R. K. Bayer

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.

Correlation of microhardness and morphology in injection-moulded poly(ethylene terephtalate)

Microindentation hardness has been applied to a series of injection-moulded poly(ethylene terephtalate) samples prepared using a range of mould temperatures, Tc. The morphology of the samples was characterized by X-ray diffraction and differential scanning calorimetry. Depending on Tc′, it is shown that microhardness is lower at the surface than in the core of the mouldings. Results are discussed in terms of the volume fraction of spherulites filling the mouldings which is shown to be dependent upon Tc. The influence of an annealing treatment on the properties of the mouldings is examined. The microhardness values are correlated with the thickness and with the surface free energy of the lamellar crystals. The results obtained indicate that increasing annealing temperatures first leads to an increase and then to a sudden decrease of hardness. The latter can be associated with the changes occurring in the number of defects on the crystals surface.

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,Ts, and mould,Tw, 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, ΔMH, (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 ΔMH 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. ΔMH is, however, nearly independent ofTs for conventional moulding-grade PE. Here the absence of a unit-cell orientation is evident in theTs range investigated while a lamellar orientation only prevails forTs<200° C. In this latter caseMH⊥ is a linear function ofTw. 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.

Nanostructure Evolution of Oriented High-Pressure Injection-Molded Poly(ethylene) During Heating

Abstract High-pressure injection-molded polyethylene (PE) rods are studied by ultra small-angle X-ray scattering from synchrotron during the heating of the polymer. Injection of a cool melt into a cold mold yields highly oriented PE rods with a core–shell structure. Samples from both the core and the shell material are studied. The two-dimensional scattering patterns are evaluated utilizing the multi-dimensional chord distribution function (CDF) analysis. From the obvious evolution of the nanostructure during successive crystallite melting, the sequence of processes occurring during crystallization is elucidated. First, nuclei form one-dimensional lattices with short-range order along the fiber axis. From this row structure, lamellae grow with wide lateral extension. An indication of an intermediate block structure is observed. Finally two steps of insertion crystallization result in two long period halvings. Increase of the mold pressure increases the lateral extension of the inserted lamellae in the shell material. In the core material a uniform row structure is absent. Extended primary lamellae form stacks with decreasing long periods before insertion crystallization takes over. But crystallites inserted in the core material do not form extended lamellae. Each of these steps leaves its footprint in the nanostructure and the corresponding scattering pattern. After CDF interpretation of the heating series, the room temperature pattern can be explained. The strong two-point pattern is associated with the primary lamellae and the intensity ridge extending along the meridian results from irregular insertion of lamellae. When the row structure is observed in the CDF, the fiber pattern exhibits equatorial scattering. Domain roughness generates a strong background scattering, which cannot be separated in one step. For the presented material it is shown that iterative background subtraction eliminates the scattering effects of the imperfect (i.e. inserted) lamellae.

Microhardness and structure of high pressure crystallized poly(ethylene terephthalate)

Abstract The microindentation hardness (H) of PET samples crystallized isothermally at various temperatures from the melt under high pressure (4 k bar) was determined in order to establish correlations with the thermal properties and microstructure. The results reveal that these materials show density, enthalpy of fusion, and melting temperature values which are close to those corresponding to chain extended crystals. The high crystallinities obtained (α = 0.70–0.95) for high pressure crystallized PET are shown to give rise to unprecedentedly high microhardness values, ranging between 300 and 400 MPa. The data are analysed on the basis of a two-phase model which allows an estimation of the influence of crystal thickness upon H. Finally, the temperature dependence of high pressure crystallized PET is also examined.

Influence of processing methods on starch properties

Abstract Native potato starch was prepared using different processing methods. The samples were characterized by wide-angle X-ray scattering (WAXS), optical microscopy, differential scanning calorimetry (DSC), and microhardness. Compression molding of the starch granules led to sintered relatively brittle materials. Here, the amylopectin crystals of the native powder remained grossly preserved. Preparation of dry films from aqueous gels resulted in disintegration of the structure of the native starch granules and in the formation of a new semicrystalline structure comprised of crystallized amylose molecules. Injection molding of native starch was found to be a processing method that gives rise to amorphous materials with superior mechanical properties.

