A fast real time measurement system to track in and out of plane optical retardation/ birefringence, true stress, and true strain during biaxial stretching of polymer films.
An instrumented and highly integrated biaxial stretching system was designed and constructed to obtain true stress, true strain, and optical behavior of polymeric films during biaxial stretching. With programmable drive motors, any form of temporally varying biaxial deformation profiles, including linear, exponential, logarithmic as well as cyclic, can be applied to a square-shaped films. This machine allows the investigation of mechano-optical behavior of films under profiles captured in industrial processes. To overcome the edge effects, the samples are painted with a dot pattern that is imaged using a high speed video capture system. This system accurately determines the locations of the each dot matrix in subsequent images acquired and calculates the true strains in both directions. The in-plane optical retardation is determined using spectral birefringence method that uses polarized white light and optical spectrometer in the optical train. This is carried out automatically at less than 10 nm in retardation resolution with the light beam passing through the symmetry center of the sample. Out of plane retardation is measured with an identical optical train tilted 45° to the plane of the film with its light beam going through the same spot on the sample as 0° beam. The true stress and birefringences are calculated with the determined instantaneous thickness of the film. With this system, the stress optical behavior of PET's is determined up to very large deformation levels at moderate to high deformation rates. Beyond the initial linear stress optical behavior, these films exhibit sudden positive deviation from linearity and this start of nonlinearity was directly associated with the stress induced crystallization.
Structural Formation of Cyclic Olefin Copolymer (COC) Films as Revealed by Real Time Retardation Measurements during Fast Stretching Process
This work reports on an experimental study of the fast stretching of a cyclic olefin copolymer (COC) film. We measured the stress retardation of the polymer film during fast stretching and identified a relationship between stress and birefringence. This relationship indicates that the film obeys the stress optical rule (SOR) at the onset of stretching. At slower stretching speeds, the stress optical constant (SOC) settles at a certain value. With faster stretching speeds, SOC values approach the photo-elastic constant (PEC). Our study also investigated the effects of stretching temperatures and stretching modes for both uniaxial stretching and biaxial stretching. We applied techniques of thermal analysis to evaluate higher order structures after fast stretching.
Determination of polyolefin film properties from refractive index measurements
A technique is described for determining the density of polyolefin film from refractive index measurements. The procedure involves the measurement of the refractive indexes (nx, ny, and nz) along the three major axes of a film sample and conversion of the arithmetic average refractive index to a density value. The refractive index data are also used to calculate the per cent crystallinity and birefringence in the film.