Aggregation Of Polymer Chains Research Articles

Sponge-like porous polyvinyl alcohol/chitosan-based hydrogel with integrated cushioning, pH-indicating and antibacterial functions

Developing a packaging material with integrated cushioning, intelligent and active functions is highly desired but remains challenging in the food industry. Here we show that a sponge-like porous hydrogel with pH-indicating and antibacterial additives can meet this requirement. We use polyvinyl alcohol and chitosan as the primary polymers to construct a hydrogel with hierarchical structures through a freeze-casting method in combination with salting-out treatment. The synergy of aggregated polymer chains and the sponge-like porous structure makes the hydrogel resilient and efficient in energy absorption. It also enables rapid movement of molecules/particles and fast reaction due to the large specific surface area of the pore structures and the large amount of free water in it, leading to a sensitive pH-indicating function. The hydrogel shows an obvious color variation within a wide pH range in 3 min. The silver nanoparticles are fixed in the dense polymer networks, enabling a lasting release of silver ions. The porous structure makes the silver ion reach the protected item in a short time, achieving an antibacterial effect against S. aureus and E. coli with little cytotoxicity. This work paves the way for fabricating multifunctional hydrogels for diverse advanced packaging systems.

Engineering Tough and Elastic Polyvinyl Alcohol‐Based Hydrogel with Antimicrobial Properties

Hydrogels have been extensively used for tissue engineering applications due to their versatility in structure and physical properties, which can mimic native tissues. Although significant progress has been made toward designing hydrogels for soft tissue repair, engineering hydrogels that resemble load‐bearing tissues is still considered a great challenge due to their specific mechanophysical demands. Herein, microporous, tough, yet highly compressible poly(vinyl alcohol) (PVA)‐based hydrogels are reported for potential applications in repairing or replacing different load‐bearing tissues. The synergy of freeze‐thawing and the Hofmeister effect, which controlled the spatial arrangement and aggregation of polymer chains, facilitated the formation of microstructured frameworks with tunable porosity. While the maximum mechanical strength, toughness, and stretchability of the engineered hydrogel were ≈390 kPa, ≈388 kJ m−3, and ≈170%, respectively, Young's modulus based on compression testing wasfound to be in the range of ≈0.02–0.30 MPa, highlighting the all‐in‐one mechanically enriched nature of the hydrogel. Furthermore, the minimal swelling and degradation rate of the engineered hydrogel met the specific requirements for load‐bearing tissues. Finally, excellent antibacterial resistance as well as in vitro biocompatibility of the hydrogel demonstrates its potential for the replacement of load‐bearing tissues.

Tailoring the microstructure of zwitterionic nanofiltration membranes via post-treatment for antibiotic desalination

Zwitterionic nanofiltration membranes, with high rejection to organic molecules while low rejection to monovalent salts, are promising for antibiotic desalination. However, due to the heterogeneous issue of interfacial polymerization, the residual amine groups result in polymer chain aggregation via hydrogen bonds, blocking the mass transfer of water and monovalent salts. Herein, EtOH-assisted Michael-addition/Schiff-base reaction strategy was proposed to solve the issue. NaOH was initially utilized to provisionally interrupt the hydrogen bonds between polymer chains. Meanwhile, with the assistance of EtOH, gallic acid diffused into the membrane and reacted with the amine groups on the polymer chains to form covalent bonds, which thoroughly destroyed the stacking of polymer chains. As a consequence, water permeance and antibiotic desalination efficiency of the membrane are improved. For example, the pure water permeance reaches 8.9 L m−2 h−1 bar−1, 1.7-fold enhancement compared with the pristine membrane. The membrane is applied for antibiotic desalination, offering long-term running stability, antifouling ability, as well as high efficiency in concentrated antibiotics.

Degradable Nanogels Based on Poly[Oligo(Ethylene Glycol) Methacrylate] (POEGMA) Derivatives through Thermo-Induced Aggregation of Polymer Chain and Subsequent Chemical Crosslinking.

