Stefano Piccarolo

An experimental methodology to study polymer crystallization under processing conditions. The influence of high cooling rates

Abstract A new experimental route for investigating polymer crystallization under very high cooling rates (up to 2000°C/s) is described. A complete and exhaustive description of the apparatus employed for preparing thin quenched samples (100– 200 μm thick) is reported, the cooling mechanism and the temperature distribution across sample thickness is also analysed, showing that the final structure is determined only by the thermal history imposed by the fast quench apparatus. Details concerning the characterization techniques used to probe the final structure are reported, including density measurements and wide angle X-ray diffraction patterns. Experimental results concerning isotactic polypropylene, polyethylenetherephthalate and polyamide 6 are reported, showing the reliability of this experimental route to assess not only a quantitative information but also a qualitative description of the crystallization behaviour of different classes of semi-crystalline polymers.

Some experimental issues of AFM tip blind estimation: the effect of noise and resolution

The convolution of tip shape on sample topography can introduce significant inaccuracy in an AFM image, when the tip radius is comparable to the typical dimension of the sample features to be observed. The blind estimation method allows one to obtain information on the AFM tip through an unknown characterizer sample and thus to perform the deconvolution of the tip shape from an image. When applying the blind estimation method to determine the AFM tip shape, some apparently trivial issues relating to the experimental operating parameters must be taken into account. In this paper, the effects of the operating parameters, e.g., sampling intervals (resolution) and instrumental noise, have been taken into account for the practical use of blind estimation and the result is that instrumental noise tends to provide a smaller estimation of the tip size, while larger sampling intervals provide a larger value of it. This paper presents guidelines to those effects in AFM and appropriate experimental conditions for applying the blind estimation method to obtain more reliable data on tip radius and therefore on sample topography.

Accurately evaluating Young’s modulus of polymers through nanoindentations: A phenomenological correction factor to the Oliver and Pharr procedure

The Oliver and Pharr [J. Mater. Res. 7, 1564 (1992)] procedure is a widely used tool to analyze nanoindentation force curves obtained on metals or ceramics. Its application to polymers is, however, difficult, as Young’s moduli are commonly overestimated mainly because of viscoelastic effects and pileup. However, polymers spanning a large range of morphologies have been used in this work to introduce a phenomenological correction factor. It depends on indenter geometry: sets of calibration indentations have to be performed on some polymers with known elastic moduli to characterize each indenter.

Nanoscale mechanical characterization of polymers by atomic force microscopy (AFM) nanoindentations: viscoelastic characterization of a model material

The atomic force microscope (AFM), apart from its conventional use as a microscope, is also used for the characterization of the local mechanical properties of polymers. In fact, the elastic characterization of purely elastic materials using this instrument can be considered as a well-assessed technique while the characterization of the viscoelastic mechanical properties remains the challenge. In particular, one finds the mechanical behavior changing when performing indentations at different loading rates, i.e. on different time scales. Moreover, this apparent viscoelastic behavior can also be due to complex contact mechanics phenomena, with the onset of plasticity and long-term viscoelastic features which cannot be identified by the force curve alone. For this reason, a viscoelastic characterization, and thus the study of the effects of indentation rate and temperature, was done on model materials where such additional phenomena are not observed. Another time dependence originating from the instrument itself has also been identified and decoupled. In fact, the viscoelastic behavior has been found to be reproducible even if one changes the experimental set-up as far as the preliminary determinations concerning AFM nanoindentations are well performed. The effects of temperature and time scales on the mechanical behavior have also been undertaken. A check on time–temperature superposition is also attempted through the WLF equation and the apparent activation energies for the elementary motions in the rubbery and in the glass transition regions are in good agreement with the expected values.

