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Cholesterol self-assembles into weakly ordered aggregates when tethered to a crosslinked hydrogel network of poly(ethylene glycol) (PEG). PEG-diacrylate and cholesterol-PEG-acrylamide (PEG-chol) were co-polymerized in organic solvent and transferred to water for equilibrium swelling. Small-angle x-ray scattering revealed self-assembled cholesterol structures not present during network synthesis. At lower ratios of PEG-tethered cholesterol to PEG (

A class of tunable, mesostructured self-assembled hydrogels has been developed based on a library of π–π stacking ABA-type telechelic triblock copolymers prepared by organocatalytic ring-opening polymerization (ROP). Poly(ethylene glycol) (PEG) served as a macroinitiator for ROP of methylene tricarbonate-benzyl ester (MTC-OBn), a benzyl-ester functionalized cyclic carbonate monomer, resulting in polymers with well-defined length and narrow polydispersity. Self-assembled hydrogels were formed by dissolving poly(MTC-OBn)-b-PEG-b-poly(MTC-OBn) copolymers in water. Physical cross-links in the hydrogels formed through hydrophobic and π–π stacking intermolecular interactions between poly(MTC-OBn) segments. The mesoscale material properties were probed using small angle X-ray scattering (SAXS) and dynamic mechanical measurements. SAXS spectra show a scattering peak, corresponding to a molecular domain spacing of 20–25 nm, which we attribute to the formation of electron-dense poly(MTC-OBn) assemblies. Results from oscillatory shear experiments were analyzed in conjunction with SAXS data in order to develop an understanding of the influence that nanoscale structure has on the mechanical properties of self-assembled gels. Modeling of scattering peaks using the Bragg spacing model demonstrated that the hydrophilic PEG network chains connecting the scattering moieties behaved as a two-dimensional self-avoiding walk. We compared the observed microstructural features with the mechanical properties of self-assembled gels and determined that the molecular weight of the polycarbonate and PEG segments controls the gel structure on both the mesoscale and macroscale. A central contribution of this work is a synthetic strategy that utilizes ROP to control polymer structure, which in turn controls both the structure and mechanical properties of these biodegradable self-assembled hydrogels.

Polymer electrolyte membrane (PEM) fuel cells can provide alternatively sourced energy, but current PEMs lose performance under certain conditions. The current work examines composition, structure and properties of poly (ethylene glycol)-aromatic polyimide-ionic liquid systems for PEM applications, as poly (ethylene glycol) (PEG) is an ion conductor and polyimides are stable. To evaluate polymer interactions, different PEG concentrations (0–50% by weight) and different PEG molecular weights (990–6000 g/mole) were examined. Characterization techniques included Fourier transform infrared spectroscopy, thermal gravimetric analysis, differential scanning calorimetry, small-angle x-ray scattering, electrochemical impedance spectroscopy and cyclic voltammetry. By increasing the PEG amount, PEG domains and polymer flexibility are increased, which increases conductivity by two to three orders of magnitude. By increasing PEG molecular weight, PEG segmental motion and PEG-polyimide interface quality are decreased, which decreases conductivity by a factor of two. The maximum conductivity was 64 mS/cm at 80 °C and 70 %RH.

A small molecule nucleating agent, orotic acid (OA), was investigated to enhance crystallinity and crystallization kinetics in poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV) for the purpose of addressing embrittlement and reducing solidification time after thermal processing. In particular, the effectiveness of monohydrated (OA-m) and anhydrous (OA-a) forms of OA as nucleating agents for isothermal and non-isothermal crystallization of PHBV films was investigated and compared. Both forms of OA were able to increase crystallization temperature of PHBV as well as form more uniform crystal structures, based on differential scanning calorimetry. It was found that OA-a at 1–2 wt% was most effective in PHBV crystal nucleation because of the increase in overall polymer crystallinity and faster crystallization rate. Additionally, the faster crystallization of OA-a led to fibrillar film morphology of the PHBV/OA blends.

