A Combined Differential Scanning Calorimetry-Fourier Transform

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27 A Combined Differential Scanning Calorimetry-Fourier Transform Infrared Approach for the Study of Polymeric Materials David A. C. Compton, David J. Johnson, and Jay R. Powell Digilab Division, Bio-Rad, 237 Putnam Avenue, Cambridge, MA 02139

A combination of the standard laboratory techniques differential scanning calorimetry (DSC) and reflectance Fourier transform in frared (FTIR) microspectroscopy is described. Both the FTIR and DSC analyses were under the direct control of one computer, and the overall operation of the instrument is described. This simultaneous DSC—FTIR technique gives spectroscopic and thermodynamic infor mation about a solid or liquid sample undergoing thermal modifica tion. DSC measures the exothermic and endothermic responses of the samples, whereas the FTIR analysis observes changes in chemical and physical composition. The curing reaction of epoxy samples and the phase transitions of poly(ethylene terephthalate) (PET) are used to illustrate the potential of the combined DSC-FTIR method for the determination of polymer structure and properties. In the PET exam ple, changes in the infrared spectrum were used to deduce that recrystallization in one sample of PET occurred even though the DSC curve showed no strong inflection at that temperature.

ΊΓΗΕ STUDY OF POLYMERC I MATERA ILS BY THERMAL ANALYSS I

increasingly popular i n m o d e r n analytical laboratories. D i f f e r e n t i a l scanning

calorimetry ( D S C ) directly measures endothermic o r exothermic behavior o f a material as a function o f temperature a n d provides valuable information about the t h e r m a l properties a n d composition o f a sample u n d e r investiga0065-2393/93/0236-0661$06.00/0 © 1993 American Chemical Society

In Structure-Property Relations in Polymers; Urban, Marek W., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1993.

has b e c o m

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tion. Properties such as heat o f cure, glass-transition temperature, percent crystalhnity, m e l t i n g point, and degree o f cure may all be calculated w i t h the D S C technique (1). C o n c u r r e n t l y , many laboratories study the same types o f materials b y infrared spectroscopy ( 2 ) that they examined b y t h e r m a l analysis. I n particu lar, F o u r i e r transform i n f r a r e d ( F T I R ) spectroscopy is a p o w e r f u l tool for analysis o f p o l y m e r i c materials. T h e currently available F T I R spectrometers can provide a wealth o f i n f o r m a t i o n about the composition, structure, crys talhnity, a n d other properties o f samples that have sizes ranging f r o m a few centimeters d o w n to a few micrometers ( i n conjunction w i t h an infraredtransmitting microscope) (3). B o t h D S C a n d F T I R analyses are w e l l suited to the study o f the t h e r m a l behavior o f materials. T h e D S C instrument monitors changes i n heat flow as a function o f temperature, b u t is unable to identify the c h e m i c a l nature o f observed transitions a n d occasionally even may fail to observe a transition. T h e F T I R spectrometer is w e l l suited to obtain information about the c h e m i c a l properties o f a sample. T h e availability o f special accessories for F T I R allows the study o f samples at nonambient temperatures ( 2 ) , b u t these accessories generally do not allow for any t h e r m a l i n f o r m a t i o n (such as heat flow) to be r e c o r d e d about the sample, except for its temperature. T h u s , changes that take place must be i n f e r r e d f r o m changes observed i n the infrared spectrum. S u c h spectral changes may be very subtle, i n w h i c h case the physical interpretation o f the changes can be ambiguous. P e r f o r m i n g simultaneous F T I R a n d D S C analysis offers great promise to overcome the hmitations o f the individual techniques for the study o f the t h e r m a l properties o f materials. T h e c o m b i n a t i o n o f D S C a n d F T I R allows the analyst to m o n i t o r structural changes i n the material as the sample passes through various t h e r m a l transition states and, potentially, to obtain sufficient information b y spectroscopic means to assign the different states to physical or c h e m i c a l phenomena.

