Commentary on “Solid-like rheological response of non-entangled polymers in the molten state” by H. Mendil etal.

We discuss the rheology experiments on classical and liquid crystal polymer melts by Mendil et al., in the light of the old and new piezorheometry experiments we have carried out on both types of melt. The mechanical behavior we have observed in the linear and non-linear regimes are independent of the melt studied (classical or liquid crystal), and of their nature (siloxane-type, acrylate-type and styrene-type). In the linear regime, the mechanical behavior of the melts presents two components: the first one is the conventional contribution. It is due to polymer chains, and is independent of sample thickness. The second one, which can be observed only when a strong interaction between the compound and the substrate exists, is associated with the glass transition. This component displays an elastic response depending on the sample thickness, and disappears at high temperature. It can be explained by assuming the presence of long-range density fluctuations, which are associated with the glass transition, and frozen at the frequencies used in the experiments. The experiments as a function of the applied strain show that the value of the elastic component decreases when the applied strain increases. This slipping transition occurs progressively, which highlights the heterogeneous nature of the anchoring. The results on the classical polymer by Mendil et al. can be considered to be consistent with ours. In contrast, their results on the liquid crystal polymer differ markedly from ours, showing that the elastic response of this sample has not the same origin.     

Polymer thin films and surfaces: Possible effects of capillary waves

It is discussed how the proximity of a free surface or mobile interface may affect the strain relaxation behavior in a viscoelastic material, such as a polymer melt. The eigenmodes of a viscoelastic film are thus derived, and applied in an attempt to explain the experimentally observed substantial shift of the glass transition temperature of sufficiently thin polymer films with respect to the bulk. Based on the idea that the polymer freezes due to memory effects in the material, and exploiting results from mode-coupling theory, the experimental findings of several independent groups can be accounted for quantitatively, with the elastic modulus at the glass transition temperature as the only fitting parameter. The model is finally applied discussing the possibility of polymer surface melting. A surface molten layer is predicted to exist, with a thickness diverging as the inverse of the reduced temperature. A simple model of thin polymer film freezing emerges which accounts for all features observed experimentally so far. Received 8 August 2001     

聚苯硫醚熔体的压致凝固行为

采用施加压力的方法将聚苯硫醚熔体凝固,凝固后获得的聚苯硫醚样品经过降温和卸压后在常温常压下回收. X射线衍射和差示扫描量热分析表明:约20 ms时间的快速压缩过程可以抑制熔体结晶,制备出非晶态聚苯硫醚块材,样品的表面及中心都是非晶态.非晶态聚苯硫醚的玻璃化转变温度和晶化温度分别为318和362 K.常压下的退火实验表明,非晶态聚苯硫醚在425 K等温结晶的产物为正交相晶型.压致凝固法中熔体的凝固不是靠温度变化,而是靠压力变化,样品表面和内部处在一致的温度下同时受压凝固,避免了热传导对非晶尺寸的影响,因此非常有利于获得结构均匀的大尺寸非晶态材料.     

The Effect of Orientation Relaxation on Polymer Melt Crystallization Studied by Monte Carlo Simulations

We use dynamic Monte Carlo simulations to study the athermal relaxation of bulk extended chains and the isothermal crystallization in intermediately relaxed melts. It is found that the memory of chain orientations in the melt can significantly enhance the crystallization rates. The crystal orientation and lamellar thickness essentially depend on the orientational relaxation. Moreover, there is a transition of the nucleation mechanism during the isothermal crystallization from the intermediately relaxed melts. These results explain the mechanism of the self-nucleation by orientation and suggest that in flow-induced polymer crystallization, the orientational relaxation of chains decides the crystal orientation.     

