The new method effectively enhance the thermal conductivity of the polymer over 200 times
Compared to inorganic metal, ceramics, and carbon materials, organic polymeric materials of low thermal conductivity, generally considered to be a heat insulating material, which greatly limits the polymer-based heat sink material in high demand and high integration of high power the development of technology related to electricity. The main reason for the lower degree of crystallinity is poor thermally conductive polymer material, so many researchers currently employed to improve the crystallinity of the polymer way to improve thermal conductivity, but the process type polymer materials, processing methods and equipment are It has great selectivity, so it\’s hard to get large-scale application. Short and long chain macromolecular structure determines the material properties of the polymer, then, in addition to improving the degree of crystallinity, for low crystallinity polymer, whether the adjustment of the molecular chain conformation to improve the thermal conductivity achieved by it?
[research]
In response to this question, recently, Professor Chien-Lung Wang Jiao Tong University Joint Taiwan National Taiwan University Professor Lu Ming-Chang research team found that by inhibiting relaxation low crystallinity polymer molecular chains (main cranking chain of the group) to greatly enhance the thermal conductivity of the material. In the study, different molecular weights of nylon -6 (N6) and nylon -11 (N11) nanofibers were annealed at different temperatures and times, found that when the annealing temperature is lower than the relaxation temperature of nylon, a material of the heat conductivity will increase, and with the increase of the annealing time and increased: the molecular weight of the nanofiber of nylon-6, under the condition of T = 120K of annealing 2.86 × 10 6 s [ 123] (~33 days), the thermal conductivity of from 0.27 W m -1 K -1 up to 59.1 W m -1 K – 1 , improved 217-fold. The reasons for this phenomenon is that when the material is nylon amorphous region the molecular chain methylene crank movement is limited, which is advantageous to increase the pitch and reduce the overall conformation of the chain ordering of molecular chains, thereby help improve the thermal conductivity; conformational transition and take some time, and therefore improve the thermal conductivity of nylon has a strong dependence on the time. Meanwhile, the authors also found that the molecular weight of the low-temperature annealing to enhance the thermal conductivity of nylon has a greater impact – higher the molecular weight, the thermal conductivity of the faster lifting. Is calculated by the theoretical model, nanofibers molecular weight nylon-6, under the condition of T = 120K annealing treatment 46 days later, the thermal conductivity can be increased to 518 W m -1 K – 1 . This study presents a generally applicable method for low crystallinity polymer materials to enhance the thermal conductivity and the theoretical model analysis has important implications for research, preparation and development of future high thermal conductivity of the bulk polymer materials. The study, entitled \”Heat Transfer of Semicrystalline Nylon Nanofibers\” published in the (attached description link) on \”ACS Nano\” journals. Download: 
[Photo A]
FIG. 1. (a) DSC Results of nylon nanofibers (calculated crystallinity degree); (B) each species nylon nanofiber infrared polarization results (calculated degree of orientation)(Mn = 42k) , N6-B (Mn = 26k) , N6-C (Mn = 6k) and N11 (Mn = 32k) . Characterization by DSC, of the four crystalline nylon were 21.7%, 21.4%, 21.6% and 20.7%, the amorphous structure as a main molecular chain, a low crystallinity of the material. IR results showed that the four polarization nylon nanofibers have high internal molecular chain orientation, whereas the latter results also found that the molecular chains are highly oriented nylon low temperature (T < T γ ) is an important post-processing conditions to enhance the thermal conductivity.
FIG. 2. (a) thermal conductivity of nylon nanofibers on the dependence of T and annealing time t; (b) nylon nanofiber thermal conductivity changes during cooling; (cT γ <T < T β ) and 120K (T < T γ ), nylon nanofiber process. Found nylon nanofiber cooled from a high temperature to the annealing process, the thermal conductivity of nylon nanofibers essentially unchanged (N6-A, N6-B, N6-C and the thermal conductivity is N11, respectively 0.42,0.35 after prolonged treatment at 0.32 and 0.5 W m -1 K -1 ), and all kinds of nylon nanofibers conditions T = 210K, the thermal conductivity also remained unchanged, . When the annealing temperature T = 120K (T < T γ ), all of the thermal conductivity rates of nylon nanofibers processing time increases with increasing [ FIG. 2 (c) shown], and increase the process temperature, the thermal conductivity of nylon nanofibers sharp drop as shown in Figure 2 (d).
FIG. 3. (a) the thermal conductivity change with nylon fibers annealing treatment (T = 120K) time; theoretical model (b) of established inquiry nylon nanofiber thermal conductivity and molecular the relationship between relaxation time of the chain; (c) nylon nanofiber microstructure molecular chain structure before and after treatment at a temperature below the schematicMn = 6k ), treated 2.86 × 10 6 s after thermal conductivity lifting to 59 .1 W m -1 K -1 . Establishment and analysis by theoretical models, asWere found to increase the thermal conductivity of a nylon nanofiber low temperature to increase the molecular chain conformation amorphous region caused by the degree of ordering, as shown in a schematic view 3 (c): In the T < T [ 123] γ , the movement of the crankshaft methylene molecular chain nylon is greatly suppressed, causes the lifting of the chain and chain ordering reduced pitch; as time increases, the degree of ordering is gradually lifting chain and decreasing pitch, the nanofiber and thus increases the thermal conductivity of the rising time. Meanwhile, the results show that the thermal conductivity change there was a marked dependence with molecular weight – the molecular weight of nylon to improve the thermal conductivity rate can be increased with the increase in the processing time increases. On binding model was found to increase the storage modulus of the polymer phonon help to improve the transmission speed, thereby improving the thermal conductivity. Low molecular weight nylon in low-temperature annealing process the storage modulus rapidly reaches equilibrium, and thus to enhance the thermal conductivity mainly depends on the transition-chain conformation. High molecular weight nylon due to the storage modulus reached equilibrium long time, in addition to the entire annealing process may rely chain conformation change, it can rely on the transfer speed to further increase the phonon thermal conductivity; Through theoretical calculations, Analysis calculated N6-a ( Mn = 42k ) a thermal conductivity of 46 days after treatment at 120K up to 518 W m -1 K -1 [123 ] . [summary] Study found using suppression nylon molecular chain relaxation, i.e. the polymer chain conformation adjustment method may greatly enhance the thermal conductivity of the low crystallinity polymer material, which enrich the people semicrystalline understanding the polymer heat transfer, while providing a solid theoretical and experimental basis for the future preparation of a variety of semi-crystalline polymer material is a high thermal conductivity. Original link: https: //pubs.acs.org/doi/10.1021/acsnano.9b07493
