I am using CF/epoxy (thermoset) and CF/PEEK (thermoplastic) composite for some thermal related research. The glass transition temperature of the epoxy is 165 ℃ and that of PEEK is 143℃. According to the literature, both thermoset and thermoplastic polymer can undergo a 'softening' period when the temperature reaches Tg. But what is the difference of the thermal response of thermoplastic and thermoset polymer when the temperature is around Tg? Is there any quantitative or qualititive conclusion with regard to the module loss of the two material? Looking forward to some discussion.
Dear Ge Jia, this is a basic question treated in any textbook on mechanical properties of polymers. Concerning thermosets, Tg is an increasing function of the curing degree, and may reach the decomposition temperature. Some properties go up such the elastic modulus and chemical resistance, other go down such as brittelness. Thermoplastiocs show three different behaviors whether: crystalline, semi-crystalline, or amorphous. Theoritically speaking, crystalline polymers don't have any Tg, so no transition until Tm (melting) is reached, where a drastic decerase in mechanical properties is noticed. Semicrystallines show show two stages of response, specific to both amorphous and crystalline phases. Amorphous polymers have only Tg. Their melting (softening) is over a wide interval of temperatures. My Regards
Abdelkader BOUAZIZ Hi Abdelkader, thanks for your help. I am using some carbon fiber reinforced epoxy composite. During the molding process, the epoxy is fully cured. For the fully cured material, if the temperature is rised up to the Tg again, I believe the material would experience some 'softening'. The elastic module of the material should decrease. I am wondering whether there is a specific comparison of the elastic modulus reduction at around Tg of thermoset and thermoplastic.
Yes this is true for low degree of cure, fully crosslinked don't show appreciable softening. May be you are using a special type of epoxy, some bioressource oils based epoxy have such a feature. My Regards
Dear Ge, your question is a basic question concerning the nature and origin of Tg and its relations to structural relaxation. For many years, Tg is defined/measured by many people in different way. If you take into consideration ' a three phase model', then you willsee that the answer given by Abdelkader is only a part of true. I recomend to read about 'paracrystlinity' and Hosemann model/definitione. It is also a question of a supermolecular struture of polymeric material. It is also interesting to combine DMTA, DSC, PALS and DIL methodes for the structure study. However then you must be carefule with samples preparation. Regards, Adam
Dear Adam Danch, what you have said is true for free molecules such as liquid crystals, the question is about a crosslinked structures, no such freedom of molecules is possible. Tg is related to the amorphous phase of a material. My Regards
Glass transition in polymers is related to dynamics of macromecules in amorphous phase. Glass transition temperature on heating corresponds to onset of cooperative segmental motions, hence above this temperature deformability of a polymer increases drastically, transition from glassy to rubbery state takes place. Glass transition temperature depends both on chain rigidity and intermolecular interactions. Restrictions of the molecular mobility increase glass transition temperature. In thermosets the restrictions are due to chemical crosslinks. The more crosslinkings the higher is the glass transition temperature and the lower is the modulus drop. In semicrystalline thermoplastics crystalline regions act as physical crosslinks. The higher is the crystallinity the higher is the glass transition temperature and the lower is the modulus drop. In amorphous thermoplastic homopolymers there are no crosslinks, modulus drop during glass transition is very large.
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