材料中玻璃化转变灵敏度表征检测方案(差示扫描量热)

ONSET AND PEAK DETERMINATIONSWITHINCORRECT CURSOR PLACEMENT GLASS TRANSITION OF POLYCARBONATE (13mg)by DSC AT 20°C/min Thermal Analysis & Rheology Exploring the Sensitivity ofThermal Analysis Techniquesto the Glass Transition J. Foreman， S. R. Sauerbrunn and C. L. Marcozzi TAInstruments，Inc. 109 Lukens Drive New Castle， DE 19720 ABSTRACT Five thermal analysis techniques (DSC， modulated DSCTM， TMA，DMA， and DEA) are used to characterize the glasstransition temperature (Tg) for amorphous polymers. Each of these thermal techniques detects the Tg based onchanges in a different material property during the glass transition. Hence， the relative sensitivities ofthe differenttechniques for detecting the Tg vary depending on the nature of the material being evaluated as well as on experimentalvariables such as the heating rate. This study compares the Tg information obtained from the five common thermaltechniques on a typical amorphous thermoplastic. The results illustrate the types of issues that can arise. INTRODUCTION The glass transition is the temperature region where an amorphous material changes from a glassy phase to a rubberyphase upon heating， or vice versa if cooling. The glass transition is very important in polymer characterization as theproperties of a material are highly dependent on the relationship of the polymer end-use temperature to its Tg. Forexample， an elastomer will be brittle if its Tg is too high， and the upper use temperature of a rigid plastic is usuallylimited by softening at Tg. Hence an accurate and precise measure of Tg is a prime concern to many plastics manufac-turers and end use designers. Thermal analysis， which is a generic term used to describe a family of analytical techniques that measure changes inthe physical properties of a material with temperature， provides a convenient means of measuring the glass transition.Each of the thermal analysis techniques senses the glass transition based on changes in a specific material property.Table 1 summarizes the different thermal techniques， the property change(s) measured for each during the glasstransition， and an indication of the relative sensitivity (relative signal change) of each for detecting the Tg. TABLE1Properties Measured by TA Techniques Relative Signal Technique Property Measured Change at Tg Differential Scanning Calorimetry Heat flow (heat capacity) 0.2 Thermomechanical Analysis Expansion coefficient or softening 3 Dielectric Analysis Permittivity and dielectric loss 100 Dynamic Mechanical Analysis Mechanical strength and energy loss 200 In this study， a typical amorphous thermoplastic polymer is evaluated by all of the techniques shown in Table 1， as wellas modulated DSCTM， to illustrate some of the experimental trade-offs and considerations commonly encountered. Auser’s guide based on the results， and those for other types of polymers， is included as a summary. EXPERIMENTAL Materials Two unfilled amorphous thermoplastic materials were used in this study， polycarbonate and polystyrene. Unfilledamorphous thermoplastics undergo larger property changes at the glass transition than any other polymer type ormorphology. In materials where the amorphous content is reduced， such as highly crystalline or highly filled thermo-plastics， or where crosslinking reduces the size of mobile polymer chains， the property changes at the glass transitionwill generally be reduced. The smaller signal changes decrease the sensitivities of the respective thermal techniques tothe glass transition， though each technique is affected to a differing degree， as will be discussed. The polycarbonate used was TUFFAK A from Rohm & Haas Co. The polystyrene was Aldrich Chemicals CatalogNumber 18242-7. Polycarbonate samples for DMA evaluation were prepared by cutting bars 12mm wide by 60mmlong from the 3mm thick sheets received. Samples for evaluation by the other techniques were prepared by pressing0.15mm films from the original 3mm sheets in a 155℃ hot press. Polystyrene samples for DSC and TMA evaluationwere prepared by pressing the polymer beads originally received into 0.2-0.5mm films in a 200℃ hot press. Instrumentation All experiments were performed on the following thermal analysis equipment from TA Instruments： DSC 2910 withautosampler and MDSCTM upgrade， TMA 2940， DMA 983， and DEA 2970. All specimens were run under a nitrogenatmosphere. The results reported in this study were obtained from heating experiments only. Results from coolingexperiments， although similar， are the topic for another study and hence are not included here. RESULTS ANDDISCUSSION Sensitivity to Transition Representation There are two basic representations of the glass transition which are commonly used. They are onset or step changeand peak maximum. In general， the