Vladimír Hynek

Gas permeability and mechanical properties of polystyrene–polypropylene blends

Permeability to water vapor and oxygen, elastic modulus, tensile strength, and impact strength of polystyrene–polypropylene and high-impact polystyrene–polypropylene blends were determined as functions of blend composition and morphology. Three types of styrene–butadiene block copolymers were tested as compatibilizers and found to improve mechanical properties of blends. The experimental data on permeability and modulus were compared with the predictions for the studied binary and ternary blends. The predictive scheme employs a two-parameter equivalent box model and the data on phase continuity of constituents calculated using general equations derived from percolation theory. Blends with decreased permeability and plausible mechanical properties were proposed with regard to intended applications in food packaging.

Measurements of excess enthalpies at high temperature and pressure using a new type of mixing unit

A flow mixing unit (calorimetric cell and auxiliary devices) has been designed for measuring the enthalpy of mixing or reaction of two fluids (gas+liquid or liquid+liquid). The indicator of the heat effect is a differential heat flux calorimeter, SETARAM C-80, allowing measurements at temperatures up to 300°C. The mixing cell is made of a stainless-steel capillary (o.d 1.6 mm, length 2.4m) which is coiled in a cylindrical form and tightly fitted in the thermopile well of the calorimeter. The fluids are delivered from the high pressure piston pumps and circulated through the system at controlled flow rates ranging from 100 to 1500 μL-min−1. The tests were carried out at pressures up to 20 MPa. Special care was taken to allow good thermostatting of fluids entering the mixing cell. Check measurements were made with one liquid-liquid system (C2H5OH+H2O) and one gas-liquid system (CO2+C6H5CH3); our enthalpies of mixing agreed with the literature values in most cases to 2%. For the system ethanol+water the experiments have been also performed at temperature of 250°C and pressures of 15 and 20 MPa. The endothermal mixing effect was higher than expected indicating an increase in the excess heat capacity.

Ultra-thin polyimide film as a gas-separation layer for composite membranes

Abstract Composite membranes with an ultra-thin polyimide separation layer have been prepared by the deposition of dimethylalkylammonium salt of polyamic acid on a poly(phenylene oxide) porous support layer by the Langmuir—Blodgett technique and subsequent thermal cyclization of the polyimide precursor. In spite of a relatively mild thermal treatment, complete cyclization was achieved as observed by Fourier transform infra-red spectroscopy. The composite membrane with polyimide ultra-thin separation layer exhibited a considerably high permeation rate, maintaining a good selectivity.

Combined Flow-Mixing Power-Compensation Calorimeter and Vibrating Tube Densimeter for Measurements at Superambient Condition

A new instrument combining a flow-mixing calorimeter and a vibrating tube densimeter inside a thermostatted environment has been constructed. The instrument is designed for simultaneous measurements of calorimetric and volumetric properties at superambient conditions on mixtures for which both pure components are liquid at room temperature or one component is a liquid and the other is gaseous. The system yields simultaneously two properties: (1) the enthalpy of mixing determined in a heat-leak calorimeter with power compensation; (2) the density difference between a mixture (solution) and a reference liquid measured as a change in frequency of a tube vibrating in a field of permanent magnets. The instrument was tested in the full concentration range using aqueous ethanol and aqueous methanol. The results are presented at temperatures between 348 and 573 K and pressures from 5 to 20 MPa for mixing enthalpies ΔHmix ranging from −160 to 5700 J-mol−1. The corresponding heating powers are between −60 and 850 mW, respectively. The results for ΔVmix of mixing volumes were measurable from 348 K to 523 and between 5 and 13 MPa with the maximum volume change being −4.0 cm3-mol−1. The errors in ΔHmix and ΔVmix near mole fraction of 0.5 are believed to be less than 5% over the temperature and pressure ranges given above.

Hyperbranched Polyimide-Silica Hybrid Materials: Synthesis, Structure, Dynamics, and Gas Transport Properties

A series of organic–inorganic hybrid materials were prepared from a hyperbranched polyimide precursor (hyperbranched polyamic acid), tetramethoxysilane, and/or 3-glycidyloxypropyl-trimethoxysilane via a sol-gel process. The hyperbranched polyimide-silica hybrids, whose polyimide moieties were based on commercially available monomers 4,4′,4″-triaminotriphenylmethane and 4,4′-oxydiphthalic anhydride taken in molar ratio 1:1, contained from 10 to 30 wt% silica. Their morphology and dynamics were characterized by using scanning electron microscopy, differential scanning calorimetry, dynamic mechanical analysis, laser-interferometric creep rate spectroscopy, and wide-angle X-ray diffraction. Attention was also focused on the relation between morphology/dynamics and gas transport properties of these materials.

Flow unit for measuring heats of mixing at subambient conditions

A type of mixing flow unit for BT 2.15D SETARAM calorimeter was designed for measuring heats of mixing of two liquids or a gas and a liquid. A cooling system using a cryostat circulating a cooling agent (ethanol) through the instrument was proposed and tested for temperatures down to −50°C. In addition, a special heat exchanger was constructed for cooling down fluids entering the calorimeter when experiments are performed at subambient temperatures. In comparison with cooling by liquid nitrogen, the operation is less expensive and no tedious manipulations with liquid nitrogen are needed. A procedure for measuring with the mixing unit was developed and the assembly was successfully tested in the temperature range from −30to75°C and at pressures up to 14MPa using several binary systems (liquid-liquid) where reliable literature data were available. The proposed flow arrangement is much more efficient and convenient to use at subambient temperatures compared to conducting experiment in a batch mode, which is ...

Evaluation of Two Methods for Measuring Vapor Sorption in Polymers

In this paper, two methods for measuring the equilibrium vapor sorption in polymers are critically compared and data on sorption of toluene, p-xylene, hexane, cyclohexane, and heptane in low density polyethylene are reported. The vapor phase calibration method (VPC) was used to measure vapor sorption at low vapor activities in air (below 0.01), and the gravimetric method was used to measure sorption over wide range of activities of pure vapors (0.1–0.9). The Flory-Huggins interaction parameter (in amorphous phase) varied between 1.00 for cyclohexane and 1.19 for toluene. The resulting confidence intervals are conjunctive, indicating that both methods provide consistent results.