Viscoelastic properties of crosslinked solutions of poly(2-hydroxymethyl methacrylate) in diethylene glycol. VI. Comparison with poly(butyl methacrylate) and poly(butyl acrylate) networks crosslinked to very low degrees

On the basis of our own experimental and some literature data, the contributions of slow relaxation mechanisms to the shear modulus, (G_eN — G_e), and the parameter C₂ of the Mooney-Rivlin equation have been examined for lightly crosslinked poly(butyl methacrylate), poly(butyl acrylate), poly(2-hydroxyethyl methacrylate), and some rubber networks. For the rubbers, increasing degree of crosslinking caused a decrease in G_eN — G_e and an increase in C₂; for the other networks, both G_eN — G_e and C₂ diminished with increasing crosslinking. The effectiveness of the crosslinking polymerization, and also the absolute values of the physical crosslinking degree, decreased in the order of poly(2-hydroxyethyl methacrylate), poly(butyl methacrylate), and poly(butyl acrylate). The values of the equilibrium compliances J of the networks studied, obtained by various methods, have also been compared, and good agreement has been found.     

Dielectric γ relaxation in poly(2-hydroxyethyl methacrylate)

The dielectric constant and loss were measured for poly(2-hydroxyethyl methacrylate) from ?90 to 34°C over the frequency range 30 to 3 × 10⁵ Hz in the γ relaxation region. The relaxation was also characterized using activation energies.     

Dynamic mechanical properties of poly(2-hydroxyethyl methacrylate) hydrogels

Homogeneous and heterogeneous poly(2-hydroxyethyl methacrylate) hydrogels were prepared by copolymerization of 2-hydroxyethyl methacrylate and small amounts of ethylene glycol dimethacrylate in the presence of water; concentrations of water (V₁) in the polymerization mixture and the volume fractions of water (v₁) of the gels swollen to equilibrium were 40, 50, 60, 70% by volume, and 0.475, 0.541, 0.669, 0.778, respectively. From the homogeneous (clear) hydrogel (V₁ = 40%) preparation, four hydrogels were prepared with v₁ = 0.434, 0.418, 0.378, 0.326. Tensile dynamic moduli were measured in the frequency range from 0.006 to 0.6 cps and the temperature range from 0 to 40°C. In these cases, the influence of swelling on the shape of the relaxation spectra and on the monomeric unit friction coefficient was studied. The dependence of the friction coefficient on water concentration was interpreted in terms of free volume. In the heterogeneous (opaque) gel preparations (V₁ = 50, 60, 70%), the effect of the aqueous phase in the system on the mechanical behavior was described by a modification of the blending law of Ninomiya. For the systems with V₁ = 60% and 70% the shapes of the storage and loss moduli in the main transition region and the friction coefficient were similar to those of the homogeneous gel with V₁ = 40%, except for the decrease in absolute value of the storage moduli. For the system with V₁ = 50% the shape of the relaxation spectrum changes appreciably and the wedge distribution does not hold.     

Dilute solution properties of tactic poly(2-hydroxyethyl methacrylate)

Isotactic and syndiotactic poly(2-hydroxyethyl methacrylate) (PHEMA) have been prepared. Intrinsic viscosity–molecular weight relationships were established for the isotactic and syndiotactic PHEMA in N,N-dimethylformamide (DMF) at 25°C by solution viscometry and light scattering. The unperturbed dimensions and interaction parameters were examined in DMF, water, methanol, ethanol, and water–methanol (1:7 by volume) mixture for isotactic PHEMA and in DMF, methanol, and water–methanol (1:7 by volume) mixture for syndiotactic PHEMA using the Stockmayer–Fixman representation. The results suggest that the compact random coil structure for isotactic PHEMA occurs in water solvent and the isotactic PHEMA is more highly extended in polar solvents.     

Imparting antifouling properties of poly(2-hydroxyethyl methacrylate) hydrogels by grafting poly(oligoethylene glycol methyl ether acrylate)

The antifouling properties of poly(2-hydroxyethyl methacrylate- co-methyl methacrylate) hydrogels were improved by the surface grafting of a brush of poly(oligoethylene glycol methyl ether acrylate) [poly(OEGA)]. The atom-transfer radical polymerization (ATRP) of OEGA (degree of polymerization = 8) was initiated from the preactivated surface of the hydrogel under mild conditions, thus in water at 25 degrees C. The catalytic system was optimized on the basis of two ligands [1,1,4,7,10,10-hexamethyl-triethylenetetramine (HMTETA) or tris[2-(dimethylamino)ethyl]amine (Me6TREN)] and two copper salts (CuIBr or CuICl). Faster polymerization was observed for the Me 6TREN/CuIBr combination. The chemical composition and morphology of the coated surface were analyzed by X-ray photoelectron spectroscopy, attenuated total reflectance Fourier transform infrared spectroscopy, contact angle measurements by the water droplet and captive bubble methods, scanning electron microscopy, and environmental scanning electron microscopy. The hydrophilicity of the surface increased with the molar mass of the grafted poly(OEGA) chains, and the surface modifications were reported in parallel. The antifouling properties of the coatings were tested by in vitro protein adsorption and cell adhesion tests, with green fluorescent protein, beta-lactamase, and lens epithelial cells, as model proteins and model cells, respectively. The grafted poly(OEGA) brush decreased the nonspecific protein adsorption and imparted high cell repellency to the hydrogel surface.     

