To explore the nature of mechanical stress of polymeric glass by stress relaxation tests (original) (raw)

Illustrating the Molecular Origin of Mechanical Stress in Ductile Deformation of Polymer Glasses

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.

On the origin of strain hardening in glassy polymers

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:

Origin of mechanical stress from tensile extension of polymer glasses

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.

Brittle-ductile transition under compression of glassy polymers

Bulletin of the American Physical Society, 2016

Polymeric glasses of high molecular weight are always ductile in compression. Even the most brittle (in tensile extension) polystyrene is ordinarily ductile in uniaxial compression. Thus, it seems that theoretical studies only need to develop a description of yielding and post-yield plastic deformation for polymer glasses. But can yielding take place in compression if the molecular weight is sufficiently reduced? In other words, can alpha processes be greatly accelerated during external deformation in absence of chain networking? Must a new paradigm account for the role of chain networking that only takes place in polymers of high molecular weight? To address these questions, we systematically explored the response over a range of temperature to uniaxial compression at different rates of polystyrene with various molecular weights and molecular weight distributions. Our preliminary results [1] show that PS of low molecular weight is brittle in compression and chain networking is necessary (but not sufficient) to ensure a ductile response. [1]

Modifying Fragility and Length Scales of Polymer Glass Formation with Nanoparticles

2012

Polymer-nanoparticle composites play a vital role in ongoing materials development. The behavior of the glass transition of these materials is important for their processing and applications, and also represents a problem of fundamental physical interest. Changes of the polymer glass transition temperature T g due to nanoparticles have been fairly well catalogued, but the breadth of the transition and how rapidly transport properties vary with temperature Ttermed the fragility m of glass-formationis comparatively poorly understood. In the present work, we calculate both T g and m of a model polymer nanocomposite by molecular dynamics simulations. We systematically consider how T g and m vary both for the material as a whole, as well as locally, for a range of nanoparticle (NP) concentrations and for representative attractive and repulsive polymer-NP interactions. We find large positive and negative changes in T g and m that can be interpreted in terms of the Adam-Gibbs model of glass-formation, where the scale of the cooperative motion is identified with the scale of string-like cooperative motion. These results provide a molecular perspective of fragility changes due to the addition of NPs and for the physical origin of fragility more generally. We also contrast the behavior along isobaric and isochoric approaches to T g , since these differing paths can be important to compare with experiments (isobaric) and simulations (very often isochoric). Our findings have practical implications for understanding the properties of nanocomposites and have fundamental significance for understanding the properties glass-forming materials more broadly.

Mechanical Heterogeneities in Model Polymer Glasses at Small Length Scales

Physical Review Letters, 2004

Molecular simulations of a model, deeply quenched polymeric glass show that the elastic moduli become strongly inhomogeneous at length scales comprising several tens of monomers; these calculations reveal a broad distribution of local moduli, with regions of negative moduli coexisting within a matrix of positive moduli. It is shown that local moduli have the same physical meaning as that traditionally ascribed to moduli obtained from direct measurements of local constitutive behaviors of macroscopic samples.

Modeling the relaxation of polymer glasses under shear and elongational loads

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.

Viscoplasticity and large-scale chain relaxation in glassy-polymeric strain hardening

Physical Review E, 2010

A simple theory for glassy polymeric mechanical response that accounts for large scale chain relaxation is presented. It captures the crossover from perfect-plastic response to Gaussian strain hardening as the degree of polymerization N increases, without invoking entanglements. By relating hardening to interactions on the scale of monomers and chain segments, we correctly predict its magnitude. Strain activated relaxation arising from the need to maintain constant chain contour length reduces the characteristic relaxation time by a factor ∼ǫN during active deformation at strain rateǫ. This prediction is consistent with results from recent experiments and simulations, and we suggest how it may be further tested experimentally.

A thermodynamic approach to the fragility of glass-forming polymers

The Journal of chemical physics, 2006

We have connected the dynamic fragility, namely the steepness of the relaxation time variation upon temperature reduction, to the excess entropy and heat capacity of a large number of glass-forming polymers. The connection was obtained in a natural way from the Adam-Gibbs equation, relating the structural relaxation time to the configurational entropy. We find a clear correlation for a group of polymers. For another group of polymers, for which this correlation does not work, we emphasise the role of relaxation processes unrelated to the  process in affecting macroscopic thermodynamic properties. Once the residual excess entropy at the Vogel temperature is removed from the total excess entropy, the correlation between dynamic fragility and thermodynamic properties is re-established.

Examining an Alternative Molecular Mechanism To Toughen Glassy Polymers

Macromolecules, 2019

Contrasting the conventional rubber-toughening mechanism, we show that the new PMMA-based rubbertoughened nanocomposite (PMMA-rt/nc) achieves the desired rubber toughening through molecular-level interactions between the glassy PMMA chains and the nanosized rubbery domains. PMMA-rt/nc, as an "inverted" polymer nanocomposite, is found to be sufficiently rigid to be treated as a polymer glass, yet ready to undergo plastic deformation like a ductile polymer glass. In parallel, we study high-impact polystyrene (HIPS) and acrylonitrile-butadiene-styrene (ABS) to elucidate the conventional rubber-toughening mechanism. Unlike PMMA-rt/nc, HIPS and ABS cannot undergo yielding and plastic deformation at room temperature. The delayed brittle fracture in HIPS and ABS is accomplished through crazinginitiated rubbery cavitation, manifested in the form of whitening of the specimens at the point of apparent yielding. Further experiments show that HIPS and ABS can be made to avoid crazing and consequently whitening as well as brittle fracture when they have undergone adequate premelt stretching that can enhance the chain networks in the glassy polymer matrix. The evidence of plastic deformation in the premelt-stretched HIPS and ABS comes from the fact that such specimens, unlike the untreated counterparts, contract in their transverse dimensions during the extension.