Glass transition investigated by a combined protocol using thermostimulated depolarization currents and differential scanning calorimetry

Relaxation times of bisphenol A polycarbonate around the glass transition temperature are estimated using the combination of differential scanning calorimetry (DSC) and thermostimulated depolarization currents (TSDC). These measurements are performed using samples with different thermal histories below and above the vitrification transformation. This protocol enables the extension of the range of equilibrium relaxation times measured by dielectric spectroscopy. By this mean we may recalculate the values of the Kauzmann temperature and fragility index

Structural Dependence of the Molecular Mobility in the Amorphous Fractions of Polylactide

Fragility index and cooperativity length characterizing the molecular mobility in the amorphous phase are for the first time calculated in drawn polylactide (PLA). The microstructure of the samples is investigated from wide-angle X-ray scattering (WAXS) whereas the amorphous phase dynamics are revealed from broadband dielectric spectroscopy (BDS) and temperature-modulated differential scanning calorimetry (TMDSC). The drawing processes induce the decrease of both cooperativity and fragility with the orientation of the macromolecules. Post-drawing annealing reveals an unusual absence of correlation between the evolutions of cooperativity length and fragility. The cooperativity length remains the same compared to the drawn sample while a huge increase of the fragility index is recorded. By splitting the fragility index in a volume contribution and an energetic contribution, it is revealed that the amorphous phase in annealed samples exhibits a high energetic parameter, even exceeding the amorphous matrix value. It is assumed that the relaxation process is driven in such a way that the volume hindrance caused by the thermomechanical constraint is compensated by the acceleration of segmental motions linked to the increase of degrees of freedom. This result should also contribute to the understanding of the constraint slackening in the amorphous phase during annealing of drawn PLA, which causes among others the decrease of its barrier properties

Fragility of a thermoplastic polymer. Influence of main chain rigidity in polycarbonate

We present new data on a vitreous polycarbonate (PC) and its fragility index. Measurements have been performed by means of differential scanning calorimetry (DSC). A comparison with other data already published in this field and concerning other linear polymers is made. We show that when experiments are performed by means of DSC, the use of the glass-forming liquid fragility concept does not lead to large enough variations of the fragility index values. Thus, any correlation with structural characteristics of linear polymers, is not possible, except in the case of main chain rigidity

Study of poly(bisphenol A carbonate) relaxation kinetics at the glass transition temperature

In this work, the variations of the relaxation times are investigated above and below the glass transition temperature of a model amorphous polymer, the polycarbonate. Three different techniques (calorimetric, dielectric and thermostimulated currents) are used to achieve this goal. The relaxation time at the glass transition temperature was determined at the temperature dependence convergence of the relaxation times calculated with dynamic dielectric spectroscopy (DDS) for the liquid state and thermostimulated depolarisation currents (TSDC) for the vitreous state. We find a value of s(Tg) = 110 s for PC samples. The knowledge of the temperature dependence, s(T), and the value s(Tg) enables to determine the glass-forming liquid fragility index, m. We find m = 178 ± 5

From Monomers to Self-Assembled Monolayers: The Evolution of Molecular Mobility with Structural Confinements

The effect of structural constriction on molecular mobility is investigated by broadband dielectric spectroscopy (BDS) within three types of molecular arrangements: monomers, oligomers and self-assembled monolayers (SAMs). While disordered monomers exhibit a variety of cooperative and local relaxation processes, the constrained nanodomains of oligomers and highly ordered structure of monolayers exhibit much hindered local molecular fluctuations. Particularly, in SAMs, motions of the silane headgroups are totally prevented whereas the polar endgroups forming the monolayer canopy show only one cooperative relaxation process. This latter molecular fluctuation is, for the first time, observed independently from other overlapping dielectric signals. Numerous electrostatic interactions among those dipolar endgroups are responsible for the strong cooperativity and heterogeneity of the canopy relaxation process. Our data analyses also revealed that the bulkiness of dipolar endgroups can disrupt the organization of the monolayer canopy thus increasing their ability to fluctuate as temperature is increased

Enhancement of the physical and chemical stability of amorphous drug-polymer mixtures via cryogenic comilling

Amorphous dispersions of the Biclotymol antiseptic are obtained in polyvinylpyrrolidone (PVP), for different drug loadings, by comilling at temperatures below the glass transition of both components. Characterization of the dispersions by scanning differential calorimetry and temperature-dependent broadband dielectric spectroscopy shows them to be homogeneous amorphous molecular mixtures. A single glass transition with pronounced enthalpy recovery peak is observed, indicative of the formation of a homogeneous glass state characterized by substantial aging. The polymer has a considerable antiplasticizing effect on the drug, increasing its glass transition temperature (Tg) to well above room temperature. The Tg of the mixture increases linearly with the macromolecular mass fraction. Three (macro)molecular relaxations are identified in the isothermal dielectric spectra of the mixtures. The slowest process corresponds to the cooperative (a relaxation) dynamics of the polymer chains and drug molecules simultaneously, and its relaxation time becomes longer the higher the polymer content. The fastest one is an intramolecular relaxation mode of Biclotymol and is virtually unaffected by the presence of the polymer. Finally, the intermediate relaxation, which is also observed in pure PVP, is assigned to the motion of the polymer side groups. Its activation barrier increases with increasing drug content, indicative of a direct interaction of the drug with the pyrrolidone moieties, likely via hydrogen bonding to the carbonyl oxygen. Contrary to pure PVP, the mixtures are not hygroscopic. Mechanical amorphization by cryomilling thus yields glassy molecular mixtures at arbitrary drug concentration, with enhanced physical stability against crystallization and enhanced chemical stability against hydrolysis reactions.Peer ReviewedPostprint (author's final draft

Amorphous rigidification and cooperativity drop in semi−crystalline plasticized polylactide

Plasticization of amorphous polylactide shifts the glass transition and extends its temperature range of crystallization to lower temperatures. In this work, we focus on how low−temperature crystallization impacts the mobility of the amorphous phase. Plasticizer accumulates in the amorphous phase because it is excluded from the growing crystal. The formation of rigid amorphous fraction is favored by the low crystallization temperature. It reaches values up to 50% in plasticized polylactide. The increase in the content of rigid amorphous fraction coincides with both the increase of free volume quantified by positron annihilation lifetime spectroscopy, and the decrease in the cooperativity length obtained from the temperature fluctuation approach. The drop of cooperativity is interpreted in terms of mobility gradient due to the amorphous rigidification

Etude de l'hétérogénéité et de la métastabilité de phases amorphes polymères par analyse des transitions et des phénomènes relaxationnels lents

TOULOUSE3-BU Sciences (315552104) / SudocSudocFranceF

Etude de l'hétérogénéité et de la métastabilité de phases amorphes polymères par analyse des transitions et des phénomènes relaxationnels lents

TOULOUSE3-BU Sciences (315552104) / SudocSudocFranceF

Secondary Retardation/Relaxation Processes in Bisphenol A Polycarbonate: Thermostimulated Creep and Dynamic Mechanical Analysis Combined Investigations

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