Dipolar motions and phase transitions in a side‐chain polysiloxane liquid crystal. A study by thermally stimulated depolarization currents

The relaxation mechanisms present in a side‐chain liquid crystalline polymer have been studied by Thermally Stimulated Depolarization Currents (t.s.d.c.), in a wide temperature range covering the glassy state, the glass transition region, and the liquid crystalline phase. The thermal sampling procedure was used to decompose the complex relaxations into its narrowly distributed components. Three relaxation mechanisms were observed in this polymer: a relaxation below the glass transition temperature that is broad and extends from −150°C up to −110°C, the glass transition relaxation whose maximum intensity appears at ∼20°C, and a relaxation above the glass transition temperature, in the liquid crystalline phase. The attribution of these relaxations at the molecular level is discussed. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 227–235, 1999     

Dynamic mechanical relaxations of polyterephthalates based on trimethylene glycols

The dynamic mechanical relaxations of poly(trimethylene glycol terephthalate) (PTMT), poly(ditrimethylene glycol terephthalate) (PDTMT) and two copolymers obtained from them have been studied between ? 150 and 200°C with a dynamic viscoelastometer. The four polymers show three relaxations that are designated α, β, and γ in order of decreasing temperature. The α relaxation is considered to be the glass transition of the polymers. The β relaxation is wider and weaker than the α, as normally occurs in the polyester series. The γ relaxation takes place at temperatures below ? 100°C and is usually overlapped by the β relaxation. The influence of thermal and mechanical histories on the nature, location, and intensity of the three relaxations is studied and discussed.     

Relaxations during the postcure of unsaturated polyester networks by ultrasonic wave propagation,dynamic mechanical analysis,and dielectric analysis

Ultrasonic wave propagation, dynamic mechanical analysis, and dielectric analysis were used to monitor relaxation phenomena during the nonisothermal postcure of unsaturated polyester networks. The measurements covered 6 decades of frequency. As a result, the residual reactive groups, immobilized in the glassy state by vitrification during an isothermal cure step, gained molecular mobility, which promoted the formation of additional crosslinks. After the postcure, the reaction was complete, and the maximum achievable glass‐transition temperature was reached. Moreover, the frequency and temperature dependence of the two relaxations, one related to the glass‐transition temperature of the partially cured sample and the other to the glass transition of the fully cured sample, was evaluated. The Williams–Landel–Ferry equation was used to model the frequency dependence of the main α‐relaxation data obtained with the different techniques. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 596–602, 2005     

Transitions/relaxations in polyester adhesive/PET system

The correlations between the transitions and the dielectric relaxation processes of the oriented poly(ethylene terephthalate) (PET) pre-impregnated of the polyester thermoplastic adhesive have been investigated by differential scanning calorimetry (DSC) and dynamic dielectric spectroscopy (DDS). The thermoplastic polyester adhesive and the oriented PET films have been studied as reference samples. This study evidences that the adhesive chain segments is responsible for the physical structure evolution in the PET-oriented film. The transitions and dielectric relaxation modes’ evolutions in the glass transition region appear characteristic of the interphase between adhesive and PET film, which is discussed in terms of molecular mobility. The storage at room temperature of the adhesive tape involves the heterogeneity of the physical structure, characterized by glass transition dissociation. Thus, the correlation between the transitions and the dielectric relaxation processes evidences a segregation of the amorphous phases. Therefore, the physical structure and the properties of the material have been linked to the chemical characteristics.     

Dynamic mechanical and dielectric properties of phosphonylated polypentenamers and their hydrogenated derivatives

Dynamic mechanical and dielectric properties of substituted polypentenamers with phosphonate side groups and their hydrogenated derivatives have been studied. Methyl esters, acids, and salts were investigated at two concentrations, 6.5 and 11.1 mole percent. In the unhydrogenated derivatives, one principal relaxation, labeled β, is observed mechanically in the temperature range from ?160 to 100°C. This β relaxation arises from micro-Brownian segmental motion accompanying the glass transition. Its temperature is substantially affected by the substituent concentration while its breadth is affected by the chemical nature of the substituent. An extended “rubbery plateau” region exists in the acid and salt derivatives. The dielectric results generally reinforce the mechanical assignments. In the hydrogenated derivatives, three relaxations labeled α, β, and γ in order of decreasing temperature occur mechanically in this temperature range. The temperatures at which the α and β relaxations occur depend greatly on the chemical nature of the substituents, the substituent concentration, and the thermal history of the sample; while the γ relaxation appears to be independent of these variables. Suggested assignments for the relaxations observed in these polymers, based on the dual glass transition theory of Boyer for semicrystalline polymers, have been proposed. The dielectric results are consistent with the proposed assignments.     

