Shear stress relaxation and physical aging study on simple glass-forming materials

Relaxation and aging behaviors in three supercooled liquids: m-toluidine, glycerol, and sucrose benzoate have been studied by shear stress relaxation experiments in the time domain above and below their nominal glass transition temperatures. For the equilibrium state, the current study provides new data on the behavior of organic complex fluids. The shape of the relaxation function as characterized by the stretching exponent beta is discussed considering that a time-temperature master curve can be constructed even though the beta's for the individual response curves at each temperature vary systematically. In the nonequilibrium state, isothermal physical aging experiments at different glassy structures reveal that the effect of the aging process on the mechanical shear relaxation in these simple glass formers is similar to that observed in polymeric and other systems. Departure from the Vogel-Fulcher-Tamman behavior after the samples have aged back to equilibrium in the glassy state is observed for m-toluidine and, less strongly, for glycerol but not for sucrose benzoate. An inherent structure-based energy landscape concept is briefly discussed to account for the slow dynamics during the physical aging process.     

Metal nanoparticles on polymer surfaces: 6. Probing of non-glassy polystyrene surface layer

Peculiarities of the state of the surface layer of the amorphous glassy polymer polystyrene are studied with a specially developed experimental approach. The essence of the method consists in the observation via atomic force microscope for the depth and rate of embedding of gold nanoparticles in a polymer after their preliminary adsorption on the polymer surface from hydrosol. It is shown that the polymer glass-transition temperature near the boundary with air is substantially lowered relative to its bulk value. “Equilibrium” thickness of the non-glassy (“melted”) surface layer is determined through analysis of the data on the kinetics of nanoparticle embedding, and it is revealed that the layer thickness increases with temperature, reaching, near the “bulk” glass-transition temperature, the magnitude that is close to the diameter of the macromolecular coil. The results obtained are analyzed with allowance for published data, and the semi-empirical formula describing variations in the thickness of the non-glassy surface layer as a function of temperature in the interval between the “surface” and “bulk” glass-transition temperatures of a polymer is proposed.     

Yielding of a notched poly(vinyl chloride) sheet

The development of plastic deformation around the crack tip of poly(vinyl chloride), a ductile glassy polymer, has been studied in relation to the Dugdale–Barenblatt model of ductile yielding. Three-dimensional observations reveal that the plastic deformation ahead of the crack tip consists of crazes, shear bands, and their intersections. The formation of the craze is due to a state of plane strain at the immediate vicinity of the crack tip and restricted to early steps of loading. The size and shape of the fully developed plastic zone can be described by the model. The influence of strain hardening beyond the yield point is discussed on the basis of comparison of the plastic zone lengths of poly(vinyl chloride) with those of polycarbonate which always shows shorter lengths than the model predicts.     

Monte Carlo simulation studies of the interfaces between polymeric and other solids as models for fiber-matrix interactions in advanced composite materials

As a coarse-grained model for dense amorphous polymer systems interacting with solid walls (i.e., the fiber surface in a composite), the bond fluctuation model of flexible polymer chains confined between two repulsive surfaces is studied by extensive Monte Carlo simulations. Choosing a potential for the length of an effective bond that favors rather long bonds, the full temperature region from ordinary polymer melts down to the glass transition is accessible. It is shown that in the supercooled state near the glass transition an “interphase” forms near the walls, where the structure of the melt is influenced by the surface. This “interphase” already shows up in static properties, but also has an effect on monomer mobilities and the corresponding relaxation behavior of the polymer matrix. The thickness of the interphase is extracted from monomer density oscillations near the walls and is found to be strongly temperature dependent. It is ultimately larger than the gyration radius of the polymer chains. Effects of shear deformation on this model are simulated by choosing asymmetric jump rates near the moving wall (large jump rate in the direction of motion, and a small rate against it). It is studied how this dynamic perturbation propagates into the bulk of the polymer matrix.     

Craze fibril structure and coalescence by low-angle electron diffraction

Brown has shown that low-angle electron diffraction (LAED) may be used to determine fibril diameters D and spacings D₀ of crazes in thin polymer films. He found, however, that the D and D₀ determined for air crazes in polystyrene (PS) thin films were larger by about a factor of 3 than those in PS bulk crazes determined by using small-angle x-ray scattering (SAXS). We have repeated Brown's LAED experiments and find that the discrepancy may be caused by an aging effect. Our fresh crazes have D and D₀ values from LAED that are comparable to those of bulk PS crazes determined by SAXS. As the craze ages, however, fibrils retract and coalesce in wide regions of the craze, leading eventually to an observable “skin.” Aged crazes thus have much larger D and D₀ values than do fresh crazes. The large molecular mobility of the PS molecules in the fibrils necessary for this aging to occur at room temperature has important implications for fibril failure.     

