Circular dichroism of poly{γ-[2-(9-carbazolyl)ethyl] L-glutamate} in dilute solution and in solid state

Circular dichroism (CD) and ultraviolet absorption (UV) spectra were measured for poly{γ-[2-(9-carbazolyl)ethyl] L -glutamate} (PCLG), whose side chain chromophore has three π – π^* transitions in a wavelength region not overlapping with the n – π^* transition of the main chain peptide group. For the dilute solution in any given helicogenic solvent, the observed CD sign had a good correlation to the polarization direction of the π – π^* transitions. On the other hand, a concentrated solution with a concentration of ca. 15 wt.-% was found to form a liquid crystalline phase, which was probably nematic at room temperature. The CD spectral profiles of the cast film of the polypeptide were quite different from that of the dilute solution. Origins of the optical activity are discussed.     

An investigation into the temperature dependence of the rheological synergy between xanthan gum and locust bean gum mixtures

The interaction between xanthan gum (XG) and locust bean gum (LBG) in water has been investigated using texture analysis, thermorheological analysis and high sensitivity differential scanning calorimetry. For the batches of XG and LBG used in the present study and at a total polymer concentration of 1% w/v, texture analysis indicated that the greatest synergy occurred at approximately 10% w/w XG, while the technique also allowed measurement of the increase in strength resulting from heating the mixes to 70 and 80°C and cooling to room temperature. Thermorheological studies showed a maximum in viscosity for the xanthan gum gels at approximately 45°C; this maximum became less pronounced as the concentration of XG decreased. High sensitivity differential scanning calorimetry indicated that XG undergoes two transitions at approximately 30 and 80°C on heating, but on cycling a single transition is seen at approximately 74°C. It was also noted that the transitions were diminished or were absent in the presence of LBG.     

Conformational transition of block copolymers in dilute solution and their morphology in the solid state

Block copolymers poly(methyl methacrylate)-block-polystyrene-block-poly(methyl methacrylate) and polystyrene-block-poly(tert-butyl methacrylate) were prepared by anionic polymerization and were studied in dilute solution as well as in the bulk state. The viscometric study revealed that the block copolymers exhibit a conformational “transition” occurring in a narrow temperature range. Films were obtained with the same copolymers by casting at different temperatures, chosen below and above the “transition”. From the investigation of these films by differential scanning calorimetry, using the entropy relaxation method, and by dynamic mechanical measurements, it was shown that the block copolymers exhibit a rather segregated structure if they are obtained below the “transition” temperature, and that they adopt a quasi-Gaussian conformation (implying a better miscibility of the two kinds of blocks) if the solvent is evaporated above the “transition” temperature.     

Study of conformational transitions of graft copolymers by UV spectrophotometry, 2. The behaviour of poly(vinyl acetate) grafted hydroxyethyl-cellulose in selective solvents

The behaviour of poly(vinyl acetate) grafted hydroxyethylcellulose (PVAc/HEC) in the binary solvent mixture ethanol-water (90:10 by vol.) was studied by absorption spectrophotometry. The macromolecules of PVAc/HEC graft copolymer exhibit a conformational transition at ca. 55°C where they pass from a monomicelle conformation to a segregated conformation. This intramolecular transition occurs both below and above the critical micellar concentration (c.m.c). In dilute solutions a critical concentration was evidenced, which is one order of magnitude higher than the c.m.c.     

Solution properties of poly(methyl methacrylate) in methyl methacrylate, 1. Viscosities from the dilute to the concentrated solution regime

The viscosities of samples of poly(methyl methacrylate), PMMA, with narrow molecular weight distributions and with molecular weights in the range 1,75·10⁴ < M?_w < 1,60.10⁶ were measured in methyl methacrylate, MMA, as solvent at 20, 40, and 60°C in the dilute and in the semi-dilute concentration regime. By extrapolation to zero polymer concentration the limiting viscosity numbers and therefrom the Mark-Houwink-Kuhn-Sakurada parameters for PMMA in MMA were obtained. In addition, unperturbed dimensions were determined from dilute solution viscosity data. Together with some published results for more concentrated solutions of PMMA in MMA the viscosities of the semi-dilute systems were analysed in terms of scaling theories. It was found that irrespective of temperature, polymer concentration, and molecular weight all data could be quite well fitted by a single master curve when the relative viscosity η/η₀ was plotted as a function of the product of the intrinsic viscosity and mass concentration [η]·c. No sharp transitions between different concentration regimes were observed, however, and the scaling law exponents for the entangled solution are significantly higher than predicted by theory.     

