Isothermal Crystallisation of PCL Studied by Temperature Modulated Dynamic Mechanical and TMDSC Analysis

Temperature modulated dynamic mechanical analysis (TMDMA) was performed in the same way as temperature modulated DSC (TMDSC) measurements. As in TMDSC TMDMA allows the investigation of reversible and non-reversible phenomena during crystallisation of polymers. The advantage of TMDMA compared to TMDSC is the high sensitivity for small and slow changes in crystallinity, e.g. during re-crystallisation. The combination of TMDMA and TMDSC yields new information about local processes at the surface of polymer crystallites. It is shown that during and after isothermal crystallisation the surface of the individual crystallites is in equilibrium with the surrounding melt. This revised version was published online in August 2006 with corrections to the Cover Date.     

Reversible Melting During Crystallization of Polymers Studied by Temperature Modulated Techniques (TMDSC, TMDMA)

Quasi-isothermal temperature modulated DSC and DMA measurements (TMDSC and TMDMA, respectively) were performed to determine heat capacity and shear modulus as a function of time during crystallization. Non-reversible and reversible phenomena in the crystallization region of polymers can be observed. The combination of TMDSC and TMDMA yields new information about local processes at the surface of polymer crystals, like reversible melting. Reversible melting can be observed in complex heat capacity and in the amplitude of shear modulus in response to temperature perturbation. The fraction of material involved in reversible melting, which is established during main crystallization, keeps constant during secondary crystallization for PCL PET and PEEK. This shows that also after long crystallization times the surfaces of the individual polymer crystallites are in equilibrium with the surrounding melt. Simply speaking, polymer crystals are ‘living crystals’. This revised version was published online in August 2006 with corrections to the Cover Date.     

Crystal Perfection of Poly(p-phenylene Sulfide) During Cooling Using Temperature-Modulated DSC

Temperature-modulated DSC (TMDSC) was used to enhance the perfection of crystals of different poly(p-phenylene sulfide) samples formed during slow cooling from the melt. The sample preparation was made with modulated cooling using a cool-heat mode. Re-heating the samples prepared by slow conventional and modulated coolings indicated that the melting point of the samples prepared by modulated cooling is considerably higher than the melting point of the samples crystallized with conventional cooling. Thus, the perfection of crystallites can be improved if the outer layers just deposited on their surface are re-melted and re-crystallized immediately.This revised version was published online in November 2005 with corrections to the Cover Date.     

Reversible melting in poly(butylene terephthalate)

Crystallized samples of poly(butylene terephthalate) (PBT), examined in the melting region by means of temperature modulated differential scanning calorimetry (TMDSC), show reversible fusion. The analysis of the complex heat capacity reveals that the fusion of poor crystallites can follow temperature modulation more easily than perfect crystals, in agreement with the findings recently reported in the literature, and that the amount of reversible melting decreases with increasing the modulation frequency.     

Phase behavior at low temperature of poly(tetrafluoroethylene) by temperature-modulated calorimetry

Temperature modulated differential calorimetry (TMDSC) is used to examine the crystal-crystal transitions of poly(tetrafluoroethylene). This study gives new information about the dynamic thermal behavior of such transitions. The involvement of reversible and irreversible processes during the phenomenon is observed, which are related to the order-disorder changes occurring during the transition.This study adds a new example to the response of TMDSC during first order transitions.     

Temperature Modulated DSC for the Investigation of Polymer Materials: A brief account of recent studies

Temperature modulated differential scanning calorimetry (TMDSC), the most recent development that adds periodic modulation to the conventional DSC, has recently seen a fast growth due to availability of commercial instrumentation. The use of the technique necessitates a total control of all of the experimental parameters. The paper focuses on recent applications to investigate polymers [1].This revised version was published online in November 2005 with corrections to the Cover Date.     

Crystallization of Polymers Studied by Temperature Modulated Calorimetric Measurements at Different Frequencies

Quasi-isothermal temperature modulated DSC (TMDSC) were performed during crystallization to determine heat capacity as function of time and frequency. Non-reversible and reversible phenomena in the crystallization region of polymers were distinguished. TMDSC yields new information about the dynamics of local processes at the surface of polymer crystals, like reversible melting. The fraction of material involved in reversible melting, which is established during main crystallization, keeps constant during secondary crystallization for polycaprolactone (PCL). This shows that also after long crystallization times the surfaces of the individual crystallites are in equilibrium with the surrounding melt. Simply speaking, polymer crystals are living crystals. A strong frequency dependence of complex heat capacity can be observed during and after crystallization of polymers.This revised version was published online in November 2005 with corrections to the Cover Date.     

