Fluorine magnetic resonance spectra and tacticities of poly(vinyl trifluoroacetate)

Fluorine magnetic resonance spectra of poly(vinyl trifluoroacetate) samples of different tacticities were investigated, along with those of model compounds. These polymers represent the first case where the line order in respect to stereostructure does not agree with that expected from the spectra of model compounds. More than four peaks were observed for signals from the trifluoromethyl groups of the polymers and the relative peak positions differed from those previously reported. The intensity ratios of the peaks were affected by the solvent employed. Therefore the spectra cannot be interpreted in terms of triad stereosequences. Seemingly longer stereosequences or other influences have to be considered to interpret the spectra.     

Nuclear magnetic relaxation and molecular motion in poly(vinylidine fluoride)

Pulsed NMR T₁, T₂, and T_1ρ measurements are reported for poly(vinylidine fluoride) (PVF₂). The results demonstrate clearly the presence of four relaxation processes, three amorphous and one crystalline. The α relaxation is undoubtedly a crystalline one, while β and γ are both amorphous, in agreement with earlier conclusions from dielectric and dynamic mechanical measurements. The fourth relaxation (β′) observed initially in the mechanical measurements of Kakutani, but undetected in dielectric experiments, has been confirmed in our results and the process is described by an activation energy of 15.1 kcl/mole. Motion of folds on the surface of crystal lamellae is deemed to be the responsible mechanism for the β′ relaxation. Two models have been considered in the interpretation of the α process; rotation of crystalline chains in the vicinity of defects and rotational oscillation of restricted amplitude of all crystalline chains about the main chain axes. Rotation of amorphous chains is a possible mechanism for the γ process while motions of a general nature are responsible for the β relaxation. Our experimental results again indicate that spin diffusion plays an important role in the overall NMR response of the polymer.     

Fluorine NMR spectra of poly(vinyl trifluoroacetate)

Samples of poly(vinyl trifluoroacetate) with differing stereosequences have been prepared. The NMR signals at 56.4 Mc./sec. corresponding to the three types of triadstereosequences of the trifluoroacetate groups (isotactic, heterotactic, and syndiotactic) can be resolved. The results for specific polymers are discussed.     

Nuclear magnetic relaxation in solutions of poly(fluoromethylene) and poly(vinyl fluoride); segmental mobility of fluorinated ethylene polymers

¹H and ¹⁹F spin-lattice relaxation times and nuclear Overhauser enhancement factors are reported for solutions of poly(fluoromethylene) in DMSO, and for poly(vinyl fluoride) in DMF. These results, together with literature values for other fluorinated ethylene polymers, show that substitutions of F for H on the chain backbone produce only mild changes in segmental mobility.     

Structural relaxation in poly(vinyl acetate)

Structural relaxation in poly(vinyl acetate) (PVAc) in and slightly above the glass-transition region has been studied by monitoring the time dependence of enthalpy using differential scanning calorimetry and the frequency dependence of electric polarization by dielectric loss measurements. The results have been analyzed to yield the kinetic parameters characterizing the structural relaxation and are compared with similar analyses of previously published shear compliance and volume relaxation experiments. Relaxation of enthalpy, electric polarization, volume, and shear stress in PVAc all appear to be characterized by somewhat different relaxation times. The difference between the volume and enthalpy relaxation times, coupled with the fact that PVAc exhibits a Prigogine–Defay ratio greater than unity, is evidence for a previously proposed connection between the thermodynamics and kinetics of structural relaxation in terms of an order parameter model.     

Nuclear spin relaxation in poly(diethylsiloxane)

Earlier work showed that heating causes poly(diethylsiloxane) to undergo a first-order transition from a semicrystalline solid to a more mobile viscous—crystalline material. The latter is composed of two phases and analogies between polymer and liquid crystal morphology and behavior have been made. The viscous—crystalline phase in PDES appears to be unique since the literature is devoid of other documented examples. In this study, spin—lattice and spin—spin relaxation times were measured over a wide temperature range. They show a glass transition at 138°K, a crystal—crystal transition at 206°K, and a transition around 250°K which results from translational motion of the polymer chains with respect to each other. This motion is observed in the amorphous phase at a lower temperature than in the crystalline phase. Translational motion in the crystalline phase is observed on melting of the crystallites. The spin—spin data permitted monitoring of the molecular motions in each phase and the data suggest that these phases exert some influence on the molecular motions of each other. The viscous—crystalline phase in PDES may represent a unique model for studying and understanding “precrystalline” behavior and structure in amorphous solids.     

