A Facile Strategy for Intrinsic Low-Dk and Low-Df Polyimides Enabled by Spirobifluorene Groups

Modified polyimides(MPIs)show great potential towards 5G communication applications,due to its excellent thermal stability,mechanical property and chemical stability as compared to most of polymers.Introducing fluoride groups or porous structure is favorable to ultra-low dielectric constant(D_(k))and dielectric loss(D_(f)).However,the cost of the fluorinated MPIs is high and their synthetic processes are complicated,and porous MPIs suffer poor mechanical properties.Also,increasing the fraction of free volume is a very effective way to lower D_(k)through introducing more ultra-low-D_(k)air component.However,most of this kind of MPIs lag far behind the fluorinated MPIs and the porous MPIs in terms of ultra-low D_(f),hindering the application of MPIs in high-speed communication devices.Thus,it is highly desirable to develop intrinsic ultra-low-D_(k)/D_(f)MPIs at high frequency with less fluoric groups and nonporous structure.Herein,we introduce a facile and effective strategy to lower D_(k)and D_(f)through introducing rigid and large sterically hindered aromatic groups into MPIs.On the one hand,their large steric hindrance effect leads to low D_(k)by increasing intrinsic free volume.On the other hand,the resulting highly stiff polymer chain and strong intermolecular interaction are favorable to reduce D_(f)by inhibiting dipole orientations.Based on this strategy,the spirobifluorene groups are preferred.The as-prepared MPIs show excellent dielectric performance with low D_(k)of 2.74–2.76 and low D_(f)of 0.00599 at 10 GHz,to some extent,exceeding the multiple fluorinated MPI with D_(k)/D_(f)of 2.67/0.00663 at 10 GHz.

Implications of advanced warning messages on eliminating sun glare disturbances at signalized intersections

Due to sun glare disturbances, drivers encounter fatal threats on roadways, particularly at signalized intersections. Many studies have attempted to develop applicable solutions, such as avoiding sun positions, applying road geometric re-directions, and wearing anti- glare glasses. None of these strategies have fully solved the problem. As one of the ＂Con- nected Vehicle＂ practices proposed by the U.S. Department of Transportation, advanced warning messages （AWMs） are capable of providing wireless information about traffic controls. AWM acts as a supplement to conventional signs and signals, which can be blocked by obstacles or natural disturbances, such as sun glare. The drivers＇ smart advisory system （DSAS） can provide drivers with AWM. Using a driving simulator this research explores the effects of DSAS messages on driving behaviors under sun glare disturbance. Statistical analyses were applied to assess （1） the negative impacts of sun glare, （2） the compensation of the DSAS AWM to sun glare effects, and （3） the improvement in driving performance due to DSAS AWM. Four performance indexes were measured, including （1） half kinetic energy speed, （2） mean approach speed, （3） brake response time, and （4） braking distance. The effects of the socio-demographic factors, such as gender, age, educational background, and driving experience were also studied. The analytical results illustrate that the DSAS can compensate for reduced visibility due to sun glare and improve driving performance to a normal visual situation, particularly for left turn and through movement.