Amphiphilic block copolymers as stabilizers in emulsion polymerization: Effects of molecular weight dispersity and evidence of self-folding behavior

Emulsion polymerizations, used to produce many commodity materials, require stabilizing agents to prevent phase separation. Incorporation of these stabilizers in the final polymer may have negative effects on product properties, so the design of new stabilizers is being actively pursued. Amphiphilic diblock copolymers are a promising type of emulsion polymerization stabilizer and are the focus of this work (Fig. 1). First, the tolerance of an amphiphilic diblock copolymer stabilizer’s performance to high molecular weight dispersity and homopolymer impurity has been investigated. Polystyrene-b-poly(acrylic acid) block copolymers were studied due to their previously demonstrated efficacy as stabilizers in emulsion polymerization, and their similarity to commercially important polystyrene-r-poly(acrylic acid) stabilizers. Neither greater molecular weight dispersity nor homopolymer impurity was found to negatively impact the stabilization performance of these block copolymers, suggesting that the economically unfavorable conditions required to achieve low molecular weight dispersity and homopolymer impurity may be avoided. We then examined novel polystyrene-b-[polystyrene-r-poly(acrylic acid)] block-random copolymers which were shown to stabilize emulsion polymerizations with up to 50 weight percent solids content, exceeding what was possible using the polystyrene-b-poly(acrylic acid) block copolymers. Of even greater significance and scientific value is that the block-random copolymers were also observed to have unusual solution behavior, self-folding rather than self-assembling, to give single chain nanoparticles. Emulsion polymerizations stabilized by these block-random copolymers had a total particle surface area which was directly proportional to the stabilizer concentration and was unaffected by polymerization kinetics. A novel “seeded-coagulative” emulsion polymerization mechanism has been proposed to explain these results, which were unexplainable by any known emulsion polymerization mechanism. Please click Additional Files below to see the full abstrac

Crosslinking chemistry for high-performance polymer networks

A new thermally reactive monomer has been designed and synthesized that brings novel crosslinking chemistry to high-performance polymers. This monomer (XTA) is a derivative of terephthalic acid and was based on the thermal chemistry of benzocyclobutene. Various model compounds have been synthesized to investigate substituent effects on benzocyclobutene reactivity. Irreversible reaction exotherms around 350[deg]C were observed in these model compounds using differential scanning calorimetry. Based on these studies, polyaramid and poly(aryl ether ketone) XTA copolymers were synthesized. The formation of an insoluble network resulted after heat treatment of these polymers.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/31246/1/0000152.pd

Design, synthesis, and characterization of reactive aromatic polymers.

The research presented in this manuscript is aimed towards the design and synthesis of reactive aromatic precursor polymers. These precursor polymers have been designed to contain latently reactive functional groups that can be activated for ladder and network formation to lock in the desired physical form. Ladder polymers have been based on a polyaminoketone (B) backbone that can be formed through thermal ring closure of a polyarylketone precursor polymer (A). Thermosetting polymers were based on extended-chain and engineered thermoplastic polymers. Specifically, aramid and arylate materials have been synthesized from a crosslinkable monomer XTA, which can be thermally activated for network formation. These novel synthetic methods were designed to address the insolubility and intractability that is common to ladder and thermosetting materials. The scope of a new polymerization reaction for polyarylketones was investigated and the information gained from this study was used in the design and synthesis of polyaminoketone ladder materials. This new polymerization reaction was based on palladium catalyzed cross-coupling of aromatic diacid chlorides and bis(trimethylstannane) monomers. Several high molecular weight, soluble polyketones having good thermal stability were synthesized using this polymerization reaction. It has been found that aromatic tin monomers substituted with electron withdrawing groups have low molecular weights where electron releasing groups have increased molecular weights. Results from a model compound study into the feasibility of the synthesis of polyaminoketone ladder polymers using this new polymerization reaction indicates that a new design may be necessary. A model compound study on derivatives of the new crosslinkable monomer, XTA was performed to determine the operating window for polymerization and processing of polymeric materials containing this structural unit. Excellent stability towards strong protonic acids was observed when benzocyclobutene small molecule derivatives were substituted with electron withdrawing groups. This is in contrast to the case of the parent hydrocarbon and derivatives substituted with electron donating groups and is consistent with acid degradation resulting from electrophilic aromatic substitution chemistry. Exothermic reaction maxima are typically around 350\sp\circC. ftn* Please refer to dissertation for diagrams.Ph.D.ChemistryUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/103929/1/9423173.pdfDescription of 9423173.pdf : Restricted to UM users only

Amphiphilic Block Copolymers as Stabilizers in Emulsion Polymerization: Effects of the Stabilizing Block Molecular Weight Dispersity on Stabilization Performance

Molecular weight dispersity is not typically studied as a design parameter of block copolymer stabilizers but is often assumed to impact stabilization performance; low molecular weight dispersity is generally assumed to be associated with best performance. This is the first quantitative investigation of the effects of block copolymer molecular weight dispersity with regards to stabilization performance in an emulsion polymerization. Poly­(styrene)-<i>b</i>-poly­(acrylic acid) block copolymers were synthesized by nitroxide-mediated radical polymerization and employed as stabilizers in the emulsion polymerization of styrene. The effect of the stabilizing poly­(acrylic acid) block molecular weight dispersity on stabilization behavior was studied, independent of molecular weight and composition. Block copolymer stabilizers were evaluated in terms of critical aggregation concentration, dispersed phase particle size, distribution, and zeta potential. The molecular weight dispersity of the stabilizing block affected the aggregation number and final number of particles but displayed no negative effects on stability or size distribution

Amphiphilic Block Copolymers as Stabilizers in Emulsion Polymerization: Effects of the Anchoring Block Molecular Weight Dispersity on Stabilization Performance

Poly­(sodium acrylate)-<i>b</i>-polystyrene block copolymers were employed as stabilizers in the emulsion polymerization of styrene. Previous work by our group has shown that the molecular weight dispersity of the stabilizing block is an important design parameter of block copolymer stabilizers; herein, the molecular weight dispersity of the anchoring polystyrene block, <i>Đ</i>_PS, was investigated. Stabilization performance was evaluated by the critical aggregation concentration, aggregation number, and surface activity of the block copolymers and the size, distribution, and zeta potential of the polystyrene latex particles. It was observed that <i>Đ</i>_PS had a strong effect on aggregation number, which led to a change in the number of latex particles in the seeded emulsion polymerization of styrene. Surface activity decreased with increasing <i>Đ</i>_PS due to a greater diversity of copolymer compositions, supporting the idea that copolymers of different composition play different roles in the stabilization of an emulsion. The performance of block copolymer stabilizers, evaluated by the stability and size distribution of latex particles, was indistinguishable over the range of <i>Đ</i>_PS studied; narrow stabilizer molecular weight distributions were not necessary for satisfactory performance