Enhanced far infrared emissivity, UV protection and near-infrared shielding of polypropylene composites via incorporation of natural mineral for functional fabric development.

Far infrared radiation in the range of 4-20 µm has been showed to have biological and health benefits to the human body. Therefore, incorporating far-infrared emissivity additives into polymers and/or fabrics hold promise for the development of functional textiles. In this study, we incorporated nine types of natural minerals into polypropylene (PP) film and examined their properties to identify potential candidates for functional textiles and apparels. The addition of 2% mineral powders into PP film increased the far-infrared emissivity (5-14 µm) by 7.65%-14.48%. The improvement in far-infrared emissivity within the range of 5-14 µm, which overlaps with the peak range of human skin radiation at 8-14 µm, results in increased absorption efficiency, and have the potential to enhance thermal and biological effects. Moreover, the incorporation of mineral powders in PP films exhibited favorable ultraviolet (UV) protection and near-infrared (NIR) shielding properties. Two films, specifically those containing red ochre and hematite, demonstrated excellent UV protection with a UPF rating of 50+ and blocked 99.92% and 98.73% of UV radiation, respectively. Additionally, they showed 95.2% and 93.2% NIR shielding properties, compared to 54.1% NIR shielding properties of PP blank films. The UV protection and NIR shielding properties offered additional advantages for the utilization of polymer composite with additives in the development of sportswear and other outdoor garments. The incorporation of minerals could absorb near-IR radiation and re-emit them at longer wavelength in the mid-IR region. Furthermore, the incorporation of minerals significantly improved the heat retention of PP films under same heat radiation treatment. Notably, films with red ochre and hematite exhibited a dramatic temperature increase, reaching 2.5 and 3.2 times the temperature increase of PP films under same heat radiation treatment, respectively (46.8 °C and 59.9 °C higher than the temperature increase of 20.9 °C in the PP film). Films with additives also demonstrated lower thermal effusivity than PP blank films, indicating superior heat insulation properties. Therefore, polypropylene films with mineral additives, particularly those containing red ochre and hematite, showed remarkable heat capacity, UV-protection, NIR-shielding properties and enhanced far infrared emissivity, making them promising candidates for the development of functional textiles.

Nanocomposite Foams with Balanced Mechanical Properties and Energy Return from EVA and CNT for the Midsole of Sports Footwear Application.

Polymer foam that provides good support with high energy return (low energy loss) is desirable for sport footwear to improve running performance. Ethylene-vinyl acetate copolymer (EVA) foam is commonly used in the midsole of running shoes. However, EVA foam exhibits low mechanical properties. Conventional mineral fillers are usually employed to improve EVA's mechanical performance, but the energy return is sacrificed. Here, we produced nanocomposite foams from EVA and multi-walled carbon nanotubes (CNT) using a chemical foaming process. Two kinds of CNT derived from the upcycling of commodity plastics were prepared through a catalytic chemical vapor deposition process and used as reinforcing and nucleating agents. Our results show that EVA foam incorporated with oxygenated CNT (O-CNT) demonstrated a more pronounced improvement of physical, mechanical, and dynamic impact response properties than acid-purified CNT (A-CNT). When CNT with weight percentage as low as 0.5 wt% was added to the nanocomposites, the physical properties, abrasion resistance, compressive strength, dynamic stiffness, and rebound performance of the EVA foams were improved significantly. Unlike the conventional EVA formulation filled with talc mineral fillers, the incorporation of CNT does not compromise the energy return of the EVA foam. From the long-cycle dynamic fatigue test, the CNT/EVA foam displays greater properties retention as compared to the talc/EVA foam. This work demonstrates a good balanced of mechanical-energy return properties of EVA nanocomposite foam with very low CNT content, which presents promising opportunities for lightweight-high rebound midsoles for running shoes.

In Situ Observation of Shear-Induced Jamming Front Propagation during Low-Velocity Impact in Polypropylene Glycol/Fumed Silica Shear Thickening Fluids.

