ftir spectroscopic investigation of thermal effects in ... · doi: 10.1002/polb.20334 published...

FTIR Spectroscopic Investigation of Thermal Effects in Semi-Syndiotactic Polypropylene*

MICHAEL S. SEVEGNEY,1 RANGARAMANUJAM M. KANNAN,1 ALLEN R. SIEDLE,2 PAMELA A. PERCHA2

1Department of Chemical Engineering and Materials Science, Wayne State University, Detroit, Michigan 48202-3902

23M Company, St. Paul, Minnesota 55144-1000

Received 1 June 2004; accepted 24 October 2004DOI: 10.1002/polb.20334Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: The temperature-dependent behavior of individual components within metal-locene-catalyzed semisyndiotactic polypropylenes (semi-sPP) with a wide range of stereo-regular content (26 to 96% rr) is studied using Fourier transform infrared (FTIR) spectros-copy and temperature-modulated differential scanning calorimetry (DSC). Changes insensitive, high-resolution absorbance spectra are observed as melt-slow-cooled thin filmsare subjected to stepwise temperature increases. In general, spectral bands previouslyidentified as being sensitive to ordered structures (e.g., conformed chains, crystal morphs)appear to follow overall trends of shifting to lower wavenumbers (energies), broadening,and decreasing in peak area intensity as temperature increases. Peaks that appear due to“splitting” (observed in more stereoregular materials) show a trend toward coalescence astemperature increases; this corresponds to a gradual loss of chain conformational order.Gauche-gauche-trans-trans (ggtt)n helical and all-trans (tttt)n planar zigzag-conformedchains that participate in the crystalline-amorphous interfacial region (“mesophase”) ap-pear to be more stable (i.e., they do not lose their conformational order as easily) withincreasing temperature in materials with a greater degree of syndiotacticity. Moreover, IRdata correspond well with modulated DSC endotherms located near 50 °C and 70 °C. Ateach transition temperature—thought to represent, respectively, a thermally driven chainconformation from planar zigzags to helices, and a dynamic disorder of helices marked byrapid gauche7 trans isomerization—the IR absorbance ratio, A978/A963, which representsthe relative population of helical chains, undergoes an accelerated decrease. © 2005 WileyPeriodicals, Inc. J Polym Sci Part B: Polym Phys 43: 439–461, 2005Keywords: syndiotactic polypropylene (PP); FT-IR; thermal properties; structure-property relations; metallocene catalysts

INTRODUCTION

The introduction of metallocene catalysts into ole-fin polymerization1,2 chemistry has generated re-

newed interest in research on many well-knownpolymers. Syndiotactic polypropylene (sPP) is agood example of this. There are many studies inthe literature, both theoretical and, to a lesserdegree, experimental, concerning sPP; however,the ability to synthesize, and thus to study exper-imentally, highly stereoregular sPP was onlyrealized around 1990.3,4 Since then, much hasbeen learned regarding the crystal morpholo-gy,5–20 chain conformations,21–29 blend compati-bility,30–32 elastomeric behavior,33–35 and many

*Contribution from the March 2004 Meeting of the Amer-ican Physical Society—Division of Polymer Physics, Montreal,Canada.

Correspondence to: R. M. Kannan (E-mail: [email protected])Journal of Polymer Science: Part B: Polymer Physics, Vol. 43, 439–461 (2005)© 2005 Wiley Periodicals, Inc.

439

other physical properties of sPP. Vibrational spec-troscopy has certainly contributed to the experi-mental progress. Infrared (IR) absorbance36–39

and Raman scattering40–43 have been used toidentify the presence and relative populations ofgauche-gauche-trans-trans (ggtt)n helical and all-trans (tttt)n planar zigzag-conformed chains assPP undergoes a variety of thermal and mechan-ical processing conditions. Recently, infrared lin-ear dichroism (IRLD) spectroscopy was used tomeasure structure formation and viscoelasticcomponent orientation in tensile-deformedsPP.44,45 Simultaneously collected absorbanceand IRLD spectra reveal and quantify both a he-lical-to-planar-zigzag conformation change and“mesophase”46 formation that accompany tensileyielding. The specific technique, rheo-opticalFTIR spectroscopy,47 provides a means of sensi-tively measuring stepwise changes in sPP morphol-ogy while en route to completing a crystal–crystaltransformation at high strains (� 600%).48,49

