Polymer Communication Morphology evolution in superheated crystal monolayer of low molecular weight poly(ethylene oxide) on mica surface Dun-Shen Zhu a , Yi-Xin Liu a , An-Chang Shi b , Er-Qiang Chen a, * a Department of Polymer Science and Engineering, and The Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China b Department of Physics and Astronomy, McMaster University, Hamilton, Ontario, 85 4M1, Canada Received 3 May 2006; received in revised form 5 May 2006; accepted 8 May 2006 Available online 19 June 2006 Abstract Using atomic force microscopy (AFM) coupled with a hot stage, we studied the morphological evolution of superheated poly(ethylene oxide) (PEO) crystal monolayer on the mica surface. The PEO possesses a number average molecular weight (M n ) of 4250 g/mol and a polydispersity of 1.03. The superheated monolayer was obtained when the entire periphery of a triply-folded chain crystal [IF(3)] was thickened to be a twice- folded chain crystal [IF(2)] ‘dam’. The IF(3) crystal was laterally confined by the IF(2) ‘dam’ and remained unchanged at its unconfined melting temperature (T m ). In superheated conditions, the interior IF(3) crystal unfolded, resulting in domains with a thickness in between the fold lengths of the IF(3) and the IF(2) crystals accompanied by hole formation. After its nucleation, the hole enlarged its area quickly and migrated long distances within the area bounded by the IF(2) crystal ‘dam’. q 2006 Elsevier Ltd. All rights reserved. Keywords: Superheated crystal; Monolayer; Poly(ethylene oxide) 1. Introduction Both the crystallization and melting of polymers are thermodynamic first-order transitions. For polymer crystal- lization with a small supercooling, the melt may stay as a metastable liquid for a long time, because the free energy barrier of primary nucleation is high [1,2]. Therefore, a sufficiently large supercooling needs to be applied to promote the polymer crystallization process. By contrast, the free energy barrier for polymer crystal melting is usually shallow, and thus the superheating required is minimal. Conventionally, the observations concerning superheated phenomenon of polymer crystals are related to the heating rate effect where the crystals are heated faster than their thermal conductivity [3]. The first case of genuine isothermal superheating was reported by Kovacs et al. [4]. Using a poly(ethylene oxide) (PEO) fraction with a number average molecular weight (M n ) of 6000 g/mol, they grew the twice-folded chain single crystals [IF(2)] followed by the once-folded chain crystals [IF(1)]. The resultant lamellar crystals were composed of the IF(2) crystal bounded around the edges by the IF(1) crystal. At temperatures higher than the IF(2) melting point [T m (2)], the unstable crystal interior remained unaffected for fairly long periods of time. Another example was observed in the single crystals of syndiotactic polypropylene (st-PP) grown from thin film melt that exhibited two sectors with different thicknesses [5,6]. The st-PP crystal in the thinner (010) sectors possesses a melting temperature (T m ) lower than that in the thicker (100) sectors. It was observed that the melting of the (010) sector could stop as soon as thicker crystals formed along the crystal/melting boundaries via recrystallization/reorganization. The just developed thicker lamellae created ‘dams’ to confine the rest (010) sectors in a two-dimensional space [6]. In this communication, we report our experimental observations on the superheated PEO crystal monolayer on the mica surface. It is known that the low molecular weight (LMW) PEO fractions can form flat-on monolayer lamellae on the hydrophilic substrates [7–11]. Being identical to the LMW PEO lamellae formed in the bulk [4,12–16], the monolayers are integral folding chain crystals [IF(n)], of which the fold number n is supercooling dependent. Using a crystallization procedure similar to that utilized by Kovacs described in [4], we were able to obtain a composite monolayer of LMW PEO with an IF(3) crystal interior confined by IF(2) crystal at the Polymer 47 (2006) 5239–5242 www.elsevier.com/locate/polymer 0032-3861/$ - see front matter q 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2006.05.013 * Corresponding author. Tel./fax: C86 10 6275 3370. E-mail address: eqchen@pku.edu.cn (E.-Q. Chen).