Thermal transport and thermal structural domain in microfibers

Abstract

I would like to thank my committee chair, Xinwei Wang, and my committee members, Xianglan Bai, Nastaran Hashemi, Guowen Song, Yue Wu for their guidance and support throughout this research. I would also like to thank my family, friends, colleagues, and the department staff for making my time at Iowa State University a wonderful experience.

Thermal Transport in Polymers: A Review

“…Interestingly, this reduction in crystallinity was found to increase thermal conductivity. 89 After heat stretching, the thermal conductivity of UHMWPE nearly tripled (from 19-22 W/mK to 51 W/mK with only a small draw ratio of 6.6) although crystallinity decreased from 92 to 83% (Fig. 5b).…”

“…Furthermore, a drawing speed too high can also be detrimental for thermal conductivity improvement, which will lead to internal stress buildup and limit the achievable elongation ratio at a given temperature. 54,89 Heat treatment (physical aging) of drawn fiber will allow the free movement of chain segments, giving them additional time to rearrange themselves when the crystallization process is not enough for chain alignment. Both local heating and hot stretching are processes that involve heating part/whole section of the fiber above the glass transition temperature but lower than their melting temperature.…”

“…However, it is shown that factors like orientation, crystalline-amorphous interfaces, and phase distribution need to be considered, and thus a high crystallinity does not always guarantee high thermal conductivity if the crystallites are not well aligned in the polymer. 68,98 While the crystalline domains should have much higher thermal conductivity than their amorphous counterparts, 89 the existence of amorphous phases between these crystalline domains can present large thermal resistance. 7,87 In addition, the crystalamorphous interface can also scatter phonons.…”

“…7,87 In addition, the crystalamorphous interface can also scatter phonons. 89 It is not clear how large a role such interfacial thermal resistance plays in the overall thermal conductivity. It has been shown that if the amorphous region consists of linker molecules, the crystal-amorphous interface is connected by covalent bonds, which present much smaller resistance than the amorphous region itself.…”