Brittle-ductile transition under compression of glassy polymers

Physical review letters, 2018

New experiments show that tensile stress vanishes shortly after preyield deformation of polymer glasses while tensile stress after postyield deformation stays high and relaxes on much longer time scales, thus hinting at a specific molecular origin of stress in ductile cold drawing: chain tension rather than intersegmental interactions. Molecular dynamics simulation based on a coarse-grained model for polystyrene confirms the conclusion that the chain network plays an essential role, causing the glassy state to yield and to respond with a high level of intrachain retractive stress. This identification sheds light on the future development regarding an improved theoretical account for molecular mechanics of polymer glasses and the molecular design of stronger polymeric materials to enhance their mechanical performance.

Physical Review Letters, 2013

This Letter reports elastic-driven internal yielding in strained ductile polymer glasses. After cold drawing of two different polymer glasses to neck at room temperature, we show that the samples display considerable retractive stress when warmed up above the storage temperature but still considerably below their glass transition temperatures. We conclude that the elastic yielding arises from the distortion of backbones leading to chain tension in the network.

ACS Macro Letters, 2014

The origin of high mechanical stresses in large deformation of polymer glasses has been elusive because both plasticity and elasticity take place. In this work on the nature of the mechanical responses, we carry out uniaxial compression experiments to make simultaneous mechanical and thermal measurements of polycarbonate. Our results confirm that two factors contribute to the growing mechanical stress in the postyield regime, which is known as "strain hardening". Besides plastic deformation that is intersegmental in origin, chain tension as an intrasegmental component contributes considerably to the measured stress in post-yield. Such a conclusion modifies the previous consensus regarding the nature of strain hardening in mechanical deformation of polymer glasses.

Polymer, 2003

The influence of network density on the strain hardening behaviour of amorphous polymers is studied. The network density of polystyrene is altered by blending with poly(2,6-dimethyl-1,4-phenylene-oxide) and by cross-linking during polymerisation. The network density is derived from the rubber-plateau modulus determined by dynamic mechanical thermal analysis. Subsequently uniaxial compression tests are performed to obtain the intrinsic deformation behaviour and, in particular, the strain hardening modulus. At room temperature, the strain hardening modulus proves to be proportional to the network density, irrespective of the nature of the network, i.e. physical entanglements or chemical cross-links. With increasing temperature, the strain hardening modulus is observed to decrease. This decrease appears to be related to the influence of thermal mobility of the chains, determined by the distance to the glass-transition temperature ðT 2 T g Þ:

Bulletin of the American Physical Society, 2015

Submitted for the MAR15 Meeting of The American Physical Society What deformation does and does not do in ductile polymer glasses JIANNING LIU, SHI-QING WANG, Department of Polymer Science, University of Akron-Entangled polymeric liquids have so far only shown strain softening, signified by stress overshoot upon startup shear. We show for the first time that entangled solutions of polymers with high glass transition temperature undergoes non-Gaussian chain stretching, losing its ability to yield through chain disentanglement. The stronger than linear increase of the shear stress with the strain ends with a sharp decline, forming a cusp. In situ visualization by particle-tracking velocimetry confirms that the solution undergoes abrupt rupture at a shearing plate in the sample interior. The rapid sample recoils elastically, producing the observed stress drop.

2012

Polymers are increasingly used in impact and complex high rate loading applications. Generally, the mechanical response of glassy polymers under high strain rates has been determined in compression. Some research programs have studied the combined effects of temperature and strain rate, still primarily in compression, providing better understanding of the physics behind the observed response and enhancing the models for these materials. However, limited data are available in tension, and even more limited are data describing both the compressive and tensile response of the same glassy polymer. This paper investigates the compressive and tensile response of glassy polymers across a range of stain rates from quasi-static to dynamic. Experimental results from dynamic mechanical analysis, quasi-static compression and tension, and split Hopkinson tension/pressure bars on several representative glassy polymers will be presented. The pressure dependant yield in these materials will be discussed through comparison of the tensile and compressive yield stresses.

The Journal of Chemical Physics, 2013

Glassy polymers show "strain hardening": at constant extensional load, their flow first accelerates, then arrests. Recent experiments under such loading have found this to be accompanied by a striking dip in the segmental relaxation time. This can be explained by a minimal nonfactorable model combining flow-induced melting of a glass with the buildup of stress carried by strained polymers. Within this model, liquefaction of segmental motion permits strong flow that creates polymer-borne stress, slowing the deformation enough for the segmental (or solvent) modes to then re-vitrify. Here we present new results for the corresponding behavior under step-stress shear loading, to which very similar physics applies. To explain the unloading behavior in the extensional case requires introduction of a 'crinkle factor' describing a rapid loss of segmental ordering. We discuss in more detail here the physics of this, which we argue involves non-entropic contributions to the polymer stress, and which might lead to some important differences between shear and elongation. We also discuss some fundamental and possibly testable issues concerning the physical meaning of entropic elasticity in vitrified polymers. Finally we present new results for the startup of steady shear flow, addressing the possible role of transient shear banding.

Bulletin of the American Physical Society, 2013

During uniaxial extension, polymer glasses undergo elastic deformation, yielding, strain softening, neck propagation, and "strain hardening". Both plasticity and anelasticity emerge under the large deformation, making the origin of the mechanic stress elusive to identify. The present work employs an IR camera to make in situ temperature measurements on the extending specimen along with the conventional force measurements. To demonstrate the generality of our findings we studied the ductile polycarbonate as well as two brittle polymers, i.e., PS and PMMA, which can be made ductile by melt extension [1]. We found that the rate of heat generation is only a small fraction of the mechanical power involved in the uniaxial extension of these polymer glasses. Thus, it seems that the origin of the tensile stress is largely intrachain, stemming from straining of the chain network.

Bulletin of the American Physical Society, 2017

Impact of melt-deformation on molecular structure and mechanical behavior of glassy polymers 1 JIANNING LIU, XIAOXIAO LI, ZHICHEN ZHAO, SHI-QING WANG, Department of Polymer Science, University of Akron -This work studies effects of melt deformation such as extension and compression on mechanical behavior of glassy polymers. Depending on how the entanglement network is altered during melt deformation, mechanical properties of polystyrene and poly(methyl methacrylate) are changed at temperatures below Tg. Conversely, the observed mechanical behavior below Tg reveals how molecular structures at segmental levels have undergone distortion due to melt stretching or shear. This research expands well beyond our previous investigations that have demonstrated how and why melt-stretched PS and PMMA turns ductile at room temperature.... 1 and why a cold-drawn ductile polymer glass produces significant retractive stress upon annealing above the cold-drawing temperature.. 2 .1.Wang, S.-Q.; Cheng, S.; Lin, P.; Li, X. A phenomenological molecular model for yielding and brittle-ductile transition of polymer glasses.

Polymer, 2005

This study is concerned with the finite, large strain deformation behavior of polymeric glasses. True stress-strain curves in uniaxial compression obtained for five different polymeric glasses: polycarbonate, polystyrene, poly(2,6-dimethyl-1,4-phenylene oxide), and linear and cross-linked poly(methylmethacrylate), revealed a strain-hardening response during plastic deformation that is strain-rate dependent and deviates from neo-Hookean behavior. An empirical modification of the so-called compressible Leonov model by a strain dependent activation volume is suggested, which describes the strain-rate dependent large strain behavior of these glassy polymers in good agreement with experimental data. q