Strength of Elastomeric Adhesive Films in the Triaxial Stress State

Abstract

In model experiments, the triaxial stress state is realized in thin polymer films placed between two solid surfaces and subjected to extension under the action of a force oriented perpendicular to the interface. In this case, the ultimate (fracture) stresses in adhesive joints of uncured flexible-chain polymers of narrow and wide molecular-mass distributions with solids of various nature have been studied as a function of the rate of loading. The results are in good agreement with the data obtained by investigating the durability of the same materials under the conditions of the triaxial stress state. In the region of cohesive fracture the strength of adhesives of narrow MMD is substantially dependent on molecular mass and temperature. In adhesive tear-off, the strength is noticeably affected by the nature of the support, but it is not practically influenced by the molecular mass of the adhesive. The results of the investigation of the strength properties of thin polymer films in the triaxial stress state have been compared with the data obtained for the same polymers subjected to uniaxial extension and shear flow.     

Properties and Phase Structure of the Poly(propylene)-Low-Density Poly(ethylene) Blends

Abstract

The phase structures of PP-LDPE blends in isotropic and oriented states were studied by the methods of DSC and polarization IR spectroscopy. The composition intervals were established that correspond to the formation of interpenetrating network structures. The presence of LDPE increases the degree of PP orientation and the mechanical strength of the blends. This indicates that LDPE acts as a structure-modifying agent, while PP plays the role of strengthening (reinforcing) filler. A relationship between the deformation and strength properties of the oriented films of the PP-LDPE blend and the phase structure of the PP component is demonstrated. The stability of the blend to oxidation in the ozone-oxygen medium is determined by the structure of a less stable system component (PP). The oxidation rate reaches maximum in samples with an isotropic PP structure and is minimum in the blends with highly oriented PP component.     

Autooxidation of Oriented Polyolefins

Abstract

The effect of the chemical structure of the polymer on its reactivity in the oriented state is studied, and causes of the effect of orientation drawing of polyolefins on the oxidation kinetics are identified.

The orientation drawing of the polymer is shown to be dependent on the nature of the latter which can either delay or accelerate the oxidation process. The effect of a delayed rate of oxidation for the samples of one drawing degree is determined by the nature of the side substitute in the monomeric link of the polymer.

The effect of the orientation drawing of the polymer on the change of parameters of oxidation (k ₂ k ₆ ^?0.5 [RH]), a decrease of the coefficients of solubility and diffusion of oxygen as well as the change in the polymer structure depending on its nature are considered.

Analysis of structural changes in the polymers due to orientation has shown that drawing of the polymer can result in localization of oxidation in defected zones of amorphous areas isolated from each other by transition chains in extremely straightened conformation. In this case, the oxidation kinetics of oriented polyolefins can be described with the use of postulates of the “zone” model developed by Yu. A. Shlyapnikov. Evaluation of the kinetic parameters of oxidation (k _2eff, k _6eff, θ) with the use of this model is made.

It is shown that the effect of the nature of the polymer is due to a change in activity of the C?H bonds participating in the oxidation process, peculiar features of changes in the structure and physical properties of the polymer in the process of orientation drawing, in terms of the “zone” model, to the rate of the expansion of the oxidation center, that is, the nature of the polymer determines the parameters k _2eff, k _6eff, θ in the oriented sample.     

On the Nature of the Induction Stage in Polypropylene Oxidation

Abstract

Propagation of fracture sites in stressed inhibitor containing polypropylene and the macrokinetics of oxidation of isotropic and oriented polypropylene films were studied. Localization of oxidation in the sites of primary initiation was considered. The induction period is given a new interpretation as the time of oxidation localized within the sites of primary high rate initiation at catalytic admixtures. The end of the induction stage is marked by the expansion of the oxidation reaction from the initiation sites and its propagation over the entire polymer. The mechanism of initiation of the kinetic chain process at this stage becomes controlled by the decomposition of hydroperoxides. An increase in induction period observed during orientation is explained by enhanced structure-related localization of the oxidation reaction within the initiation sites owing to morphological changes and by increased rigidity of the polymeric matrix.     

