Zbigniew Stachurski

Kinetics of polymer crystallisation

Abstract The methods, models, theoretical developments and techniques for the study of kinetics of polymer crystallisation are described and discussed. Some of the pertinent fundamental issues and recent results, especially of polymer crystallisation under non-isothermal conditions and crystallisation of polymer blends, are surveyed. It is demonstrated that non-isothermal crystallisation under varying cooling rates can be described and predicted through an integral method. Hotstage microscopy, improved by the technique of digital image processing and analysis, is useful for the study of the kinetics and mechanisms of polymer crystallisation through establishing a relationship between nucleation, spherulite growth and overall crystallisation rate. The effect of polymer blending on crystallisation is discussed on the basis of two fundamental factors: nucleation and spherulite growth.

Fasciclin-like arabinogalactan proteins: specialization for stem biomechanics and cell wall architecture in Arabidopsis and Eucalyptus.

The ancient cell adhesion fasciclin (FAS) domain is found in bacteria, fungi, algae, insects and animals, and occurs in a large family of fasciclin-like arabinogalactan proteins (FLAs) in higher plants. Functional roles for FAS-containing proteins have been determined for insects, algae and vertebrates; however, the biological functions of the various higher-plant FLAs are not clear. Expression of some FLAs has been correlated with the onset of secondary-wall cellulose synthesis in Arabidopsis stems, and also with wood formation in the stems and branches of trees, suggesting a biological role in plant stems. We examined whether FLAs contribute to plant stem biomechanics. Using phylogenetic, transcript abundance and promoter-GUS fusion analyses, we identified a conserved subset of single FAS domain FLAs (group A FLAs) in Eucalyptus and Arabidopsis that have specific and high transcript abundance in stems, particularly in stem cells undergoing secondary-wall deposition, and that the phylogenetic conservation appears to extend to other dicots and monocots. Gene-function analyses revealed that Arabidopsis T-DNA knockout double mutant stems had altered stem biomechanics with reduced tensile strength and a reduced tensile modulus of elasticity, as well as altered cell-wall architecture and composition, with increased cellulose microfibril angle and reduced arabinose, galactose and cellulose content. Using materials engineering concepts, we relate the effects of these FLAs on cell-wall composition with stem biomechanics. Our results suggest that a subset of single FAS domain FLAs contributes to plant stem strength by affecting cellulose deposition, and to the stem modulus of elasticity by affecting the integrity of the cell-wall matrix.

Deformation mechanisms and yield strength in amorphous polymers

Abstract The paper starts with essentials of continuum mechanics, including solutions for tension, compression, simple shear, pure shear, Poissons ratio, ideal elasto-plastic body and the definition of yield strength. The phenomenology of yield is considered in terms of: state of stress and dependence on hydrostatic component; temperature effects and limits; thermo-mechanical history effect; change of intrinsic properties (volume, heat capacity and molecular conformations). Molecular theories of yield are reviewed, including state transition and conformational change theories (Eyring, Robertson, and others), free volume theories (Rush and Beck), dislocation/ disclinations theories (Bowden and Raha, and Argon) and segmental motion theories (Yannas and Stachurski). The most recent computer modelling results (Suter, Theodorou, Argon, Roe) are described and discussed in terms of the microstructure of the amorphous polymers and in relation to the previously proposed theories.

Nano-indentation and nano-scratch of polymer/glass interfaces. II: model of interphases in water aged composite materials

The interphase region in dry and water aged polymer/glass fibre composite materials was investigated by means of the nano-indentation and the nano-scratch techniques. The indentations as small as 30 nm in depth were produced along 14 μm path starting from the matrix and ending on the fibre. This was done in order to detect water degradation of material properties in the transition region between the fibre and the matrix. The distinct properties of the interphase region were revealed by a sequence of two to three indents in dry materials and up to 15 indents in water aged, degraded materials. These results indicated interdiffusion of water into the interphase regions. The nano-scratch test results showed that the interphase region width increased and the material properties degraded during water aging. Doubts about the presence of the interphase, raised over the hardness results in the vicinity of the harder glass region, were refuted with the results showing the expansion of the interphase region during water aging. These two experimental techniques showed to be a useful tool in the investigation of the size and the character of the interphase region in the fibre/polymer matrix composite materials.

Composite materials based on wood and nylon fibre

Abstract Nylon-wood fibre and polypropylene-wood fibre composite materials were manufactured without any additives to determine the effects of wood fibre on the mechanical properties of the different composites. The raw materials used were eucalypt hardwood fibre, Nylon fibre obtained from stockings, and polypropylene (PP) pellets. A hot press technique was used to manufacture the composite materials, and improvements in the manufacturing methods are suggested. Tests were carried out on the manufactured boards to determine tensile strength and modulus of elasticity. Fracture surfaces were examined using scanning electron microscopy to investigate failure mechanisms. An increase in tensile strength and modulus of elasticity was observed in wood fibre/Nylon matrix composites, indicating that interfacial bonding occurred between these two phases. Bundles of wood fibres with internal voids prevent achieving maximum mechanical properties. The tensile strength of the PP based composites decreased significantly with increasing wood fibre content.

