Conformation of syndiotactic poly(styrenesulfonic acid)

A conformational energy map for syndiotactic PSSA indicates that a planar trans sequence of chemical repeat units with a side-chain orientation of χ ? 10° is favored over helix or glide-plane sequences. As the electrostatic potential of this planar sequence is greater than for the (0°,?120°) or (0°, ?80°) sequences it is especially favored under conditions of high ionic strength. Only a very small energy is needed to cause rotational transitions from a planar trans sequence, so that many different sequences exist at any one time in solution. Sequences such as (0°,?20°) are calculated to be hypochromic with respect to the other probable sequences, and this seems consistent with some experimental evidence on the apparent hypochromicity of the band near 261.5 nm under conditions of high ionic strength.     

Multiple melting transitions in fractions of partially isotactic poly(propylene oxide)

Three transitions are detected dilatometrically when partially isotactic poly(propylene oxide) melts. One transition, the temperature of which is independent of the crystallization temperature over a wide range below 60°C, is ascribed to the melting of lamellar crystallites which are limited in thickness by the average isotactic sequence length alone. The other two transitions, the temperatures of which vary with the crystallization temperature, are ascribed to the melting of lamellar crystallites with thickness determined predominantly by three- and two-dimensional primary nucleation acts. The theory of Flory is adapted and applied quantitatively to the melting points of three crystalline fractions of poly(propylene oxide), obtained from a polymer produced via the zinc diethyl and water catalyst system. This method leads to a thermodynamic melting point of isotactic poly(propylene oxide) near 82°C.     

Characterization of annealed isotactic polypropylene in the solid state by 2D time‐domain 1H NMR

Two‐dimensional time‐domain ¹H NMR was used to investigate annealed isotactic polypropylene in the solid phase. The spin–lattice relaxation in the laboratory frame and in the rotating frame were correlated with the shape of the free induction decay to identify and characterize relaxation components over the temperature range −120 to 120 °C. Several phase transitions were observed, and three distinct solid phases, with different chain mobilities, were detected. Two of these phases were identified as regions with different mobilities within the crystalline phase. The third phase was characterized by a high degree of isotropy in molecular motion. This phase, identified as the amorphous phase, appeared as the polymer was heated above a low‐temperature (−45 °C) phase transition. All transitions observed at higher temperatures occurred exclusively in this phase. About one‐third of the polymer chains reside between crystalline lamellae, whereas the majority form amorphous regions outside fibrils of multilamellar structure. Furthermore, the glass‐to‐rubber transition, occurring above −15 °C, consists of three stages. During the first stage, between −15 °C and 15 °C, regions with an increased segment mobility (labeled intermediate phase) appear gradually within the amorphous phase. At 15 °C, the intermediate phase consists of ∼10% of the polymer units, or one‐third of the polymer units constituting the amorphous phase. Between 15 °C and 25 °C, the intermediate phase increases rapidly to 18%. This is associated with the appearance of semiliquid and liquid regions, likely within the intermediate phase. Polymer chain segments (and possibly entire chains) involved in the liquidlike phases exhibit heterogeneous molecular motion with a correlation frequency higher than 10⁶ Hz. These two stages of glass‐to‐rubber transition occur within amorphous regions outside multilamellar structures. The third stage of the glass transition, appearing above 70 °C, is associated with the upper glass transition and occurs within the interlamellar amorphous phase. Finally, on a timescale of 100 ms or less, spin diffusion does not couple the amorphous regions outside fibrils with crystalline and amorphous regions within multilamellar fibrils. However, on a timescale of hundreds of milliseconds to seconds, all different regions within isotactic polypropylene are partially coupled. It is proposed that the relative magnitude of the crystalline magnetization, as observed in the T_1ρ experiment, is a good measure of polymer crystallinity. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2487–2506, 2000     

Characterization and evaluation of commercial poly (vinylidene fluoride)‐g‐sulfonatedPolystyrene as proton exchange membrane

