Origin of toughness in β-polypropylene: The effect of molecular mobility in the amorphous phase
Graphical abstract
Introduction
Isotactic polypropylene (i-PP) is known for its extensive polymorphism, which is attributed to the packing variation of polymer chains adopting the same, threefold 31-helical conformation [1], [2]. Six different crystal phases have been identified to date. Among them the monoclinic (α), trigonal (β) and orthorhombic (γ) phases occupy a privileged position. The toughness of the β-modification has been demonstrated to be considerably higher than that of the α-modification. Thus far, no generally accepted structural mechanism that explains the observed effect of the β-modification on the toughness of i-PP has been proposed. The first published model suggested that a stress-induced transformation from a less dense (trigonal β-modification) to a more dense (monoclinic α-modification) crystalline structure at the root of a growing crack results in crack blunting and toughness enhancement [3]. However, the phase transformation in PP is associated with a volume decrease, in contrast to the volume increase in zirconia ceramics. Therefore, the greater toughness of the β-modification appears to be an inherent property of the β-modification itself. Indeed, a majority of structural explanations ascribe the toughness enhancement to a specific arrangement of the material structure. Several authors have suggested that greater toughness reflects a distinct spherulitic morphology: an organisation of β-phase lamellae in a bundled structure [4], [5], [6]. In contrast, the authors of previous studies [7], [8], [9] have proposed that the particular arrangement of the crystalline phase and a greater number of tie molecules in the amorphous phase may contribute to the greater toughness of the β-phase. The micromechanical processes initiated by the amorphous material have been confirmed to dominate in the initial stages of deformation [10]. Recently, Luo et al. showed [11], [12], [13] that toughening in β-PP arises from improvements in the chain mobility of the amorphous portion of the material and not from changes in the crystalline morphology. However, the chain mobility was revealed indirectly through the dynamic mechanical analysis spectra. The studies of the mechanism affecting the toughness in PP systems have not produced clear answers; therefore, we decided to devote our effort to further investigating the questions concerning the diversity in PP toughness.
For probing the structure and mobility of i-PP chains with atomic-level resolution, solid-state NMR spectroscopy (ss-NMR) is one of the most efficient experimental methods. This is particularly because the different crystal modifications of i-PP have considerable impact on NMR resonance frequencies. In the thermodynamically stable monoclinic α-form, the 31-left- and right-handed helices are enmeshed in the pairs [14]. The upward and downward orientations of methyl groups allow the stems to adopt either ordered or disordered packing structures. Complete upward or complete downward orientation of the methyl groups is a typical feature of the ordered α2-phase; these orientations allow for a compact arrangement of helical molecules and generate two non-equivalent packing environments for each carbon site. Consequently, the resulting 13C CP/MAS NMR spectrum shows well-resolved splitting (2:1) of the corresponding resonances (CH2, CH and CH3). On the contrary, the α1-phase formed by rapid crystallisation from melting displays the statistical disorder of the up- and down-orientations, leading to a more open structure [14]. Because the pairs of helices are separated to a greater extent, the environment of the carbon sites is less distinct and the resonance splitting is considerably reduced. The asymmetrically broadened single lines are observed in the 13C CP/MAS NMR spectra instead of doublets.
Left- and right-handed 31-helices of the less thermodynamically stable β-form are arranged in groups with the same handedness [14]. This arrangement, however, does not allow close-packing. Consequently, the inter-chain interactions are weakened, resulting in the absence of any resolvable splitting in the 13C CP/MAS NMR spectra. Other polymorphic forms, such as the γ- and δ-phases and the recently prepared ε-phase, are rather exotic structural modifications of i-PP that represent singularities in polymer crystallography [15], [16], [17]. Although the presence of these exotic forms can be neglected in ordinary i-PP samples, a straightforward interpretation of ss-NMR spectra is complicated by the existence of two distinct amorphous phases. In addition to the amorphous phase containing molecules with a random-coil (r.c.) conformation, another non-crystalline component adopting the 31-helical conformation has been identified [18], [19]. Generally mobile amorphous fraction (MOF) and rigid amorphous fraction (RAF) are being distinguished [20]. At room temperature, these non-crystalline components give broad and largely overlapping signals. With increasing temperature, however, a more vigorous molecular motion is activated in the r.c. amorphous phase, and new well-resolved CH2 and CH resonance lines appear in the high-frequency region [21]. Therefore, ss-NMR experiments at elevated temperatures are important for understanding the molecular origin of the physicochemical and mechanical properties of i-PP systems.
The aim of this work was to elucidate the effect of molecular mobility in the amorphous phase on the toughness in α- and β-isotactic PP and to investigate the material characteristics in greater depth. Three samples from our previous research [22], [23], [24] were used for this study: α-PP and two β-nucleated samples that differ in the concentration of the nucleating agent. Two concentrations of the nucleator were chosen: 0.03 and 0.1 wt%. The first concentration represents the critical nucleator concentration, which is necessary to obtain a significant percent of crystalline β-modification. Further increasing the nucleator concentration only slightly increases the percentage of the β-modification [22]. Detailed structural and dynamic surveys were performed using high-resolution solid-state NMR (ss-NMR) spectroscopy in combination with Wide-angle X-ray scattering (WAXS).
Section snippets
Materials and specimens
Isotactic polypropylene homopolymer Mosten 52.412 (Chemopetrol, Czech Republic) was used as a starting material throughout this study. To obtain a material rich in the β-phase, the polymer was modified with a selective β-nucleator NJ-Star NU-100 (RIKA, UK), N,N′-dicyclohexylnaphthalene-2,6-dicarboxamide. On the basis of our previous studies [22], [23], [24], two concentrations of the nucleator were chosen: 0.03 and 0.1 wt%. The nucleator was premixed with a PP powder and compounded with neat PP
Toughness and WAXS analyses
As evident from the results in Table 1, the addition of 0.03 wt% of the nucleating agent led to a marked increase in the β-modification content from 10% (non-nucleated polymer) to 89%. For the highest nucleator concentration (0.10 wt%), the value of K remained identical. At the same time, the overall crystallinity, Xc, increased slowly and steadily from 40 to 45% as the nucleating-agent content was increased from 0 to 0.10 wt%. These facts thus confirm that the samples were prepared in such a
Conclusions
A range of variable-temperature (VT) ss-NMR experiments were used to investigate the molecular origin of enhanced toughness in β-polymorphic variants of i-PP. Elevated temperatures amplified spectroscopic differences between various phases of i-PP, thereby allowing exploration of the segmental dynamics of polymer chains in detail via line-shape analyses of ss-NMR spectra and 13C-detected T1ρ(1H) relaxation experiments.
Three distinctly different populations of i-PP were identified. The free
Conflict of interest
The authors declare no competing financial interest.
Acknowledgement
The work was supported by the Czech Science Foundation (project 13-29009S).
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