Structure, processing and performance of ultra-high molecular weight polyethylene (IUPAC Technical Report). Part 2: crystallinity and supra molecular structure

Test methods including OM, SEM, TEM, DSC, SAXS, WAXS, and IR were used to characterise supra-molecular structure in three batches of polyethylene (PE), which had weight-average relative molar masses ¯¯¯¯ M w of approximately 0.6 × 106, 5 × 106, and 9 × 106. They were applied to compression mouldings made by the polymer manufacturer. Electron microscopy showed that powders formed in the polymerization reactor consisted of irregularly shaped grains between 50 and 250 μm in diameter. Higher magnification revealed that each grain was an aggregate, composed of particles between 0.4 and 0.8 μm in diameter, which were connected by long, thin fibrils. In compression mouldings, lamellar thicknesses ranged from 7 to 23 nm. Crystallinity varied between 70 and 75 % in reactor powder, but was lower in compression mouldings. Melting peak temperatures ranged from 138 to 145 °C, depending on processing history. DMTA showed that the glass transition temperature θg was −120 °C for all three grades of polyethylene. IR spectroscopy found negligibly small levels of oxidation and thermal degradation in mouldings. Optical microscopy revealed the presence of visible fusion defects at grain boundaries. It is concluded that relatively weak defects can be characterized using optical microscopy, but there is a need for improved methods that can detect less obvious fusion defects

A brief guide to polymer characterization: structure (IUPAC technical report)

To bolster the series of Brief Guides released by International Union of Pure and Applied Chemistry (IUPAC), here we introduce the first Brief Guide to Polymer Characterization. This article provides a concise overview of characterization methods for teachers, students, non-specialists, and newcomers to polymer science as well as being a useful manual for researchers and technicians. Unlike pure low molar mass chemical substances, polymers are not composed of identical molecules. The macromolecules which comprise a single polymer sample vary from one another, primarily in terms of size and shape, but often also in the arrangement or positioning of atoms within macromolecules (e.g., chain branching, isomerism, etc.). Polymer properties are often drastically different from those of other substances and their characterization relies on specialist equipment and/or common equipment used in a specialized way (e.g., particular sample preparation or data analysis). This Brief Guide focuses uniquely on the structural characterization (i.e., analyzing the molecular and multi-molecular aspects) of polymers. The complex nature of the structural variables possible in macromolecular materials often presents a challenge with regard to the detailed structural characterization of polymers. This Brief Guide provides a useful starting point to direct the reader to the most commonly used and useful techniques to characterize these structural variables

Improve Requirement Prioritization By End-user Demands : Model Building and Evaluation

Background: The selection and prioritizing of requirements is the most difficult challenge insoftware development. Prioritizing requirements is a difficult task. Due to the importance of thepriority of requirements, many methods have been developed on how to prioritize requirements.However, with the increase of software modules and the expansion of software platforms, thesingle requirement prioritization method can no longer match the increase in the number ofrequirements. Little is know in how to find and develop integrated requirement prioritizationmethod. Objectives: The main purpose of this research is to explore the main challenges and successcriteria that practitioners consider when determining the priority of product requirements. Builda good requirement prioritization model to tackle these challenges. And evaluate the strengthsand limitations of this model. Method: We conducted a questionnaire survey to learn more about the major problems andsuccess criteria for prioritizing product requirements. After that, we presented a model thatcombined the KANO model and Analytic Hierarchy Process (AHP), and we examined its practicality. Finally, using Focus Group Research, we analyzed the benefits and limitations of theintegrated model and improved solutions. Result: The results show that practitioners face many challenges in product requirement prioritization. The model we developed is suitable for a variety of scenarios. It helps practitionersmanage priorities and improve end-user satisfaction, which can solve these challenges to a certain extent. Conclusion: Our research collected many major challenges encountered by requirement analysts and product managers in the process of requirement prioritization. And developed a newrequirement prioritization model, got a better understanding of requirement prioritization whichcan inspire practitioners to build more better requirement prioritization models.

Free volume changes, crystallization, and crystal transition behavior of syndiotactic polystyrene in supercritical CO2 revealed by positron annihilation lifetime spectroscopy

The free volume of syndiotactic polystyrene (sPS) conditioned in supercritical CO2 was investigated by using positron annihilation lifetime spectroscopy (PALS). Supercritical CO2 increased the free volume cavity size of amorphous sPS and made the formation of gamma crystal occur. The orthopositronium (o-Ps) intensity I-3 was correlated well with the crystallinity of gamma form sPS, revealed by its gradual decrease with the increase of crystallinity. Furthermore, the treatment atmosphere affected the crystallization behavior of amorphous sPS. The increased free volume cavity size of amorphous sPS treated in supercritical CO2 was larger than those treated in ambient air and supercritical N-2. Among these treatment conditions, supercritical CO2 increased the free volume size of amorphous sPS more effectively and thus resulted in the formation of gamma crystals. PALS measurements indicated that supercritical CO2 helped remaining a constant high free volume level of the amorphous parts of the semicrystalline sPS. Likely, the crystalline parts were also affected by the scCO(2) as a transition from the gamma form to the beta form was revealed at high temperatures by WAXD

Nucleation Enhancement in Stereodefective Poly(l-lactide) by Free Volume Expansion Resulting from Low-Temperature Pressure CO2 Preconditioning

Nucleation enhancement in a highly stereodefective poly(l-lactide) (PLLA) with an optical purity of 88% by low-temperature pressure (0 and 35 °C under 2 MPa) CO2 preconditioning was investigated using differential scanning calorimetry (DSC), infrared (IR) spectroscopy, polarized optical microscopy (POM) as well as positron annihilation lifetime spectroscopy (PALS). Despite the preconditioning of the melt-quenched films for 2 h, IR results indicated that no trace of mesophase was generated and the samples remained in the glassy state. However, judging from the results of DSC, IR, and POM, when compared to the untreated sample, both the treated ones showed a significantly enhanced crystal nucleation effect, resulting in the corresponding greatly enhanced crystallization kinetics. Moreover, owing to the existence of the retrograde vitrification, the conditions of the previous low-pressure CO2 conditioning affected the nucleation enhancement effect. When compared to the case of 35 °C, the much lower temperature of 0 °C was more effective for nucleation enhancement. The PALS results indicated that the enlarged free volume, which resulted from the CO2 conditioning, largely accounted for the formation of locally ordered structures, providing many more potential nucleation sites for forming critical nuclei and thus the resulting enhanced crystallization kinetics in glassy PLLA. The present results have implications in understanding the nucleation enhancement effect, in particular in stereodefective PLLA systems, which possess extremely low crystallization ability and are thus probably too problematic to be evaluated by conventional methods

Two-dimensional active raypath separation using examination of the roots of the spectrum polynomial

International audienceThe application of the root multiple signal classification algorithm for raypath separation was motivated by the dramatic reduction in processing time of the multiple-signal classification algorithm. However, the algorithm provides classification only in the direction of the arrival domain and fails to separate raypaths arriving at the array with similar directions of arrival. Moreover, for many applications in shallow water (such as ocean acoustic tomography and active sonar), the emitted signal is known and can be used as a priori information to improve the resolution. Thus, in this study, a two-dimensional active wideband classification algorithm is developed using the examination of the roots of the spectrum polynomial in the angle versus time domain. A two-step strategy is developed to enable extension to the two-dimensional case. The results of simulations confirm that the proposed algorithm achieves almost identical resolution as the existing two-dimensional algorithms while offering a significant reduction in computation time. (C) 2017 Acoustical Society of Americ