1,677 research outputs found
Calorimetric study of block-copolymers of poly(n-butyl acrylate) and gradient poly(n-butyl acrylate-co-methyl methacrylate)
Anti-Arrhenius cleavage of covalent bonds in bottlebrush macromolecules on substrate
Spontaneous degradation of bottlebrush macromolecules on aqueous substrates was monitored by atomic force microscopy. Scission of CâC covalent bonds in the brush backbone occurred due to steric repulsion between the adsorbed side chains, which generated bond tension on the order of several nano-Newtons. Unlike conventional chemical reactions, the rate of bond scission was shown to decrease with temperature. This apparent anti-Arrhenius behavior was caused by a decrease in the surface energy of the underlying substrate upon heating, which results in a corresponding decrease of bond tension in the adsorbed macromolecules. Even though the tension dropped minimally from 2.16 to 1.89 nN, this was sufficient to overpower the increase in the thermal energy (kBT) in the Arrhenius equation. The rate constant of the bond-scission reaction was measured as a function of temperature and surface energy. Fitting the experimental data by a perturbed Morse potential V = V0(1 - e-βx)2 - fx, we determined the depth and width of the potential to be V0 = 141 Âą 19 kJ/mol and β-1 = 0.18 Âą 0.03 âŤ, respectively. Whereas the V0 value is in reasonable agreement with the activation energy Ea = 80â220 kJ/mol of mechanical and thermal degradation of organic polymers, it is significantly lower than the dissociation energy of a CâC bond De = 350 kJ/mol. Moreover, the force constant Kx = 2β2V0 = 1.45 Âą 0.36 kN/m of a strained bottlebrush along its backbone is markedly larger than the force constant of a CâC bond Kl = 0.44 kN/m, which is attributed to additional stiffness due to deformation of the side chains
Monte Carlo Simulations of Atom Transfer Radical (Homo)polymerization of Divinyl Monomers: Applicability of FloryâStockmayer Theory
It
is well known that free radical (co)Âpolymerization of multivinyl
monomers (MVMs) leads to insoluble gels even at a low monomer conversion,
and the gelation point can be predicted by FloryâStockmayer
theory (FâS theory) based on two assumptions: (1) equal reactivity
of all vinyl groups and (2) the absence of intramolecular cyclization.
This theory has been experimentally studied and verified with conventional
free radical (co)Âpolymerization (FRP) of several MVMs (e.g., divinylbenzene,
DVB). However, it is still debatable whether this theory is applicable
for the polymerization of MVMs using reversible deactivation radical
polymerization (RDRP) approaches, such as atom transfer radical polymerization
(ATRP). Herein, Monte Carlo simulations using two statistical modelsî¸with
cyclization (<b>w.c.</b>) and without cyclization (<b>wo.c.</b>, corresponding to FâS theory)î¸and dynamic lattice
liquid (DLL) models were conducted to study ATRP of divinyl monomers.
The simulated gel points using <b>w.c.</b> and <b>wo.c.</b> models were compared with those obtained from ATRP experiments,
from calculation using FâS theory, and from simulations using
DLL models. The molecular weights, dispersity, and extent of intermolecular/intramolecular
cross-linking were calculated as a function of double bond and cross-linker
conversion. The results demonstrated that the gel points obtained
from both <b>w.c.</b> and <b>wo.c.</b> models were lower
than the values from DLL models and experiments. This indicates that
FâS theory cannot be used to accurately predict the polymerization
of divinyl monomers via ATRP. Our study shows that the limitation
of FâS theory in predicting ATRP reaction of divinyl monomers
is not only due to neglecting intramolecular cyclization but also
due to spatial restrictions which can cause the reactivity and accessibility
of vinyl groups becoming nonequivalent in ATRP of divinyl monomers
High-density information storage in an absolutely defined aperiodic sequence of monodisperse copolyester
Synthesis of a polymer composed of a large discrete number of chemically distinct monomers in an absolutely defined aperiodic sequence remains a challenge in polymer chemistry. The synthesis has largely been limited to oligomers having a limited number of repeating units due to the difficulties associated with the step-by-step addition of individual monomers to achieve high molecular weights. Here we report the copolymers of ??-hydroxy acids, poly(phenyllactic-co-lactic acid) (PcL) built via the cross-convergent method from four dyads of monomers as constituent units. Our proposed method allows scalable synthesis of sequence-defined PcL in a minimal number of coupling steps from reagents in stoichiometric amounts. Digital information can be stored in an aperiodic sequence of PcL, which can be fully retrieved as binary code by mass spectrometry sequencing. The information storage density (bit/Da) of PcL is 50% higher than DNA, and the storage capacity of PcL can also be increased by adjusting the molecular weight (~38???kDa)
Synthesis and characterization of polystyrene-blockpoly(vinylbenzoic acid): a promising compound for manipulating photoresponsive properties at the nanoscale
"Published online: 27 January 2015"Using reversible addition-fragmentation chain
transfer (RAFT) polymerization, the effect of PSt macroRAFT
and 4VBA ratio on the synthesis of a carboxylic
acid functional block copolymer (PSt-b-P4VBA) was
studied. PSt macroRAFT polymer was initially prepared
followed by the insertion of 4-vinylbenzoic acid (4VBA)
monomer. The chemical structure of the diblock copolymer
was confirmed by NMR and FTIR. The effect of PSt
macroRAFT and 4VBA ratio on copolymerization yield
and on molecular weight distribution was assessed by gel
permeation chromatography. The rate of polymerization
did not change as the 4VBA/PSt macroRAFT ratio
increased, indicating an ideal amount of 4VBA insertion.
An optimal ratio of [PSt macroRAFT]:[AIBN]:[4VBA]
was 1.2:1:180. DSC and XRD confirmed the amorphous
structure of homo and copolymer. Thermal stability was
higher for PSt-b-P4VBA forming activated porous carbon
char by dehydration, carbonization and oxidation. SEM
and STEM observations showed a morphological evolution
between PSt macroRAFT and the correspondent
copolymer.The authors acknowledge the n-STeP-Nanostructured systems for Tailored Performance, with reference NORTE-07-0124-FEDER-000039, supported by the Programa Operacional Regional do Norte (ON.2), PEst-C/CTM/LA0025/2013 (Strategic Project-LA 25-2013-2014)
Influence of Graft Density on Kinetics of Surface-Initiated ATRP of Polystyrene from Montmorillonite
Ferritin immobilization on patterned poly(2-hydroxyethyl methacrylate) brushes on silicon surfaces from colloid system
In this paper, we describe a graft polymerization/solvent immersion method for generating poly(2-hydroxyethyl methacrylate) (PHEMA) brushes in various patterns. We used a novel fabrication process, involving very-large-scale integration and oxygen plasma treatment, to generate well-defined patterns of polymerized PHEMA on patterned Si(100) surfaces. We observed brush- and mushroom-like regions for the PHEMA brushes, with various pattern resolutions, after immersing wafers presenting lines of these polymers in MeOH and n-hexane, respectively. The interaction between PHEMA and ferritin protein sheaths in MeOH and n-hexane (good and poor solvent for PHEMA, respectively) was used to capture and release ferritins from fluidic system. The âtentaclesâ behaver for PHEMA brushes was found through various solvents in fluidic system. Using high-resolution scanning electron microscopy, we observed patterned ferritin Fe cores on the Si surface after pyrolysis of the patterned PHEMA brushes and ferritin protein sheaths, which verify the âtentaclesâ behaver for PHEMA brushes
Design and Use of Organic Nanoparticles Prepared from Star-Shaped Polymers with Reactive End Groups
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