Influence of glass transition temperature of crosslinked unsaturated polyester resin/styrene formulations on the final conversion after an isothermal curing at 100°C

SMC (sheet molding compound) is a composite based on fibers‐reinforced unsaturated polyester (UP) resin molded usually at 140°C to 170°C under a pressure of 60 to 100 bars. In order to develop new SMC formulations that can be molded at lower temperature (100°C) for economic and environmental reasons, the formulation of the composite had to be completely modified, both to allow a rapid reaction at 100°C, but also to avoid a vitrification phenomenon due to the fact that the glass transition temperature (Tg) of the SMC parts becomes, during the molding process, higher than the mold temperature. In this paper, the relation between the molding temperature, the glass transition temperature, and the final conversion of UP resin/styrene formulations has been underlined. The Tg of the cured resin was decreased by two different ways. The first way involved the reduction of the crosslinking density of the UP resin by using a blend of two resins, a pure maleic and a more flexible one. This blend allows to adjust the Tg over a temperature range from 197°C (Tg of the pure UP resin) to 75°C (Tg of the pure flexible resin). The second way consisted in the addition of butyl methacrylate (BuMA), a reactive plasticizer, to the formulation, allowing a decrease of the final material's Tg from 197°C to 130°C by replacing 35 wt% of styrene by BuMA. These two methods allow to obtain a final conversion of 99% after 8 minutes of molding at 100°C.     

Effect of the initiator content and temperature on the curing of an unsaturated polyester resin

The influence of temperature and the initiator concentration on the curing of an unsaturated polyester resin was studied by means of differential scanning calorimetry (DSC) and Fourier‐transform infrared spectroscopy (FTIR). It was established that there is an isoconversional relationship of the type lnt = a − bln[I]₀ between the curing time, t, and the initial initiator concentration, [I]₀, at a given temperature. This relationship indicates that the degree of conversion curves vs. the logarithm of the curing time at different [I]₀ may be superposed by displacement relative to a reference curve. It was confirmed that the reaction mechanism varies throughout the whole curing process, although it does not vary with the temperature and the [I]₀ at each degree of conversion. It was established that there is a universal isoconversional relationship of the type lnt = d − bln[I]₀ + E/RT that expresses the dependency of the curing time on the temperature, T, and the [I]₀. The parameters a, b, and d depend on the reaction mechanism, and can be calculated on the basis of isothermal experiments at different temperatures and with different [I]₀. The adjustment lnt = d − bln[I]₀ + E/RT shows that there is an equivalence between the effect on the curing kinetics of the temperature and the initiator concentration. The same curing process can be achieved by using different combinations of curing temperatures and the [I]₀. In the two adjustments established, it is not necessary to know the reaction mechanism, and the only assumption made is that for a given conversion the reaction mechanism is invariant with respect to the [I]₀ and the temperature. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 751–768, 1999     

Reversibility of the low‐temperature transitions of polytetrafluoroethylene as revealed by temperature‐modulated differential scanning calorimetry

Temperature‐modulated differential scanning calorimetry reveals distinct differences in the kinetics of the low‐temperature phase transitions of polytetrafluoroethylene. The triclinic to trigonal transition at 292 K is partially reversible as long it is not complete. As soon as the total sample is converted, supercooling is required to nucleate the reversal of the helical untwisting involved in the transition. The trigonal phase can be annealed in the early stages after transformation with a relaxtion time of about 5 minutes. The dependence of the reversing heat capacity on the modulation amplitude, after a metastable equilibrium has been reached, is explained by a non‐linear, time‐independent increase of the heat‐flow rate, perhaps caused by an increased true heat capacity. The order‐disorder‐transition at 303 K from the trigonal to a hexagonal condis phase is completely reversible and time‐independent. It extends to temperatures as low as the transition at 292 K or even lower. Qualitatively, the thermal history and crystallization conditions of polytetrafluoroethylene do not affect the transition kinetics, that is, melt‐crystallized film and as‐polymerized powders show similar transition behaviors, despite largely different crystallinities. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 750–756, 2001     

The Use of Modulated Temperature Programs in Thermal Methods

Temperature modulation has long been used in various aspects of thermal methods. Historically, the principle areas of application have been the determination of kinetic parameters using variants of the temperature jump method and the measurement of heat capacity by AC calorimetry. More recently the introduction of temperature modulation in a variety of techniques has been used in combination with deconvolution algorithms to separate sample responses that are dependent on rate of change of temperature from those principally dependent on temperature. Finally, temperature modulation is important in the new field of micro-thermal analysis. This revised version was published online in July 2006 with corrections to the Cover Date.     

