Molecular weight distribution of polystyrene by sedimentation velocity analysis

Abstract

The problem of how the distribution of molecular weight affects the properties of a polymer has led to the need for a rapid and accurate method for determining molecular weight distributions. Most of the commonly employed methods for determining how the mass is distributed over different molecular weights are either very time-consuming or give inferior results. Recent a d v a n c e~l -~ in the analysis of the progressive boundary spreading in sedimentation velocity experiments indicate that the velocity ultracentrifuge can be used to obtain such rapid and accurate determinations of molecular weight distribution. The present work will report on the use of such sedimentation velocity experiments to determine the molecular weight distribution of polystyrene. EXPERIMENTAL MaterialsPolystyrene samples of narrow molecular weight distribution were prepared by anionic polymerization of styrene as described by the method of S~w a r c .~ A styrene solution in tetrahydrofuran was added slowly to a sodium naphthalene complex in tetrahydrofuran under a nitrogen atmosphere at OOC. The molecular weight of the polymer was established by the ratio of monomer to complex. The polymer formed was precipitated from solution in methanol and dried in a vacuum oven.Polystyrene A having a broad distribution was commercial material produced by thermal polymerization over a varying temperature range. Polystyrene B having a somewhat narrower distribution was produced by thermal polymerization under isothermal conditions. Methods Sedimentation Velocity. A Spinco Model E ultracentrifuge with thePhilpot-Svensson optical system utilizing a phase plate was used for sedimentation velocity experiments. Runs were made a t 35OC. in a 0 solvent, cyclohexane, a t polymer concentrations between 0.1 and 0.75 g./100 ml. All runs were made a t a speed of 59,780 r.p.m. A cell having a double-341

Dynamic measurements on polymer solutions and melts

“…Investigated as melts were a high density polyethylene, ttDPE-NMWD, a polydimethylsiloxane (Silopren RS supplied by Bayer) and a polystyrene S 111. This last material has a very narrow molecular weight distribution, and was kindly supplied by McCormick (2). Table 2 gives a list of the number average (Mn) and weight average (Mw) molecular weights of the polymers used (3).…”

Mittelwerte des Molekulargewichtes und anderer Eigenschaften

“…Kendall (1940) angegebenen empirischen, statistischen N~iherungsformel das Gewichtsmittel gw des Sedimentationskoeffizienten berechnen (Jullander, 1945a): 3g5o~ -gmax (129) gw--2--g5o70 ist dabei der Sedimentationskoeffizient ftir die Stelle der Kurve, die der Fl~ichenhatbierenden senkrecht zur Abszisse entspricht. Bei symmetrischen Gradientenkurven wird gmax = gSO~ und folglich gw = gm~x-Die Massenverteilung der Sedimentationskoeffizienten liiBt sich durch Sedimentationsgeschwindigkeitsmessungen im Theta-Zustand (Cantow, 1959;McCormick, 1959;Baldwin, 1959;Billick, 1962aBillick, , 1962bMcIntyre etal., 1962;Wales u. Rehfeld, 1962;Hommaet al, 1963) …”

“…Die Dehnung h~tngt von einem Mittel zwischen Zahlen-und Gewichtsmittel ab (McCormick et al, 1959), w~ihrend der Modul unabh~ingig vom Molekulargewicht oder der Molekulargewichtsverteilung ist.…”

Procedures for homogeneous anionic polymerization

“…Such distributions have not been achieved, or even approached, until quite recently with monomers such as s tyrene [4][5][6][7][8][9][10][11][12][13][14][15], O'-methylstyrene [16,17], isoprene [18][19][20][21], and butadiene [21 -24]. This was due principally to various factors which combined to veil the true possibilities of these systems.…”

“…Addition of an electron transfer initiator to an ether solution of monomer results in the formation of an ion-radical [1,2] which eventually undergoes a combination reaction with the resultant generation of a polymeric species possessing a carbanion at each end. In preparing the difunctional species, the accidental deactivation of a fraction of the carbanions will result in a bimodal molecular weight distribution [5,7,16], i.e., the material will possess a portion that will have onehalf the molecular weight of the remaining polymer.…”