A literature review is presented regarding the synthesis, and physicochemical, chemical, and mechanical properties of poly(lactic acid)(PLA). Poly(lactic acid) exists as a polymeric helix, with an orthorhombic unit cell. The tensile properties of PLA can vary widely, depending on whether or not it is annealed or oriented or what its degree of crystallinity is. Also discussed are the effects of processing on PLA. Crystallization and crystallization kinetics of PLA are also investigated. Solution and melt rheology of PLA is also discussed. Four different power-law equations and 14 different Mark–Houwink equations are presented for PLA. Nuclear magnetic resonance, UV–VIS, and FTIR spectroscopy of PLA are briefly discussed. Finally, research conducted on starch–PLA composites is introduced.

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A Literature Review of Poly(Lactic Acid)

Poly(lactic acid) crystallization

The discovery of graphene and other two-dimensional (2D) materials together with recent advances in exfoliation techniques have set the foundations for the manufacturing of single layered sheets from any layered 3D material. The family of 2D materials encompasses a wide selection of compositions including almost all the elements of the periodic table. This derives into a rich variety of electronic properties including metals, semimetals, insulators and semiconductors with direct and indirect band gaps ranging from ultraviolet to infrared throughout the visible range. Thus, they have the potential to play a fundamental role in the future of nanoelectronics, optoelectronics and the assembly of novel ultrathin and flexible devices. We categorize the 2D materials according to their structure, composition and electronic properties. In this review we distinguish atomically thin materials (graphene, silicene, germanene, and their saturated forms; hexagonal boron nitride; silicon carbide), rare earth, semimetals, transition metal chalcogenides and halides, and finally synthetic organic 2D materials, exemplified by 2D covalent organic frameworks. Our exhaustive data collection presented in this Atlas demonstrates the large diversity of electronic properties, including band gaps and electron mobilities. The key points of modern computational approaches applied to 2D materials are presented with special emphasis to cover their range of application, peculiarities and pitfalls.

https://www.rsc.org/suppdata/cs/c4/c4cs00102h/c4cs00102h1.pdf

An atlas of two-dimensional materials

Summary: Poly(lactide)s [i.e. poly(lactic acid) (PLA)] and lactide copolymers are biodegradable, compostable, producible from renewable resources, and nontoxic to the human body and the environment. They have been used as biomedical materials for tissue regeneration, matrices for drug delivery systems, and alternatives for commercial polymeric materials to reduce the impact on the environment. Since stereocomplexation or stereocomplex formation between enantiomeric PLA, poly(L-lactide) [i.e. poly(L-lactic acid) (PLLA)] and poly(D-lactide) [i.e. poly(D-lactic acid) (PDLA)] was reported in 1987, numerous studies have been carried out with respect to the formation, structure, properties, degradation, and applications of the PLA stereocomplexes. Stereocomplexation enhances the mechanical properties, the thermal-resistance, and the hydrolysis-resistance of PLA-based materials. These improvements arise from a peculiarly strong interaction between L-lactyl unit sequences and D-lactyl unit sequences, and stereocomplexation opens a new way for the preparation of biomaterials such as hydrogels and particles for drug delivery systems. It was revealed that the crucial parameters affecting stereocomplexation are the mixing ratio and the molecular weight of L-lactyl and D-lactyl unit sequences. On the other hand, PDLA was found to form a stereocomplex with L-configured polypeptides in 2001. This kind of stereocomplexation is called “hetero-stereocomplexation” and differentiated from “homo-stereocomplexation” between L-lactyl and D-lactyl unit sequences. This paper reviews the methods for tracing PLA stereocomplexation, the methods for inducing PLA stereocompelxation, the parameters affecting PLA stereocomplexation, and the structure, properties, degradation, and applications of a variety of stereocomplexed PLA materials.

http://www.mate.tue.nl/~peters/WimBras/LuigiBalzano/2011_03_11%20LB/Tsuji%20StereoComplex.pdf

Poly(lactide) Stereocomplexes: Formation, Structure, Properties, Degradation, and Applications

