Thermally stimulated depolarization currents of films obtained from supernatant layer of mixed solutions of stereoregular polymethyl methacrylates

Summary Thermally stimulated depolarization currents were investigated for films obtained from supernatant layer of mixed solutions ofi- ands-PMMA ini/s weight ratio of 1/1 and 1/2. The results showed that the degree of stereocomplex formation is not uniform: the mixed solutions contain the insoluble and the soluble stereocomplex aggregates and in some case the residual (free)i-PMMA. The stereocomplex is formed ini/s weight ratio of 1/2.     

Synthesis and characterization of stereoregular poly(methyl methacrylate)–silica hybrid utilizing stereocomplex formation

Stereoregular poly(methyl methacrylate) (PMMA)–silica nanocomposites were prepared using stereocomplex formation between i-PMMA and s-PMMA by an in situ method. The methodology adopted here is the simultaneous formation of organic gel and inorganic gel, the so-called interpenetrating polymer network (IPN) formation. The gelation of i- and s-PMMA were performed by stereocomplex formation with the associated segments forming the crosslinking points in the presence of tetramethoxysilane (TMOS). The effects of the i/s-ratio, PMMA concentration, molecular weight, and solvent nature on the hybrid materials formation were addressed. The presence of the stereocomplex in the silica matrix was confirmed by DSC and solvent extraction methods. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 785–794, 2004     

Investigation of thermally stimulated charge relaxation mechanism in SiO<Subscript>2</Subscript> filled polycarbonate nanocomposites

The thermally stimulated charge relaxation properties of polycarbonate (PC) filled with SiO₂ nanofiller were studied by means of thermally stimulated discharge current (TSDC). The nanocomposite samples were further characterized by UV–vis spectroscopy, scanning electron microscopy, energy dispersive X-ray spectra, and differential scanning calorimetry (DSC) techniques to investigate the dispersion of nanofillers in polymer matrix and glass transition temperature. All pristine and nanocomposites samples of thickness about 25 μm were prepared using solution mixing method. The suitable weight percentage of SiO₂ nanofillers has been chosen to prevent the nonuniform dispersion. TSDC measurement of PC (Pristine) and PC+ (7% SiO₂) shows the single peak, while TSDC characteristic of other nanocomposites are showing two peaks. The higher temperature TSDC peak of pristine and nanocomposites samples is originated due to the charge relaxation from shallower and deeper trapping sites, however, low temperature peak is caused by dipolar relaxation of charge carriers. Since the position of higher temperature TSDC peak is generally an analysis of glass transition temperature of polymer/polymer nanocomposites. The authors have observed that the temperature of this peak is almost same as the T _g measured by DSC with 0 to ±5% variation. This article presents the deeper understanding of charge relaxation mechanism caused by SiO₂ nanofillers in polycarbonate.     

Solid matrix polymerization and stereoisomeric complexes of poly(methyl methacrylate)

The polymerization of methyl methacrylate within solid matrices of stereoregular poly(methyl methacrylate) has been studied by proton NMR and wide angle X-ray diffraction. The semi-crystalline isotactic (i-) PMMA matrix was synthesized in the laboratory by anionic polymerization initiated by phenylmagnesium bromide, and the syndiotactic (s-) PMMA matrix was synthesized through a Ziegler–Natta reaction. Matrix polymerization of the monomer was initiated through the redox activation of benzoyl peroxide with N,N-dimethyl-p-toluidine. NMR measurements of triad distributions in matrix-polymerized chains suggest that the well-known stereospecific replica polymerization in PMMA (syndiotactic sequences promote isotactic sequences and vice versa) plays only a limited role in the systems studied. Experimental results indicate that chains grown within the i-PMMA or s-PMMA solid matrices have greater degrees of configurational disorder. The greater concentration of atactic triads in these chains could be the result of limited free volume or steric effects during polymerization in a highly condensed environment. X-ray diffraction studies of solution cast blends of isotactic PMMA and PMMA with conventional tacticity reveal some crystallinity with a structure characteristic of the stereocomplex formed by isotactic and syndiotactic PMMA from suitable solvents. Evidence was obtained for the presence of this complex in solidified mixtures of the i-PMMA solid matrix and liquid monomer. This observation is an example of special intermolecular structures that can form under conditions of in situ growth of chains within a pre-polymerized matrix.     

