Dynamic birefringence of oligostyrene: A symptom of ?polymeric? mode

Autor: Takayuki Onogi, Kunihiro Osaki, Tadashi Inoue
Rok vydání: 2000
Předmět:
Zdroj: Journal of Polymer Science Part B: Polymer Physics. 38:954-964
ISSN: 1099-0488
0887-6266
DOI: 10.1002/(sici)1099-0488(20000401)38:7<954::aid-polb6>3.0.co;2-f
Popis: The dynamic birefringence and the dynamic viscoelasticity of an oligostyrene, A1000, whose molecular weight (Mw = 1050) was comparable to the Kuhn segment size, MK, were examined near and above the glass-transition temperature in order to characterize polymeric features of very short chains with M ∼ MK. The complex shear modulus, G*(ω), was similar to that for supercooled liquids: No polymeric modes such as the Rouse mode were detected at low frequencies of viscoelastic spectrum. On the other hand, the strain-optical coefficient was found to be negative in the terminal flow zone and positive in the glassy zone. Because the negative birefringence of polystyrene is originated by polymeric modes associated with chain orientation, the present results indicate that polymeric modes exist and become dominant for birefringence in the terminal flow. The data were analyzed using a modified stress-optical rule: The modulus and the strain-optical ratio were separated into polymeric (rubbery) and glassy components. The total modulus, G*(ω), was mostly due to the glassy component, GG*(ω), resulting in the positive birefringence. GG*(ω) for A1000 agreed with that for high M polystyrenes when compared at a comparable reduced frequency scale. The polymeric component, GR*(ω), giving rise to the negative birefringence was lower than GG*(ω) over the whole frequency range but its contribution to the birefringence exceeded that of the glassy component at low frequencies because of the larger optical anisotropy and longer characteristic relaxation time of the former. The limiting modulus of GR* at high frequencies was about 3 times lower than that for high M polystyrenes, indicating that the main-chain orientation of the oligostyrene on instantaneous deformation was reduced compared with that of high M polystyrenes. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 954–964, 2000
Databáze: OpenAIRE
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