Structural Evolution of Highly Crosslinked Polymer Networks
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phenylacetophenone (DMPA) in each monomer. The samples were deoxygenated by bubbling nitrogen through the solutions for approximately 10 minutes under a nitrogen atmosphere. The samples were then placed in ESR tubes and sealed. A Bruker ESP 300E ESR spectrometer was used to monitor the radical concentrations during a photopolymerization process. The photopolymerizations were initiated on-line with a 365 nm UV light source with gave an intensity of 0.1 mW/cm 2 at the ESR sample cavity. Polymerizations were conducted at 25 0 C. Differential scanning photocalorimetry of identical samples was used to determine the 6 corresponding polymerization rate profile and conversion. Modeling The simulation results are from a kinetic gelation model based on a face centered cubic lattice with periodic boundary conditions. Monomer molecules were allowed to occupy multiple lattice sites, so the effects of distance between the double bonds on the structural features of the polymer could be examined. For example, the differences between a network formed from the polymerization
of diethylene
glycol dimethacrylate
and poly
(ethylene glycol
600)
dimethacrylate could be simulated. Initiator molecules were distinct species decaying with a first order rate constant. Thus, the effects of rate of initiation on the network microstructure could be determined. Discrete void sites were incorporated into the model to simulate the effects of increased free volume or inert solvent. Finally, limited mobility of all species was introduced through a crank-shaft type movement. The set-up of the kinetic gelation model is described in 7 great detail elsewhere. RESULTS AND DISCUSSION Figure 1 shows a 2-dimensional slice from the 3-dimensional kinetic gelation simulation at 10% conversion of double bonds in the system. Simulation parameters were for a 2-site monomer molecule polymerized in the presence of 15% void volume and 1% initiator molecules. The decay time of the initiator molecules was 1/5000. From this figure, the heterogeneous nature of the polymerization and the presence of microgel regions are observed even at low double bond conversions. These microgel regions were also examined using transmission electron microscopy, and the experimentally determined size scale of the heterogeneities ranged from 50100 nm in polymers produced from multiethylene glycol dimethacrylates.
Figure 1. A two-dimensional slice from a kinetic gelation simulation of a two-site monomer
molecule polymerized to 10% conversion of double bonds. Void volume sites are white, polymer segments are gray, and unreacted double bonds are black.
620
To quantify different aspects of the system heterogeneity during these multifunctional monomer polymerizations, a heterogeneity index was defined and studied. 8 The heterogeneity index, H.I., is defined as the ratio of the actual probability of finding two identical species as nearest neighbors on the lattice to the probability of finding these species next to each other in a completely homogeneous system. Therefore, the closer
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