Birefringence control of semicrystalline block copolymers by crystallization under confinement

A series of semicrystalline block copolymers (BCPs), poly(4-vinylpyridine)-block-poly(ε-caprolactone) (P4VP-PCL), with lamellar phases have been synthesized. P4VP-PCL BCP thin films with large-scale, oriented lamellar microdomains were obtained by rimming coating process followed by oscillated shearing using a homemade shear device. Owing to the vitrified P4VP microdomains and strongly segregated microphase separation, specific PCL crystalline chain orientation can be formed from the growth of anisotropic PCL crystallites under confinement so as to uniformly increase the birefringence of the BCP thin films. The enhanced birefringence corresponds well with the increase of PCL crystallinity. Consequently, the birefringence of the P4VP-PCL thin-films can be fine-tuned by PCL crystallization. The variation on the birefringence of the BCP thin films attributed to crystallization and melting is a reversible process with respect to temperature. The BCP thin films can thus be used as temperature-stimulated materials with controllable birefringence via crystallization kinetics.     

Thermoforming process of semicrystalline polymeric sheets: Modeling and finite element simulations

A micromechanically-based elastic-viscoplastic model for the thermoforming process of semicrystalline polymer materials is proposed and implemented in a finite element code. This model takes into account the temperature and strain rate dependence. In this process the applied temperature is taken uniform throughout the sheet and its variation is due only to the adiabatic heating. The simulations are conducted for isotactic polypropylene using the finite element method. The polymer sheet thickness, the orientation of the polymer molecular chains, and the percent crystallinity show an important dependence on the process temperature (polymer softening) and the geometry of the mold. Based on recent experimental results in the literature, amorphization (decrease of crystallinity) is taken into account. Published in Russian in Vysokomolekulyarnye Soedineniya, Ser. A, 2008, Vol. 50, No. 5, pp. 841–849. This article was submitted by the authors in English.     

Solute diffusion in swollen membranes VII. Diffusion in semicrystalline networks

A model is developed to express the solute diffusion coefficient through semicrystalline polymeric networks. The crystallites create impermeable diffusional barriers around the amorphous regions. Solute diffusion is determined by applying a transport model to the amorphous phase and incorporating the crosslinked polymer structure characteristics. This model is tested with theophylline and vitamin B₁₂ permeation experiments through semicrystalline poly(vinyl alcohol) membranes prepared by annealing of amorphous PVA membranes. The degree of crystallinity varies between 23.1 % and 40.5 % on a dry basis. The solute diffusion coefficients correlate well with various parameters of the model.     

Anisotropic polymeric networks with a controlled nanostructure

The synthesis and properties of anisotropically ordered polymer networks with a controlled (molecular) nanostructure are reported. Polymer networks with only a single rigid segment (based on 1, 4-distyrylbenzene or hydroquinone) between crosslink points were prepared by a simple polyesterification procedure. Unswollen polymer networks containing only one of these rigid segments or a combination of the two were birefringent and exhibited a reversible thermotropic phase transition. The ordering within these networks is attributed to a microphase separation of rigid aromatic and flexible aliphatic segments rather than strictly to a shape-anisotropy driven liquid crystalline phase. The nature of ordering within these networks when swollen in 1, 2-dichlorobenzene is also discussed.     

Birefringence and glass transition temperatures of poly(diethylene glycol terephthalate) networks

A series of polymer networks were prepared by trifunctionally endlinking poly(diethylene glycol terephthalate). The elastomeric properties of these materials were studied at constant temperature using experiments that involve both the elastic force and birefringence. Whereas the stress-strain isotherms show an anomalous increase in the modulus at very high elongation ratios, a downturn appears in the birefringence-strain isotherms at the same extensibilities. These results suggest that the upturn that appears in the force should be attributed to maximum chain extensibility rather than to strain-induced crystallization. A variety of additional thermoelastic experiments were carried out on these networks, to elucidate the dependence of the glass transition temperature on strain. It was found that for the elongation ratios at which the networks exhibit Gaussian behavior, the free-volume effects on the glass transition temperature T_g (decreasing T_g with increasing free volume) offset the conformational effects (increasing T_g with decreasing entropy). However, the contrary occurs in the region where the stress increases anomalously with increasing strain.     

Redox-responsive supramolecular polymeric networks having double-threaded inclusion complexes

Stimuli-responsive hydrogels have attracted attention as soft actuators that act similarly to muscles. In this work, hydrogel actuators controlled by host–guest interactions have been developed. The introduction of a 1:1 inclusion complex into a hydrogel is a popular design for achieving a change in cross-linking density. To realize faster and larger deformation properties, the introduction of a 1:2 inclusion complex is effective because the alteration in cross-linking density in a hydrogel with 1:2 complexes is larger than that in a hydrogel with 1:1 complexes. A redox-responsive hydrogel actuator cross-linked with 1:2 inclusion complexes is designed, where γ-cyclodextrin (γCD) and viologens modified with an alkyl chain derivative (VC11) were employed as the host and guest units, respectively. γCD includes two VC11 molecules in its cavity. The obtained γCD–VC11 hydrogel cross-linked with the 1:2 complex showed faster and larger deformation behaviour than the αCD–VC11 and the βCD–VC11 hydrogels cross-linked with a 1:1 complex. The deformation ratio and response speed of the γCD–VC11 hydrogel, which forms a supramolecular cross-linking structure by stimuli, are 3 and 11 times larger, respectively, than those of our previous hydrogel consisting of a βCD/ferrocene 1:1 inclusion complex.

