Glass transition behavior and dynamic fragility in polylactides containing mobile and rigid amorphous fractions

The segmental dynamics of polylactide chains covering the T_g − 30 °C to T_g + 30 °C range was studied in absence and presence of a crystalline phase by dynamic mechanical analysis (DMA) using the framework provided by the WLF theory and the Angell's dynamic fragility concept. An appropriate selection of stereoisomers combined with a thermal conditioning strategy to promote crystallization (above T_g) or relaxation of chains (below T_g) was revealed as an efficient method to tune the ratio of the rigid and mobile amorphous phases in polylactides. A single bulklike mobile amorphous phase was taken for poly(d,l-lactide) (PDLLA). In turn three phases, comprising a mobile amorphous fraction (MAF, X_MA), a rigid amorphous fraction (RAF, X_RA) and a crystalline fraction (X_c) were determined in poly(l-lactide) (PLLA) by modulated differential scanning calorimetry (MDSC) according to a three-phase model. The analysis of results confirms that crystallinity and RAF not only elevate the T_g and the breadth of the glass transition region but also yields an increase in dynamic fragility parameter (m) which entails the existence of a smaller length-scale of cooperativity of polylactide chains in confined environments. Consequently it is proposed that crystallinity is acting in polymeric systems as a topological constraint that, preventing longer range dynamics, provides a faster segmental dynamics by the temperature dependence of relaxation times according to the strong-fragile scheme.     

Crystallization behavior of linear 1-arm and 2-arm poly(l-lactide)s: Effects of coinitiators

Linear 1-arm and 2-arm poly(l-lactide) [i.e., poly(l-lactic acid) (PLLA)] polymers having relatively low number-average molecular weights (M_n) (≤5 × 10⁴ g mol⁻¹) were synthesized by ring-opening polymerization of l-lactide initiated with tin(II) 2-ethylhexanoate (i.e., stannous octoate) and coinitiators of l-lactic acid, 1-dodecanol (i.e., lauryl alcohol), and ethylene glycol (these PLLA polymers are abbreviated as LA, DN, and EG, respectively). For M_n below 1.5 × 10⁴ g mol⁻¹, non-isothermal crystallization during heating and isothermal spherulite growth were disturbed in linear 2-arm PLLA (EG) compared to those in linear 1-arm PLLA (LA and DN). This finding indicates that the chain directional change, the incorporation of the coinitiator moiety as an impurity in the middle of the molecule, and their mixed effect disturbed the crystallization of linear 2-arm PLLA compared to that of linear 1-arm PLLA, in which the chain direction is unvaried and the coinitiator moiety is incorporated in the chain terminal. Also, the finding strongly suggests that the reported low crystallizability of multi-arm PLLA (arm number ≥ 3) compared to that of linear 1-arm PLLA is caused not only by the presence of branching points but also by the chain directional change, the incorporation of the coinitiator moiety in the middle of the molecule, and their mixed effect. The effects of the chain directional change and the position of the incorporated coinitiator moiety on the crystallization and physical properties of linear 1-arm and 2-arm PLLA decreased with an increase in M_n.     

Poly(l-lactide) brushes on magnetic multiwalled carbon nanotubes by in-situ ring-opening polymerization

Biodegradable poly(l-lactide) (PLLA) has been covalently grafted onto the surface of magnetic multiwalled carbon nanotubes (m-MWCNTs) by in-situ ring-opening polymerization of lactide. The content of grafting PLLA can be controlled by adjusting the feed ratio of monomer to m-MWCNTs. FT-IR and Raman spectroscopy confirm that PLLA have been covalently attached to the sidewalls of m-MWCNTs. Thermal gravimetric analysis (TGA) indicates that the composites of PLLA grafted m-MWCNTs have a polymer weight percentage of ca. 25.6-33.7 wt%. The scanning eletron microscopy (SEM) and transmission electron microscopy (TEM) are utilized to image the PLLA grafted m-MWCNTs, showing relatively uniform polymer layer coated on the surface of m-MWCNTs. The composites of PLLA grafted m-MWCNTs exhibit superparamagnetic behavior at room temperature and are aligned under a low magnetic field.     

