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Caltech
Kornfield Lab

Past Projects

Associative telechelic polymers – dynamics and thermodynamics

RESEARCHERS: Boyu Li, Jeremy Wei

Science paper: Megasupramolecules for safer, cleaner fuel by end association of long telechelic polymers

The desired application in mist control motivated us to dig deeper into the association of these telechelic polymers, expanding the range of chain length (10 to 700 kg/mol) and concentration (dilute to semi-dilute), and comparing pair-wise associative with self-associative telechelics.

Building on the prior literature on self-associative systems (like hydrophobically end-associative polymers1), new phenomena can arise by virtue of the nature of hydrogen-bonding end groups. Acid-ended telechelic poly(1,5-cyclooctadiene)s (A-PCODs, self-associative) demonstrate a highly temperature and chain-length dependent sol-gel transition in non-polar solvents.  Furthermore, we discovered that chain length dramatically affects the phase behavior of A-PCOD solutions. To explain the temperature and chain length dependence, we hypothesize that the aggregation number (p: number of chain ends in one micelle core) decreases with both increasing temperature and increasing chain length, despite prior literature mostly assume constant p.

In contrast to self-associative system, which forms "flower-like" micelles at low concentration and form gels through bridging at higher concentration, pair-wise associative polymers are expected to build linear supramolecules, which should resemble covalent linear chains to certain extent but exhibit new properties due to end association. Our preliminary data indeed show this phenomenon.

References:

  1. C. Chassenieux, T. Nicolai & L. Benyahia, Current Opinion in Colloid & Interface Science 16, 18-26 (2011).

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Conformation of bottlebrush polymers in solution

RESEARCHER: Joey Kim

bottlebrush polymers

Bottlebrush polymers (or polymacromonomers) are polymers in which the monomeric constituents are also polymers resulting in a hierarchal structure with two polymer domains:  side chains (purple) and backbone (blue).  Linear polymers adopt a conformation the maximizes the configuration entropy, however, bottlebrush polymers take on an extended conformation due to the excluded volume interactions of the densely grafted side chains counteracting the entropic driving force.  The overall shape resembles that of a bottlebrush as can be seen in the figure taken from simulations. There have been theoretical, experimental, and, more recently, simulation efforts to elucidate the behavior of these molecules in dilute solutions.  These results led to a controversy in the past three decades.  Our goal is to resolve this controversy using various scattering techniques:  static light scattering (SLS), multi-angle light scattering (MALS), and small-angle neutron scattering (SANS).  With the new insight, we aim to study the semi-dilute behavior with small-angle X-ray scattering (SAXS).

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The Role of Macromolecular Architecture in Flow-Induced Crystallization

RESEARCHER: Bo Shen

Semicrystalline polymers are processed predominantly from the melt, for example in fiber-spinning, injection molding, and film-blowing. During processing, the polymers are subject to intense shear and elongational flow, which can orient and stretch the chains. Oriented crystallization can result, which significantly alters the crystallization kinetics and final structure, and thus material properties. If the stress is sufficiently high, threadlike precursors may grow from these point-like precursors.  These threadlike precursors, "shish", propagate along the flow direction. The shish nucleate chain-folded lamellar crystals, "kebabs", which grow radially from them.  The longest, slowest relaxing chains play a disproportionate role in the formation of the threadlike precursors. However, their exact role remains unclear. 
Our goal is to discover the "mechanism of action" of the slowest-relaxing chains in the tremendous enhancement of formation of oriented precursors during flow-induced crystallization of polymers and understand the mechanism on the molecular level. 

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Deformation induced morphological changes in poly (L-lactic acid) (PLLA) Vascular scaffolds

RESEARCHER: Karthik Ramachandran

Vascular scaffolds

Cardiovascular disease is the main cause of death in Europe and America claiming over two million lives each year. Over one million patients in the US alone receive stents each year to open coronary arteries and maintain cardiac health.  Current stents are made from metal alloys which induce critical side effects such as Late Stent Thrombosis (LST) and chronic angina. These complications occur as metal stents are permanent fixtures in the body. Therefore, there is a need to move beyond metals towards a transient material to overcome LST and angina. Bioresorbable vascular scaffolds (BVS) function like stents after implantation, holding the artery open, but completely disappear in 2 years, restoring vasomotion and vasoresponse. Clinical trials of BVSs have reported promising results with no incidence of LST. The material which has made transient vascular scaffolds possible is the polymer poly (L-lactic acid) (PLLA).