Synthesis and characterization of poly(DL-lactide)-grafted gelatins as bioabsorbable amphiphilic polymers

A series of poly(DL-lactide) grafted gelatins, as new bioabsorbable amphiphilic polymers useful in parenteral drug delivery systems and in tissue engineering, were synthesized by the ring opening polymerization of DL-lactide onto a fractionated gelatin with the molecular weight of 1.02 × 105. Using tin(II) bis(2-ethylhexanoate) as catalyst, the bulk copolymerization at 140°C and solution copolymerization in dimethylsulfoxide (DMSO) at 80°C were firstly performed in the presence of gelatin. The results showed that the solution copolymerization in DMSO could afford the expected copolymers but the bulk copolymerization would result in an insoluble crosslinked product. The number of grafting sites on gelatin chain could be adjusted by the partial trimethylsilylation of side hydroxy, amino and carboxylic groups in gelatin. The solution copolymerization of DLlactide on the partially protected gelatin in DMSO was also successful in providing copolymers with different molecular weights. The synthesized copolymers were characterized on the basis of elemental analysis, IR, 1H-NMR and thermal analysis. The IR and 1H-NMR data of these produced copolymers suggested that polylactide branches could be grafted onto gelatin via the side groups such as hydroxyl and amino groups in the solution copolymerization as well as carboxylic groups in bulk copolymerization. The molecular weights of the copolymers could be calculated from the difference of nitrogen contents between a copolymer and free gelatin. The results indicated that molecular weight of the copolymers and those of polylactide branches were increased with the feeding ratio of DL-lactide to gelatin in the copolymerization. However, because of the steric hindrance of some grafting sites on gelatin and the transesterifications of the propagating polylactide branches on gelatin with possibly formed homo-polymeric polylactide chains, the finally formed polylactide branches on gelatin were not very large and the highest average molecular weight of a polylactide branch was not over 4500 in any solution copolymerizations. The results from the thermal analysis of some copolymers, including thermogravimetry and differential scanning calorimetry, showed that the absorbed water in the samples could be lost at a temperature range below 150°C and melting point decreased with increase of polylactide branches in the poly(DL-lactide)-grafted gelatins.     

Thermal properties of ethylene-propylene block copolymers

A precision, adiabatic, vacuum calorimeter and a differential scanning calorimeter have been used to measure the heat capacities of two ethylene-propylene block copolymers containing 12 and 17 mole per cent ethylene, respectively, over the temperature range 80 to 500°K. An estimate has been made of heat capacity values below 80°K. Values of the heat capacity, entropy, enthalpy, and free energy are listed at lO°C intervals. Two first order transitions at 399 ± 3°K and 435 ± l°K were observed in both copolymers. These are attributed to the melting of ethylene and isotactic propylene blocks. Values for the heats of fusion of the copolymers are listed.     

Melting behavior of syndiotacticity-rich poly(vinyl alcohol)-water gel

The melting temperatures of a syndiotacticity-rich poly(vinyl alcohol)-water gel were measured for concentrations below 70g.dm^-3. In the gel chilled at a temperature below about 15°C the melting point first decreased monotonously then sharply in a jump and again monotonously with decreasing concentration, whereas in the gel chilled above 20°C the melting point decreased monotonously through-out. The change of the shear modulus of a gel chilled at 0°C with the rise of temperature from 0°C to the melting point was also measured. Initially the shear modulus decreased, then increased to a maximum value, and at last decreased towards the melting temperature, whereas a gel chilled at 40°C kept an almost constant value during heating from 40°C up to a high temperature and then decreased with further rise of temperature. The turbidity of dilute solutions chilled at 0°C passed a minimum and maximum with the rise of temperature in accord with the minimum and maximum of the shear modulus. It is thus concluded that in the gel chilled at temperatures below 15°C the junctions grow to some extent with the rise of temperature.     

