Session E42: Bi-Component Systems: Composites and Blends

Broader Understanding of Multiple Component Dynamic Processes in Miscible Polymer/Polymer Blends

Utilizing two different experimental techniques, isothermal frequency sweeps and isochronal temperature sweeps, in broadband dielectric spectroscopy can allow for the identification of multiple processes derived from the same relaxation mechanism in certain polymer/polymer blends. A study of poly(vinyl methyl ether) (PVME) in bulk, miscible blends with polystyrene (PS) gives evidence of two separate relaxation processes associated exclusively with the segmental dynamics of PVME; the $\alpha_{0}$ process from the temperature sweep, related to average segmental dynamics, and the $\alpha $' process from the frequency sweep, related to relaxations confined within ``frozen'' domains. The appearance of multiple processes is driven by compositional heterogeneity, mainly chain connectivity and concentration fluctuation effects. Analysis of the breadth and intensity of the dielectric loss curves gives insight into the structure and thermodynamics of the blend, which in turn can explain temperature and composition dependent dynamic trends. These results are contrasted with other miscible blend systems, polyisoprene (PI)/poly(4-tert-butylstyrene) (P4tBS) and polyisoprene (PI)/polyvinyl ether (PVE).   

Thermal and Mechanical Properties of Poly(methyl methacrylate)/Poly(vinylidene fluoride-r-hexafluoro propylene) Blends

Poly(vinylidene fluoride), PVDF, is a highly crystalline and rigid polymer, and is used in many applications where chemical inertness, resistance to various solvents and environmental degradation are required. Copolymerization of PVDF with hexafluoropropylene (HFP) tends to decrease the amount of crystallinity, thereby, resulting in a more flexible polymer, which provides new applications for PVDF. Various studies were undertaken to investigate the structure and properties of miscible blends of PVDF and poly(methyl methacrylate), PMMA; however, no studies were performed on the blends of P(VDF–HFP) random copolymers and PMMA. In the current study, we investigate the miscibility, and thermal and mechanical properties of P(VDF–HFP)/PMMA blends via differential scanning calorimetry, thermogravimetric analysis, and nanoindentation. Results indicate that increasing PMMA concentration leads to decreased crystallinity and shifting of the crystallization onset temperature during cooling to lower values.   

Engineering thermal conductivity in polymer blends

Weak inter-chain bonding in polymers is believed to be a bottleneck for both thermal conductivity and mechanical strength. Most polymers have low thermal conductivity (\textasciitilde 0.1 W/mK), hindering their performance in applications for which thermal management is critical (e.g., electronics packaging). In this work, we use computational methods to study how hydrogen bonding between polymer chains as well as water content can be used to engineer thermal transport in bulk polymers. We examine how changes in the number of hydrogen bonds, chain elongation, density, and vibrational density of states correlate with changes in thermal conductivity for polymer blends composed of different relative constituent fractions. We also consider the effects of bond strength, tacticity, and polymer chain mass. For certain blend fractions, we observe large increases in thermal conductivity, and we analyze these increases in terms of modifications to chain chemistry (e.g., inter-chain bonding) and chain morphology (e.g., chain alignment and radius of gyration). We observe that increasing the number of hydrogen bonds in the system results in better packing as well as better chain alignment and elongation that contribute to enhanced thermal conductivity.   

Field-theoretic study on colloidal interaction in solutions of adsorbing homopolymers

Using self-consistent field theory, we study the free polymer induced (FPI) interaction between colloidal particles in a homopolymer solution. In the colloid limit, the Derjaguin approximation can be applied to relate the potential of mean force between two spherical objects to that between two parallel plates. A field-theoretic model is formulated in the grand canonical ensemble to compute the potential of mean force for the system consisting of two solid plates with polymer solution confined in between. The relationship between polymer-colloid affinity and FPI interaction is investigated through simulations. Three distinct regimes, including depletion interaction, steric repulsion and bridging interaction, are identified. A transition through these interaction regimes with increasing polymer-colloid affinity is shown as a consequence of competing entropic and enthalpic effects. We also study the impact of varying the degree of polymerization of the homopolymer, the solvent quality and the polymer concentration on the FPI interaction.   

