This work focuses on the mechanical behavior and the associated structural evolution of Polylactides homopolymers blends, i.e. PLLA and PDLA blends, with the aim of determining the ability to form ...the sterecomplex crystalline form upon stretching. The investigations carried out on initially amorphous samples revealed that only the crystalline α′-form, i.e. the one encountered in the case of the homopolymers, can be induced upon stretching indicating that the stereocomplex form cannot be strain-induced. Semi-crystalline samples initially under the stereocomplex form were also studied. Besides it was evidenced that while the strain-induced α′ crystalline form is strongly oriented toward the tensile axis, the sterecomplex crystals are almost insensitive to the mechanical treatment as a result of the presence of hydrogen bonds. Finally a brittle behavior was observed in the case of initially semi-crystalline samples under the α-form. This brittleness is ascribed to a phase separation process between the L-PLA and D-PLA macromolecules during the crystallization which involves a lack of cohesion between the crystalline lamellas, particularly due to the absence of tie molecules.
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•Mechanical behavior and strain-induced structural evolution of PLLA-PDLA blends.•No possibility to induce the stereocomplex form upon stretching.•High cohesiveness of the stereocomplex crystals.•Brittle behavior induced by the presence of α-form crystals.•Crystallization induced phase separation.
This work deals with the study of the mesomorphic form or mesophase induced by tensile drawing from the amorphous state of a polylactide material containing 4 mol % of d-stereoisomer units. ...Investigations have been carried out over the draw temperature domain 45−90 °C, i.e. an interval spanning roughly ±20 °C about the glass transition temperature. In situ WAXS experiments during drawing, stress relaxation, and/or heating of stretched samples invariably showed the strain-induced occurrence of the mesophase as far as temperature did not exceed 70 °C. This seems to be the upper stability temperature of the mesophase identified in a previous study. DSC traces upon heating of drawn samples exhibit a post glass transition endothermic peak similar to the enthalpy relaxation phenomenon observed for aged polymers. The amplitude of this strain-induced endotherm proved to be strongly dependent on draw temperature and draw ratio. Draw ratio also appeared to strongly influence the temperature domain of cold crystallization. The quite different structural evolution of the drawn samples as a function of temperature, depending whether cold crystallization occurred close or far from the strain-induced endotherm, led us to the conclusion that this endotherm results from neither physical aging nor orientation relaxation but from “melting” of the mesophase. This proposal is thoroughly supported by the insensitivity of the endotherm enthalpy to the DSC scanning rate that gives evidence of a first order thermodynamic transition in contrast to the case of aging-induced endotherm. WAXS as a function of temperature on drawn samples annealed with free ends enabled to probe the persistence of chain orientation and the stability of the strain-induced structural changes in relation to drawing conditions.
Isoprene chain shuttling polymerisation between the 1,4-trans regulating Cp*La(BH
)
(THF)
/Mg
BuEt (Cp* = C
Me
) and the 1,4-cis regulating NdCl
(THF)
/AlR
(R = Et or
Bu) systems provides access to ...stereoblock polyisoprene. Under selected experimental conditions, the trans segments are semi-crystalline, leading to a thermoplastic elastomer-like material.
This study deals with the tensile drawing behavior of a polylactide material containing 4% of d-stereoisomer units, in the amorphous state. The draw temperature domain spanned from the glass ...transition to the onset of thermal crystallization, namely 70−100 °C. The stress−strain curves exhibit a strain-hardening strongly sensitive to the draw temperature regarding both the onset and the slope of the phenomenon. A detailed structural investigation reveals that various strain-induced phase changes take place depending on the draw temperature. For T d = 70 °C, a mesomorphic form develops from the strain-oriented amorphous chains, starting at a strain level ε ≈ 130%. In the case T d = 90 °C, a well-defined crystalline phase grows beyond the strain ε ≈ 250%. In the midtemperature range, i.e. T d = 80 °C, both the mesomorphic and the crystalline phases are generated in parallel. In all cases, the final weight content of ordered phases at rupture was roughly 30%, irrespective of their form. The observed evolution with increasing draw temperature of the strain-induced structure from mesomorphic to crystalline is quite surprising with regard to the concomitant drop of the strain-hardening. Indeed, if the latter finding is consistent with the thermal activation of plasticity, it also means that the mesomorphic form is almost as much cohesive as the crystalline form in spite of its imperfect ordering. The occurrence of the mesomorphic form is specifically discussed in terms of both chain mobility and thermodynamic metastability.