Multiple mechanical relaxations in ethylcyclohexane above the glass transition temperature

The mechanical response of ethylcyclohexane has been investigated at ultrasonic frequencies in a large temperature range from 300 K down to the glass transition region. The results indicate the existence of a secondary relaxation not yet reported for this system. The comparison with literature data leads to a rather complex dynamic behavior. In fact, this molecular liquid exhibits three different mechanical relaxations above the glass transition temperature: a main structural process and two additional processes, both having a possible intramolecular origin.     

The β relaxation in the dynamic mechanical spectrum of polyester networks containing various structural units

The β relaxation has been observed in the dynamic mechanical spectra of a range of styrene-unsaturated polyester networks. The influences on the β relaxation of the polyester molecular weight, the nature of the diol and saturated dioic acid units and the presence of water were investigated. From these studies, the relaxation is shown to arise from the motion of the saturated dioic acid-diol grouping in the polyester segments, and appears to be closely connected with the glass transition of the uncrosslinked polyester. Studies of the influence of the styrene concentration in the network suggest that the bulky styrene units may restrict the motion of some of the potential relaxation sites.     

Two kinds of nanoscale dynamic heterogeneity in single‐phase polymer blends and their common origin

Differential scanning calorimetry (DSC) and laser‐interferometric creep rate spectroscopy (CRS) were used for kinetic and discrete analysis of segmental motion within (and close to) glass transition range in polystyrene ‐ poly(α‐methyl styrene) (PS/PMS) and polystyrene ‐ poly(vinyl methyl ether) (PS/PVME) miscible blends. Two kinds of segmental dynamics heterogeneity were found. Separate ‘unfreezing’ of PS and PMS segmental motions was observed that manifested itself in two T_gs and simultaneous large drop in the Tg s, as well as glass transition activation energy, motional event scale and cooperativity degree values, down to the β‐relaxation parameters. The wide activation energy dispersion within a single broad glass transition in PS/PVME blends was found, and this relaxation region was subdivided, by CRS, into several predicted kinds of segmental motion. Both results are treated in the framework of the concept of common segmental nature of α‐ and β‐relaxations in flexible chain polymers.     

The relationship between chemical structure and viscoelastic properties of linear aliphatic polyesters

The effects of Chemical structure on the molecular motions in linear aliphatic polyesters have been investigated with a free-oscillation torsion pendulum. Broad-line NMR provided supplementary information. In the γ relaxation which corresponds to the local-mode motions of main chains in the noncrystalline region, the polyesters which are composed of two methylene units in the diol part of the chemical repeat unit showed an extremely asymmetric loss curve with a relatively high-loss peak temperature compared with that of the other polyesters. In addition to the two relaxations (β,γ) which have been observed in earlier dielectric measurements, a new relaxation (α) was found on the high-temperature side of the glass transition of the polyesters. The α relaxation was assigned to molecular motions of methylene segments in the crystalline region. The α and β relaxations of the two-dimensional series are situated close to the temperatures found for other polyesters with rather long methylene sequence in the chemical repeat unit. The results were explained in terms of a difference on the chain mobility in the noncrystalline regions which may be related to the difference of chemical structure of the polyesters.     

Effect of substituents in the styrene ring on the dynamic mechanical relaxations of a styrene-crosslinked unsaturated polyester resin

The dynamic mechanical properties of a series of polyester resins made from a maleic/phthalic anhydride-based unsaturated polyester crosslinked with each of styrene, 4-methyl styrene, 4-ethyl styrene, 4-n-butyl styrene, 4-isopropyl styrene, tertiary butyl styrene, 4-chlorostyrene, and 3,4-dichlorostyrene were studied. The order of the α transition temperatures was as expected from that for the homopolymers, except in the case of the chlorostyrenes, for which dipolar interactions with the polyester chain may be important. The styrene bridges appeared to be involved in a steric interaction (and in the case of the chlorostyrenes, a dipolar interaction) with the β relaxing ester species. It is suggested that both the γ and γ′ relaxations involve similar interactions between the matrix and the relaxing moieties. For the 4-n-butyl styrene resin, an additional relaxation below ?170°C was observed, and is ascribed to relaxation of the n-butyl group.     

