. Heat up your sample at a temperature in the middle of region between the Tg and the expected Tmelting. Hold the sample isothermally there for 30 min - 60 min. This way, will enhance, most probably, any crystallization ability of the polymer. It is cold ''crystallization annealing. (a) at the time of isothermal stay you can follow the DSC signal (heat flow) vs time. If you record a slow exothermal event...then this is your crystallization. After that (b) you cool down the sample well above Tg and then heat it up, well above the expected melting region. Lets see then if you record a melting peak. If not, your polymer will never be crystallized, it is fully amorphous. In the case of recorded melting in the composite systems, this suggest two possible explanations: (a) the filler promotes a certain crystallization structure (you can identify it e.g. via WAXS) and/or (b) that specific sample was ''carrying on its back'' much differnt thermal history before you melted it! And question... Did you measured it in standard or modulated temperature rate i9n DSC?

In over 30 years of using various thermal analysis techniques, I have often found the same, so would recommend using a DMTA or DMA which will always see the Tmelt and Tg (glass transition), however small they may be - and we used to reckon has about 1000 times the sensitivity of DSC. However until recently you needed a large amount for DMA, but the development by an English research group of the material pockets mean that as long as you have 1 to 5 mgs of powder, we can still see both Tg and Tmelt. If you have no access to DMA then send me 20mgs or so of your material(s)* and I will run one to three off free of charge for you!

Yours sincerely John Gearing DMA specialist at Gearing Scientific Ltd, UK.

According to http://www.m-ep.co.jp/en/pdf/product/iupi_nova/physicality_04.pdf the melting point of polycarbonate would be around 230 degC.

The following PhD Thesis discusses the crystallization of PC:

http://scholar.lib.vt.edu/theses/available/etd-04192000-12580040/unrestricted/etd.pdf

This latter discusses DSC and the effect of fractionaton, heating rate etc.

I hope it helps to understand your own data.

Amourphous polymers do not contain crystalline domain and thats why you dont see any melting point. On the other hand, situation can be a lot different in an amorphous matrix based composite containing some fillers. So it is better to have a DSC analysis of the filler and then compair it with the DSC of composites. if they are in the same place, it is easy to identify the role of metal particle on the sharp transition.

Pramod, as you has seem, you have got several comments and infos from people involved in physical properties of polymers. However, I believe that you could provide us more information about composite: filler content and size, processing method of your composite, and the use of any processing aid, compatibilizer, etc. In my opinion, this information will be useful to answer your question.

Rina,

You are correct and I feel it happens to my understanding and believe but yet to be reproduced for many such polymeric systems.

The sharp glass transition point from the composite as you reported can be attributed to either polymer-metal filler matrix that formed or the metal filler alone.

Please read the attached article. It may help you.

Please read the attached article. It may help you.

Just to be clear (given the various comments): The glass transition in a polymer is a kinetic effect, while the melting point is a thermodynamic effect (phase transition). The glass transition is observed in DSC as an inflection point. TGA DOES NOT measure glass transition, it measures decomposition temperature. There is no relation between the two measurements.

Thanks to Mehra sir and Benhegyi Sir for providing informative documents

Useful reference from R. Mehra. Thanks for this paper. I am agree with comments from Rina Tannenbaum, where TGA only measures decomposition temperature and other thermal events. Also, the decomposition temperature can be observed by DSC, but this instrument is not suitable to measure this thermodynamic process of a sample.

@ Rina madam : Good explanation

Yes it is obvious that glass transition temperature is a kinetic effect over which the polymer chains are free to slide past each other more easily. At Tg there will be a step change where heat capacity of the material changes.

Any polymer shows transitions of Tg and Tm but the nature of peak depends on the crystallinity. For crystalline polymers, there will be a sharp melting point but a broad range of Tg. On the otherhand, for amorphous polymers, there will be a sharp Tg but a broad range of Melting temperature. In that case, it is advisable to take the average (at the centre of the broad peak)

@ Thomas: As I given most of information Matrix is polycarbonate, fillers are metal nanoparticles (around 30 nm), CNTs (having dia less around 8 nm), and ceramic (having particle size around 1 micron). In all the cases same thing observed (endothermic peak observed near the melting point of polycarbonate). Now if we talk about the filler content I taken 5 vol% of each filler (and wt.% is different for each due to different density). For each, endothermic peak is observed with a slight temperature difference.

Pramod,

Once polymer it is very complex situation but patiently reproduce the experimental part before commenting any study that made you recognizable to the scientific community..

