Effect of particle size on the mechanism controlling n-hexane sorption in glassy polystyrene microspheres

The effect of particle size on the rate-determining transport mechanism controlling n-hexane sorption into polystyrene spheres was determined by monitoring the kinetics of n-hexane vapour absorption in two powder samples of distinctly different narrow particle size distributions. The microspheres differed in mean diameter by a factor of three hundred. In all cases, sorption in the smaller spheres was controlled principally by Fickian diffusion. In contrast, at temperatures and activities which result in relaxation-controlled (Case II) transport in films, the larger spheres (184 μm diameter) sorb by Case II kinetics. Under these identical boundary conditions, however, the small spheres (5340 Å diameter) absorbed n-hexane by Fickian kinetics. Presumably, there is insufficient time or space in the small spheres to develop the step concentration profiles associated with limiting Case II transport. A mathematical model describing Case II sorption in spheres cylinders, and slabs is developed. The kinetics describing absorption in the larger spheres are well described by this analysis. Desorption in all cases is adequately described by concentration-dependent diffusion. Repeated experiments confirmed, moreover, that at identical temperatures and penetrant vapour activities the apparent equilibrium concentration of n-hexane in the small spheres is significantly higher than the corresponding sorption equilibria in the large spheres. Although several possible explanations for this glassy-state anomaly are presented, the most satisfying explanation involves the development of a non-sorbing core in the large spheres consequent to the prior sorption in the surrounding shell. Desorption data are consistent with the notion of partial swelling of large spheres.     

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.     

Water diffusion in unsaturated polyester films. Effect of plasticization on the glass transition

Water diffusion through Unsaturated Polyester Resin (UPR) is investigated. From differential scanning calorimetry, we observe that the plasticization effect due to water sorption leads to large variations of T_g and is a reversible phenomenon. The transport of water through the polymer is characterized with a new instrument developed in our laboratory which shows that the water transport deviates from Fickian behavior. It is found that the diffusion coefficient is not a constant and experimental data are in good agreement with D = D_oe^γC where γ is a plasticization coefficient.     

Oriented crystallization of isotactic polystyrene in films prepared by friction transfer

The ability to orient polymer chains by applying external forces opens up the possibility to obtain polymeric surfaces with ordered structures. Here, we employed a friction-transfer approach by moving a pin of isotactic polystyrene (i-PS) across a smooth silicon counterface at controlled velocity, pressure and temperature which led to the deposition of a molecularly thin layer of highly oriented i-PS chains. The observed morphology of the resulting film (ribbons oriented in the sliding direction) indicated that the transferred molecules were highly oriented. This was confirmed after isothermal crystallization which led to the formation of so-called “shish-kebab” crystals aligned in the sliding direction. Thus, after crystallization all polymers were preferentially oriented with their chain axis in the shearing direction. Our results strongly suggest that by extending the polymer chain conformations in the shearing direction we can introduce a significant reduction of the nucleation barrier. Accordingly, friction transfer allows to align not only the transferred polymer chains but also the subsequently forming crystalline domains within the transferred films.     

Confinement, composition, and spin-coating effects on the glass transition and stress relaxation of thin films of polystyrene and styrene-containing random copolymers: Sensing by intrinsic fluorescence

The glass transition temperatures (T_gs) of polystyrene (PS) and styrene/methyl methacrylate (S/MMA) random copolymer films are characterized by intrinsic fluorescence, i.e., monomer fluorescence from an excited-state phenyl ring and excimer fluorescence from an excited-state dimer of two phenyl rings. The T_g is determined from the intersection of the rubbery- and glassy-state temperature dependences of the integrated fluorescence intensity measured upon cooling from an equilibrated state. With PS, the effects of nanoconfinement on T_g and the transition strength agree with results from studies using probe fluorescence and ellipsometry. The T_g-nanoconfinement effect is “tuned” by copolymer composition. As S-content is reduced from 100 mol% to 22 mol%, the confinement effect changes from a reduction to an enhancement of T_g relative to bulk T_g. Intrinsic fluorescence is also a powerful tool for characterizing relaxation of residual stresses. Stresses induced by spin coating affect local conformations, which in turn affect excimer and monomer fluorescence and thereby integrated intensity. The heating protocol needed to achieve apparently equilibrated local conformations is determined by equivalence in the integrated intensities obtained upon heating and subsequent cooling. While partial stress relaxation occurs upon heating in the glassy state, full relaxation of local conformations requires that a film be heated above T_g for times that are long relative to the average cooperative segmental relaxation time. For example, in thin and ultrathin films, equilibration is achieved by heating slowly (∼1 K/min) to 15-20 K above T_g. Dilute solution fluorescence of PS and S/MMA copolymers is also characterized and compared to reports in the literature.     

