Basu N, Osichow A, Mecking S, Reiter G
Physikalisches Institut, Albert-Ludwigs Universität Freiburg, Germany.
Eur Phys J E Soft Matter. 2012 Mar;35(3):1-12. doi: 10.1140/epje/i2012-12018-9. Epub 2012 Mar 19.
Polymer crystals are metastable and exhibit morphological changes when being annealed. To observe morphological changes on molecular scales we started from small nanometer-sized crystals of highly folded long-chain polymers. Micron-sized stripes consisting of monolayers or stacks of several layers of flat-on oriented polyethylene nanocrystals were generated via evaporative dewetting from an aqueous dispersion. We followed the morphological changes in time and at progressively higher annealing temperatures by determining the topography and viscoelastic properties of such assemblies of nanocrystals using atomic force microscopy. Due to smallness and high surface-to-volume ratio of the nanocrystals, already at 75 °C, i.e. about 60 degrees below the nominal melting point, the lateral size of the crystal coarsened. Intriguingly, this occurred without a noticeable reduction in the number of folds per polymer chain. Starting at around 110 °C, chain folds were progressively removed leading to crystal thickening. At higher temperatures, but still below the melting point, prolonged annealing allowed for surface diffusion of molten polymers on the initially bare substrate, leading eventually to the disappearance of crystals. We compared these results to the behavior of the same nanocrystals annealed in an aqueous dispersion and to bulk samples.
聚合物晶体是亚稳态的，在退火时会表现出形态变化。为了在分子尺度上观察形态变化，我们从高度折叠的长链聚合物的小纳米尺寸晶体开始。通过从水分散体中蒸发去湿，生成了由单层或几层平面取向的聚乙烯纳米晶体堆叠组成的微米级条纹。我们通过使用原子力显微镜确定此类纳米晶体组件的形貌和粘弹性特性，随时间并在逐渐升高的退火温度下跟踪形态变化。由于纳米晶体尺寸小且表面体积比高，在75°C时，即比标称熔点低约60度时，晶体的横向尺寸就已经变粗。有趣的是，这种情况发生时，每条聚合物链的折叠数没有明显减少。从大约110°C开始，链折叠逐渐被去除，导致晶体增厚。在更高的温度下，但仍低于熔点，长时间退火使得熔融聚合物在最初裸露的基底上进行表面扩散，最终导致晶体消失。我们将这些结果与在水分散体中退火的相同纳米晶体的行为以及块状样品进行了比较。