The dependence of DSC peak intensity on metal filler concentration in composite can help to explain experimental data

Polycarbonate melt processed is normally amorphous, but in principle it can crystallise (as is well known from PC solution casting). I would propose that the nanofillers act as a nucleating agent for PC thereby giving a certain level of crystallinity. It might be fun to check with polarisation microscopy as well - maybe you can see spherulites etc. Also, you could consider isothermal annealing somewhere between Tg and Tm, to see whether the crystallinity increases further. The second DSC heating may or may not show a Tm depending on the intermediate cooling rate (also watch out for a Tc peak on cooling). As noted before TGA is useless for determining crystallinity.

Maybe you can confirm that you are seeing Tm melting peaks at roughly 230°C on heating? Tg should be at 150°C or so. Annealing is probably best done at 190-200 °C I expect. Tc should be around that T range as well.

I strongly recommend a careful reading of Rita Tannenbaum's answer. Also, please be more cautious with the terminology. No amorphous polymer can exhibit a melting point as melting is a first-order phase transition characteristic of crystalline substances or, for instance, a crystalline phase of semicrystalline polymers. Amorphous polymers, in turn, reveal the glass transition temperature. The glass transition is not a change from solid to a liquid state (or back) but is rather a point where the amplitude (and also generally the nature) of segmental oscillations of macromolecules rapidly changes: decreases or increases depending on whether you're cooling or heating up the system. Now coming back to your point: endothermic peaks you record in DSC not necessarily have to be related to the melting. Depending on polymer and the experimental conditions, the peak can stem from a phenomenon called an enthalpy relaxation. What kind of polycarbonate you're dealing with? Some of them are liquid crystalline and may also give rise to relatively sharp peaks in DSC traces upon mesophase transitions. Nanoparticles may affect the ordering in a liquid crystalline state and also the mobility of the polymer macromolecules (for the latter cf. 10.1103/PhysRevLett.84.915). Hence considering these effects is, in the worst case, not meaningless. Finally, what you consider as "sharp" and what's the temperature? If "sharp" means a peak breadth of 2-3 kelvins it cannot be a polymer due to: i) severe kinetic restrictions during transitions of these materials (entanglements etc.) ii) poor heat conductivity of polymeric materials, which additionally broadens the recorded peaks. Did you analyze nanoparticles alone? Perhaps there is a key to your puzzle.

In any case, check this book:

http://books.google.pl/books/about/The_Physics_of_Polymers.html?id=dWy9rrEsnFwC&redir_esc=y

@Pavel Sir

I mentioned earlier that the nano particles are of copper. It was purchased (having purity 99%) and used directly without any treatment.

I recommend both Adam and Pavel for their meaningful answer. Hopefully it make sense of every thing they described for real science.

I agree with Dr. Adam Kiersnowski answer.

It is because it is not a phase transition. In amorphous polymers already randomness is there and the segments are moving even at RT. By increasing temperature you are only increasing the frequeny of the chain segments so you don't see a peak.

YKB

I agree with Dr. Y.K Bhardwaj.

1)melting point is a temperature range for melting of some kinds of partial or crystalline polymers . 2)in industries, some people misuse glass transition phenomena as melting 3)in polymer physics, melting only happens from an ordered stated to an amorphous state of a substance.

Chi,

You are absolutely correct and the Industry people never try to solve the problem rather create a problem for the Industry.

I agree with Dr/Kiersnowski

I agree with all colleague who reported that amorphous polymers dont show meelting point. Amorphous polymers show only softning point which could not be considered as thermodynamic transition. It's a chain "slipping"of polymers chains that becomes possible at some thermal agitation.

Dillip,

Thank you! You are right ! Polymer concepts should be treated carefully.

Pramod,

Let me know the real problem so that I can help you in this regard.

Dear DilIip Sir,

I discussed the problem in previous comments. In brief, I did DSC analysis of the Polycarbonate and it is just following the trend what all eminent personalities commented above (exhibiting glass transition by step change and not showing any transition at its melting point). Problem is that when different different particles (metal, CNTs, and ceramic with nano and micron size ) was added in the Polycarbonate matrix separately, in most of cases glass transition not exhibited and in all the cases near melting point a sharp endothermic peak was observed (may be of melting or may be of other changes). So here I need an expert advise to explain it. However previous comments helped me a lot. Thanks to all contributors.

Hi Pramod,

DSC is not as sensitive as DMTA for Tg detection. Try to use DMTA for tan(delta) and this term is very sensitive for distinguishing a weak change such as glass transition from a noise. Good luck!