Nanostructure of atmospheric and high-pressure crystallized poly(ethylene-2,6-naphthalate). Part II: structure-microhardness correlations

Abstract The microhardness of poly(ethylene naphthalene-2,6-dicarboxylate) (PEN), with a detailed characterized nanostructure has been investigated. PEN samples were crystallized from the glassy state at atmospheric pressure and from the melt at high pressure and were characterized using small-angle X-ray scattering (SAXS), wide-angle X-ray scattering (WAXS) and differential scanning calorimetry (DSC). Results show that the degree of crystallinity derived from WAXS, for both atmospheric and high-pressure crystallized PEN, is smaller than that obtained by density and calorimetry. For high-pressure crystallized samples, both, crystallinity and microhardness values are larger than those found for the material crystallized under atmospheric pressure. In the latter case, the hardness values depend on the volume fraction of lamellar stacks within spherulites XL that depends on the crystallization temperature Tc. For Tc

Nanostructure of atmospheric and high-pressure crystallised poly(ethylene-2,6-naphthalate)

Poly(ethylene-2,6-naphthalate) (PEN) was crystallized from the glassy state at atmospheric pressure (beyond the end of primary crystallization) and from the melt at high pressure. The structure was characterized using small-angle X-ray scattering (SAXS), wide-angle X-ray scattering (WAXS), differential scanning calorimetry (DSC) and density measurements. The SAXS patterns were analysed using the interface distribution function (IDF) method. For the materials prepared at ambient pressure the crystallinity inside the layer stacks remains nearly constant during the secondary crystallization process. On the other hand, the volume filled with the stacks increases as a function of crystallization temperature (Tc) and time (tc). For Tc > 200°C secondary crystallisation goes along with a dynamic rearrangement of the primary stacks, as concluded from variations of the layer thickness distributions in the SAXS data. For Tc < 200°C primary lamellae are stable, and both insertion of new crystal lamellae into existing stacks and generation of additional stacks is found. In contrast to PET, two different kinds of layer stacks are not observed in the PEN nano-composites. Materials prepared at 400 MPa exhibit high roughness of the crystalline domain surfaces. Depending on Tc there is a continuous transformation from the α to the β-crystal modification, but hardly any change of the long period. Crystal thickness increases, both at the expense of the amorphous thickness and of the volume filled with lamellar stacks. The structure of samples showing two melting peaks is discussed in terms of a dual lamellar contribution of correlated and uncorrelated nano-crystallites, respectively.

Elastoplastic Properties of Starch-Based Materials as Revealed by Microindentation Measurements

The elastoplastic properties of injection-molded starch and starch-cement composites were investigated by load-displacement analysis from depth-sensing experiments. The creep behavior under the indenter was studied. Hardness data from the depth-sensing and imaging methods were shown to be in good agreement. The Young modulus values derived from the compliance method were studied; results are discussed in the light of the various materials used. Finally, the influence of annealing temperature on the microhardness of starch and of the starch-cement composites, processed under a water vapor atmosphere, was investigated. Results reveal a hardening of the materials on thermal treatment. This hardness increase is associated with an equilibrium water content decrease detected within the samples.

SAXS investigation of the structure of high-pressure crystallized poly(ethylene terephthalate): a new nanostructured material?

The common crystallization conditions of poly(ethylene terephthalate) (PET) were replaced by an anabaric high-pressure crystallization at 320 °C. The PET samples were characterized by differential scanning calorimetry, density and microhardness. The resulting two-phase microstructure was studied by means of absolute small-angle X-ray scattering (SAXS). A complete SAXS analysis utilizing the interface distribution function (IDF) method was carried out. The resulting structure exhibited the presence of stacks of 10 nm thick crystalline lamellae which were separated by amorphous layers of about 1.3 nm thickness. Similar structures have been found after annealing of amorphous metals and have been termed nanocrystalline. Microhardness and structure have been discussed in analogy with the notions from the field of nanostructured materials. Theoretically, a multi-component lamellar two-phase structure has been discussed. The equations derived allow the computation of volume fractions and specific surfaces of the components (different kinds of stacks).