Structural effect of linear and star-shaped poly(L-lactic acid) on physical properties

Synthesis of star-shaped poly(lactide)s, poly(valerolactone)s and poly(caprolactone)s via ROP catalyzed by N-donor tin(ii) cations and comparison of their wetting properties with linear analogues

In this study, we report the use of N-coordinated tin(ii) cations [L1→Sn(H2O)][OTf]2·THF (1) and [L1→SnCl][SnCl3] (2) (L1 = 1,2-(C5H4N-2-CH = N)2CH2CH2) as efficient ROP catalysts, which, in combination with benzyl alcohol, afford well-defined linear poly(ε-caprolactone) (PCL) and poly(δ-valerolactones) (PVL) via an activated monomer mechanism (AMM). Thanks to the versatility of complexes 1 and 2 as catalysts, star-shaped PCL, PVL and PLA were also prepared using three-, four-, five- and six-functional alcohols. The number of arms was determined by SEC-MALS-Visco analysis. Spin-coated thin layers of linear and selected six-armed polymers were further studied in terms of their wettability to water. Attention was focused on the influence of the composition and structure of the polymers. Finally, to increase the hydrophobic properties of the studied polymers, stannaboroxines L2(Ph)Sn[(OB-(C6H4-4-CF3))2O] and L2(Ph)Sn[(OB-(C6H4-3,5-CF3)2)2O] (L2 = C6H3-2,6-(Me2NCH2)2) were applied.

Star-PCL shape memory polymer (SMP) scaffolds with tunable transition temperatures for enhanced utility

Thermoresponsive shape memory polymers (SMPs) prepared from UV-curable poly(ε-caprolactone) (PCL) macromers have the potential to create self-fitting bone scaffolds, self-expanding vaginal stents, and other shape-shifting devices. To ensure tissue safety during deployment, the shape actuation temperature (i.e., the melt transition temperature or Tm of PCL) must be reduced from ∼55 °C that is observed for scaffolds prepared from linear-PCL-DA (Mn ∼ 10 kg mol-1). Moreover, increasing the rate of biodegradation would be advantageous, facilitating bone tissue healing and potentially eliminating the need for stent retrieval. Herein, a series of six UV-curable PCL macromers were prepared with linear or 4-arm star architectures and with Mns of 10, 7.5, and 5 kg mol-1, and subsequently fabricated into six porous scaffold compositions (10k, 7.5k, 5k, 10k★, 7.5k★, and 5k★) via solvent casting particulate leaching (SCPL). Scaffolds produced from star-PCL-tetraacrylate (star-PCL-TA) macromers produced pronounced reductions in Tm with decreased Mnversus those formed with the corresponding linear-PCL-diacrylate (linear-PCL-DA) macromers. Scaffolds were produced with the desired reduced Tm profiles: 37 °C < Tm < 55 °C (self-fitting bone scaffold), and Tm ≤ 37 °C (self-expanding stent). As macromer Mn decreased, crosslink density increased while % crystallinity decreased, particularly for scaffolds prepared from star-PCL-TA macromers. While shape memory behavior was retained and radial expansion pressure increased, this imparted a reduction in modulus but with an increase in the rate of degradation.

Four-Arm Polymer-Guided Formation of Curcumin-Loaded Flower-Like Porous Microspheres as Injectable Cell Carriers for Diabetic Wound Healing

Stem cell injection is an effective approach for treating diabetic wounds; however, shear stress during injections can negatively affect their stemness and cell growth. Cell-laden porous microspheres can provide shelter for bone mesenchymal stem cells (BMSC). Herein, curcumin-loaded flower-like porous microspheres (CFPM) are designed by combining phase inversion emulsification with thermally induced phase separation-guided four-arm poly (l-lactic acid) (B-PLLA). Notably, the CFPM shows a well-defined surface topography and inner structure, ensuring a high surface area to enable the incorporation and delivery of a large amount of -BMSC and curcumin. The BMSC-carrying CFPM (BMSC@CFPM) maintains the proliferation, retention, and stemness of -BMSCs, which, in combination with their sustainable curcumin release, facilitates the endogenous production of growth/proangiogenic factors and offers a local anti-inflammatory function. An in vivo bioluminescence assay demonstrates that BMSC@CFPM can significantly increase the retention and survival of BMSC in wound sites. Accordingly, BMSC@CFPM, with no significant systemic toxicity, could significantly accelerate diabetic wound healing by promoting angiogenesis, collagen reconstruction, and M2 macrophage polarization. RNA sequencing further unveils the mechanisms by which BMSC@CFPM promotes diabetic wound healing by increasing -growth factors and enhancing angiogenesis through the JAK/STAT pathway. Overall, BMSC@CFPM represents a potential therapeutic tool for diabetic wound healing.

