Correspondence between the rigid amorphous fraction and nanoconfinement effects on glass formation

Semicrystalline polymers' properties are strongly influenced by the presence of a “rigid amorphous fraction” (RAF) of material with enhanced glass transition temperature and suppressed mobility. Here, we review striking similarities between this domain and near‐interface regions of altered dynamics in other nanostructured polymers, including thin films, nanocomposites, layered films, and ionomers. Building on these observations, we present results of molecular simulations of model nanolayered polymers in which one layer is crystalline. The results of these simulations, which bear close similarities to semicrystalline homopolymers while providing a well‐defined equilibrium crystal size, suggest that the RAF shares the same phenomenology and mechanism as dynamic interphases in these other polymeric systems. Specifically, the fraction of rigid amorphous material is driven by the scale of cooperative rearrangements characteristic of dynamics in polymers and other supercooled liquids. These results situate the RAF as a specific instance of a general tendency towards broad dynamic interphases in polymers and other glass‐forming materials. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 907–918     

Measurement of viscoelastic properties for polymers by nanoindentation

A new method has been proposed and verified to measure the viscoelastic properties of polymers by nanoindentation tests. With the mechanical response of load–displacement curves at different loading rates, the parameters of creep compliance and relaxation modulus are calculated through the viscoelastic contact model. Dynamic thermomechanical analysis (DMA) tests are conducted to compare the results by the proposed technique. The results show that the correlation coefficients between DMA tests and the new method are above 0.9 in the entire range, which verified the feasibility of the method. The loading curves fitted by the model are identical to the experimental curves within the discrete points and so it shows that this technique is more suitable for general linear viscoelastic materials. Numerical creep tests are carried out to examine the effectiveness of the proposed method by input the Prony series calculated by the three-element Maxwell model and the viscoelastic contact model. The good agreement shows that the proposed technique can be applied in practice.     

A buckling mechanics approach to elastic modulus determination of glassy polymer films

Column buckling mechanics were examined as a technique to determine the modulus of glassy polymer films that fail at very low strains in tension. As an alternative modulus measurement technique, free‐standing column buckling (FSCB) mechanics were investigated here. Given the film geometries and the critical buckling load, classical relationships can be used to determine the modulus. Several polymeric materials were tested and compared to uniaxial tensile values to determine the robustness and validity of the technique. Film geometries were varied from 4 to 18 mm in width and from 15 to 60 mm in length. The films were compressed in plane until buckling occurred and the critical buckling load was measured for each geometry. The critical buckling load increased as film width increased and decreased as film length increased, while the thickness was held constant for each material. For polyethylene terephthalate films, the elastic modulus was determined to be 3.06 ± 0.58 GPa. This FSCB‐determined modulus was compared to the elastic modulus obtained by tensile testing (3.54 ± 0.2 GPa). The modulus measurement technique presented here has the potential to be used experimentally to determine the elastic modulus of glassy polymer films that perform poorly in tension. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 15–20     

Films on amorphous polyamide nanocomposites: structure and properties

The ability of a nanoclay to improve the transport and mechanical properties of amorphous polyamide (aPA)‐based films was studied as a function of the draw ratio (DR) and the nanoclay content. The presence of nanoclay did not hinder the drawing ability as the maximum DR of the nanocomposites (NCs) and of the aPA were almost the same (51 for the aPA and from 51 to 55 for the NCs). The high degree of exfoliation and orientation along the drawing direction led to a 30% reduction in the water diffusion coefficient compared with the aPA. Moreover, the already low permeability of the aPA to oxygen was halved. The modulus of elasticity presented unusual increases both in the machine and transverse directions. Both increases of properties were attributed to the planar geometry of the oriented nanoclay sheets. The effects of the presence of nanoclay on the modulus of elasticity in the draw direction in addition to the effects caused by drawing lead to a combined modulus increase of 65% in the highly drawn 6%NC films. The nanoclay also reduced the modulus anisotropy of the films. An increase in either the nanoclay content or the DR causes a decrease in ductility due to both the stress concentrations created by the nanoclay and to the increasing number of chain segments located parallel to the drawing direction. The dimensional stability of the films greatly increased as the addition of 6% nanoclay led to a 70% decrease in creep deformation after 120 h. Copyright © 2012 John Wiley & Sons, Ltd.     

Structural aspects of deformation of amorphous polymers

Experimental and theoretical data on the inelastic deformation of amorphous glassy polymers were analyzed. The decisive role of direct structural methods in determination of the deformation mechanism of glassy polymers was established. A new mechanism of deformation and thermally stimulated recovery of strained glassy polymers was considered on the basis of structural data analysis.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 1–6, January, 2005.     