Nanospace preparation by crosslinking helical syndiotactic‐poly(methacrylic acid) in acetonitrile/water after stereocomplexation

Various molecular weights of isotactic (it‐)‐poly(methyl methacrylate) (PMMA) and syndiotactic (st‐)‐poly(methacrylic acid) (PMAA) were used to form an it‐PMMA/st‐PMAA (1/2) stereocomplex in acetonitrile/water as a suspension. The stereocomplex was tentatively crosslinked with 1,11‐diamino‐3‐6‐9‐trioxaundecane and water soluble carbodiimide at 10, 20, and 40 mol % concentrations versus MAA unit. The it‐PMMA was extracted from the crosslinked stereocomplex under alkaline conditions, and the successive re‐incorporation of the it‐PMMA was carried out. During the extraction and re‐incorporation processes, the FTIR/ATR spectra showed the absence and generation of a peak at 860 cm^?1, respectively, which was characteristic of this stereocomplex. The result of the XRD analysis also corresponded with the extraction and re‐incorporation behavior of it‐PMMA; peaks were observed at 2θ = 12 and 15°, d = 0.74 and 0.59 nm, respectively. This study showed that the nanospaces of helical st‐PMAA were available in acetonitrile/water in a suspended state using a crosslinking approach. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Macromorphology of polypropylene homopolymer tacticity mixtures

The macromorphology of isotactic/atactic (iPP/aPP) and isotactic/syndiotactic (iPP/sPP) polypropylene mixtures is examined by optical microscopy. The spherulitic macrostructure of equimolecular weight [weight‐average molecular weight (M_w) = 200k] iPP/aPP blends is volume‐filling to very high aPP concentrations when the crystallization temperature is 130 °C. Similar spherulitic macrostructures (spherulite size and volume‐filling nature) are observed for iPP homopolymer and a 50/50 iPP/aPP blend at low crystallization temperatures (115–135 °C). At higher crystallization temperatures (140–145 °C), a equimolecular weight (M_w = 200k) 50/50 iPP/aPP blend exhibits nodular texture that blurs the spherulitic boundaries. Double temperature jump experiments show that the nodular texture is due to melt phase separation that develops prior to crystallization. The upper critical solution temperature (UCST) of a 50/50 iPP/aPP blend (M_w = 200k) lies below 155 °C, and the blend is miscible at conventional melt processing temperatures. The UCST behavior is controlled by the blend molecular weight and aPP microstructure. aPP microstructures containing increased isospecific sequencing (although still noncrystalline) exhibit a reduced tendency for phase separation in 50/50 mixtures (M_w = 200k) and the absence of nodular texture at low undercoolings (140–145 °C). Equimolecular weight (M_w = 200k) 50/50 iPP/sPP mixtures exhibit phase‐separated texture at all crystallization temperatures. The size scale of the phase‐separated texture decreases with decreasing crystallization temperature because of a competition between crystallization and phase separation from a melt initially well mixed from the initial solution blending process. Extended melt annealing experiments show that the 50/50 iPP/sPP mixture (M_w = 200k) is immiscible in the melt at conventional melt processing temperatures. The iPP/sPP pair shows a much stronger tendency for phase separation than the iPP/aPP polymer pair. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1947–1964, 2000     

Miscibility and crystallization behavior in blends of poly(methyl methacrylate) and poly(vinylidene fluoride): Effect of star‐like topology of poly(methyl methacrylate) chain

