聚合物熔体注模流变学：Ⅳ,熔体的弹性及其表征

本文论述了聚合物熔体注模过程中弹性特性的几种重要的表现。对于国内外学者在熔体弹性行为的表征及过程机理的解释等方面的研究进展作了简要的述评,并综述了作者及其同事近年来在这方面所做的工作。     

PVP-LiCl-DMF溶液体系的流变学特性及相互作用

利用流变学方法, 采用核磁共振和红外光谱技术开展了聚乙烯吡咯烷酮(PVP)和LiCl相互作用研究, PVP/LiCl/DMF浓溶液的表观粘度随着LiCl含量的增加而提高, 溶液的粘流活化能也相应增加. 13C NMR结果表明, 溶液中Li+与PVP的羰基之间存在相互作用, 这种相互作用改变了PVP分子的聚集状态. 红外光谱结果证实了PVP/LiCl复合物中Li+与PVP的羰基存在相互作用.     

Investigations on a detergent system with rodlike micelles

Conductivity, kinetic, static and dynamic light scattering, electric birefringence and rheological measurements were carried out on aqueous solutions of Tetradecylpyridinium-n-Heptanesulfonate (C₁₄PyC₇SO₃) up to high concentrations. In dilute solutions between the critical micelle concentration (cmc) and another characteristic concentration (c _t) spherical micelles were detected whose radii were independent of detergent concentration and equal to the length of a detergent molecule; the aggregation numbern of these micelles of about 100 monomers per micelle was also in agreement with the existence of normal spherical micelles of aC ₁₄-detergent.Above the concentrationc _t, the spherical micelles were found to grow to rodlike aggregates whose short axis was still independent of concentration and equal to the length of a monomer, while the lengthsL of the rods increased with increasing detergent concentration. When the lengthsL of the rods became comparable with the mean distancea between them, the starting interaction between the rods slowed down their growth. In this concentration range of overlapping rods, the data could be evaluated with a recently developed theory by Doi and Edwards for stiff rods. The rods reached finally a maximum length of about 500 å and decreased again in size upon further increase of concentration when the overlap ratioL/a reached a value of about 1,5.     

Aggregate morphology and flow behaviour of micellar alkylglycoside solutions

Solutions of n-nonyl-β-D-glucoside (C₉G₁), n-decyl-β-D-glucoside (C₁₀G₁), n-dodecyl-β-D-maltoside (C₁₂G₂), n-tetradecyl-β-D-maltoside (C₁₄G₂) and C₉G₁/C₁₀G₁ mixtures have been characterised by capillary viscometry and rheology in H₂O and D₂O, in order to map the influence of surfactant characteristics on micellisation over a wide concentration range. For the maltosides, the micellar solutions are shear thinning with a zero-shear viscosity that scales with concentration according to a power law with an exponent of about 5.8. In contrast, solutions of the glucosides C₉G₁, C₁₀G₁ and their mixtures show Newtonian flow behaviour and a much lower scaling exponent (<2.4). In C₉G₁/C₁₀G₁ mixtures, the scaling exponent decreases monotonously with increasing C₁₀G₁ content. The flow behaviour correlates with the packing requirements of the various surfactants, and are compatible with the idea that the maltosides form worm-like micelles, whereas the glucosides form branched, interconnected micelles (C₉G₁) and space-filling micellar networks (C₁₀G₁).     

STRUCTURING & RHEOLOGY OF MOLTEN POLYMER/CLAY NANOCOMPOSITES

The evolution and the origin of "solid-like state" in molten polymer/clay nanocomposites are studied. Using polypropylene/clay hybrid (PPCH) with sufficient maleic anhydride modified PP (PP-MA) as compatibilizer, well exfoliation yet solid-like state was achieved after annealing in molten state. Comprehensive linear viscoelasticity and non-linear rheological behaviors together with WAXD and TEM are studied on PPCH at various dispersion stages focusing on time,temperature and deformation dependencies of the "solid-like" state in molten nanocomposites. Based on these, it is revealed that the solid-structure is developed gradually along with annealing through the stages of inter-layer expansion by PP-MA,the diffusion and association of exfoliated silicate platelets, the formation of band/chain structure and, finally, a percolated clay associated network, which is responsible for the melt rigidity or solid-like state. The network will be broken down by melt frozen/crystallization and weakened at large shear or strong flow and, even more surprisingly, may be disrupted by using trace amount of silane coupling agent which may block the edge interaction of platelets. The solid-like structure causes characteristic non-linear rheological behaviors, e.g. residual stress after step shear, abnormal huge stress overshoots in step flows and, most remarkably, the negative first normal stress functions in steady shear or step flows. The rheological and structural arguments challenge the existing models of strengthened entangled polymer network by tethered polymer chains connecting clay particles or by chains in confined melts or frictional interaction among tactoids. A scheme of percolated networking of associated clay platelets, which may in band form of edge connecting exfoliated platelets, is suggested to explain previous experimental results.     

PRELIMINARY INVESTIGATION ON THE MISCIBILITY OF ISOTACTIC POLYPROPYLENE （iPP） AND SYNDIOTACTIC POLYPROPYLENE （sPP） BLENDS

Experimental miscibility studies were performed on different compositions of iPP/sPP blends, where sPP has a low syndiotacticity （[rrrr] = 81%）. Combining optical microscopy, rheology, and solid state NMR spectroscopy, the miscibility of the blends was investigated at different scales in the traditionally thought to be ＂immiscible＂ iPP/sPP blends. For the composition of iPP/sPP （90/10） blend, it shows to be miscible in the melt, and furthermore, the existence of intermolecular chain interactions between sPP and iPP components was detected in the solid state.     

