On the Origin of the Major Peak Cluster Spacing in the Mass Spectra of Copolymers

Mass spectrometry (MS) is a uniquely informative technique in the characterization of copolymers, where spectra prominently feature peak clustering. The spacing of these clusters, in general, is dominated by the spacing of one repeat unit, and contained herein is the theory to explain this observation. Extension of this theory also explains the more subtle observation that, even though the spacing is generally that of one unit, occasionally, the spacing between the maxima of adjacent clusters shifts by that of the other unit. Furthermore, the theory predicts that, in the low molecular weight region of the spectrum, there is a total switch to the spacing of the other unit along with asymmetric peak clusters that have a “sawtooth” shape. The analysis uses the Gaussian, log–normal, and Schulz–Zimm models as well as the random coupling hypothesis to explicitly demonstrate that (1) the major peak cluster spacing naturally arises from the unit in the copolymer with the widest distribution, as measured by the scaled standard deviation, (2) the spacing shift naturally occurs due to the marginal probabilities away from the spectrum maximum, and (3) the low molecular weight switch is a natural consequence of the tail of the distribution of the unit with the widest distribution. Results are provided to predict which unit in the copolymer will govern the major peak cluster spacing, how often the spacing will shift to that of another unit in the middle and high molecular weight regions of the spectrum, the molecular weight and composition of the maximum peak in every cluster, and the molecular weight below which the spacing will be that of the another unit. We believe that our results are the first to provide tangible theory to explain the previously unknown origins of these empirically observed phenomena

Controlling Mechanical Properties of Poly(methacrylic acid) Multilayer Hydrogels via Hydrogel Internal Architecture

Hydrogel materials are crucial in many applications due to their versatility and ability to mimic biological tissues. While manipulating bulk hydrogel cross-link density, polymer content, chemical composition, and microporosity has been a main approach to controlling hydrogel rigidity, altering the internal organization of hydrogel materials through chain intermixing and stratification can bring finer control over hydrogel properties, including mechanical responses. We report on altering the mechanical properties of ultrathin poly(methacrylic acid) (PMAA) multilayer hydrogels by controlling the internal organization of the PMAA network. PMAA multilayer hydrogels were synthesized by cross-linking PMAA layers in poly(N-vinylpyrrolidone) (PVPON)/PMAA hydrogen-bonded multilayer templates prepared by dipped or spin-assisted (SA) layer-by-layer assembly using sacrificial PVPON interlayers with molecular weights of 40,000 or 280,000 g mol–1. The effect of PVPON molecular weight on PMAA hydrogel stratification and network swelling and hydration was assessed by in situ spectroscopic ellipsometry and neutron reflectometry (NR). In a new NR modeling of polymer intermixing, we have inferred nanoscopic structure and water distribution within the ultrathin-layered films from measured continuum neutron scattering length density (SLD) and related those to the mechanical properties of the hydrogel films. We have found that hydrogel swelling, the number of water molecules associated with the swollen hydrogel, and water density within the SA PMAA hydrogels can be controlled by choosing low- or high-Mw PVPON. While cross-link densities determined by ATR-FTIR were similar, greater swelling and hydration at pH > 5 were observed for SA PMAA hydrogels synthesized using higher-Mw PVPON. The enhanced swelling of these SA hydrogels resulted in softening with a lower Young’s modulus at pH > 5 as measured by colloidal probe atomic force microscopy (AFM). The effect of PMAA layer intermixing on hydrogel mechanical properties was also compared for dipped and SA (PMAA) multilayer hydrogels of similar thickness and cross-linking degree. Despite similar values of gigapascal-range Young’s modulus for dry PMAA multilayer hydrogel films, an almost twice greater softening of the SA (PMAA) hydrogel compared to that prepared by dipping was observed, with Young’s modulus values decreasing to tens of megapascals in solution at pH > 5. Our study demonstrates that, unlike simply changing bulk hydrogel cross-link density, programming polymer network architecture via controlling the nanostructured organization of SA PMAA hydrogels enables selective modulation of the cross-link density within hydrogel strata. Control of polymer chain intermixing through hydrogel stratification offers a framework for synthesizing materials with finely tuned hydrogel internal structures, enabling precise control of such physical properties as the internal architecture, hydrogel swelling, surface morphology, and mechanical response, which are critical for the application of these materials in sensing, drug delivery, and tissue engineering

