ABSTRACT
Polymer-mediated interactions among nanoparticles play a key role in many biological and technological processes such as red blood cell aggregation [7], protein crystallization [61], self-healing of the polymer composites [44], and filler reinforcement of rubbers used in tire technology [21]. The present work reviews our recent efforts to theoretically model the polymer-mediated interactions in different settings encountered in practical systems. By developing and making use of the novel potential theory [18], we investigate several important cases of these interactions acting between
nanocolloids in the diverse model filled polymer systems. As a main result of the reported work, we prove that the type of the interactions between the polymers and colloid surfaces is a decisive factor that determines the main features of selected static and dynamical properties of polymer nanocomposites. 10.1 IntroductionFiller particles immersed in the polymer system change the density structure of this system, thus inducing effective interactions between the immersed fillers [4, 5, 25]. These polymer-mediated (PMF) interactions between nanoscopic particles are in the core of many biologically and technologically relevant phenomena such as red blood cell adhesion [7], DNA-mediated depletion interactions [65], size-exclusion polymer chromatography [55], and filler reinforcement of rubber [21]. Despite more than 50-year history of thorough investigations of PMF, the role of the intra-and inter-polymer interactions in the formation of the depletion/enhancement polymer layers near colloidal particles and the influence of the properties of these layers on PMF is understood rather poorly. In the first place, this lack of understanding is caused by the significant technical difficulties in the associated theoretical development. Those difficulties become especially pronounced in the case of small colloid radius R-to-polymer gyration radius RG ratios q-1 R/RG, generally referred to as the nanoparticle or protein limit [48]. This limit generally describes the case encountered in the majority of practically important situations where the colloidal particles can easily penetrate polymer coils and simultaneously interact with many polymers. From the theoretical standpoint, in the described protein limit the polymers cannot be modeled as individual soft particles interacting with colloids, which brings essential mathematical complications to the theoretical description of PMF. The origin of these complications stems from the many-body nature of the interactions of small “protein” colloids with polymer monomers inside polymer coils, which affect the local polymer density correlations inside these coils. The magnitude of the above polymer density correlations, in turn, is known [24, 26] to significantly depend on the excluded volume interactions. These excluded volume interactions affected, in particular, by the solvent screening
[24] can therefore have the decisive influence on the magnitude and range of the polymer-mediated forces acting between nanocolloids. In the first part of this chapter we review our recent work that provided quantitative understanding of PMF by developing the exact analytical approach based on the Edwards self-consistent mean-field theory (SCMFT). The developed approach, termed “potential theory” [18] is capable of describing the polymer-mediated interactions in a variety of practically important systems ranging from semi-dilute polymer solution to dense rubbers. In addition, this method can be readily applied to different types of the interactions between the polymers and filler particles, as proved by the good agreement of our theoretical results with experimental observations in a variety of the experimental settings. The aim of the second part of the present work is to investigate the effects of the presence of adsorbing and non-adsorbing polymers on the kinetic stability and the processes of the coagulation-fragmentation of colloids in polymer systems. These effects are known [51] to originate from the effective long-range interactions between colloids mediated by the polymers added to a colloidal system, investigated in the first part of the review. Since the strength and even the sign of these interactions depend on the affinity of the colloid surfaces for the polymers, they can cause different kinetic behaviors of the colloids immersed in the polymer system. In this chapter, we investigate how the presence of free polymers affects the kinetic stability of the polymer sterically stabilized colloids interacting through the van der Waals forces. Free non-adsorbing polymers are known [20] to play in favor of increasing the coagulation rate, while the irreversibly adsorbed polymers are proven to be one of the most effective stabilizers against the coagulation. The rate of colloid coagulation is therefore determined by the counterplay of the above competitive effects that both originate from the polymer-colloid adsorption and entropic interactions.
