A Rheo-Optical FTIR Spectrometer for Investigating Molecular Orientation and Viscoelastic Behavior in Polymers

Abstract

A rheo-optical Fourier transform infrared (FTIR) spectrometer has been developed by combining step-scan FTIR spectroscopy with elements of polarimetry and rheometry in order to investigate both deformationinduced morphology changes and orientation=relaxation dynamics of components within complex polymers. This versatile instrument can directly and sensitively measure quantitative infrared spectra at different temperatures and for different deformation modes. A complex, multiplemodulation=demodulation processing scheme is used to extract both absorbance and dichroism information from a single detector signal. Experimental results from a semicrystalline isotactic polypropylene and a thermoplastic polyurethane are reported in order to illustrate instrument capabilities. INTRODUCTIONEstablishing the relationships between molecular structure and macromechanical (bulk) properties in polymers is an important task in the field of materials science and engineering. Such an understanding is becoming even more critical with emerging synthesis and processing technologies, where property manipulation at microscopic, and even nanoscopic, length scales is desired. Our recent efforts focus on understanding the dynamic behavior of individual chemical functionalities and microstructures within complex polymers as they undergo important processing steps. Specifically, we are studying both changes in morphology (over a range of length scales) and molecular orientation= relaxation responses that are induced by a mechanical perturbation of the bulk material. For example, the morphology of a semicrystalline polymer depends on stereochemical regularity, thermal history, and mechanical processing history [1,2] . Consequently, chemically identical polymers can possess different microstructures (e.g., crystalline, amorphous, mesophase), each responding quite differently to macroscopic deformation.Optical methods such as birefringence [3][4][5][6] , X-ray scattering [7][8][9][10][11][12] , and vibrational spectroscopy [13][14][15][16][17] have been used to characterize morphology and molecular orientation in polymers. Each aforementioned technique presents some subtle experimental limitation(s) that can be overcome using rheo-optical Fourier transform infrared (FTIR) spectroscopy. Rheo-FTIR involves the quantitative measurement of anisotropic absorbance (i.e., dichroism) spectra in order to investigate the local polymer physics responsible for macroscopic viscoelastic behavior. Absorption of infrared radiation is sensitive to individual moieties within polymers and thus serves as a good vibrational probe. Linear dichroism-the anisotropic absorption by a material of two mutually perpendicular states of linearly polarized light-has been recognized as a measure of orientation anisotropy on a molecular or submolecular scale [18][19][20] . Changes in orientation of different structures within a polymer, as well as morphology changes from one type of structure to another, may be driven by a macromechanical perturbation.We have developed a versati...

Interrelationship between ultimate mechanical properties of variously structured polyurethanes and poly(urethane urea)s and stretching rate thereof

“…In the FTIR spectrum of PU-3 SPU, the band at 1,703 cm −1 is related to the absorption of hydrogen-bonded, ordered [31,34]) localized in hard phase of polymer. The band at 1,729-1,731 cm −1 is related to the absorption of free urethane carbonyl, and the band at 1,710-1,711 cm −1 is related to absorption of hydrogenbonded urethane carbonyl in the soft phase of the polymer [18,28,29].…”

“…Identification of C0O absorption bands was made using the results of spectral investigations of real and model compositions with urethane urea and urethane groups [12,18,[28][29][30][31][32][33]. The band at wave number ν01,640 cm −1 relates to the absorption of ordered hydrogen-bonded urea carbonyl [18,29,30,32].…”

Monitoring elasticity and orientation in syndiotactic polypropylene

“…This is achieved using a custom-developed rheo-FTIR spectrometer [21,22], which combines broadband polarization photoelastic modulation (model PEM-90/ZS37, Hinds Instruments, Inc., Hillsboro, OR) and step-scan interferometry (model FTS 6000, BioRad/Digilab, Randolph, MA) with rheometry in order to measure quantitative IR dichroism spectra directly and rapidly. For ex situ conditions (i.e.…”

New multi-block isophorone diisocyanate-based copolymers with urethane urea hard segments

“…To assign C¼O absorption bands for compositions with uniform (x 2 ¼ 0 and x 2 ¼ 1) and various HS, results of spectral studies of the polyurea model and of real materials with urethane urea groups were used. 9,20,26,[33][34][35][36][37] The band at 1638 cm À1 is assigned to the absorbance of the hydrogen-bonded ordered urea carbonyl 9,26,[34][35][36] for material obtained using an aromatic diisocyanate. Urea groups are mutually hydrogen bonded (self-associates of urea groups) and localized in the domains of hard urethane urea segments.…”

“…To assign C=O absorption bands for compositions with uniform ( x 2 = 0 and x 2 = 1) and various HS, results of spectral studies of the polyurea model and of real materials with urethane urea groups were used. 9,20,26,33 –37…”