G. Van Assche

Modulated differential scanning calorimetry : isothermal cure and vitrification of thermosetting systems

Abstract Modulated differential scanning calorimetry (MDSC) is used to study simultaneously the evolution of heat flow and heat capacity for the isothermal cure of thermosetting systems. A stepwise decrease in the heat capacity is observed. For the organic resins studied, it is shown that the glass transition temperature of the curing resin reaches the cure temperature at half of the decrease in heat capacity. Vitrification, the isothermal transition from a rubbery or liquid state to a glassy state, and the corresponding vitrification time can thus be measured in a single experiment. A mobility factor is proposed which is based directly on the observed heat capacity evolution. This mobility factor is compared to the diffusion factor which is calculated from the modelled rate of reaction and the decrease in the rate of reaction at vitrification. For the amine-cured and the anhydride-cured epoxies studied, both factors coincide and thus the mobility factor can be proposed as a direct measurement of the diffusion factor. In contrast, the rate of reaction of the low-temperature formation of an inorganic polymer glass (IPG) is (nearly) uninfluenced by the vitrification process. The conversion increases long after vitrification and a glass transition temperature largely above the cure temperature can be reached. In this case, the diffusion and mobility factors do not coincide. MDSC is shown to be a promising technique to study in more detail the effect of polymer microstructure and reaction mechanism on the interrelation between vitrification and the decrease in the rate of reaction due to mobility restrictions.

Modulated differential scanning calorimetry: Non-isothermal cure, vitrification, and devitrification of thermosetting systems

Vitrification of a reacting thermosetting system occurs when its glass transition temperature, Tg, rises to the reaction temperature, T. This phenomenon is not restricted to isothermal conditions only: for highly reactive systems, or when the applied heating rate is sufficiently small, vitrification occurs in non-isothermal conditions too. The reaction proceeds in mobility-restricted conditions. Devitrification is observed when the reaction temperature again surpasses Tg of the vitrified resin. The non-isothermal vitrification and devitrification of two epoxy thermosetting systems have been studied using modulated differential scanning calorimetry (MDSC). A normalized mobility factor, which is directly based on the experimental heat capacity evolution, is proposed. For both organic resins, it is shown that the points for which this mobility factor equals 0.5 can be used to quantify the temperatures of vitrification and devitrification. The mobility factor derived from heat capacity is compared to a normalized diffusion factor calculated from heat flow using chemical kinetics modelling. For the epoxy resins studied, both factors coincide. Therefore, the mobility factor can be used as a direct measurement of the change in the rate of reaction when Tg of the reacting system approaches T. Isothermal and non-isothermal MDSC experiments enable the reaction mechanism, the vitrification and devitrification process, and models for diffusion control to be studied, and improved processing conditions for the cure of thermosetting resins to be developed.

Modulated temperature differential scanning calorimetry: Cure, vitrification, and devitrification of thermosetting systems

Abstract Vitrification of a reacting thermosetting system occurs when its glass transition temperature, T g , rises to the reaction temperature, T . This phenomenon can occur in both isothermal and non-isothermal conditions, depending on the temperatures or heating rates used, the reactivity of the system, and the evolution of T g with conversion. After vitrification, the reaction proceeds in mobility-restricted conditions. In non-isothermal experiments devitrification is observed when the reaction temperature again surpasses T g of the vitrified resin. The cure process of two epoxy thermosetting systems and an inorganic polymer glass has been studied using modulated temperature differential scanning calorimetry (MTDSC). A normalized mobility factor, which is directly based on the experimental heat capacity evolution, is proposed. For both organic resins, it is shown that the points for which this mobility factor equals 0.5 can be used to quantify vitrification and devitrification. Preliminary results indicate a relation between the evolution of the heat flow phase and the chemorheological transformations. The mobility factor derived from the heat capacity is compared to a normalized diffusion factor calculated using the non-reversing heat flow and chemical kinetics modelling. For the organic resins studied, both factors coincide. Therefore, the mobility factor can be used as a direct measurement of the change in the rate of reaction due to mobility restrictions when T g of the reacting system approaches T . Isothermal and non-isothermal MTDSC experiments enable the reaction mechanism, the vitrification and devitrification process, and models for diffusion control to be studied, and temperature-time transformation or continuous-heating transformation diagrams for improved thermoset processing conditions to be developed.

Characterization of Reacting Polymer Systems by Temperature-Modulated Differential Scanning Calorimetry

The benefits of temperature-modulated differential scanning calorimetry to characterize reacting polymers are illustrated for different experimental systems. The effects of combined isothermal and non-isothermal cure paths on (de)vitrification, mobility-restricted reactions, and relaxation during vitrification are discussed for anhydride- and amine-cured epoxies. The simultaneous measurement of heat capacity, heat flow, and heat flow phase provides an excellent tool for mechanistic interpretations. The influence of the metakaolinite particle size on the production of inorganic silicate-metakaolinite polymer glasses is treated as an example. These principles are further illustrated for primary and secondary amine-epoxy step growth reactions, and for styrene-cured unsaturated polyester chain growth reactions with ‘gel effect’. Finally, the effects of isothermal cure and temperature on reaction-induced phase separation in a polyethersulfone modified epoxy-amine system are highlighted.