Controlling Internal Organization of Multilayer Poly(methacrylic acid) Hydrogels with Polymer Molecular Weight

We report on tailoring the internal architecture of multilayer-derived poly­(methacrylic acid) (PMAA) hydrogels by controlling the molecular weight of poly­(<i>N</i>-vinylpyrrolidone) (PVPON) in hydrogen-bonded (PMAA/PVPON) layer-by-layer precursor films. The hydrogels are produced by cross-linking PMAA in the spin-assisted multilayers followed by PVPON release. We found that the thickness, morphology, and architecture of hydrogen-bonded films and the corresponding hydrogels are significantly affected by PVPON chain length. For all systems, an increase in PVPON molecular weight from <i>M</i>_w = 2.5 to 1300 kDa resulted in increased total film thickness. We also show that increasing polymer <i>M</i>_w smooths the hydrogen-bonded film surfaces but roughens those of the hydrogels. Using deuterated <i>d</i>PMAA marker layers in neutron reflectometry measurements, we found that hydrogen-bonded films reveal a high degree of stratification which is preserved in the cross-linked films. We observed <i>d</i>PMAA to be distributed more widely in the hydrogen-bonded films prepared with small <i>M</i>_w PVPON due to the greater mobility of short-chain PVPON. These variations in the distribution of PMAA are erased after cross-linking, resulting in a distribution of <i>d</i>PMAA over about two bilayers for all <i>M</i>_w but being somewhat more widely distributed in the films templated with higher <i>M</i>_w PVPON. Our results yield new insights into controlling the organization of nanostructured polymer networks using polymer molecular weight and open opportunities for fabrication of thin films with well-organized architecture and controllable function

Thermosensitive Multilayer Hydrogels of Poly(<i>N</i>‑vinylcaprolactam) as Nanothin Films and Shaped Capsules

We report on nanothin multilayer hydrogels of cross-linked poly­(<i>N</i>-vinylcaprolactam) (PVCL) that exhibit distinctive and reversible thermoresponsive behavior. The single-component PVCL hydrogels were produced by selective cross-linking of PVCL in layer-by-layer films of PVCL-NH₂ copolymers assembled with poly­(methacrylic acid) (PMAA) via hydrogen bonding. The degree of the PVCL hydrogel film shrinkage, defined as the ratio of wet thicknesses at 25 to 50 °C, was demonstrated to be 1.9 ± 0.1 and 1.3 ± 0.1 for the films made from PVCL-NH₂-7 and PVCL-NH₂-14 copolymers, respectively. No temperature-responsive behavior was observed for noncross-linked two-component films because of the presence of PMAA. We also demonstrated that temperature-sensitive PVCL capsules of cubical and spherical shapes could be fabricated as hollow hydrogel replicas of inorganic templates. The cubical (PVCL)₇ capsules retained their cubical shape when temperature was elevated from 25 to 50 °C exhibiting 21 ± 1% decrease in the capsule size. Spherical hydrogel capsules demonstrated similar shrinkage of 23 ± 1%. The temperature-triggered capsule size changes were completely reversible. Our work opens new prospects for developing biocompatible and nanothin hydrogel-based coatings and containers for temperate-regulating drug delivery, cellular uptake, sensing, and transport behavior in microfluidic devices

Stratified Temperature-Responsive Multilayer Hydrogels of Poly(<i>N</i>‑vinylpyrrolidone) and Poly(<i>N</i>‑vinylcaprolactam): Effect of Hydrogel Architecture on Properties

