Hugo Berghmans

Phase behaviour of poly(N-vinyl caprolactam) in water

Abstract The solution behaviour of the system poly(N-vinyl caprolactam)/water has been studied. Experiments and theoretical calculations indicate a typical Flory–Huggins (Type I) demixing behaviour with a lower critical solution temperature (LCST). The critical concentration and LCST shift to lower values with increasing molar mass of the polymer. The phenomenological description of such a system predicts a continuous temperature controlled swelling behaviour of the corresponding network. A comparison with literature data confirms this prediction. A calorimetric study of the crystallisation and melting behaviour of water and the concentration dependence of the glass transition temperature, gives no evidence for the formation of a polymer/solvent complex.

Investigation of thermal properties of glassy itraconazole: identification of a monotropic mesophase

Abstract The purpose of the present work is the elucidation of two endothermic transitions at 74 and 90°C, respectively, observed during differential scanning calorimetry of glassy itraconazole. Modulated temperature DSC (MTDSC), hot-stage microscopy (HSM), HPLC and high temperature X-ray diffraction (HT-X ray) were used to examine the thermal properties of glassy itraconazole. It was found that the preparation mode of the glass does not seem to influence the appearance of both endothermic transitions since they were present during heating of glassy itraconazole which was prepared by cooling the melt or by rapid solvent evaporation of an itraconazole solution. These observations suggest that the appearance of the two endothermic transitions require the liquid state prior to glass formation. The transitions are not due to impurities in the starting material, nor are they caused by thermal decomposition. This was further confirmed by HPLC-analysis. HSM showed structure formation following cooling of the melt, at approximately 87°C; cooling the product further showed a second change in optical contrast. HT-X ray confirmed and identified the formation of a nematic mesophase. The appearance of the two endothermic signals during scanning of glassy itraconazole points to the formation of a mesophase. Due to the nature of itraconazole, it appears as a chiral nematic phase of which the mobility is frozen into a glass upon cooling below 59°C thereby impeding further crystallization.

Molecular complex formation in the system poly(vinyl methyl ether)/water

The behaviour of poly(vinyl methyl ether) (PVME) in water has been studied in detail. A molecular complex with maximum two molecules of water per repeating unit is formed. The glass transition temperature of this complex is -58°C. Additional water plastisises and hydrates the complex. This conclusion is based on the detailed analysis of the concentration dependence of the glass transition of the system and melting behaviour of water. A similar behaviour is found with the chemical networks, obtained by radiation cross-linking.

Phase-behavior and structure formation in solutions of poly(2,6-dimethyl-1,4-phenylene ether)

The phase behaviour and the structure formation of solutions of poly(2,6-dimethyl-1,4-phenylene ether) (PPE) in cyclohexanol and decalin have been studied. Interesting structures were formed and the morphologies depended on the rate at which the solutions were cooled. This was caused by the competition between melting—crystallization, vitrification and phase separation. Differential scanning electron microscopy (SEM) was used to study the different morphologies. Porous materials can be used as membranes. Hollow porous PPE fibres were spun.

Expansion of polystyrene using water as the blowing agent

Heating of polystyrene beads containing pentane isomers as the blowing agent traditionally produces polystyrene foam. Undesirable emissions of the blowing agent and its high flammability are the complications of this process. A new process for the production of expandable polystyrene has been developed, using water as the blowing agent. Water is trapped inside the polystyrene matrix through the use of starch that is introduced as a separate phase during the suspension polymerization. The problems created by the incompatibility of starch with the organic phase can be partially overcome by “compatibilization” with maleic anhydride. The type of starch can influence the foam morphology of the pre-expanded beads, while the density is changed only in the range of the experimental error. The density of the pre-expanded beads is influenced by the blowing technique used (hot air or high frequency electric field). The use of these different blowing techniques does not influence significantly the foam morphology.

