Awl t~tuQ STI AbstractRecent investigations have implicated cage-liie precursors in the unusually high gelation conversion (W 82Yo) of acid-catalyzed tetraethoxysilane. However, the statistical models used so far cannot capture kinetic or composition-dependent features of alkoxysilane polycondensation.Here we take a first step towards unified modeling of the kinetics and structure of silica gelation.Dynamic Monte Carlo simulations [J. Somv&rsky and K. DuSek, Polym. BuJ1. 1994 33:369] are developed which permit competition between extensive cyclization and growth. The model includes well-established kinetic trends (hydrolysis pre-equilibrium and first shell substitution effects). As a first approximation, unimolecular-like terms for cyclization reactivity follow the experimental pattern of bimolecular rate coefficients. The present simulations allow unlimited formation of 3-site rings, giving rise to many structures which are not those of real silicates (where 4-site rings dominate). However, the level of cyclization (both cycles per molecule and per site) is consistent with that of real silicates, and is enough to delay gelation to 82% conversion or higher. These simulations also display a broader range of gelation behavior than prior kinetic models. At high to moderate monomer concentrations, competition between cyclization and growth causes the expected delay of gelation. Upon further dilution, we discover a third regime, absent from prior kinetic gelation models but important for siloxanes: formation of a distribution of polycyclic precursors which still rettin enough functionalisty to gel.

We present nonlinear shear and uniaxial extensional measurements on a series of polybutadienes with varying amounts of long-chain, random branching. Startup of steady shear experiments is used to evaluate the damping function of the melts. The damping function is found to show a trend toward decreased dependency on strain with increasing branching content. Interior chains, which are believed to be responsible for changing the damping function, are calculated to comprise less than 3 wt % of the melt. Extensional measurements are used to investigate the role of branching in strain hardening. We show that samples with increased branch contents do show larger deviations of the transient Trouton ratio from the linear viscoelastic limit of three. However, we also show that the extensional data can be fit using parameters determined solely by the shear measurements. Furthermore, we show that the changes in the damping function seen in shear have little impact on extensional behavior. The extensional behavior of the melt is found to be most affected by changes in the relaxation spectra which can result from both branching and increases in the high end of the molecular weight distribution. This statement runs contrary to the often expressed view that strain hardening behavior in extension is exclusively produced by branching.

We compare two approaches in modeling first shell substitution effects (FSSE) coupled with cyclization in acid-catalyzed sol−gel polymerization. First, an approximate, statistically based, kinetic-recursive model (KR) is developed that is computationally inexpensive for investigating trends in the polymerization. Second, an exact Monte-Carlo model (MC) that tracks a finite pool of growing polymer clusters is constructed for comparison to the KR model. The two models agree well prior to gelation when using rate constants typical of sol−gel polymerization. However, near the gel point, discrepancies between the two models arise because of the KR model's inability to account for correlations in the growing structure beyond the site distribution. We show that both FSSE and cyclization cause the polymer's structure distribution to be history dependent. We also show that the inclusion of both FSSE and cyclization in the model is capable of increasing gel conversions above the 0.50 limit of previous exclusive FSSE models. We show that FSSE aids cyclization by increasing the concentrations of oligomers that are candidates for intramolecular reaction and that a strong FSSE with cyclization causes a local maximum to occur in the polydispersity index as a function of conversion. Both models fall short of predicting experimentally observed gel conversions; indicating that, in addition to the small cycles allowed in the present work, cage formation may also be significant.

ABSTRACT:The hydrolysis of a cyanate ester network made from the monomer 2,2 -bis(4-cyanatophenyl)isopropylidene (bisphenol A dicyanate homopolymer) was studied. Hydrolysis reactions were performed isothermally at temperatures from 150 to 180ЊC under conditions of excess water. The kinetics of the reaction were characterized by the decrease in T g as measured by differential scanning calorimetry. The rate of change of T g was found to be adequately described as first order in T g , which is an indirect measure of the concentration of crosslink junctions. The activation energy of the reaction was found to be 115 kJ/mol. In addition, moisture-conditioned, glassreinforced laminate samples were heated and the time to delamination or blistering was recorded as a function of temperature. The blister time at solder temperatures ( T Å 220-260ЊC) was modeled using the above kinetic results. Heat transfer to the laminate was considered and the criteria used for blister time was the time at T Å T g of the sample. At lower temperatures ( T õ 220ЊC), loss of water from the laminate is sufficiently fast to prevent blistering.

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