Polymer nanogels-considered as nanoscale hydrogel particles-are attractive for biological and biomedical applications due to their unique physicochemical flexibility. However, the aggregation or accumulation of nanoparticles in the body or the occurrence of the body's defense reactions still pose a research challenge. Here, we demonstrate the fabrication of degradable nanogels using thermoresponsive, cytocompatible poly[oligo(ethylene glycol) methacrylate]s-based copolymers (POEGMA). The combination of POEGMA's beneficial properties (switchable affinity to water, nontoxicity, non-immunogenicity) along with the possibility of nanogel degradation constitute an important approach from a biological point of view. The copolymers of oligo(ethylene glycol) methacrylates were partially modified with short segments of degradable oligo(lactic acid) (OLA) terminated with the acrylate group. Under the influence of temperature, copolymers formed self-assembled nanoparticles, so-called mesoglobules, with sizes of 140-1000 nm. The thermoresponsive behavior of the obtained copolymers and the nanostructure sizes depended on the heating rate and the presence of salts in the aqueous media. The obtained mesoglobules were stabilized by chemical crosslinking via thiol-acrylate Michael addition, leading to nanogels that degraded over time in water, as indicated by the DLS, cryo-TEM, and AFM measurements. Combining these findings with the lack of toxicity of the obtained systems towards human fibroblasts indicates their application potential.

Shape memory hydrogels with remodelable permanent shapes and programmable cold-induced shape recovery behavior.

Most shape memory polymers apply glass transition or crystallization of domains to fix temporary shapes and shape recovery is induced by heating, which hinders their application under heat-intolerant conditions. Moreover, the permanent shapes of polymers normally cannot be altered arbitrarily after fabrication. Herein, we present a novel shape memory hydrogel with a remodelable permanent shape and programmable cold-induced shape recovery behavior. Poly(acrylic acid) (PAA) hydrogel is prepared in the presence of diethylenetriamine (DETA) and subsequently treated with calcium acetate (Ca(Ac)2). The charge-assisted hydrogen bonding between PAA and DETA imparts the hydrogel with remodelability, while the heat-induced hydrophobic aggregation of polymer chains and acetate groups results in shape fixation by heating and shape recovery by cooling. Afterwards, programmable deformable devices are obtained by assembling hydrogel blocks with different concentrations of Ca(Ac)2. This design strategy promotes the development of shape memory polymers with diverse potential applications.

A skeletal randomization strategy for high-performance quinoidal-aromatic polymers.

Enhancing the solution-processability of conjugated polymers (CPs) without diminishing their thin-film crystallinity is crucial for optimizing charge transport in organic field-effect transistors (OFETs). However, this presents a classic "Goldilocks zone" dilemma, as conventional solubility-tuning methods for CPs typically yield an inverse correlation between solubility and crystallinity. To address this fundamental issue, a straightforward skeletal randomization strategy is implemented to construct a quinoid-donor conjugated polymer, PA4T-Ra, that contains para-azaquinodimethane (p-AQM) and oligothiophenes as repeat units. A systematic study is conducted to contrast its properties against polymer homologues constructed following conventional solubility-tuning strategies. An unusually concurrent improvement of solubility and crystallinity is realized in the random polymer PA4T-Ra, which shows moderate polymer chain aggregation, the highest crystallinity and the least lattice disorder. Consequently, PA4T-Ra-based OFETs, fabricated under ambient air conditions, deliver an excellent hole mobility of 3.11 cm2 V-1 s-1, which is about 30 times higher than that of the other homologues and ranks among the highest for quinoidal CPs. These findings debunk the prevalent assumption that a random polymer backbone sequence results in decreased crystallinity. The considerable advantages of the skeletal randomization strategy illuminate new possibilities for the control of polymer aggregation and future design of high-performance CPs, potentially accelerating the development and commercialization of organic electronics.

