Improved Polypropylene Thermoformability through Polyethylene Layering.

Due to its low cost, stiffness, and recyclability, isotactic polypropylene (iPP) is an excellent candidate for packaging applications. However, iPP is notoriously difficult to thermoform due to its low melt strength. The addition of just 10 thin layers of high-molecular-weight, linear low-density polyethylene (LLDPE) into iPP sheets by coextrusion significantly increased extensional viscosity and reduced sag. Both LLDPE and iPP were metallocene-catalyzed with excellent adhesion as measured in our previous work. We performed a series of hot tensile tests and sheet sag measurements to determine the properties of the iPP sheet and the multilayer sheet between 130 and 180 °C. To evaluate the thermoformability of these multilayer sheets, truncated conical cups were positive vacuum formed at different temperatures and heating times, and the crush strength was measured. Cups that released easily from the mold with good shape retention and a crush strength within 80% of the maximum value were used to define a temperature-time thermoformability window. We estimated the maximum stress that occurred during the thermoforming process to be 5 MPa. Layer thicknesses before and after thermoforming were used to estimate an average strain of 0.78. The thin LLDPE layers decreased the yield stress below 5 MPa. This enabled thermoforming at sheet temperatures as low as 150 °C. The immiscible LLDPE interfaces increased extensional viscosity, which decreased sag in the multilayer sheets compared to iPP. This broadened the thermoforming range to temperatures as high as 180 °C and allowed longer heating times. These highly thermoformable, layered sheets may be recycled as iPP since they contain only 8% of LLDPE.

A15, σ, and a Quasicrystal: Access to Complex Particle Packings via Bidisperse Diblock Copolymer Blends.

A renewed focus on the phase behavior of nominally single-component, compositionally asymmetric diblock copolymers has revealed a host of previously unanticipated Frank-Kasper (FK) and quasicrystalline phases. However, these periodic and aperiodic particle packings have thus far only been reported in low molecular weight, highly conformationally asymmetric diblock copolymers, leaving researchers with a relatively small library of polymers in which these phases can be studied. In this work, we report on a simple approach to access these morphologies: blending two diblock copolymers with the same corona block length and varied core block lengths. Compositionally symmetric and asymmetric polystyrene-b-1,4-polybutadiene (SB) diblock copolymers with constant corona block lengths were blended together and shown via small-angle X-ray scattering and transmission electron microscopy to order into the FK A15 and σ phases, as well as a dodecagonal quasicrystal, providing a route to various particle packings in high molecular weight diblock copolymer melts.

Role of Chain Length in the Formation of Frank-Kasper Phases in Diblock Copolymers.

The phase behavior of poly(styrene)-b-poly(1,4-butadiene) diblock copolymers with a polymer block invariant degree of polymerization N[over ¯]_{b}≈800 shows no evidence of Frank-Kasper phases, in contrast to low molar mass diblock copolymers (N[over ¯]_{b}<100) with the same conformational asymmetry. A universal self-concentration crossover parameter N[over ¯]_{x}≈400 is identified, directly related to the crossover to entanglement dynamics in polymer melts. Mean-field behavior is recovered when N[over ¯]_{b}>N[over ¯]_{x}, while complex low symmetry phase formation is attributed to fluctuations and space-filling constraints, which dominate when N[over ¯]_{b}

Entropy-driven segregation of polymer-grafted nanoparticles under confinement.

The modification of nanoparticles with polymer ligands has emerged as a versatile approach to control the interactions and organization of nanoparticles in polymer nanocomposite materials. Besides their technological significance, polymer-grafted nanoparticle (PGNP) dispersions have attracted interest as model systems to understand the role of entropy as a driving force for microstructure formation. For instance, densely and sparsely grafted nanoparticles show distinct dispersion and assembly behaviors within polymer matrices due to the entropy variation associated with conformational changes in brush and matrix chains. Here we demonstrate how this entropy change can be harnessed to drive PGNPs into spatially organized domain structures on submicrometer scale within topographically patterned thin films. This selective segregation of PGNPs is induced by the conformational entropy penalty arising from local perturbations of grafted and matrix chains under confinement. The efficiency of this particle segregation process within patterned mesa-trench films can be tuned by changing the relative entropic confinement effects on grafted and matrix chains. The versatility of topographic patterning, combined with the compatibility with a wide range of nanoparticle and polymeric materials, renders SCPINS (soft-confinement pattern-induced nanoparticle segregation) an attractive method for fabricating nanostructured hybrid films with potential applications in nanomaterial-based technologies.

Tricontinuous Nanostructured Polymers via Polymerization-Induced Microphase Separation.

