Jinliang Yang

Temperature controlled cationic photo-curing of a thick, dark composite

In this research, the cationic photopolymerization of 1,4-butanediol diglycidyl ether was found to have an infinitely long induction period due to the sustained stability of the secondary oxonium ions species at low temperature. Based on this, the system could be separated into two steps: photolysis of the photoinitiator without polymerization under irradiation and polymerization in the dark. Under irradiation, only the secondary oxonium ion species was generated at low temperature, whereas the polymerization could proceed and auto-accelerate at room temperature without irradiation. The two steps could be monitored by a significant change of the temperature. The temperature controlled cationic mechanism resolved the issue of light penetration in colored thick composites. Through temperature control, the infinitely thick and dark material could be prepared by cationic photopolymerization.

Reasons for the yellowness of photocured samples by the benzophenone/1,3-benzodioxole photoinitiating system

The benzophenone (BP)/1,3-benzodioxole (BDO) bimolecular photoinitiating system was developed in recent years. However, the obvious yellowness of the samples cured by the BP–BDO photoinitiating system would limit its wide application in the photocuring field. To study the reasons for the yellowness of the cured samples, the photoreaction products of the mixture of BP and BDO were analyzed by infrared spectroscopy (IR) and GC-MS. Electron spin resonance (ESR) spectroscopy was also used to gain insight into the formation of the initiating radicals with 5,5-dimethyl-1-pyrroline-1-oxide (DMPO) as a spin trap. The results of IR and GC-MS indicated that after the UV exposure of the BP–BDO mixture, four other main photoreaction products (ingredient 1–4) were generated via the interaction of excited BP and BDO. From the ESR hyperfine spectrum, only phenyl radical could be observed in the absence of air, which was significant to initiate the photopolymerization of HDDA and to form the three dimensional network. However, a minor amount of hydroxyl radical could also be generated besides the phenyl radical in the presence of air. The generation of hydroxyl radical in the photoreaction system is attributed to the formation of the phenol compound ingredient 1, which greatly contributed to the obvious yellowness of cured samples photoinduced by BP–BDO in practical applications.

Electro-initiated cationic polymerization in the presence of potassium hexafluoroantimonate

This work presents the electrochemically-induced polymerization of vinyl ether monomers by using potassium hexafluoroantimonate (KSbF6) as the initiating species. Static characterization of the resulting polymers was investigated by ATR-FT-IR. Real-time FT-NIR was used to study the kinetics of the electro-initiated polymerizations. Under certain conditions, rapid cationic polymerization kinetic profiles and high final conversion of monomer to polymer could be obtained. The reaction rate could be readily adjusted based on the applied voltage or by the introduction of an alkaline additive, which along with the observation of extended post curing clearly demonstrated the cationic nature of the electrochemical process. The thermo-stability of the polymerized product was investigated by TGA.