半結晶單根尼龍11奈米纖維之熱傳導率

Abstract

本論文主要在研究半結晶單根尼龍-11奈米纖維之熱傳導率。之前的研究中發現，尼龍-6奈米纖維在110 K退火的情況下，其熱傳導率有140倍的上升。實驗結果發現尼龍-6奈米纖維內部主要為非晶相型態，因為聚合物之二級相變影響，使得熱傳導率不僅與溫度有關也與時間有關。由於尼龍-11內部主要為非晶相型態，因此非晶相區的型態是我們主要探討的重點。1959年Kawaguchi分析尼龍-11在350 K、230 K與150 K時，分別有α、β、γ等二級相變化，當溫度高於350 K時發生α相變，分子鏈開始做大規模運動，而造成分子型態有巨大的改變；當溫度高於230 K時則發生β相變，此時存在於非晶相區域的分子鏈中的未鍵結醯胺基團，將開始做局部運動；而當溫度高於150 K則為發生γ相變，此時分子鏈的 開始做局部的運動，稱為曲軸型運動(Crankshaft motion)。當溫度低於150 K時，高分子奈米纖維成固態彈體特性，其熱傳導率隨著溫度上升而上升，當溫度高於150 K分子鏈有足夠的能量作曲軸型運動(Crankshaft motion)則變成黏體，進而使熱傳導率驟降。當溫度低於150 K時，奈米纖維內之分子鏈呈現凍結狀態，故原先提供分子鏈做運動的空間會因為分子鏈在低溫持續時間越久，空間會越小，造成空隙減小使缺陷變少，導致熱傳導率會隨著時間增加而增加。然而根據之前的研究發現，尼龍-11分子鏈之γ運動現象較尼龍-6劇烈，主要原因是尼龍-11分子鏈有兩種曲軸型運動，因此我們選用尼龍-11當我們的研究材料，想探討曲軸型運動的數量對熱傳導率的上升機制影響。 在此研究中，我們應用微機電製程製作出一微元件，並應用電紡織將尼龍-11高分子溶液拉伸成尼龍-11奈米纖維並橫跨於微元件上，將微元件置於一真空腔體系統量測尼龍-11 奈米纖維之熱傳導率，透過差式掃描熱分析儀(DSC)儀器發現尼龍-11奈米纖維內部80%為非晶相型態，接著我們量測尼龍-11奈米纖維之熱傳導率隨溫度變化，分別在不同溫度點進行退火，其溫度點為50 K、80 K、110 K及190 K，發現位於溫度點190 K(低於β轉換溫度)退火時，其熱傳導率為定值0.4 W/m-K，其原因之一可能為很少部分的醯胺基團被抑制住了，導致分子鏈間能靠近的空間很少，另一方面是雖然我們抑制住了β相變，γ相變的運動還是會阻止醯胺基團靠近彼此。當溫度低於150 K，其熱傳導率在固定退火溫度時，熱傳導率(k)與時間(t)會呈現k∝t^α，α為熱傳導上升速率，接著我們分別在50 K、80 K及110 K退火6320分鐘，發現溫度越高，熱傳導上升速率α越大，其原因可能為分子鏈有較高的能量，能更快速縮短彼此間的距離。目前量測尼龍-11奈米纖維之熱傳導率為9.8 ± 0.1999 W/m-K，此值為其塊材之49倍，這是目前尼龍-11所量測出最高之值。

This study is mainly about measuring the thermal conductivity of single Nylon-11 nanofibers. It has been found that the thermal conductivity of a single amorphous Nylon-6 nanofiber is 140 times that of bulk Nylon 6 in the previous study. The thermal conductivity is not only dependent on temperature but also time-dependent because of the second order phase transition in the amorphous nylon-6 nanofiber. Given that nylon-11 is also amorphous, we expect that nylon-11 nanofibers can also exhibit similar thermal conductivity enhancement. Kawaguchi analyzed the Nylon-11 and found out that Nylon-11 would occur the α, β, and γ relaxation at 350 K, 230 K, and 150 K respectively. α relaxation means the molecular chain will do large scale movement, which will cause its structure have a great change when the temperature is above 350 K. β relaxation means the amide groups in the amorphous which are not hydrogen-bonded will do segmental motion when the temperature is above 230 K. γ relaxation means when the temperature is above 150 K, the molecular chain in the amorphous will do the crankshaft motion. When the temperature is above 150 K, the molecular chain will start the crankshaft motion and the thermal conductivity will be reduced largely. If the temperature is below 150 K, the molecular chain will be frozen. Therefore the original space for the molecular chain to do the crankshaft motion will be reduced with time. Once space becomes smaller, the number of a defect will be reduced. It will lead to the thermal conductivity increase with time. And according to the previous research, the gamma relaxation of Nylon-11 is stronger than Nylon-6’s. The main reason is that Nylon-11 has two crankshaft motion. However, we choose Nylon-11 to be our experimental material to study the number of crankshaft motion on this heat transfer mechanism. In this study, nylon-11 nanofibers’ thermal conductivities were measured. DSC (Differential scanning calorimetry) was applied to measure the crystallinity of the nylon-11 nanofibers and it was found that the nanofibers contain 80% amorphous part. The thermal conductivities of the nylon-11 nanofiber at different annealing temperature (50, 80, 110, and 190 K) were measured. We found that the thermal conductivity is constant at approximately 0.4 W/m-K when annealing at 190 K (below β but above  transition temperature). One of the reasons for this phenomenon might be the amide groups which is not hydrogen-bonded in the amorphous are very few. The other reason is that although we restrain the β relaxation, the γ relaxation still stops the amide groups come closer to each other. The thermal conductivity will increase with time k∝t^α (t for time, k for thermal conductivity, and α for thermal conductivity rise rate) when we annealed the nylon-11 nanofiber below 150 K. The results showed that when the temperature is higher, thermal conductivity rise rate to be larger. This might be due to that the molecular chain will have a higher energy to come closer to each other at a higher temperature, which leads to the higher thermal conductivity. In the work, the maximum thermal conductivity of nylon-11 nanofiber we measured was 9.8±0.1999 W/m-K, which is 49 times larger than the bulk one. This value is the largest one for the nylon-11 nanofibers in the literature.