Synthesis of Supramolecular A8 Bn Miktoarm Star Copolymers by Host-Guest Complexation

We report a new synthetic method to construct supramolecular A₈ B_n (n=1, 2, 4) miktoarm star copolymers by host-guest complexation between a resorcinarene-based coordination capsule possessing eight polystyrene chains and 4,4-diacetoxybiphenyl guest molecules that retain one, two or four polymethyl acrylate chains. The formation of the supramolecular A₈ B_n (n=1, 2, 4) miktoarm star copolymers was confirmed by dynamic light scattering (DLS), size-exclusion chromatography (SEC), and diffusion-ordered NMR spectroscopy (DOSY). Differential scanning calorimetry (DSC) measurements revealed that the miktoarm copolymers were phase-separated in the bulk. The micro-Brownian motion of the A₈ B₄ structure was markedly enhanced in the bulk due to a weak segregation interaction between the immiscible arms.

Toughening Poly(l-lactide) Blends: Effectiveness of Sequence-Controlled Six-Arm Star-Branched Block Copolymers of Poly(l-lactide) and Poly(ε-caprolactone)

Well-defined six-arm star-branched bio-degradable block copolymers of l-lactide and ε-caprolactone were prepared using controlled ring-opening polymerization and a sequential monomer addition method using dipentaerythritol as the initiator core and organocatalysts at low temperatures in solution. Sequence of enchainment was changed by reversing the order of monomer addition giving, either, a crystalline PLA block or an amorphous PCL block as the outer segment. Well-defined six-arm poly(ε-caprolactone-b-l-lactide, 6s-PCL-b-PLA) block copolymers were obtained with a range of segment molecular weights. However, in the case of six-arm poly(l-lactide-b-ε-caprolactone, 6s-PLA-b-PCL), disruption of the block structure was observed on account of competing transesterification reactions accompanying a chain-growth reaction. Such sequence-controlled block copolymers showed interesting phase morphologies, as evidenced by differential scanning calorimetry (DSC) studies. 6s-PCL-b-PLA showed two glass-transition temperatures and two melting temperatures characteristic of the amorphous and crystalline blocks. 6s-PCL-b-PLA and 6s-PLA-b-PCL with different segment chain lengths were solution blended (10 wt %) with a commercially sourced PLA. All the blends were highly transparent. The structure and properties of the blend were examined by DSC, measurement of mechanical properties, and scanning electron microscopy. The results show that a phase-separated 6s-PCL-b-PLA copolymer results in two- to three-fold improvement in tensile toughness without the loss of modulus. A possible hypothesis for the mechanism of tensile toughness in the blend has been proposed.

Recent progress of preparation of branched poly(lactic acid) and its application in the modification of polylactic acid materials

Poly (lactic acid) (PLA) with branched structure has abundant terminal groups, high melt strength, good rheological properties, and excellent processability; it is a new research and application direction of PLA materials. This study mainly summarizes the molecular structure design, preparation methods, basic properties of branched PLA, and its application in modified PLA materials. The structure and properties of branched PLA prepared by ring-opening polymerization of monomer, functional group polycondensation, and chain extender in the processing process were introduced. The research progress of in situ formation of branched PLA by initiators, multifunctional monomers/additives through dynamic vulcanization, and irradiation induction was described. The effect of branched PLA on the structure and properties of linear PLA materials was analyzed. The role of branched PLA in improving the crystallization behavior, phase morphology, foaming properties, and mechanical properties of linear PLA materials was discussed. At the same time, its research progress in biomedicine and tissue engineering was analyzed. Branched PLA has excellent compatibility with PLA, which has important research value in regulating the structure and properties of PLA materials.

