Plastic deformation of spherulitic semi-crystalline polymers: An in situ AFM study of polybutene under tensile drawing
Graphical abstract
Introduction
In recent years, the demand for predictive approaches of the mechanical behavior of semi-crystalline polymers has aroused increasing efforts for introducing physical concepts in the mechanical modeling (see Ref. [1] and refs cited therein). Benefiting from the drastic increase of computing capacities, this resulted in the development of complex strategies that can yet only account for a limited number of factors of the structural hierarchy of semi-crystalline polymers. Moreover, the deformation processes at the various scale levels of the structure are far for being thoroughly understood, so that numerous assumptions have to be used in the modeling.
Poly(1-butene), otherwise PB, is a kind of “model system” for studying the incidence of the various structural factors on the mechanical behavior, keeping constant the physico-chemistry and the crystallography of the system. Indeed, the morphological habits of PB are strongly sensitive to the crystallization conditions [2]. The spherulite size can be as large as 500 μm down to 3 μm, the crystallinity being in the range 60–40%. The crystal thickness, in the range 20–8 nm, is particularly convenient for performing investigations by Atomic Force Microscopy (AFM). We recently reported [3] the quasi in situ AFM study of the plastic deformation mechanisms of PB films having large spherulites of diameter 200 ± 20 μm. Owing to the isothermal crystallization of the films from the melt with a free surface, a good number of adjacent spherulites could grow with their core located right on the free surface of the film. This made it possible capturing the plastic deformation processes at a nanometric scale within the various regions of the spherulites, without any chemical treatment. Under tensile drawing, crazing appeared to be the predominant process of plastic deformation in the equatorial regions, whereas profuse lamella fragmentation developed in the polar regions. Combinations of the two processes occurred in the diagonal regions. The material broke in a relatively brittle manner at a strain of about 15%.
In the present paper, we address the plastic behavior of PB films having small size spherulites, i.e. below 20 μm in diameter. Such films display much greater ductility than the ones of the former study. Besides, considering that the film thickness (about 100 μm) is much greater than the spherulite size, we can easily assume that the AFM surface observations of the deformation processes are representative of the bulk-operating processes that emerge at the sample surface. This point was already argued in the previous work [3] from experimental findings that gave evidence of stress triaxiality. We will point out some particular features of the deformation processes that must be ascribed to free surface effects but do not modify the general conclusions.
Section snippets
Experimental
The poly(1-butene) material (PB) from Shell has number- and weight-average molar weights Mn ≈ 28 kDa and Mw ≈ 174 kDa. The polymer pellets were compression-molded into 100 μm thick sheets between steel plates at 170 °C for 2 min before cooling at about 20°/min. Borrowing from Weynant's isothermal crystallization study [2], two kinds of polymer films were prepared: the initial sheets were melted again at 170 °C on a steel plate with free-upper surface for 5 min, before to be quenched into a water bath at
Results
The structural and physical characteristics of the PB-20 and PB-5 films are reported in Table 1. The crystalline lamella thickness was estimated from the long period of the lamellar stacking determined from AFM height images and the DSC crystal volume fraction, taking the densities ρc = 0.95 g/cm3 and ρa = 0.87 g/cm3 for the crystalline and the amorphous phase, respectively [8]. In spite of quite large departure in spherulite diameter, the two kinds of films have very close crystallinity and crystal
Concluding discussion
The present study of the plastic deformation processes of polybutene having small size spherulites, namely PB-20 and the PB-5, is complementary to the previous study of polybutene having large spherulites, i.e. PB-200 [3]. It definitively shows that the structural factors in the size range between a few nanometers and several tens of microns have a tremendous incidence on the development of the plastic processes. As a matter of fact, large spherulites display intense crazing and cavitation
Acknowledgments
The authors are indebted to the European FEDER Program and the Région Nord/Pas-de-Calais (RNPdC) for funding to the Digital AFM equipment. The Centre National de la Recherche Scientifique and the RNPdC are also deeply acknowledged for the grant of a doctoral fellowship to C. Thomas.
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Present address: Mines Paristech, Centre des Materiaux, 91003 Evry, France.