Dielectric relaxation in concentrated solutions of poly(methyl methacrylate)

Measurements of dielectric relaxation have been made for a series of solutions of poly(methyl methacrylate) in toluene. A range of concentrations between 2% and 20% was used for a number of polymers of differing molecular weights. The dispersion was studied over the frequency range 10³-10⁶ Hz for temperatures between ?90°C and +20°C. It has been found that both the distribution of relaxation times and the activation energy increase with increasing concentration. The magnitudes of the effects vary with molecular weight. A significant observation is that the increase in dielectric constant with concentration is not linear.     

Compatibility and crystallization in poly(ethylene oxide)-poly(methyl methacrylate) semi-interpenetrating polymer network

Differential scanning calorimetry together with dynamic mechanical analysis were employed to investigate the crystallinity and the miscibility in poly(ethylene oxide)/crosslinked poly(methyl methacrylate) semi-IPN (interpenetrating polymer networks). The crystallinity of poly(ethylene oxide) in the semi-IPN is found to depend on the crosslink density of PMMA as well as the overall content of PEO. Of special interest is that an increase in the crosslink density tends to increase the crystallinity contrary to our expectation, indicating crystallization and phase separation may proceed simultaneously during IPN formation. The investigation of glass transition behaviors with dynamic mechanical analysis suggests phase separation (i.e., there exist two amorphous phases: one PEO-rich phase, the other a PMMA-rich phase). © 1993 John Wiley & Sons, Inc.     

Surface structure relaxation of poly(methyl methacrylate)

Surface structure relaxations caused by temperature changes at the free surface of poly(methyl methacrylate) were studied using IR-visible sum-frequency generation (SFG). A polarization-rotating technique was introduced to enhance the sensitivity of SFG for monitoring the surface structure relaxations during a cooling process. A new surface structure relaxation was observed at 67 degrees C. This temperature does not match any known structure relaxation temperatures for the bulk and is 40 degrees C below the bulk glass transition temperature. As expected for a free-surface phenomenon, the surface relaxation temperature was found to be independent of film thickness in the range of 0.1-0.5 microm.     

Polyepichlorohydrin polyurethane/poly(methyl methacrylate) interpenetrating polymer networks

Polyurethane (PU) based on polyepichlorohydrin/poly(methyl methacrylate) (PECH/PMMA) interpenetrating polymer networks (IPNs) was synthesized by a simultaneous method. The effects of composition, hydroxyl group number of PECH, NCO/OH ratio and crosslinking agent content in IPNs were investigated in detail. Some other glycols, such as poly(ethylene glycol), poly(propylene glycol) and hydroxyl-terminated polybutadiene, were also used to obtain PU/PMMA IPNs. The interpenetrating and fracture behaviors of the IPNs are explained briefly.     

Dehydration-induced structural relaxation effects in poly(methyl methacrylate)

The Thermally Stimulated Depolarization (TSD) dielectric technique and Dielectric Relaxation Spectroscopy (DRS) have been used in order to investigate aging phenomena in poly(methyl methacrylate) (PMMA). Earlier TSD studies on amorphous PMMA report peculiar dielectric relaxation signals within the range of the glass transition (at ∼378 K) and the secondary relaxation (∼230 K). In the present study, an intense TSD current relaxation band maximizing around 310 K is tentatively attributed to the molecular mobility due to a residual free volume below the glass transition temperature, T_g, that allows structural recovery at the free volume released from the desorption of H₂O molecules during evacuation. Limited motions in the main backbone provoke dipole (re)orientation of the ester carbonyl pendant groups with an activation energy E=0.85±0.05 eV, being responsible for the latter dielectric relaxation effect. Alternative attributions based on the short-range jump relaxation of electric charges and boundary effects are also discussed.     

Thermodynamic characterization of interpenetrating polymer networks of poly (carbonate-urethane) and poly (methyl methacrylate)

Crosslinked poly (methyl methacrylate) and poly (carbonate-urethane) as well as full interpenetrating network on their base (IPN) were characterized by precise heat capacity measurements in the temperature interval 1.2–150 K. As judged by the positive sign of the excess Gibbs free energy in the whole temperature interval above 30 K, the apparently single-phase state of IPN is thermodynamically unstable; however, its kinetic stability is ensured by permanent chamical crosslinks prohibiting the incipient phase separation. © 1994 John Wiley & Sons, Inc.     

