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Polymer Tacticity : Isotactic, syndiotactic, semi-syndiotactic polypropylenes

KEY ASPECTS:
Isotactic, syndiotactic, atactic, semi-isotactic, and semi-syndiotactic polypropylenes are being studied using a wide range of experimental tools

Rheo-optical FTIR spectroscopy is sensitive enough to monitor orientation responses at more than 10 infrared peaks simultaneously.

In a quenched, highly syndiotactic polypropylene, large deformation appears to cause a helical to trans-planar conformation transition, perhaps causing a highly elastomeric response of the polymer at high strains

Semi-syndiotactic polypropylenes show extremely slow crystallization rates, mesophase formation, and strong tacticity dependent properties.

Blends of these materials are currently being explored

Collaborators: Allen Siedle (3M), Gerald Fuller and Robert Waymouth (Stanford)

For more details please see the following publications:
  1. 'A rheo-optical FTIR spectrometer for the investigation of deformation behavior in complex polymers', M. Sevegney, G. Hofmann, R. M. Kannan, ,Int. J. Poly. Anal. Char. 9, 245-274 (2004)


  2. 'Monitoring elasticity in syndiotactic polypropylene films', G.parthasarathy, M. Sevegney, R. M. Kannan, accepted , POLYMER 46 (17): 6335-6346 AUG 8 2005 [full pdf]


  3. 'Vibrational spectroscopic investigation of stereoregularity effects in syndiotactic polypropylene structure and morphology', M. Sevegney, R.M. Kannan, R. Naik, V. Naik, Vibrational Spectroscopy (in press) (Jan 2006) [full pdf]


  4. 'FTIR Spectroscopic Investigation of Thermal Effects in Semi-Syndiotactic Polypropylene', M. Sevegney, R. M. Kannan, P. Percha, A. Siedle, J. Poly.Sci. (PP), 43(4), 493-461 (2004) [full pdf]

  5. 'Deformation-induced morphology changes and orientation behavior in syndiotactic polypropylene’, M. Sevegney, G. Parthasarthy, R. M. Kannan, Macromolecules (2003), 36, 6472-6483 [full pdf]

  6. 'Rheo-optical FTIR Spectroscopy of the deformation behavior in quenched and slow-cooled isotactic polypropylene films’, G. Parthasarthy, M. Sevegney, R. M. Kannan, J. Poly. Sci. (Poly. Phys.) (October, 2002) [full pdf]




         Understanding the relationships between tacticity and morphological behavior in semicrystalline polymers is vital for their processing and product engineering. Metallocene catalysts allow for control of tacticity in polyolefins, making it possible to synthesize materials with a wide range of properties from a single monomer. We are studying isotactic, syndiotactic, and semi-syndiotactic polypropylenes (ss-PP) with varying degrees of ‘syndiotacticity’, ranging from nearly atactic to nearly syndiotactic. We are also studying semi-isotactic polypropylenes (elastomeric polypropylenes synthesized and chracterized by Waymouth and Fuller).
         The have been custom-synthesized using asymmetric, bridged cyclopentafluorenyl catalysts, and characterized using NMR, modulated DSC, mechanical testing, AFM, rheology, and rheo-optical FTIR spectroscopy. Semi-syndiotactic polypropylenes with syndiotacticity index (based on 13C NMR racemic rr triad content) varying from 24 to 67.0 % are being investigated.

Isotactic Polypropylene: At large irreversible strains, we are able to measure the dichroic response for twelve different peaks simultaneously and quantitatively. The dichroism from the crystalline peaks is strong comparable to amorphous peaks. In the quenched sample, the dichroism from the crystalline region saturates at around 50% strain, accompanied by a significant increase in the amorphous region dichroism. This is consistent with the notion that the crystalline regions respond strongly before the yield point, whereas the majority of post-yielding orientation occurs in the amorphous region. Our results also suggest that the 841 cm-1 peak may be especially sensitive to the ‘smectic’ region orientation in the quenched sample.




Syndiotactic Polypropylene: Rheo-FTIR, WAXD, and Raman Scattering are used to characterize the morphology and orientation response to tensile strain. In quenched s-PP, there is a gradual transition in the chain conformation from (gauche-gauche-trans-trans) helical to all-trans planar. This is being viewed as a stage in the overall reversible Form I- Form III crystalline trnaformation, previously observed for s-PP fibers. We see a disappearance of Form-I crystals and the emergence of a trans-planar mesophase, the morphological precursor to Form III. Results from rheo-FTIR, Raman, and WAXD are consistent with each other. This figure shows the helical to trans-planar conformation changes induced by large strain deformation in s-PP film. The rhe-FTIR data agrees well with the Raman and WAXD data.


Static IRLD spectra for syndiotactic polypropylene films tensile deformed to various strain points. Spectra are qcquired in step-scan mode at room temperature (~30o C) with a resolution of 4 cm-1 and 32 scans co-added.


Semi-syndiotactic Polypropylene: The semi-sPPs show a significant amount of melt and solid state elasticity. These materials also exhibit a drastic change in both morphology and mechanical properties upon aging over a period of weeks. In contrast to the highly syndiotactic PP, semi-syndiotactic polypropylenes exhibit a ‘para-crystalline mesophase’ when quenched from the melt. This ‘mesophase’ that slowly grows as the sample ages at room temperature. AFM and infrared linear dichroism (IRLD) are used both to monitor the time evolution of the mesophase (at 824 and 900 cm-1) and to define its influence on mechanical properties.


‘Mesophase’ like peaks show up in the FTIR spectrum after weeks of aging!



AFM Image of Aged semi-syndiotactic PP – ‘micron-size’ structures are observed in the aged sample


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