Two Simple Approaches to Sol-Gel Transition

We represent a theory of polymer gelation as an analogue of liquid-glass transition in which elastic fields of stress and strain shear components appear spontaneously as a consequence of the cross-linking of macromolecules. This circumstance is explained on the basis of obvious combinatoric arguments as well as a synergetic Lorenz system, where the strain acts as an order parameter, a conjugate field is reduced to the elastic stress, and the number of cross-links is a control parameter. Both the combinatoric and synergetic approaches show that an anomalous slow dependence of the shear modulus on the number of cross-links is obtained.Comment: 10 pages, LaTe

Structure of plasma (re)polymerized polylactic acid films fabricated by plasma-assisted vapour thermal deposition

Plasma polymer films typically consist of very short fragments of the precursor molecules. That rather limits the applicability of most plasma polymerisation/plasma-enhanced chemical vapour deposition (PECVD) processes in cases where retention of longer molecular structures is desirable. Plasma-assisted vapour thermal deposition (PAVTD) circumvents this limitation by using a classical bulk polymer as a high molecular weight “precursor”. As a model polymer in this study, polylactic acid (PLA) has been used. The resulting PLA-like films were characterised mostly by X-ray photoelectron spectroscopy (XPS) and nuclear magnetic resonance (NMR) spectroscopy. The molecular structure of the films was found to be tunable in a broad range: from the structures very similar to bulk PLA polymer to structures that are more typical for films prepared using PECVD. In all cases, PLA-like groups are at least partially preserved. A simplified model of the PAVTD process chemistry was proposed and found to describe well the observed composition of the films. The structure of the PLA-like films demonstrates the ability of plasma-assisted vapour thermal deposition to bridge the typical gap between the classical and plasma polymers. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.Czech Science FoundationGrant Agency of the Czech Republic [GA17-10813S]; Charles University [SVV 260 579-2020]Univerzita Karlova v Praze, UK; Grantová Agentura České Republiky, GA ČR: GA17-10813

SANS study on influence of temperature on nanophase separation in epoxy-based hydrogels

Effect of temperature on nanophase separated structure of epoxy-based hydrogels containing polyoxyethylene (POE), polyoxypropylene (POP) and diglycidyl ether of Bisphenol A propoxylate (PDGEBA) was studied using small-angle neutron scattering (SANS). At the macroscopic level, increase of temperature causes an expulsion of water from the hydrogels. At the microscopic level, the expulsion of water is accompanied by redistribution of water and POE inside the hydrogels. Two kinds of structure have been revealed in the hydrogels by SANS. The structure with shorter characteristic length measured by Bragg’s distance (ca 70–80 Å) is observed in all hydrogels and at all investigated temperatures (5–80 °C). It is attributed to a distribution of water into water-rich and water-poor domains caused by nanophase separation of the crosslinked system due to differences in interaction of individual network building blocks with water. The water-rich domains consist of a part of POE blocks mixed with water. The water-poor domains consist of POP and PDGEBA blocks mixed with a small amount of water and remaining POE blocks. The domains are separated by a diffuse interface with effective thickness ca 7–10 Å as estimated from deviations from Porod’s law. Formation of a new structure with longer characteristic length (ca 240 Å) is observed in the highly non-stoichiometric hydrogels. Its contribution to SANS becomes more pronounced with increasing temperature. This contribution is attributed to the formation of regions of pure water (water pools) as a new hydrogel phase that is enabled by low network density and high amount of pending chains in the highly non-stoichiometric epoxy networks subject to swelling

Epoxy networks and hydrogels prepared from a,x-diamino terminated poly(oxypropylene)-b-poly(oxyethylene)-bpoly( oxypropylene) and polyoxypropylene bis(glycidyl ether)

A series of hydrophilic epoxy networks was prepared by reaction of a,x-diamino terminated polyoxypropylene-b-polyoxyethylene-b-polyoxypropylene of average molar mass 2000 g/mol with polyoxypropylene bis(glycidyl ether) of average molar mass 640 g/mol. Absence of microphase separation in the networks at ambient and higher temperatures was proved by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). Cooling of the networks to subambient temperatures induces microphase separation via crystallization: the crystallites of polyoxyethylene formed act as a reinforcing filler in the surrounding amorphous network phase. Hydrogels obtained by swelling of the networks in water deswell continuously with increasing temperature. The deswelling becomes less pronounced with increasing POE content and decreasing network density of the epoxy network. In cooling of the hydrogels to low temperatures the main part of water crystallizes. Water crystallites are dispersed in the amorphous network swollen with remaining no-crystallized water. The hydrogels prepared can found applications in sensors and actuators working in aqueous environment.The result was developed within the CENTEM project, reg. no. CZ.1.05/2.1.00/03.0088, co-funded by the ERDF as part of the Ministry of Education, Youth and Sports OP RDI programme. The authors appreciate a generous gift of the Jeffamines used in this study, by Huntsman Corporation. RSS acknowledge the support of the Spanish Ministry of Economy and Competitiveness through the project MAT2012-38359-C03-01 (including the FEDER financial support).Krakovský, I.; Martínez Haya, R.; Gallego Ferrer, G.; Sabater I Serra, R.; Mohan Dodda, J. (2015). Epoxy networks and hydrogels prepared from a,x-diamino terminated poly(oxypropylene)-b-poly(oxyethylene)-bpoly( oxypropylene) and polyoxypropylene bis(glycidyl ether). European Polymer Journal. 62:19-30. https://doi.org/10.1016/j.eurpolymj.2014.11.005S19306

