Homogeneous nucleation of isotactic polypropylene in the glassy state

摘要

The crystallization behavior of isotactic polypropylene (iPP) has in the past fully quantified regarding the effect of cooling rate on the crystallization temperature, enthalpy of transition, and the crystal/mesophase polymorphism in non-isothermal experiments [1-5], and the effect of temperature on the crystallization kinetics/half-time of crystallization in isothermal experiments [6-8]. Major progress in quantification of the crystallization or ordering behavior of iPP was achieved by application of sophisticated instrumentation including fast scanning chip calorimetry (FSC) [1, 2, 6-8] or fast temperature-resolved X-ray scattering [9, 10]. As such it was possible to analyze in particular the crystallization/ordering at temperatures where the crystallization rate and mesophase formation rate is of the order of magnitude of less than a second, not assessable by standard calorimetry. In addition, non-isothermal FSC permitted to gain information about critical cooling rates, e.g., to suppress crystallization at high temperature, or to obtain fully amorphous samples. In short, the isotropic relaxed melt of iPP transforms to monoclinic α-crystals at temperatures between the equilibrium melting point and about 340 K; the rate of crystallization is maximum at about 350-360 K. Crystallization can be suppressed by cooling the melt at a rate faster than about 50-100 K s~(-1) in the temperature range between 330-340 K and the melting temperature. If the cooling rate, however, is lower than 1000 K s~(-1) then mesophase forms between 330-340 K and the glass transition temperature at about 270 K. The mesophase of iPP has been classified as a conformationally disordered glass which is metastable below its glass transition temperature of about 340 K [11]. Heating of the mesophase above this temperature permits its reorganization to crystals by healing of the conformationally defects like helix reversals [11, 12]. It is important to note that mesophase formation at low temperature is distinctly faster than the crystallization process at high temperature. Since the growth rate of both crystals and mesophase, however, is assumed to decrease with increasing supercooling of the melt due to its increasing viscosity, the observed phenomenon of distinctly increasing rate of ordering at low temperature can only be explained by a qualitative change of the nucleation mechanism [13, 14]. Accordingly, it has been suggested that the slower crystallization at high temperature proceeds via heterogeneous nucleation, while the faster mesophase formation at low temperature proceeds via homogeneous nucleation [11]. Note that it is a peculiarity of iPP that heterogeneous and homogeneous nucleation seem connected with the particular crystal/mesophase polymorphism. The detection of heterogeneous and homogeneous nucleation in iPP by analysis of the kinetics of the ordering process using calorimetry ultimately has been confirmed by evaluation of the morphology of samples crystallized/ordered at low/high supercooling. Heterogeneous nucleation at low supercooling is connected with formation of lamellae and spherulites [15-17], with the number of spherulites per unit volume providing information about the rather low nucleation density. Mesophase formation at high supercooling via homogeneous nucleation is connected with development of small, particle-like domains which, due to their large number, are unable to grow in lateral, that is, cross-chain direction [5, 18-20]; the number of ordered, individual domains formed at high supercooling is several orders of magnitude higher than the number of spherulites formed at low supercooling, proving different nucleation mechanisms.