Ultra-Durable and Transparent Self-Cleaning Surfaces by Large-Scale Self-Assembly of Hierarchical Interpenetrated Polymer Networks

In nature, durable self-cleaning surfaces such as the Lotus leaf rely on the multiscale architecture and cohesive regenerative properties of organic tissue. Real-world impact of synthetic replicas has been limited by the poor mechanical and chemical stability of the ultrafine hierarchical textures required for attaining a highly dewetting superhydrophobic state. Here, we present the low-cost synthesis of large-scale ultradurable superhydrophobic coatings by rapid template-free micronano texturing of interpenetrated polymer networks (IPNs). A highly transparent texture of soft yielding marshmallow-like pillars with an ultralow surface energy is obtained by sequential spraying of a novel polyurethane-acrylic colloidal suspension and a superhydrophobic nanoparticle solution. The resulting coatings demonstrate outstanding antiabrasion resistance, maintaining superhydrophobic water contact angles and a pristine lotus effect with sliding angles of below 10° for up to 120 continuous abrasion cycles. Furthermore, they also have excellent chemical- and photostability, preserving the initial performance upon more than 50 h exposure to intense UVC light (254 nm, 3.3 mW cm(-2)), 24 h of oil contamination, and highly acidic conditions (1 M HCl). This sprayable polyurethane-acrylic colloidal suspension and surface texture provide a rapid and low-cost approach for the substrate-independent fabrication of ultradurable transparent self-cleaning surfaces with superior abrasion, chemical, and UV-resistance.

On Structure and Properties of Amorphous Materials

Mechanical, optical, magnetic and electronic properties of amorphous materials hold great promise towards current and emergent technologies. We distinguish at least four categories of amorphous (glassy) materials: (i) metallic; (ii) thin films; (iii) organic and inorganic thermoplastics; and (iv) amorphous permanent networks. Some fundamental questions about the atomic arrangements remain unresolved. This paper focuses on the models of atomic arrangements in amorphous materials. The earliest ideas of Bernal on the structure of liquids were followed by experiments and computer models for the packing of spheres. Modern approach is to carry out computer simulations with prediction that can be tested by experiments. A geometrical concept of an ideal amorphous solid is presented as a novel contribution to the understanding of atomic arrangements in amorphous solids.

Micromechanics of stress relaxation in amorphous glassy PMMA. Part I. Molecular model for anelastic behaviour

This paper describes the development of a theoretical model for the strength of mechanical relaxation in terms of micromechanics of deformation. Polymethyl methacrylate (PMMA) is used as the model amorphous polymer. The internal molecular rearrangements during relaxation are identified and accounted for by the rotation of specific atomic groups. Voronoi tessellation is used as a method to characterise the nanostructure in amorphous glassy polymer below Tg. A theoretical model is postulated and shown to provide a limited quantitative prediction capacity of the anelastic deformation and the corresponding stress relaxation based on measurable molecular parameters without adjustable factors.

Strength and deformation of rigid polymers: the stress-strain curve in amorphous PMMA

Abstract Poly(methyl methacrylate) (PMMA) is used to model the relaxation and plastic flow mechanisms of deformation, and the characteristic stress–strain response, relating it to its structure as much as it is known. The scope of this work is to identify and quantify the micro-mechanisms and the corresponding stress–strain relationships, and to assemble these into a coherent and self-consistent model for the observed mechanical behaviour. Detailed relationship between relaxation strength and time constants has been derived for some of the secondary motions. It is proposed that plastic events occur when the tension in chain segments pulls the chains out of/through constriction points. The scheme for simulating the isothermal true stress–strain curve is carried out under the following limitations: (i) geometrical effects, such as elastic instability and necking, (ii) thermodynamic adiabatic effects, and (iii) structural and kinetic effects, such as may arise from quenching or annealing, are neglected. Qualitative agreement achieved here is considered satisfactory in view of the simplicity of the model and only a few adjustable parameters.

The microdroplet test: experimental and finite element analysis of the dependance of failure mode on droplet shape

The microdroplet technique is usually designed as a fibre embedded in a drop of resin and subsequently pulled out while the drop is being supported by two knife edges, resulting in either debonding of the droplets from the fibres, or breakage of the fibres before debonding can occur. In this study, the microdroplet technique was performed using a platinum ring with a 40 μm hole instead of the usual two knife edges, giving an axisymmetric geometry, load and stress distribution. Glass/phenolic and glass/polyester composite systems were tested experimentally and subsequent finite element modelling studies were performed to assess the variation of droplet size, and contact angle between the droplet and fibre. It was found that contact angle is of major influence in the proposed failure model. This study characterizes the influence of the contact angle between the droplet and the fibre on the subsequent stress distribution in the microdroplet specimen.