Cure state detection for pre-cured carbon-fibre epoxy prepreg (CFC) using Temperature-Modulated Differential Scanning Calorimetry (TMDSC)

Carbon-fibre prepregs have found widespread use in lightweight applications. They are based on a carbon-fibre fabric impregnated with reactive epoxy resin. Prepreg materials are generally pre-cured so that they have a higher molecular weight than typical resins in order to reduce resin flow, which facilitates storage and later processing properties.The measurements were carried out using commercially available materials and follow the published DMA investigations of the same material [1]. TMDSC was used to find the correlation between curing conditions, the degree of cure and glass transition temperature. TMDSC has the advantage over standard DSC that it enables better determination of the glass transition temperature, which is often accompanied by an exothermic curing reaction, and thus overshadowed. The influence of the amplitude of temperature modulation was tested in preliminary experiments. For non-cured material a glass transition temperature of approximately 0 °C was determined; whereas for the totally cured material it was approximately 230 °C. The changes in degree of cure, temperature of actual glass transition and post-reaction are given as a function of curing time at 180 °C. The correlation between actual glass transition temperature and degree of cure is derived.     

Phase separation in a poly(ether sulfone) modified epoxy-amine system studied by temperature modulated differential scanning calorimetry and dielectric relaxation spectroscopy

The phase separation induced by the curing reaction of an epoxy based on diglycidylether of bisphenol A (DGEBA) with methylene dianiline (MDA) modified with poly(ether sulfone) (PES) at a concentration of 20 wt% was studied by temperature modulated differential scanning calorimetry (TMDSC) and dielectric relaxation spectroscopy (DRS). The effect of phase separation on the curing kinetics and vitrification phenomena is analysed. The dependence of the log of the measuring frequency on the degree of conversion allows the correlation between the dipolar relaxation of each phase and the vitrification observed by TMDSC to be established.     

Temperature modulated DSC study of the kinetics of free radical isothermal network polymerization

Temperature modulated differential scanning calorimetry (TMDSC) is used to study the kinetics of the free radical isothermal polymerization of triethyleneglycol dimethacrylate (TEGDMA). Azo-bis-isobutironitrile was used as initiator. The polymerization’s temperature is lower than the final glass transition temperature of the polymer network. The measurement of the average heat flow released and the heat capacity during the reaction allows identifying the different stages of the reaction. The presence of double peaks in the heat flow is ascribed to the autoacceleration. The influence of temperature, measuring conditions and oxygen are described. Vitrification is detected by the drop in heat capacity. It occurs at increasing conversion rates for increasing temperatures. After vitrification, the diffusion-controlled reaction continues.     

Vitrification during the isothermal cure of thermosets

The process of vitrification that occurs during the isothermal cure of a cross-linking system at temperatures below T _g∞, the glass transition temperature of the fully cured resin, has been studied by TOPEM, a new temperature modulated DSC (TMDSC) technique based upon the use of stochastic temperature pulses. A comparison is made between TOPEM and another TMDSC technique, and some advantages of TOPEM are considered. The TOPEM technique is used to show that the mobility factor is not always a reliable approach to predicting the cure rate during vitrification, in view of its frequency dependence. Also, the dependence of the apparent vitrification time on frequency is examined. There appears to be a non-linear relationship between the apparent vitrification time and log(frequency), which is further discussed in the second part of this series.     

Kinetic study of an epoxy system badge (n=0)/1, 2 dch modified with an epoxy reactive diluent

The curing reaction of an epoxy system consisting of a diglycidyl ether of bisphenol A (n=0) and 1, 2 diaminecyclohexane (DCH) with an epoxy reactive diluent vinylcyclohexane dioxide was studied by temperature modulated differential scanning calorimetry (TMDSC). The models proposed by Kamal and by Horie et al. were employed in the kinetic study. From these studies reaction orders, rate constants, and activation energies were determined. The technique of TMDSC allows to include in the kinetic study the effect of diffusion by means of the mobility factor, calculated from the curves of the complex heat capacity registered during the curing isothermal experiments. The results were compared to those obtained for the same system employing the reaction rate data. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermal characterization of poly-l-lactide by dielectric analysis and modulated DSC

