Enthalpy Relaxation Kinetics Research Articles

A comparative DSC method for physical aging measurement of polymers

A method of comparative DSC is proposed for the study of physical aging. The essentials of the comparative DSC technique consists of comparing the enthalpy relaxation behaviour of an aged sample with a quenched sample as reference. The accuracy of comparison is improved because of simultaneous measurement of the sample and reference which makes the noise of the apparatus subtractive. This technique has been applied to study the influence of crosslinking on the kinetics of enthalpy relaxation of epoxy resins.

Kinetics of enthalpy relaxation upon physical aging in glassy main-chain nematic polymers

Kinetics of enthalpy relaxation upon physical aging of glassy thermotropic nematic main-chain polyesters is followed by DSC. Aging temperatures T a are typically in the range T g -T a =10-20 K. Two fully aromatic semiflexible polymers and two inherently flexible polymers based on mesogens connected via aliphatic spacers are studied. In the latter case, the chains are randomly coiled in the isotropic melt but behave as semileflexible, rodlike materials in the nematic phase. A fractionated polystyrene is used as a reference conventional glass

Glass transition and enthalpy relaxation process in coal tar pitches

Coal tar pitches generally have very complex chemical compositions and structures. Their physico-chemical properties depend on the mutual molecular interactions of their various components. In order to characterize these materials further the properties related to their glass transition behaviour have been investigated. As for polymeric materials, enthalpy relaxation can provide useful information on the interactions occurring in a glassy-type structure. Glass transition characteristics have been determined by means of differential scanning calorimetry. The kinetics of enthalpy relaxation have been analysed with the model developed by Moynihan et al. Mean molecular size and molecular size distribution have been determined for a coal tar and different types of pitches. The glass transition characteristics have been compared in some cases to results obtained by other techniques. It is shown that the investigation of tars and pitches by means of their glass transition properties leads to a good description of their structure and changes occurring during thermal treatment.

Physical aging studies in amorphous poly(ethylene terephthalate) (PET) blends

Physical aging phenomena of amorphous poly(ethylene terephthalate) (PET) blends have been investigated. Blends modified with dioctylphthalate (DOP), a miscible plasticizer, have shown a significant reduction in the glass transition temperature, while rubber modified blends exhibit no change in T g. Enthalpy relaxation kinetics are well represented by Δ H= Δ H a[κ—exp(— t/τ]. The apparent recoverable enthalpy Δ H a reaches a maximum at about 18°C below T g The slope of the relaxation curves decreases exponentially with T g – T a Addition of rubber modifiers strongly suppresses the extent of enthalpy relaxation. High pressure treatment of fully aged samples leads to temporary erasure of the aging endotherm. Concomitant changes in mechanical properties have been detected by dynamic mechanical analysis and mechanical testing coupled with scanning electron microscopy.

The structural “memory” of amorphous polystyrene

Data on the temperature dependence of effective viscosity, specific volume, compressibility and thermal conductivity of the melt, kinetics of enthalpy relaxation in the glass transition range and heats of solution of amorphous polystyrene (PS) samples with various prehistories have been analyzed. It was conducted the incomplete restoration of the initial tangled network in a sample prepared from good solvents is perhaps due to the appearance of a fraction of rheologically less active entanglements.

Kinetics of enthalpy relaxation in glass transition of flexible-chain polymers

Heat capacity has been measured between 308 and 523 K for a number of polymers: four narrow-distribution polystyrene fractions and a non-fractionated polystyrene sample; statistical copolymers of styrene with methyl methacrylate and styrene with acrylonitrile; polycarbonate and polysulphone. The samples were first cooled from the melt at different rates. Parameters in the Moynihan model of enthalpy relaxation accompanying the glass transition were determined for the studied polymers. Empirical correlations were established between a phenomenological parameter characterizing the width of spectrum of relaxation times and the polymer molecular weight and, on the other hand, the dimensionless ratio ΔE/ RT g .