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We report on the inverse thermogelation of aqueous solutions of high molecular weight HydroxyPropylCellulose (HPC). The gelation process is investigated at different polymer concentrations, in the range 4÷16% (w/w). For each concentration, different heating rates are considered, in order to explore the effects on the sol-gel transition. Our findings corroborate the scenario proposed in literature for the thermoreversible gelation of cellulose derivatives, according to which the sol-gel transition is governed by an interplay of liquid-liquid phase separation and gelation. Gelation occurs in the polymer-rich phase via enhanced hydrophobic interactions between polymer chains at high temperature. At low heating rates, the phase separation proceeds significantly before the gelation occurs, creating more coarsened phases. As a consequence, the polymer network is less percolated and results in a weaker gel. Such an effect is more evident in less concentrated solutions characterized by a shorter terminal relaxation time, that is, faster polymer diffusion. As HPC concentration and heating rates are increased, the dissipative capacity of the final gel, expressed by the value of the phase angle at high temperatures, becomes nearly independent on polymer concentration and heating rate. However, the absolute values of the viscoelastic moduli of the formed gel increase with both HPC concentration and heating rate. The temperature window where transition occurs slightly narrows down with increasing concentration.

In this work a design tool to control cell-opening in gas foaming of thermoplastic polymers is developed. The sequence of events following bubble nucleation, namely, bubble growth and impingement, is modeled to gain a comprehensive, perspective view on the mechanisms of bubble wall rupture and on the conditions for achieving a fully open-cell morphology. In particular, unlike the previously published literature, the polymer elastic recovery is recognized as an important factor for wall retraction, which is typically considered as solely driven by surface tension. The new approach is experimentally validated on poly(e-caprolactone) (PCL), foamed with CO 2 , as a model polymer/gas system.

Poly(3-hydroxybutyrate) (PHB) is a biodegradable polymer, whose applicability is limited by its relatively poor mechanical properties and narrow processing window. In this paper, a natural polyphenolic additive, tannic acid (TA) was used as thermal and processing stabilizer for PHB. The thermal stability of both neat and TA-doped PHB samples was studied by rheology and calorimetry. The experimental results show that the neat PHB massively degrades and that the addition of tannic acid enhances the thermal stability of PHB, thus widening the processing window of the polymer. Physical and chemical interactions between the polymer and the additive were considered as key factors to interpret the experimental data. The described results are of interest for the development of sustainable alternatives to synthetic polymer additives, by increasing the applicability of bio-based materials. (C) 2014 Elsevier Ltd. All rights reserved.

During laboratory practice, it is often necessary to perform rheological measurements with small specimens, mainly due to the limited availability of the investigated systems. Such a restriction occurs, for example, because the laboratory synthesis of new materials is performed on small scales, or can concern biological samples that are notoriously difficult to be extracted from living organisms. A complete rheological characterization of a viscoelastic material involves both linear and nonlinear measurements. The latter are more challenging and generally require more mass, as flow instabilities often cause material losses during the experiments. In such situations, it is crucial to perform rheological tests carefully in order to avoid experimental artifacts caused by the use of small geometries. In this paper, we indicate the drawbacks of performing linear and nonlinear rheological measurements with very small amounts of samples, and by using a well-characterized linear polystyrene, we attempt to address the challenge of obtaining reliable measurements with sample masses of the order of a milligram, in both linear and nonlinear regimes. We demonstrate that, when suitable protocols and careful running conditions are chosen, linear viscoelastic mastercurves can be obtained with good accuracy and reproducibility, working with plates as small as 3 mm in diameter and sample thickness of less than 0.2 mm. This is equivalent to polymer masses of less than 2 mg. We show also that the nonlinear start-up shear fingerprint of polymer melts can be reliably obtained with samples as small as 10 mg.