Aqueous solutions of poly(ethylene glycol) (PEG) exhibit some remarkable properties, among which is the small changes in water activity compared to the volumes occupied by the PEG: For example, the water in a 20% mass fraction solution of 6000 Da PEG has an activity of 0.9939. We have investigated PEGs with molecular weights 200, 400, 1000, 2000, 4000, and 8000 Da in the concentration range 1% to 17% mass fraction at neutral pH and with added KCl concentrations of 10 mmol L−1 in aqueous solutions–conditions near those for promoting protein crystallization. These solutions exhibit a structural change at around 6% mass fraction as seen in the solution viscosities, compressibilities, and infrared spectra. Raman spectroscopy shows that the PEGs remain in the same structural form over the concentration range, and the infrared spectra indicate that the change must be due to a local shift in the water structure. Modeling of the results from small-angle neutron scattering (SANS) on the solutions suggests that the structures of the PEGs in the molecular mass range 2000 Da to 8000 Da are paired in the solution, and the separation distance decreases with increasing PEG concentration. From the structure, it becomes clear that the small effect on water activity occurs because of screening by the more weakly bound outer layers. From the bulk measurement of aw and with reasonable assumptions, a free energy ΔG° can be assigned to each of the fourth, third, and second hydration layers.

The governing kinetic behavior of water transport in a biopolymeric composite material derived from poly(β-hydroxybutyrate)-co-poly(β-hydroxyvalerate) and lignocellulosic wood flour were investigated along with the influence of temperature, wood flour content, and chemical modification (silane, maleic anhydride) on polymer and composite diffusivity. The water absorption process in both untreated and treated composites was found to follow the kinetics of Fickian diffusion theory. Diffusion coefficients for neat polymer and composite samples were experimentally determined, and the thermodynamics of diffusive water transport were observed to exhibit Arrhenius rate-law behavior. A model for predicting equilibrium moisture content in wood-polymer composites is presented and substantiated by obtained results and cited experimental data. Isodiffusion plots are presented to evaluate the effectiveness of chemical modifications, which were found to reduce the rates of water uptake. Both in- and out-of-plane dimensional changes were monitored during the absorption process, permitting the determination of moisture-dependent hydroexpansion coefficients.

Films of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (P3HB-co-3HHx) containing 3.8–10 mol% of 3-hydroxyhexanoate (3HHx) comonomer were subjected to anaerobic biodegradation to explore the effects of copolymer composition, crystallinity, and morphology on biodegradation. As biodegradation proceeded, samples with higher HHx fraction tended to have faster weight loss; on Day 7 of the degradation experiment, P3HB-co-10 mol%-3HHx lost 80% of its original weight, while P3HB-co-3.8 mol%-3HHx lost only 28%. Scanning electron microscopy (SEM) images revealed that the anaerobic biodegradation proceeded at the surface of the samples, with preferential erosion of the amorphous regions, exposing the crystalline spherulites formed inside the copolymer films. It was observed that copolymers with higher HHx fraction had smaller diameter spherulites, ranging from roughly 40 μm for P3HB-co-3.8 mol%-3HHx to 10 μm for P3HB-co-10 mol%-3HHx. A banded spherulite morphology was observed for P3HB-co-6.9 mol%-3HHx and P3HB-co-10 mol%-3HHx, with much wider band spacing (2 μm) for the former than the latter (0.3 μm). Different thermal history seemed to affect the morphological properties and, thus, the biodegradability of the P3HB-co-3HHx samples as well. When comparing copolymers with the same copolymer composition, P3HB-co-3HHx annealed at 70 °C had 5–30% more weight loss after the same duration of incubation in active sludge compared to the quenched samples. We suggest that annealing of P3HB-co-3HHx likely induces void formation in the semi-crystalline structure, facilitating the movement of water or perhaps enzymes to a higher degree of penetration into the sample and subsequently enhancing microbial degradation.