Dielectric relaxation behavior of poly(methyl methacrylate) under high‐pressure carbon dioxide

An in situ dielectric measurement for atactic poly(methyl methacrylate) (at‐PMMA) was performed under high‐pressure CO₂ under various pressures and temperatures. The at‐PMMA has the acetate side group with a large dipole moment. In the glassy state, a local relaxation process (β‐process) can be observed using dielectric measurement. In the rubbery state, the micro‐Brownian motion of main chain (α‐process) occurs, and the β‐process changes into αβ‐process coordinated with the α‐process. The dielectric loss (ε″) spectrum of at‐PMMA in the glassy state is asymmetric because of the density fluctuation for the amorphous structure. The loss peak frequency shifted to higher frequencies, and the relaxation strength increased with increasing CO₂ pressure. In the glassy state, the shape of ε″ spectrum became more symmetric with increasing CO₂ pressure. These show that the molecular mobility enhanced by the plasticization effect of CO₂ allows the dipolar side groups in the high‐density region to contribute to the relaxation process. We also found that the apparent activation energy decreased under high‐pressure CO₂. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2951–2962, 2005     

Melting point depression in poly (ethylene oxide)-poly (methyl methacrylate) blends

Thermo-optical analysis of solution cast mixtures of poly (ethylene oxide) (POE) and poly (methyl methacrylate) (PMMA) has been carried out. Melting point depression was observed for increasing proportion of PMMA in the mixture. An analytical expression appropriate to the crystallineamorphous polymer pair has been applied to explain the melting point depression in terms of thermodynamic quantities. From this expression, the interaction parameter for the mixture has been evaluated. The influence of $\text{M}$_w of POE on the binary interaction parameter has been studied.     

Study of thermal aging effect on space charge in poly(methyl methacrylate)

Charges evolution in poly(methyl methacrylate) (PMMA) samples under thermal aging effect has been studied by means of two complementary techniques, thermal step method (TSM) and thermally stimulated depolarization current (TSDC). For the first method, measurements reveal that injected charges, whose quantity is found depending on the number of applied temperature cycles, remain close to the surface sample. TSDC measurements have been carried out for different temperatures ranging from 25 °C to 140 °C. Three distinguishable dipolar relaxations (β₁, β₂ and α) have been highlighted. In the same way, the presence of polarization and injected charges has been confirmed. In support of electric characterization, X-ray reflectometry has been used. The obtained results equally emphasized the thermal aging effect on the material.     

Interpenetrating polymer networks of poly(dimethyl siloxane–urethane) and poly(methyl methacrylate)

Simultaneous IPNs of poly(dimethyl siloxane-urethane) (PDMSU)/poly(methyl methacrylate) (PMMA) and related isomers have been prepared by using new oligomers of bis(β-hydroxyethoxymethyl)poly(dimethyl siloxane)s (PDMS diols) and new crosslinkers biuret triisocyanate (BTI) and tris(β-hydroxylethoxymethyl dimethylsiloxy) phenylsilane (Si-triol). Their phase morphology have been characterized by DSC and SEM. The SEM phase domain size is decreased by increasing crosslink density of the PDMSU network. A single phase IPN of PDMSU/PMMA can be made at an M_c = 1000 and 80 wt % of PDMSU. All of the pseudo- or semi-IPNs and blends of PDMSU and PMMA were phase separated with phase domain sizes ranging from 0.2 to several micrometers. The full IPNs of PDMSU/PMMA have better thermal resistance compared to the blends of linear PDMSU and linear PMMA. © 1993 John Wiley & Sons, Inc.     

Diffusion of alcohols and relaxation in poly(methyl methacrylate): effect of thermal history

The sorption behavior of alkyl alcohols in poly(methyl methacrylate) is analyzed after predetermined annealing treatments of the polymer matrix. Weight uptake, penetration depth and sample dimensions are reported versus time. Dimensionless parameters are used, through which master curves are obtained for both weight uptake and the area perpendicular to the main penetration direction. Contrary to what is observed in polystyrene—n-alkane systems, in the present case either the specific plasticization of the penetrant must be accounted for, or a non-localized swelling model must be used to explain the behavior.     

Synthesis and characterization of poly (n-butyl acrylate)-poly (methyl methacrylate) latex interpenetrating polymer networks by radiation-induced seeded emulsion polymerization

A series of latex interpenetrating polymer networks (LIPNs) were prepared via a two-stage emulsion polymerization of methyl methacrylate (MMA) or mixture of MMA and n-butyl acrylate (n-BA) on crosslinked poly(n-butyl acrylate)(PBA) seed latex using ⁶⁰Co γ-ray radiation. The particles of resultant latex were produced with diameters between 150 and 250 nm. FTIR spectra identified the formation of crosslinked copolymers of PMMA or P(MMA-co-BA). Dynamic light scattering (DLS) showed that with increasing n-BA concentration in second-stage monomers, the particle size of LIPN increased. Transmission electron microscope(TEM) photographs showed that the morphology of resultant acrylate interpenetrating polymer network (IPN) latex varied from the distinct core-shell structure to homogenous particle structure with the increase of n-BA concentration, and the morphology was mainly controlled by the miscibility between crosslinked PBA seed and second-stage copolymers and polarity of P(MMA-co-BA)copolymers. In addition, differential scanning calorimeter (DSC) measurements indicated the existence of reinforced miscibility between PBA seed and P(MMA-co-BA)copolymer in prepared LIPNs.     

