Shape memory effect of ethylene–vinyl acetate copolymers

Crosslinked ethylene–vinyl acetate (EVA) copolymers with VA content of 28% by weight were prepared by a two‐step method by evenly dispersing the crosslinking agent (dicumyl peroxide) into the EVA matrix and then crosslinking at elevated temperatures. The crosslinking features of the samples were analyzed by Soxhlet extraction with xylene and dynamic mechanical measurements. All the samples were crystalline at room temperature, and the chemical crosslinks seemed to have little effect on the melting behavior of polyethylene segment crystals in the EVA copolymers. The shape recovery results indicated that only those specimens that had a sufficiently high crosslinking degree (gel content higher than about 30%) were able to show the typical shape memory effect, a large recoverable strain, and a high final recovery rate. The degree of crosslinking can be influenced by the amount of the peroxide and the time and temperature of the reaction. The response temperature of the recovery effect (about 61°C) was related to the melting point of the samples. The EVA shape memory polymer was characterized by its low recovery speed that resulted from the wide melting range of the polyethylene segment crystals. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1063–1070, 1999     

Enhancement of radiation‐resistant effect in ethylene–vinyl acetate copolymers by the formation of ethylene–vinyl acetate copolymers/clay nanocomposites

Ethylene–vinyl acetate (EVA) copolymers/clay nanocomposites, prepared by using nonreactive organophilic clay and reactive organophilic clay, were characterized by X‐ray diffraction and by high‐resolution transmission electron microscopy. The influence of gamma irradiation on the structure and properties of the pure EVA and EVA/clay nanocomposites was systematically investigated. In the presence of gamma radiation, the clay can effectively restrain the increase of the storage modulus of EVA/clay nanocomposites, which was supported by dynamical mechanical analysis. Gamma irradiation had almost no effect on the thermal properties of EVA/clay nanocomposites by using nonreactive organophilic clay, but it obviously improved the thermal stability of EVA/clay nanocomposites by using reactive organophilic clay. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 2532–2538, 2005     

Study of elastic and thermoelastic properties of ethylene–propylene and ethylene–vinyl acetate copolymers. I. Elastic properties

The elastic properties of ethylene–propylene and ethylene–vinyl acetate copolymers crosslinked to different degrees were studied. A correction of the front factor with respect to temperature has been proposed for calculation of the concentration of network chains from shear modulus G. Deviations from the Gaussian approximation of the dependence of force on deformation were evaluated.     

Study of elastic and thermoelastic properties of ethylene–propylene and ethylene–vinyl acetate copolymers. II. Thermoelastic properties

The thermoelastic properties of ethylene–propylene and ethylene–vinyl acetate copolymers crosslinked to different degrees were studied. An equation was proposed for calculating the relative contribution of the internal energy, f_U/f, from the temperature dependence of shear modulus G. Analysis of a relation for calculating f_U/f derived on the basis of the Mooney-Rivlin equation was made.     

The conversion of ethylene–vinyl acetate copolymers with aluminum alcoholates

If ethylene–vinyl acetate copolymers are converted with aluminum alcoholates, crosslinked products will form. This reaction was studied on the basis of the viscosity variation and the amount of acetic acid ester formed. The constants of the reaction velocity for the conversion with various aluminum alcoholates were determined and a reaction mechanism is discussed.     

Nitric acid digestion and crystallinity of ethylene–vinyl acetate copolymers

Nitric acid digestion studies of ethylene–vinyl acetate copolymers indicated that copolymers containing identical amounts of vinyl acetate but varying in melt index differed in crystallinity. These results were confirmed by x-ray analysis. The differences in crystallinity were interpreted as showing a variation in the degree of short-chain branching in the polyethylene segments of the copolymer chain. This variation was correlated with the conditions of synthesis.     