In this work we analyze the crystallization kinetics under steady shear flow conditions of different samples obtained by blending two isotactic poly(1-butene)s with different average molecular weights. It is observed that the addition of a small amount of high MW-polymer (

We investigated the linear melt viscoelasticity and the crystallization kinetics of a series of model poly(ethylene oxide) stars with different functionalities (f=4-32 arms) and moderately entangled arms (their molecular masses ranging from 5.5 to 12 kg/mol). The limited data in the homogeneous state indicated that the zero-shear viscosity eta(0) was adequately described by the Milner-McLeish model for functionalities f < 32, where the core effect is insignificant; a similar behavior was observed for the recoverable compliance J(e)(0) which depended on both the molecular weight and the number of the arms. Below the melting point, the isothermal crystallization was measured with differential scanning calorimetry and rheology, and analyzed in terms of the Avrami theory, expanding over a wide range of temperatures. The results were supported by additional polarizing optical microscopy data and indicated a slower crystallization kinetics of the stars compared to their linear analogues. They showed a strong dependence of the crystallization rate on the arm molecular weight, whereas the available experimental evidence is suggestive of some functionality dependence as well. (c) 2007 The Society of Rheology. Journal of Rheology

The phase transition of a series of thermoplastic polyurethanes has been investigated by means of rheological and calorimetric techniques. Five different samples of varying Molecular Weight (MW) and hard-to-soft segment ratio have been studied. Thermal histories included isothermal annealing after cooling from the melt and heating cycles following isothermal annealing. The coupled use of the two different techniques allows for a quantitative description of the kinetics of phase transition and for a good understanding of the corresponding microstructural changes. At least for the family of polymers here investigated, such a process is a combination of two distinct stages, a faster one due to the activation of hard segment interactions, and a subsequent slower one, related to the microphase separation between soft and hard segments. A decrease of MW is found to moderately accelerate the phase transition kinetics, whereas an increase of the hard segment fraction shifts the phase transition to higher temperatures. When the critical gel behavior is considered, experiments clearly indicate the equivalence of either decreasing MW or increasing the hard segment content. Some practical implications of the combined effect of MW, hard segment fraction and phase transition temperature on the polymer processing behavior are also discussed.

This study investigates the coupled effects of mild shear flow and temperature on the crystallization behavior of two thermoplastic polymers, namely, an isotactic polypropylene and an isotactic poly(1-butene). Rheological experiments are used to measure the crystallization induction time under isothermal, steady shear flow conditions. The experimental results clearly show the effects of the degree of undercooling on flow-induced crystallization (FIC). As temperature decreases, the corresponding increase in chain orientation at a given shear rate leads to an absolutely faster crystallization. At the same time, however, a temperature decrease makes the flow-induced driving force to crystallization relatively less influent with respect to the intrinsic kinetics. A FIC model based on the Doi–Edwards micro-rheological theory is shown to successfully describe the quantitative details of the observed experimental behavior.

Poly(3-hydroxybutyrate) (PHB) is a biodegradable polymer, whose applicability is limited by its relatively poor mechanical properties and narrow processing window. In this paper, different natural phenol-based additives, including tannic acid (TA), grape bagasse extract (EP), and a lignocellulosic biomass (LC) were used as thermal and processing stabilizers for PHB. The thermal stability of both neat and doped PHB samples was studied by rheology and calorimetry. The experimental results showed that neat PHB massively degrades and that the addition of phenol additives enhances the thermal stability of PHB, preserving the polymer molecular weight after processing. This finding was in agreement with the slower decay in viscosity observed through rheological tests. Physical and chemical interactions between polymer and additive were considered as key factors to interpret the experimental data. LC affected the melt crystallization kinetics of PHB enhancing crystallization upon cooling. This finding suggests th...

A detailed rheological study of aqueous solutions of methylhydroxyethylcellulose has been carried out in the presence of different acrylate-based graft polymer used as additive contents. Both polymer components are used in cement formulations to improve the flow performances of the concretes, but no physicochemical studies can be easily found in the literature. The content of the graft polymer has been varied between 0.1 and 2.7 wt% in an aqueous solution with a fixed content of 6.5 wt% of methylhydroxyethylcellulose. Creep curves were performed at different stresses in order to build up the flow curves for the various solutions. We found that the addition of the graft polymer triggers a phase transition, which is made more dramatic by the presence of an external flow. A “flow-phase diagram” has been obtained, which could be used as a guide for determining the critical conditions for the onset of the flow-induced instability.

