Glassy Polymer Matrix Research Articles

Sigmoidal swelling profiles for temperature-responsive poly( N-isopropylacrylamide- co-butyl methacrylate) hydrogels

Temperature-responsive poly( N-isopropylacrylamide- co-butyl methacrylate) gels exhibit “on—off” regulation of drug release in response to temperature. In a previous study, swelling kinetics of these gels from deswollen to swollen states at several temperature were investigated. It was demonstrated that the swelling behavior of the gel changed at various temperatures, yielding several patterns of drug release profiles. At 20°C, gel swelling increased with time, which was explained using a Case-II transport mechanism. In this mechanism, the glassy polymer matrix core acts to suppress the swelling of the outer region when swelling forces dominate. By reducing the experimental temperature to 10°C and utilizing the greatly enhanced hydration of polymer chains after disappearance of the glassy core, a sigmodal swelling pattern gives rise to novel drug release profiles. In this study, these swelling mechanisms have been verified in detail by theoretical analysis. The existence of a swelling front was confirmed by observation of the colored gel using a dye. When the thickness of gel was changed, the acceleration of swelling was delayed with increasing thickness, and the acceleration times agreed with theoretical values predicted from the model. The observed changes in diameter and thickness of the gel also supported the model. This results demonstrate validity of the model presented with the previous paper.

Guest-Host Polymers for Nonlinear Optics

Abstract Much recent interest in second-order nonlinear optical materials and devices has focussed on poled polymer films, which derives mainly from their large nonlinear optical coefficients, ease of fabrication, and high optical quality. Progress has been rapid in producing stable, efficient materials, and in building demonstration devices. The second-order nonlinear optical properties arise from the orientational order induced in a collection of highly nonlinear molecules incorporated in a glassy polymer matrix. In this paper, the physics of alignment is presented, as well as progress in the development of new polymeric materials. Electro-optic devices have been fabricated and tested confirming the promise of polymeric nonlinear optical materials. An approach to phase-matched second harmonic generation which is highly compatible with molecular-polymer materials based on anomalous dispersion in nonlinear optical molecules is also presented.

Conductance and relaxations in the glassy and rubber states of aqueous poly( vinyl pyrrolidone): A cryofixation medium

AbstractThe dielectric permittivity and loss of poly(vinyl pyrrolidone), molecular weight 40,000, containing 40% (by weight) water have been measured over the temperature range 77–325 K and frequency range 12 Hz to 0.1 MHz. A prominent relaxation due to rotational diffusion of water molecules in a hydrogen‐bonded structure occurs at T < Tg (237 K). The half‐width of the dipolar relaxation spectra is 2.27 decades and is temperature independent, which is strikingly different from the corresponding features of pure polymers. It is concluded that H‐bonded amorphous solid water persists in the glassy polymer matrix and that the H‐bonded structure contains the pyrrolidone side groups of the randomly oriented chain. The relaxation peak at T near Tg is masked by a large dc conductivity which, when expressed in terms of electric modulus, has a spectrum of half‐width 1.37 instead of 1.14 decades expected for dc conductivity alone. The contribution from dipolar reorientation in the glass‐rubber range of the PVP‐H2O solution is smaller than that in its sub‐Tg relaxation.

Free‐radical polymerization of methyl methacrylate at very high conversions, 2. Study of the kinetics of initiation by benzoyl peroxide

AbstractThe initiation kinetics of the free‐radical polymerization of methyl methacrylate at very high monomer conversions was investigated at 80, 100, and 120°C. Using a HPLC (high performance liquid chromatography) analytical technique with UV detection, the consumption of both monomer and initiator as well as the formation of reaction products of the latter could be followed. It was found that (besides very small amounts of biphenyl and benzoic acid) phenyl benzoate is the main decomposition product. The experimental results were analyzed quantitatively with the help of a reaction scheme which takes into account the most important pathways of that complex chemical reaction system. Thus, it was found that, due to the very strong cage effect in the present polymerization system, the experimentally accessible (apparent) first order rate constant of initiator decomposition is substantially lower than that for the primary cleavage of a benzoyl peroxide molecule. Efficiencies of chain initiation were calculated to be in the range of 0,1 to 0,2 in the (nearly) glassy polymer matrix.

A Generalized Model for Swelling‐Controlled Release Systems

A generalized model has been developed for simultaneous transport of an active agent and an organic penetrant in a planar glassy polymer matrix. The active agent diffuses out of the matrix undergoing macromolecular chain relaxation and volume expansion due to absorption of the organic penetrant from the environment into the matrix. The swelling behavior of the polymer is characterized by a stress-induced drift velocity term "v" corresponding to the so-called case-II velocity. The change of volume due to the relaxation phenomenon is assumed instantaneous. The model incorporates convective transport of the two species induced by volume expansion and by stress gradient. The proposed phenomenological model reduces to a number of special cases depending on polymer-penetrant system behavior.

Diffusion of organic vapors at low concentrations in glassy PVC, polystyrene, and PMMA

Diffusion coefficients of various C 1 to C 6 organic vapors, at concentrations 0.5 wt. percent, have been determined by gravimetric sorption rate measurements on emulsion and suspension-polymerized powder samples of PVC, polystyrene, and PMMA. Fickian diffusion kinetics were observed at the lowest concentrations, with a second-stage, relaxation-controlled sorption appearing at higher concentrations. In conjunction with published data for diffusivities of fixed gases in these polymers, the results indicate that diffusivity decreases exponentially, and that diffusion activation energy ( E D) increases linearly, with increasing diameter of “spherical” penetrant molecules (e.g., the noble gases, CH 4, SF 6, CCl 4, and neopentane). Much of the observed scatter in these correlations is attributable to uncertainty in the molecular diameters. For C 4 and larger n-alkanes and other elongated or flattened molecules, diffusivities are higher, and E D lower, than for spherical molecules of similar molar volume. This finding suggests that anisometric molecules are oriented and move along their long axes during diffusion through the glassy polymer matrix. Correlations of diffusivities with molecular dimensions suggests that transport of anisometric molecules is governed by a diameter smaller than the mean (equivalent sphere) diameter but larger than the minimum dimension of their extended-chain conformation. Among the three polymers studied, diffusivity of each penetrant, at a given temperature, decreases in the order polystyrene > PVC ≥ PMMA.