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Glass transition phenomena of four binary systems composed of simple hydrocarbons were studied by means of the differential thermal analysis (DTA). For all the systems, a definite glass transition was observed and a monotonous relation between the glass transition temperature (T g) and composition (x) was obtained. The composition dependence ofT g was analyzed in terms of the entropy theory based on the regular solution model. The theoretical prediction could not reproduce our results other than (1-butene)x(1-pentene)1−x system. This disagreement is considered to be due to deviations of the present systems from the regular solution, and the accompanying excess configurational entropy Sc E was estimated as a function of composition. Extraordinarily large values of Sc E? were obtained for (propene)x(propane)1−x and (propene)inx(1-pentene)1−x systems.

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Journal of Thermal Analysis and Calorimetry
Authors:
N. Delpouve
,
C. Lixon
,
A. Saiter
,
E. Dargent
, and
J. Grenet

Abstract  

Temperature modulated differential scanning calorimetry (TMDSC) and dynamic mechanical analysis (DMA) are used to calculate cooperative rearranging region (CRR) average sizes for drawn poly(ethylene terephthalate) (PET) with different draw ratios (λ) ranging from λ=1 to 4, according to Donth’s approach. It is shown for both studies that the CRR size decreases when increases, due to the amorphous phase confinement by the crystals generated during the drawing. However, differences observed between the values calculated from TMDSC and DMA investigations are explained by the differences between a mechanical uniaxial dynamic solicitation (DMA) or a thermal solicitation (TMDSC) in terms of cooperative rearrangements at the glass transition.

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Abstract  

We have used molecular simulations to study the properties of nanocomposites formed by the chemical incorporation of polyhedral oligomeric silsesquioxane (POSS) particles in the cross-linked epoxy network. The particular POSS molecule chosen—glycidyloxypropyl-heptaphenyl POSS—can form only one bond with the cross-linker and thus was present as a dangling unit in the network. Four epoxy-POSS nanocomposites containing different fractions (up to 30 mass/%) of POSS particles were studied in this work. Well-relaxed atomistic model structures of the nanocomposites were created and then molecular dynamics simulations were used to characterize the density, glass transition temperature (T g), and the coefficient of volume thermal expansion (CVTE) of the systems. In addition to the effect of nanoparticle loading, the effect of nanoparticle chemistry on the nanocomposite properties was also characterized by comparing these results with our previous results (Lin and Khare, Macromolecules 42:4319–4327, 2009) on neat cross-linked epoxy and a nanocomposite containing a POSS nanoparticle that formed eight bonds with the cross-linked network. Our results showed that incorporation of these monofunctional POSS particles into cross-linked epoxy does not cause a measurable change in its density, glass transition temperature, or the CVTE. Furthermore, simulation results were used to characterize the aggregation of POSS particles in the system. The nanofiller particles in systems containing 11, 20, and 30 mass/% POSS were found to form small clusters. The cluster-size distribution of nanoparticles was also characterized for these systems.

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At temperatures above 350° polypyromellitimides exhibit continuously increasing deformations. The temperatures of glass transition and melting were determined in their homologous series.

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Abstract  

An inherent challenge with polymer blends is the difficulty in resolving the glass transition, T g, for the smaller mass fraction component. The objective of this work was to determine the practical scanning conditions for identifying the dual T g’s for a 75:25 polycarbonate/acrylonitrile-butadiene-styrene (PC/ABS) blend using a thermomechanical analyzer (TMA). Scanning rates up to 20°C min−1 using dilatometer and expansion modes were studied. Heating and cooling rates were found to affect both T g values but the effects were not simple relationships. T g values could either increase or decrease depending on the scanning rate applied. Higher rates resulted in large thermal lags which opened the accuracy of measurements to question. Generally, higher rates tended to display only one T g but the duality of T g’s can be detected with scanning rates between 0.5 and 5°C min−1 for both modes.

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Abstract  

The thermoelastic behaviour of an unfractionated polystyrene was studied in the temperature interval 353–453 K in the regimes of isobaric cooling and of isothermal quasi-adiabatic loading, respectively. The main experimental results can be summarized as follows. 1. In the temperature interval far above the glass transition temperature Tg, both the temperature and volume relaxations of the polystyrene melt after sudden pressure jumps were completely reversible and proved to be simple exponential functions of the time. Therefore, by a straightforward application of Eqs (1) and (2) to the relevant thermoelastic data obtained in a single experimental run one can arrive at the reasonable values of the specific volume, specific heat capacity, thermal diffusivity and heat conductivity of the polymer in the equilibrium melt state. 2. In the temperature interval close to Tg, both the temperature and volume relations of the supercooled polystyrene melt in compression/expansion cycles became markedly asymmetric and non-exponential. The low values of the exponent β in the fractional-exponent Eq. (5) for the volume relaxation suggest a broad spectrum of relaxation times indicating the high degree of coupling between different mechanisms of the molecular motions involved.