Expansion and Shrinkage of Fibers: Load- and temperature modulated TMA measurements temperature calibration of fiber attachments

The thermal behavior of a drawn PET fiber has been investigated by thermomechanical analysis, TMA, and by differential scanning calorimetry, DSC. Above the glass transition temperature of 79°C, the fiber shrinks to a maximum of 8% of the initial length. Temperature modulated TMA enabled the separation of the thermal expansion from the overlapping shrinkage during the first heating and to calculate the expansivity, _e and the shrinkage coefficient, _s, independent of each other. Young's modulus, E, was measured by TMA with modulation of the tensile stress. Hence, it was possible to record the behavior of _e, _s and E during the structural changes by combining both modulations in a single measurement.A new technique was developed to calibrate the sample temperature. With this, accurate control of the modulated temperature of the specimen was achieved, independent of the changing heating rate.This revised version was published online in November 2005 with corrections to the Cover Date.     

差示扫描量热法测定对乙酰氨基酚原料药纯度

建立差示扫描量热(DSC)法测定对乙酰氨基酚原料药纯度的方法。考察升温速率、称样量、坩埚类型对测定结果的影响,确定最佳测定条件:升温速率为1.0℃/min,称样量为2.0~2.2 mg,选用Tzero密封铝坩埚作为样品盘。DSC法测定对乙酰氨基酚原料药纯度为99.91%,测定结果的相对标准偏差为0.03%(n=6),DSC法测定结果与紫外可见分光光度法测定结果(99.85%)基本一致,且DSC法测定结果的相对标准偏差较小。该方法简便、快速、准确,无需标准品,可用于对乙酰氨基酚原料药纯度的测定。     

Characterisation of different inulin samples by DSC: Influence of polymerisation degree on melting temperature

Melting behaviour of powder inulin has been studied by differential scanning calorimetry. DSC curves show two endothermic peaks, relative to water elimination and to inulin melting, respectively. The second peak is dependent on inulin type and a shift to a higher temperature is observed with increasing average polymerisation degree (DP) of the sample. For similar crystallinity index, linear relations have been underlined and so predicting inulin mean DP can be done by DSC analysis. The study shows that a relatively high heating rate (25°C min^–1) can be used and brings a supplementary interest by an important reduction of analysis time.This revised version was published online in November 2005 with corrections to the Cover Date.     

Determination of the stability of omeprazole by means of differential scanning calorimetry

Differential scanning calorimetry was used to study the stability of omeprazole in two forms: granules and powder. The drug was subjected to light, elevated temperature (40 and 60°C) and different pH values. The greatest alterations in stability were caused by pH, followed by light.     

Synthesis and nonlinear optical properties of novel Y‐type polyester containing tricyanovinylthiazolylazoresorcinoxy group with enhanced thermal stability of dipole alignment

Novel Y‐type polyester 4 containing 5‐methyl‐4‐{5‐(1,2,2‐tricyanovinyl)‐2‐thiazolylazo}resorcinoxy groups as nonlinear optical (NLO) chromophores, which are parts of the polymer backbone, was prepared, and its NLO properties were investigated. Polyester 4 is soluble in common organic solvents such as N,N‐dimethylformamide and dimethylsulfoxide. Polymer 4 shows a thermal stability up to 250 °C from thermogravimetric analysis with glass‐transition temperature obtained from differential scanning calorimetry of approximately 94 °C. The second harmonic generation (SHG) coefficient (d₃₃) of poled polymer film at 1560‐nm fundamental wavelength is 8.12 × 10^?9 esu. The dipole alignment exhibits a thermal stability even at 6 °C higher than glass‐transition temperature (T_g), and no significant SHG decay is observed below 100 °C due to the partial main‐chain character of polymer structure, which is acceptable for NLO device applications. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Synthesis and investigation of polyphenyldimethylsiloxane block copolymers with bimodal chain length distribution of oligodimethylsiloxane blocks

Siloxane block copolymers containing linear dimethylsiloxane (DMS) and cyclolinear phenylsilsesquioxane blocks were synthesized. A peculiarity of the copolymers is a bimodal distribution of linear DMS blocks in the polymer chain. The results of X-ray diffraction. thermomechanical, and DSC studies of bimodal block copolymers indicate a higher degree of microphase separation of the blocks as compared to unimodal block copolymers. Published inIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1542–1546, September, 2000.     

