PMMA Melt Research Articles

Surface Characteristics of Poly(alkyl methacrylate)s from Molecular Dynamics Simulations Using All-Atom Force Field.

Molecular dynamics (MD) simulations of melt films of poly(alkyl methacrylate)s (PAMAs) with methyl, ethyl, and n-butyl substituents, respectively, have been performed using an all-atom model to investigate their surface and thin film properties. The applied all-atom force fields predict the bulk densities of PAMAs in good agreement with experiments. Moreover, predictions of the surface tensions of PMMA, PEMA, and Pn-BMA melts are in reasonably good agreement with experiments. The density profiles and orientational-order parameters of chain segments show atomic-scale characteristics in the air/polymer interfacial region. In the surface region, the backbone segments of PAMAs form a well-defined layer structure with the chain vectors oriented parallel to the surface, while the ester side-chains strongly segregate to the surface region and show perpendicular orientation to the surface, with the most pronounced surface segregation noted for Pn-BMA. Such surface segregations of chain segments make it difficult to apply a simple relationship between the cohesive energy density and the surface tension of polymers, for example, and should be taken into account in relating the surface/thin film characteristics to the bulk properties of polymers in general.

Electric Field Enhances Shear Resistance of Polymer Melts via Orientational Polarization in Microstructures.

In this paper, we studied the alteration of viscoelastic properties of a neat poly(methyl methacrylate) (PMMA), induced by an applied external electric field. The rheological properties of PMMA are measured using a rotational rheometer at elevated temperatures. The electric field effect on the shear resistance of the polymer was studied by examining rheological responses under difference experimental scenarios. We find that the external electric field can remarkably enhance shear resistance and prevent flow of PMMA melt, enabling it to behave more predictably at high temperatures. Dynamic rheological analysis illustrates that the external electric field speeds up the recovery of mechanical properties of the PMMA melt after large deformations, whereas the PMMA melt exhibits thixotropic behaviors. The recovery velocity is influenced by the strength of the electric field, specifically, and is found to be proportional to the electric field strength. Our experimental characterization may provide new evidence on the tuning mechanical properties of polymer melts via controlling segmental polarization.

On the universality in the extensional rheology of monodisperse polymer melts and oligomer dilutions thereof

The startup and steady extensional viscosities were measured on two narrow molar mass distributed (NMMD) poly(methyl methacrylates) (PMMA) diluted in 57% 2.1 kg/mole oligomer methyl methacrylates. The oligomer is short enough to be random configured and un-entangled though it is still a Kuhn chain. The weight-based average molecular weights of the PMMAs are 86 kg/mole and 270 kg/mole with polydispersites of 1.08 and 1.09 respectively. The extensional viscosities were in theoretical agreement with a constant ‘interchain pressure’ model, representing the maximal level of strain hardening in a Kuhn fluid. This has been observed for similar (styrene) oligomer diluted NMMD polystyrenes before, when the styrene oligomers were Kuhn chains. The original ‘interchain pressure’ model by Marrucci and Ianniruberto (Macromolecules 37(10):3934–3942, 2004), represents the minimal level of strain hardening in a Kuhn fluid. It has been shown previously to predict the extensional viscosities of NMMD polystyrene melts, and it is also in agreement with the extensional viscosities of the 86 kg/mole NMMD PMMA melt as well.

Grafted polymer chains suppress nanoparticle diffusion in athermal polymer melts.

We measure the center-of-mass diffusion of poly(methyl methacrylate) (PMMA)-grafted nanoparticles (NPs) in unentangled to slightly entangled PMMA melts using Rutherford backscattering spectrometry. These grafted NPs diffuse ∼100 times slower than predicted by the Stokes-Einstein relation assuming a viscosity equal to bulk PMMA and a hydrodynamic NP size equal to the NP core diameter, 2Rcore = 4.3 nm. This slow NP diffusion is consistent with an increased effective NP size, 2Reff ≈ 20 nm, nominally independent of the range of grafting density and matrix molecular weights explored in this study. Comparing these experimental results to a modified Daoud-Cotton scaling estimate for the brush thickness as well as dynamic mean field simulations of polymer-grafted NPs in athermal polymer melts, we find that 2Reff is in quantitative agreement with the size of the NP core plus the extended grafted chains. Our results suggest that grafted polymer chains of moderate molecular weight and grafting density may alter the NP diffusion mechanism in polymer melts, primarily by increasing the NP effective size.

Size-Dependent Particle Dynamics in Entangled Polymer Nanocomposites.

