Rheological characterization of cross-linked poly(methyl methacrylate)

Poly(methyl methacrylate) (PMMA) with various degrees of cross-linking were prepared from methyl methacrylate and a cross-linker, and the effect of dilution of the polymerizable mixture by a thermoplastic PMMA on the cross-linked PMMAs was evaluated. The rheological properties were characterized in linear viscoelasticity and in uniaxial extensional flow. A critical gel is formed at concentrations of the cross-linking agent neopentyl glycol dimethacrylate (NPG) of approximately 250 mol ppm both in the case of PMMAs, which are not diluted by an addition of thermoplastic PMMA to the monomer (Recipe-A), and of PMMAs, which were obtained by an addition of 25 wt% low molecular weight thermoplastic PMMA to the monomer (Recipe-B). Significant strain hardening is observed for concentrations of NPG at and above 100 mol ppm for PMMAs based on Recipe-A and for all PMMAs produced by Recipe-B. At the same NPG concentration of 30 mol ppm, PMMA produced by Recipe-A shows very little strain hardening, while PMMA produced by Recipe-B shows significant strain hardening. This is due to the difference in the molecular weight distribution: PMMA from Recipe-A is mono-modal with M _w /M _n = 2.5, while PMMA from Recipe-B is bimodal with M _w /M _n = 5.6. Surprisingly, the strain-hardening tendency is strongly increasing with increasing NPG concentration, and at the same NPG concentration, the strain hardening of PMMAs produced by Recipe-B is higher than that of PMMAs produced by Recipe-A. This difference can be attributed to the dilution effect of the (unreacted) thermoplastic PMMA in Recipe-B PMMAs. The elongational flow behavior was also analyzed by the Molecular Stress Function (MSF) model.     

Viscoelastic properties of semidilute poly(methyl methacrylate) solutions

Viscoelastic properties were examined for semidilute solutions of poly(methyl methacrylate) (PMMA) and polystyrene (PS) in chlorinated biphenyl. The number of entanglement per molecule, N, was evaluated from the plateau modulus, G _N . Two time constants, _s and ₁, respectively, characterizing the glass-to-rubber transition and terminal flow regions, were evaluated from the complex modulus and the relaxation modulus. A time constant _k supposedly characterizing the shrink of an extended chain, was evaluated from the relaxation modulus at finite strains. The ratios ₁/ _s and _k / _s were determined solely by N for each polymer species. The ratio ₁/ _s was proportional to N ^4.5 and N ^3.5 for PMMA and PS solutions, respectively. The ratio _k / _s was approximately proportional to N ^2.0 in accord with the prediction of the tube model theory, for either of the polymers. However, the values for PMMA were about four times as large as those for PS. The result is contrary to the expectation from the tube model theory that the viscoelasticity of a polymeric system, with given molecular weight and concentration, is determined if two material constants _s and G _N are known.     

Constant interchain pressure effect in extensional flows of oligomer diluted polystyrene and poly(methyl methacrylate) melts

The constant ‘interchain pressure’ idea has been addressed, to evaluate if it is an adequate quantitative assumption to describe the fluid mechanics of oligomer diluted entangled NMMD polymer systems. The molecular stress function constitutive framework has been used with the constant interchain pressure assumption. Furthermore, the maximal extensibility based on the number of Kuhn steps in an entanglement has been used based on the relative Padé inverse Langevin function. The model predictions agree with the extensional measurements on all previously published poly(methyl methacrylate)s and almost all published oligomer diluted NMMD polystyrenes. The only deviation is on the most diluted and largest molecular weight case of an 18% 1880 kg/mol polystyrene in oligomer diluent. In this case, the maximal extensibility is not needed.     

