F. Ania

Physical ageing and glass transition in amorphous polymers as revealed by microhardness

Microhardness (MH) data as a function of temperature for two amorphous polymers [poly(methylmethacrylate) and poly(vinylacetate)] and two semicrystalline polymers [poly(ethyleneterephthalate) and poly(arylether ether ketone)] quenched into the amorphous state are presented. It is shown that MH can conveniently detect the glass transition temperature (Tg) for the above mentioned polymers. Molecular rearrangements taking place above and belowTg, such as physical ageing leading to a more compact molecular packing, and thermal expansion can also be followed by means of MH measurements. Finally, the presence of a crystalline phase in these materials has been shown to shift theTg value towards higher temperatures.

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 (Tm=144 and 210 °C). SAXS patterns from the highly oriented regions show the presence of either one axial long period (L1) or two (L1 and L2) depending on molecular weight (¯Mw) and Tm. It is shown that L1 and L2 increase with ¯Mw up to a given critical molecular weight ¯Mc. Above ¯Mc, L1 and L2 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.

Microhardness studies of chain-extended PE: II. Creep behaviour and temperature dependence

The variation of hardness with indentation time has been investigated for chain-extended polyethylene (PE), other PE samples crystallised under different conditions and paraffins. Hardness is shown to decrease with indentation time for all the samples investigated according to a power-law. Chain-extended PE, produced by high pressure crystallization or annealing, flows at the lowest rate under the indenter of all the PE samples considered. On the other hand, paraffins creep at the highest rate. Creep behaviour depends markedly on the crystal thickness of the material. The mechanical properties at long indentation times seem to be determined primarily by the deformation modes of the crystals. The temperature dependence of hardness and that of the creep behaviour has also been investigated. In chain-extended PE, the softening of the sample and the higher rate of creep with increasing temperature are discussed in terms of the thermal expansion of the unit cell.

Numerical-experimental method for the identification of plastic properties of polymers from microhardness tests

A computational method in conjunction with indentation measurements is presented to determine the yield stress and the tangential modulus of different polymeric materials. A novel approach is developed to select representative points at which finite element calculations are performed. Yield stress and tangential modulus can be obtained by minimizing the deviation between the calculated and the experimental data. Hardness values are also derived using finite element calculations and compared to those obtained from the fitting to the loading curve of depth-sensing experiments.

Simultaneous birefringence, small- and wide-angle X-ray scattering to detect precursors and characterize morphology development during flow-induced crystallization of polymers

An experimental configuration that combines the powerful capabilities of a short-term shearing apparatus with simultaneous optical and X-ray scattering techniques is demonstrated, connecting the earliest events that occur during shear-induced crystallization of a polymer melt with the subsequent kinetics and morphology development. Oriented precursors are at the heart of the great effects that flow can produce on polymer crystallization (strongly enhanced kinetics and formation of highly oriented crystallites), and their creation is highly dependent on material properties and the level of stress applied. The sensitivity of rheo-optics enables the detection of these dilute shear-induced precursors as they form during flow, before X-ray techniques are able to reveal them. Then, as crystallization occurs from these precursors, X-ray scattering allows detailed quantification of the characteristics and kinetics of growth of the crystallites nucleated by the flow-induced precursors. This simultaneous combination of techniques allows unambiguous correlation between the early events that occur during shear and the evolution of crystallization after flow has stopped, eliminating uncertainties that result from the extreme sensitivity of flow-induced crystallization to small changes in the imposed stress and the material. Experimental data on a bimodal blend of isotactic polypropylenes are presented.

Basic aspects of microindentation in multilayered poly(ethylene terephthalate)/polycarbonate films

The microhardness H of multilayered poly(ethylene terephthalate) (PET)/polycarbonate (PC) films, produced by continuous layer multiplying coextrusion has been determined. These materials present rather uniform laminates up to thousands of layers in the micrometre and submicrometre range. The micromechanical properties have been investigated as a function of layer thickness of the single polymer components, the total number of layers, the film thickness and the influence of heat treatment. The microhardness of the microlayered structure has also been determined across the profile in the parallel direction to the packing of the layers. The hardness in the vicinity of the PET/PC phases has been examined. Results reveal that the influence of the interphase on the H values for the samples with a large number of layers is rather small. The most important parameter in determining the final hardness of the multilayered films is the ratio of the penetration depth to the thickness of the layer. Upon heating, a microhardness increase is observed as a consequence of a double contribution: the crystallization of the PET layers, on the one hand, and the physical ageing of the PC zones on the other.

