Measuring the Glass Transition of Thermosets by Alternating Differential Scanning Calorimetry

The glass transition temperature of thermosets is determined by alternating differential scanning calorimetry (ADSC), which is a temperature modulated DSC technique. The different values of the glass transition obtained from heat flow measurements (total and reversible) and heat capacity (modulus of the complex heat capacity) are analysed and compared with the values obtained by conventional DSC. The effect of the sample mass on the values of T_g, heat capacity and phase angle has been analysed. The effect of the thermal contact between sample and pan has been studied using samples cured directly inside the pan and disc-shaped samples of different thickness. The results obtained for the thermal properties and the phase angle are compared and analysed. The modulus of the complex heat capacity enables the determination of the dynamic glass transition, T_g, which is frequency dependent. The apparent activation energy ofthe relaxation process associated with the glass transition has been evaluated from the dependence of T_g on the period of the modulation.This revised version was published online in November 2005 with corrections to the Cover Date.     

Modulated Differential Scanning Calorimetry Observation of Physical Ageing in Polymers

Modulated-temperature differential scanning calorimetry (M-TDSC) is becoming a useful tool in the characterisation of thermal behaviour of polymers. In this paper, we discuss whether the non-reversing M-TDSC signal can be used, quantitatively and directly, to study the process of physical ageing in polymers. Difference exists between the values of relaxation enthalpy determined by using average heat capacity, <C _p>, signal as for conventional DSC and non-reversing heat capacity, C ⁿ _p, signal. When the signal of reversing heat capacity of unaged sample is considered as baseline for <C _p> and C ⁿ _p signals, the difference disappears. It is concluded that non-reversing M-TDSC signal can be used to observe the process of physical ageing semi-qualitatively and directly. With increasing annealing time, the peak of the imaginary part, C″_p, of the complex heat capacity becomes narrow, but peak area changes little. This indicated that C″_p is not correlated with relaxation enthalpy. It may be related to entropy change during the modulation. However, the entropy change is quite small. This revised version was published online in August 2006 with corrections to the Cover Date.     

调制差示扫描量热法在高分子材料中的应用

调制差示扫描量热法(MDSC)是在传统线性变温基础上叠加一个正弦震荡温度程序,将总热流量分解为可逆热流(热容成分)和不可逆热流(动力学成分),同时具有较高的灵敏度和分辨率,在研究高分子材料复杂相变时具有独特的优势。近年来,MDSC在研究高分子材料的玻璃化转变、结晶-熔融、热容变化等领域得到了较为广泛的应用。本文对MDSC近年来在高分子材料中的最新研究应用做了详细介绍。     

Conjugated Oligomers and Polymers Sheathed with Designer Side Chains

Conjugated polymers (CPs) are often referred to as molecular wires because of their quasi one‐dimensional electronic wavefunctions delocalized along the polymer chains. However, in the solid state, CPs tend to self‐assemble through π‐stacking, which greatly attenuates the one‐dimensional nature. By molecular design, CPs can be molecularly insulated just like electric power cords, resulting in so‐called “insulated” molecular wires (IMWs). In this Focus Review, we will discuss their unique photophysical, electronic, and mechanical properties which originate from the absence of π‐stacking.     

High Glass Transition Temperature Renewable Polymers via Biginelli Multicomponent Polymerization

A novel and straightforward one‐pot multicomponent polycondensation method was established in this work. The Biginelli reaction is a versatile multicomponent reaction of an aldehyde, a β‐ketoester (acetoacetate) and urea, which can all be obtained from renewable resources, yielding diversely substituted 3,4‐dihydropyrimidin‐2(1H)‐ones (DHMPs). In this study, renewable diacetoacetate monomers with different spacer chain lengths (C3, C6, C10, C20) were prepared via simple transesterification of renewable diols and commercial acetoacetates. The diacetoacetate monomers were then reacted with renewable dialdehydes, i.e., terephthalaldehyde and divanillin in a Biginelli type step‐growth polymerization. The obtained DHMP polymers (polyDHMPs) displayed high molar masses, high glass transition temperatures (T_g) up to 203 °C and good thermal stability (T_d5%) of 280 °C. The T_g of the polyDHMPs could be tuned by variation of the structure of the dialdehyde or the diacetoacetate component.

     

Perdeuterated Conjugated Polymers for Ultralow-Frequency Magnetic Resonance of OLEDs

The formation of excitons in OLEDs is spin dependent and can be controlled by electron-paramagnetic resonance, affecting device resistance and electroluminescence yield. We explore electrically detected magnetic resonance in the regime of very low magnetic fields (<1 mT). A pronounced feature emerges at zero field in addition to the conventional spin- Zeeman resonance for which the Larmor frequency matches that of the incident radiation. By comparing a conventional π-conjugated polymer as the active material to a perdeuterated analogue, we demonstrate the interplay between the zero-field feature and local hyperfine fields. The zero-field peak results from a quasistatic magnetic-field effect of the RF radiation for periods comparable to the carrier-pair lifetime. Zeeman resonances are resolved down to 3.2 MHz, approximately twice the Larmor frequency of an electron in Earth's field. However, since reducing hyperfine fields sharpens the Zeeman peak at the cost of an increased zero-field peak, we suggest that this result may constitute a fundamental low-field limit of magnetic resonance in carrier-pair-based systems. OLEDs offer an alternative solid-state platform to investigate the radical-pair mechanism of magnetic-field effects in photochemical reactions, allowing models of biological magnetoreception to be tested by measuring spin decoherence directly in the time domain by pulsed experiments.     