I n 1986, two groups o f workers reported studies that c o m b i n e d FTJR a n d D S C (4-6), but such w o r k was l i m i t e d b y the e q u i p m e n t available at that time. T o obtain the i n f r a r e d spectral data, b o t h groups used transmission spectroscopy, w h i c h p l a c e d severe restrictions o n the types o f samples that c o u l d be studied. Improvements to F T I R instruments, i n b o t h software a n d hardware terms, enable us to report experiments that show great promise for routine operation. Reflectance i n f r a r e d spectroscopy allows examination o f a broader variety o f samples a n d makes more experimental information obtain able. I n all o f the D S C - F T I R experiments reported, i n c l u d i n g those i n this report, the D S C was a hot-stage microscopy c e l l ( M e t t l e r F P 8 4 ) . This particular D S C was used because it is very small, is designed to fit u n d e r a microscope objective, a n d has a sample c u p accessible to i n c o m i n g i n f r a r e d


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radiation. T h e various reported D S C - F T I R studies differed significantly i n the manner i n w h i c h the microscopy c e l l was m o u n t e d i n the F T I R spec trometer. K o b e r s t e i n a n d co-workers ( 6 ) reported a n experiment w h e r e the D S C was simply placed directly i n the m a i n i n f r a r e d b e a m o f the spectrometer sample compartment. T o accomplish this direct sample placement, the D S C sample c u p was r e m o v e d a n d a t h i n sample o f polyurethane film ( m o u n t e d o n a K B r w i n d o w ) was p l a c e d over the hole. T h e simultaneous D S C data w e r e r e c o r d e d o n a thicker sample o f polyurethane p l a c e d i n the D S C reference c u p . T h i s approach, unfortunately, means that the D S C a n d the F T I R experiments examined specimens i n different environments, a n d , as is w e l l k n o w n , the t h e r m a l properties o f a sample are often dependent o n the sample morphology. T o obtain a better signal-to-noise ratio o n the F T I R data, M i r a b e l l a used an i n f r a r e d transmitting microscope accessory (4) to condense the b e a m into the small o p e n i n g o f the D S C sample c u p , w h i c h y i e l d e d m u c h higher optical throughput. T h e i n f r a r e d b e a m was transmitted through the sample a n d the D S C c u p , facilitated b y special cups fashioned f r o m s o d i u m chloride o r potassium chloride crystals. A g a i n , it was necessary to examine a t h i n film o f each sample to avoid excessive infrared absorbances. T h i s approach made it possible to m o n i t o r the structural changes i n polypropylene (4) a n d poly ethylene ( 5 ) d u r i n g melt a n d recrystallization, a n d to study the degradation o f p o l y (ethylene v i n y l alcohol) ( 5 ) . D u r i n g measurement o f a transmission i n f r a r e d spectrum, the ab sorbance o f the strongest bands o f interest s h o u l d not exceed about 0.8 absorbante units. T o maintain acceptable absorbance levels, a typical p o l y m e r sample must b e ~ 10 μ ι η o r less i n thickness. Samples this t h i n generally have insufficient mass to generate an adequate D S C curve. Consequently, m u c h thicker sample o f the same material i n the c u p that is n o r m a l l y the reference c u p . T h i s procedure resulted i n a D S C trace w i t h the y axis reversed f r o m the n o r m a l convention. O u r present research was p e r f o r m e d i n a significantly different manner. T h e same D S C ( M e t t l e r F P 8 4 ) was e m p l o y e d , b u t w e obtained the spectral data using a n i n f r a r e d microscope operating i n the reflectance m o d e . T h e infrared b e a m was d i r e c t e d d o w n f r o m the microscope objective onto the sample i n the D S C c u p . T h e n the reflected energy was collected b y the same microscope objective a n d focused onto the i n f r a r e d detector. Infrared spectra were r e c o r d e d continuously d u r i n g a standard D S C experiment. I n addition, one data station a n d to c o m b i n e t h e experimental data i n o n e set o f data files. T h e r e are several advantages to this experimental arrangement. It was not necessary to fashion D S C sample cups f r o m a material (like potassium b r o m i d e ) that transmits i n f r a r e d radiation; instead, standard a l u m i n u m cups



664


were e m p l o y e d . I n m a n y cases, it was possible to study samples that h a d the n o r m a l thickness for the D S C experiment, a n d the design o f m o d e r n i n frared-transmitting microscope accessories allowed spectra to be collected i n the reflectance m o d e w i t h excellent signal-to-noise ratio. Finally, the use o f a single data station for the data collection f r o m b o t h D S C a n d F T I R signifi cantly improves the ability to correlate the results f r o m b o t h instruments. Because only one sample is studied b y b o t h techniques simultaneously, the D S C curve can b e c o m p a r e d directly to various spectroscopic parameters, temperature.