Rheology of critical LCST polymer blends: poly(styrene-co-maleic anhydrite)/poly(methyl methacrylate)

The shear rheology of a binary polymer blend exhibiting a lower critical solution temperature (LCST) phase diagram and a small dynamic asymmetry (difference of glass transition temperatures between its constituents) has been investigated in the vicinity of phase separation; it is a mixture of a random copolymer of styrene and maleic anhydrite and poly(methyl methacrylate). In the linear viscoelastic regime, the material functions are sensitive to phase separation, and the effects of critical concentration fluctuations, which dominate the mechanical response, are quantified, yielding both the binodal and spinodal curves. The weak dynamic asymmetry is apparently responsible for the reduced magnitude of the observed effects, compared to blends exhibiting much larger contrast in glass transition; therefore, this property affects to some degree the accuracy of the rheologically determined phase diagram. The steady shear properties are weakly sensitive to phase separation, and suggest that shear-induced demixing may be possible. They also indicate the importance of the amount of strain energy introduced to the blend in controlling the effects of flow on phase behavior.This investigation demonstrates that the universal effects of concentration fluctuations can be detected in LCST binary polymer blends, provided that some dynamic asymmetry exists, and further they can be quantified in order to characterize the interplay between rheology and thermodynamics of these systems.Dedicated to the memory of Professor Tasos C. Papanastasiou     

Effect of nanofillers on the phase separation and rheological properties of poly(methyl methacrylate)/poly(styrene-co-acrylonitrile) blends

The viscoelastic characteristics of the blends of poly(methyl methacrylate)/poly(styrene-co-acrylonitrile) (PMMA/SAN) were investigated at various temperatures below, near, and above the phase separation temperature. The investigated polymer system is characterized by a lower critical solution temperature. Rheological behavior of the blends in the region of a phase separation was compared with change of the light scattering intensity. The presence of nanofillers in the blend results in that the phase separation occurs at a higher temperature. At the isothermal conditions, the phase separation begins earlier and proceeds with a higher rate as compared with the same blend without filler. The results of the study show the considerable change of the viscoelastic characteristics of PMMA/SAN when the polymer system passes from the homogeneous state to the heterogeneous one. Such characteristics as the dependence of the storage modulus (G ^′) on the loss modulus (G ^″), the dependence of the loss viscosity (η ^″) on the dynamic viscosity (η ^′), the dependences of the complex viscosity (η*), and the free volume fraction (f) on the blend composition are the most sensitive to the phase separation. The phase separation affects the characteristics G ^′(ω), where ω is the frequency only in a low-frequency range. Temperatures of phase separation were estimated using dependence G ^′(T) at ω, which is the constant in the range of low frequencies.     

Ultrasonic investigation of interpenetrating networks of poly(methyl methacrylate) and polyurethane

Interpenetrating networks are the most recent development in polymeric blend materials. Due to the crosslinking of both the continuous and dispersed phases, a high degree of molecular mixing is achieved in these materials. Notwith-standing that poly(methyl methacrylate)-polyurethane (PMMA-PUR) interpene-trating and semi-interpenetrating networks have been extensively investigated by Meyer et al., ultrasonic relaxation technique has been applied here for the first time. These materials were found to be highly ultrasound absorbing.It is observed that ultrasonic absorption has a peak at a particular composition of PMMA-PUR interpenetrating network. The absorption coefficient increases with frequencyf. The absorption is of relaxational nature and is not due to the scattering of ultrasonic waves by the domains of the dispersed phase. At every composition of the interpenetrating network, the/f ² vs.f curve indicates the presence of a relaxation frequency below 2 MHz and that the absorption increases with the temperature at some compositions which indicates the presence of thermal relaxation. An attempt is made to relate the absorption with the relaxation of pendent groups of polyurethane in the continuous phase.     

