Basically, Tg can be defined as the temperature of molecular chains start to move by external force. So take normal polymer (semi-crystalline polymer) as an example, when polymer molecular structure becomes more ordering, the movements of molecular chains are constrained, which reflecting as the increase of Tg. On the contrast, Tg decreases. As you said, in order to change Tg without changing polymer composition, you can try to change polymer crystallinity, morphology (i.e. molecular size), and also the structure of single molecular chain (introduce branches to main molecular chains). These can be done by controlling cooling rate and isothermal time during heat treament, and polymer modification, respectively. On the other hand, increasing pressure to polymer will also increas molecular crowing and interaction, resulting increased Tg.

May be it is a codoping materials that will shift the Tg depending on the bleding ratio. Actually, It should be measure by DSC, TMA that will find heat fllow clearly.

Can change...thru molecular structure modification. Eg. normal polyethylene (PE) modified to crosslinking PE

I suspect there may be additional ways to change Tg. Similar to Mr. Said's cross-linking idea (which may require a chemistry change), deformation in a single direction (extruding, stretching) could align the molecules to promote crystallization. Irradiation might be another method to accomplish the change.

Thank you for your answer.

eng. Ruaa Aldogaili

It could be make some fifferent, especially in 1st heating, 2nd heating....the Tg will different,. It depend on what you select the turning point.

But the most important thing is that the realy Tg will indicate after choseion realease.

If you just still want to change the Tg vaule, 2 ways:

(1). Change another polymer materials.

(2). Modify your polymer structure or molecules strain in 3D space.

Heating at 20-40oC/sec? Hmmm did you mean 20-40oC/min? I assume your thermal excursions are longer than 5 seconds.

Heating at the larger rates will create non-equilibrium heating effects, and due to the low thermal conductivity of polymers. In other words, you can heat the sample such that the region closest to the pan is at one temperature and the bulk is at a different temp. Yes the DSC will present a graph of output and its possible that the thermogram will look different, but it may not represent anything other than transients in the system. These effects are magnified with larger sample sizes where you overwhelm the capacity to heat the polymer quickly. Its possible that on the cooling side, you might find skin-core effects where for example with polyethylene, the outside surfaces are quenched with a lower degree of crystallinity and the inside core-region is cooled much more like it was cooled under equilibrium cooling conditions.

Yes it can be done by heating cycles in slow heating and longer holding time

Thank you for answer.

eng. Ruaa Aldogaili

Glass transition depending different variable

structure of monomer for example if you have benzene in main chain you have a rigid structure and tg reduce

or if you use a monomer that have long branch relative type of monomer can reduce or increase tg.

Polarity of monomer effect in your tg.increase of polarity interactions of between chain polymers is more and reduce of tg.use of plastisizer can reduce tg.

And ..........

Interesting discussion. I'm not sure what is trying to be accomplished. From my simple view, Tg asymtotically approaches a fixed value for a specific polymer composition as molecular weight rises or dispersivity changes. Incomplete reaction , impurities, plasticizers all can greatly impact the observed Tg. Subtle changes in molecular weight will move the obsered Tg around. When thermally cycling, mild degradation products or smaller oligomers will often act as plasticizers. As stated earkier DSC is an excellent way to track Tg changes. However, polymer systems represent a population of molecular weights. You need to also look at changes in dispersivity and molecular weights to understand if the populations are changing and consistent with any observed Tg changes.

Thanks, your point view is very helpful.

eng. Ruaa Aldogaili

Slower the heating rate..safer it is so that you do not miss the elusive tg. Usually, if the polymer behaves like glass (most of them do) and tg takes place over a temperature range. Only specifc composition determines this temp. range. Therefore, if one is just tiptoe-ing just go slow. And repeat the expt. to verify results.

Yes, Tg is defined as a range of temperature. Tg is also dependent on the thermal history of the materials. With change in cooling rate, one can get different Tg value.

How many degrees do you want to change the Tg value? It is possible that you change it but you can not see it. Be carefull!!!!

