成核剂对聚丙烯熔融行为的影响

成核剂对聚丙烯熔融行为的影响陈彦徐懋（中国科学院化学研究所高分子物理开放实验室北京１０００８０）关键词成核剂，聚丙烯，平衡熔点聚合物的熔融过程受分子量大小、分子量分布、分子链构型（等规度、分子结构单元的键接序列）、不同的结晶晶型、不同的热历史等...     

Orientation mechanisms during biaxial drawing of polymer films

Summary Second drawing of monoaxially drawn PP and PET films perpendicular to the first drawing direction has been investigated. At low draw ratios structural systems having an angle of 45° with regard to the second draw direction are formed. By increasing the draw ratio the structures rotate into the new preferred direction. The results lead to the impression that the structural transformations do not turn out molecule by molecule, but domain by domain.

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Zusammenfassung Die Zweitverstreckung monoaxial verstreckter PP- und PET-Folien senkrecht zur Erstverstreckrichtung wird untersucht. Bei kleinen Verstreckgraden bilden sich Struktursysteme unter 45° zur Zweitverstreckrichtung; bei weiterer Verdehnung drehen sich die Strukturen in die Vorzugsrichtung. Die Ergebnisse vermitteln den Eindruck, daß die Strukturänderung nicht Molekül für Molekül, sondern Bereich für Bereich abläuft.

With 10 figures and 2 tables     

Using Neural Networks or Linear Models to Predict the Characteristics of Microcapsules Containing Phase Change Materials

Summary: Microcapsules with large amount of PRS® paraffin wax encapsulated and narrow size distribution were prepared by shirasu porous glass (SPG) emulsification technique and a subsequent suspension like polymerization process and then examined by DSC, laser diffraction and SEM analyses. An experimental design approach, based on a central composite design, was used to determine quantitatively the effect of PRS® paraffin wax/styrene mass ratio (PRS/St), percentage of polyvinylpyrrolidone/styrene mass ratio (%PVP/St) and water/styrene mass ratio (H₂O/St) on the microparticles properties. The results were fitted using two black-box models. The empirical equations allow the prediction of the amount of the paraffin wax encapsulated and the mean particle size in number as a function of aforementioned synthesis variables. It was observed that both models allowed to drawn the same conclusions. %PVP/St mass ratio was the most important parameter affecting the particle size distribution decreasing the average particle size with the increase of %PVP/St. On the other hand, PRS/St mass ratio has a direct influence on the latent heat of fusion.     

ON MORPHOLOGY AND MULTIPLE MELTING IN POLYPROPYLENE

A series of polypropylenes, including two modern high crystallinity materials (HCPP) were subjected to a stepwise crystallization procedure as a guide to their properties. The results were complicated by the development of double melting endotherms at the highest crystallization temperature. Both HCPP, when grown at the temperature of 145°C, give a double melting peak, but the proportions of the two peaks varied according to the density of nucleation. By partially melting a specimen between the two peaks, it was possible to assign the higher peak to radial dominant lamellae and the lower peak to mostly tangential subsidiary lamellae filling the space in between. Uniformly cross-hatched lamellae at the center of spherulites melt along with the lower melting population in the outer regions of the spherulites. Even if differences in crystallization temperature are eliminated, the properties of nucleated as opposed to nonnucleated PP may therefore be influenced by differences between the center and periphery of spherulites, with “central” properties much more in evidence in nucleated material. The development of these double endotherms is related to morphological constraint, rather than molecular fractionation. Their form is strongly influenced by cross-hatching, but the phenomenon is still found at 160°C where cross-hatching does not form.     

Rheological and Mechanical Characterization of Multi-Walled Carbon Nanotubes/Polypropylene Nanocomposites

The rheology and morphology of multi-walled carbon nanotube (MWNT)/polypropylene (PP) nanocomposites prepared via melt blending was investigated. The minor phase content of MWNT varied between 0.25 and 8 wt%. From morphological studies using a scanning electron microscopy technique a good dispersion of carbon nanotubes in the PP matrix was observed. The rheological studies were performed by a capillary rheometer, and mechanical properties of the nanocomposites were studied using a tensile and flexural tester. Both PP and its nanocomposites showed non-Newtonian behavior. At low shear rates the addition of MWNT content causes an increase in viscosity; however, viscosity is less sensitive to addition of MWNT content at higher shear rates. Flow activation energy for the nanocomposites was calculated using an Arrhenius type equation. From this calculation it was concluded that the temperature sensitivity of nanocomposites was increased by increasing of nanotube content. An increase in tensile and flexural moduli and Izod impact strength was also observed by increasing the MWNT content. From rheological and mechanical tests it was concluded that the mechanical and rheological percolation threshold is at 1.5 wt%.     

NUCLEATING EFFECT OF MONTMORILLONITE NANOPARTICLES IN POLYPROPYLENE

The nucleation effect of two layered montmorillonite silicates of different origin was studied in polypropylene (PP). Composites were prepared as a function of composition in an internal mixer or by homogenization in a twin-screw compounder. Melting and crystallization characteristics were determined by differential scanning calorimetry. The gallery distance of the silicates and the structure of the composites were studied by WAXS. The results prove that montmorillonite may considerably influence the crystallization of PP. The effect depends very much on the origin and treatment of the filler. Both treatment and composite preparation lead to considerable changes in the separation distance of the silicate layers. Even though the composites contain fillers with more than one population of layer distances, the nucleating effect is related to the completely collapsed galleries of 1 nm distance. Besides gallery distance, organophillization also modifies the surface tension of the filler, but this apparently does not influence its nucleating efficiency. The efficient gallery distance is twice as large as the characteristic matching lattice dimension determined by Lotz et al. Although a few observations could not be explained and the tentative explanation given earlier may need further verification, the results prove that nucleation does not occur at the flat surface of the filler but between its galleries, where polymer molecules may adsorb preferentially.     

Rheological Behaviors of Polypropylene/Poly(1-butene) Blends

The rheological behaviors of polypropylene (PP)/poly(1-butene) (PB) blends with homo-polypropylene (PP₁) or impact polypropylene (PP₂), a poly(propylene-co-ethylene) as the PP component were studied. With increasing of PB resin content for both PP/PB blends, the blends showed higher G'(ω), G''(ω) and η*(ω) at low frequencies but lower values at high frequencies which implied that the processability was improved. A two-phase morphology was observed through the various rheological responses, including G'(ω)-ω terminal region curves, Cole-Cole plots and the weighted relaxation spectra with the PB contents up to 40?wt%. With the same PB content, the rheological parameters of the PP₂/PB blends were quite different from those of the PP₁/PB, which can be attributed to the stronger interaction between PB chains and the ethylene-co-propylene copolymer in PP₂. The impact strength of the PP₂/PB blends was improved dramatically over that of the PP₁/PB. The more significant toughening effect for the PP₂/PB blends can be attributed to the special responses of its rheological behaviors.