Ahmad Zeinolebadi

Studying nanostructure gradients in injection-molded polypropylene/ montmorillonite composites by microbeam small-angle x-ray scattering

Abstract The core–shell structure in oriented cylindrical rods of polypropylene (PP) and nanoclay composites (NCs) from PP and montmorillonite (MMT) is studied by microbeam small-angle x-ray scattering (SAXS). The structure of neat PP is almost homogeneous across the rod showing regular semicrystalline stacks. In the NCs the discrete SAXS of arranged crystalline PP domains is limited to a skin zone of 300 μm thickness. Even there only frozen-in primary lamellae are detected. The core of the NCs is dominated by diffuse scattering from crystalline domains placed at random. The SAXS of the MMT flakes exhibits a complex skin–core gradient. Both the direction of the symmetry axis and the apparent perfection of flake-orientation are varying. Thus there is no local fiber symmetry, and the structure gradient cannot be reconstructed from a scan across the full rod. To overcome the problem the rods are machined. Scans across the residual webs are performed. For the first time webs have been carved out in two principal directions. Comparison of the corresponding two sets of SAXS patterns demonstrates the complexity of the MMT orientation. Close to the surface (< 1 mm) the flakes cling to the wall. The variation of the orientation distribution widths indicates the presence of both MMT flakes and grains. The grains have not been oriented in the flowing melt. An empirical equation is presented which describes the variation from skin to core of one component of the inclination angle of flake-shaped phyllosilicate filler particles.

Extruded blend films of poly(vinyl alcohol) and polyolefins: common and hard-elastic nanostructure evolution in the polyolefin during straining as monitored by SAXS

Abstract Straining of PVA/PE and PVA/PP blends (70:30) is monitored by small-angle x-ray scattering (SAXS). Sheet-extruded films with different predraw ratio are investigated. The discrete SAXS of predrawn samples originates from polyolefin nanofibrils inside of polyolefin microfibrils immersed in a PVA matrix. PE nanofibrils deform less than the macroscopic strain without volume change. PP nanofibrils experience macroscopic strain. They lengthen but their diameter does not decrease. This is explained by strain-induced crystallization of PP from an amorphous depletion shell around the core of the nanofibril. The undrawn PVA/PE film exhibits isotropic semicrystalline nanostructure. Undrawn PVA/PP holds PP droplets containing oriented stacks of semicrystalline PP like neat precursors of hard-elastic thermoplasts. Respective predrawn films are softer than the undrawn material, indicating conversion into the hard-elastic state. Embedding of the polyolefin significantly retards neck formation. The polyolefin microfibrils can easily be extracted from the water-soluble matrix.

Summary and Future Works

In-situ small angle X-ray scattering measurements during mechanical testing is a strong tool for constructing structure-property-relationship of polymers. The evolution of nanostructure can be monitored with sufficient precision to correlate it with macroscopic response of the sample. Even without any precise pre-knowledge about the nanostructure the 2-dimensional scattering data can be transformed into the real space resulting in a chord distribution function (CDF). The peak analysis of the CDF reveals abundant information about the size, shape and correlation of the nanostructural entities. Thus deformation and rupture of the multi-phase morphology (e.g. semi-crystalline stacks), strain-induced crystallization/melting, crazing and other nanostructural transitions can be monitored. These information are vital when designing multi-phase materials with tailored properties is aimed at. In addition, the local nanoscopic strain can be estimated. Such quantitative experimental data combined with theory can improve our ability of simulating the mechanical behavior of multi-phase polymer materials[1-3].

A novel method for determination of occurrence order of stabilization reactions in carbon fiber precursor

Thermal stabilization is an important step in production of carbon fiber from polyacrylonitrile (PAN) precursors. During thermal stabilization step different thermochemical reactions take place almost simultaneously. Understanding the onset and temperature range of the stabilization reactions is a key for adjusting processing parameters such as tension, stretching, etc. However, stabilization reactions are very complex and overlap with each other. In order to separate the stabilization reactions, we combined the results of different thermal analysis techniques, namely Differential Scanning Calorimetry (DSC), Thermogravimetry (TGA) and Thermomechanical Analysis (TMA), to study behaviour of PAN precursors during stabilization. By means of combining the results of these techniques, we were able to determine the temperature range and occurrence order of each of stabilization reactions regardless of the composition of initial PAN fibers and history of fiber formation.

