Session Q48: Polymer Blends and Crystallization

How Pure Components Control Polymer Blend Miscibility

We present insight into some intriguing relationships revealed by our recent studies of polymer mixture miscibility. Applying our simple lattice-based equation of state, we discuss some of the patterns observed over a sample of experimental blends. We focus on the question of how much key information can one determine from a knowledge of just the pure components only, and further, on the role of separate enthalpic and entropic contributions to the miscibility behavior. One interesting correlation connects the value of the difference in pure component energetic parameters with that of the mixed segment interactions, suggesting new possibilities for predictive modeling. We also show how in some cases these two parameter groupings act as separate controls determining the entropy and enthalpy of mixing. Also discussed are the different patterns exhibited for UCST-type and LCST-type blends, these being revealed in some cases by simple examination of the underlying microscopic parameters.   

Tailoring Co-continuous Nanostructured Morphologies in Polymer Blends

We describe a simple approach to prepare thin films of polymer mixtures with co-continuous morphologies having characteristic length scales down to tens of nanometers based on spinodal phase separation. The degree of immiscibility between polystyrene (PS) and poly(2-vinylpyridine) (P2VP) is tuned by incorporating styrene monomers into the P2VP backbone to yield a random copolymer, thereby tailoring the nonfavorable interactions between the two components. The size scale of the co-continuous morphology is controlled by varying the molecular weights of the components and the film thickness. This strategy is shown to be robust in that the process involves a simple solution-casting; the co-continuity of the morphology occurs provided the solvent dissolves both components; and the co-continuous morphology is insensitive to the substrate surface chemistry. Porous membranes with continuous channels and gradient co-continuous structures can also be generated from the phase separated blend films.   

Structure and elasticity of crosslinked polymer blends

We consider a blend of mutually incompatible homopolymer species, A and B, that are randomly crosslinked to form a network. In such a network there is a competition between the repulsion of the A and B polymers (which favors the demixing of the two species) and the crosslinking (which prohibits complete demixing) [1,2].~ We treat the system by means of a model of flexible polymers, which are permanently crosslinked (with statistics modeled by the Deam-Edwards distribution) and have species-dependent excluded-volume interactions [3].~ As expected, the model shows that at sufficiently low temperatures the demixing tendency drives microphase separation, with a characteristic scale set by the network localization length.~ It also shows that if the system is strained after crosslinking, correspondingly anisotropic microdomains are generated in the pattern of A-B polymer concentration fluctuations trapped in by the network and, furthermore, allows the impact of microphase separation on the elastic properties of the network to be determined.\\[4pt] [1] P.G. de Gennes, J. Phys. (Paris) Lett., 40 (1979) L-69. \newline [2] D.J. Read, M.G. Brereton and T.C.B. McLeish, J. Phys. II, 5 (1995) 1679. \newline [3] C. Wald et al., Europhys. Lett. 70, 843--849 (2005); J. Chem. Phys. 124, 214905 (2006).   

Assessing the Strength Enhancement of Heterogeneous Networks of Miscible Blends of 1,2-Polybutadiene and Polyisoprene

At typical crosslink densities of elastomers, failure properties vary inversely with mechanical stiffness, so that compounding entails a compromise between stiffness and strength. Our approach to circumvent this conventional limitation is by forming networks of two polymers that: (i) are thermodynamically miscible, so that the chemical composition is uniform on the nm level; and (ii) have markedly different reactivities for network formation. The resulting elastomer consists of one highly crosslinked component and one that is lightly or uncrosslinked. This disparity in crosslinking causes their respective contributions to the network mechanical response to differ diametrically. Earlier results showed some success with this approach for thermally vulcanized blends of 1,2-polybutadiene and polyisoprene, taking advantage of their differing reactivities to sulfur. In this work we explore networks of this miscible blend formed via UV irradiation with a photoinitiator. The vinyl group in 1,2-polybutadiene has a much greater photo-reactivity than the double bond in polyisoprene, resulting in a disparity in respective degrees of crosslinking, while the thermodynamic miscibility is retained. Mechanical properties of the radiation crosslinked blend are compared to conventional networks.   

Compatibilization of polymer blends 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). The mechanism of compatibilization involves threading of the CD core by a second polymer and solubilization of the threading polymer into a PS matrix by the PS star arms. In principle, the same CD-star polymer can be used to compatibilize blends of several different polymers with PS, provided that the second polymer is able to thread the CD core. We have taken the first step toward demonstrating the generality of this approach by producing compatibilized blends of PS with poly(dimethyl siloxane) (PDMS) or poly(methyl methacrylate) (PMMA) using the same CD-star polymer. Thin spun-cast films of these blends exhibit a nanoscale level of mixing, while spun-cast films of the same blends without CD-star exhibit large-scale phase separation. The number of CD-star molecules that must be threaded onto the polymer chain to achieve compatibilization is larger for PMMA than for PDMS.   

Theory of Hydrogen-Bonding in Diblock Copolymer/Homopolymer Blends

The phase behavior and physical properties of hydrogen-bonding diblock copolymer/ homopolymer (AB/C) blends are examined using self-consistent field theory. We consider two methods for modeling the formation of hydrogen bonds between polymer chains. The first using a large negative (attractive) interaction parameter and the second using polymer-polymer complexation. Despite the success of the first model in describing the phase behavior of the system, it fails to correctly predict the change in lamella spacing induced by addition of homopolymer chains, previously shown in many experiments. Using polymer-polymer complexation we were able to qualitatively show the order-order phase transitions and decrease in lamella spacing for the case of strong hydrogen bonding. Our analysis of both methods show that hydrogen bonding of polymer chains should be described by polymer-polymer complexation.   