Shear-Induced Crystallization of Star and Linear Poly(L-lactide)s

The influence of macromolecular architecture on shear-induced crystallization of poly(L-lactide) (PLLA) was studied. To this aim, three star PLLAs, 6-arm with Mw of 120 and 245 kg/mol, 4-arm with Mw of 123 kg/mol, and three linear PLLAs with Mw of 121, 240 and 339 kg/mol, were synthesized and examined. The PLLAs were sheared at 170 and 150 °C, at 5/s, 10/s and 20/s for 20 s, 10 s and 5 s, respectively, and then cooled at 10 or 30 °C/min. Shear-induced crystallization during cooling was followed by a light depolarization method, whereas the crystallized specimens were examined by DSC, 2D-WAXS, 2D-SAXS and SEM. The effect of shear depended on the shearing conditions, cooling rate and polymer molar mass but it was also affected by the macromolecular architecture. The shear-induced crystallization of linear PLLA with Mw of 240 kg/mol was more intense than that of the 6-arm polymer with similar Mw, most possibly due to its higher Mz. However, the influence of shear on the crystallization of the star polymers with Mw close to 120 kg/mol was stronger than on that of their linear analog. This was reflected in higher crystallization temperature, as well as crystallinity achieved during cooling.

Shape memory polymer (SMP) scaffolds with improved self-fitting properties

"Self-fitting" shape memory polymer (SMP) scaffolds prepared as semi-interpenetrating networks (semi-IPNs) with crosslinked linear-poly(ε-caprolactone)-diacrylate (PCL-DA, M_n∼10 kg mol^-1) and linear-poly(l-lactic acid) (PLLA, M_n∼15 kg mol^-1) [75/25 wt%] exhibited robust mechanical properties and accelerated degradation rates versus a PCL-DA scaffold control. However, their potential to treat irregular craniomaxillofacial (CMF) bone defects is limited by their relatively high fitting temperature (T_fit∼55 °C; related to the T_m of PCL) required for shape recovery (i.e. expansion) and subsequent shape fixation during press fitting of the scaffold, which can be harmful to surrounding tissue. Additionally, the viscosity of the solvent-based precursor solutions, cast over a fused salt template during fabrication, can limit scaffold size. Thus, in this work, analogous semi-IPN SMP scaffolds were formed with a 4-arm star-PCL-tetracryalate (star-PCL-TA) (M_n∼10 kg mol^-1) and star-PLLA (M_n∼15 kg mol^-1). To assess the impact of a star-polymer architecture, four semi-IPN compositions were prepared: linear-PCL-DA/linear-PLLA (L/L), linear-PCL-DA/star-PLLA (L/S), star-PCL-TA/linear-PLLA (S/L) and star-PCL-TA/star-PLLA (S/S). Two PCL controls were also prepared: LPCL (i.e. 100% linear-PCL-DA) and SPCL (i.e. 100% star-PCL-TA). The S/S semi-IPN scaffold exhibited particularly desirable properties. In addition to achieving a lower, tissue-safe T_fit (∼45 °C), it exhibited the fastest rate of degradation which is anticipated to more favourably permit neotissue infiltration. The radial expansion pressure exerted by the S/S semi-IPN scaffold at T_fit was greater than that of LPCL, which is expected to enhance osseointegration and mechanical stability. The intrinsic viscosity of the S/S semi-IPN macromer solution was also reduced such that larger scaffold specimens could be prepared.

