THE EFFECT OF NANO- AND MICROPARTICLES ON PHASE SEPARATION AND THE STRUCTURE OF SEMIDILUTE BIOPOLYMER MIXTURES - Biochemistry and Biotechnology: Research and Development - page 82

1. I NTRODUCTION
The stabilization of emulsions by incorporating colloidal particles is known since one
century with the pioneer works of Ramsden[1] and Pickering[2]. Stabilization and phase
inversion in different oil-water emulsions by nano-silica particles has been recently
intensively studied the research group of Binks[3-6]. Several mechanisms for stabilization of
water-oil and oil-water emulsions by particles have been suggested, including steric (bilayer)
stabilization of fully covered droplets[3], stabilization by particle monolayer's which can
bridge droplets[4,7,8], effects of surface rheological properties[9,10] and even flocculation in
the bulk[5].Vermant and co-workers[11], and Thareja and Velankar[12] has shown that
particles at interfaces can be used to influence the structure and properties of polymer
mixtures.The effects are similar to those in particle-stabilized water-oil emulsions. The size
and shape of the dispersed phase of polymer blends depends on several processing parameters
including rheology, interfacial properties, and the composition of the blend. This invariant
morphology is due to a rapid establishment of equilibrium between drop breakup and
coalescence. Recently, the effect of low-volume fractions of nanoparticles on the
morphological processes of immiscible polymer blends was studied by Vermant and co-
workers[13] and suppress coalescence for blends of poly-isobutylene and
polydimethylsiloxane by silica particles was shown. More theoretically, Nesterov and
Lipatov[14,15] studied the influence of fumed silica particles on the phase diagram behavior
of polymer blends with lower critical solution temperature. They came to the conclusion that
the total free energy of a blend system should also include the interaction parameters between
the polymers and the inorganic filler surface. In other words, addition of a filler S to A-B
blend stabilizes the morphology. Actually, the solid particles act as a compatibilizer by
adsorbing A and/or B polymers on their surface. To play this role the inorganic phase should
have the largest possible surface area and should be able to disperse very well in the two
phases.
The reverse situation i.e. possibility of the phase separation in polymer mixtures induced
by nano or micro particles is less clear because the experimental observations in this field are
absent. This conclusion also concern multicomponent biopolymer systems. From a
technological point of view, especially important are biopolymer systems which undergo
liquid-liquid phase separation in a wide concentration range, starting from low
concentrations[16,17]. Although the majority of biopolymer mixtures show phase separation,
in most cases the phase separation takes places at critical total concentrations, which are
much higher (7-12 wt%) compared with those of synthetic polymers (less than 1-2 wt%)[15].
In our resent paper[18] we described approach for inducing demixing of semidilute
biopolymer mixtures by physical interactions of the constituents. The addition of sulfate
polysaccharide to the semidilute protein-acid polysaccharide systems (dextran sulfate sodium
salt/DSS/or carrageenans), even in trace concentrations (10 -3 wt %), lead to segregative
liquid-liquid phase separation of the later , and a substantial increase in storage and loss
moduli of the system.
The phase separation observed is the result of formation at pH 7.0 (i.e. far away from the
iep of the caseins /4.4-4.6/) of DSS/SC water soluble charged associates (1:10 mol/mol),
having R H =0.26 um and electrostatic nature. The minimal compatibility of SC and SA was
observed at the DSS/SC weight ratio of 0.14, which corresponds to an equality of the cationic