Entanglement Length in Miscible Blends of \textit{cis}-Polyisoprene and Poly(\textit{ptert}-butylstyrene)

In miscible polymer blends, the entanglement length is common for the components, but its changes with the composition $w$ remain unclear. For this problem, this study analyzed viscoelastic data for miscible blends of \textit{cis}-polyisoprene (PI) and poly(\textit{ptert}-butylstyrene) (PtBS), considering the basic feature that the local relaxation is determined only by $w_{\mathrm{PI}}$. On the basis of this feature, a series of unentangled low-$M$ PI/PtBS blends having various $M$ and a given $w_{\mathrm{PI\thinspace }}$were utilized as references for well-entangled high-$M$ PI/PtBS blends having the same $w_{\mathrm{PI}}$, and the modulus data of the references were subtracted from the high-$M$ blend data. For an optimally chosen reference, the storage modulus $G_{\mathrm{e}}' $of the high-$M$ blends obtained after the subtraction exhibited a clear entanglement plateau $G_{\mathrm{N}}$ and the corresponding $G_{\mathrm{e}}''$ decreased in proportion to 1/$\omega $ at high frequencies $\omega $. Thus, the onset of entanglement relaxation was detected. The $G_{\mathrm{N\thinspace }}$values were well described by a linear mixing rule of the entanglement length with the number fraction of Kuhn segments of the components being utilized as the averaging weight. This result, not explained by a mean-field picture of entanglement, is discussed in relation to local packing of bulky PtBS chains and skinny PI chains.   

Effect of twist-orientation on mechanical properties of self-reinforced poly(lactic acid) screws in simulated body environment

Poly(lactic acid) (PLA) attracts much attention as a typical biodegradable polymer, and has been applied as a bone fixation device. As one of the methods to improve mechanical properties of PLA bone fixation device, orientations of molecular chains have been investigated. However, conventional uniaxial drawing could not improve mechanical properties along the other loading direction than the drawing direction, such as torsion. In this study, screw is treated as a bone fixation device. In order to improve torsional strength of a bioabsorbable PLA screw, twist-orientation method has been developed. PLA screw is prepared through a series of routes including extrusion molding, extrusion drawing, twist-orientation and forging. This screw was immersed in the phosphate buffer solution for 0, 8, 16 and 24 weeks, then shear strength, orientation function, crystallinity and molecular weight were measured. As a result, twist-orientation improves the initial torsional strength of PLA screw without the decrease in initial shear strength. In addition, the shear strength on twist-oriented screw is equivalent that of non-twist oriented screw during immersion until 24 weeks. This result shown that the twist-orientation does not decrease shear strength after immersion.   

PVC-OH Functionalized SWCNT Nanocomposites

Nanocomposites of polyvinylchloride loaded with various amounts of OH functionalized Single Walled Carbon Nanotubes (SWCNT-OH) have been obtained by melt mixing using a Haake Rheomixer. The polymeric matrix has been loaded by various amount of SWCNT-OH ranging between 0 and 15 {\%} wt. The as obtained nanocomposites have been measured by Raman spectroscopy using a InVia Renishaw spectrometer. The Raman lines have been deconvoluted into a superposition of extended Breit-Wigner-Fano line shapes. The effect of nanofiller concentration on the stress transfer from the polymeric matrix to SWCNTs has been analyzed and the Radial Breathing Mode was investigated. Differential Scanning Calorimetry revealed modest shifts of the melting and crystallization temperatures upon loading with SWCNT-OH. Additional information has been obtained by X-Ray measurements. The as obtained nanocomposites have not a very good thermal stability due to the thermally induced dehydrochlorination process. The thermogravimetric data are analyzed in detail and related to Raman results. Preliminary data on the thermal stability of these nanocomposites are reported.   

Bottlebrush Polymer Additives for Binary Polymer Blends

Bottlebrush polymers are highly branched polymers that have been used in applications such as self-assembling photonics, drug delivery and stimuli-responsive surface coatings. However, they have not been widely studied as compatibilizers for polymer blends. In this study, bottlebrush polymers with poly(styrene-r-methyl methacrylate) side chains were used as additives for thin film blends of polystyrene (PS) and poly (methyl methacrylate) (PMMA). The blends were heated above the glass transition temperature to drive phase separation, and the resulting morphology was characterized with atomic force microscopy and optical microscopy. Outcomes were compared with PS/PMMA blends that contain conventional compatibilizers such as linear random copolymers of poly(styrene-r-methyl methacrylate) and diblock PS-PMMA copolymers. The bottlebrush additive accumulates at the PS/PMMA interface and drives the formation of vesicle-like droplets that assemble into longer chains. The continuity of the chains depends on the blend composition, where a network structure is achieved close to the critical composition. This unusual microstructure was not observed with the other additives, and may be a consequence of preferential wetting of the bottlebrush by the PS phase.   

