Polymers in 2D confinement: A nanoscale mechanism for thermo‐mechanical reinforcement

Abstract

Acrylate-clay nanocomposites, a 2D confined system, exhibited unusual increase of thermo-mechanical properties. The nature of this reinforcement can be ascribed to chain dynamics modification and therefore investigated via dynamic mechanical analysis. Transmission electron microscopy and dynamic light scattering showed a strong nanoconfined regime, 2R h ≫ d 001 , where R h is the polymer's hydrodynamic radius and d 001 is the clay gallery spacing.The geometrical constraints to polymer dynamics led to significant enhancement of the thermo-mechanical properties. Adding only 1 wt% nanoclay, the glass transition temperature increased significantly, ΔT g = T g − T g,bulk~1 0°C, and the dynamic modulus E′ increased 10-fold.Analysis of dynamic mechanical spectra showed an increase of relaxation time τ, ie, polymer dynamics retardation. Furthermore, the mechanical damping tan δ was strongly attenuated evidencing the reduction of viscous dissipation. The activation energy E a of the α-transition increased as the confined macromolecules needed to overcome higher energy barriers to achieve configurational rearrangements. The considerable increase of mechanical modulus cannot be explained by polymer composite models, rather it was associated to a "nano-effect," scaling with the degree of confinement as E/E matrix~( 2R h /d 001 )n . This study paves the road for further understanding of polymer dynamics under 2D confinement and the reinforcement mechanism of thermo-mechanical properties. 1-3 Hybrid nanoparticles are also successful to produce nanocomposites with more robust physical properties and are also being used to produce smart, shape memory polymeric networks with tailored thermo-mechanical properties. 4,5The addition of nanoclays, ie, layered silicates, to glassy and semicrystalline polymers has produced nanocomposites with enhanced mechanical, thermal, and barrier properties. For recent reviews, see Paul and Robeson 3 and Ray and Okamoto. 6 It is noted however that polymers (glassy and semicrystalline) with nanoclay concentrations in excess of 10 wt% led to relatively modest enhancement of mechanical properties, and the degree of property enhancement varies broadly according to the type of nanoclay used and the surface treatment. Furthermore, the concentration of nanoclays used also varies greatly and higher concentrations (>10%) appear contrary to the assumption that rather small concentrations of nanofiller would enhance the physical properties. 2,3It is believed that the macromolecular nanoconfinement found in, for instance, intercalated morphologies of layered silicate nanocomposites would be responsible to a larger extent for the changes of thermomechanical properties. For instance, confinement has induced lowering or increasing of the glass transition temperature, T g , in glassy polymer nanocomposites. The reduction of T g in confined small molecule glassy materials was first documented by McKenna et al. 7 The confinement in polymer nanocomposites is understood as (1) polymer chains constrained to layere...

Viscoelasticity and Dynamics of Confined Polyelectrolyte/Layered Silicate Nanocomposites: The Influence of Intercalation and Exfoliation

“…The behavior of Δ T g agrees with other reports on model polymer nanocomposites with interacting polymer‐nanoparticles, and results obtained from highly confined acrylic/clay nanocomposites . The results shown in Figure a clearly demonstrated the influence of degree of confinement on T g where the intercalated morphology and ammonia‐based surfactant induced the largest increases of T g .…”

“…The apparent molecular weight was reduced (see Table ) thus denoting a denser system. The same result was observed in surfactant‐free acrylate‐bentonite nanocomposites …”

“…Recently, it was reported that acrylate‐bentonite nanocomposites showed considerable enhancement of physical properties. That is, only 1 wt% of bentonite nanoclay was enough to produce ≈10 °K increase of T g , fivefold increase of Young's modulus, E , and tenfold increase of dynamic tensile modulus E ′ . Traditional composite theories (e.g., Halpin–Tsai–Nielsen (HTN) model) failed to explain the dramatic increase of mechanical modulus.…”

“…Due to the current availability of nanoparticles with well‐defined size and morphology different states of confinement can be achieved for polymer nanocomposites: (i) polymer chains constrained by nanoclay, (ii) polymer chains inside nanopores or nanocylinders, or (iii) inorganic nanoparticles and hybrids (i.e., silica, alumina, POSS, and so on) dispersed in a polymer matrix …”

“…Traditional composite theories (e.g., Halpin–Tsai–Nielsen (HTN) model) failed to explain the dramatic increase of mechanical modulus. Rather, it was shown that a “nanoeffect,” the degree of confinement 2 R h / d 001 ( d 001 denotes nanoclay gallery spacing), was the relevant factor driving the mechanical modulus …”