Spectroscopic determination of the in-situ composition of epoxy matrices in glass fiber-reinforced composites

Computerized infrared analysis is applied to the characterization of a glass-reinforced crosslinked polyester. The method of factor analysis determines the number of independent components which constitute the polymeric matrix. Subsequently, the spectra of those components are fitted by a least-squares criterion to spectra of the multicomponent matrix, or, if the glass spectrum is included as an additional component, to the spectra of composites. The least-squares coefficients yield the matrix composition in terms of the initial reactant composition and the extent of crosslinking.     

Surface composition,bonding, and morphology in the nucleation and growth of ultra-thin,high quality nanocrystalline diamond films

The morphology, composition, and bonding character (carbon hybridization state) of continuous, ultra-thin (thickness ∼ 60 nm) nanocrystalline diamond (NCD) membranes are reported. NCD films were deposited on a silicon substrate that was pretreated using an optimized, two-step seeding process. The surface after each of the two steps, the as-grown NCD topside and the NCD underside (revealed by etching away the silicon substrate) is examined by X-ray PhotoElectron Emission spectroMicroscopy (X-PEEM) combined with X-ray absorption near edge structure (XANES) spectroscopy, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). The first step in the seeding process, a short exposure to a hydrocarbon plasma, induces the formation of SiC at the diamond/Si interface along with a thin, uniform layer of hydrogenated, amorphous carbon on top. This amorphous carbon layer allows for a uniform, dense layer of nanodiamond seed particles to be spread over the substrate in the second step. This facilitates the growth of a homogeneous, continuous, smooth, and highly sp³-bonded NCD film. We show for the first time that the underside of this film possesses atomic-scale smoothness (RMS roughness: 0.3 nm) and > 98% diamond content, demonstrating the effectiveness of the two-step seeding method for diamond film nucleation.     

Spectroscopic determination of the in-situ composition of epoxy matrices in glass fiber reinforced composites

Computerized infrared analysis is applied to the characterization of a glass-reinforced crosslinked polyester. The method of factor analysis determines the number of independent components which constitute the polymeric matrix. Subsequently, the spectra of those components are fitted by a least-squares criterion to spectra of the multicomponent matrix, or, if the glass spectrum is included as an additional component, to the spectra of composites. The least-squares coefficients yield the matrix composition in terms of the initial reactant composition and the extent of crosslinking.     

Investigation of the Tll (>Tg) transition in polystyrene by torsional braid analysis

Investigation of the T_ll (>T_g) transition in amorphous poly-styrene by the technique of torsional braid analysis is reviewed. For the most part, the relaxation behaves like the glass transition (T_g) in its dependence on molecular weight, on average molecular weight in binary polystyrene blends, and on composition in a compatible plasticized system.     

Rheological and processing characteristics of polystyrene in the vicinity of Tll transition

Rheological and processing parameters have been measured for a commercial polystyrene at temperatures from 155°C to 210°C resulting in no clear evidence for the existence of liquidliquid transition. Zero shear viscosities obtained from dynamic measurements and ends corrections and extrudate swell from capillary rheometry are analyzed, with the conclusion that the mechanism of flow is the same throughout the range of temperatures considered.     

Co-operative diffusion of semi-dilute solutions, concentrated solutions and swollen networks for polystyrene in dibutylphthalate

The diffusion constant of solutions of polystyrenes of molecular weight ranging from 110 000 to 3.6 × 10⁶ in straight-chain-dibutylphthalate has been measured by photon correlation spectroscopy as a function of polymer volume fraction and temperature. In the semi-dilute range the co-operative diffusion constant D_c exhibits a much smaller increase with the polymer volume fraction Φ than theoretically predicted, except at high temperatures (T 100°C) where it follows a Φ^1/2 law characteristic of a marginal solvent. This effect can be described to an enhancement of the friction factor, which is also demonstrated by a decrease in D_c occurring at a volume fraction which increases with temperature. The same effect is observed in swollen networks but it is strongly reduced for swelling equilibrium conditions.     