This work focuses on the characterization ofthe IR absorbance spectrum of sPP as stereo-regularity is decreased. The effects of varyingtacticity alone have been investigated recent-ly.50 Increasing syndiotacticity is found to causeintensity change, wavenumber shift, and “split-ting” in certain characteristic IR peaks. Thesephenomena can be linked to an overall changein the local molecular environment of vibra-tional dipole moments, specifically in the degreeof configurational and conformational order insPP chains. It is hypothesized here that varyingtemperature will also have some effect on mo-lecular order, and thus, on the IR spectrum ofsPP as well.

The impetus of this work is two-fold:

1. Semisyndiotactic polypropylene (semi-sPP) with such a broad range of stereo-regular content (26 to 67% rr) has, to date,not been analyzed with FTIR spectros-copy. The microstructural consequences ofstereoregularity could be manifested inthe IR spectrum.

2. The spectral changes listed above havealso been observed in the course of rheo-optical FTIR dichroism experiments onhighly stereoregular sPP.44 Results fromthe present work can be helpful in distin-guishing thermal effects from other fac-tors that may influence IR spectra in asimilar fashion.

Thus, a thorough investigation of semi-sPP be-comes necessary to distinguish between tacticity,thermal, and mechanical effects on IR spectra,and thus, to analyze and interpret spectral effectsin dichroism studies accurately. In general, thevariations observed (intensity change, broaden-ing, shifting, and splitting) are small with respectto the overall magnitude of the spectra measured.Thus, high-resolution IR spectroscopy must beemployed. The following work investigates sixmaterials, each with a different degree of stereo-regularity. As temperature is varied, subtlechanges in absorbance spectra are detected sen-sitively, and careful analysis is performed, inlight of what has been learned from a previousstudy where tacticity alone was varied.50

EXPERIMENTAL

Materials

Five semisyndiotactic polypropylenes with vari-ous degrees of stereoregularity, synthesized usingan unsymmetrical ansa-fluorenyl metallocenecatalyst,51 are analyzed: aPP, s-sPP32, s-sPP49,s-sPP65, and s-sPP67.52 A highly stereoregularmaterial (sPP), obtained from the Fina Oil andChemical Company, is also studied. 13C-NMRspectroscopy and high-temperature GPC triple-detection are used to determine stereoregularityand molecular weight distributions, respectively,of each material. These properties are reported inTable 1. Thin film samples are obtained by com-pression molding melted material, followed byslow cooling. Bulk material is heated to 180 °Cinside a hot press (Hydraulic Unit model 3912,Carver, Inc.) and held for 20 min to ensure com-plete melting. Melts are then sandwiched be-

Table 1. Syndiotacticity and Molecular WeightDistributionsa of Semi-Syndiotactic PolypropyleneSamples Studied

Material % rr % mr % mm Mw Mn PDI

aPP 26.5 49.0 24.5 1160000 417000 2.78s-sPP32 32.0 41.4 26.6 230000 61000 3.78s-sPP49 49.0 37.8 13.2 350000 140000 2.5s-sPP65 65.4 24.3 10.3 790000 220000 3.6s-sPP67 67.0 24.0 9.0 230000 89000 2.6sPP 96 228500 60600 3.77

a Obtained using trichlorobenzene solvent and polystyrenestandards.

440 SEVEGNEY ET AL.

tween polytetrafluoroethylene sheets and com-pression molded into films of nearly uniform150-�m thickness. The pressing load is then re-moved, and films are maintained at 180 °C for30 min to allow relaxation of any processingstresses.53 Samples are then allowed to air-coolslowly to ambient temperature (� 30 °C) over aperiod of 4 h. This step encourages the formationof any thermodynamically favorable orderedstructures (e.g., conformed chains, aggregates,crystallites). It is expected that any structuralorder in each material will be composed mostly ofchains adopting a (ggtt)n helical bond sequence, aportion of which may arrange to create crystallineForm I domains in the more stereoregular spe-cies.7,9,11 Each sample film is maintained at am-bient conditions for at least one month of resi-dence prior to testing. Under these processingconditions, it is unlikely that a majority of mac-romolecular chains will adopt the less stable(tttt)n planar zigzag conformation. Nevertheless,it is also unreasonable to expect that this confor-mation will be absent entirely, especially in morestereoregular samples.