Fluorine substitutent effects on poly(2,6-diphenylphenylene ether)

A series of fluoro-substituted poly(2,6-diphenylphenylene ether)s (P₃O) with systematically varied structures were prepared. The properties of the polymers change significantly with the variations in the polymer structures. By increasing the number of fluorine substituents on P₃O polymer, the melting points and the tendency to crystallize for these polymers decrease significantly. Random copolymers from fluoro-substituted 2,6-diphenylphenols and 2,6-diphenylphenol were also prepared. The influence of copolymer structure on the transition temperatures and crystallinity of the resulting copolymers is described. © 1993 John Wiley & Sons, Inc.     

Viscoelastic relaxation phenomena in poly(mono-n-alkyl itaconates)

Dynamic mechanical relaxation measurements were carried out on two poly(monoalkylitaconates). Three relaxation peaks between −120 and 140°C were present. A prominent subglass relaxation (β) near −60°C was studied by means of the Fuoss-Kirkwood formalism. A slight viscoelastic activity (β′) was observed at room temperature, and then a high temperature peak (α), probably related to the glass transition, was found. A biparabolic model, double Cole constant phase element was used to model the experimental data in this zone. © 1996 John Wiley & Sons, Inc.     

Side-chain relaxation in stereoregular poly(isobutyl methacrylate)

The effects of stereoregularity on the low-temperature relaxation processes were studied by dynamic mechanical measurements on isotactic and syndiotactic polyisobutyl methacrylates (iso-PiBMA and syn-PiBMA). The α, β, and γ relaxation processes were observed in both stereoregular forms. Both the α, and β loss peaks were at lower temperatures for iso-PiBMA than for syn-PiBMA. The γ loss peak was observed at about ?155°C at 30 Hz for both forms, and the apparent activation energy of this process was same for both samples within experimental error (6.7 ± 0.5 kcal/mole). It was reduced from these results that the α and β processes are both considerably influenced by the isotactic configuration but the γ process is not.     

CPS K 221 Model of proton longitudinal magnetic relaxation in hydrated crosslinked poly(methacrylic acid)

Proton longitudinal magnetic relaxation time (T₁) measurements have been made at 30 MHz over a wide range of temperature for crosslinked poly(methacrylic acid), PMA, hydrated with H₂O as well as with D₂O. From the point of view of nuclear magnetic relaxation, PMA hydrogel is a multiregion system in which three proton regions (a, b, c) can be distinguished. Region a is regarded as to be formed by the nonexchangeable polymer protons, region b by the protons of -COOH · H₂O combinations, and region c by the protons of remaining water molecules. Cross relaxation between polymer and water protons and a log normal distribution of correlation times have been assumed to take place. Temperature dependences of the T₁ time for the particular regions have been determined, from which the distribution width parameter, the second moment and the intramolecular proton-proton distance for sorbed water have been calculated.     

Dielectric relaxation and molecular motion in poly(vinylidene fluoride)

The dielectric properties of poly(vinylidene fluoride) have been studied in the frequency range 10 Hz to 100 kHz at temperatures between ?196 and 150°C. Three dielectric relaxations were observed: the α relaxation occurred near 130°C, the β near 0°C, and the γ near ?30°C at 100 kHz. In the α relaxation the magnitude of loss peak and the relaxation times increased not only with increasing lamellar thickness, but also with decrease of crystal defects in the crystalline regions. In the light of the above results, the α relaxation was attributed to the molecular motion in the crystalline regions which was related to the lamellar thickness and crystal defects in the crystalline phase. In the β relaxation, the magnitude of the loss peak increased with the amount of amorphous material. The relaxation times were independent of the crystal structure and the degree of crystallinity, but increased slightly with orientation of the molecular chains by drawing. The β relaxation was ascribed to the micro-Brownian motions of main chains in the amorphous regions. The Arrhenius plots were of the so-called WLF type, and the “freezing point” of the molecular motion was about ?80°C. The Cole-Cole distribution parameter of the relaxation time α increased almost linearly with decreasing temperature in the temperature range of the experiment. The γ relaxation was attributed to local molecular motions in the amorphous regions.     