Lactone polymers. V. Viscoelastic properties of poly-ε-caprolactone and poly-ε-thiocaprolactone

The viscoelastic properties of crystalline poly-ε-caprolactone and poly-ε-thiocaprolactone have been characterized and compared by stress relaxation and dynamic mechanical studies. The glass transition temperature of poly-ε-thiocaprolactone was shown to be -40°C at 1 Hz and appeared to be independent of the degree of crystallinity. The rate of viscoelastic relaxation for each polymer was independent of linear strain rate of a decade range. The density of each polymer over a wide temperature range was used to reduce the individual time-dependent modulus values to an arbitrary reference temperature. This reduction of stress relaxation data to a standard mechanical state obviated the requirement of vertical shift factors for construction of the respective master curves. The distribution of relaxation times was correlated with the glass transition and the crystalline melt temperature range for each polymer.     

Glass transition and relaxation following PhotoPerturbation in thin polymeric films

In this letter we employ null ellipsometry on Langmuir Blodgett multilayers of a glass forming photosensitive side chain polymer to investigate both the structural changes and the related time relaxation as a function of temperature and sample thickness following isothermal optical perturbation. Below a thickness of a few layers the glassy multilayer system collapses to a smecticlike crystal. This effect is discussed in the context of the glass transition in restricted dimensionality.     

The synergetic theory of the glass transition in liquids

The glass transition is treated as a spontaneous emergence of the shear components of strain and stress elastic fields upon cooling a liquid at a rate exceeding the critical value. The stationary elastic strains and stresses and the effective relaxation time are determined within the adiabatic approximation. It is shown that the glass transition process occurs through the mechanism of a first-order kinetic transition with allowance made for the strain dependence of the shear modulus. The critical cooling rate turns out to be proportional to the thermal diffusivity and unrelaxed shear modulus and inversely proportional to the temperature derivative of the relaxed shear modulus and the square of the heat conductivity length of the sample.     

Reply to the “Commentary by D. Collin & P. Martinoty”

We would like to thank the authors of the commentary [1] for their efforts to reproduce in the range of the narrow gap geometry (10–40 μm), our experiments [2] using the same polymer (PBuA – poly(n-butyl acrylate) – for the ordinary polymer) and a cyano-biphenyl substituted polyacrylate for the liquid crystal polymer sample. The comparison of their results with ours is very instructive.     

Dynamic rheological and dynamic thermomechanical properties of poly(trimethylene terephthalate)/short carbon fibre composites

The dynamic rheology and thermomechanical properties of poly(trimethylene terephthalate) (PTT)/short carbon fibre (CF) composites at different mechanical states were investigated by a rotational rheometer and a dynamic mechanical analyzer (DMA). At molten state, the composite melts were pseudo-plastic fluids, and the complex viscosity of the composite melts decreased much with increasing CF content because of the poor adhesion at the fiber/matrix interface. The viscous behavior was predominant rather than elastic behavior in the composites melt and viscous behavior was increased with increasing CF at low shearing frequency. An apparent slope change in storage modulus and loss modulus plot suggested that a structure change occurred in the melt that was dependent on shearing frequency. At glassy state, the storage modulus increased with increasing CF content, suggesting that CFs had good reinforcing effect on PTT. At glass transition region, the increasing loss modulus indicated a better toughness of the composites, and the elastic behavior was predominant rather than viscous behavior. Moreover, the glass-transition temperatures of the composites increased with 10% CF content. The composites have larger cold-crystallization rate than pure PTT.     

电控聚合物分散液晶变焦全息透镜制作

介绍了相位型全息聚合物分散液晶(PDLC)材料全息透镜,在电场作用下液晶微滴折射率逐渐与聚合物折射率匹配,实现透镜电控变焦。研究了微米尺寸和纳米尺寸液晶微滴聚合物分散液晶材料配方特性和微观结构。采用优化纳米尺寸材料配方制作5~6μm聚合物分散液晶盒,采用离轴式平面波和球面波干涉全息写入光路,成功制作电控变焦聚合物分散液晶全息透镜样品。该透镜样品焦距为20 mm,能够正一级衍射放大成像。实现“0”,“1”变焦的驱动电压阈值为60 V。并进一步提出了基于聚合物分散液晶电控变焦元件集成叠加技术实现电控变焦光学成像系统的技术思路。     

Structure and properties of polybutylene crystallized from the glassy state. I. X-ray scattering,DSC, and torsion pendulum

