Nanostructure Evolution of Isotropic High-Pressure Injection-Molded UHMWPE during Heating

Abstract

Ultrahigh molecular weight polyethylene (UHMWPE) is injection molded under high pressure and studied by ultra-small-angle X-ray scattering (USAXS) during melting in a time-resolved synchrotron radiation experiment. Results concerning melting and recrystallization of crystalline lamellae are compared to data obtained by differential scanning calorimetry (DSC). USAXS analysis reveals a coupled process of melting and crystallization which is not accompanied by external heat flow. Nine isotropic samples differing in molecular weight and molding pressure are heated at a rate of 5 °C/min. 2D USAXS images integrate over temperature intervals between 3 and 7 °C. The materials are considered two-phase semicrystalline polymers. Scattering curves obtained by azimuthal averaging are transformed to interface distribution functions (IDF) which are perfectly fitted by a nanostructural model comprising an ensemble of thick, uncorrelated layers (50 nm thickness) and stacks of short-range correlated crystalline lamellae (20 nm). Crystalline layers are identified from their narrower layer thickness distribution and their melting behavior. After the scattering effect of amorphous layers is eliminated, a composite crystallite thickness distribution is obtained. Its variation is studied as a function of temperature, molecular mass and molding pressure. In DSC thermograms samples prepared at high pressure exhibit a single strong melting peak, whereas the other samples show an additional melting peak at lower temperature. This might lead to the conclusion that the high-pressure samples predominantly contain extended chain crystals. USAXS shows that high-pressure materials contain considerable amounts of imperfect thin crystal lamellae that melt at lower temperature, while thick lamellae are formed. With low-pressure samples, this coupled process of nanostructure transformation during annealing is found to be negligible.

Nanostructure Evolution of Homogeneous Poly(ethylene‐co‐1‐octene) as a Function of Strain

“…In order to discriminate between the two possibilities, it would have been necessary to carry out the SAXS measurements on an absolute scale and then to analyze the patterns quantitatively. A viable route would start from computing the longitudinal scattering curve30 from the 2D pattern and then to fit it using one‐dimensional paracrystalline stacking models, as has been published earlier 31,32. Crystallinity in the microfibrils could be approximated from the analysis of the longitudinal structure as well.…”

Nanostructure Evolution of SIS Thermoplastic Elastomers During Straining as Revealed by USAXS and Two-Dimensional Chord Distribution Analysis

“…While for one-dimensional structures suitable model functions are at hand [33,34] which describe and link together long-period distributions and particle size distributions, the observed variety of multidimensional peaks reveals the challenge in the task of fitting a model to the multidimensional CDF. Interpretation of these rich functions, on the other hand, may add to the understanding of nanostructure in polymer materials.…”

Improving the integration of HDPE/UHMWPE blends by high temperature melting and subsequent shear

“…Because of the ultra-long chains and peculiar semi-crystalline network, UHMWPE can confer many excellent properties on HDPE materials [1][2][3], such as outstanding mechanical properties, low friction and wear rate as well as excellent fatigue resistance. Therefore, the modification of HDPE with UHMWPE always gains great interest among researchers [4][5][6][7][8][9][10][11][12][13][14][15].…”