Phase behaviour of syndiotactic polystyrene-decalin

Abstract The phase behaviour of syndiotactic polystyrene-decalin is studied by differential scanning calorimetry, wide-angle X-ray scattering and thermogravimetric analysis. Both the trans and cis isomers of the solvent have been used. The polymer chain can adopt a helix or a zigzag conformation. The formation of these conformations and their relative contribution to the crystalline structure depend on the initial concentration, the scanning rate and the thermal history of the samples. High cooling rates and low polymer concentrations tend to favour the formation of the helix structure. The conformational transition from the helix to the zigzag conformation can take place on heating. This depends on the experimental conditions.

Ordering and structure formation in triblock copolymer solutions. Part I. Rheological observations

The linear viscoelastic properties of a poly(styrene)-poly(ethylene, butylene) triblock copolymer in the presence of a hydrocarbon oil have been investigated. The samples were followed rheologically, during annealing at different temperatures. In a relatively small temperature interval, tan δ-curves, recorded at different frequencies, evolved in a pattern which is typical for a physical gelation. It allowed determination of the gel time. The kinetics of gel formation were found to be affected by the annealing temperature, and the rate was maximum at an intermediate temperature that depended on the polymer content of the sample. Increasing the polymer content shifted this maximum to higher temperatures. At lower temperatures, a glass transition of the polystyrene end-blocks was detected. The microphase separation temperature however, could not be determined. In addition, the influence of strain and the applicability of time-temperature superposition was analysed.

Crystallization of isotactic polystyrene induced by organic vapours

Abstract The crystallization of isotactic polystyrene induced by dichloromethane and acetone was studied. From equilibrium absorption data, the critical concentration for crystallization was deduced. The crystallization kinetics were followed and diffusion coefficients were calculated. It was found that the crystallization was diffusion controlled. The increase in the rate of crystallization is explained by the high rate of diffusion and nucleation in the presence of the organic solvents. Evidence for the high nucleation probability was found in morphological observations (SAXS and electron microscope observations). The melting behaviour of the solvent crystallized samples suggests formation of a very unstable crystalline structure which re-organizes rapidly on heating.

Thermal studies on homogeneously synthesized cellulose p‐toluenesulfonates

The thermal behavior in argon of homogeneously synthesized cellulose p-toluenesulfonates (tosylates) with a degree of substitution (DS) ranging from 0.4 to 2.3 was studied by means of thermogravimetry and derivative thermogravimetry from ambient temperature up to 500°C. For comparison, the thermal behavior of the starting celluloses used (pulps, linters, bacterial cellulose) was also examined. The thermal degradation of cellulose tosylates was initiated at lower temperature than cellulose itself and proceeds in two main stages. The temperature of the first one (169 196°C) increases with increasing DS and is independent of the molecular weight. Activation energies calculated following the method of Broido, FTIR, and ultimate analysis as well as mass spectroscopy show that the first stage of degradation is closely associated not only with the scission of tosyl ester groups but also with a partially degradation of the polymer backbone. Further, the temperature-concen-tration diagram for the system cellulose tosylate 20/o-dichlorobenzene was studied by optical observations and calorimetric investigations. A liquid-liquid demixing interferes with the glass transition of the cellulose tosylate-solvent system. It results in the solidification of the solution.

Phase behaviour of syndiotactic polystyrene-o-xylene

Abstract The study of the phase behaviour of syndiotactic polystyrene (sPS) in o -xylene is reported. Differential scanning calorimetry, microcalorimetry, infra-red spectroscopy, wide angle X-ray scattering, transmission electron microscopy and thermogravimetric analysis have been used. The polymer chains can adopt a T 2 G 2 helix or a planar T 4 zig-zag conformation, depending on the experimental conditions. The initial concentration, the cooling rate and the thermal history strongly influence the formation and the relative contribution of the different conformations, leading to a complex phase behaviour. A decrease of the polymer concentration, and/or an increase of the scanning rate, tends to favour the formation of the helical structure. This helical conformation is always formed when a vitrified, concentrated solution is heated. The transformation of the helical conformation into the planar zig-zag conformation proceeds by a two-step mechanism of melting and recrystallization. Solvent-induced crystallization of amorphous, glassy sPS results in the formation of the helical conformation. The zig-zag conformation is always formed by crystallization from the pure melt. The formation of an incongruent polymer-solvent compound is proposed to explain the complex phase behaviour.