Small-angle neutron scattering determination of nonrandomness in cross-linked polystyrene at midrange conversions

Abstract

Polystyrene was photopolymerized by bulk, free-radical methods in a three-step synthesis to yield a sample containing a portion of deuterated chains at a specific conversion of interest. The initial portion of protonated polystyrene, fraction I, was cross-linked with 1 mol % divinylbenzene and polymerized up to conversions of 40-50%. The unreacted monomer was replaced with deuterated styrene monomer, and fraction II was polymerized for an additional 4-20% conversion. Fraction III was again protonated, which brought the total conversion to 100%. Fractions II and III were either linear or cross-linked. All of the compositions with fraction II greater than 4 wt % were found to have unusually high molecular weights and radii of gyration when studied by small-angle neutron scattering (SANS). SANS gave weight-average molecular weights between 110000 and 760000 for fraction II, while GPC indicated molecular weights from 110000 to 240000, yielding an apparent enhancement of SANS molecular weight of up to 5 times the GPC value. The enhancement was thought to be caused by the presence of "excluded volume" arising from previously inhomogeneously polymerized cross-linked fraction I and leads to a model suggesting a nonrandom distribution of labeled polymer, which results in several chains scattering like one large chain.

Insights into Free Volume Variations across Ion-Exchange Membranes upon Mixed Solvents Uptake by Small and Ultrasmall Angle Neutron Scattering

“…The microstructure of ion-exchange polymers is somewhat similar to that of polymer gels and is usually regarded as a polyelectrolyte gel able to swell in three dimensions upon solvent absorption . However, unlike uniform polymer gel network, styrene-divinylbenzene copolymers exhibit highly heterogeneous microstructures due to the higher reaction rate of divinylbenzene, used as a cross-linking agent, compared to the styrene monomer polymerization rate. − Therefore, starting from the initiator molecules, the microstructure exhibits a higher degree of cross-linking at the beginning of the polymerization reaction and a lower one toward the end of the reaction . The resulting structure can be represented by a styrene-divinylbenzene copolymer network containing dense zones with a high cross-linking degree and less cross-linked flexible polystyrene chains further away from the polymerization initiation point (Figure a).…”

Small-angle neutron scattering from amorphous polymers

“…Bates, Berney, Cohen & Wignall (1983) speculated on the possibility of non-ideal mixing of deuterated and protonated poly(1,4-butadiene) based on an anomalously high SANS-determined molecular weight for the polybutadiene blocks of contrast-matched polystyrenepolybutadiene diblock copolymers. Weissman & Sperling (1985) have also obtained a considerably higher molecular weight for deuterated polystyrene polymerized in a protonated interpenetrating polystyrene network using SANS than they determined by conventional analytical methods. In spite of the interpretation by the authors, isotope effects are almost certainly responsible for this behavior; the combination of high concentration labeling and a high-(essentially infinite) molecular-weight network makes these effects unavoidable.…”

Applications and limitations of deuterium labeling methods to neutron scattering studies of polymers

“…Another investigation in which the isotope effect seems to have played a significant role is in the SANS evaluation of interpenetrating polymer networks (lPN's) (80,81). Here the polymerization of deuterated styrene in a protonated polystyrene network was observed to result in a SANS-determined molecular weight in excess of that measured by chromatography.…”

“…Another investigation in which the isotope effect seems to have played a significant role is in the SANS evaluation of interpenetrating polymer networks (lPN's) (80,81). Here the polymerization of deuterated It is our strong conviction that this phenomenon can be of great utility in investigating the thermodynamics and dynamics of polymer-polymer mixing.…”