Synthesis and characterization of thermosensitive chitosan copolymer as a novel biomaterial

Novel water-soluble thermosensitive chitosan copolymers were prepared by graft polymerization of N-isopropylacrylamide (NIPAAm) onto chitosan using cerium ammonium nitrate (CAN) as an initiator. The physicochemical properties of the resulting chitosan-g-NIPAAm copolymers were characterized by Fourier transform infrared (FT-IR) spectroscopy, ¹H-nuclear magnetic resonance, X-ray diffraction measurement, thermogravimetric analysis (TGA) and solubility test. Sol–gel transition behavior was investigated by the cloud point measurement of the chitosan-g-NIPAAm aqueous solution. The gelling temperature was examined using the vial inversion method. The percentage of grafting (%) and efficiency of grafting (%) were investigated according to concentrations of monomer and initiator. The maximum grafted chitosan copolymer was obtained with 0.4 M NIPAAm and 6×10^-3 M CAN. Water-soluble chitosan-g-NIPAAm copolymers were prepared successfully and they formed thermally reversible hydrogel, which exhibits a lower critical solution temperature (LCST) around 32°C in aqueous solutions. A preliminary in vitro cell study showed nontoxic and bio- compatible properties. These results suggest that chitosan-g-NIPAAm copolymer could be very useful in biomedical and pharmaceutical applications as an injectable material for cell and drug delivery.     

Preparation and Properties of Polysaccharide-Based Epoxy Vitrimers

Traditional chemically cross-linked polymer materials may cause severe environmental pollution and economic losses due to the difficulty of recycling. Vitrimers containing reversible dynamic covalent bonds are processable, recyclable, and self-healing. In this article, a bio-based epoxy resin curing agent is synthesized by hydrolysis using cellulose as raw material. The results show that the epoxy vitrimers with the best ratio have a lower topological transition temperature and a fast relaxation time at 140 °C. After soaking in solvent for 90 days, the gel content of the material is greater than 81%. The glass transition temperature of the sample measured by differential scanning calorimetry （DSC) is 83 °C. It also has a high initial decomposition temperature (350 °C) with good flexibility and solvent resistance, showing its good performance as a coating paint.     

Collagen-Poly(methyl methacrylate) graft copolymers: Effect of aqueous organic solvent system and salts on copolymer composition

Collagen-poly(methyl methacrylate) (PMMA) graft copolymers were prepared in a number of aqueous/organic solvent systems and in the presence of neutral salts using ceric ammonium nitrate as initiator. The composition of the collagen-methyl methacrylate graft copolymers was studied by hydrolyzing the collagen backbone and measuring the molecular weights of the grafted PMMA branches. In water/methanol grafting media the molecular weights of the PMMA branches showed a maximum at a methanol concentration of 25% and then decreased with further increase in methanol concentration. Even though the percent grafting registered a sharp fall with increasing concentration of methanol, the number of grafting sites remained more or less constant and was almost the same as that obtained in aqueous medium. Other aqueous/organic solvent systems gave more grafted chains than those prepared in water alone, and these were of lower molecular weight. Anions such as sulphate and chloride were found to have more influence than nitrate in decreasing the number of grafting sites.     

Compatibility of poly(ethylene oxide) and poly(methyl methacrylate) chains in solutions of their blends,diblock and triblock copolymers

Solvent activity data were obtained from vapour pressure measurements at 50°C, 70°C, 100°C and reduced to χ-functions in systems consisting of benzene or toluene on the one hand and blends or block copolymers of poly(ethylene oxide) and poly(methyl methacrylate) on the other. The χ_BC-values are negative for all polymer subsystems, but miscibility on a segmental scale is somewhat different for block copolymers and blends. χ_BC depends on the concentration of solvent and is mainly influenced by the Δχ-effect in solution and correlated with the syndiotacticity of the PMMA chains. There is only a small influence of temperature and blend composition or block ratio.     

