Mechanical Rejuvenation in Poly(methyl methacrylate) Glasses? Molecular Mobility after Deformation

Structural Relaxation Time of a Polymer Glass during Deformation

In order to determine the structural relaxation time of a polymer glass during deformation, a strain rate switching experiment is performed in the steady-state plastic flow regime. A lightly cross-linked poly(methylmethacrylate) glass was utilized and, simultaneously, the segmental motion in the glass was quantified using an optical probe reorientation method. After the strain rate switch, a nonmonotonic stress response is observed, consistent with previous work. The correlation time for segmental motion, in contrast, monotonically evolves toward a new steady state, providing an unambiguous measurement of the structural relaxation time during deformation, which is found to be approximately equal to the segmental correlation time. The Chen-Schweizer model qualitatively predicts the changes in the segmental correlation time and the observed nonmonotonic stress response. In addition, our experiments are reasonably consistent with the material time assumption used in polymer deformation modeling; in this approach, the response of a polymer glass to a large deformation is described by combining a linear-response model with a time-dependent segmental correlation time.

Personal Perspective on Strain-Induced Polymer Crystallization

Semicrystalline polymer materials are commonly strong yet tough after processed through fiber spinning, film stretching (or blowing), and plastic molding (or foaming), which are fundamentally related with strain-induced crystallization. This paper provides a personal perspective on thermodynamics and kinetics aspects of strain-induced polymer crystallization, mainly based on the author's recent research experience. The thermodynamic studies include homopolymers, random copolymers, solution polymers, and blend polymers. The kinetic studies cover three sequential crystallization stages, i.e., crystal nucleation, crystal growth, and postgrowth. The thermodynamic driving forces join with the kinetic barriers to determine the crystal nucleation mechanisms and the structure evolution at the molecular level, which yield unique polymer crystal morphologies from lamellar crystals to shish-kebab crystals and eventually fibril crystals. The resulting semicrystalline structures were discussed with their implications for the mechanical properties of products. Some future studies were briefly proposed.

Molecular rotors to probe the local viscosity of a polymer glass

We investigate the local viscosity of a polymer glass around its glass transition temperature using environment-sensitive fluorescent molecular rotors embedded in the polymer matrix. The rotors' fluorescence depends on the local viscosity, and measuring the fluorescence intensity and lifetime of the probe therefore allows to measure the local free volume in the polymer glass when going through the glass transition. This also allows us to study the local viscosity and free volume when the polymer film is put under an external stress. We find that the film does not flow homogeneously, but undergoes shear banding that is visible as a spatially varying free volume and viscosity.

Role of Local Structure in the Enhanced Dynamics of Deformed Glasses

External stress can accelerate molecular mobility of amorphous solids by several orders of magnitude. The changes in mobility are commonly interpreted through the Eyring model, which invokes an empirical activation volume. Here, we analyze constant-stress molecular dynamics simulations and propose a structure-dependent Eyring model, connecting activation volume to a machine-learned field, softness. We show that stress has a heterogeneous effect on the mobility that depends on local structure through softness. The barrier impeding relaxation reduces more for well-packed particles, which explains the narrower distribution of relaxation time observed under stress.

Simultaneous memory effects in the stress and in the dielectric susceptibility of a stretched polymer glass

We report experimental evidence that a polymer stretched at constant strain rate λ[over ̇] presents complex memory effects after λ[over ̇] is set to zero at a specific strain λ_{w} for a duration t_{w}, ranging from 100s to 2.2×10^{5}s. When the strain rate is resumed, both the stress and the dielectric constant relax to the unperturbed state nonmonotonically. The relaxations depend on the observable, on λ_{w} and on t_{w}. Relaxation master curves are obtained by scaling the time and the amplitudes by ln(t_{w}). The dielectric evolution also captures the distribution of the relaxation times, so the results impose strong constraints on the relaxation models of polymers under stress and they can be useful for a better understanding of memory effects in other disorder materials.

