Simultaneous Differential Scanning Calorimetry and Infrared

21 Simultaneous Differential Scanning Calorimetry and Infrared Spectroscopy Francis M . Mirabella, Jr. Quantum Chemical Corporation, Rolling Meadows, IL 60008-4070

The simultaneous measurement of thermal response and infrared spectra on specimens of microscopic dimensions was demonstrated for polymer melting and crystallization of polymer blends. The measurement technique was achieved by combining a differential scanning calorimeter (DSC) having provision for microscopic observation of the specimen with an infrared microsampling accessory (IRMA) fitted into a Fourier transform infrared (FTIR) spectrometer. This combined technique was shown to provide important insight into structural changes associated with thermal response for a wide range of systems including polymers. In particular, the structural changes of polypropylene during melting were observed, and the separate crystallization of polypropylene and polyethylene in two-part blends was observed and correlated with the crystallization exotherms of the blends.

SIMULTANEOUS

M E A S U R E M E N T S O F T H E R M A L P R O P E R T Y R E S P O N S E and in-

frared spectra on specimens of microscopic dimensions can provide important insight into structural changes associated with thermal response for a wide range of systems. The simultaneous measurement of the thermal response in a differential scanning calorimeter (DSC) and the infrared spectra in a Fourier transform infrared (FTIR) spectrometer can provide this capability. In particular, the use of a D S C cell adapted for microscopic observation during thermal treatment, combined with an infrared microsampling accessory (IRMA) in an F T I R spectrometer, permits the simultaneous collection of such data. The range of applications of such a technique is undoubtedly large and varied. The various phase transitions and thermal responses of

0065-2393/90/0227-0357$06.00/0 © 1990 American Chemical Society

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POLYMER CHARACTERIZATION

crystalline and amorphous polymers, polymer blends, copolymers, and liquid-crystalline polymers, as well as small molecules, could be correlated with the structural information derivable from the simultaneously collected infrared spectra. This approach would permit studies of reaction and crystallization kinetics, oxidation, efficacy of additives, etc.

Experimental Details A Nicolet 6000 FTIR spectrometer was used. All spectra were recorded at 4 - c m resolution. Acceptable signal-to-noise ratios (S/N) were obtained by the coadding of 10 spectra at a mirror velocity of 0.586 cm/s. A Spectra-Tech IR-Plan III transmittance-reflectance microscope with a 250- X 250-u.m mercury-cadmium telluride (MCT) detector was used to focus the infrared radiation through the DSC cell. The DSC cell used was a Mettler FP84 thermal analysis microscopy cell. A Mettler FP80 central processor with recorder was used to control the temperature in the FP84 DSC cell. Typical DSC thermograms and infrared spectra could be simultaneously recorded with this combined apparatus. A schematic diagram of the IRMA is shown in Figure 1.

1

A schematic diagram of the DSC microscopy cell is shown in Figure 2. The DSC microscopy cell was placed on the stage of the IRMA. The infrared beam was required to pass through a 2.5-mm hole in the bottom of the DSC cell, travel 3.0 mm (thereby passing through the sample and sample cup), and exit from the DSC cell through a 3.0-mm hole on the way to the M C T detector. Thus, the infrared beam must pass through a cylinder of 2.5-mm diameter and 3.0-mm length. This situation requires a careful alignment procedure prior to operation. To permit both microscopical observation and transmission infrared spectroscopy, sample cups of potassium bromide were used to hold the sample. For simultaneous optical microscopy and DSC, sapphire sample cups were used. The thermal conductivity of KBr is 1.3 g-cal/s-cm-°C, that of sapphire is 0.6 g-cal/s-cm-°C. Thus, KBr is a reasonably good conductor of heat, but it is also transparent to visible light and infrared radiation in the wavelength range of 0.35-15 u,m. It is, therefore, an excellent material for use in the present application. The combination of temperature program rates in the DSC and spectral collection rates in the FTIR spectrometer offers a vast range in which to collect data simultaneously. Of course, the FTIR spectrometer has the capability of rapidly collecting spectra over the entire DSC thermogram. Spectra can be collected at the rate of about one per second and thereby a large number of spectra can be accumulated over a 15-20-min DSC experiment. These spectra may be important for full elucidation of the thermal response under investigation. For the purposes of this work and for ease of presentation, fewer spectra, collected over longer time, will be presented. The DSC was programmed at 10 °C/min. The FTIR spectrometer collected 10 scans in 10 s at the aforementioned conditions. Polymer blends were prepared from Quantum USI PP 8001-LK polypropylene (melt flow rate 5 g/10 min) and N H D 6180 high-density polyethylene (melt index 1.15 g/10 min and density 0.96+ g/cm ). Blends were prepared by repetitive melt blending in a hot press. Four blends at 20:80, 40:60, 60:40 and 80:20 wt% of polypropylene-polyethylene were prepared. 3