Glass Transition Temperature and Value of the Relaxation Time at Tg in Vitreous Polymers

Summary: Many works focused on glassy polymers determine values of glass transition temperature (T_g) and an overview of the literature shows that depending on the method used, values of T_g are found different for the same material. In this paper, a review of data collected on different materials are used and interpreted in term of molecular mobility characterized by relaxation time functions. By using three independent experimental procedures (dielectric, thermally depolarized current and calorimetric), we show that the value of the glass transition and the value of the relaxation time at T_g can be correctly determined. It is also shown that the assumption: τ _(Tg) = 100 s is constant, is not correct. The protocol proposed also allows the determination of the value of the fragility index “m” of the glass forming liquid with a great accuracy.     

Experimental Evaluation of Procedures for Heat Capacity Measurement by Differential Scanning Calorimetry

Experimental evaluation of the procedures adopted for heat capacity measurements employing differential scanning calorimetry (DSC) has been carried out by taking nickel and sapphire as test samples. Among the various methodologies reported in literature, the absolute dual step method was chosen for this purpose due to its simplicity and minimum number of measurements required. By proper temperature and heat flux calibration employing indium as reference, it was possible to obtain the calibration factor independent of temperature. This was ascertained by analysing other pure metals namely Sn, Zn, Cd, and Pb and determining their melting temperatures and heats of melting. Various operator- and sample-dependent parameters such as heating rate, sample mass, the structure of the sample, reproducibility and repeatability in the measurements were investigated. Heat capacities of both nickel and sapphire have been determined using the above method. Further, the heat capacity of nickel has also been determined using the widely employed three-step method taking sapphire as the heat flux calibration standard. Both methods yielded the comparable heat capacity values for nickel. Based on the parameters investigated and their influence, it could be concluded that reasonably precise and accurate heat capacity measurements are possible with DSC. One advantage of this method is the elimination of a separate calibration run using a reference material of known heat capacity. This revised version was published online in August 2006 with corrections to the Cover Date.     

调制式DSC在高聚物研究中的应用

综述了一种新的热分析技术－－调制式差示扫描量热示（Modulated Differentail Scanning Calorimetry）。对其工作原理，优点及在高聚物研究中的应用作了简介。     

Covalently Scaffolded Inter‐π‐System Orientations in π‐Conjugated Polymers and Small Molecule Models

Organic π‐conjugated polymers have emerged as one of the most fascinating classes of materials as they have found utility in a host of plastic electronics technologies. The distance between π‐systems and their relative orientation dictate energy/charge transfer, conductivity, and photophysical properties of these materials in bulk. This Feature Article discusses π‐conjugated polymers and model compounds in which specific inter‐π‐system interactions are covalently enforced and the effect that the scaffolding has on optoelectronic properties.

     

呋喃稠环类共轭有机分子和聚合物研究进展

目前,噻吩稠环衍生物及相关共轭聚合物作为有机半导体材料已经得到了较多的研究和应用.呋喃作为一种与噻吩杂原子同主族的五元环体系,与噻吩具有类似的化学结构和电子性质,但其具有芳香性更小、载流子迁移率高、荧光量子效率高和溶解性好的特点,得到了越来越多的研究和关注.本综述介绍了呋喃稠环共轭有机分子与呋喃稠环共轭聚合物的合成方法、性质及应用.     

Effect of Side Chain Substituent Volume on Thermoelectric Properties of IDT-Based Conjugated Polymers

A p-type thermoelectric conjugated polymer based on indacenodithiophene and benzothiadiazole is designed and synthesized by replacing normal aliphatic side chains (P1) with conjugated aromatic benzene substituents (P2). The introduced bulky substituent on P2 is detrimental to form the intensified packing of polymers, therefore, it hinders the efficient transporting of the charge carriers, eventually resulting in a lower conductivity compared to that of the polymers bearing aliphatic side chains (P1). These results reveal that the modification of side chains on conjugated polymers is crucial to rationally designed thermoelectric polymers with high performance.     

