Microscopic flow and failure processes in polymer glasses

The microscopic flow and failure processes, the structural parameters controlling these processes, and how such processes are modified by the service environment are presented for three classes of polymer glasses: polycarbonate, polyimides, and epoxies. The microscopic flow characteristics of polycarbonate are controlled by (a) the ease of shear band deformation which depends on free-volume and previous thermal history and (b) surface crazes whose characteristics depend on exposure to organic compounds, thermal history, and to surface crystallization and fabrication stresses. Polyimides deform and fail in the bulk by crazing with extensive fibrillation. The growth and development of shear bands from microvoids in polyimides are discussed. Epoxies predominantly deform and fail by crazing with regions of high crosslink density remaining intact during the flow processes. Craze cavitation and growth are enhanced in epoxies by the presence of absorbed moisture.     

Phenomenology of plastic recovery in high polymer glasses

Deformations in isotropic, strain-free polymer glasses are usually completely recoverable (at the test temperature or after warming to T_g), in sharp contrast with the behavior of low molecular weight glasses and crystals. The apparent ‘plastic strain’ which remains at the end of a creep or stress relaxation experiment does not recover at the test temperature, but only after the sample is heated. It is proposed that the long time scales needed for entanglement reorganization in the glass are responsible for this delayed recovery. A phenomenological network model for thermally activated strain recovery in polymer glasses is analyzed. A superposition relation between the stress and the strain history using a KWW (stretched exponential) memory kernel is employed. The recovery of plastic (i.e. residual) strain in non-crosslinked amorphous thermoplastics is a two-step process that may be interpreted in terms of the network model. In particular, recovery at sub-T_g temperatures is associated with entanglement slippage, while recovery near-T_g is believed to involve reorganization at or near chain ends.     

Statistical entropy model for relaxation in stressed polymer glasses

The molecular kinetic theory near the glass transition, bused on the existence of free volume distribution, is extended to incorporate the effects of stress and stress rate. The fundamental equations for the volume relaxation and recovery in stressed amorphous polymers are derived in accordance with the balance of nonequilibrium statistical entropy. Using these kinetic equations, an earlier nonequilibrium criterion for the glass transition temperature, T_g, is generalized to include the effects of stress and stress rate. In contrast to the prevalent thinking toward free volume theories, an explicit expression between T_g and stress is developed and reveals that T_g does not continue to increase at all pressures but levels off to a “universal” asymptote at very high pressure (>10 K bars). The expression is applicable to any tension and compression stress conditions. A comparison between theory and experiment under constant stresses determines the activation volume tensor which reveals the molecular mechanism relating T_g and the plastic yield of glassy polymers.     

Chemical structure—normal force relationships in polymer glasses

In previous work, we found that the normal force responses in poly(methyl methacrylate) (PMMA) and poly(ethyl methacrylate) (PEMA) subjected to torsional deformations were strongly influenced by their prominent β-relaxation mechanisms which are related to the n-alkyl methacrylate side group motions. Using the concept of an elastic stress relaxing material we were able to extract the derivatives of the strain energy density function with respect to the first and second invariants of the deformation tensor (W₁ and W₂). In the present work, we report results from two materials, polysulfone (PSF) and polycarbonate (PC), with weak β-relaxations that are due to main chain motions. We find that for the two materials, the difference from neo-Hookean behavior is real though very small, contrary to the large deviations from neo-Hookean behavior observed for PMMA and PEMA. The results support the contention that the large normal force responses and the consequent strongly non neo-Hookean behavior of the glassy n-alkyl methacrylates is due to the side chain.     

Non-linear, rate-dependent strain-hardening behavior of polymer glasses

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.     

Structural aspects of physical aging of polymer glasses

Published data concerning the problem of the physical aging of glassy polymers are surveyed. Basic attention is given to an analysis of structural rearrangements that accompany the physical aging of glassy polymers. The processes of aging (spontaneous change in the properties of polymer glasses during the storage at a temperature below the glass transition temperature) can be classified into the two following categories: first, the aging of undeformed polymer glasses and, second, the mechanical stress-induced aging of a polymer. It is shown that in the former case, the processes occur throughout the entire body of the polymer and, in the latter case, the aging processes is concentrated in microscopic zones (shear bands) that emerge during polymer deformation. The current concepts of the aging of polymer glasses are discussed.     

Theory of slow,steady state crack growth in polymer glasses

Cracks in polymer glasses can grow slowly preceded by a craze, a narrow zone of plastic cavitation. The craze widens by drawing more polymer from its surfaces into its fibrils but the fibrils themselves fail by local creep. When the crack tip moves at velocity v the loading at the crack tip can be described by a local stress intensity factor K which is the sum of the ‘apparent’ stress intensity factor K_A and a plastic contribution K_p (usually negative). K_p is found to be $\text{?KP(K)}\text{v}$ where P(K) is an integral over the craze boundary displacement law. Fibril failure by local creep leads to a power law, v ∞ K^m. From these relations K and v can be determined as a function of K_A. The plot of K vs. K_A is multiple-valued with a stable branch (at high K) and an unstable branch (at low K) separated by a minimum value of K_A which represents a threshold for stable, steady state crack growth. There is also a v threshold, below which cracks will not grow steadily. These predictions, the form of the v?K_A curve and implications for slip-stick crack growth are compared with recent experiments.     

