New dynamical effects in spherulitic growth D. Villers*, M. Dosi&re and L. Paternostre Universit6 de Mons-Hainaut, D6partement des Matdriaux et Proc6dds, Laboratoire de Chimie-Physique, 20 place du parc, B-7000 Mons, Belgium (Received 28 May 1993; revised 14 September 1993) Two unexpected phenomena have been observed during the study of the growth of ~t and fl spherulites of a poly(ethylene oxide)-resorcinol complex: an overgrowth rate (denoted as Gi) of the fastest, namely the or-form, along the interface of the r-form, and a transformation of the E-form into the or-form, which propagates as a dynamic front at a constant rate G t. The evidence for these two phenomena is demonstrated, and possible explanations are discussed. Measurements of the two rates, G i and Gt, are also reported. (Keywords: spherulitic growth; interspherulitic limit; PEO-resorcinol complex) INTRODUCTION types of spherulites during the same sample preparation. It is well known 1-3 that under normal conditions of Isotactic polypropylene is a good example, of this, since crystallization, semicrystalline polymers crystallize from four distinctly different types of spherulites can be grown the melt in the form of spherulites, i.e. spheres whose from the melt, and at least two of these at the same radii increase with time, and which originate from nuclei, temperature s'9. Generally, a particular form can be favoured by using the appropriate operating conditions. Their sizes range from a few nm to several/~m, depending upon the density of the nuclei. The chain axis of the In addition, different growth rates can be measured for the different allotropic forms, and this gives a good macromolecules is always found to be tangentially implication of typical shapes for the interspherulitic oriented within these crystalline aggregates. At a boundaries4,1o,11. microscopic level, spherulites are composed of a lamellar Poly(ethylene oxide) and resorcinol form a molecular type morphology, consisting of a two-phase system with crystalline and amorphous components 4'5. As complex with a 2/1 stoichiometry (i.e. two PEO monomer units for one resorcinol molecule) 12 14. This paper deals sample preparations for polarized optical microscopy with two different morphologies (denoted hereafter as the observations are usually in the form of thin films of polymer which are sandwiched between glass slides, the ct- and E-form) for the molecular complex formed between spherulites are therefore diametral cross-sections of the these two components. The experimental preparation normally three-dimensional objects. Sometimes, this conditions have been described elsewhere 15. In these procedure allows us to obtain quite large spherulites, experiments, we used PEO (M,=35000) which was since the production of thinner cross-sections involves a obtained from Hoechst, and resorcinol, which was natural reduction in the number of nuclei. Nucleation obtained from Aldrich, to prepare the molecular complex. In addition to the classical experimental study of the can be induced by quenching in liquid nitrogen, in order thermal dependence of the growth rates, which provides to obtain spherulites with sizes ranging from 10 #m to fascinating results arising from the different polymorphs, 10 mm. In melt crystallization, the growth rate is defined and the experiments have also revealed two unexpected measured as the velocity of advancement of circular phenomena. First, an interfacial growth rate was observed fronts. Hence, the larger the spherulite, the more precise which can be described as an overgrowth rate of the ~-form spherulites when they are in contact with the is its growth rate determination. Some semicrystalline slower growing E-form spherulites. The relative rate polymers can exhibit several crystal modifications, depending upon the experimental conditions. For increase depends upon the temperature. This can example, poly(ethylene oxide) (PEO) usually crystallizes be several tens of percents at low temperatures, which in a 7/2 helical conformation 6, but can crystallize in a becomes increasingly less significant at elevated tempera- trans-planar conformation 7 in a stretched sample. It is tures, and eventually becomes essentially undetectable also possible to reversibly, or irreversibly, transform above a certain temperature. Secondly, a transformation one form into another by changing the experimental of the E-form into the ~t-form, which occurs when a E-form spherulite encounters an ~-form spherulite, was found. conditions (such as stretching, heating, drying, etc.), e.g. as seen with poly(oxacyclobutane), which can exist in any At high temperatures, this transformation process follows one of three crystal modifications 7. In more common a dynamic front, which propagates at a constant examples, it is possible to obtain at least two different rate within the r-phase. This rate increases with temperature. Some results about spherulitic growth and interspherulitic limits will be recalled prior to the * To whom correspondence should be addressed presentation of these specific new results. 0032-3861/94/08/1586-07 © 1994 Butterworth-Heinemann Ltd 1,585 POLYMER Volume 35 Number 8 1994