Processing and properties of an ethylene–vinyl acetate blend foam incorporating ethylene–vinyl acetate and polyurethane waste foams

Waste polyurethane foam (w‐PU) and waste ethylene–vinyl acetate foam (w‐EVA) were used as fillers for the production of an ethylene–vinyl acetate (EVA) blend foam. Two different foaming techniques (single‐stage and heat–chill processes) were used for this purpose. The waste foam concentration was varied up to 30 wt % of the original EVA. The physical, mechanical, and morphological properties of the filled foam were studied. The single‐stage process produced blend foams with a lower density and a greater cell size than the foams obtained by the heat–chill process. The density and compression strength of the blend foam increased as the percentage of w‐PU foam increased. However, for the w‐EVA/EVA blend foams, the addition of w‐EVA foam did not significantly affect the density or compression strength compared to the original EVA foams. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44708.     

Kinetics and extent of low-temperature crystallization of ethylene–vinyl acetate copolymers

The enthalpies of fusion of two types of EVA copolymers containing 9 and 16% of vinyl acetate, respectively, were investigated by DSC. After melting, the samples were cooled down and held at 10, 15, 20, and 25°C for different periods of time from 15 min to 2.5 months (only at 20°C). The enthalpy of fusion increased over the 2.5-month period for 9.3 and 11.3 J/g, respectively. There was a new small melting peak on the endotherm of the aged sample whose position and size depended on aging temperature and aging time. During 2.5 months, the peak shifted toward higher temperature for 8°C. The enthalpy of fusion and corresponding degree of crystallinity changed linearly with the logarithm of time, as is the case in high-temperature annealing or secondary crystallization at high temperatures. The rate and the extent of low-temperature crystallization of ethylene copolymers depend on the comonomer content, sequence length distribution, and temperature. © 1996 John Wiley & Sons, Inc.     

The rheological properties of vinyl chloride–vinyl acetate copolymers

Copolymers of vinyl chloride–vinyl acetate have been prepared with different vinyl acetate contents and molecular weights and under different polymerization conditions. A rheological study of these copolymers indicates that they behave in some ways like externally plasticized PVC. For instance, as the vinyl acetate content increases, the melt viscosity decreases, the flow activation energy decreases, and the copolymer becomes more Newtonian. However, the critical shear rate for melt fracture increases, resembling the addition of elastic polymers to PVC. An increase in copolymer molecular weight has a similar effect on the rheological behavior as in PVC, except that the flow activation energy is observed to increase rather than decrease. Decreasing the polymerization temperature affects the flow properties of the copolymer, probably due to changes in degree of branching and crystallinity. A copolymer made by the delayed addition of vinyl chloride, having a more random structure than one made by the conventional batch method, exhibited quite different flow behavior. It had a lower melt viscosity, higher critical shear rate, and lower flow activation energy.     

Preparation and characterization of the ethylene‐vinyl acetate copolymer partially hydrolyzed assisted by microwave radiation

Ethylene‐vinyl acetate copolymer (EVA) was modified by hydrolysis using dielectric heating. The modified EVA was characterized by Fourier transform infrared spectroscopy, differential scanning calorimetry, thermogravimetric analysis (TG), ¹H Nuclear magnetic resonance (NMR), gel permeation chromatography, and small‐angle X‐ray scattering. The results showed that the EVA was hydrolyzed with degree between 36.1 and 42.6% according to ¹H NMR and TG results. The relative reaction rate for the sample prepared in the oil bath was ～9%/h, while in the dielectric was ～150%/h for 15 min. This significant improvement was due to the specific effect of microwave‐assisted reactions through dipole rotation and ionic polarization contributions. The hydrolysis reaction promoted a reduction of the pendent group size of EVA backbone. This change directly contributed to increasing melt temperature, crystallization temperature, melt and crystallization enthalpies. Also, decreased the long period, increased the volume crystallinity, and caused reductions amorphous/crystalline interface and two‐phase model deviations. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44558.     