This work deals with the influence of biaxial orientation on the mechanical behavior of Polylactide (PLA) films. Comparisons between a crystallizable grade and a non-crystallizable one have been made ...in order to separate the effect of macromolecular orientation from the potential influence of the crystalline phase induced during the thermomechanical treatment. While unstretched PLAs exhibited brittle behavior at room temperature, it was observed for both types of PLA the strains at break for biaxially drawn films were remarkably enhanced to values over 100%. This study highlights for the first time, in the case of PLA, that a critical molecular chain orientation of the amorphous phase is the necessary condition to induce a ductile behavior. In-situ small-angle X-ray scattering (SAXS) experiments and post-mortem microscopic observations have revealed that it is a change of elementary plastic deformation mechanisms that is at the origin of this Brittle-to-Ductile (B-D) transition.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, ODKLJ, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
The crystallization kinetics and the resulting structure and morphology of polylactide (PLA) were investigated in the presence of carbon nanotubes (CNTs). Nanocomposite samples prepared by solution ...and melt mixing present homogeneous filler dispersion, as observed by scanning electron microscopy. Calorimetric characterization of the nonisothermal and isothermal crystallization behavior analyzed according to Avrami’s theory provides evidence of the significant impact of CNTs on the crystallization kinetics of the PLA matrix. The nucleating effect of the nanofillers is confirmed by Raman spectroscopy experiments. Indeed, during isothermal crystallization, the nanotube characteristic vibrations are strongly affected by the development of polymer crystalline phase. Additionally, CNTs increase the number of nucleation sites and thereby decrease the average spherolite size as observed by optical microscopy. The PLA crystal structure is not modified by the presence of CNTs, as probed by X-ray diffraction.
Blends of entirely biosourced polymers, namely polylactide (PLA) and polyamide11 (PA11), have been melt-compounded by twin-screw extrusion without the use of any compatibilizing agent. The ...crystallization and melting behavior, the morphology and mechanical properties of the melt-compounded binary blends have been investigated over the whole composition range. Albeit immiscibility prevails in all blends, the micronic and sub-micronic dispersion of the minor phase reveals a self-compatibilization behavior of the PLA/PA11 system as directly evidenced via scanning electron microscopy. For PLA compositions below 50%, PLA appears to be dispersed as globules in the PA11 matrix. With increasing PLA content beyond 50%, the blends exhibit co-continuous intertwined phases, then thread-like PA11 phase dispersed in a PLA matrix. For PLA composition greater than 80%, PA11 displays globular dispersion. In the case of PLA-rich blends, the crystallization of PA11 from the melt displayed a contribution of “homogeneous nucleation” corroborating the high degree of dispersion of the minor phase. However, the earlier crystallization of PA11 upon cooling did not promote that of PLA suggesting low interfacial free energy at the boundaries of the phase-separated domains,
i.e. roughly neutral interactions between the unlike species. The non-symmetric structural behavior of the blends over the whole composition range was found to influence the mechanical properties. If the elastic modulus of the blends roughly obeys an additive mixture law at room temperature (RT), this is not the case above the Tg of both PLA and PA11. More particularly in the PLA-rich range, the thread-like dispersion of the stiff PA11 component plays the role of
in situ fibrillar reinforcement of the soft PLA matrix. The strain at break and the yield stress also do not obey a simple mixture law, both at RT and above Tg. Perspectives of morphological and mechanical improvements of PLA/PA11 blends are discussed.