We report on the effects of hydrophilicity and architecture on the temperature-responsive behavior and surface morphology of nonionic double-stack hydrogels prepared from cross-linked hydrogen-bonded layer-by-layer films. A hydrophilic poly­(<i>N</i>-vinyl­pyrrolidone) (PVPON)_<i>n</i> multilayer hydrogel is integrated with a relatively hydrophobic temperature-sensitive poly­(<i>N</i>-vinyl­caprolactam) (PVCL)_<i>m</i> network as either a top or bottom stratum, where <i>n</i> and <i>m</i> represent numbers of layers for each individual stratum. Neutron reflectometry revealed that all double-stack films in the dry state are well stratified with two distinct (PVPON) and (PVCL) strata of higher and lower scattering density, respectively, unlike highly mixed alternating (PVCL/PVPON) hydrogels. We have found that the order of stacking and stack thickness significantly influence hydration of the (PVPON)_<i>n</i>(PVCL)_<i>m</i> and (PVCL)_<i>m</i>(PVPON)_<i>n</i> networks at ambient temperature and above the LCST of PVCL. The hydration of the hydrogels consistently increases with PVPON amount within the network, resulting in suppressed temperature response. This effect is more pronounced for (PVPON)_<i>n</i>(PVCL)_<i>m</i> as compared to its mirror counterpart as explained by the two adjacent aqueous interfaces in which the (PVCL)_<i>m</i> stack is sandwiched between the hydrophilic (PVPON)_<i>n</i> stack below and the bulk of water above it. Our results yield new insights into controlling the temperature response and surface properties of nanostructured polymer networks, which is relevant to both fundamental and applied research where the dynamics of hydration, thickness, and control of surface hydrophobicity are important

Thermoresponsive Micelles from Double LCST-Poly(3-methyl‑<i>N</i>‑vinylcaprolactam) Block Copolymers for Cancer Therapy

We present synthesis and assembly of novel thermoresponsive block copolymers with double LCST precisely controlled within the physiological temperature range. Two separate phase transition temperatures were achieved by RAFT polymerization of structurally similar monomers with varied hydrophobicity. The LCST1 was varied from 19 to 27 °C by copolymerization of <i>N</i>-vinylcaprolactam with a novel hydrophobic monomer, 3-methyl-<i>N</i>-vinylcaprolactam, while the LCST2 at 41–42 °C was attained by copolymerization of <i>N</i>-vinylcaprolactam with hydrophilic <i>N</i>-vinylpyrrolidone. The LCST1 facilitates micelle formation and entrapment of anticancer drug doxorubicin or hydrophobic dye Nile Red into the micelle core surrounded with hydrophilic yet temperature-sensitive corona. The LCST2 induces collapse of the micelle corona and the consequent drug release. The second elevated temperature is typical for tumors and can trigger the drug-loaded micelle aggregation/accumulation within the tumor resulting in the enhanced passive targeting

Hydrogen-Bonded Multilayers of Silk Fibroin: From Coatings to Cell-Mimicking Shaped Microcontainers

We present a novel type of all-aqueous nonionic layer-by-layer films of silk fibroin with synthetic macromolecules and a natural polyphenol. We found the multilayer growth and stability to be strongly pH-dependent. Silk assembled with poly­(methacrylic) and tannic acids at pH = 3.5 disintegrated at pH ∼ 5, while silk/poly­(<i>N</i>-vinylcaprolactam) interactions were stable at low and high pH values but resulted in thinner films at a high pH. The results suggest that the intermolecular interactions are primarily driven by hydrogen bonding with a considerable contribution of hydrophobic forces. We also demonstrated that cubical, spherical, and platelet capsules with silk-containing walls can be constructed using particulate sacrificial templates. This work sets a foundation for future explorations of natural and synthetic macromolecules assemblies as biomimetic materials with tunable properties

Tailoring Architecture of Nanothin Hydrogels: Effect of Layering on pH-Triggered Swelling

We have tailored the internal architecture of ultrathin poly­(methacrylic acid) (PMAA) hydrogels from well stratified to highly intermixed by controlling the internal structure in layer-by-layer templates used for hydrogel fabrication. We have found pH-triggered swelling properties of these hydrogels to be significantly affected by hydrogel architecture. Well-stratified hydrogels exhibited a dramatic 10-fold increase in thickness when transitioned between pH = 5 and 7.5, unlike the 2-fold swelling observed in less-organized hydrogels

Polyphenolic Polymersomes of Temperature-Sensitive Poly(<i>N</i>‑vinylcaprolactam)-<i>block</i>-Poly(<i>N</i>‑vinylpyrrolidone) for Anticancer Therapy

