Solvent rotation polymer concentration

The deposition of thin solid polymer films by the SCO process is illustrated in Figure 1. A fixed volume of a viscous solution of the polymer to be deposited is placed on a flat substrate, such as a silicon wafer, and then rapidly rotated. Depending on the solvent volatility from the solution and such parameters as initial volume of solution, initial concentration of solids, the substrate temperature relative to the boiling point of the solvent, rotational speed, and spinning duration, a wet or dry polymer film results. 

An interface between gel permeation chromatography (GPC) and Fourier transform infrared (FTIR) spectrometry has been developed. With this system it is possible to collect solvent free polymer deposition and to measure their infrared spectra as a function of molecular weight. The mobile phase from the GPC effluent is converted into an aerosol and removed using a pneumatic nozzle. The sample is collected on a Ge disc that rotates below the nozzle. After the sample is collected, the disc is transferred to an FTIR spectrometer where the infrared spectrum of the sample is collected. Normal GPC sample concentrations (0.1-0.25 wtJvol%) give sufficient sample for useable FTIR signals. All normal GPC solvents can be effectively removed, and the interface works with both low temperature and high temperature GPC applications. 

Proteins, on the other hand, are characterized by one or two well-defined relaxation times, which are of the magnitude to be expected if the protein molecule rotates as a fairly compact and rigid unit in the electric field. If the protein molecule is denatured and caused to unfold in a solvent such as concentrated aqueous urea, it is possible that it would behave more like the synthetic polymers in an electric field but such studies stiU remain to be carried out. 

The latest advance in the design of this system is to replace the rotating cylinder with a thin stationary wire, shown in Fig. 4.2(a). The polymer solution is deposited on the wire via a reciprocating applicator. In this way, the polymer solution is only presented to the electrode immediately prior to spinning, reducing the impact of solvent evaporation, which can lead to increases in polymer concentration. 

Yoshizaki, et a/. (46) (note also Ref. (47)) measured solvent rotational relaxation times in the presence of very-low-molecular-weight polymer for polyisobutylene benzene and poly(dimethylsiloxane) bromocyclohexane. Both polymers were studied as the dimer an 8.26 kDa PDMS oligomer was also examined. At 500 g/1, the polyisobutylene had no effect on t similar concentrations of PDMS reduced tr of the bromocyclohexane, though by less than 20%. The dimer and c. 75-mer PDMS had the same effect on tr to within experimental error, showing that polymer molecular weight and chain end effects each have minimal effects on the reduction in tr, at least for short polymers. 

Early work on NMR of polymers in dilute solution was reviewed by Heatley(29). It was already clear in that early review that relaxation times of dilute polymers were independent of polymer molecular weight, at least for molecular weights above a few to ten thousand, and were nearly independent of polymer concentration for concentrations up to 100-150 g/1 or so. A revealing exception to this rule was provided by polymers plausibly expected to rotate as nearly rigid bodies, for which Ti continued to depend on M up to much larger M. From these observations, it was plausibly inferred that local chain motions are primarily responsible for the observed relaxation times. Dependences of Ti on solvent temperature and viscosity were concluded to scale linearly with solvent viscosity, at least in most systems, a matter treated in more detail below. Heatley also considers correlations between Ti and chain structure. 

The addition of a polymeric solute to a small-molecule solvent affects translational diffusion, viscosity, and rotational diffusion of solvent and other small molecules in solution. For polymer concentrations 4> < 0.4, the solvent selfdiffusion coefficient follows D exp(—a). The constant a is linear in the probe s molecular volume but is independent of polymer molecular weight. At larger concentrations (/) > 0.4, the simple-exponential dependence of D on concentration is replaced by a stretched-exponential concentration dependence, D and dD/dc both appearing continuous through the transition. The effects of added polymer on solvent self-diffusion and on the diffusion of small-molecule probes are clearly not the same. 

The basic design of electrostatic spinning and its realization in most research laboratories is very simple (Fig. 15.2). A polymer solution is fed to a nozzle by a defined low pressure. An electrical field is applied between the nozzle and a collector (rotating mandrel or a conveyor belt). The application of the voltage on the nozzle or the carrier is not important initially. The application of positive or negative charges is mandatory. Independent of the polymer, solvent and the concentration of the solution, a voltage of 1 kV per centimeter between the nozzle and the collector of the spun fibers is useful. The nozzle diameter is usually about 100 pm. 

An alternative approach for determining the molecular weight of a polymer in theta solvents includes the determination of the polymer s concentration at the meniscus (c ,) and at the bottom ic, ) (or alternatively at two other positions Xi and X2) in the cell. These two outstanding positions have a distance of x ix ) and Xh(x2), respectively, from the center of rotation. Then, one obtains the weight-average molecular weight of a polydisperse polymer sample via the equation ... 

For a specific polymer, critical concentrations and temperatures depend on the solvent. In Fig. 15.42b the concentration condition has already been illustrated on the basis of solution viscosity. Much work has been reported on PpPTA in sulphuric acid and of PpPBA in dimethylacetamide/lithium chloride. Besides, Boerstoel (1998), Boerstoel et al. (2001) and Northolt et al. (2001) studied liquid crystalline solutions of cellulose in phosphoric acid. In Fig. 16.27 a simple example of the phase behaviour of PpPTA in sulphuric acid (see also Chap. 19) is shown (Dobb, 1985). In this figure it is indicated that a direct transition from mesophase to isotropic liquid may exist. This is not necessarily true, however, as it has been found that in some solutions the nematic mesophase and isotropic phase coexist in equilibrium (Collyer, 1996). Such behaviour was found by Aharoni (1980) for a 50/50 copolymer of //-hexyl and n-propylisocyanate in toluene and shown in Fig. 16.28. Clearing temperatures for PpPTA (Twaron or Kevlar , PIPD (or M5), PABI and cellulose in their respective solvents are illustrated in Fig. 16.29. The rigidity of the polymer chains increases in the order of cellulose, PpPTA, PIPD. The very rigid PIPD has a LC phase already at very low concentrations. Even cellulose, which, in principle, is able to freely rotate around the ether bond, forms a LC phase at relatively low concentrations. 

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