Experimental A l l spectroscopic data w e r e collected using an F T I R spectrometer ( B i o - R a d F T S 40) e q u i p p e d w i t h a K B r b e a m splitter, high-temperature ceramic source, a n d infrared transmitting microscope accessory ( U M A 300A). T o give sufficient w o r k i n g d e p t h w i t h the D S C c e l l i n place o n the microscope stage, v i e w i n g w i t h visible fight. Spectra w e r e collected continuously at 8 - c m resolution d u r i n g the D S C experiment. - 1

A microscopy c e l l ( M e t t l e r F P 8 4 T A ) was used to heat the samples a n d to obtain the D S C data. T h e temperature was p r o g r a m m e d to ramp f r o m 25 to 280 °C at 10 °C p e r m i n u t e . O p e r a t i o n o f the microscopy c e l l was controlled b y the data station ( B i o - R a d 3200) w h i c h passed instructions to a central processor ( M e t t l e r F P 8 0 ) . T h e central processor p e r f o r m e d direct control o f the D S C c e l l a n d f e d data f r o m the D S C experiment back to the data station. T h u s , the data station was collecting data f r o m b o t h the F T I R optical b e n c h a n d the D S C microscopy c e l l simultaneously, w h i c h a l l o w e d for the direct comparison o f the optical a n d t h e r m a l results o n the same t i m e or temperature basis. T o allow for the i n f r a r e d b e a m to reach the sample i n the D S C c u p , the glass cover slip that is n o r m a l l y positioned above the c e l l was removed. I f necessary, this cover c o u l d b e replaced w i t h a t h i n w i n d o w fashioned f r o m an i n f r a r e d transmitting material, b u t that was not done for this w o r k .

Results and Discussion D u r i n g collection o f a reflected i n f r a r e d b e a m , a variety o f p h e n o m e n a that d e p e n d o n the surface, geometry, a n d phase o f the material u n d e r investiga tion may occur. T w o interactions b e t w e e n the infrared b e a m and sample were observed i n this research. I n some cases, the sample is relatively transmissive to i n f r a r e d radiation, a n d the b e a m transmits d o w n through the sample, reflects off the a l u m i n u m c u p , a n d passes back t h r o u g h the material.



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Approach

665

T h i s type o f analysis is c a l l e d r e f l e c t i o n - a b s o r p t i o n spectroscopy, a n d it can only be a p p l i e d to a t h i n film o f material i n the sample p a n . A s previous workers (4-6) have done, the b u l k o f the sample must be p l a c e d i n the reference side o f the sample cup. R e f l e c t i o n - a b s o r p t i o n behavior was exhib i t e d b y the samples of epoxy resin that w e examined. T w o advantages are gained by use o f the reflectance mode o f collection instead o f the transmis sion m o d e (4-6) for this class o f sample: (1) a special sample c u p ( w h i c h may require disposal after a single c u r i n g experiment) n e e d not b e fashioned a n d (2) formulations w i t h varying amounts o f filler a l l can be studied i n reflectance. A different b e h a v i o r — s p e c u l a r reflectance o f the l i g h t — w a s n o t e d w i t h a sample o f p o l y (ethylene terephthalate) ( P E T ) that was optically opaque. I n that case, because the b u l k o f the energy was reflected f r o m the top surface o f the sample, the w h o l e sample was p l a c e d i n the sample cup. T h e advantage o f this procedure is that b o t h the i n f r a r e d spectrum a n d D S C trace can b e obtained f r o m the same sample. I n addition, experiments can be p e r f o r m e d o n samples that cannot be physically examined as a t h i n film, such as samples that are heavily filled. H o w e v e r , the i n f r a r e d spectrum that is obtained b y specular reflectance exhibits distorted or derivatized b a n d shapes a n d requires a software correction to p r o d u c e a n o r m a l absorbance spectrum. T h i s correction, the K r a m e r s - K r o n i g transformation (3), w i l l be described i n m o r e detail later d u r i n g the discussion o f the results obtained for P E T .

