Session V50: Structure and Properties of Copolymers

Designing 100 K Glass Transition Breadths in Bulk Polymer Systems: Effects of Architecture in Homopolymers, Copolymers, and Copolymer Blends

Gradient copolymers have attracted interest as vibration or acoustic damping materials due to their extremely broad, tunable glass transition temperature (Tg) responses, up to 100 K in breadth. This behavior is caused by large compositional heterogeneity resulting from sinusoidal composition profiles in nanophase-separated systems. We have also found that some homopolymers and random copolymers exhibit large Tg breadths caused by incompatible main- and side-chain interactions. For example, the Tg response broadens with increasing side-chain length in the poly(n-alkyl methacrylate) series by more than a factor of 2 in going from poly(methyl methacrylate) to poly(n-hexyl methacrylate). We have also blended weakly-segregating styrene/n-butyl acrylate random copolymers of different compositions to allow for tunable Tg breadths over a 100 K temperature range. Finally, we have shown that blending a selective plasticizer into a styrene/4-vinylpyridine gradient copolymer results in a dramatic shift in the Tg response of a single nanophase region, increasing the Tg breadth above 100 K.   

Structure and Mechanical Behavior of Elastomeric Multiblock Terpolymers Containing Glassy, Rubbery, and Semicrystalline Blocks

Multiblock terpolymers containing poly(cyclohexylethylene) (C), poly(ethylene-\textit{alt}-propylene) (P), and poly(ethylene) (E) were synthesized. The CECPCEC (denoted XPX) and CECP (XP) each contain 50 v{\%} P and equal amounts of C and E. These materials have been studied by DSC, DMS, TEM, SAXS, WAXS, and tensile deformation to characterize the morphology, phase behavior, and mechanical properties. Microphase separation is induced by crystallization of E and/or chemical incompatibility between the three blocks, leading to a morphology which contains continuous region of P and continuous region of microphase separated X, resulting in mechanically resilient materials. High $M_{w}$ block copolymers microphase separate with two different length scales associated with segregation between C and E, and X and P. These structural features produce a non-classical scaling relationship for the C-E domain spacing, d $\sim $ N$^{0.31}$. The role of semicrystalline E domains during uniaxial deformation has been exposed with WAXS experiments, which support a two-step mechanism involving recoverable and non-recoverable deformation to different extents. Strain hardening is observed in double-anchored XPX, but not in single-anchored XP, at large tensile strains.   

Energy Storage and Dissipation in Random Copolymers during Biaxial Loading

Random copolymers composed of hard and soft segments in a glassy and rubbery state at the ambient conditions exhibit phase-separated morphologies which can be tailored to provide hybrid mechanical behaviors of the constituents. Here, phase-separated copolymers with hard and soft contents which form co-continuous structures are explored through experiments and modeling. The mechanics of the highly dissipative yet resilient behavior of an exemplar polyurea are studied under biaxial loading. The hard phase governs the initially stiff response followed by a highly dissipative viscoplasticity where dissipation arises from viscous relaxation as well as structural breakdown in the network structure that still provides energy storage resulting in the shape recovery. The soft phase provides additional energy storage that drives the resilience in high strain rate events. Biaxial experiments reveal the anisotropy and loading history dependence of energy storage and dissipation, validating the three-dimensional predictive capabilities of the microstructurally-based constitutive model. The combination of a highly dissipative and resilient behavior provides a versatile material for a myriad of applications ranging from self-healing microcapsules to ballistic protective coatings.   

X-ray scattering studies of ordered block copolymer melts during uniaxial extensional flow

We present the design and implementation of a new apparatus for in situ x-ray scattering studies of polymer melts during homogenous uniaxial extensional flow. The instrument is based on the commercial SER extensional flow fixture, which employs counter-rotating drums to deform a strip of polymer melt, which is incorporated into a custom-built convection oven designed to facilitate x-ray access to the sample and operation in a synchrotron environment. Here we report measurements of extensional flow-induced structural changes in a cylindrically ordered styrene-ethylene butylene-styrene triblock copolymer melt. At early stages, SAXS data reveal that the ordered microstructure deforms affinely until Hencky strains of $\sim$ 0.2. A global re-orientation process leads to alignment of microdomains predominantly along the stretching direction after Hencky strains of $\sim$ 1. Further stretching does not lead to further qualitative changes in 2-D SAXS patterns. Relaxation of both microdomain orientation and d-spacing is observed following cessation of extensional flow, albeit with different characteristic time scales. In situ x-ray scattering data are compared with off-line measurements of transient extensional viscosity, performed using the SER fixture in a rotational rheometer.   