But the value from DMTA is usually higher than that from a DSC if it exists.

But the value of tan(delta) obtained from DMTA is usually higher than that from a DSC if it exists.

As Dr.Kiersnowski's answer,it is the good explaination. Let me add the further information. You can check liquid crystalline state by polarized microscope. You will see the liquid crystalline state of the sample with and without nanoparicle ( if you your polymer is LCP).

REgarding DSC, it measures the heat flow or hear capacity changes for materials tested. For amorphous polymer, if glass tansition is in the tested range, you will see the the scan curve changes in this area. Since amourphous materials only changes from sloid state to flow state, it does not change it heat capacity in this zone, therefore, you will observe any change from DSC. However, if you use crystal or hemi-crystal materials, such as PE, PP, we will observe the peak because the crystal disappear when it melted and also change the heat capacity of the materials. This is why you observe peak there. Hope it helps.

The results of a DSC scan depend very much on the thermal history or the treatment of the sample. Not all polymer exhibit melting transition when you scan without any treatment because their crystallization rate could be very slow. To observe a crystallization transition, it is a common practise to heat the sample to 40-50oC above the melting temperature to erase all the thermal history and than cool to a temperature where the crystallization rate is the maximum and anneal for a period of time, depending on the rate of crystallization. Upon completion of annealing, quence it to 40-50oC below Tg temperature. From there, scan up slowly, e.g. 10oC/min. You should be able to observe

I have to argue against Adam's statement that no amorphous polymer can exhibit a melting point. While melting of crystals is certainly a first order phase transition, the melting of amorphous solids can occur by second order or continuous phase transitions. The terms solid, liquid, and gas are well defined in terms of observables like long range order, local order, diffusion, flow, and elasticity. I refer you to a discussion of a solid-liquid transition we reported earlier this year, where a hybrid polymeric solvent-free nanofluid produced a second order (lambda) transition on freezing [“Organosiloxane supramolecular liquids – Surface energy driven phase transitions;” J. Texter, K. Bian, D. Chojnowski, and J. Byrom, Angew. Chemie Int. Ed. 52, 2511-2515 (2013); ]. The magnitude of this transition was accounted for by the loss in surface free energy of the nanogel (nanofluid) droplets when they coalesced to solidify. In a non-cross-linked polymeric solid that exhibits solid-liquid transitions, the lack of DSC-measurable transition enthalpy can be accounted for on the very marginal magnitudes of enthalpy and entropy change exhibited across the melting temperature range. We normally do DSC at scan rates to insure forming glasses and seeing glass transitions. I suggest studying amorphous solid-liquid transitions using SLOW scan rates; even better, study them using MDSC, and do not focus on a single temperature, but on a range of temperatures. Second order transitions are better thought of as chemical equilibria spread over a range of temperatures (or other intensive variable). It is important to differentiate amorphous glasses that provide bases for glass transitions, and amorphous materials that do not provide glass transitions. One can produce the latter by making suitable mixtures of oligomers to frustrate the possibility of forming an isolable crystalline phase. Almost any liquid or highly thermally activated polymer can be thermally quenched into a glass that will exhibit a glass transition. However, sufficiently frustrating materials, when cooled SLOWLY enough, become amorphous solids that do not exhibit glass transitions, but that still melt on warming.

No polymer is 100% crystalline (no Tg ) and no polymer is 100% amorphous (no Tm). It all depends on sample treatment, i.e. how fast you quence cool (to detect Tg) and how long you anneal (to detect Tm)

No Tg detected for neat (amorphous) polymer suggest your polymer is amorphous in state. However addition of metal fillers lead to formation of nucleation sites during processing conditions (depend upon experimental procedure) you followed to incorporate them. These sites lead to formation of crystalline zones in polymer matrix composite. Hence you observe a transition from melting of crystalline domains. Please also cross-check to ensure this is melting by comparison with Themogravimetry trace for same sample.

No Tm detected ....

Dr. Kiersnowski is correct regarding amorphous polymers not having a melting point by definition. However, if you're looking to examine various thermal reactions occuring within a sample that is not giving very good resolution, you may want to consider running the sample against another sample with known DSC characteristics, rather than against a blank reference pan.

For example, if your sample shows miniscule (but detectable) changes in the DSC at certain temperatures and let's hypothetically say 2mW heat flow, find a compound that you can run against that where at those same temperatures has a constant/steady heat flow of, hypothetically, -2mW.

Simply increasing the difference between the sample you'd like to analyze and the reference can improve your resolution greatly.