Viscoelastic properties of single poly(ethylene glycol) molecules.

The viscoelastic properties of single poly(ethylene glycol) (PEG) molecules were measured by analysis of thermally and magnetically driven oscillations of an atomic force microscope (AFM) cantilever/molecule system. The molecular and monomer stiffness and friction of the PEG polymer were derived using a simple harmonic oscillator (SHO) model. Excellent agreement between the values of these two parameters obtained by the two approaches indicates the validity of the SHO model under the experimental regimes and the excellent reproducibility of the techniques. A sharp minimum in the monomeric friction is seen at around 180 pN applied force which we propose is due to a force induced change in the shape of the energy landscape describing the conformational transition of PEG from a helical to a planar state, which in turn affects the timescale of the transition and therefore modifies the measured internal friction. A knowledge of the viscoelastic response of PEG monomers is particularly important since PEG is widely used as a linker molecule for tethering groups of interest to the AFM tip in force spectroscopy experiments, and we show here that care must be exercised because of the force-dependent viscoelastic properties of these linkers.     

The conformational stability of tactic poly(2-hydroxyethyl methacrylate) in aqueous salt solutions

The conformational stability of tactic poly(2-hydroxyethyl methacrylate) (PHEMA) in aqueous salt solutions was investigated by measurements of swelling, surface-free energy, and differential scanning calorimetry, as this polymer in water is sensitive in various electrolytes. In the case of inorganic salts, the major role for the salt effect is played by the anions, and the exposure of hydrophobic components at the PHEMA surface can be correlated with the increase of the degree of swelling. The influence of cations is considerably weaker. In the case of organic salts, tetraalkylammonium halides cause the chain extension more effectively with the increase of alkyl chain length in the cations. This result indicates that the breakdown of the hydrophobic parts in PHEMA provides an important clue on conformational stability. The amount of water molecules bound with the hydrophilic sites of tactic-PHEMA mainly depends on the chain extension and the hydration of cations. Since the sites in tactic-PHEMA influenced by the cation and the anion are different, their effects cause the conformational transition at a specific range of salt concentration. © 1993 John Wiley & Sons, Inc.     

Polymerization of β-cyanopropionaldehyde. II. Crystalline poly(cyanoethyl)oxymethylene

Additional investigation was made on the polymerization of β-cyanopropionaldehyde at ?78°C. with triethylaluminum and triethylaluminum—titanium tetrachloride complexes as initiators. The complexes give a higher polymer yield than triethylaluminum alone. The yield—Ti/Al plot also has a maximum at a Ti/Al mole ratio of about 0.2 at constant Al(C₂H₅)₃ concentration. The rate of polymerization seems to be increased in the following order: toluene < methylene chloride < tetrahydrofuran. This order is reversed with regard to the content of DMF-insoluble fraction mentioned below. The polymer obtained consists of two fractions: one is soluble in dimethylformamide (DMF) and the other is not. The former consists of an amorphous polymer and the latter of crystalline polymer. It was found that the infrared absorption bands at 790, 1258, and 1375 cm.^-1 were characteristic of crystalline polymer and were assigned to crystalline bands. Those at 1270 and 1345 cm.^-1 are characteristic of amorphous bands. The crystalline bands and C? O? C bands show very intense infrared dichroism, whereas the nitrile band does not. The crystal data obtained from the analysis of the x-ray diffraction pattern, including the fiber repeat distance of 4.95 A. and other unit cell dimensions in a triclinic system, were compared with those reported for various aldehyde polymers. The unit cell dimension a′ or the maximum interplanar distance is somewhat smaller, suggesting that the molecules are more tightly packed than poly(n-butyraldehyde), in which the side chain has the same carbon number as that of poly-(cyanoethyl)oxymethylene. Internal rotation angles and a radius of helix were calculated for an isotactic fourfold helical model of the polymer. Some other characterizations of the polymer were also made.     