Characterization and degradation of poly(methylphenylsiloxane)–poly(methyl methacrylate) interpenetrating polymer networks

Poly(methylphenylsiloxane)–poly(methyl methacrylate) interpenetrating polymer networks (PMPS–PMMA IPNs) were prepared by in situ sequential condensation of poly(methylphenylsiloxane) with tetramethyl orthosilicate and polymerization of methyl methacrylate. PMPS–PMMA IPNs were characterized by infrared (IR), differential scanning calorimetry (DSC), and ²⁹Si and ¹³C nuclear magnetic resonance (NMR). The mobility of PMPS segments in IPNs, investigated by proton spin–spin relaxation T₂ measurements, is seriously restricted. The PMPS networks have influence on the average activation energy E_a,av of MMA segments in thermal degradation at initial conversion. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1717–1724, 1999     

Time-domain dielectric relaxation of poly(methyl methacrylate) and nitrile rubber

The time-domain dielectric response (after-effect) of poly(methyl methacrylate) (PMMA) and nitrile butadiene rubber (NBR) to a voltage step (100 V) was measured at varying temperatures. From the variation of the sample capacitanceC with time, we determined the ratioF _d/C, withF _d=(dC/dlnt)_max and C denoting the difference between the initial and the extrapolated equilibrium capacitance values. For PMMA around room temperature (RT) this ratio assumed values similar to those reported for mechanical stress relaxation. With NBR, such values were observed only at temperatures significantly below RT. A modified Kohlrausch-Williams-Watts (stretched exponential) function provided a good fit to the measuredC(t) data.     

Thermal degradation of poly(vinyl bromide), vinyl bromide-methyl methacrylate copolymers,and poly(vinyl bromide)-poly(methyl methacrylate) blends

Degradation behavior has been compared for PVB, five VB-MMA copolymers which span the composition range, PMMA, and PVC by using thermogravimetry in dynamic nitrogen and thermal volatilization analysis (TVA) under vacuum for programmed heating at 10°C/min. Volatile products have been separated by subambient TVA and identified. PVB is substantially less stable than PVC but shows inmost respects analogous degradation behavior. The introduction of VB into the PMMA chain leads to intramolecular lactonization with release of methyl bromide at temperatures a little above 100°C; after this reaction is complete, however, the polymer is more stable toward volatilization than PMMA. Copolymers with moderate and high VB contents also lose hydrogen bromide. Carbon dioxide is a significant product at intermediate compositions. The variation of product distribution with copolymer composition is discussed in relation to the several reactions involved and comparisons are made with VC-MMA copolymers. PVB-PMMA blends snow some features of degradation behavior in common with the PVC-PMMA system but also very important differences. The effect of PVB is only to stabilize the PMMA; the mechanism is discussed. The role of PVB as an additive and VB as a comonomer for fire-retardant PMMA compositions is briefly considered in relation to earlier studies.     

Effect of kinetics on morphology and mechanical properties of poly(allyl diglycol carbonate)-poly(urethane) interpenetrating polymer networks

Simultaneous interpenetrating polymer networks (SINs) of polyallyl diglycol carbonate (ADC) and polyurethane (PU) were prepared by differing modes of synthesis. The kinetics of the network formation of each constituent component was investigated by gel time studies and infra-red spectroscopy. The effect of different rates of network formation of each component on the morphology and mechanical properties were studied by transmission electron microscopy (TEM) and scanning electron microscopy (SEM), dynamic mechanical analysis (DMA), stress-strain, and single edge notch tension. TEM and DMA studies showed a two-phase separated morphology. The extent of phase eparation was dependent on the relative rate of formation of component networks. Thus, simultaneous gelation of both networks showed a fine morphology and exhibited improved toughness over neat ADC resin.     

Fourier transform infrared study of orientation and relaxation in poly(methyl methacrylate)

Infrared measurements of the dicroic ratio of poly-(methyl methacrylate) absorption bands provide a valuable method for determining the orientation as well as the relaxation of chains during stretching. Different strain rates and temperatures of stretching were used. Orientation relaxation was determined and a master curve was obtained at a reference temperature T_O = 135°C. The master curve shows that orientation relaxation behaves similarly to mechanical relaxation.     

Brillouin scattering of oriented films of poly(vinylidene fluoride) and poly(vinylidene fluoride) -poly( methyl methacrylate) blends

Temperature dependent Brillouin scattering studies of PVF₂ films stretched to various stretch ratios have been carried out. Elastic constants for unstretched and stretched films have been obtained as functions of temperature. The elastic constant C₃₃ of the stretched films has a greater temperature dependence than that of unstretched films. To elucidate the effect of the surrounding amorphous matrix on the Brillouin spectrum of semicrystalline PVF₂ film, we carried out Brillouin scattering studies of films made from blends of PVF₂ and PMMA.