Shear jamming, a relatively new type of phase transition from discontinuous shear thickening into a solid-like state driven by shear in dense suspensions, has been shown to originate from frictional interactions between particles. However, not all dense suspensions shear jam. Dense fumed silica colloidal systems have wide applications in the industry of smart materials from body armor to dynamic dampers due to extremely low bulk density and high colloid stability. In this paper, we provide new evidence of shear jamming in polypropylene glycol/fumed silica suspensions using optical in situ speed recording during low-velocity impact and explain how it contributes to impact absorption. Flow rheology confirmed the presence of discontinuous shear thickening at all studied concentrations. Calculations of the flow during impact reveal that front propagation speed is 3-5 times higher than the speed of the impactor rod, which rules out jamming by densification, showing that the cause of the drastic impact absorption is the shear jamming. The main impact absorption begins when the jamming front reaches the boundary, creating a solid-like plug under the rod that confronts its movement. These results provide important insights into the impact absorption mechanism in fumed silica suspensions with a focus on shear jamming.

Complex coacervates of oppositely charged co-polypeptides inspired by the sandcastle worm glue.

Sandcastle worms secrete a water-resistant proteinaceous glue that is used to bind mineral particulates into their protective tubing. Previous proteomics studies have shown that the constitutive proteins of the glue are oppositely charged co-polypeptides that form a complex coacervate precursor phase, which is critical for stable underwater delivery of the adhesive. Using ring-opening polymerization (ROP) from N-carboxyanhydride (NCA) monomers, we synthesized oppositely charged co-polypeptides that mimic the amino acid composition and molecular weight of the native glue-forming proteins. The synthesis strategy enabled the incorporation of non-standard phosphoserine (pSer) and 3,4-Dihydroxyphenylalanine (Dopa) amino acids in the co-polypeptides, thereby duplicating chemical functionalities of the native glue that are key for electrostatic complexation and adhesion. Complex coacervates were obtained from these oppositely charged co-polypeptides, thus mimicking the self-assembly process of the native adhesive secreted by the sandcastle worm. Varying the relative ratio of the co-polypeptides enabled the fine-tuning of coacervation conditions such as pH and ionic strength. Wetting and rheological characterization demonstrated that our oppositely charged co-polypeptide complexes exhibited the key features associated with coacervates, namely, low surface tension, shear thinning behaviour, and viscoelastic response, making these sandcastle worm glue-inspired polypeptide coacervates a suitable modality for water-resistant bioadhesives.

Hyaluronic acid-based nanocomposite hydrogels for ocular drug delivery applications.

Hyaluronic acid (HA) is a widely investigated biomaterial for many therapeutic applications owing to its unique properties of biocompatibility, biodegradation, and viscoelasticity. HA being a natural component of eye tissue with significant role in wound healing is a natural choice as a carrier for ocular drug delivery, provided the incorporated drugs are released in a sustained manner. However, localized sustained release of drugs inside eye has been difficult to achieve because of the inability to retain carriers for long periods in the eye. Using noncrosslinked (soluble) HA offers limited control over site retention of drugs. In order to obtain prolonged sustained delivery, two HA-based composite hydrogels incorporating nanocarriers, have been synthesized and characterized for swelling, rheology, degradation, and in vitro release of latanoprost, a drug used to reduce intraocular pressure. The HA is first chemically modified, mixed with drug-loaded liposomes, and then crosslinked to obtain nanocomposite hydrogels. In vitro release study shows longer sustained release of latanoprost from composite hydrogels as compared to liposomes or hydrogels alone indicating additional resistance to drug diffusion because of the incorporation of liposomes inside the hydrogels. It is believed that these nanocomposite hydrogels, with controlled degradation properties and sustained release, could serve as potential drug delivery systems for many ocular diseases.

Characterization and degradation of elastomeric four-armed star copolymers based on caprolactone and L-lactide.

Although biodegradable polymers have found extensive applications in medical areas, there are limited reports that show elastomeric behavior. In this work, a biodegradable, elastomeric polymer is demonstrated from a four-armed star copolymer. With a fixed middle core composition, comprising caprolactone (CL) and L-lactide (LA), an elastomer is obtained by increasing the polylactide (PLA) end block lengths to obtain sufficient end block crystallinity. This increase suppressed the middle core's crystallinity yet ensured cocrystallization of the PLA ends of individual star copolymer chains to form a three-dimensional network via physical crosslinking. Cyclic and creep test of the star copolymers showed that at least 75% of recovery was achieved. Degradation study of the copolymer showed that degradation first occurred in the caprolactone-co-lactide (CLLA) core, followed by degradation in the PLA ends. Chain scission in the middle core resulted in immediate formation of CL crystals within the core and increased crystallinity over time, in both CLLA core and PLA ends.

