Dorian A. Canelas

Design of Nonionic Surfactants for Supercritical Carbon Dioxide

Interfacially active block copolymer amphiphiles have been synthesized and their self-assembly into micelles in supercritical carbon dioxide (CO2) has been demonstrated with small-angle neutron scattering (SANS). These materials establish the design criteria for molecularly engineered surfactants that can stabilize and disperse otherwise insoluble matter into a CO2 continuous phase. Polystyrene-b-poly(1,1-dihydroperfluorooctyl acrylate) copolymers self-assembled into polydisperse core-shell-type micelles as a result of the disparate solubility characteristics of the different block segments in CO2. These nonionic surfactants for CO2 were shown by SANS to be capable of emulsifying up to 20 percent by weight of a CO2-insoluble hydrocarbon into CO2. This result demonstrates the efficacy of surfactant-modified CO2 in reducing the large volumes of organic and halogenated solvent waste streams released into our environment by solvent-intensive manufacturing and process industries.

Top-down particle fabrication: control of size and shape for diagnostic imaging and drug delivery

This review discusses rational design of particles for use as therapeutic vectors and diagnostic imaging agent carriers. The emerging importance of both particle size and shape is considered, and the adaptation and modification of soft lithography methods to produce nanoparticles are highlighted. To this end, studies utilizing particles made via a process called Particle Replication In Non-wetting Templates are discussed. In addition, insights gained into therapeutic cargo and imaging agent delivery from related types of polymer-based carriers are considered.

Polymerizations in Liquid and Supercritical Carbon Dioxide

In the past few years, remarkable progress has been made in defining the scope and limitations of carbon dioxide (CO2) as an inert polymerization medium. It has appear that CO2 represents a viable solvent choice for a variety of propagation mechanisms including both chain growth and step growth polymerizations. When the environmental advantages of CO2 are combined with its ability to be used as a solvent/dispersing medium for a wide variety of chemical reactions, it becomes clear that CO2 may be the solvent of the future for the polymer industry. In addition, the design and synthesis of micelle-forming surfactants for CO2 opens the doors for use of surfactant-modified CO2 as the medium for heterogeneous polymerizations. This review will focus on the use of CO2 as an inert solvent for the synthesis and processing of polymers.

Neutron scattering characterization of homopolymers and graft-copolymer micelles in supercritical carbon dioxide

Abstract Superficial fluids (SCF) are becoming an attractive alternative to the liquid solvents traditionally used as polymerization media [1]. As the synthesis proceeds, a wide range of colloidal aggregates form, but there has hitherto been no way to measure such structures directly. We have applied small-angle neutron scattering (SANS) to characterize such systems, and although SCF polymerizations are carried out at high pressures, the penetrating power of the neutron beam means that typical cell windows are virtually transparent. Systems studied include polymers soluble in CO 2 such as poly(1,1-dihydroperfluorooctyl acrylate) (PFOA), poly(hexafluoropropylene oxide) (PHFPO) and poly(dimethyl siloxane) (PDMS). PFOA has previously [2] been shown to exhibit a positive second virial coefficient ( A 2 ), though for PHFPO, A 2 appears to be close to zero ( 10 4 A 2 ⋍ 0 ± 0.2 cm 3 g −2 mol ). PDMS is soluble on the molecular level only in the limit of dilute solution and seems to form aggregates as the concentration increases (c > 0.01 g cm −3 ). Typical hydrocarbon polymers are insoluble in CO 2 , but it has been found that polymerizations may be accomplished via the use of a stabilizer [3]. For example, styrene has been polymerized in CO 2 by means of a polystyrene-b-PFOA block copolymer surfactant, which forms micelles in CO 2 and is also amenable to SANS characterization. Other amphiphilic surfactant molecules that form micelles include PFOA-g-poly(ethylene oxide) (PFOA-g-PEO) graft copolymers, which swell as the CO 2 medium is saturated with water. These systems have been characterized by SANS, by taking advantage of the different contrast options afforded by substituting D 2 O for H 2 O. This paper illustrates the utility of SANS to measure molecular dimensions, thermodynamic variables, molecular weights, micelle structures etc. in supercritical CO 2 .

The Morphology of Block Copolymer Micelles in Supercritical Carbon Dioxide by Small‐Angle Neutron and X‐ray Scattering

Above its critical point, carbon dioxide forms a super-critical fluid, which promises to be an environmentally responsible replacement for the organic solvents traditionally used in polymerizations. Many lipophilic polymers such as polystyrene (PS) are insoluble in CO2, though polymerizations may be accomplished via the use of PS-fluoropolymer stabilizers, which act as emulsifying agents. Small-angle neutron and X-ray scattering have been used to show that these molecules form micelles with a CO2-phobic PS core and a CO2-philic fluoropolymer corona. When the PS block was fixed in length and the fluorinated corona block was varied, the number of block copolymer molecules per micelle (six to seven) remained constant. Thus, the coronal block molecular weight exerts negligible influence on the aggregation number, in accordance with the theoretical predictions of Halperin, Tirrell & Lodge [Adv. Polym. Sci. (1992), 100, 31–46]. These observations are relevant to understanding the mechanisms of micellization and solubilization in supercritical fluids.

Cooperative learning in organic chemistry increases student assessment of learning gains in key transferable skills

Science and engineering educators and employers agree that students should graduate from college with expertise in their major subject area as well as the skills and competencies necessary for productive participation in diverse work environments. These competencies include problem-solving, communication, leadership, and collaboration, among others. Using a pseudo-experimental design, and employing a variety of data from exam scores, course evaluations, and student assessment of learning gains (SALG) surveys of key competencies, we compared the development of both chemistry content knowledge and transferable or generic skills among students enrolled in two types of large classes: a lecture-based format versus an interactive, constructive, cooperative learning (flipped classroom) format. Controlling for instructor, as well as laboratory and recitation content, students enrolled in the cooperative learning format reported higher learning gains than the control group in essential transferable skills and competency areas at the end of the term, and more growth in these areas over the course of the term. As a result of their work in the class, the two groups of students reported the most significant differences in their gains in the following areas: “interacting productively to solve problems with a diverse group of classmates,” “behaving as an effective leader,” “behaving as an effective teammate,” and “comfort level working with complex ideas.” Our findings clearly show that cooperative learning course designs allow students to practice and develop the transferable skills valued by employers.