TY - JOUR
T1 - Rheo-optical evidence of a flow-induced isotropie-nematic transition in a thermotropic liquid-crystalline polymer
AU - Mather, Patrick T.
AU - Romo-Uribe, Angel
AU - Han, Chang Dae
AU - Kim, Seung Su
PY - 1997/12/15
Y1 - 1997/12/15
N2 - A direct rheo-optical characterization of the flow-induced isotropic-nematic (I-N) transition in a semiflexible thermotropic liquid-crystalline polymer (TLCP) was investigated, using a specially constructed apparatus enabling in-situ optical microscopic observations at elevated temperatures, along with cone-and-plate rheometry. For the investigation, an aromatic polyester, poly[(phenylenesulfonyl)-p-phenylene 1,10-decamethylenebis(4-oxybenzoate)] (PSHQ10), was synthesized via solution polymerization. Above the equilibrium isotropic-nematic transition temperature for this polymer, T = 170.5 °C, application of steady-state shear flow above a certain critical value of shear rate, γ + ̇c, produces a first-order I-N transition, with γ + ̇c increasing with temperature. This transition is evidenced by the formation of elongated nematic (birefringent) domains in the isotropic matrix, accompanied by a drastic decrease in shear viscosity (η). Remarkably, the nematic domains that form for γ + ̇ > γ + ̇c are optically uniform under cross-polarized optical microscopy; i.e., they are apparently free of disclinations (defects), typical of textured TLCPs. The flow-induced I-N transition in PSHQ10 is found to be reversible; i.e., upon cessation of shear flow, the domains melt to the original isotropic phase and the dynamic moduli rise toward the pretransition values. The observed flow-induced I-N transition may find important applications, such as envisaging new routes for processing TLCPs with better mechanical properties and helping to understand bioprocesses such as silk thread spinning.
AB - A direct rheo-optical characterization of the flow-induced isotropic-nematic (I-N) transition in a semiflexible thermotropic liquid-crystalline polymer (TLCP) was investigated, using a specially constructed apparatus enabling in-situ optical microscopic observations at elevated temperatures, along with cone-and-plate rheometry. For the investigation, an aromatic polyester, poly[(phenylenesulfonyl)-p-phenylene 1,10-decamethylenebis(4-oxybenzoate)] (PSHQ10), was synthesized via solution polymerization. Above the equilibrium isotropic-nematic transition temperature for this polymer, T = 170.5 °C, application of steady-state shear flow above a certain critical value of shear rate, γ + ̇c, produces a first-order I-N transition, with γ + ̇c increasing with temperature. This transition is evidenced by the formation of elongated nematic (birefringent) domains in the isotropic matrix, accompanied by a drastic decrease in shear viscosity (η). Remarkably, the nematic domains that form for γ + ̇ > γ + ̇c are optically uniform under cross-polarized optical microscopy; i.e., they are apparently free of disclinations (defects), typical of textured TLCPs. The flow-induced I-N transition in PSHQ10 is found to be reversible; i.e., upon cessation of shear flow, the domains melt to the original isotropic phase and the dynamic moduli rise toward the pretransition values. The observed flow-induced I-N transition may find important applications, such as envisaging new routes for processing TLCPs with better mechanical properties and helping to understand bioprocesses such as silk thread spinning.
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U2 - 10.1021/ma970737h
DO - 10.1021/ma970737h
M3 - Article
AN - SCOPUS:0031342452
SN - 0024-9297
VL - 30
SP - 7977
EP - 7989
JO - Macromolecules
JF - Macromolecules
IS - 25
ER -