Anisotropic random networks of semiflexible polymers

Motivated by the organization of cross-linked cytoskeletal biopolymers, we present a semimicroscopic replica field theory for the formation of anisotropic random networks of semiflexible polymers. The networks are formed by introducing random permanent cross-links which fix the orientations of the corresponding polymer segments to align with one another. Upon increasing the cross-link density, we obtain a continuous gelation transition from a fluid phase to a gel where a finite fraction of the system gets localized at random positions. For sufficiently stiff polymers, this positional localization is accompanied by a continuous isotropic-to-nematic (IN) transition occurring at the same cross-link density. As the polymer stiffness decreases, the IN transition becomes first order, shifts to a higher cross-link density, and is preceded by an amorphous solid where the average polymer orientations freeze in random directions.     

Definition and properties of ideal amorphous solids

It is proposed that two ideal amorphous structures, type I and type II, based on maximally random jammed packing of spheres of equal size, form a distinct class of ideal amorphous solids. The ideal amorphous structures contain wide variations in local density, limited by the condition of solidity. Four distinct characteristics, based on statistical geometry and topology, are shown to define this class. Voronoi tessellations carried out on simulated cells of random packed spheres and amorphous polymers give a broad distribution of individual volumes, skewed, with a tail at the high volume end.     

典型高分子材料的固体核磁共振研究

本论文通过固体核磁共振（NMR）谱及动力学参量的测量,并结合X-射线衍射技术和DSC测量等研究了两种典型高分子材料的相结构、链的运动以及相与相之间的关系.  乙烯-醋酸乙烯共聚物( EVA) 是最主要的乙烯共聚物之一. 研究发现,EVA的相组成非常复杂,共有5个不同的组分. 除了PE中所观察到的常规单斜晶相和刚性的正交晶相外,我们发现还存在第三个晶相分量－运动性较强的晶相（SOCP,可能是转动相）. 它不仅拥有自己的熔点,而且它的化学位移和分子运动性不同于刚性正交晶相（LOCP）. 另一方面,非晶相也由两种不同的分量组成：运动受限的各相异性的非晶界面相和高度可动的橡胶型的非晶相. 我们进一步详细研究了EVA中的晶区链动力学和非晶区的低温冻结行为. 实验发现,在正交晶相中,高分子链以180° flip-flop方式运动,同时伴随沿链方向的平移型跳跃运动,并引起正交晶相和非晶相之间的长程链扩散,通过NOE的测量证实了这种相间链扩散的存在,并进一步通过实验证实这种相间链扩散是一种受限扩散而不是自由扩散. 同时非晶相的两个组分具有不同的低温冻结行为：当温度低于-弛豫转变温度时,橡胶型的非晶相中的长程分子运动被冻结,但仍存在分子的局域运动;而界面非晶在低温时冻结成一种有序取向结构,并用质子自旋扩散实验证实该有序结构与正交晶相相邻近.  少量纳米级片层状粘土分散在聚合物中就可赋予材料许多优异的性能,我们用固体NMR技术对EVA/REC复合材料的结构和其中粘土的分散性质进行研究,发现上述复合材料中所形成的晶体类型不仅依赖于各组分的性质还依赖于所形成的复合材料的类型.  偏氟乙烯/三氟乙烯共聚物（P（VDF-TrFE））是最主要的铁电高聚物之一. 我们利用变温固体¹⁹F MAS NMR 谱及弛豫数据的测量详细研究了电子辐照对P(VDF-TrFE)共聚物的分子结构、构型、运动性以及相变等的影响. 发现,电子辐照不仅改变了分子链段的构型和运动性,同时也改变了局部分子化学结构. 电子辐照促使铁电相向顺电相(或者非晶相)转变,与此同时诱发了富含VDF和含-TrFE链段从全反式的构型到混合的反式-旁式构型的转变. 电子辐照加剧顺电区域中的分子运动而在高温熔融态中(>100 ℃）,分子的运动反而受限.     

Structural conditions of deformation and fracture of oriented crystalline polymers

Deformation of supermolecular structure elements of oriented crystalline polymers and nucleation of initial submicroscopic cracks induced by stress have been studied by the small-angle x-ray scattering technique. It is shown that the intrafibrillar amorphous interlayers have low strength and high deformability. The rupture of the weakest amorphous interlayers leads to nucleation of initial submicrocracks. The influence of submicrocracks on deformation around such cracks is revealed. The micromechanics of deformation and fracture of polymers is discussed.     

