Calorimetric and Dielectric Investigations of Glass Transition in Intermediate Glass-forming Liquid

The calorimetric glass transition and dielectric dynamics of -relaxation in propylene glycol (PG) and its five oligomers (polypropylene glycol, PPG) have been investigated by the modulated differential scanning calorimetry (MDSC) and the broadband dielectric spectroscopy. From the temperature dependence of heat capacity of PPGs, it is clarified that the glass transition temperature (T_g) and the glass transition region are affected by the heating rate. The kinetic changes of PG and PPGs near T_g strongly depend on the underlying heating rate. With increasing the molecular mass of PPGs, the fragility derived from the relaxation time against temperature also increases. The PG monomer is stronger than its oligomers, PPGs, because of the larger number density of the —OH end group which tends to construct the intermolecular network structure. Adam-Gibbs (AG) theory could still hold for MDSC results due to the fact that the dielectric relaxation time can be related to the configurational entropy.This revised version was published online in November 2005 with corrections to the Cover Date.     

Relaxation Processes and Glass Transition in Polymer Filled with Nanoparticles

We report the results of the investigations of the influence of filling of polymer with Aerosil nanosize particles on the glass transition and dynamics of the α- and the β-relaxation processes in poly(n-octyl methacrylate) by dielectric spectroscopy and differential scanning calorimetry (DSC). The polymer was filled with hydrophilic and hydrophobic Aerosil particles of 12 nm diameter. In filled polymers the characteristic frequency of the alpha-process was shifted to higher frequencies in comparison with pure bulk polymer at the same temperature. This suggests that the filling of the polymer with nanoparticles has resulted in the shift of its glass transition temperature T_g. This change in T_g was mainly due to the existence of a developed solid particle-polymer interface and the difference in the dynamic behavior of the polymer in the surface layers at this interface compared to the bulk behavior. This result was in agreement with DSC experiments.     

Glass Transition Temperature and Value of the Relaxation Time at Tg in Vitreous Polymers

Summary: Many works focused on glassy polymers determine values of glass transition temperature (T_g) and an overview of the literature shows that depending on the method used, values of T_g are found different for the same material. In this paper, a review of data collected on different materials are used and interpreted in term of molecular mobility characterized by relaxation time functions. By using three independent experimental procedures (dielectric, thermally depolarized current and calorimetric), we show that the value of the glass transition and the value of the relaxation time at T_g can be correctly determined. It is also shown that the assumption: τ _(Tg) = 100 s is constant, is not correct. The protocol proposed also allows the determination of the value of the fragility index “m” of the glass forming liquid with a great accuracy.     

Simulation of the Glass Transition

Enthalpic relaxation has been used to model the development of the glass transition in polymers, using kinetic parameters determined separately. For this purpose the Kohlrausch-Williams-Watt stretched exponential function, relating the extent of relaxation, Φ(t), to time t and an average relaxation time, τ_a, i.e.

Dielectric relaxation of polystyrene with side-chain- and chain-end-labels

Polystyrenes with different concentrations of side groups with cyano groups were prepared and complex dielectric constants were measured in the range of the glass transition temperature and the frequency range of 10^–2–10⁷ Hz.The GPC and DSC measurements showed that the molecular weight of these polystyrenes was about 10500 g/mole and the glass transition temperatures were 89.5°C for all samples.The dielectric relaxation spectra obtained for the side group polystyrene labels and also the chain-end polystyrene labels prepared before [9] were analyzed to find out the degree of coupling of the chain-end and side-group labels with the cooperative reorientation of the polymeric matrix. The analysis of the spectra was carried out using the analysis method developed by Mansour and Stoll [6].The results obtained showed that both end- and side-group labels are strongly coupled with the segmental reorientation and relax with relaxation times longer than that of the segments.The value of logf _m = (logf _m(label)) – logf _m(matrix)) was obtained from the recently designed comparison diagram suggested by Mansour and Stoll [6, 14]. The value of logf _m depends on the label length in the case of chain-end labels.It was surprising to find that the side groups relax slower than the segments by only 0.9 decades. These results obtained implied that the label relaxes through a multistep relaxation mechanism of the side and end groups and not through a diffusion mechanism of the whole chain. In addition, the effective lengths of the relaxing units were determined using the empirical equation obtained before in the case of rodlike molecules in polyisoprene [7].     

