Session W40: Polymer Blends

Entanglement Dynamics in Miscible Polyisoprene / Poly(p-{\it tert}-butyl styrene) Blends

Viscoelastic and dielectric behavior was examined for well entangled, miscible blends of high-$M$ {\it cis}-polyisoprene (PI) and poly(p-{\it tert}-butyl styrene) (PtBS). The dielectric data of the blends, reflecting the global motion of the PI chains having the type-A dipoles, indicated that PI and PtBS were the fast and slow components therein. At high temperatures $T$, the blends exhibited two-step entanglement plateau. The high frequency ($\omega$) plateau height was well described by a simple mixing rule of the entanglement length based on the number fraction of the Kuhn segments. At low $T$, the blend exhibited the Rouse-like power-law behavior of storage and loss moduli, $G' = G'' \sim \omega^{0.5}$, in the range of $\omega$ where the high-$\omega$ plateau was supposed to emerge. This lack of the high-$\omega$ plateau was attributed to retardation of the Rouse equilibration of the PI chain over the entanglement length due to the hindrance from the slow PtBS chains: The PI and PtBS chains were equilibrated cooperatively, and the retardation due to PtBS shortened the plateau for PI to a width not resolved experimentally. A simple model for this cooperative equilibration formulated on the basis of the dielectric data described the viscoelastic data surprisingly well.   

Local Relaxation Behavior and Dynamic Fragility in Hydrogen Bonded Polymer Blends

The dynamics of intermolecularly hydrogen bonded polymer blends of poly(p-(hexafluoro-2-hydroxyl-2-propyl)styrene) with poly(vinyl acetate), poly(ethylene[30]-co-vinyl acetate[70]) and poly(ethylene[55]-co-vinyl acetate[45]) are investigated by broadband dielectric relaxation spectroscopy and Fourier transform infrared spectroscopy. Each blend component exhibits a glassy state (beta) relaxation, and these relaxations are affected by the formation of intermolecular associations. The glassy state behavior of the blends can be modeled using the Painter-Coleman association model. All blends exhibit a single Tg and a single dielectric segmental (alpha) relaxation, indicative of strong segmental-level coupling. The fragility of the glass-formers depends on the volume fraction of intermolecularly associated segments, and the association model predicts which compositions have the highest fragilities. A relaxation related to the breaking and reforming of hydrogen bonds is observed at temperatures above the alpha process, and its temperature dependence varies systematically with ethylene content.   

Partial Miscibility in Copolymer Blends

Copolymers can be used to affect the miscibility of otherwise immiscible polymer blends by acting as compatibilizers. To better understand the energetics of these types of systems, we use a simple lattice model to study phase separation in binary copolymer/homopolymer blends. We focus on a copolymer that contains both A and B type monomers and a homopolymer that contains purely A type monomer. An example of a system that we are investigating is polyethylene mixed with either random or alternating poly(ethylene-co-propylene). The sequence effect on miscibility as the copolymer microstructure is varied from random to alternating is investigated as well.   

Properties of polystyrene/poly(dimethyl siloxane) blends partially compatibilized with star polymers containing a gamma-cyclodextrin core and polystyrene arms

Cyclodextrins (CDs) are cyclic starch molecules having a hollow central cavity which can be threaded by a polymer to form an inclusion compound. This characteristic is exploited in a new type of compatibilizer: a star polymer with a gamma-CD (g-CD) core and polystyrene (PS) arms (CD-star). Spun-cast thin films of PS containing up 20 weight percent poly(dimethyl siloxane) (PDMS) are compatibilized by CD-star. The mechanism of compatibilization involves threading of the CD core by PDMS and solubilization of the resulting slip-ring graft copolymer via the PS star arms. Thin spun-cast films of these blends exhibit a a nanoscale level of mixing and remain well-mixed after annealing at 125 C for three days. In contrast, thicker solution-cast films of these blends exhibit larger-scale phase separation since the film solidification process occurs over a period of days rather than seconds. This allows some of the PDMS to de-thread from the CD-star and phase separate. However, DSC, DMA and PDMS leaching data show that PS and PDMS remain partially compatibilized in the solution-cast films.   

Crystallization in the Binary Blends of Crystalline-Amorphous Diblock Copolymers Bearing Chemically Different Crystalline Block

The crystallization behavior of a series of lamellae-forming blends of a shorter PS-$b$-PEO (SEO) and a longer PS-$b$-PLLA (SLLA) has been studied. In SLLA-rich blends, the junction point constraint coupled with the poor chain mobility at low $T_{c}$ ($\le $45 $^{\circ}$C) hampered the formal crystallization of PLLA. In this case, a local demixing between a fraction of PEO and PLLA chains took place, yielding the PLLA crystalline domains in which the PLLA crystalline stems were intervened by the PEO chains. This crystalline species gave rise to a relatively broad peak at 2$\theta $= 15.92$^{\circ}$ in the WAXS profile and displayed a much lower melting point of ca. 100 $^{\circ}$C compared to that of the typical $\alpha $-form crystal of PLLA. It was suggested that the inserted PEO chains served as the molecular defects which induced an expansion of the $a$-axis and $b$-axis of the $\alpha$-form PLLA unit cell and lowered the crystal melting point due to introduction of defect free energy.   