Effect of sucrose on molecular and interaction parameters of sodium caseinate in aqueous solution: relationship to protein gelation

The effect of sucrose on molecular and interaction parameters of sodium caseinate in aqueous medium has been investigated using static and dynamic multi-angle laser light scattering over a wide range of sucrose concentration (from 10 to 78 w/v%) and pH values (from 7.0 to 3.5). Measurements have been made of the molar mass, the radius of gyration, the hydrodynamic radius, and the second virial coefficient of sodium caseinate in aqueous solution. Pronounced dissociation of sodium caseinate sub-micelles¹ was found in the presence of sucrose at a pH above the protein's isoelectric point. The effect of sucrose at a pH near the isoelectric point is very different. This is reflected in the pronounced increase in molar mass, radius of gyration, and the difference between the radius of gyration and the hydrodynamic radius. It was found that the extent of the protein association, caused by the presence of sucrose, is a key factor contributing to the hydrophobic–hydrophilic balance of the protein surface, and hence to the thermodynamic affinity of the caseinate sub-micelles for the aqueous medium and for each other. Analysis of light-scattering data using structure-sensitive plots shows a clear transition from Gaussian to wormlike chain/rod behaviour for sodium caseinate on pH lowering. Apparent relationships between the effects of sucrose on the self-association of sodium caseinate and a marked enhancement of the viscoelasticity of acid-induced casein gels have been revealed. Moreover, the dissociation of sodium caseinate sub-micelles is in excellent agreement with the more homogeneous microstructure of acid-induced protein gels in the presence of sucrose as detected by confocal laser scanning microscopy. We discuss likely molecular mechanisms underlying the observed effects of sucrose on the interactions and rheology in acidified caseinate systems.     

Synthesis and characterization of carboxymethylcelluloses from non-wood pulps II. Rheological behavior of CMC in aqueous solution

This paper concerns the rheological behavior of carboxymethylcelulloses(CMC) derived after one and two successive steps from different non-woodbleached cellulose pulps. CMC rheological characterization was achieved in0.1M NaCl solution, as a function of polymer concentration. Theevidence of a critical concentration (C* < 1 g/L) is discussedfromsteady shear and dynamic experiments. Rheological properties of the CMC werefound to depend on the cellulose source reactivity and on their degree ofsubstitution (DS). Higher molecular weight of initial cellulose was accompaniedby higher apparent intrinsic viscosity of the CMC produced. Depending on theCMCconcentration and on the degree of etherification, the system behaves as asolution or as a gel. In the case of abaca CMC sample, it is shown that afteronly one step of chemical modification and above a polymer concentration of20 g/L, the system behaves as a gel. The gel behavior was studied asafunction of temperature. In the temperature range from 25 to 45°C, the rheological behavior was found to remain almostconstant due to the existence of dispersed swollen aggregates. This unusualcharacteristic represents an advantage for applications such as oil recovery inthe petroleum industry, where viscosity of the recovered fluid should not diminishwith temperature.     

Effects of structural rearrangements on the rheology of rennet-induced casein particle gels

During ageing of casein or skim milk gels, structural changes take place that affect gel parameters, such as pore size and storage modulus. These changes can be explained in terms of rearrangements of the gel network at various length scales. In this paper, rheological experiments on rennet-induced casein gels and a general model on rearrangements are presented. The results of experiments (e.g. microscopy, permeametry) and computer simulations, the model, and recent literature on casein gels and other types of particle gels are compared to each other. Experiments presented include measurements of storage and loss moduli and maximum linear strain of the casein gels. Parameters varied were pH (5.3 and 6.65) and temperature (25 and 30 degrees C). In addition, the casein volume fraction (5-9 vol.%) was varied, which enables application of fractal scaling models. For rennet-induced casein gels, it is demonstrated that at the lower pH, all types of rearrangements proceed significantly faster. The rearrangements include: an increase in the size of compact building blocks; partial disappearance of fractal structure; and the formation of straightened strands, some of which eventually break. All of these rearrangements seem to be a consequence of particle fusion. There are indications of universality of the relation between particle fusion and gel syneresis for gels composed of viscoelastic particles.     

Miscibility and compatibilization of poly(trimethylene terephthalate)/acrylonitrile-butadiene-styrene blends

Poly(trimethylene terephthalate)/acrylonitrile-butadiene-styrene (PTT/ABS) blends were prepared by melt processing with and without epoxy or styrene-butadiene-maleic anhydride copolymer (SBM) as a reactive compatibilizer. The miscibility and compatibilization of the PTT/ABS blends were investigated by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), capillary rheometer and scanning electron microscopy (SEM). The existence of two separate composition-dependent glass transition temperatures (T_gs) indicates that PTT is partially miscible with ABS over the entire composition range. In the presence of the compatibilizer, both the cold crystallization and glass transition temperatures of the PTT phase shifted to higher temperatures, indicating their compatibilization effects on the blends.The PTT/ABS blends exhibited typical pseudoplastic flow behavior. The rheological behavior of the epoxy compatibilized PTT/ABS blends showed an epoxy content-dependence. In contrast, when the SBM content was increased from 1 wt% to 5 wt%, the shear viscosities of the PTT/ABS blends increased and exhibited much clearer shear thinning behavior at higher shear rates. The SEM micrographs of the epoxy or SBM compatibilized PTT/ABS blends showed a finer morphology and better adhesion between the phases.