Effect of temperature on the compressive stress-strain properties of polystyrene

The compressive stress-strain behavior of a commercial polystyrene has been studied and the effect of deformation temperature on modulus, yield stress, percent yield strain and yield energy was determined. Yield energy is the only one of these parameters that is linear with temperature in the ductile region. A change in the mode of failure from ductile to brittle occurs between 5–30°C at a strain rate of O.1/in./in./min. At all temperatures studied, the yield or fracture stress varied linearly with the rate of deformation for strain rates ranging from 0.1 to 1.0 in./in./min. The yield data as a function of temperature were analyzed via a rate expression modified to incorporate the Coulomb-Navier yield criterion, Activation energy was found to be a function of deformation temperature with a change in slope occurring near the β transition. Activation volume increased linearly with deformation temperature, for the range studied. Agreement of dynamic mechanical and yield activation energies imply that the type of motion and the height of the energy barrier are similar for both. However, an increase in activation volume for stressed vs unstressed conditions suggests that a greater number of chain segments move as a result of stress biasing. Also the increase of both activation volume and activation energy with temperature implies that the correlated length of chain movement increases as temperature is increased. Similar to activation energy, yield stress exhibits a change in temperature dependence near the β transition. Data on other glassy polymers suggest that the highest temperature sub-T_g, transition is related to the change in the temperature dependence of yield stress.     

Chain conformation of polystyrene in bulk by small-angle X-ray scattering

Small-angle x-ray scattering from solid solutions of different molecular weight fractions of partially brominated polystyrene in polystyrene has been studied. The results of this study indicate that polymer chains in bulk have a radius of gyration proportional to the square root of molecular weight, which is consistent with the random coil model. However, these radii are approximately 20% larger than the unperturbed radii deduced by light scattering in dilute solutions.     

Theoretical investigation of SANS from elastomeric networks with topological constraints

Summary This paper presents a theoretical investigation of SANS from labeled chains in polymer networks which are characterized by the topology of tube-like deformation dependent configurational constraints. The deformation dependence of the constraints is assumed to relax and to be determined by a microscopic deformation law. The results lead to the conclusion that the change of the radius of gyration of a network chain may be much less than the deformation of the macroscopic sample. It is pointed out that network defects favour the effect of constraint release.     

分子动力学模拟预测壳聚糖的玻璃化转变温度

为了预测壳聚糖的玻璃化转变温度,在COMPASS力场和恒温恒压（NPT）系综条件下,利用分子动力学模拟方法,在343～543 K温度范围内研究了壳聚糖的玻璃化转变行为,通过模拟体系在不同温度下的比体积、回转半径和能量参数,获得了壳聚糖的玻璃化转变温度(T_g)。研究结果表明,壳聚糖的比体积、回转半径、内能随温度有规律的变化并在T_g处发生转折。模拟计算得到的壳聚糖的T_g与实验方法获得的值基本相符,分子动力学方法可用于壳聚糖玻璃化转变温度的预测。其中,通过回转半径-温度曲线获得的T_g与实验值最相符,回转半径是影响玻璃化转变的一个重要因素,可用于预测聚合物的玻璃化转变温度。     

Deformation-induced highly oriented and stable mesomorphic phase in quenched isotactic polypropylene

Changes of structure and morphology of quenched isotactic polypropylene (i-PP) film during tensile deformation at room temperature have been investigated by applying in situ synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) techniques. SAXS patterns show that the isotropic diffuse scattering peak in the initial quenched i-PP film disappears after the film experiences necking and a strong streak, which accompanies obvious film stress-whitening, appears when strain reaches 600% and above. Explanation to the former is the dropping of density contrast between amorphous and mesomorphic phases due to deformation and that to the latter is the formation of microfibrils and/or microvoids in the film. WAXD patterns indicate that i-PP chains in the mesomorphic phase become highly oriented along the drawing direction with the strain value more than 5% and the three-fold helical chain conformation keeps in the deformed mesomorphic phase. The mesomorphic structure in the initial quenched i-PP film does not transform into crystal phase although high orientation appears during tensile deformation and the highly oriented mesomorphic phase is considered to be quite stable. Thermal behaviors of the initial quenched i-PP and deformed i-PP with the strain value of 650% are examined by modulated differential scanning calorimetry (MDSC). MDSC results show that the highly oriented i-PP chains in deformed mesomorphic phase are stable, which brings about restriction to chain mobility and prohibits from phase transformation of mesophase to crystal phase in the temperature range normally measurable for the initial quenched i-PP. A model about tensile deformation of the quenched i-PP is outlined to distinctly illuminate the changes of structure and morphology in both the amorphous and mesomorphic phases.     

