Browsing by Subject "polymer mechanochemistry"

When mechanical force is applied to synthetic materials, polymer chains become

highly strained, leading to bond scission and ultimately material failure. Over the last

decade or so, work in the field of polymer mechanochemistry has coupled this tension to

desired covalent chemical reactions. These functionalities, known as mechanophores,

react to unveil a new molecular structure and triggering a constructive response. This

strategy has been explored for a variety of purposes, including stress sensing, stress

strengthening, small molecule release, catalysis, and development of soft devices.

Additionally, the effect of force on a reaction coordinate, through biasing and probing

reaction pathways and trapping of transition states and intermediates, has been well–

studied experimentally and in theory. This work reports on understanding structure property

relationships for the spiropryan mechanophore and expanding our control of mechanochromism

from the single-molecule to device scale.

First, we report the effect of substituents on spiropyran derivatives substituted

with H, Br, or NO2 para to the breaking spirocyclic C− O bond using single molecule

force spectroscopy. The force required to achieve the rate constants of ~ 10 s−1 necessary

to observe transitions in the force spectroscopy experiments depends on the substituent,

with the more electron withdrawing substituent requiring less force. Rate constants at

375 pN were determined for all three derivatives, and the force coupled rate dependenc

eon substituent identity is well explained by a Hammett linear free energy relationship

with a value of ρ = 2.9, consistent with a highly polar transition state with heterolytic,

dissociative character. The methodology paves the way for further application of linear

free energy relationships and physical organic methodologies to mechanochemical

reactions.

The development and characterization of new force probes has enabled

additional, quantitative studies of force-coupled molecular behavior in polymeric

materials. The relationship between strain and color change has been measured for

these three spiropyran derivatives. The color appears at around the same strain and the

ratio of color intensities remains constant for all three derivatives. This result was not predicted by

previously reported computational work and motivates future studies of

force distribution within filled silicones.

On the material and device scale, we have utilized mechanochromism for soft

and stretchable electronics, which are promising for a variety of applications such as

wearable electronics, human− machine interfaces, and soft robotics. These devices,

which are often encased in elastomeric materials, maintain or adjust their functionality

during deformation, but can fail catastrophically if extended too far. Here, we report

new functional composites in which stretchable electronic properties are coupled to

molecular mechanochromic function, enabling at-a-glance visual cues that inform user

control. These properties are realized by covalently incorporating a spiropyran

mechanophore within poly(dimethylsiloxane) to indicate with a visible color change that

a strain threshold has been reached. The resulting colorimetric elastomers can be molded

and patterned so that, for example, the word “STOP” appears when a critical strain is

reached, indicating to the user that further strain risks device failure. We also show that

the strain at color onset can be programmed through the layering of silicones with

different moduli into a composite. As a demonstration, we show how color onset can be

tailored to indicate a when a specified frequency of a stretchable liquid metal antenna

has been reached. The multi-scale combination of mechanochromism and soft

electronics offers a new avenue to empower user control of strain-dependent properties

for future stretchable devices.

Through the study of the reaction that converts spiropyran into merocyanine, we

are able to teach and connect a number of standard general chemistry course topics

while also introducing students to polymer concepts. By framing a number of different

concepts including molecular orbital theory, quantum mechanics, equilibrium,

hydrogen bonding, mechanical work, and polymer chemistry with the same reaction,

our goal is to allow students to see connections in seemingly disparate sections of

general chemistry.

The reactivity of a mechanically active functional group is determined by the

activation energy of the reaction (ΔG‡) and the force-coupled change in length as the

reaction proceeds from the ground to transition state (Δx‡). Finally, we report a combination

of both principles enhances the mechanochemical reactivity of epoxides:

placing alkenes adjacent to cis-epoxide mechanophores along a polymer backbone

results in ring-opening to carbonyl ylides during sonication, whereas epoxides lacking

an adjacent alkene do not. Upon release, tension-trapped ylides preferentially close to

their trans-epoxides in accordance with the Woodward-Hoffman rules. The reactivity of

carbonyl ylides is exploited to tag the activated species with spectroscopic labels for

force-induced cross-linking through a reaction with pendant alcohols. Even with alkene

assistance, mechanochemical reactivity remains low; single molecule force spectroscopy

establishes a lower limit for ring-opening ca. 1 sec-1 at forces of ~2600 pN.

Covalent polymer mechanochemistry has attracted significant attention in the last decade due to its broad potential in stress-responsive materials. Force-sensitive chemical moieties (mechanophores) embedded into polymer backbones are triggered by mechanical loads, with demonstrated responses that include mechanochromism, small-molecule release, mechano-catalysis, enhancements in material toughness and even electrical conductivity. While many qualitative studies have demonstrated the structure-activity correlation between mechanophore designs and the force-responsive properties of bulk materials, quantitative investigations remain rare. Gaining quantitative perspectives is advantageous for the rational, molecular design of polymer materials with tailored mechanical properties, which ultimately could lead to cost-effective and less wasteful materials. Here, we probe quantitative structure-activity relationships on three fronts: 1) stereochemical effects in the mechanochemical reactivity of endo- vs. exo- furan-maleimide Diels-Alder adducts; 2) the mechanochemical reaction pathways of dichlorocyclopropane diester ring opening reaction; 3) enhanced mechanics of hydrogels through stress-responsive covalent extension of single strands within the polymer network. Chapter 2 presents a systematic study of a structure-activity correlation in polymer mechanochemistry, specifically investigating how stereoisomerism affects the mechanochemical scission of furan-maleimide Diels-Alder adducts. In this study, we evaluated the internal competition between the mechanically triggered retro-DA reaction and the mechanochemical ring opening of gem-dichlorocyclopropane (gDCC) mechanophores in the pulsed sonication of polymer solutions. The relative extent of the two mechanochemical reactions in the same polymer shows that the endo DA isomer exhibits greater mechanical lability than its exo isomer. This result contrasts with recent measurements of the relative rates of scission in a similar system and points to potential enhanced sensitivity obtained through the use of internal competition as opposed to absolute rates in assessing mechanical reactivity in sonication studies. Chapter 3 investigates how the mechanically accelerated ring opening of gDCC mechanophores is influenced by the use of ester, rather than alkyl, handles. We find that when ester groups are used to link the mechanophore to the attached polymer chains, both cis- and trans- gDCC pulling lead to reaction rates and outcomes that are consistent with the symmetry-allowed disrotatory pathways at forces relevant to either single-molecule force spectroscopy and sonication experiments. Finally, Chapter 4 presents a quantitative study of how the properties of a bulk materials can be enhanced by embedding mechanophores capable of covalent reactive strand extension (RSE) into the constituent strands of a polymer network. RSE allows constituent strands to lengthen through force-coupled reactions that are triggered as the strands reach their nominal breaking point. Reactive strand extensions of up to 40% lead to hydrogels that stretch 40-50% further than, and exhibit tear energies twice that of, networks made from analogous control strands. The enhancements are synergistic with those provided by double network architectures, and complement other existing toughening strategies.