Mechanical properties, morphologies, and crystallization behavior of plasticized poly(l-lactide)/poly(butylene succinate-co-l-lactate) blends

The blends of poly(l-lactide) (PLLA) with poly(butylene succinate-co-l-lactate) (PBSL) containing the lactate unit of ca. 3 mol% and Rikemal PL710 (RKM) which is a plasticizer mainly composed of diglycerine tetraacetate were prepared by melt-mixing and subsequent injection molding. The studied RKM content of the PLLA/PBSL/RKM blends was 0-20 wt%, and the PLLA/PBSL weight ratio was 100/0 to 80/20. Although elongation at break in the tensile test did not increase by the addition of 10 wt% RKM to PLLA, the addition of a small amount of PBSL to the PLLA/RKM blend caused a considerable increase of the elongation. The SEM and DSC analyses revealed that all the PLLA/PBSL/RKM blends are immiscible blends where the PBSL particles are finely dispersed, and that there is some compatibility between PLLA-rich phase and PBSL-rich phase in the amorphous state when the RKM content is 20 wt%. As a result of investigation of the crystallization behavior by DSC and polarized optical microscopic measurements, it was revealed that the addition of RKM causes the acceleration of crystalline growth rate at a lower annealing temperature, and the addition of PBSL mainly enhances the formation of PLLA crystal nucleus.     

Synthesis and characterization of poly(l-glutamic acid)-block-poly(l-phenylalanine)

Poly(γ-benzyl l-glutamate)-block-poly(l-phenylalanine) was prepared via the ring opening polymerization of γ-benzyl l-glutamate N-carboxyanhydride and l-phenylalanine N-carboxyanhydride using n-butylamine·HCl as an initiator for the living polymerization. Polymerization was confirmed by ¹H-nuclear magnetic resonance spectroscopy and matrix assisted laser desorption ionization time of flight mass spectroscopy. After deprotection, the vesicular nanostructure of poly(l-glutamic acid)-block-poly(l-phenylalanine) particles was examined by transmission electron microscopy and dynamic light scattering. The pH-dependent properties of the nanoparticles were evaluated by means of ζ-potential and transmittance measurements. The results showed that the block copolypeptide could be prepared using simple techniques. Moreover, the easily prepared PGA-PPA block copolypeptide showed pH-dependent properties due to changes in the PGA ionization state as a function of pH; this characteristic could potentially be exploited for drug delivery applications.     

Telechelic polymers by living and controlled/living polymerization methods

Telechelic polymers, defined as macromolecules that contain two reactive end groups, are used as cross-linkers, chain extenders, and important building blocks for various macromolecular structures, including block and graft copolymers, star, hyperbranched or dendritic polymers. This review article describes the general techniques for the preparation of telechelic polymers by living and controlled/living polymerization methods; namely atom transfer radical polymerization, nitroxide mediated radical polymerization, reversible addition-fragmentation chain transfer polymerization, iniferters, iodine transfer polymerization, cobalt mediated radical polymerization, organotellurium-, organostibine-, organobismuthine-mediated living radical polymerization, living anionic polymerization, living cationic polymerization, and ring opening metathesis polymerization. The efficient click reactions for the synthesis of telechelic polymers are also presented.     

Polymer vectors via controlled/living radical polymerization for gene delivery

The design of efficient gene delivery vectors is a challenging task in gene therapy. Recent progress in living/controlled radical polymerizations (LRPs), in particular atom transfer radical polymerization (ATRP) and reversible addition-fragmentation chain transfer (RAFT) polymerization providing a means for the design and synthesis of new polymeric gene vectors with well-defined compositions, architectures and functionalities is reviewed here. Polymeric gene vectors with different architectures, including homopolymers, block copolymers, graft copolymers, and star-shaped polymers, are conveniently prepared via ATRP and RAFT polymerization. The corresponding synthesis strategies are described in detail. The recent research activities indicate that ATRP and RAFT polymerization have become essential tools for the design and synthesis of advanced, noble and novel gene carriers.     

pH-dependent self-assembly of amphiphilic poly(l-glutamic acid)-block-poly(lactic-co-glycolic acid) copolymers

