Experimental methods in polymer melt rheology

Experimental developments in different areas of polymer melt rheology and recent results are reviewed: Shear oscillations were performed with mixtures of polyisobutylenes (PIB) of relatively small molecular mass distributions and with blends of polystyrene (PS) and polymethylmethacrylate (PMMA). For the latter, the interfacial tension between the melts of PS and PMMA can be derived from the results. - In constant shear rate flow, the measurement of the first normal stress difference N₁ in a commercial cone-and-plate rotational rheometer is simple if the test temperature is kept extremely constant. By a further modification a partial integral of the pressure distribution is measured from which the second normal stress difference N₂ can be determined. - The molecular orientation in shear flow results in a rheological anisotropy that can be studied in a parallel-plate shear rheometer in which the polymer melt sample can arbitrarily be sheared in two perpendicular directions. - For melt elongation by rotary clamps constant, arbitrary ratios of the strain rate components can be applied. Planar elongations of the PIB samples indicate an influence of the width of the molecular mass distribution. More general elongations with a change of the strain rate ratio during the test are of especial interest as is documented by the comparison of the resulting stresses with predictions from different network theories. - In a rheometer for simple elongation at 170°C, recovery after melt extension of the PS/PMMA blends reveals a value of the interfacial tension equal to the one obtained from shear oscillations.     

On-line analysis of polymer melt processes

Molten polymer process streams are difficult to analyze either in- or on-line because of sampling problems due to the high temperature and viscosity of the molten state. Real-time monitoring of chemical compositions in these processes can significantly improve safety and product quality and minimize process costs and waste. The information content of the mid-infrared spectrum combined with the recent development of rugged process Fourier transform (FT) IR spectrometers is stimulating the application of process FT-IR to industrial polymer melt processes. Sampling considerations for polymer melts are reviewed. Also, the use of FT-IR spectrometry for on-line measurements of the polymer composition for polymer blends and copolymers in the melt, and the question of how this information could be used to monitor and control the quality of the product given by the process are discussed.     

Estimation of melt elasticity of degraded polymer from melt flow index

An effective method, proposed earlier by the authors, has been extended to generate normal stress difference versus shear rate curves from the melt flow index of polymer waste. This would be a useful tool for the plastics processor to assess the elastic response of the waste material and adjudge its potential and conditions for reprocessing.     

Designing polymer additives to minimise loss

The solubility of additives in polymers is shown to determine the state of the additive in a processed sample and the mechanisms by which it may be lost. The loss of additives is described in terms of a model which considers the effects of solubility, diffusion rate and volatility, and it is shown that the rate and mechanism of loss depends upon these quantities and upon the sample geometry. The effects of additive structure on loss mechanism and rate are discussed and the implications for additive design are reviewed.     

Small changes with big effects: Tuning polymer properties with supramolecular interactions

For biological polymers like DNA and proteins, supramolecular interactions dictate the folding and assembly of the polymer chains. Advances in synthetic polymer chemistry enable the synthesis of polymers of defined length and composition, but the field has yet to reach the same level of sophistication as nature's polymers. However, the incorporation of just a few supramolecular interactions into a synthetic polymer chain can drastically change the manner in which the polymer assembles and interacts, thereby altering the properties of a polymeric material. This highlight will focus on approaches wherein a low‐density of supramolecular functionalities (<10 wt %) were used per polymer chain. How the selection of the appropriate supramolecular functionality (based on the directionality and strength of the interaction), along with the location of these groups on a polymer chain, can afford a spectrum of material properties has been highlighted. At one end, the supramolecular motif can dramatically alter the elasticity of a material, and at the other, the motif can have a more subtle effect like increasing the stability of a micelle. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 457–472     

Tuning the interactions in cyclodextrin polymer nanoassemblies

The aim of this paper was to study the associations between a neutral β-cyclodextrin polymer and amphiphilic dextrans substituted with two kinds of groups able to make inclusion complexes with β-cyclodextrin. The first kind of substituent is purely hydrophobic (dodecyl C₁₂ or adamantyl Ada groups) and the second one, cyclohexancarboxylic acid group CHX, brings pH-dependent charge density on the dextran chains. Synthesis and characterization of the di-substituted dextrans have been detailed in a first part where it has been shown that the incorporation of CHX groups (up to 7 mol.%) did not modify the self assembling properties of the amphiphilic dextrans. Affinity of the amphiphilic dextrans for β-cyclodextrin polymer has been studied by different methods including isothermal titration calorimetry, dynamic light scattering and zeta potential. An original pH sensitive behaviour has been obtained for polymer–polymer associations at low concentrations. Nanoassemblies are formed at pH around 4 which are destabilized at lower pH and swollen into soluble aggregates at higher pH. The nanoassemblies formed at pH 4 have also been evidenced by transmission electron microscopy and have revealed a spherical shape of few hundred nanometers.     