Skin-like cryogel electronics from suppressed-freezing tuned polymer amorphization

The sole situation of semi-crystalline structure induced single performance remarkably limits the green cryogels in the application of soft devices due to uncontrolled freezing field. Here, a facile strategy for achieving multifunctionality of cryogels is proposed using total amorphization of polymer. Through precisely lowering the freezing point of precursor solutions with an antifreezing salt, the suppressed growth of ice is achieved, creating an unusually weak and homogenous aggregation of polymer chains upon freezing, thereby realizing the tunable amorphization of polymer and the coexistence of free and hydrogen bonding hydroxyl groups. Such multi-scale microstructures trigger the integrated properties of tissue-like ultrasoftness (Young’s modulus <10 kPa) yet stretchability, high transparency (~92%), self-adhesion, and instantaneous self-healing (<0.3 s) for cryogels, along with superior ionic-conductivity, antifreezing (−58 °C) and water-retention abilities, pushing the development of skin-like cryogel electronics. These concepts open an attractive branch for cryogels that adopt regulated crystallization behavior for on-demand functionalities.

A facile method to fabricate multifunctional polyvinyl alcohol mineralized hydrogels with high strength and adaptable shape

AbstractHydrogel‐based materials are widely used in many fields, but most of them compared to tendons with the same water content do not exhibit high toughness, strength or fatigue resistance, which limits their application. The Hofmeister effect can adjust the mechanical properties of hydrogels by ions affecting the aggregation of polymer chains. Herein, we prepared a bioactivated hydrogel through simply mixing polyvinyl alcohol (PVA) and Na2CO3 aqueous solutions. PVA chains are aggregated by strong hydrogen bonds between CO32− and OH, at which stage the hydrogel is soft and easy to shape. Further salting out promotes the formation of crystal microphase of PVA chain, resulting in high mechanical strength and excellent toughness. The existence of Na2CO3 endow the hydrogel with conductivity, exhibits high and stable sensitivity in strain sensing, and could be used to monitor human actions. On the basis of this method, Ca2+ was added to prepare mineralized PVA hydrogels, which had high stability and long‐term swelling resistance in different ionic solutions. In vitro cell experiment, hydrogel had obvious effect of promoting bone cell proliferation and calcium deposition. This work provides a facile and feasible method for preparing bionic hydrogels with high strength, toughness, and adaptable shape.

Low-Temperature Alignment of Conjugated Polymers by Plasticizer-Aided Physical Rubbing.

Rubbing-induced alignment of conjugated polymers issystematically investigated in terms of intra- and inter-molecular interaction. Various polymer films with a broad range of polymer chain rigidity arerubbed, and the degree of polymer chain alignment isquantitatively characterized. The rubbing technique effectively aligns crystalline domains in conjugated polymer films when the temperature approaches the critical rubbing temperature ( ), at which the rearrangement and the slip of polymer chains are possible. A polymer with significant intra-/inter-molecular interactions exhibits higher , though quantitative analysis reveals an intermediately aligned state at temperature Tr ' lower than . This state originates from polymer chain aggregation in an amorphous domain. The intermediately aligned state can be controlled by plasticizer, which enables low-temperature alignment of high-mobility polymer film by reducing Tr ' to near 100 °C, increases the crystallinity, and improves the alignment effect at this state comparable to that of the completely aligned state obtained at extremely high temperatures.

Methacrylate Silatrane: New Building Block for Development of Functional Polymeric Coatings through Controlled Silanization.

