Using Changes in Initiation and Chain Transfer Rates To Probe the Kinetics of Cross-Linking Photopolymerizations:  Effects of Chain Length Dependent Termination

Abstract

The importance of incorporating chain length dependent termination (CLDT) behavior into the interpretation of the kinetics of cross-linked systems has been examined. Kinetic chain length distributions were varied in a variety of di(meth)acrylate photopolymerizations via the manipulation of the initiation rate and the chain transfer rate. Shifting the kinetic chain lengths toward shorter chains has little visible effect on multiacrylate systems. In contrast, similar changes in the corresponding multimethacrylate polymerizations changed the kinetics significantly. Shorter kinetic chains led to the delayed onset of reaction−diffusion-controlled termination behavior, as well as an increase in the ratio of k t/k p[M] at all conversions prior to the onset of reaction−diffusion control. Additionally, the magnitude of the kinetic constant ratio in the reaction−diffusion-controlled regime was affected by the kinetics at low conversion in the polymerization of a rubbery system, PEG(600)DMA. This behavior was independent of the method used to alter the kinetic chain length distribution and thus implies that CLDT may potentially impact the network formation in polymerizations occurring above the T g of the system. These results illustrate that, although counterintuitive, CLDT is an important factor in cross-linking free radical polymerizations.

Modeling the Effects of Chain Length on the Termination Kinetics in Multivinyl Photopolymerizations

“…[43] c) Ref. [32,33] The three figures clearly show that the model predicts autoacceleration, autodeceleration, and a CLDT region; the same non-classical behaviors observed experimentally when polymerizing multivinyl monomer systems. The qualitative agreement between previous experimental results and model predictions is attributed to the incorporation of CLDT into the model termination kinetic equations.…”

“…Anseth et al [33] experimentally investigated PEG(600)DMA and DEGDMA using DSC. When a 10-fold difference in initiation rate was induced, the examined scaling exponent was again 0.3, in agreement with the observations of Berchtold et al [31,32] Additionally, Cook experimentally investigated a series of oligomeric dimethacrylates using DSC. [34,35] He reported a similar non-classical kinetic behavior, where the scaling exponent varied from 0.3 to 0.6.…”

“…[3,8] The reaction-diffusion parameter, R, is taken from experimental work by Anseth et al and Berchtold et al [32,33] The model equations include parameters for the ultraviolet photoinitiator, DMPA. The molar absorptivity, e, for DMPA at 365 nm, the peak initiating wavelength in the majority of commercial UV curing applications, is approximately 150 L N mol -1 N cm -1 .…”

“…Berchtold et al performed studies using differential scanning calorimetry (DSC) and Fourier transform infrared (FT-IR) spectroscopy [31,32] to investigate poly-(ethylene glycol 600)dimethacrylate, (PEG(600)DMA), and diethylene glycol dimethacrylate (DEGDMA) polymerization kinetics. These systems were initiated with 0.1 wt.-% of the ultraviolet initiator 2,2-dimethoxy-2-phenylacetophenone (DMPA).…”

Photo-patterning of porous hydrogels for tissue engineering

“…Bridging occurs when the partially polymerized monomer solution is not completely removed prior to the onset of crosslinking. It is well known that these polymerizations will continue in the dark, i.e., after the light is turned off [21]. The partially polymerized, yet soluble regions will proceed to form a crosslinked gel if not dissolved and washed away immediately following the 35 s exposure.…”