Solutions of different polymers in the same solvent are incompatible as a rule and show phase separation when they are mixed. If incompatibility is also to be observed in systems where one of the polymer components is replaced by colloidal particles, sterically stabilized by a cover of polymer chains, will be discussed in this lecture. After a discussion of the applicability of statistical thermodynamical criteria for colloid stability we focus attention on the potential of average force between two particles, V(r), and the second virial coefficient, B2. First it is shown from general arguments that V(r) and B2 always decrease in magnitude upon addition of particles identical to the particle pair considered. The decrease is particularly large for high molecular weight polymers. Subsequently, the analysis is extended, with the help of simple models, to mixtures of polymer colloid and polymer. It is predicted that B2 should decrease and may become negative when the molecular weight and concentration of the polymer are sufficiently large. For high molecular weight polymer this is of the order of a per cent or less. More polymer is needed for low molecular weights. The destabilization is intimately connected with the expulsion of polymer from the interstitial spaces between approaching particles because of “volume restriction”- and “osmotic” effects. The predictions are in accordance with some experiments that were available. Finally the applicability of light scattering as an experimental tool in these stability problems is stressed. Results are also given of the incompatibility of two polymers in a single solvent in which one of the polymers is masked i.e. does not scatter light.

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Polymers at Interfaces and the Interactions in Colloidal Dispersions

We have analyzed the influence of different amounts ω be of two diblock copolymers, poly(styrene-b-methyl methacrylate) (sm blend series) and poly(cyclohexyl methacrylate-b-methyl methacrylate) (cm blend series), on the morphological and rheological characteristics of a blend containing w = 7.5 wt % polystyrene in poly(methyl methacrylate) matrix. The morphological analysis is based on the sphere size distribution function, which was determined from the image analysis of the transmission electron micrographs. Using this function and assuming that all block copolymers are located at the interface, the interfacial area per copolymer joint, Σ, was calculated. From its hyperbolic dependence on ω bc the value at the critical micelle concentration, Σ cmc , was found to be about 10 nm 2 for both systems. The rheological analysis reveals that in addition to the form relaxation process, well-known for polymer blends, a new relaxation process is observed for these systems. Its relaxation time, τ β , has been studied in dependence on the amount of added block copolymers. The observed phenomena for each blend series, i.e. constant blend viscosity, slight shift of the form relaxation times τ 1 , and systematic shift of the interface governed relaxation time τ β (τ β > τ 1 ), have been interpreted quantitatively. In contrast to τ 1 , τ β is less influenced by the interfacial tension but is mainly governed by an additional contribution, the interfacial shear modulus. Formulas were derived from an expanded version of the Palierne emulsion model which allows the determination of the proposed interfacial properties from rheological measurements. In general, the interfacial tension decreases with increasing amount of block copolymer, and the decrease is more pronounced for the cm blend series. The interfacial shear modulus increases during compatibilization from 0 to amounts which are in the range of 20-30% of the interfacial tension. The decrease of interfacial tension is in good agreement with predictions from Leibler's brush model extended by Dai et al. In conclusion, it was found that the Palierne model with an nonisotropical interfacial stress state is quantitatively correct to describe the observed phenomena for those blends.

Interpretation of a New Interface-Governed Relaxation Process in Compatibilized Polymer Blends

The structure and contact faces of epitaxially crystallized isotactic polypropylene (iPP) in its α and γ phases are investigated by electron microscopy, electron diffraction, and atomic force microscopy (AFM). AFM results with methyl group resolution help recognize which one of two possible contact faces interacts with the substrate. Structural analysis makes it possible (at least in the a phase) to determine the hand of helices in contact with the substrate

Contact faces of epitaxially crystallized .alpha.- and .gamma.-phase isotactic polypropylene observed by atomic force microscopy

Structural and scanning microscopy studies of layered compounds MCl3 (M=Mo, Ru, Cr) and MOCl2 (M=V, Nb, Mo, Ru, Os)

Correlation between rheology and morphology of compatibilized immiscible blends

Contact faces of thin films of α-phase isotactic polypropylene epitaxially crystallized on benzoic acid are examined by atomic force microscopy (AFM). The AFM images reveal the lamellar structure as well as the methyl side group pattern, thus enabling the discrimination between two structurally different contact surfaces in favour of the ‘four’ face type, with one methyl group per helix turn exposed. Furthermore it has been verified, that the observed helices are right-handed.

H. J. Cantow

Papers

Interpretation of a New Interface-Governed Relaxation Process in Compatibilized Polymer Blends

Contact faces of epitaxially crystallized .alpha.- and .gamma.-phase isotactic polypropylene observed by atomic force microscopy

Structural and scanning microscopy studies of layered compounds MCl3 (M=Mo, Ru, Cr) and MOCl2 (M=V, Nb, Mo, Ru, Os)

Correlation between rheology and morphology of compatibilized immiscible blends

Atomic force microscopy on epitaxially crystallized isotactic polypropylene