measurement based on onset or step change is subject to greater uncertainty thanthe measurement based on a peak maximum because the former measurement relies on the ability to accurately definebaselines and tangents surrounding the transition. This is illustrated in Figures 1 and2. Figure 1 shows the correctcursor placement for onset and peak determinations. Figure 2 shows similar determinations where cursor placementfor drawing the baseline tangent has been shifted 3℃. Note that this shift results in a 1.5℃ decrease in the Tg deter-mined from the onset while no change occurs in the measured peak maximum. In general， the glass transition temper-ature is less subject to operator interpretation when the property measured by the specific thermal analysis techniquerelies on a signal peak (e.g. DMA damping， DSC heat flow derivative). ONSET AND PEAK DETERMINATIONSWITH CORRECT CURSOR PLACEMENT 80 Temperature (°C) Figure 1 Figure 2 DSC Sensitivity DSC， which measures heat flow to and from a specimen relative to an inert reference， is the most common thermalanalysis method used to measure the glass transition. The heat capacity step change at the glass transition yields threetemperature values： onset， midpoint and endset. The midpoint is usually calculated as the peak maximum in the firstderivative of heat flow (see Figure 3) although it can also be calculated as the midpoint of the extrapolated heatcapacities [1] before and after the glass transition. The DSC thermal curve for polycarbonate heated at 20℃/minute is shown in Figure 3. Most DSC experiments areperformed at 5 or 10°℃/minute. A higher heating rate is beneficial in detecting Tg， however， because the heat flowsignal associated with heat capacity change during the glass transition is enhanced with very little correspondingincrease in noise， thereby increasing sensitivity. Nevertheless， this increase in sensitivity with increased heating rate does have penalties. Both the temperature andbreadth of the glass transition are affected by increased heating rates. Figure 4 shows the glass transition of polysty-rene (18 mg specimen) by DSC at various heating rates. Notice that Tg shifts to higher temperatures and the transitionbroadens as the heating rate is increased， especially at 20 and 50℃/min. It is therefore important to report heating ratealong with Tg values. GLASS TRANSITION OF POLYSTYRENE (18mg)by DSC VERSUS HEATING RATE Figure 4 Three components contribute to this heating rate -related shift： instrument effects， sample thermal conductivity， andtransition kinetics. The instrument effects associated with cell mass and heat transfer properties are relatively small，typically amounting to about 0.1℃ per decade change in heating rate [2，3]. Thermal conductivity effects on the otherhand are more significant particularly with larger samples or with samples of low thermal conductivity. In those cases，a thermal gradient develops across the sample which broadens and shifts the Tg to higher temperature. Figure 5illustrates that effect for polystyrene. The width of the glass transition (the difference between the onset () and theendset (()) is rather constant until 5℃/min where the width increases dramatically with increasing heating rate. Thusat heating rates below 5C/min the sample thermal conductivity has little effect on the Tg measurement. Below 5°C/min， though， the Tg measurements do continue to change with heating rate. That shift (approximately 2°℃ per tenfoldchange in heating rate) can be attributed to the kinetics of the glass transition. Analysis of this data using Arrheniuskinetics yields an activation energy of 890kJ/mole. Analysis of DMA multi-frequency data by time-temperaturesuperpositioning yields a similar activation energy， confirming the kinetic effect on Tg. GLASS TRANSITION ONSET， MIDPOINT AND ENDSETOF POLYSTYRENE (18 mg) BY DSC VERSUS HEATING RATE Figure5 Volume relaxation peaks are another phenomenon observed in DSC which can affect accurate Tg measurement. Thesepeaks occur in materials when the cooling rate encountered by the polymer during processing (thermal history) ismuch slower than the heating rate used during DSC evaluation (Figure 6). By imposing the proper thermal history onthe material or by using modulated DSCTM， the effects of volume relaxation can be reduced. Finally， DSC measurement of Tg is significantly affected by high crystallinity in the polymers and by adding reinforc-ers or fillers to the polymer. Since these additional materials do not contribute to the heat capacity change measured atthe glass transition， they act as “dilutants”，adding sample weight without increasing sensitivity. EFFECT OF COOLING RATE ON VOLUME RELAXATIONBY DSC Figure6 MDSCTM Sensitivity Modulated DSC (MDSC) is a high performance version of conventional DSC