Epoxy Study. T h i n films o f u n c u r e d amine-activated epoxy f o r m u l a tions were p l a c e d i n the sample pan o f the microscopy c e l l a n d heated f r o m 25 to 280 °C at 10 °C p e r m i n u t e . Changes i n the structure o f the epoxy as a function o f temperature w e r e simultaneously r e c o r d e d b y i n f r a r e d spec troscopy. T h e reaction mechanism o f cure initially involves the reaction o f a cyelo aliphatic p r i m a r y a m i n e activator (4-amino-4-methyl-eyclohexenemethaneamine) w i t h the epoxide group o f the resin [2-di-[4-(2, 3 - e p o x y - l - p r o poxy)-l-phenyl] propane] to p r o d u c e a secondary amine. T h e secondary amine further reacts w i t h an additional epoxide group to f o r m a tertiary amine. F u r t h e r reaction, catalyzed b y water, hydroxyl, a n d tertiary amine concentration, continues the cross-finking activity. T h i s same system has b e e n studied extensively b y c o m b i n e d thermogravimetric analysis ( T G A ) and F T I R (7), as w e l l as by D S C - F T I R (8). T h e research using T G A - F T I R d e m o n strated utility to quantitate the a m i n e - r e s i n ratio a n d to determine qualita tively the t h e r m a l history o f the p o l y m e r after cure. T h e c u r i n g o f an epoxy is an exothermic reaction. Research (8) shows that the shape o f the D S C exothermic peak changes considerably w h e n the a m i n e - r e s i n ratio is varied. T h e D S C trace observed w h e n a mixture o f 35 parts p e r h u n d r e d parts resin (phr) amine ( 4 1 % over the stoichiometric p r i m a r y and secondary amine concentration) is heated shows a single-peak exotherm at about 140 °C. W h e n the same epoxy is p r e p a r e d w i t h 17 p h r


666



amine ( 3 1 % u n d e r the stoichiometric level o f p r i m a r y a n d secondary amines) a n d is heated, a double-peak exotherm that has a second, broader peak centered at about 200 °C is generated. T h e extra peak i n the D S C trace for the second sample indicates a significant change i n the reaction m e c h a n i s m . B y c o u p l i n g the F T I R to the D S C , these changes w e r e studied a n d differentiated. D u r i n g each r u n , 128 scans w e r e coadded p e r spectrum a n d 30 spectra were collected. F o u r spectra generated at various temperatures d u r i n g a typical experiment are s h o w n i n F i g u r e 1. C o m p a r i s o n o f the spectra shows a n u m b e r of regions where differences are observed as a function of tempera ture. These differences can be studied easily as either peak intensities or frequencies. A s examples o f typical results, the relative intensities o f a pair o f bands at 3030 a n d 3048 c m change significantly d u r i n g the reaction and, hence, can be used to m o n i t o r the degree o f cross-linking. It is not an easy task to assign these bands to particular components o f the reaction mixture, but they are almost certainly due to epoxy ( 3 0 5 0 - 3 0 3 0 c m ) a n d aromatic C — H stretch i n g ( 3 0 8 0 - 3 0 1 0 c m " ) (9). T h e 3 0 4 8 - c m b a n d decreases i n intensity as the reaction proceeds, so it probably arises f r o m the epoxy that is b e i n g c o n s u m e d . A plot o f this peak ratio as a function of temperature for b o t h mixtures shows that the rate o f reaction is slightly faster w i t h the overstoichiometric mix. B a n d shifts i n the i n f r a r e d spectra may also be used to monitor the progress o f the reaction. F o r example, F i g u r e 2 shows a plot o f the frequency o f the absorbance b a n d near 1295 c m . This b a n d shifts steadily to lower frequencies as a function o f cure. H o w e v e r , it is interesting that b o t h sets o f i n f r a r e d data (peak ratios a n d peak frequencies) show a steady change throughout the reaction, whereas the D S C curve indicates a strong reaction at certain temperatures. - 1

- 1

1

- 1

- 1

N o t only does the rate o f reaction vary, but the reaction mechanism itself changes as a function o f activator-resin ratio. F i g u r e 3 shows a series o f three spectra generated w h e n the over-stoichiometric mix system was heated. E v e n though this b a n d sits o n the n o r m a l broad, featureless b a n d due to hydroxyl, o f a secondary amine ( 3 5 0 0 - 3 3 0 0 c m " ; reference 9). T h e presence o f this b a n d at 139 and 210 °C indicates that even at elevated temperatures, the dominant reaction is p r i m a r y amine to epoxy-producing secondary amine. F i g u r e 4 shows that w h e n less activator is used i n the mix, as w i t h the 17 p h r activator system, the 3 3 5 0 - c m b a n d is noticeably absent at elevated t e m peratures. W e interpret this to indicate that the reaction between secondary amine and r e m a i n i n g epoxide groups becomes significant due to the lack o f p r i m a r y amine. 1

- 1

T h e p r e c e d i n g interpretation explains the differences between the D S C data obtained for the two samples. T h e single-peak character of the sample that contains the over-stoichiometric mix is almost completely due to an


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C O M P T O N ET AL.

