Effect of silica nanoparticles on the morphology of polymer blends

Weizhen Li

Research output: ThesisPhd Thesis 1 (Research TU/e / Graduation TU/e)

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Abstract

Polymeric materials are often a combination of different polymers and plasticizers, stabilizers, and organic/inorganic additives to tailor the properties. The type and fineness of the morphology is the key factor for the ultimate properties of polymer blends. Recently, the use of inorganic nanoparticles, such as carbon black, organoclay, carbon nanotubes, and silica, has come up to control the morphology of polymer blends. The objective of the research described in the thesis is to investigate the effect of the silica nanoparticles on the morphology of polymer blends. Since polymer blends are classified into several categories based on their miscibility, the effect of silica nanoparticles is studied with different blend categories. The first category is called a fully miscible blend, in which the polymers are miscible over a wide range of temperatures and at all compositions due to specific interactions. The second category is called a partially miscible blend, for which miscibility is only observed in a specific temperature and/or concentration window. The third category is called an immiscible blend, in which the polymers are not miscible at any temperature or concentration. Since complete miscibility among polymer pairs is exceptional, this study is focused on partially miscible and immiscible blends. For the category of partially miscible polymer blends, a blend consisting of poly(methyl methacrylate) (PMMA) and poly(styrene-co-acrylonitrile) (SAN) with a lower critical solution temperature (LCST) was used as the model system. The interaction between the surface of the particles and the polymer components was found to be the key factor to control the distribution of the silica nanoparticles, which can either be in one of the polymer phases or at the PMMA/SAN interface after phase separation. Hydrophilic silica nanoparticles preferentially migrate to the PMMA phase due to the strong interaction of the hydroxyl groups on the surface of silica with the carbonyl groups of the PMMA. The migration of the particles leads to a slow down of the coarsening rate and a lower phase separation temperature. Three explanations were considered for this effect: i) local increase of the viscosity because of an increase of the silica concentration; ii) reduction of the interfacial tension; iii) selective adsorption of low molar mass PMMA chains on the surface of the silica nanoparticles, thereby increasing the average molar mass of the bulk, which is consistent with the shift of the phase diagram. This is the most probable explanation for this blend. The hydrophobic silica nanoparticles were localized at the PMMA/SAN interface, which might act as a solid barrier between the polymers which influences the interfacial mobility. For the category of immiscible polymer blends, a blend consisting of PMMA and poly(carbonate) (PC) was used as the model system with two types of silica particles, i.e. hydrophilic and hydrophobic. For the hydrophilic silica, selective distribution of the nanoparticles in the PMMA phase was observed, which was independent of the compounding sequence. The stabilization of the finer morphology can be attributed to the local increase of the viscosity and a concomitant reduction of the mobility of the PMMA phase. For the hydrophobic silica, localization of the nanoparticles at the PC/PMMA interface is the thermodynamically preferred state, but the kinetics of coarsening can be influenced by the compounding sequence. The observed stabilization effect of the hydrophobic silica particles might be related to the presence of an immobilized layer of nanoparticles around the polymer droplets. This mechanism is very efficient to control the morphology. For immiscible polymer blends containing block copolymers, macrophase separation between the homopolymer and di- or triblock copolymers occurs for systems with NAh > NAc (the degree of the polymerization of polymer A in both the homopolymer, NAh, and the copolymer, NAc). The silica nanoparticles show a suppression effect on the extent of macrophase separation between the PMMA homopolymer and poly(styrene)-b-poly(butadiene)-b-poly(methyl methacrylate) (SBM) triblock copolymer, which is related to the strong hydrogen bonding interaction between the hydroxyl groups on the surface of silica nanoparticles with the carbonyl groups of the PMMA. By using different molar mass distributions of the PMMA homopolymer, the suppression effect of nanoparticles can be attributed to selective adsorption of the high molar mass PMMA on the surface of the silica particles, which may force the system into the ‘wet-brush’ regime. For blends of SBM with poly(styrene) (PS), silica nanoparticles with different surface characteristics were used. The location of the silica particles depends on the interaction between the silica surface and the polymer, which can also be influenced by the compounding procedure. Upon adding hydrophilic silica to the PS/SBM blend, the silica nanoparticles are found preferentially in the core (PMMA phase) of the core-shell structures without macrophase separation due to the strong hydrogen bonding interaction between the silica surface and PMMA. On the other hand, hydrophobic silica nanoparticles suppress the extent of macrophase separation between the homopolymer and block copolymer blend based on a selective distribution within the PS phase. The suppression effect on the phase separation and the concomitant kinetics can be controlled by the preparation method, i.e. solvent-, melt processing or in-situ polymerization. The toughness of brittle amorphous, glassy polymers can be improved by the addition of ABA or ABC block copolymers containing one rubbery block and one semi-crystalline block, which form micellar or cylindrical structures within the matrix. Upon cooling, additional internal stresses can build up during fractionated crystallization, i.e. homogeneous and heterogeneous nucleation, which induce pre-cavitation, as shown in a previous study on systems with a cylindrical morphology. After adding the silica nanoparticles, the morphology of the PMMA/poly(methyl methacrylate)-b-poly(butyl acrylate)-b-poly(e-caprolactone) (MBC) blend shows a transition from spherical to spherical/cylindrical structure, which arises from the separation of the triblock copolymer MBC and the diblock copolymer BC, together with the localization of the silica particles in the spherical PCL domains, leading a fractionated crystallization of PCL. In this thesis, it was shown that silica nanoparticles have a significant effect on the morphology of partially miscible, immiscible polymer blends and blends with block copolymers. The distribution of the nanoparticles is governed by the interaction between the polymers and the silica surface and can be at the interface or preferentially in one of the phases. The kinetics of (re)distribution can be influenced by the preparation method, i.e. solution processing, melt compounding and in-situ polymerization in the presence of nanoparticles.
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Department of Chemical Engineering and Chemistry
Supervisors/Advisors
  • Lemstra, Piet, Promotor
  • Goossens, Han, Copromotor
Award date23 Nov 2011
Place of PublicationEindhoven
Publisher
Print ISBNs978-90-386-2889-9
DOIs
Publication statusPublished - 2011

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