Gamma Radiation-induced Graft Copolymerization of – Phenylmaleimide onto Polypropylene Films

Polypropylene (PP) films were grafted with N-phenylmaleimide using γ-radiation. The effect of both monomer concentration and dose rate on the degree of grafting was measured. Characterization of the grafted PP films included IR spectroscopy, X-ray spectroscopy, electrical conductivity, mechanical properties and thermal stability has been studied. © of SCI.     

Chemilumunescence of Isotactic Polypropylene Induced by UV Irradiation

Abstract

The kinetics of the weak Chemilumunescence (CL) and peroxyl radical decay after UV irradiation of isotactic polypropylene (PP) have been studied. The correlation has been found between CL and the reaction of chain termination under the PP post-oxidation at room temperature in air. Kinetics of CL decay, its compliance with kinetics of peroxyl radical decay, relative quantum yield of CL per one act of radical termination and effects of the plasticizer, accelerating both radical termination and CL decay, have been determined. Kinetic features of CL decay indicates the distributions of peroxyl radicals on both rate constants of termination and quantum yields of CL in the acts of the disproportionation of two peroxyl radicals. In terms of these distributions it is possible to describe quantitatively observed kinetics of CL decay under the post oxidation of PP after the UV irradiation in air.     

Determination of fracture toughness of poly (ethylene terephthalate) film by essential work of fracture and J integral measurements

Abstract

The plane stress fracture toughness of a semicrystalline poly (ethylene terephthalate) (PET) film of thickness 0·125 mm has been measured as a function of specimen size, specimen geometry, loading rate, and temperature using the essential work of fracture (EWF) approach. It was found that the specific essential work of fracture w _e was independent of specimen width, specimen gauge length, and loading rate, but was dependent upon specimen geometry and test temperature. Below the glass transition temperature (93°C), w _e for double edge notched tension (DENT) type specimens was temperature insensitive, but increased with temperature for single edge notched tension (SENT) type specimens. The w _e value for SENT specimens was consistently higher than for DENT specimens. Estimation of w _e via crack opening displacement was reasonable using the relationship w _e = σ_n e _0,y; estimations made via similar type equations were either too high or too low and were generally unsatisfactory. It was found that values of J integral obtained by power law regression and linear extrapolation of the J–R curves to zero crack growth were lower than w _e. The power law regression of the J–R curves with ?a taken as half the crack opening displacement value at maximum load gave J _c values which agreed reasonably well with w _e.     

Polypropylene blends with a phenoxy

Polypropylene (PP) blends with a phenoxy have been prepared in various compositions using a Brabender Plasticorder. Scanning electron microscopy (SEM) micrographs showed a clean two-phase morphology. The domains were larger when PP formed the dispersed phase, probably due to the greater melt viscosity of PP. The melting peak temperature (T_m) of PP decreased marginally, whereas the glass transition temperature (T_g) of phenoxy increased by 5-10°C, in the blends. X-ray diffraction measurements indicated that PP crystallizes in a hexagonal β-form in 50/50 and 30/70 (PP/phenoxy) blends. Mechanical properties of the blends generally showed negative deviation from the additivity rule, perhaps due to the inherently immiscible nature of the blends. © 1996 John Wiley & Sons, Inc.     

Original antifouling strategy: Polypropylene films modified with chitosan-coated silver nanoparticles

A new coating strategy of polypropylene (PP) films with silver nanoparticles (AgNPs) is proposed to obtain surfaces with antifouling properties. As a first step, the photograft polymerization is used to produce polyacrylic acid-grafted PP (PAA-grafted PP) films. A green AgNP synthesis is used by thermal reduction of AgNO₃ with amino groups of chitosan (CS), which controls ion diffusion and stabilizes nanoparticles. AgNP/CS complexes are adsorbed on PAA-grafted PP by electrostatic interactions, yielding AgNP/CS-coated PP films. These films show an excellent antimicrobial activity, even for AgNP contents as low as 0.08 wt %, reducing more than 4 log units in the viable Staphylococcus aureus concentration or inducing Escherichia coli death. This trend is consistent with an adequate amount of small AgNP adsorbed in an organized manner within a thin surface layer. Therefore, the antimicrobial activity of this film seems to be more than promising, used as an active surface for a wide range of applications. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48448.     