The rheological properties of a series of microbially synthesized poly(3-hydroxybutyrate-co-3-hydroxyhexanoate)s (PHB-HHxs), with varying comonomer (HHx) content, were systematically investigated. Shear viscosities show dependence on the rate of deformation, temperature, molecular weight, and copolymer compositions. The zero-shear viscosity η0 follows the classical M w 3.4 power-law relationship with the weight average molecular weight Mw. The characteristic relaxation time λ, which indicates the onset of shear thinning, ranges from 0.02 to 0.2 s for different PHB-HHxs and is roughly linearly related to η0. The temperature dependence of rheological properties follows an Arrhenius form. Activation energies for flow Ea are obtained from the slope of the natural logarithm of the shift factor αT plotted against the inverse of temperature curve, and the values for PHB-HHxs are found to be in the range of 27–36 kJ/mol Ea decreases with HHx content in the copolymer, a trend that can be related to the difference in chemical structure between HHx and HB, according to the method of Vankrevelen and Hoftyzer. A Generalized Maxwell model models the viscoelastic behavior of the PHB-HHx melt well. The value of the plateau modulus G N 0 obtained suggests a highly entangled configuration. The molecular weight between entanglements Me decreases from 11,600 to 9400 as HHx content increases from 3.8 to 10.0 mol%. Our results suggest that the presence of propyl groups in HHx increases the steric hindrance of the PHB-HHx chains, thus resulting in increased segmental friction and entanglement density. As a result, viscoelastic parameters for PHB-HHx copolymers, such as η0 and G N 0 , are readily tunable by varying the HHx content, making them attractive as “green” substitutes for non-degradable thermoplastics.

In this paper, we present the systematic development of mechanically enhanced interpenetrating polymer network (IPN) hydrogels with Young's moduli rivaling those of natural load-bearing tissues. The IPNs were formed by synthesis of a crosslinked poly(acrylic acid) (PAA) network within an end-linked poly(ethylene glycol) (PEG) macromonomer network. The strain-hardening behavior of these PEG/PAA IPNs was studied through uniaxial tensile testing and swelling measurements. The interaction between the independently crosslinked networks within the IPN was varied by (1) changing the molecular weight of the PEG macromonomer, (2) controlling the degree of PAA ionization by changing pH, and (3) increasing the polymer content in the PAA network. Young's moduli and the maximum stress-at-break of the swollen hydrogels were normalized on the basis of their polymer content. Strain hardening in the IPNs exhibited a strong dependence on the molecular weight of the first network macromonomer, the pH of the swelling buffer, as well as the polymer content of the second network. The results indicate that the mechanical enhancement of these IPNs is mediated by the strain-induced intensity of physical entanglements between the two networks. The strain can be applied either by mechanical deformation or by changing the pH to modulate the swelling of the PAA network. At pHs below the pKa of PAA (4.7), entanglements between PEG and PAA are reinforced by interpolymer hydrogen bonds, yielding IPNs with high fracture strength. At pHs above 4.7, a “pre-stressed” IPN with dramatically enhanced modulus is formed due to ionization-induced swelling of the PAA network within a static PEG network. The modulus enhancement ranged from two-fold to over 10-fold depending on the synthesis conditions used. Variation of the network parameters and swelling conditions enabled “tuning” of the hydrogels' physical properties, yielding materials with water content between 58% and 90% water, tensile strength between 2.0 MPa and 12.0 MPa, and initial Young's modulus between 1.0 MPa and 19.0 MPa. Under physiologic pH and salt concentration, these materials attain “biomimetic” values for initial Young's modulus in addition to high tensile strength and water content. As such, they are promising new candidates for artificial replacement of natural tissues such as the cornea, cartilage, and other load-bearing structures.

