Periodic crazing on polymethylmethacrylate film by localized bending

Crazing was induced on a polymethylmethacrylate film using an original, low‐cost, simple process in which the film was bent using a wedge‐shaped plate (referred to herein as a crazing edge, or an edge). In this process, stress concentration and stress opening occur spontaneously by feeding the film past the edge, and crazing is periodically induced (periodic craze). The craze interval, width, and depth depend on the localized bending conditions at the tip of the edge. Two different types of crazing mechanism were suggested. It was assumed that the films were either crazed by bending along the tip of the edge or by bending according to the curvature induced by the three‐point bending system. The calculated maximum stress determined by the three‐point bending process and the experimental values of the craze interval and width showed similar tendencies. These results suggested that the craze depth could be determined using this bending process and known mechanical properties and crazing conditions. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Kinetics of crazing in polybutadiene/polystyrene blends

The crazing behavior of blends of polystyrene (PS) and a low molecular weight polybutadiene (PB) was examined as part of a continuing study of toughening mechanisms in thermoplastic polymers. These PS/PB blends attain high levels of toughness from the stress-activated plasticizing action of the polybutadiene (PB), a mechanism that is active only in the region of a growing craze. The plasticization is therefore localized and leads to enhanced toughness without loss in stiffness. The net result of this plasticization is a reduction in craze flow stress accompanied by an increase in craze velocity, which, in turn, allows the specimen to reach substantial strains-to-fracture in uniaxial deformation under an imposed strain rate. The ability of the PB to plasticize a growing craze is expected to be a function of the mobility of the PB. To investigate the role of diluent mobility, tensile tests and craze velocity measurements were conducted at ?20°C and compared with previously published results collected at 23°C. Although the blends displayed high levels of toughness at 23°C, the blends tested at ?20°C showed brittle behavior. Craze velocities measured at ?20°C were 2 orders of magnitude lower than the results at 23°C. Addition of 3 wt % PB at ?20°C led to craze velocities only as large as those found in pure PS at 23°C. Comparison of the craze velocities with an asymptotic theoretical model describing the dependence of the craze velocity on the PB content showed good agreement with the results at ?20°C and only fair agreement with the room-temperature results. © 1994 John Wiley & Sons, Inc.     

Effect of injection pressure and crazing on internal stresses in injection-moulded polystyrene

The influence of injection pressure on the internal stress distribution in injection-moulded polystyrene bars has been found to be rather small using the layer removal method. On the other hand, substantial differences in the stress relaxation behaviour have been discovered in specimens moulded at pressures in the range 37–143 MPa. The Kubát and Rigdahl internal stress parameter for all sets of specimens was numerically fairly small, but changed from negative to positive on increasing the injection pressure. The parameter most sensitive to variations in injection pressure appears to be the index in the power law expression used to describe the stress relaxation behaviour. Specimens were surface-crazed by bending around a cylindrical former and tested by the same techniques. A further increase in the magnitude of stress at all positions was indicated by the layer removal procedure while the gradients of the Kubát and Rigdahl plots from stress relaxation data were unchanged.     

Solvent stress crazing in PMMA: 1. Geometrical effects

Single crazes were propagated in poly(methyl methacrylate) (PMMA) by loading sheet specimens containing ‘starter’ cracks and immersed in various alcohols. Craze lengths and propagation velocities were measured (as functions of the applied load) using an ultrasonic scanning technique, for three types of behaviour. These were: (I) where craze arrest occurs after some growth; (II) where the craze grows at a constant velocity for an unlimited distance; and (III) where constant velocity growth continues after removal of the original ‘starter’ crack and re-application of the load. The results are analysed using the concept of an ‘equivalent crack’, whose length I turns out to be related to the length c₀ of the starter crack. At temperatures above a previously identified ‘characteristic temperature’, I is commensurate with c₀, but at lower temperatures, I?c₀. These results are discussed in terms of the strain hardening properties of the craze matter which fills the craze.     

