Tailored Silica Polymer Composites and ABA Type Copolymers: Polymerization Kinetics, Structural Design, and Mechanical Properties

Abstract

In this work, kinetic and mechanistic aspects of the radical polymerization from the surface of fumed silica particles were examined. The surface-initiated polymerization technique was complemented by reversible addition fragmentation chain transfer (RAFT) polymerization in order to synthesize silica polymer composites and ABA type copolymers containing hydrogen bonds for mechanical analysis. An approach for the determination of propagation rate coefficients,kp, of surface-initiated radical polymerizations is presented. The main feature of this approach is the application of a silica-immobilized photoinitiator in pulsed-laser polymerization size-exclusion chromatography (PLP SEC). The determined values for styrene and n-butyl acrylate (nBA) are noticeably higher compared with the International Union of Pure and Applied Chemistry (IUPAC) benchmark data for polymerizations in solution. The surface-initiated radical polymerization of styrene triggered by thermal decomposition of the silica-anchored azo initiator was examined for both the conventional and the RAFT approach. The conventional radical surface-initiated polymerization of styrene yielded grafted polystyrene whose molar masses were independent of total monomer conversion. The loading of grafted polymer on the silica surface increased steadily during the course of polymerization and reached a maximum value of 48 %. This specific sample of maximum polymer loading further exhibited several eye-catching sites in scanning electron microscopy (SEM), where the anchored polystyrene seemed to be visible as filaments of 1 ìm length at the site of cracks in the surface. The addition of RAFT agent to the interstitial solution induces living behavior for both the initiator-derived chains on the surface and the RAFT-derived chains in solution. As a result, the molar masses increased linearly towards higher conversion and polymers with narrow molar-mass distributions (MMDs) were obtained. The MMDs of the anchored polystyrene were slightly broader than the MMDs of the free polystyrene and additionally displayed a high-molar-mass shoulder. This shoulder was identified in PREDICI® (Polyreaction Distributions by Countable System Integration) simulations, which further indicate a reduced addition rate coefficient of the main equilibrium between free and grafted species. The surface-initiated RAFT polymerization was used to synthesize silica-filled styrene nBA copolymers in a one-step procedure. The resulting composites were tailored with respect to the molar mass of the copolymer, its monomeric composition, the silica content and the loading of anchored copolymer on the corresponding silica particles. The latter was achieved by the addition of pure, initiator-modified and RAFT agent-modified silica particles, respectively, to the polymerization system. This approach allowed for tuning the amount of surface-grafted copolymer on the silica surface that was formed in situ during polymerization. Tensile testing was used to evaluate the mechanical properties of the composite materials. The silica content and the surface modification of the silica particles display crucial composite properties that largely affected the tensile performance. Copolymers, in which hydrogen bonds were introduced in a controlled manner, were synthesized to systematically study a recently detected secondary relaxation mode at temperatures below the glass transition. This was achieved by RAFT polymerization of the two monomers tert-butyl acrylate (tBA) and acrylic acid (AA) to yield ABA type copolymers, in which the inner B-block consists of pure poly(tBA). The two outer A-blocks, on the other hand, contain a mixture of tBA and AA and can thus form hydrogen bonds. Dynamic mechanical analysis (DMA) revealed the occurrence of this secondary relaxation mode termed chemical confinement (cc). The glass transition temperature was thoroughly examined as well as a high-temperature relaxation that is absent in pure poly(tBA). This relaxation at high temperatures was further accompanied by a distinctive flow behavior of the copolymers in the rubbery region. The impact of the AA content and the AA location in the polymer chain on the tensile properties was probed for copolymers of methyl acrylate (MA) and AA. The AA content largely affected the tensile properties of the copolymers. The insertion of AA into the outer chain parts of poly(MA), which resembles the ABA type arrangement of the tBA AA copolymers, yielded a copolymer with enhanced tensile modulus and tensile strength compared with random MA AA copolymers.