The composition of semi-interpenetrating polymer networks (semi-IPNs) based on responsive N-isopropylacrylamide (NIPAAm) hydrogels has been shown to affect the kinetics of the volume phase transition. Several N-alkyl-substituted acrylamides were used as the linear polymers in a crosslinked NIPAAm network, and the kinetics was observed as a function of crosslinking density and linear polymer concentration. The time required for collapse of the network could be reduced by as much as 90%, with little change to the corresponding swelling ratio and volume phase transition temperature. However, the underlying changes in network morphology are not known, and here we present kinetics data in combination with imaging of the resulting hydrogel networks. The crosslinked networks and the linear polymers were fluorescently labeled, and the resulting morphology was imaged with confocal fluorescence microscopy and two-photon laser scanning microscopy. The most hydrophilic of the linear polymers was acrylamide, which was shown to phase separate during polymerization. The hydrophilic domains become more interconnected at higher concentrations of the crosslinker and the linear polymers. This correlates well with the kinetics of the volume phase transition for the corresponding networks. The semi-IPNs containing more hydrophobic linear polymers had very similar morphology, but some domains were present, ranging from 500 nm to 2 μm and increasing in size with increased linear polymer concentration. The time scale of collapse was an order of magnitude faster than expected, based on size of the hydrophobic N-alkyl group, when the linear polymer had the same lower critical solution temperature as the hydrogel network. This is an indication that the simultaneous collapse of the linear polymer and the crosslinked network contributes to the fast response of these semi-IPNs.

Inclusion of a polymer cushion between a lipid bilayer membrane and solid surface has been suggested as a means to provide a soft, deformable layer that will allow for transmembrane protein insertion and mobility. In this study, we evaluate the properties of a disulfide- and lipid-modified polyacrylamide polymer cushion. High molecular weight random copolymers with various degrees of disulfide and lipid substitution were synthesized. X-ray photoelectron spectroscopy (XPS) was used to determine quantitatively the percentage of disulfide groups bound to gold and silver surfaces. A quartz crystal microbalance with dissipation (QCM-D) was used to study the adsorption process and resulting film properties in situ. The presence of backbone–surface interactions leads to a competition between physisorption of the acrylamide backbone and chemisorption of the disulfide side-chains. This competition limits the degree of chemisorption to gold and silver surfaces. For a polyacrylamide with 10 mol% disulfide side-chains, 78% of the side-chains covalently bind to silver and only 41% bind to gold. The undesired physisorption of the acrylamide backbone leads to adsorption of the homopolymer itself. In addition, film thicknesses, as indicated by both XPS and QCM-D, are limited to 15–30 A. The QCM-D results for all films indicate the formation of relatively rigid films, rather than the soft, deformable films desired for a lipid membrane polymer cushion.

We have evaluated the mechanical strength of a series of composites consisting of carbon particles bound together by poly(vinylidene fluoride) (PVDF), which is closely related to the carbonaceous anode in a lithium ion battery. We used a balanced beam scrape adhesion tester and evaluated the influence of carbon particle structure, the chemical properties of PVDF, and the processing parameters of annealing temperature and casting solvent on the adhesion of the composite film to a copper substrate. The composite prepared with amorphous carbon shows over 10 times higher adhesion strength than those fabricated from other graphite materials. This results from chemical binding that is intermediate between semi-ionic and covalent C–F bonds, as detected by X-ray photoelectron spectroscopy. To address the effect of the crystalline phase of the binder on the adhesion strength, we investigated PVDF crystallinity in the composite films using differential scanning calorimetry. Samples with higher crystallinity show higher adhesion strength, independent of annealing temperature and casting solvent. The scratch adhesion was also measured for swollen electrodes immersed in 3:7 volume ratio of ethylene carbonate:ethyl methyl carbonate (EC:EMC) at different temperatures. After being swollen, the composite films prepared from PVDF modified with hydroxyl functional groups show higher adhesion strengths than the others due to their low uptake of the electrolyte solvent.