Effect of maleic anhydride content on the rheology and phase behavior of poly(styrene-co -maleic anhydride)/ poly(methyl methacrylate) blends

 We investigated the thermo- rheological behavior of high glass transition, high molecular weight and small dynamic asymmetry blends of poly(styrene-co-maleic anhydride) (SMA) and poly (methyl methacrylate) (PMMA) with varying amounts of maleic anhydride (MA) content, namely 8 wt%, 14 wt% and 32 wt%, in the SMA component. The phase separation (binodal) temperature of each blend was determined rheologically using a combination of dynamic frequency and temperature sweeps in parallel plate geometry; it was marked by a change in slope of the elastic modulus and the occurrence of a peak in tan δ in temperature sweeps. Failure of the time-temperature superposition principle and observation of two peaks in the Cole-Cole plots corroborated these findings. The blends displayed lower critical solution temperature (LCST) behavior with the critical temperatures exhibiting a non-monotonic dependence on the MA content. From rheological and thermal measurements it was concluded that SMA/PMMA blends containing 14% MA were more miscible than those containing 8% or 32% MA, a finding attributed to the compositional dependence of the interplay between SMA-SMA and SMA-PMMA interactions in the different samples. MA also influenced the dynamic asymmetry and pretransitional concentration fluctuations. The phase diagrams corresponding to each blend were modeled using a two-parameter temperature dependent interaction parameter, based on the concept of generalized Gibbs free energy of mixing. The fitted values of interaction parameter were in good agreement with values calculated explicitly using the Flory-Huggins theory. Received: 16 February 2001 Accepted: 11 July 2001     

Method for generating and realizing replicates of randomly roughened surfaces,tested on poly methyl (methacrylate)

Experimental Techniques - In experimental studies the need of having replicates of specimens is crucial. This requirement becomes even more critical when performing statistical analysis in...     

Interactive effect of oxygen diffusion and volatiles advection on transient thermal degradation of poly methyl methacrylate (PMMA)

A transient, one-dimensional model has been presented to formulate the substantial role of polymer gasification in the early stages of fire growth. The present model comprises the interaction between the oxygen diffusion and the released volatiles on the rate of polymer gasification, when the polymeric sample is subjected to an external radiative source. The model also includes different mechanisms affecting the degradation process such as in-depth thermal and oxidative decomposition, in-depth absorption of radiation and heat and mass transfer in the both gas and solid phases. The results for two different radiative heat sources (17 and 40 kW m⁻²) are reported and yielded realistic results, comparing to the published experimental data. It was found that an increase in the oxygen concentration will lead to a considerable decrease in the surface temperature as well as significant increase of gasification rate at 17 kW m⁻²; nevertheless this effect is less apparent at 40 kW m⁻².     

Electrical fluctuations and entry flow instabilities during capillary flow of aqueous poly (ethylene oxide) solutions

Capillary flow of poly (ethylene oxide) solutions generates voltage fluctuations (noise) between electrodes placed on both sides of the capillary. The noise has a 1/f type spectral distribution, the value of increasing with (shear rate) to a limiting level. Within certain ranges, two sets of harmonic peaks appear in the spectra. It is demonstrated that these peaks are related to the frequency components of the instabilities in the entry flow region. The lower frequency set of harmonics corresponds to axial oscillations (pulsations) of the flow, while the higher frequency peaks are associated with the transverse oscillation of the stream lines in the vicinity of the entry. The corresponding frequencies were measured by visual counting and by spectral analysis of laser light transmitted through the entry region during injection of a coloured solution.The noise measurements were carried out using both platinum and reversible Ag/AgCl electrodes. In the latter case the streaming potential was also measured; its variation due to pressure fluctuations in the instable flow region appears to provide a plausible explanation of flow-induced noise phenomena as observed in elastic solutions.Dedicated to Prof. Dr. J. Schurz on the occasion of his sixtieth birthday.     

