Tg of the polymer material is a second order transition because it takes place some temperature range. But LC transitions like rubbery to smectic, smectic to nematic and nematic to isotropic sometimes gives a sharp peak and in some cases as broad peaks and sometimes we arent able to observe. In these LC transitions can we say 1st order or second order?
Dear Tota, dear all,
Sorry to disagree with some people here. I will try to keep it shorter and easy…
Let's say that a first order transition involves an exchange of heat (latent heat) when going from order-to-disorder phases. Nevertheless, there are some peaks (hydrogen bonding, and other supramolecular interactions or even decompositions/reactions) which are not fully reversible and might induce errors of interpretation. Sometimes, TGA is required for that together with NMR or FTIR analysis.
For a second order transition, there is no exchange of heat involved. So, a glass-to-melt transition does not need energy - nowadays, this point is under discussion... the main difference with respect to the first order transition is that you should see a change in the slope in the DSC curve. That change is due to the change in the heat capacity of the material (volume). In first order transitions, it might also occur a change in the slope since the packing of molecules might differ and that the heat capacity. You can visualize it when analyzing the tangents on the left and right side of the peak.
Said this, a first order transition should show reversibility: endo and exo peaks upon heating and cooling, respectively, and also a shift as function of the heating/cooling rate. A second order transition strongly depends on factors like the degree of crystallinity and heating/cooling rate.
If you want to see whether your transitions are first or second order, you must run modulated DSC, where the non-reversing and reversing part of the heat flow is related to the first and second order transitions, respectively.
Now, in the case of polymers, elastomers or glassy networks, which are liquid-crystalline, if you see broad peaks is due to... POLYDISPERSITY! You do not have a single molecule whit a defined molecular weight. You have a sort of Gaussian distribution of chains. That's the reason. The short molecules melt first, and the largest molecules later. The maximum in the melting peak is related to the most probable molecular weight of your liquid-crystalline polymer chains.
To make it even more complicated, liquid-crystalline elastomers might also show order-to-disorder transitions which are… second order transitions! This unusual behavior comes from the fact of external fields or due to the presence of crosslinkers. In other words, at high temperatures where isotropic phase is expected, the material is stil nematic or smectic: the so-called para-nematic and para-smectic phases.
Please, check my papers if you have any doubt, or contact me any time.
Hope this is clear now.
SALUT!
Tony
I am not in expert in LC transitions. Nevertheless, the following example perhaps applies to your question.
In the case of melting transitions, which are 1st order transitions, you will always observe a more or less broadened melting peak in the DSC measurement unless the material is not an ideal crystal. That means, the more impurities or lattice defects your sample have, the broader the melting peak will be. (In addition, the peak width is influenced by the heating rate and the sample mass.)
Therefore, very sharp melting peaks can be observed e.g. for metals of high purity. In the cases of metals, the high thermal conductivity narrows the peak even more.
Please find attached a relevant introduction
http://plc.cwru.edu/tutorial/enhanced/files/lc/phase/phase.htm
smectic/nemtaic and similar transitions are first order transitions with endothermal peaks in the DSC curves - like meting.
Dear all;
first and second orders transitions are well defined mathematically and thermodynamically. The shapes are totally different. The shape of a second order transition (e.g., Tg: the glass transition temperature) is similar to a curve of a titration (e.g., acid-base), and for the first order it is a parabolic-like (upward or downward depending on the direction of the exotherm and the endotherm). Regards
I agree with Maity. The second order transition does not involve much energy. The first order is associated with large amount of energy.
Strictly speaking there is no energy change involved in second order transiiton. The enthlaypy changes continuously,the derivative of enthlapy (Cp) or that volume (expansion coefficient) changes abruptly.
You can save your data in ASCI format and open it with origin or any other software. Differentiate it twice and plot both the curves. You will be able to see, if it is first order or second order transition.
Dear Tota, dear all,
Sorry to disagree with some people here. I will try to keep it shorter and easy…
Let's say that a first order transition involves an exchange of heat (latent heat) when going from order-to-disorder phases. Nevertheless, there are some peaks (hydrogen bonding, and other supramolecular interactions or even decompositions/reactions) which are not fully reversible and might induce errors of interpretation. Sometimes, TGA is required for that together with NMR or FTIR analysis.
For a second order transition, there is no exchange of heat involved. So, a glass-to-melt transition does not need energy - nowadays, this point is under discussion... the main difference with respect to the first order transition is that you should see a change in the slope in the DSC curve. That change is due to the change in the heat capacity of the material (volume). In first order transitions, it might also occur a change in the slope since the packing of molecules might differ and that the heat capacity. You can visualize it when analyzing the tangents on the left and right side of the peak.
Said this, a first order transition should show reversibility: endo and exo peaks upon heating and cooling, respectively, and also a shift as function of the heating/cooling rate. A second order transition strongly depends on factors like the degree of crystallinity and heating/cooling rate.
If you want to see whether your transitions are first or second order, you must run modulated DSC, where the non-reversing and reversing part of the heat flow is related to the first and second order transitions, respectively.
Now, in the case of polymers, elastomers or glassy networks, which are liquid-crystalline, if you see broad peaks is due to... POLYDISPERSITY! You do not have a single molecule whit a defined molecular weight. You have a sort of Gaussian distribution of chains. That's the reason. The short molecules melt first, and the largest molecules later. The maximum in the melting peak is related to the most probable molecular weight of your liquid-crystalline polymer chains.
To make it even more complicated, liquid-crystalline elastomers might also show order-to-disorder transitions which are… second order transitions! This unusual behavior comes from the fact of external fields or due to the presence of crosslinkers. In other words, at high temperatures where isotropic phase is expected, the material is stil nematic or smectic: the so-called para-nematic and para-smectic phases.
Please, check my papers if you have any doubt, or contact me any time.
Hope this is clear now.
SALUT!
Tony
If thermodynamic equilibrium can be assumed ─ while following Ehrenfest's classification ─ the first derivative of Gibbs free energy with regards to temperature is discontinuous at the phase transition temperature. If it is continuous, but the second one is discontinuous, phase transition is said of second order. For a very low cooling rate from the melt ─ what can not be achieved in practice ̶ it has been hypothesized that the glass transition might approximate a second order phase transition. However, for any practical achievable cooling rate, glass transition does not occur at a definite temperature, but rather within a temperature range, and presents a cooling rate dependency, what is incompatible with a true (equilibrium) phase transition. For extended discussion, also focusing specific heat response, you may check: C.A. Queiroz, J. Šesták, Phys. and Chem. of Glasses - Eur. J. of Glass Sci. and Technol. Part B, 51(3) 2010, 165-172; https://www.researchgate.net/publication/231124196_Aspects_of_the_non-crystalline_state
Article Aspects of the non-crystalline state
a first order transition involves an exchange of heat (latent heat) , For a second order transition, there is no exchange of heat involved. So, a glass-to-melt transition does not need energy
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