Effect of the molecular structure of the polymer and nucleation on the optical properties of polypropylene homo- and copolymers.

Two soluble nucleating agents were used to modify the optical properties of nine PP homo- and random copolymers. The ethylene content of the polymers changed between 0 and 5.3 wt%. Chain regularity was characterized by the stepwise isothermal segregation technique (SIST), while optical properties by the measurement of the haze of injection molded samples. Crystallization and melting characteristics were determined by differential scanning calorimetry (DSC). The analysis of the results proved that lamella thickness and change in crystallinity influence haze only slightly. A model was introduced which describes quantitatively the dependence of nucleation efficiency and haze on the concentration of the nucleating agent. The model assumes that the same factors influence the peak temperature of crystallization and optical properties. The analysis of the results proved that the assumption is valid under the same crystallization conditions. The parameters of the model depend on the molecular architecture of the polymer. Chain regularity determines supermolecular structure and thus the dependence of optical properties on nucleation

On clarification of haze in polypropylene

The mechanism of reducing light scattering in isotactic polypropylene (i-PP), through the addition of so-called clarifying agents, is studied with small-angle light scattering (SALS) and scanning electron microscopy (SEM). The clarifying agents used in this study depict monotectic phase behavior with i-PP, crystallizing in a relatively narrow concentration range in a nanofibrillar network, providing an ultrahigh nucleation density in the i-PP melt. It is found that the clarifying effect, a dramatically increased transparency and reduced haze, that occurs within the aforementioned additive concentration range, coincides with a change in morphology from strongly scattering spherulites to shish-kebab-like crystalline structures, as evidenced by in situ SALS measurements and confirmed by SEM images. A simple scaling law, relating the diameter of the shish-kebab structures to the fibril diameter and volume fraction of the clarifying agent is proposed, suggesting that the performance of a (fibril-forming) clarifying agent will improve by reducing the fibril diameter and/or increasing the volume concentration of the clarifying agent

Tuning the self-assembled 1,3:2,4-di(3,4-dimethylbenzylidene) sorbitol nanoarchitectures using the phase inversion method

[[abstract]]1,3:2,4-Di(3,4-dimethylbenzylidene) sorbitol (DMDBS) molecules can self-assemble into nanoscaled structures in organic solvents and polymer melts. The nanofibril structures were the mostly found. In this study, we used two phase inversion methods, i.e., dry and wet methods, to obtain different DMDBS nanoarchitectures. Poly(vinylidene fluoride) (PVDF) was chosen as polymer matrix, and the DMDBS structures were tuned by the process of PVDF membrane formation (crystallization and liquid–liquid demixing). When the membrane was prepared using the dry method, the DMDBS structure is controlled by the PVDF crystallization. Fewer DMDBS nanofibrils formed on the surfaces, and no nanofibrils were found in the cross-sections. On the other hand, when the membrane was prepared using the wet method, the liquid–liquid demixing (nonsolvent induced phase separation) occurred simultaneously as PVDF crystallized, and thus influenced the aggregation of DMDBS molecules. DMDBS is an amphiphilic molecule with two hydrophilic hydroxyl groups. The addition of nonsolvent (water) caused a large number of DMDBS molecules to aggregate outside the hydrophobic PVDF. In addition, a new structure “nanomat” was found. The mat was composed of DMDBS nanofibrils with diameters of 10–20 nm, similar to those observed in the dry method membranes. Fourier transform infra-red spectroscopy indicates that the DMDBS molecules self-assembled (aggregated) mainly through intermolecular hydrogen bonding in the presence of PVDF. The more intermolecular hydrogen bonding between DMDBS existed, the more excessive amounts of DMDBS molecules were, leading to the formation of nanomats.[[incitationindex]]SCI[[booktype]]紙本[[booktype]]電子

Non-isothermal crystallization of semi-crystalline polymers : the influence of cooling rate and pressure

During industrial processing, polymer melts are exposed to local high cooling rates, strong deformation rates and high pressures. Nowadays, research in the field of semi-crystalline polymers still strives towards an accurate prediction of the evolution and final appearance of the crystalline morphology in polymer products. After all, the amount, number, phase and orientation of the crystallites act in a combined way and control the final optical and mechanical properties. This chapter discusses recent experimental and model developments concerning the influence of industrially relevant cooling rates and pressures on the non-isothermal crystallization of both an isotactic polypropylene and a linear low-density polyethylene grade. The influence of flow gradients is discussed in Chapter (Roozemond et al., Adv Polym Sci, 2016

Modeling Flow-Induced Crystallization

A numerical model is presented that describes all aspects of flow-induced crystallization of isotactic polypropylene at high shear rates and elevated pressures. It incorporates nonlinear viscoelasticity, including viscosity change as a result of formation of oriented fibrillar crystals (shish), compressibility, and nonisothermal process conditions caused by shear heating and heat release as a result of crystallization. In the first part of this chapter, the model is validated with experimental data obtained in a channel flow geometry. Quantitative agreement between experimental results and the numerical model is observed in terms of pressure drop, apparent crystallinity, parent/daughter ratio, Hermans’ orientation, and shear layer thickness. In the second part, the focus is on flow-induced crystallization of isotactic polypropylene at elevated pressures, resulting in multiple crystal phases and morphologies. All parameters but one are fixed a priori from the first part of the chapter. One additional parameter, determining the portion of β-crystal spherulites nucleated by flow, is introduced. By doing so, an accurate description of the fraction of β-phase crystals is obtained. The model accurately captures experimental data for fractions of all crystal phases over a wide range of flow conditions (shear rates from 0 to 200 s−1, pressures from 100 to 1,200 bar, shear temperatures from 130°C to 180°C). Moreover, it is shown that, for high shear rates and pressures, the measured γ-phase fractions can only be matched if γ-crystals can nucleate directly on shish