The effect of orientation on the morphology and kinetics of solvent crazing in polystyrene

Unoriented, uniaxially oriented, and biaxially oriented polystyrene films were crazed by immersing samples in liquid n-hexane at 45°C. The craze morphology and crazing kinetics were studied as a function of the preorientation and thermal histories of the polymer films. The shape of the micropores was related to the degree of orientation of the film. Ellipsoidal microvoids were formed on the surfaces of uniaxially oriented films containing a residual glassy core. Unoriented samples displayed spherical microvoids 0.1 to 3 μ in diameter. The major-to-minor axis ratio of the micropores increased monotonically from 1/1 to 10/1 as the sample orientation was increased from 0% to 200%. The kinetics of the crazing process similarly increased with uniaxial orientation. Surfaces of unannealed films which crazed mainly during the sorption of liquid n-hexane displayed numerous micropores when examined by the SEM. Conversely, films which were extensively annealed crazed mainly during desorption regardless of subsequent orientation. These extensively annealed films exhibited surfaces with many fine cracks and few, if any, micropores.     

Effects of orientation on the penetration,crazing, and dissolution of polystyrene by n-hexane

Extruded, drawn, and quenched samples of polystyrene were subsequently immersed in liquid n-hexane at temperatures between 35° and 55°C for various time intervals. Samples were removed from the immersion bath, quenched and fractured. Subsequent microscopic examination of the cross-sections revealed a distinct boundary between a crazed outer shell and an essentially unpenetrated central core. The time dependence of the depth of penetration of the advancing craze front was measured at various temperatures for several draw ratios. The initial rate of penetration increased monotonically with draw ratio (orientation) and the advance of the penetrant front was completely controlled by diffusion for drawn samples at 55°C. More complex kinetics, involving relaxations at the moving boundary, describe the penetration at lower temperatures; a slight systematic variation in the relative contribution of diffusion and relaxation was observed with increasing draw ratio. An activation energy of 23.7 k cal/g-mole characterized the temperature dependence of the initial penetration rate, independent of sample orientation. Gravimetric swelling experiments were confounded by sample dissolution in the case of the oriented samples. Intriguing swelling patterns, including discernable differences between the pronounced edge effects in the draw and tranverse directions, were apparent. Conversely, diffusion transverse to the orientation direction was accelerated by the orientation resulting in an increasing component of relaxation control in the penetration experiments and increased rates of dissolution in the oriented samples.     

利用废弃聚苯乙烯泡沫塑料制备溶剂型胶粘剂

以废弃聚苯乙烯(EPS)泡沫塑料为原料,甲苯、乙酸乙酯和丙酮为混合溶剂,制备溶剂型EPS胶粘剂。着重探讨了增黏树脂含量、热塑性弹性体SBS(苯乙烯-丁二烯-苯乙烯嵌段共聚物)含量对EPS胶粘剂剪切强度、黏度和热稳定性等影响。研究结果表明:当V(甲苯)∶V(乙酸乙酯)∶V(丙酮)=52∶48∶3、m(EPS)∶m(松香)=100∶2时,相应EPS胶粘剂对PVC(聚氯乙烯)板的剪切强度(2.54 MPa)相对最大;当V(甲苯)∶V(乙酸乙酯)∶V(丙酮)=52∶48∶3、m(EPS)∶m(酚醛树脂)∶m(SBS)=100∶4∶6时,相应EPS胶粘剂对木材的剪切强度(7.60 MPa)相对较大;适量的增黏树脂和SBS可有效降低EPS胶粘剂的黏度,并且相应EPS胶粘剂的润湿性(对基材)、操作性、粘接强度和EPS胶膜的热稳定性等均有所提高。     

Mechanical α-dispersion and interaction in filled polystyrene and polymethylmethacrylate

Dynamic mechanical properties of polystyrene (PS) and polymethylmethacrylate (PMMA) filled with glass beads and mica flakes have been investigated. The α-dispersion temperature (T_α) increases with filler content in each system. The increase in T_α is larger for PMMA than for PS in the case of the same filler; the increase in T_α is larger for mica than for glass in the case of the same polymer. Polymer-filler interaction energies have been estimated from heats of adsorption of the monomers of the filler surfaces determined by solid gas adsorption chromatography. The increase in T_α can be related to the extent of polymer-filler interaction energy. The thickness of the partially immobilized layer on the filler surface has been estimated to be the orders of 0.1 and 1.0 μm, respectively, for the mica and the glass-filled systems.     

