The production and properties of poly(aryletherketone) (PEEK) rods oriented by drawing through a conical die

Oriented poly(aryletherketone) (PEEK) rods have been produced by drawing isotropic polymer through a conical die. Room temperature Young's moduli were measured by three-point bending and ranged from 5 GPa at a draw ratio of about 2 to 11 GPa at a draw ratio of about 4. Dynamic mechanical properties were explored in the range ?150 to 200°C; two loss peaks were observed, with the higher corresponding to T_g.     

Conjugation-induced lysis of Schizosaccharomyces pombe

About 15% of the conjugating cells of Schizosaccharomyces pombe were observed to lyse spontaneously during the conjugation process. Lysis occurred at the site of union.     

Study of blends based on recycled polyethylene wastes

A calorimetric study of binary blends based on recycled, low and high density polyethylene, wastes is presented. The physical properties of these blends were reported previously [1]. A comparison between the first and second differential scanning calorimetry (DSC) traces of blended samples reveals the presence of an intermediate endothermic peak which can be associated to the melting of crystals from mixed molecular species of the single components. Further thermal treatments after the second DSC run at temperatures just below and above the intermediate peak for two compositions (_B = 10 and 30% of high density component) highlight the dependence of the various crystal populations upon annealing. A quantitative analysis of a series of consecutive thermograms for each sample has allowed us an estimation of the relative amount of material associated to the three main endothermic peaks.     

Comparative study of size and distribution of lamellar thicknesses and long periods in polyethylene with a shish-kebab structure

This study contains a combined application of three different techniques for the study of injection moulded polyethylene (PE), showing an oriented shish-kebab structure: small angle X-ray scattering (SAXS), low frequency Raman spectroscopy (LAM) and transmission electron microscopy (TEM), A series of linear PEs and molecular weights in the range 51000–478000 has been investigated and two injection temperatures have been used (T _m=144 and 210 °C). SAXS patterns from the highly oriented regions show the presence of either one axial long period (L ₁) or two (L ₁ and L ₂) depending on molecular weight (¯M _w) and T _m. It is shown that L ₁ and L ₂ increase with ¯M _w up to a given critical molecular weight ¯M _c. Above ¯M _c, L ₁ and L ₂ remain constant. Raman results qualitatively confirm the existence of two separate distributions of straight-length chain segments for those samples having molecular weights above the critical value. Shorter segments are shown to be more abundant than the longer ones. In the lowest molecular weight sample, results from SAXS, TEM and Raman spectroscopy seem to be consistent with each other, although in some cases a tilted molecular arrangement within the lamellae has to be invoked. On the other hand, in case of the highest molecular weight sample, the length of the short straight-chain segments derived from Raman spectroscopy agrees well with the double periodicity obtained from SAXS. On the contrary, long periods measured from TEM only correspond to the shorter SAXS periodicity. This result is discussed by assuming the occurrence of crystalline bridges among adjacent lamellae.     

Hydrological classification of non-perennial Mediterranean rivers and streams: A new insight for their management within the water framework directive

Classification of natural flow regimes of non-perennial rivers and streams (NPRS) is an incipient field of research. NPRS represent approximately 70% of the total Mediterranean rivers and are expected to increase in the next decades as a result of climate change. Due to the ecological importance of NPRS and the need to improve national ecological assessment methods within the scope of the Water Framework Directive (WFD), this paper aims to classify the hydrological regime of 69 non-regulated streams, testing several hydrological indices related to the magnitude, frequency, duration, timing, and rate of change in periods of flow cessation. Using daily flow records, a total of 315 indices were calculated and their relationships were examined with Principal Component Analysis (PCA) for different thresholds used to define zero-flow condition (0, 1, 2, and 5 L/s). Redundancy analysis identified five indices that better describe the patterns of hydrological variability in Mediterranean NPRS: number of days per year without flow, annual percentage of months without flow, mean daily annual flow, coefficient of variation of Julian date of the annual start of zero-flow and annual rise rate. Using these indices, a self-organizing map (SOM) was trained in order to categorize the NPRS into three groups with similar hydrological features. The results of this study provide a statistically-based hydrological classification of NPRS in Mediterranean environments. We expect that this classification will provide useful insights to water authorities to improve the assessments of the ecological status in this type of water bodies.     