Crystallisation, melting, recrystallisation and polymorphism of n-eicosane for application as a phase change material

Phase change materials (PCM) provide thermoregulation originating from the latent heat exchanged during melting or crystallisation. Linear hydrocarbons have weak interactions, but high symmetry, providing an effective quantity of latent heat over the most acceptable temperature range for applications. The ability to both melt and crystallise over a narrow range is made complex by nucleation, polymorphism and the kinetic nature of these changes. Differential scanning calorimetry (DSC), optical microscopy and temperature modulated DSC (TMDSC) was used to study the melting of n-eicosane. This PCM has a low degree of supercooling and conversion to the most stable crystalline state (triclinic) that occurs rapidly from a metastable phase (rotator) state on cooling. TMDSC revealed a small, yet similar degree of thermodynamic reversibility in the melting of each of the crystalline phases.     

Some elements in specific heat capacity measurement and numerical simulation of temperature modulated DSC (TMDSC) with R/C network

One important application of temperature modulated DSC (TMDSC) is the measurement of specific heat of materials. In this paper, a thermal resistance/capacitance (R/C) numerical model is used to analyze the effects of experimental parameters and calibration on the measurement of specific heat in TMDSC under isothermal conditions. The actual TMDSC experiments were conducted with sapphire and pure copper samples, respectively. Both simulation and experiments showed that in TMDSC, the measured sample specific heat is a non-linear function of many factors such as sample mass, the heat transfer properties of the TMDSC instrument, temperature modulation period, the heat capacity difference between calibration material and the test material, but modulation amplitude has very little effect on the results. The typical behavior of a heat flux type TMDSC can be described as a low pass filter in terms of specific heat capacity measurement when the instrument heat transfer properties are taken into account. At least for metallic materials, where the temperature gradient inside the sample can normally be ignored, the sample should be chosen in such a way that its total heat capacity (mass times specific heat) is close to that of the calibration material in order to get a more accurate result. Also, a large modulation period is beneficial to improving the test accuracy.     

On the kinetics of melting and crystallization of poly(l-lactic acid) by TMDSC

The crystallization and melting process of poly(l-lactic acid), PLLA, is investigated by temperature modulated differential scanning calorimetry, TMDSC. The sample is cooled from the melt to different temperatures and the crystallization process is followed by subjecting the material to a modulated quasi-isothermal stage. From the average component of the heat flow and the application of the Lauritzen–Hoffman theory two crystallization regimes are identified with a transition temperature around 118 °C. Besides, the oscillating heat flow allows calculating the crystal growth rate via the model proposed by Toda et al., what gives, in addition, an independent determination of the transition temperature from modulated experiments. Further, the kinetics of melting is studied by modulated heating scans at different frequencies. A strong frequency dependence is found both in the real and imaginary part of the complex heat capacity in the transition region. The kinetic response of the material to the temperature modulation is analyzed with the model proposed by Toda et al. Finally, step-wise quasi-isothermal TMDSC was used to investigate the reversible surface crystallization and melting both on cooling and heating and a small excess heat capacity is observed.     

Experimental Considerations for Temperature Modulated DSC at Low Temperature

Temperature modulated DSC (TMDSC) at low temperatures requires attention to the selection of experimental parameters that are within the capability of the instrumentation as well as special care in calibration of heat capacity measurement when high precision is required. Data are presented to facilitate selection of appropriate modulation periods and amplitudes at low temperature when using a mechanical cooling accessory. The standard error of the mean heat capacity measurement for a sapphire standard increased with decreasing temperature, decreasing period, and increasing pan mass. For ice in hermetically sealed pans, the standard error of the mean heat capacity measurement was larger than for sapphire and did not follow a predictable trend with changes in temperature and period of modulation. This was attributed to changes in sample geometry between successive measurements due to melting and resolidification. A simple one-point temperature calibration by TMDSC may be unsuitable for precise measurement of heat capacity because of the random error caused by sample placement and the systematic error caused by cell asymmetry, temperature dependence of the calibration constant, and different sample thermal conductivities. An alternative calibration procedure using standard DSC and either a linear or second order fit of the calibration constant over the temperature range of interest is proposed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Thermal memory of poly(3‐hydroxybutyrate) using temperature‐modulated differential scanning calorimetry