Viscoelastic interfacial properties of compatibilized poly(ε‐caprolactone)/polylactide blend

Poly(ε‐caprolactone)/polylactide blend (PCL/PLA) is an interesting biomaterial because the two component polymers show good complementarity in their physical properties. However, PCL and PLA are incompatible thermodynamically and hence the interfacial properties act as the important roles controlling the final properties of their blends. Thus, in this work, the PCL/PLA blends were prepared by melt mixing using the block copolymers as compatibilizer for the studies of interfacial properties. Several rheological methods and viscoelastic models were used to establish the relations between improved phase morphologies and interfacial properties. The results show that the interfacial behaviors of the PCL/PLA blends highly depend on the interface‐located copolymers. The presence of copolymers reduces the interfacial tension and emulsified the phase interface, leading to stabilization of the interface and retarding both the shape relaxation and the elastic interface relaxation. As a result, besides the relaxation of matrices (τ_m) and the shape relaxation of the dispersed PLA phase (τ_F), a new relaxation behavior (τ_β), which is attribute to the relaxation of Marangoni stresses tangential to the interface between dispersed PLA phase and matrix PCL, is observed on the compatibilized blends. In contrast to that of the diblock copolymers, the triblock copolymers show higher emulsifying level. However, both can improve the overall interfacial properties and enhance the mechanical strength of the PCL/PLA blends as a result. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 756–765, 2010     

Thermal and photo-induced phenomena in γ-irradiated poly(methyl methacrylate)

ESR spectra and optical absorption spectra of γ-irradiated poly(methyl methacrylate) (PMMA) at 77°K and the photo- and thermal bleaching behavior have been studied. γ-Irradiation of PMMA yields a singles spectrum with a line width of 6 G which is bleachable with visible light. This species is assigned to the polymer cation. The unbleached species consists mainly of a four-line spectrum and is assigned to a polymer radical having a different CH₂ conformation from the usual nine-line spectrum. On subsequent warming to room temperature, the spectrum changes into the nine-line spectrum. γ-Irradiation of PMMA containing biphenyl as an additive at 77°K gives biphenyl cation. Yield of polymer radical is reduced by the addition of biphenyl and 2-methyltetrahydrofuran. A mechanism is proposed which is consistent with the experimental results.     

Mass transfer of oxygen in poly(2-hydroxyethyl methacrylate)

The diffusion coefficient of oxygen in poly(2-hydroxyethyl methacrylate) has been explicitly measured using an optical technique based on fluorescence quenching. This measurement represents the first explicit determination of D in PHEMA. A diffusion coefficient of oxygen in PHEMA of 1.36 × 10^?7 cm²/s at 20°C was obtained from this measurement. This value is shown to agree well with permeability data for PHEMA, the free volume theory of diffusion, and with values of D that have been explicitly measured in other methacrylate hydrogels.     

Rheology of concentrated solutions of poly(γ-benzyl-glutamate)

Steady shear viscosity, dynamic viscosity, dynamic modulus, and normal force were measured via rotational rheometry for concentrated solutions of racemic mixtures of poly(benzyl-glutamate) and poly(benzyl-D-glutamate) in m-cresol. A transition from the isotropic state to liquid-crystalline order with increase in concentration was indicated by optical anisotropy and maxima in all four material functions. This occurred at a critical concentration higher than the Flory prediction. Over a well-defined range of concentrations and shear stresses, some of the liquid-crystalline solutions exhibited negative first normal-stress differences that were not due to inertial effects.     

Poly(β-amino acid)s. IV. Synthesis and conformational properties of poly(α-isobutyl-L-aspartate)

Poly(α-isobutyl-L -aspartate) was prepared by the polycondensation reaction of p-nitrophenyl ester of α-isobutyl-L -aspartate and the conformation of the poly(β-amino acid) was investigated by X-ray diffraction, polarized infrared, circular dichroism (CD), optical rotatory dispersion (ORD), and NMR spectroscopy. α-Isobutyl β-p-nitrophenyl-L -aspartate hydrochloride and hydrobromide were used as monomers and dimethylformamide, chloroform, and chlorobenzene, as solvents. A high-molecular-weight polymer with [η] 1.0 dl/g (dichloroacetic acid, 25°C) was formed in the polymerization of the hydrochloride in chloroform at 25°C. The X-ray diagram and polarized infrared spectrum of the stretched polymer film obtained from a chloroform solution suggested a cross-β-form as the most probable structure in the solid state. The CD spectra of the polymer in a 2,2,2-trifluoroethanol (TFE) solution and its film cast from the solution showed a peak at 205 nm and a trough at 190 nm which were assigned to a β-structure. The polymer was associated in chloroform. The NMR and ORD spectra in chloroform were similar to those in TFE, which suggests that the polymer also exists in the β-structure in chloroform. The addition of small amounts of dichloroacetic acid and sulfuric acid to chloroform and TFE solutions, respectively, destroyed the β-structure. A random copolymer of α-isobutyl-L -aspartate with β-alanine was also prepared by polycondensation reaction. The copolymer apparently did not form an ordered structure in the solid state or in solution.     

