Our structural understanding of semi-crystalline polymers is based on descriptions of what is happening at a molecular scale. However, until recently, imaging techniques were not able to probe to this length scale, so the pictures we have are, by necessity, built up from a combination of microscopy, scattering and spectroscopy methods. The problem with this approach is that many of these techniques inherently obtain an average behaviour across the whole sample, and the individual, molecular, detail is lost. We have recently shown that, using a new atomic force microscopy technique, torsional tapping AFM (TTAFM), with ultra-sharp carbon whisker tips, it is possible to obtain true molecular resolution images of commodity polymers such as polyethylene [1]. Here we will present new data building upon these preliminary results. In polyethylene we explore the nature of the crystal-amorphous interface in detail at the single chain level. We also analyse the data in terms of an effective density, and compare this to the results obtained from x-ray scattering that averages over the entire sample. We use our data to compare with the predictions of existing theory and computer simulation, with the aim of providing a new insight into the fundamentals of polymer crystallization. In polypropylene we are able to obtain similar, single chain resolution images, but here we find that, because of the complexity of chain conformation within the crystal, inter chain interactions play a stronger role in the surface topography. Insights into the vibrational freedom of the molecules, obtained from imaging, will also be discussed. Finally, the wider applicability of this approach, and the possibility of obtaining comparable data on standard instruments, will be explored. [1] N Mullin and JK Hobbs, (2011) “Direct Imaging of Polyethylene Films at Single-Chain Resolution with Torsional Tapping Atomic Force Microscopy.” Phys Rev Lett, 107 (19), 197801

Molecular scale imaging of semicrystalline polymers

249th National Meeting (2015)

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