Molecular Mobility In Polymers Research Articles

Molecular Mobility of Polymers at the Melting Transition.

Melting of crystals is an archetypical first order phase transition. Albeit extensive efforts, the molecular origin of this process in polymers is still not clear. Experiments are complicated by the tremendous change in mechanical properties and the occurrence of parasitic phenomena masking the genuine material response. Here, we present an experimental procedure permitting to circumvent these issues by investigating the dielectric response of thin polymer films. Extensive measurements on several commercially available semicrystalline polymers allowed us to identify a genuine molecular process associated with the newly formed liquid phase. In line with recent observations of amorphous polymer melts, we show this mechanism─known as the slow Arrhenius process (SAP)─involves time scales longer than those characteristics of segmental mobility and has the same energy barrier of the flow of the melt.

A control system for performing automated time-domain NMR measurements in Bruker Minispec spectrometers

Time-Domain Nuclear Magnetic Resonance (TD-NMR) is a powerful method used for general characterization of a variety of materials as well as to online or in-situ monitoring of process. Because of that industries increasingly benefit from its use in process analytical technology (PAT) for production control and quality assurance. However, there is an increasing number of applications that requires automated signal acquisition as a function of temperatures, a resource that is not always available in many of the commercial equipments. We describe an automation procedure using the Bruker Minispec mq20 spectrometer with the BVT3000 temperature control, building software and hardware to execute TD-NMR experiments as a function of the temperature as well as the temperature calibration in an automatic form. Despite being developed for this specific equipment, the general idea can be used in other TD-NMR equipments that could benefit from this type of automation. As a demonstration, the system was used to monitor the temperature dependence of molecular mobility in polymers. All developed codes are shared in an open repository on GitHub.

Температурная эволюция взаимодействия релаксационных процессов с локальной динамикой на терагерцевых частотах в полимерах с водородными связями

At terahertz frequencies, the torsional-vibrational motion is associated with dielectric relaxation in disordered solids with hydrogen bonds. The interaction between these processes is still poorly understood, especially at temperatures below the glass transition temperature, which is especially important for molecular mobility in polymers. We have studied polymers with hydrogen bonds (polyamide-6 and polyvinyl chloride) at temperatures from 90 to 400 K in the range 0.25 - 4 THz using far-infrared spectroscopy. Three general features were observed in the spectrum of dielectric losses, ”(a) at temperatures well below the glass transition (Tg), these losses are represented by the low-frequency wing of the absorption peak due to the libration of polymer monomer units. (b) At 0.7Tg <T <Tg, additional temperature-dependent losses are observed, which can be associated with the manifestation of secondary relaxation processes. (c) At temperatures above Tg, primary α-relaxation processes become the dominant contribution to terahertz losses. The results obtained show that the evolution of terahertz losses with temperature is caused by a change in the structure of hydrogen bonds, which, apparently, is universal for systems with similar intermolecular interactions.

Temperature evolution of the interaction of relaxation processes with local dynamics at terahertz frequencies in polimers with hydrogen bonds

At terahertz frequencies, the torsional-vibrational motion is associated with dielectric relaxation in disordered solids with hydrogen bonds. The interaction between these processes is still poorly understood, especially at temperatures below the glass transition temperature, which is especially important for molecular mobility in polymers. We have studied polymers with hydrogen bonds (polyamide-6 and polyvinyl chloride) at temperatures from 90 to 400 K in the range 0.25-4 THz using far-infrared spectroscopy. Three general features were observed in the spectrum of dielectric losses, ε''(ν) (a) at temperatures well below the glass transition (Tg), these losses are represented by the low-frequency wing of the absorption peak due to the libration of polymer monomer units. (b) At 0.7Tg<T<Tg, additional temperature-dependent losses are observed, which can be associated with the manifestation of secondary relaxation processes. (c) At temperatures above Tg, primary α-relaxation processes become the dominant contribution to terahertz losses. The results obtained show that the evolution of terahertz losses with temperature is caused by a change in the structure of hydrogen bonds, which, apparently, is universal for systems with similar intermolecular interactions. Keywords: polymers, terahertz IR-spectra, temperature dependence, α- and β-relaxations, hydrogen bonding.

