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45416-AC7
Cure and Properties of Thermosetting Materials at the Nanoscale

Sindee L. Simon, Texas Tech University

Nanoscale constraint is known to have a significant impact on the thermal properties of materials.  Such changes are particularly important for the use of thermosetting materials in nanoelectronic applications, as well as for the use of nanoparticle-thermoset composites.  The objective of the proposed work is to investigate these effects under the well defined nanoscale constraints imposed by controlled pore glass (CPG). 

In the first year of the grant, we have investigated the isothermal curing under nanoscale constraint of a thermosetting resin, bisphenol M dicyanate ester (BMDC), which trimerizes to form a polycyanurate network material.  Differential scanning calorimeter was used to monitor the evolution of the glass transition temperature (designated T_f' since measurements were performed on heating) and the conversion during cure as a function of the diameter of the silanized control pore glass matrix which is used for confinement. 

A T_g depression is observed for both the bisphenol M dicyanate ester monomer and the polycyanurate networks; the depression is only a few degrees for the monomer.  On the other hand, a 56 K depression is observed for the "fully-cured" network in 11.5 nm pores. Perhaps more interesting, the T_g depression of nanoconfined polycyanurates with pore size appears to be nonlinear, decreasing initially, then leveling off in the range of 100 nm to 25 nm, and then decreases again below 25 nm.  This apparent nonlinear behavior is consistent with the experimental data on small molecules and polymer solutions confined in silanized CPGs by McKenna and coworkers and by Jonas and coworkers. On the other hand, when only pore sizes  below 15 nm are studied, linear relationships of T_g versus inverse pore radius are generally observed, presumably due to the result of the narrow pore size range investigated.  It is also noted that we observe a second T_g both for the fully-cured polycyanurate and for the monomer confined to the smallest pores.

The cure kinetics of bisphenol M dicyanate ester confined in the controlled pore glasses has also been investigated by following the evolution of the glass transition temperature during cure.  The results are shown in the figure as a function nanopore size.  As the pore size decreases from infinity (i.e., the bulk state) to 11.5 nm, cure is accelerated.  It has been suggested by Vyazovkin that the enhanced reactivity observed at nanoscale in this system may be attributed to the increased collision efficiency of the molecules in the vicinity of the surface due to decreased mobility.  The accelerated cure under nanoconfinement is also corroborated by the fact that the DSC onset temperature for the reaction of initially uncured monomer confined in the CPGs decreases as pore size decreases.   Furthermore, based on the similarity found for the nanocomposites and ultrathin film (see for example, work by Glotzer and coworkers and by Schadler and coworkers), this observation may be analogous to the curing of epoxy resin in the presence of  carbon nanotubes where the reaction rate was also found to increase with increasing nanotube loading (Xie et al.).  It must be noted, however, that catalytic (or reaction inhibiting) groups on surfaces could be more important than the effects associated with reduced degrees of freedom at the surface.   Similar to the bulk reaction studied earlier by Simon and Gillham, the reaction rate for the confined material is well described by a second-order plus second-order autocatalytic mechanism, allowing the rate constants to be a function of pore size.  The T_g (or T_f') versus conversion relationship for the material cured in the nanopores is identical to that of the bulk; hence, it is hypothesized that network structure is not perturbed by cure under nanoscale constraint. 

Work is currently being performed for the monomer cured in native pores to evaluate the influence of surface chemistry on the results.  In addition, the hypothesis concerning the effect of nanoscale constraint on network structure will be further examined in the coming year.

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