The effect of penetrant activity and temperature on the anomalous diffusion of hydrocarbons and solvent crazing in polystyrene part I: Biaxially oriented polystyrene

The effects of temperature and penetrant activity on the sorption kinetics and equilibria of a series of alkanes in glassy, biaxially oriented polystyrene were studied. Normal isomers of pentane, hexane, and heptane cause crazing of polystyrene film samples at high penetrant activities (> 0.85). Crazing kinetics are identical to the kinetics of Case II transport. Transport of these normal hydrocarbons in glassy polystyrene in the temperature range 25 to 50°C is markedly non-Fickian; limiting Case II transport is observed at activities in exces of 0.6. Sorption appears to be controlled by highly activated relaxation processes including primary bond breakage at these high penetrant activities. Fickian diffusion behavior is approached, however, as penetrant activity is reduced. Sorption of the branched isomers of these compounds does not result in polymer microfailure. The sorption kinetics of the branched isomers, although time dependent, appear to be controlled primarily by thermally activated diffusion rather than large scale polymer relaxations which control Case II transport.     

Solvent effects on mutual diffusion in concentrated polystyrene solutions

In a previous paper, mutual diffusion coefficients for concentrated solutions of a monodisperse polystyrene in 14 solvents were reported. In this paper, these data are reinterpreted in terms of new thermodynamic data for these solvents. These new results permit evaluation of the activity derivative, ? In a₁/? In v₁, and therefore enable calculation of the diffusional friction coefficient from the mutual diffusion coefficient. The friction coefficient was found to be directly proportional to solvent viscosity to within the experimental errors involved in the combined transport and thermodynamic measurements. The molecular implications of this result are discussed.     

Preparation of silane‐terminated polystyrene and polymethylmethacrylate self‐assembled films on silicon wafer

End silane functionalized polystyrene and polymethylmethacrylate were prepared through radical chain‐transfer reaction and characterized with gel‐permeation chromatography. The chain‐transfer constants of mercaptopropyltrimethoxysilane for the polymerization of styrene and methylmethacrylate were determined to be 8.48 and 0.67, respectively, from the slopes of 1/DP_n versus [S]/[M] lines. The ultrathin films of the end silane–terminated polymers were prepared by self‐assembly onto hydroxylated silicon wafers. The water contact angle on the resulting ultrathin films and the film thickness were measured. The morphology and chemical features of the films were observed and investigated by means of atomic force microscopy and X‐ray photoelectron spectroscopy. Results indicated that the chain‐transfer agent played a key role in making it possible for the silane‐terminated polystyrene and polymethylmethacrylate to be self‐assembled on Si(111), whereas the thickness and surface quality of the ultrathin films were dependent on the molecular weights of the polymers. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 1695–1701, 2004     

Use of functionalized WS2 nanotubes to produce new polystyrene/polymethylmethacrylate nanocomposites

Multiwall WS₂ nanotubes of 40-50 nm diameter were functionalized with n-octadecyl phosphonic acid by sonication in toluene and blended with mixtures of polystyrene (PS) and polymethylmethacrylate (PMMA) to form new nanocomposite (NC) materials. The surface and domain structures were studied by atomic force microscopy (AFM), scanning transmission X-ray microscopy (STXM) and transmission electron microscopy (TEM) for various levels of loading of nanotubes up to 20 wt%. Phase-separated domain size and surface roughness of the nanocomposite films were found to be dramatically reduced relative to the pure homopolymer blend and good dispersal of the nanotubes in the blend matrix was attained.     

Solvent power effect on graessley's time constant in concentrated polystyrene solutions

Results of our measurements of polystyrene solutions and those reported in the literature demonstrate the common dependence of Graessley's relaxation time on the product of concentration and molecular weight, irrespective of the solvent power and its viscosity, and of temperature. Such behavior is in agreement with the conclusions reached in our previous paper, namely, that the shear stress dependence of viscosity of this polymer in solvents differing considerably in viscosity and solvent power may be unified by plotting relative viscosity against the product of velocity gradient and Rouse's relaxation time.     

Thermochemical stress crazing and cracking of thermoplastics

A new method of inducing crazing and cracking in thermoplastics is discussed. The method involves the thermal quenching of hot thermoplastics into simple organic liquids or organic solutions. Data are presented for several different thermoplastics and a number of different liquids. The results are interpreted in terms of a model based on the work of Manning and Lineback that incorporates thermal strains and stresses, thermal conductivity, specific interfacial energies, and defects. The specific crazing energy for bisphenol-A polycarbonate when quenched into isopropyl alcohol is determined to be approximately 3 × 10⁵ erg/cm² from this model. © 1994 John Wiley & Sons, Inc.     

