In-situ SAXS study of the plastic deformation behavior of polylactide upon cold-drawing
Graphical abstract
Introduction
During the past decade polymers issued from renewable resources have been the subject of an increasing interest. Among them, poly(lactic-acid) or polylactides appear as the most promising candidates for the substitution of polyolefins, namely in the field of packaging. Moreover in addition to its bio-based origin, PLA is biodegradable and biocompatible. Regarding its properties, PLA appears to be better than poly(styrene) and in fact quite similar to poly(ethylene terephtalate) [1]. It was also shown that PLA exhibits relatively slow crystallization kinetics, thus allowing to get an amorphous material, and that due to its stereo-isomerism, its crystallinity strongly depends on the d-isomer content [2]. Regarding the mechanical behavior, most studies have focused on the temperature domain beyond the glass transition temperature with special attention to the deformation-induced structural evolution [3], [4], [5], [6].
Actually, one the main drawback of PLA lies in the fact that it exhibits a brittle behavior at room temperature, the origin of which being attributed to the occurrence of crazing [7], [8]. The inherent brittleness of polylactide (PLA) poses considerable scientific challenges and limits its large-scale applications. To overcome this drawback, mainly two routes have been proposed. On the one hand the formation of hyper-branched structures [9], [10], [11], [12], [13] has been developed in order to inhibit crazing by increasing the entanglement density of the polymer. On the other hand, an alternative route consists in blending PLA with an elastomeric component [14] such as poly(ε-caprolactone) [15], acrylate rubber particles [16], [17], isoprene [18], poly(butyl acrylate) [19], [20], poly(hydroxyl butyrate) [21] or nanofillers [22], [23], [24]. The gain in toughness observed using this route is explained by the fact that the reinforcing phase allows the activation of the shear yielding mechanism in addition to crazing.
Although the latter two topics are well documented, the intrinsic plastic deformation of PLA in the glassy state remains poorly addressed. This is probably due to the complex behavior exhibited by this polymer. Besides Renouf-Glauser et al. showed that PLA brittleness increases when crystallinity increases as well as when the molecular weight decreases [25]. Regarding the effect of crystallinity, previous authors also highlighted that crystalline morphology affects the ultimate properties of PLA rather than the deformation micromechanism itself [25]. In other words elongation at break decreases when crystallinity increases but crazing is always involved whatever the PLA crystal content is. Occurrence of crazing during cold-drawing of PLA originates both from the high stiffness of PLA chains as indicated by the high value of the characteristic ratio C∞ [26], [27], and from its relatively low entanglement density, with Me around 8000 g/mol [28]. Regarding the effect of draw temperature, Rezgui et al. have shown that when stretched just below Tg, slightly crystalline PLA shows extensive ductility even if a crazing process is still activated [7]. Consequently, even if the degree of crystallinity is clearly identified as a key parameter, the drawing conditions obviously have a strong effect on the mechanical properties of PLA as well. Nevertheless limited information is available regarding the latter, particularly in the case of initially amorphous PLA.
Cold drawing of glassy amorphous polymers has always been a topic of interest. Indeed, significant research efforts have been devoted in the 1980's to the understanding of elementary plastic deformation mechanisms involved. The pioneering studies carried out by E.J. Kramer, A.M. Donald, A.S. Argon and J.C.M. Li for instance have shed light on the two main plastic deformation mechanisms involved during cold-drawing of amorphous polymers, namely shear banding and crazing. On the one hand, shear banding consists of localized shear of the amorphous matter along slip planes roughly oriented at ±45° to the direction of the applied stress. One of the main features is that this mechanism does not involve changes in sample volume. Shear processes may be localized or diffuse. By contrast, crazing is a localized cavitational mechanism that implies sample dilatation through formation of micro-voided zones bridged by elongated polymer fibrils. Unlike shear bands, this defect nucleates and grows perpendicular to the draw direction.
In most cases a single mechanism is activated for a given polymer drawn at fixed conditions. For example, polycarbonate (PC) is well known to deform rather by shear banding while polystyrene (PS) or polymethylmethacrylate (PMMA) display a crazing mechanism. Extensive studies of these “textbook cases” have allowed considerable advances regarding the understanding of both the origin and the role of these mechanisms. In particular it has been demonstrated that although strongly dependent on polymer structure, the occurrence of either one mechanism also depends on the deformation conditions (draw temperature, stretching speed…). In some cases, a transition from crazing to shearing may be observed when the temperature is increased. A crazing to shear banding transition has also been reported in the case of pre-oriented polystyrene samples. Indeed Sultan et al. have shown that in such a situation, the crazing mechanism is inhibited and plastic deformation occurs by shear banding [29]. Finally, in some peculiar cases, a coexistence of the two deformation mechanisms can be observed during cold drawing. For example, G'Sell et al. reported in the case of PET that crazing and shear banding are both activated during drawing and that their interactions are at the origin of specific structural features [30]. Many questions remain however regarding the interactions between these mechanisms and the origin of their coexistence is still unclear [31].
In this context, the present work is aimed at studying the plastic deformation mechanisms involved during cold drawing of PLA as a function of the drawing conditions. For this purpose, in-situ SAXS experiments were performed, taking advantage of the fact that this technique has been recognized for long as the most adapted for the characterization of crazes [32], [33] together with the opportunity of avoiding any relaxation effects owing to the in-situ character.
