Molecular Conformation at the Crystal–Amorphous Interface in Polyethylene

R C Savage,J K Hobbs,N Mullin

doi:10.1021/ma5025736

Abstract

Torsional-tapping AFM with supersharp carbon-whisker tips is used to explore the molecular conformations at the surface of a semicrystalline polymer. Images of the crystal–amorphous interface of oriented polyethylene have allowed us to measure hitherto inaccessible parameters that can be directly compared to polymer crystallization theories and molecular simulations, such as the length of stem-to-stem overhang. It has also been possible to identify both first- and second-neighbor folds and to determine the surface roughness of lamellae which we find approximately doubles the interfacial area. Finally, we calculate the interfacial density profile from the images and find it to be sigmoidal but narrower than values reported by SAXS measurements.

Highlights

Most semicrystalline polymers consist of hard, elastic, crystalline lamellae with a typical thickness of the order of 10 nm, surrounded by a matrix of rubbery amorphous material, with individual polymer chains having parts in both states.[1]
We have used atomic force microscopy to characterize the crystal−amorphous interface of the archetypal semicrystalline polymer polyethylene
We find that at a chain-by-chain level it is rough but that the maximum step height from the end of one stem to that of its neighbor tends to be less than a nanometer

Summary

Introduction

Most semicrystalline polymers consist of hard, elastic, crystalline lamellae with a typical thickness of the order of 10 nm, surrounded by a matrix of rubbery amorphous material, with individual polymer chains having parts in both states.[1]. Experimental evidence has come from methods that average over many molecules, such as scattering and spectroscopy, which rely on the application of a model to interpret the raw data. This difficulty of directly measuring the behavior of individual molecules has severely limited the development of a verifiable molecular theory of polymer crystallization. Though computer simulations have provided valuable insights into the crystallization process on the molecular scale, validation of their results is hampered because of the inaccessibility of directly comparable experimental data, to the point that such information is rarely even reported, with authors concentrating instead on larger scale and more readily measurable properties such as average lamella thickness or growth rate

Methods

Results

Conclusion