Intrachain Contraction Research Articles

Kinetically Controlled Star Copolymer Self-Assembly for Rapid Fabrication of Nanoparticles with High Encapsulation Capacity.

Rapid and scalable self-assembly of an amphiphilic 21-arm star copolymer, (polystyrene-block-polyethylene glycol)21 [(PS-b-PEG)21 ] in aqueous solution has been performed by reverse solvent exchange procedure. Transmission electron microscope (TEM) and nanoparticle tracking analysis (NTA) reveal the formation of nanoparticles with narrow size distribution. Further investigation indicates a kinetically controlled self-assembly mechanism of the copolymers, in which the star topology of the amphiphilic copolymer and deep quenching condition by reverse solvent exchange are key to accelerate intrachain contraction of the copolymer during phase separation. When interchain contraction dominant over interchain association, nanoparticles with low aggregation number could be formed. Thanks to the high hydrophobic contents of the (PS-b-PEG)21 polymers, the resulted nanoparticles could encapsulate a high capacity of hydrophobic cargo up to 19.84 %. The kinetically controlled star copolymer self-assembly process reported here provides a platform for the rapid and scalable fabrication of nanoparticle with high drug loading capacity (LC), which may find broad range of applications in, for example drug delivery, nanopesticide.

Selectively Deuterated Poly(ε-caprolactone)s: Synthesis and Isotope Effects on the Crystal Structures and Properties

Selective deuteration is an important tool for many analytical techniques including neutron scattering and spectroscopies. However, the availability of deuterated materials is limited because of the challenges in their synthesis. Here, we report the synthesis of partially and fully deuterated ε-caprolactone monomers and their corresponding polymers, poly(ε-caprolactone)s (PCLs), and the investigation of isotope effects on their crystalline structures and physical properties. Deuteration of PCLs leads to smaller crystal lattices and volumes compared to protiated PCLs by the amount proportional to the deuteration levels. The linear trend suggests that the volume isotope effect in PCL is primarily governed by the vibrations of C–D and C–H bonds. The large intrachain contraction of deuterated PCLs compared to that of polyethylene reported in the literature can be ascribed to the presence of polar ester groups in PCLs. Deuterated PCLs also display lower melting temperatures than protiated PCLs proportional to their deuteration levels because of weaker intermolecular interactions in deuterated polymers. FTIR spectroscopy, with support from density functional theory calculations, shows large red shifts of the stretching and bending frequencies of C–D versus C–H bonds as dictated by their relative reduced masses 13/7, and to a smaller degree for the C═O stretching frequency. This work is among the very few studies comparing the effects of partial versus full deuteration on the structures and properties in semicrystalline polymers. These results not only advance our understanding of isotope effects in polymeric materials but also provide an important avenue to design polymers with desirable properties.

Mechanisms of polyaniline film formation via solution casting: Intra-chain contraction versus inter-chain association

Two different mechanisms of formation of solution-cast films of emeraldine base (EB) of polyaniline are shown. The first mechanism was found in films drop-cast from the concentrated solution of EB. This mechanism is suggested to be due to entanglement of different polymer chains when the film is formed. It results in the relatively loose film morphology with fractal dimensionality of 1.68 and blue color. This film has a relatively good ability to absorb analyte vapors from the environment and a better ability to be protonated from the acidic medium as compared to films formed by the second mechanism. It is suggested that the polymer has an extended coil conformation in such a film, similar to that in a good solution. The second mechanism takes place when the film is drop-cast from a solution of low concentration and it is controlled by intramolecular processes which play the major role before the molecules aggregate into the film. This mechanism is suggested to be due to prevailing intramolecular tendency to compact-coil conformation. It results in formation of films of the grained morphology with fractal dimensionality of 1.56 and violet color. These films have a poor ability for absorption of analyte vapors and protonation as well. Various factors that affect the above mechanisms are discussed. The different properties of the films resulted from the different formation mechanisms allow us to specify them as different polymer phases, where either inter- or intra-chain processes play a crucial role, respectively.

