Nanovoid relaxation in a series of copolyester glasses under cyclic loading using synchronous PALS (original) (raw)
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Macromolecules, 1999
A correlation between the yield behavior of copolymers based on poly(ethylene terephthalate) and poly(1,4-cyclohexylenedimethylene terephthalate) (PCT) and their secondary relaxation motions is established. The yield stress decreases as the cyclohexylene content increases for different temperatures and strain rates. The activation volume based on Eyring's model of yielding increases as the copolymers become more PCT-like, while the activation energy does not exhibit any significant change. We speculate that the conformational changes of the cyclohexylene rings reduce the barriers between chain segments sufficiently to facilitate chain slippage. This is supported by the increased dynamic fluctuation found by positron annihilation lifetime spectroscopy. The crazing stress of the copolyesters increases with increasing cyclohexylene content. We propose that the formation of stable microvoids is impeded by local stress relaxation; hence craze formation is retarded. The ductile/brittle transition is viewed as a competition between yielding and crazing with changes in the transition temperature dependent on activation of molecular motions of the cyclohexylene groups.
The Journal of chemical …, 2005
In this study, polycarbonate (PC) and polystyrene (PS) are subjected to plastic deformation by means of cold rolling and the resulting variation of the free volume and its subsequent time evolution after rolling is investigated by means of positron annihilation lifetime spectroscopy (PALS). The value of the long lifetime component that is attributed to the decay of ortho-positronium (tau o-Ps) and its intensity (I o-Ps) are used to characterize, respectively, the size and the concentration of the free-volume holes. In addition to the PALS experiments, the effect of plastic deformation on the dynamic tensile modulus is investigated. The PALS results show that both for well-aged PC and PS an increase of to-Ps and a decrease of Io-Ps occur upon plastic deformation. During the subsequent aging, to-Ps tends to return to the value assumed before plastic deformation, while Io-Ps remains constant with time. These results corroborate the idea of an amorphous-amorphous transition, rather than that of a “mechanical rejuvenation” as proposed in the past to explain the ability of plastic deformation to reinitiate physical aging. Finally, a linear relation between the size of the free-volume holes and the dynamic tensile modulus is found, which suggests that the stiffness of amorphous glassy polymers is fully determined by their nanoscopic structure.
Craze initiation in glassy polymers: Quantifying the influence of molecular orientation
Polymer, 2011
A study has been made of crazing stress in oriented glassy polystyrene. The aim was to develop a methodology for prediction of crazing stress in glassy polymers with frozen-in molecular orientation. Oriented specimens of two grades of monodisperse polystyrene (PS) and one grade of polydisperse PS were produced by uniaxial melt-drawing and subsequent quenching of compression-moulded bars. Birefringence and crazing stress parallel to the draw direction (in the presence of diethylene glycol) were measured on miniature beam specimens cut from them. The crazing stress increased substantially with orientation, and the magnitude of the increase relaxed approximately on a timescale associated with the longest Rouse time. Specifically, a linear correlation was found, to within experimental scatter, between the increase in crazing stress and the orientation expressed in terms of frozen-in conformational stress, as predicted by the theory of Maestrini and Kramer . The inverse gradient (constant b in the theory) was found to be 0.059 AE 0.002, when inferring the conformational stress from the measured birefringence. Crazing was found to be suppressed in favour of yielding in the most highly oriented specimens, and this could be explained in terms of the differing sensitivities of crazing and yield to molecular orientation.
