Jesper de Claville Christiansen

Rabbit: A New High-Performance Stream Cipher

We present a new stream cipher, Rabbit, based on iterating a set of coupled non-linear functions. Rabbit is characterized by a high performance in software with a measured encryption/decryption speed of 3.7 clock cycles per byte on a Pentium III processor. We have performed detailed security analysis, in particular, correlation analysis and algebraic investigations. The cryptanalysis of Rabbit did not reveal an attack better than exhaustive key search.

Model for Anomalous Moisture Diffusion through a Polymer-Clay Nanocomposite

Experimental data are reported on moisture diffusion and the elastoplastic response of an intercalated nanocomposite with vinyl ester resin matrix and montmorillonite clay filler at room temperature. Observations in diffusion tests showed that water transport in the neat resin is Fickian, whereas it becomes anomalous (non-Fickian) with the growth of the clay content. This transition is attributed to immobilization of penetrant molecules on the surfaces of hydrophilic clay layers. Observations in uniaxial tensile tests demonstrate that the response of vinyl ester resin is strongly elastoplastic, whereas an increase in the clay content results in a severe decrease of plastic strains observed as a noticeable reduction in the curvatures of the stress-strain diagrams. This is explained by slowing down the molecular mobility in the host matrix driven by confinement of chains in galleries between platelets. Constitutive equations are developed for moisture diffusion through and the elastoplastic behavior of a nanocomposite. Adjustable parameters in these relations are found by fitting the experimental data. Fair agreement is demonstrated between the observations and the results of numerical simulation. A striking similarity is revealed among changes in diffusivity, ultimate water uptake, and the rate of plastic flow with an increased clay content.

The effect of annealing on the elastoplastic response of isotactic polypropylene

Abstract Four series of tensile loading–unloading tests are performed on isotactic polypropylene in the sub-yield domain of deformations at room temperature. In the first series, injection-molded specimens are used as produced, whereas in the other series the samples are annealed for 24 h at 120, 140 and 160 °C, which covers the low-temperature region and an initial part of the high-temperature region of annealing temperatures. A constitutive model is developed for the elastoplastic behavior of a semicrystalline polymer. The stress–strain relations are determined by five adjustable parameters that are found by fitting the experimental data. The effect of annealing is analyzed on the material constants.

Atomic-scale structure of dislocations revealed by scanning tunneling microscopy and molecular dynamics.

The intersection between dislocations and a Ag(111) surface has been studied using an interplay of scanning tunneling microscopy (STM) and molecular dynamics. Whereas the STM provides atomically resolved information about the surface structure and Burgers vectors of the dislocations, the simulations can be used to determine dislocation structure and orientation in the near-surface region. In a similar way, the subsurface structure of other extended defects can be studied. The simulations show dislocations to reorient the partials in the surface region leading to an increased splitting width at the surface, in agreement with the STM observations. Implications for surface-induced cross slip are discussed.

Deformation and structure evolution of glassy poly(lactic acid) below the glass transition temperature

Poly(lactic acid) (PLA) is a bio-based and compostable thermoplastic polyester that has rapidly evolved into a competitive commodity material over the last decade. One key bottleneck in expanding the field of application of PLA is the control of its structure and properties. Therefore, in situ investigations under cooling are necessary for understanding the relationship between them. The most intriguing feature of a supercooled liquid is its dramatic rise in viscosity as it is cooled toward the glass transition temperature (Tg) though accompanied by very little change in the structural features observable by typical X-ray experiments. The deformation behaviors and structure evolution of glassy PLA during uniaxially stretching below Tg were investigated in situ by synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) techniques. The stretched samples were measured by differential scanning calorimetry (DSC). The obtained results showed that the deformation and yield stress of glassy PLA are strongly dependent on the stretching temperatures together with the transition from mesophase to mesocrystal and the formation of cavities. With the increase in drawing temperature, the onset of the mesocrystal formation is delayed to a higher strain value, whereas corresponding to the same critical orientation degree of amorphous chains (fam ≈ 0.45). The DSC results indicated that the post-Tg endothermic peak corresponding to the melting of mesocrystal appears and shifts to a higher temperature with increasing stretching temperature, followed by the down-shifts (to a lower temperature) of the exothermic peak of cold crystallization of PLA. The appearance of a small exothermic peak just before the melting peak related to the transition of the α′ to α crystal implies the formation of an α′ crystal during cold crystallization in the drawn PLA samples. The structure evolution of glassy PLA stretched below Tg was discussed in details.

Polypropylene/organoclay/SEBS nanocomposites with toughness–stiffness properties

Polypropylene nanocomposites with a different amount of styrene-ethylene-butylene-styrene block copolymer (SEBS)/clay were prepared via a melt mixing technique. To improve the dispersion of commercial organoclay (denoted as OMMT), various amounts of SEBS were incorporated. At a fixed content of OMMT, the mechanical properties were improved with increasing SEBS content. The obtained nanocomposites were characterized through X-ray diffraction (XRD), differential scanning calorimetry (DSC-TG) and mechanical tests. The thermal–morphological–mechanical properties were investigated. The nanomaterials presented an improved decomposition temperature, a small decrease in tensile strength, a higher Youngs modulus and a spectacular increase of 300% in impact strength.