Glass Transition Dynamics of Poly(L‐lactic acid) during Isothermal Crystallisation Monitored by Real‐Time Dielectric Relaxation Spectroscopy Measurements

Summary: Dielectric relaxation spectroscopy was used to monitor the development of crystallinity in poly(L ‐lactic acid) (PLLA) at 80 °C. The continuous shifting of the main loss peak towards lower frequencies was modelled considering the evolution of three relaxation processes: the α‐process of the bulk‐like (non‐restricted) amorphous phase, the α‐process of the amorphous fraction influenced by the crystalline structure and the β‐relaxation. It was found that the shape parameters and the position of the loss peaks are essentially constants during crystallisation, just with their dielectric strengths varying. This indicates that (i) the two segmental dynamics process develops independently and (ii) the dynamics features of the (slower) confined amorphous phase do not change during crystallisation. Optical microscopy results showed that a correlation exists between the evolution of the spherulitic morphology and the dielectric strength of the α‐processes.

Dielectric loss in the frequency domain for PLLA during crystallisation at 80 °C and polarised optical micrographs showing the spherulitic morphology in PLLA after 45 min during isothermal crystallisation.     

An Investigation of Structure Transition in Sodium Dodecyl Trioxyethylene Sulfate／n-Butanol／n-Octane／Water System by Dielectric Relaxation Spectroscopy Method

IntroductionDielectricrelaxationspectroscopy(DRS) possess esanimportantpositioninmodern physicochemicalanalysis .Dielectricmeasurementscanbedoneinsuchawidefrequencyrangeof 10 - 5— 10 11Hz ,makingitpossibletodetectallkindsofprocessesofvarioussys tems[1] .DRSisbasedonthepolarizationtheory ,in cludingdipolereorientation polarization ,interfacialpolarization ,ionicpolarizationandelectronicpolariza tion ,ofwhichinterfacialpolarizationis prevalent.Whenthereexistsaclearphaseinterface ,aninterfa c…     

Time‐Temperature Independence of Aging‐Induced Relaxation Peak in the Glassy State

Cooling a polymer glass through the glass transition temperature and then holding the material temporarily at an aging temperature produces a localized relaxation peak on its dynamic mechanical spectra. Clear evidence is provided that the aging‐induced structural relaxation is frequency‐temperature insensitive. The aging‐associated retardation time, if there is any, should therefore not relate to an activation process over temperature barriers or originate from a manipulation of the retardation spectra of the glass and the β‐transition.     

Relations between Glass Transition Temperatures in Miscible Polymer Blends and Composition: From Volume to Mass Fractions

The use of volume fractions in the empirical mixing laws to predict the glass transition temperatures (Tg) of polymer blends provides good agreement with experimental values, even for polymer systems with different densities. No adjustment parameter is therefore required whereas Gordon-Taylor and Kwei equations based on weight fractions need the use of a fitting parameter which has to be determined from experimental data. This assumption was validated from Tg measurements through DSC experiments conducted on PMMA /PVDF blends which have significantly different densities.     

