The effect of tensile drawing on the structure and relaxation processes in vinyl alcohol–ethylene copolymers (original) (raw)
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Polymer, 2000
Viscoelastic relaxations of three samples of vinyl alcohol-ethylene copolymers, richer in the former comonomer, were studied in a wide range of temperature. The temperature location, intensity and apparent activation energy of the distinct relaxations found are discussed and compared with those of the homopolymers, poly(vinyl alcohol) and polyethylene. Differential scanning calorimetry and X-ray diffraction results of the specimens are also discussed in the frame of the dynamic mechanical analysis, showing that the polymorphism exhibited in some copolymers is a result of the thermal treatment. ᭧
The influence of a tensile drawing on the crystalline organisation of ethylene-vinyl acetate copolymers (EVA) of different molecular weights has been investigated by differential scanning calorimetry (d.s.c.). The consequence of the crystalline organization changes on the mobility of the amorphous phase has been also investigated, through the study of the glass transition. The results show that more disorganized crystals appear after the tensile test, specially for the higher molecular weight. These more disorganized crystals constitute physical ties reducing the mobility of the amorphous chains which link them, as shown by the significant increase of the glass transition temperature of these high molecular weight EVA after tensile drawing.
Journal of Polymer Science Part B: Polymer Physics, 2000
Vinyl alcohol-ethylene (VAE) copolymers, commercially manufactured by hydrolysis of the corresponding vinyl acetate-ethylene copolymers, can contain small amounts of unhydrolyzed vinyl acetate. This article shows the influence of these residual groups on the structure of the resulting copolymers, studied by nuclear magnetic resonance and wide-angle X-ray scattering. Thermal and mechanical properties of these materials were investigated by differential scanning calorimetry, thermogravimetry, drawing behavior, birefringence measurements, and dynamic mechanical analysis. The structure of the copolymers is considerably affected by the volume of the residual acetate groups, bigger than that of the hydroxyl ones, which hinders the crystallization process. In relation to the thermal and mechanical properties, the temperature and enthalpy of melting as well as the Young's modulus and yield stress, decrease as vinyl acetate molar fraction increases. Moreover, the ␣ and  relaxations are shifted to lower temperatures as residual content in the copolymer is raised.
Polymer, 2004
Some structural details and the viscoelastic and mechanical response of blends of a vinyl alcohol-ethylene (VAE), and a metallocenic ethylene-1-octene copolymer (CEO), have been analyzed. Both copolymers exhibit crystalline lattices whose diffraction peaks and long spacings appear at very similar spacing intervals. More information about the crystalline region is obtained from differential scanning calorimetry (DSC) measurements due to the difference in melting temperatures found for each of them. In addition, DSC results point out an inhibition of the VAE crystallization with CEO presence that is cooling-rate dependent. A decrease of rigidity and yield stress is observed as CEO content increases in the blends. However, the changes found in the mechanical parameters are not as significant as the variation in oxygen permeability. This feature seems to be due to the disruption of intra and intermolecular hydrogen interactions. q
The role of chain structure in the rheological behavior of vinyl acetate-vinyl alcohol copolymers
Polymer Science Series A, 2014
A comparative study of the viscoelastic properties of melts of vinyl acetate-vinyl alcohol copolymers with equimolar compositions characterized by different statistical distributions of chain units has been performed. It has been shown that the principle of temperature-frequency superposition is obeyed by copolymers close to a random copolymer, but is violated by copolymers with the block distribution of units. Unlike amorphous random copolymers, a multiblock copolymer is characterized by weak crystallinity, the absence of the relaxation flow state, and a more pronounced tendency to form interchain hydrogen bonds both between two hydroxyl groups and between hydroxyl and ester groups.
Mechanical properties and dynamic mechanical relaxations of ethylene/alpha‐olefin copolymers
British Polymer Journal
The tensile drawing behaviour and the viscoelastic relaxations of three different copolymers of ethylene, containing I-butene, 1-hexene and 4-methyl-lpentene as comonomer, have been studied. The composition of the copolymers is increasingly poorer in comonomer following the above order. However, they show similar mechanical properties, both large-strain and small-strain dynamic mechanical, because the minor differences which have been found can generally be explained by the thermal and mechanical histories of the samples. Thus, it seems that the higher disrupting effect produced in the copolymer structure by the comonomers of greater size and volume is counterbalanced by the corresponding smaller concentration, and that only thermal treatment produces appreciable differences in properties.
