共聚物组成对线形苯乙烯-丁二烯-苯乙烯嵌段共聚物粘弹弛豫与相形态的影响

研究了不同组成的苯乙烯-丁二烯-苯乙烯嵌段共聚物(SBS)的相形态与粘弹弛豫.用透射电子显微镜(TEM)表征了SBS的形态,结果显示,几种SBS均呈层状结构,随着苯乙烯含量的降低,聚苯乙烯(PS)相的尺寸稍有减小,而聚丁二烯(PB)相尺寸明显增大.用动态流变学方法考察了不同温度下SBS嵌段大分子的弛豫行为,结果表明,苯乙烯含量减少,PS相玻璃化转变和有序-无序转变温度均降低;苯乙烯含量少的,在有序-无序转变过程中呈现出高且宽的损耗峰,表明有序-无序转变过程中能量的耗散主要由两相溶合时分子链间的内摩擦所决定,分子链越长,内摩擦越大,能量耗散越大.

FFECT OF COMPOSITON ON THE VISCOELASTIC RELAXATION AND PHASE MORPHOLOGY OF LINEAR STYRENE-BUTADIENE-STYRENE TRIBLOCK COPOLYMERS

The influence of copolymer composition on the viscoelastic relaxation and the morphology of styrene-butadiene-styrene triblock copolymers (SBS) were investigated by using dynamic rheological measurements and transmission electron microscope (TEM).Through the TEM observation,it is found that the three kinds of SBS with various styrene contents present lamellar structures and the domain size of styrene (PS) phase decreases slightly,however the size of butadiene (PB) phase increases with the decrease of styrene contents.It has been accepted that the period of microdomains of block copolymer is at the same order with the size of the polymer chain coil in molten state,i.e.R_g∝N~ 1/2 .The N~ 1/2 of PS block and PB block in each SBS were calculated cursorily,which was coincided with the TEM observation.But the exact size of each phase could not be confirmed only by means of the TEM micrographs.Three relaxation peaks in the plot of loss factor (tanδ) and temperature (T) at different frequencies(ω) were observed,which should owe to the relaxation of various motorial unit of SBS.The glass transition temperature(T_g or T_2) of PS phase and the order-disorder transition (ODT) temperature (T_3) for various SBS were obtained from the plot of tanδ-T.The results exhibit that T_2 reduces slightly with the styrene contents decrease.The decrease of T_2 is mainly caused by the reducing of the molecular weight of styrene blocks of SBS.Comparing with T_2,T_3 descends rather remarkably,which is considered to be concerned with PB block.In addition,it revealed that the ODT peak could not be observed if ω was too high during dynamic measurements.Only when ω was lower than 0.1 rad/s,a notable ODT peak could be gained.Two plateaus in the G′(or G″)-T-ω three dimension graph and two humps in the tanδ-T-ω three dimension graph were observed.The hump "b" at high temperatures and low ω region is considered to be related to the ODT behavior.An interesting thing is that SBS with low styrene contents shows a huge hump "b"compared with the SBS with high styrene contents in spite of almost identical polymer molecular weight.High quantity of tanδ in dynamic measurements means drastic internal friction and will lead to large energy dissipation.Usually,tanδ is inversely proportional to the diffusion coefficient (D) of a reptating chain.D is inversely proportional to N~2.Therefore,N~2 of PS block,PB block and the whole SBS chain were calculated roughly.Due to the small molecular weight of butadiene monomer,the SBS with low styrene content actually possesses longer chain length and exhibits larger N~2 compared with the SBS with high styrene content.Consequently,long chain gives rise to large inner friction during ODT and huge tanδ peak on account of the viscoelastic nature of polymer.In summary,although the temperature of ODT is mainly decided by the styrene contents,however,the energy dissipation during the ODT transition mostly comes from the inner friction of PB block as well as the whole SBS chain.