Combined

DSC-FTIR

667

Approach

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Figure 2. A plot of the absorbance maximum of the infrared band near 1290 cm as a function of temperature for epoxy samples containing 35 phr amine curing agent (+) and 17 phr actio ator (®). -1

absence o f any other reaction except that o f the p r i m a r y amine, w h i c h adds to epoxide a n d generates a secondary amine group. T h e D S C trace for the other sample, however, has a double-peak nature w h e r e the first peak is due to c o n s u m p t i o n o f p r i m a r y amine. Because excess epoxide groups r e m a i n after the majority o f the p r i m a r y amine has b e e n consumed, the less reactive secondary amine is able to react a n d give rise to the second D S C peak. Similar preference for the p r i m a r y amine reaction w i t h the use o f aromatic c u r i n g agents is w e l l d o c u m e n t e d (JO). It has b e e n shown that i n aromatic amine c u r i n g agents, the secondary amine reaction w i t h epoxide has approxi mately one-tenth the reaction rate constant o f that for a p r i m a r y amine reacting w i t h epoxide. I n o u r w o r k , it is apparent that the cycloaliphatic amine shows the same preference for p r i m a r y amine reaction. Results f r o m this study indicate that the simultaneous D S C - F T I R technique may b e used to successfully m o n i t o r changes i n reaction m e c h a nisms d u r i n g the cure o f epoxy systems as a f u n c t i o n o f activator-resin ratio. Relative changes i n the rates o f reaction may also be m o n i t o r e d .



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3600

Combined DSC-FTIR

3400

669

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3200 3000 Wavenumber

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Figure 3. Expanded portion of the IR spectra generated during the cure of 35 phr amine curing agent mixture at 53, J39, and 210 °C.

3600

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3200 3000 Wavenumber

2800

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Figure 4. Expanded portions of the IR spectra generated during the cure of the 17 phr amine curing agent mixture at 53, 139, and 210 °C.

P E T Research. T w o samples o f P E T were analyzed b y D S C - F T I R to m o n i t o r the structural changes o f the material as it was heated a n d c o o l e d through glass-transition temperature a n d m u l t i p l e m e l t i n g e n d o t h e r m . These samples contained different types o f nucleating agent, w h i c h was believed to lead to different t h e r m a l properties. T h e samples were heated i n the



670


microscopy c e l l f r o m about 40 to 280 °C at 10 °C p e r m i n u t e . Infrared spectra (256 scans c o a d d e d p e r spectrum) were simultaneously collected b y the specular reflectance m o d e o f analysis. A s m e n t i o n e d earlier, d u e to the reflective surface o f the P E T , the generated I R spectra exhibited distorted o r derivitized b a n d shapes. T h i s p h e n o m e n o n occurs w h e n a material undergoes a significant change i n refractive index i n frequency regions o f strong I R absorbances. T h e l o w e r p o r t i o n o f F i g u r e 5 shows an observed spectrum ( i n units o f percent reflectance). T h e u p p e r p o r t i o n o f F i g u r e 5 shows the observed spectrum after a K r a m e r s - K r o n i g correction was applied. T h e K r a m e r s - K r o n i g correction separated the index-of-refraction c o m p o n e n t f r o m the extinction coefficient component to p r o d u c e a " K " spectrum, w h i c h has the appearance o f a n o r m a l absorbance spectrum. D S C curves f o r P E T n o r m a l l y show a glass-transition temperature near 80 °C (11) a n d a m u l t i p l e m e l t i n g e n d o t h e r m near 255 °C. O n e sample showed these two transitions as w e l l as a sharp exotherm at 115 °C, d u e to recrystalfization. T h e D S C curve f o r this sample, labeled sample X , is s h o w n as the top plot i n F i g u r e 6. T h e D S C curve o f sample Y showed the n o r m a l melt, b u t n o other inflections d u e to either the glass transition o r recrystal fization (see top o f F i g u r e 7). Significant changes i n the spectra w e r e observed w h e n the two samples were heated. A s an example, the temperature behavior f o r the 1100-cm"" region o f the spectrum o f sample X is shown i n F i g u r e 8, w h e r e the bands at 1118 a n d 1098 c m are p l o t t e d i n ascending order w i t h temperature increase. T h e 1 1 1 8 - c m shoulder increases rapidly i n intensity d u r i n g 1