Nonisothermal Crystallization Modeling and Simulation for Polypropylene/Nano CaSO4 Composites with Variations in Nanosizes and Wt.% of Loading

Avrami and Ozawa's combined analysis was employed to study the nonisothermal crystallization kinetics of Polypropylene (PP): CaSO₄ (of 12 and 22 nm) composites using a Differential Scanning Calorimeter (DSC). The parameters, such as Avrami's exponent (n) and growth rate constant (Z_t), that characterized the system of different nanosize composites and virgin PP, were determined. The relative degree of crystallinity as a function of temperature for PP/nano CaSO₄ composites at the same cooling rate and the Sigmoidal shape of curves indicate a strong interaction between PP molecules and the nanolayer, which leads to greater nucleation with a reduction in nanosizes. The theoretical combination of kinetic equations was found to be suitable to describe the physical phenomena of real system. The values of parameters n, Z_t and predicted time t for crystallization at a single cooling rate were obtained from the mathematical model.     

Grafting of Hindered Amine Stabilizer in Poly(propylene) Films under γ‐Irradiation

Summary: In this paper, the grafting of a hindered amine stabilizer (HAS) is studied in isotactic poly(propylene) (PP) films under γ‐irradiation. The HAS used has a definite structure that combines a hindered amine functionality and a UV‐absorbing unit (benzylidene malonate ester group) detectable at 308 nm in the UV spectrum of PP film and 314 nm in chloroform. The stabilizer is added to the polymer at various concentration ratios: 0.1, 0.2, and 0.3 wt.‐%. The percentage of HAS grafting in the PP film at various additive concentrations is determined as a function of γ‐radiation dose in the range of 0–100 kGy by direct spectroscopic measurements through the absorption band of the stabilizer in the UV spectra of the PP film. The percentage of free HAS extracted with chloroform from the PP film versus the radiation dose is determined by UV spectroscopy for all the additive concentrations used. This study reveals that only 80% of the HAS is grafted on the 100 kGy irradiated PP matrix independent of the additive concentration used. However, the percentage of HAS grafted on PP films displays an exponential dependence on γ‐radiation dose. These results are consistent with the data obtained on the free HAS content. γ‐Irradiation grafting of HAS in the PP is accompanied by the oxidative degradation of the polymer substrate that is evaluated by increasing the carbonyl index and reducing significantly the oxidation induction time of the PP films.

The percentage of hindered amine stabilizer grafted to the PP film as a function of γ‐radiation dose.     

Photooxidation of polypropylene under natural and accelerated weathering conditions

The environmental and accelerated photodegradation of two polypropylene (PP) films (high and low crystallinity) were investigated. FT-IR measurements coupled with derivatization reactions (NO and SF₄), elongation at break tests and gel permeation chromatography technique were used to monitor the degree of oxidation during the UV exposure. No stoichiometry changes were observed under both UV conditions for the PP films. The dominant photoproduct, i.e., ketone, was suggested to be produced from peroxy radicals. The low concentration of tert-alcohol detected questioned the validity of its formation from hydroperoxide decomposition as suggested by several authors. Under natural exposure, the effect of degradation on mechanical property was different for high and low PP. High PP was useless after a short exposure time; low PP retained its physical property for a longer period of time. In the latter case, the analysis of the changes in elongation at break, the polydispersity P = M_w/M_n, the number of chain scissions N_t, and the kinetic accumulation of photoproducts provided strong indications on the mechanisms of acid and ester production and shed some light on the competition between crosslinking and chain scission reactions. The estimated acceleration factors were 7.5 and 8.5 for high and low PP, respectively, values that were much lower than those obtained for linear low-density polyethylene. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 2497–2503, 1997     