The possibility of developing low‐cost commercial grafted and sulfonated Poly(vinylidene fluoride) (PVDF‐g‐PSSA) membranes as proton exchange membranes for fuel cell applications have been investigated. PVDF‐g‐PSSA membranes were systematically prepared and examined with the focus of understanding how the polymer microstructure (degree of grafting and sulfonation, ion‐exchange capacity, etc) affects their methanol permeability, water uptake, and proton conductivity. Fourier transform infrared spectroscopy was used to characterize the changes of the membrane's microstructure after grafting and sulfonation. The results showed that the PVDF‐g‐PSSA membranes exhibited good thermal stability and lower methanol permeability. The proton conductivity of PVDF‐g‐PSSA membranes was also measured by the electrochemical impedance spectroscopy method. It was found that the proton conductivity of PVDF‐g‐PSSA membranes depends on the degree of sulfonation. All the sulfonated membranes show high proton conductivity at 92°C, in the range of 27 to 235 mScm⁻¹, which is much higher than that of Nafion212 (102 mScm⁻¹ at 80°C). The results indicated that the PVDF‐g‐PSSA membranes are particularly promising membranes to be used as polymer electrolyte membranes due to their excellent stability, low methanol permeability, and high proton conductivity.     

Molecular modeling of polymers. 10. Characterization of conformational transitions of poly(acrylic acid) and poly(methacrylic acid) using dyad structures

The conformational profiles of nearest side-chain neighbors, methylene-dyad structures, of poly(acrylic acid), PAA, and poly(methacrylic acid), PMA, were determined as a function of tacticity, extent of ionization, and presence of counterion. The dominant backbone conformer states are quite similar for both isotactic and syndiotactic diads in a common charge state. Thus, the overall dimensional properties of isotactic syndiotactic and atactic chains of PAA or PMA, based upon dyad interactions, are predicted to be alike for a given charge state. Significant deviations from precise t, g⁺, and g^? states are found for the dyad minimum energy conformations. The rod-to-coil and coil-to-rod transitions observed in PAA and PMA, respectively, as a function of increasing counterion concentration can be explained, to a large extent, by the conformational profiles of the corresponding dyad model structures. © 1994 John Wiley & Sons, Inc.     

New understanding on the memory effect of crystallized iPP

In this study, recovery processes of isotactic polypropylene(iPP) melted spherulites at 135 °C after melting at higher temperatures(170 °C–176 °C) were investigated with polarized optical microscopy and Fourier transform infrared spectroscopy. The recovery temperature was fixed to exclude the interference from heterogeneous nuclei. After melting at temperatures between 170 °C and 174 °C, the melted spherulite could recover back to the origin spherulite at low temperatures. Interestingly, a distinct infrared spectrum from iPP melt and crystal was observed in the early stage of recovery process after melting at low temperatures, where only IR bands resulting from short helices with 12 monomers or less can be seen, which indicates that the presence of crystal residues is not the necessary condition for the polymer memory effect. Avrami analysis further indicated that crystallization mainly took place in melted lamellae. After melting at higher temperatures, melted spherulite cannot recover. Based on above findings, it is proposed that the memory effect can be mainly ascribed to melted lamellae, during which crystalline order is lost but conformational order still exists. These conformational ordered segments formed aggregates, which can play as nucleation precursors at low temperatures.     

Geometrical and energetical feasibility of a highly extended chain conformation for isotactic polystyrene

Highly extended, four-atom helices with $\text{h = 5.1}\text{A}\text{?}$ and t = 60° are geometrically feasible for a vinyl polymer. The energetical feasibility for the case of isotactic polystyrene (in crystalline gels) is discussed on the basis of appropriate conformational maps.     

Stereoregularity of poly(methyl methacrylate) prepared by aqueous polymerization and catalyzed by corundum

The aqueous polymerization of methyl methacrylate was carried out in the absence and in presence of corundum or carborundum at 25 and 80°C. In the absence of corundum and carborundum, it has been found that rising the polymerization temperature from 25 to 80°C resulted in changing the tacticity of the obtained polymers. At 25°C the isotactic triad was 26% while the heterotactic triad was 33.5% and the syndiotactic one was 40.5%. Increasing the polymerization temperature to 80°C resulted in a decrease of the isotactic structure to 0% and increased the heterotactic structure and syndiotactic structure to 48 and 52% respectively. Polymerizing at 25°C in presence of corundum (0.5 g) an increase in the syndiotactic triad took place from 40.5 to 50.7% while the isotactic triad decreased from 26 to 22.2% and the heterotactic structure decreased from 33.5 to 27%. Raising the polymerization temperature to 80°C in the presence of the same amount of corundum resulted in an increase in both the isotactic and heterotactic triads to 35 and 32.7%, respectively. Polymerizing at 80°C in presence of corundum (0.5 g) resulted in nearly an equal percentage of each triad 33%.     