Self-Assembly Assisted Polypolymerization (SAAP) of Diblock Copolymer Chains with Two Reactive Groups at Its Insoluble End

Preparation of diblock copolymers, (≡,N3)-poly(N-isopropylacrylamide)-b-poly(N,N-dimethylacrylamide) [(≡,N3)-PNIPAM-b-PDMA] and (≡,N3)-polystyrene-b-PNIPAM [(≡,N3)-PS-b-PNIPAM], with reactive alkyne and azide at one end using a trifunctional agent enables us to study how their self-assembly in a selective solvent affects interchain coupling, i.e., the self-assembly assisted polypolymerization (SAAP). As expected, (≡,N3)-PNIPAM-b-PDMA chains self-assemble into a micelle-like core–shell structure with a PNIPAM core in water at 50 °C. The coupling of as many as 17 PNIAPM ends together led to star-like chains, independent of the copolymer concentration, while the coupling efficiencies at lower temperatures (with no self-assembly) and in good solvents are much lower. These star-like chains remember their “birth” state in water and undergo the intrachain contraction to form single-chain micelles instead of large multichain aggregates. On the other hand, (≡,N3)-PS-b-PNIPAM exists as individual chains in THF, a mixture of unimers and micelles in 2-propanol, the core–shell micelles in methanol, and irregular aggregates in water. Only in methanol, the coupling efficiency is notably improved. The addition of water into 2-propanol enhances the self-assembly and so does the interchain coupling. The current study shows that even the solvophobic interaction makes the insoluble blocks less mobile inside the core and decreases the collision probability of reactive chain ends, the self-assembly still concentrates the reactive ends together and assists the coupling if the selective solvent is properly chosen.

Intrachain Folding and Interchain Association of Hyperbranched Chains with Long Uniform Subchains Made of Amphiphilic Diblock Copolymers

Combining atom transfer radical polymerization and copper-catalyzed azide–alkyne “click” chemistry, hyperbranched copolymers with uniform poly(tert-butyl acrylate-b-styrene-b-tert-butyl acrylate) triblock copolymer subchains, denoted as hyper-(PtBA-PS-PtBA)n, were successfully prepared and characterized by size exclusion chromatography (SEC), FT-IR, and 1H NMR. Using laser light scattering, we first studied the intrachain contraction of ultralarge hyper-(PtBA36-PS55-PtBA36)600 chains in cyclohexane, a solvent selectively poor for PS at lower temperatures. We found that at temperatures lower than 34 °C each PS block collapses into a small globule that was stabilized by its three neighboring PtBA blocks with no intrachain or interchain association in a dilute solution. Further, after hydrolyzing those tert-butyl (tBA) moieties into acrylic acids (AA), we comparatively studied the interchain association of linear triblock PAA23-PS14-PAA23 and its resultant hyper-(PAA23-PS14-PAA23)n with different overall mol...

Thermoresponsive Self-Assemblies of Cyclic and Branched Oligosaccharide-block-poly(N-isopropylacrylamide) Diblock Copolymers into Nanoparticles

This paper discusses the thermoresponsive nanoparticles obtained by self-assemblies of nonlinear oligosaccharide-based diblock copolymer systems. These diblock copolymers were synthesized by Cu(I)-catalyzed 1,3-dipolar azide/alkyne cycloaddition ("click" reaction) of propargyl-functionalized β-cyclodextrin (βCyD) and xyloglucooligosaccharide (XGO) with poly(N-isopropylacrylamide) (PNIPAM) having a terminal azido group prepared by atom transfer radical polymerization (ATRP). Elastic and quasi-elastic light scattering analysis of the dibock copolymers in H(2)O indicated that thermodynamic phase transitions of the PNIPAM blocks at their cloud points (T(cp)s ≈ 34 °C), around lower critical solution temperatures (LCSTs), triggered their self-assemblies into the nanoparticles. These nanoparticles had narrow size distributions and small interphases (i.e., sharp boundaries). The mean hydrodynamic radii (R(h)s) of the βCyD and XGO-based nanoparticles were determined to be around 150 and 250 nm upon slow heating (i.e., step-by-step heating), and 364 and 91.5 nm upon fast heating, respectively, depending on a predominance of the interchain association or the intrachain contraction. Transmission electron microscope (TEM) and field emission gun-scanning electron microscopy (FEG-SEM) images of the nanoparticles clearly showed compact spherical nanoparticles whose cores are mainly made with the PNIPAM blocks, whereas the rough shells consist in the oligosaccharidic blocks.