Spectroscopic investigations of the microstructure changes induced by substrate temperature in nitrogen-substituted amorphous carbon thin films

The effect of substrate temperature (T_S) on the microstructure of amorphous nitrogen-substituted carbon thin films (a-C_1−xN_x) has been studied. The a-C_1−xN_x films were deposited on Si(1 0 0) using reactive radio-frequency (RF) magnetron sputtering of a high-purity graphite target in a pure nitrogen plasma using different RF powers and different substrate temperatures T_S. Combined Fourier transform infrared (FTIR), Raman spectroscopy and residual stress measurements are used to fully characterise the films in the as-deposited state. FTIR spectra showed the existence of NCsp² and CN triple bonding, and suggested the presence of N-Csp³ bonding in the deposited films. The analysis of the spectra versus T_S reveals two rearrangement mechanisms of the microstructure. Up to 150 °C, the reversion of NCsp² to N-Csp³ and CCsp²respectively, and the increase the connectivity of the C-C network for higher T_S. The Raman features reveals that this progressive graphitisation of the material, with increasing T_S, is accompanied by a high disorder form of Csp² sites. These results are used to describe the stress variation that accompanies nitrogen evolution within the deposited films.     

Ternary diffusion of gases in capillaries in the transition region between knudsen and molecular diffusion

Diffusion of a ternary gas mixture in the transition region of diffusion was experimentally investigated in an isobaric open system of fine glass capillaries in parallel. The flux data for HeNeAr were obtained over a 673/1 pressure range from 0·450 to 303·2 mm Hg abs. in the transition region between molecular (ordinary) and Knudsen diffusion and confirm the integrated theoretical equations in true capillaries for a ternary mixture. This appears to be the first experimental confirmation of these equations over a wide pressure range. The use of binary transition equations to approximate the ternary flux values throughout the transition region is demonstrated. Approximate methods are presented to calculate the ternary flux values at different boundary conditions or driving forces.     

Fabrication of polystyrene/multiwalled carbon nanotube composite films synthesized by in situ microemulsion polymerization

A microemulsion polymerization method was used to achieve better compatibility between polystyrene (PS)/multiwalled carbon nanotubes (MWCNTs) nanocomposites and the host PS matrix to form films with excellent electrical and thermal properties. The films were prepared by embedding the PS/MWCNTs nanocomposite into the PS matrix. The MWCNTs were functionalized with PS nanoparticles to avoid the phase separation problem between the filler and host matrix and to enhance the good dispersibility of MWCNTs in the PS host matrix. The confirmation of the synthesis was analyzed by Fourier transform infrared spectroscopy, nuclear magnetic resonance, and Raman spectroscopy. The variation effect of the PS‐linking density on the MWCNT was revealed by scanning electron microscopy and transmission electron microscopy. An enhancement of the thermal and mechanical properties was revealed by thermal gravimetric analysis, differential scanning colorimetric analysis, and dynamic mechanical analysis. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers     

气相色谱法测定胶粘剂中丙酮、正己烷、苯、甲苯、二甲苯含量

用Agilent 7890气相色谱仪,FID检测器,DB-1301 60m×0.32mm×0.25μ毛细管色谱柱,以乙酸乙酯为溶剂。外标法测定胶粘剂中丙酮、正己烷、苯、甲苯、二甲苯含量,丙酮的线性相关系数为0.9997,平均回收率100.26%,样品测定的标准偏差0.170,相对标准偏差为2.6%;正己烷的线性相关系数为0.9998,平均回收率99.80%,样品测定的标准偏差0.038,相对标准偏为2.0%;苯的线性相关系数为0.9999,平均回收率98.30%,样品测定的标准偏差0.043,相对标准偏差为2.3%;甲苯的线性相关系数为0.9999,平均回收率101.94%,样品测定的标准偏差0.070,相对标准偏差为2.2%;间对二甲苯的线性相关系数为0.9999,平均回收率97.97%,样品测定的标准偏差0.075,相对标准偏差为3.1%;邻二甲苯的线性相关系数为1.0000,平均回收率97.20%,样品测定的标准偏差0.046,相对标准偏差为2.1%。     

The study of diffusion of media in the organic coating films by BAW admittance analysis

The permeation of water and corrosive species in coating films is very important for the corrosion protection by organic coatings. As a new method for the study of diffusion of media in coating films, bulk acoustic wave (BAW) admittance analysis has been proposed in this article. Real-time information on the change of the film mass, mechanic property of the layer, and the permeation process of the media can be provided simultaneously by the parameters of BAW admittance analysis, f_s (series resonant frequency), ΔL₁ (inductance variance), R₁ (resistance), Q (quality factor), and C_s (film capacitance). The diffusion behavior of media and the corresponding film stability can be described by the following parameters: D (diffusion coefficient), P (permeation coefficient), S (the solubility of media), and SC (the swelling coefficient of the coating layer). © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 70: 2283–2290, 1998