Experimental study on the surface characteristics of polymer melts

Interfacial Forces in Free-Standing Layers of Melted Polyethylene, from Critical to Nanoscopic Thicknesses

Molecular dynamics simulations of ultrathin free-standing layers made of melted (373.15–673.15 K) polyethylene chains, which exhibit a lower melting temperature (compared to the bulk value), were carried out to investigate the dominant pressure forces that shape the conformation of chains at the interfacial and bulk liquid regions. We investigated layer thicknesses, tL, from the critical limit of mechanical stability up to lengths of tens of nm and found a normal distribution of bonds dominated by slightly stretched chains across the entire layer, even at large temperatures. In the bulk region, the contribution of bond vibrations to pressure was one order of magnitude larger than the contributions from interchain interactions, which changed from cohesive to noncohesive at larger temperatures just at a transition temperature that was found to be close to the experimentally derived onset temperature for thermal stability. The interchain interactions produced noncohesive interfacial regions at all temperatures in both directions (normal and lateral to the surface layer). Predictions for the value of the surface tension, γ, were consistent with experimental results and were independent of tL. However, the real interfacial thickness—measured from the outermost part of the interface up to the point where γ reached its maximum value—was found to be dependent on tL, located at a distance of 62 Å from the Gibbs dividing surface in the largest layer studied (1568 chains or 313,600 bins); this was ~4 times the length of the interfacial thickness measured in the density profiles.

Measurement of nanoscale molten polymer droplet spreading using atomic force microscopy

We present a technique for measuring molten polymer spreading dynamics with nanometer scale spatial resolution at elevated temperatures using atomic force microscopy (AFM). The experimental setup is used to measure the spreading dynamics of polystyrene droplets with 2 μm diameters at 115-175 °C on sapphire, silicon oxide, and mica. Custom image processing algorithms determine the droplet height, radius, volume, and contact angle of each AFM image over time to calculate the droplet spreading dynamics. The contact angle evolution follows a power law with time with experimentally determined values of -0.29 ± 0.01, -0.08 ± 0.02, and -0.21 ± 0.01 for sapphire, silicon oxide, and mica, respectively. The non-zero steady state contact angles result in a slower evolution of contact angle with time consistent with theories combining molecular kinetic and hydrodynamic models. Monitoring the cantilever phase provides additional information about the local mechanics of the droplet surface. We observe local crystallinity on the molten droplet surface, where crystalline structures appear to nucleate at the contact line and migrate toward the top of the droplet. Increasing the temperature from 115 °C to 175 °C reduced surface crystallinity from 35% to 12%, consistent with increasingly energetically favorable amorphous phase as the temperature approaches the melting temperature. This platform provides a way to measure spreading dynamics of extremely small volumes of heterogeneously complex fluids not possible through other means.

Calculation of the Contact Angle of Polymer Melts on Tool Surfaces from Viscosity Parameters

It is of great importance for polymer processing whether and how viscosity influences the wettability of tool surfaces. We demonstrate the existence of a distinct relationship between the contact angle of molten polymers and zero shear viscosity in this paper. The contact angle of molten polypropylene and polymethylmethacrylate on polished steel was studied in a high temperature chamber using the sessile drop method. A high pressure capillary rheometer with a slit die was employed to determine the shear viscosity curves in a low shear rate range. A linear relation between the contact angle and zero shear viscosity was obtained. Furthermore, the contact angle and the zero shear viscosity values of the different polymers were combined to one function. It is revealed that, for the wetting of tool surfaces by molten polymers, a lower viscosity is advantageous. Furthermore, a model based on the temperature shift concept is proposed which allows the calculation of the contact angle of molten polymers on steel for different temperatures directly from shear viscosity data.

Modeling Temperature and Time Dependence of the Wetting of Tool Steel Surfaces by Polymer Melts

Beside the surface properties of the tool material, its temperature is an important parameter influencing the wettability of the tool surface by polymer melts in extrusion technology and injection molding. The temperature and time dependence of the contact angle of a polypropylene and a polymethylmethacrylate melt on polished tool steel was studied in this work at close to process conditions. The experiments were conducted by placing the polymeric sample on the hot tool material substrate in a high temperature chamber and recording the drop shape dependence on time. Based on the experimental results, a novel model was developed which allows a description of the contact angle dependent on temperature and time. The contact angle of the investigated polymer melts exhibits a linear decrease with rising temperature, which means that the wettability of the tool material by the polymer melt is improved with increasing temperature. Furthermore, the model proposed herein enables a complete mathematical description of the contact angle of polymer melts on the tool material dependent on temperature and time. The parameters of this function are the initial contact angle θ0, the contact angle when time approaches infinity θ∞ and a characteristic material time B. The time dependency is incorporated by an exponential function. The characterizing contact angle parameters (θ0, θ∞) follow a linear decrease with rising temperature. The characteristic material time B obeys an exponential law dependent on the reciprocal value of temperature similar to Arrhenius' law.

Self-assembly at air/water interfaces and carbohydrate binding properties of the small secreted protein EPL1 from the fungus Trichoderma atroviride

The protein EPL1 from the fungus Trichoderma atroviride belongs to the cerato-platanin protein family. These proteins occur only in filamentous fungi and are associated with the induction of defense responses in plants and allergic reactions in humans. However, fungi with other lifestyles also express cerato-platanin proteins, and the primary function of this protein family has not yet been elucidated. In this study, we investigated the biochemical properties of the cerato-platanin protein EPL1 from T. atroviride. Our results showed that EPL1 readily self-assembles at air/water interfaces and forms protein layers that can be redissolved in water. These properties are reminiscent of hydrophobins, which are amphiphilic fungal proteins that accumulate at interfaces. Atomic force microscopy imaging showed that EPL1 assembles into irregular meshwork-like substructures. Furthermore, surface activity measurements with EPL1 revealed that, in contrast to hydrophobins, EPL1 increases the polarity of aqueous solutions and surfaces. In addition, EPL1 was found to bind to various forms of polymeric chitin. The T. atroviride genome contains three epl genes. epl1 was predominantly expressed during hyphal growth, whereas epl2 was mainly expressed during spore formation, suggesting that the respective proteins are involved in different biological processes. For epl3, no gene expression was detected under most growth conditions. Single and double gene knock-out strains of epl1 and epl2 did not reveal a detectable phenotype, showing that these proteins are not essential for fungal growth and development despite their abundant expression.