(a) Chemical structure of -bis(2,5-di-tert-butylphenyl)- 3,4,9,10-perylenedicarboximide (BTBP). The arrow indicates the direction of the absorption transition dipole. (b) Mold for preparation of the PMMA film. (c) Dimensions of the test film and definition of coordinate system.
Schematic of the creep apparatus. A freestanding PMMA film is placed in a temperature-controlled cell on a piezostage. The film is clamped between two rubber pieces at one end and glued between polyetherimide (PEI) sheets in another end. A constant stress is provided by a pulley and weight system. A window in the bottom of the cell allows light to pass between the microscope and the sample.
Schematic of optical apparatus: , polarizer; , shutter; , mirror; BS, beam splitter; , lens; , half-(quarter-) wave retardation plate; DM, dichroic mirror; , pinhole; PBS, polarizing beam splitter; , fiber coupling; APD, avalanche photodiode. The dotted line indicates the Nikon TE2000 microscope.
(a) Photobleaching measurements of BTBP reorientation in undeformed PMMA at . (●) Fluorescence intensity for polarization parallel to the bleaching beam polarization. (○) Fluorescence intensity (after being multiplied by the factor) for polarization perpendicular to the bleaching beam polarization. (b) Anisotropy decays for BTBP in undeformed PMMA at the temperatures indicated. The solid lines are KWW fits to the data.
Local measurement of strain and BTBP reorientation. The four photobleached lines define the region where local strain and mobility measurements are performed. The local strain is defined as . The small bleach patterns are used for measurements of BTBP reorientation; one of these regions is shown in detail in the lower left. These images were obtained with wide field illumination; photobleached regions appear dark because less fluorescence originates from these regions.
Test of correction scheme for dye alignment. (●) Anisotropy decay for BTBP in PMMA during deformation using a bleaching beam polarized along the axis. (▿) Anisotropy decay for BTBP in PMMA during deformation using a bleaching beam polarized along the axis. (◼)Anisotropy decay for BTBP in undeformed PMMA as a reference; the solid line is a KWW fit to the data. After applying the correction described in the text, the two anisotropy decay curves are identical within experimental error.
Rotational correlation times for BTBP in undeformed PMMA at different temperatures. The solid curved line above is the temperature dependence of the dielectric relaxation time from Ref. 27, shifted vertically. The solid straight line below is a guide to the eye.
(a) Three trials of creep and recovery experiments on PMMA at with an engineering stress of . (b) Shift factor for the rotational correlation time, showing a hundredfold increase in mobility during creep. The solid lines are guides to the eye. The labels A, B, C, and D, refer to Fig. 9.
(a) Normalized anisotropy decays obtained during the creep experiment shown in Fig. 8(a). These curves were obtained from photobleaching experiments starting (◇), (◼), (▿), and (●) after applying the stress. (b) Normalized anisotropy decays obtained during recovery. Curves obtained at (●), (▿), (■), (◇), and (▴) after applying the stress. The A, B, C, and D anisotropy decay curves were performed at times A, B, C, and D in Fig. 8(b). The solid lines are KWW fits to the data. During creep, from time A to time B, as strain and strain rate increase, higher mobility (faster anisotropy decays) is observed. After removing the stress, the enhanced mobility slowly disappears.
(a) Creep experiment on PMMA at with an engineering stress of . (b) Shift factor for the rotational correlation time. Very little change in mobility is observed.
The shift factor for the rotational correlation time as a function of the true stress for PMMA at .
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