Hole growth as a microrheological probe to measure the viscosity of polymers confined to thin films

We review recent hole growth measurements performed at elevated temperatures in freely-standing polystyrene (PS) films, using optical microscopy and a differential pressure experiment (DPE). In the hole growth experiments, which were performed at temperatures close to the bulk glass-transition temperature of PS, T = 97 °C, we find evidence for nonlinear viscoelastic effects, which markedly affect the growth of holes in freely-standing PS films. The hole radius R initially grew linearly with time t before undergoing a transition to exponential growth characterized by a growth time τ. The time scale τ₁ for the decay of the initial transient behavior prior to reaching steady state was consistent with the convective constraint release mechanism of the tube theory of entangled polymer dynamics, while the characteristic hole growth times τ of the holes were consistent with significant reductions in viscosity of over eight orders of magnitude with increasing shear strain rate due to shear thinning. DPE measurements of hole growth on very thin freely-standing films revealed that hole formation and growth occurs only at temperatures that are comparable to or greater than T, even for films for which the T_g value was reduced by many tens of degrees Celsius below the bulk value. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B:Polym Phys 44: 3011–3021, 2006     

Investigation of the viscoelastic properties of 4-propoxy-biphenyl-4-carbonitrile

ABSTRACT

We report on the temperature-dependent measurements of dielectric permittivity, birefringence, elastic constants and rotational viscosity for 4-propoxy-biphenyl-4-carbonitrile in the nematic region. The temperature dependence of the three elastic constants was determined from studies of the Freédericksz transition. The thermal dependence of elastic constants shows features similar to the literature (bend > splay > twist). Elastic constants are proportional to the square of the order parameter. Temperature-dependent dielectric characterisation was carried out at a frequency of 10 kHz. The compound shows positive dielectric anisotropy in the nematic phase. The rotational viscosity is found to be relatively low. Temperature dependence of order parameter is estimated using Haller’s method. The figure of merit was also calculated as a function of temperature.     

Nanomechanical properties in ultrathin polymer films: Measurement on rectangular versus circular bubbles

Prior studies of inflation of circular membranes of ultrathin polystyrene (PS) films have evidenced a reduced glass transition temperature (T_g) and rubbery stiffening, whose origins remain unclear. Here, we describe results from inflation of rectangular, ultrathin films of the same PS material. The bubble shapes obtained from the experiment are consistent with finite element (FE) simulations. The accuracy of three approximate solutions for modulus obtained from the inflation of the thin, rectangular films was evaluated by comparison with FE analysis. The best among the three solutions was used to determine the creep compliance and rubbery stiffness of the thin films. It is found that the reduction of T_g and the rubbery stiffening for rectangular bubbles are consistent with results obtained using circular bubbles, although there is some indication that the rectangular bubbles give somewhat greater rubbery stiffening. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Enhanced mobility and increased gas sorption capacity in thin film and nanoconduit confined polymers

Poly(tert butyl acrylate) (PTBA) is found to exhibit enhanced mobility when spun cast into thin films or impregnated into cylindrical anodic aluminum oxide (AAO) nanoscale pores. In a thin film configuration, the glass transition temperature of 20 nm thick PTBA is found to decrease almost 20 °C compared to the bulk. Consistent with this mobility increase, an increased volume fraction of interphase polymer leads to at least a 2.4 times viscosity reduction when PTBA is impregnated in 100 nm pores versus 200 nm pores. Such increases in mobility result in a 15‐fold increase in CO₂ permeability for an AAO confined geometry compared to a bulk film. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 434–441, 2010     

Chain entanglement and viscoelastic properties of molten polymers

Linear low-density polyethylenes (LLDPES) and polypropylene (PP) have been recovered from solutions of varying initial polymer concentration. Melts of these polymers show significant reductions in viscosity and elasticity, and the effects are attributed to changes in the entanglement density of the polymer. Measurements of entanglement densities have been attempted from experimental values of the apparent zero-shear melt viscosity. These indicate that solution treatments in trichlorobenzene at 135°C reduce the entanglement density more effectively in PP than in LLDPE. In all cases the observed effects are reversible by annealing at elevated temperatures. Analytic data point to entanglement changes as the true origin of changes in viscoelastic properties, since solution treatments produce no changes in molecular weights and weight distributions, and the samples tested are free of solvent residues.     

CdS和CdSe薄膜的制备及其光电化学性质

本文采用简易的化学水浴沉积法和自牺牲模板法制备CdS、CdSe薄膜,对两种薄膜进行了XRD表征,比较了两种薄膜的紫外吸收光谱并研究了CdS、CdSe薄膜作为太阳能电池中的光阳极时所产生的光电流和光电压,对两种薄膜的电化学性能进行了比较.     