Aqueous solutions of hydroxypropylcellulose (HPC) have been widely used as a model system to study liquid crystalline behavior in polymers. The HPC limiting concentration for mesophase formation in water is about 40% wt, quite independent of molecular weight. Most rheo-optical investigations have been carried out in the concentration range from 50% to 65% wt, on the assumption that only the liquid crystalline phase was present. In this study, by using video-enhanced contrast light microscopy, we show that an isotropic phase in form of tiny droplets is also present at concentrations up to 60% wt, both in quiescent and in sheared samples at room temperature. The isotropic phase can be made to disappear by lowering the temperature. The effects of phase separation on the rheology of the HPC/water system are studied by measuring viscosity as a function of temperature and concentration. A slope of Region I of the viscosity curve close to −0.5 is found only at low temperatures, when the sample is fully anisotropic, whereas an anomalous dependence of the viscosity on temperature is observed when phase separation is significant. This study shows that special care is needed when interpreting experimental results from the HPC/water system in terms of theories for liquid crystalline polymers.

In this paper, we report on the synthesis and detailed characterization of a new semiflexible nematic liquid crystalline polyester which could serve as a ‘model’ polyester for a variety of physical and physico-chemical investigations. The polymer is a nematic liquid over a wide temperature range–from the glass transition temperature at ∼95°C to the isotropic transition at ∼240°C. We expect this polyester to be particularly useful for studying the effect of flow on the orientation of liquid crystalline polymers, as well as the production and removal of disclinations.

The rheological behaviour of liquid crystalline polymers differs considerably from that of ordinary polymers as well as of low-molecular-weight liquid crystals. The only theoretical approach which has reached a significant development is based on the rigid rodlike model of the polymer molecule. In spite of the crudity of the model, the theoretical predictions based on it prove able to qualitatively describe the most peculiar features of the observed rheological behaviour. A comparison between the relevant experimental peculiarities and the qualitative predictions of the theory is presented.

In this paper we analyze the crystallization kinetics under steady shear flow conditions of different samples obtained by blending two isotactic poly(1-butene)s with different average molecular weights. It is observed that the addition of a small amount of high molecular weight (MW) polymer (

Cone and plate rheological measurements have been performed on aqueous solutions of hydroxypropylcellulose in the liquid crystalline phase. The time‐ dependent experiments (start‐up of shear and oscillatory shear flow) suggest the presence of a supramolecular structure which is modified by the flow. The characteristic time of structure evolution is estimated, and a tentative explanation of such a process is suggested in terms of polydomain rearrangement. Steady‐state measurements confirm some peculiar properties of liquid crystalline polymers, such as the presence of a shear thinning regime at the lowest shear rates and the occurrence of negative values of the first normal stress difference. Finally, an anomalous, yet reproducible, behavior of the dynamical properties is reported and discussed.

Flow Induced Crystallization (FIC) is the common term to indicate the acceleration in polymer crystallization kinetics due to the action of flow. When modeling FIC, two major challenges are encountered. On the one hand, the model must be able to produce quantitative reliable results, while correctly describing the coupling between the intrinsic (quiescent) crystallization kinetics and the rheological response of the polymer. On the other hand, the model must be able to describe the complex kinematics taking place in real industrial processes. In this paper, we present the predictions of a recently proposed model for FIC in the case of a mixed flow, where both shear and extensional components are present at the same time. In particular, the effects of the overall flow intensity and of relative weight between shear and extension on the enhancement in nucleation rate are presented and discussed. Some guidelines for future development are also proposed.

The effects of recycling on the processability of two injection molding grade thermoplastic polymers has been investigated. Recycling of a Polypropylene (PP) and a Polyamide (Nylon 6, PA6) has been simulated by repeated injection molding cycles, followed by blending of the processed materials with the virgin polymer. The flow properties of both the recycled materials and the blends have been evaluated by different rheological techniques, namely, capillary viscosity and MFI measurements, linear viscoelastic tests and solution viscosity number determination. The experimental results clearly indicate a qualitatively different behaviour of the two polymers upon recycling. PP shows a reduction of both viscosity and viscoelastic properties upon increasing the number of processing cycles, thus confirming the thermo-mechanical degradation of this polymer. PA6, conversely, shows an increase of both the melt and the number viscosity as the number of injection molding passages is increased. The results on PA6 suggest also that the thermally-induced re-gradation is accompanied by an increase of the degree of branching, that may be responsible for some peculiar features of the Virgin/Recycled blend rheology.