Dielectric analysis (DEA) is a very sensitive technique, which allows for detection of small structural changes at the low scale. An advantage of DEA, with respect to other modulated techniques, is the possibility of using a wider frequency range. Molecular relaxations of the order of only a few nanometers are not observed by any other thermoanalytic method. Nevertheless, these small relaxations involve dipole changes that can be observed by DEA. Thus, this technique is used here, in combination with temperature-modulated differential scanning calorimetry (TMDSC) to obtain insightful information about the thermal transitions of poly-l-lactic acid (PLLA), one of the stereo-isomers of polylactide. Its complex thermal behavior is the subject of ongoing debate, with several overlapping crystallization and melting processes. The combined use of TMDSC and DEA provides a better insight of three important transitions of this polymer: the alpha relaxation, the enthalpic relaxation, and the cold crystallization. The dependences of the enthalpy relaxation on the dynamic glass transition relaxation and on the glass transition as a thermal event are evaluated. On the other hand, it will be shown how the cold crystallization can be identified by TMDSC, and DEA helps us understand the effect of crystallization on the dipole movements. The shape of the dielectric permittivity curve at low frequencies is compared to that of the reversing heat capacity to check whether both signals are sensitive or not to the same events. It is also verified how the experimental results of alpha relaxation of PLLA follow an Arrhenius or a Vogel trend.     

基于松香酸酐的环氧树脂体系固化动力学及TTT图绘制

研制了基于松香酸酐固化剂的生物质环氧树脂体系,采用全动态DSC法研究了树脂体系的固化反应动力学,通过半经验的唯象模型拟合得到了固化反应参数,活化能Ea为59.68 kJ/g,指前因子A0为1.28×1015s-1,反应级数n为2.483,由此建立了体系固化温度/时间/固化度间的关系;采用恒温DSC及DMA方法测试玻璃化转变温度,应用DiBenedetto经验方程拟合得到了玻璃化转变温度与固化度间的关系.应用锥板旋转黏度计测试了树脂体系不同温度下的凝胶时间,通过线性回归分析得到了凝胶时间与温度之间的关系.由唯象模型和DiBenedetto方程分别计算得到凝胶时的固化度为0.386,玻璃化转变温度为26.22°C.由上述工作绘制了基于松香酸酐生物质树脂体系的TTT(time-temperature-transition diagram)固化图,可确定树脂体系在不同温度任意时间下的状态.     

Reversible and irreversible heat capacity of poly[carbonyl(ethylene‐co‐propylene)] by temperature‐modulated calorimetry

The heat capacity of poly[carbonyl(ethylene‐co‐propylene)] with 95 mol % C₂H₄? CO? (Carilon EP®) was measured with standard differential scanning calorimetry (DSC) and temperature‐modulated DSC (TMDSC). The integral functions of enthalpy, entropy, and free enthalpy were derived. With quasi‐isothermal TMDSC, the apparent reversing heat capacity was determined from 220 to 570 K, including the glass‐ and melting‐transition regions. The vibrational heat capacity of the solid and the heat capacity of the liquid served as baselines for the quantitative analysis. A small amount of apparent reversing latent heat was found in the melting range, just as for other polymers similarly analyzed. With an analysis of the heat‐flow rates in the time domain, information was collected about latent heat contributions due to annealing, melting, and crystallization. The latent heat decreased with time to an even smaller but truly reversible latent heat contribution. The main melting was fully irreversible. All contributions are discussed in the framework of a suggested scheme of six physical contributions to the apparent heat capacity. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1565–1577, 2001     

DSC Studies of Thermally Induced Phase Separation of Hydrogen Bonded Polymer Blends

The thermally induced phase separation behavior of hydrogen bonded polymer blends, poly(n-hexyl methacrylate) (PHMA) blended with poly(styrene-co-vinyl phenol) (STVPh) random copolymers having various vinyl phenol contents, was studied by temperature modulated differential scanning calorimetry (TMDSC).The enthalpy of phase separation was determined to be about 0.5 cal g^–1 for one of the blends. A phase diagram was constructed from the TMDSC data for one of the blends. The kinetics of phase separation was studied by determining the phase compositions from the glass transition temperatures of quenched samples after phase separation. Subsequently, the phase separated samples were annealed at temperatures below the phase boundary to observe the return to the homogeneous state.This revised version was published online in November 2005 with corrections to the Cover Date.     