Effect of the Level of Preliminary Deformation on the Stiffness of Synthetic Fibres

The deformation stiffness of synthetic fibres increases significantly (by 3 and more times) as a result of preliminary stretching. The stress—strain diagram of the initial synthetic fibre can be used to predict the character of the change in the deformation stiffness in different stages of preliminary elongation. In the stretching segments where destructive processes are weakly manifested, the stiffness of the stretched fibres increases with an increase in the deformation. In the region of intensive molecular destruction processes, the deformation stiffness of the stretched fibres almost does not change, which is due to overlap of competing processes: structure ordering and molecular chain breaking.     

Dynamics of irreversible uncoiling of a polymer chain

Using a simple dynamic model, the growth of rigidity and of the mean size of the polymer chain induced by a chemical reaction irreversibly fixing extended conformations of units has been investigated. When the reaction rate is controlled by the rotational diffusion of segments, the change in the mean length of a rigid segment and mean radius of gyration of the chain with time are described by power dependencies. For a macromolecule in a constant viscosity medium, the time needed to complete the reaction (i.e. to transform the coil into an extended rigid rod) is t_comp N³, where N is the initial number of segments in the chain. The cases of a kinetically controlled reaction and a mixed type of reaction have also been considered. The slowing down of the reaction caused by increasing the viscosity of the reaction medium has been evaluated, giving t_comp N⁷.     

不同结晶度纤维素的酶解历程的SEC-MALLS研究(英文)

The reactions of exo-cellulase (cellobiohydrolase, CBH) and endo-cellulase (endoglucanase, EG) were investigated by analyzing the insoluble residues of microcrystalline cellulose (MCC) and filter paper cellulose (FPC) during enzymatic hydrolysis. Molecular parameters including molecular weight and its distribution, degree of po-lymerization, and radii of gyration were measured by size exclusion chromatography coupled with multi-angle laser light scattering. No significant change in MCC chains was found during the whole reaction period, indicating that CBH digestion follows a layer-by-layer solubilization manner. This reaction mode might be the major reason for slow enzymatic hydrolysis of cellulose. On the other hand, the degree of polymerization of FPC chains decreases rapidly in the initial reaction, indicating that EG digestion follows a random scission manner, which may create new ends for CBH easily. The slopes of the conformation plots for MCC and FPC increase gradually, indicating stronger chain stiffness of cellulose during hydrolysis.     

Poly(ethylene glycol)s 2000-8000 in water may be planar: A small-angle neutron scattering (SANS) structure study

Poly(ethylene glycol)s of all molecular weights are highly soluble in water. Small-Angle Neutron Scattering (SANS) data has been collected on buffered ionic D₂O solutions of PEGs of nominal molecular weights 2000, 4000, and 8000 Da at a low concentration of 0.5% (w/v). The radii of gyration R_g vary as approximately M_w^0.5, which is consistent with both a Gaussian chain and a flat plate. Numerous types of experiments indicate flaws in the interpretation as a Gaussian chain. When the alternative interpretation is used, the structures in these dilute solutions are found to have a constant area stoichiometry of 1:1 water:-CH₂CH₂O- and are consistent with the data when monomeric molecules are plates of packed chains one chain thick.     

Internal stresses and activation volumes from the stress relaxation behavior of polyethylene at large deformations

The internal stress level in polyethylene has been determined as a function of strain by analyzing the stress relaxation behavior at room temperature. Also the influence of annealing on the magnitude of the internal stresses was studied, both with HD and LD polyethylene. The maximum initial strain was comparatively high: 20% and 40% for HDPE and LDPE, respectively. The internal stresses were found to be introduced during the initial deformation of the sample prior to the relaxation experiment. For both LDPE and HDPE the internal stress varied with the initial deformation in a manner resembling that of a stress–strain curve. In no case was a permanent residual internal stress (due to the molding process, etc.) observed. The activation volume v, evaluated from the stress relaxation kinetics, was found to be related to the effective stress σ^* according to vσ^* ≈ 10 kT, where k is the Boltzmann constant and T is the absolute temperature. This is in agreement with results reported earlier for both metals and polymers.     

Failure mechanisms in polypropylene with glass beads

The effect of glass beads on the stress-strain behavior of isotactic polypropylene has been examined. Poisson's ratio and secant compliance as a function of strain have been measured. Both sets of data are consistent with interfacial debonding as the initial damage mechanism. Interfacial debonding is then followed by extensive plastic yielding of the matrix at the debond sites. The maximum stress and strain to failure decrease with glass bead content and glass bead diameter. Impact properties correlate with the ability of the composites to reach high strain to failure. The proposed failure mechanisms are supported by fractography and in-situ deformation studies by scanning electron microscopy.     