The effect of molecular weight on the crystallization behavior of isotactic poly(1-butene) (iPB) under quiescent, isothermal conditions is investigated by rheological and optical microscopy techniques. Small-amplitude oscillatory shear experiments are used to determine the critical gel behavior of the crystallizing melt. The resulting critical gel time is related to the crystallization kinetics during the early stages of the process. While molecular weight does not substantially affect the gel characteristic time, the rheological parameters of the crystalline critical gel are found to be strongly dependent upon molecular weight, thus suggesting that the viscoelastic behavior in the early stages of crystallization is dictated by the molecular properties of the amorphous phase. Optical microscopy observations provide quantitative information on the growth rate and the morphology of the crystallites. It is found that the molecular weight has no effect on the iPB growth rate. Conversely, molecular weight affects the morphological transition from spherulites to square-shaped (hedrites) crystallites. The coupled effects of temperature and molecular weight on the spherulite-to-hedrite transition are presented in the form of a phase diagram which quantitatively confirms recent observations made by Fu et al.

The flow-induced transition of Liquid Crystalline Polymers (LCPs) is studied by rheological techniques. Aqueous solutions of Hydroxypropylcellulose (HPC) in water are adopted as a model LCP system. Non-isothermal oscillatory tests are first used to quantitatively determine the “rheological” phase diagram of the HPC/Water system under quiescent conditions. The phase diagram compares well with those obtained by other, more conventional techniques. Superposition of oscillatory and steady shear flow is then used to describe the non-isothermal flow-induced transition. In this case, it is shown that a critical shear stress must be reached to effectively induce the isotropic/mesophase transition. Stress-loop experiments are also used to identify the isothermal flow-induced transition, and to provide information on the transition kinetics

Liquid Crystal Polymers (LCP) exhibit a complex rheological behaviour which is mainly due to the intrinsic anisotropy of the macromolecules. The Doi (1981) theory is able to capture many of the essential features of LCP rheology. The main limit of this theory is the assumption that the system is spatially homogeneous. This assumption precludes the theory from accounting for the generation and the proliferation of textures.

This work shows the application of several experimental methods to the measurement of the interfacial tension between two immiscible polymers. A quantitative knowledge of the interfacial tension is important in view of the crucial role that this parameter plays in polymer blend processing. Common to all methods presented here are two main points. The first is that the interfacial tension is obtained from experiments where the shape of the interface between the liquids is directly observed by means of optical microscopy techniques. The second point is that the interface geometry is controlled by a balance between the interfacial force and the viscous stresses generated by some flow applied to the system. Measurements have been carried out on a model polymer blend, whose constituents are a poly-isobutylene and a polydimethylsiloxane, both transparent and liquid at room temperature. When compared with each other, the values of interfacial tension obtained from the different methods show a good quantitative agreement. Excellent agreement is also found with results for the same system previously published in the literature

The motion of aqueous solutions of hydroxypropylcellulose in a shallow channel was visualized by means of an electrochemical technique. Isotropic and anisotropic solutions of approximately the same viscosity were used. The velocity profiles of the liquid crystalline solution were found to be qualitatively very different from those of the isotropic one. In particular, anomalous maxima in the velocity near the side walls were observed.

An expression for the rotational diffusivity of a rod in a polydisperse system of similar rods in the general case of anisotropic conditions is derived. This result is a prerequisite for extending the Doi-Edwards and Doi theories to the polydisperse case. As a simple application, the zero-shear viscosity in the isotropic phase is calculated explicitly.

A tracer particle technique has been used to investigate the deformation profiles of a liquid-crystalline polymer under shear between parallel plates. The system was an aqueous solution of hydroxypropylcellulose (HPC), the good transparency of which allows for optical observations over thicknesses of the order of millimetres, as used here. The results indicate that the deformation profile deviated considerably from linearity, whereas a linear profile was confirmed for isotropic liquids, including polymeric ones. For the HPC liquid-crystalline solution most of the deformation was concentrated close to the walls.