Kinetically Driven Intra- and Interchain Association of Hydrophobically and Hydrophilically Modified Poly(acrylic acid) in Dilute Aqueous Solutions

Effects of pH, dodecyl, and PEO contents as well as the method of preparing the solution on the formation of the unimolecular micelles (unimer micelle) or aggregates made of poly(acrylic acid)-graft-poly(ethylene oxide)-graft-dodecyl (PAA-g-PEO-g-dodecyl) and PAA-g-dodecyl have been studied by a combination of static and dynamic laser light scattering (SLS and DLS). It revealed that: in most cases, these copolymers tend to form interchain associates; while low pH value, proper high dodecyl content and fast switch rate of solvent quality promote intrachain association. These phenomena of the system being trapped in a metastable state can be mainly attributed to its kinetic pathway: the factors mentioned earlier can enhance the initial intrachain contraction, leading to τc (interaction time) ≪ τe (entanglement time) for the two interaction intrachain globules. So they behave like tiny “elastic balls” and their further merge/fusion become nearly impossible during their interaction time (τc). In this way, the...

Multistep Thermosensitivity of Poly(N-n-propylacrylamide)-block-poly(N-isopropylacrylamide)-block-poly(N,N-ethylmethylacrylamide) Triblock Terpolymers in Aqueous Solutions As Studied by Static and Dynamic Light Scattering

Poly(N-n-propylacrylamide)-block-poly(N-isopropylacrylamide)-block-poly(N,N-ethylmethylacrylamide) (PnPA-b-PiPA-b-PEMA) triblock terpolymers of low polydispersity have been synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization via sequential monomer addition. The temperature-induced formation and dissociation of terpolymer micelles in aqueous solutions in a heating−cooling cycle were investigated by a combination of static and dynamic laser light scattering. In the heating process, the folding of PnPA at ca. 25 °C led to the formation of polymeric micelles with a collapsed hydrophobic PnPA core and a hydrophilic swollen PiPA−PEMA shell. Further increase of the temperature to ca. 32 and 53 °C led to the successive dehydration of PiPA and PEMA on the periphery, respectively. Our results revealed that when PEMA on the periphery of the micelle is too short to stabilize the hydrophobic core, individual micelles tend to aggregate into large micellar clusters, corresponding to the intrachain contraction and interchain association of the PiPA chains. In the cooling process, the complete dissolution temperature was lower than the association temperature in the heating process. At a given temperature, the aggregates in the cooling process had a higher mass than those in the heating process, clearly indicating a hysteresis.

FORMATION OF MESOGLOBULES BY POLY(N,N-DIETHYLACRYLAMIDE) CHAINS IN DILUTE SOLUTIONS

The association of poly(N,N-diethylacrylamide) (PDEA) chains has been investigated by a combination of static and dynamic laser light scattering (LLS). Unlike poly(N-isopropylacrylamide) (PNIPAM), PDEA chains can not form the C=O⋯H–N hydrogen bonds. Our results demonstrate that a limited number of PDEA chains collapse and associate into metastable globules in dilute solutions. The heating rate has great effect on the mesoglobule size. The formation of such mesoglobules is attributed to the competition between intrachain contraction and interchain association. The stabilization has been explained in terms of viscoelastic effect.

Can Coil-to-Globule Transition of a Single Chain Be Treated as a Phase Transition?

