Relaxation of the surface energy of solid polymers

Summary The surface energy of a polymer can be increased by compression moulding against a metal substrate. After removal of the substrate relaxation to the equilibrium value sets in. We determined the rate of polymer surface energy relaxation as a function of temperature. For a vinyl chloride-vinyl acetate copolymer we determined an activation energy of the relaxation process that could be correlated to segmental motions in this polymer. For a plasticized polyvinylchloride we found a lower activation energy and a larger rate of relaxation, which is the result of the action of plasticizers on segmental motions. In the case of polyethylene the results indicate segmental motions in amorphous regions in the polymer. With polyethylene the activation energy drops when nearing the melt temperature. The movements of molecular segments correspond to a desorption process at the polymer surface, after removal of the substrate. This agrees with the adsorption during compression moulding, as repotted in earlier work.

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Zusammenfassung Die Oberflächenenergie eines Kunststoffs kann erhöht werden durch Schmelzen auf einer Metalloberfläche. Nach Ablösen von der Metallfläche beginnt eine Relaxation zu dem Gleichgewichtswert für die Oberflächenenergie. Wir bestimmten die Geschwindigkeit der Relaxation als Funktion der Temperatur. Bei einem Vinylchlorid-Vinylazetat-Copolymer fanden wir eine Aktivierungsenergie für den Relaxationsprozeß entsprechend der Segmentbeweglichkeit im Polymeren. Bei einem plastifizierten PVC fanden wir eine geringere Aktivierungsenergie wegen der Mitwirkung des Weichmachers. Bei Polyäthylen wirkt sich die Segmentbewegung in den amorphen Anteilen des Polymers aus. Bei Polyäthylen nimmt die Aktivierungsenergie bei höherer Temperatur ab. Die Segmentbewegungen sind zu erklären durch einen Desorptionsprozeß an den Polymer-Oberflächen nach Entfernung des Substrats. Diese Ergebnisse bestätigen vorangehende Experimente, in denen wir Adsorption zum Substrat beim Spritzguß fanden.

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List of symbols A rate of degradation - k rate of relaxation - M number average molecular weight - T temperature - T _p highest temperature during compression moulding - T _R temperature during relaxation - t time after removal of substrate - _P polymer surface energy - _P ^O equilibrium value of polymer surface energy - t) _P – _P ^O excess polymer surface energy after compression moulding and removal of substrate With 6 figures and 1 table     

Two-dimensional NMR methods for studying solid polymers

A survey is given about two-dimensional (2D) NMR experiments on solid polymers involving ²H- and ¹³C-NMR. 2D exchange NMR spectra of static samples directly reflect the distribution of rotational angles resulting from ultraslow molecular motions. Typical examples are the chain motion above the glass transition or rotations around a helix axis in semi-crystalline polymers. 2D-Magic angle spinning not only allows the detection of molecular order and motion. By combining rotor synchronized MAS with rotations in spin space the correlation of order and mobility can be studied.     

Length scales in heterogeneous polymers from solid state NMR

New methods for probing structure and dynamics of heterogeneous polymers by multidimensional nuclear magnetic resonance (NMR) are described. On the molecular level high resolution multiple quantum spectroscopy of solids probes connectivities of different functional groups on length scales below 1 nm. On a mesoscopic scale NMR spin diffusion techniques probe phase separation and interfacial effects in polymer blends and block copolymers. On a macroscopic scale NMR imaging techniques allow to spatially resolve differences in order and mobility in the necking region or in shearbands of deformed polymers. The techniques are illustrated with amorphous polymers, elastomers, and core-shell systems.     

Chemically resolved solid state NMR images of polymers

Polymers are generally spatially heterogeneous, either in terms of morphology or as blends of various components. Solid state NMR imaging provides a means of characterizing both the chemical/morphological composition and its spatial variation. Here we discuss multiple-pulse line-narrowing approaches to acquiring high resolution NMR images and how these sequences can be modified so as to be sensitive to the chemical composition of a sample.     