We have evaluated the potential use of thermoresponsive hydrogels based on N-isopropylacrylamide as actuators in microfluidic and lab-on-a-chip devices. This required fabrication of hydrogel actuators on the μm length scale, anisotropic swelling of the resulting materials, and control over the kinetics of the hydrogel volume phase transition. The fabrication procedure combined gel polymerization and casting techniques from the life sciences with more traditional semiconductor fabrication protocols for spin-coating, patterning, and etching. The actuator design used a PDMS membrane to separate the hydrogel actuator from the microfluidic channel and a separate reservoir for fluid to swell the actuator. As a result, the actuator could control flow for organic as well as aqueous solutions over a wide range of pH and ionic strength. The presence of a fixed substrate causes the gel swelling to be highly anisotropic, and the actuating motion is perpendicular to the substrate. The anisotropic swelling also limits the degree of swelling for the responsive hydrogel, and the total volume change is lower than the corresponding bulk materials by as much as an order of magnitude. The resulting actuators conform easily to the shape of the microfluidic channel, and the rate of the hydrogel response could be increased by using a series of semi-interpenetrating hydrogel networks. The microfluidic channels ranged in diameter from 180 to 380 μm, and the typical actuator was between 100 to 500 μm in diameter. The time scale of the actuator response was approximated by fitting with a single exponential (∼exp[−t/τ]). The time scale (τ) varied as a function of the actuator size and composition, ranging from 10 min to less than 10 s.

A membrane-based chiral separation system was developed by the modification of poly(vinylidene fluoride) (PVDF) ultrafiltration membranes with polyglutamate derivatives. Both physisorbed and chemisorbed poly(γ-benzyl- l -glutamates) (PBLG) were vapor-deposited on the membranes, and the resulting poly(amino acids) were modified through debenzylation or ester exchange reaction to produce poly( l -glutamic acid) (PLGA) and polyglutamates with triethylene glycol monomethyl ether side chains (PLTEG). The enantioselectivities for chiral α-amino acids (tryptophan (Trp), phenylalanine (Phe), and tyrosine (Tyr)) and chiral drugs (propranolol, atenolol, and ibuprofen) were determined by performing permeation cell experiments, with enantioselectivities ranging from 1.04 to 1.47. The selectivity of PLGA increased as its helical content increased, which occurred at high ethanol concentrations and at low pH. In addition, an increase in enantioselectivity was observed for chemically grafted polymers compared to physisorbed polypeptides. This may be attributed to an increase in molecular weight (MW) and density of the polymer chains, which may enhance the interaction between the chiral compounds and the surface-bound polypeptides.

Interface characteristics of polystyrene (PS) melts in free-standing thin films and on a graphite surface were investigated by molecular dynamics simulations employing an explicit all-atom force field. The calculated surface tension is in good agreement with experiment, which provides good support for the force field parameters employed. In the polymer/vacuum free-surface region, the density profile exhibits an enrichment of phenyl groups relative to the backbone alkyl groups at the outermost low-density free surface, but this free surface is followed by a layer of relatively depleted phenyls and enriched alkyls of ca. 7 A thickness. In the free surface, the phenyl-ring normal vectors and backbone chain vectors are both preferentially oriented along the film surface, in agreement with available experiments. At the polymer/graphite interface, the backbone chain vectors are strongly oriented along the graphite surface whereas the orientation distribution of phenyl-ring normal vectors exhibits two maxima along the nearly parallel (20°) and the perpendicular direction to the graphite-surface normal. A densely packed structure is formed at the PS-graphite interface, which strongly decreases the segmental chain mobility, in contrast to the enhanced segmental mobility in the free-surface region.

