Cryogenic scanning electron microscopy (cryo-SEM) is a powerful technique of visualizing the state of microstructure, or nanostructure, of colloidal polymer suspensions, or dispersions, after they have been immobilized by fast-freezing and fractured for imaging. The fracture is done at −196 °C, the normal boiling point of liquid nitrogen, which is far below the glass transition temperature of both bulk and fully coalesced particles of the polymers examined. The cryo-SEM images show a range of responses of particles to the fracture that propagated past them through ice:  cleanly sliced through, i.e., broken, partially drawn and unbroken, drawn to breakage, and neither drawn nor broken. The drawn particles often display curious features called “pullouts” that have been generated by plastic deformation of parts of the particles that remain in a fracture surface, or sometimes both surfaces near the tip of a fracture that halted. The main morphologies of pullouts are mushroom shaped, spool shaped, and awl shaped. Pullouts form when the molecular weight (M w) of the polymer exceeds twice the entanglement molecular weight (M e); they do not form when cross-linking density exceeds a certain level. We hypothesize that nonbulk molecular organization of submicron colloidal particles, perhaps most disturbed in a surface zone, promotes the formation of pullouts. Pullouts have been seen in particles from 500 nm in diameter, the largest examined, down to 30 nm, the smallest. At least when M w is greater than twice M e, T g does not affect the formation of pullouts. The pullout features indicate that the yield behavior, and thus the mechanical properties, of submicron particles synthesized by emulsion polymerization differs from that of polymer crazing in bulk and in thin films; the molecular-level explanation stands as a challenge to macromolecular science.

Despite its industrial importance, the subject of freeze-thaw (F/T) stability of latex coatings has not been studied extensively. There is also a lack of fundamental understanding about the process and the mechanisms through which a coating becomes destabilized. High pressure (2100 bar) freezing fixes the state of water-suspended particles of polymer binder and inorganic pigments without the growth of ice crystals during freezing that produce artifacts in direct imaging scanning electron microscopy (SEM) of fracture surfaces of frozen coatings. We show that by incorporating copolymerizable functional monomers, it is possible to achieve F/T stability in polymer latexes and in low-VOC paints, as judged by the microstructures revealed by the cryogenic SEM technique. Particle coalescence as well as pigment segregation in F/T unstable systems are visualized. In order to achieve F/T stability in paints, latex particles must not flocculate and should provide protection to inorganic pigment and extender particles. Because of the unique capabilities of the cryogenic SEM, we are able to separate the effects of freezing and thawing, and study the influence of the rate of freezing and thawing on F/T stability. Destabilization can be caused by either freezing or thawing. A slow freezing process is more detrimental to F/T stability than a fast freezing process; the latter actually preserves suspension stability during freezing.

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