Glassy Poly Research Articles

Intramolecular energy migration in polyesters from 2,6-naphthalene dicarboxylic acid: polarization of fluorescence in the polymers and in bichromophoric model compounds

The fluorescence has been measured in dilute solution and in glassy poly(methyl methacrylate) for polyesters in which 2,6-naphthalene dicarboxylate is the rigid unit, and (CH 2) m ( m=2–6) is the flexible spacer. Bichromophoric compounds in which the same flexible spacer connects two molecules of 2-naphthoate were also studied. The anisotropy in the rigid medium demonstrates the existence of intramolecular energy migration, which becomes more important as m decreases from 6 to 2. The Förster radius is ∼ 12 A ̊ in the bichromophoric compounds and 14 Å in the polyesters.

Effect of initial sample anisotropy on the solvent sorption kinetics of glassy poly(2-hydroxyethyl methacrylate)

When moving solvent fronts meet at the centre of a glassy polymer during sorption of solvent, making the sample entirely rubbery, sudden changes in the area, thickness and sorption rate typically occur. Examination of the sorption of water and methanol by glassy poly(2-hydroxyethyl methacrylate) showed that the magnitudes and directions of these changes can be readily modified by creating anisotropic stresses within the glassy polymer by controlled deformation of the sample prior to solvent sorption. However, the transport mechanism and sorption kinetics were not significantly altered by differing levels of initial sample anisotropy.

Synthesis and Nonlinear Optical Characteristics of Chromophore-Functionalized Polymers Having Chromophore-Centered Hydrogen-Bonding and Crosslinking Groups

ABSTRACTIn this study, we report the synthesis and characterization of a new, systematically designed class of second-order NLO polymers in which the appended chromophoric substituent is modified (hydroxylated) to allow direct chromophore involvement in hydrogen-bonding and crosslinking. Thus, the glassy polymer poly(p-hydroxystyrene) has been functionalized to varying levels with the N-(3-hydroxy-4-nitrophenyl)-(S)-prolinoxy (HNPP) high-β chromophore. The time-dependent second harmonic generation characteristics of thin films of this material have been studied at λ = 1.064 μm as a function of chromophore functionalization level, hydrogen-bonding capacity, and thermal crosslinking. These materials are found to possess high d33 values (up to 34 × 10−9 esu) and long characteristic SHG decay times (longer than a year). Analysis of the decay of the second harmonic signal shows that increasing hydrogen-bonding density substantially increases relaxation times; it is further shown that introduction of crosslinks is yet more effective in imparting SHG temporal stability to these polymeric nonlinear optical materials.

Solvent mixtures sorption in amorphous peek

The sorption kinetics and equilibrium isotherms of methylene chloride, n-heptane and of methylene chloride/n-heptane mixtures in glassy amorphous Poly(aryl-ether-ether-ketone) (PEEK) have been investigated. Ideal Fickian diffusion, anomalous non-ideal Fickian diffusion, “Case II” sorption mechanism and diffusion controlled swelling were observed depending on temperature, on solvent type and on external solvent activity. Gaschromatographic analysis performed on PEEK samples contacted with methylene chloride/n-heptane mixtures indicated that the presence of methylene chloride enhances n-heptane mobility and equilibrium sorbed amount.

Energetics of stress relaxation of glassy polymers at high pressure

AbstractKinetic changes of relaxation of the shift stress of glassy poly(methyl methacrylate) were studied when temperatures and pressures were changed in isobaric, isothermal, and isochoric conditions. In isochoric conditions the relaxation rate depends slightly on temperature. The volume of the polymer is the main parameter for the rate of stress relaxation. Isobaric and isochoric activation energies and activation vohime of stress relaxation were determined. The results are shown to satisfy the thermodynamic relation of the activation parameters of the relaxation process. The dimensions of elementary kinetic unit of the stress relaxation process were estimated.

Physical ageing of amorphous polymers. Theoretical analysis and experiments on poly(methyl methacrylate)

AbstractStructural relaxation of glassy poly(methyl methacrylate) (PMMA) was studied with several techniques: differential scanning calorimetry (DSC), volumetry by thermomechanical analysis (TMA) and mechanical spectroscopy. A theory is proposed in order to interpret the data in a unique theoretical framework. This theory is developed on the basis of two main physical assumptions: (i) the existence in amorphous polymers of quasipunctual defects whose concentration decreases during structural relaxation, and (ii) the occurrence of hierarchical correlation effects conditioning molecular mobility. As a first approximation a self‐consistent semiquantitative description of the results is obtained, i. e., the experimental results obtained for the different quantities on PMMA can be accounted for employing always the same set of model parameters. However, it seems that a significant difference exists between the relaxation times associated with the different experimental techniques.

