Effects Of Stereocomplexation Research Articles

The Effect of Stereocomplexation and Crystallinity on the Degradation of Polylactide Nanoparticles.

Polymeric nanoparticles (PNPs) are frequently researched and used in drug delivery. The degradation of PNPs is highly dependent on various properties, such as polymer chemical structure, size, crystallinity, and melting temperature. Hence, a precise understanding of PNP degradation behavior is essential for optimizing the system. This study focused on enzymatic hydrolysis as a degradation mechanism by investigation of the degradation of PNP with various crystallinities. The aliphatic polyester polylactide ([C3H4O2]n, PLA) was used as two chiral forms, poly l-lactide (PlLA) and poly d-lactide (PdLA), and formed a unique crystalline stereocomplex (SC). PNPs were prepared via a nanoprecipitation method. In order to further control the crystallinity and melting temperatures of the SC, the polymer poly(3-ethylglycolide) [C6H8O4]n (PEtGly) was synthesized. Our investigation shows that the PNP degradation can be controlled by various chemical structures, crystallinity and stereocomplexation. The influence of proteinase K on PNP degradation was also discussed in this research. AFM did not reveal any changes within the first 24 h but indicated accelerated degradation after 7 days when higher EtGly content was present, implying that lower crystallinity renders the particles more susceptible to hydrolysis. QCM-D exhibited reduced enzyme adsorption and a slower degradation rate in SC-PNPs with lower EtGly contents and higher crystallinities. A more in-depth analysis of the degradation process unveiled that QCM-D detected rapid degradation from the outset, whereas AFM exhibited delayed changes of degradation. The knowledge gained in this work is useful for the design and creation of advanced PNPs with enhanced structures and properties.

Synergistic effect of stereo-complexation and interfacial compatibility in ammonium polyphosphate grafted polylactic acid fibers for simultaneously improved toughness and flame retardancy

Flammability and poor toughness of unmodified PLA limit its applications in various fields. Though ammonium polyphosphate (APP) is a green and effective flame retardant, it has poor compatibility with the matrix, leading to a decrease in mechanical properties. Stereo-complexation greatly improves the strength and heat resistance of traditional PLA. However, the effect of flame retardants on the formation of stereo-complexed crystals and the impact of stereo-complexation on flame retardancy have not been studied previously. In this research, PDLA chains were first in-situ reacted with APP particles for improved interfacial compatibility. By utilizing the characteristic of PLA enantiomers that can form stereo-complexed crystals, near-complete stereo-complexed PLA fibers with flame retardancy were produced via clean and continuous melt spinning. The compatibility between PDLA-g-APP and PLLA matrix was studied by SEM, rheological analyses and DSC. Strength and flexibility of the fibers were simultaneously enhanced compared to traditional PLA due to the synergistic effect of interfacial compatibility and stereo-complexation. Compared to traditional PLA, the peak heat release rate and total heat release in microcalorimetry test were reduced by 33 % and 22 %, respectively. The flame-retardant fibers achieved a V-0 rating in the UL-94 test, and an increase in LOI value from 19.4 % to 28.2 %.

Fractionated crystallization and fractionated melting behaviors of poly(ethylene glycol) induced by poly(lactide) stereocomplex in their block copolymers and blends

Stereocomplexation has been an effective way to improve the physical properties of block copolymers or blends. However, the effect of stereocomplexation on the confined and fractionated crystallization of other crystallizable block has not been comprehensively investigated so far. In this work, we used poly(l-lactide) (PLLA) and poly(d-lactide) (PDLA) as stereocomplexable higher melting component and studied the confined crystallization and melting behaviors of poly(ethylene glycol) (PEG) in their block copolymers and blends. Unexpectedly, marked fractionated crystallization and fractionated melting behaviors of PEG component occurred during the two-step crystallization process, because of the constrained effect of the early formed stereocomplex (SC) crystals. The fractionated crystallization and melting behaviors were further facilitated with decreasing the isothermal crystallization temperatures of PLLA and PDLA (Tic,PLA). As revealed by polarized optical microscopy (POM), the increase of Tic,PLA favors the transformation of PEG component from the interlamellar to interspherulitic regions due to the reduced nucleation density of PLA. We proposed that fractionated crystallization and fractionated melting behaviors of PEG impacted by SC crystals were attributed to PEG component dispersed in and out interlamellar regions of PLA matrix. Moreover, small-angle X-ray scattering (SAXS) results indicated that two scattering peaks were identified for copolymers after two-step crystallization process, one consisted of mixed PEG and PLA lamellae and the other contained almost pure PEG lamellae originated from the crystallization of PEG dispersed in interfibriller/interspherulitic regions. This study has shed light on the unique fractionated crystallization and fractionated melting behaviors of the lower melting component in polymer systems with the higher melting component preferentially co-crystallized into SC crystals.