Organosilicons are prevalent for the development of nanostructures, adhesives, fillers, and surface functionalization due to their ease of operation, availability, and effective modification on various surfaces. 3-(Trimethoxysilyl) propyl methacrylate (TMSPMA) is a widely used commercial silane for designing hybrid polymers via radical polymerization for a wide spectrum of applications. However, the chemical stability and processibility of TMSPMA encounter burdens due to its susceptibility to hydrolysis and aggregation, resulting in limiting its functionality and implementations. In this work, methylacrylate silatrane (MAST) was newly developed to bear a chemically stable tricyclic caged silatranyl ring and a transannular N → Si dative bond for excellent stability, processability, and progressive deposition. A complete, uniform, and thin assembled adlayer of MAST on a silicon wafer was verified by a contact angle goniometer, ellipsometry, atomic force microscopy, X-ray photoelectron microscopy, and Fourier transform infrared spectroscopy. The good homogeneity and molecular orientation of the MAST coatings are attributed to controlled silanization on oxide surfaces and strong intermolecular hydrogen bonds between internal urea groups. Moreover, the zwitterionic monomer of 2-methacryloyloxyethyl phosphorylcholine (MPC) was employed to copolymerize with MAST or TMSPMA to afford macromolecular modifiers of p(MPC9-co-MAST1) and p(MPC9-co-TMSPMA1), respectively, for surface modification and antifouling properties on silicon substrates. p(MPC9-co-MAST1) well preserved the reactivity of the silatrane groups after the polymerization process, whereas the hydrolysis of silane groups of p(MPC9-co-TMSPMA1) obviously occurred, giving rise to aggregation of polymer chains. Therefore, the p(MPC9-co-MAST1) film on surfaces exhibited superior wettability, grafting density, and antifouling properties compared to p(MPC9-co-TMSPMA1). Accordingly, we envision the great potential of the MAST building block for the development of functional hybrid polymers, well-defined polymeric thin films, and nanomaterials.

Electro-Responsive Aggregation and Dissolution of Cationic Polymer Using Reversible Redox Reaction of Electron Mediator.

Stimuli-responsive aggregation of polymer chains in water has found a variety of applications in polymer science, biology, and chemical engineering. To date, the majority of the phase transitions between the aggregated and dissolved forms has been observed by changing the solution temperature, and an active and precise control on the phase transition with a high time resolution has been challenging. Herein, a reversible phase transition of poly(allylamine-co-allylurea) (PAU) in an aqueous electrolyte is achieved by electrochemical redox cycling of hexacyanoferrate(II/III) ([Fe(CN)6 ]4-/3- ) ion pair. The aggregation and dissolution cycle can be completed in a high-resolution time frame of as short as 5s. The strong electrostatic interaction between the protonated primary amino group of PAU and the tetravalent [Fe(CN)6 ]4- anion induces the aggregation, while the oxidation to the trivalent [Fe(CN)6 ]3- anion reduces the attractive force, and the polymer chain redissolves in solution. The ureido group of PAU helps the chain-folding process through the formation of inter/intrachain hydrogen-bonding networks, resulting in the sharp phase transition. By using [Fe(CN)6 ]4-/3- as the electron mediator, the electrochemical control on the large transparency change of polymer aqueous solution is realized for the first time.

Tough Hydrogel Electrolytes for Anti-Freezing Zinc-Ion Batteries.

As the soaring demand for energy storage continues to grow, batteries that can cope with extreme conditions are highly desired. Yet, existing battery materials are limited by weak mechanical properties and freeze-vulnerability, prohibiting safe energy storage in devices that are exposed to low temperature and unusual mechanical impacts. Herein, a fabrication method harnessing the synergistic effect of co-nonsolvency and "salting-out" that can produce poly(vinyl alcohol) hydrogel electrolytes with unique open-cell porous structures, composed of strongly aggregated polymer chains, and containing disrupted hydrogen bonds among free water molecules, is introduced. The hydrogel electrolyte simultaneously combines high strength (tensile strength 15.6MPa), freeze-tolerance (<-77 °C), high mass transport (10× lower overpotential), and dendrite and parasitic reactions suppression for stable performance (30 000 cycles). The high generality of this method is further demonstrated with poly(N-isopropylacrylamide) and poly(N-tertbutylacrylamide-co-acrylamide) hydrogels. This work takes a further step toward flexible battery development for harsh environments.