The effects of the concentration (C) and heating rate on the collapse and association of poly(N-isopropylacrylamide) chains in water have been investigated by use of ultrasensitive differential scanning calorimetry. In the dilute solutions, both the phase transition temperature (Tp) and enthalpy change (DeltaH) increase with the heating rate but decrease with concentration. By extrapolation to zero heating rate and zero concentration, Tp and DeltaH for coil-to-globule transition of a single chain in thermodynamic equilibrium can be obtained. In semidilute solutions, both Tp and DeltaH increase with the heating rate but slightly vary with the concentration. Tp and DeltaH for pure interchain association in equilibrium are obtained by extrapolation to zero heating rate. Our experiments reveal that only intrachain contraction occurs when the concentration is infinitely close to zero. When the concentration is above the overlap concentration (C*), only interchain association exists. In the range 0<C<C*, both intrachain contraction and interchain association coexist.

Comparative Study of Temperature-Induced Association of Cyclic and Linear Poly(N-isopropylacrylamide) Chains in Dilute Solutions by Laser Light Scattering and Stopped-Flow Temperature Jump

We have comparatively studied the association of cyclic- and linear-poly(N-isopropylacrylamide) (c-PNIPAM and l-PNIPAM) chains with varying chain lengths and end groups in dilute aqueous solutions by laser light scattering (LLS) and stopped-flow temperature-jump measurements. Dynamic and static LLS results reveal that the heating leads to a microphase transition. The resultant structures of interchain aggregates depend on the heating rate and the chain topologies. In comparison with l-PNIPAM chains, a slow heating of c-PNIPAM chains in the solution results in stable mesoglobules with a lower average aggregation number, a looser structure, and a smaller average size (∼290 nm). The temperature-jump-induced association of c-PNIPAM chains in the stopped-flow measurement reveals two kinetic growth stages, which were tentatively ascribed to the loose packing of contracted c-PNIPAM chains and further contraction-induced fragmentation of initially packed c-PNIPAM chains due to the lack of interchain entanglements. On the other hand, for l-PNIPAM chains, the intrachain contraction and interchain penetration/entanglement simultaneously occur as the temperature increases, leading to larger and more compact aggregates whose size increases with the solution temperature.

Microcalorimetric Investigation on Association and Dissolution of Poly(N-isopropylacrylamide) Chains in Semidilute Solutions

The concentration (C) effect on association and dissolution of linear poly(N-isopropylacrylamide) (PNIPAM) chains in dilute and semidilute aqueous solutions was investigated by using ultrasensitive differential scanning calorimetry (US−DSC). When C is below the overlap concentration (C*), both the phase-transition temperature (Tp) and enthalpy change (ΔH) in the heating process decrease as the solution becomes more dilute, while Tp and ΔH become independent of C in the semidilute regime (C > C*). Such a difference can be attributed to different contributions of interchain association and intrachain contraction in dilute and semidilute solutions. In the cooling process, Tp nearly remains a constant, but ΔH decreases as C increases in dilute solutions, but both of them slightly vary in the range C > C*. A hysteresis was observed in one heating-and-cooling cycle. Such a hysteresis becomes more obvious as C increases. The effect of scanning rate on both Tp and ΔH further confirms that the hysteresis is due to incomplete disruption of additional hydrogen bonds formed when the chains are in the collapsed state.

LLS and FTIR Studies on the Hysteresis in Association and Dissociation of Poly(N-isopropylacrylamide) Chains in Water

Using a combination of static and dynamic laser scattering, we examined the association and dissociation of linear poly(N-isopropylacrylamide) (PNIPAM) chains in dilute aqueous solutions. There exists a hysteresis in the temperature dependence of the average hydrodynamic radius (〈Rh〉), average radius of gyration (〈Rg〉), and apparent weight-average molar mass (Mw,app) in one heating-and-cooling cycle. In the heating process, the chains first undergo intrachain contraction before interchain association to form stable aggregates at temperatures much higher than the lower critical solution temperature (LCST ∼ 32 °C) of PNIPAM in water. In the cooling process before the solution temperature approaches the LCST, Mw,app remains a constant and both 〈Rg〉 and 〈Rh〉 increase, but the ratio of 〈Rg〉/〈Rh〉 decreases. In other words, the aggregates undergo an unevenly swelling; namely, the periphery swells more than the center, and there is no chain dissociation. FTIR spectra reveal that as the temperature increases, the ...

