scholarly journals Polymer Degradation through Chemical Change: A Quantum-based Test of Inferred Reactions in Irradiated Polydimethylsiloxane

2021 ◽  
Author(s):  
Matthew Kroonblawd ◽  
Nir Goldman ◽  
Amitesh Maiti ◽  
James Lewicki
Keyword(s):  
Ad Hoc ◽  
Chemical Change ◽  
Polymer Degradation ◽  
Chain Scission ◽  
Reaction Networks ◽  
Oh Groups ◽  
Chemical Damage ◽  
Causal Connections ◽  
Silicone Polymers ◽  
Reaction Schemes

Chemical reaction schemes are key conceptual tools for interpreting the results of experiments and simulations, but often carry implicit assumptions that remain largely unverified for complicated systems. Established schemes for chemical damage through crosslinking in irradiated silicone polymers comprised of polydimethylsiloxane (PDMS) date to the 1950's and correlate small-molecule off-gassing with specific crosslink features. In this regard, we use a somewhat reductionist model to develop a general conditional probability and correlation analysis approach that tests these types of causal connections between proposed experimental observables to reexamine this chemistry through quantum-based molecular dynamics (QMD) simulations. Analysis of the QMD simulations suggests that the established reaction schemes are qualitatively reasonable, but lack strong causal connections under a broad set of conditions that would enable making direct quantitative connections between off-gassing and crosslinking. Further assessment of the QMD data uncovers a strong (but nonideal) quantitative connection between exceptionally hard-to-measure chain scission events and the formation of silanol (Si-OH) groups. Our analysis indicates that conventional notions of radiation damage to PDMS should be further qualified and not necessarily used ad hoc. In addition, our efforts enable independent quantum-based tests that can inform confidence in assumed connections between experimental observables without the burden of fully elucidating entire reaction networks.

Occasionalism

Philosophy ◽  
2014 ◽  
Author(s):  
Walter Ott
Keyword(s):  
Forest Fire ◽  
David Hume ◽  
Ad Hoc ◽  
18Th Century ◽  
David Lewis ◽  
Human Action ◽  
Natural World ◽  
Lightning Strike ◽  
Causal Connections ◽  
Modern Period

Occasionalism is the doctrine that God is the only true cause. What appear to be causes in the natural world—a lightning strike that sets off a forest fire, for example—are only “occasions” for God to act. Such natural events are merely correlated, not causally connected. The same holds true for human action: One’s desire to move one’s arm toward the chocolate ice cream is only an occasion for God to move one’s arm. Such a view seems outlandish, at best a historical curiosity. Since the modern period, occasionalism has been dismissed as an ad hoc answer to the Cartesian problem of interaction: If minds are not physical things, how can they act on bodies? Occasionalism simply denies that there is any interaction at all, hence the problem is dissolved. But in fact, occasionalism antedates the Cartesian problematic by many centuries. And even in the modern period, its real motivations have more to do with the nature of divine creation and causation itself than with the problem of interaction. While occasionalism has few adherents today, its rejection of genuine causal connections in the sublunary world was an important source for the views of David Hume in the 18th century and David Lewis in the 20th.

Occasionalism and the Cartesian Metaphysic of Motion*

1975 ◽  
Vol 1 (1) ◽  
pp. 29-40 ◽  
Author(s):  
T. M. Lennon
Keyword(s):  
Body Problem ◽  
Ad Hoc ◽  
Body Interaction ◽  
General Account ◽  
Causal Connections ◽  
Mind Body Problem ◽  
The Mind ◽  
Special Case

Occasionalism is often taken by historians of philosophy to have been an ad hoc hypothesis to establish the mind-body causal connections which on Cartesian principles are thought otherwise impossible. My aim in this paper is to show that this view is utterly without historical foundation, that, on the contrary, the view that only God can be a real cause of mind-body interaction was but a special case of a claim argued on grounds transcending the mind-body problem, and, what will be part of this, that the logical character of occasionalism anyhow precluded it from the role into which it was later miscast. More specifically, I shall show that occasionalism was but a consequence of the metaphysics adopted by the Cartesians in their general account of change. Though the same case could be made for the views of Clauberg and Geulincx, my concern will be with the occasionalism of Malebranche. My case here will be that his view is the historical and logical dénouement of principles more or less explicit both in Descartes and in two of his lesser known disciples, LaForge and Cordemoy.

