Energy calculations for isotactic polypropylene: a comparison between models of the α and γ crystalline structures

The role of nucleating agents in high-pressure-induced gamma crystallization in isotactic polypropylene

Nucleation of the γ-form in isotactic polypropylene (PP) under high pressure was investigated. Three nucleating agents were used to nucleate crystallization of PP under atmospheric pressure: commercial Hyperform HPN-20E from Milliken Chemical, poly(tetrafluoroethylene) particles nucleating the α-form, and calcium pimelate nucleating the β-form. Crystallization of neat PP and PP with addition of 0.2 wt% of the nucleating agents was studied. Specimens were either kept at 200 °C under pressure of 200 MPa for time ranging from 2 min to 4 h or for 15 min under pressure ranging from 1.3 to 300 MPa. After cooling to ambient temperature and releasing the pressure, the specimens were analyzed by DSC, WAXD, and PLM to have an insight into the structure and to determine a crystallinity level and contents of crystallographic forms. Both α-nucleating agents strongly nucleated crystallization of PP under high pressure in the γ-form, whereas the β-nucleating agent had only a slight effect. The results show the possibility to use nucleating agents to nucleate the γ-form of PP under high pressure.

High-pressure crystallization of isotactic polypropylene droplets

Dispersions of isotactic polypropylene (PP) particles in polystyrene (PS) were produced by interfacially driven breakup of nanolayers in multilayered systems that were fabricated by means of layer-multiplying coextrusion. The droplet size was controlled by the individual PP layer thickness ranging from 12 to 200 nm. In addition, PP was melt blended with PS to produce PP droplets larger than those formed by breakup of nanolayers. The dispersions of PP particles in the PS matrix were melted and annealed under high pressure of 200 MPa. Only the largest PP droplets, with average sizes of 170 μm, crystallized predominantly in the γ form. In the 42-μm droplets obtained by breakup of 200 nm layers, a minor content of the γ form was found whereas the smaller droplets obtained by breakup of the thinner nanolayers contained the α form and/or the mesophase. The results showed that the γ phase formed only in the droplets sufficiently large to contain the most active heterogeneities nucleating PP crystallization under atmospheric pressure. It is concluded that the presence of nucleating heterogeneities is necessary for crystallization of PP in the γ form under high pressure.

A novel dispersion method comprising a nucleating agent solubilized in a microemulsion, in polymeric matrix

This is the first of a two-part study focusing on a novel dispersion method which enables increasing the crystallization rate of polypropylene (PP) through the incorporation of nucleating agent HPN-68 into the molten polymer using a microemulsion as a nanovehicle. The cycle time for processing the PP is significantly reduced and thus the effectiveness of its production is increased. Our concept is based on creating an advantage in dispersion capability of the nucleator that is dissolved in a nanoreactor vehicle in comparison with its conventional introduction as a crystalline powder. The microemulsions were introduced to the target PP using a mixer. By the end of the mixing, when the water phase had evaporated, only the nucleator and the surfactant remained in the matrix. The microemulsion components that solubilized the HPN-68 were mineral oil, alcohol, surfactant, and water. DSC results showed a 24% improvement in nucleation efficiency of PP by this method. WAXS results showed that HPN-68 is a gamma-nucleator. It causes polymorphism by significantly raising the gamma-phase concentration in the PP. SEM results showed a four-fold decrease in the PP spherulite size due to the improved dispersion of HPN-68 within the matrix via microemulsion compared to conventional nucleator incorporation.

Aplicaciones de los métodos computacionales al estudio de la estructura y propiedades de polímeros

En este trabajo se revisan las técnicas de simulación molecular más habituales y potentes para la descripción de los polímeros a escala atómica y molecular, las cuales se han clasificado en cuánticas o clásicas dependiendo de cómo se describen las interacciones entre las partículas. Se presentan asimismo diversas aplicaciones de dichas metodologías, realizadas en nuestro laboratorio, en el contexto del estudio de la estructura y propiedades de polímeros. En particular, se muestran aplicaciones de las técnicas clásicas a la determinación de estructuras cristalinas, a estudio del plegamiento lamelar de los nylons, a la estabilidad de las estructuras supramoleculares observadas en algunos complejos tensioactivo·polielectrolito y a la difusión de gases en matrices poliméricas, mientras que el uso de técnicas cuánticas se ha ilustrado presentando estudios dedicados a la predicción de efectos cooperativos, interacciones específicas y parámetros espectroscópicos.