Amorphous PLLA is subjected to tube expansion and laser cutting to make a scaffold. The laser-cut scaffold is then crimped onto an inflatable balloon and coated with anti-inflammatory drugs. The surgeon guides the crimped scaffold towards the affected artery and deploys it by the expansion of a balloon. PLLA is known to be a stiff polymer but it survives the intense deformation during tube expansion, crimping and deployment. We hypothesized that the strength needed to survive this deformation must come from the underlying morphology of the scaffold. My previous research used a combination of polarized light microscopy and synchrotron X-ray beamlines to investigate the structural changes that occur in PLLA scaffolds during crimping and deployment. We discovered that crimping creates a multiplicity of morphologies in a single PLLA scaffold. This imparts the scaffold with the strength necessary to undergo deployment and to support the occluded artery. The morphology developed during the processing of PLLA vascular scaffolds governs the therapeutic function and in-vivo performance of the scaffolds. 

Reference: Oberhauser, J. P., Hossainy, S., & Rapoza, R. J. (2009). Design principles and performance of bioresorbable polymeric vascular scaffolds. EuroIntervention: journal of EuroPCR in collaboration with the Working Group on Interventional Cardiology of the European Society of Cardiology, 5, F15-22.

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Inorganic layered nanomaterials as reinforcement of bioerodible polymeric vascular scaffolds

RESEARCHER: Tiziana Di Luccio

My research at Caltech focuses on the investigation of novel nanocomposites of biocompatible and biodegradable polymers and inorganic nanostructures. Incorporated at a few percentage amounts into the polymer, inorganic layered materials (such as metal dichalcogenide nanotubes) have great potential to promote polymer crystallinity and reduce brittleness which is a major limitation for application to bioerodible polymeric vascular scaffolds.

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Associative telechelic polymers – application as mist-control additives, dynamics and thermodynamics

Jeremy Wei, Boyu Li

Science paper: Megasupramolecules for safer, cleaner fuel by end association of long telechelic polymers

Liquid fuels, such as gasoline, diesel and kerosene, are the world's dominant power source, representing 34% of global energy consumption.  Transportation relies on such liquids, presenting the risk of explosive combustion in the event of impact, such as the 1977 Tenerife airport disaster—an otherwise-survivable runway collision that claimed 583 lives in the post-crash fireball.  The UK and the U.S. responded with a multi-agency effort to develop polymeric fuel additives for "mist control." Ultra-long, associative polymers (e.g., ICI's "FM-9," >3•106 g/mol copolymer, 5 mol% carboxyl units) increased the drop diameter in post-impact mist1,2, resulting in a relatively cool, short-lived mist fire. However, the polymers interfered with engine operation3, and their ultra-long backbone—essential for mist control—degraded upon pumping4. They were abandoned in 1986.  A quarter century later, the post-impact fuel fireball involved in the collapse of the World Trade Center motivated us to revisit polymers for mist control.  Building on recent advances in supramolecular assembly as a route to emergent functional materials5-9, particularly assembly of complex polymer architectures10,11, we discovered an unexplored class of polymers that is both effective and compatible with fuel systems.  Long (>4•105g/mol) end-associative polymers form "mega-supramolecules" that control post-impact mist without adversely affecting power, efficiency or emissions of unmodified diesel engines.  They also reduce turbulent drag, hence, conserving energy used to distribute fuel.  The length and end-association strength of the present polymers were designed using statistical mechanical considerations. In comparison with ultra-long polymers for mist control4, the present polymers are an order of magnitude shorter; therefore, they are able to resist shear degradation. In contrast to prior randomly-functionalized associative polymers3, these end-associative polymers also avoid chain collapse12. We found simple carboxylic-acid/tertiary-amine end-association to be effective, and the unprecedented length10 of these telechelic polymers to be essential for their potent rheological effects.