-

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- 1

~I 3000

! 1 2500 2000 Wavenumber

1 1500

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Figure 5. Observed reflectance IR spectrum of PET at 234 °C prior to Kramers-Kronig correction (bottom) and absorbance spectrum obtained after the correction (top).



27.

COMPTON ET AL.

—I

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Combined DSC-FTIR

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Approach

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671

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40 60 80 100 120 140 160 180 200 220 240 260 280 TEMPERATURE C

Figure 6. DSC curve for PET sample X heated at 10 °C per minute (top) and the ratio of the infrared peaks at 1118 and 1098 cm" as a function of temperature (bottom). 1

recrystallization at about 120 °C. F i g u r e 9 shows the effect o f further heating, ture f o r convenience, because t h e shoulder at 1118 c m

- 1

was observed t o

decrease d u r i n g the melt. T o show the temperature behavior o f this spectral region m o r e clearly, the 1 1 1 8 - 1 0 9 8 intensity ratios f o r b o t h samples are plotted as a f u n c t i o n o f temperature i n the lower curves i n F i g u r e s 6 a n d 7 (below the corresponding D S C curve). T h e 1 1 1 8 - 1 0 9 8 intensity ratio is very revealing. It shows that b o t h samples d o i n d e e d go t h r o u g h a recrystalhzation near 120 °C. I n t h e case o f sample X , this transition is fast a n d is accompanied b y a n exotherm. I n t h e case o f sample Y , however, the transition is slower a n d less w e l l defined, a n d the D S C curve shows n o evidence o f an exotherm. Originally these changes i n relative intensity o f t h e b a n d pair at 1100 cm

- 1

were b e l i e v e d to b e related to rotational isomerism o f t h e ethylene

glycol segments i n P E T (12). T h e two bands at 1118 a n d 1100 c m assigned to t h e trans a n d gauche conformers o f the glycol segment,

- 1

were

respec

tively. A l t h o u g h the position is not conclusive, n o r m a l coordinate calculations indicate that the observed changes i n relative intensity are due to symmetry a n d resonance characteristics o f the aromatic ring framework instead (12).




672

Figure 8. Corrected infrared spectra for PET sample X at 85, 95, 105, 115, 125,



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673

T h e calculations indicate that the P E T framework is relatively rigid, even i n the melt. O u r observation that the 1 1 0 0 - c m

- 1

b a n d increases i n intensity

d u r i n g the recrystalhzation a n d t h e n decreases to its original level o n m e l t i n g upholds the argument that these bands are not due to different conforma tions, but that their intensities arise f r o m a m o r e complex molecular mecha n i s m . T h e classical way that conformational energy differences are calculated shows that the change i n relative intensity o f a conformer pair should be a smooth function o f Γ

- 1

(temperature). I n any case, whatever causes these

changes, examination o f spectra d u r i n g the recrystalhzation f r o m the melt show that these changes are reversible.

Summary and Conclusions It is apparent f r o m this research a n d earlier w o r k p e r f o r m e d b y K o b e r s t e i n and M i r a b e l l a (4-6)

that the D S C - F T I R technique provides a p o w e r f u l tool

for the characterization o f p o l y m e r i c materials. B y c o u p l i n g an F T I R spec trometer to the D S C , structural i n f o r m a t i o n about the sample as it passes through t h e r m a l transitions may be gained i n the same w a y that T G A - F T I R gives information about evolved gases. T h e s e types o f i n f o r m a t i o n s h o u l d provide additional insight into p o l y m e r behavior i n various t h e r m a l environ ments. I n this w o r k all o f the spectra w e r e collected b y use o f the reflectance m o d e o f i n f r a r e d microspectroscopy instead o f the transmission m o d e used previously (4-6).