A hydrogel actuator with a 1:2 host–guest complex controlled by redox stimuli has been developed to realize faster and larger deformation.     

Cholesteric polymeric networks with steroids as chiral components

Induced cholesteric polymeric networks with steroids as chiral components were prepared with the aim of finding steroid esters which show distinctly higher helical twisting power (HTP) than cholesteryl 3,4-di-(2-acryloyloxyethoxy)benzoate. Isothermal photopolymerization led to crosslinked networks. The HTPs of the networks were determined by measuring the reflection wavelengths and their dependence on the molar fraction of the chiral compound. The relationship between the HTP of the cholesteric polymeric networks and the molecular structure of the chiral steroid ester is discussed. A high HTP can be attributed to the definite alignment imposed by the large substituents on the long axis of the steroid nucleus.     

Cholesteric polymeric networks with steroids as chiral components

Induced cholesteric polymeric networks with steroids as chiral components were prepared with the aim of finding steroid esters which show distinctly higher helical twisting power (HTP) than cholesteryl 3,4-di-(2-acryloyloxyethoxy)benzoate. Isothermal photopolymerization led to crosslinked networks. The HTPs of the networks were determined by measuring the reflection wavelengths and their dependence on the molar fraction of the chiral compound. The relationship between the HTP of the cholesteric polymeric networks and the molecular structure of the chiral steroid ester is discussed. A high HTP can be attributed to the definite alignment imposed by the large substituents on the long axis of the steroid nucleus.     

Stress–strain behavior of swollen polymeric networks

The network parameters of swollen, solution-crosslinked polymer filaments can be collected from deswelling measurements in solutions of nonpermeating polymer or, as shown in this paper, from the stress–strain relation when in equilibrium with the surrounding solvent. The degree of swelling, at which the partial molar free energy of elasticity equals zero, is found to vary with solvent power in agreement with earlier findings on other systems. Comparison with results of studies on rubber networks crosslinked in the absence of diluent show that previously observed discrepancies between theory and experiment can be attributed to the deficiency of the single term involving the one-third power of the volume fraction of polymer in the swollen network to describe the contribution of the partial elastic free energy.     

Entrapment of living microbial cells in covalent polymeric networks

The kinetics of oxidative phenol degradation with microbial cellsCandida tropicalis, immobilized in a polyacrylamide and polymethacrylamide matrix, were mathematically simulated assuming zero-order and Michaelis-Menten rate equations. For zero-order kinetics an expanded equation for catalytic effectiveness as a function of the Thiele modulus, Biot number, and partition coefficients was derived and compared with numerical solutions for Michaelis-Menten kinetics. Errors with regard to the zero-order approximation become negligible ifc _o/K _M >2. Experimentally determined catalyst activities as a function of particle size and cell concentration were compared to calculated ones. Additional experiments to determine the diffusion and oxygen consumption ratios have been carried out in an effort to resolve the physical parameters to be used in the above mentioned calculations. Furthermore, experiments on cell growth during reincubation with nutrients and oxygen are reported; an increase in activity up to a factor of ten was observed. These experiments demonstrate that the microbial cells are entrapped in the polymer matrix in the living state.     

Some properties of intermediate regions in interpenetrating polymeric networks

The molecular mobility in an interpenetrating polymeric network (IPN) based on crosslinked polyurethane (1) and styrene–divinylbenzene copolymer (2) was studied by broad-line NMR spectroscopy and reversed gas chromatography. NMR and infrared investigations show the absence of chemical interaction between the two constituent networks. For an IPN in the temperature range ?196 to 120°C transition regions corresponding to the individual networks and to an intermediate region were found, the latter being characterized by an additional transition at 25–60°C (NMR spectroscopy) and 44–78°C (gas chromatography). The existence of an intermediate region, presumably of loosely packed structure, leads to a shift in the beginning of the temperature transition in IPN to lower temperature and to a decrease in activation energies of relaxation in comparison with the individual networks. The activation energies in IPN are decreased with increasing weight fraction of the second network.     

A theory of the stress-induced crystallization of crosslinked polymeric networks

A non-Gaussian theory of the stress-induced crystallization of polymeric networks is presented. It is predicted that for uniaxial extension of crosslinked polyethylene, a perfectly oriented, extended-chain crystal is formed initially, changing to one-fold crystal oriented perpendicular to the stretch direction at low extension and to a two-fold crystal having nearly perfect orientation at high extension. The stress is predicted to decay initially and then to rise as the network chains switch from an extended- to a folded-chain morphology, the rise being delayed and finally suppressed by additional crosslinking. The final, equilibrium birefringence is calculated and found to be negative at low extension and positive at high extension. The initial rate of crystallization is calculated using irreversible thermodynamics and is found to increase with extension and decrease with increasing crosslinking and temperature. All of the theoretical predictions are in qualitative agreement with experiment.     