Probing the mechanisms of drug release from hydrogels

The swelling and drug release kinetics in suspension polymerized glassy poly(hydroxyethyl methacrylate/ (PHEMA) beads have been studied in detail using model drugs of both high and low water solubilities: oxprenolol HCl (sol. in H 2O ≈ 77%) and diclofenac sodium (sol. in H 2O ≈ 2.65%). The current results verify our previous findings that the drug loading has a definite effect on the drug release mechanism from hydrogels. The initial swelling front penetration has been observed to behave more Fickian as drug loading increases. Such a transition can be considered as a change of relative importance of the diffusion process versus the polymer relaxation as a function of drug loading. The swelling fronts have also been observed to accelerate near the core. This is shown to be the natural outcome of the associated moving boundary problem in spherical geometry and not, as proposed by some authors, a super-Case II transport behaviour. In all cases, the swelling bead dimension goes through a maximum followed by a gradual approach to an equilibrium value during the drug release. In addition, a doublefront formation (a swelling front and a drug dissolution front) has been observed for diclofenac sodium during the drug release due to the low drug solubility. We have also shown that by considering the increase in hydrogel bead dimension due to swelling and the decrease in dimension due to drug release, the swelling maximum in the transient dimensional changes can be qualitatively predicted. The osmotic contributions due to the presence of the drug can then be estimated from the differences between the experimental and calculated transient dimensional changes.

PH dependent zero order release from glassy hydrogels: penetration vs. diffusion control

Release of low molecular weight solutes from glassy hydrogels is known to follow zero order kinetics. Enhanced swelling brought about by the polymer-penetrant interaction enables the release of bulkier drugs such as caffeine to take place at a constant rate. The release in both the cases has been attributed in the past to the case II transport controlled penetration of the surrounding medium. This work reports pH dependent release of theophylline and carbamazepine from glassy poly(2-hydroxyethyl methacrylate-co-4-carboxy styrene), P(HEMA-4CS) hydrogels. Zero order release observed at pH 11.0 is consistent with an increase in the diffusivity of the active ingredient due to the enhanced swelling rather than case II transport controlled penetration of the medium.

Energetics of stress relaxation in glassy polymers at high pressures

Kinetics of relaxation of the shear stress in glassy poly(methyl methacrylate) were studied in isobaric, isothermal and isochoric conditions. In isochoric conditions the relaxation rate depends only on temperature and polymer volume is the main parameter for the rate of stress relaxation. Isobaric and isochoric activation energies and activation volume of the stress relaxation were determined. The results are shown to satisfy the thermodynamic relation of activation parameters of the relaxation process. The size of the area in which the elementary stress relaxation process occurs in glassy poly(methyl methacrylate) was evaluated.

Gas sorption in poly(butylene terephthalate). A critical test of the influence of molecular gas data

AbstractTo test the molecular parameters concerning gas sorption in polymers, the concentration of CO2, N2O, CO, N2, CH4 and the noble gases Ne, He in glassy poly(butylene terephthalate) films (PBTP) has been studied gravimetrically with a recording microbalance at 25°C. The sorption isotherms exhibit downward curvature to the pressure axis. As neither solubility nor adsorption can explain the experimental results, analysis was carried out based on the dual‐sorption model: gas dissolution and microvoid filling are considered as independent sorption mechanisms. The parameters of the dual‐sorption model for the mentioned penetrants are determined. The results indicate that for parameter correlation the Lennard‐Jones potential parameters give a rough idea, but size exclusion of gases in small diameter microvoids is proposed and special chemical interactions must be considered.

Stress-strain curves of glassy poly(arylate sulphone) described in terms of rubber elasticity

Stress-strain curves of films prepared by casting polyblock poly(arylene sulphone) from various solvents and measured between 293 and 443 K have been interpreted on the basis of equations developed for describing rubber elasticity. The concept of molecular entanglements allows one to describe practically the whole course of the stress-strain curve of the polymer in the glassy state and to explain the effect of solvent on its fundamental mechanical characteristics.