Crystalline structures and crystallization behaviors of poly(L-lactide) in poly(L-lactide)/graphene nanosheet composites

Poly(L-lactide) (PLLA)/graphene nanosheet (GNS) composites and pure PLLA were prepared by the solution blending method. Crystalline structures and crystallization behaviors of PLLA in the composite were investigated by XRD, POM, SAXS, and DSC. It was found that α′ form PLLA formation seemed to be more preferred than α form PLLA formation in PLLA/GNS composites at crystallization temperatures Tcs within the α′–α crystal formation transition region due to the existence of GNSs, resulting in an obvious shift of the α′–α crystal formation transition of PLLA in PLLA/GNSs towards high Tcs compared with that of pure PLLA. At Tcs below α′–α crystal formation transition, the formed α′ crystal turned to be more imperfect due to GNS addition, while at Tcs above α′–α crystal formation transition, the crystal structure of α form PLLA was not affected by GNSs. Further POM observations at high Tcs with only α crystal formed showed that PLLA spherulites were well formed in both PLLA/GNSs and pure PLLA, however with very different crystallization kinetics while isothermally crystallizing at different Tcs. The PLLA crystallization process of PLLA in PLLA/GNSs was accelerated by GNSs with both the nucleation rate and spherulite growth rate increased mainly because of the increasing segmental mobility of PLLA chains due to GNS addition; whereas, GNSs showed no observable influence on the determined zero growth temperature Tzg of α form PLLA and the Tzg was estimated lower than the equilibrium melting point of PLLA, indicating that the crystal growth of PLLA is mediated by a transient mesophase with the transition temperature of Tzg between the mesophase and melt not influenced by GNSs in PLLA. Synchrotron on-line SAXS results revealed that the long periods of PLLA in PLLA/GNS composites isothermally crystallized at different Tcs are much smaller than those in pure PLLA. The GNSs are helpful in forming more perfect recrystallized α form PLLA after the α′ form PLLA is melted with increasing Tcs. The presence of GNSs resulted in imperfect α form PLLA from melt directly when it is isothermally crystallized at different Tcs within the temperature range of α′–α crystal formation transition.

Enhancement of mechanical properties of polypropylene by blending with styrene-(ethylene-butylene)-styrene tri-block copolymer

Abstract Observations are reported in impact tests, uniaxial tensile tests with various strain rates, relaxation tests with various strains and cyclic tests with a mixed deformation program and various maximum strains per cycle on neat polypropylene (PP) and a blend of PP with styrene-(ethylene-butylene)-styrene copolymer (SEBS). Experimental data demonstrate a pronounced enhancement of impact resistance of PP due to the presence of an impact modifier, accompanied by improvement of its properties under low-speed loading, observed as a decrease in relaxation rate and residual strain under cyclic deformation. Material constants in constitutive equations are determined by matching the experimental data. Correlations are established between changes in the viscoelastoplastic response of PP and evolution of its microstructure induced by the presence of an impact modifier.

Nonlinear time-dependent response of isotactic polypropylene

Tensile creep tests, tensile relaxation tests and a tensile test with a constant strain rate are performed on injection-molded isotactic polypropylene at room temperature. A constitutive model is derived for the time-dependent behavior of semicrystalline polymers. A polymer is treated as an equivalent network of chains bridged by permanent junctions. The network is modeled as an ensemble of passive mesoregions (with affine junctions) and active mesodomains (where junctions slide with respect to their reference positions with various rates). The distribution of activation energies for sliding of junctions in active mesoregions is described by a random energy model. Adjustable parameters in the stress–strain relations are found by fitting the experimental data. It is demonstrated that the concentration of active mesodomains monotonically grows with strain, whereas the average activation energy for sliding of junctions and the standard deviation of energies suffer substantial down jumps in the vicinity of the yield point. With reference to the concept of dual population of lamellae, these changes in material parameters are attributed to transition from breakage of subsidiary (thin) lamellae in the subyield region to fragmentation of dominant (thick) lamellae in the postyield region of deformations.Tensile creep tests, tensile relaxation tests and a tensile test with a constant strain rate are performed on injection-molded isotactic polypropylene at room temperature. A constitutive model is derived for the time-dependent behavior of semicrystalline polymers. A polymer is treated as an equivalent network of chains bridged by permanent junctions. The network is modeled as an ensemble of passive mesoregions (with affine junctions) and active mesodomains (where junctions slide with respect to their reference positions with various rates). The distribution of activation energies for sliding of junctions in active mesoregions is described by a random energy model. Adjustable parameters in the stress–strain relations are found by fitting the experimental data. It is demonstrated that the concentration of active mesodomains monotonically grows with strain, whereas the average activation energy for sliding of junctions and the standard deviation of energies suffer substantial down jumps in the vicinity of th...

Temperature dependence of poly(lactic acid) mechanical properties

The mechanical properties of polymers are not only determined by their structures, but also related to the temperature field in which they are located. The yield behaviors, Youngs modulus and structures of injection-molded poly(lactic acid) (PLA) samples after annealing at different temperatures were measured during stretching within 30–150 °C. The in situ photographs were recorded with a camera to observe deformation behavior during stretching. The less ordered α′ form crystal of PLA can be formed in the un-annealed PLA samples and those that were annealed at 70 °C with low crystallinity. The crystallinity increases with increasing annealing temperature and α form crystal is formed when the annealing temperature is higher than 100 °C. The stretched samples with low crystallinity show the first yield at draw temperatures below the glass transition temperature (Tg) and the second yield above Tg. For the samples annealed between 80 and 120 °C, a peculiar double yield appears when stretched within 50–60 °C and only the first or the second yield can be found at the lower and higher draw temperatures. The yield strain and yield stress together with Youngs modulus were obtained and discussed in terms of the effects of the draw temperature and crystalline structure of PLA samples.