Radiation chemistry of poly(propylene oxide)

Atactic, isotactic, and optically active poly(propylene oxides), PPOx, were irradiated with both γ-rays and electron beams. Up to a dose of 37 Mrad no change could be detected in the optical activity. G values for hydrogen evolution decreased as compared to polypropylene in about the same ratio as G(H₂) of polyoxymethylene decreased as compared to polyethylene. G values for crosslinking and scission, estimated by means of gelation theories of Saito and Inokuti, were found to be greater for isotactic than for atactic PPOx. The behavior of transient infrared and ultraviolet absorption bands is discussed. Intrinsic viscosity data indicate a rapid initial chain degradation whereas CO gas and OH group production is linear with dose. Evidence for the conversion of one type of free radical to another on heating an irradiated sample from 77°K to room temperature is based on the behavior of transient infrared and ultraviolet absorption bands.     

Luminescence studies in polymers—IV. Effect of orientation,tacticity and crystallinity on polystyrene and polyvinylcarbazole fluorescence

The fluorescence spectra of amorphous atactic, amorphous isotactic and crystallized isotactic polystyrene films have been compared. The effect of chain orientation has also been analysed on amorphous atactic samples. The results show that the fluorescence yield increases with crystallinity at room temperature and 77°K. The contribution of excimer fluorescence at 77°K increases according to the sequence: atactic < atactic oriented < isotactic amorphous < isotactic crystallized. An increase of the fluorescence yield with crystallinity was also observed for polyvinylcarbazole samples although the contribution of excimer fluorescence at 77°K is independent of crystallinity for this polymer. The results are interpreted in terms of energy migration.     

Chemical modification of poly(vinyl chloride): A conclusive study of the influence of tacticity

Three samples, A, B and C, of poly(vinyl chloride) (PVC) were prepared at 90, 60, and 0°C, respectively, to give them different isotactic content measured by ¹³C-NMR spectroscopy. A kinetic study of the nucleophilic substitution with sodium thiophenate, carried out for the three samples, showed that even at temperatures as high as 60°C a fraction of the units remain unreactive and that the extent of this fraction depends on the syndiotactic content of the polymer. This was also supported by a comparison of the behavior of samples B and C in substitution experiments at 5 and 60°C. In contrast the substitution experiments at ?30°C demonstrated that, as suggested, a small fraction of extremely reactive units exists in PVC, the content of which is higher as the isotactic content of the polymer increases. In this connection, and even though a slight contribution of some defect structures cannot be ruled out, a ¹³C-NMR analysis of sample B after modification at 40°C to various degrees demonstrates that the reactivity of the isotactic triads is high in relation to the heterotactic. On the basis of the results obtained and the possible conformations in PVC the substitution mechanism is related to the occurrence of isotactic TT conformations. The results, as discussed in terms of the various ways in which isotactic TT conformations appear open new prospects in the field of PVC chemical modification and stabilization mechanisms.     

Infrared spectra of polymer solutions. I. Conformational stability of isotactic polymer chains in solution

The infrared spectra of isotactic polystyrene, polypropylene, and poly-p-chlorostyrene were measured in dilute solutions or gels at various temperatures ranging from room temperature to ?100°C. For isotactic polystyrene and polypropylene, all the absorption bands characteristic of the helical conformation of the molecules increase in intensity with decreasing temperature, and show intensities at low temperature as strong as in the highly crystallized samples. This suggests that the molecules can assume very regular conformations even in solution. Similar temperature dependence of the spectra was observed for a homogeneous gel of isotactic poly-p-chlorostyrene, which is believed to be noncrystalline. These experimental facts lead to the conclusion that the stability of the TG type helical conformation of these isotactic polymers may be ascribed to the intramolecular forces within a molecule. The fact that these spectral changes are reversible has permitted a thermodynamic treatment of the conformational regularity in solution on the basis of the infrared data. The temperature dependence of the absorption intensities has been interpreted quantitatively by a simple statistical mechanical model, and the enthaply and the entropy differences between the helical and the random states of the monomeric residue in the molecular chain have been estimated.     