Dielectric relaxations of chitosan: The effect of water on the α‐relaxation and the glass transition temperature

In this work thermal relaxations of chitosan are reported by using a novel methodology that includes subtraction of the dc conductivity contribution, the exclusion of contact and interfacial polarization effects, and obtaining a condition of minimum moisture content. When all these aspects are taken into account, two relaxations are clearly revealed in the low frequency side of the impedance data. We focus on the molecular motions in neutralized and non‐neutralized chitosan analyzed by dielectric spectroscopy in the temperature range from 25 to 250 °C. Low and high frequency relaxations were fitted with the Havriliak and Negami model in the 10^?1 to 108 Hz frequency range. For the first time, the low frequency α‐relaxation associated with the glass‐rubber transition has been detected by this technique in both chitosan forms for moisture contents in the range 0.05 to 3 wt % (ca. 18–62 °C). A strong plasticizing effect of water on this primary α‐relaxation is observed by dielectric spectroscopy and is supported by dynamic mechanical analysis measurements. In the absence of water (<0.05 wt %) the α‐relaxation is obscured in the 20–70 °C temperature range by a superposition of two low frequency relaxation processes. The activation energy for the σ‐relaxation is about 80.0–89.0 kJ/mol and for β‐relaxation is about 46.0–48.5 kJ/mol and those values are in agreement with that previously reported by other authors. The non‐neutralized chitosan possess higher ion mobility than the neutralized one as determined by the frequency location of the σ‐relaxation. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2259–2271, 2009     

Physical aging in polycarbonate nanocomposites containing grafted nanosilica particles: A comparison between enthalpy and yield stress evolution

Understanding and controlling physical aging below the glass transition temperature (T_g) is very important for the long‐term performance of plastic parts. In this article, the effect of grafted silica nanoparticles on the physical aging of polycarbonate (PC) below the T_g is studied by using the evolution of the enthalpy relaxation and the yield stress. The nanocomposites were found to reach a thermodynamic equilibrium faster than unfilled PC, implying that physical aging is accelerated in presence of grafted nanosilica particles. The Tool‐Narayanaswamy‐Moynihan model shows that the aging is accelerated by the grafted silica nanoparticles, but the molecular mechanism responsible for physical aging remains unaltered. Furthermore, dynamic mechanical analysis shows that the kinetics of physical aging can be related to a free volume distribution or a local attraction‐energy distribution as a result of the change in mobility of the polymer chain. Finally, a qualitative equivalence is observed in the physical aging followed by both the enthalpy relaxation and yield stress. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 2069–2081     

Molecular motion during physical aging in polystyrene: Investigation using probe reorientation

A photobleaching method has been used to observe the reorientation of tetracene and rubrene in polystyrene during physical aging. Rotation times change more than an order of magnitude during isothermal aging after a temperature quench from above T_g. Down‐ and up‐jumps of the temperature show the expected asymmetry due to the nonlinearity of the aging process. The rotation times of tetracene and rubrene require the same amount of time to reach equilibrium after a temperature change (10³ − 10⁵ s in the range 93–99 °C). These equilibration times are the same order of magnitude as equilibration times for volume and enthalpy relaxation, but have a somewhat weaker temperature dependence. Very near equilibrium, the rates of aging are different for the two probes, with rubrene approaching equilibrium more rapidly at very long times. This may be understood if the aging process is spatially heterogeneous, that is, if aging occurs more rapidly in some small regions of the sample than in others. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 68–79, 2000     

Microwave Accelerated Polymerization of 2‐Phenyl‐5,6‐dihydro‐4H‐1,3‐oxazine: Kinetics and Influence of End‐Groups on Glass Transition Temperature

Summary: The ring‐opening polymerizations of 2‐phenyl‐5,6‐dihydro‐4H‐1,3‐oxazine (PhOZI) with methyl tosylate (MeOTs) and butyl iodide (BuI) as initiators were performed in refluxing butyronitrile. Reaction kinetics under microwave irradiation was compared with conventional oil bath heating. The polymerization rates, under microwave irradiation, showed an acceleration by a factor of 1.8 (independently from the used initiator). The investigation of the thermal properties of the obtained poly(N‐benzoyl‐trimethyleneimine) showed the influence of molecular weight and end‐groups on the glass transition temperature.

The ring‐opening polymerizations of 2‐phenyl‐5,6‐dihydro‐4H‐1,3‐oxazine performed in refluxing butyronitrile.