Neither being a polymer chemist, I lightheaded would assume the following.

HDA polymer leads to irregular packing of the polymer chains, i.e. relatively lower melting point. Initially adding AA this, similar to the usual melting point reduction adding a second compound, would initially lead to more disorder. AA polymer itself allows for more regular packing of the chains, thus a considerably higher melting point. Obviously there must be a minimum in between. Not my topic, but would assume that you should find this behaviour also for mixtures of non-polymer materials.

I assume there is absolutely no role of moisture in this process of synthesis. If there is, then explanation wold tend to support molecular weight effect. Otherwise, equilibrium concentration of monomers are playing significant effect in determining the rate of reaction or the kinetics of the process. Explanation provided above by Titus would make more sense in the later case. Packing effect must be in line with the changes in the crystallinity. A sequential analysis of time dependent sampling with crystallinity determination will shed more lights.

I guess you don't make a blend but a co-polymer. First a compliment, for not beeing a polymer chemist your samples look very nice. Titus Sobisch is right. The polyamide from HDA (not optimal choice of abbreviation for a publication because polyurethane chemists chose this abbreviation for hexamethylene diamine = hexane diamine) and hexamethylene diamine hat at least some order. This order is disturbed if AA is added. Near 50:50 you reach the maximum of disorder and only at higher AA content the potential of AA to form ordered semi-crystalline polyamides dominates. You can observe this phenomenon for many other cases in polymer chemistry - nearly in every case when the both homo-polymers are semi-crystalline. I want to give you two example from my own experience. If you make a polyphenylene sulfide (PPS, melting point 270°C) from p-dichlorobenzene and Na2S and put more and more 1,4-dichloro biphenyl in it as co-monomer, first the melting point drops and near 50:50 ratio the polymer is even amorphous (without melting point). At higher 1,4-dichloro biphenyl contents the melting point raises and raises until it is in the rage of 350°C. A similar behavior shows the monomer couple Bisphenol-F and 4,4-dihydroxybiphenyl in polycarbonate synthesis. Both homo-polymers are semi-crystalline, the melting point is to high for processing the polymers and they precipitates from solution during preparation. A 60:40 mixture is however amorphous with only a glass transition temperature of about 150°C, exhibits no melting point and stays soluble in dichloromethane. If you make these kind of experiments with a monomer couple whose homo-polymers are amorphous you can observe a different behavior, the glass transition temperature of the co-polymers is in nearly all cases  between both homo-polymers (there are very few exceptions where the glass transition temperature of the co-polymers run through a maximum).

Thank you all.

Burkhard Koehler, Thank you for the compliment. I must admit that these were actually prepared by one of my colleagues, but he also had no experience with polymers until this. I shall pass along your compliments.

Hello Jack

 Indeed you samples look good. I agree with Titus' explanation about difference in the melting point of pure polyamide made of HDA vs AA. You call them blends but I am wondering how they were actually prepared. Depending on the preparation they may be block-copolymers or fully miscible blends/alloys since the structure of these two polyamide are very close to each other. In any case, it is normal to notice a discontinuity in the melting point as mentionned due to the great difference in crystallinity of these two polyamide. However, since we do not have any of your DSC curve, nor SEC curves, nor XRD, nor TEM of any of the samples it is hard to make any conclusion on this.

At one end of your graph essentially you report  the melting pointy of the HDA polyamide crystals disrupted by the presence of the AA polyamide and at the other end you report the melting point of the AA polyamide crystals disrupted by the presence of the HDA polyamide.

The shape of the endotherm would have been quite interesting to have and/or the heat of fusion of the samples. Would you mind to share some example at say 20AA/80HDA and 80AA/20HDA. or any of the other test mentionned above?

The melting point has already been discussed in detail. When two types of acids are used there is possibility of depression of melting point in the initial stage. Further, the presence of double bond in one diacid might prevent good packing. With increasing AA, the regularity is improved and the melting point increases.

Hello Guy,

I believe my calling this a blend as opposed to a block copolymer or something else only illustrates my ignorance to this field. Hence, my disclaimer at the beginning of the question.

To clarify, these materials were prepared by precipitating AA/HMDA and HDA/HMDA salts. The salts were physically mixed and polymerized under nitrogen in a tube furnace at 250 C.

We do have XRD on pure AA and pure HDA polyamide samples. They both have signals near 2theta ~ 20 degrees. However, the signal is much noisier and broader with HDA. I have attached below some of the DSC curves.

Jack in you legend you may have inverse the labels. I suppose the black curve is the 100% HDA/ 0% AA and the green curve is the  0%HDA/100%AA

From your preparation description, it is indeed a copolymer that may just be random. Your colleagues may like to repeat the experiments since there maybe some variability from the way the salts are precipitated and then dispersed.

What you call the "melting point" of 100%HDA on the black curve is so close to the possible Tg that it could just be an artefact. If you could have access to some MDSC you could separate what percent of signal belongs to the Tg versus the  melting behavior.

Another practice used for DSC of polymer is to do a heat cool followed by a rerun heat cool.   You could also anneal the 100%HDA just below the "melting point", rerun a DSC and check whether the "melting endotherm" would have grown or you just have a glass temperature transition. 

If you carry the DSC experiments please keep me posted with the observations. 

Guy,

Those DSC are 3 runs of heating and cooling. Each time they fall very closely on top of each other. If I understand what you're saying here correctly: then if this were a glass transition temperature then I would not see it in multiple heat/cool cycles? Or am I misinterpreting what you are saying?

Jack,

You are interpreting correctly, however below you'll find a more detailed explanation.

The last two graphs looked like a zoom on the low temperature zone of the first graph which is good to see what's going on but not reruns . In the cooling of the samples you cannot see a recrystallisation exotherm for the high concentration HDA but just a shoulder that looks like the glass transition temperature. It  may also be possible that the HDA doesn't recrystallize due to the defects and the weakness of the heat of fusion.  On the second heat cool, if it didn't have a chance to recrystallize the melting endotherm wouldn't appear. However, at this point since these samples with a high concentration of HDA are highly amorphous the "endotherm" may just be a strong Tg that is changing position due to the change of concentration of AA.  By the way, It will be good to run a faster scan to spot the value of the Tg of the pure AA or quench the other samples to have a better signal of the Tg and see how the Tg keeps increasing with increasing concentration of AA toward the Tg of pure AA.

Therefore, what I was suggesting is to try to increase the crystallinity of the HDA or remove the artefact by annealing the sample before running the first scan DSC. In addition, in this case the use of a MDSC would be most appreciated if indeed it is a melting behavior that it is overlapping with the glass transition temperature. If indeed you are able to improve the crystallinity of your 100% HDA sample then you'll also get a better XRD spectrum.

There are other options but I don't think that at this point you are interested to get involved in more experiments.

Jack, the glass transition temperature is not a thermodynamic change of state but a change from a glassy mechanical behavior to a rubbery behavior. The presence of the double bond in the HDA chain  is less flexible than the straight C-C bonds chain of the AA. It  will increase the temperature at which this transition occurs.

You may want to check my posts elsewhere at this forum: https://www.researchgate.net/post/How_do_I_maintain_the_same_viscosity_of_a_co-polymer_made_of_DADMAC_and_Acrylamide