Studies of Micromechanical Deformation Processes in Particle-Filled Glassy Polymer

The deformation behavior of rubber-toughened polymer, which was prepared by incorporating soft, core-shell rubbery particles into a glassy polymer such as poly (methyl methacrylate) (PMMA), has been investigated by means of mechanical tests, optical monitoring (OM), and scanning electron microscopy (SEM). By mechanical testing, the neat PMMA reveals a 2% strain with high yield stress. After inclusion of 17.5 and 35 vol%rubber particles, the softened-PMMA samples exhibit corresponding strain of 20% and 38%, showing an increase of strain along with the relative decrease of yield stress, resulting in a toughening behavior of PMMA. Clear shear bands and stress whitening develop in the rubber-toughened PMMA after deformation, as observed by OM. Investigation by SEM shows crazes/cracks in the stretched, rubber-softened PMMA samples in which the core-shell particles are found to be cavitated. The mechanism of this deformation has been explained based on the void formation in the rubbery shell as well as the initiation and propagation of crazing.     

High-temperature mechanical behavior of polycrystalline yttrium-doped barium cerate perovskite

The high-temperature mechanical properties of the mixed ionic-electronic conductor perovskite BaCe_0.95Y_0.05O_3−δ with average grain size of 0.40 μm have been studied in compression between 1100 and 1300 °C in air at different initial strain rates. The true stress-true strain curves display an initial stress drop, followed by an extended steady-state stage. As the temperature decreases and/or the strain rate increases, there is a transition to a damage-tolerant strain-softening stage and eventually to catastrophic failure. Analysis of mechanical and microstructural data revealed that grain boundary sliding is the primary deformation mechanism. The strength drop has been correlated with the growth of ultrafine grains during deformation, already present at grain boundaries and triple grain junctions in the as-fabricated material.     

Effect of cavitation on the plastic deformation and failure of isotactic polypropylene

To clarify the effect of cavitation, which is mostly induced by crystalline phase, on the plastic deformation and failure of isotactic polypropylene, solid‐state annealing at 160°C for 1.5 h is adopted to change the crystalline phase only while the amorphous phase keeps nearly intact. With aid of a special video setup, the relation of true stress and strain as well as the evolution of volume strain with axial strain has been derived. Enhancing crystalline phase due to annealing increases the yield stress and volume strain simultaneously. Moreover, the strain corresponding to steep increasing of volume strain is comparable with that related to yield, indicating that cavitation at early stage is accompanied with process of yield. With knowledge of toughness derived from impact tensile stretching and essential work of fracture (EWF), respectively, the relationship between cavitation and toughness has been correlated to some degree. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Ideal crosslinked-polymers and their characterization in free-radical monovinyl-divinyl copolymerizations

The critical conditions in which the classical Flory-Stockmayer gelation theory (F-S theory) is applicable to monovinyl-divinyl copolymerizations were pursued in detail. The resulting prepolymers or precursors of ideal crosslinked-polymers were characterized as standard polymers for the discussion of network formation in free-radical monovinyl-divinyl copolymerizations. Methyl methacrylate was copolymerized with a small amount of ethylene dimethacrylate, butylene dimethacrylate or nonapropyleneglycol dimethacrylate in the presence of lauryl mercaptan, a chain transfer agent to reduce the occurrence of a thermodynamic excluded volume effect and intramolecular crosslinking as the primary and secondary factors, respectively, for the greatly delayed gelation in the free-radical monovinyl-divinyl copolymerizations and, moreover, to keep the primary chain length constant by inhibiting a gel effect. The ratio of the actual gel point to the theoretical one reached 1.1, supporting the validity of F-S theory. The resulting prepolymers were subjected to SEC-MALLS analysis to determine the molecular weights, the molecular-weight distributions and the radii of gyration; the correlations of molecular weight vs. elution volume and radius of gyration vs. molecular weight were useful for the characterization of the precursors of ideal network-polymers.     