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Journal of Thermal Analysis and Calorimetry
Authors:
Yanni Qi
,
Jian Zhang
,
Shujun Qiu
,
Lixian Sun
,
Fen Xu
,
Min Zhu
,
Liuzhang Ouyang
, and
Dalin Sun

Abstract  

Polyaniline/NiO (PANI/NiO) composites were synthesized by in situ polymerization at the presence of HCl (as dopant). FTIR, TEM and XRD were used to characterize the composites. Thermogravimetry (TG)–mass spectrometer (MS) and temperature modulated differential scanning calorimetry (TMDSC) were used to study the thermal stability, decomposition and glass transition temperature (T g) of the composites, respectively. FTIR and XRD results showed that NiO nanoparticles connected with PANI chains in the PANI/NiO composites. TEM results exhibited that the morphologies of PANI/NiO composites were mostly spherical, which were different from the wirelike PANI. TG–MS curves indicated that the products for oxidative degradation of both PANI and PANI/NiO composite were H2O, CO2, NO and NO2. TG curves showed that with NiO contents increased in PANI/NiO composites, thermal stability of PANI/NiO composites increased firstly and then decreased when the NiO content was higher than 66.2 wt%. T g of PANI/NiO composites also increased from 163.19 to 252.36 °C with NiO content increasing from 0 to 50 wt%, and then decreased with NiO content increasing continuously.

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Differential scanning calorimetry (DSC) measurements have been carried out on a series of ABA poly(styrene-b-isoprene) triblock copolymers with 30% polyisoprene content and various molecular weights. The DSC data show an inward shift for the glass transition temperatures (T g) of the blocks compared to the corresponding homopolymers. As a function of the molecular weight, one to three transitions were found. The additional thirdT g gives some further evidence of the existence of an interphase between the microdomains.

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Abstract  

The miscibility of poly(N-isopropylacrylamide) (PNIPA) with poly(vinyl pyrrolidone) (PVP) and a cross-linked poly(acrylic acid) (Carbopol 971P) was evaluated from the rheological data of aqueous dispersions and the temperature of glass transitions of films made of binary mixtures. PNIPA has a low critical solubility temperature (LCST) of about 33C, below which 1% dispersion behaves as a viscous system. At temperatures above LCST, the hydrophobic interactions among the isopropyl groups initially provide transient networks of greater elasticity. The LCST of PNIPA as well as its T g (144C, estimated by DSC and MTDSC of films) were not modified by the presence of PVP. The immiscibility of PNIPA and PVP was confirmed by the absence of interaction between both polymers as shown by FTIR analysis of the films. In contrast, PNIPA and carbopol were miscible and the behaviour of their mixtures differed significantly from that of the parent polymers; i.e. a strong synergistic effect on the viscoelasticity of the dispersions was observed below the LCST. As temperature increased, the blends showed a decrease in the loss and storage moduli, especially those with greater PNIPA proportions. The fall was smoother as the PNIPA proportion decreased. This behaviour may be explained as the result of the balance between PNIPA/carbopol hydrogen bonding interactions (as shown in the shift of C=O stretch in FTIR spectra) and PNIPA/PNIPA hydrophobic interactions. The T g values of the films of the blends showed a positive deviation from the additivity rule; the mixtures containing more than 1:1 amide:carboxylic acid groups have a notably high Tg (up to 181C). This increase is related to the stiffness induced in the films by the PNIPA/carbopol interactions.

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The glass transition temperature,T g is a sensitive and practical parameter for following cure of reactive thermosetting systems. A new equation was developed for predicting theT g -conversion relationship based on the Dillman-Seferis viscoelastic compliance model. It assumes that the changes inT g are primarily due to changes in relaxation time as chain extension and crosslinking reduce the mobility of a polymer network. Such information is essential in combining kinetic and viscoelastic measurements, which monitor transformations of thermosets during cure. The equation derived from the viscoelastic model was shown to be applicable for a variety of experimental data. The success of the methodology was further demonstrated by comparing well-established relations, such as the Fox equation and the Di-Benedetto equation, to predictions made possible by adjusting two viscoelastic model parameters. Finally, the fitting power of the proposed equation was shown by fitting published epoxy data from the literature as well as experimental data on a relatively new resin system such as dicyanates used as a model in this study.

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