Electrical noise generated during capillary flow of aqueous poly(ethylene oxide) solutions

 The relation between the electrical noise generated during flow of aqueous poly(ethylene oxide) PEO solutions in three capillaries of different lengths and entry shape but the same diameter and the flow behaviour was investigated. Flow into the square cut off capillary entry was characterised by a low pressure loss regime followed at higher shear rates by a high pressure loss regime while flow into the trumpet shaped capillary end exhibited only the low pressure loss regime. The high pressure loss flow was associated with strong electrical noise generation. The results suggest that the noise generation mechanism includes a current source supplied by the flow through the capillary volume with an associated streaming potential and a modulation of the current by mixing and turbulence at the capillary entrance. The noise measuring technique can potentionally be used to study disturbances in the entry flow to capillaries. Received: 22 July 1996/Accepted: 9 June 1997     

Thermorheological properties near the glass transition of oligomeric poly(methyl methacrylate) blended with acrylic polyhedral oligomeric silsesquioxane nanocages

Two distinct oligomeric species of similar mass and chemical functionality (M _w≈2,000 g/mol), one a linear methyl methacrylate oligomer (radius of gyration R _g≈1.1 nm) and the other a hybrid organic–inorganic polyhedral silsesquioxane nanocage (methacryl-POSS, r≈1.0 nm), were subjected to thermal and rheological tests to compare the behaviors of these geometrically dissimilar molecules over the entire composition range. The glass transition temperatures of the blends varied monotonically between the glass transition temperatures of the pure oligomer (T _g=−47.3°C) and the pure POSS (T _g=−61.0°C). Blends containing high POSS contents (with volume fraction φ _POSS≥0.90) exhibited enhanced enthalpy relaxation in differential scanning calorimetry (DSC) measurements, and the degree of enthalpy relaxation was used to calculate the kinetic fragility indices m of the oligomeric MMA (m=59) and the POSS (m=74). The temperature dependences of the viscosities were fitted by the free-volume based Williams–Landel–Ferry (WLF) and Vogel–Fulcher–Tammann (VFT) framework and a dynamic scaling relation. The calculated values of the fragility from the WLF–VFT fits were similar for the POSS (m=82) and for the oligomer (m=76), and the dynamic scaling exponent was similar for the oligomeric MMA and the POSS. Within the range of known fragilities for glass-forming liquids, the temperature dependence of the viscosity was found to be similarly fragile for the two species. The difference in shape of the nanocages and oligomer chains is unimportant in controlling the glass-forming properties of the blends at low volume fractions (φ _POSS<0.20). However, at higher volume fractions, adjacent POSS cages begin to crowd each other, leading to an increase in the fractional free volume at the glass transition temperature and the observed enhanced enthalpy relaxation in DSC.     

Elastic and viscous properties of dilute solutions of poly(methyl)methacrylate in certain solvent/non-solvent mixtures

Investigation of certain rheological properties (viscosity, shear-elasticity, Spinnbarkeit, Weissenberg Effect) of solutions ofpoly(methyl)methacrylate shows that, for a given concentration of the polymer (e. g. 3%), large changes of the observed properties can be produced by altering the composition of the solvent and/or temperature. Such differences appear to be associated particularly with the magnitude of the stress relaxation time: when this is relatively large (presumably when conditions favour cohesion between the macromolecules), the polymer solutions exhibit markedly both Spinnbarkeit and the Weissenberg Effect.

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Zusammenfassung Die Untersuchung gewisser Theologischer Eigenschaften (Viskosität, Scherelastizität, Spinnbarkeit, Weissenbergeffekt) von Lösungen des Polymethylmethacrylates zeigt, daß für eine gegebene Konzentration des Polymeren (z. B. 3%) große Änderungen der beobachteten Eigenschaften durch Verändern der Zusammensetzung des Lösungsmittels und/oder der Temperatur hervorgerufen werden können. Diese Unterschiede scheinen besonders die Werte der Spannungsrelaxationszeit zu beeinflussen: Wenn diese relativ lang ist (vornehmlich, wenn die Bedingungen die Kohäsion zwischen den Makromolekülen untereinander begünstigen), zeigen die polymeren Lösungen ausgeprägt beides: Spinnbarkeit undWeissenbergeffekt.

