High molecular weight polystyrene separations

Figure 7.15 shows that the hole volume decreases with smaller additions of mineral oil to the low molecular weight polystyrene to rise again above 6 vol% mineral oil which separates two opposite behaviors in flexural modulus in Figure 7.14. High molecular weight polystyrene does not follow this behavior and the hole volume remains unchanged in the fiill range of studied concentra-...

Field flow techniques have been reviewed in a number of articles [148-150]. Sedimentation field flow fractionation has found use in the separation of PVC [151, 152], polystyrene [151-153], poly(methyl methacrylate) [153, 154], poly (vinyl toluene) [155] and poly(glycidyl methacrylate) latexes [156] to produce particle-size distributions and particle densities. It has also been applied in polymer-aggregation studies [157], pigment [157] quality control and in the separation of silica particles [158] and its performance has been compared with that of ultracentrifugation [159]. Thermal field flow fractionation has been used successfully in the characterisation of ultra-high-molecular-weight polystyrenes [160, 161], poly(methyl methacrylate), polyisoprene, polysulphane, polycarbonate, nitrocellulose, polybutadiene and polyolefins [162]. In the difficult area of water-soluble polymers, poly(ethylene glycol), poly(ethylene oxide), poly(vinyl pyrrolidone) and poly(styrene sulphonate) have been analysed [163, 164]. In addition, compositional separations have been achieved for polystyrene-poly(methyl methacrylate) mixes [165] and comparisons between TFFF and SEC have been made [166]. 

Initially, styrene monomer is completely miscible with the oil prepolymer, but as the styrene polymerizes to high molecular weight polystyrene, the two components phase separate from each other. At this point, it is thought that the oil-rich phase is continuous, and the polystyrene-rich phase discontinuous. If the oil is not prepolymerized, a phase inversion will occur, as in Figures 4-A and 4-B. For extensive prepolymerization, the oil remains continuous, but the domains are smaller. As described below, the morphology of Figure 4-C yielded the highest impact resistance. 

A spectacular phenomenon was described by another of our students, Ed Vanzo. Uniform block polymers of styrene and butadiene, swollen by a small amount of solvent and spread on the surface, form parallel layers of polystyrene and polybutadiene of uniform thickness. For polymers of a sufficiently high molecular weight the separation of the layers becomes comparable to the wavelength of visible light. Since the uniformly distributed layers act as a grating, the intrinsically colorless material acquires brilliant colors due to the interference of light. 

Small particle size resins provide higher resolution, as demonstrated in Fig. 4.41. Low molecular weight polystyrene standards are better separated on a GIOOOHxl column packed with 5 /u,m resin than a GlOOOHg column packed with 10 /Ltm resin when compared in the same analysis time. Therefore, smaller particle size resins generally attain a better required resolution in a shorter time. In this context, SuperH columns are best, and Hhr and Hxl columns are second best. Most analyses have been carried out on these three series of H type columns. However, the performance of columns packed with smaller particle size resins is susceptible to some experimental conditions such as the sample concentration of solution, injection volume, and detector cell volume. They must be kept as low as possible to obtain the maximum resolution. Chain scissions of polymer molecules are also easier to occur in columns packed with smaller particle size resins. The flow rate should be kept low in order to prevent this problem, particularly in the analyses of high molecular weight polymers. 

In summary, silica gel can be an excellent stationary phase for use in exclusion chromatography in the separation of high molecular weight, weakly polar or polarizable polymers. It cannot be used for separating mixtures that require an aqueous mobile phase or operate at a pH outside the range of 4-8. Examples of the type of materials that can be separated by exclusion chromatography using silica gel are the polystyrenes, polynuclear aromatics, polysiloxanes and similar polymeric mixtures that are soluble and stable in solvents such as tetrahydrofuran. 

Gel permeation chromatography of polystyrene separated from blend samples indicates that high-molecular-weight polymer is formed. Polystyrene removed from a 43 wt % polystyrene/HDPE specimen exhibited Mw = 628 K with polydispersity index, PDI = 2.7. The corresponding polystyrene formed outside this specimen exhibited Mw = 48 K with PDI = 3.1. 

Hydrogenated SBCs are often used to modify polyolefins such as polypropylene, polybutylene and polyethylene. One of the unique characteristics of strongly phase-separated block copolymers such as high molecular weight (>50 000) SEBS and SEPS is their response to shear in the melt. These polymers retain their phase-separated structure well above the Tg of the polystyrene because their order-disorder transition temperatures are above processing temperatures. This phase separation strongly inhibits flow in the absence of shear resulting in infinite viscosity at zero shear rates. The application of shear... 

Fig. 3-9. Gel permeation chromatography elution curve.s for anionic polystyrene standards used for calibi ation. The polystyrene standard samples were measured separately use of a mixture of polymers may cause elution volumes of very high molecular weight standards to be erroneously low [I8J.

---