Effect of mid-block on the morphology and properties of PDLA-softblock-PDLA/PLLA Blends

A novel method to overcome the brittleness of PLLA is by kinetically trapping a continuous low Tg amorphous phase. This morphology has been accomplished by exploiting the significant difference in the crystallization temperatures of the neat PLLA versus its stereocomplex with the PDLA isomer. This morphology is formed by blending and co-crystallizing triblock copolymer with a configuration of the form PDLAn- Soft Blockm- PDLAn with PLLA. The type of morphology formed and the improvement in the sample toughness strongly depends on the miscibility of the midblock in the triblock copolymer with the matrix PLLA. This work explores the effect of the chemical nature of the midblock on the stereocomplex crystallization between the PDLA end-blocks and the PLLA matrix polymer and the blend morphology formed. It is found that miscible midblocks give rise to a soft continuous amorphous phase morphology while in case of immiscible midblocks a glassy phase separated amorphous phase morphology is formed. Dramatically different physical properties can be obtained for various PLLA/tri-block copolymer blends giving access to tough, semicrystalline PLLA blends.   

Molecular Dynamics Simulations of Polypropylene: Crystallization and Melting

The crystallization of polypropylene is studied using molecular dynamics (MD), using an all atom force field. Starting from basic ordered arrangements of oligomers, the $\alpha $ and $\beta $ phases for isotactic polypropylene are recovered, with cell parameters in excellent agreement with crystallographic data. A high pressure phase similar to the smectic phase is also observed. The melting temperatures of these crystals, obtained by isobaric heating, match with the experiments. Oligomers of syndiotactic polypropylene mirror all structural features observed in experiments. The all-trans state is found to be the most stable. The characteristic 2/1 helical structure of syndiotactic polypropylene is also formed along with the all-trans ``zigzag'' conformation.   

Molecular simulation of plastic deformation of semicrystalline polyethylene

The detailed structure and the high anisotropy of semicrystalline polymer at the lamellar length scale play an important role in determining its mechanical response. In this study, we performed molecular dynamics simulation of semicrystalline polyethylene under various industrially important deformations, such as extension, compression, and shear, to characterize the plastic deformation response. The semicrystalline polyethylene model empolyed in this study consists of a 1-D alternating stack in the longitudinal z direction of crystalline lamellae and interlamellar noncrystalline domains, that are infinite in the lateral x and y directions. The molecular dynamics simulations are carried out at the temperature of 350K using united atom model and two deformation strain rates (fast and slow) are considered in each deformation. Stress-strain curves, elastic moduli, yield stresses and so on, are examined under each deformation and compared to each other. In addition, the etanglement statistics of semicrystalline polyethylene under various deformation modes are investigated using the Z-code.   

Digestive Ripening: A Quantitative Thermodynamic Analysis of Stable Nanocrystals

Previous studies have shown that stable, monodisperse nanocrystals (NCs) have been produced using strongly binding surfactants, e.g. Au NCs with alkylthiols or Co NCs with oleic acid, to name a few. Through a first-principles theoretical investigation, we develop the full quantitative thermodynamic expression for the state of these surfactant-coated NCs. The general free energy expression allows for the crystal free energy, the surfactant binding energy, surfactant conformational entropy, and the surfactant interactions with other surfactants through excluded volume, solvent depletion, and energetic interactions between surfactant molecules. The energetics of the surfactant chains are treated quantitatively through Single Chain Mean Field theory, which determines the optimal number of grafted surfactants on the surface of an R-sized NC. Then, the size distribution function f(R) is calculated to determine the most favorable NC size and the equilibrium polydispersity. The theoretical conclusions will be compared quantitatively with experimental results. The full thermodynamic expression allows a parametric study of experimentally relevant conditions that govern whether Ostwald ripening vs. digestive ripening vs. dissolution will occur in a given nanocrystal-surfactant system.   

Morphology and Shear Viscosity for a Phase-separating Polymer Blend of Polybutadiene and Polyisoprene under Simple Shear Flow

The domain structure and shear viscosity of a phase-separated polymer blend of polybutadiene /polyisopreneare investigated with optical microscopy, light scattering, and rheometry. At the steady shear state, the shear-induced structures can be the nearly spherical droplets, the partially interconnected domains, the typical string-like domains, or the string-like domains with blurred interface, depending on the shear rate. The steady shear viscosity displays a rather non-Newtonian fluid behavior. In the transient flow experiments, the time dependence of viscosity and morphology after a stepwise increase of shear rate is studied and found to mainly depend on the final shear rate. In particular, as long as the final structures are the partially interconnected domains, the morphology evolution proceeds in the same way and the behaviors of the corresponding shear viscosity are similar.   

Dynamics of Polymer Blend Film Formation During Spin Coating

Spin casting is a process broadly used to obtain a uniform film on a flat substrate. A homogeneous film results from the balance between centrifugal and viscous forces. Here we revisit the Meyerhofer model of the spin casting process by taking in account the centrifugal forces, a uniform time dependent evaporation rate, and account for the changes in viscosity using the Huggins intrinsic viscosity. Time resolved light reflectometry is used to monitor the thickness changes of a polystyrene-poly(methyl methacrylate)(which we denote as PS and PMMA) film initially dissolved in toluene and spin cast for ten seconds at 1000 rpm. The experimental data are in good agreement with the model. We also investigate how the volume fraction of PS and PMMA influences the thinning of the film during spin casting. A distinct change in the temporal evolution of thickness as a function of time delimits the first phase of the spin casting process where centrifugal forces are dominant from a second phase dominated by the solvent evaporation. This hypothesis is supported by in-situ off specular scattering data. The time at which this change from centrifugal to evaporation-dominated behaviour is delayed as the volume fraction of PMMA increases.