Morphological and kinetic analyses of regime transition for poly[(S)-lactide] crystal growth

Young's modulus of the different crystalline phases of poly (l-lactic acid)

Young's modulus of α'- and α-crystals of poly (l-lactic acid) (PLLA), more precisely, of aggregates of isotropically arranged lamellae, has been estimated based on dynamic-mechanical analysis of sets of isotropic film samples containing largely different though well-defined amounts of crystals. Evaluation of the modulus of elasticity of these film samples yielded the dependence of Young's modulus as a function of the enthalpy-based crystallinity, increasing with the crystal fraction in the assessed range, from zero to about 75% crystallinity. Extrapolation towards 100% crystallinity suggests values of Young's modulus of around 3.7 and 4.6 GPa for isotropic aggregates of α'- and α-crystals, respectively, being only slightly higher than the modulus of the unaged glassy amorphous phase of 3.0 GPa. Noting the inherent anisotropy of the crystal modulus, suggested in the literature, the average modulus determined in this work seems to be controlled by weaker interchain secondary bonding but not the modulus in chain direction. Great effort has been undertaken to minimize errors by keeping the lamellar thickness in samples of different crystallinity constant, and by providing evidence for independence of the moduli on the spherulitic superstructure.

Crystallization of Long-Spaced Precision Polyacetals III: Polymorphism and Crystallization Kinetics of Even Polyacetals Spaced by 6 to 26 Methylenes

In this paper we extend the study of polymorphism and crystallization kinetics of aliphatic polyacetals to include shorter (PA-6) and longer (PA-26) methylene lengths in a series of even long-spaced systems. On a deep quenching to 0 °C, the longest even polyacetals, PA-18 and PA-26, develop mesomorphic-like disordered structures which, on heating, transform progressively to hexagonal, Form I, and Form II crystallites. Shorter polyacetals, such as PA-6 and PA-12 cannot bypass the formation of Form I. In these systems a mixture of this form and disordered structures develops even under fast deep quenching. A prediction from melting points that Form II will not develop in polyacetals with eight or fewer methylene groups between consecutive acetals was further corroborated with data for PA-6. The temperature coefficient of the overall crystallization rate of the two highest temperature polymorphs, Form I and Form II, was analyzed from the differential scanning calorimetry (DSC) peak crystallization times. The crystallization rate of Form II shows a deep inversion at temperatures approaching the polymorphic transition region from above. The new data on PA-26 confirm that at the minimum rate the heat of fusion is so low that crystallization becomes basically extinguished. The rate inversion and dramatic drop in the heat of fusion irrespective of crystallization time are associated with a competition in nucleation between Forms I and II. The latter is due to large differences in nucleation barriers between these two phases. As PA-6 does not develop Form II, the rate data of this polyacetal display a continuous temperature gradient. The data of the extended polyacetal series demonstrate the important role of methylene sequence length on polymorphism and crystallization kinetics.

Crystallization-induced mechanofluorescence for visualization of polymer crystallization

The growth of lamellar crystals has been studied in particular for spherulites in polymeric materials. Even though such spherulitic structures and their growth are of crucial importance for the mechanical and optical properties of the resulting polymeric materials, several issues regarding the residual stress remain unresolved in the wider context of crystal growth. To gain further insight into micro-mechanical forces during the crystallization process of lamellar crystals in polymeric materials, herein, we introduce tetraarylsuccinonitrile (TASN), which generates relatively stable radicals with yellow fluorescence upon homolytic cleavage at the central C-C bond in response to mechanical stress, into crystalline polymers. The obtained crystalline polymers with TASN at the center of the polymer chain allow not only to visualize the stress arising from micro-mechanical forces during polymer crystallization via fluorescence microscopy but also to evaluate the micro-mechanical forces upon growing polymer lamellar crystals by electron paramagnetic resonance, which is able to detect the radicals generated during polymer crystallization.

Homocrystallization and Stereocomplex Crystallization Behaviors of As-Spun and Hot-Drawn Poly(l-lactide)/Poly(d-lactide) Blended Fibers During Heating

