Refractive index polymer solvents

The concentration of the polymer molecules eluting from SEC columns is continuously monitored by a detector. The most widely used detector in SEC is the differential reftactometer (DRI), which measures the difference in refractive index between solvent and solute. Other detectors commonly used for SEC are functional group detectors ultraviolet (UV) and infrared (IR), and absolute molecular weight detectors low angle laser light scattering (LALLS) and in-line continuous viscometers. Applications of these detectors to SEC analysis will be discussed later in the Multiple Detectors Section. Other detectors also being used are the densimeter (11-19) and the mass detector (20-23). 

Polymerization was carried out at 60 °C for 24 h in chlorobenzene. [M]o (initial monomer concentration) = 0.25. Monomer to catalyst mole ratio. Methanol-insoluble polymer. " Values were obtained by GPC analysis with polystyrene standards calibration (Waters high-pressure GPC assembly Model M590 pump /<-Styragel columns of 10 , 10, 10, 500, and 100 A refractive index detectors, solvent, THF). 

Inevitably the choice of solvent must reflect the type of detector used in the analysis. If a refractometer is used, then clearly there must be sufficient difference in refractive index between solvent and polymer solution for good response at the low concentrations required for good molecular separation. Alternatively, an appropriate window in the spectra of the solvent for an IR or UV detector is required. The exact choice of the solvent depends on the polymer and the required sensitivity of the detectors. 

Most fillers and fibers are inorganic materials of high density, polarity, modulus, melting point, refractive index, and solvent resistance, so incorporating them into organic polymers produces major changes in properties. 

The value of Anr depends on the shape of the boundary between the phases as well as the difference in the average refractive indexes of the two phases. If that difference is small enough one could neglect its contribution. One way to determine its value is to swell the oriented polymer in solvents of varying refractive index. The solvent should swell the amorphous phase only and alter its refractive index. Thus the refractive index difference between the crystalline and the swollen amorphous phase may be altered (47). 

Light scattering is a classical technique used to study solutions (with the necessary difference in refractive index between solvent and solute). This is described in Chapter 2. It is also used for characterization of superstructures (spherulitic, axialitic, etc.) in semicrystalline polymers. The technique is called small-angle light scattering (SALS) and information is obtained about type and size of superstructure (Chapter 7). 

The factor 1 - p/p2 cannot be too close to zero, nor can the refractive index of the polymer and the solvent be too similar. These additional considerations limit the choice of solvents for a synthetic polymer, while their values are optimal for aqueous protein solutions. 

Working with different samples of the same polymer, other researchers have published conflicting values for the refractive index gradient in these solvents ... 

Using the original Hc2/r values, recalculate M using the various refractive index gradients. On the basis of self-consistency, estimate the molecular weight of this polymer and select the best value of dn/dc2 in each solvent. Criticize or defend the following proposition Since the extension of the Debye theory to large particles requires that the difference between n for solute and solvent be small, this difference should routinely be minimized for best results. 

Solution Polymers. Acryflc solution polymers are usually characterized by their composition, solids content, viscosity, molecular weight, glass-transition temperature, and solvent. The compositions of acryflc polymers are most readily determined by physicochemical methods such as spectroscopy, pyrolytic gas—liquid chromatography, and refractive index measurements (97,158). The solids content of acryflc polymers is determined by dilution followed by solvent evaporation to constant weight. Viscosities are most conveniently determined with a Brookfield viscometer, molecular weight by intrinsic viscosity (158), and glass-transition temperature by calorimetry. 

Eor amine-containing polymers, DMF is often a good choice of solvent. DMF can also be a good choice for polymers of higher carboxylic acid content. However, DMF does present some experimental difficulties. It must be run at an elevated temperature, typically 60°C, because of its viscosity. Also, because most polymers have a much lower refractive index response in DMF, the signal-to-noise ratio for a polymer in this solvent is diminished versus the same ratio for common acrylates in THF. 

One can read letters through the porous PVA-PVAc film in benzene, but one cannot do so in cyclohexane nor in the case of the blank. This is supported by the fact that the refractive indices of benzene are close to that of PVA, but the refractive index of cyclohexane is far from that of PVA. When the porous film was dipped in a mixed solvent of benzene and cyclohexane (8.0 2.0 in weight), it became semi-transparent. To make this point clearer, the refractive index and the dispersive power of polymers and organic solvents were measured. The results are shown in Table 3, which shows that the refractive index of PVA is near that of benzene and that the dispersion power of aliphatic compounds is lower than that of aromatic compounds. 

The subscripts 1,2,3 refer to the main solvent, the polymer, and the solvent added, respectively. The meanings of the other symbols are n refractive index m molarity of respective component in solvent 1 C the concentration in g cm"3 of the solution V the partial specific volume p the chemical potential M molecular weight (for the polymer per residue). The surscript ° indicates infinite dilution of the polymer. 

Mixed solvents are generally unsatisfactory for use in the determination of polymer molecular weights owing to the likelihood of selective absorption of one of the solvent components by the polymer coil. The excess of polarizabilit f of the polymer particle (polymer plus occluded solvent) is not then equal to the difference between the polarizabilities of the polymer and the solvent mixture. For this reason the refractive increment dn/dc which would be required for calculation of K, or of i7, cannot be assumed to equal the observed change in refractive index of the medium as a whole when polymer is added to it, unless the refractive indexes of the solvent components happen to be the same. The size Vmay, however, be measured in a mixed solvent, since only the dissymmetry ratio is required for this purpose. 

In this study, four Styragel columns were utilized one column had a nominal porosity rating of 10, two colvtmns of 10, and the fourth column of 10 A. The refractometer was maintained at 37°C. A 5 ml syphon was used to monitor a solvent flow rate of 1 ml/min. The instrviment was run at the highest sensitivity setting because the refractive index difference between our solvent and polymer was only moderate and because a number of samples analyzed had a broad molecular weight distribution (MWD). 

Detection is also frequently a key issue in polymer analysis, so much so that a section below is devoted to detectors. Only two detectors, the ultra-violet-visible spectrophotometer (UV-VIS) and the differential refractive index (DRI), are commonly in use as concentration-sensitive detectors in GPC. Many of the common polymer solvents absorb in the UV, so UV detection is the exception rather than the rule. Refractive index detectors have improved markedly in the last decade, but the limit of detection remains a common problem. Also, it is quite common that one component may have a positive RI response, while a second has a zero or negative response. This can be particularly problematic in co-polymer analysis. Although such problems can often be solved by changing or blending solvents, a third detector, the evaporative light-scattering detector, has found some favor. 

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