Conformational changes of amorphous poly(ethylene terephthalate) films during uniaxial stretching

Abstract

Rapid scanning Fourier transform infra-red spectroscopy was used to follow the changes in the population of trans conformers and in the infra-red dichroic functions of the bands characterizing the trans (T) and gauche (G) conformers and the chains in the amorphous regions during stretching of a solution-cast amorphous poly(ethy1ene terephthalate) (PET) film A in the glassy state (67 "C) and in the rubbery state (88 "C). Stretching at 67 "C showed a yield stress peak followed by strain softening in the stress-strain curve. The population of trans conformers in the amorphous and crystalline regions, T, + T,, decreased in the pre-yielding region and then increased continuously after yielding. The T conformers formed during post-yield stretching showed high orientation, while the G conformers remained in low degree of orientation.Stretching at 88 "C showed no pronounced yield stress peak in the stress-strain curve, and the population of T conformers did not change until the beginning strain-induced crystallization. A solvent-cast PET film annealed at 67 "C for 24 h, the film N, showed a sharp and high yield stress peak in the stress-strain curve, and strain-induced crystallization occurred at lower draw ratio as compared to the film A. The film A showed a non-equilibrium high population of T, conformers, part of which reverted to G conformers during sub-T, annealing. 0 1991, Hiithig & Wepf Verlag, Basel CCC 0025-1 16X/91/$03.00

The concept of cohesional entanglement

“…It is well known that in the engineering stress-strain curve of an amorphous polymer in its glassy state a stress peak appears on yielding during uni-axial drawing (Fig 7) Sub-T, annealing will enhance the stress peak [23,24]. Even for rubbers of high molar mass at room temperature, being well above their T,, a stress peak on yielding also appeared during uni-axial drawing, for examples natural rubber, cis-1,4-polyisoprene, cis-1,4-polybutadiene and styrene-butadiene rubber [25][26][27].…”

Two-Dimensional Correlation Study of Uniaxially Drawn Poly(Ethylene Terephthalate) Films by Using Attenuated Total Reflection Based Dynamic Compression Modulation Step-Scan Fourier Transform Infrared in Combination with Spectral Simulation Analysis by Density Functional Theory

“…Infrared and Raman spectroscopies are powerful tools for the characterization of polymers and are extensively used for PET analysis. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] Dynamic infrared linear dichroism (DIRLD) 16,17 is a technique for characterizing polymeric systems under a smallamplitude oscillatory mechanical strain. This novel method, also combined with two-dimensional (2D) correlation analysis, 18 has been applied to various polymeric materials including PET 19,20 and other materials.…”

Coil extension stage in the cold drawing of glassy polymers

“…Some of the data in Figure 4 are replotted from Shen et al's research 16. Surprisingly, before the yield point, the trans fraction of the ethylene glycol has a downward trend.…”

“…For the experimental curve, this early occurring transition point observed demonstrates that the coil extension almost stops after the point of λ = 2.3. An earlier study16, 23, 24 showed that under such stretching conditions, PET samples could have 100% recovery in their dimensions at higher temperatures if the drawn ratio was less than the critical value (λ = 2.3). The preceding result also supports the conclusion that the coil extension nearly stops at λ = 2.3 for the case being studied.…”

“…After the yielding and before a critical value (λ ∼ 1.4), the fraction increases and reaches a value that is almost the same as that at the beginning. The pronounced decrease in the trans fraction of the ethylene glycol during preyielding deformation is thought to originate from the nonequilibrium conformation state during the film formation 16…”