Glass transition in polyacrylonitrile: Analysis of dielectric relaxation data

Dielectric relaxation of polyacrylonitrile (PAN) samples, in which the presence of an amorphous phase is evidenced by x-ray diffraction, has been studied over the temperature range 30–150°C and frequency range 10²–10⁵ Hz. These data as well as those reported by other authors, reveal several points useful to the understanding of the dielectric relaxations of PAN in relation to its structure. A glass transition in PAN is evidenced by at least two of the four data sets investigated; the third shows combined effect of two relaxations, whereas the fourth clearly shows a relaxation process different from the glass transition. The glass-transition behavior of the dielectric relaxation data is confirmed by Williams-Landel-Ferry theory and a recent theory of Phillips, both of which lead to consistent conclusions.     

Dielectric spectroscopy of biodegradable poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) films

In this paper, we report a systematic study of the dielectric relaxation spectroscopy of biodegradable poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx) polyester which has potential applications as a “green” dielectric material in electronic devices. The dielectric spectra was measured over a wide frequency range (10⁰ ～ 10⁷ Hz) from ?100 to 60 °C. A glass and a sub-glass transition relaxations were observed in the dielectric spectra of PHBHHx. In addition, a nearly constant loss behavior was found by analyzing the dielectric and conductivity spectra.     

Structure and dielectric relaxations of antibacterial sulfonated polystyrene and silver nanocomposites

Structure and dielectric relaxations of antibacterial sulfonated polystyrene (SPS) and silver nanocomposites (SPS/Ag) were investigated via broadband dielectric spectroscopy, Fourier transform infrared spectroscopy, ultraviolet–visible spectroscopy, differential scanning calorimetry, scanning electron microscopy, and wide‐angle X‐ray diffraction. SPS/Ag nanocomposites were prepared from SPS containing 2, 4, and 7 mol% of acid contents, followed by ion exchange and a reduction process. Silver nanoparticles were formed in the structural cavities of SPS films. The single glass transition temperature of the SPS copolymers was observed and increased with increasing acid contents and more enhanced with embedded silver nanoparticles because of the restriction of the polymer chain movement. The particle size of embedded silver nanoparticles was about 10 nm and well dispersed in SPS matrices. Four dielectric relaxations were observed above the glass transition temperature, and they were attributed to the fast segmental relaxation, the slow‐hindered segmental relaxation, relaxations associated with Maxwell–Wagner–Sillars interfacial polarization and electrode polarization. Weak local relaxations were observed due to the motion of sulfonated phenyl groups. Copyright © 2014 John Wiley & Sons, Ltd.     

Dynamic mechanical thermal analysis of polyamide 6/thermoplastic polyurethane blends

Blends obtained from polyamide 6 and polyester or polyether polyurethanes were investigated by means of DMTA. The blends were prepared by compounding in a twin-screw Brabender —Plasticorder. Changes in composition did not influence the glass temperature of the amorphous fraction of the polyamide, but also no distinct transition for separated polyurethane soft segment was visible. Therefore the blends seem to be multiphase systems, where the elastomeric polyurethane phase is dispersed in a continuous polyamide phase. From changes in the β relaxation region of the polyamide better miscibility of polyester polyurethanes comparing to polyether polyurethanes was explained by hydrogen bonding in the common amorphous phase.     

Surface structure relaxation of poly(methyl methacrylate)

Surface structure relaxations caused by temperature changes at the free surface of poly(methyl methacrylate) were studied using IR-visible sum-frequency generation (SFG). A polarization-rotating technique was introduced to enhance the sensitivity of SFG for monitoring the surface structure relaxations during a cooling process. A new surface structure relaxation was observed at 67 degrees C. This temperature does not match any known structure relaxation temperatures for the bulk and is 40 degrees C below the bulk glass transition temperature. As expected for a free-surface phenomenon, the surface relaxation temperature was found to be independent of film thickness in the range of 0.1-0.5 microm.     

