Phase behaviour of polyethylene knotted ring chains

The phase behaviour of polyethylene knotted ring chains is investigated by using molecular dynamics simulations. In this paper, we focus on the collapse of the polyethylene knotted ring chain, and also present the results of linear and ring chains for comparison. At high temperatures, a fully extensive knot structure is observed. The mean-square radius of gyration per bond ² / (Nb²) and the shape factor <δ^*> depend on not only the chain length but also the knot type. With temperature decreasing, chain collapse is observed, and the collapse temperature decreases with the chain length increasing. The actual collapse transition can be determined by the specific heat capacity C_v, and the knotted ring chain undergoes gas-liquid-solid-like transition directly. The phase transition of a knotted ring chain is only one-stage collapse, which is different from the polyethylene linear and ring chains. This investigation can provide some insights into the statistical properties of knotted polymer chains.     

Nature of Shear-Induced Primary Nuclei in iPP Melt

Although observations of molecular processes in the formation of primary nuclei prior to actual crystallization are beyond the detection limits of current instrumentation, we attempted to probe the nature of primary nuclei in sheared isotactic polypropylene (iPP) polymer melt. In situ rheo-SAXS (small-angle X-ray scattering) and -WAXD (wide angle X-ray diffraction) experiments using synchrotron radiation were carried out to evaluate the effects of an addition of a high molecular weight atactic polypropylene (aPP) (5 wt%), which is compatible with the iPP matrix but does not crystallize, on the evolution of oriented structures in the sheared iPP melt and its crystallization kinetics. It is unlikely that the aPP chain segments can be incorporated into iPP nuclei or crystal; hence, its addition effects, if any, would be seen only in the amorphous melt prior to crystallization. The results showed stonger orientation and improved crystallization kinetics in the iPP/aPP blend compared to pure iPP. Observations that the presence of long chains of an amorphous polymer aid in nucleation and crystallization kinetics of iPP, combined with our previous synchrotron results of sheared iPP melts at high temperature (165°C), lead us to conclude that primary nuclei in iPP most likely consist of liquid-crystalline or mesomorphic bundles of aligned chain segments prior to the formation of crystals.     

Equilibrium shapes of flat knots

We study the equilibrium shapes of prime and composite knots confined to two dimensions. Using scaling arguments we show that, due to self-avoiding effects, the topological details of prime knots are localized on a small portion of the larger ring polymer. Within this region, the original knot configuration can assume a hierarchy of contracted shapes, the dominating one given by just one small loop. This hierarchy is investigated in detail for the flat trefoil knot, and corroborated by Monte Carlo simulations.     

Growth of polymer crystals during annealing

Observations by transmission electron microscopy are reported on the processes involved in polymer crystal growth during annealing. The observations suggest that crystal growth occurs by two processes. One process involves the melting of those regions of the crystals in which the melting point is lower than the annealing temperature. The polymer melt due to the melting process gradually becomes incorporated into the unmolten crystals, resulting in crystal growth. The alternative process is solid-state crystal growth by the migration of the amorphous region between crystallites.     

Enhancing nucleation and controlling crystal orientation by rubbing/scratching the surface of a thin polymer film

We used the tip of an atomic force microscope (AFM) in the contact mode to scratch/rub the surface of a glassy polymer thin film, i.e., isotactic polystyrene (i-PS) at room temperature. After subsequent isothermal crystallization, an extremely high nucleation density of edge-on crystals within the rubbed region or at the edge of the scratched area was observed. Furthermore, a transition from edge-on to flat-on lamellae occurred beyond a certain distance from the edge of the scratched region. Our results demonstrate that both, soft rubbing or hard scratching, allow to lower the nucleation barrier for polymer crystallization and to control the orientation of the resulting crystalline lamellae. The role of scratching/rubbing on chain deformation and its relation to nucleation and crystal orientation in polymer thin films is discussed.     

The Effect of Orientation Relaxation on Polymer Melt Crystallization Studied by Monte Carlo Simulations

We use dynamic Monte Carlo simulations to study the athermal relaxation of bulk extended chains and the isothermal crystallization in intermediately relaxed melts. It is found that the memory of chain orientations in the melt can significantly enhance the crystallization rates. The crystal orientation and lamellar thickness essentially depend on the orientational relaxation. Moreover, there is a transition of the nucleation mechanism during the isothermal crystallization from the intermediately relaxed melts. These results explain the mechanism of the self-nucleation by orientation and suggest that in flow-induced polymer crystallization, the orientational relaxation of chains decides the crystal orientation.     