Degradation of Rubber by Chemical Agents in Solution

1961 ◽  
Vol 34 (4) ◽  
pp. 1212-1219
Author(s):  
G. H. Foxley
Keyword(s):  
Free Radicals ◽  
Low Temperatures ◽  
Model Compound ◽  
Polymer Degradation ◽  
Chain Scission ◽  
Redox Systems ◽  
Chemical Agents ◽  
The Subject ◽  
Partial Fulfillment

Abstract It is clear that the subject of polymer degradation by chemical agents is an active branch of polymer chemistry. This is reflected in the large number of patents applying to polymer degradation and no attempt has been made to include every appropriate patent. Much of the comparative work is based on equal weights, rather than equal numbers of molecules, so that the true comparisons of the efficiency of peptizers are often difficult. It has been shown that polymer degradation can proceed via several mechanisms all of which involve free radicals and the main points can be summarized as follows :— In solutions at low temperatures the initiatory free radicals come from the added peptizer such as benzoyl peroxide or bis-azoisobutyronitrile. Although oxygen accelerates the reaction, it is not essential, and there is appreciable degradation in the absence of oxygen. Thiols are active only when oxygen is present even at high temperature. This is somewhat surprising, since the rubber radicals produced by thermal scission should be just as active as those produced by mastication and be capable of reaction with thiols, and serves to emphasize the importance of the role of oxygen in peptization by thiols and disulfides. Oxygen is also necessary for degradation by redox systems and in its absence structurizing takes place. The lack of work on triphenyl methane derivatives is somewhat surprising in view of the ease with which they undergo homolysis to give free radicals. However, it is not sufficient to introduce any type of free radical and expect degradation: stabilized free radicals are the best peptizers, unstable radicals can add to olefinic bonds and cause crosslinking rather than chain scission. Squalene has been used as a model compound for the study of the reactions of natural rubber with free radicals in a similar manner to the use of methylcyclohexene as a model compound for oxidation studies. This review forms part of a dissertation submitted in partial fulfillment of the requirements for the London University M.Sc. (External) Examination.

scholarly journals Organocatalytic Control over a Fuel-Driven Transient Esterification Network

2020 ◽  
Author(s):  
Michelle van der Helm ◽  
Chang-Lin Wang ◽  
Mariano Macchione ◽  
Eduardo Mendes ◽  
Rienk Eelkema
Keyword(s):  
Chemical Change ◽  
Polymer System ◽  
Reaction Networks ◽  
Polymer Conformation ◽  
Controlled Processes ◽  
Full Control ◽  
Ester Formation ◽  
Driven System ◽  
Ester Yield ◽  
Non Equilibrium

<p>Signal transduction in living systems is the conversion of information into a chemical change and the principal process by which cells communicate. This process enables phenomena such as time-keeping and signal amplification. In nature, these functions are encoded in non-equilibrium (bio)chemical reaction networks (CRNs) controlled by enzymes. While these catalytically controlled processes are an integral part of biocatalytic pathways, man-made analogs are rare. Here, we incorporate catalysis in an artificial fuel driven out-of-equilibrium CRN. The study entails the design of an organocatalytically controlled fuel driven esterification CRN, where the forward (ester formation) and backward reaction (ester hydrolysis) are controlled by varying the ratio of two different organocatalysts: pyridine and imidazole. This catalytic regulation enables full control over ester yield and lifetime. The fuel-driven strategy is subsequently used in the design of a responsive polymer system, where transient polymer conformation and aggregation can be controlled through variation of fuel and catalysts levels. Altogether, we show how organocatalysis is an important tool to exert control over a man-made fuel driven system and induce a change in a macromolecular superstructure, as ubiquitously found in natural non-equilibrium systems. </p>

scholarly journals Organocatalytic Control over a Fuel-Driven Transient Esterification Network

2020 ◽  
Author(s):  
Michelle van der Helm ◽  
Chang-Lin Wang ◽  
Mariano Macchione ◽  
Eduardo Mendes ◽  
Rienk Eelkema
Keyword(s):  
Chemical Change ◽  
Polymer System ◽  
Reaction Networks ◽  
Polymer Conformation ◽  
Controlled Processes ◽  
Full Control ◽  
Ester Formation ◽  
Driven System ◽  
Ester Yield ◽  
Non Equilibrium

<p>Signal transduction in living systems is the conversion of information into a chemical change and the principal process by which cells communicate. This process enables phenomena such as time-keeping and signal amplification. In nature, these functions are encoded in non-equilibrium (bio)chemical reaction networks (CRNs) controlled by enzymes. While these catalytically controlled processes are an integral part of biocatalytic pathways, man-made analogs are rare. Here, we incorporate catalysis in an artificial fuel driven out-of-equilibrium CRN. The study entails the design of an organocatalytically controlled fuel driven esterification CRN, where the forward (ester formation) and backward reaction (ester hydrolysis) are controlled by varying the ratio of two different organocatalysts: pyridine and imidazole. This catalytic regulation enables full control over ester yield and lifetime. The fuel-driven strategy is subsequently used in the design of a responsive polymer system, where transient polymer conformation and aggregation can be controlled through variation of fuel and catalysts levels. Altogether, we show how organocatalysis is an important tool to exert control over a man-made fuel driven system and induce a change in a macromolecular superstructure, as ubiquitously found in natural non-equilibrium systems. </p>