Isotactic polybutene (PB) can be quenched into a completely glassy state by quenching molten films into a solid-liquid mixture of isopentane, Freon, or ethanol. The crystallization of PB from the glass form was studied by x-ray scattering, differential scanning calorimetry (DSC), and dynamic mechanical spectroscopy (torsion pendulum). As for crystallization from the melt, PB crystallizes from the glass into a tetragonal crystal structure (Form II) at ca. 0°C, depending on sample thickness, and then transforms to the twinned hexagonal structure (Form I) upon aging at room temperature. In the presence of isopentane, PB crystallizes partially from the glass into the untwinned hexagonal (Form 1′) structure at ca. -70°C; the rest of the sample starts to transform to tetragonal structure at ca. -30°C and nearly completes crystallization at ca. 0°C. The exact temperatures of both transformations depend on the amount of isopentane present and sample thickness. Upon aging at room temperature the tetragonal structure converts to the twinned hexagonal structure even faster than in the absence of isopentane. Dynamic mechanical experiments show the presence of two relaxation-like peaks for the ultraquenched samples: T_r (L) = -27°C and T_r (U) = -15°C. X-ray diffraction, DSC, and torsion pendulum experiments show that PB crystallizes from the glass at T_r (U).     

Viscoelastic dynamics of polymer thin films and surfaces

The strain relaxation behavior in a viscoelastic material, such as a polymer melt, may be strongly affected by the proximity of a free surface or mobile interface. In this paper, the viscoelastic surface modes of the material are discussed with respect to their possible influence on the freezing temperature and dewetting morphology of thin polymer films. In particular, the mode spectrum is connected with mode coupling theory assuming memory effects in the melt. Based on the idea that the polymer freezes due to these memory effects, surface melting is predicted. As a consequence, the substantial shift of the glass transition temperature of thin polymer films with respect to the bulk is naturally explanied. The experimental findings of several independent groups can be accounted for quantitatively, with the elastic modulus at the glass transition temperature as the only fitting parameter. Finally, a simple model is put forward which accounts for the occurrence of certain generic dewetting morphologies in thin liquid polymer films. It demonstrates that by taking into account the viscoelastic properties of the film, a morphological phase diagram may be derived which describes the observed structures of dewetting fronts. It is demonstrated that dewetting morphologies may also serve to determine nanoscale rheological properties of liquids.Received: 1 January 2003, Published online: 14 October 2003PACS: 47.50. + d Non-Newtonian fluid flows - 68.47.Mn Polymer surfaces - 68.60.Dv Thermal stability; thermal effects     

The Great Myths of Rheology Part III: Elasticity of the Network of Entanglements

The dynamic data of polymer melts are analyzed in a novel way, presenting new correlations between the viscosity, G′ and G′′ (the elastic and loss moduli), and strain rate and the implications of the new formulas on our understanding of melt entanglement network elasticity are discussed. In the two previous articles of this series, Part I and Part II, we showed that the existing models valid in the linear viscoelastic deformation range were not adequate to extrapolate to the nonlinear regime, suggesting that the stability of the network of entanglements was at the center of the discrepancies. In this article, we introduce new tools for the analysis of the dynamic data and suggest new ideas for the understanding of melt deformation based on this different focus. In particular, we express classical concepts, such as shear-thinning, melt diffusion or melt elasticity and viscosity, in a different context, that of the existence of a dual-phase interaction, essential to our treatment of the statistics of interaction of the bonds responsible for the system coherence and cohesion. It is within this framework that viscoelasticity parameters emerge and the new view of the deformation of a polymer melt results in a different definition of the entanglement network.     