Poly(alkyl α-chloroacrylates). V. Preparation and properties of methyl,ethyl, and isopropyl polymers of varied tacticity

Polymers of different tacticities, from highly isotactic to highly syndiotactic, were prepared from methyl, ethyl, and isopropyl α-chloroacrylates. These polymers were characterized for tacticity by infrared spectroscopy and 100 and 300 MHz nuclear magnetic resonance (NMR) and for thermal properties by differential scanning calorimetry (DSC). After corrections were made for molecular weight effects, the observed glass temperature-tacticity results were analyzed, and it was determined that the maximum differences in glass temperatures of the purely isotactic compared to the purely syndiotactic polymers should be 92°C for the methyl ester, 86°C for the ethyl ester, and 68°C for the isopropyl ester polymers. The highly isotactic polymers of all three esters were crystalline. Possible polymerization reaction mechanisms are discussed on the basis of the triad and tetrad tacticity values observed and the calculated propagation statistics.     

Isotactic sequence lengths by ozonation of poly(propylene oxide)

Ozonation followed by lithium aluminum hydride reduction cleaved high molecular weight isotactic poly(propylene oxide) to crystalline polyglycols. From the melting point and molecular weight of the latter, the molar freezing point depression produced by end groups is found to be ca. 18°C./mole, as compared to that estimated for poly(ethylene glycols), K_f = 12°C./mole, from earlier data. By assuming syndiotactic placements (or other irregularities) would produce the same molar depression, the melting point of isotactic poly(propylene oxides) produced by various catalysts has been used to estimate the isotactic sequence lengths.     

γ-Ray-induced polymerization of α-haloacrylic acids

The γ-ray induced polymerizations of α-chloroacrylic acid, mp 66°C, and α-bromo-acrylic acid, mp 72°C, were investigated in the temperature range from 35°C to 85°C. An analysis of polymerization kinetics was made, and results were similar to those reported in the literature for other vinyl monomers. On heating of the polymer obtained, elimination of hydrogen halide takes place, and intramolecular lactone formation is observed. The rate of lactone formation of poly(α-chloroacrylic acid) obtained in the solid-state polymerization was found to be higher than that in the liquid state, because a highly isotactic configuration of polymers, tends to be formed in the solid-state polymerization, and elimination of hydrogen chloride is facilitated with an isotactic 5₂ helix structure.     

Proton‐conducting polymer membrane based on sulfonated polystyrene microspheres and an amphiphilic polymer blend

A new type of amphiphic polymer blend comprising polystyrene (PS), polyethylene oxide (PEO) and microspheres of crosslinked polystyrene sulfonic acid (PSSA) was prepared by solution blending and followed by casting. Besides providing protons, PSSA plays a role in enhancing the miscibility of polystyrene (PS) and polyethylene oxide (PEO) according to the IR and the DSC studies. The resulting polymer blend is a proton electrolyte. The influence of the mixing extent between PS and PEO on the proton conductivity has been studied. It is also found that for those samples in which PEO and PS mix well, the hydrophobic PS component can effectively prevent water evaporation from the hydrophilic components at elevated temperatures, and therefore preserve the proton conductivity (10⁻⁴ S/cm) at the temperature as high as 80 °C. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1530–1538, 2000     

Studies of the Polydiacetylene Bis-p-chlorocinnamate of 10,12-Docosadiyn-1,22-diol

The bis-p-chlorocinnamate of 10,12-docosadiyn-1,22-diol polymerized with ⁶⁰Co gamma radiation to give a soluble polydiacetylene (PDA), PDA-ClCIN-22. We have studied PDA-ClCIN-22 in both crystallographic powder and solution coated films. Thin films of solution coated PDA-ClCIN-22 exhibit an absorption maximum of 544 nm, similar to the concentrated solution in chlorobenzene. Irradiation of these films with 254 nm UV light in air leads to loss of intensity in the visible spectrum. The thin films of PDA-ClCIN-22 are amorphous by X-ray powder diffraction. Irradiation of both PDA-ClCIN-22 powder and film with 254 nm light did not result in photochemical reactivity of the chlorocinnamate group as recorded by FTIR spectra. Differential scanning calorimetry (DSC) studies of PDA-ClCIN-22 reveal that presence of monomer and endothermic transitions near 116°C and 134°C on a first heating. On a second heating of a sample of PDA-ClCIN-22 taken to 150°C on first heating, the 116°C and 134°C endotherms are absent. These endotherms are also lost by monomer extraction with boiling cyclohexane. One possible source of the endotherms would be conformational transitions in the side chain.     