Healing of interfacial surfaces in polymer systems

Literature data on the problems related to the healing of interfacial surfaces in polymers are revisited. Specific features and behavior of the contacting surfaces of polymer films in the rubbery and glassy states, as well as in heterophase polymer systems, are analyzed. Particular details associated with the healing of interfacial surfaces in polymers, which are capable of chemical interactions with each other, are considered. Special attention is focused on the analysis of the phenomena taking place on the newborn interfaces formed owing to the deformation of polymers in different physical states. Processes providing healing of shear bands and crazes during annealing of deformed polymer glasses are discussed. The above phenomena are shown to present evident practical interest from the viewpoint of the development of advanced nanocomposites based on polymer matrices.     

Nanocomposites on the Basis of Crazed Polymers

Analysis of published data on the mechanism of structural rearrangements in solid polymers on their crazing in liquid media is presented. The experimental evidence characterize crazing not only as a kind of spontaneous polymer dispersion under joint action of a mechanical stress and an active liquid medium, but also as the method of colloidal dispersion of low-molecular substances in a polymer. In the process of crazing, active liquid fills the porous structure of crazes, thereby transporting various low-molecular substances to the polymer volume. Crazing is believed to open the ways for preparing various nanocomposites on the basis of a wide variety of glassy and crystalline polymers, on the one hand, and target additives on the basis of practically any low-molecular substances, on the other.     

The effects of cyclic loading on polymers in a glassy state

The effects of cyclic loading on several physical properties of some polymers have been investigated. The results indicate that internal damping, shear modulus, heat capacity, and density of poly(methyl methacrylate) and alkali-polymerized polyamide 6 in the glassy state undergo changes during cyclic loading well before the appearance of crazes. It is suggested that cyclic loading induces a nonuniform volume contraction in the structure of the material.     

Craze initiation of glassy polymers under the action of crazing agent

This paper deals with the formation of crazes that may be caused by an external load on glassy polymers wetted with kerosene. First, the orientation of crazes has been determined when applying a uniaxial tension to a specimen of cold-rolled polyvinyl chloride sheet at various angles to the rolling direction. The critical stress for craze initiation in poly(methyl methacrylate) and polyvinyl chloride rods has been investigated under combined tension–torsion loading. It is shown that: (1) in an anisotropic, as well as an isotropic polymer, the direction of crazes is perpendicular to that of the maximum strain calculated by taking into account the internal stress due to rolling; and (2) under the action of a crazing agent, crazing may occur even under the pure torsional load, i.e., in the absence of dilatational stress.     

Electron microscopy of crazes in glassy polymers: Use of reinforcing impregnants during microtomy

Attempts to prepare undamaged microtomed sections of crazes without reinforcement have failed. Several methods of reinforcing crazes in glassy polymers with impregnants prior to microtomy have been tried. Generalized characteristics of successful impregnant systems are suggested on the basis of this experience. The most successful system has involved the infusion of liquid sulfur into crazes in poly(2,6-dimethyl-1,4-phenylene oxide). After quenching, the solid sulfur reinforces the crazes successfully during microtomy but subsequently sublimes away under vacuum. The resultant, largely undamaged craze structure is seen by transmission electron microscopy to resemble an open-cell foam, the holes and polymer elements of which uniformly average ～200 Å in diameter. A moderate degree of orientation in the original tensile stress direction is observed. Implications drawn from craze structure for the existence of order in the glassy state are discussed.     

Physical aging phenomena in silica and glass beads filled elastomers (EPDM)

The physical aging concept is generally used to explain the typical behavior of amorphous glassy materials such as amorphous polymers. It can be easily evidenced by measuring the effect of a static deformation on the dynamic mechanical properties. In this paper, an attempt is made to determine the “glassy” behavior of elastomeric EPDM chains when they are confined in the vicinity of the filler (glass beads, silicas) surface. It is demonstrated that glassy behavior and physical aging phenomena are detected even with a filled elastomer. Furthermore, the influence of the filler volume fraction, the filler nature and of filler surface treatments with silanes were studied. Finally, an original attempt is made to explain filler-rubber reinforcement by a kind of bimodal network created from linkages between a densely packed interfacial region and the outer loose matrix.     