Journal of Materials Science, 1990
The tensile drawing behaviour and the dynamic mechanical properties of four ethylene-1butene copolymers, with nearly the same comonomer content but obtained with slightly different catalyst systems, have been analysed. It was found that the large strain mechanical properties and the strain-hardening behaviour seem to be affected by the degree of homogeneity in the distribution of comonomer along the different chains, reflecting the differences in the catalyst systems used. However, a parallel trend was not so clear for the small-strain dynamic mechanical properties for a particular thermal history, although the dependence of the relaxations on the crystallization conditions is evident.
Viscoelasticity Evolution of Ethylene-vinyl-acetate Copolymers During Crystallization
Chemical Engineering Transactions, 2019
In polymer processing as fibre spinning, injection moulding, film casting and so on, the forming phase of the melt is followed by its solidification due to cooling. This implies the crystallization of the polymer during the processing. Typically, crystallites are formed and an ordered structure can be detected, if the cooling rate is not too high. As a consequence, during processing the rheological response of the material significantly changes from a liquid-like to a solid-like behaviour. The knowledge of the mechanical response of the material during crystallization is therefore of seminal importance for process control and modelling. We here focus on a random copolymer, the Ethylene vinyl acetate (EVA), made of ethylene and varying amounts of vinyl acetate that interfere with poly-ethylene chain packing reducing the crystallinity, improving the transparency and lowering the melting temperature. This allows studying the crystallization kinetics at relatively low temperatures so to...
Macromolecules, 2014
Bulk and solution studies revealed a strongly pronounced effect of chain structure on the rheological and relaxation behavior of well-characterized vinyl acetate–vinyl alcohol copolymers of similar composition and polymerization degree. The frequency–temperature superposition principle is fully applicable to the random copolymers, which demonstrate all expected relaxation states, whereas a divergence of the reduced dynamic moduli–frequency dependences is observed for the multiblock copolymers. In the latter case, the terminal zone is sensitive to the self-assembling of vinyl alcohol blocks into (depending on the copolymer composition) crystalline or amorphous microstructures. The monomer unit distribution particularly affects properties of the copolymer solutions in N,N-dimethylformamide (DMF). 5% solutions behave as simple viscoelastic liquids at 20 °C and show viscoplastic behavior at −20 °C, where more blocky chains are characterized by up to 4 orders of magnitude higher yield stress values. The multiblock copolymer solutions demonstrate a pronounced viscosity hysteresis in the heating–cooling cycle, being absent in the random copolymers. 10% solutions of multiblock copolymers are practically gelatinous even at room temperature. The observed effects are explained by examining the peculiarities of hydrogen bonding in vinyl acetate–vinyl alcohol copolymers using FTIR spectroscopy. The multiblock copolymers are characterized by stronger hydroxyl–hydroxyl H-bonds and greater fraction of interchain hydroxyl–acetyloxy H-bonds providing aggregation of chains and high viscosity of the corresponding samples, whereas the random copolymers more strongly interact with the residual solvent. Dynamic light scattering studies prove that the relaxation of concentration fluctuations is completely diffusive, being bimodal in the random copolymers and trimodal in the multiblock ones. The fast mode in the latter case demonstrates anomalous concentration behavior. In the dilute regime, up to very low concentrations, multiblock copolymer chains form stable aggregates, and this fact correlates with an unusual growth of the reduced viscosity in the corresponding rheological experiments.
2001
Low-density polyethylene (LDPE) has been the subject of several rheological studies trying to correlate rheological features with molecular parameters. As the ethylene vinyl acetate copolymer (EVA-c) and LDPE are obtained through the same polymerization process, it has been long assumed that both materials would exhibit a similar viscoelastic response. However, a recent work has put forward the question as to whether the presence of vinyl acetate (VAc) units could modify the expected viscoelastic behaviour. In order to further explore this effect, we have carried out a dynamic viscoelastic study in a series of seven EVA-c. At a ®rst step, it will be shown that EVA-c presents a thermorheological complex behaviour. It will be shown that Newtonian viscosity (h 0) values are conspicuously higher than those reported for LDPE of similar molecular weight (M w). Furthermore, a concurrent lower elastic character of the EVA-c samples is observed. The results are discussed in terms of molecular architecture and molecular weight distribution (MWD). However, a ®nal explicit VAc content dependence of the viscoelastic properties up to 30% by weight of this comonomer is not observed.