Interrelations between mechanism, kinetics, and rheology in an isothermal cross-linking chain-growth copolymerisation

The free radical chain-growth cross-linking copolymerisation of an unsaturated polyester resin with styrene is investigated in isothermal conditions using different techniques. By modulated temperature differential scanning calorimetry (MTDSC) the rate of polymerisation is observed simultaneously with the (partial) vitrification of the reacting material. An important autoacceleration closely before vitrification corresponds to an accelerating styrene consumption. The autoacceleration occurs long after the gelation of the material, which implies that termination reactions involving small radicals are still important at an advanced conversion. The deceleration of these termination reactions causes the autoacceleration, which is stopped when the propagation reaction slows down due to vitrification. MTDSC proves to be very suitable for studying the interrelations between vitrification and the polymerisation kinetics.

TMDSC and Dynamic Rheometry, Gelation, Vitrification and Autoacceleration in the Cure of an Unsaturated Polyester Resin

The free radical cross-linking copolymerization of an unsaturated polyester resin with styrene is studied in isothermal conditions using temperature modulated differential scanning calorimetry (TMDSC) and dynamic rheometry. The dynamic rheometry measurements show that gelation occurs at a conversion below 5%, while TMDSC measurements show that an important autoacceleration starts near 60% conversion, giving rise to a maximum cure rate closely before the (partial) vitrification of the system near 80%. This indicates that the autoacceleration is not due to the sharp increase in bulk viscosity at gelation, but rather to a change in molecular mobilities at higher conversion.

Modulated temperature differential scanning calorimetry

The influence of temperature modulation and signal treatment (deconvolution procedure) of modulated temperature differential scanning calorimetry is discussed with respect to the investigation of cure kinetics of thermosetting systems. The use of a ‘dynamic’ heat capacity calibration is not important for this purpose due to normalization of the heat capacity signal in all cure experiments. The heat flow phase during isothermal and non-isothermal cure is always small, giving rise to negligible corrections on the heat capacity and reversing heat flow signals in-phase with the modulated heating rate. The evolution of the heat flow phase contains information on relaxation phenomena in the course of the chemical reactions.

Software NoteOPTKIN—Mechanistic modeling by kinetic and thermodynamic parameter optimization

Abstract A program package OPTKIN is presented for the user-friendly development and analysis of complex reaction mechanisms on personal computers. OPTKIN can (1) simulate the evolution with time of the composition of a reaction mixture, (2) optimize rate parameters and/or thermodynamic properties, (3) search for redundant reaction steps, and (4) perform sensitivity analyses.

RheoDSC Analysis of Hardening of Semi-Crystalline Polymers during Quiescent Isothermal Crystallization

Abstract The crystallization of semi-crystalline polymers is often analyzed by rheometry and calorimetry. By rheometry the viscosity evolution during crystallization can be followed, whereas from a calorimetric measurement, the evolution of the degree of crystallinity can be calculated. The time evolution of these material properties is valuable input for polymer processing simulation software and in order to combine the data in a reliable manner, hardening curves are used as a characterization tool. Such a hardening curve correlates the relative increase of the viscosity resulting from crystallization, to the advancing degree of crystallinity. In this study, these are extracted from simultaneous measurements on one sample using a RheoDSC device. The RheoDSC technique allows for the direct combination of the rheological and calorimetric signal without the need of combining separate stand-alone measurement results. In this study, isothermal crystallization experiments are used to discuss the benefits of this approach. This will lead to the recommendation that measuring the hardening effect in steady shear measurements at very low shear rates in a direct combined RheoDSC setup is the most reliable method to compile unambiguously a material specific hardening curve for semi-crystalline polymers.

A time dependent DFT study of the efficiency of polymers for organic photovoltaics at the interface with PCBM

The interface between donor and acceptor material in organic photovoltaics is of major importance for the functioning of such devices. In this work, the singlet excitation schemes of six polymers used in organic photovoltaics (P3HT, MDMO-PPV, PCDTBT, PCPDTBT, APFO3 and TBDTTPD) at the interface with a PCBM acceptor were studied using TDDFT in combination with the range-separated CAM-B3LYP exchange–correlation functional. By comparing with the excitations in the pure polymer and analyzing the excitation intensities and a measure for orbital overlap, it was possible to identify excitations as either excitation of the polymer or as a charge transfer between donor and acceptor. By combining orbital overlaps between the molecular orbitals involved in charge transfer and the intensity of the polymer excitation a broad correlation was seen with the record efficiencies found in the literature.