Study of Radiation-Chemical Structuring of Compositions Based on Epoxy Oligomers

The processes of radiation-chemical structuring of modified epoxy acrylic compositions were studied depending on the nature of epoxy oligomers and modifiers. ED-20 and ED-22 grades epoxy oligomers were chosen as epoxidian oligomers; 3,4-epoxyhexahydrobenzal-3,4-epoxy-1,1-bis(hydroxymethyl)cyclohexane (UP-612 grade), 3,4-epoxycyclohexylmethyl-3,3-epoxycyclohexanecarboxylate (UP-632 grade) were chosen as cycloaliphatic oligomers; diethylene glycol diglycidyl ether (DEG) was chosen as aliphatic oligomer. To modify the epoxy oligomers, unsaturated acrylic monomers, such as acrylic acid and methyl acrylic acid ester; aromatic and aliphatic oligoester acrylates, such as α,ω-methacryl(bis-diethylene glycol)phthalate (MDP-1 grade), α,ω-methacryldi(diethyleneglycolphthalate) (MDP-2 grade), α,ω-methacryl(bis-triethyleneglycol)phthalate (MGP-9 grade) and atri-(oxyethylene)-α,ω-dimethacrylate (TGM-3 grade); condensation product of linseed oil dimerized fatty acids and polyethylenepolyamine, such as oligoamide L-20 grade; condensation product of ricinoleic acid, castor oil and maleic anhydride, such as unsaturated polyester PE-220, were used. The thermophysical and relaxation properties of cross-linked polymers, obtained under exposure to γ-radiation of Co60 and electrons accelerated by the absorbed radiation dose of 50–150 kGy at the beam current of 2‑4 mA and electron energy of 240-300 keV, were studied. The distance from the exhaust window of the accelerator to the irradiated surface of the samples was 63-80 mm. The thermophysical properties of the cured polymers were evaluated using thermomechanical studies on a thermomechanograph with the temperature rise of 1о/min and the pressure of 0.54 MPa in the temperature range of 293‑673 K, as well as the differential thermal and thermogravimetric analyzes on a system derivatograph by L. Paulik, R. Paulik, L. Erdei in the temperature range of 293–973 K with the rise rate of 7о/min. The relaxation properties and molecular mobility of the cross-linked polymers were studied by the dielectric method. The dielectric loss tangent was determined in the temperature range of 143‑393K at the frequency of 1 kHz using a digital automatic bridge of alternating current R-589. The test sample temperature was measured using a potentiometer on a copper-constantan thermocouple, which was placed in the measuring cell along with the test sample. The samples were cooled by placing the cell in a vessel with liquid nitrogen. As a result of the studies, the influence of the chemical nature of epoxy oligomers and unsaturated modifiers on the processes of structure formation and molecular mobility of polymers, obtained under conditions of radiation-chemical curing, was established. It was determined, that when modifying epoxy oligomers with acrylic acid, methyl acrylic acid ester, oligoester acrylates of MGP-9 ((α,ω-methacryl(bis-triethyleneglycol)phthalate)) or TGM-3 ((tri-(oxyethyleneglicol)-α,ω-dimethacrylate)) grades, the structuring of compositions, based on the DEG-1 aliphatic epoxy oligomer and UP-612 cycloaliphatic oligomer, was more efficient than the compositions based on unsaturated ETO oligomers grade such as 2-ethylhexylepoxytallate oligomer and epoxidian oligomers of ED-20 and ED-22 grades. It was established, that the modifier oligoamide of L-20 grade, containing primary and secondary amino groups, increased the radiation sensitivity of the compositions, based on epoxidian and cycloaliphatic oligomers, and reduced the inhibitory effect of atmospheric oxygen. The optimal composition and the main technological parameters were determined, what allowed to obtain materials with high physical and mechanical properties and adhesive strength to various metals. The application of the developed materials under industrial conditions will make it possible to organize a continuous high-speed radiation-chemical process for producing polymer coatings on metals, providing improved working conditions, lower energy consumption, as compared to the thermochemical process of the coatings production, and will increase the corrosion resistance of metals.

Dielectric Spectroscopy of Small Molecule Pharmaceuticals—Effect of Sample Configuration

In dielectric spectroscopy, a technique traditionally used to characterize molecular mobility in polymers, the sample is usually analyzed as a thin film. In recent years, the technique has been extended to characterize both drug substances and drug products and has revealed a correlation between molecular mobility and stability. However, for pharmaceutical systems, analysis of powders is strongly preferred over films. Therefore, the dielectric behavior of several compounds of pharmaceutical interest-nifedipine, indomethacin, itraconazole, and griseofulvin-obtained using powder and film samples were compared. The magnitudes of the intrinsic dielectric properties were affected by the sample configuration with the powder samples consistently yielding lower values. The use of effective medium theories enabled us to account for the effect of air in the powder samples. The relaxation time, a property of immense importance to the pharmaceutical community, was not influenced by the sample configuration.