that provides information about thereversing and nonreversing characteristics of thermal events， in addition to traditional DSC heat flow and temperatureinformation. In MDSC a sinusoidal temperature ripple (modulation) is overlaid on the conventional linear temperatureramp to yield a modulated heating ramp. This modulated heating causes the heat flow to oscillate. Fouriertransformation deconvolution converts the modulating heat flow signal into reversing and nonreversing componentsand directly calculates specimen heat capacity. The details of this technique are described elsewhere [4]. Briefly，reversing components of heat flow are those which are both thermodynamically reversible and kinetically feasible. Theremainder of the heat flow， which includes events which are not reversible or are not kinetically rapid， is grouped intothe nonreversing heat flow signal. MDSC has value in measuring the Tg in three areas. First， it improves interpretation by separating the volumerelaxation endotherm (a nonreversing phenomenon) from the reversing heat capacity change at the glass transition.Second， MDSC can separate nonreversing transitions such as cold crystallization and curing from the glass transitionwhich is impossible by traditional DSC. Finally， sensitivity for heating rate dependent transitions such as the glasstransition is increased over traditional DSC due to the high effective heating rates achieved during modulation. Thefollowing examples illustrate each of these situations. Figure 7 is Modulated DSC scan for polycarbonate. The total heat flow (solid line) shows a Tg (inflection point) at146.3°C. Note that this temperature is lower than that seen in Figure 3 because of the lower underlying heating rate ofthe MDSC experiment. The nonreversing curve (long dashes) shows a small (0.8 J/g) endothermic volume relaxationpeak. The Fourier transform deconvolution separates this relaxation from the heat capacity curve (short dashes).The resulting Tg， 148.6C， is an improved value because interference from the volume relaxation is removed. Figure 8 shows results of an MDSC experiment on a bilayer film containing polycarbonate (PC) and amorphouspolyethylene terephthalate (PET). The conventional DSC curve (solid line) shows a single broad transition between130°℃ and 150℃ which is difficult to interpret because it represents overlap of the polycarbonate glass transition andthe PET cold crystallization exotherm. The MDSC results， on the other hand， clearly separate these two phenomenabased on the fact that the polycarbonate glass transition is a reversing transition， while the PET crystallization is anonreversing phenomenon. GLASS TRANSITION OF POLYCARBONATE BY MDSCTM(12 MG.5°C/min， 0.5°℃ AMPLITUDE AND 50 SECOND PERIOD) Figure 7 GLASS TRANSITION OF POLYCARBONATE BY MDSCTM IN APOLYCARBONATE/POLYETHYLENETEREPHTHALATE BILAYERFILM Figure 8 Figures 9 and 10 illustrate the increased glass transition sensitivity available from MDSCTM. In MDSC， a low underly-ing heating rate (1-2°C/minute) is used， thus providing good resolution of transitions but very low sensitivity. Thesuperimposed modulated heating profile， however， results in instantaneous heating rates which are much higher than theunderlying rate， thereby increasing sensitivity to transitions such as the glass transition. The overall result is a combi-nation of resolution and sensitivity not available from conventional DSC. Figure 9 shows a high temperature epoxyduring isothermal curing at 90°℃. The solid line represents the nonreversing heat flow with time and shows theexotherm associated with curing. This peak is identical to what would be observed in the conventional DSC experi-ment. A second signal can be measured in MDSC， however. That is the heat capacity (dashed line)， which clearlyshows a step decrease associated with the reduction in polymer free volume and molecular motion as the thermosetgoes from a rubbery phase to a glassy phase (vitrifies). This decrease occurs after the exothermic peak maximumwhich implies that heat capacity changes more dramatically during crosslinking than during linear polymerization.Evaluation by DMA supports this conclusion since the storage modulus (dash-dot curve) increases at exactly the sametime as heat capacity begins to decrease. The breadth of the reaction results from the diffusion control of the processduring vitrification[5]. This ability to measure heat capacity and vitrification at an underlying heating rate of 0°C/minute (isothermal) is not possible in conventional DSC. Figure 10 illustrates another example where the MDSCreversing heat flow curve (broken line) indicates a Tg not observable in conventional DSC. ISOTHERMAL CURING OF EPOXY RESIN BY MDSCTMAND DMA Figure9 TMA Sensitivity TMA is generally used to measure the glass transition