Interactions between high-temperature deformation and crystallization in zirconium-based bulk metallic glasses

High-temperature deformation of a ZrTiCuNiBe bulk metallic glass (BMG) is investigated by compression tests in the supercooled liquid region. When the temperature is decreased or strain rate increased, the amorphous alloy exhibits the usual Newtonian/non-Newtonian transition behaviour. Using specific heat treatments, partially crystallized alloys are produced, the associated microstructures characterized and the volume fractions of the crystal measured. The interaction between high-temperature deformation and crystallization is investigated by appropriate mechanical testing. According to these measurements, partial crystallization is responsible for a significant increase in flow stress and the promotion of non-Newtonian behaviour. Deformation does not significantly change the volume fraction, composition or size of the crystal. The flow-stress increase with crystallization is analyzed under different hypotheses. We conclude that the flow-stress increase cannot be interpreted through a compositional change in the residual amorphous matrix, either by reinforcement due to hard crystallites or by connections between crystals. It appears that the effect is due to the nanometric size of the crystals alone.     

A microscopic insight into the deformation behavior of semicrystalline polymers: the role of phase transitions

The deformation behavior of semicrystalline polymers associated with polymorphic transformations under tensile deformation is discussed in the case of syndiotactic polypropylene. We report a phase diagram of this polymer where the regions of stability of the different polymorphic forms are defined as a function of the degree of stereoregularity and deformation. The values of critical strain corresponding to the structural transformations depend on the stereoregularity that affects the relative stability of the involved polymorphic forms and the state of the entangled amorphous phase.     

Stress relaxation in entangled melts of unlinked ring polymers

Stress relaxation in unlinked ring polymer melts poses an important challenge to our theoretical understanding of entangled polymer dynamics. Recent experiments on entangled unlinked ring melts show power-law stress relaxation with no hint of a rubbery plateau, usually the hallmark of entangled polymers. Here we present a theory for stress relaxation in rings analogous to the successful approach for star polymers. We augment our theory with mesoscale Monte Carlo dynamics simulations of equivalent "lattice animal" configurations. We find a stress relaxation function G(t)～t(-α) with α≈1/2 consistent with experiment, emerging ultimately from the disparate relaxation times of more- and less-central portions of ring conformations.     

Exploring the melting of a semirigid-chain polymer with temperature-resolved small-angle <Emphasis Type="Italic">X</Emphasis>-ray scattering

The thermal behavior of semirigid semicrystalline polymers differs significantly from that of flexible-chain polymers. The origin of the differences is believed to lie in the higher energy expenditure associated with the formation of adjacent re-entry folds at the crystalline surface in the case of semirigid chains. The effect of constraints imposed by the interlamellar amorphous regions on the neighboring crystals was studied with temperature-resolved synchrotron radiation small-angle X-ray scattering (SAXS). The analysis of SAXS patterns with a generalized paracrystalline lamellar stack model indicates that melting of a semirigid-chain polymer is not a random process but that the crystals grown in the smallest amorphous gaps melt first. This suggests that the hitherto largely neglected geometrical confinement effects may play an important role in determining the thermodynamic stability of semirigid-chain polymer crystals.Received: 5 March 2004, Published online: 4 May 2004PACS: 61.41. + e Polymers, elastomers, and plastics - 64.70.Dv Solid-liquid transitions - 81.10.Aj Theory and models of crystal growth; physics of crystal growth, crystal morphology and orientation     

The mechanisms of recrystallization after melting in syndiotactic polypropylene and isotactic polystyrene

Several semicrystalline polymers show a recrystallization after melting during a heating scan. We have studied the mechanisms of such recrystallization processes for two different polymers, namely syndiotactic polypropylene (sPP) and isotactic polystyrene (iPS). This was done by monitoring the structure evolution during the recrystallization process and its changes during a subsequent heating scan via time- and temperature-dependent SAXS measurements, respectively. The results of this study showed that the sPP samples exhibited a recrystallization mechanism similar to the multi-stage route found upon initial crystallization of semicrystalline polymers from an entangled melt. Meanwhile, a different recrystallization mechanism was shown by the iPS samples. In this case, the recrystallization process proceeded as a direct growth into the melt in a one-step process. This is the first time we have observed such a mechanism which resembles the picture presented by the classical models for crystallization from an entangled polymer melt. The reason for such different mechanisms may be related to the initial melt state prior to crystallization. It seems as though, when crystallization sets in an entangled polymer melt, it follows the multi-stage route, whereas if the melt is locally disentangled, it proceeds by a direct growth mechanism. Received 23 July 2001 and Received in final form 4 October 2001     

