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Many physicochemical processes depend on the diffusion of small molecules through solid materials. While crystallinity in polymers is advantageous with respect to structure performance, diffusion in such materials is difficult to predict. Here, we investigate the impact of crystal morphology and organization on the diffusion of small molecules using a lattice Monte Carlo approach. Interestingly, diffusion determined with this model does not depend on the internal morphology of the semi-crystalline regions. The obtained insight is highly valuable for developing predictive models for all processes in semi-crystalline polymers involving mass transport, like polymer degradation or drug release, and provide design criteria for the time-dependent functional behavior of multifunctional polymer systems.
Collagen-based biomaterials with oriented fibrils have shown great application potential in medicine. However, it is still challenging to control the type I collagen fibrillogenesis in ultrathin films. Here, we report an approach to produce cohesive and well-organized type I collagen ultrathin films of about 10 nm thickness using the Langmuir-Blodgett technique. Ellipsometry, rheology, and Brewster angle microscopy are applied to investigate in situ how the molecules behave at the air-water interface, both at room temperature and 37 degrees C. The interfacial storage modulus observed at room temperature vanishes upon heating, indicating the existence and disappearance of the network structure in the protein nanosheet. The films were spanning over holes as large as 1 mm diameter when transferred at room temperature, proving the strong cohesive interactions. A highly aligned and fibrillar structure was observed by atomic force microscopy (AFM) and optical microscopy.
Inspired by the application of ultrasonic cavitation based mechanical force (CMF) to open small channels in natural soft materials (skin or tissue), it is explored whether an artificial polymer network can be created, in which shape-changes can be induced by CMF. This concept comprises an interconnected macroporous rhodium-phosphine (Rh-P) coordination polymer network, in which a CMF can reversibly dissociate the Rh-P microphases. In this way, the ligand exchange of Rh-P coordination bonds in the polymer network is accelerated, resulting in a topological rearrangement of molecular switches. This rearrangement of molecular switches enables the polymer network to release internal tension under ultrasound exposure, resulting in a CMF-induced shape-memory capability. The interconnected macroporous structure with thin pore walls is essential for allowing the CMF to effectively permeate throughout the polymer network. Potential applications of this CMF-induced shape-memory polymer can be mechanosensors or ultrasound controlled switches.
Two-Level Shape Changes of Polymeric Microcuboids Prepared from Crystallizable Copolymer Networks
(2017)
Polymeric microdevices bearing features like nonspherical shapes or spatially segregated surface properties are of increasing importance in biological and medical analysis, drug delivery, and bioimaging or microfluidic systems as well as in micromechanics, sensors, information storage, or data carrier devices. Here, a method to fabricate programmable microcuboids with shape-memory capability and the quantification of their recovery at different levels is reported. The method uses the soft lithographic technique to create microcuboids with well-defined sizes and surface properties. Microcuboids having an edge length of 25 mu m and a height of 10 mu m were prepared from cross-linked poly[ethylene-co-(vinyl acetate)] (cPEVA) with different vinyl acetate contents and were programmed by compression with various deformation degrees at elevated temperatures. The microlevel shape-recovery of the cuboidal geometry during heating was monitored by optical microscopy (OM) and atomic force microscopy (AFM) studying the related changes in the projected area (PA) or height, while the nanolevel changes of the nanosurface roughness were investigated by in situ AFM. The shape-memory effect at the microlevel was quantified by the recovery ratio of cuboids (R-r,R-micro), while at the. nanolevel, the recovery ratio of the nanoroughness (R-r,R-nano) was measured. The values of R-r,R-micro,,micro could be tailored in a range from 42 +/- 1% to 102 +/- 1% and Rr,nano from 89 +/- 6% to 136 +/- 21% depending on the applied compression ratio and the amount of vinyl acetate content in the cPEVA microcuboids.
Bio-interactive hydrogel formation in situ requires sensory capabilities toward physiologically relevant stimuli. Here, we report on pH-controlled in situ hydrogel formation relying on latent cross-linkers, which transform from pH sensors to reactive molecules. In particular, thiopeptolide/thio-depsipeptides were capable of pH-sensitive thiol-thioester exchange reactions to yield a,co-dithiols, which react with maleimide-functionalized multi-arm polyethylene glycol to polymer networks. Their water solubility and diffusibility qualify thiol/thioester-containing peptide mimetics as sensory precursors to drive in situ localized hydrogel formation with potential applications in tissue regeneration such as treatment of inflamed tissues of the urinary tract.
Defined chemical reactions in a physiological environment are a prerequisite for the in situ synthesis of implant materials potentially serving as matrix for drug delivery systems, tissue fillers or surgical glues. ‘Click’ reactions like thiol Michael-type reactions have been successfully employed as bioorthogonal reaction. However, due to the individual stereo-electronic and physical properties of specific substrates, an exact understanding their chemical reactivity is required if they are to be used for in-situ biomaterial synthesis. The chiral (S)-2-mercapto-carboxylic acid analogues of L-phenylalanine (SH-Phe) and L-leucine (SH-Leu) which are subunits of certain collagenase sensitive synthetic peptides, were explored for their potential for in-situ biomaterial formation via the thiol Michael-type reaction.
