Hollow latex particles

There are various morphologies of latex particles available these include core-shell and other complex morphologies within the latex particles, and also hollow latex particles. The traditional route to hollow latex particles is the production of core-shell latexes whore the inno core of the latex particles can be removed in a post-polymerization process [8]. These hoUow latex particles have a varied of uses in surface coatings, controlled release tedmologies and as opacifiers. Recently a new approach to the production of hollow latex particles has been developed [9]. In tiiis approach a surfactant structure is stabilized by the polymerization of a vinyl monomer via free radicals in the walls of the vesicles. A vesicle, which is usually a meta-stable structure is converted into a hollow latex particle . This process, while not strictly an emulsion polymerization, has been optimized by use of emulsion polymerization procedures [10]. In the future it is possible that many other unique surfactant structures may be maintained by the in situ introduction of polymer. 

The formation of nanocapsules was achieved by a variety of approaches. One of the earliest processes for making hollow latex particles was developed in the research laboratories of the Rohm and Haas Company [51-54]. Their concept involved making a structured particle with a carboxylated core polymer and one or more outer shells. Ionization of the carboxylated core with base under the appropriate temperature conditions expands the core by osmotic swelhng to produce hollow particles with water and polyelectrolyte in the interior. In addition to this approach, a number of alternative processes have also been patented that are complex in terms of process stages and chemistry [55-57]. 

Beach, E., Keefe, M., Heeschen, W, and Rothe, D. (2005). Cross-sectional analysis of hollow latex particles by focused ion beam-scanning electron microscopy. Polymer 46(25), 11195-11197. 

Me Donald CJ, Devon MJ. Hollow latex particles synthesis and applications. Adv Colloid Interface Sci 2002 99 181-213. 

Figure 6.11. FIB-SEM images of cross-sections of latex particles, 500nm in diameter. (A) Solid core with shell latex particles. (B) Hollow latex particles with a small central void. (C) Set of particles with large central voids and thin shell material. Scanning electron microscopy images at 5kV time to cut through particles, 5-15 s. (From Beach et al. [240] reproduced with permission.)...

The foregoing examples show that hollow polymer capsules with varying composition and sizes of ca. 2-20 micrometers can be produced, either by templating charged (latex particles and biocrystals) or uncharged (organic microcrystals), and that different core removal procedures can be employed. Nanometer-size polymer capsules have also been produced by employing smaller particle templates [107]. 

O. Velev, K. Furusawa, and K. Nagayama Assembly of Latex Particles by Using Emulsion Droplets as Templates. . Micro structured Hollow Spheres. Langmuir 12, 2374 (1996). 

Velev OD, Eumsawa K, Nagayama K. Assembly of latex particles by using emulsion droplets as templates. 1. Microstractured hollow spheres. Langmuir 1996 12 2374-2384. 

Following a related approach, Castelvetro et al. reported the formation and properties of hybrid latex films resulting from the coalescence of low 7 poly(BA-co-MMA-co-MPTMS) terpolymer latex particles coated by a silica shell [78], The latex was synthesized at neutral pH by semi-continuous emulsion polymerization under starved-feed conditions in order to protect the MPTMS monomer from premature hydrolysis and condensation reactions. A substantial amount of free silanols were therefore available for further reaction with the silica precursor. In order to avoid the formation of a densely crosslinked silica network around the latex core, which may significantly alter film formation, the pH was kept at around 2 (at this pH, hydrolysis is promoted and condensation is significantly retarded). TEM and AFM studies of the nanocomposite film indicated that the silica shell formed a continuous percolating network throughout the polymer matrix. A porous film of interconnected hollow silica spheres was next elaborated by thermo-oxidative decomposition of the organic phase. 

Fig. 28 TEM images of (a) the polystyrene latex spheres used as the core and (b) the same particles coated with a thin 25 nm titania shell, (c) SEM image of hollow titania spheres obtained by calcination of the polystyrene/Ti02 core-shell particles at 600°C under ain (d) Eidaiged TEM view of some hollow titania particles showing the small anatase crystallites that compose the sheU. Reproduced from [231] with permission of the American Chemical Society...

Organic spheres are predominantly polymeric, consisting of synthetic or natural polymers. The field of polymeric nano- and microparticles is vast, comprising, for instance, latex particles for coatings, hollow particles for syntactic foams, and microcapsules for foaming and additive release. In addition, there are core-shell microbeads and coated polymeric particles, where the particles can exhibit multiple functionalities, thanks to the individual features of their different layers 1]. As fillers in thermosets and thermoplastics, hollow microspheres and expandable microcapsules are among the most frequently used in commercial applications. 

Polymers for Advanced Technologies 8, No.ll, Nov.1997, p.627-30 SYNTHESIS AND CHARACTERISATION OF HOLLOW POLYMER LATEX PARTICLES... 

A series of multi-hollow structure core-shell latex particles in the presence and absence of acrylic acid was synthesised by seeded emulsion copolymerisation, then... 

Multilayers may also be used for their permeation properties. Accordingly, membranes covered by multilayers have been employed for gas permeability measurements and for pervaporation studies [88,181,185,333-335]. These measurements showed, for example, 02/H20,C02/02, or toluene/ heptane selectivity. The permeation properties of polyelectrolyte multilayers are also important when they are used for the encapsulation of enzymes [210] or living cells [336,337]. The deposition of polyelectrolyte multilayers or of hybrid polyelectrolyte/inorganic multilayers on latex particles, and the subsequent dissolution or calcination of the latex beads leads to the fabrication of hollow spheres [94-96,338-340]. Potential applications of such hollow spheres are numerous, for example, for the controlled release and targeting of drugs. 

Fig. 6.6 SBS-latex particles dried at moderate air inlet temperatures, such as 120°C. The particles are hollow and show indentations due to collapse of the skin.

Figure 6 shows that the soft, powdery coagulum comprises 10-50ym diameter particles, many of which appear to be hollow. All particles comprise aggregates of the ca. 0.05ym-size original latex... 

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