Effect of Organoclays on Immiscible Polymer Blends

The effect of adding organoclays on the phase behavior, rheological properties and bulk mechanical properties of immiscible polymer blends of polystyrene (PS) and poly(methyl methacrylate) (PMMA) is investigated. Traditional organoclays, prepared using alkyl ammonium chains, display a preference to segregate to the PS phase for high PS volume fraction blends where the PS forms the continuous matrix. On the other hand, for blends with low PS volume fractions, the organoclay segregates to the interface between the PS and PMMA domains and leads to a decrease in the domain size that does not change much with organoclay concentration variations from 0.1 to 2 wt {\%}. Linear dynamic rheological data of these samples show significant increase in the low-frequency modulus of the blends with added organoclay. A thermodynamic model for estimating the interfacial modulus is proposed and the results agree well with the interfacial modulus calculated by Palierne's emulsion model. The toughness of the blends increases at low concentrations of added organoclays with the optimal improvements observed for less than 0.5 wt {\%} added organoclay.   

Characterization of the Early Stages of Phase Separation in PS/PVME Blends Using Fluorescence

Controlling the early stages of phase separation in polymer blends provides a potentially easy route towards obtaining interconnected nanostructured domains. We present results of thermally induced phase separation in polystyrene (PS) / poly(vinylmethylether) (PVME) blends using different fluorophores covalently attached to the PS component. Fluorescence identifies the phase separation temperature Tc at earlier stages then the more traditional method of light scattering. At Tc, a large increase in fluorescence intensity is observed due to a strong reduction in the fluorescence quenching caused by the intimate presence of the more polar PVME component. We discuss the spectral red shifts of pyrene associated with the dissolution of the weak hydrogen bonding in this blend and the change in polarity of the local environment during phase separation.   

Non-Isothermal Crystallization of PET/PLA Blends

Binary blends of poly(ethylene terephthalate) with poly(lactic acid), PET/PLA, were studied by differential scanning calorimetry. The solution cast blends were miscible in the melt over the entire composition range. We report the non-isothermal crystallization of: a.) PET, with and without presence of PLA crystals, and b.) PLA, with and without presence of PET crystals. PET can crystallize in all blends, regardless of whether PLA is amorphous or crystalline, and crystallinity of PET decreases as PLA content increases. PLA crystallization is strongly affected by the mobility of the PET. When PET is wholly amorphous, PLA can crystallize weakly even in 70/30 blends. When PET is crystalline, PLA cannot crystallize when its own content is below 0.90. The different behaviors may be related to the tendency of each polymer to form constrained chains, i.e., to form rigid amorphous fraction, RAF. PET is capable of forming a large amount of RAF, whereas relatively smaller amount of RAF forms in PLA. Like the crystals, rigid amorphous fraction of one component may inhibit growth of crystals of the other blend partner.   

Critical Micelle Concentrations for Different Micelle Shape in Diblock Copolymer/Homopolymer Blends

Diblock copolymers (AB) blended with homopolymers (A) may self-assemble into lamellar, cylindrical and spherical micelles. The critical micelle concentrations for different micelle shape are determined using self-consistent field theory. The effect of varying copolymer block asymmetry, homopolymer molecular weight and monomer-monomer interaction are considered.   

Ordered Materials via Additive Driven Assembly and Reaction using Surfactant-Based Templates

We recently reported (1) the ordering behavior of Pluronic surfactant melts through the addition of aromatic additives with hydrogen bond donating groups, which exhibit selective interactions with the polyethylene oxide (PEO) block. The ordered blends had domain sizes ranging from 12 to 16 nm at additive loadings up to 80{\%}.The goal of this work is to utilize condensation chemistries based on the functionality of similar additives, to yield ordered composite materials that could be used for applications involving membranes or dielectric materials. The structure of the blends and composites are determined by small angle x-ray scattering, which indicates that the ordered structure is preserved following reaction of the additives. Differential scanning calorimetry indicates that an increase in additive loading causes a decrease in the melting temperature and enthalpy of melting of the PEO, which demonstrates that the interaction between the PEO segments and the additive is strong. (1) Daga, V.K., Watkins, J. J. Macromolecules, ASAP.   

Interfacial Structure and Dynamics of the liquid/liquid interface between Polydimethylsiloxane and Polystyrene

Many important phenomena in biology, chemistry and in various fields involve processes that occur at the interface between two immiscible liquids. A molecular level understanding of such interfaces is crucial for insight into the complex dynamics that are observed at such interfaces. In this study, atomistic molecular dynamics simulations were performed to study the structural and dynamical properties of the liquid/liquid interface between two immiscible polymers, polydimethylsiloxane (PDMS) and polystyrene (PS). A series of simulations is carried out to examine the temperature and molecular weight dependence of the orientation of molecules at and away from the interface, intermolecular correlation at the interface, interfacial tension and interfacial thickness. The results from these detailed simulations will be presented.   

Effect of linear contaminants on the dynamics and rheology of ring polymer melts

Understanding the behavior of ring polymer melts remains a challenge. Early experimental efforts to characterize the rheological behavior of pure ring polymer melts have led to controversial results most likely because the samples were contaminated with linear chains. Recent studies found that stress relaxation follows a simple power law with no sign of a plateau. To further investigate these systems we have conducted molecular dynamics simulations for a semiflexible bead-spring model for chain lengths up to 14 entanglement lengths. The structure, dynamics and rheology of these systems are investigated for different concentrations of linear chains. We find that the viscosity of a ring melt increases dramatically when trace quantities of linear contaminants are present. The rings are found to swell slightly and diffuse more slowly with increasing linear concentration while the linear chains mostly behave as if in a pure linear melt. We use the concept of threading, analogous to thread passing through the eye of a needle, to explain the response of the ring melts to linear contaminants.