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

Abstract

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...

A structural interpretation of the two components governing the kinetic fragility from the example of interpenetrated polymer networks

“…They observed that a decrease in the vinyl acetate content leads to a decrease in the cooperativity length, and attributed it to a reduction of the intermolecular interactions established between the vinyl acetate groups because of the presence of ethylene. A very different behavior was reported in the same study for the value of the fragility index, as m was found to stay close to 90 for the whole series of samples.…”

“…Recent studies highlighted some discrepancies between the variations of fragility and cooperativity length with structural changes. For example, Puente et al studied the segmental dynamics of amorphous poly(ethylene–vinyl acetate) copolymers by investigating a whole series of samples going from pure poly(vinyl acetate) system to EVA50 (50% vinyl acetate and 50% ethylene). They observed that a decrease in the vinyl acetate content leads to a decrease in the cooperativity length, and attributed it to a reduction of the intermolecular interactions established between the vinyl acetate groups because of the presence of ethylene.…”

Correlated and cooperative motions in segmental relaxation: Influence of constitutive unit weight and intermolecular interactions

“…The T g values range from 41.5°C for the PVAc, down to −27.5°C for the EVA50. The details of the calorimetric investigations are given in a previous work [23]. It can be observed that the α-relaxation peak shifts towards high frequency with the increasing temperature.…”

“…For this goal, the N C and N α with time and temperature variations have been studied in amorphous copolymers of poly(ethylene-co-vinyl acetate) (EVA) with different vinyl acetate content (VAc): from PVAc (100 wt % VAc) to EVA50 (50 wt % VAc). This sample family presents the main advantage to have the same backbone but a different number of dipolar pending groups, i.e., different interchain dipolar interactions having a clear influence on the CRR size [23].…”

Fabrication of Dielectric Elastomer Composites by Locking a Pre-Stretched Fibrous TPU Network in EVA

“…Actually, EVA comprises random copolymers composed of vinyl acetate polar units and ethylene apolar units. The interchain dipolar interactions are expected to decrease as a consequence of the decreasing vinyl acetate content [ 42 ]. By coating EVA on an E-TPU fiber membrane, pores were nearly eliminated as shown in Figure 3 d. Thus, the EVA/E-TPU composite presented an enhanced dielectric constant of about 4.5, which was higher than that of both E-TPU and EVA.…”