A novel orientation technique for semi-rigid polymers. 2. Mechanical properties of cellulose acetate and hydroxypropylcellulose films

The networks of cellulose acetate and hydroxypropylcellulose prepared in the first part of this investigation were studied with regard to their mechanical properties. The quantities of particular interest were increases in tensile modulus and tensile strength obtained by drying the swollen films under strain, both uniaxial and equi-biaxial. These increases or improvements in mechanical properties were determined as a function of polymer concentration during cross-linking, polymer molecular weight, degree of cross-linking, and elongation during drying. In all cases, the improvements increased with increase in elongation during drying, and the largest increases were obtained in the case of the highest molecular weight polymer which had been lightly cross-linked in dilute (isotropic) solutions. The extent of ordering in these systems was gauged approximately by measurements of birefringence, which were correlated with their tensile moduli and tensile strengths.     

Mechanical properties of the Ni filler metal layer in Si3N4 joints measured by nanoindentation

Nanoindentation experiments were done on Si₃N₄/Ni/Si₃N₄ specimens bonded at different temperatures. Tests were carried out in both the ceramic adherent and the metal filler. Results from nanoindentation experiments done across the interlayer verify that a solution of Si atoms in the Ni interlayer did not modify the mechanical properties of the metal (elastic modulus and hardness). The increase in bonding temperature produced an increase in hardness in the metal interlayer but did not affect its elastic modulus. This hardening of the Ni interlayer was related to the decrease in the shear strength of these joints. Copyright © 2004 John Wiley & Sons, Ltd.     

Mechanical properties of recycled polymers

Modifications of structure and morphology of the polymers induced by the thermomechanical stress (or by the presence of water for some polymers) during reprocessing can cause a drastic deterioration of mechanical properties of the recycled material. The property - reprocessing relationships are an important tool to determine not only the properties of the recycled polymers but also the strategies to use (processing conditions, stabilizers, fillers, compatibilizers, etc) in order to obtain recycled polymers with good mechanical properties. In this work, typical behaviour of some mechanical properties as a function of the number of reprocessing operations is discussed and correlated with their structure and morphology. The influences of the type of apparatus and of the processing conditions are also considered.     

From chemical structure to viscoelastic properties of polymers

The basic result of the coupling model consists of a cross-over from exp-(t/τ_o) to exp-(t/τ*)^1-n at a temperature insensitive microscopic time t_c. We indicate how this basic result can be derived from chaotic, interacting Hamiltonian systems which include densely packed polymer molecules. Recent quasielastic neutron scattering experiments and molecular dynamics simulations are discussed and the results are shown to support this result as well. An application of the coupling model to find how the viscoelasticity of a polymer depends on the chemical structure of the monomer through the coupling parameter of the local segmental motion is given to illustrate the utility of the model.     

Some viscoelastic properties of siloxane polymers during irradiation

The dynamic moduli, E′ dyn, and loss tangents, tan δ, of polydimethylsiloxane and polydimethyldiphenylsiloxane polymers have been investigated by an in situ technique during γ-irradiation. These viscoelastic properties were calculated and plotted as a function of irradiation exposure time by measuring the free end displacements and resonance frequencies of polymeric cantilever reeds. The reeds were swept through a small frequency range from about 20 to 100 cycles/sec. The moles of effectively elastic chains per unit volume (v) of the two unfilled polysiloxahes were calculated from in situ modulus data and compared to values obtained utilizing the swelling technique. The approximate molecular weights between entanglements, M_e, of these unfilled polymers were determined by extrapolation of moduli data to zero radiation exposure. The addition of a large silica filler, SiO₂, into the polymers did not alter the crosslinking rates, and the filler did not enter into polymer—filler bonding.     

Real-time neutron reflectivity study of the early stages of diffusion into and dissolution of glassy polymers

The early stages of the penetrant behavior of a series of phthalate plasticizers into thin films of glassy, high-molecular-weight deuterated poly(methyl methacrylate) (dPMMA) have been studied with in situ real-time neutron reflectivity. After an initial induction phase, both dioctyl phthalate and diisononyl phthalate penetrate the dPMMA films, as indicated by an increase in the thickness. In both cases, a fast linear rate of swelling of the polymer is followed by another behavior that is much slower. The slowdown in the velocity of the plasticizers at or near the transition point is assumed to occur because of a balancing of the misfit-induced pressure and the osmotic pressure, which is responsible for the initial plasticizer ingress. In contrast, and unexpectedly, lower molecular weight dibutyl phthalate does not swell dPMMA, but after an initial induction period, the polymer film begins to dissolve. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3267–3281, 2004