A tetraarmed star‐shaped poly(methyl methacrylate) (s‐PMMA) was synthesized via atom transfer radical polymerization with 2‐bromoisobutyryl pentaerythritol as the initiator. For comparison, a linear PMMA with the identical molecular weight (l‐PMMA) was also prepared. The blends of the two PMMA samples with poly (vinylidene fluoride) (PVDF) were prepared to investigate the effect of macromolecular topological structure on miscibility and crystallization behavior of the binary blends. The behavior of single and composition‐dependent glass transition temperatures was found for the blends of s‐PMMA with PVDF, indicating that the s‐PMMA is miscible with PVDF in the amorphous state just like l‐PMMA. The miscibility was further evidenced by the depression of equilibrium melting points. It is found that the blends of s‐PMMA and PVDF displayed the larger k value of Gordon–Taylor equation than the blends of l‐PMMA and PVDF blends. According to the depression of equilibrium melting points, the intermolecular parameters for the two blends were estimated. It is noted that the s‐PMMA/PVDF blends displayed the lower interaction parameter than l‐PMMA/PVDF blends. The isothermal crystallization kinetics shows that the crystallization of PVDF in the blends containing s‐PMMA is faster than that in the blends containing the linear PMMA. The surface‐folding free energy of PVDF chains in the blends containing s‐PMMA is significantly lower than those in the blends containing l‐PMMA. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2580–2593, 2007     

Specific recognition of syndiotactic poly(methacrylic acid) in porous isotactic poly(methyl methacrylate) thin films based on the effects of stereoregularity,temperature, and solvent

The effects of stereoregularity, temperature, and solvent on the specific recognition of syndiotactic (st)‐poly(methacrylic acid) (PMAA) in macromolecularly porous isotactic (it)‐poly(methyl methacrylate) films were investigated to give important insights into the regularity and stability of nanospaces in the it‐PMMA films as well as template polymerization. The porous it‐PMMA films were fabricated on quartz crystal microbalance (QCM) substrates via the layer‐by‐layer (LbL) assembly of it‐PMMA/st‐PMAA, plus the st‐PMAA extraction from the assembly. QCM analysis and infrared spectroscopy revealed the first case of stereocomplex formation using st‐PMAA with lower stereoregularity (rr = 73%) in the LbL films, while st‐PMAA obtained with conventional free radical polymerization (rr = 62%) was barely incorporated into the porous it‐PMMA films. The maximum st‐PMAA incorporation increased from 25 to 40 °C, but there were almost no difference between 40 and 55 °C, indicating that the it‐PMMA crystallization would also be accelerated with increasing temperature. The studies on st‐PMAA incorporation with various complexing solvents revealed that the host it‐PMMA in the porous films could only form the original stereocomplex with 2/1 unit‐molar stoichiometry (st‐PMAA/it‐PMMA) in acetonitrile/water or ethanol/water. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3651–3657, 2010     

Study of ordered structures of syndiotactic poly(methyl methacrylate) in solution and in the solid state

From analysis of infrared spectra it was found that in syndiotactic (s) poly(methyl methacrylate) (PMMA) in solution, long s sequences contain an increased population of diads with a skeletal conformation tt (in the staggered approximation). Self-aggregation of s-PMMA in solution leads to a further increase of the fraction of long s sequences in the extended chain conformation, and to an ordering of easter groups. When solid s-PMMA is isolated from a solution in which it exists in the aggregated state, these characteristics are preserved in the solid. The polymer appears partially crystalline by x-ray scattering, and it exhibits fibrillar morphology under the electron microscope. Ordered structures of s-PMMA melt at temperatures about 150°C, while the presence of residual solvent decreases the temperature of melting. Solid s-PMMA obtained from solutions in which aggregation of the polymer does not take place, like s-PMMA which did not come into contact with solvent, contains a higher proportion of syndiotactic diads with a skeletal conformation tg; these samples are amorphous and morphologically structureless. Analogies between the structure of ordered s-PMMA and the structure of the PMMA stereocomplex are also discussed.     