Formation of Mesoglobular Phase of PNIPAM-g-PEO Copolymer with a High PEO Content in Dilute Solutions

Free radical copolymerization of N-isopropylacrylamide (NIPAM) and poly(ethylene oxide) (PEO) macromonomer (Mw = 5000) end-capped with methacrylate in water at 45 °C, higher than PNIPAM's lower critical solution temperature (∼32 °C), resulted in (PNIPAM-g-PEO) copolymer with 48 wt % of PEO. As expected, the grafting of PEO on the PNIPAM chain backbone was heterogeneous because PEO is hydrophilic and stayed on the periphery of the PNIPAM segments collapsed at the reaction temperature. The copolymer is soluble in water at lower temperatures. When the solution temperature is raised to ∼34 °C, a limited number of the copolymer chains can collapse and associate to form stable narrowly distributed mesoglobules with a collapsed PNIPAM core and a swollen PEO shell, different from either individual single-chain globules or macroscopic precipitation. The formation of such mesoglobules was studied by a combination of static and dynamic laser light scattering. We found that the mesoglobular formation has two transition stages, occurring at ∼34 and ∼47 °C. The transition at ∼34 °C is related to the contraction of the PNIPAM segment between two grafted PEO chains. The high-temperature transition is attributed to the stretch and collapse of the grafted PEO chains on the periphery. The fast heating generally resulted in smaller mesoglobules, which is due to a competition between intrachain contraction and interchain association as well as the viscoelastic effect.

Formation of Mesoglobular Phase of Amphiphilic Copolymer Chains in Dilute Solution: Effect of Comonomer Distribution

Poly(N-isopropylacrylamide) (PNIPAM) is a thermally sensitive polymer with a lower critical solution temperature (∼32 °C) in water. Its copolymerization with hydrophilic vinylpyrrolidone (VP) at temperatures higher and lower than its LCST respectively resulted in segmented and random VP distributions. As the solution temperature increases, PNIPAM changes from hydrophilic to hydrophobic at ∼32 °C so that copolymer P(NIPAM-co-VP) becomes amphiphilic in water at higher temperatures. The association of two pairs of such copolymers with a similar chain composition and length, but different comonomer distributions, in water at temperatures higher than 32 °C was studied by laser light scattering. A limited number of neutral P(NIPAM-co-VP) chains can associate to form narrowly distributed stable mesoglobules existing between single-chain collapsed globules and macroscopic precipitates. The copolymer chains with a segmented VP distribution aggregate more readily to form larger mesoglobules than their counterpart with a random VP distribution. The formation of mesoglobules is related to a competition between intrachain contraction and interchain association. Besides thermodynamic consideration, the formation of a stable mesoglobular phase is also affected by chain viscoelasticity inside each aggregate, even in dilute solution. A proper variation of the chain structure and association temperature can result in mesoglobules with different sizes, molar masses, and structures.

Self-Assembly of Narrowly Distributed Carboxy-Terminated Linear Polystyrene Chains in Water via Microphase Inversion

We found by surprise that ω-metal carboxy-terminated linear polystyrene chains could self-assemble into stable surfactant-free colloidal nanoparticles if the polymer solution in THF or DMF is added dropwise into an excess of water. Our results showed that the self-assembly was controlled by competition between the intrachain contraction and interchain association. There is a delicate balance between thermodynamic and kinetics. Inside the self-assembled micelle-like nanostructure, the polymer chains were in a metastable state and kinetically frozen. The self-assembly resembles the initial stage of surfactant-free emulsion polymerization. The size of the nanostructures was essentially governed by the surface area occupied per ionic end group. A variation of the initial conditions, such as the polymer concentration in THF and DMF, solvent, and counterions, can make either the intrachain contraction or interchain association dominant so that the self-assembly can be tailored.