Anomalous transport and diffusion versus extreme value theory

In the present work we match the biased hierarchical continuous-time random flight (HCTRF) on a regular lattice (based on hierarchical waiting-time distribution) and the extreme event theory (EVT). This approach extends the understanding of the anomalous transport and diffusion (for example, found in some amorphous, vitreous solids as well as in conducting and light-emitting organic polymers). Both independent approaches were developed in terms of random-trap or valley model where the disorder of energy landscape is exponentially distributed while the corresponding mean residence times in traps obey the power-law. This type of disorder characterizes several amorphous (even used commercially) materials which makes it possible to apply the HCTRF formalism. By using the EVT we additionally show that the rare (stochastic) events are indeed responsible for the transport and diffusion in these materials.     

A structural model for the amorphous state of polymers: Folded-chain fringed micellar grain model

A more realistic, two-phase structural model is proposed for the amorphous solid state of polymers. The model is based on structural evidence reported to date, in particular the evidence from our own electron diffraction and microscopy studies of crystallizable and non-crystallizable amorphous polymers, including near-molten rubbers. This model is different from either Kargin's chain-packet or Hose-mann's paracrystalline model. The two major elements of the model are the grain, which consists of an ordered domain and a distinct grain boundary, and the intergrain region, which consists of molecules that are in a more truly random conformation. In the ordered domain (～20～40 Å) the molecular chain segments are aligned parallel to one another with a nearly constant spacing. A natural consequence of the present model is the concentration of excess free volume in the intergrain region. The amount of excess free volume has been estimated for typical polymers in their glassy amorphous state and found to be large for polymers that possess good ductility, but small for brittle polymers. Phenomena, such as the contributions of changes in entropy and energy to the rubbery elastic force, the increasingly extended rubbery plateau with increasing molecular weight, the non-Newtonian behavior of viscous flow and lamella crystallization and formation, are also discussed and appear to be consistent with the proposed two-phase granular model.     

Mössbauer study of amorphous Fe2RE (RE=Ce,Er) alloys

Among amorphous Fe₂RE (RE=Er, Ce, Gd, La, Pr, Sm, Dy, Ho) alloys, Fe₂Ce exhibits a tendency toward short range order, while the other Fe₂RE compounds show clustering. However, we have almost no information about environments around Fe atoms. Using Mössbauer spectroscopy we have determined the quadrupole splitting distributionsP(QS) of two representative amorphous Fe₂RE (RE=Ce, Er) alloys, leading to local environments of Fe atoms. The analysis of the mixed magnetic dipole and quadrupole interactions in Fe₂Er shows two kinds of electrical field gradients (EFT) with the positive and negative signs in the sample, indicating a random packing of Fe atoms. Furthermore, the analyzed quadrupole splitting distributionP(QS) of Fe₂Er also supports random packing in this amorphous alloy. On the other hand, the amorphous Fe₂Ce alloy shows two kinds of distributions of quadrupole splitting; the major component indicating random packing and the minor component Ce-rich Ce-Fe clusters.     

铁基软磁非晶/纳米晶合金研究进展及应用前景

非晶合金通常是将熔融的金属快速冷却、通过抑制结晶而获得的原子呈长程无序排列的金属材料.由于具有这种特殊结构,铁基软磁非晶合金具有各向同性特征、很小的结构关联尺寸和磁各向异性常数,因而具有很小的矫顽力H_c,但可和晶态材料一样具有高的饱和磁感强度B_s.优异的软磁性能促进了铁基软磁非晶合金的应用研究.目前,铁基软磁非晶/纳米晶合金带材已实现大规模工业化生产和应用,成为重要的高性能软磁材料.本文回顾了软磁非晶合金的发现和发展历程,结合成分、结构、工艺对铁基非晶/纳米晶合金软磁性能的影响,介绍了相关基础研究成果和工艺技术进步对铁基软磁非晶/纳米晶合金研发和工业化应用的重要贡献.并根据结构、性能特征将铁基软磁非晶合金研发与应用分为三个阶段,指出了目前铁基软磁非晶合金研发与应用中面临的挑战和发展方向.     

Smoothing of depinning transitions for directed polymers with quenched disorder

We consider disordered models of pinning of directed polymers on a defect line, including (1 + 1)-dimensional interface wetting models, disordered Poland-Scheraga models of DNA denaturation, and other (1 + d)-dimensional polymers in interaction with columnar defects. We consider also random copolymers at a selective interface. These models are known to have a (de)pinning transition at some critical line in the phase diagram. In this work we prove that, as soon as disorder is present, the transition is at least of second order: the free energy is differentiable at the critical line, and the order parameter (contact fraction) vanishes continuously at the transition. On the other hand, it is known that the corresponding nondisordered models can have a first order (de)pinning transition, with a jump in the order parameter. Our results confirm predictions based on the Harris criterion.     