A novel biodegradable AB-type diblock copolymer poly(L-lactic- co-glycolic acid)-block-poly(l-glutamic acid) (PLGA-b-PGA) was synthesized by a macromolecular coupling reaction between carboxyl-terminated PLGA and amino-terminated poly(γ-benzyl-glutamate) (PBLG) and the subsequent elimination of the protecting benzyl group. The structures of PLGA-PGA and its precursors were confirmed by Fourier transform infrared spectroscopy (FT-IR), ¹H nuclear magnetic resonance (¹H NMR) spectroscopy and gel permeation chromatography (GPC). This synthetic strategy simplified a former synthesis process of polypeptide-poly(l-lactic acid)(PLA); by using this new synthetic route the molecular weight and block ratio of PLGA-PGA could be easily controlled by adjusting the chain length of PLGA/PGA. The pH sensitivity and self-assembly behavior of PLGA-PGA copolymer were investigated by environmental scanning electron microscopy (ESEM), transmission electron microscopy (TEM) and dynamic light scattering (DLS). The results showed that the copolymer exhibited high pH responses, and the morphologies of the copolymer aggregates underwent four stages orderly with the pH increase (pH = 3-9): a disorganized form, micelles, semi-vesicles with thick walls and vesicles. Such a pH-dependent self-assembly process of the copolymer is promising for drug control release and bio-applications.     

Phase behaviour of l-lactic acid based polymers of low molecular weight in supercritical carbon dioxide at high pressures

The results of investigations of phase behaviour in the systems l-lactic acid based polymers + carbon dioxide at high pressures are presented. The measurements have been performed in wide temperature and composition ranges. Two samples of the polymer differing in molecular weight (M_n: 1080 and 3990 g/mol) have been investigated. Both samples of the polymer were characterized with the gel permeation chromatography and NMR spectroscopy. The influence of the structure of the polymer on the solubility in supercritical carbon dioxide has been discussed. The results obtained suggest that the solubility of low molecular weight l-lactic acid based polymers in supercritical carbon dioxide is not controlled by its size, but to a large extent by the character of its terminal groups. The phase behaviour in the system l-lactic acid + carbon dioxide has been also investigated and the results were compared with those for the systems composed of l-lactic acid based polymers and carbon dioxide.     

Controlled accommodation of metal nanostructures within the matrices of polymer architectures through solution-based synthetic strategies

Controlled accommodation of metal nanostructures (MNSs) into the matrix of a well-defined polymer architecture offers an effective approach to achieve hierarchically structured nanocomposites with tunable synergistic properties to broaden application potentials in the emerging fields of energy, environmental science, and medicine. This review focuses on the recently developed zero-dimensional and one-dimensional MNSs@polymer hybrid nanostructures obtained by solution-based synthetic strategies. Progress in the controlled synthesis of those hybrid nanostructures in terms of the number (e.g., monomer, dimer and trimer), organization manner (e.g., linear alignment or confined assembly in certain domains), and spatial arrangement (e.g., in the core and shell) of the MNSs within the distinct polymer matrices are detailed. The synergistic properties and potential applications of those MNSs@polymer hybrids associated with their compositions and morphologies are also reviewed.     

Spherulite growth of l-lactide copolymers: Effects of tacticity and comonomers

The spherulite growth behavior and mechanism of l-lactide copolymers, poly(l-lactide-co-d-lactide) [P(LLA-DLA)], poly(l-lactide-co-glycolide) [P(LLA-GA)], and poly(l-lactide-co-ε-caprolactone) [P(LLA-CL)] have been studied using polarization optical microscopy in comparison with poly(l-lactide) (PLLA) having different molecular weights to elucidate the effects of incorporated comonomer units. The incorporation of comonomer units reduced the radius growth rate of spherulites (G) and increased the induction period of spherulite formation (t_i), irrespective of the kind of comonomer unit. Such effects became remarkable with the content of comonomers. At a crystallization temperature (T_c) of 130 °C, the disturbance effects of comonomers on the spherulite growth decreased in the following order: d-lactide>glycolide>ε-caprolactone, when compared at the same comonomer unit or reciprocal of averaged l-lactyl unit sequence length (l_l). The t_i estimation indicated that the glycolide units have the lowest disturbance effects on the formation of spherulite (crystallite) nuclei. The PLLA having the number-average molecular weight (M_n) exceeding 3.1×10⁴ g mol⁻¹ showed the transition from regime II to regime III at T_c=120 °C, whereas PLLA with the lowest M_n of 9.2×10³ g mol⁻¹ crystallized solely in regime III kinetics and the copolymers excluding P(LLA-DLA) with 3% of d-lactide units crystallized solely according to regime II kinetics. The nucleation and front constant for regime II and III [K_g(II), K_g(III), G₀(II), and G₀(III), respectively] estimated with each (not with a fixed for high-molecular-weight PLLA) decreased with increasing the amount of defects per unit mass of the polymer for crystallization, i.e. with increasing the comonomer content and the density of terminal group through decreasing the molecular weight.     