Topographical transition of submicron pillar array of azo molecular glass induced by circularly polarized light

The well-aligned submicron patterns on surfaces have attracted wide attention from scientific curiosity to practical applications. Understanding their formation and transition is highly desirable for efficient manufacture of the patterns for many usages. Here, we report a unique observation on self-organized topographical transition of submicron pillar array of an azo molecular glass, induced by irradiation with circularly polarized light. During gradual erasure of the patterns upon exposure to the light, which is a property of this material, a new set of pillars unexpectedly emerge with new one in middle of each triangle cell of the original array. The highly regular pillar array with triple area density is formed and finally stabilized in the process, as revealed by thorough investigation reported here. This unusual observation and its rationalization will be of benefit for deep understanding of the light–matter interaction and can be expected to be applied in different areas.

Comparative study of photoinduced surface-relief-gratings on azo polymer and azo molecular glass films

Photoinduced surface-relief-gratings (SRGs) on azo polymer and azo molecular glass films, caused by trans–cis isomerization of azo chromophores, have attracted wide interest for their intriguing nature and many possible applications in recent years. Understanding the mechanical properties of SRGs at the nanoscale is critically important for elucidating their formation mechanism and exploring their applications. In this work, a representative azo polymer (BP-AZ-CA) and a typical azo molecular glass (IAC-4) were comparatively studied for the first time concerning their properties related to SRG formation through a variety of methods. The results indicate that when inscribing SRGs on the films, IAC-4 shows a much higher efficiency for forming SRGs relative to that of BP-AZ-CA. The overall average moduli of SRGs measured by nanomechanical mapping techniques are obviously smaller compared with the moduli of the corresponding films of both materials. The moduli at different regions of SRGs are periodically varied along the grating vector direction for both BP-AZ-CA and IAC-4 gratings. The moduli at the trough regions of SRGs are always larger than those of the crests, while the moduli at the hillsides are the smallest. Distinct from BP-AZ-CA, even the moduli at the trough regions of IAC-4 SRG are smaller compared with that of the original film, and the ratio between the trough and crest moduli is significantly larger for IAC-4. These results provide deep understanding of the SRG formation mechanism and reveal the clear distinction between these two types of glassy materials for their SRG-forming behavior, which are important for future applications.

A representative azo polymer (BP-AZ-CA) and a typical azo molecular glass (IAC-4) were studied for their surface-relief-grating formation behavior to provide a deep understanding of the clear distinction between these two types of glassy material.

Effect of Polymer Chain Length on the Physical Stability of Amorphous Drug-Polymer Blends at Ambient Pressure

Rational selection of polymers for amorphous drug stabilization is necessary for further successful development of solid dispersion technology. In this paper, we investigate the effect of polymer chain length on the inhibition of amorphous drug recrystallization. To consider this problem, we prepared a drug-polymer blend (in 10:1 drug to polymer ratio) containing bicalutamide (BIC) and polyvinylpyrrolidone (PVP) with different chain lengths K10, K30, and K90. We applied broadband dielectric spectroscopy to compare the molecular dynamics of investigated samples and thoroughly recognize their crystallization tendencies from supercooled liquid state. Despite the lack of differences in molecular dynamics, we noticed significant changes in their crystallization rates. To rationalize such behavior, we performed positron annihilation lifetime spectroscopy measurements. The results showed that the value of free volume was the highest for blend with PVP K90, which at the same time was characterized by the greatest tendency to crystallize. We postulate that the polymer chain, depending on its length, can have different configurations in the space, leading to better or worse sample stabilization. Our results highlight how important is detailed understanding of physical properties of polymers for judicious selection of the best stabilization approach.

Mapping Brittle and Ductile Behaviors of Polymeric Glasses under Large Extension

We have carried out a series of tensile extension tests on the two most common polymer glasses to describe their generic mechanical responses as a function of deformation rate at different temperatures. The essentially defect-free polystyrene and poly(methyl methacrylate) both show remarkable re-entrant failure: being ductile at intermediate rates and showing diminishing toughness at both higher and lower rates. We draw phase diagrams to map out the relationship between brittle-like and yield-like states in terms of temperature, rate, and stress. A coherent understanding of the rich phenomenology requires us to describe in more detail the interplay between the chain network and the primary structure bonded by intersegmental van der Waals forces.