Calculation of smoothing factors for the comparison of DSC results

Complex mixtures of long chain organic compounds often show overlapping glass transition temperatures (T _gs) when analyzed by differential scanning calorimetry (DSC) or modulated DSC (MDSC). In such cases, subjective and inconsistent smoothing of data acquired under different conditions can lead to the misinterpretation of results. A quantitative method for the selection of smoothing factors for the analysis and comparison of (M)DSC results is presented. The method is most useful for the analysis of the derivative of the heat capacity, dC _p/dt or dC _p/dT, plots which best highlight overlapping T _gs. Four equations are shown to relate the heating rate and the smoothing factor. The equations allow a comparison of data acquired i) at different heating rates and plotted vs. temperature, ii) at a single heating rate and plotted vs. both time and temperature, i.e., dC _p/dt vs. dC _p/dT, iii) at different heating rates and plotted vs. both time and temperature, and iv) at different heating rates, and shown exclusively in the time domain. Examples of the use of the equations are provided for the analysis of bitumen, a complex mixture of natural origin.     

Determination of dual glass transition temperatures of a PC/ABS blend using two TMA modes

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.     

Glass transition temperature of thin polycarbonate films measured by flash differential scanning calorimetry

Flash differential scanning calorimetry was used to study the glass transition temperature T_g of polycarbonate ultrathin films. The investigation was made as a function of film thickness from 22 to 350 nm and over a range of cooling rates from 0.1 to 1000 K/s. Polycarbonate spin cast films were floated on a layer of grease on the calorimetric chip. The results show a greatly reduced glass temperature for the thinnest films relative to the macroscopic value. We also observed that the magnitude of the glass temperature reduction decreases as the cooling rate increases with the highest cooling rates showing little thickness dependence of the T_g. Dynamic fragility and activation energy at T_g were found to decrease with decreasing film thickness. The results are discussed in the context of literature reports for supported and freely standing polycarbonate films. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 1462–1468     

Sub-zero temperature behaviour of non-ionic microemulsions in the presence of propylene glycol by DSC

The five-component system is quite unique since it allows formation of reverse micelles with hydrophilic ethoxylated alcohol in the presence of ethanol and it facilitates dilution by water/propylene glycol (1,2-propanediol, PG) aqueous phase, all the way from a water-in-oil (W/O) microemulsion via a bicontinuous phase to an oil-in-water (O/W) microemulsion.The surfactant/alcohol/PG can strongly bound water in the inner phase so that it freezes below –10°C and acts in part as bound water and in part as non-freezable water. Upon dilution to >30 mass% aqueous phase (water/PG at constant mass ratio of 1/1) the system becomes bicontinuous and the aqueous layers are composed again from bound water. Even after complete inversion to O/W microemulsions the water in the continuous phase is strongly interacting with the PG/surfactant and remains bound or non-freezable. Water/PG/ethanol have a strong effect on the head groups (freezing below -10°C) and also on the hydrophobic tails (recrystallizing and melting) at lower temperature when dilution exceeds 45 mass% water/PG (1/1).No free water was detected neither in the W/O microemulsion's inner droplet domains nor when the microemulsion was either bicontinuous or when it was inversed to O/W. Continuous phase of resulting O/W microemulsion apparently is based on water/PG at a mass ratio of 1/1.This revised version was published online in November 2005 with corrections to the Cover Date.     

