Recently, the desire for new polymeric materials with tailorable properties has directed interest toward multi-component syntheses. In polymer science, multi-component chemistry is most commonly used in the field of interpenetrating polymer networks (IPNs). IPNs contain two or more polymer networks that are cured simultaneously using separate mechanisms to physically interlock. In contrast to IPNs, a hybrid polymer network (HPN) contains a monomer with functional groups that are capable of reacting with both networks, resulting in chemical crosslinking. Here, novel HPNs were synthesized using thiol-, epoxide- and acrylate-functionalized starting materials. While past research efforts on such systems have employed a combination of sequential UV and thermally initiated reactions of thiols with (meth)acrylate and epoxide functional groups, we have created hybrids using room temperature reactions and base catalysis with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). Both simultaneous and sequential curing approaches have been employed. Kinetics analysis demonstrates that thiol-acrylate reactions are considerably faster than thiol-epoxy reactions in the presence of DBU. (In the absence of DBU, thiol-epoxy reactions exhibit little to no conversion at room temperature). Three-component system curing conversions were tracked both isothermally and non-isothermally using Fourier Transform Infrared Spectrscopy (FTIR) in order to gain insight on multi-component kinetics. The hybrid system kinetics are easily tailored and controlled through variables such as monomer structure and molecular weight, catalyst loading, monomer composition, addition sequence and reaction conditions.
Thermal and mechanical properties for these three-component hybrid polymers have been investigated using differential scanning calorimetry (DSC), scanning electron microscopy (SEM), dynamical mechanical analysis (DMA) and tensile testing. Linear polymers were produced with difunctional reactants to produce thermoplastic elastomeric materials. Hybrid crosslinked materials were synthesized using a trifunctional thiol with difunctional acrylate and epoxide containing monomers. The materials, dependent on monomer composition, achieved intermediate glass transition temperatures (Tg) when compared to the thiol-acrylate or thiol-epoxy two-component controls. The highest Tg was achieved with equal mole percent epoxide and acrylate mixtures. Young’s modulus and tensile strengths increased with increasing weight percent of epoxide-containing reactants. Materials exhibited a wide range of mechanical properties dependent on monomer structure (including Young’s modulus values varying from 2 – 180 MPa with chosen reactants). With increasing molecular weight of the reactants, elastomeric behavior is observed with high elongation and moderate strength values.
With some low molecular weight reactants, single-phase materials are produced which exhibit the same thermal and mechanical properties when reactions are run simultaneously or sequentially. With some higher molecular weight reactants, microphase-separated materials are produced, as evidenced by the presence of two glass transition temperatures. The microphase-separated HPNs exhibit properties that are highly tunable by sequential reaction order. The time scales of microphase separation were examined for chosen reacting systems and correlated with reaction conversions in order to consider the competitive relationship between network formation and microphase separation.