Movie captions:

Apparatus for impact/flame propagation experiments. An aluminum canister (outer diameter = 23 mm, height = 100 mm) was used as a miniature fuel tank to hold ~30 mL of a test sample. The cap was tightly sealed with superglue and electrical tape. A stainless steel cylinder (diameter = 24 mm, length = 50 mm) was used as a projectile to impact the sample canister and disperse the fuel. To the left of this image: Compressed air at 6.89×105 Pa was used to propel the projectile through a 1.66 m-long barrel (inner diameter = 25.4 mm), resulting in a muzzle speed of 63 m/s measured by time of flight between two flush-mounted sensors in the barrel. An array of three continuously burning propane torches was placed in the path of the ejected fuel. To prevent the torches from being extinguished by the burst of air from the gun, a shield was placed between the torch tip and the gun. The impact, misting, subsequent ignition and flame propagation were captured using a high-speed camera (Photron SA1.1, frame rate 10 kHz). Image acquisition was triggered by a laser-motion detector attached to the end of muzzle.

Movie 1 High-speed impact movie for pure Jet-A.

Movie 2 High-speed impact movie for Jet-A treated with 4.2M PIB (0.35% wt) "unsheared"

Movie 3 High-speed impact movie for Jet-A treated with 4.2M PIB (0.35% wt) "sheared" 

Movie 4 High-speed impact movie for Jet-A treated with 430k TA (0.3% wt) "unsheared"

Movie 5 High-speed impact movie for Jet-A treated with 430k TA (0.3% wt) "sheared"

References:

  1. McKinley, G. H. & Sridhar, T.  Filament-stretching rheometry of complex fluids. Annu. Rev. Fluid Mech. 34, 375-415 (2002).
  2. Peng, S. T. J. & Landel, R. F. Rheological behavior of FM-9 solutions and correlation with flammability test-results and interpretations. J. Non-Newton Fluid 12, 95-111 (1983).
  3. National Research Council (U.S.). Committee on Aviation Fuels with Improved Fire Safety. Aviation fuels with improved fire safety:a proceedings. (National Academy Press, Washington, D.C., 1997).
  4. Paterson, R. W. & Abernathy, F. H. Turbulent flow drag reduction and degradation with dilute polymer solutions. J. Fluid Mech. 43, 689-710 (1970).
  5. Lehn, J. -M. Toward self-organization and complex matter. Science 295, 2400-2403 (2002).
  6. Aida, T., Meijer, E. W. & Stupp, S. I. Functional Supramolecular Polymers. Science 335, 813-817 (2012).
  7. Boal, A. K. et al. Self-assembly of nanoparticles into structured spherical and network aggregates. Nature 404, 746-748 (2000).
  8. Tayi, A. S. et al. Room-temperature ferroelectricity in supramolecular networks of charge-transfer complexes. Nature 488, 485-489 (2012).
  9. Ikkala, O. & ten Brinke, G. Functional materials based on self-assembly of polymeric supramolecules. Science 295, 2407-2409 (2002).
  10. Yang, S. K., Ambade, A. V. & Weck, M. Main-chain supramolecular block copolymers. Chem. Soc. Rev. 40, 129–137 (2011).
  11. Li, S.-L., Xiao, T. X., Lin, C. & Wang, L. Y. Advanced supramolecular polymers constructed by orthogonal self-assembly. Chem. Soc. Rev. 41, 5950-5968 (2012).
  12. David, R. L. A. et al. Effects of pairwise, self-associating functional side groups on polymer solubility, solution viscosity, and mist control. Macromolecules 42, 1380-1391 (2009).

Mixed Matrix Polymer Films for Sustainable Chemistry  

Rachel Ford

Polymeric membranes are critical for applications in sustainable chemistry, engineering and materials (SusChEM)1–2.  The Diallo Group at KAIST1–3 has made major advances in the preparation of mixed matrix porous membranes that combine the durability of polyvinylidene fluoride (PVDF) with the fouling resistance and diverse functionality of neutral, hydrophilic polymers (e.g., polyethylene glycol).  The Diallo Group recently discovered a new class of membranes—PVDF membranes with embedded polymeric particles (Figure 1)—and these materials show broad potential applications.

In collaboration with Prof. Diallo, we seek to understand the relationship between the preparation conditions and the resulting structure of these membranes, using small-angle neutron scattering (SANS), electron microscopy, x-ray photoelectron spectroscopy, and permporometry.  Two exciting applications of the membrane will also be explored. The first one is water purification, particularly desalination of seawater.  Our goal is to develop ultrafiltration and nanofiltration membranes for the pretreatment of seawater. Furthermore, we will exploit a few key features of these membranes for use in electrochemical cells.  One such feature is the functionality of the embedded polymeric particles; this functionality enables them to bind metal ions and encapsulate metal clusters.  We are utilizing this chelating ability to develop novel copper-containing electrode materials for the electrochemical reduction of carbon dioxide.

