Abstract
Crystallization of isotactic polypropylene (iPP) at the interface with crystalline films of two commercially employed nucleating agents (sodium benzoate (NaBz) and sodium 2,2’-methylene bis-(4,6-di-tert-butylphenyl)phosphate (NA-11)) and with a glass fiber (GF) was investigated using a polarized optical microscope. The analysis of the light intensity evolution during the crystallization process enabled the successful estimation of the time at which the crystal growth began, i.e., the induction time (ti), at various crystallization temperatures. Meaningful differences in the ti values were observed between the investigated systems. Moreover, the ti data have been analyzed according to different nucleation models proposed in the literature, which consider either the time to form the first crystalline layer in contact with the substrate or the time required to grow a cluster of critical size. It has been found that the two models are applicable in different temperature ranges depending on the efficiency of the given substrate. Therefore, in order to obtain the value of the surface free energy difference function, Δσ, which is directly related to the nucleation energy barrier and useful for the definition of a universal nucleating efficiency scale, a model that considers both the above-mentioned times was fitted to the overall data. The values of Δσ for the nucleation of iPP on the surface of the different substrates are thus obtained and discussed in the framework of the literature results.
Highlights
In modern polymer technology, the use of additives has a very important role because it allows for tuning and improvement of the properties of materials
A qualitative method consists of the observation of the morphology of crystallizing polymers induced by different substrates; a spherulitic morphology is representative of a low nucleation efficiency, while a transcrystalline one is characteristic of highly efficient nucleating surfaces [2]
According to this model, the small difference between the two Nucleating agents (NAs), Given that ∆σ is proportional to the slope of the fitting lines, the results shown in Table 1 do not would predict that nucleating agents were sodium benzoate (NaBz) is more efficient than NA-11 and that glass fiber (GF) would be conform to expectations
Summary
The use of additives (nucleating agents, pigments, fillers, etc.) has a very important role because it allows for tuning and improvement of the properties of materials. In the case of nucleation, an extremely small number of foreign particles may be responsible for heterogeneous nucleation, in which the crystalline embryo can form on solid surfaces already present in the polymer melt. Several different types of heterogeneity (extraneous solids, cavities, surfaces wetted by the nucleus, container walls, already formed crystal surfaces, etc.) can enhance the formation of stable embryos and have important implications for polymer crystallization [1]. Nucleating agents (NAs) are sometimes voluntarily added to the polymers in order to speed up processing times or improve their final properties. A qualitative method consists of the observation of the morphology of crystallizing polymers induced by different substrates; a spherulitic morphology is representative of a low nucleation efficiency, while a transcrystalline one is characteristic of highly efficient nucleating surfaces [2].
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