Preparation and characterization of PLA foam chain extended through grafting octa(epoxycyclohexyl) POSS onto carbon nanotubes

Improving foamability of poly (lactic acid) (PLA) resin is a key issue for its critical foaming applications with high-performance and ultralow density. However, owing to the rheological nature of linear PLA chain structure with relatively low molecular weight, the overall foamability of PLA resin cannot meet the processing requirements of foaming purpose. Here, we describe a simple and versatile technique to prepare high foamability PLA resin by inducing chain extender through grafting octa(epoxycyclohexyl) polyhedral oligomeric silsesquioxanes (POSS) on carbon nanotubes (CNT). After the orderly assemble of the two nanoparticles, an obvious increase in melt elasticity of PLA is observed. The enhanced melt elasticity of PLA had a significant effect on controlling subsequent foaming behavior. Thus, a homogeneous and finer cellular morphology of PLA rigid foam was obtained with a proper content of CNT-POSS. Eventually, the expansion ratio of chain-extended PLA foam was 13 times higher than that of unmodified PLA foam. The proposed design methodology will potentially pave a way for designing and preparing high-performance PLA rigid foam products.

Fast and controlled ring-opening polymerization of δ-valerolactone catalyzed by benzoheterocyclic urea/MTBD catalysts

New benzoheterocyclic urea/MTBD catalysts are highly efficient and controllable in the ring-opening polymerization of δ-valerolactone under solvent-free conditions or in solution.

Star polymerization of norbornene derivatives using a tri-functionalized Blechert's olefin metathesis catalyst

A tri-functionalized initiator (IV) based on Blechert's catalyst as a highly efficient ROMP initiator for the living polymerization of norbornene derivatives.

Facile Synthesis of an Eight-Armed Star-Shaped Polymer via Coordination-Driven Self-Assembly of a Four-Armed Cavitand

The polystyrene chains were installed at the lower rim of a resorcinarene-based cavitand via reversible addition-fragmentation (RAFT) polymerization to form a four-armed star-shaped polymer. A star-shaped polystyrene-functionalized supramolecular capsule was prepared through the coordination-driven self-assembly of the four-armed start-shaped polymer with silver ions. The eight-armed start-shaped supramolecular capsule encapsulated 4,4'-diacetoxybiphenyl as did a cavitand-based self-assembled capsule. A DOSY measurement indicated that the eight-armed star-shaped polymer was twice as large as the four-armed star-shaped polymer. The solution behaviors of these compounds resulted in a difference in their zero-shear viscosities.

Cell proliferation influenced by matrix compliance of gelatin grafted poly(d,l-Lactide) three dimensional scaffolds

Surface and mechanical properties of the biomaterials are determinants of cellular responses. In our previous study, star-shaped poly(d,l-Lactide)-b-gelatin (ss-pLG) was reported for possessing improved cellular adhesion and proliferation. Here, we extended our investigation to establish the cellular compatibility of gelatin-grafted PDLLA with respect to mechanical properties of biological tissues. In this view, linear PDLLA-b-gelatin (l-pLG) was synthesized and tissue-level compatibility of 1-pLG and ss-pLG against fibroblasts (L929), myoblasts (C2C12) and preosteoblasts (MG-63) was examined. The cell proliferation of C2C12 was significantly higher within l-pLG scaffolds, whereas L929 showed intensified growth within ss-pLG scaffolds. The difference in cell proliferation may be attributed to the varying mechanical properties of scaffolds; where the stiffness of l-pLG scaffolds was notably higher than ss-pLG scaffolds, most likely due to the variable levels of gelatin grafting on the backbone of PDLLA. Therefore, gelatin grafting can be used to modulate mechanical property of the scaffolds and this study reveals the significance of the matrix stiffness to produce the successful 3D scaffolds for tissue engineering applications.

Stereocomplexation, Thermal and Mechanical Properties of Conetworks Composed of Star-Shaped l-Lactide, d-Lactide and ε-Caprolactone Oligomers Utilizing Sugar Alcohols as Core Molecules