Solventless processing of conjugated polymers—A review

The molecular mobility of polymers in their solid or molten states allows their processing without the need for toxic, “non-friendly” solvents. In this work, the main features of solvent-free processing methods applied to conjugated polymers are reviewed taking into consideration that these materials are largely used in a broad range of (opto-)electronic applications, including organic field-effect transistors, polymer light-emitting diodes and polymer photovoltaic devices. This review addresses the main advantages of processing such materials in their solid state offering an alternative fabrication scheme for several opto-electronic devices. Moreover, the properties of polymer-based opto-electronic devices are largely dependent on the conformations and orientations of polymer molecules in the corresponding polymer thin film layers; especially the ordering of conjugated polymer molecules in the active layer has been a topic of intense research in organic electronics. Therefore, this review also highlights the control of conjugated polymer molecule ordering when using solvent free processing techniques. Finally, conclusions and perspectives of solventless processed conjugated polymer molecules in organic electronics are addressed.

Study of the Molecular Mobility in Polymers with the Thermally Stimulated Recovery Technique—A Review

Thermally stimulated recovery (TSR) is a non‐conventional mechanical spectroscopy technique that allows to analyse in detail the relaxation processes of polymeric systems in the low frequency region. This work reviews the main aspects and potentialities of this technique. The different kinds of TSR experiments that can be performed, global and thermal sampling (TS) experiments, are described and illustrated with several examples. Also, the different methods for the determination of the thermokinetic parameters (activation energy and pre‐exponential factor) of the thermal sampling (TS) procedure are explained and compared. In this context, the compensation phenomenon, which always appears in TSR results when the studies are performed in the glass transition region of a given system, is discussed. Examples of the application of this technique to different polymeric systems during the last 20 years are provided. An emphasis will be made on the analysis of the effect of crystallinity degree and crosslink density on the TSR response. A comparison between the results (characteristic times and activation energies) obtained by different techniques, namely TSR, dynamic mechanical analysis (DMA), and differential scanning calorimetry (DSC), is made.

Macroscopic Phenomena in the Molecular Mobility of Polymers under External Force Fields

Macroscopic Phenomena in the Molecular Mobility of Polymers under External Force Fields

Molecular mobility in polymers studied with thermally stimulated recovery. II. Study of the glass transition of a semicrystalline PET and comparison with DSC and DMA results

The glass transition of a semicrystalline PET is studied by dynamic mechanical analysis (DMA) and thermally stimulated recovery (TSR). The DMA results allowed us to construct a master curve for E′ and D′, where the shift factors were modelled with the Vogel–Fulcher–Tamman–Hesse (VFTH) equation above Tg. Using the thermal sampling procedure, the TSR technique allowed us to decompose the complex distribution of retardation times into a set of quasi-elementary processes. At the compensation point a broad distribution of retardation times is observed using DSC data. The TSR results show a typical increase of the activation energies in the glassy state, followed by a decrease above Tg that may be modelled with the VFTH equation. This observation was explained taking into account both the typical time scale range of TS experiments and the evolution of the retardation times distribution with temperatures below and above Tg, under the Adams–Gibbs theory. The evolution of the activation energies from the TSR results was shown to be compatible with the DMA and DSC data.

Manifestation of Molecular Mobility of Polymers of Different Classes over a Wide Temperature Range

Manifestation of Molecular Mobility of Polymers of Different Classes over a Wide Temperature Range

Thermally stimulated creep spectroscopy for the study of DGEBA-DDM networks

During the past two decades, Thermally Stimulated Creep Recovery (TSCR) has been used successfully to study molecular mobility in polymers. The main feature of this technique is its quite good sensitiveness: the sample undergoes a shear stress, so that the subsequent strain is linear in a range larger than in elongation or compression experiments. Furthermore, the low equivalent frequency of TSCR permits the resolution of normally overlapping peaks.In this study, the TSCR technique has been used to investigate a series of DGEBA-DDM networks differing from the value of the amine-to-epoxy ratio which was systematically varied below stoichiometric composition. The magnitude of theΒ-mode (glassy region) is seen to decrease unexpectedly as the network gets looser, while its peak-temperature decreases. This result is interpreted in terms of ‘internal antiplasticization’. Moreover, we show that the important shift of theα-peak associated with the glass transition has an essentially entropic origin.

Effect of simultaneously applied external force fields on molecular mobility in polymers

The processes of molecular mobility in polymers under conditions when various force fields are simultaneously applied are quantitatively described by means of the methods of nonequilibrium thermodynamics. Considering the changes in the height of potential barriers resulting from the interaction of the kinetic units of a given subsystem with external force fields, theoretical relations were obtained in sufficiently good agreement with experimental data.

