Early stages of polymer crystallization—a dielectric study Andreas Wurm, Ragab Soliman, Christoph Schick * Deparment of Physics, University of Rostock, Universita ¨tsplatz 3, 18051 Rostock, Germany Received 8 July 2003; received in revised form 15 September 2003; accepted 17 September 2003 Abstract The existence and the formation of pre-ordered structures as the initial step during the complex process of polymer crystallization are discussed controversially. Most of the findings and interpretations are based on scattering experiments, which test small density differences between the assumed precursors of the crystals and the surrounding melt. Because of the low contrast the interpretation of experimental results become often speculative. In contrast relaxation experiments are probing motions in the sample and are therefore independent on density contrast. During crystallization, material is transformed from the liquid to the solid state. Consequently, motions typical for a liquid become impossible and do not longer contribute to the measured signal. For pre-ordered structures we expect some changes in mobility too because of the changes in conformation on pre-ordering. We performed dielectric relaxation experiments during isothermal crystallization of PCL. Pronounced effects in e 0 are observed long before changes in crystallinity can be detected. The observations strongly support the idea of pre-order in the polymer melt before the formation of crystals. q 2003 Elsevier Ltd. All rights reserved. Keywords: Polymer crystallization; Early stages; Dielectric spectroscopy 1. Introduction The process of polymer crystallization is a matter of debate since polymer crystals were first mentioned by Staudinger in 1927 [1]. Over the years the whole spectrum of available experimental methods was used to study the process of morphology development in polymers. Since the beginning of the 60th the Hoffmann–Lauritzen theory and several modifications and extensions of it dominate the discussion in the scientific community [2–4]. These theories assume the transition from the entangled polymer melt to the crystal, having the final thickness and stability, as a process occurring at the growth front. But there is increasing evidence that these theories do not describe the process correctly. Especially the observation of ordered structures at very early times forced the development of new theories and models [5–13]. All these theories assume a multistep process from the entangled melt via different metastable structures to the final polymer crystal. Often only the first or initiating step, as the key step for the whole process, is discussed and described. This step is assumed as spinodal decomposition [14–17] or as nucleation followed by growth [18,19]. Recently Strobl [20] introduced a model for polymer crystallization, which covers the whole process with a few specific steps. These steps are considered to be universal for polymer crystallization. The first step is assumed to be the formation of a metastable pre-ordered structure in the super cooled melt. This structure should be in thermodynamic equilibrium with the surrounding melt and should undergo different annealing stages to a stable lamella step by step. As the last step a stabilization process of the lamellae is assumed. Evidence for that comes from scattering experiments [21] and techniques probing proper- ties like shear modulus or melting temperature rather than morphology directly [22]. In parallel to the development of new theories and models specific experiments were performed to demonstrate the existence of pre-ordered structures at the beginning of the crystallization process. Most of the experimental techniques used are scattering techniques [8,12,23 – 25]. But there remain a lot of open questions regarding the interpretation of the data. Interpretation is only possible if a certain structural model is assumed. Therefore up to now it was not possible to prove explicitly the existence of 0032-3861/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2003.09.028 Polymer 44 (2003) 7467–7476 www.elsevier.com/locate/polymer * Corresponding author. Tel.: þ49-381-498-6880; fax: þ 49-381-498- 6702. E-mail address: christoph.schick@physik.uni-rostock.de (C. Schick).