Rheo-optical measurements

Rheo-optical Studies. One of the most useful techniques for studying the crystallization behavior of stereoregular polymers is rheo-optical measurements (11). 

Van Puyvelde, P., Antonov, Y.A., Moldenaers, P. (2002). Rheo-optical measurement of the interfacial tension of aqueous biopolymer mixtures. Food Hydrocolloids, 16, 395-402. 

By exploiting the automated information processing capability of the dedicated computer in the FTIR system much of the routine analj s of spectra series produced in rheo-optical measurements has been alleviated. In this respect BASIC software was developed for the evaluation and representation of the spectroscopic data in terms of various parameters ... 

The rheo-optical measurements have been primarily performed with the intention to obtain further experimental evidence to support or reject spedflc features of the existing models. The FTIR polarization spectra taken during elongation up to 130% strain are shown in Fig. 55. The evaluation of these spectra in terms of the state of order and the orientation functions of the crystalline regicms and the average polymer... 

A rheo-optical measurement is considered to be a combination of a mechanical test with a simultaneous optical measurement. In rheo-optical FTIR... 

The first measurements of this type were reported by Palmer et al. [18], who noted the similarity of the experimental setup for dynamic polymer rheo-optical measurements and liquid-crystal electroreorientation. However, because the changes in the polarized spectmm on switching the electric field can be very large, the measurement of liquid-crystal electroreoiientation tends to be much easier than the DIRLD measurements described in Section 21.2. Two-dimensional correlation analysis is frequently used to aid in interpreting the results. 

Most of the rheo-optical measurements have so far been performed by IR spectroscopy ° on film samples <50 pm thickness. With the more efficient exploitation of the NIR region, sample thickness and sample preparation have become less of an issue and polymer films up to 1 mm thickness can now be readily investigated by rheo-optical spectroscopy. The potential of FT-NIR spectroscopy can be readily illustrated by rheo-optical investigations of PDMS/polycarbonate (PC) copolymers. The mechanical properties of these segmented block copolymer networks composed of alternating PDMS and PC soft and hard blocks, respectively, such as elasticity, modulus, and stretchability are attributed to the order of the physical... 

In conclusion, rheo-optical measurements provide a multiplicity of stmctural informations that contribute toward an improved understanding of the technological properties of the investigated polymer. 

An interesting feature of polarized IR spectroscopy is that rapid measurements can be performed while preserving molecular information (in contrast with birefringence) and without the need for a synchrotron source (X-ray diffraction). Time-resolved IRLD studies are almost exclusively realized in transmission because of its compatibility with various types of tensile testing devices. In the simplest implementation, p- and s-polarized spectra are sequentially acquired while the sample is deformed and/or relaxing. The time resolution is generally limited to several seconds per spectrum by the acquisition time of two spectra and by the speed at which the polarizer can be rotated. Siesler et al. have used such a rheo-optical technique to study the dynamics of multiple polymers and copolymers [40]. 

Since the tack and green strength of these rubbers may depend on their ability to undergo strain induced crystallization, this behavior was studied and compared with that shown by NR. Information on the strain induced crystallization behavior of these experimental rubbers was obtained from x-ray diffraction measurements and from rheo-optical studies. 

A.W. Chow, and G.G. Fuller, The rheo-optical response of rod-like chains subject to transient shear flow. Part I Model calculations on the effects of polydispersity, Macromolecules 18, 786 (1985) A.W. Chow, G.G. Fuller, D.G. Wallace and J.A. Madri, The rheo-optical response of rod-like chains subject to transient shear flow. Part II. Two-color flow birefringence measurements, Macromolecules 18,793 (1985) A.W. Chow, G.G. Fuller, D.G. Wallace and J.A. Madri, The rheo-optical response of rod-like shortened collagen protein to transient shear flow, Macromolecules, 18, 805 (1985). 

A review of rheo-optical techniques by Sherman et al. (1996) notes that there has been an increase in the use of rheo-optic set-ups both for FT-IR dichroism and for dynamic IR dichroism spectroscopies for polymer melts and polymer blends. Skytt et al. (1996) highlight the use of simultaneous measurement of the transient or steady-state rheological properties and IR dichroism to characterize orientation in polymer melts. However, there is little reference to dual spectroscopic-rheological techniques for reactive polymer systems in the literature. 

An area that will require attention in the future involves in-line analysis of polymer blends during processing potentially allowing feedback information to improve uniformity of the resultant blend. In-line morphology determination, rheological measurements, and shear-induced phase behavior are subjects for consideration. Rheo-optical studies under shear for low molecular weight PS/PIB blends near phase separation conditions were... 

Rheo-optical techniques (46 8) afford information on the strain dependence not only of stress but also of optical quantities associated directly with the structure or molecular morphology. The techniques were developed extensively for crystalline polymers to investigate the molecular deformation mechanism underlying the tensile elongation. In this part, the chain orientation behavior is characterized by infrared dichroism measured simultaneously with tensile deformation at a constant rate of elongation. 

Rheo-optical FTIR spectroscopy was used to measure orientation and orientational relaxation in binary blends of long- and short-chain linear PS. Films were stretched at constant strain rate at various temperatures above Tg. 17 refs. 

This test may then represent a method for determining the time for loss of orientation which is difficult to measure using rheo-optical techniques. 

Generally, from spectroscopic data such as frequency position, band shape, intensity and dichroism of specific absorption bands conclusions can be derived in terms of the applied mechanical stress and the state of order and orientation of the polymer under investigation. An extremely powerful method for the study of transient phenomena in polyuMr deformation and relaxation is rheo-optics which describes the relation between stress, strain and an optical quantity (for example birefringence, infrared absorption, light scattering. X-ray diffraction) measured simultaneously with stress and strain as a function of time In a given rheo-optical method therefore, a mechanical test is combined with one of these various types of optical measurements. 

In this context rheo-optical FTIR spectroscopy has proved a valuable technique to study the phenomenon of strain-induced crystallization on-line to the deformation process of the elastomer under investigation. Whith the aid of an appropriate absorption band which is characteristic of the threedimensional order in the crystalline phase the onset and progress of strain-induced crystallization during elongation and its disappearance upon recovery can be unambigously monitored simultaneously to the mechanical measurements. Representative for several rubber-like materials which have been investigated by this technique in our laboratory the results obtained with sulfur-crosslinked (1.8 % S) natural rubber (100% 1,4-ds-polyisoprene) and a radiation-crosslinked synthetic polyisoprene (93% 1,4-ds-isomer) lall be discussed in some detail here. 

Here, the results of rheo-optical investigations of the three abovementioned polyester urethanes in combination with elevated temperature measurements and deuterium exchange will be discussed... 

A. Chow, G. Fuller, D. Wallace, and J. Madri, Rheo-optical response of rod-like chains subject to transient shear flow 2. Two-color flow birefringence measurements on collagen protein. Macromolecules, 18,793-804,1985. 

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