Perdeuterated Conjugated Polymers for Ultralow‐Frequency Magnetic Resonance of OLEDs

The formation of excitons in OLEDs is spin dependent and can be controlled by electron‐paramagnetic resonance, affecting device resistance and electroluminescence yield. We explore electrically detected magnetic resonance in the regime of very low magnetic fields (<1 mT). A pronounced feature emerges at zero field in addition to the conventional spin‐ Zeeman resonance for which the Larmor frequency matches that of the incident radiation. By comparing a conventional π‐conjugated polymer as the active material to a perdeuterated analogue, we demonstrate the interplay between the zero‐field feature and local hyperfine fields. The zero‐field peak results from a quasistatic magnetic‐field effect of the RF radiation for periods comparable to the carrier‐pair lifetime. Zeeman resonances are resolved down to 3.2 MHz, approximately twice the Larmor frequency of an electron in Earth's field. However, since reducing hyperfine fields sharpens the Zeeman peak at the cost of an increased zero‐field peak, we suggest that this result may constitute a fundamental low‐field limit of magnetic resonance in carrier‐pair‐based systems. OLEDs offer an alternative solid‐state platform to investigate the radical‐pair mechanism of magnetic‐field effects in photochemical reactions, allowing models of biological magnetoreception to be tested by measuring spin decoherence directly in the time domain by pulsed experiments.     

Investigations of the Sub-Ambient Transitions in Frozen Sucrose by Modulated Differential Scanning Calorimetry (MDSC®)

40% w/w sucrose/water solutions were analyzed by Modulated Differential Scanning Calorimetry [1] in the sub-ambient temperature region. At these temperatures, the solutions exhibit a complex, two-step thermal event. The lower-temperature event is believed to be the glass transition of the amorphous sucrose phase. The nature of the higher-temperature event is the subject of controversy. This event has been shown to have distinct second-order characteristics, and as such is believed to be a second T_g. Others feel that this event is the onset of melting. The temperature region between these events contains a devitrification exotherm. Through the use of MDSC, both in scanning and stepwise quasi-isothermal modes, improved sensitivity and resolution of MDSC provides new insight into the nature of these transitions.This revised version was published online in November 2005 with corrections to the Cover Date.     

Differential Scanning Calorimetry at Hydrothermal Conditions of Amorphous Materials Prepared by Drying Sodium Silicate Solutions

Concentrated sodium silicate solutions with a molar SiO₂:Na₂O ratio of 3.3 and a SiO₂ content of 27 mass% were dried up to 63 d at temperatures between 40 and 100°C to residual water contents between 12 and 38 mass%. The dried solid materials were investigated by differential scanning calorimetry (DSC). Pressure tight autoclave crucibles were applied to suppress evaporation and boiling of the samples during heating. Two thermal events are discussed with respect to glass transition. This revised version was published online in August 2006 with corrections to the Cover Date.     

Temperature Modulated Differential Scanning Calorimetry Part IV. Effect of heat transfer on the measurement of heat capacity using quasi-isothermal ADSC

An analysis developed in previous work has been further refined in order to study the effect of heat transfer on the heat capacity and phase angle measurements by TMDSC. In the present model, a temperature gradient within the sample has been taken into account by allowing for heat transfer by thermal conduction within the sample. The influence of the properties of the sensors, the heat transfer conditions between the sensor and sample,and the properties of the sample have been investigated by varying each parameter in turn. The results show that heat capacity measurements are reliable only within a restricted frequency range, for which the experimental conditions are such that the heat transfer phase angle depends linearly on the modulation frequency. This revised version was published online in July 2006 with corrections to the Cover Date.     

Studying the Glass Transition by DSC and TMDSC

First, the principal features of the glass transformation process in polymers are reviewed, and then it is shown how they are manifest in conventional DSC, and the quantitative analysis of typical DSC data is discussed in terms of the Tool-Narayanaswamy-Moynihan (TNM) model. Subsequently, the way in which the glass transition is manifest in Temperature Modulated DSC is presented, and the effects of both experimental and material parameters are discussed. In conclusion, the two techniques are compared in terms of the information they provide about the glass transformation process. This revised version was published online in July 2006 with corrections to the Cover Date.     

Pd‐Catalysed Direct Arylation Polymerisation for Synthesis of Low‐Bandgap Conjugated Polymers and Photovoltaic Performance

Low‐bandgap conjugated copolymers based on a donor–acceptor structure have been synthesised via palladium‐complex catalysed direct arylation polymerisation. Initially, we report the optimisation of the synthesis of poly(cyclopentadithiophene‐alt‐benzothiadiazole) (PCPDTBT) formed between cyclopentadithiophene and dibromobenzothiadiazole units. The polymerisation condition has been optimised, which affords high‐molecular‐weight polymers of up to M _n = 70 k using N‐methylpyrrolidone as a solvent. The polymers are used to fabricate organic photovoltaic devices and the best performing PCPDTBT device exhibits a moderate improvement over devices fabricated using the related polymer via Suzuki coupling. Similar polymerisation conditions have also been applied for other monomer units.     

Crystalline Conjugated Polymers for Organic Solar Cells: From Donor,Acceptor to Single‐Component

Organic solar cells have made rapid progress in the last two decades due to the innovation of conjugated materials and photovoltaic devices. Microphase separation that connects with materials and devices plays a crucial role in the charge generation process. In this account, we summary our recent works of developing new crystalline conjugated polymers to control the microphase separation in thin films in order to realize high performance in solar cells, including crystalline diketopyrrolopyrrole‐based donor polymers, perylene bisimide‐based electron acceptors, and “double‐cable” conjugated polymers that contain covalently‐linked crystalline donor and acceptor in one material for single‐component organic solar cells.