Results and Discussion The combined D S C - m i c r o s c o p y - F T I R technique was applied to the study of polymer melting and crystallization. Thermal analysis of polymers is a

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Simultaneous DSC and IR Spectroscopy

359

Figure 1. Schematic diagram of infrared microsampling accessory. useful technique for obtaining qualitative and quantitative information. However, the thermal analysis technique is, by its nature, limited by the fact that polymers do not have well-defined melting points or heats of fusion. Real polymer crystals are virtually never in thermodynamic equilibrium but rather are formed on the basis of kinetic factors during crystallization. For this reason polymer crystals may exhibit a large range of melting tempera-


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1 2 3 4 5

Microscope objective 6 Microscope light source DSC measuring sensor Sapphire sample crucible Heat protection filter & Metal plate with heating wires and Pt 100 resistance sensor Metal plate with heating wires Sapphire reference crucible 7

Figure 2. Photograph and schematic diagram of DSC microscopy cell. (Reproduced with permission from ref. 1. Copyright 1986 Society for Applied Spectroscopy.)

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hires and heats of fusion as a result of the thickness and perfection of crystal lamellae produced under the kinetic conditions of crystallization. These facts may compromise the quantitative value of thermal analysis data and, furthermore, may inhibit the use of such data for qualitative purposes because different polymers may exhibit similar melting endotherms when subjected to particular thermal histories. Typically, these considerations are realized, and thermal analysis is quite useful in spite of these limitations. However, this situation may be significantly improved by the simultaneous recording of the D S C thermogram and the corresponding infrared spectra as a function of temperature. The melting of polypropylene is a particularly interesting case because of the drastic changes in the infrared spectrum as this transition occurs. The changes in the infrared spectrum have been explained on the basis of particular bands arising from a particular phase, crystalline or noncrystalline (2), or on the basis of particular bands arising from a critical minimum number of monomer units arranged helically and necessary for observation of the band (3, 4). The melting of a 1-mg sample of polypropylene that had been slowly cooled from the melt over a 5.5-h period was studied by the simultaneous D S C microscopy-FTIR technique. The D S C thermogram is shown in Figure 3, and some of the infrared spectra collected over this temperature range are shown in Figure 4. Changes in the infrared spectra can be first observed in the 110 °C spectrum. This spectrum was taken over the temperature interval from 100 to 110 °C. These changes correspond to the first evidence of heat uptake observed in the D S C thermogram in Figure 3. Large changes in the infrared spectra can be observed in Figure 4 as the melting endotherm is traversed. Careful study of the thermograms and infrared spectra with the use of the simultaneous D S C mieroscopy-FTIR technique may aid in the

165 Figure 3. DSC thermogram of polypropylene cooled slowly from the melt over 5.5 h. The program rate was 10 °Clmin. (Reproduced with permission from ref 1. Copyright 1986 Society for Applied Spectroscopy.)

362


Wavenumber (cm ) -1

Wavenumber (cm"" ) 1

Figure 4. Selected infrared spectra taken over DSC thermogram in Figure 3 at the following temperatures (°C): A , 25; B , 82; C , 110; D , 155; £, 160; and F, 165. (Reproduced with permission from ref. 1. Copyright 1986 Society for Applied Spectroscopy.)

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Figure 4. Continued.

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explanation of the structural changes that occur in polypropylene during melting. Polymer blend studies are particularly appropriate for the application of the simultaneous D S C microscopy-FTIR technique. The difficulty in understanding what is happening during crystallization of two blended polymers is especially evident in blends of polyethylene and polypropylene. Figures 5 and 6 show the crystallization exotherms of the polyethylene and polypropylene homopolymers, respectively. The peak crystallization temperatures are similar, and the crystallization exotherm envelopes are largely superimposed on one another. The crystallization exotherms of the 20:80, 40:60, 60:40 and 80:20 wt% blends of polypropylene-polyethylene are shown in Figures 7-10, respectively. These figures show a single peak for each blend, a result indicating simultaneous crystallization of the two polymers in the blends. Therefore, it is not possible to understand the separate crystallization processes of each polymer in the blends by using the D S C data alone. A l l of the blends are initially assumed to involve no cocrystallization of polypropylene and polyethylene because these polymers are highly incompatible (5). However, the simultaneous D S C microscopy-FTIR apparatus was used to record infrared spectra over each of the crystallization exotherms in Figures 5-10. Spectral quality was excellent, as shown in Figures 11 and 12. Three spectra are

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5. DSC crystallization exotherm for a 0:100 wt% polypropylene-polyethylene blend.