It is important to develop tailor-made biodegradable/biocompatible polymer networks usable for biomaterials whose thermal and mechanical properties are easily controlled by changing the composition. We synthesized sugar-alcohol-based polymer networks (SPN-mscLAO/3CLO, m = 4, 5 or 6) by the crosslinking reactions of erythritol, xylitol or sorbitol-based m-armed star-shaped l-lactide and d-lactide oligomers (HmSLLAO and HmSDLAO), a glycerol-based 3-armed star-shaped ε-caprolactone oligomer (H3SCLO) and hexamethylene diisocyanate (HDI) at the weight ratios of HmSLLAO/HmSDLAO = 1/1 and (HmSLLAO + HmSDLAO)/H3CLO = 100/0, 75/25, 50/50, 25/75 or 0/100). The influence of the arm number on the crystallization behavior, thermal and mechanical properties of SPN-mscLAO/3CLOs were systematically investigated by comparing with those of sugar-alcohol-based homochiral polymer network (SPN-mLLAO, m = 4, 5 or 6) prepared by the reaction of HmSLLAO and HDI. Stereocomplex (sc) crystallites are dominantly formed for SPN-mscLAO/3CLOs 100/0⁻25/75, whereas SPN-mLLAOs were amorphous. The higher order of melting temperature of sc-crystals for SPN-mscLAO/3CLOs 100/0⁻25/75 was m = 5 > m = 6 > m = 4. The sc-crystallinities of SPN-4scLAO/3CLOs 100/0⁻50/50 were significantly lower than those of SPN-mscLAO/3CLOs 100/0⁻50/50 (m = 5 and 6). The larger order of the sc-spherulite size at crystallization temperature of 110 °C was m = 5 > m = 6 > m = 4 for SPN-mscLAO/3CLO 100/0. The size and number of sc-spherulites decreased with increasing crystallization temperature over the range of 110⁻140 °C and with increasing CLO fraction. Among all the networks, SPN-5scLAO/3CLOs 75/25 and 50/50 exhibited the highest and second highest tensile toughnesses (21.4 and 20.3 MJ·m-3), respectively.

Evolution of mechanics in α-helical peptide conjugated linear- and star-block PEG

We have designed a peptide conjugated poly-ethylene glycol (PEG) bioconjugate system that allows us to examine the intra- and inter-molecular dynamics of gelation. We measure the kinetics of gelation for end-functionalized linear- and star-architectures, and we correlate the gelation behavior with the molecular structure and self-association. The 23-amino acid peptide sequence is known to form a coiled-coil structure as a function of the solution's electrolyte concentration, and the two topologies of the PEG are peptide end-functionalized to examine formation of supramolecular assemblies. Subsequently, microrheology is used to evaluate the dynamics of self-assembly and the gelation time-scales. This study shows that the dynamics of peptide folding and assembly for linear-PEG conjugated systems yield a percolated network, but the star-PEG conjugated systems yield discrete assemblies and remain viscous. The results suggest that the degree of intra- and inter-molecular folding defines the critical gel behavior of the supramolecular system.

The Effect of Branching (Star Architecture) on Poly(d,l-lactide) (PDLLA) Degradation and Drug Delivery

This study focuses on the comparative evaluation of star (branched) and linear poly(l,d-lactic acid) (PDLLA) as degradable materials employed in controlled release. The polymers were prepared via ring-opening polymerization initiated by decanol (linear), pentaerythritol (4-armed star) and dipentaerythritol (6-armed star), and processed both in the form of films and nanoparticles. Independent of the length or number of their arms, star polymers degrade slower than linear polymers, possibly through a surface (vs bulk) mechanism. Further, the release of a model drug (atorvastatin) followed zero-order-like kinetics for the branched polymers, and first-order kinetics for linear PDLLA. Using NHOst osteoblastic cells, both linear and star polymers were devoid of any significant toxicity and released atorvastatin in a bioavailable form; cell adhesion was considerably lower on star polymer films, and the slower release from their nanoparticles appeared to be beneficial to avoid atorvastatin overdosing.