Crazing in semicrystalline thermoplastics

The deformation processes in crystalline polymers have been studie ever since the discovery of chain folding in 1957. Since then, scientists have been intrigued by the different steps of the transformation of the folded-chain lamellar structure of single crystals or of macroscopically isotropic, often spherulitic, polymers into fibrous morphologies (see Refs. 1 and 2 for early reviews). The importance of molecular tilt, of inter- and intralamellar slip, and of micronecking were rapidly recognized [1–4]. In this paper, we discuss the analogies and differences with respect to crazing of glassy amorphous polymers. Obviously, there is an extensive body of literature on the micromechanics of crazing (see the reviews in Refs. 5–9). On the basis of these studies, it has been established that crazes in amorphous polymers are well-defined regions with approximately planar boundaries that extend perpendicular to the direction of maximum principal tensile stress and that contain highly stretched and voided material [7]. However, crazelike features have also been observed in many semicrystalline polymers (polyethylene [PE], isotactic polypropylene [IPP], isotatic polystyrene [IPS], polyoxymethylene [POM], polyamide 6 and 66 [PA6 and PA66], polycarbonate [PC], polyethylene terephthalate [PET], polybutylene terephthalate [PBT], polyvinylidene fluoride [PVDF], and polyether ether ketone [PEEK]). They are designated in the literature [3–10] as micronecks, true crazes, fibrillar deformation zones (DZs), or simply as crazes since they correspond well to the above definition.     

Morphology of amorphous polymers

It has been suggested that amorphous polymers consist of small domains (30 to 100 A) in which there is local ordering or alignment of neighboring segments. Although free volume remains the primary physical parameter useful in characterizing the properties of amorphous polymers (as percent crystallinity for crystalline polymers), the distribution of the free volume, as determined by the size, interconnection, internal order, etc. of the domains is proposed to also be of importance. The paper reviews electron microscope and electron diffraction evidence for the presence of the domain structure, while also pointing out significant remaining problems.     

Evidence of SiO at the Si-oxide interface by surface soft X-ray absorption near edge spectroscopy

Using synchrotron radiation a new surface sensitive spectroscopy has been applied to determine the local structure of the first surface oxide layer formed on the Si(111) surface. The Surface Soft X-ray Absorption (SSXA) spectra have been measured. From the analysis of the X-ray Absorption Near Edge Structures (XANES) we have extracted structural information. We have first determined that bulk amorphous SiO has a characteristic microsopic structure, which cannot be described by the random alloy or microcrystalline (Si + SiO₂) mixture models. The oxide layer formed on the Si(111) surface by ground-state molecular excitation in ultra high vacuum at temperatures (～700°C) approaching the oxide dissociation point has this unique SiO local structure. Such SiO layer not formed at room temperature is expected to be present in the SiSiO₂ interface grown at high temperature. An electronic transition to empty states at the SiSiO₂ interface has been observed.     

A geometric theory for Lévy distributions

Lévy distributions are of prime importance in the physical sciences, and their universal emergence is commonly explained by the Generalized Central Limit Theorem (CLT). However, the Generalized CLT is a geometry-less probabilistic result, whereas physical processes usually take place in an embedding space whose spatial geometry is often of substantial significance. In this paper we introduce a model of random effects in random environments which, on the one hand, retains the underlying probabilistic structure of the Generalized CLT and, on the other hand, adds a general and versatile underlying geometric structure. Based on this model we obtain geometry-based counterparts of the Generalized CLT, thus establishing a geometric theory for Lévy distributions. The theory explains the universal emergence of Lévy distributions in physical settings which are well beyond the realm of the Generalized CLT.     

基于缠结微结构的非晶态玻璃态高分子材料屈服行为的分子动力学研究

非晶态玻璃态高分子材料作为结构材料在工程领域应用广泛,其机械力学性能特别是屈服变形行为受到热处理、加载应变率和环境温度的影响.采用分子动力学模拟方法研究非晶态玻璃态高分子材料不同工况下的单轴拉伸变形,基于分子链缠结微结构的概念,阐明了非晶态玻璃态高分子材料屈服和应变软化过程的内在变形机制.结果表明,拓扑缠结具有较为稳定的空间结构,难以发生解缠,决定了非晶态高分子材料屈服后的软化平台.由相邻分子链的局部链段相互作用形成的次级缠结在一定外界条件下可发生破坏或重新生成,次级缠结微结构及其演化是非晶态高分子材料发生屈服及软化的内在物理原因.     