The influence of thermal history on morphology, melting, and crystallization behavior of bacterial poly(3‐hydroxybutyrate) (PHB) has been investigated using temperature‐modulated DSC (TMDSC), wide‐angle X‐ray diffraction (WAXRD) and polarized optical microscopy (POM). Various thermal histories were imparted by crystallization with continuous and different modulated cooling programs that involved isoscan and cool–heat segments. The subsequent melting behavior revealed that PHB experienced secondary crystallization during heating and the extent of secondary crystallization varied with the cooling treatment. PHB crystallized under slow, continuous, and moderate cooling rates were found to exhibit double melting behavior due to melting of TMDSC scan‐induced secondary crystals. PHB underwent considerable secondary crystallization/annealing that took place under modulated cooling conditions. The overall melting behavior was interpreted in terms of recrystallization and/or annealing of crystals. Interestingly, the PHB analyzed by temperature modulation programs showed a broad exotherm before the melting peak in the nonreversing heat capacity curve and a multiple melting reversing curve, verifying that the melting–recrystallization and remelting process was operative. WAXRD and POM studies supported the correlations from DSC and TMDSC results. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 70–78, 2006     

Molecular dynamics revealed from frequency dependent heat capacity

Temperature modulated DSC (TMDSC) measurements at reasonably high frequencies allow for the determination of baseline heat capacity. In this particular case vitrification and devitrification of the rigid amorphous fraction (RAF) can be directly observed. 0.01 Hz seems to be a reasonably high frequency for Bisphenol‐A Polycarbonate (PC). The RAF of PC is established during isothermal crystallization. Devitrification of the RAF seems to be related to the pre‐melting peak. For PC the melting of small crystals between the lamellae is thought to yield the pre‐melting peak.     

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.     

Temperature modulated DSC at intermediate-low temperatures

In this paper we present a new cooling system for temperature modulated DSC (TMDSC) working down to about 60 K. In order to demonstrate the features of this new system in combination with commercial TMDSC apparatus, we present measurements of the specific heat capacity (c_p) around the phase transitions of betaine borate and betaine phosphate. For SilGel 604 we report c_p and sound velocity data around the melt, as well as around the glass transition.     

Data Analysis Without Fourier Transformation for Sawtooth-type Temperature-modulated DSC

The response of a differential scanning calorimeter (DSC) to sawtooth-type temperature modulation has been analyzed in the time domain using a standard treatment of the DSC data without Fourier transformation into the frequency domain. This method has some of the advantages of a temperature-modulated DSC (TMDSC) and may achieve a reasonable accuracy with more transparent and less time-consuming data analysis than the current TMDSC. The limits of linearity and stationarity of the thermal response, a prerequisite for the validity of the calculation of the reversing heat capacity by Fourier transformation, can be easily recognized in standard DSC. In contrast to the common handling of TMDSC, where the non-reversing contributions are calculated as difference between the total and reversing parts, we define a new, directly measured quantity, called the imbalance in heat capacity. It represents the difference between heating and cooling due to the non-reversing thermal process. This quantity is also of value for the representation of irreversible contributions inquasi-isothermal processes, such as cold crystallization and the annealing of crystallites in the melting range. This revised version was published online in July 2006 with corrections to the Cover Date.     

Structural Studies on PEK/TPI Blends

Blends of poly(ether ketone) (PEK) with poly(terephthaloyl-imide) (a thermoplasticpolyimide, TPI) were studied by temperature-modulated DSC (TMDSC) and X-ray diffraction. Samples were prepared by compression moulding of the premixed materials at 400°C and quenched to prevent crystallisation.The amorphous blends showed a single glass transition but with a jump in the temperature value at 60 mass% of PEK, indicating limited miscibility of the system at both sides of the composition series in the quenched, glassy state. Two cold crystallisation peaks over the concentration range 30 to 70 mass% of PEK were observed, but only one for all other compositions. A single melting peak was observed in all systems.Blends crystallised from the glassy state showed eutectic behaviour with the presence of the crystals of both pure components. This is the first reported case of two semicrystalline polymers exhibiting eutectic co-crystallisation. The formation of eutectic crystals is proof of full miscibility of the two polymers in their liquid state, i.e. at a temperature of 400°C and above. Blends cooled from the melt at a cooling rate of 2 K min^–1 showed a single glass transition and an extended melting range.Crystallisation during a second melting run generally starts at a different temperature then during the first run indicating chemical changes occurred in the molten state. This change was also verified by an exothermic peak above the melting temperature using TMDSC.This revised version was published online in November 2005 with corrections to the Cover Date.     