Pressure dependent excimer fluorescence and dynamic light scattering measurements are made on poly(styrene-b-ethylene propylene) (PSPEP) diblock copolymers that form micelles in the selective solvent n-heptane. We determine the critical micelle temperature (cmt) as a function of block copolymer concentration and applied pressure up to 200 MPa by monitoring the excimer emission band energy, which passes through a minimum at the cmt. The photophysical determination of cmt agrees with simple turbidity measurements taken under the same conditions. The hydrodynamic radius, as determined by dynamic light scattering, increases approximately 5% for a PSPEP diblock copolymer having PS Mw = 35 000 and PEP Mw = 61 000, denoted PSPEP 35 61 , with increasing pressure. At 298 K, the hydrodynamic radius decreases approximately 4% for the PSPEP diblock copolymer with PS Mw = 21 000 and PEP Mw = 66 000, denoted PSPEP 21 66 . These size changes are consistent with a negative volume change upon micellization for PSPEP 35 61 and a positive value for PSPEP 21 66 . For temperatures greater than 308 K, both block copolymers exhibit positive volume changes upon micellization. Using a simple corresponding states thermodynamic model, we relate the pressure effects to a decrease in the PS/heptane interaction parameter, i.e. an increase in the solvent quality with increasing pressure.

Diffusion coefficients of small oligosaccharides within high strength poly(ethylene glycol)/poly(acrylic acid) interpenetrating network (PEG/PAA IPN) hydrogels were measured by diffusion through hydrogel slabs. The ability of hindered diffusion models previously presented in the literature to fit the experimental data is examined. A model based solely on effects due to hydrodynamics is compared to a model based solely on solute obstruction. To examine the effect of polymer volume fraction on the observed diffusion coefficients, the equilibrium volume fraction of polymer in PEG/PAA IPNs was systematically varied by changing the initial PEG polymer concentration in hydrogel precursor solutions from 20 to 50 wt./wt.%. To examine the effect of solute radius on the observed diffusion coefficients, solute radii were varied from 3.3 to 5.1 A by measuring diffusion coefficients of glucose, a monosaccharide; maltose, a disaccharide; and maltotriose, a trisaccharide. Both the hydrodynamic and obstruction models rely on scaling relationships to predict diffusion coefficients. The proper scaling relationship for each of the hindered diffusion models is evaluated based on fits to experimental data. The scaling relationship employed is found to have a greater significance for the hydrodynamic model than the obstruction model. Regardless of the scaling relationship employed, the obstruction model provides a better fit to our experimental data than the hydrodynamic model.

The effects of various casting parameters were measured for films of novolacs (cresol-formaldehyde resins) containing pyrene groups either covalently bound to the polymer chain or physically dispersed in the novolac matrix. Aggregation of the dispersed pyrene was observed, as measured by excimer fluorescence and crystallite formation of the pyrene. The phase separation was enhanced in static-cast films over spin-cast films. Pyrene aggregation in films was also affected by the choice of casting solvent even though no significant solvent effects were observed for the tagged polymer or free pyrene in solutions of the casting solvents. A minimum in the plot of the excimer-to-monomer ratio for the films versus the dipole moment of the casting solvent occurred at about the dipole moment expected for the novolac. Pyrene aggregation appears to be more dependent on the interactions of the casting solvent and the host polymer rather than on the interaction of either with the pyrene itself.