Toughening effects of ethylene–vinyl acetate copolymers with different vinyl acetate content and viscosity on nylon 1010 blends

Nylon 1010 blends with ethylene–vinyl acetate copolymer (EVA) and maleated ethylene–vinyl acetate (EVA‐g‐MAH) were prepared through melt blending. The vinyl acetate (VA) content and viscosity of EVA significantly affected the notched impact strength of nylon/EVA/EVA‐g‐MAH (80/15/5) blends. The nylon/EVA/EVA‐g‐MAH blends with high notched impact strength (over 60 kJ/m²) were obtained when the VA content in EVA ranged from 28 to 60 wt%. The effect of VA content on the notched impact strength of blends was related to the glass transition temperature for EVA with high VA content and crystallinity for EVA with low VA content. For nylon blends with EVA with the same VA content, low viscosity of EVA led to high notched impact strength. Fracture morphology of nylon/EVA/EVA‐g‐MAH (80/15/5) blends showed that blends with ductile fracture behavior usually had large matrix plastic deformation, which was the main energy dissipation mechanism. A relationship between the notched impact strength and the morphology of nylon/EVA/EVA‐g‐MAH (80/15/5) blends was well correlated by the interparticle distance model. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Photoinitiated crosslinking of ethylene‐vinyl acetate copolymers and characterization of related properties

Photoinitiated crosslinking of EVA in the presence of benzophenone (BP) as photoinitiator and triallyl isocyanurate (TAIC) as crosslinker and characterization of the related properties have been studied by gel determination, heat extension, DSC, DMTA, TGA, and mechanical measurements. The photoinitiated crosslinking efficiency of the EVA‐BP‐TAIC system and various factors affecting the crosslinking process, such as photoinitiator and crosslinker and their concentrations, irradiation temperature, and irradiation atmosphere were studied in detail and optimized by comparison of gel contents. The results show that the EVA samples with a thickness of 1 mm are readily crosslinked to a gel content of above 80% with 5 s UV‐irradiation under optimum conditions. The data from the heat extension and DSC show that the crosslinking density of photocrosslinked EVA increase and their crystallinities decrease with increasing the UV‐irradiation time. At the same time, photocrosslinking of EVA leads to a lowering of the melt temperature and a decrease of heat of fusion. The DMTA results show that photocrosslinking increases the amorphous phase and storage modulus of the crosslinked EVA, but does not change the glass transition temperature. The data from TGA and mechanical tests give evidence that the thermal stability and mechanical properties of photocrosslinked EVA samples are much better than those of the uncrosslinked EVA. POLYM. ENG. SCI., 47:1761–1767, 2007. © 2007 Society of Plastics Engineers     

Hansen solubility parameter: from polyethylene and poly(vinyl acetate) homopolymers to ethylene–vinyl acetate copolymers

The Hansen solubility parameters (HSPs ) of two ethylene–vinyl acetate (EVA ) copolymers (with 18 and 33 wt% of vinyl acetate) and their corresponding homopolymers (polyethylene, PE , and poly(vinyl acetate), PVAc ) have been studied at various temperatures, employing the previously obtained Flory–Huggins parameters. From these latter values, a procedure based on the Hansen solubility spheres theory was employed to determine the HSPs , as well as the radius of interaction. The procedure was validated with literature data, with deviations of around 3%. The HSP values (dispersion, polar and association terms, respectively, all in MPa^{1 /2}) at 333.15 K are 14.84, ?3.88 and 1.78 for PE , 17.65, ?1.24 and 2.76 for EVA410 (with 18 wt% of vinyl acetate), 17.52, 0.15 and 3.61 for EVA460 (with 33 wt% of vinyl acetate) and 19.45, 10.59 and 5.76 for PVAc . The main characteristic of the obtained HSP values is that the high polar term of PVAc tends to increase the solubility character of the pure PE , and thus the EVA copolymers, allowing them to solubilize dispersion and polar compounds. Finally, it was also demonstrated that it is possible to predict the HSPs of EVA copolymers using the vinyl acetate content and the HSPs of pure PE and PVAc as input data. © 2017 Society of Chemical Industry     