A USAXS study of melt processed PE with a shish-kebab structure: the influence of temperature on the long periods

Abstract The changes in the two axial long periods of injection moulded oriented linear polyethylene (PE) with a shish-kebab structure have been investigated as a function of temperature by ultra-small angle X-ray scattering (USAXS) using synchrotron radiation. The aim of the present study is to investigate the origin of both periodicities and to establish whether both scattering maxima are mutually related. The gradual transformation of the initial two long periods, L 1 = 36.4 and L 2 = 65.7 nm, measured at room temperature to the final single long period L f = 120 nm at T = 130°C has been demonstrated on high molecular weight oriented PE samples. The detection of such high long periods (up to 150 nm for these samples) has been possible, for the first time, thanks to the use of the new high resolution USAXS beam line at the DORIS-bypass, DESY. On the other hand, low molecular weight PE shows, at room temperature, one long period corresponding to an oriented structure with a wide lamellar distribution. The coherently diffracting domains in the chain direction and the mean orientation of the lamellae derived from the azimuthal scanning of the SAXS profiles have also been examined as a function of temperature. The changes of the long period L 1 with temperature are shown to be independent from the changes of the L 2 periodicity. From the results it is concluded that the oriented shish-kebab structure, in the high molecular weight samples, consists of two separate populations of lamellar stacks with the layer normals parallel to the injection direction. During annealing only the thinner lamellae increase in thickness until they reach the size of the thicker ones at about T = 130°C. For higher temperatures than T = 137°C the scattering maxima vanish suggesting that the lamellae melt. Finally, after cooling from the melt the lamellae crystallize epitaxially on the preserved oriented shish fibrils and both stacking periodicities L 1 and L 2 appear again.

Reversible changes in the solid state of HBA/HNA liquid crystalline copolyesters studied by X-ray diffraction

Abstract Real time X-ray diffraction patterns were recorded as a function of heating and cooling cycles using a synchrotron radiation source for copolyesters of 4-hydroxybenzoic acid (HBA) and 2-hydroxy-6-naphthoic acid (HNA) with monomer ratios 75 25 , 58 42 and 30 70 . The angular positions of the two main interchain X-ray diffraction reflections 110 and 200 were obtained at temperatures below the melting point with a powder diffractometer, and the unit cell dimensions of the copolymers were measured. For the 30 70 and 58 42 compositions the cell dimensions vary linearly and reversibly with temperature. For the 75 25 copolymer the cell dimensions show a clear bend with temperature, which can be associated with a transformation from an orthorhombic to a quasi-hexagonal phase. It is further shown that at higher temperature the dimensions of the D 110 and D 200 coherently diffracting domains increase while the scattering intensity of the diffraction peaks decreases with temperature. These changes are reversible with temperature. Results are discussed in terms of partial melting of smaller crystallites contributing in a reversible manner to the occurrence of a mobile high temperature quasi-hexagonal phase.

Structure–microhardness correlation in blends of nylon 6/nylon 66 monofilaments

The microstructure (crystallinity, long spacing) and the micromechanical properties (microhardness H) of two series of nylon 6 and nylon 66 monofilaments and their blends were investigated as a function of annealing temperature TA and uniaxial deformation in a wide composition range. In case of the homopolymers, the gradual rise of microhardness with TA is interpreted in the light of the increasing values of the crystallinity a and the hardness of the crystals Hc. The depression of the hardness values of the blends from the additive behavior of the hardness of individual compo- nents is discussed in the basis of the crystallinity depression of one component by the second one and viceversa. Finally, the influence of drawing and pressing the blends at 130°C which leads to a hardness increase is also explained in the light of an increase in the Hc value of nylon 66 due to orientation.

Novel aspects of microstructure of liquid crystalline copolyesters as studied by microhardness: influence of composition and temperature

Abstract The microhardness of a series of random copolyesters of 4-hydroxybenzoic (HBA) and 2-hydroxy-6-naphthoic acid (HNA) has been investigated as a function of composition and temperature. The results reveal that, at room temperature, the microhardness of non-oriented materials deviates from the linear additivity of the hardness of single homopolymers. Such a deviation is shown to be mainly related to changes in the molecular packing of the rigid chains. This packing, and as a consequence microhardness, can be characterized by an average cross-sectional area which includes crystalline and non-crystalline regions. The fact that the microhardness behaviour and the temperature dependence of the cross-sectional area are closely related is emphasized. An analytical expression embracing the coefficient of thermal softening, β, and the thermal expansion coefficient of the cross-sectional area, αA, is proposed.