     

Indentation lifetime of swollen poly (β-hydroxyethyl methacrylate) networks

Summary An original indentation lifetime method was used in the investigation of the ultimate properties of crosslinked poly(-hydroxyethyl methacrylate) gels swollen with water to equilibrium. The method consists in the determination of the time (indentation lifetime,) which is needed to produce a failure in a polymer sample by means of a thin cylindrical indentor under constant load. The curves obtained by plotting the indentation lifetimex in dependence on stress at various temperatures can be superimposed to yield a universal dependence; the temperature dependence of the shift factorsa _T obeys theWLF equation. It was found that thea _T values corresponded to the shift factors of the viscoelastic statical measurements at much shorter times.From the temperature dependence of the indentation lifetime, the valuesT _s from theWLF equation were determined for samples swollen to equilibrium and prepared at various concentrations of the crosslinking agent or diluent at polymerization. The difference found between the dependence of theT _s values on the concentration of the crosslinking agent and diluent and the same dependence determined in dry samples by measuring the creep compliance was elucidated. The non-monotonous dependence of the indentation lifetime values on stress was explained in terms of the specific deformation and stress geometry of the indentation test.

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Zusammenfassung Zur Untersuchung des Festigkeitsverhaltens im Wasser gequollener vernetzter Gele des Poly--Hydroxymethylmethacrylates wurde eine Methode der Penetrationslebensdauer verwendet. Die Methode beruht in der Bestimmung der Zeit (Penetrationslebensdauer), die zur Zerstörung eines Prüfkörpers durch einen konstant belasteten dünnen zylindrischen Indentor nötig ist. Die Kurven der Penetrationslebensdauer in Abhängigkeit von der Druckspannung, die verschiedenen Temperaturen entsprechen, können in eine gemeinsame Abhängigkeit superponiert werden; die Temperaturabhängigkeit der Temperaturreduktionsfaktorena folgt derWLF-Gleichung. Diea _T -Werte entsprechen den statischen viskoelastischen Temperaturreduktionsfaktoren bei viel kürzeren Zeiten.Für gleichgewichtsgequollene Proben, die mit verschiedenen Konzentrationen des Vernetzungs- bzw. Verdünnungsmittels bei der Polymerisation hergestellt wurden, wurdenT _s -Werte aus derWLF-Gleichung festgestellt. Es wurde der Verlauf derT _s -Werte in Abhängigkeit von der Vernetzungsmittelkonzentration diskutiert im Vergleich mit derselben Abhängigkeit, die bei trockenen Proben mit Hilfe der Kriechmessung gefunden wurde. Der amonotone Verlauf der Penetrationslebensdauerwerte in Abhängigkeit von der Spannung wurde der spezifischen Verformungs- und Spannungsgeometrie zugeschrieben.

     

Solidification behavior of the theta system 2-propanol/poly(n-butyl methacrylate) I. Influences of thermoreversible gelation on stationary flow

Zero shear viscosities, ₀, were determined by means of a magnetoviscometer for melts of poly(n-butyl methacrylate) (M = 8.7 to 450 kg/mol, T =53.5 to 200°C) and for concentrated solutions of the highest molecular weight sample in isopropanol (T = 34.8 to 131.5 °C). Master curves can be constructed in both cases if the reference temperature is set proportional to the gelation temperature of the particular fluid. Special intersegmental interactions (eventually leading to thermoreversible gelation) can above all be felt in ₀ (T) and in M _c , the critical molecular weight determined in plots of log ₀ vs. log M. As the temperature is lowered, the behavior changes from WLF to Arrhenius, and M _c declines considerably. The former observation is explained by analogy to the transition from fragile to hard glasses resulting from increased crosslinking. The latter effect can also be rationalized in terms of a physical network reducing the molecular weight that is required for the formation of entanglements.