F. Rietsch

The compression yield behaviour of polycarbonate over a wide range of strain rates and temperatures

Abstract The compression yield stress behaviour of engineering polycarbonates in the glassy state has been investigated over a wide range of strain rates (10−4–4500 sec−1) and temperatures (−40 to 60°). In this region, the Ree-Erying and Bauwens approach takes into account a distribution of two different relaxation times (α and β). At high strain rate in the glassy range, this model provides evidence on the secondary transition linking the yield behaviour with the β mechanical loss peak.

Thermal and mechanical behaviour of crystalline poly(ethylene terephthalate): effects of high temperature annealing and tensile drawing

Abstract The melting behaviour and several mechanical properties of crystallized poly(ethylene terephthalate) have been investigated paying particular attention to the effect of thermal annealing at high temperature. Although the occurrence of a double melting peak in the differential scanning calorimetry (d.s.c.) curves of isothermally annealed samples can be undeniably ascribed to a recrystallization process during the heating scan, it is suggested that the chain-folded lamellar crystals grown at a moderate crystallization temperature undergo a morphological change into fringed-micelle crystals when submitted to a high temperature annealing. This conclusion is inferred from the striking changes of the mechanical properties of the samples which result from the annealing treatment, in conjunction with the thermal behaviour of drawn samples. This metamorphosis is attributed to ester interchange reactions which turn the chain folds of the amorphous layers into intercrystalline tie molecules at high temperature.

Tensile drawing of poly(aryl ether ether ketone): 1. Birefringence, infra-red dichroism and shrinkage-stress measurements

Abstract Birefringence, infra-red dichroism and shrinkage-stress data of poly(aryl ether ether ketone) (PEEK) uniaxially drawn above the glass transition have been analysed in the framework of the theory of rubber elasticity. It is shown that PEEK chains having a molecular weight Mn ≈ 13000 only contain three entangelements per chain on average. The very low level of shrinkage stress of the samples having a draw ratio below the threshold of strain-induced crystallization is perfectly consistent with a weakly entangled network. The birefringence versus stress curve discloses an orientation-dominated regime at low strains and a deformation-dominated regime starting from the onset of the strain-induced crystallization. The length of the random link, L= 1.8 nm, is in very good agreement with the one that can be theoretically predicted for the stiff-chain polymer relatives, poly(ethylene terephthalate), polycarbonate and poly(phenylene oxide). A value for the intrinsic birefringence of the amorphous phase, Δn°a = 0.38±0.04, is also reported.

Molecular deformation processes in an ethylene copolymer under tensile drawing: static and dynamic study by means of Fourier transform infra-red spectroscopy

Abstract Static and dynamic orientation measurements have been performed by means of Fourier transform infra-red spectroscopy in order to characterize the uniaxial drawing behaviour of an ethylene/1-butene copolymer. The orientational evolution of the crystalline bands in the static study has shown that the plastic deformation probably proceeds by untwisting of the original lamellar ribbons, before the onset of the fibrillar transformation. The assignment of some infra-red-active amorphous conformations with various kinds of chains has been discussed by considering their orientational behaviour. The dynamic study has revealed relaxation processes of the amorphous chains together with strain-induced crystallization, in a range of time up to a few tens of milliseconds after neck completion.

Strain-induced crystallization of oriented poly(ethylene terephthalate): influence on the tensile yield stress behaviour

Abstract The influence of pre-orientation and molecular weight (various viscosities: [η] on the yield stress behaviour has been studied for poly(ethylene terephthalate) by means of uniaxial elongation tests and birefringence measurements. The results show that the amorphous pre-orientation birefringence achieved in hot drawing seems to be the most sensitive parameter to describe the influence of the strain history on the yield stress.

Mechanical behaviour of linear low density polyethylene fibres

Abstract The mechanical behaviour of linear low density polyethylene fibres has been investigated by means of uniaxial elongation tests and thermomechanical measurements. A mechanical model initially introduced by Grubb for HDPE fibres is proposed to account for the experimental results. This model is built on the basis of a high modulus Takayanagi-type model in which the crystalline continuity is interrupted by adjunction of a small fraction of amorphous phase in series with the mechanically active crystals. This fraction is determined by means of an original theoretical treatment of the stress-strain curves of the fibres. Correlations have been set up between this additional parameter and the mechanical properties of the fibres.

A mechanical model for fibres of linear polyethylenes over a wide range of crystallinities

Abstract The Takayanagi-like mechanical model introduced by Grubb for semicrystalline high-modulus fibres is used for linear polyethylenes covering the range of crystal weight fraction 0.28–0.66. This model assumes an interruption of crystal continuity in the mechanically active part of the fibre, which is consistent with the relatively high elasticity of the fibres from medium- and low-crystallinity materials. A novel method is proposed for determining the fraction f of the interrupting amorphous phase from the stress-elongation curves of the fibres assuming a Gaussian behaviour at low strains. A universal relationship is found between f and the compliance of the fibres. The volume fraction of the mechanically active part of the fibres is shown to decrease along with the decreasing crystallinity of the starting materials, indicating a gradual reduced capability to undergo a fibrillar transformation.

Tensile drawing of poly(aryl ether ether ketone): 2. Analysis of the stress-strain-orientation relationships through the Brown and Windle model

The deformation behaviour of amorphous poly(aryl ether ether ketone) films above the glass transition temperature is studied by analysing the amorphous orientation function and shrinkage stress variations versus draw ratio in the framework of the two component model of Brown and Windle which considers separate orientational and extensional contributions to the overall strain. This analysis is in line with our previous conclusion of the existence of two regimes dominated by orientation of rigid structural units and extension of the chains before and after the threshold of the strain-induced crystallization, respectively. Also supported is the disentanglement trend of the chains prior to the crystallization which brings about a physical crosslinking effect to the macromolecular network. The identification of the rigid structural unit with a single monomer unit is remarkably consistent with our previous determination of the size of the random link. The study of isothermally-crystallized samples supports the conclusion about the efficiency of the crystallites for preventing chain slippage. The estimation of the molecular weight between entanglements as a function of draw ratio for both amorphous and crystallized samples suggests that the physical crosslinking effect of the crystallites is much more efficient than the intertwining of the chains.