•Hydrophobic chain end of PNIPAM is critical in the SDS/PNIPAM interaction.•SDS is preferred to interact with the dodecyl chain end of PNIPAM.•SDS can weaken the association of alkyl-ended PNIPAM chains.•SDS endows the PNIPAM chains some polyelectrolyte characteristics.•The chain length-dependent cloud point can be reversed at high SDS concentration.The studies on the interaction between thermo-responsive polymer PNIPAM and various ionic surfactants have been extensively investigated in the past few years. Unfortunately, the role of the hydrophobic end-group which is inevitably introduced by the initiator or chain transfer agent was generally overlooked in related studies. In this work, to show the interplay between the hydrophobic end-group and the surfactant molecule, five n-dodecyl-terminated poly(N-isopropylacrylamide)s (PNIPAMs-C12H25) with different chain lengths were prepared by RAFT polymerization. The unambiguous molecular parameters were obtained by GPC, LLS, 1H-NMR and UV–Vis measurements. Further study on the association behaviors in SDS aqueous solutions showed that, these PNIPAM-C12H25 chains can form uniform spherical micelles accompanied with a trace amount of large irregular aggregates in pure water at 20 °C, and the size-molar mass relation of aggregates presents a scaling law. The addition of SDS could continuously weaken the association of these alkyl-terminated PNIPAMs, reflected in the decrease of average hydrodynamic radius () and average aggregation number () of aggregates. Also, it could endow the PNIPAM chains the polyelectrolyte characteristics, reflected in the emergency of a slow relaxation mode in dynamic LLS measurement. More interestingly, the reversal of the dependence of cloud point on the chain length was observed at high SDS concentration, which may be attributed to the preferentially van der Waals interaction between the SDS dodecyl tail and the PNIPAM dodecyl chain end.

•How the chain parameter affects the translocation of a polymer chain through a cylindrical pore under an elongational flow field was elucidated.•The application of ultrafiltration technique in the separation and characterization of polymer chains was discussed.•The outlook for future research emphasis and challenge in this research field was given.How a polymer chain translocates through a cylindrical pore with its pore size smaller than the chain size (ultrafiltration behavior) is a fundamental question in polymer physics. Answering this question can provide broader implications and lead to potential applications for many applicable processes, such as gene transfection, protein transportation, and separation of a mixture of polymer chains. In the process of an electroneutral polymer chain passing through a nanopore, generally, an external flow with a sufficient high shear stress is needed to apply at the entrance of pore to induce a conformation change from a coil-like to a rod-like shape which is referred to as the “coil-to-stretch” transition, to squeeze into the pore. Up to last decade, many theoretical models have been built and carried out to predict how polymer chains pass through a nanopore. By contrast, rather limited experimental investigations have been performed to validate these theoretical predications, which is mainly because this kind of experimental study demands polymer chains with explicit topologies and nanopores with well-defined structures. Namely, 1) the polymer samples with narrow molecular-weight distributions, well-defined chain configurations, as well as hydrodynamic sizes larger than the pore radii; and 2) membranes with well-defined pore structure and isolated pore channels to prevent possible interaction between neighboring shearing flows.In recent years, the development of polymer synthetic technology and the improvement of membrane manufacturing technology have stimulated a mass of research work on understanding the ultrafiltration behavior of polymer chains under an elongational flow field. In this feature article, the authors would like to mainly focus on the ultrafiltration behavior of flexible polymer chains with various topologies in dilute solution. More specifically, we will elucidate how the structural parameters of a polymer chain are related to the critical volumetric flow rate and the shape of polymer retention curve. Further application of this ultrafiltration method to the separation of polymer chain mixture and the rapid transformation among various polymer chain aggregated structures will be discussed. It is hoped that this perspective can provide a better view in understanding the translocation behavior of (bio)macromolecules in various practical processes and offer some guidance for the design and reality of commercially available ultrafiltration separation apparatus in the future.Figure optionsDownload full-size imageDownload as PowerPoint slide

A trifunctional initiator with one alkyne, one hydroxyl, and one bromine group was used to construct Br-polystyrene-alkyne-poly(ε-caprolactone)-OH (Br–PS–≡–PCL–OH) diblock copolymer precursor with one terminal alkyne group located at the junction between the two blocks. Further bromination and azidation substitution of the precursor led to a seesaw-type macromonomer azide-polystyrene-alkyne-poly(ε-caprolactone)-azide (N3–PS–≡–PCL–N3). Subsequently, novel hyperbranched copolymers [HB-(PS-b-PCL)n] with independently adjustable PS and PCL branching subchains were prepared by “click” chemistry. All of the linear precursors and hyperbranched copolymers were characterized by FT-IR, 1H NMR, and GPC with triple detectors in detail. It was found that such hyperbranched copolymers are self-similar objects; namely, their intrinsic viscosities ([η]) are scaled to the weight-average molar masses (Mw) as [η] ∼ Mwν, where ν = 0.45 ± 0.01 and 0.48 ± 0.01 for the longer and shorter PS block, respectively. Moreover, the study on the crystallization behavior of unfractionated and fractionated HB-(PS-b-PCL)n copolymers indicated both the crystal size and the degree of crystallinity decrease with the PS subchain length and the overall degree of polymerization, and the remaining oligomer and macromonomer components could facilitate the crystallization of the unfractionated sample. Finally, it was found that the degree of crystallinity decreases dramatically when the weight fraction of fractionated hyperbranched copolymer in macromonomer/hyperbranched copolymer blend films exceeds ∼67%, indicating that the uncrystallizable hyperbranched chains may impose some extra restriction on the crystallization of the macromonomer chains.