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The cold drawing behavior of PLA has been investigated with special attention given to the identification of the elementary plastic deformation mechanisms involved. A combination of SAXS and AFM ...studies proved to be the most adapted and complementary approaches for this purpose, providing relevant information at a pertinent mesoscopic scale. Regarding the mechanical response, the present work shows that a brittle to ductile transition occurs when the draw temperature is increased and/or when the stretching speed is decreased. Morphological studies reveal that the brittle behavior of PLA is correlated with the occurrence of crazing, whereas in the case of a ductile, several plastic deformation mechanisms are involved and interact in a rather unusual way. In particular, the formation of “shear band crazes” was identified and a tentative explanation of its origin is proposed. In addition this study shows shear banding, activated at high temperature, plays a dual role. While shear bands promote the formation of crazes, they also stabilize them during their growth. Finally some correlations between chain dynamics and the observed mechanical behavior of PLA are also discussed.
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Among “green” alternatives for oil-based commodity plastics like Polyethylene, Polyamides, and Poly(Ethylene Terephthalate), a new class of polyesters synthesized from furandicarboxylic acids (FDCAs) ...currently is in the spotlight of both the academic and industrial communities. Of particular interest is Poly(Ethylene-2,5-Furanoate) (PEF), which can be synthesized from monomers derived from sugars and exhibits physical properties that imply large substitution potential for PET in bottle, fiber and packaging applications. In this study the structure-thermal properties relationship of PEF have been investigated in order to explain the complex behavior exhibited by PEF during isothermal crystallization. Secondary crystallization and melt-recrystallization processes have been identified in order to explain the observed behavior. The existence of a critical crystallization temperature around 170 °C marking a transition in terms of crystalline phase induced is also pointed out. Moreover, data such as the molecular weight between entanglements (Me) and standard melting enthalpy (ΔHm°) have been determined. In addition the crystalline morphology was characterized by means of SAXS and WAXS. The influence of the crystallization conditions on this morphology is given. As a main result it has been found that depending on the crystallization temperature (Tc), a disordered or an ordered crystalline phase (called α′ and α respectively) are formed at low (Tc ≤ 170 °C) and high (Tc ≥ 170 °C) temperatures, respectively. Finally, evidence for the formation of a rigid amorphous phase during crystallization is presented, indicating that a three-phase model has to be considered in order to provide an appropriate description of the morphology in semi-crystalline PEF.
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•First evidence of a defective crystalline form in PEF depending on the crystallization temperature.•Explanation of the multiple melting phenomenon.•ΔHm° = 140 J/g.•Mass between entanglements = 3500 g/mol.•Existence of a rigid amorphous phase in PEF.
This work focuses, for the first time, on the structural evolution of Polylactide (PLA) and PLA-Talc nanocomposites upon biaxial stretching. Biaxial stretching is a widely used technique to improve ...the end-use properties of polymer films. Encountered in a large number of elaboration processes it involves important structural changes into the material which directly govern the properties gain. In this work the influence of both stretching conditions and clay content were addressed. Besides it is observed that while relatively low clay content, i.e. below 10 wt%, has a limited impact on the mechanical behavior, higher contents dramatically modify the latter and particularly decreases the stretchability of the material. It was also evidenced that both the stretching conditions and clay content influence the strain-induced structure. Particularly the presence of talc favors the formation of a crystalline structure upon stretching due to its nucleating ability.
The mechanical behavior of the biaxially stretched samples has also been investigated. As a key point it was observed that while the as elaborated materials are brittle when uniaxially stretched at room temperature, the biaxially stretched ones exhibit a ductile behavior with achieved strains at break up to 100%. The origin of this brittle to ductile transition, assessed by means of in situ SAXS experiments, was found in the inhibition of the crazing mechanism for samples biaxially oriented under appropriate conditions.
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•Influence of talc content on the mechanical behavior of PLA upon biaxial stretching.•Nucleating effect of talc on the strain-induced crystallization.•Strain-induced structural evolution upon biaxial stretching.•Brittle to ductile transition induced by texturation•Macromolecular orientation involves crazing inhibition.
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