T h e r e are several advantages to the reflectance experimen-


674


tal arrangement. It was not necessary to fashion D S C sample cups f r o m a material (like potassium b r o m i d e ) that transmits infrared radiation; instead, samples that have the n o r m a l thickness for the D S C experiment, even w h e n they are opaque to the i n f r a r e d b e a m . Finally, the design o f m o d e r n i n f r a r e d transmitting microscope accessories allows for spectra to be collected i n the reflectance m o d e w i t h excellent signal-to-noise ratio. A further advance was gained b y p e r f o r m i n g b o t h D S C a n d F T I R experiments u n d e r the c o n t r o l o f Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 26, 2016 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/ba-1993-0236.ch027

a single computer, w h i c h enabled easy direct comparison o f the

types

of experimental data. A s demonstrated here, spectroscopic data, such as peak ratios a n d peak frequencies, can be directly c o m p a r e d to the D S C curve, w h i c h avoids any ambiguity about the correlation o f the various plot abscissae. B y c o u p l i n g the F T I R to a D S C w e w e r e able to gain some insight into the final molecular structure a n d c u r i n g steps o f cycloaliphatic polyamine cross-linked D G E B A (diglycidyl ether of b i s p h e n o l A ) polymers. O u r results suggest that the system u n d e r study reacts i n steps, m u c h the same way as for most e p o x y - a r o m a t i c amine polymers. I n such systems, it is thought that steric hindrance effects, due to an aromatic substituent adjacent to the amine nitrogen, makes the secondary a n d p r i m a r y amines react at different rates. M o s t systems using aliphatic

p r i m a r y a n d secondary amines seem to have

reaction rates that are indistinguishable (9). I n no way do w e dispute that most aromatic a n d aliphatic a m i n e - c u r e d epoxies show these differences. H o w e v e r , b y theorizing a distinct two-step reaction ( 8 ) i n o u r study, w e suggest that steric hindrance factors influence epoxy-cycloaliphatic polyamines i n m u c h the same m a n n e r as e p o x y - a r o m a t i c polyamine reactions.

References 1. Thermal Characterization of Polymeric Materials; Turi, E., E d . ; Academic: Or lando, FL, 1981. 2. Laboratory Methods in Vibrational Spectroscopy, 3rd ed.; Willis, Η. Α.; van der Maas, J. H.; Miller, R. G. J., Eds.; Wiley: Chichester, England, 1987. 3. Krishnan, K.; H i l l , S. L. In Practical Fourier Transform Infrared Spectroscopy: Industrial and Laboratory Chemical Analysis; Ferraro, J. R.; Krishnan, K., Eds.; Academic: San Diego, C A , 1990; Chapter 3, pp 103-165. 4. Mirabella, F. M. Appl. Spectrosc. 1986,40, 417-420. 5. Mirabella, F. M. In Infrared Microspectroscopy; Messerschmidt, R. G.; Harthcock, Μ. Α., Eds.; Marcel Dekker: New York, 1988; Chapter 6, pp 85-92. 6. Koberstein, J. T.; Gancarz, I.; Clarke, T. C . J. Polym. Sci., Polym. Phys. Ed. 1986,24, 2487-2498. 7. Johnson, D. J.; Compton, D. A . C.; Cass, R. S.; Canale, P. L. In Proceedings of the 17th North American Thermal Analysis Society Conference; North American Thermal Analysis Society: Colonia, N J , 1988; V o l . II, pp 574-579.


27.

COMPTON ET AL.

Combined DSC-FTIR

Approach

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8. Johnson, D . J. Compton, D . A . C . ; Canale, P. L . Thermochim. Acta 1992,195, 5-20. 9. Socrates, G. Infrared Characteristic Group Frequencies; Wiley: Chichester, Eng land, 1980. 10. Epoxy Resins, Chemistry and Technology; M a i , C . Α., E d . ; Marcel Dekker: N e w York, 1988; pp 301-304. 11. Polymer Handbook, 3rd ed.; Brandrup, Α., E d . ; Wiley: New York, 1989. 12. Painter, P. C . ; Coleman, M . M.; Koenig, J. L . The Theory of Vibrational Spectroscopy and Its Application to Polymeric Materials; Wiley-Interscience: New York, 1982; pp 505-510. Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 26, 2016 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/ba-1993-0236.ch027

;

RECEIVED for review July 1 5 , 1991. ACCEPTED revised manuscript M a y 28, 1992.


A Combined Differential Scanning Calorimetry-Fourier Transform

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