Deformation and annealing behavior. III. thin films of polycarbonate,isotactic polymethyl methacrylate,and isotactic polystyrene

The morphology of thin films of polycarbonate (PC), isotactic polymethyl methacrylate (i-PMMA), and isotactic polystyrene (i-PS) were studied by bright field, diffraction, and dark field transmission, electron microscopy. The films were thermally annealed both before and after uniaxial deformation. The undrawn i-PMMA did not develop a crystalline texture upon annealing whereas the undrawn i-PS developed a random arrangement of lamellae. Thermal crystallization of undrawn PC has been shown to occur through the growth and merging of a nodular structure into fibrillar spherulites. Deformation of gold-decorated samples indicated a nonhomogeneous structure in the amorphous polymer on the 200-500 A scale. Thermal annealing of drawn samples resulted in fibrillar lamellae arranged perpendicular to the draw direction for all three polymers, indicating the straining has a marked influence on the crystalline morphology. This morphology is compared to similar morphologies reported in the literature for crystallization from stressed melts. High-resolution dark field micrographs confirm this morphology and indicate variations in crystallite size, depending upon the polymer and deformation conditions.     

A Ubiquitous Thermal Conductivity Formula for Liquids,Polymer Glass,and Amorphous Solids

The microscopic mechanism of thermal transport in liquids and amorphous solids has been an outstanding problem for a long time.There have been several approaches to explain the thermal conductivities in these systems, for example, Bridgman's formula for simple liquids, the concept of the minimum thermal conductivity for amorphous solids, and the thermal resistance network model for amorphous polymers.Here, we present a ubiquitous formula to calculate the thermal conductivities of liquids and amorphous solids in a unified way, and compare it with previous ones.The calculated thermal conductivities using this formula without fitting parameters are in excellent agreement with the experimental data.Our formula not only provides a detailed microscopic mechanism of heat transfer in these systems, but also resolves the discrepancies between existing formulae and experimental data.     

Deformation and entanglement in semicrystalline polymers

Transmission electron microscopy (TEM) of amorphous and semicrystalline isotactic polystyrene (iPS) thin films deformed well below T _g, suggests the same crazing mechanism to operate in both cases. Therefore, by analogy with the amorphous case, highly entangled semicrystalline polymers, such as poly(ether ether ketone) (PEEK) should craze less readily in the glassy-semicrystalline state than iPS, which has a low degree of entanglement. Since this is confirmed by observation, it is reasonable to extend this analogy, and invoke entanglement in order to account for the high fracture resistance of highly entangled semicrystalline polymers, such as polyoxymethylene, using models previously applied to glassy amorphous polymers. There are nevertheless often significant decreases in fracture toughness in polyoxymethylene as the crystallization temperature is raised and/or the molecular weight is reduced, which we attribute to entanglement loss during lamellar folding.     

Corrosion effects of nuclear tracks in polymers

Usually ion tracks are studies ex-situ, i.e. after removal of the ion-irradiated sample from the evacuated irradiation vessel. The exposure to ambient air leads to degradation effects which are frequently neglected and consequently have been studied rarely. They have never been explained consistently. In this work we compile and discuss them in the frame of today's general understanding of ion tracks in polymers. It appears that some pristine polymers such as polyethylene are covered by a surface layer which is badly permeable for gaseous contaminants. Ion irradiation leads to destruction of this protective layer so that oxygen and moisture from the ambient air can easily penetrate through the ion tracks, and eventually even into the surrounding pristine material. Moisture absorption along the tracks gives rise to some faint conductivity. Oxygen uptake adds new trapping centers to the existing ones along the tracks, so that on one hand an increased dopant uptake capability is recorded, and on the other hand oxygen-sensitive mobile dopants are immobilized along the ion tracks. Photooxidation processes contribute to the fading of the optical blackening of irradiated polymers.     

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     

Short-range order,large-scale potential fluctuations,and photoluminescence in amorphous SiN<Subscript>x</Subscript>

The short-range order and electron structure of amorphous silicon nitride SiN_x (x<4/3) have been studied by a combination of methods including high-resolution X-ray photoelectron spectroscopy. Neither random bonding nor random mixture models can adequately describe the structure of this compound. An intermediate model is proposed, which assumes giant potential fluctuations for electrons and holes, caused by inhomogeneities in the local chemical composition. The characteristic scale of these fluctuations for both electrons and holes is about 1.5 eV. The photoluminescence in SiN_x is interpreted in terms of the optical transitions between quantum states of amorphous silicon clusters.