Nonisothermal Crystallization Kinetics of Polypropylene/Polyethersulfone Blend

The nonisothermal crystallization kinetics of PP and PP/PES (80/20 wt%) blend was investigated by using differential scanning calorimetry (DSC). It was observed that the crystallization peak temperature (T_p) and the half time (t _1/2) of crystallization of PP/PES blend are slightly but consistently lower than those of PP at various cooling rates. The nonisothermal crystallization data were analyzed by using Avrami equation, Ozawa and Mo method. The validity of the different kinetics models to the nonisothermal crystallization process of two samples is discussed. The Mo method can successfully explain the overall nonisothermal crystallization process of PP and PP/PES blend. The activation energy (ΔE) for nonisothermal crystallization of PP and PP/PES blend is determined by using the Kissinger method. The result shows that the ΔE value of PP is slightly higher than that of PP/PES blend.     

Crystallographic Orientation Dependence on Electrical Properties of (Bi,Na)TiO3‐based Thin Films

Orientation‐engineered (La, Ce) cosubstituted 0.94(Bi_0.5Na_0.5)TiO₃–0.06BaTiO₃ thin films were epitaxially deposited on CaRuO₃ buffered (LaAlO₃)_0.3(Sr₂AlTaO₆)_0.35 single‐crystal substrates by pulsed laser deposition. The ferroelectric, piezoelectric, dielectric, and leakage current characteristics of the thin films were significantly affected by the crystallographic orientation. We found that the (001)‐oriented film exhibited the best ferroelectric properties with remnant polarization P_r = 29.5 μC/cm² and coercive field E_c = 7.4 kV/mm, whereas the (111)‐oriented film demonstrated the largest piezoelectric response and dielectric permittivity. The bipolar resistive switching behavior, which is predominantly attributed to a combined effect of ferroelectric switching and formation/rupture of conductive filaments, was observed. The conduction mechanisms were determined to be ohmic conduction and Poole–Frenkel emission at high‐ and low‐resistance states, respectively, in all the films.     

Impact testing of polypropylene blends and comosites

Polypropylene (PP) composites containing 0.30 vol% of talc filler, in addtion to blends modified with an ethylene-propylene copolymer (EPR) elastomer were prepared and their fracture resistance was determined by the standard Ixod impact test and by a fracture mechanics technique. Effects of composition, type of modification, specimen size, and temperature were studied. The validity of linear elastic freacture mechanics (LEFM) conditions were checked: It was shown that under the conditions applied they can be satisfied even twith specimens of reasonable size (4 × 10 × 80 mm) prepared by conventional processing techniques. Calculations of minimum specimen thickness must be carried out, with material properties obtained under the conditions of impact. For heterogeneous blends and composites yield stress should be corrected for the effect of decreasing load-bearing cross section. Linearity of the fracture energy (U) vs. BD? or U vs. B(D - a) plots is not a proof for either elastic of plastic fracture. The composition dependence of fracture properties proved to be practicaly independent of specimen size, temperature, or measurement technique.     

Effect of Laser Radiation on Dyed Polymeric Films

Abstract

Destruction of the thin dyed polymeric films under the action of resonant and non-resonant laser radiation has been investigated. It is shown thai laser stability of dyed polymers depends on the type of incorporated dye and it is independent of film thickness and dye concentration at the same powers of absorbed laser radiation. Moreover, laser stability of polymeric films with chemically bonded dye is higher than the polymer stability with dissolved dye. Comparative analysis showed that the polymer laser stability is determined by the kinetics of the polymer thermodegradation. which results in the appearance and accumulation of the products absorbing laser radiation. It is established that resonant irradiation of dyed polymers is considerably more efficient for polymer destruction than the nonresonant one.     

Chemiluminescence of polyethylene induced by UV irradiation

Summary The chemiluminescence (CL) of polyethylene (PE) film induced by UV irradiation was investigated. CL parameter, I₀, which indicates the oxidation rate under UV irradiation was obtained from a kinetic analysis. The I₀ was found to be valuable to evaluate the durability of PE films.     