Fabrication and characterization of aligned nanofibrous PLGA/Collagen blends as bone tissue scaffolds

Aligned nanofibrous blends of poly (d, l-lactide-co-glycolide) (PLGA) and collagen with various PLGA/collagen compositions (80/20, 65/35 and 50/50) were fabricated by electrospinning and characterized for bone tissue engineering. Morphological characterization showed that the addition of collagen to PLGA resulted in narrowing of the diameter distribution and a reduction in average diameter. Differential scanning calorimetric (DSC) studies showed that the triple helix structure of the native collagen was not destroyed during the fabrication process. However, the blending had a marked effect on the overall enthalpy of the blends, whereby the total enthalpy decreased as the collagen content decreased. Thermogravimetric analysis showed the addition of collagen increased the hydrophilicity of the scaffolds. The crosslinking of collagen to increase the biostability was done using 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC) in ethanol and an overall ∼25% degree of crosslinking was achieved. The EDC crosslinking had little effect on the nanofibrous morphology of the 80/20 blend system; however, the nanofibrous features were compromised to some extent at higher collagen concentrations. The mechanical characterization under dry and wet conditions showed that increasing collagen content resulted in a tremendous decrease in the mechanical properties. However, crosslinking resulted in the increase in elastic modulus from 47 MPa to 83 MPa for the wet PLGA/Collagen 80/20 blend system, with little effect on the tensile strength. In conclusion, the aligned nanofibrous scaffold used in this study constitutes a promising material for bone tissue engineering.     

Functionalized micelles from new ABC polyglycidol-poly(ethylene oxide)-poly(d,l-lactide) terpolymers

New ABC type terpolymers of poly(ethoxyethyl glycidyl ether)/poly(ethylene oxide)/poly(d,l-lactide) were obtained by multi-mode anionic polymerization. After successive deprotection of the ethoxyethyl groups from the first block, highly hydroxyl functionalized copolymers of polyglycidol/poly(ethylene oxide)/poly(d,l-lactide) were obtained. These copolymers form elongated ellipsoidal micelles by direct dissolution in water. The micelles consist of a poly(d,l-lactide) core and stabilizing shell of polyglycidol/poly(ethylene oxide). The hydroxyl groups of polyglycidol blocks situated at the micelle surface provide high functionality, which could be engaged in further chemical modification resulting in a potential drug targeting agents. The micellization process of the copolymers in aqueous media was studied by hydrophobic dye solubilization, static and dynamic light scattering, and transmission electron microscopy.     

The reduction of l-cystine hydrochloride at stationary and rotating disc mercury electrodes

The kinetics of l-cystine hydrochloride reduction have been studied at a mercury-plated copper rotating disc electrode (RDE) and at a stationary mercury disc electrode (SMDE) in 0.1 mol dm⁻³ HCl at 298 K. The reduction of the disulphide is irreversible and hydrogen evolution is the major side reaction. In contrast to steady state electrode kinetic studies at a mercury drop electrode (which shows a well-defined limiting current), the mercury-plated Cu RDE shows overlap between disulphide reduction and hydrogen evolution. These effects are attributable to strong reactant adsorption with a calculated surface coverage close to 100%. A Tafel slope of −185 mV per decade is found with a cathodic transfer coefficient of 0.32 and a formal rate constant of 6.7 × 10⁻⁹ m s⁻¹. The relative merits of steady state voltammetry at a mercury-plated copper RDE and linear sweep voltammetry at the SMDE are discussed, as is the mechanism of l-cysteine hydrochloride formation.     