Investigation of the Influence of Diffusion Control on the Curing Reaction Using DSC and Temperature Modulated DSC

The curing reaction of a thermosetting system is investigated by DSC and temperature modulated DSC (TMDSC). When the material vitrifies during curing, the reaction becomes diffusion controlled. The phase shift signal measured by TMDSC includes direct information on the reaction kinetics. For long periods the phase shift is approximately proportional to the partial temperature derivative of the reaction rate. This signal is very sensitive for changes in the reaction kinetics. In the present paper an approach to determine the diffusion control influence on the reaction kinetics from the measured phase shift is developed. The results are compared with experimental data. Further applications of this method for other reactions are proposed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Correlation study among oxygen permeability,molecular mobility,and amorphous structure change of poly(ethylene‐vinylalcohol copolymers) by moisture

Oxygen permeability and the free‐volume hole size (cavity size) of ethylene‐vinylalcohol copolymers (EVOH) indicate abnormal humidity dependence, that is, they have minimum values in the range of around 20–40% RH, not showing a monotonic increase with humidity. To clarify this abnormal phenomenon, we investigated its molecular mobility and amorphous structure change by means of solid‐state NMR and temperature‐modulated differential scanning calorimetry (TMDSC). The glass transition temperature (T_g) decreased with humidity. Specimens stored at 15–60% RH showed large enthalpy relaxation, and it was found that the amorphous structure became more compact and the molecular conformation became more stable by ageing within this range of humidity. Under these conditions, solid‐state NMR measurement showed a component with intermediate relaxation time in the amorphous region. The results obtained by TMDSC and solid‐state NMR showed a reduction in molecular mobility by densifying in the amorphous region under the condition of 15–60% RH. The fact that the oxygen permeability and the cavity size of EVOH indicate minimum values at low humidity are attributed to the reduction in molecular mobility by enthalpy relaxation. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1181–1191, 2009     

The Relationship Between the TMDSC Curve of Frozen Sucrose Solutions and Collapse During Freeze-Drying

This study compares measurements of the collapse temperature of sucrose solutions by freeze drying microscopy with features of TMDSC curves both in the scanning and quasi-isothermal modes. The objective was to determine which feature of the TMDSC curve is predictive of collapse and to provide additional evidence for recent interpretations of the physical significance of the low temperature transitions for sucrose solutions. Interpretations based on the heat capacity signal and the kinetic heat flow using TMDSC are consistent with previous reports based on total heat flow measurement, where the lower temperature transition is the glass transition and the higher temperature transition is associated with the onset of ice melting. Quasi-isothermal experiments further support these conclusions, since additional crystallization of ice is observed only in the region of the lower temperature transition. Collapse of sucrose solutions during freeze-drying begins at the approximate midpoint between the end of the glass transition region and the onset of ice melting. This revised version was published online in July 2006 with corrections to the Cover Date.     

Matching Cure Characteristics of Automotive Rubber Compound and Polyurethane Coating

The present investigation focuses on matching cure characteristics of EPDM rubber compound and polyurethane (PU) coating using temperature modulated and pressure differential scanning calorimetry (TMDSC, PDSC). TMDSC provides a detailed and better understanding of the curing process of model rubber system as well as complex automotive rubber compounds. The low level of unsaturation present in EPDM, results in the small heat of vulcanization (2–5 J g^–1), which is difficult to accurately measure using conventional differential scanning calorimetry (DSC). Thus, curing of highly filled EPDM compound was investigated using TMDSC. The kinetics of PU curing was monitored using pressure DSC (PDSC), and heat of curing was determined as 4.2 J g^–1 at 10°C min^–1 heating rate. It is found that complex automotive compounds and the PU coating are curing simultaneously. This revised version was published online in July 2006 with corrections to the Cover Date.     

Melting and crystallization of poly(butylene terephthalate) by temperature‐modulated and superfast calorimetry

Quantitative temperature‐modulated differential scanning calorimetry (TMDSC) and superfast thin‐film chip calorimetry (SFCC) are applied to poly(butylene terephthalate)s (PBT) of different thermal histories. The data are compared with those of earlier measured heat capacities of semicrystalline PBT by adiabatic calorimetry and standard DSC. The solid and liquid heat capacities, which were linked to the vibrational and conformational molecular motion, serve as references for the quantitative analyses. Using TMDSC, the thermodynamic and kinetic responses are separated between glass and melting temperature. The changes in crystallinity are evaluated, along with the mobile–amorphous and rigid–amorphous fractions with glass transitions centered at 314 and 375 K. The SFCC showed a surprising bimodal change in crystallization rates with temperature, which stretches down to 300 K. The earlier reported thermal activity at about 248 K was followed by SFCC and TMDSC and could be shown to be an irreversible endotherm and is not caused by a glass transition and rigid–amorphous fraction, as assumed earlier. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1364–1377, 2006     

Enthalpy relaxation and fragility in polychlorinated biphenyls

We employ temperature modulated DSC (TMDSC) to determine the dependence of the fictive temperature on cooling rate for a series of polychlorinated biphenyls (PCB). From the slopes of semi-logarithmic plots of cooling rate vs. fictive temperature, the latter normalized by the fictive temperature for an arbitrary cooling rate, we determine the enthalpic fragilities. Despite significant differences in glass transition temperature and chemical structure (specifically chlorine content), the PCB have the same fragility. The value of the fragility determined using TMDSC is consistent with the fragility previously determined using dielectric relaxation spectroscopy.     

