Structural Dependence of the Molecular Mobility in the Amorphous Fractions of Polylactide

A Saiter, et al.. Segmental mobility and glass transition of poly(ethylene-vinyl acetate) copolymers : Is there a continuum in the dynamic glass transitions from PVAc to PE?. Polymer, Elsevier, 2015, 76, pp.213-219. 10.1016/j.polymer.2015 The segmental dynamics of amorphous poly(ethylene-vinyl acetate) copolymers (from PVAc to EVA50) were studied. In that sample set with similar backbone stiffness and different amount of dipoles, the dynamic glass transition was investigated by Modulated Temperature Differential Scanning Calorimetry and Broadband Dielectric Spectroscopy measurements. A decrease of the cooperativity length scale was obtained with the vinyl acetate (VAc) content decreasing. On the other hand, there was no modification of the temperature dependence of the relaxation time. Thus, the fragility value is quite constant whatever the VAc content. These results show that fragility and cooperativity have two different origins. An extrapolation to nonconstrained polyethylene amorphous phase was proposed and new glass transition temperature and fragility values were determined. Highlights 1.Amorphous phase mobility is studied for poly(ethylene-vinyl acetate) copolymers 2.Fragility is constant from PVAc to EVA50 while cooperativity length decreases 3.New extrapolated T g and fragility values are proposed for PE 4.Fragility and cooperativity are not governed by the same macromolecular properties Keywords Amorphous phase, dynamic glass transition, poly(ethylene-vinyl acetate) . Therefore, structural relaxation temperature dependences can be defined as Super-Arrhenius, due to a possible behavior between two extreme limits: "strong" glass forming liquids for which the viscosity variations (or relaxation time) are very slow and follow an Arrhenius law, and the "fragile" glass forming liquids for which a very abrupt and steep non-Arrhenian variations can be observed.The fragility index (m) quantifies the steepness of the temperature dependence of the relaxation time () close to T g (defined at τ = 100s) and can be calculated as follows:The fragility values for different materials such as polymers, metallic glasses, organic and inorganic ionic glasses, and for small organic molecules were summarized Several approaches have been proposed to explain the correlation between the fragility and the molecular mobility near T g [10,16,17]. According to the theory proposed by Adam and Gibbs [18], it is well accepted that the α relaxation process is cooperative in nature: a structural unit can move only if a certain number of neighboring structural units move also. Besides, the molecular motions are mainly governed by the intermolecular interactions having important effects in the viscous slowing down of molecular dynamics when the glass-forming liquid is cooled-down close to T g . Thus, the notion of Cooperative Rearranging Region (CRR) was introduced, and the CRR size can be estimated according to different models and theories in terms of characteristic length scale or in terms of structural unit number [16,19-22]. Accordi...

Section: Thermal Analysismentioning

confidence: 99%

Segmental mobility and glass transition of poly(ethylene-vinyl acetate) copolymers: Is there a continuum in the dynamic glass transitions from PVAc to PE?

Puente

Rijal

Delbreilh

et al. 2015

“…They suggested that it constituted therefore an accelerated pathway for transport of penetrants around crystalline lamellae, counteracting the tortuosity increase occasioned by the same lamellae. Delpouve et al [25] worked on the cooperativity length and the fragility index of semicrystalline bi-axially stretched and post-stretching annealed PLA samples. They showed that the amorphous phase of the annealed samples had accelerated segmental motions, which would be beneficial for the gas permeability and not for the barrier properties.…”

Section: Introductionmentioning

confidence: 99%

Multi-scale analysis of the impact of polylactide morphology on gas barrier properties

Nassar

Guinault

Delpouve

et al. 2017

Self Cite

scale analysis of the impact of polylactide morphology on gas barrier properties. Polymer, Elsevier, 2017, 108, pp.163-172. 10.1016/j.polymer.2016 Multi-scale analysis of the impact of polylactide morphology on gas barrier properties Samira Fernandes Nassar t r a c tSemicrystalline polylactide (PLA) films with controlled morphology were produced by thermal crystallization to optimize the oxygen barrier properties. The crystalline morphology of PLA at the scales of the lamella and the spherulite was investigated and the mobile amorphous phase dynamics were studied. The crystalline morphology had a negligible impact on the oxygen diffusion coefficient. The occurrence of a rigid amorphous fraction (RAF) in the amorphous phase due to its insufficient decoupling from the crystalline phase provided an accelerated pathway for diffusion, though. As a conclusion, for reaching optimal barrier properties, semicrystalline PLA should be pre-nucleated and rapidly crystallized from the glass in the a-polymorph in the aim to reach a high crystallinity degree and decoupling of the amorphous and the crystalline phase. These recommendations can benefit to industry for the optimization of PLA annealing treatments.

“…Because of the strong dependence of PLA macroscopic properties on its microstructure [15] and molecular dynamics [16], the cooperativity of the alpha relaxation process has been particularly investigated when the relaxation dynamics of the amorphous phase are modified by orientation [17], crystallization [13], or even plasticization [11]. However, the impact of several microstructural modifications on the amorphous phase dynamics remains unknown.…”

Section: Introductionmentioning

confidence: 99%

Molecular dynamics in electrospun amorphous plasticized polylactide fibers

Monnier

Delpouve

Basson

et al. 2015

is an open access repository that collects the work of Arts et Métiers ParisTech researchers and makes it freely available over the web where possible. The molecular dynamics in the amorphous phase of electrospun fibers of polylactide (PLA) has been investigated using the cooperative rearranging region concept. An unusual and significant increase of the cooperativity length at the glass transition induced by the electrospinning has been observed. This behavior is attributed to the singularity of the amorphous phase organization. Electrospun PLA fibers rearrange in a pre-ordered metastable state which is characterized by highly oriented but non-crystalline polymer chains, and the presence of highly cohesive mesophase which plays the role of an anchoring point in the amorphous phase. The successful processing of electrospun fibers of plasticized polylactide is also demonstrated. It is shown that the plasticizer remains in the polymer matrix of the nanofiber after electrospinning. When PLA is plasticized, the loosening of the macromolecules prevails over the preferential orientation of the chains; therefore no mesophase is formed during the electrospinning and the cooperativity length remains the same. When the content of plasticizer increases, the inter-chain characteristic distances estimated from wide angle X-ray scattering (WAXS) are redistributed, suggesting a change in the level of interactions between macromolecules. It is assumed that the resulting decrease of the cooperativity length is driven by the progressive reduction of the number of inter-chain weak bonds. It is shown that in a non-confined environment, the number of structural entities involved in the alpha relaxation is strongly dependent on the level of physical interactions in the amorphous phase.