Interfacial and Annealing Effects on Primary α-Relaxation of Ultrathin Polymer Films Investigated at Nanoscale

Nguyen, Hung K.; Labardi, M.; Capaccioli, S.; Lucchesi, M.; Rolla, Pierangelo; Prevosto, D.

doi:10.1021/ma202757q

Cited by 51 publications

(69 citation statements)

References 45 publications

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“…On the contrary, we verified that capped films of constant thickness, in contact with the same supporting medium, and annealed for times much longer than the reptation time (longest relaxation time in a bulk melt) can exhibit different T g 's [18]. Similar results were extended by Nguyen et al to supported films deposited onto a larger number of different substrates [32]. These findings are in line with recent models [33,34] suggesting that a third key parameter, namely the interfacial free volume, should also be considered.…”

Section: Introductionsupporting

confidence: 87%

Glassy dynamics of soft matter under 1D confinement: How irreversible adsorption affects molecular packing, mobility gradients and orientational polarization in thin films

2013

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The structural dynamics of polymers and simple liquids confined at the nanometer scale has been intensively investigated in the last two decades in order to test the validity of theories on the glass transition predicting a characteristic length scale of a few nanometers. Although this goal has not yet been reached, the anomalous behavior displayed by some systems--e.g. thin films of polystyrene exhibit reductions of Tg exceeding 70 K and a tremendous increase in the elastic modulus--has attracted a broad community of researchers, and provided astonishing advancement of both theoretical and experimental soft matter physics. 1D confinement is achieved in thin films, which are commonly treated as systems at thermodynamic equilibrium where free surfaces and solid interfaces introduce monotonous mobility gradients, extending for several molecular sizes. Limiting the discussion to finite-size and interfacial effects implies that film thickness and surface interactions should be sufficient to univocally determine the deviation from bulk behavior. On the contrary, such an oversimplified picture, although intuitive, cannot explain phenomena like the enhancement of segmental mobility in proximity of an adsorbing interface, or the presence of long-lasting metastable states in the liquid state. Based on our recent work, we propose a new picture on the dynamics of soft matter confined in ultrathin films, focusing on non-equilibrium and on the impact of irreversibly chain adsorption on the structural relaxation. We describe the enhancement of dynamics in terms of the excess in interfacial free volume, originating from packing frustration in the adsorbed layer (Guiselin brush) at t(*) ≪ 1, where t(*) is the ratio between the annealing time and the time scale of adsorption. Prolonged annealing at times exceeding the reptation time (usually t(*) ≫ 1 induces densification, and thus reduces the deviation from bulk behavior. In this Colloquium, after reviewing the experimental approaches permitting to investigate the structural relaxation of films with one, two or no free surfaces by means of dielectric spectroscopy, we propose several methods to determine gradients of mobility in thin films, and then discuss on the unexploited potential of analyses based on the time, temperature and thickness dependence of the orientational polarization via the dielectric strength.

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Section: Introductionsupporting

confidence: 87%

Glassy dynamics of soft matter under 1D confinement: How irreversible adsorption affects molecular packing, mobility gradients and orientational polarization in thin films

2013

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“…Such a residue is no more soluble in the polymer solvent and can be considered strictly interacting with the clay. A similar phenomenon, i.e., the adsorption of polymer chain at the interface with an inorganic component, has been also evidenced to occur at the polymer-metal surface in the polymer film [22,23,57]. Moreover, a higher-density region of polymer close to MMT has been found in simulation [58].…”

Section: Discussionsupporting

confidence: 57%

Cooperativity length scale in nanocomposites: Interfacial and confinement effects

Saiter

Prevosto²,

Passaglia³

et al. 2013

Phys. Rev. E

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Interfacial and confinement effects on the evolution of cooperativity on approaching the glass transition have been studied in poly(propylenecoethylene) functionalized with diethylmaleate, polyethylene 1,4-cyclohexylenedimethylene terephthalate glycol and their nanocomposites with montmorillonite. A small increase of the structural dynamic cooperativity, a weak alteration of the temperature dependence of the characteristic relaxation frequency, and no changes in the glass transition temperature observed in poly(propylenecoethylene)-based samples can be rationalized in terms of interfacial interactions between polymer and exfoliated clay. On the other hand, confinement of polymer chains in the galleries of clay (intercalated nanocomposite) produces a strong reduction of cooperativity, of the temperature dependence of the characteristic relaxation frequency, and of the glass transition temperature in polyethylene 1,4-cyclohexylenedimethylene terephthalate glycol samples. Finally, by investigating the temperature dependence of a generalized fragility and of cooperativity, we evidenced that fragility of glass formers is determined not only by cooperativity.

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“…A similar result has been reported in silica-filled PVAc composites by Roland et al, 14 but differs from the ones confined in ultrathin polymer films. 40,56 It is generally accepted that the peak intensity of dielectric loss tangent (tan δ) is dependent on the polymer chain mobility and proportional to the relative number of relaxing elements. Figure 3b shows the tan δ peak values as a function of silica loading for different PVAc/silica nanocomposites.…”

Section: ■ Results and Discussionmentioning

confidence: 99%

Interfacial Interactions and Segmental Dynamics of Poly(vinyl acetate)/Silica Nanocomposites

Lin

Liu

et al. 2015

J. Phys. Chem. C

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We investigate the interfacial interactions and segmental dynamics of hydrophilic and hydrophobic (calcined) silica nanoparticle (NP) filled poly(vinyl acetate) (PVAc) composites via a combination of Fourier transform infrared spectroscopy (FTIR) and broadband dielectric spectroscopy measurements. For hydrophilic silica NP filled composites, an increase in the amount of bound carbonyl groups with increasing silica loading can be noted due to hydrogen bonding interactions with hydroxyls on the NP surfaces and carbonyl groups of PVAc. It is surprising that the apparent glass transition temperature (T g ) increases very slightly (∼1 K) compared to pure PVAc, but the glass transition process becomes much broader, indicating the existence of a slower relaxation mode. In addition to the bulklike α-relaxation assigned to polymer segments away from the NP surfaces, the nanocomposites also exhibit a slower interfacial α′-relaxation by 3−4 orders of magnitude compared to bulk polymers. However, in the case of hydrophobic (calcined) silica NP filled composites, T g shifts to higher temperatures by 4−5 K even if there is the absence of strong polymer-NP interfacial interactions confirmed by the FTIR results. Consequently, the overall α-relaxation dynamics are suppressed in the presence of silica NPs, which is attributed to an increase of steric hindrance and decrease of free volume. More importantly, in the nanocomposites with high NP loadings, it is worth noting a weak physical adsorption interfacial layer for which the segmental mobility is on average slower by 1−2 orders of magnitude relative to that for bulk polymers. However, the dielectric strength and interfacial bound fraction is much smaller than that of hydrophilic silica NP filled composites, indicating the fact that physical adsorption is much weaker compared to hydrogen bonding.

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Interfacial and Annealing Effects on Primary α-Relaxation of Ultrathin Polymer Films Investigated at Nanoscale

Cited by 51 publications

References 45 publications

Glassy dynamics of soft matter under 1D confinement: How irreversible adsorption affects molecular packing, mobility gradients and orientational polarization in thin films

Glassy dynamics of soft matter under 1D confinement: How irreversible adsorption affects molecular packing, mobility gradients and orientational polarization in thin films

Cooperativity length scale in nanocomposites: Interfacial and confinement effects

Interfacial Interactions and Segmental Dynamics of Poly(vinyl acetate)/Silica Nanocomposites

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