On the intramolecular ordering and the mobility mechanism in solutions of comb-like polyalkyl methacrylates

The dipole polarization relaxation method was used to investigate molecular mobility in solutions of polyhexadecylmethacrylate (PMA-16) in toluene. The previously obtained data on molecular mobility in polymers of type PMA- n ( n = 1, 4, 8, 10, 16 and 22) have been analysed. It is shown that a new relaxation process appears as a result of the partial aggregation of side-chains in the case where n > 10.

Relationships between the kinetic parameters of radical reactions and the state parameters, macro-characteristics and molecular mobility of polymers and solids

Relationships between the kinetic parameters for free-radical recombination in polymers and low-molecular weight crystals and the pressure, volume and temperature have been studied. Experimental values for the activation volumes and energies of chemical reactions in solids have been shown to coincide with the corresponding V∗ and E values in small-scale molecular dynamics. A relationship is established between the activation recombination parameters and the compound's coefficients of compressibility and thermal expansion. Equations are obtained relating the state parameters, reaction activation parameters and macro-characteristics of condensed media.

Molecular dynamics in polymers and the hydrogen bond

The results of IR spectroscopic investigations of the formation and rupture of hydrogen bonds and intermolecular proton transfer in complexes involving carboxyl groups and hydrogen bonds in polymeric matrices have been generalized. Most attention has been given to consideration of the effect of molecular mobility in polymers on the thermodynamic condition of the system of hydrogen bonds. Deviation of the hydrogen bond system and the proton-transfer complex from the equilibrium state occurs at temperatures corresponding to the characteristic relaxation transitions of the polymeric matrix.

Molecular mobility in polymers under a load and its influence on the kinetics of radical reactions. Review

The authors examine and systematize the experimental data on the dynamics of movements of spin probes in polymers exposed to uniaxial stretching or all-sided compression. It is shown that these results may be explained both in terms of the thermodynamic approach to diffusion and within the framework of the model of the static (rigid) cell of the polymer matrix. The data on the influence of the molecular mobility in polymers on the kinetics of radical reactions are analysed.

Analysis of the molecular mobility of polymers in the bulk state by the fluorescence anisotropy decay technique

The basic phenomena of fluorescence and fluorescence anisotropy decay (FAD) are briefly presented. The FAD technique is first applied to polybutadiene labelled with anthracene in the middle of the chain and embedded in a matrix of unlabelled polybutadiene; experiments are performed in the temperature range −50°C- +80°C. It is shown that the experimental data can be very well accounted for using the orientation autocorrelation function proposed by Hall and Helfand (6) for polymer chains. Studies performed on 9,10-dialkylanthracene probes (with various lengths of the alkyl substituents) molecularly dispersed in a polybutadiene matrix, have shown that the dynamic behavior of the 9,10-dihexadecylanthracene probe is very close to that of the labelled polybutadiene chain. This indicates that the 1-dimension diffusion of orientation correlation along the chain, observed by FAD on labelled polybutadiene in bulk, involve rather short chain portions (about 6 monomer units). Furthermore, the temperature dependence of the correlation times of both anthracene labelled chains and anthracene probes with long alkyl substituents, is in agreement with the WLF equation used to describe the glass-transition phenomena. Thus, these local reorientation processes observed in FAD correspond to segmental motions of polybutadiene chains which are involved in the macroscopic phenomenon of the glass transition.

Cage effects in a polymer matrix

The experimental data on investigations of the cage effect in polymers are examined for the decomposition of 2,2′-azoisobutyronitrile and lauroyl peroxide. The kinetic schemes taking into account only translational motion of radicals are shown not to describe the cage effects in polymers. The experimental dependence of the cage effect on molecular mobility in polymers may be explained on the basis of schemes taking into account both the rotational and translational motions of particles. The study of cage effects in oriented polymeric films proves an important role of the form of the cage. In polymers the molecular mobility influences the rate of slow reactions. This is explained for the scope of the model of a rigid cage where orientation of the particles depends on the form of the cage.

Molecular mobility in polymers with aromatic mesogenic groups in their side chains

The dielectric relaxation parameters of two polymers of the methacrylic series having pendant mesogenic groups of the phenylbenzoate class have been investigated in the audible frequency range at temperatures below the glass transition. In studies of these polymers in the bulk state, several relaxation processes involving dipole polarization have been observed; these can be related to the mobility of various kinetic units in the pendant group. A study of the intramolecular mobility of the polymers in solution has shown that there is structural order in the positioning of the side groups.