Dielectric relaxation studies of dipolar aromatics in polyethylene. II. Oriented low-density polyethylene

Stretched polyethylene has been used for several years by organic spectroscopists as a means of orienting isolated aromatic molecules. Dielectric relaxation studies are reported which consider dipolar aromatic molecules dissolved in stretched polyethylene in order to learn more about the environment of these oriented molecules. The research builds on earlier studies of the dielectric relaxation behavior of dipolar aromatic molecules dissolved in unoriented low density polyethylene. Studies demonstrate that molecules in the amorphous phase are oriented at temperatures below the glass transition, both the β and γ relaxations being orientation dependent. It is shown through studies of oriented rods that large numbers of the orientable molecules are immobilized by the oriented polyethylene and cannot relax. An essential criterion for immobilization to occur is that molecules exhibit geometrical symmetry.     

Enthalpic relaxation of silica–polyvinyl acetate nanocomposites

Although the effects of filler nanoparticles size and surface treatment on the glass transition temperature of the matrix phase have received well-deserved attention, nanofiller effects on other physical parameters associated with the glass transition have received less interest. To better understand how the incorporation of nanofillers affects the enthalpic relaxations associated with the glass transition, differential scanning calorimeter measurements were carried out on silica–polyvinyl acetate nanocomposites with respect to filler content, annealing temperature, and annealing period. As expected, longer annealing periods below the glass transition temperature resulted in an increase of the subsequent enthalpic relaxations. However, the presence of filler substantially reduces the enthalpic relaxation relative to that of the neat polymer at longer annealing periods only. The underlying enthalpic relaxations and the effects suppressed by the fillers are specific to the annealing temperature. These results suggest a significant alteration in the physical state of the matrix because of the presence of the filler particles. However, this does not imply the existence of a glassy layer or layers with a glass transition gradient near the filler surface. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2733–2740, 2008     

The nearly constant loss,Johari‐Goldstein β‐relaxation,and α‐relaxation of 1,4‐polybutadiene

From high‐resolution dielectric spectroscopy measurements on 1,4‐polybutadiene (1,4‐PB), we show that in addition to the structural α‐relaxation and higher frequency secondary relaxations in the spectra, a nearly constant loss (NCL) is observed at shorter times/lower temperatures. The properties of this NCL are compared to those of another chemically similar polymer, 1,4‐polyisoprene. The secondary relaxations in 1,4‐PB include the well‐known Johari‐Goldstein (JG) β‐relaxation and two other higher‐frequency peaks. One of these, referred to as the γ‐relaxation, falls between the JG‐relaxation and the NCL. Seen previously by others, this γ‐relaxation in 1,4‐PB is not the JG‐process and bears no relation to the glass transition. At very low temperatures (<15 K), we confirm the existence of a very fast secondary relaxation, having a weak dielectric strength and an almost temperature‐invariant relaxation time. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 342–348, 2007     

Primary and secondary relaxations in bis-5-hydroxypentylphthalate

Broadband dielectric spectroscopy was used to study the relaxation dynamics in bis-5-hydroxypentylphthalate (BHPP) under both isobaric and isothermal conditions. The relaxation dynamics exhibit complex behavior, arising from hydrogen bonding in the BHPP. At ambient pressure above the glass transition temperature T(g), the dielectric spectrum shows a broad structural relaxation peak with a prominent excess wing toward higher frequencies. As temperature is decreased below T(g), the excess wing transforms into two distinct peaks, both having Arrhenius behavior with activation energies equal to 58.8 and 32.6 kJmol for slower (beta) and faster (gamma) processes, respectively. Furthermore, the relaxation times for the beta process increase with increasing pressure, whereas the faster gamma relaxation is practically insensitive to pressure changes. Analysis of the properties of these secondary relaxations suggests that the beta peak can be identified as an intermolecular Johari-Goldstein (JG) process. However, its separation in frequency from the alpha relaxation, and both its activation energy and activation volume, differ substantially from values calculated from the breadth of the structural relaxation peak. Thus, the dynamics of BHPP appear to be an exception to the usual correlation between the respective properties of the structural and the JG secondary relaxations.     

A new insight in the dynamo-mechanical behavior of poly(ethylene terephthalate)

Dynamic mechanical experiments performed on a semicrystalline sample of poly(ethylene terephthalate) evidenced the relaxation processes (α and β) that occur in the polymer. Multifrequency experiments were used both in order to calculate the apparent activation energy of relaxation and to point out how the dynamic conditions alter not only the main relaxations, but the overlapping melting/recrystallization events that succeed after the glass transition. Heating rate seems to affect these phenomena in a way similar to frequency. Also, a consecutive heating step was carried out to support the statements.