Vickers indentation anisometry on thin cylindrical materials

The required anisometric correction to be made when using Vickers indentations on filaments with radial dimensions of the order of those of the microindenter, is evaluated. For an isotropic plastic material the anisometry of indentation. l=l{-l ₁ increases with decreasing cross-section of the filament. If the calculated correction, /^c, is in agreement with experiment the interest lies in the fact that the material preserves the indentation shape after removal of the indenter. Values of //^c reveal the presence of a mechanical anisotropy on metallic filaments due to processing. Furthermore, in the case of thin highly oriented polymeric fibres the inherent microindentation anisotropy is masked by the anisometry effect. Consequently, the proposed correction is very important and particularly valuable when anisotropic cylindrical material surfaces are being studied.     

Transient charging currents in annealed bulk polyethylene

Transient charging currents resulting from the application of a low constant voltage, up to 2×10⁴ V cm^–1, are investigated in melt crystallized samples of high density polyethylene. Three distinct regions can be clearly distinguished in the current-field characteristic curves at a given time after application of a voltage: (a) a linear region at a low electric field, (b) a negative resistance region in which the slope is negative at a medium electric field, (c) a superlinear region at a high electric field. At low electric fields and for charging times less than 10³ sec the law of dielectric response, l t ^–n , is obeyed. It is shown that the type of cooling process, after annealing at temperatures near 75° C, markedly influences the current decay. Annealing at this temperature is associated with a partial rearrangement of the lamellar microstructure. The current is enhanced after quenching while it decreases after slow cooling. Various mechanisms to explain the obtained data are considered.     

Influence of processing conditions on the structure and surface microhardness of injection-moulded polyethylene

Linear polyethylene (PE) was injection-moulded into Standard tensile bars using a range of melt,T _s, and mould,T _w, temperatures. Microhardness testing and X-ray small- and wide-angle diffraction techniques were used to investigate the changes in mechanical properties, microstructure and crystalline orientation, occurring throughout the range of mouldings. A correlation was shown to exist between microstructure and processing variables. Thus, a clear increase in hardness anisotropy, MH, (from 15 up to about 30%), corresponding to a well-developed molecular and lamellar orientation, and hardening (for high molecular grade PE), especially when decreasing the melt temperature below 200° C, has been detected. This increase in MH favours the view of an increase of a substantial fraction of tie molecules contributing to the local instant elastic recovery beneath the indenter along the injection direction. MH is, however, nearly independent ofT _s for conventional moulding-grade PE. Here the absence of a unit-cell orientation is evident in theT _s range investigated while a lamellar orientation only prevails forT _s<200° C. In this latter caseMH is a linear function ofT _w. This result is consistent with the fact that microhardness increases with the fraction of crystallized material. The obtained results suggest that the three-dimensional molecular network existing in the material plays a relevant role in steering the mechanical behaviour of the final lamellar moulded material.     

Strain-induced β-α polymorphic transition in iPP as revealed by microhardness

The microhardness (H) technique was used for characterization of the - polymorphic transition in isotactic polypropylene (iPP). For this purpose the microhardness in the damage zone of a tensile loaded deeply edge-notched (DEN-T) -iPP specimen was mapped. Mapping of H was performed, both along the loading direction (central) and close to the shorter fracture edge. Around half-length of the plastic zone a sharp increase of the H values in both cases was observed. The H increase is related to the polymorphic transition. Microvoid formation in the central part results in lower H values. However for the edge zone close to the top of the fracture surface unusually high H values (around 200 MPa) are obtained. The latter are explained in terms of the formation of microfibrils due to crazing during deformation which are characterized by very high molecular orientation as reported from X-ray analysis.