Synthesis of Block Copolymers of Varying Architecture Through Suppression of Transesterification during Coordinated Anionic Ring Opening Polymerization.

Well-defined di- and triblock copolymers consisting of ε-caprolactone (CL), L-lactide (LA), and trimethylene carbonate (TMC) were synthesized via "PLA first route" in coordinated anionic ring opening polymerization/copolymerization (CAROP) with tin (II) octoate as catalyst. The desired block structure was preserved by use of protective additive α-methylstyrene by preventing the transesterification side-reactions. MALDI-TOF analysis revealed that the protection mechanism is associated with α-methylstyrene and tin (II) octoate complexation. Additionally, it was shown that use of α-methylstyrene in ring opening polymerization allowed the formation of polyesters with high molar mass.

Triblock copolymers of ε-caprolactone, trimethylene carbonate, and L-lactide: effects of using random copolymer as hard-block.

A series of triblock copolymers comprising end block of PLLA modified with PCL, and random copolymer of PCL and PTMC as soft segment were synthesized. DSC data show that PCL disrupted the crystallinity of PLLA, making the hard block to be completely amorphous when the PCL content is 50%. Correspondingly, the addition of PCL into PLLA block enhances the elongation of the triblock considerably. With regards to the elasticity, however, creep test results show that adding PCL to PLLA block seems to reduce the "equilibrium" recovery, while cyclic test results shows that the instantaneous recovery increased significantly with more PCL inside PLLA block. It was also observed that the degradation rate of triblock with added PCL inside the PLLA was slower compared to triblock with pure PLLA hard block. Compared to biodegradable polyurethane, these polymers are expected to yield less harmful degradation products, and offer more variables for the manipulation of properties. These polymers are also processable from the melt at temperatures exceeding about 130 °C. We expect to use these polymers in a variety of applications, including stent coatings, fully-degradable stents and atrial septal defect occluders.

Triblock copolymers of ε-caprolactone, L-lactide, and trimethylene carbonate: biodegradability and elastomeric behavior.

For the triblock copolymer of ε-caprolactone, trimethylene carbonate, and L-lactide, where L-lactide blocks form the two ends, there is a range of compositions over which elastomeric behavior is obtained. Within this composition range, these polymers show good creep and recovery at ambient temperature, and exhibit high elongations to break. Additionally, we demonstrate that the recovery is independent of stress and strain for the elastomer compositions. The range of compositions that yield elastomeric character is rationalized based on the structure; specifically, there must be a minimum crystallinity of the end blocks and no crystallinity in the midblock, in addition to molar mass requirements. These polymers degrade by simple hydrolysis, and the rate of degradation is potentially programmable by manipulation of the molar ratio of hard segment to soft segment. Compared to biodegradable polyurethane, these polymers are expected to yield less harmful degradation products, and offer more variables for manipulation of properties. These polymers are also processable from the melt at temperatures exceeding about 130 °C. We expect to use these polymers in a variety of applications, including stent coatings, fully-degradable stents, and atrial septal defect occluders.

Thermoplastic biodegradable elastomers based on ε-caprolactone and L-lactide block co-polymers: a new synthetic approach.

Although biodegradable polymers have found extensive application in medical devices, there are very few commercially available elastomeric biodegradable polymers. In this work, starting with the well-known monomers L-lactide and ε-caprolactone, we developed elastomers using a multiblock co-polymer approach. This ensures that the degradation products of such elastomers are also acceptable from a cytotoxicity standpoint. A series of polymers with various structures was synthesized utilizing a design of experiment approach. The basic structure is that of a diblock, with each block being modified by the addition of co-monomer. The synthesized polymers exhibited a range of mechanical properties from a typical thermoplastic polymer to that approaching a good thermoplastic elastomer. 13C nuclear magnetic resonance analysis, size exclusion chromatography and differential scanning calorimetry measurements have been utilized to relate the observed range of mechanical properties to the structure. In addition, the elastomeric nature has been established with the use of creep and recovery measurements. Such elastomers may find a variety of biomedical applications, ranging from stent coatings to atrial septal defect occluders.