Properties of l-tyrosine based polyphosphates pertinent to potential biomaterial applications

Since their introduction by Kohn and Langer et al. in 1984, l-tyrosine based ‘pseudo’ poly(amino acids) have undergone extensive research in the area of polymeric biomaterials. Starting from l-tyrosine based diphenolic monomers, polyiminocarbonates, polycarbonates and polyarylates have been developed by Kohn and co-workers and are being investigated for potential orthopedic biomaterial applications. Mao et al. have reported development of l-tyrosine based polyphosphates and polyphosphonates in a patent, however, detailed structural and physico-chemical characterization studies on such polymers have not been reported yet. For the purpose of the current paper, using a novel solid phase process for synthesis of l-tyrosine based diphenolic monomers and adapting the polymerization process described by Mao et al., l-tyrosine based polyphosphates were developed and investigated for their pertinent bioengineering properties. The properties investigated consist of chemical solubility, hydrophilicity and hydrolytic degradation. The results of this investigation are crucial to validate further investigation of biomaterial applications of these polymers.     

Crystallization and morphology of stereocomplexes in nonequimolar mixtures of poly(l-lactic acid) with excess poly(d-lactic acid)

Crystallization of nonequimolar compositions of poly(d-lactic acid) with low-molecular-weight poly(l-lactic acid) (PDLA/LM_w-PLLA) blends leads to formation of various fractions of stereocomplexed PLA (sc-crystallites) and homocrystallites (PDLA or PLLA). For the PDLA/LM_w-PLLA blends within the composition window of LM_w-PLLA content between 30 and 50 wt%, only sc-crystal exists and no homocrystal is present. On the other hand, for PDLA/LM_w-PLLA blends with excess PDLA, e.g. PDLA/LM_w-PLLA = 90/10, atomic-force microscopy (AFM) characterization on various stages of crystallization of sc-PLA crystal with PDLA homocrystal shows a repetitive stacking of excess PDLA on pre-formed sc-PLA crystal serving as crystallizing templates. The crystallization initially begins with string-like (fibril-like) PDLA lamellae, followed with PDLA aggregating on sc-PLA crystal into a bead-on-string crystal, then growing to thicker irregularly-shaped dough-like lamellae. Repetitive growth cycle from strings to bead-on-string lamellae continues on top of the dough-like lamellae as new substrates, until ending impingement of the PDLA spherulites.     

Poly(d,l-lactide)/poly(methyl methacrylate) interpenetrating polymer networks: Synthesis, characterization, and use as precursors to porous polymeric materials

The use of semi-hydrolyzable oligoester-derivatized interpenetrating polymer networks (IPNs) as nanostructured precursors provides a straightforward and versatile approach toward mesoporous networks. Different poly(d,l-lactide) (PLA)/poly(methyl methacrylate) (PMMA)-based IPNs were synthesized by resorting to the so-called in situ sequential method. The PLA sub-network was first generated from a dihydroxy-telechelic PLA oligomer via an end-linking reaction with Desmodur^® RU as a triisocyanate cross-linker. Subsequently, the methacrylic sub-network was created by free-radical copolymerization of methyl methacrylate (MMA) and a dimethacrylate (either bisphenol A dimethacrylate or diurethane dimethacrylate) with varying compositions (initial MMA/dimethacrylate composition ranging from 99/1 to 90/10 mol%). Both cross-linking processes were monitored by real-time infrared spectroscopy. The microphase separation developed in IPN precursors was investigated by differential scanning calorimetry (DSC). Furthermore, the quantitative hydrolysis of the PLA sub-network, under mild basic conditions, afforded porous methacrylic structures with pore sizes ranging from 10 to 100 nm -at most- thus showing the effective role of cross-linked PLA sub-chains as porogen templates. Pore sizes and pore size distributions were determined by scanning electron microscopy (SEM) and thermoporometry via DSC measurements. The mesoporosity of residual networks could be attributed to the good degree of chain interpenetration associated with both sub-networks in IPN precursors, due to their peculiar interlocking framework.     