Largely improved mechanical properties of a biodegradable polyurethane elastomer via polylactide stereocomplexation

Thermoplastic polyurethane elastomers (PUEs) consisting of biodegradable poly (ε-caprolactone) (PCL) and low toxic aliphatic diisocyanates are of great potential in biomedical applications, but the inferior mechanical strength and elastic recovery at large strain have limited their practical utilization. Inspired by the outstanding thermal and mechanical properties of stereocomplex (SC) PLA, a thermoplastic PUE that contains SC crystallites (SC-PU) was prepared via racemically blending the PLA stereoisomers-based polyurethanes. The formation of SC crystallites in SC-PU was confirmed by WAXD and ATR-FTIR techniques. The effects of stereocomplexation on its thermal, morphological and mechanical properties were investigated. The results show that the SC crystalline domains, acting as ‘additional reinforcing phases’, endow the improved performance of SC-PU in tensile strength, Young's modulus, elastic recovery, and other properties. This study may provide a new method to improve the inferior mechanical properties of biodegradable PUEs based on PCL and aliphatic diisocyanates, which can contribute to the expansion of their applications in widespread fields.

Enhanced crystallization rate and mechanical properties of poly(l‐lactic acid) by stereocomplexation with four‐armed poly(ϵ‐caprolactone)‐block‐poly(d‐lactic acid) diblock copolymer

AbstractPoly(l‐lactic acid) (PLLA) was blended with a series of four‐armed poly(ϵ‐caprolactone)‐block‐poly(d‐lactic acid) (4a‐PCL‐b‐PDLA) copolymers in order to improve its crystallization rate and mechanical properties. It is found that a higher content of 4a‐PCL‐b‐PDLA copolymer or longer PDLA block in the copolymer lead to faster crystallization of the blend, which is attributed to the formation of stereocomplex crystallites between PLLA matrix and PDLA blocks of the 4a‐PCL‐b‐PDLA copolymers. Meanwhile, the PDLA block can improve the miscibility between flexible PCL phase and PLLA phase, which is beneficial for improving mechanical properties. The tensile results indicate that the 10% 4a‐PCL5k‐b‐PDLA5k/PLLA blend has the largest elongation at break of about 72% because of the synergistic effects of stereocomplexation between enantiomeric PLAs, multi‐arm structure and plasticization of PCL blocks. It is concluded that well‐controlled composition and content of 4a‐PCL‐b‐PDLA copolymer in PLLA blends can significantly improve the crystallization rate and mechanical properties of the PLLA matrix. © 2017 Society of Chemical Industry

Enantiomeric blends of high-molecular-weight poly(lactic acid)/poly(ethylene glycol) triblock copolymers: Enhanced stereocomplexation and thermomechanical properties