FTIR spectroscopy

Transmission mode IR absorbance spectra arecollected using a Bio-Rad model FTS 6000 FTIRspectrometer operating under rapid-scan (contin-uous) interferometry. A silicon carbide source pro-vides broadband excitation in the wavenumberrange from 4000 cm�1 to 400 cm�1. The Michel-son-interferometer-modulated beam is redirectedto an external chamber that contains a custommodified tensile rheometer (MiniMat 2000, Rheo-metric Scientific, Inc.) capable of near isothermaltemperature control from ambient conditions to200 °C. The rheometer is fitted with potassiumbromide windows in an aluminum block housingto provide both a transparent medium for themid-IR beam and good control of the samplechamber temperature. Spectra are collected witha resolution of 0.5 cm�1 and 1024 scans coadded(i.e., averaged) using a liquid-nitrogen-cooled fastmercury-cadmium-telluride (MCT) detector. Be-tween loading a sample film and spectral acqui-sition, 45 min are allowed to elapse in order forsamples to equilibrate thermally and for ambientcarbon dioxide and moisture to be purged fromthe external chamber. Spectra are acquired at 30°C and at increasing intervals of 20 °C until thefilm can no longer stand freely under gravity (i.e.,it “sags” out of the beam path). Thus, more spec-

tra are collected for certain materials than forothers. In general, the maximum temperatureachievable for scanning films using this experi-mental setup increases with syndiotactic content,with the exception of aPP—probably due to itsnoticeably greater molecular weight. Includingthe allotment for thermal equilibration andchamber purging, spectral acquisition requiresapproximately 3 h.

Differential scanning calorimetry

Specimens are analyzed using either a TA Instru-ments 2920 or a TA Instruments Q1000 Modu-lated® Differential Scanning Calorimeter(MDSC). A linear heating rate of 5 °C/min is ap-plied with a perturbation amplitude of � 0.796 °Cevery 60 s. Samples are subjected to a heat-cool-heat profile using a temperature range from ei-ther –60 °C or –80 °C to � 200 °C. n-Decane,indium, and tin standards are used for tempera-ture calibration. Calorimeters are also calibratedfor modulated work using a sapphire standard forheat capacity calibrations. Thermogram base-lines are normally examined and corrected forcurvature if present using manufacturer-sug-gested methods during calibration; however,these are not included in the data presented.Sample amounts of 10 to 14 mg are used foranalysis; sample mass does not change apprecia-bly over the course of the DSC experiments.

RESULTS AND DISCUSSION

FTIR spectroscopy

IR absorbance spectra of all six materials at dif-ferent temperatures are reported in Figures 1through 6. Peak assignments, summarized in Ta-ble 2, are compiled from three sources: valenceforce calculations and observations of sPP vibra-tional spectra54; rheo-optical infrared linear di-chroism studies of sPP44; and a recent work, com-plementary to the present work, characterizingthe effects of stereoregularity on sPP vibrationalspectra.50 Since each spectrum contains manypeaks over the wavenumber range scanned (1300cm�1 to 800 cm�1), the analysis to follow willfocus more narrowly on those peaks exhibitinggreatest change with temperature. Each figure(Figs. 1–6) consists of:

THERMAL EFFECTS IN SEMI-SYNDIOTACTIC POLYPROPYLENE 441

Figure 1. High-resolution FTIR absorbance spectra of aPP, reported over variouswavenumber ranges and at different temperatures: (a) fully measured spectrum (1300cm�1 to 800 cm�1) at 30 °C; (b) 905 cm�1 to 800 cm�1, from 30 °C to 150 °C; (c) 1000cm�1 to 967.5 cm�1, from 30 °C to 150 °C; (d) 1265 cm�1 to 1150 cm�1, from 30 °C to150 °C.

