Papers by Keyword: Glass Transition

Abstract: In order to improve the thermal stability of PLA based materials it was followed the strategy of blending it with a polymer having a higher glass transition temperature such as poly (carbonate) of bisphenol A (PC) . PLA/PC blends with different compositions were by melt extrusion produced also in the presence of an interchange reaction catalyst, tetrabutylammonium tetraphenylborate (TBATPB) and triacetin. The dynamical mechanical thermal characterization showed an interesting change of the storage modulus behavior in the PLA glass transition region, evident exclusively in the catalyzed blends. In particular, a new peak in the Tan δ trend at a temperature in between the one of PLA and the one of PC was observed only in the blends obtained in the presence of triacetin and TBATPB. The height and maximum temperature of the peak was different after the annealing of samples at 80°C. The data showed an interesting improvement of thermal stability above the PLA glass transition, this was explained keeping into account the formation of PLA-PC copolymer during the reactive extrusion. Furthermore, the glass transition temperature of the copolymer as a function of composition was studied and the obtained trend was discussed by comparing with literature models developed for copolymers.

Abstract: Since the first paper by Keddie et al. published on 1994 [21], the glass transition of polymer systems on surface/thin film has been an active research field and attracted many groups interests. Numerous works have been done, in both experimental and computation approaches, to investigate this subject. In this paper we reviewed the milestone findings in the last twenty years. Generally with only minor disagreements in the mechanism all the mainstream works are consistent in the conclusions that: 1) Geometric confinement in thin film or on surface reduces the glass transition temperature T_g comparing to the bulk behavior; 2) For supported film the substrate-film interaction is critical and its effect may surpass the geometry effects and rise increase on T_g; 3) Chain mobility and molecular weight are critical but the detailed phenomena vary with systems. Notwithstanding the achievement has been made, due to the controversy of glass transition itself and technology limitation on characterization on glass transitions on thin film, the research in this field is still a long-marching effort and breakthrough findings are expected for the development in materials science and engineering and feedback knowledge to understand the glass transition on the theoretical base.

Abstract: We demonstrate that that a quartz crystal resonator (QCR) is applicable to investigate the glass transition of the Poly (vinyl butyral) (PVB) thin films by the simultaneous measurement of both resonant frequency and bandwidths on several overtones. The response of resonant frequency and bandwidths of film coated quartz crystal directly reflects the information about surface mass and film shear modulus. This enable us to explicitly deduce the glass transition temperature and the shear modulus parameter of PVB thin films on different overtones from the frequency shift and the bandwidths shift. These results indicate the feasibility of using QCR technique to characterize glass transition temperature and viscoelastic properties of the PVB thin films.

Abstract: The modified PFA via silica nanoparticle insertion was studied under different experimental conditions. In the key step, thermal and mechanical properties variations of the material (thermal stability, vitrification) were closely correlated with the type of the silica nanoparticles employed. To reach the results, thermal analytical experiment were conducted such as TGA or DMTA. Moreover, the silica nanoparticles surface modifications were highlighted from TGA and FTIR. Their morphology and dispersion into the polymer were shown by TEM measurements.

Abstract: By means of full atomistic molecular dynamics (MD) simulation, the solubility parameters for hydroxyl-terminated polybutadiene (HTPB), dioctyl sebacate (DOS), dioctyl adipate (DOA), dibutyl phthalate (DBP), dioctyl phthalate (DOP), nitrated esters nitroglycerine (NG) and diethylene glycol dinitrate (DEGDN) are calculated and the results are in agreement with the literature values. Furthermore, in order to reveal the HTPB/plasticizer blend property, the specific volume vs. temperature curves of the blend systems are simulated by employing MD simulation to obtain the glass transition temperature (T_g). From the specific volume vs. temperature curve, the T_g of HTPB, HTPB/DOS, HTPB/DOA, HTPB/DBP, HTPB/DOP, HTPB/NG and HTPB/DEGDN are 197.54, 176.30, 183.11, 189.27,187.40, 200.03 and 205.31 K, respectively. It should be pointed out that as for HTPB and DOS, DOA, DBP, DOP, the solubility parameters are similar and there is only one glass transition of the blend system, these indicate that these studied blend systems are miscible, but HTPB/NG and HTPB/DEGDN are not miscible.

Abstract: Glass transition temperature T_g is the most important parameter affecting the mechanical properties of amorphous and semi-crystalline polymers. However, the atomistic origin of glass transition is not yet well understood. Using Polyethylene (PE) as an example, this paper investigates the glass transition temperature T_g of PE with the aid of molecular dynamics (MD) simulation. The effects of PE chain branches, crystallinity and carbon-nanotube (CNT) additives on the glass transition temperature are analyzed. The MD simulations render a good agreement with the relevant experimental data of semi-crystalline PE and show the significant effects of crystallinity and addition of CNTs on T_g.

Abstract: Mechanical testing is foremost a means to measure material performance, however it provides a probe into the complex elastic, viscoelastic and viscoplastic behavior of polymer morphologies. The techniques in this work utilize variables of time/frequency, temperature, stress and strain with emphasis on dynamic and modulated implementation. Several instruments were used since a particular instrument does not provide all of the capabilities. The material response is complex and it has been resolved into typically instantaneous and time-dependent components. Some of the techniques are widely used and these have been extended, while other techniques introduce control over alternate variables. Polycarbonate was chosen as the main example with support from similar polymers, though the techniques are applied to many polymer types.

Abstract: The effect of the addition of 0.2-0.65 at% of V on the glass forming ability and mechanical properties of Al88Y7Fe5 alloy were investigated. The addition of V in this range had little effect on the glass forming ability of the alloy, but lowered the tensile strength of the amorphous ribbon. The fracture surface of Al88Y7Fe5 amorphous ribbons was typical vein pattern for ductile metallic glasses, however, that of the alloy with 0.5% V addition changed to two different regions, i.e., vein pattern region and smooth region. At high magnification, the smooth region was composed of nanometer sized corrugations, which is typical for brittle metallic glasses.

Abstract: The subnanoscopic structures of polyrotaxanes, prepared from α-cyclodextrins, poly(ethylene glycol), and bulky adamantane end groups, were examined by means of the positron annihilation lifetime technique, in consideration of the free-volume hole, quantified from the long-lived ortho-positronium (o-Ps) lifetimes. The influence of the chemical structure on the temperature dependence of the o-Ps lifetimes are discussed.

Abstract: We give a description of the phase transition in an ensemble of the electric dipole with internal degrees of freedom in dielectric glass model.The model predicts a freezing of the random projections of the electric dipole moments. The low temperature phase transition from disordered paraelectric phase to the electric dipole orientational like glass phase is considered