The solubility of hydrogen in glassy poly(vinyl acetate) at elevated pressures

The solubility of H 2 in glassy poly(vinyl acetate) (PVAc) has been determined by a gravimetric technique at 5.0 and −5.0°C in the pressure range from 1 to 20 atm. The solubility is a nonlinear function of the gas pressure and can be described by the “dual-mode” sorption model. The solubility decreases with increasing temperature. The values of the solubility coefficient ( S) vary from a maximum of 6.5 x 10 −2 cm 3 (STP)/(cm 3 polym.-atm) at 1.5 atm and -5.0°C to a minimum of about 1.9 x 10 −2 (in the same units) at 19 atm and 5.0°C. The fact that the H 2/PVAc system exhibits dual-mode sorption behavior was anticipated from the observation of Meares that a plot of log S versus 1/ T, the reciprocal absolute temperature, exhibits changes in slope in the glass-transition region of the polymer. The possible effect of the size of penetrant gas molecules on dualmode sorption is discussed.

Zero-order release from glassy hydrogels. I. Enigma of the swelling interface number

In vitro release kinetics of structurally related benzoic acids and other active ingredients from glassy hydrogels are reported. It is shown that a zero-order release from swellable glassy poly(2-hydroxyethyl methacrylate) (PHEMA) hydrogels can be achieved by a judicious choice of the active ingredient. The criteria for the swelling controlled zero-order release proposed in the past have been examined. The equilibrium swelling interface number ( Sw e) is a rather conservative criterion. It is shown that swelling controlled zero-order release can be realized for values of the swelling interface number up to 0.6. The Deborah numbers for release ( De R) for the systems which follow zero-order kinetics are in good agreement with the predictions based on the time dependent diffusion coefficient to account for the role of the molecular relaxation processes.

Sorption and transport of organic vapors in poly[1-(trimethylsilyl)-1-propyne

Sorption kinetics and apparent equilibrium isotherms were determined gravimetrically at 35°C for organic vapors in glassy poly[1-(trimethylsilyl)-1-propyne] (PMSP). The sorption kinetics are characterized by diffusion coefficients on the order of 10 −8 cm 2/sec for the normal hydrocarbons (C 3-C 9). These diffusivities are more than one million times larger than corresponding diffusivities characterizing the transport of corresponding penetrants in conventional glassy polymers such as polystyrene, poly(vinyl chloride), and poly(methyl methacrylate). The sorption capacity of this polymer is also quite high compared with values observed for more conventional polymeric glasses. The sorption isotherms of the organic vapors in PMSP follow a generalized dual mode behavior which is often observed for vapors in polymeric glasses. Specifically, at low activities the isotherms are concave toward the activity axis, then become convex to the activity axis at higher activities. The transition separating these two portions of the isotherms does not become evident until the sorption level in PMSP reached more than 30 weight percent, compared with less than 10 weight percent for other glassy polymers. These unusual properties can be related to the low packing density of PMSP. However, the results also reveal that although the quantitative values of diffusivities and solubilities are extraordinary, the qualitative behavior regarding sorption and diffusion is consistent with comparable observations on conventional glassy polymers.

Reinforcement of elastomeric poly(dimethylsiloxane) by glassy poly(diphenylsiloxane)

Some novel approaches were taken to provide the improvements in mechanical properties that are almost always necessary to prepare a commercially useful elastomer from poly(dimethylsiloxane) (PDMS) [-Si(CH3)2O-]. The reinforcement was provided by poly(diphenylsiloxane) (PDPS) [-Si(C6H5)2O-], a hard glassy polymer, which was introduced into the PDMS by two rather different techniques. In the first, the PDPS was prepared separately by condensation polymerization of diphenylsilanediol and then solution-blended into the PDMS. In the second, it was generated by in situ polymerization of the same monomer absorbed into the PDMS network. The resulting materials were characterized by scanning electron microscopy and by stress-strain isotherms in elongation. At least under some conditions both techniques were found to be successful, leading to increases in ultimate strength by a factor of two or more.