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8. DSC crystallization exotherm for a 40:60 wt% polypropylene-polyethylene blend. + —

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Figure 10. DSC crystallization exotherm for an 80:20 wt% polypropylene-polyethylene blend.

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Figure 11. Infrared spectra of polyethylene recorded simultaneously with the DSC crystallization exotherm in Figure 5 at the following temperatures (°C): A , 170; B, 115; andC, 90.

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Figure 12. Infrared spectra of polypropylene recorded simultaneously with the DSC crystallization exotherm in Figure 6 at the following temperatures CC): A , 180; B 120; and C , 80. y

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shown in each figure during the crystallization of polyethylene and polypropylene homopolymers at 170, 115, and 90 °C and 180, 120, and 80 °C, respectively. It is not practical to show the large number of spectra recorded over each one of the crystallization exotherms in Figures 5-10. However, a wealth of information can be gleaned from these spectra, especially concerning the comparison of the crystallization of each polymer separately, as compared to crystallization of each polymer in the presence of the other in the blends. One of the methods used to analyze the data was to identify a crystallineand noncrystalline-phase infrared band for polyethylene and polypropylene. The bands chosen are as follows: Polymer

Infrared

Polyethylene Polypropylene

Band

(cm ) j

Phase

731 720

crystalline

998

crystalline noncrystalline

noncrystalline

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The ratios of the 731- to 720-cm" bands in polyethylene and the 9 9 8 - 9 7 3 - c m bands in polypropylene were used as measures of the relative proportion of crystalline and noncrystalline material at any particular tern1

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perature. The infrared absorbance ratios A

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ethylene and polypropylene, respectively, are plotted in Figure 13 for the crystallization of each polymer, as shown in Figures 5 and 6, respectively. As each of these ratios increases in magnitude, the proportion of crystalline material is increasing. The ratios are seen in Figure 13 to increase rapidly _i.00

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between 115 and 120 °C for polyethylene and polypropylene, in agreement with the peak crystallization temperatures in Figures 5 and 6, respectively. Similar plots can be drawn for each of the blend compositions in which the absorbance ratios for each polymer can be plotted as a function of temperature. These plots are shown in Figures 14 and 15 for polypropylene and polyethylene, respectively. Figure 14 is a plot of each of the blends and 100% polypropylene showing the A

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Figure 15. Absorbance ratios for polyethylene (A731IA720) versus temperature for the crystallization of blends of polypropylene with 0 (%)> 20 (O), 40 (•), 60 and 80 polypropylene.

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indicates the crystallization of polypropylene. Figure 15 is a plot of each of the blends and 100% polyethylene showing the A /A m

m

ratio versus tem-

perature, which indicates the crystallization of polyethylene. Figures 14 and 15 indicate that one of the polymers has no effect on the crystallization of the other polymer in the blends. That is, each polymer crystallizes in the same 115-120 °C temperature range whether the polymer is in a blend or not. A slower cooling rate and more detailed analysis of the crystallization in this narrow temperature range might yield more subtle effects in the blends. Furthermore, comparisons of this type can yield separate crystallization rates and crystallization temperature profiles for each polymer in the blends.

Acknowledgments I am grateful to Timothy M i n v i e l l e for collecting the simultaneous D S C - F T I R data and to Marlene Moellenkamp for typing the manuscript.

References 1. Mirabella, F. M . A p p l . Spectrosc. 1986, 40{3), 417. 2. Samuels, R. J. Makromol. Chem. Suppl. 1981, 4, 241.

3. Kissin, Y. V.; Rishina, L. Eur. Polym. J. 1976, 12, 757. 4. Glotin, M.; Rahalkar, R. R.; Hendra, P. J . ; Cudby, M . E . A . ; Willis, H . A. Polymer 1981, 22, 731.

5. Krause, S. In Polymer Blends; Paul, D. R.; Newman, S., Eds.; Academic: New York, 1978; "Polymer-Polymer Compatibility," Chapter 2. RECEIVED

1989.

for review February 14, 1989.

ACCEPTED

revised manuscript August 14,

Simultaneous Differential Scanning Calorimetry and Infrared

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