Surface crazing in PVC and other polymers

Initiation and growth of surface crazes in rigid PVC, subjected to tensile creep loads, has been studied experimentally for a wide variety of conditions. The tests were carried out at room temperature, at stress levels ranging from 10 to 50 MPa, in air or in the vapour of natural gas condensate (a crazing agent). PVCs with different molecular weights (K-values) and degrees of gelation were tested. Physical aging of the sample was considered by comparing quenched and aged samples. The behaviour under continuous loading was compared with that under cyclic loading. In cyclic loading, crazing behaviour differs markedly from that observed in continuous loading. The defects grow linearly with the number of load periods, i.e. linearly with the loading time, and they appear to be associated with cracks. In continuous loading, the craze length grows linearly with the logarithm of the loading time, after some initiation time has been passed. This logarithmic growth in constant loading is bounded by two processes of growth cessation. The first, at high stresses, coincides with gross yielding of the sample; the second, at low stress levels, is less well understood. The effects of stress level, crazing agent, K value, degree of gelation and physical aging on the processes of initiation and cessation of crazing under constant stress are discussed. The growth rate was shown to depend only on the stress; the crazing agent, molecular weight and degree of gelation have little influence. For other polymers, such as PS, PMMA and PC, the growth kinetics are essentially the same as for PVC. For these materials, the growth rate for crazing sharply decreases with increasing stress, according to the inverse second to inverse third power of the stress.     

Nanocomposites based on MWCNT and polystyrene,styrene‐acrylonitrile copolymer,or polymethylmethacrylate,obtained by miniemulsion polymerization

Free radical miniemulsion polymerization of styrene (St), St/acrylonitrile 3 : 1 mixture or methylmethacrylate in the presence of multiwalled carbon nanotubes (MWCNT) was proven as a convenient way to obtain homogenous hybrids with perspectives in associated applications like foams specialties materials. Miniemulsion polymerization was viable up to 2% wt. MWCNT to monomer, without agglomerations. The grafting on MWCNT during the polymerization occurs without the need for supplementary functionalization and the polymer grafted nanotubes showed stable dispersions in the polymer solvent. Monomer polarity affected the grafting ability during the polymerization process. The nanocomposites obtained after purification and drying were used in foaming process. MWCNT presence in the related nanocomposites decreased the pore sizes in foam‐like materials (for all three different matrices). At 1 wt % MWCNT content, low density (< 0.3 g/cm³), low pore size (< 10 μm) and high cell density (>10⁹ cell/cm³) were achieved. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41148.     

The residual stress of magnet wire and crazing

It is well known that crazing occurs sometimes when a magnet wire is immersed in an impregnating varnish after coiling. A quantitative study between residual stress of a magnet wire film and crazing is presented in this paper by using a magnet wire having poor adhesive strength between the film and the conductor. The residual stress of a magnet wire was found to be maximum in the range of 3–5% elongation. A morphological study of crazing which occurred in crosslinked and linear polymers is also described in this paper.     

Solvent and relative humidity effect on highly ordered polystyrene honeycomb patterns analyzed by Voronoi tesselation

Highly ordered honeycomb‐patterned polystyrene surfaces are efficiently prepared by static breath figure method. The structured arrays can be obtained by casting a dilute solution polymer on glass substrates under various conditions. Tetrahydrofuran and chloroform are used as solvent to form cavities of several micrometers. The analysis of the surfaces indicates nonlinear relation between concentration and pore size in this system. Voronoi tessellations of the polystyrene surfaces at different relative humidity (RH) are achieved, and each conformational entropy determined. Optimum parameters of concentration and RH are obtained for both solvents. Analysis of hole size distribution and conformational entropy demonstrates the high order of the films obtained. This is a promising method for the fabrication of homogeneous and highly porous films from polystyrene. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44004.     

Thermal crazing phenomena in epoxy resins

Hot-setting epoxy systems, consisting of a bisphenol A diglycidyl ether resin cured with phthalic anhydride and dibutyl phthalate as plasticizing agent (mainly for photoelastic applications), exhibit crazing phenomena when processed at moderately high temperatures. Chemical analysis and i.r. spectroscopy reveal that crazing is due to rejection of dibutyl phthalate from the surface layers of the specimens. The same phenomenon appears with triethylenetetramine (TETA)-cured cold-setting systems, where dibutyl phthalate undergoes transamidification with TETA before rejection.