Abstract Poly( l -lactic acid)-poly(ethylene glycol)-poly( l -lactic acid) (PLLA-PEG-PLLA) and poly( d -lactic acid)-poly(ethylene glycol)-poly( d -lactic acid) (PDLA-PEG-PDLA) triblock copolymers having relatively high molecular weights (Mn > 60 kDa) and different lengths of PEG midblock (2–20 kDa) and PLLA, PDLA end blocks (20–45 kDa for each block) were prepared and blended in equal mass. The competing crystallization kinetics, polymorphic crystalline structure, mechanical and thermomechanical properties of PLLA-PEG-PLLA/PDLA-PEG-PDLA blends were investigated and compared with the PLLA/PDLA blend. Both the crystallization rate and stereocomplexation ability of PLLA/PDLA blends were improved after the incorporation of PEG midblocks. The crystallization half-time decreases and the crystallinity, content of stereocomplexes (SCs) enhance as the length of PEG midblock increases or the lengths of PLLA, PDLA end blocks decease. The melt recrystallization from homocrystallites (HCs) to SCs becomes more pronounced with increasing the length of PEG midblocks in the enantiomeric blends. The enhanced SC formation and crystallization rate of PLLA-PEG-PLLA/PDLA-PEG-PDLA blends are attributed to the increased diffusion ability and decreased diffusion pathway of enantiomeric chains. The synergistic effects of stereocomplexation and plasticization of flexible PEG midblocks offer the PLLA-PEG-PLLA/PDLA-PEG-PDLA blended materials better mechanical and thermomechanical properties. The enantiomeric blends exhibit higher tensile strength, larger elongation-at-break, and better thermal resistance than the corresponding PLLA-PEG-PLLA copolymers.

Competitive Effects of Stereocomplexation and Hyper‐Conjugation of Triethoxysilyl‐Terminated Poly(d‐lactide) in Poly(l‐lactide) matrices

Triethoxysilyl‐terminated poly(d‐lactide)s (eSi‐PDLA) were successfully prepared by hydrosilylation of allyl‐terminated PDLAs (A‐PDLA‐ac) which were readily prepared by ring‐opening polymerization of d‐lactide and the following terminal protection by acetylation. Their acid‐catalyzed hydrolysis produced hyper‐conjugated PDLAs with terminal silanol coupling. Blend PLLA films containing eSi‐PDLA (Mn = 2.1 kDa) in 5‐25 wt.% were successfully prepared by the solution casting method. The blend films were found to take a nano‐porous or micro‐phase separation structure, being different from that of the blend PLLA films containing trimethoxysilyl‐terminated poly(d‐lactide) (mSi‐PDLA) which exhibited particle distribution structure. This difference was reasonably attributed to the different rate of hydrolytic polycondensation of the trimethoxy‐ and triethoxy‐silyl terminals. These results revealed that the competing sterecomplexation/ hyper‐conjugation/phase separation can afford specific morphologies, allowing fine control of properties.

Polylactide stereocomplexation leads to reduced migration during microwave heating in contact with food simulants

The effect of stereocomplexation on the stability and migration resistance of polylactide during microwave and conventional heating in contact with different food simulants was evaluated. The heating effects were followed through mass loss measurements, molecular weight measurements and identification of the individual migrants by electrospray ionization-mass spectrometry (ESI-MS). Increased mass losses were observed as a function of time and temperature, but approximately 50% smaller mass losses were always measured for PLA stereocomplex as compared to the corresponding regular PLLA material. The stability of the stereocomplex material was, thus, significantly higher. Microwave heating increased the mass loss as compared to the conventional heating at the same time and temperature. This effect was especially significant when 10% ethanol was used as food simulant instead of water. The amount of water-soluble migrants was in most cases under the detection limits, but when heating temperature was increased to 95°C, ESI-MS revealed the formation of homologous series of linear lactic acid oligomers. Results indicate that PLA stereocomplex materials could have potential in single-use microwave applications.

Improved mechanical and thermal properties of PLLA by solvent blending with PDLA-b-PEG-b-PDLA

In this article, two types of poly (d-lactide)-b-poly (ethylene glycol)-b-poly (d-lactide) (pdla-b-peg-b-pdla) were added into plla matrix by solution casting method. the obtained films were characterized by wide-angle x-ray diffractomery (waxd), differential scanning calorimetry (dsc), tensile testing and thermal gravimetric analyzer (tga). results indicated that the heat resistance of these blends could be improved when the amount of copolymer exceeded 30 wt%, because the high melting point stereocomplex was preferentially formed at that addition amount, whenever from solutions or from melt. both the tensile strength and elongation at break of the blends were enhanced when 30 wt% copolymer was added, which were partly caused by the synergistic effects of stereocomplexation between enantiomeric plas and plasticization of peg blocks. the blends showed higher thermal stability than neat plla at temperature above 370 degrees c. these results showed that the toughness and heat resistance of plla were improved, which made the application of pla probable. (c) 2014 elsevier ltd. all rights reserved.