Stereoregular uniform polymers

Progress in stereospecific living polymerizations of methacrylate monomers and the concept and realization of “uniform polymers” and “uniform polymer architectures” are described, with particular emphasis on the fusion of polymer synthesis and characterization and their interactive stimulation, which are inevitable not only for the formation of polymers with highly controlled structures but also for the development of polymer characterization, thereby bringing about the spiral progress of both fields. First, three types of stereospecific living polymerizations are described, including formation of 100% isotactic poly(methyl methacrylate) (PMMA), 98% syndiotactic PMMA, and 96% heterotactic PMMA. Supercritical fluid chromatography (SFC) has proven useful for isolating uniform polymers from these stereoregular PMMAs with narrow molecular weight distributions. Living nature of these stereospecific polymerizations is further utilized for the synthesis of end‐functionalized stereoregular polymers, which are separated into uniform end‐functionalized polymers and used to construct more elaborated uniform polymer architectures such as stereoblock, star, and comb polymers. The uniform polymers have proven quite useful for the studies on the relationship between structures and properties such as glass transition temperature, melting temperature, and solution viscosity. In addition to this, stereoregular uniform polymers are particularly important to understand stereocomplex formation between isotactic and syndiotactic uniform PMMAs. On‐line GPC/NMR measurement at 750 MHz and −15°C in acetone/acetone‐d₆ allowed definitive determination of the compositions of the complexed species and noncomplexed species separately, but not in average. Also interesting is the stereocomplex formation of uniform stereoblock PMMA, where intramolecular complexation in addition to an intermolecular complexation was distinctively observed by GPC analysis in acetone. Uniform star and comb PMMAs were also prepared and found useful to discuss the effect of branching on the solution viscosity. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 245–260, 1999     

The glass transition in linear low density polyethylene determined by thermally stimulated depolarization currents

New thermally stimulated depolarization currents (TSDC) results on LLD polyethylene functionalized with diethylmaleate polar groups are precisely computer fitted with the direct signal analysis technique. It is shown that the TSDC spectrum consists, with increasing temperatures, of a sub-γ peak, a sharp γ peak, and a β and an α relaxation. The first peak is analyzed in terms of Arrhenius relaxation times, whereas the γ and β transitions could only be fitted by using Vogel-Fulcher temperature dependence for the relaxation times. The best value for T_o obtained from both fittings is 69.7 K. This is a quantitative proof for the identification of the γ transition as one of the dielectric manifestations of the glass-rubber transition for polyethylenes, T_g = 136.5 K, which has been discussed extensively in the literature. The β relaxation, T_gβ = 237 K, has also the expected characteristic of a glass transition; the existence of two T_gs in polyethylene could explain our results. © 1996 John Wiley & Sons, Inc.     

Dielectric relaxation behavior of C5+ (70 MeV) swift heavy ion irradiated polyetheretherketone (PEEK) using TSDC technique

The dielectric relaxation characteristics of polyetheretherketone (PEEK) irradiated with C⁵⁺ (70 MeV) ion, have been investigated in the temperature range 60–230° as a function of poling temperature T _P (50, 100, 150, and 200°C), poling field E _P (200, 300, 400, and 500 kV/cm), and storage time t _s (2, 24, 48, and 120 h), using thermally stimulated discharge current (TSDC) technique. The TSDC spectra show a prominent maximum around glass transition temperature (T _g ～ 143°C) named as α-peak. This peak is attributed to the movement of ketone dipoles linked with the main chain. It is observed that the magnitude of α-peak increases with the increase in poling temperature and poling field. The peak current and area under the α-peak are found to be diminished with the increase of storage time t _s for electrets. The β-peak (space charge peak) is absent in irradiated PEEK samples as compared to pristine PEEK samples. The results obtained, in the present studies, are compared with the results on pristine samples. The activation energies and pre-exponential factor for PEEK samples determined using Bucci plot method.     