Prediction of DNA separation by capillary electrophoresis with polymer additives

The paper is focused on the powerful prediction ability of the quantitative DNA sieving model in DNA separations by capillary electrophoresis, which was proposed by us previously. First, the DNA resolution can be predicted by the theory. The model predicts that the most difficult and easiest separation will be 184bp/192bp and 234bp/267bp respectively, which is consist with experimental results. Furthermore, the average relative differences of predicted and experimental resolution values (R(S)) for ssDNA 184b/192b or dsDNA 184bp/192bp were all smaller than 2.8% if the diffuse parameter D considered was 8×10(-5) cm(2)/s. Secondly, the optimum polymer concentrations for DNA separation were also calculated by the model, and the results show that polymer concentration should be as high as possible in DNA separation. Thirdly, the sieving ability of polymer will be predicted by the model. Polymer with smaller k, a polymer parameter calculated by the model, is prior to use as DNA sieving media.     

Polymer conformation at the polymer melt surface

Due to the symmetry which characterizes the surroundings of each chain segment in the amorphous melt all (but end) segments are equivalent in their embedment in space. Energetic interactions between segments are confined to segments in contact. Hence, in the presence of a surface only the segments actually in the surface layer will be affected and will have a statistical weight and Boltzmann factor different from segments in the bulk. The number of segments so involved is fixed, since the extent of the surface is controlled. The partition function is comprised of factors which depend neither upon the order of the segments in the chain nor upon the conformation of any particular chain. Only segments finding themselves in the surface contribute to the surface tension, with the largest contribution deriving from the density transition in the interface. This, at most temperatures, is confined to a lamella one segment layer, or less, thick. For polymer of high enough molecular weight end-effects can be neglected and no allowance for the presence of ends is made here. It is then possible to reach a representation of surface tension in reduced coordinates, as shown already by Posner and Sanchez. Changes in chain conformation near the surface are calculated.     

Statistical thermodynamics of polymer crystal and melt

A crystalline-state theory recently developed by Midha and Nanda is commented on and applied to the isobars of polyethylene, poly(vinylidene fluoride), and poly(chlorotrifluoroethylene) at atmospheric pressure, and to an isotherm of polyethylene. Satisfactory agreement with experiment results. This includes the volume change at the melting point T_m and the volume difference ΔV between crystal and melt below T_m, when crystal and the earlier liquid-state theory are combined. A similar agreement is noted with respect to the results at high pressure. The scaling parameters obtained indicate the approximate role of melt temperature and volume as reducing quantities. An inverse proportionality between T_m and α_l, the expansivity of the melt at T_m, derived much earlier for low-molecular-weight solids, is recovered with an identical numerical coefficient. The thermodynamic functions of polyethylene are investigated in both phases. For this purpose contributions of internal harmonic modes are considered within the framework of the equivalent s-mer model. One or, at most, two average frequencies are adequate to represent the temperature dependence of the excess free energy and entropy over the value at absolute zero, when the external contributions are included for the crystal. A similar representation of the hard modes can be adopted for the melt. However, the free energy of segmental disorientation computed either from a constant entropy for the s-mer or a rotational isomeric state model for the isolated chain does not appear to be an adequate representation over a sufficient temperature range. An additional temperature-dependent term in the entropy and free energy is introduced and tentatively attributed to a volume- and temperature-dependent short-range ordering. Good agreement with experiment, including the entropy and temperature of fusion, ensues.     

Influence of additives on recycled polymer blends

Polymer systems based on polymer waste offer promising way to increase recycling in the society. Since fillers play a major role in determining the properties and behavior of polymer composites, recycled polymers can also be combined with fillers to enhance the stiffness and thermal stability. In this study, blends of recycled polyethylene and recycled polypropylene with mica and glass fiber were prepared by melt blending technique. The effect of the particle loading, filler type, and filler–matrix interaction on thermal degradation and thermal transition of processed systems were investigated. Thermogravimetric analysis, differential thermogravimetric analysis, and differential scanning calorimetry were used in this investigation. Comparative analysis shows that both fillers produced different effects on thermal properties of the processed systems. These results were confirmed by calculating the activation energy for thermal degradation and thermal transition using Kissinger and Flynn–Wall expressions.     

