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.     

Temperature modulated dynamic mechanical analysis

Temperature modulated dynamic mechanical analysis (TMDMA) was performed in the same way as temperature modulated DSC (TMDSC) measurements. Temperature modulation with amplitude 0.5 K and period 20 min was realised by a series of linear heating and cooling cycles (saw-tooth modulation). As in TMDSC TMDMA allows for the investigation of reversible and non-reversible phenomena in the melting and crystallisation region 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.     

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.     

Recent developments in thermal analysis of polymers: Calorimetry in the limit of slow and fast heating rates

This review focuses on new insights into the crystal melting transition and the amorphous glass transition of polymers that have been gained through recent advances in thermoanalytical methods. The specific heat capacity can now be studied under two extreme limits, that is, under quasi‐isothermal conditions (limit of zero heating rate) and, at the other end of the scale, under rapid heating conditions (heating rates on the order of thousands of degrees per second), made possible through nanocalorimetry. The reversible melting, and multiple reversible melting, of semicrystalline polymers is explored using quasi‐isothermal temperature modulated differential scanning calorimetry, TMDSC. The excess reversing heat capacity, above the baseline, measured under nearly isothermal conditions is attributed to locally reversible surface melting and crystallization processes that do not require molecular nucleation. Observations of double reversible melting endotherms in isotactic polystyrene suggest existence of two distinct populations of crystals, each showing locally reversible surface melting. The second subject of the review, nanocalorimetry, is utilized to study samples of small mass under conditions of very fast heating and cooling. The glass transition properties of thin amorphous polymer films are observed under adiabatic conditions. The glass transition temperature appears to be independent of film thickness, and is observed even in ultra‐thin films. Recrystallization and reorganization during rapid heating are studied by nanocalorimetry of semicrystalline polymers. The uppermost endotherm seen under normal DSC scanning of poly(ethylene terephthalate) is caused by reorganization, and vanishes under the rapid heating conditions (3000K/s) provided by nanocalorimetry. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 629–636, 2005     

Temperature Modulated DSC of Irreversible Melting of Nylon 6 Crystals

A new method is presented to analyze the irreversible melting kinetics of polymer crystals with a temperature modulated differential scanning calorimetry (TMDSC). The method is based on an expression of the apparent heat capacity, , with the true heat capacity, mc_p, and the response of the kinetics, . The present paper experimentally examines the irreversible melting of nylon 6 crystals on heating. The real and imaginary parts of the apparent heat capacity showed a strong dependence on frequency and heating rate during the melting process. The dependence and the Cole-Cole plot could be fitted by the frequency response function of Debye's type with a characteristic time depending on heating rate. The characteristic time represents the time required for the melting of small crystallites which form the aggregates of polymer crystals. The heating rate dependence of the characteristic time differentiates the superheating dependence of the melting rate. Taking account of the relatively insensitive nature of crystallization to temperature modulation, it is argued that the ‘reversing’ heat flow extrapolated to ω → 0 is related to the endothermic heat flow of melting and the corresponding ‘non-reversing’ heat flow represents the exothermic heat flow of re-crystallization and re-organization. The extrapolated ‘reversing’ and ‘non-reversing’ heat flow indicates the melting and re-crystallization and/or re-organization of nylon 6 crystals at much lower temperature than the melting peak seen in the total heat flow. This revised version was published online in July 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     

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.     

