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.
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.
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.
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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Ultrahigh-molecular-weight polyethylene (UHMWPE) has been processed by means of sintering of a nascent powder. Particular attention was paid to the precompaction of the powder just below the melting ...point (T m) under vacuum. The particle welding was subsequently carried out under pressure at various temperatures above T m for various durations. Tensile drawing experiments performed on sintered samples either at room temperature or above T m were specifically aimed at discriminating the role of chain interdiffusion through the particle interfaces from that of cocrystallization in the mechanism of particle welding. It turned out that efficient welding occurred within a very short time. One of the novel results of the work is the much weaker influence of sintering time as compared with temperature, giving evidence that chain interdiffusion is not governed by a reptation process. The entropy-driven melting explosion over distances much larger than the chain length between entanglements is suggested to be the main mechanism of the fast chain re-entanglement and particle welding in the present case of a nascent powder consisting of nonequilibrium chain-disentangled crystals. Another major aspect of this study is the demonstration of the huge cocrystallization efficiency in the interface consolidation in the solid state that significantly hides the kinetics of chain intertwining occurring in the melt.
A mapping of the initiation of the three major plastic deformation processes in polyethylene is established as a function of temperature and microstructure. In this aim, a collection of polyethylene ...samples covering large ranges of crystallinity 0.50 < X c < 0.80 and crystal thickness, 8 nm < L c <29 nm was prepared. The strain onsets of crystal shear, cavitation, and martensitic-like transformation were determined from in situ WAXS and SAXS experiments during uniaxial tensile tests. The construction of the map was made via the determination of the critical L c values at which a competition exists between two processes for various temperatures in the range 25–100 °C. These critical L c values were reported on a temperature versus crystal thickness diagram where the various regions are assigning the order of occurrence that the three plastic processes should obey during tensile testing. Examples are given for illustrating the use of the map for predicting the order of occurrence of the three processes as a function of temperature for a given microstructure. Extrapolation is proposed for the behavior of materials having a crystal thickness out of the range of the present study.
This study focuses on the first occurrence of either cavitation or crystal shear in relation to temperature and microstructure during the tensile drawing of polyethylene. Four high density ...polyethylenes covering a range of crystallinity have been thermally treated to generate different microstructures displaying a large range of crystal thickness from 8 to 29 nm. The testing temperature spanned the domain 25–100 °C. In-situ SAXS measurements on synchrotron have been performed to capture the initiation of cavitation in parallel with stress-strain measurements. Depending on microstructure and temperature the strain onset of cavitation proved to be either before or after yielding associated with homogeneous or localized cavitation regimes respectively. The transition between the two regimes can be defined by a critical value of lamella thickness at each temperature. A physical modeling based on a thermally activated nucleation process has been developed for predicting the macroscopic stress for generation of cavities as well as the one for initiating crystal shearing. This modeling accounts for both temperature and microstructure effects on yielding. It allows describing successfully the delayed apparition of cavitation with increasing temperature and decreasing crystal thickness. The observation of complete disappearance of cavitation at high temperature is also predicted by the model in relation to crystal thickness. The more relevant aspects as well as the shortcomings of the model are discussed in the conclusion.
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This work addresses the question of the intimate coupling of plastic and damaging processes during the deformation of semi-crystalline polymers at small strains. The evolution of the spherulitic ...structure in the pre-yield strain range under tensile testing is investigated by atomic force microscopy for three semi-crystalline polymers, namely polycaprolactone, poly(1-butene) and polyamide 6. These materials have different spherulite size, crystallinity index and lamella thickness, and different glass transition temperature of the amorphous phase. Strain-induced damage is clearly evidenced through the gradual loss of elastic properties upon cyclic tensile tests, since the early stage of stretching. In parallel, volume strain appears to be about nil up to the yield point for the three polymers. AFM reveals that fragmentation of the crystalline lamellae occurs well before the yield strain at room temperature, starting about the core region of the spherulites and extending towards the periphery, for all polymers. This is claimed as evidence that lamella fragmentation is a basic mechanism of damage without significant cavitation at low strain. An approach of damage modeling is carried out via preliminary assessment of the viscoelastic contribution from low strain dynamic mechanical analysis using a generalized Maxwell model. It is shown that computing the viscoelastic contribution in the strain range up to yielding, in the assumption of linearity, fairly account for the loading-unloading hysteresis of the tensile cycles. A phenomenological plasticity/damage coupling law is established from the elastic modulus drop with increasing plastic strain, both assessed from the “relaxed” tensile cycles. The same kind of law is shown to apply for the three polymers. A physical meaning to the phenomenological law is proposed via a simple model of fiber rupture in single-fiber-reinforced composite.
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This work deals with the
in situ and
ex situ WAXS study of the strain-induced structural changes in relation to the mechanical behavior of a semi-crystalline polylactide containing 4% of
d-stereomer ...units. The crystals isothermally generated at 120
°C were in the α form. The draw temperature was in the range 65
°C
<
T
d
<
90
°C,
i.e. just above
T
g. In this
T
d range isothermal crystallization was so slow that only strain-induced crystals could be generated during the time scale of the experiments. This allowed studying samples with various initial levels of crystallinity,
X
cr. For
X
cr
=
5%, the early occurrence of strain-hardening contrasted with the case of the amorphous material. This was due to the physical crosslinking of the macromolecular network by the crystallites that prevented chain relaxation since the onset of drawing. Strain-induced α′ crystals developed in parallel with the mesophase, the respective amount of the two phases being dependent on
T
d. For
X
cr
=
24%, the drawing behavior was that of a crosslinked and filled elastomer. Again, both strain-induced α′ crystals and mesophase occurred in various proportions in relation to
T
d. For
X
cr
=
40%, it was shown that the initial α crystals underwent partial destruction and subsequent reorganization into mesophase and/or α′ crystals. Whatever
T
d and
X
cr, the total amount of ordered phases,
i.e. crystal
+
mesophase, did not exceed the value of 40% that was the maximum accessible crystal content for the present material. A mechanism of plastic deformation involving twinning is proposed from the WAXS analysis of the crystalline texturing in the case
X
cr
=
40%. AFM observations as a function of strain corroborate the proposed deformation scheme.
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The tensile behavior of nylon 6 films has been investigated in relation to water content. Modification of chain mobility in the amorphous phase via water plasticization appears to have a determining ...impact on the stress–strain response. More specifically, both yield stress value and hardening behavior over a large strain domain are strikingly equivalent for samples drawn at same Δ
T between draw temperature
T
d and main amorphous relaxation temperature
T
α. This apparent lack of thermal activation of crystal plasticity in the fibrillar transformation suggests that crystal block fragmentation proceeds via H-bond unzipping through water penetration at defective crystal interfaces.