Polymer-grafted nanoparticles with diameter d homogeneously dispersed in entangled polymer melts with varying random coil radius R0, but fixed entanglement mesh size a(e), are used to study particle motions in entangled polymers. We focus on materials in the transition region between the continuum regime (d > R0), where the classical Stokes-Einstein (S-E) equation is known to describe polymer drag on particles, and the noncontinuum regime (d < a(e)), in which several recent studies report faster diffusion of particles than expected from continuum S-E analysis, based on the bulk polymer viscosity. Specifically, we consider dynamics of particles with sizes d ≥ a(e) in entangled polymers with varying molecular weight M(w) in order to investigate how the transition from noncontinuum to continuum dynamics occur. We take advantage of favorable enthalpic interactions between SiO2 nanoparticles tethered with PEO molecules and entangled PMMA host polymers to create model nanoparticle-polymer composites, in which spherical nanoparticles are uniformly dispersed in entangled polymers. Investigation of the particle dynamics via X-ray photon correlation spectroscopy measurements reveals a transition from fast to slow particle motion as the PMMA molecular weight is increased beyond the entanglement threshold, with a much weaker M(w) dependence for M(w) > M(e) than expected from S-E analysis based on bulk viscosity of entangled PMMA melts. We rationalize these observations using a simple force balance analysis around particles and find that nanoparticle motion in entangled melts can be described using a variant of the S-E analysis in which motion of particles is assumed to only disturb subchain entangled host segments with sizes comparable to the particle diameter.

The effect of flake diameter on the reinforcement of few-layer graphene–PMMA composites

The effect of the lateral dimensions of the graphene reinforcement on the physical properties of poly(methyl methacrylate) (PMMA) nanocomposites was studied. Electrochemically-exfoliated, few-layer graphene with average diameters of 5 μm (5-FLG) and 20 μm (20-FLG) were incorporated into a PMMA matrix at loadings from 0.5 to 20 wt.% by melt mixing using a twin screw compounder. The smaller graphene flakes were found to have only a slight effect on the rheological properties of the PMMA, whereas the larger flakes significantly increased the viscosity and dynamic moduli of the PMMA melt. Mechanical testing showed that the 5 μm flakes behaved in a similar way to short fibers and reinforced the PMMA matrix poorly compared to the 20 μm flakes, in line with previous predictions on the critical length of graphene. Significantly, these larger flakes gave a linear increase in the modulus with loading even at high loadings, without any of the detrimental aggregation effects seen in most other graphene systems.

Double Glass Transition Temperatures of Poly(methyl methacrylate) Confined in Alumina Nanotube Templates

Recently, confinement of polymers with different geometries has become a research hotspot. Here, we report the dramatic deviation of glass transition behaviors of poly(methyl methacrylate) (PMMA) confined in cylindrical nanopores with diameter significantly larger than chain’s radius of gyration (Rg). Fast cooling a PMMA melt in the nanopores results in a glass with one single glass transition temperature (Tg). But two distinct Tgs are detected after slow cooling the melt. The deviation in Tg could be as large as 45 K. This phenomenon is interpreted by a two-layer model. During vitrification under slow cooling two distinct layers are formed: a strongly constrained interfacial layer showing an increased Tg as compared to that of the bulk polymer and a core with a decreased Tg. By thermal annealing experiments, we find that these two Tgs are inherently correlated. In addition, the deviation of Tg for PMMA confined in nanopores reveals a dependence on molecular weight.

Origins of the Process-Parameter Effects in Fabricating Polymer Micromixers Using Microinjection Molding

Based on our previous work, the present work intends to reveal the origin of the process-parameter effects in fabricating polymer micromixers using microinjection molding. First of all, the effects of the process variables including the melt temperature, injection velocity, packing pressure and packing time, on replication quality of the micromixers were numerically investigated using Moldex3D software. The simulation results were found to agree well with the reported experimental ones, that’s, among the investigated variables, the melt temperature was the only significant factor influencing the replication quality of the micromixers. Moreover, the rheology behavior and the cooling behavior of the PMMA melt were analyzed and identified as the origins of the process-parameter effects.

