Crystalline Lamellae Fragmentation during Drawing of
Artur Rozanski* and Andrzej Galeski*
Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, Sienkiewicza 112, 90-363 Lodz, Poland
ABSTRACT: Filling free volume pores of the amorphous phase with the molecules of low molecular weight modifier leads to a complete elimination of the cavitation during tensile drawing. Such way of modification of a solidified material makes the polypropylene/modifier system a model system that enables the analysis of the influence of cavitation on thermomechanical properties and the mechanisms activated during its deformation. In this paper we have presented the influence of cavitation on the intensity of the lamellae fragmentation. In the case of cavitating material, on the basis of X-ray measurements and scanning electron microscopy, we have observed substantial decrease of the undisturbed crystallites lengths during its deformation up to 50−55% of their initial value. The deformation of noncavitating material proceeded with smaller decrease of average crystallites lengths by only 15−20% of their initial value. The changes of the SAXS’s long period of noncavitating polypropylene indicated that only a small fraction of lamellae stacks that are oriented parallel to the tensile direction undergo fragmentation. This type of fragmentation is connected with excessive lamellae thinning and interfacial instabilities but by no means by cavitation. ■ INTRODUCTION
At a molecular scale level, yield in semicrystalline polymers1 involves the disruption of the crystalline phase in an irreversible deformation process. Upon yielding, the spherulitic structure is deformed and eventually destroyed and transformed into a fibrillar one as the plastic deformation increases.2 There are numerous other phenomena accompanying deformation of crystalline polymers. Apart from the crystalline lamella being broken into small crystalline blocks upon yielding, Liu et al. also observed that the crystalline phase is transformed into amorphous one.3 Ferreiro and Coulon4 evidenced the role of the amorphous phase on the plastic deformation at yield of a polyamide 6. Shear bands are developed in the amorphous phase originating crystalline nanoblocks, whose size increases with the strain rate. Hughes et al.5 related the onset of both intense microvoiding and stress-induced martensitic phase transitions to the yield point. Rault proposed that the yield of semicrystalline polymers involves some collective chain motions taking place in the crystalline phase.6 Nitta et al. explained the yield behavior by the disintegration of lamellar clusters (before deformed by bending due to the action of active tie molecules) accompanying lamellar fragmentation.7
Strobl et al.8−11 studied the deformation behavior of various polyethylenes under an applied tensile load based upon measurements of true stress−strain curves, elastic-recovery properties, and texture changes at different stages of the deformation process. Although Strobl et al. did not observe cavitation in their experiments and did not consider it in their analysis, cavitation was observed by others in many crystalline polymers.
The first model to deal with the formation of cavities in a crystalline polymer during its deformation was a “micronecking” model proposed by Peterlin.2,12,13 Even though the model explained many phenomena taking place during plastic deformation of polymers, it contained significant inconsistency.
Further research conducted in many laboratories allowed to examine and better understand the mechanisms accompanying deformation of crystalline polymers.14−18 However, the phenomenon of cavitation was only treated as an effect accompanying deformation process, related to distribution of stresses inside the polymer material, a result of operation of other deformation mechanisms. Moreover, cavitation was frequently regarded as a phenomenon which not only does not influence the course of deformation but also masks the true mechanisms activated during deformation of a crystalline polymer, thereby making it difficult to examine. Pawlak et al.19 demonstrated that cavitation of the crystalline polymers not only is responsible for material whitening but also influences the mechanical parameters such as yield stress. We have observed also substantial influence of the cavitation phenomenon on the amount of heat generated during uniaxial stretching.20 The description of the cavitation phenomenon and its influence on thermomechanical properties of semicrystalline polymers based on the papers published during the recent three decades and the newest literature data were collected in our last review.21
Received: June 1, 2015
Revised: July 10, 2015
Article pubs.acs.org/Macromolecules © XXXX American Chemical Society A DOI: 10.1021/acs.macromol.5b01180
Macromolecules XXXX, XXX, XXX−XXX
In crystalline polymers the cavitation occurs preferentially in amorphous layers. It is evident that the physical parameters of the amorphous phase control the course and intensity of a cavitation process. The effect of stabilizers, additives, and low molecular weight fractions on cavitation during tensile drawing was studied in polypropylene22,23 and polyethylene.23 The additives were extracted from compression-molded samples by critical CO2 and also by a mixture of nonsolvents. The extract was an oily liquid composed of antioxidant, processing stabilizer, and a spectrum of low molecular weight fractions of polypropylene. Purified polypropylene exhibited surprisingly more intense cavitation than pristine polypropylene as it was determined by small-angle X-ray scattering and volume strain measurements. Intensification of the cavitation process in the purified samples was explained by the changes in the amorphous phase, namely the changes in free volume by eliminating low fractions and soluble additives. Increase in free volume was also confirmed by positron annihilation lifetime spectroscopy.22,23 The dominant role of the free volume of amorphous phase, which is an integral part of unordered regions of all crystalline polymers, in formation of cavitation pores proves that initiation of the phenomenon is of a homogeneous nature. It is meant that the nucleation occurs within the material itself in contrast to heterogeneous nucleation on foreign substances. That was proven by removing most of heterogeneous nuclei: impurities, additives, and gas. It appears also that in crystalline polymers a heterogeneous nucleation of cavitation is nearly inactive. More intense formation of cavitation pores in purified polypropylene proves that initiation of cavitation in polypropylene has a homogeneous nature. This anticipation is strongly supported by unusually high negative pressure necessary for cavitation for several commodity crystalline polymers at ambient conditions: polypropylene (−13.7 MPa),19 poly(methylene oxide) (−35.8