Transition mechanism from elastic deformation to plastic flow in poly(methyl methacrylate)

AbstractSudden changes of compressive strain‐rate on glassy poly(methyl methacrylate) lead to the conclusion that the post‐yield state under a constant stress is a state of steady flow in the polymer. Non‐linearity between the stress divided by temperature and the logarithm of strain rate for this steady plastic flow can successfully be analyzed using the Eyring equation with structural factors variable depending on stress and temperature. This analysis gives a unique functional relationship between the activation entropy and the activation enthalpy, which agrees fairly well with that for the melt derived from the WLF equation. This agreement provides a direct evidence verifying structural change of the glass into liquid‐like structure and enables us to recognize the conformational arrangement as an essential structural parameter controlling molecular mobility. Another experimental relation between the Eyring factors—the activation volume and the activation enthalpy—permits us to estimate the magnitude of an elementary volume for the molecular movement in the melt as a function of temperature. Moreover, the experimental finding of the steady flow in the glassy polymer motivated us to propose a transition mechanism from elastic deformation to plastic flow in which the internal viscosity or the activation volume is introduced as a single parameter representing the transient state of the structure.

Evidence of dual-mode diffusion of small molecules in glassy poly(1-trimethylsilyl-1-propyne) from fluorescence photobleaching recovery

Evidence of dual-mode diffusion of small molecules in glassy poly(1-trimethylsilyl-1-propyne) from fluorescence photobleaching recovery

Angular-dependent photoselection and anisotropy of optical absorption for photochromic molecules in glassy poly(methyl methacrylate)

It is shown that films of poly(methyl methacrylate) which contain the thermally-stable photochrome 7,7a-dihydro-2,4,7,7,7a-pentamethylbenzo-[b]-furan-5,6-dicarboxylic anhydride II may be rendered optically dichroic using polarized laser light (λ = 514·5nm). The dichroism is produced by angular-dependent photoselection of photochromic molecules in the photoconversion of photochrome II to fulgide I. The absorption anisotropy has been measured as a function of the angle between the planes of polarization of the pump and probe beams, and it is found to be time-invariant for this photochrome/glassy polymer system at room temperature. It is suggested that the time-dependence of absorption anisotropy of thermally-stable photochromic molecules in host polymer or liquid media should provide a direct means of studying their reorientational motions, and should be applicable over the time-range 10-11 to 10+3 s.

Analysis of the permeation of CO 2 through glassy polymers

Dual-mode sorption and mobility parameters for the transport of CO 2 in glassy poly (ethylene terephthalate) were determined from the pressure dependence of the apparent diffusion coefficients, D a, below 1 atm at various temperatures. Experimental values of these diffusion coefficients were obtained from permeation time-lag measurements. Values of D a calculated by means of the above parameters are in excellent agreement with the experimental data. Moreover, the permeation time-lag curves calculated by the finite-difference technique in conjunction with the dualmode sorption parameters are consistent with the experimental time-lag curves. These results appear to support the validity of the dual-mode sorption model of gas transport in glassy polymers. However, equally good agreement between the calculated and experimental curves was obtained by using the apparent solubility and diffusion coefficient without the dual-mode sorption parameters. Similar results were obtained with poly(vinyl acetate)/CO 2 system at temperatures both above and below the glass transition temperature.

The effect of sorbed penetrants on the aging of previously dilated glassy polymer powders. Part III: The effect of exposure to lower alcohols on enthalpy relaxations in poly(methylmethacrylate)

Enthalpy relaxations in glassy poly(methyl-methacrylate) have been studied through the endothermic, sub-Tg, aging peak observed in differential scanning calorimetry (DSC) thermograms. Powder samples were swollen by exposure to high activity methanol vapor and then aged in vacuum and in the presence of low activities of methanol, ethanol, and n-propanol at 308 K. The position and size of the DSC aging peak, which developed during aging, were monitored as a function of aging time. The aging peaks which developed for the alcohol-aged samples were smaller than those observed for similar samples aged in vacuum. These results suggest that the presence of dissolved penetrant in the samples retarded or arrested the relaxations which were observed in otherwise identical experiments performed in vacuo. Contacting the powder samples with 0.05 activity n-propanol or 0.10 activity ethanol during aging also appeared to arrest the shift in the temperature of the peak with aging time, whereas aging in the presence of 0.10 activity methanol or 0.05 activity ethanol appeared to have no effect on the temperature of the aging peak. These results are qualitatively consistent with a complementary study of the effect of the presence of lower alcohols on the time dependence of sorption capacity.