Macroporous Thermosets via Chemically Induced Phase Separation

Abstract

We have explored a new technology based on chemically induced phase separation that yields porous epoxies and cyanurates with a closed cell morphology and micrometer sized pores with a narrow pore size distribution. When the precursor monomers are cured in the presence of a low molecular weight liquid, the desired morphology results from a phase separation and a chemical quench. After phase separation, the porosity is achieved by thermal removal of the secondary liquid phase, specifically by diffusion through the crosslinked matrix. In respect to the thermodynamics and kinetics, the origin of the phase separation process can be identified as nucleation and growth. The influence of internal and external reaction parameters, such as chemical nature of the low molecular weight liquid, its concentration and the curing temperature on the final morphology are presented. Thus, the morphology can be controlled ranging from a monomodal to bimodal pore size distribution with pore sizes inbetween 1 to 10 μm. These porous thermosets are characterized by a significantly lower density, without any loss in thermal stability compared to the neat matrix. Such new materials demonstrate great interest for lowering the dielectric constant and for improving the fundamental understanding of the role of voids in stress relaxation and toughening.

Nanoporous Cyanate Ester Resins: Structure-Gas Transport Property Relationships

“…On the other hand, Kiefer et al described the synthesis of CER foams using cyclohexane as a porogen [ 9 , 10 ]. Foam creation as well as full network curing were accomplished by chemically induced phase separation realized through heating of cyanate ester/cyclohexane mixture up to 280 °C in vacuum.…”

Synthesis of solvent-modified epoxies via Chemically Induced Phase Separation: A new approach towards void toughening of epoxies

“…The solvent remaining in the matrix leads to a plasticization only resulting in a slight increase of the fracture energy as it can be conluded by comparing the non-modified epoxy to the sample prepared with 13 wt% cyclohexane just below the critical concentration to induce a phase separation. The substantial increase in fracture energy of up to around 400% compared to the neat matrix is a consequence of the buildup and interaction of internal stresses resulting from the generation of a randomly dispersed phase, as concluded from (38,39), thus enabling extensive shear band formation. Several micromeehanisms might be associated with this observation.…”

“…Even though, this model allowed to predict the possibility of void toughening, the finite element approach does not allow to take into account the multiple interactions between voids. Therefore a refined model has been developed, which is based on a zero order approximation and allows to visualize the stress distributions in porous epoxies (38,39). This model predicts the buildup of internal stresses which can initiate multiple shear banding, thus leading to effective toughening.…”

“…The experimental verification of the above theoretical predictions of void toughening requires a technique which yields porous thermosets having closed pores with sizes and distributions in the Ixrn-range similar to those commonly used for toughening with rubber or thermoplastic particles. Therefore, we have developed a new technology, termed Chemically Induced Phase Separation (CIPS) (39,(41)(42)(43)(44), for the generation of the desired morphology which is governed by a phase separation process resulting from a chemical quench (45). With this method, the epoxy precursor and curing agent are cured in the presence of a low molecular weight liquid, which turns into a non-solvent upon curing, thus initiating a phase separation proceeding via a nucleation and growth mechanism.…”

Grafted Epoxide Functionalized Polypropylene Carbonate Porogen for Low Dielectric Constant Epoxy Films

“…There are several reported techniques to form porous films, including gas expansion by using gas-blowing agent such as nitrogen, 12 concentrated emulsion by polymerizing one phase and selectively removing the dispersed liquid phase, 13 thermally induced phase separation by freeze-drying the solvent below the glass transition temperature (T g ) of the polymer, 14 and chemically induced phase separation by crosslinking the polymer to form two phases followed by removal of the solvent to create pores. 15,16 Despite the large pore volume created using these methods, the pores are either large or open-pores, leading to the formation contiguous pathways through the polymer film. This can significantly deteriorate the mechanical property of the film, making them undesirable in preparation for PWB substrate.…”