Peroxide crosslinking of ethylene–vinyl acetate copolymer

The work reported here concerns the peroxide crosslinking of ethylene–vinyl acetate rubber. Calculated values for scission-to-crosslinking ratios are higher for EVA than for low-density polyethylene. In the temperature range from 150 to 200°C at a constant peroxide content, a rise in temperature results in a decrease in the obtained gel content. Some tensile and modulus–temperature results on crosslinked EVA samples are also reported on.     

Flow properties of molten ethylene–vinyl acetate copolymer and melt fracture

In an investigation of the behavior and formation mechanism of melt fracture the flow properties of molten ethylene–vinyl acetate (EVA) copolymer in the region of high shear rate were measured with a capillary-type rheometer. EVA copolymer differs slightly in flow curve from low-density polyethylene (LDPE); it seems, however, that the difference is due to the difference in molecular weight distribution (MWD) rather than to the materials themselves. The fluidity of molten EVA copolymer having a narrow MWD is equivalent to that of LDPE having a broad MWD and, generally, EVA copolymer has a higher fluidity than LDPE. It is expected that the fluidity increases with incorporation of vinyl acetate at the same MWD and the same M?_w. The critical shear rate increases with melt index and temperature. It cannot be found that the materials themselves and the MWD directly influence the critical point of melt fracture formation when the melt index is taken as a parameter. The critical viscosity (η_c) at which melt fracture forms decreases in an almost straight line with an increase of melt index. It was found from the studies of end correction and behavior of melt fracture formation that melt fracture occurs at the inlet of the die, and it is supposed that the melt fracture formation is caused by the elastic turbulence in the flow pattern due to a failure of recoverable shear strain at the die inlet.     

Rheological characterization of ethylene vinyl acetate copolymers

A series of ethylene vinyl acetate (EVA) copolymers was studied by dynamic mechanical spectroscopy to understand the relative influence of composition, structure, and molecular weight distribution on their rheological behavior in the melt. The examination of their viscoelastic properties in a large temperature range showed that the glass transition temperature is nearly independent on their composition because of the statistical nature of the copolymers, though some long sequence of polyethylene homopolymer may exist at low vinyl acetate (VA) content. The successful use of the time temperature superposition for oscillatory experiments in the melt confirmed the previous remarks, because the application of the Williams Landel Ferry (WLF) equation leads to a unique set of WLF coefficient, whatever the composition of the EVA. This enables the comparison of the rheological behavior in the melt at the same temperature, in the same free volume condition, and at last it was shown that in the terminal zone, the molecular weight distribution is more influent on the behavior of EVA copolymers than their composition. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2625–2630, 1999     

Surface photocrosslinking of ethylene–vinyl acetate copolymer films

Surface photocrosslinking of ethylene–vinyl acetate (EVAc) copolymer films containing benzophenone (BP) was investigated for the purpose of replacing a poly(vinyl chloride) floor. The photogelatin in the EVAc films was effectively observed after UV radiation in the presence of oxygen. The crosslinking reaction was initiated from the surface of the irradiated film, which was mainly due to the dehydrogenation and generation of macroradicals of polymer by the light absorption of BP. The experiments of polyethylene–VAc with BP showed that the VAc‐rich amorphous part in the EVAc copolymer works as a crosslinking site. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 1741–1745, 2000     

Influence of the molecular weight of ethylene vinyl acetate copolymers on the flow and mechanical properties of uncompatibilized polystyrene/ethylene–vinyl acetate copolymer blends

