Physically crosslinked low-temperature elastomers were prepared based on linear polydimethylsiloxane (PDMS) elastic chains terminated on both ends with mesogenic building blocks (LC) of azobenzene type. They are generally (and also structurally) highly different from the well-studied LC polymer networks (light-sensitive actuators). The LC units also make up only a small volume fraction in our materials and they do not generate elastic energy upon irradiation, but they act as physical crosslinkers with thermotropic properties. Our elastomers lack permanent chemical crosslinks—their structure is fully linear. The aggregation of the relatively rare, small, and spatially separated terminal LC units nevertheless proved to be a considerably strong crosslinking mechanism. The most attractive product displays a rubber plateau extending over 100 °C, melts near 8 °C, and is soluble in organic solvents. The self-assembly (via LC aggregation) of the copolymer molecules leads to a distinctly lamellar structure indicated by X-ray diffraction (XRD). This structure persists also in melt (polarized light microscopy, XRD), where 1–2 thermotropic transitions occur. The interesting effects of the properties of this lamellar structure on viscoelastic and rheological properties in the rubbery and in the melt state are discussed in a follow-up paper (“Part II”). The copolymers might be of interest as passive smart materials, especially as temperature-controlled elastic/viscoelastic mechanical coupling. Our study focuses on the comparison of physical properties and structure–property relationships in three systems with elastic PDMS segments of different length (8.6, 16.3, and 64.4 repeat units).
In bio-nanocomposites with a poly(lactic acid) (PLA)/poly(ε-caprolactone) (PCL) matrix with neat and polydopamine (PDA)-coated cellulose nanocrystals (CNCd), the use of different mixing protocols with masterbatches prepared by solution casting led to marked variation of localization, as well as reinforcing and structure-directing effects, of cellulose nanocrystals (CNC). The most balanced mechanical properties were found with an 80/20 PLA/PCL ratio, and complex PCL/CNC structures were formed. In the nanocomposites with a bicontinuous structure (60/40 and 40/60 PLA/PCL ratios), pre-blending the CNC and CNCd/PLA caused a marked increase in the continuity of mechanically stronger PLA and an improvement in related parameters of the system. On the other hand, improved continuity of the PCL phase when using a PCL masterbatch may lead to the reduction in or elimination of reinforcing effects. The PDA coating of CNC significantly changed its behavior. In particular, a higher affinity to PCL and ordering of PLA led to dissimilar structures and interface transformations, while also having antagonistic effects on mechanical properties. The negligible differences in bulk crystallinity indicate that alteration of mechanical properties may have originated from differences in crystallinity at the interface, also influenced by presence of CNC in this area. The complex effect of CNC on bio-nanocomposites, including the potential of PDA coating to increase thermal stability, is worthy of further study.
The multiple roles of organic nanofillers in biodegradable nanocomposites (NC) with a blend-based matrix is not yet fully understood. This work highlights combination of reinforcing and structure-directing effects of chitin nanowhiskers (CNW) with different degrees of deacetylation (DA), i.e., content of primary or secondary amines on their surface, in the nanocomposite with the PCL/PLA 1:1 matrix. Of importance is the fact that aminolysis with CNW leading to chain scission of both polyesters, especially of PLA, is practically independent of DA. DA also does not influence thermal stability. At the same time, the more marked chain scission/CNW grafting for PLA in comparison to PCL, causing changes in rheological parameters of components and related structural alterations, has crucial effects on mechanical properties in systems with a bicontinuous structure. Favourable combinations of multiple effects of CNW leads to enhanced mechanical performance at low 1% content only, whereas negative effects of structural changes, particularly of changed continuity, may eliminate the reinforcing effects of CNW at higher contents. The explanation of both synergistic and antagonistic effects of structures formed is based on the correspondence of experimental results with respective basic model calculations.
The structural–dynamic states of n-butanol (n-BuOH) and tert-butanol (t-BuOH) isomers as representatives of a linear or globular protic polar medium in the bulk and confined states in the Mobile Composition Matter-41 (MCM-41) matrix obtained from the free volume and phase behavior using positron annihilation lifetime spectroscopy (PALS) or differential scanning calorimetry (DSC), respectively, together with the spectral properties and related mobility and interaction of the spin probe (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) using electron spin resonance (ESR), are presented. In the bulk t-BuOH, the typical τ3 vs T response for strongly crystallizing globular organics with a stepwise effect in the vicinity of T m is found. On the other hand, the bulk n-BuOH exhibits a complicated course depending on thermal cycling due to the distinct crystallization ability of the linear constituents due to their intermolecular H-bonding. Under confinement, both n-BuOH and t-BuOH media in the MCM-41-SIL matrix were amorphized and heterogenized with larger mean free volume sizes and strong broadening of their dispersion with respect to the corresponding bulk states. In addition, very distinct temperature dependences in τ3 vs T/T g plots with some anomalous effect in the subplateau region in the linear isomer correlating with the DSC response are observed. In the ESR experiment, a drastic difference in the most pronounced characteristic ESR temperature marking a transition from the slow-to-fast-motion regime with the following relations was found: T 50G(b) ≅ T m DSC(b) for n-BuOH against T 50G(b) ≪ T m DSC(b) for t-BuOH, where T m DSC(b) is the respective melting point. Further, several changes in the TEMPO mobility in both BuOH media are related to the numerous dynamic and thermodynamic transitions as obtained from measured DSC, PALS, and literary viscosity data. Finally, the changes in hyperfine splitting constants of TEMPO sensitively reflect the altered structural–dynamic relationships in both confined BuOH isomers with close coincidences between all three characteristic PALS and ESR temperatures, indicating the same origin of the underlying processes behind the changes in spin probe mobility or free volume expansion.
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