Structure And Decoloration Behaviors Of Polymer/photochromic Compound Composites

Photochromic materials which can conduct reversible photochemical reaction induced by electromagnetic radiation have been attracting significant attention in the development of data storage, data processing, ophthalmic lenses, photodeformation, etc. Many applications require both high mechanical strength and fast isomerization behavior. However, such two requirements are usually contradictory. Photochromic molecules can be copolymerized with monomer or grafted to the polymer chain by covalently bonding or dispersed into polymer matrices by physically doping. Unfortunately, in some cases, their isomerization rates were largely slowed down due to steric restrictions. So, it still remains a challenge to achieve high switching speeds of photochromic molecules in polymer matrices. In the present dissertation, we put a plasticizer into polymer matrix to balance a fast photochromic switching speed and good mechanical properties. More importantly, a facile method of polymer dispersed photochromic organogel was used to achieve concurrent solution-like decoloration rate and high mechanical strength. Systematical analysis had been done to clarify the effects of the plasticizer or dispersion of organogels on the fading rate and mechanical properties of polymer films.Firstly, thin films of polystyrene (PS) dispersed spiropyran (SP) were fabricated by solution casting with di-n-octyl phthalate (DOP) as a plasticizer. The effects of the plasticizer on the photochromism, fading rate, and mechanical properties of polymer materials were studied. The results showed that a blue shift of the maximum absorption wavelength (λmax) of merocyanine was observed as the polarity of matrix increased from DOP, while the fading rate of merocyanine increased and a better mono-exponential decay behavior of merocyanine with increasing of the content of DOP. When the content of DOP was more than30wt%, the toughness of film was remarkably improved, while its elastic modulus and tensile strength dropped sharply. Therefore, photochromic switching rates can be favored in polymer matrix through plasticization, while this compromises the mechanical properties of the resulted polymer films.Secondly, photochromic organogels were prepared through the self-assembly of benzylidene-D-sorbitol derivative (DBS) as a gelator in SP/propanol solution. The results showed that the organogels exhibit good self-supporting ability with high storage modulus due to the three-dimensional brillar networks of DBS. The improvement of rigidity and thermal resistance were observed as well as the loss of deformation resistance of the organogels when the concentration of DBS increased. Since the SP molecules were not involved in the formation of DBS three-dimensional fibrillar networks, the isomerization of SP was independent on the surrounding network, i.e. the decoloration rate of the photochromic organogels with different concentration of DBS was similar to SP/propanol solution.Then, in order to provide SP with a solution-like local environment within polymer matrix, while retaining high mechanical strength of the glassy polymer matrix, we proposed a facile strategy of cosolvent-dispersion-evaporation method to prepare polymer dispersed photochromic organogel. Polyvinyl alcohols (PVA) and liquid polypropylene glycols (PPG) with negligible vapor pressure were used as the polymer matrix and the solvent for SP and organogel. Compared with the traditional elevated temperature dissolution-cooling, the novel method could avoid the destruction of photochromic molecules and retained their intrinsic photochromism. Photochromic molecules could freely rotate in organogels as in solutions. The fibrillar network held the SP/PPG solution through capillary forces and/or surface tension and thus improved the stability of SP/PPG within the polymer matrix. As the dispersed micro-organogel just slightly reduced the glass transition temperature (Tg) of PVA, polymer matrix showed excellent mechanical strength.Finally, in order to prove the universality of above-mentioned method and to prepare a photochromic material with good fatigue resistance, spirooxazine (SO) was used instead of SP. The results showed that the decoloration rate of SO within the thin film of PVA dispersed photochromic organogel was also solution-like. Interestingly, SO showed a much better fatigue resistance than SP within the glassy thin film of PVA dispersed photochromic organogel.