The concentration dependence of micelle formation by a diblock copolymer of polystyrene-poly(ethylene propylene) (PSPEP) in n-heptane is examined by turbidity, viscosity and photon correlation spectroscopy (p.c.s.). The critical micelle concentration (c.m.c.) as determined by p.c.s. is estimated to be 0.002 wt%. At c = 0.003%, an unusually large hydrodynamic radius is found, in agreement with the anomalous increase in hydrodynamic size near the c.m.c. previously reported by Tuzar et al. in 1985. The average micellar weight obtained from turbidity measurements and the hydrodynamic radius for non-interacting micelles as measured by p.c.s. were relatively constant for c = 0.01–0.5%. A comparison of the second virial coefficients from the turbidity and viscosity measurements suggests that the relatively large Huggins constant of k H = 2.4, obtained from viscosity measurements, may be explained in terms of an excluded-shell model. At a critical concentration above 0.5%, an increasing amount of freely dispersed polymers is present in the solution. The driving force for this change in the free chain-micelle equilibrium is believed to arise from the concentration gradient of PEP segments between the micelle corona and the dispersed phase. The amount of free chains relative to micelles is found to increase from 0.7% up to 2% based on p.c.s. results. It is not clear, however, if the equilibrium is further shifted in favour of the free chains from 3 to 5%, since the weight fractions of free chains and micelles cannot be deduced from p.c.s. data at such high concentrations. However, turbidity and viscosity data seem to suggest that the relative amount of free chains at 3% is lower than that at 2%. At even higher concentrations (4–5%), the viscosity of the micelle solutions increases drastically, presumably because of the formation of a macrolattice.

Excimer fluorescence in aryloxyphosphazene cyclic trimers and polymers has been measured as a function of substituent, solvent and temperature. While only one type of excimer forming site (EFS) is possible in the cyclic trimers, the phosphazene polymers exhibit three possible types of EFS which are responsible for a blue shift and broadening of their excimer bands relative to the trimers. Excimer band assignment for intramolecular EFS formed between two aryloxy groups bonded to the same phosphorus atom (type I) is possible from a comparison of trimer and polymer spectra. The type II EFS, which results from intramolecular interactions between aryloxy groups appended to adjacent phosphorus atoms, is considered less favourable than the type III EFS, which is the intermolecular EFS. Increasing the size of substituents from poly[bis(phenoxy)phosphazene] (PBPP) to poly[bis(p-cresoxy)phosphazene] (PBCP) increases the steric effect of the side groups while changing the main chain conformation of the polymers. We observe a strong dependence of excimer formation on solvent and temperature in terms of the excimer-to-monomer intensity ratio, I D I M , excimer bandwidth and excimer band maximum position. The binding energies for excimer formation in dichloromethane were found to be 14.0 and 15.5kJmol−1 for PBPP and PBCP, respectively, and are considerably greater than those for type I EFS previously reported. The activation energy for PBPP in dioxane was found to be 2.6kJmol−1, whereas the corresponding activation energy for PBCP was not determined since no Arrhenius behaviour was observed. These results suggest that intermolecular excimer forming sites may be pervasive in the polymer solutions and are responsible for part of the diffusive and deactivation processes.

A series of pyrene end-labelled polystyrene (PS) samples having molecular weights ranging from 2000 to 40 000 have been synthesized anionically. Complete characterization data are provided. The end-labelling reaction is performed by coupling the living polystyryl anion with 1-bromobutylpyrene, with coupling efficiencies ranging from 72 to 90%. The resulting pyrene end-labelled PS has no ester linkage in the backbone, a common structure resulting from alternative end-labelling reactions proceeding via terminal hydroxyl units. This leads to a potential enhancement in thermal and hydrolytic stability; in fact, the samples have the same thermal stability as unlabelled PS. The glass transition temperatures (Tg) of our samples vary linearly with M −1 n , consistent with the Fox-Flory equation. However, the Tg values of the labelled PS samples are higher than the unlabelled ones with the difference widening as M n decreases. This difference is explained by a decrease of chain end mobility and hence of free volume caused by bulky end groups.

The effects of uniaxial and biaxial extension on the intrinsic fluorescence of poly(ethylene terephthalate) (PET) films have been investigated. A power law relationship, valid for both uniaxially and biaxially deformed samples, was found between the fluorescence emission at 368 nm and the planar extension, which is defined as the product of the extension ratios in the transverse and machine directions. Dimethyl terephthalate (DMT) model compound studies indicate that the fluorescent species in the PET films is not the monomeric unit. The dependence of emission intensity at 368 nm on orientation was greater when the films were excited with 300 nm light than with 340 nm. However, the absolute emission intensity was an order of magnitude larger when the excitation wavelength was 340 nm as compared to 300 nm. Transient fluorescence measurements show that the 368 nm emission has an average lifetime of 1.3 ns when using 340 nm excitation, and an average lifetime of 3.8 ns when exciting at 300 nm. A model incorporating energy migration in the non-crystalline region has been proposed to explain these results.