Effects of carbon nanotubes on crystallization and melting behavior of poly(L‐lactide) via DSC and TMDSC studies

In this work, multiwalled carbon nanotubes (MWNTs) were surface‐modified and grafted with poly(L ‐lactide) to obtain poly(L ‐lactide)‐grafted MWNTs (i.e. MWNTs‐g‐PLLA). Films of the PLLA/MWNTs‐g‐PLLA nanocomposites were then prepared by a solution casting method to investigate the effects of the MWNTs‐g‐PLLA on nonisothermal and isothermal melt‐crystallizations of the PLLA matrix using DSC and TMDSC. DSC data found that MWNTs significantly enhanced the nonisothermal melt‐crystallization from the melt and the cold‐crystallization rates of PLLA on the subsequent heating. Temperature‐modulated differential scanning calorimetry (TMDSC) analysis on the quenched PLLA nanocomposites found that, in addition to an exothermic cold‐crystallization peak in the range of 80–120 °C, an exothermic peak in the range of 150–165 °C, attributed to recrystallization, appeared before the main melting peak in the total and nonreversing heat flow curves. The presence of the recrystallization peak signified the ongoing process of crystal perfection and, if any, the formation of secondary crystals during the heating scan. Double melting endotherms appeared for the isothermally melt‐crystallized PLLA samples at 110 °C. TMDSC analysis found that the double lamellar thickness model, other than the melting‐recrystallization model, was responsible for the double melting peaks in PLLA nanocomposites. Polarized optical microscopy images found that the nucleation rate of PLLA was enhanced by MWNTs. TMDSC analysis found that the incorporation of MWNTs caused PLLA to decrease the heat‐capacity increase (namely, ΔC_p) and the C_p at glass transition temperature. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1870–1881, 2007     

Investigation of Progesterone Loaded Poly(D,L-Lactide) Microspheres Using TMDSC,SEM and PXRD

Poly(d,l-lactide) microspheres with progesterone loadings of 0, 10, 20, 30 and 50% w/w were manufactured using an interrupted solvent evaporation process. Spherical microspheres with loadings close to the theoretical values were produced. The glass transition of the polymer could be identified by a step change in the heat capacity measured by TMDSC. Progesterone was found to plasticise the glass transition temperature at contents of 20% w/w or less. At a 30% loading, cold crystallisation of progesterone was seen indicating that an amorphous form of the drug was present; these microspheres were found to exhibit a pitted surface. TMDSC of the 50% progesterone samples suggested that most of the drug was present as crystals. This was supported by the SEM and PXRD results. This revised version was published online in July 2006 with corrections to the Cover Date.     

TMDSC analysis of single-site copolymer blends after thermal fractionation

Temperature-modulated differential scanning calorimetry (TMDSC) has been used to study the melting of a series of blends containing linear low-density polyethylene (LLDPE) and very low-density polyethylenes (VLDPE) with long chain branches. After the blends were subjected to different thermal histories including thermal fractionation by stepwise isothermal cooling, they were examined by TMDSC. TMDSC curves have been interpreted in terms of a combination of the reversing and non-reversing specific heats that result from reversible and irreversible events at the time and temperature, which they are detected, respectively. It was found that crystals formed at different crystallisation conditions had different internal order; hence they showed different amounts of reversing and non-reversing contributions. There is no exothermic activity seen in the non-reversing signal for the thermally fractionated polymers and their blends suggesting formation of crystals approaching equilibrium. In contrast, polymers and blends cooled at 10°C min^-1 cooling rate showed large exothermic contributions corresponding to irreversible effects. In addition, a true reversible melting contribution is also detected for both fast-cooled and thermally-fractionated samples during the quasi-isothermal measurements. This revised version was published online in July 2006 with corrections to the Cover Date.