The Synthesis Of Low Dielectric Loss Cross-linking Polystyrene

In recent years, the signal band of information communication apparatuses, such as PHS and portable telephones, and the CPU clock time of computer have reached the GHz band, and the trend toward higher frenquency is increasingly obvious. The dieletric loss of an electric signal is proportional to the square root of dielectric consant of the insulating material constituting the circuit and to the product of the dieletric loss tangent and the frenquency of the signal used. Therefore, the higher the frequency of the signal, the larger is the dielectric loss. Since dielectric loss causes attenuation of electric signals to lower the reliability of the signals, it has been necessary to select as an insulator a material which has a low dielectric constant and low dielectric loss tangent. The cross-linking polysterne is a sort of thermosetting resin, which is characteristic of the low dielectic constant, low dielectric loss even in the high frenquency, high voltage and radiation resistance, rigidity and dimensional stability, easy handling in all machining operations, good chemical and heat-resistant, and excellent mechanical properties. So the cross-linking polystyrene is a proper material for low dielectric constant and loss in the band of microwave frenquency.In the thesis, the low dielectric loss cross-linking polystyrene is prepared using styrene(St) and divinylbenzene(DVB) as monomers, azo-bis-isobutyronitrile(AIBN) as initiator, via the bulk polymerization under a controled temperature programming.Firstly, it is necessary to know the exothermal characteristic of the polymerization. The gel effect may be the main feature of bulk polymerization. So the method that is measuring the history of the gelation and the solation can be used to investigate the exothermic law of the bulk polymerization. It is found that when the ratio of DVB is small, the temperature of the system should be low after the gel point, and when the ratio of DVB is large, the temperiture of the system in the pre-gelation period should not be too high, while after the gel piont the temperature should be higher and the reaction time should be prolonged.Secondly, based on the exothermic rule of copolymerization of styrene and divinylbenzene, the proper polymerization process is designed to prepare for low dielecltric loss cross-linking polystyrene. Four recipes for preparing such polystyrene plank are used. The corresponding the ratios of St:DVB:AIBN are (a) 99:1:0.05, (b)97.5:2.5:0.05, (c)98.5:1.5:0.05 and(d)98:2:0.05.Finally, the planks were tested and chatacterized. FITIR spectroscopic study of the broad surface of cross-linking polystyrene indicates that benzene ring and double band are located on the network structure. The dielectric and mechanical properties of the plank was characterized by Network Analyzer, Izod Impact Tester and Dynamic Mechanic Analyzer. The result shows that the impact intensity and the loss modulus are decreased gradually and the glass transition temperature is increased slowly, as increasing of the content of DVB in the copolymer. Meanwhile, the tensile strength and the breaking elongation of the cross-linking polystyrene firstly increases and then decreases with the increase of DVB content, and the maximum is present at around 1.8% of DVB content. Finally, from the SEM photographs of the tension fracture of the planks, the heterogenous interior structure is observed, except of recipe (d). So the polymerization process is a applicable to the (d) systeme. The polymerization process has a great effect upon the interior structure of the plank, namely, the temperature in each step of the polymerization process can definitively affect the interior structure of the plank.