Microstructure and its relationship to deformation processes in amorphous polymer glasses

The microdeformation morphology of a number of vinyl polymers with bulky side chains (type I) and arylene polymers with flexible oxygen linkages (type II) was studied by electron microscopy. The polyarylenes crazed only near the glass transition while the polyvinyls exhibited a crazing regime that extended to liquid nitrogen temperatures. In addition significantly less plastic strain was localized in type II glass crazes relative to those in type I glasses. In compatible blends of polystyrene (PS) and 2,6-dimethyl poly(phenylene oxide) (2MPPO), ca. 30% 2MPPO was sufficient to induce a transition from type I to type II crazing behavior. Small amounts of PS suppressed the low-temperature 2MPPO β relaxation but enhanced the intermediate transition of 2MPPO at higher temperatures. Blending increased the conformational energy of the 2MPPO chain and improved interchain packing. The propensity for the polymer glass to form sharp shear bands at the expense of diffuse bands was increased by a decrease in the conformation energy of the polymer chain and an improvement in the glassy state packing.     

Intermolecular interaction in polymer alloys as studied by crack healing

The “method of crack healing” permits measuring the mechanical effects resulting from an interdiffusion of chain molecules across an interface. Having shown that in highly cross-linked ruptured styrene-acrylonitrile (SAN) a two-stage healing mechanism is active, a similar phenomenon was expected in blends of polystyrene (PS) with a more flexible component such as poly(vinyl methyl ether) (PVME), or with a more rigid component such as poly(?2,6 dimethyl 1,4 phenylene oxide) (PPO). However, in both cases crack healing at a temperature 10 K above the respective glass transition temperature (T_g) was linear with the healing time to the power of 1/4 and slower than with pure PS. This unexpected observation is explained by analysis of the thermal and viscoelastic behavior of these blends and by the different contribution to the stress intensity factor resulting from the entanglements formed by the interdiffusing molecules.     

Predicting embrittlement of polymer glasses using a hydrostatic stress criterion

In this study, the aging-induced embrittlement of three polymer glasses is investigated using a previously developed hybrid experimental–numerical method. The evolution of yield stress of unnotched tensile bars upon aging is coupled to the evolution of embrittlement of notched tensile bars using a numerical model combined with a critical hydrostatic stress criterion that determines the onset of failure. The time-to-embrittlement of notched tensile bars with a different notch geometry is predicted and in good agreement with the experimentally determined value. Next to that, the approach is extended to three polysulfone polymers, and it is shown that the value of the critical hydrostatic stress correlates well with the polymers entanglement density: : polymers with a denser entangled network display higher values, that is, a higher resistance against incipient cavitation. © 2019 The Authors. Journal of Applied Polymer Science published by Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47373.     

Craze kinetics for PMMA in liquids

Observations are reported on the growth of crazes in PMMA immersed in a range of alcohols, some less active crazing agents and some mixtures of methanol and inactive agents. When there is substantial plasticization of the craze, the growth is adequately described by the fluid flow model. Mixtures are also explicable in these terms since the active agent is preferentially absorbed. For lower degrees of plasticization, the craze growth is small and probably relaxation controlled.     

Craze failure by midrib creep

It has been known for some time that crazes thicken during growth mainly by drawing in fresh material from the craze-bulk interface, keeping the average craze fibril draw ratio approximately constant. Since Creep effects contribute only negligibly to craze growth rates these effects have generally been considered unimportant regarding craze breakdown. However, it is also known that the first stage of fracture is failure of the craze midrib, which is a highly drawn, very thin region down the middle of a craze. Because of the very low thickness of the midrib it has little influence on craze behavior, and information on midrib behavior is difficult to obtain. It is the purpose of this paper to attempt to rationalize what information is available.     

Craze dissemination during fatigue fracture in polystyrene

Results of studies on fatigue fracture in polystyrene are reported. Experiments were carried out on tension-tension single edge notched specimens, 0.25 mm thick. Macroscopic studies involved the propagation of the crack and its associated active zone (or the so-called process zone) evolution. Microscopic studies consisted of fracture surface examination and quantitative characterization of the crazing distribution along the trailing edge and within the active zone. The width and length of the active zone increased monotonically during the quasi-static phase of crack layer growth. The crack growth kinetics followed an S-shaped curve. Analysis of crazing distribution showed that; (i) the distribution of crazing along the trailing edge of the active zone is related by a scaling parameter, (ii) the average crazing density along the trailing edge as well as within the active zone is constant, (iii) the specific energy of crazing evaluated here compares well with previously reported data. The results of this work support the form of self similarity of damage evolution adopted by the crack layer model, and that the specific energy of damage is a material constant.     