Stereospecific Cyclic Poly(methyl methacrylate) and Its Topology‐Guided Hierarchically Controlled Supramolecular Assemblies

In this study, the stereocomplexation between a novel stereospecific cyclic vinyl polymer, that is, cyclic syndiotactic poly(methyl methacrylate) (st‐PMMA), with the complementary linear isotactic (it‐) PMMA was investigated. Surprising new insight into the effects of the topology (i.e., end groups), size, and tacticity of the assembling components on stereocomplex formation was obtained. Characterization of the stereocomplexes revealed that the self‐assembly of cyclic st‐PMMAs and linear it‐PMMAs resulted in the formation of an unprecedented “polypseudorotaxane‐type” supramolecular assembly. This stereocomplex exhibited remarkably different physical properties as compared to the conventional PMMA triple‐helix stereocomplex as a result of the restricted topology imposed by the cyclic st‐PMMA assembling component.     

Investigation of liquid–liquid transition process in atactic polystyrene by means of thermally stimulated depolarization current

Thermally stimulated depolarization current (TSDC) measurement was used to investigate the liquid–liquid transition process of atactic polystyrene (aPS). There are four distinct peaks (α, ρ₁, LL and ρ₂) showed in TSDC spectrum in the range of 300–480 K. Compared with the result of differential scanning calorimetry for aPS, the TSDC spectrum showed that the peak LL located at 422 K could be attributed to the movement of entire chain related to liquid–liquid transition. The isolated peak LL was separated from TSDC spectrum of aPS by curve fitting procedure of the kinetics equation. The distribution of relaxation time of liquid–liquid transition was obtained in terms of the principle of Debye relaxation process. Based on analysis of calculated results, it was confirmed that the relaxation time during the liquid–liquid transition of aPS changed gradually from following Vogel–Fulcher equation to Arrhenius rule with increasing temperature.     

Chromatographic investigation of the effect of dissolved carbon dioxide on the glass transition temperature of a polymer and the solubility of a third component (additive)

The effect of dissolved carbon dioxide on the glass transition temperature of a polymer, PMMA, has been investigated using molecular probe chromatography. The probe solute was iso-octane, and the specific retention volumes of this solute in pure PMMA and mixtures of PMMA with CO₂ were measured over a temperature range of 0 to 180°C and CO₂ pressures from 1 to 75 atm. The amount of CO₂ dissolved in the polymer was calculated from a model fit to previously published solubility data determined chromatographically. Classical van't Hoff-type plots were used to determine the glass transition temperature of CO₂-impregnated PMMA from low pressure up to 46 atm of CO₂. Solvent-induced plasticization was observed with the glass transition temperature decreasing by about 40°C. At some pressures, glass transitions at low temperatures could not be determined from the van't Hoff plots because of the proximity of the polymer glass transition temperature to the gas–liquid transition temperature for CO₂. For these pressures, a new method was developed to determine the glass transition composition. The glass transition pressure was then calculated from the measured composition and temperature using an isotherm model. In every case, the glass transition temperature decreased linearly with increasing concentration of CO₂ in the polymer. However, at higher compositions, the glass transition pressure decreased with increasing composition and decreasing temperature. The observed retention volume of iso-octane with PMMA in a glassy state was correlated with an adsorption model developed from a theory for liquid–solid chromatography derived by Martire. This model accurately described the observed decrease in retention of iso-octane by adsorption on the surface of glassy PMMA with increasing concentration of CO₂ dissolved in the polymer. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. B Polym. Phys. 36: 2537–2549, 1998     

NMR study of structure and dynamics of stereocomplexes of poly(methyl methacrylate) in C6D6 and CD3CN

Summary Stereocomplexes of isotactic (i) and syndiotactic (s) poly(methyl methacrylate)(PMMA), with different stereoregularity of thes-PMMA component, in C₆D₆ and CD₃CN solution were investigated by means of¹H NMR spectra measured with magic angle spinning (MAR-NMR), broadline (BL) NMR spectra and¹³C NMR spectra measured with strong proton decoupling. This study has shown that in associated segments of the stereocomplexes withp= 30–40% (p is the fraction of associated monomer units) the-CH₃ and CH₂ groups exhibit equal mobility as in solutions of onlyi ors PMMA, but in consequence of interaction of the ester groups these motions are restricted in space. In complexes with high density of interactions (p 75%) the motion of-CH₃ and CH₂ groups in associated segments is retarded. The network model of the stereocomplex, and the possibility of the formation of a double helix are discussed.