In model reactions were investigated the kinetics, the specificity and influence of stereochemistry of this reaction. We could show that only reactions involving SH-Leu yielded the expected thiol-Michael product. The inability of SH-Phe to react was attributed to the steric hindrance of the bulky phenyl group. In aqueous media, successful reaction using SH-Leu is thought to proceed via the sodium salt formed in-situ by the addition of NaOH solution, which was intented to aid the solubility of the mercapto-acid in water. Fast reaction rates and complete acrylate/maleimide conversion were only realized at pH 7.2 or higher suggesting the possible use of SH-Leu under physiological conditions for thiol Michael-type reactions. This method of in-situ formed alkali salts could be used as a fast approach to screen mercapto-acids for thio Michael-type reactions without the synthesis of their corresponding esters.
Among the high-performance and engineering polymers, polyimides and the closely related polyetherimide (PEI) stand out by their capability to react with nucleophiles under relatively mild conditions. By targeting the phthalimide groups in the chain backbone, post-functionalization offers a pathway to adjust surface properties such as hydrophilicity, solvent resistance, and porosity. Here, we use ultrathin PEI films on a Langmuir trough as a model system to investigate the surface functionalization with ethylene diamine and tetrakis(4-aminophenyl)porphyrin as multivalent nucleophiles. By means of AFM, Raman spectroscopy, and interfacial rheology, we show that hydrolysis enhances the chemical and mechanical stability of ultrathin films and allows for the formation of EDC/NHS-activated esters. Direct amidation of PEI was achieved in the presence of a Lewis acid catalyst, resulting in free amine groups rather than cross-linking. When comparing amidation with hydrolysis, we find a greater influence of the latter on material properties.
Shape-memory hydrogels (SMH) are multifunctional, actively-moving polymers of interest in biomedicine. In loosely crosslinked polymer networks, gelatin chains may form triple helices, which can act as temporary net points in SMH, depending on the presence of salts. Here, we show programming and initiation of the shape-memory effect of such networks based on a thermomechanical process compatible with the physiological environment. The SMH were synthesized by reaction of glycidylmethacrylated gelatin with oligo(ethylene glycol) (OEG) alpha,omega-dithiols of varying crosslinker length and amount. Triple helicalization of gelatin chains is shown directly by wide-angle X-ray scattering and indirectly via the mechanical behavior at different temperatures. The ability to form triple helices increased with the molar mass of the crosslinker. Hydrogels had storage moduli of 0.27-23 kPa and Young's moduli of 215-360 kPa at 4 degrees C. The hydrogels were hydrolytically degradable, with full degradation to water-soluble products within one week at 37 degrees C and pH = 7.4. A thermally-induced shape-memory effect is demonstrated in bending as well as in compression tests, in which shape recovery with excellent shape-recovery rates R-r close to 100% were observed. In the future, the material presented here could be applied, e.g., as self-anchoring devices mechanically resembling the extracellular matrix.
The incorporation of inorganic particles in a polymer matrix has been established as a method to adjust the mechanical performance of composite materials. We report on the influence of covalent integration of magnetic nanoparticles (MNP) on the actuation behavior and mechanical performance of hybrid nanocomposite (H-NC) based shape-memory polymer actuators (SMPA). The H-NC were synthesized by reacting two types of oligo(ω-pentadecalactone) (OPDL) based precursors with terminal hydroxy groups, a three arm OPDL (3 AOPDL, Mn = 6000 g mol•1−1 ) and an OPDL (Mn =3300 g • mol−1 ) coated magnetite nanoparticle (Ø = 10 ± 2 nm), with a diisocyanate. These H-NC were compared to the homopolymer network regarding the actuation performance, contractual stress (σcontr) as well as thermal and mechanical properties. The melting range of the OPDL crystals (ΔTm,OPDL) was shifted in homo polymer networks from 36 ºC − 76 ºC to 41ºC − 81 °C for H-NC with 9 wt% of MNP content. The actuators were explored by variation of separating temperature (Tsep), which splits the OPDL crystalline domain into actuating and geometry determining segments. Tsep was varied in the melting range of the nanocomposites and the actuation capability and contractual stress (σcontr) of the nanocomposite actuators could be adjusted. The reversible strain (εrev) was decreased from 11 ± 0.3% for homo polymer network to 3.2±0.3% for H-NC9 with 9 wt% of MNP indicating a restraining effect of the MNP on chain mobility. The results show that the performance of H-NCs in terms of thermal and elastic properties can be tailored by MNP content, however for higher reversible actuation, lower MNP contents are preferable.
The interplay of an enzyme with a multiblock copolymer PDLCL containing two segments of different hydrophilicity and degradability is explored in thin films at the air-water interface. The enzymatic degradation was studied in homogenous Langmuir monolayers, which are formed when containing more than 40 wt% oligo(epsilon-caprolactone) (OCL). Enzymatic degradation rates were significantly reduced with increasing content of hydrophobic oligo(omega-pentadecalactone) (OPDL). The apparent deceleration of the enzymatic process is caused by smaller portion of water-soluble degradation fragments formed from degradable OCL fragments. Beside the film degradation, a second competing process occurs after adding lipase from Pseudomonas cepacia into the subphase, namely the enrichment of the lipase molecules in the polymeric monolayer. The incorporation of the lipase into the Langmuir film is experimentally revealed by concurrent surface area enlargement and by Brewster angle microscopy (BAM). Aside from the ability to provide information about the degradation behavior of polymers, the Langmuir monolayer degradation (LMD) approach enables to investigate polymer-enzyme interactions for non-degradable polymers. (C) 2016 Elsevier Ltd. All rights reserved.