Crystallization Atmosphere and Substrate Effects on the Phase and Texture of Chemical Solution Deposited Strontium Niobate Thin Films

Strontium niobate (Sr:Nb  =  1:1) thin films were prepared via chemical solution deposition on (001)‐oriented SrTiO₃, (001)_p‐oriented LaAlO₃, (0001)‐oriented sapphire, and polycrystalline alumina substrates. Crystallization in oxygen at 1000°C yielded Sr₂Nb₂O₇ films on all substrates with strong (010) orientation. Films on LaAlO₃ and SrTiO₃ single‐crystal substrates possessed a small amount of preferred in‐plane orientation, whereas films prepared on sapphire and polycrystalline alumina substrates were fiber textured. Films crystallized at 900°C in a low oxygen atmosphere (~10 ^{? 21} atm pO₂) formed a randomly oriented polycrystalline perovskite, SrNbO_3?δ on all substrates. A similar set of films crystallized at 900°C at a slightly higher oxygen partial pressure (~10^?15 atm pO₂) was comprised of Sr₂Nb₂O₇ and SrNbO_3?δ phases, exposing the dependence of phase formation on oxygen partial pressure. When subjected to a high‐temperature anneal in oxygen, the SrNbO_3?δ phase is shown to transform into Sr₂Nb₂O₇, however, Sr₂Nb₂O₇ did not significantly reverse transform into SrNbO_3?δ after annealing in low oxygen partial pressure atmospheres.     

Evaluation of impact fracture toughness of polymeric materials by means of the J‐integral approach

The most commonly accepted method of determining impact fracture toughness of polymeric materials that exhibit small scale yielding and negligible influence of dynamic effects is given by the ISO/DIS 17281 Standard, which states that for brittle behavior, basically a linear relationship exists between the fracture energy, U, and the energy calibration factor, ?. This relationship allows calculation of the critical strain energy release rate, G_IC from the slope of the U vs. BW? plot. This paper describes a simpler alternative methodology capable of evaluating impact fracture toughness using the J_c parameter. The J‐integral is evaluated at the instability load point, by calculating the fracture energy required to produce cleavage behavior of a pre‐cracked specimen. The methodology is limited to single edge notched three‐point‐bending specimens with a crack to depth ratio equal to 0.5. Tests were carried out on an instrumented falling weight impact testing machine on the following materials: PP (polypropylene), HDPE (high‐density polyethylene), MDPE (mid‐density polyethlene) and RT‐PMMA (rubber toughened polymethylmetacrylate). Results are in excellent agreement with the critical values determined by the ISO/DIS 17281 Standard.     

Polypropylene/calcium carbonate nanocomposites

Polypropylene (PP) and calcium carbonate nanocomposites were prepared by melt mixing in a Haake mixer. The average primary particle size of the CaCO₃ nanoparticles was measured to be about 44 nm. The dispersion of the CaCO₃ nanoparticles in PP was good for filler content below 9.2 vol%. Differential scanning calorimetry (DSC) results indicated that the CaCO₃ nanoparticles are a very effective nucleating agent for PP. Tensile tests showed that the modulus of the nanocomposites increased by approximately 85%, while the ultimate stress and strain, as well as yield stress and strain were not much affected by the presence of CaCO₃ nanoparticles. The results of the tensile test can be explained by the presence of the two-counter balancing forces—the reinforcing effect of the CaCO₃ nanoparticles and the decrease in spherulite size of the PP. Izod impact tests suggested that the incorporation of CaCO₃ nanoparticles in PP has significantly increased its impact strength by approximately 300%. J-integral tests showed a dramatic 500% increase in the notched fracture toughness. Micrographs of scanning electron microscopy revealed the absence of spherulitic structure for the PP matrix. In addition, DSC results indicated the presence of a small amount of β phase PP after the addition of the calcium carbonate nanoparticles. We believe that the large number of CaCO₃ nanoparticles can act as stress concentration sites, which can promote cavitation at the particle-polymer boundaries during loading. The cavitation can release the plastic constraints and trigger mass plastic deformation of the matrix, leading to much improved fracture toughness.