Kloxin Christopher J, Scott Timothy F, Bowman Christopher N
Department of Chemical and Biological Engineering, University of Colorado, Boulder, CO 80309-0424, USA.
Macromolecules. 2009 Apr 14;42(7):2551-2556. doi: 10.1021/ma802771b.
Allyl sulfide addition-fragmentation chain transfer was employed concurrently with the radical-mediated formation of a thiol-ene network to enable network adaptation and mitigation of polymerization-induced shrinkage stress. This result represents the first demonstration of simultaneous polymerization and network adaptation in covalently crosslinked networks with significant implications for the fabrication of low stress polymer networks. For comparison, analogous networks incorporating propyl sulfide moieties, incapable of addition-fragmentation, were synthesized and evaluated in parallel. At the highest irradiation intensity, the allyl sulfide-containing material demonstrated a more than 75% reduction in the final stress when compared with the propyl sulfide-containing material. Analysis of the conversion evolution revealed that allyl sulfide addition-fragmentation decreased the polymerization rate owing to thiyl radical sequestration. Slow consumption of the allyl sulfide functional group suggests that intramolecular homolytic substitution occurs by a step-wise, rather than concerted, mechanism. Simultaneous stress and conversion measurements demonstrated that the initial stress evolution was identical for both the allyl and propyl sulfide-containing materials but diverged after gelation. While addition-fragmentation chain transfer was found to occur throughout the polymerization, its effect on the stress evolution was concentrated towards the end of polymerization when network rearrangement becomes the dominant mechanism for stress relaxation. Even after the polymerization reaction was completed, the polymerization-induced shrinkage stress in the allyl sulfide-containing material continued to decrease, exhibiting a maximum in the stress evolution and demonstrating the potential for continuing, longer term stress relaxation.
烯丙基硫醚加成-断裂链转移与自由基介导的硫醇-烯网络形成同时进行，以实现网络自适应并减轻聚合诱导的收缩应力。这一结果首次证明了在共价交联网络中同时进行聚合和网络自适应，对低应力聚合物网络的制造具有重要意义。为了进行比较，同时合成并评估了含有不能进行加成-断裂的丙基硫醚部分的类似网络。在最高辐照强度下，与含丙基硫醚的材料相比，含烯丙基硫醚的材料最终应力降低了75%以上。转化率演变分析表明，烯丙基硫醚加成-断裂由于硫自由基的螯合而降低了聚合速率。烯丙基硫醚官能团的缓慢消耗表明分子内均裂取代是通过逐步而非协同的机制发生的。同时进行的应力和转化率测量表明，含烯丙基和丙基硫醚的材料的初始应力演变相同，但在凝胶化后出现分歧。虽然发现加成-断裂链转移在整个聚合过程中都发生，但其对应力演变的影响在聚合后期集中，此时网络重排成为应力松弛的主要机制。即使在聚合反应完成后，含烯丙基硫醚材料中的聚合诱导收缩应力仍继续降低，在应力演变中出现最大值，表明存在持续的、长期的应力松弛潜力。
