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.Rg∝N1/2.The N1/2 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(Tg or T2) of PS phase and the orderdisorder transition (ODT) temperature (T3) for various SBS were obtained from the plot of tanδ T.The results exhibit that T2 reduces slightly with the styrene contents decrease.The decrease of T2 is mainly caused by the reducing of the molecular weight of styrene blocks of SBS.Comparing with T2

T3 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 - 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 isinversely proportional to the diffusion coefficient (D) of a reptating chain. D is inversely proportional to N2.Therefore

N2 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 N2 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.