Ok, thanks. I will be careful.

eng. Ruaa Aldogaili

Basically, Tg can be defined as the temperature of molecular chains start to move by external force. So take normal polymer (semi-crystalline polymer) as an example, when polymer molecular structure becomes more ordering, the movements of molecular chains are constrained, which reflecting as the increase of Tg. On the contrast, Tg decreases. As you said, in order to change Tg without changing polymer composition, you can try to change polymer crystallinity, morphology (i.e. molecular size), and also the structure of single molecular chain (introduce branches to main molecular chains). These can be done by controlling cooling rate and isothermal time during heat treament, and polymer modification, respectively. On the other hand, increasing pressure to polymer will also increas molecular crowing and interaction, resulting increased Tg.

Thanks.

eng. Ruaa Aldogaili

The key to understanding the glass transition in polymers is the change of the microscopic dynamics, and any structural changes are therefore a consequence of it.

I suggest everyone read the article published in Science devoted by The Microscopic Basis of the Glass Transition in Polymers from Neutron Scattering Studies. It is very useful information. I hope it will provide answers to many questions that are discussed.

http://www.lehigh.edu/imi/docs_relaxation/papers/Frick_Science_1995.pdf

Agreeing with the previous colleagues I try to list shortly some factors that may influence the measured Tg value

- test method

- test frequency

- coolingheating rate

- phyiscal aging (volume relaxation, annealing below Tg)

- crosslinking

- copolymerization (main chain and grafting)

- plasticizers

- blending

- filler addition

In order to be more specific you should provide some details on the effects/porperties to be achieved

I have experience with getting different results by changing the heating rates when I do DSC measurements. As rightfully, pointed out Tg is defined as a range of temperature - changing the heating or cooling rate changes the 'temperature-resolution' of the result.

So I believe it can change.

I think that the other thing is that Tg is not an intrinsic property - how you measure it as well as the material's thermal history can have a huge impact on the realized result.

Dear Ruaa,

A Tg variation can be induced by moisture absorption.

Regards

I do agree with the comments of Feng Zhao. I my point of view Your may change the Tg of a polymer by once heating in the Tg range and sudden quenching in water. This practice could lower the Tg range.

repeated thermal cycling also alters the Tg of a polymer

Tg actually can be obtained by DSC and DMA, and they are different, also if you use different rate of rising temperature Tg will different, so what is your mean to change it.

Btw, you can change it by put nano particles inside and make the polymer into composite or change the monomer and composition of polymer, then you will also can change the Tg

Tg of a polymer may be changed on heating if it undergoes some change like crosslinking or degradation. If there is no change taking place on heating then there may not be a change in Tg.

Moisture absorption, plasticizers, contamination with impurities will influence the Tg.

If the polymer undergoes any thermal treatment, maybe even below Tg, the molecular dynamics change and Tg also changes.

So, it is possible.

Any additives to epoxies (normal ambient room temperature settings ) possible ?

@ Mee: in epoxies usually so-called active diluents are used to adjust the mobility and the Tg value. These are monofuncntional epoxies which reduce the crosslink density.

It depends on type of polymer. For example, Tg of thermosets relates with curing content. More it is cured, higher will be its Tg until max Tg (100% curing) is achieved. For basic plz check this link : http://csacs.mcgill.ca/francais/docs/CHEM634/DSC_Hunt.pdf

we will be able to help if you precise your polymer

In epoxy adhesives, Tg depends also on the curing temperature: Tg will always be lower than the temperature at which the adhesive was cured. Apart from this, resin and hardener control Tg - by far the most important influence for these materials.

Another approach: by attaching the polymer to a substrate will limit the mobility of that chain-end and thus increase the Tg of the polymer. By that you will change the Tg of the polymer without affecting the composition.

I am also searching for polymers whose Tg could be more than the present level ie. 90 deg C (application method at normal temp).  But could not find an answer.

TG foundation is based on molecular flexibility and free volume (both types fixed free volume and free volume that goes through the bulk of the sample). Whatever increase or decreases each of these factors its going to change TG. You want to change TG without changing polymer composition you can play with crystallinity level cool it off fast or cool it off slow from the melting point.