Arkadiusz Gradys

Optimization of melting conditions for the analysis of crystallization kinetics of poly(3-hydroxybutyrate)

Abstract Studies of kinetics of polymer crystallization are generally performed by heating the material above the melting point, in order to erase previous thermal and mechanical history, followed by rapid cooling to the desired crystallization temperature or by cooling at a constant rate. For poly(3-hydroxybutyrate) this procedure implies some degradation of the polymer chain, which starts below the onset of melting. In this article the effects of melting conditions on the subsequent crystallization kinetics are discussed. It is shown that in order to sufficiently cancel memories of previous crystalline order of the analyzed PHB, it is necessary to bring the material at a temperature higher than 192 °C. Thermal treatments conducted at lower temperatures are not sufficient to destroy all solid aggregates, and crystallization of PHB has an anticipated onset of crystallization due to nucleation occurring via self-seeding. The chain degradation attained upon exposure at high temperatures has much lesser influence on crystallization kinetics than incomplete melting, with some effects detectable on the spherulitic morphology and on the final degree of crystallinity.

Thermomechanical properties of polyurethane shape memory polymer–experiment and modelling

In this paper extensive research on the polyurethane shape memory polymer (PU-SMP) is reported, including its structure analysis, our experimental investigation of its thermomechanical properties and its modelling. The influence of the effects of thermomechanical couplings on the SMP behaviour during tension at room temperature is studied using a fast and sensitive infrared camera. It is shown that the thermomechanical behaviour of the SMP significantly depends on the strain rate: at a higher strain rate higher stress and temperature values are obtained. This indicates that an increase of the strain rate leads to activation of different deformation mechanisms at the micro-scale, along with reorientation and alignment of the molecular chains. Furthermore, influence of temperature on the SMP’s mechanical behaviour is studied. It is observed during the loading in a thermal chamber that at the temperature 20 °C below the glass transition temperature (Tg) the PU-SMP strengthens about six times compared to the material above Tg but does not exhibit the shape recovery. A finite-strain constitutive model is formulated, where the SMP is described as a two-phase material composed of a hyperelastic rubbery phase and elastic-viscoplastic glassy phase. The volume content of phases is governed by the current temperature. Finally, model predictions are compared with the experimental results.

On the metastability of β phase in isotactic polypropylene: experiments and numerical simulation

Abstract Phase transitions in isotactic polypropylene were investigated during isothermal crystallization and heating after isothermal crystallization using various experimental techniques. The results obtained by wide angle x-ray scattering (WAXS), light depolarization technique (LDT), differential scanning calorimetry (DSC) and optical microscopy show that crystallization of isotactic polypropylene can result in simultaneous formation of two crystal modifications, α and β. There is clear experimental evidence that β phase tends to convert into α modification during crystallization as well as during subsequent heating. Experimental results are compared with numerical simulation performed according to the model of nucleation-controlled phase transitions in multiphase systems. The results of simulation show that b phase is not thermodynamically stable in any temperature range. The reason for the appearance of β phase is related to low interfacial tension of melt vs β. It has been also shown that maximum crystallinity reached in experiments does not exceed 40-50% in agreement with the concept of constrained amorphous phase.

Determination of the melting enthalpy of β phase of poly(vinylidene fluoride)

Abstract Wide Angle X-ray Scattering (WAXS), Differential Scanning Calorimetry (DSC) and Fourier Transform Infrared spectroscopy (FTIR) analyses of phase composition and of thermal properties of PVDF samples, crystallized at temperatures 27 - 155 °C by casting from N,N-dimethyl formamide (DMF) solution, are reported. Samples obtained at 27 oC contain only β crystal phase and with increase of casting temperature content of β phase decreases in favor of α phase. Evaluation of combined: phase content (WAXS) and melting heat (DSC), leads to two fold higher than for 100 % α phase value of 100% β melting enthalpy, ΔHβ0= 219.7 J.g-1, which may be justified by strong polar interactions in β phase TTT conformation. The relation ΔHβ 0 > ΔHα0 leads either to the thermodynamic stability of β phase in whole temperature range (if Tmβ0 ≥ Tmα0) or to the limited temperature range of thermodynamic stability of α phase (if Tmβ0 < Tmα0).

Transient nucleation in isothermal crystallization of poly(3- hydroxybutyrate)

Abstract The time dependence of nucleation rate in isothermal crystallization of poly(3-hydroxybutyrate) was experimentally shown, both in heterogeneous and homogeneous nucleation. The time dependence of nucleation rate is one of the important limitations for the applicability of the simplified form of Kolmogoroff- Avrami-Evans model with time independent kinetic characteristics. The presented results are interpreted in terms of non-steady-state cluster size distribution underlying transient nature of nucleation. The relaxation time needed for reaching a steady-state cluster size distribution and thus steady-state nucleation rate is relatively long, exceeding the time of exhaustion of heterogeneities. The relaxation time estimated from homogeneous process was tens of seconds in the temperature range between 83 and 120 oC. Application of Arrhenius law allows estimation of relaxation time in broader temperature range, showing an increase of relaxation time with decreasing temperature.

OPTIMIZATION OF MELTING CONDITIONS OF POLY(3‐HYDROXYBUTYRATE)

Studies of kinetics of polymer crystallization are generally performed by heating the material above the melting point, in order to erase previous thermal and mechanical history, followed by rapid cooling to the desired crystallization temperature or by cooling at a constant rate. For poly(3‐hydroxybutyrate) (PHB) this procedure implies some degradation of the polymer chain, which starts below the onset of melting. In this contribution the effects of melting conditions on the subsequent crystallization kinetics are discussed. It is shown that in order to sufficiently cancel memories of previous crystalline order of the analyzed PHB, it is necessary to bring the material at a temperature higher than 192 °C. Thermal treatments conducted at lower temperatures are not sufficient to destroy all solid aggregates, and crystallization of PHB has an anticipated onset of crystallization due to nucleation occurring via self‐seeding. The chain degradation attained upon exposure at high temperatures has a much minor i...