單根尼龍6奈米纖維之熱傳導率

Abstract

本論文旨在研究單一根尼龍-6奈米纖維之熱傳導率。以前的研究中發現，將聚乙烯經過超拉伸之後而形成之奈米纖維，其熱傳導率遠高於其塊材之熱傳導率。然而目前對於高分子奈米纖維熱傳導率的提升尚無法有完整解釋。由於尼龍-6與聚乙烯同為結構簡單的線性高分子，故我們認為其奈米纖維有與聚乙烯相同之熱傳增強特性。在此研究中，我們應用微機電製程製作出一微元件，並以一真空腔體系統量測尼龍-6奈米纖維之熱傳導率。實驗結果中發現尼龍-6奈米纖維內部為非晶相型態，並因為此聚合物之二級相變之影響，使得其熱傳導率不僅與溫度有關，且與時間相關(在此稱為熱變)。在低溫時，奈米纖維呈固態彈體特性，故其熱傳導率隨著溫度上升而上升，當溫度高於150 K時則變成黏體而使熱傳導率劇降。且由於此奈米纖維內部之黏體非平衡特性，故奈米纖維內之聚合物鍊隨著在低溫持續的時間愈久而排列越緊密，造成熱傳導率隨時間增加而增加。此外，尼龍-6奈米纖維有嚴重的熱遲滯現象，所得熱傳導率在升溫路徑與降溫路徑呈現極大差異。此尼龍-6奈米纖維在溫度約為150 K時有一最大熱傳導率42 W/m-K，此值為其塊材之140倍，這是目前所發現擁有最高熱傳導率之非結晶高分子材料。

This study aims to investigate the thermal conductivity of a single nylon-6 nanofiber. It has been shown that an ultra-drawn polyethylene nanofiber has a thermal conductivity much larger than its bulk value. However, the physical mechanisms which result in the thermal conductivity enhancement in the polymer nanofiber are not fully understood. In this study, the thermal conductivity of a single nylon-6 nanofiber was investigated. Nylon-6 has a simple molecular structure as that in polyethylene. Thus, it is expected that it might exhibit similar thermal conductivity enhancement during nano-structuring. A micro-device array was made to measure the thermal conductivity of the a nylon-6 nanofiber and an ultra-high-vacuum cryostat with a temperature ranged from 30 K to 200 K was adopted to study the heat transfer at low-to-moderate temperature conditions. Thermal conductivity of the nanofiber not only depends on temperature but also depends on time (called thermal rheology). The secondary phase transition in the amorphous nylon-6 nanofiber is presumably responsible for the observed phenomena. At low temperature, the thermal conductivity of the nylon-6 nanofiber increases with temperature due to its elastic characteristic, whereas the viscous characteristic of the nanofiber causes the large thermal conductivity reduction at approximately 150 K. Moreover, at low temperature, thermal conductivity of the nanofiber increases with annealing time because of the physical-aging of the polymer. A maximum thermal conductivity of approximately 42W/m-K was obtained at 150 K, which is 140 times that of bulk nylon. This is the first work demonstrating that the amorphous polymer can have a large thermal conductivity comparable to steel.