A new approach to network heterogeneity: Polymerization induced phase separation in photo-initiated, free-radical methacrylic systems

Abstract

Non-reactive, thermoplastic prepolymers (poly- methyl, ethyl and butyl methacrylate) were added to a model homopolymer matrix composed of triethylene glycol dimethacrylate (TEGDMA) to form heterogeneous networks via polymerization induced phase separation (PIPS). PIPS creates networks with distinct phase structure that can partially compensate for volumetric shrinkage during polymerization through localized internal volume expansion. This investigation utilizes purely photo-initiated, free-radical systems, broadening the scope of applications for PIPS since these processing conditions have not been studied previously. The introduction of prepolymer into TEGDMA monomer resulted in stable, homogeneous monomer formulations, most of which underwent PIPS upon photo-irradiation, creating heterogeneous networks. During polymerization the presence of prepolymer enhanced autoacceleration, allowing for a more extensive ambient cure of the material. Phase separation, as characterized by dynamic changes in sample turbidity, was monitored simultaneously with monomer conversion and either preceded or was coincident with network gelation. Dynamic mechanical analysis shows a broadening of the tan delta peak and secondary peak formation, characteristic of phase-separated materials, indicating one phase rich in prepolymer and another depleted form upon phase separation. In certain cases, PIPS leads to an enhanced physical reduction of volumetric shrinkage, which is attractive for many applications including dental composite materials.

Nanostructured Multiphase Polymer Networks

“…The refractive index increases with polymerization, so it is expected that at the maximum light transmission reduction, either phases capable of scattering the incident light are being formed or the refractive index mismatch between the phase that polymerizes first and the surrounding monomeric phases is being observed. The latter explains the light transmission recovery observed in some systems after a maximum drop in transmission is observed [67].…”

“…In other words, even if there is thermodynamic instability past gelation, diffusion of macromolecular chains to allow for rearrangement in phase-separated domains is impeded by network formation. This has been demonstrated in a study using IR-coupled rheometry for a methacrylate network to which pre-polymerized methyl methacrylate was added [67]. In that study, the decrease in light transmission through a sample (used as an indication of onset of phase separation, as will be discussed later) took place earlier in conversion than did the gel point (as determined by rheometry).…”

“…In addition, the use of thermo-activated systems in general produces greater internal stresses due to post-polymerization curing contraction effects that can generate poorly controlled micro-/macro-void formation in phase-separated polymers. Alternatively, the use of photopolymerizable systems, either purely free-radical [67,73,74] or free-radical/cationic hybrids [41], has the advantage of room-temperature processing, which limits the temperature rise to the exotherm of the reaction. Moreover, the polymerization can be started on demand after the material is in place.…”

Recent Advances in Photo-Induced Free-Radical Polymerization

“…7 Polymerization induced phase separation can also be realized for systems monomer/dissolved polymer. 8,9 In this way photopolymerization kinetics may provide a convenient approach to control heterogeneity. The polymerization of such systems shows a multistage kinetics, which results from the appearance of two compositionally different phases, polymerizing more or less independently.…”

Microstructural Features of Current Resin Composite Materials

“…PIPS leads to the formation of a multi-phase polymer structure with heterogeneous domains distributed co-continuously or in droplet-matrix fashion [57,58]. By tailoring the monomer systems (or additives) it is possible to combine desirable properties of different networks, as well as create opportunities for shrinkage and stress relaxation at interfacial regions between the domains [57,58]. For example, the use of liquid crystals has been recently proposed as one way to effectively drive phase separation in dental materials [59,60].…”