Dielectric relaxation in concentrated solutions of poly(γ-benzyl-L-glutamate)

Measurements of dielectric constant and loss, broad-line nuclear magnetic resonance, and differential thermal analysis of concentrated solutions of poly(γ-benzyl-L -glutamate) (PBLG) were carried out to determine the effects of intermolecular interactions on the mobility of the side group and the solvent (dichloromethane or dioxane). Only one dielectric loss peak due to the cooperative motion of the side group and the solvent was found. The activation energy of this relaxation process varied from 3 to 47 kcal/mole with increasing concentration of PBLG from 20 to 100% by weight accompanied with steep increases at about 40 and 80%. This result is explained as due to entanglement of neighboring side groups. In NMR, narrowing of the line was observed near the temperature where the dielectric loss was observed. The glass transition was also observed by differential thermal analysis. From these results it was concluded that the relaxation observed in PBLG solution and in pure PBLG have the nature of primary or α relaxations.     

Viscoelastic properties of poly(ε‐caprolactone) – hydroxyapatite micro‐ and nano‐composites

Various composites have been proposed in the literature for the fabrication of bioscaffolds for bone tissue engineering. These materials include poly(ε‐caprolactone) (PCL) with hydroxyapatite (HA). Since the biomaterial acts as the medium that transfers mechanical signals from the body to the cells, the fundamental properties of the biomaterials should be characterized. Furthermore, in order to control the processing of these materials into scaffolds, the characterization of the fundamental properties is also necessary. In this study, the physical, thermal, mechanical, and viscoelastic properties of the PCL‐HA micro‐ and nano‐composites were characterized. Although the addition of filler particles increased the compressive modulus by up to 450%, the thermal and viscoelastic properties were unaffected. Furthermore, although the presence of water plasticized the polymer, the viscoelastic behavior was only minimally affected. Testing the composites under various conditions showed that the addition of HA can strengthen PCL without changing its viscoelastic response. The results found in this study can be used to further understand and approximate the time‐dependent behavior of scaffolds for bone tissue engineering. Copyright © 2012 John Wiley & Sons, Ltd.     

Dual‐responsive supramolecular inclusion complexes of block copolymer poly(ethylene glycol)‐block‐poly[(2‐dimethylamino)ethyl methacrylate] with α‐cyclodextrin

A new class of temperature and pH dual‐responsive and injectable supramolecular hydrogel was developed, which was formed from block copolymer poly(ethylene glycol)‐block‐poly[(2‐dimethylamino)ethyl methacrylate] (PEG‐b‐PDMAEMA) and α‐cyclodextrin (α‐CD) inclusion complexes (ICs). The PEG‐b‐PDMAEMA diblock copolymers with different ratio of ethylene glycol (EG) to (2‐dimethylamino)ethyl methacrylate (DMAEMA) (102:46 and 102:96, respectively) were prepared by atom transfer radical polymerization (ATRP). ¹H NMR measurement indicated that the ratio of EG unit to α‐CD in the resulted ICs was higher than 2:1. Thermal analysis showed that thermal stability of ICs was improved. The rheology studies showed that the hydrogels were temperature and pH sensitive. Moreover, the hydrogels were thixotropic and reversible. The self‐assembly morphologies of the ICs in different pH and ionic strength environment were studied by transmission electron microscopy. The formed biocompatible micelles have potential applications as biomedical and stimulus‐responsive material. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2143–2153, 2010     

Stability of heterocharges in poly(methyl methacrylate) thermoelectrets. II. Effect of γ-irradiation on thermally stimulated current spectra

Thermally stimulated currents (TSC) of γ-irradiated poly(methyl methacrylate) thermoelectrets have been investigated in the high-temperature range. The two main peaks of the TSC spectrum, appearing, respectively, at about 100 (α) and 120°C (ρ), show marked differences in stability under irradiation: while the α peak is little affected by doses up to 10⁷ rad, the ρ peak vanishes following doses of 10⁵ to 10⁶ rad. On the other hand, a study of the rate of decrease of the ρ peak as a function of irradiation conditions reveals the important role of the surrounding atmosphere and the existence of a flux effect, probably related to irradiation-accelerated diffusion of gas into the polymer. All these facts confirm the respective attributions of α and ρ peaks to a dipolar volume polarization and to an ionic space charge polarization, showing that the TSC study of irradiated thermoelectrets can be a useful method for identification of polarization processes in polymers.