Structure investigations of solid organosilicon polymers by high resolution solid state Si NMR

The high resolution ²⁹Si NMR spectra of five solid silicon polymers of different structure have been studied and the ²⁹Si chemical shifts of characteristic structure units determined. ²⁹Si---¹H cross-polarization in combination with high speed magic angle sample spinning and high power proton decoupling was used to achieve high resolution in the solid state spectra. Comparison of the latter with the results obtained in the liquid state clearly indicates that no special solid state effects on ²⁹Si chemical shifts arise and the relations between δ(Si) and the molecular structure, well known from investigations of liquids, can be used for interpretation of the solid state spectra. It is shown that high resolution solid state ²⁹Si NMR spectroscopy offers detailed information about the structural units of the siloxane resin framework, and this opens up new possibilities for structural determinations of solid organosilicon polymers.     

Pulsed deuteron NMR investigations of structure and dynamics of solid polymers

Pulsed deuteron NMR is described, which recently has been developed to become a powerful tool for studying structure and molecular dynamics in solid polymers. The techniques that have been developed in this area are described, analyzing the response of the I = 1 spin system to the solid echo two-pulse and the Jeener-Broekaert three-pulse sequence, respectively. By applying these techniques to selectively deuterated polymers, slow rotational motions involving different segments of the monomer unit can be monitored over a range of approximately 8 orders of magnitude of characteristic frequencies. In addition, motional heterogeneities can be detected. In drawn fibres the complete orientational distribution of the polymer chains can be determined from the analysis of deuteron NMR line shapes. The techniques are illustrated by experimental examples including order and chain mobility in the amorphous regions of $\text{linear polyethylene}$, chain dynamics of $\text{polystyrene}$ in the vicinity of the glass transition and the phenyl motion in $\text{polycarbonate}$.     

Molecular dynamics of solid polymers as revealed by deuteron NMR

Pulsed deuteron NMR spectroscopy is described, which has recently been developed to become a powerful tool for studying molecular dynamics in solid polymers. It is shown that by analyzing the line shapes of²H absorption spectra and spectra obtained via solid echo and spin alignment, respectively, both type and timescale of rotational motions can be determined over an extraordinary wide range of characteristic frequencies, approximately 10 MHz to 1 Hz. By applying these techniques to selectively deuterated polymers, motional mechanisms involving different segments of the monomer unit can be monitored. In addition, motional heterogeneities in glassy polymers can be detected. The information about polymer dynamics available now is illustrated by a number of experimental examples. The chain motion in the amorphous regions of linearpolyethylene is discussed in detail and it is shown that it can clearly be distinguished from the chain motion of an amorphous polymer above the glass transition, wherepolystyrene is used as an example. Localized motions in the glassy state are illustrated through the jump motion phenyl groups exhibit both in the main chain (polycarbonate) and as a side group (polystyrene). The latter polymers also serve as examples for detecting motional heterogeneity. Finally, the mobility in novel classes of systems,liquid crystalline polymers andpolymer model membranes as revealed by²H NMR are described.     

NMR imaging of solid polymers: Strategies for chemical-shift selectivity

One reason that the solid-state imaging field is exciting is because of its potential to provide spatially resolved chemical information from deep inside a solid sample. To achieve this goal, methods are needed which exploit the many NMR parameters to generate image contrast. We describe two strategies for employing the chemical-shift anisotropy as a contrast mechanism in images of abundant nuclei in solid polymers. We illustrate these approaches with images which map spatial variations of molecular orientation in samples of polytetrafluoroethylene.     

Relaxation time measurements and molecular dynamics of dipalmitoyl-α-lecithin in the solid phase

The relaxation times T₁ and T_D were measured for dipalmitoyl-L-α-lecithin and dipalmitoyl-DL-α-lecithin at various temperatures. It was possible to separate the contribution of the slow motions to the relaxation rates from the fast ones. For some types of motions the activation energies could be estimated.     

Modern solid state NMR techniques and concepts in structural studies of synthetic polymers

In this mini‐review, we present the solid state nuclear magnetic resonance (NMR) methods which trace the new tendency in structural studies of synthetic polymers. The review is organized into three sections. In the first part, short theoretical background and introduction to very fast magic angle spinning NMR technique with sample rotation 60 kHz are shown. The second part presents method for enhancing the sensitivity of NMR experiments by application of dynamic nuclear polarization magic angle spinning technique. In the third section, the power of the NMR crystallography approach which can be used for fine refinement of polymers structure on the atomic level is discussed. Copyright © 2016 John Wiley & Sons, Ltd.     

Deuteron NMR investigations of structure and dynamics in solid polymers,liquid crystalline polymers and polymer model membranes

Pulsed deuteron NMR offers unique possibilities for studying molecular order and dynamics in polymers. The different techniques employed in this area and the results obtained so far are briefly reviewed. Current developments involving two-dimensional NMR are also outlined.     