Amphiphilic Conetworks and Gels Physically Cross-Linked via Stereocomplexation of Polylactide

Amphiphilic conetworks (APCNs), consisting of hydrophilic poly[poly(ethylene glycol) methyl ester acrylate] (PPEGMEA) and hydrophobic stereocomplex of poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA), were prepared by free radical copolymerization of PEGMEA with acrylate macromonomer of the PLA stereocomplex. The effects of stereocomplexation and the amount of PLA stereocomplex on the rheology properties of APCNs were investigated. The results indicated that the APCNs was stronger in the presence of stereocomplexation compared with the that of nonstereocomplex system, and the strength of the APCNs increased with the increasing of the amount of PLA stereocomplex. The storage modulus of the APCNs could be easily tuned from 1200 to 4300 Pa by incorporating 2-10% of stereocomplex PLA. On the other hand, the swelling behavior of APCNs decreased with the increasing content of hydrophobic PLA cross-linker.

Effects of stereocomplexation on the physicochemical behavior of PLA/PEG block copolymers in aqueous solution

A series of polylactide/poly(ethylene glycol) (PLA/PEG) block copolymers were synthesized by ring-opening polymerization of L- or D-lactide in the presence of mono- or di-hydroxyl PEG. The effects of stereocomplexation on the physicochemical behavior of PLA/PEG copolymers in aqueous solution were investigated by varying the degree of stereocomplexation or PLLA/PEG to PDLA/PEG ratio. In mixture solutions of insoluble and soluble copolymers, stereocomplexation strongly affects the solubility of the copolymers. In mixture solutions of soluble copolymers, both the size and aggregation number (Nagg) of the aggregates vary as a function of the degree of stereocomplexation. It is suggested that the size variation of the aggregates with increasing the degree of stereocomplexation is dependent on Nagg changes which are determined by two effects: the self-adjusting of the aggregates so as to minimize the free energy and thus to increase the Nagg, and the kinetics of aggregation which tend to form more aggregates and thus to decrease the Nagg. Combination of the two opposite effects well explains the diverse variations of Nagg and size of the aggregates as a function of the degree of stereocomplexation. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012

Stereocomplexation Effects Induced by Mixtures of Enantiomeric Polylactides on Hydrolytic Behaviors and Surface Structure

ABSTRACT The effects of stereocomplexation of enantiomeric polylactide (PLA) mixture films, (l-PLA/d-PLA), on hydrolytic behaviors and surface structure were investigated by Langmuir technique and X-ray photoelectron spectroscopy, respectively. The alkaline hydrolysis rate of stereocomplexed monolayers from equimolar l-PLA and d-PLA was much slower than those of their homopolymers. The (l-PLA-b-PDMS-b-l-PLA/l-PLA, 50/50 by wt.%) mixture film showed the surface segregation of PDMS at the surface layer while the surface composition of PDMS in (l-PLA-b-PDMS-b-l-PLA/d-PLA, 50/50 by wt.%) was similar to its bulk composition. These are likely due to strong interaction between d- and l-lactide unit sequences, which prevents the penetration of water into the bulk and surface segregation of PDMS.

Triblock Copolymer Based Thermoreversible Gels. 3. Effect of Stereocomplexation with iPMMA

Thermoreversible gelation of syndiotactic poly(methyl methacrylate) (sPMMA)−polybutadiene (PBD)−sPMMA, triblock copolymers in o-xylene has been studied in the presence of isotactic PMMA (iPMMA) in a large range of syndio/iso mixing ratio (s/i). Differential scanning calorimetry (DSC) shows that two types of association contribute to the gel formation, i.e., the self-aggregation of the sPMMA outer blocks and the stereocomplexation of the sPMMA outer blocks with iPMMA. At low iPMMA contents (s/i = 30/1), self-association of the outer blocks accounts for the gelation process, although stereocomplexation occurs. Stereocomplexation becomes the dominant gelation mechanism at a high enough iPMMA content (s/i = 2/1). Thus, gelation is observed at a copolymer concentration as low as 1 wt %. The time dependence of the storage (G‘) and loss (G‘‘) moduli has been measured in the 0.08−1Hz frequency range in order to probe the gelation process. At the gel point, where the loss angle (tan δc = G‘‘/G‘) is independent of ...