| Rheo-optical Fourier Transform infrared (rheo-FTIR) spectroscopy, a novel characterization technique, is used to characterize polypropylene homopolymers with very well defined tacticity (stereochemical) distributions. Elements of step-scan interferometry, polarimetry, rheometry, and digital signal processing are combined to create an instrument capable of measuring orientation anisotropy and detecting morphological moieties in polymers over a broad range of temperatures. Small changes in linear dichroism and absorbance spectra are measured simultaneously and sensitively as solid-state thin films undergo tensile perturbation. Two modes of deformation are employed: small-amplitude, oscillatory strain, used to characterize component viscoelastic orientation behavior, and large-amplitude, irreversible strain, which induces overall morphology transformation in the materials studied. Semi-crystalline, stereospecific polypropylenes, synthesized using Ziegler-Natta and metallocene catalysts, are characterized with rheo-FTIR as tacticity, temperature, and processing history are varied. Under large deformation, highly isotactic polypropylene (iPP) shows, sequentially, orientation of crystalline domains, a "saturation" of crystal orientation (corresponding to mechanical yielding), and finally, orientation of amorphous domains. Results are qualitatively similar for melt-quenched iPP, which contains more disordered "smectic" crystals, and melt-slow-cooled iPP, which possesses ordered crystals. Moreover, dynamic dichroism spectra of iPP manifest bimodal peaks and quantitative out-of-phase responses. These reveal the viscoelastic and multi-component nature of characteristic IR bands. Highly syndiotactic material (sPP) exhibits mechanical behavior somewhat similar to iPP, but also a very complex polymorphism under large tensile strain. Rheo-FTIR spectra detail gradual deformation-induced transformations in chain conformation and morphology, from helical-rich crystalline to planar-zigzag-rich mesomorphic, which correlate with yielding and elastomeric behavior in sPP. The effects of stereoregularity are also studied closely using metallocene-catalyzed semi-syndiotactic materials (s-sPP). In addition to affecting subtle changes in IR spectral profiles, tacticity decrease appears to retard (or, in extreme cases, to completely eliminate) the crystallization and deformation-induced morphological transformation processes in sPP. Tacticity, however, appears to be a potentially useful variable in polymer materials engineering, as it seems able to influence a compromise between material strength and elasticity in sPP. Undoubtedly, this work gives only a limited illustration of the utility and scope of rheo-FTIR spectroscopy; thus, it concludes with a discussion of future research objectives involving this powerful new instrument. |
