Brittle‐ductile transition in uniaxial compression of polymer glasses

Abstract

We carried out a large set of tests to establish a correlation between the molecular (network) structure (influenced by molecular weight, molecular weight distribution, and melt predeformation) and mechanical responses of several glassy polymers to uniaxial compression at different temperatures and different compression speeds. The experimental results show that to have ductile responses there must be an adequate chain network, afforded by the interchain uncrossability among sufficiently long chains. Specifically, polystyrene (PS) and poly(methyl methacrylate) of sufficiently low molar mass do not have chain network and are found to be very brittle. Binary PS mixtures are brittle at room temperature when the volume fraction of the high-molecular-weight component is sufficiently low (e.g., at and below 27.5%). Moreover, sufficiently meltstretched PS mixtures show brittle fracture when compressed along the same direction, along which melt stretching was made. All the experimental findings confirm that a robust chain network is also a prerequisite for yielding and ductile cold compression of polymer glasses, as is for extension.

Structural Evolutions of Initially Amorphous Polymers during Near‐Tg Stretching: A Minireview of Recent Progresses

“…Besides chain mobility, one should note the role of overall polymer network evolution on altering mechanical properties of glassy polymers. Wang and coworkers emphasize in a few studies combining experimental studies with modeling simulations [ 43,46,51,52 ] that the key for ductile behavior of polymer glasses is effect of chain networking. They proposed that stretching increases the density of load‐bearing strands in cross‐section area through geometric condensation, which induces the strengthening of mechanical response in the stretching direction, as illustrated in Figure .…”

Double Yielding in Deformation of Semicrystalline Polymers

“…According to our recent molecular model, [ 21 ] yielding of a glassy polymer (polymer glass) is defined as activation of chain network associated with increase in the segmental mobility and it turns out that the condition for activation of chain network is more readily provided in compression deformation compared to tensile deformation. [ 22,23 ] Considering a semicrystalline polymer as a composite structure of glassy and crystalline phases below T g , we predict that the yielding of a semicrystalline polymer should involve two yielding, yielding of glassy phase followed by yielding of crystalline phase. Yielding of glassy phase should not be different than that of a pure glassy polymer, on the other hand yielding of crystalline phase is due to destruction of crystalline structure which itself can be combination of “fragmentation” and “force‐induced melting”.…”

Universal Strain‐Life Curve Exponents for Thermoplastics and Elastomers under Tension‐Tension and Torsion

“…[12,15,16] Depending on the frequency and temperature, materials (like polymers) can mechanically behave as brittle or ductile. [17][18][19][20][21] The different failure mechanisms depend on the plastic deformation before fracture and can be distinguished, for example, via optical analysis of the failed specimen. In torsion, brittle materials break along the plane of maximum tension; that is, along a plane with a 45°angle to the length side of the specimen, while ductile materials break along the plane of maximum shear; that is, along a plane with a 90°angle to the length side of the specimen.…”