Preparation of mixed matrix polyvinylidene fluoride (PVDF) membrane with in-situ synthesized polyethyleneimine (PEI) particles.  Particles are made by crosslinking hyperbranched PEI with epichlorohydrin (ECH).4

References

1) Kotte, M. R.; Hwang, T.; Han, J-I. and Diallo, M. S. J. Memb. Sci. 2015, 474, 277–287.

2) Kotte, M. R., Cho, M. and Diallo, M. S. J. Memb. Sci. 2014, 450, 93–102.

3) Kotte, M. R., Kuvarega, A., Mamba, B. B, Cho, M. and Diallo, M. S. Mixed Matrix PVDF Membranes With In-Situ Synthesized Dendrimer-Like PAMAM Particles: A New Class of Sorbents for Cu(II) Recovery from Aqueous Solutions by Ultrafiltration. Environ. Sci. Technol. Accepted.

4) Figure 4B from Diallo, M. S.; Kotte, M. R.; and Cho, M. Mining Critical Metals and Elements from Seawater: Opportunities and Challenges. Environ. Sci. Technol. [Article ASAP]. DOI: 10.1021/acs.est.5b00463. Published Online: April 20, 2015.

Identifying the molecular mechanism of action of an investigational ocular drug

Zach Zixuan Shao, Dan Zhou

In collaboration with a local pharmaceutical company, we are currently investigating a therapeutic oligopeptide that is potent in treating several blinding eye diseases (eg. age-related macular degeneration and diabetic retinopathy). Clinical studies using the drug showed significant vision improvement, yet its molecular mechanism of action remains unknown. Our research focuses on identifying its locus of binding and the downstream molecular pathways responsible for the therapeutic effects. We are currently employing ligand capture and cell labeling methods to identify the binding partners as well as bioinformatic tools to discover the affected cellular pathways. Results from our studies will shed light on the pathophysiology of the relevant retinal diseases as well as better treatment of these and other related complications. 

OCT Scan of the back of the eye showing drug treatment reduced inflammatory edema and restored vision

Seeing clearly: Using protein nanofibers to promote orderly corneal wound healing

Amy Fu

The study evaluates the ability of electrospun protein nanofibers to display topographical and biochemical cues to support epithelial closure, foster fibroblast recruitment and mitigate myofibroblast phenotype. Gelatin electrospun nanofibers were used to present integrin binding sites and mechanical cues. By control of the electrode geometry, we prepare nanofiber mats with three types of orientation: isotropic, radial, and uniaxial. Rates of epithelial and fibroblast cell migration in vitro, measured using a mock wound healing assay, showed a result for epithelial cells. While fibroblasts (and a variety of cells in the literature) migrate rapidly on oriented nanofibers, corneal epithelial cells migrate faster on isotropic nanofibers than on aligned nanofibers or planar controls. Of particular importance, production of smooth muscle actin (αSMA, green in image) by myofibroblasts (TGF-β transformed fibroblasts) was reduced in cells cultured on oriented nanofiber substrates. Nanofiber diameter over the range 100–220 nm does not measurably affect migration or αSMA expression. The molecular mechanism by which myofibroblasts respond to topographical cues was explored using siRNA transfected cells with knockdown of kinases associated with integrin mediated response (FAK and Raf-1) and transcriptional regulators associated with mechanotransduction (YAP and TAZ). We were surprised that the response does not involve integrin-mediated transduction; it involves YAP and TAZ (indicative of mechanostranduction). Preliminary in vivo experiments in mice and rabbits showed reepithelialization occurs as quickly over nanofiber scaffolds as it does over the native corneal stroma. The population of inflammatory cells was reduced in wound beds treated with nanofiber scaffolds relative to untreated controls. Oriented nanofiber scaffolds do not elicit an inflammatory response and have an anti-inflammatory effect. Oriented nanofiber substrates are well suited for corneal wound applications due to their transparency, non-cytotoxicity and ability to modulate the myofibroblast phenotype.