Reaction-induced phase separation in polyethersulfone-modified epoxy-amine systems studied by temperature modulated differential scanning calorimetry

In epoxy-amine systems with a thermoplastic additive, the initially homogeneous reaction mixture can change into a multi-phase morphology as a result of the increase in molecular weight or network formation of the curing matrix. Temperature modulated DSC (TMDSC) allows the real-time monitoring of this reaction-induced phase separation. A linear polymerizing epoxy-amine (DGEBA–aniline) and a network-forming epoxy-amine (DGEBA–methylene dianiline), both with an amorphous engineering thermoplastic additive (polyethersulfone, PES), are used to illustrate the effects of phase separation on the signals of the TMDSC experiment. The non-reversing heat flow gives information about the reaction kinetics. The heat capacity signal also contains information about the reaction mechanism in combination with effects induced by the changing morphology and rheology such as phase separation and vitrification. In quasi-isothermal (partial cure) TMDSC experiments, the compositional changes resulting from the proceeding phase separation are shown by distinct stepwise heat capacity decreases. The heat flow phase signal is a sensitive indication of relaxation phenomena accompanying the effects of phase separation and vitrification. Non-isothermal (post-cure) TMDSC experiments provide additional real-time information on further reaction and phase separation, and on the effect of temperature on phase separation, giving support to an LCST phase diagram. They also allow measurement of the thermal properties of the in situ formed multi-phase materials.     

Ultraviolet (UV) curing of phosphated polyurethane-acrylic dispersions

Phosphate-containing polyurethane-acrylic dispersions were synthesised for UV curing studies. The effects of light intensity, substrate-dependent temperature increase, soft-segment content and water on conversion were investigated. The effect of the light intensity on conversion was twofold. At first, conversion increased with light intensity. This was attributed to the inability of shrinkage to keep pace with the polymerisation and crosslinking, resulting in the creation of free volume, thereby facilitating reaction and enhanced conversion. At higher intensities, conversion was found to be reversed or, at least, it remained constant, owing to increasing radical-radical termination reactions. Phosphated-polyurethane coatings with high soft-segment content show improved conversion with exposure time. This was found to be related to the chain mobility, caused by the low glass transition temperature of the soft segment. The effect of water on conversion was also twofold. On the one hand, water had a plasticising effect on the UV curing and the polymerisation rate was fast. On the other hand, the gel content was found to be lower when films were cured before the evaporation of water.     

Annealing behavior of gel‐spun polyethylene fibers at temperatures lower than needed for significant shrinkage

The annealing at 373 K of ultrastrong, gel‐spun polyethylene (PE) has been studied. At this temperature, the fibers show no significant shrinkage. Still, a significant decrease in the mechanical properties is observed. The fibers have been analyzed with differential scanning calorimetry (DSC), temperature‐modulated differential scanning calorimetry (TMDSC), atomic force microscopy (AFM), and small‐angle X‐ray scattering (SAXS). During the annealing, the glass transition of the intermediate phase is exceeded, as shown by DSC. When split for structure analysis by AFM, the annealed fibers undergo plastic deformation around the base fibrils instead of brittle fracture. The quasi‐isothermal TMDSC experiments are compared to the minor structural changes seen with SAXS and AFM. The loss of performance of the PE fibers at 373 K is suggested to be caused by the oriented intermediate phase, and not by major changes in the structure or morphology. The overall metastable, semicrystalline structure is shown by TMDSC to posses local regions that can melt reversibly. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 403–417, 2003     

Thermal analysis of the influence of water content on glass transitions

Starch is an important natural substance in which the water content has a significant influence on its structure and properties. In the present study, the effect of the water content on glass transition temperatures T _g and heat capacities C _p of wheat, maize and potato starches were investigated by high-sensitivity differential scanning calorimetry (temperature modulated TMDSC and conventional DSC). Thermal analysis measurements were performed on starch samples with different water contents. The exact water mass percentage of each sample was determined using the Karl-Fischer method. The obtained results show that the water content does influence the starch thermal properties in a systematic and measurable trend, the higher the water% the lower the glass transition temperature, and the higher the heat capacity jump during gelatinisation. At this stage possible interpretations of the results are just put forward and should be confirmed through complementary measurements.