Atomistic simulations of isotactic and atactic poly(methyl methacrylate) melts: exploring the backbone conformational space

Molecular dynamics simulations were performed separately on an isotactic poly(methyl methacrylate) (PMMA) melt and on an atactic PMMA melt. These simulations allow for a detailed atomistic exploration of the conformational space about the polymers' backbone at a temperature above the glass transition for both polymers, which is experimentally difficult to accomplish. In agreement with previous experimental and theoretical studies, we found the trans–trans backbone conformation to be the most energetically stable, followed by the trans–gauche conformations. Unique in this study is the ability to attribute how the underlying meso and racemic diad pairs contribute to the overall backbone population. Additional simulations were performed on methyl methacrylate, the compound that forms PMMA through radical reactions. These latter simulations help to validate our recently created force field for use in condensed-phase simulations.

Slow dynamics of the α and α′ relaxation processes in poly(methyl methacrylate) through the glass transition studied by mechanical spectroscopy

In this paper, low-frequency mechanical spectroscopy has been used to study poly(methyl methacrylate) (PMMA) melt around the glass transition temperature Tg for shedding light on its unique relaxation behaviors. The mechanical spectra show an asymmetrical broad structure with a maximum peak on the high-temperature side and a shoulder peak on the low-temperature side. The shoulder peak corresponds to the α relaxation due to the local segmental motion; while the maximum peak is assigned to the α′ relaxation due to the slow motion of longer chain segments or chains. The α relaxation mode has a stronger temperature dependence of relaxation time than the α′ mode, causing the α′ mode to merge with the α mode with decreasing temperature toward Tg. Time-temperature superposition (TTS) breaks down in the entire temperature range due to the different friction coefficients of the α and α′ relaxations. For only the α′ process, TTS is found to hold for PMMA melt, where the high-frequency decay of the mechanical loss decreases like a power law of the frequency with an exponent of −1/3. Moreover, compared with the dielectric results, the relaxation times of β and α relaxations from mechanical measurements are about one order of magnitude slower than those from dielectric experiments.

Low-Frequency Internal Friction Study on the Structural Changes in Polymer Melts

With the help of the low-frequency internal friction method, we investigate the structural properties of polymer melts, such as amorphous polystyrene (PS), poly(methyl methacrylate) (PMMA), and semi-crystalline poly(ethylene oxide) (PEO). An obvious peak of relaxation type is found in each of the internal friction curves. The peak temperature Tp follows the relation Tp ≈ (1.15 – 1.18) Tg for PS and PMMA melts, while it follows Tp ≈ 1.22Tm for PEO melt, with Tg being the glass transition temperature and Tm the melting temperature. Based on the analysis of the features of this peak, it is found that this peak is related to the liquid-liquid transition temperature Tu of polymer melts. Mechanism of the liquid-liquid transition is suggested to be thermally-activated collective relaxation through cooperation. This finding may be helpful to understand the structural changes in polymer melts. In addition, the internal friction technique proves to be effective in studying dynamics in polymer melts.

Biomimetic Fabrication of Vaterite Film from Amorphous Calcium Carbonate on Polymer Melt: Effect of Polymer Chain Mobility and Functionality

We demonstrate that amorphous calcium carbonate (ACC) thin film crystallization on poly(e-caprolactone) (PCL), poly(bisphenol A carbonate) (PC), poly(tert-butyl acrylate) (PtBA), and poly(methyl methacrylate) (PMMA) substrates is affected by the chain mobility and functionality of the polymers above and below their melting and glass transition temperatures. ACC films are transformed into metastable (004) vaterite films on PCL and PMMA melts and are transformed into thermodynamically stable calcite (110) films on PC and PtBA substrates at 100 °C. On the basis of our results, we suggest that, in the melt phase, polymer chains containing ester groups trigger the formation of oriented vaterite films through interactions between the ester functionalities and ionic species present in ACC. In contrast, less reactive ester groups in polymer substrates, as well as esters in the solid phase, were found not to affect the kinetic growth of CaCO3 crystals in ACC in air. A comparison of the ACC film crystallization pro...

Interfacial Properties of Asymmetric Polymer Mixtures

AbstractUsing a Monte‐Carlo simulation of a continuous space Rod Bead Model the interface properties of systems of flexible polymer chains with different sizes of monomers are investigated. An immiscible polymer blend in the strong segregation state is modeled by a double sandwich system of chains differing by an factor of two in the size of the beads and the interfacial tension is calculated by a virial theorem method. The simulation data are compared to self‐consistent mean field and experimental data. The results show that the simulation data agree very satisfactory with mean‐field results. The interfacial tension decreases for asymmetric systems in comparison to symmetric systems with comparable volume contents of monomers and interaction strengths due to a decrease of the effective interaction. The parameters of the investigated systems are close to the properties of PS, PMMA and PI melts. A comparison with experimental results yields a very good agreement with data for PS/PMMA and less satisfactory for PS/PI. Additionally to the interfacial tension we have studied the interfacial width, the deformation of polymer chains near the interface, distributions of chain ends, monomer densities and distributions of centers of mass of chains.Snapshot of a typical configuration for chains with different monomer sizes and equal number of monomers per chain.imageSnapshot of a typical configuration for chains with different monomer sizes and equal number of monomers per chain.