Front Cover: Unusual switching of ionic conductivity in ionogels enabled by water‐induced phase separation

Zhang et al. develop an unprecedented humidity-responsive ionogel. At low humidity, the ionogel exhibits a high ionic conductivity due to the high mobility of the ions within the swollen polymer networks. When the ambient humidity rises, the ion motion is restricted by the strong polymer chain aggregation induced by water, resulting in a lower ionic conductivity. (e249)

Contributions of Polymer Chain Length, Aggregation and Crystallinity Degrees in a Model of Charge Carrier Transport in Ultrathin Polymer Films

Herein we report a systematic study on the quantitative relationships between molecular structure, crystallinity, aggregation, and charge carrier transport in films of poly(3-hexylthiophene-co-thiophenes) (P3HTTs) with thickness ranging from 5.7 to 8.0 nm. The P3HTTs contained 10, 20, or 30 mol % of thiophene monomer units. Our UV–Vis, X-ray diffraction, and atomic force microscopy (AFM) studies indicated that the variable thiophene content in the P3HTTs as well as the solution aging prior to film casting permitted a control over aggregation density (Xa), crystallinity degree (Xc), crystal fragmentation, and overall film morphology. Increasing the thiophene content in P3HTT caused a linear increase in the Xa and a linear drop in the Xc as well as crystal sizes. Aging the solution prior to casting resulted in a change of film morphology from granular to fibrillar and a noticeable increase in both Xa and Xc. The hole mobility determined from field-effect transistor characteristics grew nonlinearly with increasing thiophene content in P3HTT and was boosted by the aggregation-induced increase in Xa or Xc. For the 7.5 nm-thin P3HTT film (30 mol % of thiophene), the μh was as high as 0.017 cm2 × V–1 × s–1. In order to explain the observed μh variation, we have employed a model of charge transport through a series circuit, where crystals, aggregates, and the amorphous phase were considered units with different resistance to charge carrier transport. A systematic analysis of the experimental dataset revealed a quantitative correlation of the μh with the polymer end-to-end distance, interaggregate distance, and crystallinity index. Also, the correlation expressed with a single equation derived in this work enabled estimation of the minimum mobility of the amorphous phase and maximum mobility of the crystal phase in the quasi-two-dimensional films. Our original findings suggest that in quantitative considerations of charge transport in the ultrathin polymer films, the morphology should be considered secondary to molar mass and phase composition.

A Facile yet Versatile Strategy to Construct Liquid Hybrid Energy-Saving Windows for Strong Solar Modulation.

Smart windows with light management and indoor solar heating modulation capacities are of paramount importance for building energy conservation. Thermochromic poly(N-isopropylacrylamide) (PNIPAm) hydrogel smart windows exhibit advantages of the relatively suitable transition temperature of 32 °C, high cost-effective and automatic passive sunlight regulation, but sustain slow response rate and unsatisfactory solar modulation efficiency. Herein, a strategy of one-step copolymerization of NIPAm and different olefine acids (OA) using reverse atom transfer radical polymerization method is developed to fabricate various chain/microparticle hybrids (CMH) for liquid energy-saving windows. Synergetic mechanisms of thermal-induced dissolution and aggregation of linear polymer chains integrated with water capture and release of microgel particles contribute to tunable light-scattering behaviors and adaptive solar modulation. Without any post-treatment, the as-prepared poly(N-isopropylacrylamide-co-acrylic acid) (P(NIPAm-co-AA))-based CMH suspension is injected into sandwich glass to construct energy-saving windows, which exhibits appreciated near-room-temperature transition (26.7 °C), rapid response (5 s), extraordinary luminous transmittance (91.5%), and solar modulation efficiency (85.8%), resulting in a substantial decline of indoor temperature of 24.5 °C in simulation experiment. Combining the versatile strategy with flexible adjustment on transition temperature, multifarious P(NIPAm-co-OA)-based CMH windows with eminent light management capacity are obtained. This work will powerfully promote the development and renovation of energy-efficient windows.