Relaxation times,diffusion coefficients and high frequency NMR measurements of methane dissolved in MBBA

The measurements of proton T₁, T₂ and diffusion coefficients of CH₄ and deuterium T₁ in CD₄, dissolved in MBBA are reported as a function of the temperature in the nematic and solid phases. Also reported are cw measurements of CH₄ in MBBA at 250 MHz which definitely prove that it is oriented. The results are discussed in terms of their relation to previously published cw NMR measurements on these systems.     

Open-path laser-induced plasma spectrometry for remote analytical measurements on solid surfaces

Open-path laser-induced plasma spectrometry has been studied for elemental analysis at a distance of 45 m from the target. The 230-mJ pulsed radiation of a Q-switched Nd:YAG laser at 1064 nm has been used to produce a plasma on the sample and light emission has been collected under an off-axis open-path scheme. Under such conditions, the main variables influencing the signal response such as beam focal conditions, laser incidence angle and laser penetration depth have been identified and diagnosed on the basis of spectral signal-to-noise ratio considerations. The incidence angle is critical beyond 60°. Crater morphology and ablation rates have been studied also. A semi-quantitative analysis of several stainless steel grades has been implemented using a pattern recognition algorithm, which allowed to discriminate successfully the samples on the basis of their variable content in alloying elements.     

Relaxation spectroscopy on amorphous high polymers

Summary Measurements of the dynamic Youngs modulus and shear modulus of Polymethylmethacrylate (PMMA) in the frequency range from 10^–3 cps to 10^–1 cps and in the temperature range from –20°C to 100°C up to stress amplitudes of 4 kN/cm² are presented. The measured nonlinear viscoelastic behavior is discussed with regard to nonlinear elasticity and nonlinear effects caused by the shift of relaxation times due to the reaction rate theory.

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Zusammenfassung Es wird über Messungen des dynamischen Elastizitätsmoduls und Schubmodells von Polymethacrylsäuremethylester (PMMA) im Frequenzbereich von 10^–3 Hz bis 10^–1 Hz und im Temperaturbereich von –20°C bis +100°C bei Spannungsamplituden bis zu 4 kN/cm² berichtet. Das gemessene nichtlineare viskoelastische Verhalten wird hinsichtlich einer nichtlinearen Elastizität und hinsichtlich nichtlinearer Effekte, die durch eine Verschiebung der Relaxationszeiten nach der Platzwechseltheorie verursacht sind, diskutiert.

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With 21 figures     

Relaxation of entangled polymers in melts

The stress relaxation function of a monodisperse or polydisperse melt, and the corresponding viscosity, have been calculated in the entanglement domain by applying the Doi-Edwards theory which relies on the reptation concept introduced by P. G. de Gennes. Though the theory has been considered as successful, the agreement with precise experiments is only qualitative, and strong anomalies remained to be explained. A new theory is presented here: it is obtained by introducing simple approximations derived from elementary but novel considerations. This theory is shown to be in good agreement with experiments on monodisperse and polydisperse melts. In particular, it explains the well-known fact that the viscosity of a monodisperse polymer melt of molecular mass M seems to increase proportionally to M^3,4 when M is large.     

Isotopic abundance measurements on solid nuclear-type samples by glow discharge mass spectrometry

A double-focusing Glow Discharge Mass Spectrometer (GDMS) installed in a glovebox for nuclear sample screening has been employed for isotopic measurements. Isotopic compositions of zirconium, silicon, lithium, boron, uranium and plutonium which are elements of nuclear concern have been determined. Interferences arising from the matrix sample and the discharge gas (Ar) for each of these elements are discussed. The GDMS results are compared with those from Thermal Ionization Mass Spectrometry (TIMS). For boron and lithium at microg/g-ng/g levels, the two methods gave results in good agreement. In samples containing uranium the isotopic composition obtained by GDMS was in agreement with those from TIMS independently of the enrichment. Attempts for the determination of plutonium isotopic composition were also made. In this case, due to the interferences of uranium at mass 238 and americium at mass 241, the GDMS raw data are complementary with those values obtained from physical non-destructive techniques.     

EPR measurements in irradiated polyacetylene

Dependence of the EPR spectra on the radiation dose in samples of irradiated polyacetylene was studied.