Mechanistic modeling of the reaction kinetics of phenyl glycidyl ether (PGE) + aniline using heat flow and heat capacity profiles from modulated temperature DSC

Modulated temperature DSC (MTDSC) has been performed on phenyl glycidyl ether (PGE) + aniline in order to obtain the non-reversing heat flow and heat capacity profiles simultaneously in a wide range of cure temperatures and mixture compositions. The epoxy (PGE) conversion as determined from the former signal corresponds to the one obtained from separate high performance liquid chromatography (HPLC), while the latter signal contains information on the individual reaction steps. Optimized kinetic parameters using a mechanistic approach, including both reactive and non-reactive complexes can successfully simulate MTDSC measurements for isothermal reaction temperatures ranging from 50 to 120 °C and for non-isothermal experiments with mixture compositions corresponding to concentrations of aniline in a range from 1.68 to 6.53 mol kg⁻¹. Concentration profiles for three mixture compositions as obtained from HPLC are also well predicted. The activation energies for the primary amine and secondary amine-epoxy reaction catalyzed by hydroxyl groups are 50 and 52 kJ mol⁻¹, respectively, while the initiation of the reaction corresponds to the primary amine-epoxy reaction catalyzed by primary amine groups with an activation energy of 72 kJ mol⁻¹. A negative substitution effect can be calculated at 0.18 from the ratio of secondary amine to primary amine-epoxy reaction rate constants.     

Synthesis of POSS–polyurethane hybrids using octakis(m‐isoprenyl‐α,α′‐dimethylbenzylisocyanato dimethylsiloxy) octasilsesquioxane (Q8M8TMI) as a crosslinking agent

A series of polyurethane hybrid networks have been synthesized using octakis(m‐isoprenyl‐α,α′‐dimethylbenzylisocyanato dimethylsiloxy) octasilsesquioxane (Q₈M₈^TMI) as a crosslinking agent. The formation of the urethane linkages within the polyurethane hybrids was confirmed by photoacoustic FTIR spectroscopy. The TGA and DSC studies demonstrated that the incorporation of the POSS crosslinking agents altered the thermal properties of the polyurethanes, and that this was dependent on the length of the polyethylene glycol chain. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 5038–5045     

Fast method for the experimental determination of vaporization enthalpy by differential scanning calorimetry

A simple method is proposed to estimate the vaporization enthalpy of the palmitic acid (hexadecanoic acid) at its normal boiling temperature. Differential scanning calorimetry (DSC) was the technique used to directly measure these thermodynamic properties. The advantages of this method are its speed and small amount of sample required. In order to avoid evaporation and to ensure equilibrium conditions, the experiments were carried out including a-alumina in contact with the fatty acid. The effect of the alumina concentration is discussed. The obtained experimental data (T_bp=625.4±0.5 K, D_vap H=237.6±5.9 J g^-1) is compared with that obtained by using thermodynamic equations. This revised version was published online in August 2006 with corrections to the Cover Date.     

Influence of the styrene–maleate ratio on the structure–property relationships of unsaturated polyester networks

Several structural series of unsaturated polyester networks which had different isophthalic acid–maleic anhydride ratios or different precentage rates of styrene incorporated in the network were studied by means of thermomechanical tests. It was demonstrated that the transition zone (α and β) did not correspond specifically to a polytyrene phase but could be linked to the presence of microdomains of different structures within the polyester network. It is the successive setting in motion of these microdomains, under the effect of a rise in temperature, which creates a broad transition zone. Structural chain formations based on the resin components (prepolymer and styrene) were proposed and linked to the transition temperatures detected. The improved knowledge of the structure and the various microdomains present within the networks will provide a better apprehension of the hydrolytic stability of unsaturated polyester materials.     

Synthesis,characterization and evaluation of new fluorene‐based dimethacrylates for formulating dental composites

This study focused on the preparation and evaluation of several new fluorene‐based dimethacrylates, as possible intermediates to formulate improved dental composites. As the first step towards modification of the composite formulation, the new monomers and unfilled resin matrix were prepared. The formulated and visible light‐cured materials were found to be more hydrophobic, along with having higher wet (water saturated) glass transition temperatures (T_g) than the commonly used matrix resins based on 2,2‐bis(4(2‐hydroxy‐3‐methacryloxy‐propyloxy))‐phenylpropane (Bis GMA). In addition, the new matrix resins exhibited greater compressive strength, along with greater resistant to creep and fracture. Finally, the results suggested a correlation may exist between creep and the homologous temperature (T/T_g) of the light‐cured neat resins, where the T/T_g may be used to compare the mechanical properties of different materials at various temperature. Copyright © 1999 John Wiley & Sons, Ltd.