I agree with Aik Hwee Eng

I believe it is very difficult to find out the sample with same but negative heat flow change (say plus minus 2mW, as quoted by M. Webster). I have not come across any such literature, Many publications do not report the heat flow change. They only quote imp. parameters Tg, Tm, To, Tp, Delta H etc....

I propose to use rheology to characterize precisely the transition in polymers. With this technics one can measure the effect of transition on rheological properties and we often obtain a more precise response (than DSC) for Tg or Tm and so and particularly transitions which are not thermodynamic ones (as Tg for example).

In the case of amorphous polymers, the terminal domain allows to characterize by limit parameters as zero-shear viscosity, creep-compliance...

Amorphous polymers have a sharp Tg but a broad range of Tm where as reverse is the trend for crystalline polymers. For this reason, when DSC is run, one gets broad Tm peak for amorphous polymers and sharp peak for crystalline polymer.

Tm of filler can alter the results..

Promod, No such Tg is associated with any amorphous systems, may it be polymer or not.

100% amorphous polymers exhibit solely glass transition temperature.

100% crystalline polymers exhibit solely crystallization and melting temperatures.

Anything that is in between these two limits exhibits both glass, melting, and crystallization. However DSC technique is not very sensitive and hence the contribution of small content phase can be lost in the baseline. Thermal treatment may change eventually the ratio between crystalline and amorphous phases.

I agree with Adam Kiersnowski

Yes very true, rheological studies truly assist the analysis and provide reasoning for this type of behavior. I do not know how you introduced filler in the polymer matrix. Was it by melt extrusion or solution processing. Surely rheology along with TEM/SEM provide you great insight in dispersion of metal fillers.

Having just read all this correspondence, I would agree with an earlier answer that DMA or DMTA will give in our experience perhaps 1000 times the sensitivity of DSC or STA.

I personally have never used modulated DSC so am not sure how much more sensitive this is than the older linear DSC heating - but would be interested to hear from others who have used both. It appears that some DMAs are much more sensitive than others should also be considered too. We use the Triton DMA which is now P-E 8000 I believe. If the researcher does not have access to DMA then we could run a sample for him / her free of charge, but they may need to wait a few days for the results as our paid for work takes priority. We have been using DMA since the late 1970s and seen quite a few changes in the hardware and particularly the software since then! john gearing

Gearing Scientific Ltd Ashwell England.

If a material is 100% amorphous, it will not show a melting point transition at all - it technically is ALREADY liquid after undergoing the glass transition, only a very undercooled liquid. Hence, it can not have a transition.

If you are sure your metal fibres aren't undergoing any transition at that temperature (not only melting, but for example also a change of crystalline phase), then it's possible that the insertion of fibres altered the crystalline nature of your polymer matrix - in other words, that the fibres acted as nucleation sites that allowed some of the adjacent polymer to become crystalline. In that case, that part of the polymer would melt and cause a peak. You can easily esteem how much of the polymer did that by measuring the latent heat released during the transition and compare it to the known thermodynamic data for that specific compound. If it's less than 10% of the total, that could be a likely explanation.

An amorphous polymer has no crystalline regions which may undergo melting. Your DSC results of the composite prove that the polymer has crystellized in part at least.

All about this was already said by Adam Kiersnowski and by Gyorgy Banhegyi. It seems to me that it should be something that is not completely correct in the observations described in the answer.

Some amorphous polymers do exhibit melting points. Melting can be a first order OR a second order phase transition. Crystallization is a first order transition. Polymer materials have been documented that exhibit lambda-type melting and freezing transitions, characteristic of second order transitions. Did you ever see an amorphous wax melt? Similar things happen for polymers. Solid-liquid states are not defined by crystalinity, necessarily. Many are confusing these physical states with pacing motifs. Read my first effort in this string from last year, and try reading the reference if you have any more questions, ask!

Dear All ,

Thank you for your useful comments,

I found Tg for polymer-GNP compsoites is lower than polymer matrix by 5 C on the DSC but On the DMA there is no signifcant difference btween the nanocompsoites and PET. Could you please advise me if there is any suggestion or comments ###

thanks

In my opinion it might be due to two reasons:

1-errors in experiment (5C)

2-poor dispersion of nanoparticles in polymer matrix which lead in decrease of glass transition temperature of your nanocomposite

you fillers are able to recrystallize your polymer.

You can also do different DCS cycles. For instance a first heating, then a colling at slow speed (5ºC/ min) and then a second heating. During the colling the crystals, if any, will reorderer and thus during the second heting you may be can see the melting.