based on changes in coefficient of thermal expansion whichresult as the free volume of the material changes at the glass transition. Figure 11 shows the Tgof polycarbonateheated at 3℃/min where the Tg is defined as the onset of change in rate of expansion (slope) at 140.2℃. For thepolycarbonate specimen， as with many thermoplastics， the expansion coefficient above Tg is not as well behavedbecause material softening counteracts the probe motion due to thermal expansion. Another TMA approach for determining glass transition is penetration in which a substantial force is applied to a smallpoint on the material surface. The penetration mode detects Tg as a downward probe movement as the materialbecomes less rigid in the transition from glassy to rubbery behavior. This mode is used for materials which cannotsupport much force after Tg， or which exhibit very broad ill-defined expansion changes. Figure 12 shows the TMApenetration results for polystyrene. Measurement of Tg by TMA is better for filled， highly crystalline， or crosslinked materials than measurement by DSCbecause the dimensional changes observed at Tg are usually fairly significant. On the other hand， the Tg profiles fromTMA are often broad， can be affected by probe loading conditions， and may be complicated by volume relaxationeffects Figure11 GLASS TRANSITION OF POLYSTYRENE BY TMA(PENETRATION PROBE，3°C/min AND 5 grams) DMA Sensitivity DMA measures mechanical stiffness (modulus) and energy absorption by subjecting a specimen to oscillating mechan-ical stress and strain within the linear viscoelastic region. At the glass transition， the increase in molecular motionwithin the polymer results in a dramatic step decrease (up to four decades) in the storage modulus (E') making DMAprobably the most sensitive thermal technique for Tg determinations. In fact， the change in modulus signal is readilydetectable even in the most highly filled， crystalline， or crosslinked material where the amorphous fraction is verysmall. Energy is also absorbed as the molecular motion increases. As the specimen proceeds through the glass transition， therate of energy absorption goes through a maximum resulting in peaks in the loss modulus (E") and Tan Delta curves.The temperatures of the three DMA events can be used to define the range of the glass transition. For polycarbonate (3°℃/min， 1 Hz)， the Tg values are 141.8℃ (E'onset)， 147.1℃ (E" peak) and 151.5℃ (Tan Deltapeak)， see Figure 13. Each value has meaning relative to specific applications. The E' (storage modulus) onset definesthe temperature at which the material’s strength will begin to decrease， such that the material may no longer be able tobear a load without deforming. The peak in the loss modulus (E") represents the temperature at which the material isundergoing the maximum change in polymer mobility， which corresponds to the chemical definition of the Tg. Theloss tangent (tan delta) peak describes the damping characteristics of a material and also has historical significance，since it was the first DMA property quantified and much of the DMA Tg reference data is based on the tan delta peaktemperature. The Tg measured by DMA is dependent on the oscillation frequency， because the glass transition is a second order，kinetically limited transition. That is， Tg is both time and temperature dependent. Hence， when comparing resultsfrom different DMA evaluations or when comparing DMA results to those from other thermal techniques， oscillationfrequency must be appropriately chosen and kept constant. The frequency of 1 Hz is usually chosen as a vcompromialue whseich gives Tg values comparable to other thermal methods while allowing collection of data at a sufficient rateto permit reasonable experimental times. DMA experiments using multiple frequencies provide additional informationuseful for defining kinetic parameters for the glass transition， which permit prediction of material properties overbroader frequency ranges [6] such as those the material could encounter in actual end-use. GLASS TRANSITION OF POLYCARBONATE BY DMA DEA Sensitivity DEA measures the ability of dipoles and trace ions present in a material to align with an oscillating electric field. Theincrease in molecular motion associated with the glass transition allows the dipoles or ions in an amorphous polymerto more freely align with the electric field and dissipate energy. The sensitivity of the DEA to the Tg is dependent onthe strength of permanent dipoles and the amount and mobility of ions in the polymer. The glass transition of polycar-bonate (3°C/min， 1Hz) is measured as the onset of the increase in the permittivity signal (e') at 144.9℃， and as thepeak (151.1℃) in the loss factor (e")， (see Figure 14). The Tg measured by DEA is frequency dependent as with the