442 SEVEGNEY ET AL.

a. The full spectrum acquired at 30 °C. Sig-nificant peaks are labeled with centerwavenumber and, parenthetically, peakarea magnitudes relative to the conforma-

tion-insensitive reference peak near 1157cm�1.55

b. the region from 920 cm�1 to 800 cm�1 atvarious temperatures.

Figure 1. (Continued from the previous page)


Figure 2. High-resolution FTIR absorbance spectra of s-sPP32, reported over variouswavenumber ranges and at different temperatures: (a) fully measured spectrum (1300cm�1 to 800 cm�1) at 30 °C; (b) 915 cm�1 to 800 cm�1, from 30 °C to 90 °C; (c) 1005 cm�1

to 967.5 cm�1, from 30 °C to 90 °C; (d) 1270 cm�1 to 1150 cm�1, from 30 °C to 90 °C.

444 SEVEGNEY ET AL.

c. the region from 1010 cm�1 to 920 cm�1 atvarious temperatures.

d. the region from 1270 cm�1 to 1010 cm�1 atvarious temperatures.

920 cm�1 to 800 cm�1

In approximately atactic materials (aPP in Fig. 1,s-sPP32 in Fig. 2), the region from 850 cm�1 to 800



Figure 3. High-resolution FTIR absorbance spectra of s-sPP49, reported over variouswavenumber ranges and at different temperatures: (a) fully measured spectrum (1300cm�1 to 800 cm�1) at 30 °C; (b) 920 cm�1 to 800 cm�1, from 30 °C to 110 °C; (c) 1010cm�1 to 930 cm�1, from 30 °C to 110 °C; (d) 1270 cm�1 to 1150 cm�1, from 30 °C to 110 °C.

446 SEVEGNEY ET AL.

cm�1 is dominated by a broad, nearly featurelessplateau. Two very weak peaks appear around 828cm�1 and 843 cm�1, and have been assigned to

planar zigzag and helical chains, respectively, in theinterfacial mesophase using rheo-FTIR spectrosco-py.44 These peaks are understandably weak due to:




448 SEVEGNEY ET AL.

1. Low stereoregularity, and therefore, the in-ability to form helices and planar zigzags.

2. The complete absence of a crystalline

phase (confirmed using X-ray diffrac-tion50), and thus, of a crystalline-amor-phous interface.




450 SEVEGNEY ET AL.

These peaks remain relatively weak, even formore stereoregular samples (Figs. 3–6). This be-havior is curious, especially considering that the

strongest mesophase response in IR dichroism (insPP) occurs at 838 cm�1 (a band to which 828cm�1 and 843 cm�1 contribute). With increasing



Figure 6. High-resolution FTIR absorbance spectra of sPP, reported over variouswavenumber ranges and at different temperatures: (a) fully measured spectrum (1300cm�1 to 800 cm�1) at 30 °C; (b) 910 cm�1 to 800 cm�1, from 30 °C to 150 °C; (c) 1010cm�1 to 930 cm�1, from 30 °C to 150 °C; (d) 1270 cm�1 to 1150 cm�1, from 30 °C to 150 °C.

452 SEVEGNEY ET AL.

temperature (Figs. 1b and 2b), these peaks simplydecrease in intensity, coalescing into the broadplateau.

Peaks representing crystalline helices (“FormI”7) appear at 812 cm�1 and 867 cm�1 as tacticityincreases. As temperature increases, these two



peaks decrease in relative intensity and growbroader, as observed in Figures 3b through 6b.Also, the 867 cm�1 peak, which has been charac-terized as most sensitive to Form I crystals,56

undergoes a wavenumber shift from 867.3 cm�1

to 866.6 cm�1 in Figure 6b. This is a commonfeature that is observed to some extent in eachmaterial. As thermal energy is added, intramolec-ular and intermolecular forces will be overcomegradually. Thus, less energy is required to excitecorresponding vibrational dipoles, and spectralpeaks will appear at lesser wavenumbers as ther-mal energy drives materials toward increasingdisorder. This trend does not always hold, how-

ever, as will be demonstrated for peaks that splitwith increasing tacticity.

