Search results for "THERMODYNAMICS"

Cure monitoring of catalysed cyanate ester resins

2000

The cure behaviour of two bisphenol A-based cyanate ester resins, AroCy B10 and B30, catalysed by copper acetylacetonate and nonylphenol was studied. For this purpose, differential scanning calorimetric (DSC) and rheological measurements were carried out at temperatures between 130 and 170 °C. The cyanate conversion profiles are fitted with a second-order rate law in the kinetically controlled regime where a good time–temperature superposition is attained. However, it is necessary to add an empirical kinetic term to give a good description of the entire range of curing. Simultaneously, times to gelation and vitrification have been determined by dynamic rheological measurements over the same…

Specific features of the interfacial tension in the case of phase separated solutions of random copolymers

2000

Abstract Phase diagrams (cloud point curves, critical points, tie lines for constant critical composition) and interfacial tensions as a function of temperature were measured for solutions of two random copolymers: poly(dimethylsiloxane- ran -methylphenylsiloxane) [I] and poly(styrene- ran -acrylonitrile) [II]. Acetone and anisole served as solvents for I and toluene for II; all solutions exhibit UCSTs between 300 and 310 K. The phase separation behavior can be well modeled if one accounts for the molecular and chemical non-uniformities of the random copolymers used in this study. The interfacial tensions σ differ most markedly from that of comparable homopolymer solutions in their correlat…

On the factors governing the pressure dependence of the viscosity of moderately concentrated polymer solutions

1982

Viscosity measurements were carried out as a function of pressure and temperature with solutions of polystyrene in eight (endothermal) θ-solvents at the respective critical composition by means of a Searle-type apparatus. A rolling-ball viscometer was used for the investigation of the pure solvents. In all cases the viscosity coefficient increases in a more or less exponential manner when the pressure is raised. For θ-conditions, the volumes of activation of the solutions exceed that of the pure solvent by typically 10–15%. The exact amount of this extra efffect stemming from the presence of the polymer and its variation with temperature can be qualitatively correlated with the heats of mix…

A method for determination of time- and temperature-dependences of stress threshold of linear-nonlinear viscoelastic transition: Energy-Based Approach

2011

A methodology for determination of time- and temperature-dependences of stress threshold of linear–nonlinear viscoelastic transition is proposed and validated by example of uniaxial creep of epoxy resin. Energy approach is applied for characterization of the region of linear viscoelasticity (LVE) and the threshold of LVE is given in the stress–strain representation as the master curve independent of time and temperature. Time- and temperature-dependences of the stress threshold are calculated by extending LVE theory and time–temperature superposition principles (TTSP) to the energy relations. Reasonable agreement between experimental data and calculations is obtained. It is shown that numbe…

1997

We have determined by forced Rayleigh scattering the diffusion coefficients of photo-labeled polystyrene micronetwork spheres (radii ≤ 10 nm) in melts of linear polyvinylmethylether (M W ≤ 40000 g/mol) at temperatures of 20-80°C. An expected slippage of the spheres through the meshes of the entanglement network appears possible but is still within the experimental uncertainty of our results.

Interfacial tension and interaction parameters

1994

In view of various contradictory theoretical equations relating the interfacial tension σ between phase-separated polymer solutions to the Flory-Huggins interaction parameter of the system, the idea that σ should depend on the extent of the ‘hump’ in the concentration dependence of the Gibbs energy of mixing was studied. This investigation is based on 12 series of σ( T ) measurements reported in the literature and on additional experiments for the system cyclohexane/polystyrene. To quantify the extent of the ‘hump’, a reduced ‘hump energy’ ɛ was introduced and its value calculated for different temperatures from the measured critical data of the systems. The analysis of the entire experimen…

Calorimetric and Volumetric Investigations of the Effect of the Hydrophobicity of the Surfactant on the Binding between (Ethylene oxide)13-(propylene…

2004

The enthalpy and the volume of transfer (DeltaY(t)) of the unassociated (ethylene oxide)(13)-(propylene oxide)(30)-(ethylene oxide)(13) (L64) from water to the aqueous sodium alkanoate solutions as functions of the surfactant concentrations (m(S)) were determined at 298 K. The surfactants studied are sodium hexanoate, sodium heptanoate, sodium octanoate, sodium undecanoate, and sodium dodecanoate. As a general feature, for the short alkyl chain surfactants, DeltaY(t) describes an S-shaped curve in the range of m(S) analyzed whereas for the more hydrophobic surfactants the DeltaY(t) vs m(S) trends exhibit maxima which appear at ms values very close to the critical micellar concentration in w…

Application of time-temperature superposition to energy limit of linear viscoelastic behavior

2009

The energy approach for evaluation of the limits of linear viscoelastic (LVE) behavior is considered. The approach of Foux and Bruller based on the Reiner-Weissenberg dynamic theory of strength is developed for the temperature effect. Value of the stored energy at the limit of LVE is considered as the material characteristic independent on loading conditions and temperature. Time–temperature superposition principle is extended for the energy calculations. Curves of the stored energy calculated for different temperatures are shifted to each other in the logarithmic time axes similarly as creep compliance and relaxation modulus curves in creep and tension tests, respectively. Temperature is c…

Statistical Analysis of a Method to Predict Drug–Polymer Miscibility

2015

In this study, a method proposed to predict drug-polymer miscibility from differential scanning calorimetry measurements was subjected to statistical analysis. The method is relatively fast and inexpensive and has gained popularity as a result of the increasing interest in the formulation of drugs as amorphous solid dispersions. However, it does not include a standard statistical assessment of the experimental uncertainty by means of a confidence interval. In addition, it applies a routine mathematical operation known as "transformation to linearity," which previously has been shown to be subject to a substantial bias. The statistical analysis performed in this present study revealed that t…

Temperature-controlled poly(propylene) glycol hydrophobicity on the formation of inclusion complexes with modified cyclodextrins. A DSC and ITC study.

2011

The study highlighted the main forces driving the formation of hydroxypropyl-cyclodextrins (HP-CDs) + poly(propylene) glycol 725 g mol(-1) inclusion complexes. The temperature parameter was chosen as the variable to modulate the hydrophobicity of the polymer, and consequently ITC experiments as functions of temperature as well as DSC measurements were done in a systematic way. The polymer is not included into HP-α-CD, it is strongly bound to HP-β-CD and it is floating in HP-γ-CD. The stability of the inclusion complexes is entropy controlled. The gain of the entropy is a unique result compared to the opposite literature findings for inclusion complexes based on polymers and CDs. This peculi…