Reversible melting of high molar mass poly(oxyethylene)

The heat capacity, C_p, of poly(oxyethylene), POE, with a molar mass of 900,000 Da, was analyzed by temperature-modulated differential scanning calorimetry, TMDSC. The high molar mass POE crystals are in a folded-chain macroconformation and show some locally reversible melting, starting already at about 250 K. At 335 K the thermodynamic heat capacity reaches the level of the melt. The end of melting of a high-crystallinity sample was analyzed quasi-isothermally with varying modulation amplitudes from 0.2 to 3.0 K to study the reversible crystallinity. A new internal calibration method was developed which allows to quantitatively assess small fractions of reversibly melting crystals in the presence of the reversible heat capacity and large amounts of irreversible melting. The specific reversibility decreases to small values in the vicinity of the end of melting, but does not seem to go to zero. The reversible melting is close to symmetric with a small fraction crystallizing slower than melting, i.e., under the chosen condition some of the melting and crystallization remains reversing. The collected data behave as one expects for a crystallization governed by molecular nucleation and not as one would expect from the formation of an intermediate mesophase on crystallization. The method developed allows a study of the active surface of melting and crystallization of flexible macromolecules.     

Reversible and irreversible heat capacity of poly(trimethylene terephthalate) analyzed by temperature‐modulated differential scanning calorimetry

The heat capacity of poly(trimethylene terephthalate) (PTT) has been analyzed using temperature‐modulated differential scanning calorimetry (TMDSC) and compared with results obtained earlier from adiabatic calorimetry and standard differential scanning calorimetry (DSC). Using quasi‐isothermal TMDSC, the apparent reversing and nonreversing heat capacities were determined from 220 to 540 K, including glass and melting transitions. Truly reversible and time‐dependent irreversible heat effects were separated. The extrapolated vibrational heat capacity of the solid and the total heat capacity of the liquid served as baselines for the analysis. As one approaches the melting region from lower temperature, semicrystalline PTT shows a reversing heat capacity, which is larger than that of the liquid, an observation that is common also for other polymers. This higher heat capacity is interpreted as a reversible surface or bulk melting and crystallization, which does not need to undergo molecular nucleation. Additional time‐dependent, reversing contributions, dominating at temperatures even closer to the melting peak, are linked to reorganization and recrystallization (annealing), while the major melting is fully irreversible (nonreversing contribution). © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 622–631, 2000     

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.     

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.     

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.     

Melting of polymers by non-isothermal, temperature-modulated calorimetry: analysis of various irreversible latent heat contributions to the reversing heat capacity

This paper provides an analysis of contributions to the apparent, reversing heat capacity when measured by temperature-modulated differential scanning analysis (TMDSC) with an underlying heating rate in the temperature range where irreversible transitions with latent heats occur. To deconvolute the data of a TMDSC scan into a total and reversing part, it is common practice to use the sliding averages and the first harmonics of the Fourier series of temperature and heat-flow rate. Under certain conditions, this procedure produces erroneous reversing contributions which are detailed by experiment and simulation. Unless the response to the temperature modulation is linear, the total heat-flow rate is stationary, and the transition is truly reversible and occurs only once during the temperature scan, one cannot expect a true deconvolution of total and reversible effects. In the presence of multiple, irreversible transitions within a modulation period, however, each process involving latent heat can increase the modulation amplitude, as demonstrated by computer-simulation of polymer melting. As a result, the multiple transitions may give erroneously high latent heats when integrating the apparent reversing heat capacity with respect to temperature.     

Linear Relations of Activation Energy in Non-isothermal Kinetics

Temperature-modulated DSC (TMDSC) measurements at reasonably high frequencies allow for the determination of base-line 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 lowest endotherm. For PC the melting of small crystals between the lamellae is expected to yield the lowest endotherm. This revised version was published online in July 2006 with corrections to the Cover Date.     

Periodically Modulated Driving Force Applied with TMDSC to the Crystallization and Melting Kinetics of Ice Crystals Confined in a Porous Silica Gel

The application of a periodically modulated driving force has been examined in the melting and crystallization kinetics of ice crystals confined in a porous media. The kinetic response of transformation gives the real and imaginary parts of the ‘apparent’ heat capacity obtained with a temperature modulated differential scanning calorimetry (TMDSC). Based on a modelling of the kinetics, the detailed examination of the frequency dispersion and its dependence on underlying heating/cooling rate enables us to evaluate the transformation rate and the dependence of the rate coefficient on the driving force, i.e. the degree of supercooling or superheating. The experimental results indicate that the transformation processes are limited by heat diffusion from the growth interface of each crystallite to surroundings. This revised version was published online in July 2006 with corrections to the Cover Date.     