SAXS/WAXS study of the annealing process in quenched samples of isotactic poly(propylene)

The structural rearrangement in samples of quenched isotactic poly(propylene) (iPP) submitted to different annealing treatments has been studied using simultaneous small- and wide-angle X-ray scattering (SAXS/WAXS) at the synchrotron radiation source of DESY, Hamburg. From a quantitative analysis of the WAXS profiles the values of the α-monoclinic, mesomorphic, and amorphous mass fractions coexisting in the material were determined. It is demonstrated that the SAXS patterns were characterized by two different long-period values that are attributed to α- and mesomorphic periodicity, respectively. The related α- and mesomorphic volume phase fractions, calculated from the analysis of the SAXS data, are compared with the corresponding WAXS results. The rearrangement of the initial structure involves i) the thickening of the already existing α-phase lamellae at the expenses of the amorphous regions and ii) the structural rearrangement of the mesomorphic phase leading to its transformation into the α-monoclinic one.

Role of thermal history on quiescent cold crystallization of PET

Abstract The cold crystallization of poly(ethylene terephthalate) (PET) has been studied as a function of the initial structure of the glass using density, microhardness, wide angle X-ray scattering, small angle X-ray scattering and DSC measurements. Glassy PET samples varying from slightly crystalline to completely amorphous phase were investigated. Results reveal that differences in the inner structure of the starting glassy material induce different crystallization rates from the glassy amorphous state. Thus, it is observed that crystallization rate decreases with the increasing cooling rate used to quench the samples. Results have been analyzed using the Kolmogroff–Avrami–Evans theory. A good agreement between theoretical and experimental data is obtained providing accurate values for kinetic constants. The different crystallization rates obtained are explained in terms of differences in nucleation density.

Influence of morphology and chemical structure on the inverse response of polypropylene to gamma radiation under vacuum

Abstract In this work the influence of the chemical structure and of the morphology on the gamma-radiation effects on polypropylene based polymers is studied on the basis of a previously discussed kinetic model [1] . For this aim an isotactic polypropylene and a random ethylene–propylene copolymer were irradiated under vacuum at one dose rate and several absorbed doses after well defined solidification conditions. We show that the model is reliable varying both the chemical structure and the morphology of the polypropylene based polymer. An inversion of the response of the material to gamma radiation under vacuum is always observed, and the inversion conditions depend on the irradiation parameters. In particular at a fixed dose rate we check an absorbed dose, depending on the ethylene and mesomorphic phase content in the polymer, in correspondence of which a significant change in the molecular response to gamma irradiation occurs.

Injection Molding of iPP

Abstract An experimental study of iPP injection molding is carried out. Pressure distribution during both filling and post filling stages, mass entering the mold during each stage of the process, morphology and crystallinity distribution of the final object are analysed. Interesting features are evidenced and discussed in relation to gate geometry and filling flow rate. Evidence of flow crystallization inside the cavity is also reported.

Structure development in poly(ethylene terephthalate) quenched from the melt at high cooling rates: X-ray scattering and microhardness study

The structure and microhardness of poly(ethylene terephthalate) (PET) cooled from the melt, using a wide range of cooling rates, was studied. PET thin films rapidly cooled from the melt (cooling rates larger than 5°C/s) show a continuous variation of structure and properties depending on cooling rate. Results highlight differences in the micro-mechanical properties of the glass suggesting the occurrence of amorphous structures with different degrees of internal chain ordering. The comparative X-ray scattering study of two glassy PET samples (7500 and 17°C/s) reveals the occurrence of frozen-in electron density states giving rise to an excess of scattering for the amorphous sample solidified at a lower cooling rate. The initial glassy structure and its evolution, during isothermal cold crystallization at 117°C of these two samples can be interpreted by assuming an improvement in the state of internal order. The differences in the incipient molecular ordering, which are detected by SAXS but not by WAXS, could be responsible for the hardening observed in the glassy PET samples.

Crystallization Behaviour at High Cooling Rates of Two Polypropylenes

Phase distribution of quenched samples of two isotactic polypropylenes, having different molecular weight distributions, was evaluated by a deconvolution procedure of WAXD spectra. The dependence on cooling rate of the two resins shows the low molecular weights rich polymer is characterized by a faster kinetics with an α-monoclinic to mesomorphic transition taking place at higher cooling rates.