DMA. Dielectric properties can easily be measured overa very wide frequency range (more than 8 orders of magnitude)， since high electrical frequency is much more readilyachieved than its mechanical counterpart. The wide frequency range allows measurement of subtle transitions as wellas multiple-mechanism transitions that are sensitive to the frequency of measurement. Dielectric analyzers can measure Tg on a wide variety of samples including films， liquids and powders. Furthermore，dielectric loss is a sensitive probe that allows measurement of properties of filled materials. DEA on the other hand， islimited to materials that have a reasonable level of dipole groups or trace ions present (polyolefins for example are noteasily evaluated by DEA)， and DEA does not work for materials which are electrically conductive. GLASS TRANSITION OF POLYCARBONATE FILM BY DEA(1 Hz，3°C/min) Figure 14 SUMMARY In this study， DMA is the most sensitive technique for measuring the Tg， followed by DSC which is about one half assensitive as DMA. TMA and DEA are less sensitive techniques for the polycarbonate studied since these methodsexhibit larger signal to noise ratios. The order of these sensitivities is different than expected based solely upon therelative signal strengths in Table 1. The fact that DSC is more sensitive than expected can be attributed to the superiorbaseline performance (noise reduction) of modern DSC instrumentation combined with the use of a high heating rate.DEA， on the other hand， is less sensitive than expected because of the absence of strong molecular dipoles in polycar-bonate. Despite sensitivity differences， however， the Tg temperatures observed for polycarbonate by the five thermalanalysis techniques showed good agreement (Table 2). TABLE 2Glass Transition of Polycarbonate by TA Techniques Method Thermal Conditions Onset Midpoint Endset (C) (C) (C) DSC 20°C/min. 144.0 148.0 149.1 MDSCM 5℃/min， 0.5℃ amp， 50 sec period 144.0 148.6 153.5 TMA 30°C/min，0.5 g load 140.2 N/A N/A DMA 3°C/min. 1 Hz 141.8 147.1 151.5 DEA 3°C/min. 1 Hz 144.9 151.1 N/A Although there is insufficient space in this brief overview to study a broad spectrum of polymeric materials， a largeamount of work has already been done in our applications laboratory on evaluating glass transitions. Table 3 is asummary based on that work. This table indicates the relative utility of the different thermal techniques for evaluatingTg in different materials and should be a useful starting point when selecting techniques for evaluating new materials. TABLE 3Preferred Thermal Methods Polymer Type DSC MDSCM TMA DMA DEA Amorphous best best best best better Semi-crystalline better best best best better Highly crystalline good good best best better Plasticized better best better best better Thermosetting resin best best better better best Cured thermoset good better better best better Elastomer better best better best better Glass Filled good good better best better Carbon Filled good good better best good Volume Relaxation Present (a) best (a) better better a) Volume relaxation may interfere with Tg precision. REFERENCES 1) B. Wunderlich， Thermal Analysis， Acad. Press， p 101 (1990). 2) ASTM Test Method E-698. 3) J. H. Flynn， et. al.， Thermochimica Acta， 134， pp 401-406 (1988). 4) S. R. Sauerbrunn， B. S. Crowe and M. Reading， NATAS Proc.， 21，pp 137-144(1992). 5) R. B. Prime in Thermal Characterization of Polymeric Materials， E. Turi ed.，Academic Press， p 438 (1981). 6) A.V. Tobolsky， Properties and Structures of Polymers， Wiley， New York (1960). For more information or to place an order， contact： TAInstruments，Inc.， 109 Lukens Drive，New Castle，DE 19720，Telephone： (302)427-4000，Fax：(302)427-4001TAInstruments S.A.R.L.，Paris，France， Telephone： 33-01-30489460，Fax：33-01-30489451 TAInstrumentsN.V./S.A.，Gent，Belgium， Telephone： 32-9-220-79-89，Fax：32-9-220-83-21 TAInstruments GmbH，Alzenau，Germany， Telephone： 49-6023-30044，Fax：49-6023-30823 TAInstruments， Ltd.，Leatherhead，England，Telephone： 44-1-372-360363，Fax：44-1-372-360135 TAInstruments Japan K.K.， Tokyo，Japan， Telephone： 813-3450-0981，Fax：813-3450-1322 Figure Five thermal analysis techniques (DSC, modulated DSC™, TMA, DMA, and DEA) are used to characterize the glass transition temperature (Tg) for amorphous polymers. Each of these thermal techniques detects the Tg based on changes in a different material property during the glass transition. Hence, the relative sensitivities of the different techniques for detecting the Tg vary depending on the nature of the material being evaluated as well as on experimental variables such as the heating rate. This study compares the Tg information obtained from the five common thermal techniques on a typical amorphous thermoplastic. The results illustrate the types of issues that can arise.