Radical products of poly(triphenylcarbinol) thermolysis

At least three kinds of stable paramagnetic species are formed in films of poly(triphenylcarbinol) 1 upon thermolysis in vacuo up to 500°C. “Low temperature” radicals of triarylmethyl type (RTAMT1) are created in the 120 ÷ 220°C temperature region. These radicals give rise to a symmetric ESR signal (g = 2.0026 ± 0.0003; ΔH ≈ 1.0 mT) with poorly resolved hyperfine structure and to an absorption band (AB) of allowed transition with λ_max = 407 nm in UV spectra. The spectral region of forbidden transitions, assumed for the RTAMT1, is masked by a nonradical AB, in particular by that of quinoid structure of Chichibabin hydrocarbon type with λ_max = 595 nm. From 220°C to 280°C the “high temperature” radicals RTAMT2 appear, with a strong bathochromic shifted AB of forbidden transition (λ_max = 710 nm). Maximum concentration of RTAMT2 (≈?10²⁰ spin/g) is achieved at ≈?280°C. Possible structures of both kinds of RTAMT are discussed. At T ≥ 300°C paramagnetic species that differ from RTAMT are formed in 1 .     

Effects of ethanol on the temperature profile of Saccharomyces cerevisiae

Ethanol at concentrations above 3% (w/v) decreased the maximum temperature for growth of Saccharomyces cerevisiae in batch culture. At 9% (w/v), the highest concentration tested, the maximum temperature suffered a decrease of about 10 degrees centigrade. At effective concentrations ethanol shifted the ARRHENIUS plots of growth and death in the superoptimal temperature range to lower temperatures while an associative temperature profile was maintained. Thus at a concentration of 6% (w/v), ethanol depressed the optimum temperature for growth from 37 °C to 25 °C, the final maximum temperature for growth from 40 °C to 33 °C and the initial maximum temperature for growth from 44 °C to 36 °C. The results indicate that during alcoholic batch fermentation these three cardinal temperatures are variables, the values of which decrease with increasing ethanol concentration. When the ethanol concentration becomes high enough to depress them successively below the process temperature, the yeast population becomes increasingly subject to ethanol-enhanced thermal death. Implications of the findings for the production of fermentation ethanol in batch and continuous processes are discussed.     

Reactive blending of poly(ethylene terephthalate) and polycarbonate, 2. A novel route to copoly(ester-ether)s

Copoly(ester-ether)s have been prepared by reacting poly(ethylene terephtalate) (PET) and bisphenol A polycarbonate (PC) at high temperature, under vacuum, in the presence cf titanium tetrabutoxide as catalyst. The polymeric materials obtained in this way, which contain terephthalate, bisphenol A and ethylene glycol moieties, are the result of exchange reactions followed by the elimination of cyclic ethylene carbonate and by a decarboxylation reaction. These copolymers have a higher glass transition temperature and thermal resistance than PET, and a better solvent resistance than PC; their dynamic mechanical spectra are similar to that of PC in the ?150 to +50°C temperature range.     

Phase diagram and structural study of polystyrene — poly(ethylene oxide) block copolymers, 1. Systems polystyrene/poly(ethylene oxide)/diethyl phthalate

By means of low angle X-ray scattering and differential scanning calorimetry, we have drawn the phase diagrams of poly(ethylene oxide)-polystyrene block copolymers in the presence of diethyl phthalate as a preferential solvent of polystyrene. Such systems exhibit two liquid-cristalline structures in terms of temperature and solvent concentration. At temperatures below about 50°C, a lamellar structure (LC) with crystallized poly(ethylene oxide) chains exists. Between 50 and about 170°C, we find a structure with melted poly(ethylene oxide) chains. Like for amorphous block copolymers, the type of the latter structure is governed by the copolymer composition. We have shown that in the LC structure, the poly(ethylene oxide) chains crystallize folding in two superposed layers, and we have studied the number of folds and the crystallinity of the poly(ethylene oxide) blocks as a function of solvent concentration, composition and rel. mol. mass of the copolymer, and of the crystallization temperature.     

