Mechanical Properties Of Amorphous Polymers Research Articles

A viscoelastic-viscoplastic constitutive model and its finite element implementation of amorphous polymers

The amorphous polymers present remarkable temperature- and rate-dependent deformation behaviors. Based on a combination of the multiple relaxation viscoelastic-viscoplastic model and the three-element viscoelastic model, a constitutive model was constructed to describe the changes in mechanical properties of amorphous polymers from below to above glass transition temperature (θg). In this model, an exponential evolution equation of volume fraction was constructed to reflect the changes in glassy and rubbery phases at different temperatures. The proposed model was implemented into ABAQUS using the user-defined material subroutine (VUMAT). The strain-softening after yield and rate-dependent behaviors above and below θg were reasonably captured by the present model. Meanwhile, the creep and relaxation behaviors of the material were described. Finally, the processes of the tensile deformation of a dumbbell plate with a circular hole and the rate-dependent pressed film molding were simulated by the VUMAT. The results show that the implemented model can reasonably predict the structural responses of amorphous polymers.

Impact of structural relaxation on mechanical properties of amorphous polymers

Fusion based methods, such as hot-melt extrusion, are a common way of preparing amorphous solid dispersions. Since the amorphous glass, however, is not in a configurational equilibrium, the molecular arrangement of the obtained material can differ in dependence of the preparation conditions. Although the changes in the configuration of an amorphous material, which are commonly referred to as structural relaxation or physical aging, are well investigated, the impact on mechanical properties of amorphous solid dispersions have widely been neglected so far. The presented study investigated copovidone as a model polymer commonly used in amorphous solid dispersions and revealed that structural relaxation was already introduced into the polymer during hot-melt extrusion while its degree was cooling rate dependent. The degree of structural relaxation significantly affected the mechanical properties of copovidone as assessed by diametral compression tests, macroindentation and nanoindentation. An increase in Young’s modulus and indentation hardness was observable with a higher degree of structural relaxation, which, during tablet compression, translated into tablets with significantly lower tensile strength. Furthermore, evaluation of the force-displacement curves during tablet compression revealed a decreased proportion of irreversible deformation with higher degree of structural relaxation correlating well with the increased indentation hardness during macroindentation. Thus, understanding structural relaxation and its impact on material properties is of utmost importance to assess the processability and compaction performance of amorphous solid dispersions in dependence of their preparation conditions and thermal history.

Nonmonotonic dependence of polymer-glass mechanical response on chain bending stiffness.

We investigate the mechanical properties of amorphous polymers by means of coarse-grained simulations and nonaffine lattice dynamics theory. A small increase of polymer chain bending stiffness leads first to softening of the material, while hardening happens only upon further strengthening of the backbones. This nonmonotonic variation of the storage modulus G^{'} with bending stiffness is caused by a competition between additional resistance to deformation offered by stiffer backbones and decreased density of the material due to a necessary decrease in monomer-monomer coordination. This counterintuitive finding suggests that the strength of polymer glasses may in some circumstances be enhanced by softening the bending of constituent chains.

The Influence of Crosslink Density on the Failure Behavior in Amorphous Polymers by Molecular Dynamics Simulations.

The crosslink density plays a key role in the mechanical response of the amorphous polymers in previous experiments. However, the mechanism of the influence is still not clear. In this paper, the influence of crosslink density on the failure behavior under tension and shear in amorphous polymers is systematically studied using molecular dynamics simulations. The present results indicate that the ultimate stresses and the broken ratios (the broken bond number to all polymer chain number ratios) increase, as well as the ultimate strains decrease with increasing crosslink density. The strain concentration is clearer with the increase of crosslink density. In other words, a higher crosslink density leads to a higher strain concentration. Hence, the higher strain concentration further reduces the fracture strain. This study implies that the mechanical properties of amorphous polymers can be dominated for different applications by altering the molecular architecture.