Self-assembled, mesoporous polymeric networks for patterned protein arrays

A facile, self-assembly approach to the fabrication of a robust, mesoporous, biocompatible polymeric network for the spatial organization of proteins is described. Surface-deposited poly(styrene) (PS) beads that assemble into a two-dimensional (2-D) hexagonal array are used to template cross-linked poly(vinyl alcohol) (PVA), yielding an inverse opal structure. The porous, water insoluble network is used to entrain a model, soluble protein, green fluorescent protein (GFP). The polymeric network is characterized by atomic force microscopy (AFM) and optical microscopy, and the spatial localization of the incorporated GFP is determined by fluorescence microscopy. The results demonstrate that this system may constitute a versatile platform for the lateral organization of biomolecules.     

Toughness of polymeric networks: The limited importance of crosslink density

Four series of polymeric model networks were prepared with bimodal chain length distribution between crosslink points and two types of dangling chains as network defects. In the last series the crosslink density was changed without a large change in the chemical composition. The fracture toughness of those networks were compared with that of the defect–free networks. The fracture toughness of the various networks is surprisingly little influenced by the introduction of defects. Neither bimodality, nor dangling chains, nor a high sol fraction alters the toughness of the network. A good correlation between K_Ic and the weight fraction of polyether is observed. A much smaller dependence of K_Ic on the strand density can be deduced. The yield stress is high and approximately invariant for all systems studied. It is concluded that the toughness of a polymeric network does not seem to be influenced by its perfection and only to a small extent by its degree of crosslinking.     

Nonlinear optically active polymeric coordination networks based on metal m-pyridylphosphonates

A family of polymeric coordination networks based on meta-pyridylphosphonate bridging ligands has been synthesized and characterized by single-crystal X-ray crystallography. Compounds [M(2)(L-Et)(4)(mu-H(2)O)] (M = Mn, 1; Co, 2; Ni, 3; L-Et = ethyl-4-[2-(3-pyridyl)ethenyl]phenylphosphonate) were obtained by hydro(solvo)thermal reactions between diethyl-4-[2-(3-pyridyl)ethenyl]phenylphosphonate (L-Et(2)) and corresponding metal salts, while [Cd(L-H)(2)], 4 (L-H is monoprotonated 4-[2-(3-pyridyl)ethenyl]phenylphosphonate), was obtained by a hydro(solvo)thermal reaction between (L-H(2)).HBr and Cd(CF(3)SO(3))(2).6H(2)O. Compounds 1-3 are isostructural and crystallize in noncentrosymmetric space group Fdd2, and they adopt a complicated 3D framework structure composed of [M(2)(L-Et)(4)(mu-H(2)O)] building units, while compound 4 adopts a centrosymmetric 3D network structure resulted from linking 1D sinusoidal cadmium phosphate chains with L-H bridging ligands. Consistent with their polar structures, compounds 1-3 exhibit powder second harmonic generation signals larger than that of potassium dihydrogen phosphate.     

Birefringence of filled rubbers

The birefringence change occuring upon stretching a rubber containing spherical isotropic filler particles is calculated as a function of the volume fraction of the filler. The stress concentration arising from the presence of the filler particles leads to an enhanced birefringence of the rubber. From a consideration of the detailed birefringence pattern about a spherical particle within a stretched rubber, the enhancement of the retardation is calculated as a function of the volume fraction of the filler particles.     

Integration of polymeric membranes with microfluidic networks for bioanalytical applications.

The concept of microfluidics has significantly influenced the design and the implementation of modern bioanalytical systems due to the fact that these miniaturized devices can handle and manipulate samples in a much more efficient way than conventional instruments. In an analogy to the development of microelectronics, increasingly sophisticated devices with greater functionalities have become one of the major goals being pursued in the area of micrototal analysis systems. The incorporation of polymeric membranes into microfluidic networks has therefore been employed in an effort to enhance the functionalities of these microfabricated devices. These commercially available membranes are porous, flexible, mechanically robust and compatible with plastic microfluidic networks. The large surface area-to-volume ratio of porous membrane media is particularly important for achieving rapid buffer exchange during microdialysis and obtaining ultrahigh concentration of adsorbed enzymes for various biochemical reactions. Furthermore, the membrane pore diameter in the sub-microm range eliminates the constraints of diffusional mass-transfer resistance for performing chiral separation using adsorbed protein as the chiral stationary phase. A review on the recent advancement in the integration of polymeric membranes with microfluidic networks is presented for their widespread applications in bioanalytical chemistry.     

Radiation modification of semicrystalline polymers

Three problems of the radiation modification of polymers are dealt with. Different methods of grafting are compared with respect of the formation of true graft copolymer and homopolymer in the polyethylene-styrene system; the prevention of the penetration of monomer by grafting polytetrafluoroethylene is shown; the mechanical properties of crosslinked ethylene polymers below and above the melting point are discussed.