A weak peak at 898.4 cm�1 in aPP (Fig. 1b)simply diminishes in peak area intensity (from0.040 to 0.024) with temperature increase. Atmore moderate levels of stereoregularity (s-sPP32and s-sPP49), this peak shifts to greater wave-numbers with additional shoulder peaks appear-ing on either side (e.g., 908.6 cm�1 and 888.8cm�1 in Fig. 3b). It has been suggested that this“split” of 900 cm�1 is caused by some as yet un-identified, perhaps highly disordered, modifica-tion of Form I crystals that is generated only inmoderately stereoregular sPP.50 A more thorough

Table 2. Characteristic Infrared Peak Assignments for Syndiotactic Polypropylenea

Position(cm�1) Vibrational mode(s)b Morphology Comment

812 �(CH2) Helical (crystalline)c

828 �(C™CH3) Planar (mesophase)c Very weak in absorbance; strong in dichroismc

843 �(CH2) Helical (mesophase)c Very weak in absorbance; strong in dichroismc

867 �(CH2) Helical (crystalline)c

889 Shoulderd of 900 cm�1

900 �(CH3)Helical (crystalline)c �Amorphousd

909 Shoulderd of 900 cm�1

936 �(CH3), �(C™C) Helical (crystalline)d

963 �(CH3), �(C™C) Planar zigzag (interfacial)cSplitd from 972 cm�1; helical population indexwith 977 cm�1

972 Amorphousd Parentd of 963, 977 cm�1

977 �(CH3), �(C™C) Helical (interfacial)cSplitd from 972 cm�1; helical population indexwith 963 cm�1

992–997 Helical (amorphous)d

1005 �(C™CH3), �(CH3) Helical (crystalline)c

1035 �(C™C), �(CH2) Helical (crystalline)d

1061 �(C™CH3) Helical (crystalline)d

1085 �(C™C), �(C™H) Helical (crystalline)d

1131 �(CH2), �(CH2) Planar zigzagc (amorphous)c

1153 �(CH2), �(CH3) Planar zigzag (amorphous)d Splitd from 1157 cm�1

1157 Conformation insensitive“Reference peak”; Parentd of 1153, 1161, and1169 cm�1

1161 �(C™C)Non-conformed(amorphous)d Splitd from 1157 cm�1

1169 �(C™C) Helical (amorphous)d Splitd from 1157 cm�1

1201 �(C™H) Helical Syndiotacticity indexd (position)

1232 �(C™H), �(CH2)Planar zigzag �Amorphousc

Syndiotacticity indexd (intensity) with 1249cm�1

1249 �(CH2) AmorphousdSyndiotacticity indexd (intensity) with 1232cm�1

1264 �(C™H), �(CH2) Helical (crystalline)d

a Assignment taken from valence force calculations (Ref 54), unless otherwise noted.b Key: � � rock, � � stretch, � � twist, � � bend, � � wag.c Assignment taken from rheo-FTIR spectroscopy of sPP (Ref 44).d Assignment suggested based on results in vibrational spectroscopic studies (Ref 50 and the present work).

454 SEVEGNEY ET AL.

crystallographic analysis is required to compose amore definite picture of what is happening. Thefact that the shoulders do not coalesce with theparent peak at 900 cm�1 as temperature in-creases (Fig. 3b) suggests that the shoulders arenot formed from true “splitting.” That is, thepeaks at 889, 900, and 909 cm�1 are distinct, anddo not represent sPP chains that participate in acommon domain or microstructure, as is the casewith peak splitting at 972 cm�1 and 1157 cm�1.50