Molecular single crystals induce chain alignment in a semiconducting polymer

The blending of π-conjugated molecules with polymeric semiconductors into a composite system is an effective strategy to promote charge carrier mobility because of the transmission path by the conductive polymers through electrical bridge connection of the small organic molecule crystalline domain. In this work, pentacene single crystal was prepared to induce the conjugated polymer chain alignment of polymeric semiconductor PDPP2T-TT-OD to form the semiconducting composite, which led to enhanced field-effect mobility of the organic thin-film transistor (OTFT) by improving the crystallinity due to nucleation and growth phase separation. Besides, with the addition of anti-solvents, the crystallization of the blend film was further improved, 27 times higher than that of a pure polymer semiconductor–based OTFT. That was because the pentacene nuclei induced polymer crystallization through π-π interactions and the addition of anti-solvent promoted the aggregation of polymer chains in solution, enabling the molecular chains packed more closely in solid films. Therefore, the chain arrangement of polymers induced via small molecular single crystals provides a new idea to improve mobility in composite semiconductor thin films for the construction of novel organic optoelectronic devices.

Improving the Resistance of Molecularly Doped Polymer Semiconductor Layers to Solvent.

The ability to form multi-heterolayer (opto)electronic devices by solution processing of (molecularly doped) semiconducting polymer layers is of great interest since it can facilitate the fabrication of large-area and low-cost devices. However, the solution processing of multilayer devices poses a particular challenge with regard to dissolution of the first layer during the deposition of a second layer. Several approaches have been introduced to circumvent this problem for neat polymers, but suitable approaches for molecularly doped polymer semiconductors are much less well-developed. Here, we provide insights into two different mechanisms that can enhance the solvent resistance of solution-processed doped polymer layers while also retaining the dopants, one being the doping-induced pre-aggregation in solution and the other including the use of a photo-reactive agent that results in covalent cross-linking of the semiconductor and, perhaps in some cases, the dopant. For molecularly p-doped poly(3-hexylthiophene-2,5-diyl) and poly[2,5-bis(3-tetradecyl-thiophene-2-yl)thieno(3,2-b)thiophene] layers, we find that the formation of polymer chain aggregates prior to the deposition from solution plays a major role in enhancing solvent resistance. However, this pre-aggregation limits inclusion of the cross-linking agent benzene-1,3,5-triyl tris(4-azido-2,3,5,6-tetrafluorobenzoate). We show that if pre-aggregation in solution is suppressed, high resistance of thin doped polymer layers to solvent can be achieved using the tris(azide). Moreover, the electrical conductivity can be largely retained by increasing the tris(azide) content in a doped polymer layer.

(Digital Presentation) Characterization of Molecular Aggregation State in Deteriorated Nafion Membrane on the Basis of X-Ray Diffraction Measurement

Nafion is widely utilized as key polymer for proton electrolyte fuel cells owing to their high chemical, mechanical, and proton-conducting properties. The molecular aggregation state in Nafion films strongly affects the performance in the fuel cells. During operation process in fuel cells, peroxide derivatives such as H2O2 is generated and the chemical species deteriorates Nafion membrane, leading to a decreasing of performance of the fuel cells. The detail deterioration process including local structure, however, does not well-understand. In this work, we evaluate the local molecular aggregation state in deteriorated Nafion using microbeam wide X-ray diffraction (WAXD) measurements.Figure 1a shows WAXD line profile of before and after deterioration of Nafion membranes. The diffraction peaks were observed in ranging from 11 to 12 nm-1, which could be assigned to crystalline and amorphous structure in Nafion. The ratio of crystalline peak in deteriorated Nafion was more striking than that of initial Nafion membrane. This phenomenon might be explained following two mechanism. ① Polymer chains in amorphous domain were disconnected and were removed from the membrane. ② Polymer chains were disconnected and molecular motion of lower molecular weight is activated, leading to a formation of new crystalline domain caused by aggregation of lower molecular weight polymer chains. It is anticipated that above mechanisms occurs individually or at the same time. Though the dominant mechanism is still not clear, we can conclude that deterioration in Nafion starts from amorphous domain.To evaluate local molecular aggregation state in deteriorated Nafion, microbeams with size of 50 μm and 7 μm were utilized for WAXD measurements. Figure 1b and c show variation of ratio in crystalline domain as function of strain. The ratio of the crystalline domain in deteriorated Nafion was more striking than that of untreated Nafion. The ratio was constant when 50 μm beam was used, but not the case in 7 μm beam. This indicates that deterioration of Nafion membrane occurs 7 to 50 μm scale. The detail will be discussed in my presentation. Figure 1

