Rheol Acta (2006) 45: 387–392 DOI 10.1007/s00397-005-0054-2 ORIGINAL CONTRIBUTION Stefano Acierno Ernesto Di Maio Salvatore Iannace Nino Grizzuti Received: 28 September 2005 Accepted: 4 October 2005 Published online: 10 November 2005 # Springer-Verlag 2005 Structure development during crystallization of polycaprolactone Abstract In this paper, the quiescent crystallization of polycaprolactone (PCL) melts is studied by rheological measurements coupled to calorimetry and optical microscopy. Based on a comparison between the different techniques, we find that the increase in viscoelastic properties during crystal- lization starts only when a relatively high degree of crystallinity is reached, which corresponds to a much devel- oped crystalline microstructure. Like other semicrystalline thermoplastic polymers, the crystallization of PCL can be seen as a gelation process. In this case, however, we find a peculiar critical gel behavior, as the liquid-to- solid transition takes place at a very high (~20%) relative crystallinity, and this value is independent of tempera- ture. These facts, and the comparison with optical microscopy observations, suggest that the microstructure at the gel point is controlled by the interac- tions between the growing crystallites. The gel time (from rheometry) and the half-crystallization time [from differ- ential scanning calorimetry (DSC)] both show an Arrhenius-like behavior and have the same pseudoactivation energy. A practical implication of this parallel behavior of t gel and t 0.5 is that the rheological measurements can be used to extend to higher tempera- tures the study of crystallization kinetics where DSC is not sufficiently sensitive. Keywords Crystallization . Gelation . Polycaprolactone Introduction In the last decades, the use of rheological techniques in the study of polymer crystallization has gained increasing consensus. Khanna (1989, 1993) was the first to propose rheological techniques to estimate polymer crystallinity and to monitor crystallization kinetics. He pointed out how the evolution of the elastic modulus (as measured at a single frequency) can be related to the evolution of relative crystallinity; a crystallization time can be calculated, and it compares well to those measured by more traditional techniques [such as differential scanning calorimetry (DSC)]. The major drawback of Khanna’ s works is that the viscoelastic moduli are themselves frequency-depen- dent. Thus, the crystallization times calculated from vis- coelastic measurements are dependent on the selected frequency as well. More recently, Winter ’ s group showed that the early crystallization stages can be viewed as a physical gelation process (Schwittay et al. 1993). Crystalline order slows down the macromolecular motion and increases the con- nectivity among molecules. The correlation length diverges This paper was presented at the second Annual European Rheology Conference (AERC) held in Grenoble, France, 21–23 April 2005. S. Acierno (*) . N. Grizzuti Dipartimento di Ingegneria Chimica, Università degli Studi di Napoli Federico II, P.le Tecchio 80, 80125 Naples, Italy e-mail: sacierno@unina.it Tel.: +39-081-7682288 Fax: +39-081-2391800 E. Di Maio Dipartimento di Ingegneria dei Materiali e della Produzione, Università degli Studi di Napoli Federico II, P.le Tecchio 80, 80125 Naples, Italy S. Iannace Istituto per i Materiali Compositi e Biomedici, CNR, P.le E. Fermi 1, 80055 Portici (NA), Italy