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     

A Detailed Comparison of First Order Transitions by DSC and TMC

Indium was analyzed with both, standard differential scanning calorimetry (DSC) and temperature-modulated DSC (TMDSC) using sinusoidal and saw-tooth modulation. Instrument and sample effects were separated during nucleated, reversible melting and crystallization transitions, and irreversible crystallization with supercooling. The changes in heat flow, time, and sample and reference temperatures were correlated as functions of heating rate, mass, and modulation parameters. The transitions involve three regions of steady state (an initial and a final region before and after melting/crystallization, a region while melting/crystallization is in progress) and one region of approach to steady state (melting peak to final steady state region). Analyses in the time domain show promise when instrument lags, known from DSC, are used for correction of TMDSC. A new method of integral analysis is introduced for quantitative analysis even when irreversible processes occur in addition to reversible transitions. The information was derived from heat-flux calorimeters with control at the heater block or at the reference temperature sensor. This revised version was published online in July 2006 with corrections to the Cover Date.     

Analysis of the complex thermal behavior of poly(L‐lactic acid) film. I. Samples crystallized from the glassy state

The melting behavior of poly(L ‐lactic acid) film crystallized from the glassy state, either isothermally or nonisothermally, was studied by wide angle X‐ray diffraction (WAXD), small angle X‐ray scattering (SAXS), differential scanning calorimetry (DSC), and temperature‐modulated differential scanning calorimetry (TMDSC). Up to three crystallization and two melting peaks were observed. It was concluded that these effects could largely be accounted for on the basis of a “melt‐recrystallization” mechanism. When molecular weight is low, two melting endotherms are readily observed. But, without TMDSC, the double melting phenomena of high molecular weight PLLA is often masked by an exotherm just prior to the final melting, as metastable crystals undergo melt‐recrystallization during heating in the DSC. The appearance of a double cold‐crystallization peak during the DSC heating scan of amorphous PLLA film is the net effect of cold crystallization and melt‐recrystallization of metastable crystals formed during the initial cold crystallization. Samples cold‐crystallized at 80 and 90 °C did not exhibit a long period, although substantial crystallinity developed. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3200–3214, 2006     

Specific reversible melting of polymers

Temperature‐modulated differential scanning calorimetry can detect a certain amount of reversible latent heat in flexible macromolecules. In short, one can identify a reversible melting in such polymers earlier thought to exhibit only fully irreversible crystallization and melting. Details of the reversible melting of isotactic polypropylene and ethylene‐1‐octene copolymers of low and medium densities have newly been measured and linked to the crystallization, annealing, or melting temperature. It is possible to assign the experimental reversibility of melting to specific crystal fractions that ultimately melt irreversibly at higher temperatures; that is, it is suggested that reversible melting mainly occurs only between the temperatures of their formation and their zero‐entropy‐production melting temperature, at which they change to a melt of the same degree of metastability. This is supported by the almost complete absence of reversibility below the temperature of crystal formation and the observation of a distinct relationship between the amount of irreversibly by annealing reorganized material and reversibility in the case of isotactic polypropylene. A given crystal fraction, characterized by its formation temperature and zero‐entropy‐production melting temperature, has a specific reversibility of the melt‐to‐crystal transition, which is represented by the ratio of the reversible latent heat to the total enthalpy change when the crystal fraction of interest ultimately melts. This specific reversibility is, for ethylene‐1‐octene copolymers, at least 25% at temperatures in the primary crystallization range, and this indicates that the reversible contribution to the total of the melting processes is much larger than expected from simple calculations by the excess apparent reversible heat capacity being referred to the heat of fusion of the polymer, as is commonly done. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2039–2051, 2003