Understanding the Dynamics of Magnetic Field Alignment for Rod-Coil Block Copolymers

Alignment of semiconducting block copolymer nanostructures is crucial to optimize charge transport in these materials. Magnetic fields can act on the liquid crystalline conjugated polymers, inducing alignment in rod-coil block copolymers. By using a combination of small angle x-ray scattering (SAXS) and transmission electron microscopy (TEM) we have studied the magnetic field alignment of poly(alkoxy phenylene vinylene-b-isoprene) (PPV-PI) rod-coil block copolymers. In situ measurements have also shown the magnetic field leads to a stabilization of the ordered phase. Furthermore, there appear to be two distinct timescales for alignment: at short times the alignment of these materials is fast likely caused by preferential growth of aligned domains, and at long times alignment increases by the very slow process of defect annihilation. Further, there is an optimum temperature where the kinetics and thermodynamic driving forces for alignment are balanced, producing very highly aligned samples. Understanding the mechanisms by which alignment occurs has lead to knowledge helping to rationally optimize the magnetic alignment of rod-coil block copolymers.   

In-situ SAXS observation of magnetic field effects on block copolymer ordering and alignment

The use of external fields to direct block copolymer self-assembly is well documented. Magnetic fields offer particular promise due to their space-pervasive nature and the ability to produce arbitrary alignments over truly macroscopic length scales in appropriate systems. We present here the results of in-situ SAXS studies of side-chain liquid crystalline diblock copolymers ordering under high magnetic fields and ex-situ GISAXS data on thin films. Despite the coincidence of the block copolymer order-disorder transition (ODT) and the LC clearing temperature in these weakly segregated materials, there is no measurable effect of the field on the ODT of the system, up to 6 T. This is in line with rough estimates based simply on the magnitudes of the relevant energy scales - the free energy of field interaction and the enthalpy of the isotropic-LC transition. We show that the alignment of the system is critically limited by the viscosity of the mesophase such that alignment can only be advanced by residence in a small temperature window near $T_{ODT}$. This residence produces a weakly aligned system which thereafter transitions to a strongly aligned state on cooling even in the absence of the field.   

Characterizing Mesoporous Block Copolymers by Resonant Soft X-ray Scattering

Mesoporous block copolymers membranes can be used as water filtration membranes and battery separators. In these applications, it is often advantageous to generate a structure where one block serves as a structural component, and the second block lines the pores. The membrane thus has 3 components: the two blocks and vacuum (or air). Small Angle X-ray Scattering (SAXS), which relies on electron density for contrast, only distinguishes between vacuum and polymer. This is because the scattering contrast between the two blocks is much less than that between the polymer and vacuum. Resonant Soft X-ray Scattering (RSoXS) experiments can be used to adjust the contrast between block copolymer phases by tuning the energy of the incident x-ray beam. We have studied mesoporous poly(styrene-block-ethylene-block-polystyrene) (SES) films, where the semicrystalline polyethylene serves as a structural phase, and the polystyrene lines the pores. At a particular energy, the scattering contrast between PS and vacuum becomes negligible, while contrast between PS and PE is enhanced. We present RSoXS data at different x-ray energies to demonstrate this contrast enhancement.   

Self-assembly Morphology and Crystallinity Control of Di-block Copolymer Inspired by Spider Silk

To obtain a fuller understanding of the origin of self-assembly behavior, and thus be able to control the morphology of biomaterials with well defined amino acid sequences for tissue regeneration and drug delivery, we created a family of synthetic silk-based block copolymers inspired by the genetic sequences found in spider dragline, HABn and HBAn (n=1,2,3,6), where B = hydrophilic block, A = hydrophobic block, and H is a histidine tag. We assessed the secondary structure of water cast films by Fourier transform infrared spectroscopy (FTIR). The crystallinity was determined by Fourier self-deconvolution of amide I spectra and confirmed by wide angle X-ray diffraction (WAXD). Results indicate that we can control the self-assembled morphology and the crystallinity by varying the block length, and a minimum of 3 A-blocks are required to form beta sheet crystalline regions in water-cast spider silk block copolymers. The morphology and crystallinity can also be tuned by annealing. Thermal properties of water cast films and films annealed at 120 C were determined by differential scanning calorimetry and thermogravimetry. The sample films were also treated with 1,1,1,3,3,3-Hexafluoro-2-propanol (HFIP) to obtain wholly amorphous samples, and crystallized by exposure to methanol. Using scanning and transmission electron microscopies, we observe that fibrillar networks and hollow micelles are formed in water cast and methanol cast samples, but not in samples cast from HFIP.   

The Effects of Blockiness on the Chemical Composition Distribution of Partially Functionalized Polystyrene

Monodisperse polystyrene has been functionalized chemically to make random copolymers with controlled sequence distribution of the unmodified and modified styrene segments. The sequence blockiness of the resulting random copolymers can be controlled via the temperature of reaction, with a high temperature reaction resulting in a ``truly random'' copolymer, and a low temperature resulting in a ``random blocky'' copolymer. Interaction chromatography has been employed to estimate the chemical composition distribution of these partially functionalized polystyrenes. Two different chemical systems will be discussed; i.e., the brominated and borylated polystyrene systems. The results of our analysis reveal that the chemical composition distribution of ``random blocky'' copolymers is narrower than that of the corresponding ``truly random'' copolymers. The chemical composition of the two systems will be compared directly, and the influence of ``chain conformation inversion'' will be discussed.   

