Elsevier

Polymer

Volume 42, Issue 23, November 2001, Pages 9617-9626
Polymer

Crystalline memory effect in isothermal crystallization of syndiotactic polypropylenes: effect of fusion temperature on crystallization and melting behavior

https://doi.org/10.1016/S0032-3861(01)00507-9Get rights and content

Abstract

In this manuscript, studies on the crystalline memory effect in syndiotactic polypropylene (s-PP) were focused on the effect of prior melt annealing on the subsequent isothermal crystallization kinetics, crystalline structure, lamellar morphology, and subsequent melting behavior using a combination of differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and small-angle X-ray scattering (SAXS) techniques. On partial melting, choices of the fusion temperature used to melt the samples played an important role in determining the overall rate of isothermal crystallization, while they had no effect on the resulting values of the apparent crystallinity content, the long period, the lamellar thickness, and the low-melting peak temperature.

Introduction

It is well established that nucleation mechanisms play an important role in crystallization of polymers either from solution or from melt. Nucleation mechanisms can be categorized into two main processes: primary and secondary nucleation (i.e. subsequent crystal growth). Primary nucleation is defined as the origination of crystalline phase from the polymer solution or the melt. It can be categorized into two types depending on the physical origins of the nucleus (i.e. chemical make-up of the critical nucleus when comparing with that of the surface onto which the critical nucleus is formed): homogeneous and heterogeneous nucleation. Secondary nucleation is defined as a surface nucleation on an existing growth face, which is responsible for further growth of the activated nucleus.

In actual processings of a semicrystalline polymer, primary nucleation mechanism and rate are characterized and controlled mainly by not only the presence of infusible heterogeneous nuclei (e.g. catalyst residues, nucleation agents, impurities, etc.), but the processing history (viz. dictated by temperature, pressure, stress, etc.) as well. Because of their importance in determining overall crystallization kinetics and morphology, it is necessary that the nucleation mechanism and rate are better understood. It is therefore very important that the influences of impurities, additives, nucleating agents, and especially ‘crystalline memory effect’ on crystallization behavior be thoroughly investigated. The latter refers to clusters of molecules that retain their crystallographic arrangement of crystals as a result of insufficient or partial melting conditions, and, upon subsequent cooling, these aggregates of clusters of molecules can act as predetermined athermal nucleation sites (provided they exceed the critical nucleus size, required for crystallization at a specific temperature) which can greatly enhance the overall crystallization rate. This phenomenon is also referred to as ‘self-nucleation effect.’

Since, in actual polymer processings, a polymer part is not only subjected to thermal treatments, but to mechanical manipulations as well. Such mechanical deformational histories can lead to preferred orientation of polymer molecules which in itself can enhance nucleation rate. This effect is referred to as ‘orientational memory effect’. Both types of memory effects can greatly affect the crystallization behavior during subsequent cooling of the polymer part. To eliminate both kinds of memory effects, it is necessary to keep the part at a sufficiently high fusion temperature Tf for a sufficiently long period of time (depending on the fusion temperature Tf used) in order to eradicate as many traces of crystalline and oriented aggregates as possible (i.e. complete melting). Practically, a fusion temperature Tf for attaining complete melting is chosen such that it is greater than the equilibrium melting temperature Tm0 of the polymer of interest (Tf>Tm0). In some cases however, we wish to use these memory effects to preferably control the overall crystallization rate or morphology of the crystallizing polymer. Thus, we need to understand the characteristics of these effects in detail.

Due to their important influence on the crystallization behavior of polymers, memory effects (i.e. either in the context of the crystalline or orientation memory effect, or both) have been of considerable interest and have been studied by several investigators [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]. Recently, Supaphol and Spruiell [16] have reported studies on the crystalline memory effects in isothermal crystallization of syndiotactic polypropylene (s-PP), which were mainly dealt with the effects of heating rate φ, fusion temperature Tf, and holding time th on the effective total concentration of predetermined nuclei Ntot and the effective average spherulite diameter D.

In this manuscript, we focus on the effect of prior melt annealing on the subsequent isothermal crystallization kinetics, crystalline structure, lamellar morphology, and subsequent melting behavior of three commercial grade s-PP resins using a combination of differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and small-angle X-ray scattering (SAXS) techniques. This can be carried out by studying subsequent isothermal and melting behavior of s-PP samples melted at various choices of the fusion temperature Tf for a fixed holding time th. Even though the effect of Tf on the crystallization kinetics of polymers and the interpretation of such an effect based on the concept of self-nucleation have been well-documented (see, for example, [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]), it is guaranteed to the best of our knowedge that there have not been prior studies of s-PP on the similar aspects.

Section snippets

Theoretical background

Isothermal bulk crystallization kinetics of semicrystalline polymers is commonly analyzed from the DSC crystallization exotherms [17], [18], [19] based primarily on the assumption that the evolution of crystallinity is linearly proportional to the evolution of heat released during the course of crystallization. Based on this notion, the relative crystallinity as a function of time θ(t) can be obtained according to the following equation:θ(t)=0tdHcdtdt0dHcdtdt∈[0,1],where t and t=∞ are the

Materials

The s-PP resins used in this study were supplied in pellet form by Fina Oil and Chemical Company (La Porte, Texas, USA). Molecular characterization of these materials was carried out by Dr Roger A. Phillips and his coworkers at Basell USA, Inc. (Elkton, Maryland, USA). The results are listed in Table 1. It should be noted that the unusually high degree of polydispersity observed for s-PP#2 resin is due to the bimodal molecular weight distribution the resin exhibits.

Sample preparation

The standard film for each

Effect of fusion temperature on crystallization behavior

Fig. 1 illustrates, respectively, the time-dependent relative crystallinity function θ(t) (after subtraction of the induction time t0) for s-PP#1 samples isothermally crystallized at Tc=85°C after being melted partially at fusion temperatures Tf of 128, 130, and 140°C and after being melted completely at Tf of 180°C for a fixed holding time th of 5 min (the raw data are shown as different geometrical points). Qualitatively, the time to reach the ultimate crystallinity (i.e. complete

Conclusions

In this manuscript, the effect of fusion temperature on isothermal crystallization, comprising the kinetics and the lamellar morphological aspects of the process, and subsequent melting behavior of three commercial grade s-PP resins was thoroughly investigated using a combination of DSC, WAXD, and SAXS techniques. On partial melting, the DSC experiment suggested that the choice of the fusion temperatures used to melt the samples plays an important role in determining the overall rate of

Acknowledgements

We would like to thank Dr Joseph Schardl of Fina Oil and Chemical Company (Dallas, Texas, USA) for supplying the s-PP resins used in this study, and Dr Roger A. Phillips and his co-workers of Basell USA, Inc. (Elkton, Maryland, USA) for performing molecular characterizations of the resin. The research at Oak Ridge was sponsored in part by the US Department of Energy under contract number DE-AC05-00OR22725 with the Oak Ridge National Laboratory (ORNL), managed by the UT Battelle, LLC. In

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