Determination of polymer additives using analytical pyrolysis

When analyzing a polymeric material using pyrolysis-GC, the majority of the peaks seen are degradation products from the polymer matrix, but there may be specific compounds present resulting from the presence of antioxidants, plasticisers, stabilizers, flame retardants and other additives. Some of these compounds may be volatile or semi-volatile and appear as intact molecules, while others are larger and only appear as fragments after the pyrolysis. In understanding the pyrolysis of the complete system, it is important to understand the behavior of such additives under the thermal conditions used to analyze the polymer matrix.This paper presents data for several polymer additives, showing their contribution to the analytical results when studying typical polymers using Py-GC/MS. Specific types of additives include phenolic antioxidants, hindered amine light stabilizers, phthalates and phosphites.It was determined that for some additives, especially when analyzing simple polymers, co-analysis of the polymer and additive was feasible. For other, more complex formulas, a multi-step approach permitted a thermal separation of compound families and simplified the analysis. For some additives, especially in the parts-per-million range, pyrolysis with selected or extracted ion mass spectrometry was the most informative.     

The diverse effect of antiplasticizer in the molecular dynamics of an organic dye-doped polymer observed at different motional lengthscales

Complementary thermal analysis techniques were used to study blending-induced perturbations in polymer dynamics pertaining to different motional lengthscales. The antiplasticizing role of common neutral and apolar fluorescent perylimides on the cooperative relaxation dynamics of poly(methyl methacrylate) (PMMA) chain segments is evidenced by a clear rise of both the glass transition (T_g) and liquid-liquid transition (T_LL) temperatures. Simultaneously, the dielectric strength, Δε_β, of the signal ascribed to restricted rotational movements of lateral groups, shows a substantial reduction. Most aspects of the observed behavior bear analogies with recent experimental observations in nanoconfined PMMAs (e.g., PMMA with homogeneously dispersed SiO₂ nanospheres, in-situ polymerized in silica nanopores or in the form of supported ultrathin films), suggesting that a common mechanism is operational. In our mixtures, confinement effects, such as a modification in the macroconformation of the polymer, and changes in the orientation and packing of the polymer random coil, provide a plausible explanation of the observed changes regardless of the motional lengthscale concerned. Consonant with this scenario are reports of advanced optical properties for perylimide + PMMA blends, ascribed to the high rigidity of the matrix together with weak intercomponent interactions.     

Interfacial tension of demixed polymer solutions: augmentation by polymer additives

The interfacial tension between phase separated polymer solutions increases pronouncedly upon the addition of small amounts of incompatible polymers. This feature is demonstrated by means of measurements with solutions of polystyrene in cyclohexane and the following additives: poly(styrene-block-dimethylsiloxane), polyisobutylene and polydimethylsiloxane. Theoretical considerations based on a correlation between the lengths of tie lines and the corresponding interfacial tension corroborate this finding.     

Tuning reversible supramolecular polymer properties through co-monomer addition

Unlike most low molar mass organogelators, which form strong gels due to the entanglement of strong molecular aggregates such as crystalline fibres, a few reversible supramolecular polymers form viscoelastic solutions due to the formation of both rigid and dynamic filaments. By using three different bisureas which self-assemble according to the same hydrogen-bonding pattern and form such rigid reversible supramolecular polymers, we have shown that it is possible to finely tune the properties of the resulting solutions. The critical temperature below which viscoelastic solutions are obtained and the viscosity of the dilute solutions can be adjusted by changing the co-monomer content. This clearly facilitates gel formulation, which is an important step as far as applications are concerned.

By using three different bis-ureas which self-assemble according to the same hydrogen bonding pattern and form rigid reversible supramolecular polymers, we show that it is possible to finely tune the properties of the resulting viscoelastic solutions.     

Towards realistic rheological models for polymer melt processing

Greatly needed improvements to rheological models of polymer fluids can best be made by understanding and modifying a model on the molecular level, where the physics governing real fluids are created. For this purpose, stochastic simulations are an indispensible tool. The usefulness of stochastic simulations for kinetic theory models are well-known. In this paper, we show that the class of Rivlin-Sawyers, or K-BKZ, integral models is also subject to a molecular interpretation and can be simulated using stochastic techniques. We also illustrate how an existing molecular model can be modified, resulting in better predictions of viscoelastic behavior. For this we formulate a stochastic simulation of a reptation model modified to account for double reptation and tube-length fluctuations.     

Neutron reflectivity studies of hot polymer melt interfaces

Neutron reflectivity data have been obtained using a specially designed cell that allows the investigation of the interfaces between polymers in the melt state. Systems which are semicrystalline at room temperature can be investigated for the first time. Example results from a polystyrene (PS)/ polyethylene (PE) interface (both with and without copolymer) and an isotactic polypropylene (PP)/ high density polyethylene interface are presented.     

Effects of polymer additives on pool boiling phenomena

The addition of small amount of soluble polymer makes the boiling behavior significantly different from that of pure water. The polymer additives reduce the tendency of coalescence between vapor bubbles. Consequently, they become smaller in size, larger in number, and tend to stay on the heating surface in a relatively orderly manner. No significant improvement in heat transfer rate caused by the polymer additives was observed in this work; the critical burn out heat flux was reduced slightly.