1010 cm�1 to 920 cm�1

A weak band appears near 936 cm�1 in s-sPP49(Fig. 3c) and grows slightly more intense withincreasing tacticity. This peak behaves very muchlike 812 cm�1 with temperature increase, dimin-ishing in intensity and broadening, but not shift-ing its position very much. This behavior mostlikely corresponds to helices found in Form I crys-tallites. The strong peak near 972 cm�1 in aPP(Fig. 1c) does not change much with temperature.Even at high temperatures, this peak shows onlya minimal shift from 971.9 cm�1 to 971.3 cm�1.This peak also increases gradually in magnitudeas temperature increases. For s-sPP32 (Fig. 2c),this peak also grows with temperature, but doesnot shift its position. This is interesting to notesince, although both materials are approximatelyatactic—certainly both are unable to form con-formed chains or crystals50—the peak appears at

a position in s-sPP32 1 cm�1 greater than in aPP.Even with similar morphologies, the subtle differ-ence in configurational order (i.e., stereoregular-ity) between these two materials is detected withFTIR. As discovered previously,50 this peak splitsinto two peaks at 963 cm�1 and 978 cm�1 assyndiotacticity increases (see Figs. 4a–6a). Rheo-FTIR analysis assigns these bands to planar zig-zag and helical chains, respectively, that partici-pate in the crystalline-amorphous interface,which transforms into the planar-rich mesophase,upon application of sufficient tensile strain.46 Ascan be observed in the more stereoregular mate-rials (Figs. 4c–6c), these split peaks do not simplydiminish and shift to lesser wavenumbers withtemperature increase as do other IR bands.Rather, each peak broadens and changes its in-tensity to match that of the other. To varyingextents in each material, the peaks noticeablyshift toward each other, as if coalescing to formthe parent peak at 972 cm�1. This behavior alsoagrees well with the assignment of the absor-bance ratio of these two split peaks (A978/A963) asan index of overall helical content in sPP.37 Astemperature increases, the value of this ratiosteadily decreases, as illustrated in Figure 7. Italso appears that tacticity affects the extent towhich the split peaks shift as temperature varies.Table 3 lists the extent of shift for both 978 cm�1

and 963 cm�1 peaks in the three most stereoregu-lar samples. The lesser degree of shifting ob-

Figure 7. Ratio of the absorbance at 978 cm�1 to the absorbance 963 cm�1—indica-tive of the overall relative population of helical chains to planar zigzag chains—forhighly syndiotactic materials as temperature varies.


served in sPP suggests that conformed chains(certainly helices) are more stable in materials ofgreater syndiotacticity. This is similar to behaviorof the parent 972 cm�1 peak in the approximatelyatactic materials. The minimal amount of split-ting observed in s-sPP49 (Fig. 3c) quickly coa-lesces into the parent peak at greater tempera-tures.

The moderately strong peak at 994 cm�1 showssplitting behavior similar to 972 cm�1 for morestereoregular samples, but behaves quite differ-ently at lower stereoregularities. In aPP (Fig. 1c),this peak decreases in intensity and broadensslightly. The same peak, which appears near 997cm�1 in s-sPP32 (Fig. 2) (again, the more ener-getic position stemming from configurational or-der), also decreases in peak area intensity andbroadens with increased temperature. However,its center position shifts from 996.9 cm�1 at 30 °Cto 995.9 cm�1 at 90 °C. We suggest that this peakshifts to a greater extent in s-sPP32 than in aPP(from 993.9 cm�1 to 993.7 cm�1) because the he-lical chains it represents experience a greater lossof order with temperature increase than do themore disordered helices in aPP. Unlike 978 cm�1,which represents interfacial helices and under-goes a lesser degree of wavenumber shift withtemperature, this peak most likely represents he-lices present in amorphous domains. 994 cm�1

does not undergo as pronounced a wavenumbershift in aPP as it does in s-sPP32 (where it ap-pears at 997 cm�1) simply because it has lessconformational order to lose to thermal effects. Ins-sPP49 (Fig. 3a), this peak appears to split intotwo bands centered around 992 cm�1 and 1005cm�1. For a given temperature, as tacticity in-creases, the relative intensities of 992 cm�1 and1005 cm�1 decrease and increase, respectively,until the former peak disappears entirely in sPP(Fig. 6a). This behavior supports a previously pro-

posed assignment of 992 cm�1 and 1005 cm�1 tohelical chains present in amorphous and Form Icrystalline domains, respectively.50

1270 cm�1 to 1010 cm�1

Several weak bands appear around 1035, 1061,1085, and 1264 cm�1 in the more stereoregularsamples (Figs. 4–6). All four peaks decrease inintensity and shift to lesser wavenumber posi-tions as temperature increases (Figs. 4d–6d).These results help to support the assignment ofeach band to helices present in Form I crystallinedomains. Yet another weak band that grows withstereoregular content appears at 1131 cm�1.While it behaves similarly to the four bands men-tioned above, both valence force calculations54

and rheo-FTIR dichroism experiments44 confirmthat 1131 cm�1 is sensitive to planar zigzagpresent in amorphous domains.