Dehydration-induced structural relaxation effects in poly(methyl methacrylate)

The Thermally Stimulated Depolarization (TSD) dielectric technique and Dielectric Relaxation Spectroscopy (DRS) have been used in order to investigate aging phenomena in poly(methyl methacrylate) (PMMA). Earlier TSD studies on amorphous PMMA report peculiar dielectric relaxation signals within the range of the glass transition (at ∼378 K) and the secondary relaxation (∼230 K). In the present study, an intense TSD current relaxation band maximizing around 310 K is tentatively attributed to the molecular mobility due to a residual free volume below the glass transition temperature, T_g, that allows structural recovery at the free volume released from the desorption of H₂O molecules during evacuation. Limited motions in the main backbone provoke dipole (re)orientation of the ester carbonyl pendant groups with an activation energy E=0.85±0.05 eV, being responsible for the latter dielectric relaxation effect. Alternative attributions based on the short-range jump relaxation of electric charges and boundary effects are also discussed.     

Thermal,dielectric, and mechanical relaxation in poly(benzimidazole)/poly(etherimide) blends

Thermal, dielectric, and mechanical relaxation measurements on miscible blends of polybenzimidazole (PBI) and a polyetherimide (PEI, Ultem 1000) prepared by solution casting from dimethylacetamide (DMAc) reveal a number of structurally related features. Annealing below the glass transition temperature induces an enthalpy relaxation process typical of single-phase glasses of nonequilibrium structure. Dielectric relaxation experiments on samples annealed at ambient conditions reveal two relaxation processes below 400°C. At lower temperatures (50–200°C), the desorption of water is observed. Above 200°C in the first run, a composition-dependent relaxation is seen at the highest frequencies (100 kHz) while a relaxation approximately independent of composition appears in the second run. The latter corresponds to the glass transition of the PEI phase. The glass transition of PBI at this frequency is above the degradation temperature. At lower frequencies a strongly dispersive electrode polarization process masks the high-temperature relaxations. Dynamic mechanical results show similar features with respect to phase separation; the main difference is that the tan δ curves in the first run are complicated by the desorption of residual solvent.     

Dielectric relaxation of poly(ester-ether-carbonate) multiblock terpolymers

The dielectric properties of a series of poly(ester-ether-carbonate) multiblock terpolymers have been investigated as a function of ether and carbonate composition in the frequency range of 10³–10⁶ Hz. The degree of polymerization of the samples was determined by viscosimetry measurements. The weight fraction, degree of crystallinity, and melting temperatures were characterized by means of x-ray diffraction and DSC methods. Dielectric behavior has been discussed in terms of Havriliak-Negami formulation. The variation of the dielectric properties with temperature has been associated with two relaxation processes: a) the -relaxation process observed at low temperature, which is associated to local motion of polar groups attached to both the soft and the hard segments, and b) the process assigned to long-range molecular motions above the glass transition temperature.     

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.     

Enthalpy relaxation of rigid poly(vinyl chloride)

The changes which take place on annealing rigid PVC in the vicinity of the glass transition have been followed by differential scanning calorimetry. The changes appear as an increase in the glass-transition temperature and a decrease in the enthalpy with time of annealing. For annealing at 75°C, the enthalpy after 50–100 hr approaches the value characteristic of the equilibrium liquid state. The results obtained for annealing at 65°C and 75°C are in accord with those expected for the relaxation of an amorphous material, and are at variance with those expected on the basis of crystallization taking place on annealing. The enthalpy relaxation process is characterized by a distribution of activation energies centered about 18.8 kcal mole^?1, and seems to reflect a multiplicity of molecular processes.     

Cross-linked magnetic nanoparticles from poly(ethylene glycol) and dodecyl grafted poly(succinimide) as magnetic resonance probes

Cross-linked magnetic nanoparticles were developed to improve the structural stability of amphiphilic polymer coated magnetic nanoparticles. These nanoparticles show strong potential for biomedical applications such as magnetic resonance imaging (MRI).