STUDY OF RELAXATION TRANSITIONS IN SOME AROMATIC POLYESTERS

Методом спинового зонда проведено систематическое изучение основных релаксационных переходов жесткоцепных стеклообразных полимеров в широком интервале температур с применением спиновых зондов разного размера на примере полисульфона. В главной области релаксации ароматических полиэфиров с помощью метода спинового зонда обнаружены два перегиба. Два перегиба, обнаруженные на температурной зависимости времени корреляции вращения, являются следствием размораживания сегментальной подвижности в областях с различной упаковкой сегментов. Показано, что высокотемпературный перегиб соответствует размораживанию сегментальной подвижности кооперативного типа (а -переходу), а перегиб при более низкой температуре р -размораживанию сегментальной подвижности локального типа (а'-процессу). Для объяснения природы данного перехода была использована теоретическая модель полимера, где предполагается, что в аморфных полимерах возможны разные уровни надмолекулярной организации, т.е. существование более упорядоченных и более плотно упакованных областей, которые чередуются с более «рыхлыми» и менее упорядоченными областями. Исследовано влияние различных факторов на положение а’-перехода на температурной шкале. Выбор объема зонда на основе строго обоснованной модели его движения позволил выявить взаимосвязь между вращательным движением зонда и движением отдельных сегментов аморфного полимера ниже температуры стеклования. The spin probe method was used to systematically study the main relaxation transitions of rigid-chain glassy polymers in a wide temperature range using spin probes of different sizes using polysulfone as an example. In the main relaxation region of aromatic polyesters, two inflections were found using the spin probe method. The two inflections found in the temperature dependence of the rotation correlation time are the result of unfreezing of segmental mobility in regions with different packing of segments. It was shown that the high-temperature inflection corresponds to the defrosting of the cooperative-type segmental mobility (a -transition), and the inflection at a lower temperature corresponds to the / -defrosting of the segmental mobility of the local type (the a' -process). To explain the nature of this transition, a theoretical model of the polymer was used, where it is assumed that different levels of supramolecular organization are possible in amorphous polymers, i.e. the existence of more ordered and more densely packed regions, which alternate with looser and less ordered regions. The influence of various factors on the position of the a' -transition on the temperature scale is investigated. The choice of the probe volume on the basis of a strictly substantiated model of its motion made it possible to reveal the relationship between the rotational motion of the probe and the motion of individual segments of the amorphous polymer below the glass transition temperature.

Effect of PMMA Molecular Weight on Its Localization during Crystallization of PVDF in Their Blends

In miscible crystalline/amorphous polymer blends, the exclusion behaviors of the latter with various molecular weights during the crystallization of the former were investigated by the combination of SAXS and DSC by taking a PVDF/PMMA blend as an example. The ratio between internal crystallinity from SAXS and overall crystallinity of the entire blend from DSC was employed to characterize the exclusion of PMMA. Our results indicate that the molecular weight of the amorphous component produces a remarkable influence on the diffusion coefficient (D) and the crystal growth rate (G) of the crystalline component. There are both inter-lamellar and inter-fibrillar structures when PVDF blended with lower-molecular-weight PMMA. With increasing molecular weight of PMMA, the decrease in crystal growth rate (G) dominates the enhanced exclusion behaviors of PMMA, resulting in bigger pores after extraction. Our results are significant not only for the basic understanding of crystallization in polymer blends, but also for the fabrication and structure control of porous structures based on crystallization templates.

Structural evolution and synthesis mechanism of ytterbium disilicate powders prepared by cocurrent chemical coprecipitation method

Ytterbium disilicate powders were synthesized by cocurrent chemical coprecipitation method. The influence of Si/Yb molar ratio and calcination temperature on compositions and structures of Yb2Si2O7 products were investigated. The formation mechanism and thermal behavior of precursor as well as the phase evolution of Yb2Si2O7 were also discussed in depth. Results show that pure β-Yb2Si2O7 powders with nanoscale size can be obtained from the precursor with Si/Yb molar ratio of 1.1 after being calcinated at temperatures above 1200 ℃. The Yb2Si2O7 precursor is an amorphous polymer cross-linked with -[Si-O-Yb]- chain segments which are formed though Yb atoms embedding in the -[Si-O-Si]- network. After a continuous dihydroxylation and structural ordering, the amorphous precursor transformed to α-Yb2Si2O7 crystals by atomic rearrangement. Elevated calcination temperature can induce to the coordination structures and environment evolutions of structural units and then converted to stable (Si2O7) groups and (YbO6) polyhedrons, which results in the formation of β-Yb2Si2O7.

Emissive Organogel Mediated Construction of Flexible Covalent Organic Polymer for the Separation of Aniline for Water Purification

A flexible covalent organic polymer (COP) has been successfully synthesized via dynamic covalent gel (DCG) formation through imine condensation reaction between 6-hydrazinonicotinic hydrazide hydrate and benzene-1,3,5-tricarboxaldehyde within 7 min under ambient condition. An emissive organogel mediated protocol has been developed for the construction of amorphous polymer (COP), selectively in N,N-dimethyl acetamide (DMA). Interestingly, two non-emissive building block units without large π-conjugated structures have been engaged for the construction of green emissive COP-gel. The green emission of COP-gel is generated by the intermolecular H-bonding assisted aggregation induced emission phenomenon. The dried COP efficiently adsorbs aniline molecules into its cavities and separate them from binary mixtures of aniline/nitrobenzene and aniline/water, respectively.

DYNAMIC THREE-POINTS-BENDING TEST MODE OF TWO AMORPHOUS POLYMER MATERIALS (PMMA, PC) FOR THEIR VISCOELASTIC AND MECHANICAL CHARACTERIZATION

This paper presents an approach to classifying amorphous polymer materials. Temperature is This classification involves the determination of mechanical and viscoelastic characteristics considered a descriptive variable to clarify the specific field of practical applications of amorphous polymers. according to the reference temperature characterizing the behaviour of polymer materials. The mechanical and viscoelastic characteristics of amorphous polymers such as methyl poly-methacrylate (PMMA), polycarbonate (PC) and imide poly ether (PEI) are determined through the three-point dynamically embedded test carried out in an adiabatic close enclosure. The complex dissipative or conservative modules according to the temperature are represented. The results obtained show that the fluidity index of these materials is linked to their viscosity, which is a determining property which is decisive for the choice of the technique of the application of the material. Our method of measuring properties is therefore, in principle, comparable to the techniques used in industrial development.

Unravelling Main- and Side-Chain Motions in Polymers with NMR Spectroscopy and Relaxometry: The Case of Polyvinyl Butyral

Polyvinyl butyral (PVB) is an amorphous polymer employed in many technological applications. In order to highlight the relationships between macroscopic properties and dynamics at a microscopic level, motions of the main-chain and of the propyl side-chains were investigated between Tg − 288 °C and Tg + 55 °C, with Tg indicating the glass transition temperature. To this aim, a combination of solid state Nuclear Magnetic Resonance (NMR) methods was applied to two purposely synthesized PVB isotopomers: one fully protonated and the other perdeuterated on the side-chains. 1H time domain NMR and 1H field cycling NMR relaxometry experiments, performed across and above Tg, revealed that the dynamics of the main-chain corresponds to the α-relaxation associated to the glass transition, which was previously characterized by dielectric spectroscopy. A faster secondary relaxation was observed for the first time and ascribed to side-chains. The geometry and rate of motions of the different groups in the side-chains were characterized below Tg by 2H NMR spectroscopy.