Craze initiation and growth in high-impact polystyrene

Quantitative transmission electron microscopy and optical microscopy is used to study craze initiation and growth in thin films of high-impact polystyrene (HIPS). Dilution of the HIPS with unmodified polystyrene reduces the craze–craze interactions, permitting equilibrium growth and craze micromechanics to be studied. It is found that the equilibrium craze length depends on the size of the nucleating rubber particle, but not the internal structure; no short crazes less than a particle diameter are observed. The long crazes can be adequately modelled by the Dugdale model for crazes grown from crack tips. The effects of particle size and particle internal occlusion structure on craze nucleation have been separated. Craze nucleation is only slightly enhanced at highly occluded particles relative to craze nucleation at solid rubber particles of the same size. There is a strong size effect, however, which is independent of particle internal structure. Crazes are rarely nucleated from particles smaller than ～1 μm in diameter, even though these make up about half the total number. These craze nucleation and growth effects may be understood in terms of two hypotheses for craze nucleation: (1) the initial elastic stress enhancement at the rubber particle must exceed the stress concentration at a static craze tip and (2) the region of this enhanced stress must extend at least three fibril spacings from the particle into the glassy matrix. Since the spatial extent of the stress enhancement scales with the particle diameter, the second hypothesis accounts in a natural way for the inability of small rubber particles to nucleate crazes.     

Craze initiation in glassy polymers - Revisited

In spite of the continued interest in the yield, flow and fracture behavior of glassy homopolymers and the important role that crazing plays in their ultimate mechanical response, the basic understanding of the molecular level processes that govern craze initiation still remains murky. Here we revisit some early experiments of Argon and Hannoosh of the 1970s on craze initiation in homo polystyrene under combinations of both deviatoric shear stress and mean normal stress and also take note of more recent computer simulations of ultimate plastic flow and cavitation response of model glassy polymers. We then formulate a new craze initiation model and compare its predictions with the earlier experiments to find much better agreement over a wider range of applicability that sheds new light on this hitherto inadequately understood phenomenon.     

Effect of strain hardening of shape memory polymer fibers on healing efficiency of thermosetting polymer composites

Recently, shape memory polymer fibers (SMPFs) have been used in a biomimetic two-step (Close-Then-Heal) self-healing system for healing macroscopic cracks. The objective of this study was to investigate the effect of cold-drawing programming of SMPFs on the healing efficiency of conventional thermosetting polymer composites and the possibility of healing wide-opened crack by localized heating. To achieve the objective, continuous SMPF strand reinforced conventional epoxy composite beam specimens, which were dispersed with thermoplastic particles, were prepared. The SMPF strands were cold-drawn to various pre-strain levels before casting the polymer matrix. Repeated fracture/healing test was conducted by uniaxial tension. It is found that the composites were able to repeatedly heal macroscopic cracks. Strain-hardening by cold-drawing increased the healing efficiency considerably. It was demonstrated that healing can be achieved by heating locally surrounding the cracked region. The mechanism for the enhanced recovery stress was due to cold-drawing induced molecular alignment and formation of some perfect crystals in the hard segment domain of the SMPF.     

Craze fibril formation and breakdown

As crazes grow in areal extent they also increase in width. The areal growth involves craze tip advance which has been shown to occur by the Taylor meniscus instability. Craze widening, at least for air crazes, occurs by drawing more fibrillar material from the craze-bulk polymer interfaces at essentially constant extension ratio. Simple arguments will be given to predict the scale of the fibrillation in terms of the stress S at the craze tip and interfaces and an effective polymer surface energy (Γ) where: which assumes that all entangled chain crossing the surface are broken [γ represents the van-der-Waals (intermolecular) surface energy, d is the entanglement mesh size, v_E is the entanglement density, and U_b is the energy required to break a single backbone bond]. These arguments also give the rate of fibrillation as a function of S, a nominal plastic resistance σ_y and Γ and can explain the fact that the stress for crazing increases relative to that for shear deformation as the entanglement density of the polymer is increased. The geometrically necessary entanglement loss (either by scission as assumed above or by disentanglement- at temperatures just below T_g) that accompanies fibril formation has important consequences for fibril stability. The probability p that a given entangled chain is lost can be computed from simple geometrical considerations knowing the fibril diameter D, its extension ratio λ and the mesh size d; p increases rapidly as Dλ^½ becomes comparable to or less than d. These concepts can be tested in blends of high molecular weight polymer with chains of the same polymer that are too short to entangle.     

某有机玻璃零件银纹开裂分析及预防

针对某轨道交通有机玻璃零件的银纹开裂,介绍了其材料特性、零件结构及装配使用过程,分析了零件的尺寸、加工工艺、裂纹显微形貌。通过耐受试验及推理论证,发现银纹开裂的原因与加工应力大和外界有机溶剂有关。根据改进验证,改进加工工艺后的零件抗银纹开裂性显著增强,提出了进一步减少银纹开裂故障的建议措施。