Fatigue models for glassy polymers

Temperature effects on fatigue crack propagation in glassy polymers can be described by the Chudnovsky and Moet model, which is based on the propagation of microdefects near the crack tip. This was shown by fitting several models to fatigue data for polycarbonate and poly (methyl methacrylate). Statistical regression analysis and a global search method were used in the model fitting.     

Measuring surface and bulk relaxation in glassy polymers

We present a comprehensive study of gold nanoparticle embedding into polystyrene (PS) surfaces at temperatures ranging from T _g + 8 K to T _g − 83 K and times as long as 10⁵ minutes. This range in times and temperatures allows the first concurrent observation of and differentiation between surface and bulk behavior in the 20nm region nearest the free surface of the polymer film. Of particular importance is the temperature region near the bulk glass transition temperature where both surface and bulk processes can be measured. The results indicate that for the case of PS, enhanced surface mobility only exists at temperatures near or below the bulk T _g value. The surface relaxation times are only weakly temperature dependent and near T _g , the enhanced mobility extends less than 10nm into the bulk of the film. The results suggest that both the concept of a “surface glass transition” and the use of glass transition temperatures to measure local mobility near interfaces may not universally apply to all polymers. The results can also be used to make a quantitative connection to molecular dynamics simulations of polymer films and surfaces.     

Periodic surface photoreliefs in glassy and hyperelastic polymers

The formation and degradation of periodic photoreliefs on the surface of polymer layers having significantly different glass-transition temperatures are investigated for various process activation temperatures. It is established that the main factor limiting the resolution of periodic relaxation photoreliefs at the surface of glassy polymer layers containing dimerizing anthracene derivatives is the presence of shear stresses. Their action is suppressed by the thermal decomposition of dimers, a process which gives rise to inverted reliefs of higher spatial frequencies. It is shown that the resolution can be enhanced by more than an order of magnitude by using a polymer matrix in the hyperelastic state. Zh. Tekh. Fiz. 69, 79–83 (August 1999)     

The fracture mechanics of glassy polymers

The application of fracture mechanics to glassy polymers, in particular crack growth in PMMA, is discussed. Particular attention is paid to two processes which modulate the energy supply to the crack tip: viscoelastic dissipation at slow crack speeds and specimen inertia at large crack speeds. The relation between fracture energy and crack speed is reviewed, and, where possible, fracture surface observations are correlated with dynamic behavior.     

Creeping friction dynamics and molecular dissipation mechanisms in glassy polymers

The dissipation mechanism of nanoscale kinetic friction between an atomic force microscopy tip and a surface of amorphous glassy polystyrene has been studied as a function of two parameters: the scanning velocity and the temperature. Superposition of the friction results using the method of reduced variables revealed the dissipative behavior as an activated relaxation process with a potential barrier height of 7.0 kcal/mol, corresponding to the hindered rotation of phenyl groups around the C-C bond with the backbone. The velocity relationship with friction F(v) was found to satisfy simple fluctuation surface potential models with F proportional to const-ln(v) and F proportional to const-ln(v)2/3.     

Determination of polymer chain conformation in amorphous polymers

We present the results of a careful examination of neutron scattering from polymer chains in bulk; we have arrived at the following conclusions: 1) Macromolecular chains (in bulk) have the same dimensions as in a θ solvent; 2) They obey the Debye formula q^?1 as low as 5 Å. In order to show this clearly, we compare the theoretical Debye curve and the experimental result for deuterated polystyrene (M_w = 21,000) in a matrix of ordinary polystyrene. The fit is remarkably good and leaves very little room for other interpretations.     

The conformation of adsorbed polyacrylamide and derived polymers

We have studied the adsorption of different polyacrylamide derivatives in water on freshly cleaved mica surfaces. Using the potential of measuring force versus distance profiles with an atomic force microscope (AFM), we investigate the conformation and the loop distribution of adsorbed chains. In particular, we explain that (i) the loop distribution of the polyacrylamide homopolymer can be described from scaling laws arguments, (ii) the loop distribution of statistical copolymers should be a signature of the distribution of the monomers along the chain backbone, and (iii) hydrophobically modified polyacrylamide copolymers form surface gels whose growing kinetics can be studied by AFM. Our results also show that the co-operative effect of H-bonding can give much larger adhesion that ionic association.     

Physical basis of distortional and dilational plastic flow in glassy polymers

A molecular theory for distortional plasticity is described based on thermally activated production of molecular kink pairs which incrementally align short molecular segments and thereby produce strain. The process of crazing is described as a micromechanical problem of elastic-plastic expansion of initially stable micropores produced by a thermally activated mechanism under stress to form a craze nucleus, followed by lateral extension of the craze nucleus by a time-dependent addition of craze matter at points of the craze boundary where the local conditions of craze matter conversion can be satisfied. The analysis of the condition of craze nucleation provides a natural framework for the derivation of the dilational yield condition of Sternstein and Ongchin by crazing. The process of fracture by propagation of cracks pushing plastic craze wedges in front of them is also briefly described qualitatively.     