Phase and structural transformations in a molecular dynamics model of iron under ultrafast heating and cooling

It is shown that a system of classical particles considered in a molecular dynamics model with Pak-Doyama pairwise interatomic potential adequately describes not only the various structural states of iron (melt, bcc crystal, metal glass) but also the complex self-organization processes occurring in first-and second-order phase transitions (crystallization and vitrification, respectively). When the temperature is varied at a constant rate of 6.6×10¹¹ K/s, crystallization sets in from both the amorphous and the liquid state; at a rate of 1.9×10¹² K/s, crystallization is observed only in the amorphous state; and when heated at a rate of 4.4×10¹² K/s, the model amorphous iron transfers to the liquid state without crystallization. The energy of homogeneous formation of a crystal nucleus in the bulk of the amorphous phase of iron is calculated to be ～0.71 eV under the assumption that there is a spectrum of activation energies.     

Thickness-dependent crystallization behavior of fast-growth phase-change amorphous films

The crystallization behavior of amorphous films embedded in substrates with thickness of several nanometers is investigated based on a thermodynamic model. It is found that there is an optimum layer thickness where the crystallization speed of the films is maximized with the lowest energy barrier for crystallization. This is induced by an energetic change from glass/substrate interface energy to crystal/substrate interface energy as the crystal size is larger than the film thickness during the crystallization. Thus, the crystallization speed in thin films is affected by its thickness.     

Characterization of structural modifications in poly-tetra-fluoroethylene induced by electron beam irradiation

The effects of electron beam irradiation doses on the poly-tetra-fluoroethylene (PTFE) have been studied. Several techniques, such as X-ray diffraction (XRD), differential scanning calorimetry (DSC), mechanical properties and Fourier transform infrared spectroscopy (FTIR) were applied to characterize the PTFE samples and to study the radiation effects on the crystal structure of the polymer.The irradiation dose up to 150 kGy showed an increase in the crystallinity degree of PTFE, which has been observed and confirmed during the DSC and XRD measurements. The increase in crystallinity was attributed to the scissions of the chain in the amorphous region. Moreover, the number-average molecular weights were estimated from the heat of crystallization measured by DSC technique. The results indicated that the molecular weights were decreased by increasing the heat of crystallization due to irradiation with doses up to 150 kGy. Radiation resistance of the irradiated and non-irradiated PTFE was investigated during its mechanical properties at room temperature. The dose at half value of the elongation at break is about 3.10 kGy while the dose at half value of the tensile strength is about 1.70 kGy.     

Effects of constrained chain conformations on polymer-solute interactions in semicrystalline polymers

The chemical potential of a solute in a solid polymer includes contributions from solute-polymer chain conformations, Flory-Huggins type interaction, and elastic energy of swelling. Presence of impermeable and rigid crystallites in such systems is expected to affect all these contributions. Theoretical calculations have been performed to check the direct effects of constrained chain conformations in the amorphous domains in semicrystalline polymers. Experimental results are used to determine Flory-Huggins coefficient and elastic modulus. From all these, the primary effects are shown to be on the entropic part of the Flory-Huggins coefficient and an increase in the elastic modulus by one or two order of magnitude. Finally, these results are used to calculate the rates of solvent-induced crystallization to show that these rates can drop to negligible values as the amount of crystals formed rises. Thus, the actual degree of crystallization can lie well below the Flory-Yoon limit.     

Electron microscopic investigation of the kinetics of the layer and island crystallization of amorphous V<Subscript>2</Subscript>O<Subscript>3</Subscript> films deposited by pulsed laser evaporation

An electron microscopic investigation was performed on the kinetics of the layer and island crystallization of amorphous V₂O₃ films deposited by pulsed laser evaporation of vanadium in an oxygen atmosphere. The crystallization was initiated by the action of an electron beam on an amorphous film in the column of a transmission electron microscope. The kinetic curves were plotted on the basis of a frame-by-frame analysis of the video recorded during the crystallization of the film. It was found that the layer crystallization of amorphous films is characterized by a quadratic dependence of the fraction of the crystalline phase x on the time t, whereas the island crystallization is described by an exponential dependence of x on t. The kinetic curves of island crystallization of amorphous films were analyzed on the basis of the α-version of the Kolmogorov model. For each type of crystallization, there are specific values of the dimensionless relative length unit δ₀, which is equal to the ratio of the characteristic length unit to the parameter characterizing the unit cell of the crystal. It was established that, for the layer crystallization, the relative length unit lies in the range δ₀ ~ 4300–4700, whereas for the fine-grained island crystallization, it amounts to δ₀ ~ 110.     