Isotactic poly(acrylonitrile) via ultraviolet irradiation of acrylonitrile-urea canal complex

Isotactic poly(acrylonitrile) (PAN) has been prepared by means of a conventional ultraviolet (UV) irradiation apparatus without γ-ray sources; an acrylonitrile-urea canal complex was directly formed at the surface of the UV (Hg) emission tube at low temperatures (～ ?78°C). When the complex was UV-irradiated at this temperature, a stereoregular polymer was formed in the canal. The ¹³C-NMR analyses indicate that (1) these PAN are rich in isotactic configuration, (2) the extent of the isotactic triad is in the range of 56?71%, and (3) the penultimate unit effect, 4 (mm) (rr)/(mr)², is linearly correlated with the ultimate unit effect, (mm)/(rr). From the plots of log{4(mm)(rr)/(mr)²} vs log{(mm)/(rr)}, the anomaly in the polymerization of AN is discussed. The molecular characteristics of the UV canal PAN such as molecular weight, etc., were briefly noted. © 1995 John Wiley & Sons, Inc.     

Dynamic mechanical and dielectric relaxations of polystyrene below the glass temperature

Dynamic mechanical and dielectric properties of various kinds of polystyrene, including bulk-polymerized, monodisperse, isotactic, and thermally degraded samples, have been measured below the glass temperature to 4°K. Five relaxation processes are found, designated β, γ, γ′, δ, and ε in order of descending temperature. The β peak (350°K at 10 kHz) is attributed to the local oscillation mode of backbone chains and the γ′ peak (180°K at 10 kHz) to rotation of phenyl groups. The δ peak (100°K at 10 kHz) is observed only in dielectric properties of the bulk-polymerized sample and is assigned to weak polar bonds, such as oxygen bonds in the chain. The δ peak (55°K at 10 kHz) which is prominent in dynamic mechanical properties is interpreted in terms of lattice defects due to a syndiotactic diad inserted between isotactic sequences in a chain or vice versa. The ε peak (ca. 25°K at 10 kHz) is first reported in the present work, but the mechanism involved is not yet clear.     

Polymerization of coordinated monomers. V. Polymerization of methyl methacrylate–Lewis acid complexes

The polymerization of the complex of methyl methacrylate with stannic chloride, aluminum trichloride, or boron trifluoride was carried out in toluene solution at several temperatures in the range of 60° to ?78°C by initiation of α,α′-azobisisobutyronicrile or by irradiation with ultraviolet rays. The tacticities of the resulting polymers were determined by NMR spectroscopy. Both the 1:1 and the 2:1 methyl methacrylate–SnCl₄ complexes gave polymers with similar tacticities at the polymerization temperatures above ?60°C. With decreasing temperature below ?60°C, the isotacticity was more favored for the 2:1 complex, whereas the tacticities did not change for the 1:1 complex. On the ESR spectroscopy of the polymerization solution under the irradiation of ultraviolet rays at ?120°C, the 1:1 SnCl₄ complex gave a quintet, while the 2:1 SnCl₄ complex gave both a quintet and a sextet. The sextet became weaker with increasing temperature and disappeared at ?60°C. This behavior of the sextet corresponds to the change of the tacticities of polymer for the 2:1 SnCl₄ complex. An intra–intercomplex addition was suggested for the polymerization of the 2:1 complex, which took a cis-configuration on the basis of its infrared spectra. The sextet can be ascribed to the radical formed by the intracomplex addition reaction, while the quintet can correspond to that formed by the intercomplex addition reaction. The proportion of the intracomplex reaction was estimated to be about 0.25 at ?75°C, and the calculated value of the probability of isotactic diad addition of the intracomplex reaction was found to be almost unity.