Preparation of multi-walled carbon nanotubes grafted with synthetic poly(l-lysine) through surface-initiated ring-opening polymerization

A surface-initiated ring-opening polymerization approach was used to functionalize multi-walled carbon nanotubes (MWNTs) with linear poly(l-lysine) (PLL). The oxidized MWNTs, which resulted from the treatment of MWNTs with mixed concentrated sulfuric acid and nitric acid, were converted into amino-functionalized MWNTs (MWNT-NH₂) by the amidation of carboxylic group on MWNT surface with excess 1,6-diaminohexane. Surface-initiated ring-opening polymerization of ?-(benzyloxycarbonyl)-l-lysine N-carboxyanhydride with MWNT-NH₂ as the initiator resulted in MWNTs grafted with poly(?-(benzyloxycarbonyl)-l-lysine) (MWNT-g-PLys(Z)). After the acidolysis of benzylcarbamate group of the grafting poly(?-(benzyloxycarbonyl)-l-lysine) chain, water-soluble MWNTs grafted with PLL (MWNT-g-PLL) were obtained. The successful grafting was confirmed by Fourier transform infrared spectroscopy, thermal gravimetric analysis, X-ray photoelectron spectroscopy, elemental analyses, high-resolution transmission electron microscopy (HRTEM) and field-emission scanning electron microscopy. HRTEM indicates that MWNTs were enveloped by PLL layer. The as-prepared MWNT-g-PLys(Z) and MWNT-g-PLL exhibited excellent ability to disperse homogeneously in THF and water, respectively.     

Ring-opening polymerization of six-membered cyclic esters catalyzed by tetrahydroborate complexes of rare earth metals

Catalytic behavior of tetrahydroborate complexes of rare earth metals, Ln(BH₄)₃(THF)_x (1: Ln = La, x = 3; 2: Ln = Pr, x = 2; 3: Ln = Nd, x = 3; 4: Ln = Sm, x = 3; 5: Ln = Y, x = 2.5; 6: Ln = Yb, x = 3), for ring-opening polymerization (ROP) of six-membered cyclic esters, δ-valerolactone (VL) and d,l-lactide (d,l-LA), was studied. The controlled polymerization of VL with 1-6 proceeded in THF at 60 °C. The catalytic activities of these complexes for the ROP of VL were observed to be in order of the ionic radii of the metals: 1(La) ≥ 2(Pr) ≥ 3(Nd) > 4(Sm) > 5(Y) > 6(Yb). The obtained polymers were demonstrated to be hydroxy-telechelic by ¹H NMR and MALDI-TOF MS spectroscopy. The controlled ROP of d,l-LA also proceeded by these complexes. The activities of these complexes for the d,l-LA ROP were also in order of the ionic radii of the metals.     

Complex polymer architectures via RAFT polymerization: From fundamental process to extending the scope using click chemistry and nature's building blocks

Reversible addition fragmentation chain transfer (RAFT) polymerization has made a huge impact in macromolecular design. The first block copolymers were described early on, followed by star polymers and then graft polymers. In the last five years, the types of architectures available have become more and more complex. Star and graft polymers now have block structures within their branches, or a range of different branches can be found growing from one core or backbone. Even the synthesis of hyperbranched polymers can be positively influenced by RAFT polymerization, allowing end group control or control over the branching density. The creative combination of RAFT polymerization with other polymerization techniques, such as ATRP or ring-opening polymerization, has extended the array of available architectures. In addition, dendrimers were incorporated either as star core or endfunctionalities. A range of synthetic chemistry pathways have been utilized and combined with polymer chemistry, pathways such as ‘click chemistry’. These combinations have allowed the creation of novel structures. RAFT processes have been combined with natural polymers and other naturally occurring building blocks, including carbohydrates, polysaccharides, cyclodextrins, proteins and peptides. The result from the intertwining of natural and synthetic materials has resulted in the formation of hybrid biopolymers. Following these developments over the last few years, it is remarkable to see that RAFT polymerization has grown from a lab curiosity to a polymerization tool that is now been used with confidence in material design. Most of the described synthetic procedures in the literature in recent years, which incorporate RAFT polymerization, have been undertaken in order to design advanced materials.