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High crystallization rate and thermomechanical stability make polylactide stereocomplexes effective nanosized physical netpoints. Here, we address the need for soft, form-stable degradable elastomers for medical applications by designing such blends from (co)polyesters, whose mechanical properties are ruled by their nanodimensional architecture and which are applied as single components in implants. By careful controlling of the copolymer composition and sequence structure of poly[(L-lactide)-co-(epsilon-caprolactone)], it is possible to prepare hyperelastic polymer blends formed through stereocomplexation by adding poly(D-lactide) (PDLA). Low glass transition temperature T-g <= 0 degrees C of the mixed amorphous phase contributes to the low Young's modulus E. The formation of stereocomplexes is shown in DSC by melting transitions T-m > 190 degrees C and in WAXS by distinct scattering maxima at 2 theta = 12 degrees and 21 degrees. Tensile testing demonstrated that the blends are soft (E = 12-80 MPa) and show an excellent hyperelastic recovery R-rec = 66-85% while having high elongation at break epsilon(b) up to >1000%. These properties of the blends are attained only when the copolymer has 56-62 wt% lactide content, a weight average molar mass >140 kg center dot mol(-1), and number average lactide sequence length >= 4.8, while the blend is formed with a content of 5-10 wt% of PDLA. The devised strategy to identify a suitable copolymer for stereocomplexation and blend formation is transferable to further polymer systems and will support the development of thermoplastic elastomers suitable for medical applications.
Crystallization and degradation behaviour of multiblock copolyester blends in Langmuir monolayers
(2021)
Supporting the wound healing of soft tissues requires fixation devices becoming more elastic while degrading. To address this unmet need, we designed a blend of degradable multiblock copolymers, which is cross-linked by PLA stereocomplexation combining two soft segments differing substantially in their hydrolytic degradation rate. The degradation path and concomitant structural changes are predicted by Langmuir monolayer technique. The fast hydrolysis of one soft segment leads to a decrease of the total polymer mass at constant physical cross-linking density. The corresponding increase of the average spacing between the network nodes suggests the targeted increase of the blend's flexibility.
The hierarchical design approach provides various opportunities to adjust the structural performance of polymer materials. Electrospinning processing techniques give access to molecular orientation as a design parameter, which we consider here in view of the shape-memory actuation performance. The aim of this work is to investigate how the reversible strain epsilon'(rev) can be affected by a morphology change from a bulk material to an electrospun mesh. epsilon'(rev) could be increased from 5.5 +/- 0.5% to 15 +/- 1.8% for a blend from a multiblock copolymer with poly(epsilon-caprolactone) (PCL) and poly(L-lactide) (PLLA) segments with oligo(D-lactide) (ODLA). This study demonstrates an effective design approach for enhancing soft actuator performance, which can be broadly applied in soft robotics and medicine.
Soft actuators have drawn significant attention due to their relevance for applications, such as artificial muscles in devices developed for medicine and robotics. Tuning their performance and expanding their functionality are frequently done by means of chemical modification. The introduction of structural elements rendering non-synthetic modification of the performance possible, as well as control over physical appearance and facilitating their recycling is a subject of a great interest in the field of smart materials. The primary aim of this thesis was to create a shape-memory polymeric actuator, where the capability for non-synthetic tuning of the actuation performance is combined with reprocessability. Physically cross-linked polymeric matrices provide a solid material platform, where the in situ processing methods can be employed for modification of the composition and morphology, resulting in the fine tuning of the related mechanical properties and shape-memory actuation capability.
The morphological features, required for shape-memory polymeric actuators, namely two crystallisable domains and anchoring points for physical cross-links, were embedded into a multiblock copolymer with poly(ε-caprolactone) and poly(L-lactide) segments (PLLA-PCL). Here, the melting transition of PCL was bisected into the actuating and skeleton-forming units, while the cross-linking was introduced via PLA stereocomplexation in blends with oligomeric poly(D-lactide) (ODLA). PLLA segment number average length of 12-15 repeating units was experimentally defined to be capable of the PLA stereocomplexes formation, but not sufficient for the isotactic crystallisation. Multiblock structure and phase dilution broaden the PCL melting transition, facilitating its separation into two conditionally independent crystalline domains. Low molar mass of the PLA stereocomplex components and a multiblock structure enables processing and reprocessing of the PLLA-PCL / ODLA blends with common non-destructive techniques. The modularity of the PLLA-PCL structure and synthetic approach allows for independent tuning of the properties of its components. The designed material establishes a solid platform for non-synthetic tuning of thermomechanical and structural properties of thermoplastic elastomers.
To evaluate the thermomechanical stability of the formed physical network, three criteria were appraised. As physical cross-links, PLA stereocomplexes have to be evenly distributed within the material matrix, their melting temperature shall not overlap with the thermal transitions of the PCL domains and they have to maintain the structural integrity within the strain ε ranges further applied in the shape-memory actuation experiments. Assigning PCL the function of the skeleton-forming and actuating units, and PLA stereocomplexes the role of physical netpoints, shape-memory actuation was realised in the PLLA-PCL / ODLA blends. Reversible strain of shape-memory actuation was found to be a function of PLA stereocomplex crystallinity, i.e. physical cross-linking density, with a maximum of 13.4 ± 1.5% at PLA stereocomplex content of 3.1 ± 0.3 wt%. In this way, shape-memory actuation can be tuned via adjusting the composition of the PLLA-PCL / ODLA blend. This makes the developed material a valuable asset in the production of cost-effective tunable soft polymeric actuators for the applications in medicine and soft robotics.