Morphological, Rheological, and Mechanical Properties of Polyamide 6/Polypropylene Blends Compatibilized by Electron-Beam Irradiation in the Presence of a Reactive Agent

An immiscible polyamide 6 (PA6)/polypropylene (PP) blend was compatibilized by electron-beam irradiation in the presence of reactive agent. Glycidyl methacrylate (GMA) was chosen as a reactive agent for interfacial cross-copolymerization between dispersed PP and continuous PA6 phases initiated by electron-beam irradiation. The PA6/PP (80/20) mixture containing GMA was prepared using a twin-screw extruder, and then exposed to an electron-beam at various doses at room temperature to produce compatibilized PA6/PP blends. The morphological, rheological, and mechanical properties of blends produced were investigated. Morphology analysis revealed that the diameter of PP particles dispersed in PA6 matrix was decreased with increased irradiation dose and interfacial adhesion increased due to high surface area of treated PP particles. Complex viscosities (η*) and storage moduli (G’) of blends increased with increasing irradiation dose and were higher than those of PA6 and PP. The complex viscosity of the blend irradiated at 200 kGy was 64 and 8 times higher than PA6 and PP, respectively. The elongation at break of blend irradiated less than 100 kGy was about twice that of PA6. Electron beam treatment improved the compatibility at the interface between PA6 and PP matrix in the presence of GMA.

Novel biodegradable star-shaped polylactide scaffolds for bone regeneration fabricated by two-photon polymerization

AIM

To assess the properties of 3D biodegradable scaffolds fabricated from novel star-shaped poly(D,L-lactide) (SSL) materials for bone tissue regeneration.

MATERIALS & METHODS

The SSL polymer was synthesized using an optimized synthetic procedure and applied for scaffold fabrication by the two-photon polymerization technique. The osteogenic differentiation was controlled using human adipose-derived stem cells cultured for 28 days. The SSL scaffolds with or without murine MSCs were implanted into the cranial bone of C57/Bl6 mice.

RESULTS

The SSL scaffolds supported differentiation of human adipose-derived stem cells toward the osteogenic lineage in vitro. The SSL scaffolds with murine MSCs enhanced the mineralized tissue formation.

CONCLUSION

The SSL scaffolds provide a beneficial microenvironment for the osteogenic MSCs' differentiation in vitro and support de novo bone formation in vivo.

The morphology and spherulite growth of PLA stereocomplex in linear and branched PLLA/PDLA blends: effects of molecular weight and structure

In the linear and three armed PLLA/PDLA blends, the G value increased firstly, then decreased as M_n,PLA increased, and G reduced as three armed PLAs added.

Stereocomplex formation in stereoblock copolymer networks composed of 4-armed star-shaped lactide oligomers and a 2-armed ε-caprolactone oligomer

A stereoblock-copolymer network prepared from 4-armed star-shaped l-lactide and d-lactide oligomers formed stereocomplex crystallites, while the corresponding homochiral networks were amorphous.

PLA architectures: the role of branching

Biobased and biodegradable polymers have become more and more interesting in view of waste management and crude oil depletion.

On stereocomplexed polylactide materials as support for PAMAM dendrimers: synthesis and properties

Synthesis of a PLA-based system capable of supporting dendritic/star-like molecules, combining the properties of the stereocomplex PLA matrix with those of PAMAM dendrimers.

Preparation and Characterization of Bioplastic-Based Green Renewable Composites from Tapioca with Acetyl Tributyl Citrate as a Plasticizer

Granular tapioca was thermally blended with poly(lactic acid) (PLA). All blends were prepared using a plasti-corder and characterized for tensile properties, thermal properties and morphology. Scanning electron micrographs showed that phase separation occurred, leading to poor tensile properties. Therefore, methylenediphenyl diisocyanate (MDI) was used as an interfacial compatibilizer to improve the mechanical properties of PLA/tapioca blends. The addition of MDI could improve the tensile strength of the blend with 60 wt% tapioca, from 19.8 to 42.6 MPa. In addition, because PLA lacked toughness, acetyl tributyl citrate (ATBC) was added as a plasticizer to improve the ductility of PLA. A significant decrease in the melting point and glass-transition temperature was observed on the basis of differential scanning calorimetry, which indicated that the PLA structure was not dense after ATBC was added. As such, the brittleness was improved, and the elongation at break was extended to several hundred percent. Therefore, mixing ATBC with PLA/tapioca/MDI blends did exhibit the effect of plasticization and biodegradation. The results also revealed that excessive plasticizer would cause the migration of ATBC and decrease the tensile properties.