Visualization of structural rearrangements responsible for temperature-induced shrinkage of amorphous polycarbonate after its deformation at different conditions

Structural rearrangements during the temperature-induced shrinkage of amorphous polycarbonate after its tensile drawing below and above the glass transition temperature, rolling at room temperature, and solvent crazing have been studied with the use of the direct microscopic procedure. This evidence demonstrates that the character of structural rearrangements during the temperature-induced shrinkage of the oriented amorphous polymer is primarily controlled by the temperature and mode of deformation. In the case of the polymer sample stretched above the glass transition temperature, the subsequent temperature-induced shrinkage is shown to be homogeneous and proceeds via the simultaneous diffusion of polymer chains within the whole volume of the polymer sample. When polymer deformation is carried out at temperatures below the glass transition temperature, the subsequent temperature-induced shrinkage within the volume of the polymer sample is inhomogeneous and proceeds via the movement of rather large polymer blocks that are separated by the regions of inelastically deformed polymer (shear bands or crazes).     

Specific features of the formation of polymer-dye systems based on nanostructured polymer matrices prepared by solvent crazing

Specific features of the formation of polymer-dye systems based on various nanostructured polymer matrices prepared by the method of solvent crazing are discussed. In the general case, the formation of polymer-dye composites includes four main stages: sorption of dye molecules by the highly disperse fibrillar material of crazes, shrinkage of the polymer composite due to the removal of the solvent, migration of dye molecules from their localized sites on the surface of fibrils, and healing of the structure of crazes (internal interfacial boundaries) under thermal treatment. Analysis of the migration of dye molecules in the polymer matrix includes the following assumptions: first, a metastable (nonequilibrium) state of the system after solvent crazing and introduction of dye molecules into the fibrillar craze material and, second, the statement according to which both the depth and direction of the above migration processes are controlled by the free energy of mixing of components. For amorphous glassy systems (PVC, PS, PC), healing of the fibrillar craze material (after shrinkage and removal of the solvent) is observed. In the case of semicrystalline polymers (PP, vinylidene fluoride-trifluoroethylene copolymer) and amorphous crystallizable polymer matrices (PET), the intensity of healing upon thermal treatment decreases due to the presence of crystalline regions, which slow down the motion of macromolecules.     

Study of the carbon fiber–Poly(Ether–Ether–Ketone) (PEEK) interfaces, 3: influence and properties of interphases

The aim of this third part is to analyze the structure and properties of the interfacial region between carbon fibers and PEEK as a function of different thermal conditioning treatments. First, it is shown by means of optical microscopy that the interfacial zone is not different from the bulk matrix when standard cooling conditions are used. On the contrary, a transcrystalline interphase is formed near the carbon fiber surface in systems that have been subjected to isothermal treatments. By comparison with previous results concerning the mechanical properties of the fiber–matrix interface, it appears that the interfacial shear strength decreases in the presence of a transcrystalline interphase or when the crystallization rate of PEEK increases. Moreover, it seems that the “constraint state” of the amorphous phase of PEEK near the fiber surface could also play a role in the interfacial shear strength. Secondly, a method is proposed in order to estimate the elastic modulus of crystalline interphases. It seems that this modulus is strongly dependent on the crystallization rate of the polymer. Finally, the determination of the stress-free temperature, defined as the temperature at which a longitudinal compressive stress just appears on the carbon fiber during the processing of the composites, is performed by recording the acoustic events corresponding to the fragmentation process in single-fiber composites. The results confirm that the crystallization rate and the “constraint state” of the amorphous phase of the matrix play an important role in the mechanical behavior of carbon fiber–PEEK interfaces.     

Crazing and fracture in crystalline,isotactic polypropylene and the effect of morphology,gaseous environments,and temperature

Stress crazing is studied in three forms of crystalline, isotactic polypropylene (PP): (1) smectic/nonspherulitic, (2) monoclinic/nonspherulitic, and (3) monoclinic/spherulitic PP. Optical and scanning electron microscopy as well as stress—strain measurements are used to characterize crazing behavior in these three forms as a function of temperature (?210 to 60°C) and of the gaseous environment (vacuum, He, N₂, Ar, O₂, and CO₂). Forms 1 and 2 are found to craze much like an amorphous, glassy polymer in the temperature range between ?210 and ?20°C, irrespective of environment. The plastic crazing strain is large close to the glass-transition range (ca. ?20°C) of amorphous PP and in the neighborhood of the condensation temperature of the environmental gas. Near condensation, the gas acts as a crazing agent inasmuch as the stress necessary to promote crazing is lower in its presence than in vacuum. A gas is the more efficient as a crazing agent, the greater is its thermodynamic activity. Spherulitic PP (form 3) crazes in an entirely different manner from an amorphous, glassy polymer, showing that the presence of spherulites influences crazing behavior much more profoundly than the mere presence of a smectic or monoclinic crystal lattice. Below room temperature, crazes are generally restricted in length to a single spherulite, emanating from the center and going along radii perpendicular, within about 15°, to the direction of stress. They never go along spherulite boundaries. Gases near their condensation temperature act as crazing agents much as in nonspherulitic PP. Above room temperature the crazes are no longer related to the spherulite structure, being extremely long and perfectly perpendicular to the stress direction. Apparently the crystals are softened enough by thermally activated segmental motion to permit easy propagation of the craze. The morphology of the fracture surfaces and its dependence on temperature and environment is described and discussed. Concerning the action of gases as crazing agents it is argued that the gas is strongly absorbed at the craze tip, where stress concentration increases both the equilibrium gas solubility and the diffusion constant. Hence, a plasticized zone is formed having a decreased yield stress for plastic flow. This is considered to be the main mechanism by which the gas acts as a crazing agent. In addition, reduction of the surface energy of the polymer by the adsorbed gas eases the hole formation involved in crazing.     