The principal feature in this range of the spec-trum is the strong peak centered near 1157 cm�1.This is the well-characterized conformation-in-sensitive reference peak of sPP. 1157 cm�1 re-tains its general shape, with some shifting tolesser wavenumbers as syndiotacticity increases,for all semi-sPP materials (Figs. 1a–5a). This be-havior can be explained using the split peaks thatappear and surround this reference peak in sPP(Fig. 6a). Valence force calculations clearly placebands sensitive to helical and planar zigzagchains at 1167 cm�1 and 1153 cm�1, respectively(near the observed split peaks).54 The succes-sively greater populations of nonconformed (1161cm�1) and planar zigzag (1153 cm�1) chains canbe described best by assigning all three splitpeaks (and the parent peak) to chains that areabsent from any ordered domain or microstruc-ture (i.e., in amorphous domains). In highly ste-reoregular sPP, it is intuitive to suggest that themajority of helices are populating either the crys-talline–amorphous interface or Form I crystal-lites. By this reasoning, it is conjectured thatamorphous domains contain a greater relativepopulation of planar zigzags than both noncon-formed chains (due to the high level of configura-tional order in sPP) and helical chains (due totheir participation in ordered structures). Fur-thermore, these three peaks exhibit true splittingbehavior, as they coalesce completely into the par-ent peak as temperature increases (Fig. 6d).

The broad, weak peak at 1201 cm�1 remainsrelatively unchanged with temperature. In fact,

Table 3. Extent of Temperature-Induced Shifting ofthe 978 cm�1 and 963 cm�1 Peaks (Split from 972cm�1) as a Function of Tacticity

Material

Maximumtemperature

achieved(°C)

978 cm�1

peak shift(cm�1)

963 cm�1

peak shift(cm�1)

s-sPP65 110 �1.4 �0.4s-sPP67 110 �1.1 �0.7sPP 150 �0.6 0.0

456 SEVEGNEY ET AL.

its only significant changes, an intensity increaseand shift to greater wavenumbers, accompany anincrease in syndiotacticity. This suggests that thehorizontal shift of 1201 cm�1 may function as asyndiotacticity index because of its low degree ofsensitivity to temperature change. Figure 8 illus-trates the overall increase in wavenumber posi-tion for this peak as tacticity increases. Whilesome decrease in position is observed as tacticity

increases, the variance is within the 0.5-cm�1

resolution of spectral measurements. The overallposition change, however, clearly exceeds this pa-rameter (e.g., 1201.0 cm�1 to 1202.8 cm�1) at 30°C. A pair of peaks at 1232 cm�1 and 1249 cm�1

also seems to function as a good syndiotacticityindex. The absorbance ratio, A1232/A1249 (Fig. 9),agrees well qualitatively with 13C-NMR tacticitymeasurements. The relative intensities of these

Figure 8. Center wavenumber position of peak near 1201 cm�1, suggested to be aqualitative index of syndiotacticity, as both tacticity and temperature are varied.

Figure 9. Ratio of the absorbance at 1232 cm�1 to the absorbance 1249 cm�1—indicative of the overall syndiotactic content in sPP—at different temperatures forlower syndiotactic materials.


Figure 10. DSC thermograms of syndiotactic polypropylene materials (aPP, s-sPP49,s-sPP65, and sPP): (a) heat flow versus temperature. (b) Modulated® thermograms,showing both reversible and irreversible components of heat flow.

458 SEVEGNEY ET AL.

peaks change steadily with tacticity until 1232cm�1 is dominant and 1249 cm�1 is only a shoul-der (in s-sPP65, Fig. 4a). These peaks do undergoa slight shift to lesser wavenumbers as tempera-ture increases, but no greater than 0.6 cm�1, ob-served in aPP (Fig. 1d). This suggests that each issensitive to some chain conformation; rheo-FTIRfirmly assigns 1232 cm�1 to amorphous planarzigzags.