Molecular conformation in the liquid phase

A brief review is made of methods used to determine the conformation of molecules by diffraction methods. Studies of various simple molecules (e.g. N₂, P₄, CC?₄) show little change of form between the vapour and condensed phases. More recent work on hydrogen-bonded liquids indicates that there is a substantial stretch in the intra-molecular OH(D) bond. Although little data is currently available the results seem to indicate that this effect is systematic for associated species and relates to the strength of the hydrogen bond. The prospects for a more detailed study using the improved flux levels of new X-ray and neutron sources are also examined.     

Raman spectroscopy of crystalline,glassy, and molten states of lead diborate

Polarized Raman spectra of single crystals of lead diborate, PbB₄O₇ (PBO), are studied in detail at 300 K. The TO-, LO-, and IO-phonon lines of the A ₁, A ₂, B ₁, and B ₂ symmetries in the Raman spectra of this compound are assigned. Changes in the Raman spectra of the internal vibrations of boron–oxygen complexes upon transition from the crystalline to the glassy and the molten states of PBO are observed. On the basis of the obtained results, the regularities in the formation of boron–oxygen complexes in glasses, melts, and crystals of the PbO · 2B₂O₃, SrO · 2B₂O₃, and Li₂O · 2B₂O₃. diborate compositions are analyzed.     

Solid-like rheological response of non-entangled polymers in the molten state

We show that non-entangled polymers display an elastic-like behaviour at a macroscopic scale (probed at some 0.100 mm thickness) up to at least hundred degrees above the glass transition temperature. This observation, found under non-slippage conditions, both for side-chain liquid crystalline polymers and ordinary polymers, is in contradiction with the typically found flow behaviour of polymer melt. Our measurements were carried out with a conventional rheometer at thicknesses of several tenths millimetres. Thus, we were probing bulk properties. The observed elasticity supposedly implies that even in the melt the chains experience a cohesive effect of macroscopic distances, involving collective motions over time scales longer than the individual relaxation time of an individual polymer chain. The detection of such a solid-like property of molten non-entangled polymers is of considerable importance for a better understanding of the polymer dynamics.     

冰晶石-氧化铝熔盐的分子动力学研究

采用分子动力学方法,对冰晶石-氧化铝熔盐的结构及其电学性质进行了研究.通过对不同氧化铝含量熔盐的计算,证明文章采用的势函数参数,可以获得与实验相近的平均键长以及相同变化规律的密度.证实了Al-F-Al和Al-O-Al桥接络合离子的存在,由于这些络合离子的存在,阻碍了铝离子的自由运动,使得在铝离子电场作用下被带到阳极,降低了电解效率.同时离子淌度的计算显示钠离子是电场作用下导电的主要载流子.     

Water sorption thermodynamics in glassy polymers endowed with hydrogen bonding interactions

In this contribution, we review and critically compare the results of the analyses we have previously performed on water sorption thermodynamics in a series of polyimides. The experimental investigation was performed by combining gravimetric tests and in situ vibrational spectroscopy. A non-equilibrium theory, based on a compressible lattice framework accounting for the glassy state of the polymer and for the occurrence of hydrogen bonding interactions, has been used to interpret data. Information at a molecular level gained by vibrational spectroscopy has been used to tailor the model equations. The main features of water sorption thermodynamics are well captured, qualitatively and quantitatively, by the adopted model which displays a remarkable agreement with experimental results.     

Experimental evidence for interplay of dynamic heterogeneity and finite-size effect in glassy polymers

Despite two decades of extensive research, direct experimental evidence of a dynamical length scale determining the glass transition of confined polymers has yet to emerge. Using a recently established experimental technique of interface micro-rheology we provide evidence of finite-size effect truncating the growth of a quantity proportional to a dynamical length scale in confined glassy polymers, on cooling towards the glass transition temperature. We show how the interplay of variation of polymer film thickness and this temperature-dependent growing dynamical length scale determines the glass transition temperature, which in our case of 2–3 nm thick films, is reduced significantly as compared to their bulk values.     

Relaxation processes in the glassy state: Molecular aspects

Molecular relaxations in the glassy state of amorphous polymers are discussed, and related to the energy requirements of the various possible molecular motions involved under two broad headings: those occurring in groups pendant to the main chain and those occurring in the main chain units themselves. A variety of motions can result in relaxations in polymers with alkane side chains, whereas only rotation (or partial rotation) is possible for pendant phenyl rings. Relaxations in polymers with cycloalkane rings in the side chain can originate within the ring itself or from motion of the ring as a unit. Restricted movements of various types of backbone chain are discussed, and possible alternatives to the postulated crank-shaft motions in polymethylene chains are presented.