Preferential adsorption of swollen polydimethylsiloxane (PDMS) networks and of linear PDMS in 2-butanone/cyclohexane

Measurements of preferential adsorption, λ(ν), by differential refractometry, and of swelling equilibrium, ν₃^?, of polydimethylsiloxane (PDMS) networks were carried out in the solvent mixture ethyl methyl ketone (MEK) + cyclohexane (Ch), at 25°C. The preferential adsorption and the total sorption were also determined, by light scattering, for linear PDMS in the same mixed solvent. The behaviour of the system crosslinked PDMS/MEK + Ch is compared with that of the system linear PDMS (dilute solution)/MEK + Ch. The experimental values of ν₃ and λ are compared with those predicted by thermodynamic theory. For this comparison, new thermodynamic equations, to calculate preferential and total sorption at finite polymer concentration, are derived using the extended Flory-Huggins model. The derived equations take into account the solvent composition dependence of interaction parameters, neglected (as an approximation) in previous works.     

The effect of polymer composition on the gelation behavior of PLGA-g-PEG biodegradable thermoreversible gels

Graft copolymers consisting of a poly(D,L-lactic acid-co-glycolic acid) backbone grafted with polyethylene glycol side chains were synthesized and formed thermoreversible gels in aqueous solutions that exhibited solution behavior at low temperature and sol-to-gel transitions at higher temperature. The composition of the polymer and relative amounts of polylactic acid, glycolic acid, and ethylene glycol were varied by controlling the precursor concentrations and reaction temperature. The gelation temperature could be systematically tailored from 15 to 34 degrees C by increasing the concentration of polyethylene glycol in the graft copolymer. The gelation temperature also depended on the polymer molecular weight and concentration. This work has importance for the development of water soluble gels with tailored compositions and gelation temperatures for use in tissue engineering and as injectable depots for drug delivery.     

Thermal properties and crystallization behavior of poly(ether ether ketone ketone)/poly(ether biphenyl ether ketone ketone) copolymers

Thermal properties of poly(ether ether ketone ketone) (PEEKK)/poly(ether biphenyl ether ketone ketone) (PEDEKK) copolymers were investigated by means of differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The glass transition temperature (T_g) increases from 154°C to 183°C as the content of PEDEKK units increases. The melting point (T_m) of the copolymers varied in the range between 314°C and 409°C and showed the behavior of eutectic type copolymer. From the investigation of the crystallization behavior of the copolymers, it was found that the cold-crystallization temperature (T_c′) of the amorphous copolymers assumes a maximum value for the copolymer with a mole fraction of the PEDEKK segment (n_B) of about 0.6, isothermally crystallized PEEKK and the PEEKK/PEDEKK copolymer exhibit double-melting behavior.     

Synthesis and characterization of a partially quaternized poly(1-vinylimidazole) bound to a ruthenium(II) complex and a viologen moiety

Three metallopolymers having both a photosensitizer and an electron acceptor on the same polymer backbone were prepared and characterized by UV-Vis absorption and fluorescence spectroscopies and thermal measurements as differential scanning calorimetry (DSC) and thermogravimetric differential analysis (TG/DTA). These metallopolymers consist of bis(2,2′-bipyridine)ruthenium(II) complexes as a photosensitizer and a viologen species as an electron acceptor linked covalently to the imidazolyl residues on partially quaternized poly(1-vinylimidazole) through an alkyl spacer. DSC curves showed a glass transition temperature at ca. 230°C and an exothermic peak around 170°C. The exothermic peak was assigned to the cleavage of the alkyl spacer between the viologen and the quaternized imidazolium segment. UV-vis absorption spectra showed that these polymers have three absorption maxima, the metal-to-ligand charge transfer band (MLCT) of the ruthenium(II) complex residue being in the visible region and two electronic transition bands deriving from the viologen residue and the imidazolyl ring in the UV region. These absorption maxima are independent of the length of the alkyl group in the viologen residues and the spacer between the imidazolyl and the viologen segment. However, the luminescence spectra depend significantly on the length of alkyl group and the spacer, demonstrating that part of the ruthenium(II) complex residues are quenched by viologen residues. Furthermore, quenching studies revealed that these polymers contain at least four ruthenium(II) complex residues on the same polymer backbone.