Pre‐necking and post‐necking relaxation and creep behavior of polycarbonate: A phenomenological study

Experiments have been performed to investigate the mechanical response of unfilled polycarbonate vis‐à‐vis the influence of prior deformation on stress relaxation and creep. Piecewise linear deformation histories, which involve strain‐controlled tensile loading of a specimen to a maximum load and partial unloading to a target strain/stress point as prologue to a relaxation test, have been shown to qualitatively influence the recorded stress‐time behavior. In particular, the stress magnitude during relaxation first increases and is then followed by a decrease. Analogously, in creep tests during unloading, the strain might decrease and then increase. Time characteristics for this U‐turn in the deformation response are influenced by the placement of the test. The influence of prior specimen conditioning on this phenomenon is investigated by comparing test data from virgin samples to that of specimens having high (～85%) inelastic strain from prior tensile elongation. Findings suggest that the observed persistence in the occurrence of this reversal effect for both types of specimens is evidence of the need to incorporate this behavior into the fold of material modeling. Additionally, this novel relaxation and creep behavior has been observed in other amorphous (poly(phenylene oxide)) and crystalline (high‐density polyethylene) polymers. Polym. Eng. Sci. 44:1783–1791, 2004. © 2004 Society of Plastics Engineers.     

Superplastic Deformation in Fine-Grained MgO 2Al2O3 Spinel

Fine-grained polycrystals of MgO 2A1₂O₃ spinel were deformed to large strains at strain rates ranging from 1O^-5 to 1O^-3 s^-1 and at temperatures from 1723 to 1885 K. These polycrystals were ductile at low strain rates and high temperatures; the ductility was especially remarkable at temperatures near the solvus. The mechanism of deformation, which was determined by measuring the change of flow stress with grain size, was dislocation creep, with the stress exponent in the power-law creep equation having a value of 2.1 ± 0.4. Despite deformation of several hundred percent, the grain size remained equiaxed, and deformation led to the evolution of a grain size which depended only on the strain rate and temperature. The initial microstructure had an influence on whether the poly-crystal would fracture or flow; a small initial grain size and a supersaturated solid solution were conducive to ductile flow. The ductility is attributed to dynamic recrystallization. It is proposed that the onset of fracture and the onset of dynamic recrystallization are competitive processes. Conditions which promote dynamic recrystallization also promote ductile flow.     

Stress softening in carbon black-reinforced vulcanizates. Strain rate and temperature effects

Stress softening of carbon black-reinforced butadiene-styrene rubber was studied as a function of the rate and temperature of the original tensile deformation. To a good approximation, stress softening depends on the product of the extension rate and a temperature function which is analytically well represented by the familiar Williams-Landel-Ferry relationship. When the elongation of the original deformation is also varied, a good correlation is obtained between stress softening and the maximum stress attained in the original extension, irrespective of the particular combination of strain, strain rate and temperature used to achieve this stress. Variables which tend to increase the stiffness of the vulcanizate, such as increased degree of crosslinking or carbon black chain structure, also increase stress softening; dilution by plasticizers decreases it. Prestressing at high strain rates and low temperatures affects the stress–strain curve of the softened vulcanizates beyond the elongation of the original extension. Connections are established between stress softening and viscoelastic and failure behavior. The evidence presented favors the contribution of several mechanisms to the general phenomenon of stress softening. These are thixotropy of transient filler structures, network chain rupture, and breakage of “permanent” filler structure. The latter appears to be most important at high strain rates, low temperatures, and with highly reticulated “structure” blacks.     

Small angle neutron scattering from ionomer gels

Small angle neutron scattering intensities for sols and gels of the physically associating ionomer 1.39 mol% sodium sulphonated polystyrene with molecular weight 10⁵ g/mole in xylene have been obtained over a broad wavevector (q) and concentration range. In the low q and concentration range the scattering behaviour of this ionomer/solvent system can quite readily be interpreted using the open association aggregation model. In more concentrated solutions and at higher q, however, interpretation of the scattering behaviour for polymers associating via an open association mechanism (OAM) is more difficult, particularly if, as in this investigation, the single chains and aggregates have varying densities and fractal parameters. In this study various methods have been developed to interpret the low and high q scattering from systems whose extent of aggregation can be modelled using the OAM. Using these methods it has been possible to confirm that the open association model can be used to interpret the extent of aggregation of the above ionomer in xylene even after the solutions appear to be gelled. Single ionomer chains within both the dilute solutions and gels were found via modelling to have a radius of gyration of 60 Å, which compares with dimensions of 25 Å and 93 Å calculated for a solid sphere of polystyrene or an unperturbed polystyrene Gaussian coil, respectively. The aggregates, however, all had radii of gyration comparable with what would be expected for polystyrene of the aggregate molecular weight in an unperturbed state. These results suggest that gelation of ionomer solutions at particular concentration thresholds is not due to an abrupt change in the aggregate structures at some critical concentration but occurs as a result of interactions between the very large aggregates that the OAM predicts should gradually form as the ionomer concentration increases.