Front. Chem. China 2007, 2(2): 174–177 DOI 10.1007/s11458-007-0035-3 RESEARCH ARTICLE ZHU Dunshen, SHOU Xingxian, LIU Yixin, CHEN Erqiang, Stephen Zhengdi Cheng AFM-tip-induced crystallization of poly(ethylene oxide) melt droplets © Higher Education Press and Springer-Verlag 2007 Abstract The AFM-tip-induced crystallization of poly(ethylene oxide) (PEO) melt droplets was studied. The melt droplets with a height of 50–100 nm and a lateral size of 2–3 μm were obtained by melting the PEO ultra-thin films on a mica surface. For the PEO samples with average molecular weights (M n ) ranging from 1.0x10 3 g/mol to 1.0x10 4 g/mol, the lateral perturbation from the AFM tip in the hard-tapping or nanoscratch modes could not induce the growth of the flat- on lamellae. In contrast, under AFM nanoindentation mode, the tip-induced crystallization occurred when a sufficiently high vertical tip force was applied to the melt droplets of PEO with M n i1.0x10 4 g/mol. Moreover, the experimental results indicated that the AFM-tip-induced crystallization of PEO in the nanoindentation process had molecular weight dependence. Keywords atomic force microscopy, poly(ethylene oxide), tip-induced crystallization, nanoindentation 1 Introduction Atomic force microscopy (AFM), as an important tool, has been widely used in the polymer crystallization field. Usually, in order to truly reflect the crystal morphology and crystalli- zation process, the influence from the AFM tip during scan should be minimized. On the other hand, AFM-tip-induced nucleation and the crystal growth of polymers have been also studied. It is expected that the considerable lateral force in the AFM contact mode may increase the probability of tip- induced nucleation. Pearce and Vancso [1,2] reported that the AFM tip could not induce poly(ethylene oxide) (PEO) crystallization. However, for poly(e-caprolactone) (PCL), Beekmans et al. [3] observed the AFM-tip-induced nucleation of the PCL chains, which further resulted in the growth of edge-on crystals normal to the scan direction. The above two different results arise from their difference in strength of lateral forces applied in the AFM experiments. In the AFM tapping mode, the tip has weaker interaction with the sample due to the small tip-to-sample force. Li and coworkers [4,5] found there was no evidence of tip-induced crystallization in the amorphous region around the crystal at a high undercool- ing (∆T = ~60°C), using tapping mode. A similar observation was reported by Godovsky and Magonov [7] in polyethylene crystallization. However, according to Magonov et al., the AFM tip could penetrate the sample surface to touch the crys- talline core when the applied force was increased from light- tapping to hard-tapping. Therefore, the AFM tip in tapping mode may also affect the polymer crystallization kinetics greatly and even induce polymer chains to pack together to form a nuclei in the melt. Here we report our investigation on AFM-tip-induced crystallization by lateral and vertical inter- actions between the AFM tip and the PEO melt droplets on a mica surface. The PEO melt droplets possessed a height ranging from 50 to 100 nm and a lateral size of 2–3 μm. The lateral perturbation was realized in hard-tapping mode or nanoscratch mode, and the vertical perturbation was done in the nanoindentation mode. The results obtained may shed some light on the control of polymer crystallization using in situ AFM technique. 2 Experimental In this paper, a series of low molecular weight (LMW) PEO fractions with –OH groups on both chain ends were purchased from Polymer Source or Polymer Laboratory, of which the M n ranged from 1.0x10 3 to 1.0x10 4 g/mol, and the polydisper- sities are less than 1.1. A relatively high molecular weight PEO sample with M n of 1.0x10 5 g/mol was purchased from Aldrich. The ultra-thin films of the PEO samples on the Translated from Acta Polymerica Sinica, 2006, (4): 553–556 (in Chinese) ZHU Dunshen, SHOU Xingxian, LIU Yixin, CHEN Erqiang (,), Stephen Zhengdi Cheng Department of Polymer and Engineering, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China E-mail: eqchen@pku.edu.cn Stephen Zhengdi Cheng Maurice Morton Institute and Department of Polymer Science, University of Akron, Akron, Ohio 44325-3909, USA