Microstructure Changes in Polyester Polyurethane upon Thermal and Humid Aging

The microstructure of compression molded Estane 5703 films exposed to 11%, 45%, and 80% relative humidity and 70 °C for 1 and 2 months has been studied by small-angle neutron scattering (SANS), Fourier transform infrared spectroscopy (FTIR), gel permeation chromatography (GPC), and differential scanning calorimetry (DSC). Scattering data indicated increase of the interdomain distance and domain size with a higher humidity and longer aging time. GPC data showed a progressive shortening of polyurethane chains with increasing humidity and aging time. The shortening of the polyurethane chains caused a drop of the glass transition temperature of soft segments, and promoted crystallization of the soft segments during long-time storage of the aged samples at room temperature. FTIR showed a substantial increase in the number of inter-urethane H-bonds in the aged samples. This correlates with the increase of the hard domain size and the degree of phase separation as measured by SANS. The data collected reveals that the reduced steric hindrance caused by hydrolysis of ester links in polybutylene adipate residues promotes the organization of hard segments into domains, leading to the increase of domain size and distance, as well as phase segregation in aged Estane. These findings provide insight into the effects of humidity and thermal aging on the microstructure of aged polyester urethane from molecular to nanoscale level

Nanostructure of hyaluronan acyl-derivatives in the solid state

Acylované deriváty hyaluronanu (acyl-HA) jsou perspektivní materiály pro biomedicínské aplikace. V závislosti na délce acylové skupiny a stupni substituce mohou mít tyto deriváty formu polymerů rozpustných ve vodě se schopností samouspořádání až po materiály ve vodném prostředí nerozpustné. Již bylo studováno chování acyl-HA v roztocích, avšak málo pozornosti jim bylo zatím věnováno v pevném stavu, přestože jsou významné pro takové aplikace, jako je například výroba zdravotnických prostředků. Proto jsme s pomocí rentgenového rozptylu a elektronové mikroskopie zkoumali nanostrukturu acyl-HA v pevném stavu. Sada vzorků zahrnovala různé substituenty, stupně substituce a molekulové hmotnosti. Získaná data ukázala, že všechny studované materiály acyl-HA obsahují struktury o rozměrech v řádu nanometrů, které nejsou přítomné v nemodifikovaném hyaluronanu. Rozměr nanostruktur se zvyšoval s délkou acylové skupiny, zatímco vliv stupně substituce a molekulové hmotnosti byl zanedbatelný. Zjištěná nanostrukura pravděpodobně odpovídá distribuci hydrofobních domén v hydrofilní matrici nemodifikovaného hayluronanuAcyl derivatives of hyaluronan (acyl-HA) are promising materials for biomedical applications. Depending on the acyl length and the degree of substitution, these derivatives range from self-assembling watersoluble polymers to materials insoluble in aqueous environments. The behaviour of acyl-HA was studied in solution, but little attention was paid to the solid state, despite its importance for applications such as medical device fabrication. We thus used X-ray scattering and electron microscopy to explore the solidstate nano-structure of acyl-HA. The set of samples included various substituents, substitution degrees and molecular weights. The obtained data showed that all studied acyl-HA materials contained structures with dimensions on the order of nanometers that were not present in unmodified HA. The size of the nanostructures increased with the acyl length, while the degree of substitution and molecular weight had negligible effects. We suggest that the observed nanostructure corresponds to a distribution of hydrophobic domains in a hydrophilic matrix of unmodified HA segment

Microphase-Separated PE/PEO Thin Films Prepared by Plasma-Assisted Vapor Phase Deposition

International audienc

Thin films of cross-linked polylactic acid as tailored platforms for controlled drug release

Drug-loaded polymers are desirable for the controlled administration of bioactive molecules to biological media because polymer viscoelasticity can be translated into benefits of tissue-contacting materials. Here, we report on plasma-assisted deposition of polyester thin films performed via thermal evaporation of polylactic acid (PLA). The films can be produced with the chemical composition and polymer topology precisely tuned by the discharge power. At low power, weakly cross-linked films are produced with the chemical motif resembling that of PLA, the molar mass distribution peaking at ~350 g × mol−1 and skewing to larger species. At high power, highly cross-linked films are produced with a worse resemblance to PLA. The films swell and dissolve in water, releasing oligomers with the dissolution kinetics spanning over a broad time scale of 10−1–104 s. The released oligomers undergo hydrolysis at the time scale of days and with the final product of lactic acid, meeting the biocompatibility demands. When dissolving, the films expose micrometre-sized pores or buckling instabilities, depending on the discharge power. The phenomenon can be used for controlled release of nisin, an antibacterial peptide so that an hour-delayed release is achieved via the pore-mediated diffusion, whereas a minute-delayed release is achieved through the buckling. Nisin-loaded polyester plasma polymer films are effective against Micrococcus luteus, the bactericidal activity correlating with the drug release kinetics. Hence, the film design holds promise for developing advanced wound dressing materials and other tissue-contacting devices with tunable therapeutic effect. © 2021 Elsevier B.V.Czech Science FoundationGrant Agency of the Czech Republic [GA17-10813S]; Charles University [SVV 260 579/2021]; Ministry of Industry and Trade of the Czech Republic [TRIO FV10400]Univerzita Karlova v Praze, UK: SVV 260 579/2021; Grantová Agentura České Republiky, GA ČR: GA17-10813S; Ministerstvo Průmyslu a Obchodu, MPO: TRIO FV1040