Large Scale Purification and Characterization of Model Poly(isopropyl methacrylate)-block-Poly(styrene) Diblock Copolymers

The development of purification techniques of block copolymers is vital for overcoming the synthetic difficulty of preparing well-defined block copolymers using various living polymerization techniques. A large scale separation technique would lead us to obtaining sufficient amounts of homopolymer-free block copolymers for subsequent physical characterization. This can potentially aid in the elucidation of the role of chemical heterogeneity on the thermodynamic transitions and viscoelastic properties of block copolymer materials. Employing an acute understanding of polymer adsorption/desorption onto nanoporous silica during solvent gradient interaction chromatography, we demonstrate the large scale purification of anionic polymerized poly(isopropyl methacrylate)-\textit{block}-poly(styrene) diblock copolymers with narrow molecular weight distribution. Additionally, we address the impact of removing early-terminated poly(styrene) homopolymers on the viscoelasticity of these model diblock copolymers, as well as their impact on block copolymer assembly as analyzed by small-angle X-ray scattering.   

Quantifying Fluctuation Effects on the Order-Disorder Transition of Symmetric Diblock Copolymers

How fluctuations change the order-disorder transition (ODT) of symmetric diblock copolymers is a classic yet unsolved problem in polymer physics.\footnote{\textit{L. Leibler}, \textbf{Macromolecules, 13}, 1602 (1980); \textit{G. H. Fredrickson and E. Helfand}, \textbf{J. Chem. Phys., 87}, 697 (1987).} Here we unambiguously quantify the fluctuation effects by direct comparisons between fast off-lattice Monte Carlo (FOMC) simulations\footnote{\textit{Q. Wang and Y. Yin}, \textbf{J. Chem. Phys., 130}, 104903 (2009).} and mean-field theory using exactly the same model system (Hamiltonian), thus without any parameter-fitting. The symmetric diblock copolymers are modeled as discrete Gaussian chains with soft, finite-range repulsions as commonly used in dissipative-particle dynamics simulations. The effects of chain discretization and finite-range interactions on ODT are properly accounted for in our mean-field theory.\footnote{\textit{Q. Wang}, \textbf{J. Chem. Phys.}, \textbf{129}, 054904 (2008); \textbf{131}, 234903 (2009).} Our FOMC simulations are performed in a canonical ensemble with variable box lengths to eliminate the adverse effects of fixed box sizes on ODT.\footnote{\textit{Q. Wang et al.}, \textbf{J. Chem. Phys.}, \textbf{112}, 450 (2000).} Furthermore, with a new order parameter for the lamellar phase, we use replica exchange and multiple histogram reweighting to accurately locate ODT in our simulations.   

Simulation of micelle lattice ordering via nucleation from disorder in a diblock copolymer melt

We examine the dynamics of the order-disorder transition (ODT) in diblock copolymer melts by simulating the nucleation of the BCC phase of spherical micelles out of the disordered phase, using the time-dependent Landau-Brazovskii model. Questions about the description of the disordered phase as a phase of disordered micelles, and the role of intermediate close-packed structures suggest a rich dynamics for this ordering transition. For a copolymer composition $f = 0.39$, we find that above a critical size, which diverges at the ODT, a spherical nucleus of the BCC micelle phase grows, forming the BCC phase directly. The growth rate varies linearly with undercooling. We also examine the growth of nuclei of the close-packed phases. For more asymmetric copolymers, $f = 0.35$ and $f = 0.3$, a region of disordered micelles surrounds a core of the BCC phase in the growing nucleus, suggesting that, for asymmetric copolymers, the ODT proceeds via a two-step mechanism.   

Order-to-order Transition in Closed-loop type Block Copolymer

Recently, the complex morphologies such as gyroid (GYR), hexagonally perforated layer (HPL) and non-cubic network phase (Fddd) observed in block copolymer (BCP) melts. In this study, we investigated the unique and unusual OOT behavior of asymmetric deuterated polystyrene-block-poly(n-pentyl methacrylate) copolymers, denoted as dPS-b-PnPMA at narrow range of volume fraction. Scattering results and transmission electron microscopy show that dPS-b-PnPMAs exhibit an unusual OOT phase behavior of GYR?HPL and GYR?HPL?GYR, whereas a general morphological transition occur from HPL to GYR with increasing temperature. The OOT process found in this work provides a physical insight into a new type of morphological transition in diblock copolymer system.   

Self-assembly Behavior of Poly(3-alkylthiophene)-\textit{block}-poly(methyl methacrylate) Block Copolymers Prepared by Anionic Coupling Reaction

We synthesized rod-coil block copolymers composed of regioregular poly(3-alkylthiopene) (P3AT) and poly(methyl methacrylate) (PMMA) via anionic coupling reaction. For poly(3-hexylthiopene)-b-PMMA, the morphology was mainly determined by self-crystallization of P3HT moieties due to strong rod-rod interaction. On the other hand, poly(3-dedecyl thiophene) (P3DDT)-b-PMMA, the self-crystallization was effectively suppressed. Detail phase behaviors were investigated at temperatures higher melting point (T$_{m})$ of P3DDT using small-angle x-ray scattering (SAXS), wide-angle x-ray scattering (WAXS) and transmission electron microscopy (TEM).