Journal of Polymer Science Part B: Polymer Physics, 1992
Positron annihilation lifetime spectroscopy (PALS) is used to probe structural changes in glassy polycarbonate in terms of changes in the hole volume and the number density of holes during fatigue (cyclic stress) aging. The ortho-positronium (0-Ps) pickoff annihilation lifetime T~, as well as the intensity 13, were measured as a function of cyclic stresses and various previous thermophysical aging histories. It is found that T~, the longest of the three lifetime components resolved in the PALS of glassy polycarbonate, increases when a cyclic stress is applied. These results indicate that there is a structural change during fatigue aging. The "holes" where 0-Ps can localize become larger upon fatigue aging. These results also suggest that a significant distinction exists between structural changes induced by thermophysical aging and fatigue aging. The 0-Ps annihilation intensity, which is a relative measure of the hole density in a material, showed a continuous decrease upon fatigue aging, indicating the possibility of hole coalescence, which could be a precursor of crazing. The interaction between thermophysical aging and fatigue aging corresponds very well with the enthalpy relaxation behavior as reported previously, uiz., a well-aged sample is much more sensitive to cyclic stress. More importantly, it is hypothesized that fatigue failure initiation is probably closely related to hole size and density fluctuation. Keywords: polycarbonate, stress-induced structural changes in positron annihilation spectroscopy of glassy polycarbonate after cyclic stress glassy polymer, structural changes in stress and aging positron annihilation study of
On the characterization of glassy polymers by positron annihilation lifetime spectroscopy
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 1991
The present paper reviews some of our recent results exemplifying the use of positron annihilation lifetime spectroscopy in evaluating the effects of thermal history on the physical properties of glassy polymers. Specifically, studies on thermal cycling, physical aging and structural relaxation in polycarbonate are discussed. It is shown that the ortho-positronium pickoff component intensity I3 is sensitive to structural transitions below the glass transition and the thermal response of I3 is a sensitive structural parameter that can be used in the study of the thermodynamics and kinetics of physical aging and structural relaxation.
The effects of cyclic loading on polymers in a glassy state
Journal of Polymer Science: Polymer Physics Edition, 1976
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.
Plastic deformation of glassy polymethylene: Computer-aided molecular-dynamic simulation
Polymer Science Series A, 2010
Molecular dynamic simulation of low temperature plastic deformation (T def = 50 K, T def /T g ≤ 0.3) is studied for glassy polymethylene under the regime of active uniaxial compression and tension for a cell composed of 64 chains containing 100 -CH 2 groups in each (as united atoms) and with periodic boundary conditions. Thirty two such cells are created, and, in each cell, polymethylene chains in the statis tical coil conformation are independently constructed. The cells are subjected to isothermal uniaxial com pression at T def = 50 K by ε = 30% and by ε = 70% under uniaxial tension. In the course of loading, a σ-ε diagram is recorded, while the mechanical work spent on deformation, the changes in the overall potential energy of the system, and the contributions from various potential interactions (noncovalent van der Waals bonds, chemical links, valence and torsional angles) are estimated. The results are averaged over all 32 cells. The relaxation of stored potential energy and residual strain after complete unloading of the deformed sample is studied. The relaxation of stored energy and residual strain is shown to be incomplete. Most of this energy and strain is stored in the sample at the deformation temperature for long period. The conformational com position of chains and the average density of polymer glass during loading are analyzed. Simulation results show that inelastic deformations commence not with the conformational unfolding of coils but with the nucleation of strain bearing defects of a nonconformational nature. The main contribution to the energy of these defects is provided by van der Waals interactions. Strain bearing defects are nucleated in a polymer glass during tension and compression primarily as short scale positive volume fluctuations in the sample. During tension, the average density of the glass decreases; during compression, this parameter slightly increases to ε ≈ 8% and then decreases. An initial increase in the density indicates that, during compression and at ε < 8%, coils undergo compactization via an increase in chain packing. During compression, the concentration of trans conformers remains unchanged below ε ≈ 8% and then decreases. During compression, it means that in a glass, coils do not increase their sizes at strains below ε ≈ 8%. During tensile drawing, coils remain unfolded below ε ≈ 35%; at higher strains, coils become enriched with trans conformers or unfold. At this stage, the concentration of trans conformers linearly increases. The development of a strain induced excess volume (strain bearing defects) entails an increase in the potential energy of the sample. Under the given conditions of deformation, nucleation of strain bearing defects and an increase in their concentration are found to be the only processes occurring at the initial stage of loading of glassy polymethylene. The results of computer aided simulation are compared with the experimental data reported in the literature.
European Polymer Journal, 2011
Aiming to develop new dielectric polymers containing CN and F groups with strong dipole moments, a novel copolymer of acrylonitrile (AN) and 2,2,2-trifluoroethyl acrylate (ATRIF) was synthesized in acetonitrile by free radical process as well as the respective homopolymer (poly(ATRIF)). The copolymer's composition and microstructure were analyzed by FTIR, 1 H and 13 C NMR spectroscopy and SEC. The molar incorporation of AN determined in the copolymer by NMR was 58 mol%. Thermogravimetric analysis of poly(AN-co-ATRIF) copolymer showed good thermal stability comparatively to the fluorinated homopolymer.