Section snippets
Material
The polylactide (PLA) of the study is the 4042D grade material from Natureworks (USA) containing 4.3 mol % of d-isomer units (supplier data). The number-average and weight-average molar weights are Mn = 116 kDa and Mw = 188 kDa, respectively, as determined from size exclusion chromatography. PLA films, having a thickness of 200 μm, were processed by extrusion blowing. In order to remove internal stresses due to the extrusion process, and to erase physical aging, PLA films were systematically
Drawing behavior
Fig. 1 depicts the mechanical behavior of PLA when stretched below its glass transition temperature (Tg).
Two distinct behaviors are evidenced. On the one hand, for Td = 25 °C, PLA exhibits a brittle behavior as revealed by the small deformation ratio achieved. In this case sample failure occurs before Yielding. This is in agreement with results previously reported in the literature about the brittle character of PLA. On the other hand for Td ≥ 35 °C PLA exhibits a ductile behavior as confirmed
Discussion
The simultaneous presence of crazes and shear bands has already been reported by G'Sell et al. in the case of PET [30]. As assumed by the authors, such a microstructure is indicative of the occurrence of interactions between crazes and shear bands. Nevertheless the chronology of these phenomena remains unclear. Particularly a main issue is to elucidate whether crazes are nucleated at the intersection of the shear bands or, in opposite view, if the shear bands are nucleated from the tips of the
Conclusion
The present study was focused on the understanding of the cold drawing behavior of PLA. In-situ high throughput SAXS experiments were carried out upon stretching. SAXS experiments appear as a very suitable tool to characterize the complex plastic deformation behavior exhibited by PLA, considering both the elementary mechanisms and the drawing kinetics. As expected, the crazing mechanism is responsible for the brittle character of PLA. In-situ SAXS experiments revealed that some crazes are
Acknowledgments
The authors are indebted to the SOLEIL Synchrotron Facility (Gif-sur-Yvette, France) for beam time allocation on the SWING line for the in situ SAXS study and to Dr. Javier Perez for his help during the experiments and for the scientific discussions. Financial support from Région Nord Pas de Calais and European FEDER for SAXS laboratory equipment is gratefully acknowledged.
References (49)
- et al.
WAXS study of the structural reorganization of semi-crystalline polylactide under tensile drawing
Polymer
(2012) - et al.
Deformation and damage upon stretching of degradable polymers (PLA and PCL)
Polymer
(2005) - et al.
Crystallization, structure and properties of plasticized poly(l-lactide)
Polymer
(2005) - et al.
Long chain branching polylactide: structures and properties
Polymer
(2010) - et al.
Utilization of ultrafine acrylate rubber particles as a toughening agent for poly(lactic acid)
Mater Sci Eng A
(2012) - et al.
Transparent and ductile poly(lactic acid)/poly(butyl acrylate) (PBA) blends: structure and properties
Eur Polym J
(2012) - et al.
Toughening of poly (lactic acid) by poly (β-hydroxybutyrate-co-β-hydroxyvalerate) with high β-hydroxyvalerate content
Eur Polym J
(2013) - et al.
Preparation and properties of nanocomposites based on poly(lactic acid) and functionalized TiO2
Acta Mater
(2009) - et al.
Comparison of polylactide/nano-sized calcium carbonate and polylactide/montmorillonite composites: reinforcing effects and toughening mechanisms
Polymer
(2007) - et al.
The effect of crystallinity on the deformation mechanism and bulk mechanical properties of PLLA
Biomaterials
(2005)
Experimental characterization of deformation damage in solid polymers under tension, and its interrelation with necking
Int J Solids Struct
Failure mechanisms in polyolefines: the role of crazing, shear yielding and the entanglement network
Polymer
Craze fibril breakdown in glassy polymers
Polymer
Craze structure and stability in oriented polystyrene
Polymer
An overview of polylactides as packaging materials
Macromol Biosci
Microstructure and thermal properties of polylactides with different l- and d-unit sequences: Importance of the helical nature of the l-sequenced segments
Macromol Mater Eng
Strain-induced molecular ordering in polylactide upon uniaxial stretching
Macromolecules
Thermal and strain-induced chain ordering in lactic acid stereocopolymers: influence of the composition in stereomers
Macromolecules
New insights on the strain-induced mesophase of poly(d, l-lactide): In situ WAXS and DSC study of the thermo-mechanical stability
Macromolecules
Modification of brittle polylactide by novel hyperbranched polymer-based nanostructures
Biomacromolecules
Free radical branching of polylactide by reactive extrusion
Polymer Engineering and Science
Reactively modified poly (lactic acid): properties and foam processing
Macromol Mater Eng
Reactive extrusion of poly(l-lactic acid) with glycidol
J Appl Polym Sci
Research progress in toughening modification of poly(lactic acid)
J Polym Sci Part B Polym Phys
Cited by (66)
Shear bands in amorphous polymers under four-point bending
2024, International Journal of Mechanical SciencesMelt stretching and quenching produce low-crystalline biodegradable poly(lactic acid) filled with β-form shish for highly improved mechanical toughness
2023, International Journal of Biological Macromolecules