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.
Glass-rubber constitutive model for amorphous polymers near the glass transition
The constitutive behaviour of amorphous polymers near the glass transition contains many features general to this class of materials. A new, physically based, three-dimensional constitutive model has been developed for simulating this wide range of features in models of polymer products and processes. In particular, the model displays glassy response at low temperatures and short time-scales, and rubber-like response at high temperatures and long time-scales, and is therefore an example of a glass-rubber constitutive model. Its basis is the assumed additivity of free energies of bond distortion and conformation perturbation. For the elastic bond distortion stress-strain law and flow model, and the conformational entropy function, the model employs linear elasticity, Eyring viscous flow and the Edwards-Vilgis entropy function, respectively. Glass structure and temperature dependence are introduced through the Vogel-Tammann-Fulcher and Arrhenius equations for viscosity, respectively. With parameters obtained for poly(ethylene terephthalate) in a companion paper, the model was solved numerically to simulate a variety of uniaxial strain sequences, and found to replicate well the characteristic patterns of behaviour of amorphous polymers in the temperature region of interest, over a wide range of experimental situations from small to large strains. The only major deficiency, resulting from the simplifying assumption of a single activation barrier height, is its being too localized in the time domain. At high temperatures and long times, applicability of the model is limited by the onset of conformational relaxation.
Comparative study of the molecular species of chloropropanediol diesters and triacylglycerols in milk fat
In an effort to establish the origin of the fatty acid esters of 3-chloropropanediol, which recently have been isolated in small amounts from goat milk, we compared the molecular species composition of the chlorohydrin diesters and of goat milk triacylglycerols. The chloropropanediol diesters were found to be composed of molecular species containing C10−C18 fatty acids and corresponded closely in carbon number to those calculated for the long chain sn-1,2-diacyl-glycerol moieties of goat milk triacylglycerols. The molecular species of goat milk total triacylglycerols contained C4−C18 fatty acids. It is suggested that triacylglycerols and chloropropanediol diesters are derived from the same pool of long chain fatty acids. A molecular distillate of bovine milk fat did not contain chloropropanediol diesters, while the available samples of human milk fat were shown to contain alkyldiacylglycerols as the major components of a neutral lipid fraction corresponding in polarity to the chloropropanediol diesters.