Amphiphilic 8-shaped cyclic-(polystyrene-b-poly(acrylic acid))2 with two rings and its linear precursor, i.e., 8-shaped cyclic- and linear-(PS–PAA)2, were successfully prepared by a combination of atom transfer radical polymerization (ATRP) and “click” chemistry. Using various methods, we characterized those intermediates and resultant copolymers and studied their association properties in solutions. As expected, the average aggregation number (⟨Nagg⟩) increases with the molar fraction of styrene for a given overall degree of polymerization. Our results reveal that the cyclization leads to a smaller ⟨Nagg⟩ but slightly larger and looser aggregates, presumably due to the topological constraint of the two rings (8-shaped). Using these amphiphilic chains as emulsifying agents, we found that 8-shaped cyclic-(PS–PAA)2 chains are less effective in stabilizing latex particles in emulsion polymerization because each cyclic chain occupies a smaller interfacial surface area than its linear counterpart. Further, using pyrene as a model hydrophobic molecule, we investigated their solubilization powers. Our results reveal that 8-shaped cyclic- and linear-(PS–PAA)2 chains have a similar ability in loading hydrophobic pyrene molecules, different from our original expectation, presumably because the hydrophobic PS block is too short and the hydrophilic PAA rings are too small. The current study provides a better understanding of the complicated topological constraint on the solution properties of 8-shaped cyclic amphiphilic copolymers.

We developed a strategy to make model hyperbranched structure with uniform subchains and controlled locations of cleavable linkages. First, a novel seesaw-type tetrafunctional initiator with one alkyne, one disulfide linkage, and two bromine groups (≡–S–S–(Br)2) was prepared. Using such an initiator, an AB2-type macromonomer (azide∼∼alkyne∼∼azide) with one disulfide linkage at its center was prepared via successive atom transfer radical polymerization (ATRP) and azidation substitution reaction, where ∼∼ represents polystyrene chains. Further interchain “clicking” coupling between the azide and alkyne groups on the macromonomers led to model hyperbranched polystyrenes with uniform subchains and controllablly located cleavable disulfide linkages. The 1H nuclear magnetic resonance spectra, Fourier transform infrared spectroscopy, and size exclusion chromatography with a multiangle laser light scattering detector confirmed the designed degradable hyperbranched structure. Armed with this novel sample, we studied its dithiothreitol (DTT)-induced degradation in various organic solvents by a combination of static and dynamic LLS. We found that the cleavage of disulfide bonds contains a fast and a slow process. The fast one reflects the degradation of disulfide bonds on the chain periphery; while the slow one involves those inside. Both the fast and slow degradation reaction rate constants (Kfast and Kslow) are a linear function of the initial DTT concentration ([DTT]0), but the relative contribution of the two processes is mainly governed by the hyperbranched chain structure, nearly independent of [DTT]0.

Combining atom transfer radical polymerization (ATRP) and “click” chemistry, a set of well-defined amphiphilic block copolymers poly(n-butyl acrylate)-b-poly(acrylic acid) (PnBA20-PAA85) with a similar chemical component, but different topological structures, i.e., linear-, cyclic-, and multiblock structures, were successfully prepared, characterized (size exclusion chromatography (SEC), FT-IR and 1H NMR), and used as surfactants in emulsion polymerization. Our further transmission electron microscopy (TEM) and laser light scattering (LLS) characterization of the resultant latex particles demonstrates all the surfactants with different topologies can effectively stabilize the latex particles but no significant difference among the solids contents was observed. Moreover, we have, for the first time, experimentally established the quantitative relation between the final number of latex particles (Np) and the concentration of polymeric surfactant with different topologies (C), i.e., Np = kCα, and the order of our measured exponent α is as follows: αmulti(1.10) > αlinear(0.81) ≥ αcyclic(0.73), indicating cyclic surfactant molecules behave more like small-molecule surfactants attributed to its strongest unimers extraction and diffusion ability; in contrast, multiblock surfactant molecules can act as seeds to directly nucleate to create latex particles. In addition, Np,multi > Np,linear ≥ Np,cyclic at higher concentration, and Np,linear > Np,cyclic ≥ Np,multi at lower concentration was observed. Similar results (αmulti(1.02) > αlinear(0.65) ≥ αcyclic(0.58)) were also observed when polystyrene-b-poly(acrylic acid) (PS9–PAA60) copolymers were used as surfactants. Further aqueous SEC characterization shows the broad size distribution of our micellar solution has no effect on obtaining narrowly distributed latex particles. Finally, interfacial tension measurement of the micellar solution indicates, compared to multiblock structure, the rate of adsorption at a hydrophobic interface is much faster for linear and cyclic-block structures, agreeing with our observed order of exponent α.

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.

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 molar masses in water. Our results reveal that larger hyperbranched chains have a less tendency to undergo the interchain association, and the average aggregation number (Nagg) is scaled to the weight-average degree of polycondensation [(DP)w], i.e., the number of linear triblock precursors inside each hyperbranched chain, as Nagg ∼ (DP)w–0.7. As expected, increasing salt (NaCl) concentration led to stronger interchain association, resulting in large aggregates, while linear precursors only form small polymeric micelles. Moreover, our rheological study shows, unlike their linear precursor, large hyper-(PAA23-PS14-PAA23)n chains can form a physical gel with a network-like structure at a concentration as low as 50 g/L, whose modulus increases with (DP)w.