A Multiscale Mixture Model for Polymer Degradation and Erosion

2010 ◽  
Author(s):  
João A. Soares ◽  
Paolo Zunino
Keyword(s):  
Polymer Degradation ◽  
Chain Scission ◽  
Polymer Chains ◽  
Local Drug Delivery ◽  
Implant Design ◽  
Tissue Engineering Scaffolds ◽  
Material Loss ◽  
Biodegradable Implant ◽  
Polymer Backbone ◽  
Polymer Bulk

Over the last 50 years, biodegradable materials have found a wide variety of applications in the medical field ranging from biodegradable sutures, pins and screws for orthopedic surgery, implants for local drug delivery, tissue engineering scaffolds, and biodegradable endovascular and urethral stents. The ability to predict the evolution of biodegradable polymers over the course of degradation would enhance the biodegradable implant design process. Polymer degradation is the irreversible chain scission process that breaks polymer chains down to oligomers and finally monomers. Extensive degradation leads to erosion, which is the process of material loss from the polymer bulk. Such materials can be monomers, oligomers, parts of the polymer backbone, or even parts of the polymer bulk. Hence, degradation and erosion are distinct but related process.

scholarly journals An accurate and efficient numerical calculation of detonation waves in multidimensional supernova simulations using a burning limiter and adaptive quasi-statistical equilibrium

2020 ◽  
Vol 493 (4) ◽  
pp. 5413-5433 ◽  
Author(s):  
Doron Kushnir ◽  
Boaz Katz
Keyword(s):  
Numerical Scheme ◽  
Ad Hoc ◽  
Statistical Equilibrium ◽  
Reaction Networks ◽  
Detonation Waves ◽  
Quasi Equilibrium ◽  
Small Length Scale ◽  
Transition Criterion ◽  
Better Than

ABSTRACT Resolving the small length-scale of thermonuclear detonation waves (TNDWs) in supernovae is currently not possible in multidimensional full-star simulations. Additionally, multidimensional simulations usually use small, oversimplistic reaction networks and adopt an ad hoc transition criterion to nuclear statistical equilibrium (NSE). The errors due to the applied approximations are not well understood. We present here a new accurate and efficient numerical scheme that accelerates the calculations by orders of magnitudes and allows the structure of TNDWs to be resolved. The numerical scheme has two important ingredients: (1) a burning limiter that broadens the width of the TNDW while accurately preserving its internal structure, and (2) an adaptive separation of isotopes into groups that are in nuclear statistical quasi-equilibrium, which resolves the time-consuming burning calculation of reactions that are nearly balanced out. Burning is calculated in situ employing the required large networks without the use of post-processing or pre-describing the conditions behind the TNDW. In particular, the approach to and deviation from NSE are calculated self-consistently. The scheme can be easily implemented in multidimensional codes. We test our scheme against accurate solutions of the structure of TNDWs and against homogeneous expansion from NSE. We show that with resolutions that are typical for multidimensional full-star simulations, we reproduce the accurate thermodynamic trajectory (density, temperature, etc.) to an accuracy that is better than a per cent for the resolved scales (where the burning limiter is not applied), while keeping the error for unresolved scales (broadened by the burning limiter) within a few per cent.

scholarly journals Deep Reaction Network Exploration at a Heterogeneous Catalytic Interface

2021 ◽  
Author(s):  
Qiyuan Zhao ◽  
Yinan Xu ◽  
Jeffrey Greeley ◽  
Brett Savoie
Keyword(s):  
Ad Hoc ◽  
Heterogeneous Systems ◽  
Reaction Network ◽  
System Size ◽  
Product Formation ◽  
Major Product ◽  
Ethylene Oligomerization ◽  
Reaction Networks ◽  
Heterogeneous Catalytic ◽  
Reaction Barriers

Characterizing the reaction energies and reaction barriers of complex reaction networks is central to catalyst development and optimization. Nevertheless, heterogeneous catalytic surfaces pose several unique challenges to automatic reaction network characterization, including large system sizes and open-ended reactant lists, that make ad hoc network construction and characterization the current state-of-the-art. Here we show how automated algorithms for exploring and characterizing reaction networks can be adapted to the constraints of heterogeneous systems using ethylene oligomerization on silica-supported single site Ga3+ catalysts as a model system. Using only graph-based rules for exploring the network and elementary constraints based on activation energy and system size for identifying network terminations, a comprehensive reaction network was generated for this system and validated against standard methods. The automated algorithm (re)discovers the classic Cossee-Arlman mechanism for this system that is hypothesized to drive major product formation while remarkably also predicting several new pathways for producing alkanes and coke precursors. This demonstration represents the largest heterogeneous catalyst (more than 50 atoms, with an open-ended pool of reactants) to be characterized using a quantum chemistry-based automated reaction method.

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