Differential scanning calorimetry

Intramolecular forces that hold the dominantpopulation of helical chains in their conformationare progressively overcome as thermal energy isadded. This conformational disorder in sPP heli-cal regions has been observed using variable tem-perature 13C-CP/MAS NMR,57 in addition to theaforementioned IR analysis. Modulated® DSCthermograms (Fig. 10) illustrate how s-sPP49 ands-sPP65 possess thermal transitions at tempera-tures intermediate to those observed for transi-tions in highly atactic and highly syndiotacticmaterials. The intermediate tacticity materialsshow two endothermic nonmelting transitionsnear 50 °C and 70 °C, which are above the glasstransition temperature of aPP (near 0 °C) andbelow the classical melting region observed forcrystalline sPP (at 129 °C; see Fig. 10a). Theformer is attributed to a thermally-driven trans-to-gauche isomerization, causing planar zigzagchains to re-conform as more thermodynamicallystable helices.57 The latter transition is thoughtto be due to helices that exist in a “dynamicallydisordered” equilibrium, characterized by rapidtrans 7 gauche conversion of bond angles alongthe main chain. The reversing and nonreversingheat flow signals summarized in Figure 10b alsosupport the interpretation that the lower temper-ature endotherms are due to a thermally driventrans-to-gauche isomerization and the motion ofthe helices as described above. Note that abovethe 50 °C and 70 °C endothermic transitions, thenonreversing signal for the highly syndiotacticpolymer evolves into the typical exothermic crys-tal rearrangement/crystal perfection behaviornormally observed prior to melting in other slowlyordering semicrystalline materials (e.g., PET). Itis also interesting to note that the characteristicabsorbance ratio, A978/A963, (Fig. 7) decreases invalue at an accelerated rate near the two afore-mentioned transition temperatures. This obser-vation reinforces the assignment of the A978/A963

ratio as proportional to the population of Form Ihelical crystals.

CONCLUSIONS

This work continues the first-ever experimentalFTIR investigation of metallocene-catalyzedsemisyndiotactic polypropylenes with a broadrange of stereoregular content (26 to 96% rr).With the spectral effects of tacticity characterizedand understood, this work examines spectral re-sults for melt-slow-cooled semi-sPP films as tem-perature is varied. These measurements help toconfirm the splitting behavior of 972 cm�1 and1157 cm�1 that occurs as syndiotacticity in-creases. The trend of split peaks to coalesce grad-ually as temperature increases corresponds nicelyto the thermally induced conformation loss of pla-nar zigzags and helices described using Modu-lated® DSC and 13C-NMR spectroscopy. Also, re-sults suggest that peak behavior around 900 cm�1

at moderate syndiotacticity levels is not true peaksplitting, due to the lack of thermally driven co-alescence. Rather, peaks at 889 cm�1 and 909cm�1 are most likely caused by some heretofore-unidentified disordered crystal morph that is ableto form at moderate tacticities. New assignmentsare proposed for peaks at 1201, 1232, and 1249cm�1. These peaks exhibit a much greater sensi-tivity toward tacticity change than toward tem-perature change, and are thus suggested to begood qualitative indices of syndiotactic content.

The spectral phenomena observed in our exper-imental studies thus far—peak area intensitychange, broadening, shifting, and splitting—havealso been observed in rheo-FTIR linear dichroismmeasurements. To be sure, a mechanical pertur-bation imposed on sample films can cause a slightvariation in the local environment of vibrationaldipole moments, thus causing one or more of thepeak behaviors listed above. Having gained moreinsight into the roles tacticity and temperatureplay in the IR spectrum of sPP, we now moveforward to characterize the anisotropic orienta-tion behavior of these novel semi-sPP materialsusing rheo-FTIR spectroscopy.

The authors acknowledge assistance from Dr. S. Venkat-aramani (3M Co.) for providing molecular weight distri-bution data, as well as financial support from NSF-CAREER (DMR9876221), NSF-IGERT (DGE9870720),and the Institute for Manufacturing Research at WayneState University.


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