The Influence of Molecular Orientation on the Yield and Post‐Yield Response of Injection‐Molded Polycarbonate

AbstractThe influence of the flow history experienced during injection molding on the mechanical properties of amorphous polymers is investigated. It is demonstrated that flow‐induced molecular orientation only causes a small anisotropic effect on the yield stress, which can be regarded as insignificant with respect to its absolute value. Its influence on the post‐yield strain‐hardening response is also shown to be imperceptible, in contrast to a orientation which is applied during deformation below the glas transition.magnified image

Near-surface mechanical properties of amorphous polymers

Abstract Polymeric material near a free surface can have properties which deviate considerably from the bulk properties. Many researchers have reported a reduced glass transition temperature in thin polymeric films and attributed this effect to an enhanced segmental mobility near a free surface. It was also reported that sufficiently thin polymeric structures show a higher ductility than the bulk material. In this paper, we therefore investigate the hypothesis that the near-surface mechanical properties of amorphous polymers differ from the bulk properties owing to the presence of an absolute length scale. Microindentations and nanoindentations are performed on polystyrene, using a range of indenter sizes and indentation loads. In addition, numerical simulations are carried out with an advanced material model for polystyrene. A comparison between the experimental and numerical results indeed indicates that a length-scale effect is present near the surface. Simulations performed at an elevated temperature indicate that our results are consistent with the observations of a reduced T g.

Near-surface mechanical properties of amorphous polymers

Near-surface mechanical properties of amorphous polymers

Stress/strain analysis of polymeric materials by molecular dynamics simulations

AbstractMechanical properties number themselves among the most important properties of any polymeric material used in engineering applications. The Molecular Mechanics approach has been used to calculate the mechanical moduli of various polymers, but this approach has the limitation of requiring experimental densities and neglecting both entropy contributions and local volume fluctuations which can be significant in condensed phase polymers such as glasses and melts. Although constant stress molecular mechanics can provide an estimate of the bulk density, constant stress molecular dynamics (CSMD) can overcome all these limitations. However, CSMD is limited by an inherently short simulation time scale. CSMD was used to calculate the bulk moduli, Young's moduli and density of various amorphous polymers, including: polyethylene, poly(propylene), polystyrene, poly(vinyl chloride), poly(ethylene terephthalate), poly(butylene terephthalate), nylon 6, poly(methyl methacrylate), polydimethylsiloxane, bisphenol‐A polycarbonate, poly(phenylene sulfide). Results indicate that CSMD is a useful tool in characterizing mechanical properties of amorphous polymers despite this time scale limitation.

Molecular interpretation of T∥, the rubbery-viscous ‘transition’ temperature of amorphous polymers

Many physical and mechanical properties of amorphous polymers show anomalous behaviour at a temperature above T g , designated T ∥, the ‘liquid-liquid transition temperature’. It is suggested here that T ∥ can be considered to be the rubbery-viscous ‘transition’ temperature of amorphous polymers, analogous to T g , the glassy-rubbery transition temperature. At T ∥, the viscous dissipation goes through a maximum due to a shift in response of the molecules to stress from a coiling-uncoiling process (elastic rubbery region) to slippage between the molecules (viscous region). It is argued here that this maximum is responsible for the occurrence of a maximum in log(decrement) traces obtained by torsional braid analysis (t.b.a.) above T g .

Polymer property enhancement by solid‐state forming

AbstractThe alteration of the physical properties of a polymer by the use of forming techniques which are applied below the melt or glass transition temperature of the polymer has been discussed. The influence of cold rolling (rolling accomplished at room temperature) on the mechanical properties of amorphous polymers was first briefly reviewed (1). The specific effect of rolling on the toughness of polycarbonates, as measured by notched Izod impact strength, was discussed in detail and is summarized in (2). The role of crystallinity in further enhancing the mechanical properties of polycarbonate and the combined effect of rolling and crystallinity was also described and is discussed in more detail in (3). Other amorphous polymers were also discussed, particularly the effect of rolling on the notched Izod impact strength (4). For most polymers studied, a large increase in impact strength occurs for small reductions in thickness and a maximum occurs at a given thickness reduction beyond which the ductility of the material is exhausted. The increase in impact strength is not only a result of molecular orientation but also a function of residual stress placed in the sheet by the rolling process.In addition to the studies on amorphous polymers, recent studies (5, 6) were described on the high‐pressure nonisothermal compression molding of linear polyethylenes. Mold pressures up to 80,000 psi were used and several different polyethylenes having various molecular weights (up to 106) were investigated. It was shown that the modulus of elasticity of polyethylene can be increased by a factor of two when the mold pressure is increased to 80,000 psi. A detailed discussion of the changes in morphology responsible for this increased stiffness is presented in (6).