Nanostructured tricontinuous block polymers allow for the preparation of single-component materials that combine multiple properties. We demonstrate the synthesis of a mesoporous material from the selective orthogonal etching of a microphase-separated tricontinuous block polymer precursor. Using the synthetic approach of polymerization-induced microphase separation (PIMS), divinylbenzene (DVB) is polymerized from a mixture of poly(isoprene) (PI) and poly(lactide) (PLA) macro-chain transfer agents. In the PIMS process in situ cross-linking by the DVB arrests structural coarsening, resulting in a disordered block polymer morphology that we posit exhibits three nonintersecting continuous domains. Selective etching of the PI domains by olefin cross metathesis and PLA domains by hydrolytic degradation produces a mesoporous polymer with two independent pore networks arising from the different etch mechanisms.

Polymer ligand-induced autonomous sorting and reversible phase separation in binary particle blends.

The tethering of ligands to nanoparticles has emerged as an important strategy to control interactions and organization in particle assembly structures. We demonstrate that ligand interactions in mixtures of polymer-tethered nanoparticles (which are modified with distinct types of polymer chains) can impart upper or lower critical solution temperature (UCST/LCST)-type phase behavior on binary particle mixtures in analogy to the phase behavior of the corresponding linear polymer blends. Therefore, cooling (or heating) of polymer-tethered particle blends with appropriate architecture to temperatures below (or above) the UCST (or LCST) results in the organization of the individual particle constituents into monotype microdomain structures. The shape (bicontinuous or island-type) and lengthscale of particle microdomains can be tuned by variation of the composition and thermal process conditions. Thermal cycling of LCST particle brush blends through the critical temperature enables the reversible growth and dissolution of monoparticle domain structures. The ability to autonomously and reversibly organize multicomponent particle mixtures into monotype microdomain structures could enable transformative advances in the high-throughput fabrication of solid films with tailored and mutable structures and properties that play an important role in a range of nanoparticle-based material technologies.

Confined Pattern-Directed Assembly of Polymer-Grafted Nanoparticles in a Phase Separating Blend with a Homopolymer Matrix.

The controlled organization of nanoparticle (NP) constituents into superstructures of well-defined shape, composition and connectivity represents a continuing challenge in the development of novel hybrid materials for many technological applications. We show that the phase separation of polymer-tethered nanoparticles immersed in a chemically different polymer matrix provides an effective and scalable method for fabricating defined submicron-sized amorphous NP domains in melt polymer thin films. We investigate this phenomenon with a view towards understanding and controlling the phase separation process through directed nanoparticle assembly. In particular, we consider isothermally annealed thin films of polystyrene-grafted gold nanoparticles (AuPS) dispersed in a poly(methyl methacrylate) (PMMA) matrix. Classic binary polymer blend phase separation related morphology transitions, from discrete AuPS domains to bicontinuous to inverse domain structure with increasing nanoparticle composition is observed, yet the kinetics of the AuPS/PMMA polymer blends system exhibit unique features compared to the parent PS/PMMA homopolymer blend. We further illustrate how to pattern-align the phase-separated AuPS nanoparticle domain shape, size and location through the imposition of a simple and novel external symmetry-breaking perturbation via soft-lithography. Specifically, submicron-sized topographically patterned elastomer confinement is introduced to direct the nanoparticles into kinetically controlled long-range ordered domains, having a dense yet well-dispersed distribution of non-crystallizing nanoparticles. The simplicity, versatility and roll-to-roll adaptability of this novel method for controlled nanoparticle assembly should make it useful in creating desirable patterned nanoparticle domains for a variety of functional materials and applications.

Soft-shear induced phase-separated nanoparticle string-structures in polymer thin films.

Application of shear stress has been shown to unidirectionally orient the microstructures of block copolymers and polymer blends. In the present work, we study the phase separation of a novel nanoparticle (NP)-polymer blend thin film system under shear using a soft-shear dynamic zone annealing (DZA-SS) method. The nanoparticles are densely grafted with polymer chains of chemically dissimilar composition from the matrix polymer, which induces phase separation upon thermal annealing into concentrated nanoparticle domains. We systematically examine the influence of DZA-SS translation speed and thus the effective shear rate on nanoparticle domain elongation and compare this with the counterpart binary polymer blend behavior. Unidirectionally aligned nanoparticle string-domains are fabricated in the presence of soft-shear in confined thin film geometry. We expect this DZA-SS method to be applicable to various NP-polymer blends towards unidirectionally aligned nanoparticle structures, which are important to functional nanoparticle structure fabrication.