A series of poly(l-lactide)/poly(d-lactide) blended chips (LD_C), as-spun LD fibers (LD_A) and hot-drawn LD fibers (LD_H) were prepared for investigating the homocrystallization and stereocomplex crystallization behaviors of LD_A and LD_H fibers during heating. Modulated differential scanning calorimetry (MDSC), hot stage polarized microscopy (HSPM), and real-time wide-angle X-ray diffraction (WAXD) were used for studying the crystallization and melting behaviors, fiber morphology, and crystalline structure evolution of the LD_A and LD_H fibers’ homocrystals and stereocomplex crystals during heating. The molecular chain orientations of the LD_A and LD_H fibers were obtained through spinning and improved through the hot drawing processes. When the molecular chain was oriented on the fiber axis, the homocrystals and stereocomplex crystals of the fibers began to form in turn as the heating temperature exceeded the glass transition temperature of the fiber. The side-by-side packing of the molecular chains was promoted by mixing the molecular chains with the extrusion screw during the spinning process, facilitating stereocomplex crystallization. When the LD_A fiber was heated above the glass transition temperature of the fiber, movement of the fiber molecular chain—including molecular chain orientation and relaxation, as well as crystallization, melting, and recrystallization of homocrystals and stereocomplex crystals—were investigated through HSPM. MDSC and real-time WAXD were used to observe the molecular chains of the melted poly(l-lactide) and poly(d-lactide) homocrystals of the fibers rearranging and transiting to form stereocomplex crystals during heating.

Lamellae Assembly in Dendritic Spherulites of Poly(l-lactic Acid) Crystallized with Poly(p-Vinyl Phenol)

Lamellar assembly with fractal-patterned growth into dendritic and ringed spherulites of crystallized poly(l-lactic acid) (PLLA), of two molecular weight (MW) grades and crystallized at (temperature of crystallization) Tc = 120 and 130 °C, respectively, are evaluated using optical and atomic-force microscopies. The results of surface-relief patterns in correlation with interior microscopy analyses in this work strongly indicate that the observed birefringence changes in PLLA polymer dendritic or ringed spherulites (from blue to orange, or to optical extinction) need not be definitely associated with the continuous helix twisting of lamellae; they can be caused by sudden and discontinuous lamellae branching at intersected angles with respect to the original main lamellae, as proven in the case of dendritic and zig-zag rough-ringed spherulites. Intersection angles between the main stalks and branches tend to be governed by polymer crystal lattices; for PLLA, the orthorhombic lattice (α-form) usually gives a 60° angle of branching and hexagonal growth. The branching lamellae then further bend to convex or concave shapes and finally make a 60⁻90° angle with respect to the main stalks. Such mechanisms are proven to exist in the straight dendritic/striped high-molecular weight (HMW)-PLLA spherulites (Tc = 120 °C); similar mechanisms also work in circularly ringed (Tc = 130 °C) HMW-PLLA spherulites.

Equilibrium Melting Temperature of Polymorphic Poly(l-lactide) and Its Supercooling Dependence on Growth Kinetics

In this study, the isothermal crystallization process of poly(l-lactide) (PLLA) has been investigated using in situ XRD, differential scanning calorimetry (DSC), and polarized optical microscopy (POM). Linear and nonlinear extrapolation methods have been deployed to estimate the equilibrium melting temperature (Tm0), which is used for analyzing the supercooling dependence of the PLLA spherulitic growth rate (G). A double-melting behavior observed for PLLA under crystallization T_c < 120 °C has been attributed to the formation of both α′ and α crystals. The Tm0 values of both α′ and α crystals have been evaluated using the linear method (172.8 °C) and nonlinear method (196.4 °C), with the nonlinear estimate being 23.6 °C higher. A discontinuity in the temperature dependence of spherulite growth rate is observed around 128.3 °C. Regime II–III transition is found to occur at 128.3 °C when Tm0 = 196.4 °C as estimated by the nonlinear extrapolation method.

Semicrystalline Block Copolymers in Rigid Confining Nanopores

We have investigated PLLA crystallization in lamellae-forming PS-b-PLLA confined to straight cylindrical nanopores under weak confinement (nanopore diameter D/equilibrium PS-b-PLLA period L0 ≥ 4.8). Molten PS-b-PLLA predominantly forms concentric lamellae along the nanopores, but intertwined helices occur even for D/L0 ≈ 7.3. Quenching PS-b-PLLA melts below TG(PS) results in PLLA cold crystallization strictly confined by the vitrified PS domains. Above TG(PS), PLLA crystallization is templated by the PS-b-PLLA melt domain structure in the nanopore centers, while adsorption on the nanopore walls stabilizes the outermost cylindrical PS-b-PLLA shell. In between, the nanoscopic PS-b-PLLA melt domain structure apparently ripens to reduce frustrations transmitted from the outermost immobilized PS-b-PLLA layer. The onset of PLLA crystallization catalyzes the ripening while transient ripening states are arrested by advancing PLLA crystallization. Certain helical structure motifs persist PLLA crystallization even if PS is soft. The direction of fastest PLLA crystal growth is preferentially aligned with the nanopore axes to the same degree as for PLLA homopolymer, independent of whether PS is vitreous or soft.