無定形高分子の動的粘弾性に及ぼす充てん材の効果

ポリメタクリル酸メチル (PMMA), ポリ酢酸ビニル (PVAc) およびポリスチレン (PS) に, ガラスビーズおよびマイカを充てんさせた系について, 動的粘弾性研究を行った. 充てんによる主分散温度の上昇 (ΔTα) は, 高分子一充てん材系によって異なる. 換算変数法によって合成曲線を求め, 分子鎖の屈曲性と関係のあるパラメーターnをlog E′ (υaT) = (log E1+log E2) /2の値における傾斜より求めた (E1およびE2は, それぞれlogυaT=6および1における値を示す). 未充てん試料からのnの減少率とΔTαは, 同じ充てん材のとき, 高分子の種類と無関係に直線関係によって与えられる. 活性化エネルギーの温度依存性および緩和スペクトルから, 充てん材による高分子運動の束縛の程度は, 充てん材界面と高分子鎖との接着に依存すること, そして充てん材界面の影響を受ける高分子マトリックスの範囲はかなり大きいことが判明した.

Deformation and failure of plastics and elastomers

AbstractThe effect of various environmental conditions, other than chemical, on the mechanical properties of amorphous polymers is considered. The response of such polymers to an applied stress or strain depends on the mobility of polymeric chains which in turn is largely a function of free volume. The free volume varies with temperature, pressure, and plasticizer concentration; in the glassy state, it also depends on thermal history and tensile strain. These concepts are reviewed and some illustrative data, primarily from the published literature, are presented. A discussion is also given of the nonlinear viscoelastic properties (in uniaxial tension) of amorphous network polymers in the rubbery state. Methods for characterizing the ultimate tensile properties of such polymers are also described. Finally, some comments are made about the mechnical characteristics of polymers in the glassy state.

Dependence of the Ultimate Properties of a SBR Rubber on Strain Rate and Temperature

Abstract The tensile strength and ultimate elongation of polymeric materials depend on both the temperature and experimental time scale. The mechanical properties of amorphous polymers at temperatures above their glass transition temperature Tg are more amenable to treatment in terms of molecular theories than are their mechanical properties at temperatures below Tg or the mechanical properties of crystalline polymers. For amorphous polymers at temperatures above Tg the viscoelastic properties in small deformations have been studied rather extensively, and several molecular theories—essentially identical—have been published. In contrast, few systematic studies have been made of the effect of time and temperature on the ultimate properties. Consequently, only a limited amount of data is available which can serve as a basis for developing and verifying molecular theories dealing with ultimate properties. A recent theory by F. Bueche treats the time and temperature dependence of tensile strength. According to his theory, the tensile strength for a given material is a universal function of a reduced time or a reduced strain rate, except at short times or high strain rates where the material approaches glasslike behavior. Also, to superpose data measured at different temperatures, a shift factor is needed which is determined by the temperature dependence of the frictional factor for polymeric segment mobility and thus is the same factor as used to superpose viscoelastic data measured in small deformations. Thus, according to Bueche's theory, the temperature dependence of the tensile strength is given by the equation of Williams, Landel, and Ferry which is applicable in the temperature range Tg<T<(Tg+100). Bueche reported some tensile data for polybutyl methacrylate which has a glass transition temperature of 8° C. These tensile data were measured under various constant loads at temperatures between 30 and 95° C, and reasonable agreement between theory and experiment was found. Although Bueche did not consider the ultimate elongation, it seems reasonable that such data can be superposed by using the same shift factor as required to superpose the tensile strength data. Other workers have not considered explicitly the effect of viscous forces on the ultimate properties but have considered the effect of such variables as molecular weight, degree of crosslinking, and plasticizers.