高分子薄膜於高分子刷表面的潤濕現象

Abstract

透過多體耗散粒子動力學（MDPD）模擬方法，研究高分子液體在不同基材上的去濕動力學。我們模擬了高分子薄膜在平滑基材上、高分子液滴在高分子刷基材上以及高分子薄膜在高分子刷基材上三種系統的潤濕現象。當高分子薄膜在平滑基材上時，存在三種去濕機制，分別是（i）Spinodal Decomposition（Spontaneous Dewet），（ii）Nucleation Decomposition（Dewet By External Disturbance）和（iii）Self-Healing。而可能造成薄膜以三種不同去濕機制的原因取決於高分子薄膜的厚度（H），薄膜高分子鏈長（N）以及外界干擾，也就是乾孔的半徑大小（R0）。對於固定的外界干擾，我們得到不同高分子薄膜的厚度（H）以及薄膜高分子鏈長（N）對潤濕現象的影響相圖。隨著薄膜高分子的鏈長越長，膜厚需要更厚(液體粒子數越多)才能穩定薄膜不受外界干擾而產生去濕現象。我們也藉由改變高分子的構造，使用星形高分子降低高分子鏈的旋轉半徑，發現隨著星形臂數的增加，旋轉半徑越小，並有效促進高分子薄膜的穩定性，不易受系統缺陷所影響。 為了增加液體在基材上的潤濕效果，我們透過高分子刷在平滑基材上作改質，高分子液滴在高分子刷基材上的潤濕性增加，並研究自疏性除潤現象(Autophobic Dewetting)的影響。影響液滴潤濕性的原因包括液滴高分子鏈長（N），高分子刷鏈長，高分子刷的接枝密度以及高分子刷的硬度。結果顯示：（i）隨著高分子液滴的鏈長越長，液滴接觸角越大，滲透深度越小，液滴的潤濕效果較弱，較易產生自疏性除潤現象。（ii）當高分子刷鏈長越長，接觸角減小，滲透深度增加，液滴的Pinning較強，潤濕效果越好。（iii）隨著高分子刷接枝密度逐漸增加，滲透深度將上升到一極大值，同時液滴所對應的接觸角則下降到極小值，此時潤濕效果最好。然而，再繼續提高接枝密度時，發現液滴因亂度效應而產生自疏性除潤現象。此外，當液滴滲入高分子刷的滲透深度越深時，遲滯現象的影響越明顯。越長的高分子刷雖然可增加潤濕效果，但仍無法達到完全潤濕，因此藉由增加高分子刷的硬度，在適當的高分子刷長度下可以有效增加液滴的潤濕效果。 最後，我們藉由模擬高分子薄膜在高分子刷上受外界干擾的孔洞變化情形，進一步探討高分子薄膜的潤濕性質，此時影響薄膜潤濕性的原因包括乾孔的半徑大小，高分子刷的接枝密度，高分子刷鏈長以及液滴高分子鏈長。模擬結果顯示高分子薄膜的孔洞有三種變化情形：（i）Self-Healing，（ii）Quasi-Stable Hole以及（iii）Dewet By External Disturbance（Nucleation Decomposition）。當高分子刷接枝密度較低時，薄膜的穩定度高，潤濕性強，即使受較大的外界干擾仍能自我修復將孔洞填回成穩定薄膜，相較之下，當高分子刷接枝密度較高時，薄膜能夠承受的外界干擾減小許多，容易產生自疏性除潤現象而破裂。在更大程度的外界干擾下，當高分子刷接枝密度較低時，薄膜無法自我修復，但也不會破裂造成孔洞變化，而是會維持孔洞的存在持續很長一段時間，即為Quasi-Stable Hole。最後，高分子刷鏈長以及液滴高分子鏈長對薄膜穩定度的影響與高分子刷上的液滴之潤濕性質相符。

The wetting behaviors of polymer films/droplets on smooth substrates or substrates modified with polymer brushes are investigated by many-body dissipative particle dynamics simulation method. Three wetting dynamics of a polymer film on a smooth, partial wetting surface can be identified, (i) spinodal decomposition, (ii) nucleation decomposition, and (iii) self-healing. The outcome depends on polymer chain length(N), polymer film thickness(H), and radius of the dry hole (R_0). The phase diagram of the dewetting mechanism as a function of H and N is obtained for a specified R_0. As chain length increases (increasing N), the critical film thickness associated with the nucleation/self-healing crossover grows so that the metastability of the film can be retained by the self-healing process. It is also found that the stability of the polymer thin film can be enhanced by employing polymers with compact structures such as star polymers, which has smaller radius of gyration as the arm number increases. Our simulation results also indicate that the modification of a substrate with a chemically identical polymer brush can significantly retard the dewetting phenomena of polymer droplets. Three wetting dynamics of a polymer film on top of a polymer brush can be identified, (i) self-healing, (ii) quasi-stable hole, and (iii) dewet by external disturbance (nucleation decomposition). The phase diagram of the dewetting behavior as a function of grafting density and radius of dry hole is obtained. When the grafting density is low, the polymer film is more stable and tends to proceed with self-healing process as an external disturbance is applied. As the grafting density becomes high, a quasi-stable hole is formed with imposed dry hole. Further increase in the grafting density leads to the effect of autophobicity and polymer film becomes high unstable even under a minor disturbance. Our simulation results can provide strategies for dewetting suppression such as the metastability of the film of polymers with large molecular weight can be promoted either by the addition of short polymers or by employing compact polymers such as star polymers.