Branched crystal morphology of linear polyethylene crystallized in a two-dimensional diffusion-controlled growth field

The branched crystal morphology of linear polyethylene formed at various temperatures from thin films has been studied by atomic-force microscopy (AFM), transmission electron microscopy (TEM), electron diffraction (ED) pattern and polymer decoration technique. Two types of branched patterns, i.e. dendrite and seaweed patterns, have been visualized. The fractal dimension d _f = 1.65 of both dendrite and some of seaweed patterns was obtained by using the box-counting method, although most of the seaweed patterns are compact. Selected-area ED patterns indicate that the fold stems tilt about 34.5^° around the b-axis and polymer decoration patterns show that the chain folding direction and regularity in two (200) regions are quite different from each other. Because of chain tilting, branched crystals show three striking features: 1) the lamella-like branches show two (200) regions with different thickness; 2) the crystals usually bend towards the thin region; 3) the thick region grows faster by developing branches, thus branches usually occur outside the thick region. The branched patterns show a characteristic width w, which gives a linear relationship with the crystallization temperature on a semilogarithmic plot. Received 15 March 2002 and Received in final form 29 April 2002     

Tight and loose shapes in flat entangled dense polymers

We investigate the effects of topological constraints (entanglements) on two-dimensional polymer loops in the dense phase, and at the collapse transition ( -point). Previous studies have shown that in the dilute phase the entangled region becomes tight, and is thus localised on a small portion of the polymer. We find that the entropic force favouring tightness is considerably weaker in dense polymers. While the simple figure-eight structure, created by a single crossing in the polymer loop, localises weakly, the trefoil knot and all other prime knots are loosely spread out over the entire chain. In both the dense and conditions, the uncontracted-knot configuration is the most likely shape within a scaling analysis. By contrast, a strongly localised figure-eight is the most likely shape for dilute prime knots. Our findings are compared to recent simulations.Received: 7 October 2003, Published online: 21 November 2003PACS: 87.15.-v Biomolecules: structure and physical properties - 82.35.-x Polymers: properties; reactions; polymerization - 02.10.Kn Knot theory     

共轭高聚物中低浓度掺杂下的杂质分布研究

基于紧束缚近似,研究了一维共轭高聚物链在链呈电中性,以及链中带有正、负电荷等不同情况下低浓度掺杂对系统稳定性的影响,并采用绝热近似下的自洽计算方法得出了系统在掺杂前后发生的总能量改变。研究发现,掺杂位置对系统稳定性的影响非常明显。根据杂质分布的特点,一条共轭高聚物链一般可分为链端区、中心区和过渡区三个明显不同的区域。系统的稳定性不仅受掺杂位置,杂质势的强度及性质影响,而且还受到高聚物链的载荷状态的影响。在链端区及过渡区,杂质分布趋向于凝聚成畴,而在中心区域,杂质趋于均匀分布。该研究表明,通过对掺杂条件的控制,可以有效控制杂质在共轭高聚物中的分布状态。     

Theoretical analysis of the effect of crystallization temperature on structure formation in flexible-chain polymers

Theoretical analysis of flexible-chain polymer crystallization was carried out over a wide temperature range from glass transition temperature T_g to melting temperature T_m. Temperature dependence of dynamic behavior manifesting in the decrease of crystallizing length with decreasing temperature was taken into account. The dependence of crystallizing length on temperature was obtained using chosen values of it at T_g and T_m. The crystallization with the formation of folded-chain crystals (type I) and uncoiled-chain crystals (type II) was considered. The analysis of thermodynamical favorability of both types of crystals with respect to temperature made it possible to obtain a flexible-chain crystallizing polymer phase diagram. This diagram shows the existence of two ranges where type II crystal formation is more favorable: a narrow range near T_m and the wider one near T_g, separated by the temperature range of crystallization with chain folding. Temperature dependences of type II crystals fraction in the system and their size were calculated. It is shown that the crystallization at considerable supercooling leads to the appearance of a great number of type II small crystals connected by tie chains. The system formed is characterized by a high degree of crystallite interconnection.