This work takes a simple phenomenological approach to the questions of when, how, and why a brittle polymer glass turns ductile and vice versa. Perceiving a polymer glass as a hybrid, we recognize that both the primary structure formed by van der Waals forces (network 1) and chain network (i.e., the vitrified entanglement network) (network 2) must be accounted for in any discussion of the mechanical responses. To show the benefit of this viewpoint, we first carried out well-defined melt-stretching experiments on four common polymer glasses (PS, PMMA, SAN, and PC) in a systematic way either at a fixed Hencky strain rate to a given degree of stretching at several temperatures or at a given temperature to different levels of stretching using the same Hencky rate. Then we attempted to preserve the effect of melt-stretching on the chain network structure by rapid thermal quenching. Subsequent room-temperature tensile extension of these melt-stretched amorphous polymers reveals something universal: (a) along the direction of the melt-stretching, the brittle glasses (PS, PMMA, and SAN) all become completely ductile; (b) perpendicular to the melt-stretching direction, the ductile glass (PC) becomes brittle at room temperature. We suggest that the transformations (from brittle to ductile or ductile to brittle) arise from either geometric condensation or dilation of load bearing strands in the chain network due to the melt-stretching. Regarding a polymer glass as a structural hybrid, we also explored two other cases where the ductile PC becomes brittle at room temperature: (1) upon aging near the glass transition temperature; (2) when blended with PC of sufficiently low molecular weight. These results indicate that (i) the strengthening of the primary structure by aging can raise the failure stress σ* to a level too high for the chain network to sustain and (ii) the PC blend becomes brittle upon weakening the chain network by dilution with short chains.

Mechanical properties of layered silicates on the nanometer scale have been associated with large uncertainty. We attempt to clarify the linear elastic properties including tensile moduli, shear moduli, and potential failure mechanisms for the minerals pyrophyllite, montmorillonite, and mica in the order of increasing cation exchange capacity (CEC) under a broad range of stress using electronic structure calculations, semiempirical classical molecular dynamics simulation, and the comparison to available macroscopic experimental data. In-plane tensile moduli (xx and yy) are ∼160 GPa independent of CEC and stress, whereas perpendicular tensile moduli (zz) range from 5 to 60 GPa as a function of CEC at low stress (0.01 to 1 GPa) and approach in-plane values at high stress. In-plane shear moduli (xy) are ∼70 GPa independent of CEC and the shear strength increases from ∼1 to ∼3 GPa with increasing cation density. Shear moduli parallel to the layers (xz and yz) are between 2 and 20 GPa as a function of CEC, with a shear strength of 0.2 to 1 GPa beyond which the layers exhibit lateral shear flow. Tensile zz moduli, shear moduli, and shear strength in the xz and yz direction reach a local minimum at a cation density of 0.3 relative to mica. The simulation suggests sliding of the layers, in-plane kinks, and cation intrusion into the layers as potential failure mechanisms equal to amorphization on the macroscale. The anisotropy and stress-dependence of the mechanical properties is determined by the presence of rigid layers and flexible interlayer spaces of variable cation density. Current classical simulation models tend to overestimate in-plane moduli (xx, yy, xy) in a systematic way relative to electronic structure (DFT) and experimental results.

Bending and failure of aluminosilicate layers are common in polymer matrices although mechanical properties of curved layers and curvature limits are hardly known. We examined the mechanism of bending, the stored energy, and failure of several clay minerals. We employed molecular dynamics simulation, AFM data, and transmission electron microscopy (TEM) of montmorillonite embedded in epoxy and silk elastin polymer matrices with different weight percentage and different processing conditions. The bending energy per layer area as a function of bending radius can be converted into force constants for a given layer geometry and is similar for minerals of different cation exchange capacity (pyrophyllite, montmorillonite, mica). The bending energy increases from zero for a flat single layer to ∼10 mJ/m2 at a bending radius of 20 nm and exceeds 100 mJ/m2 at a bending radius of 6 nm. The smallest observed curvature of a bent layer is 3 nm. Failure proceeds through kink and split into two straight layers of shorter length. The mechanically stored energy per unit mass in highly bent aluminosilicate layers is close to the electrical energy stored in batteries. Molecular contributions to the bending energy include bond stretching and bending of bond angles in the mineral as well as rearrangements of alkali ions on the surface of the layers. When embedded in polymers, average radii of curvature of aluminosilicates exceed hundreds of nanometers. The small fraction of highly bent layers (<20 nm radius) can be increased by extrusion, especially in stacked layers, and by an increase in weight percentage of layered silicates above 5%. Extrusion also promotes failure and shortening of isolated layers.

Particle tracking velocimetry (PTV) was used in conjunction with rheological measurements to investigate slip at polymer–polymer interfaces during and after startup shear using simple shear geometry, i.e., a sliding plate rheometer. Polymer pairs include styrene butadiene rubber (SBR) and polyisoprene (PI) of narrow molecular weight distribution, as well as a PI and polydimethylsiloxane (PDMS), with Flory interaction parameter χ ≈ 0.001 and χ ≈ 0.08 respectively. During startup shear, the SBR-PI pair was able to deform without interfacial failure up to a critical strain γ = 2, revealing a level of interfacial strength consistent with thermodynamic considerations. This pair also exhibited arrested interfacial slip after shear cessation for high Weissenberg numbers when the step strain exceeds unity. In contrast, the PDMS–PI interface is much weaker, and slip occurs at γ as low as 0.1. These results shed light on the nature of possible mechanical failure at interphases in multicomponent polymer systems during shear and after shear cessation.