We report a versatile synthesis for polyphenolic polymersomes of controlled submicron (<500 nm) size for intracellular delivery of high and low molecular weight compounds. The nanoparticles are synthesized by stabilizing the vesicular morphology of thermally responsive poly­(<i>N</i>-vinylcaprolactam)_<i>n</i>-<i>b</i>-poly­(<i>N</i>-vinylpyrrolidone)_<i>m</i> (PVCL_<i>n</i>–PVPON_<i>m</i>) diblock copolymers with tannic acid (TA), a hydrolyzable polyphenol, via hydrogen bonding at a temperature above the copolymer’s lower critical solution temperature (LCST). The PVCL₁₇₉–PVPON_<i>m</i> diblock copolymers are produced by controlled reversible addition–fragmentation chain transfer (RAFT) polymerization of PVPON using PVCL as a macro-chain transfer agent. The size of the TA-locked (PVCL₁₇₉–PVPON_<i>m</i>) polymersomes at room temperature and upon temperature variations are controlled by the PVPON chain length and TA:PVPON molar unit ratio. The particle diameter decreases from 1000 to 950, 770, and 250 nm with increasing PVPON chain length (<i>m</i> = 107, 166, 205, 234), and it further decreases to 710, 460, 290, and 190 nm, respectively, upon hydrogen bonding with TA at 50 °C. Lowering the solution temperature to 25 °C results in a slight size increase for vesicles with longer PVPON. We also show that TA-locked polymersomes can encapsulate and store the anticancer drug doxorubicin (DOX) and higher molecular weight fluorescein isothiocyanate (FITC)–dextran in a physiologically relevant pH and temperature range. Encapsulated DOX is released in the nuclei of human alveolar adenocarcinoma tumor cells after 6 h incubation via biodegradation of the TA shell with the cytotoxicity of DOX-loaded polymersomes being concentration-dependent. Our approach offers biocompatible and intracellular degradable nanovesicles of controllable size for delivery of a variety of encapsulated materials. Considering the particle monodispersity, high loading capacity, and a facile two-step aqueous assembly based on the reversible temperature-responsiveness of PVCL, these polymeric vesicles have significant potential as novel drug nanocarriers and provide a new perspective for fundamental studies on thermo-triggered polymer assemblies in solutions

Internalization of Red Blood Cell-Mimicking Hydrogel Capsules with pH-Triggered Shape Responses

We report on naturally inspired hydrogel capsules with pH-induced transitions from discoids to oblate ellipsoids and their interactions with cells. We integrate characteristics of erythrocytes such as discoidal shape, hollow structure, and elasticity with reversible pH-responsiveness of poly(methacrylic acid) (PMAA) to design a new type of drug delivery carrier to be potentially triggered by chemical stimuli in the tumor lesion. The capsules are fabricated from cross-linked PMAA multilayers using sacrificial discoid silicon templates. The degree of capsule shape transition is controlled by the pH-tuned volume change, which in turn is regulated by the capsule wall composition. The (PMAA)₁₅ capsules undergo a dramatic 24-fold volume change, while a moderate 2.3-fold volume variation is observed for more rigid PMAA–(poly(<i>N</i>-vinylpyrrolidone) (PMAA–PVPON)₅ capsules when solution pH is varied between 7.4 and 4. Despite that both types of capsules exhibit discoid-to-oblate ellipsoid transitions, a 3-fold greater swelling in radial dimensions is found for one-component systems due to a greater degree of the circular face bulging. We also show that (PMAA–PVPON)₅ discoidal capsules interact differently with J774A.1 macrophages, HMVEC endothelial cells, and 4T1 breast cancer cells. The discoidal capsules show 60% lower internalization as compared to spherical capsules. Finally, hydrogel capsules demonstrate a 2-fold decrease in size upon internalization. These capsules represent a unique example of elastic hydrogel discoids capable of pH-induced drastic and reversible variations in aspect ratios. Considering the RBC-mimicking shape, their dimensions, and their capability to undergo pH-triggered intracellular responses, the hydrogel capsules demonstrate considerable potential as novel carriers in shape-regulated transport and cellular uptake