長鏈高分子超薄膜在奈米尺度之除潤行為研究:分子鏈反彈力,軟基材交互作用,應力鬆弛,與自由能效應

Dewetting of Ultra-Thin Films of Long Macromolecules: Molecular Recoiling, Soft Substrate Interaction, Stress Relaxation, and Excess Free Energy Effect

楊明勳(Yang Ming Hsun)

指導教授 ： 楊長謀

國立清華大學/工學院/材料科學工程學系/博士(2006年)

https://doi.org/10.6843/NTHU.2006.00305

摘要

本論文將探討有關於高分子鏈交纏結構與分子彈力在除潤現象(dewetting)中所扮演的重要角色。經歷玻璃轉換溫度(glass transition temperature, Tg)以上退火的高分子超薄膜，會於薄膜表貌發生除潤現象。然而由長鏈高分子所組成的超薄膜，其上的作用力除了一般僅認定的分散力(dispersive force，由長距離的凡得瓦爾力和極性作用力所組成)之外，我們發現尚有分子彈力(molecular recoiling force)。此分子彈力乃因分子鏈處於超薄膜內，其分子鏈組態(chain conformation)偏離平衡態所產生的分子應變(molecular strains)所引起。藉著奈米力學，由除潤孔洞的成長，可以分別測量此分子彈力和作用在薄膜上的總力，並可據以得到分散力。我們發現分子彈力的大小和高分子的分子量和超薄膜的厚度有強烈關連，其大小介於9.0到28.2 mN/m之間，可為分散力的三倍，而不可忽略。而分散力與分子量較無關，但隨著薄膜厚度下降而增加，此與低分子量的超薄膜行為一致。另外，藉由將聚苯乙烯超薄膜製備在低表面的軟性基材上，可以發現長鏈高分子薄膜除潤會出現兩個階段的dewetting過程，此顯示薄膜與基材間的交互作用，改變了除潤的發生途徑。首先，薄膜表貌會先以異質成核(nucleation and growth)的方式發生除潤，這些剛發生的小洞成長迅速並且很快的就不再成長，並且在隨後的退火(annealing)過程中，直徑也不再增大，在此稱為第一階段除潤(1st stage dewetting)。在第一階段除潤發生時，藉由原子力顯微鏡(atomic force microscope, AFM)的觀察，可以發現低表面能軟性基材會因聚苯乙烯除潤，而產生下凹的形變。隨著退火時間的增長，可以發現原本的薄膜處，產生了與第一階段除潤迥然不同的小洞，這些小洞的邊緣不規則，並且以樹枝狀的方式向外生長，我們稱之為第二階段除潤(2nd stage dewetting)。這些延伸出去的樹枝狀最後會彼此碰觸而形成小液滴，隨著退火時間的增加，最終這些小液滴會佈滿整個試片。同時，由基材的形變，我們可以計算得到除潤總力為36mN/m。這種獨特的兩階段除潤現象僅發生在長時間的退火以及薄膜厚度趨近於分子尺寸(coil size)，而第二階段除潤更顯示了分子鏈間作用力與交纏結構的重要關聯性。再者，藉由時效高分子超薄膜，可以發現，經歷了不同時效時間、不同的時效溫度之後，分子彈力顯現出不一樣的數值。厚度為10nm，分子量為200k的聚苯乙烯薄膜，放置於80℃的真空烘箱中兩週後，其分子彈力會由原本的12.1mN/m慢慢的下降至3.6mN/m；分子量為900k的聚苯乙烯薄膜，放置於80℃的真空烘箱中兩週後，其分子彈力會由原本的13.2mN/m，慢慢的下降至4.5mN/m。而較低的時效時間與較高的分子量，均顯示出較長的分子彈力鬆弛時間。此跡象顯示出分子間之交纏結構影響除潤過程甚鉅。相較於分子彈力的變化，分散力在整個時效的過程中，均顯現出迥然不同的特性，既不與分子量的大小有關，也不與時效時間及時效溫度有關，僅與薄膜厚度有關係。藉由研究高分子超薄膜之除潤行為，將更有助於學術界釐清高分子超薄膜特性，分子運動，及分子鏈交纏結構。

關鍵字

交纏 ； 除潤 ； 玻璃轉換溫度 ； 退火 ； 分散力 ； 分子彈力 ； 總力 ； 異質成核 ； 樹枝狀

並列摘要

In this thesis, we discussed dewetting mechanism. How the polymer chain entanglements and molecular recoiling affect dewetting was carefully discussed. First of all, Molecular recoiling force stemmed from non-equilibrium chain conformation was found to play a very important role in the dewetting stability of polymer thin films. Correct measurements and inclusion of this molecular force into thermodynamic consideration are crucial for analyzing dewetting phenomena and nanoscale polymer chain physics. This force was measured using a simple method based on contour relaxation at the incipient dewetting holes. The recoiling stress was found to increase dramatically with molecular weight and decreasing film thickness. The corresponding forces were calculated to be in the range from 9.0 to 28.2 mN/m, too large to be neglected when compared to the dispersive forces ( ~ 10 mN/m) commonly operative in thin polymer films. Furthermore, Stability of high molecular weight polystyrene (PS) thin films (200k≦Mw≦2M, 20 nm≦h≦80 nm) on a soft surface of low energy at 170℃ was investigated. A unique two-stage dewetting process was discovered and the magnitude of total forces driving the dewetting was determined from the strong substrate interaction. The film dewet by first nucleating small holes which grew rapidly soon after their emergence but subsequently ran into a complete rest. With these holes remaining stationary, however, a new type of dewetting took place in the regions of intact surface when additional annealing of more than 4.5 hours was given. The newly initiated holes grew steadily in size with the characteristic dendrite-like fingers developed and lengthened from the retracting edges of the holes. The fingers finally broke down to form droplets to cover the whole surface. The magnitude of the dewetting force was estimated from the substrate deformation to be around 0.036 N/m. Substrate interactions appeared to have a significant effect of increasing surface energy, hampering the rate of dewetting. The two-stage dewetting occurred only for films of high molecular weights (≧ 900k) and with thicknesses close to the coil dimensions. Moreover, investigations of the evolution of opening of dewetting holes in ultra-thin, almost glassy polystyrene films deposited onto silicon wafers were disclosed. These films were aged at temperatures closely to its glassy transition temperature (T

並列關鍵字

Entanglement ； dewetting ； glass transition temperature ； annealing ； dispersive force ； molecular recoiling force ； total force ； nucleation and growth ； dendrite.

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