Molecular Dynamics Simulations of Nanoparticles Coated with Charged Polymers

Polymer coating is frequently used to stabilize colloidal and nano-sized particles. We employ molecular dynamics simulations to study nanoparticles coated with polymer chains that contain ionizable groups. In a polar solvent, the chains become charged with counterions dissociated. In the computational model, we treat the solvent as a uniform dielectric background and use the bead-spring model for the polymer chains. Counterions are explicitly included as mobile beads. The nanoparticle is modeled as a layer of sites uniformly distributed on a spherical surface with a certain fraction of sites serving as the tether points of the grafted polymer brush. We vary the grafting density and calculate the distribution of polymer beads and counterions around the nanoparticle. Our results indicate that charged chains adopt extended conformations because of their mutual repulsions. We further study the interactions between two polymer-coated nanoparticles and obtain the potential of mean force. We also find an interesting transition of a confined single layer of such polymer-coated nanoparticles into two layers when the confinement is removed. Results show that the brush-brush contact has a nonuniform distribution and the nanoparticles tend to form dipole-like structures.   

Raman Investigations of PVDF-BaTiO$_{3}$ Nanocomposites

BaTiO$_{3}$ nanoparticles (from Nanostructured {\&} Amorphous Materials, Inc.\textbf{)} were dispersed within PVDF powder (from Sigma Aldrich) by melt mixing using a Haake Rheomix with two counter rotating screws. The mixing consisted of 3 segments first at 190 $^{\circ }$C and 60 rotations per minute (rpm) for 30 minutes, second at 210 $^{\circ }$C and 80 rpm for 15 minutes, and the last at 180 $^{\circ }$C and 60 rpm for 30 minutes. Nanocomposites containing various fractions of nanofiller, ranging from 0 to 15 {\%} wt. have been prepared. Raman investigations on the as obtained nanocomposites have been performed by using a Renishaw InVia spectrometer operating at 532 and 785 cm$^{-1}$. Complementary Wide Angle X-Ray Scattering measurements on the same samples revealed that the as obtained nanocomposites have a dominant beta phase and provided additional information about the size of polymeric crystallites. The effect of the nanofiller on the Raman lines of the PVDF are analyzed in detail. The Raman spectra have been deconvoluted assuming a superposition distorted Lorentzian line shape. The changes of the Raman spectrum parameters (position, amplitude, width, and asymmetry factors) due to the loading with BaTiO$_{3}$ is discussed.   

Coarse-grained explicit solvent simulation of the translational and rotational diffusion of a spherical particle in a polymer solution

We use an extension of DPD model to address the dynamical properties of a colloid particle in an unentangled semi-dilute polymer solution. Solvent and monomers are represented as DPD particles.The colloid particle is represented as a larger DPD particle with the rotational degrees of freedom and tangential component of the dissipative and random DPD interactions with the solvent and monomers. This allows us to model a finite slip length boundary condition at the particle fluid interface and study translational $D_t$ and rotational $D_r$ diffusivities of a spherical particle. For zero polymer concentration our results agree with the Stokes-Einstein (SE) theory. Fore dilute and semi-dilute polymer solutions we have found that polymer dynamic follow the Zimm model in a dilute regime and the Rouse model at high polymer concentration. For particles smaller than the polymer $R_g$ observed $D_t$ is much high than SE prediction for $R>R_g$ SE prediction recovers. We have found that increase of $D_r$ relative to SE is rather correlated to the $\frac{R}{R_g}$ ratio than $\frac{R}{\xi}$, where $\xi$ is the thickness of a depletion shell around the particle. $D_r$ is very sensitive to the slip length at the particle fluid interface and insensitive to $\frac{R}{R_g}$.   

Characterization of the crosslinking reaction in high performance phenolic resins

In this study, a combination of thermal analysis, infrared spectroscopy (near and mid) in conjunction with low field NMR, was used to characterize the crosslinking reaction involving phenol formaldehyde resin and a crosslinking agent, Hexamethylenetetramine (HMTA). The strong hydrogen bonds in the resin and the completely crystalline HMTA (Tm $=$ 280 \textdegree C) severely hamper the crosslinking process. Yet the addition of a small amount of plasticizer can induce a highly efficient crosslinking reaction to achieve the desired mechanical properties needed in a number of high performance organic-inorganic composites. The infrared spectroscopy clarifies the dissolution process of the crystalline crosslinker and the specific interactions needed to achieve miscibility of the reactants. The thermal analysis enabled us to follow the changing mobility of the system as a function of temperature. The low field NMR with the T1 inverse recovery technique allowed us to monitor the crosslinking process directly. For the first time, it is now possible to identify the functionality of the plasticizer and correlate the crosslinked structure achieved to the macroscopic performance needed for high performance organic-inorganic composites.