Grandeurs caractéristiques d'un polymère

Les différentes masses molaires moyennes La synthèse des polymères aboutit le plus souvent à une distribution de chaînes de longueurs différentes, on ne peut parler de masse molaire mais bien de masses molaires moyennes. Elles s'expriment en g·mol−1. Soit : :i le degré de polymérisation ; :M_i la masse molaire des chaînes de degré de polymérisation i ; :m_i la masse de chaînes de degré de polymérisation i ; :N_i, le nombre de chaînes de masse molaire M_i ; on distingue quatre masses molaires moyennes, décrites ci-dessous. Masse molaire moyenne en nombre Il s'agit de la moyenne des masses molaires pondérée par le nombre de chaînes de chaque longueur. Cela correspond à la masse molaire d'une chaîne de longueur moyenne. \bar {M_n} = \frac {\sum_i N_i \times M_i} {\sum_i N_i} On l'obtient généralement par osmométrie, cryoscopie ou ébulliométrie. Masse molai

Design of amphiphilic nanocapsules based on hyperbranched star-block copolymers for the encapsulation and the controlled release of olfactory compounds

Céline Ternat

Effective encapsulation of small, volatile, weakly water-soluble molecules such as flavors and fragrances is necessary to protect them from degradation, to increase their lifetime, and to improve their water dispersion and fixation depending on the substrate of interest. The aim of this project has been to improve encapsulation and release in order to optimize the performance of fragrance molecules in water-based household (detergents, softeners, etc.) and body care applications (shampoos, lotions, etc.), and fine perfumery applications. More specifically, the aim has been to investigate the effectiveness of "unimolecular micelles" based on amphiphilic multi-arm star-block copolymers with a hyperbranched core with more than 26 functional groups, a hydrophobic inner and a hydrophilic outer shell. These have been prepared from a commercial hyperbranched polyester macroinitiator (HBP) by ring-opening polymerization of ε-caprolactone, followed by the atom transfer radical polymerization (ATRP) of tert-butyl acrylate (tBuA). Hydrolysis of the tert-butyl groups has then been used to convert the poly(tBuA) blocks to poly(acrylic acid) (PAA), resulting in HBP-(PCL)p-(PAA)q pH-dependent amphiphilic star-block copolymers with good control of the molecular weight distribution. These were shown to form stable nanocapsules with a well defined core-shell architecture, as confirmed by thermal and microstructural characterization. The necessity of the core-shell architecture for the effective encapsulation of fragrance molecules in aqueous dispersion has been demonstrated by NMR (nuclear magnetic resonance). The extent of encapsulation reflects the dynamic equilibrium between the free molecules and the fragrance/polymer complex and is dependent on the octanol/water partition coefficient (logP) of the fragrance compounds, as demonstrated with a similar non ionic core-shell architecture HBP-(PBMA)37-(PPEGMA)39 composed of a hydrophobic poly(butyl methacrylate) (PBMA) core and a hydrophilic poly(polyethylene glycol) methyl ether methacrylate (PPEGMA) shell (provided by the Polymer Laboratory of the EPFL). The fragrance loadings in the polymer (HBP-(PCL)p-(PAA)q and HBP-(PBMA)37-(PPEGMA)39), which reached up to 30 wt% depending on the type of the fragrance molecule, were argued to be linked to their solubility in the hydrophobic core of the star-block copolymer. Moreover, under conditions representative of real applications (fine perfumery and softener applications) the star-block copolymers significantly extended the time over which the concentration of certain volatiles remained above the human olfactory threshold. Effective encapsulation and delayed release of small hydrophobic molecules was hence demonstrated to be possible with the present systems. The straightforward synthesis, tailorable chemistry and globular architecture of the star-block copolymers investigated here therefore offer promise for the development of relatively cheap encapsulants with affinities for specific components in fragrance packages, and release triggered by selected substrates according to their surface chemistry.

EPFL2007

Poly(lactide)/cellulose nanocrystal nanocomposites by high-shear mixing

Nicolas Candau, Fatma Nalan Cetin, Mehmet Kerem Citak, Adrien Julien Demongeot, Véronique Michaud, Oguzhan Oguz

There is currently considerable interest in developing stiff, strong, tough, and heat resistant poly(lactide) (PLA) based materials with improved melt elasticity in response to the increasing demand for sustainable plastics. However, simultaneous optimization of stiffness, strength, and toughness is a challenge for any material, and commercial PLA is well-known to be inherently brittle and temperature-sensitive and to show poor melt elasticity. In this study, we report that high-shear mixing with cellulose nanocrystals (CNC) leads to significant improvements in the toughness, heat resistance, and melt elasticity of PLA while further enhancing its already outstanding room temperature stiffness and strength. This is evidenced by (i) one-fold increase in the elastic modulus (6.48 GPa), (ii) 43% increase in the tensile strength (87.1 MPa), (iii) one-fold increase in the strain at break (similar to 6%), (iv) two-fold increase in the impact strength (44.2 kJ/m(2)), (v) 113-fold increase in the storage modulus at 90 degrees C (787.8 MPa), and (vi) 10(3)-fold increase in the melt elasticity at 190 degrees C and 1 rad/s (similar to 10(5) Pa) via the addition of 30 wt% CNC. It is hence possible to produce industrially viable, stiff, strong, tough, and heat resistant green materials with improved melt elasticity through high-shear mixing.

WILEY2020

Effects of Molecular Weight on Microstructure and Carrier Transport in a Semicrystalline Poly(thieno)thiophene

Andrea Gasperini, Kevin Sivula

The ultimate control over chain self-assembly is key to unravel and optimize the relationship between film microstructure and charge carrier mobility in solution processable conjugated polymer semiconductors. Here we employ preparatory size exclusion chromatography to produce fractions of a poly(thieno)thiophene polymer, coded PBTTT-C-12, with varying number-average molecular weight, M-n, from 5.8 to 151 kDa and low polydispersity index of 1.1-1.4. Solution processing of these samples into bottom-contact, bottom gate, field effect transistors reveals a strong dependence of transistor performance on the molecular weight. Further analysis of the films' microstructure and crystallinity show three distinct regions: fiber formation (ca. 5-20 kDa), terrace formation (20-50 kDa), and a rough morphology (50-150 kDa). The performance of low-M-n films was found to increase rapidly with increasing chain length, and while the best transistor performance was found with the terrace morphology, films not exhibiting the terraced morphology (using 80 kDa polymer) were capable of similar performance. In addition, by blending only 5 wt % of a high molecular weight fraction into a low-M-n film, we demonstrate the ability to drastically increase the measured charge carrier mobility of the low-M-n material without attaining a terraced morphology. This illustration suggests a viable route to easily increase the processability and transistor performance of low molecular weight conjugated polymeric or oligomeric semiconductors. In addition, GIXRD and thermal analysis of select fractions further indicate that the films of higher molecular weight exhibit a reduced side-chain crystallinity due to chain entanglement; the degree of backbone crystallinity remains more constant.

American Chemical Society2013

Design of amphiphilic nanocapsules based on hyperbranched star-block copolymers for the encapsulation and the controlled release of olfactory compounds

Céline Ternat

EPFL2007