Tuning the Mechanical and Electric Properties of Conjugated Polymer Semiconductors: Side‐Chain Design Based on Asymmetric Benzodithiophene Building Blocks

AbstractUnderstanding molecular design rules for stretchable polymer semiconductors is important for enabling next generation stretchable electronic circuits. To simultaneously improve both electrical properties and mechanical stretchability, a design strategy is reported in introducing conjugated rigid fused‐rings with bulky side groups in semiconducting polymers. In this work, the understanding of this design concept is improved by systematically investigating the effect of different types of bulky side groups asymmetrically substituted on conjugated polymer semiconductor backbones. Specifically, four types of side groups are investigated, including naphthalene (NaPh), biphenyl (PhPh), thienylphenyl (ThPh), and alkylphenyl (C4Ph), asymmetrically substituted on benzodithiophene units, namely asy‐BDT. With the four types of side groups installed on BDT‐containing conjugated polymers in an asymmetrical fashion, it is observed that they reduced the polymer chain aggregation and film crystallinity, hence improving the film stretchability. Furthermore, the fully conjugated polymer backbone allows maintenance of good charge carrier mobilities. Specifically, polymer PDPP‐C4Ph (with C4Ph side groups) shows the highest mobility in the fully stretchable transistor and maintained its mobility even after being subjected to hundreds of stretching‐releasing cycles at 25% strain. Overall, the results provide anunderstanding of the use of asymmetrically substituted fused‐ring conjugated polymer structures to tune mechanical and charge transport properties.

Preparation of Poly(vinyl Alcohol) Microparticles for Freeze Protection of Sensitive Fruit Crops.

Poly(vinyl alcohol) (PVA) displays ice recrystallization inhibition (IRI) properties as many antifreeze proteins found in cold tolerant organisms. The molecular architecture and composition (molecular weight and distribution of pendant OH and acetate groups) have been studied to improve the antifreezing properties of PVA, suggesting that the molecular architecture of PVA plays an important role in IRI activity. The present work deals with the preparation of PVA microparticles using an alkaline treatment. The effect of PVA molecular weight on the morphology and antifreezeing properties of PVA microparticles was investigated. The antifreezeing property of PVA microparticles on the susceptibility of flower bud tissues to freeze damage was also evaluated. The alkaline treatment of an aqueous PVA solution produced stable polymer chain aggregates with spherical shapes. The average size of the PVA microparticles increased significantly with the increasing molecular weight of the PVA macromolecule precursor. The PVA microparticles inhibited the growth of ice crystals and blocked ice growth at concentrations as low as 0.01 % w/v. The effect of impeding ice crystal growth by preventing the joining of adjacent ice crystals is attributed to the larger size of the PVA particles adsorbed on the ice surface compared to the aggregated PVA macromolecules in saline solution. The thermal hysteresis activity of PVA macromolecules and microparticles was not detected by differential scanning calorimetry analysis. The PVA microparticles reduced the incidence of freeze injuries in flower bud tissues by 55% and their application, considering the low toxicity of PVA, has a high potential for freeze protection in fruit crops.