Universal rescaling of flow curves for yield-stress fluids close to jamming

The experimental flow curves of four different yield-stress fluids with different interparticle interactions are studied near the jamming concentration. By appropriate scaling with the distance to jamming all rheology data can be collapsed onto master curves below and above jamming that meet in the shear-thinning regime and satisfy the Herschel-Bulkley and Cross equations, respectively. In spite of differing interactions in the different systems, master curves characterized by universal scaling exponents are found for the four systems. A two-state microscopic theory of heterogeneous dynamics is presented to rationalize the observed transition from Herschel-Bulkley to Cross behavior and to connect the rheological exponents to microscopic exponents for the divergence of the length and time scales of the heterogeneous dynamics. The experimental data and the microscopic theory are compared with much of the available literature data for yield-stress systems.

Relaxation of a hydrophilic polymer induced by moisture desorption through the glass transition

The relaxation of a hydrophilic polymer induced by relative humidity down-jump through the glass transition contains multiple stages.

Stability of polymer glasses vitrified under stress

How stress or strain imparts mobility to glasses is a scientific issue linking ideas of jamming and the glass transition across colloids, granular materials, polymers, and molecular glasses. Here, we address for the first time how stress applied during vitrification, formation of the glassy state by a temperature quench, affects the subsequent stability of the glassy state, even after the stress has been removed. Using entangled polymers that are easily manipulated mechanically above the glass transition temperature, we find that the resulting polymer glasses become less stable, exhibiting a higher physical aging rate, when stress is applied while rapidly cooling the polymer films. The data show an initial plateau value at low stress, before transitioning rapidly to a higher aging rate at larger stress. These results are suggestive of the glassy system being left trapped in a less stable, higher energy state with faster physical aging rate when stressed above some minimum value during vitrification.

Elastic yielding in cold drawn polymer glasses well below the glass transition temperature

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 intra-segmental tension in the chain network.

Simple model for the deformation-induced relaxation of glassy polymers

Glassy polymers show "strain hardening": at constant extensional load, their flow first accelerates, then arrests. Recent experiments have found this to be accompanied by a striking and unexplained dip in the segmental relaxation time. Here we explain such behavior by combining a minimal model of flow-induced liquefaction of a glass with a description of the stress carried by strained polymers, creating a nonfactorable interplay between aging and strain-induced rejuvenation. Under constant load, liquefaction of segmental motion permits strong flow that creates polymer-borne stress. This slows the deformation enough for the segmental modes to revitrify, causing strain hardening.

Theory of aging, rejuvenation, and the nonequilibrium steady state in deformed polymer glasses

The nonlinear Langevin equation theory of segmental relaxation, elasticity, and mechanical response of polymer glasses is extended to describe the coupled effects of physical aging, mechanical rejuvenation, and thermal history. The key structural variable is the amplitude of density fluctuations, and segmental dynamics proceeds via stress-modified activated barrier hopping on a dynamic free-energy profile. Mechanically generated disorder (rejuvenation) is quantified by a dissipative work argument and increases the amplitude of density fluctuations, thereby speeding up relaxation beyond that induced by the landscape tilting mechanism. The theory makes testable predictions for the time evolution and nonequilibrium steady state of the alpha relaxation time, density fluctuation amplitude, elastic modulus, and other properties. Model calculations reveal a rich dependence of these quantities on preaging time, applied stress, and temperature that reflects the highly nonlinear competition between physical aging and mechanical disordering. Thermal history is "erased" in the long-time limit, although the nonequilibrium steady state is not the literal "fully rejuvenated" freshly quenched glass. The present work provides the conceptual foundation for a quantitative treatment of the nonlinear mechanical response of polymer glasses under a variety of deformation protocols.