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Zusammenfassung Stereokomplexe von isotaktischem (i) und syndiotaktischem (s) Polymethylmethacrylat (PMMA), mit verschiedener Stereoregularität ders-PMMA Komponente, in C₆D₆ und CD₃CN Lösung, wurden durch¹H NMR Spektra mit Magisch-Winkel-Rotation (MAR-NMR), durch Breitband (BL) NMR Spektra und durch¹³C NMR Spektra mit starker Protonenentkopplung untersucht. Es wurde herausgefunden, daß in assoziierten Segmenten des Stereokomplexes mitp = 30–40% (p ist der Bruchteil der assoziierten Monomereinheiten) die-CH³ und CH²-Gruppen die gleiche Beweglichkeit wie in Lösungen von nur s oder nuri-PMMA ausweisen, aber infolge der Wechselwirkung der Estergruppen sind diese Bewegungen raumbehindert. In Komplexen mit einem hohen Grad von Wechselwirkungen (p 75%) ist die Beweglichkeit der-CH³ und CH²-Gruppen in assoziierten Segmenten verzögert. Das Netzwerkmodell des Stereokomplexes, sowohl als auch die Möglichkeit einer Doppelhelixform, werden diskutiert.

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Dedicated to Prof. Dr. G. Rehage on the occasion of his 60th birthday.

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With 2 figures and 1 table     

Association of stereoregular poly(methyl methacrylates) in dilute solution. Proton NMR study

By measurement of integrated intensities of high-resolution proton nuclear magnetic resonance (NMR) bands, associated structures of isotactic (i) and syndiotactic (s) poly(methyl methacrylate) (PMMA) in dilute toluene-d₈ and dimethylformamide-d₇ solutions were detected and characterized. In 1% (w/v) solutions of highly stereoregular s-PMMA in toluene-d₈ at 27°C, 76% of the monomer units are present in the form of compact aggregates. Consequences of this finding for the polymerization of methyl methacrylate in toluene in the presence of s-PMMA are discussed.     

The relation between the microphase separation transition and the glass transition in diblock copolymers

The microphase separation transition (MST) has been studied for short chain diblock copolymers poly(styrene-b-isoprene) and poly(styrene-b-mma). A detailed analysis of small-angle x-ray scattering (SAXS) profiles in the homogeneous phase allows determination of the interaction parameter and the spinodal temperature T_s of the MST. T_s for the PS/PI diblocks is found to be lower than the glass transition temperature of their hard blocks. This results in a coupling of the MST and the glass transition. Using both structural (SAXS) and thermal differential scanning calorimetry (DSC) methods it is shown that an endothermal peak found in the DSC diagrams is related to the combined effect of the MST and the glass transition. © 1992 John Wiley & Sons, Inc.     

Correlation between dipolar TSDC and AC dielectric spectroscopy at the PVDF glass transition

The α_a-mode (associated to the dynamic glass transition) in PVDF-α has been studied by Thermally Stimulated Depolarization Currents (TSDC) and Dielectric Spectroscopy (DS) techniques. The distribution of relaxation parameters, reorientation energies, characteristic temperature, and preexponential factors of the Vogel–Tammann–Fulcher relaxation times have been precisely determined by using the Simulated Annealing Direct Signal Analysis applied to a partially discharged TSDC α_a peak. This distribution has been used to predict the variation of the dielectric loss, ε″(ω, T), in the temperature and frequency range where the DS measurements were made on the same material. The simulated ε′(T, ω) for various ω, are compared to the experimental values. The width of the peak is always too low, due to the restricted distribution used for the generation of the curves. A relaxation map including the TSDC results is used to determine the relaxation time variation. In the limited frequency range where the AC DS experiments are performed (10² ≤ f ≤ 10⁵ Hz) a master curve is drawn and the exponents of the frequency dependence are found at low and high frequency; also, a fitting to the Havriliak–Negami distribution is successfully performed. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 2483–2493, 1997