Thermoplastic Polyurethane Elastomers Under Uniaxial Deformation

The special rubber elastic properties of thermoplastic elastomers are the result of their two- or multi-phase nanostructure consisting of hard domains in a soft matrix. The hard domains form physical cross-links and make the material behave rubber elastic.

HDPE/PA Microfibrillar Composites Under Load-Cycling

Microfibrillar reinforced composites (MFC) are polymer–polymer composites in which both the isotropic matrix and the fibrous anisotropic reinforcements are formed in-situ during processing [1,2]. MFC materials promise both improved properties during service and low ash content after incineration [3-5]. The first feature is required to replace metals by light-weight parts in automobiles. The second feature is a European legislative request [6-8] that must be met in the future. In many practical applications that MFCs are designed for, the materials are subjected to cyclic (dynamic) load. Hence, resistance [9] to dynamic loads (i.e. low fatigue [10-12]) is required. In this chapter we examine the evolution of the nanostructure under slow load-cycling in HDPE/PA oriented MFC precursors that have not been subjected to the final compression molding processing step which removes [13,14] the orientation of the HDPE matrix.

Polypropylene/Montmorillonite Nanocomposites: Continuous Stretching and Load-Cycling

In the last two decades there have been a lot of efforts to improve mechanical properties of polypropylene [1] via compounding it with layered silicates (clay) [2–7]. For instance, enhancements of storage modulus [8–10], Young’s modulus [11, 12], impact strength [13, 14], and tensile strength [15, 16] have been reported. Layered silicates can be mixed with polypropylene in the melt state using conventional polymer processing machinery [3, 4, 6, 16]. Nevertheless, incompatibility of layered silicates with hydrophobic polypropylene chains provokes problems with dispersing them inside the matrix [17, 18].

Exploring the Thermodynamic Aspects of Structure Formation During Wet-Spinning of Polyacrylonitrile Fibres

Wet-spun polyacrylonitrile fibres are the main precursor for high strength carbon fibres. The properties of carbon fibres strongly depend on the structure of the precursor fibre. Polyacrylonitrile fibres were spun from solutions with varying solvent/nonsolvent content and different draw ratios. Wet-spinning is an immersion precipitation process, thus thermodynamic affinity of spinning dope to the coagulation medium was considered as the driving force of phase-separation, while viscosity of the solution accounted for the resistive force against phase separation and growth of the nucleated voids. Thermodynamic affinity was estimated by modifying Ruaan’s theory and viscosity of the solution was assessed on-line by measuring flow rate and back pressure at the spinneret. Hence, the parameter X (thermodynamic affinity/viscosity) was introduced to predict the porous morphology of the fibres. Generally, an increase in X led to fibres with higher porosity. A combination of electron scanning microscopy (SEM), porosimetry and thermoporometry was applied to fully characterize microstructure of fibres. Based on image analysis of SEM micrographs and data obtained from thermoporometry and porosimetry fractions of dense polymer ligament, micrometer size voids (macrovoids) and nanometer size voids (nanovoids) were estimated. Increasing polymer content or nonsolvent content in the spinning dope caused an increase in the solution viscosity and resulted in fibres with lower porosity. Imposing drawing on the as-spun fibres further decreased the porosity. Drawing also shifted the size distribution of nanovoids toward smaller values.

Exploring a pathway for time-resolved studies of polymer fatigue related to nanostructure evolution

An uniaxially oriented polymer blend based on high-density polyethylene (HDPE) that is reinforced by polyamide 6 microfibrils is subjected to slow mechanical fatigue tests. Simultaneously, the nanostructure evolution is monitored by two-dimensional (2D) patterns of the small-angle X-ray scattering (SAXS). The objective is to demonstrate that subtle changes of macroscopic strain and nanostructure parameters can be recorded with an accuracy that is sufficient to detect mechanisms that relate fatigue and nanostructure variation. The macroscopic deformation of the material is recorded by a video camera and the captured pictures are analyzed by automated image processing methods to compute the macroscopic strain. From the 2D patterns the multidimensional chord distribution function (CDF) is calculated, and the scattering power Q is determined. Peaks of the CDF that are related to the semicrystalline domain structure inside the HDPE are identified. The position of their centers of gravity is tracked automatically from the voluminous series of scattering patterns. The resulting curves demonstrate the relation between mechanical load and nanostructure evolution.