The effects of solubility parameter differences between the guest polymer and the host matrix and of guest molecular weight on the thermodynamic miscibility and chain configuration of poly(2-vinylnaphthalene) (P2VN) dispersed in polymeric or glassy solvent hosts are investigated using excimer fluorescence. For P2VN blends with a series of poly(alkyl methacrylates) in which the difference in the guest and host solubility parameters is minimized, the small increase in the excimer to monomer fluorescence ratio, I D I M , with increasing P2VN molecular weight can be rationalized by a one-dimensional energy migration model proposed by Fitzgibbon and Frank. The results indicate the possibility of small-scale phase separation or local coil contraction for P2VN with molecular weight greater than 21 000. As the thermodynamic interaction between the guest and polymeric host is altered towards immiscibility, I D I M can be related to the extent of intermolecular aggregation in the blend. To study the efficiency of intramolecular energy migration independent of intermolecular association and free of casting solvent effects, we chose a host system consisting of a series of monomeric glassy solvents. The rate of energy transfer is investigated by transient fluorescence and treated by a one-dimensional model developed by Fredrickson and Frank. The results are in excellent agreement with independently measured photostationary-state fluorescence ratios.

Polyimides are widely used in a variety of technological applications, from interlayer dielectrics in multilevel very-large-scale integrated circuits to matrix resins in high-temperature composite materials. Although data are available from most polyimide manufacturers on the electrical and physical properties of fully cured polyimide, few data are available concerning the precured or undercured polymer. Moreover, there is currently no convenient method available to monitor the degree of cure, which is determined by the highest cure temperature used in the thermal treatment of the polyimide film. In the present studies the degree of cure of a poly( N , N ′-bis(phenoxyphenyl)pyromellitimide)-type polyimide has been observed to have a profound influence on the fluorescence of films irradiated in the near-ultra-violet to visible regions. Excitation of two bands centred at 350 and 490 nm results in an emission band centred at 575 nm that undergoes a significant increase in intensity with cure temperature. We propose that this fluorescence behaviour is associated with an increase in local ordering or aggregation coincident with an increase in the final cure temperature, from 200 to 450°C. Conformational changes such as rotation around the N -phenyl bond are proposed to account for the change in morphology and fluorescence intensity with cure. In addition, the formation of a charge transfer complex between the dianhydride (acceptor) and diamine (donor) moieties is invoked to explain the low-energy absorption band that results in the observed fluorescence.

A low concentration of chloromethyl groups (up to ≈0.1% monomer unit) has been incorporated into monodisperse polystyrene (PS) at the pendant phenyl ring by reaction with chloromethyl ethyl ether and boron trifluoride-etherate in methylene chloride. This method is both easier and safer to carry out than the conventional chloromethylation procedure. If the reaction is done at a suitable concentration, the molecular weight and polydispersity of the polymer are unchanged after the reaction. The chloromethylated PS reacts readily with the caesium salts of carboxylic acid derivatives of naphthalene, anthracene, pyrene, 7-nitrobenzo-2-oxa-1,3-diazole (NBD) and the potassium salt of carbazole to yield labelled PS. At high polymer concentration the reaction gives insoluble gel product, and in very dilute polymer solution and high concentration of the ether or the catalyst, intramolecularly crosslinked ‘microgel’ is obtained. Preliminary measurements on the mutual and tracer diffusion coefficients of the NBD-labelled PS particles stabilized by a block copolymer in 20% toluene 80% methanol mixture reveal that the two diffusion coefficients are identical under our experimental conditions.