Computation of solidification problems with hydrodynamic convection resolving energetic anisotropies at the microscale quantitatively

In this work we propose a new numerical approach to solve the solidification of microstructures from a pure melt including hydrodynamic effects in the molten phase. The model is based on the classical sharp-interface model, i.e the solid–liquid interface is tracked and latent heat is released. An enhanced scheme is employed to solve fluid flow in the melt. The no-slip condition is applied on the interface by enforcing the velocities in the solid phase to be zero. The morphology evolution of the solidifying crystal microstructure under the influence of convection is compared with an existing morphology diagram for pure diffusion controlled growth (see Brener et al. [1]). The peculiarity of our approach is that it models the physical anisotropies along the solid–liquid interface with high accuracy. This allows us to report changes in the morphology diagram given by Brener et al. [1] due to the influence of forced flow. Moreover, we present some results on the scaling of the dendritic tip in such cases.     

Molecular order and dynamic mechanical behavior of polyurethanes based on liquid crystalline diol

Synthesis of the liquid crystalline (LC) diol 6,6′-[ethylenebis(l,4-phenylene-oxy)]-dihexanol (I) is described. The structure of polyurethanes prepared from diol I and 4,4′-methylenedi(phenyl isocyanate) (MDI), 4,4′-methylenedi(cyclohexyl isocyanate) (HMDI), or 2(4)-methyl-l,3-phenylene diisocyanate (TDI) at 1:1 molar ratios of isocyanate and hydroxy groups is studied by dynamic mechanical spectroscopy, differential scanning calorimetry (DSC), polarizing microscopy, and x-ray scattering. The polymer prepared from HMDI and the diol (I/HMDI) shows, on cooling, thermal behavior typical of amorphous polymers. A frequency-temperature superposition could be applied to the mechanical data, and the horizontal shift factor satisfied the Williams-Landel-Ferry (WLF) equation. A more-complex thermal behavior was found for I/HMDI polymer during subsequent heating; above 70°C, the formation of an ordered structure takes place, and this structure melts at about 120°C. Complex thermal behavior is exhibited by I/TDI polymer. On cooling its melt, the polymer forms a nematic phase at about 80°C, which freezes into the LC glassy state. On heating, the mesophase melts, and. subsequently, a better-ordered smectic phase is formed at 95°C, which melts at 120°C. This structure buildup is accompanied by a rapid increase in storage modulus G′, and the sample shows thermorheologically complex mechanical behavior. The polymer formed from the diol and MDI (I/MDI) exhibits a most-complex thermal behavior. On cooling and heating, four transitions can be detected in its thermal mechanical behavior, and the structure of the polymer is strongly dependent on its thermal history.     

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.     

不同类型偶氮材料光致双折射的温度特性研究

在不同温度下,分别测量了掺杂偶氮材料、偶氮聚合物和偶氮液晶聚合物的光致双折射行为,并利用双e指数模型对光致双折射的动力学过程进行了拟合.实验结果表明,偶氮材料的光致双折射源于偶氮分子的光致异构和光致分子取向,光致双折射大小随温度的升高表现出先增大后减小的趋势.在抽运光的作用下,含偶氮材料的光致双折射包含一个由偶氮分子取向引起的快过程和一个由偶氮分子带动大分子取向引起的慢过程.关闭抽运光后,掺杂偶氮材料和偶氮聚合物表现为可逆的弛豫,而偶氮液晶聚合物则展现出长久光存储特性.     

Crystallization Kinetics of Lamellar Crystals Confined in Polymer Thin Films

We used dynamic Monte Carlo simulations to investigate the crystallization kinetics of flat-on lamellar polymer crystals in variable thickness films. We found that the growth rates linearly reduced with decreasing film thickness for the films thinner than a transition thickness d_t , while they were constant for the films thicker than d_t . Moreover, the mean stem lengths (crystal thickness) we calculated decreased with film thickness in a similar way to the growth rates, and the intramolecular crystallinities we calculated confirmed the film thickness dependence of the crytsal thickness. Besides, the crystal melting rates in thin films were calculated and increased with decreasing film thickness. We proposed a new interpretation on the film thickness dependence of the crystal growth rate in thin films, suggesting that it is dominated by the crystal thickness in terms of the driving force term (l–l _min) expressed by Sadler, rather than the chain mobility based on experiments. The crystal thickness can determine whether a crystal grows or melts in a thin film at a fixed temperature, indicating the reversibility between the crystal growth and melting.