The effect of hydroxyapatite nanoparticles on crystallization and thermomechanical properties of PLLA matrix

In this study, hydroxyapatite (HA) was incorporated in a poly(L-lactic acid) (PLLA) matrix and the thermal properties and crystallization behavior of the derived composites were investigated. The nanocomposites, containing 0–20 wt% HA, were prepared by melt extrusion employing a twin-screw extruder. XRD experiments verified an increase in the intensity of the characteristic diffraction peak of the α-form crystalline phase of PLLA with increasing HA content. By DSC experiments it was observed that the presence of HA increased the crystallinity during cold crystallization, leading to a shift of cold-crystallization temperature to lower values and to an increase in the melting temperature of the PLLA phase. Isothermal crystallization experiments at 100, 110, 115 and 120°C, revealed a maximum in crystallization kinetic around 100°C after the addition of HA compared to 115°C for pure PLLA. The crystallization rate of PLLA matrix in the nanocomposites decreased with increasing crystallization temperature. By using the Avrami and Lauritzen-Hoffman equations the exponent n was calculated in the range 2–3 and a theoretical approach verified that the HA/PLLA systems belong to Regime II of crystallization behavior. The investigated melting behavior of PLLA was attributed to better organized crystalline structure with increasing isothermal crystallization temperatures and might be related with the longer time necessary for the completion of crystallization.

Polylactides-Methods of synthesis and characterization

Polylactides with various molar masses, microstructures and crystallinities are used as degradable and biocompatible polymers suitable for preparation of drug carriers and temporary medical implants. This paper presents state of current knowledge on synthesis of lactic acids, high purity lactide monomers and their polymerization. Syntheses of high molar mass polylactides by polycondensation of lactic acid and by ring-opening polymerization of lactides are described and their advantages and disadvantages are discussed. Mechanisms of lactide polymerization initiated by metal alkoxides are described. There are presented also results of more recent studies of polymerization initiated with the so-called "no metal" organocatalysts; both anionic and cationic. Presented are advantages and limitations of synthesis of PLA by all the major polymerization processes until now. Some properties of PLA and most important methods used for PLA characterization are also described.

Transcrystallization of poly(l-lactic acid) on the surface of reduced graphene oxide fibers

Transcrystalline layer could form between rGOF and PLLA. The good nucleating ability of rGOF could be quantitatively characterized based on the theories of heterogeneous nucleation and crystal growth rate.

Sugar-Based Polyamides: Self-Organization in Strong Polar Organic Solvents

Periodic patterns resembling spirals were observed to form spontaneously upon unassisted cooling of d-glucaric acid- and d-galactaric acid-based polyamide solutions in N-methyl-N-morpholine oxide (NMMO) monohydrate. Similar observations were made in d-galactaric acid-based polyamide/ionic liquid (IL) solutions. The morphologies were investigated by optical, polarized light and confocal microscopy assays to reveal pattern details. Differential scanning calorimetry was used to monitor solution thermal behavior. Small- and wide-angle X-ray scattering data reflected the complex and heterogeneous nature of the self-organized patterns. Factors such as concentration and temperature were found to influence spiral dimensions and geometry. The distance between rings followed a first-order exponential decay as a function of polymer concentration. Fourier-Transform Infrared Microspectroscopy analysis of spirals pointed to H-bonding between the solvent and the pendant hydroxyl groups of the glucose units from the polymer backbone. Tests on self-organization into spirals of ketal-protected d-galactaric acid polyamides in NMMO monohydrate confirmed the importance of the monosaccharide's pendant free hydroxyl groups on the formation of these patterns. Rheology performed on d-galactaric-based polyamides at high concentration in NMMO monohydrate solution revealed the optimum conditions necessary to process these materials as fibers by spinning. The self-organization of these sugar-based polyamides mimics certain biological materials.

Stereocomplexation kinetics of enantiomeric poly(l-lactide)/poly(d-lactide) blends seeded by nanocrystalline cellulose

Stereocomplexation of PLA was significantly promoted by nanocrystal cellulose (NCC) showing strong NCC-dependent stereocomplex crystal growth and crystallization regime transitions.

Crystalline structures and crystallization behaviors of poly(l-lactide) in poly(l-lactide)/graphene nanosheet composites

GNS existence in PLLA favors α′ crystal formation more than α crystal formation resulting in a shift of α′–α crystal formation transition toward high T_cs.