The “food polymer science” approach to flour functionality and ingredient technology in biscuit baking

An overview is made on the “food polymer science” approach developed by the authors. The quality and performance of flours for the production of cookies and crackers have been shown to depend upon the major functional polymeric components of flour: gluten protein, damaged starch, and pentosans. Of these, damaged starch and soluble pentosans in soft-wheat flours are detrimental to the commercial production of low-moisture cookies and crackers. The detrimental effects of soluble pentosans in flours can be eliminated through the use of pentosanase enzyme in cookie and cracker doughs. Three commercialized applications of this industrial enzyme technology have been patented by Nabisco. The “food polymer science” approach to baking technology has also been used to study finished-product attributes such as texture, in the context of the thermomechanical properties (e.g. modulus) of glassy solid and rubbery liquid matrices. Results of various studies have clearly demonstrated that products in a glassy solid physical state (at T < T_g) are hard and crisp in texture, but upon increasing plasticization by water (such that T_g is depressed below the observation T), are transformed to a rubbery or viscous liquid state, wherein textural hardness (and mechanical modulus) and crispness are dramatically reduced.     

Nanostructured Composites with Layered Morphology: Structure and Properties

Using different microscopic techniques, we investigate the morphology and the micro-deformation processes in two entirely different classes of polymer based composites: natural biocomposites and synthetic polymer composites. The emphasis has been put on the comparison of the micromechanical properties of those composite materials. In the natural layered composites exemplified by human cortical bone, analogous to the synthetic glassy polymers, craze-like deformation zones were formed. A strong dependence of deformation mechanisms (such as transition from formation of single crazes to multiple crazing behaviour) on the layer dimension was observed in the layered composites made up of different amorphous polymers.     

Sheared polymer glass and the question of mechanical rejuvenation

There has been much recent debate as to whether mechanical deformation reverses the aging of a material, and returns it to a structure characteristic of the system at a higher temperature. We use molecular dynamics simulation to address this problem by carrying out shear and temperature increase simulation on atactic glassy polystyrene. Our results show explicitly that the structure (as quantified by the torsion population) changes associated with shear and temperature increase are quantitatively--and in some cases qualitatively--different. This is due to the competition between rejuvenation and physical aging, and we show this by carrying out a relaxation simulation. The conclusion agrees with those from previous experiments and simulations, which were suggestive of mechanical deformation moving the system to structures distinct from those reached during thermal treatment.     

半晶聚对苯二甲酸乙二酯中的受限非晶相

当半晶聚对苯二甲酸乙二酯 (PET)的结晶度 (Xwc)处于一定范围内时 ,其物理老化后在差示扫描量热(DSC)曲线上的玻璃化转变区有吸热双峰出现 .通过对此吸热双峰分别与完全非晶试样和具有相当高Xwc 的半晶试样物理老化后在DSC曲线上出现的吸热单峰的比较 ,表明半晶PET中存在两种性质极为不同的非晶区 ,即自由非晶区和受限非晶区 .动态力学热分析 (DMTA)曲线上显示的损耗正切 (tanδ)双峰进一步证实了这两种不同非晶区的存在 .这两种不同非晶区的产生是由于试样中晶粒对非晶相中高分子链段活动性的不同限制作用所致 .研究发现 ,对于由冷结晶得到的半晶试样来说 ,出现两种不同非晶区所需的Xwc 上下限都随结晶温度 (Tc)的升高而增高 .还发现 ,在物理老化过程中 ,虽然非晶相的总量基本保持不变 ,但部分自由非晶区却逐渐转变为受限非晶区 .上述实验结果很好地符合Struik的“扩展玻璃化转变”模型 .