Rubber‐toughened polystyrene (PS) has been extensively studied and is a well‐established material. However, the use of thermoplastic elastomers to toughen PS is new and not well understood. In this study, three types of ethylene vinyl acetate (EVA) copolymers with the same vinyl acetate (VA) content (27.2–28.8 wt %) but with different melt flow indexes (MFI; g (10 min)⁻¹) of 365–440 (Elvax 210), 38.0–48.0 (Elvax 240) and 2.6–3.4 (Elvax 265) were used as impact modifiers for PS. The uncompatibilized blend systems at different compositions were prepared using a twin‐screw extruder and injection moulding to produce the required test pieces. The viscosity of the dispersed phase (EVA) has a significant effect on the mechanical properties of the blends. Rheological studies show that uncompatibilized PS/EVA265 blends exhibit some degree of compatibility when the amount of EVA265 added is below 30 wt %. These results indicate that EVA265 with the lowest melt flow index or highest molecular weight is the most effective impact modifier for PS. The mechanism for such behaviour is still unclear. © 2001 Society of Chemical Industry     

Rheological behavior of partially hydrolyzed poly(vinyl acetate-co-ethylene)

The rheological behavior of partially hydrolyzed poly(vinyl acetate-co-ethylene) (VAE) was investigated, using a Rheometrics Dynamic Mechanical Spectrometer. For the investigation, measurements of storage modulus (G′), loss modulus (G″) and loss tangent (tan δ) of the materials were recorded as functions of temperature at a fixed frequency (i.e., using temperature scans) and, also, as functions of frequency at a fixed temperature (i.e., using frequency scans). For the hydrolysis, three different grades of copolymer (containing 34, 47, 62 mol% of vinyl acetate) were used to yield poly(vinyl acetate-co-ethylene-co-vinyl alcohol) (VAEOH) with varying amounts of hydroxyl group. For comparison purposes, the viscoelastic properties of blends of VAE copolymer with poly(vinyl alcohol) (PVOH), having varying blend compositions, were also measured. The blends were prepared by first mixing a solution of VAE copolymer, which was dissolved in a toluene/dichloroethane mixed solvent, and an aqueous solution of PVOH, and then freeze drying the mixture to a constant weight. Also measured were the glass transition temperature (T_g) of the VAE copolymers and the T_g and melting point (T_m) of the VAEOH terpolymers, using a DuPont Thermal Analyzer equipped with a 910 DSC Module. It has been found that (1) introduction of hydroxyl group into the backbone of the amorphous VAE copolymer has made the resulting VAEOH terpolymer semicrystalline; (2) the T_m, T_g, G′, and the complex viscosity (η^*) of the VAEOH terpolymers increase with increasing amounts of hydroxyl group; (3) the physical blending of PVOH with VAE copolymer has not affected the T_g of the VAE copolymer; (4) the G′ and η^* of the VAE/PVOH blends are found to increase with the amount of PVOH in the blend. The use of logarithmic plots of G′ versus G″ has been found to be very useful for discerning the differences in the structure of the materials tested.     

Effect of damp‐heat aging on the structures and properties of ethylene‐vinyl acetate copolymers with different vinyl acetate contents

We reported herein the damp‐heat aging of ethylene‐vinyl acetate copolymers (EVA) with different vinyl acetate (VAc) contents simultaneously for weeks. The aging was carried out under temperature of 40°C and relative humidity of 93% in air atmosphere. The changes of copolymers' structures and properties were investigated by means of FTIR, wide angle X‐ray diffraction (WAXD) and differential scanning calorimetry (DSC) and mechanical tests. CI values derived from ATR‐FTIR spectra have a decrease when aging time is 1 week and then increase during damp‐heat aging process which suggests the first loss then incorporation of O?C group. WAXD infer that the narrowing trend of FWHM and increase of crystal sizes may attribute to the melting and re‐crystallization of secondary crystallization, which is also confirmed by DSC results. Mechanical tests including Shore A and Shore D hardness, modulus at 100%, tensile strength and elongation at break, are all depending on the primary crystallization and influenced little by damp‐heat aging. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009