Uniaxial and Biaxial Extension Behavior of a Lightly Cross-Linked PMMA Melt at Constant Strain Rates

For a lightly cross-linked poly (methyl methacrylate) (PMMA) with a gel fraction of 0.65, experimental studies are made on swelling, dynamic viscoelasticity, and uniaxial and biaxial extension at constant strain rates. The values of average molecular weight between cross-links, Mc, (or between a cross-link and a trapped entanglement) are determined by four methods; swelling, the equilibrium modulus, the Mooney-Rivlin plot and the Young's modulus. These values are in the same order but somewhat smaller values are obtained from the latter two methods. The density of weakly attached or untrapped entanglement strands is evaluated from the plateau modulus of (G' −Geq), where G' and G eq are the storage and equilibrium moduli. The entanglement molecular weight, Me, evaluated by this method is certainly smaller than Mc, and is slightly larger than Me of uncross-linked atactic PMMA melt. In uniaxial extension, the stress growth coefficient shows stronger strain-hardening than that of uncross-linked PMMA melts with very high molecular weight component. On the other hand, in biaxial extension, the stress growth coefficient exhibits weak strain-softening followed by small upturn.

Molecular Weight Dependence of Viscosity Compensation Effect for Non-Newtonian Viscous Flow in Poly(methyl methacrylate) Glass

Molecular weight dependence of viscosity compensation effect for non-Newtonian flow in polymer glasses was studied using low and high molecular weight poly(methyl methacrylate)(PMMA) glass samples: the weight-averaged molecular weight MW was 82 × 103 for the former sample, whereas MW = 3380 × 103 for the latter. The compensation effect functions obtained from steady viscous flow in the samples were slightly different, yet the functions were almost the same straight line in a range of low activation energy values of molecular rearrangement. An experimental point obtained from a PMMA melt sample at temperatures much higher than the glass transition temperature was rightly on the straight line for the glasses. This implies that the structures of low and high molecular weight glass samples flowing at low temperatures far below the glass transition temperature and high strain rates are identical with the equilibrium structure of the melt at temperatures much higher than the glass transition temperature.

Effect of a filler on the rheological and mechanical properties of the liquid-crystalline polyester-poly(methyl methacrylate) blends

The rheological and mechanical properties of the blends of liquid-crystalline polyester (LCP) and poly(methyl methacrylate) (PMMA) filled with aluminum borate whiskers have been studied. It was established the combined action of reinforcing LCP and filler onto the property of PMMA matrix leads to marked reinforcing of PMMA. At 10% of filler and 30% of LCP, the tensile strength of PMMA increases by 30% and elasticity modulus by 110%, the processability being no worse. The viscosity of the blend PMMA + 30% LCP + 10% filler practically is the same as the PMMA melt viscosity at 220°C. With increasing concentration of LCP up to 30%, the filler effect in binary matrix becomes more essential. The possible reason is the preferential adsorption of LCP at the filler interface (surface segregation) and additional ordering of LCP near the surface, possible, due to additional stretching of nematic phase in the convergent flow zone. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 993–999, 2000

Reversal of recovery direction during creep recovery of polymer blends

At small stresses or strains, polymeric liquids show a linear viscoelastic behavior. This can be demonstrated in creep followed by creep recovery. If the creep period 0<t<t1 is long enough, the elastic portion Je(t) of the shear creep compliance is equal to the recovered creep compliance Jr(t’), the latter follows from the recovery of the shear strain for t≳t1 with the new time scale t’=t−t1. Both functions are monotonously increasing. Such a behavior, at least approximately, is shown in this paper for melts of polystyrene (PS) and poly(methylmethacrylate) (PMMA). However, when blends of these polymers are made with the PS dispersed in the continuous phase of PMMA, the relation Je(t)=Jr(t’) is no longer valid. Unexpectedly, the recovered creep compliance shows a maximum at which the recovery direction is reversed. The height of the maxima increases with the PS concentration. We attribute this surprising phenomenon to the action of the interfacial tension and the different retardation times of the PS and PMMA melts. When the PMMA is dispersed in the continuous phase of PS, such a reversal of the recovery direction is not observed.