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Cellulose nanocrystals (CNC) were extracted from okra bahmia (Abelmoschus Esculentus) bast fibers and inserted in different tenors (1, 2, 5, and 10 wt %) as the reinforcement of a poly(vinyl alcohol) (PVA) matrix. The extraction of cellulose was carried out in a two-step procedure: the first chemical treatment led to the production of holocellulose by the gradual removal of lignin, while the subsequent sulphuric acid hydrolysis process allowed obtaining cellulose nanocrystals in an aqueous suspension. The dispersion of CNC in the composite appeared effective at low cellulose content (1 wt %), while it presented more problems for higher contents. However, a 5 wt % cellulose content proved ideal to promote a direct mechanical interaction between the PVA and cellulose structures. Thermal analysis demonstrated that the presence of okra did not have a large effect on glass transition temperature, while it sensibly modified the melting temperature of the PVA matrix, as well as the crystallization temperature, due to the nucleating action of the nanofillers. FTIR spectroscopy performed during the exposure to UV light underlined that no oxidative reactions occur after a short-time exposure and that a longer irradiation times are required to produce oxidation on neat matrix and PVA/CNC nanocomposites. The results confirmed that the presence of CNC does not affect the stability of the neat PVA matrix to the photodegradation after UV irradiation.

Micellar copolymerizations of amphiphilic ω-methoxy-α-p-styrylalkyl poly(ethylene oxide) (PEO) macromonomers 1 (C1−PEO−C m −S-n, m = 1, 4, 7 and n = 18) with a limited amount of styrene solubilized therein have been investigated. The reactions were carried out with 4,4‘-azobis(4-cyanovaleric acid) (AVA) as a water-soluble initiator and 2,2‘-azobis(isobutyronitrile) (AIBN) as an oil-soluble initiator at 60 °C. Fractional time-evolution experiments revealed that the copolymerization rate of styrene is greatly enhanced by the presence of macromonomer micelles and that the rate increases with hydrophobicity (increase of m) of α-end group of 1. The characterization of the resulting graft copolymers revealed that the organized copolymerization proceeds in the manner of a pseudo-living radical copolymerization with highly limited terminations between compartmentalized (isolated) propagating radicals. That is, the molecular weight of the graft copolymers increased with the conversion of monomers. At an equimolar styrene with 1 (n = 18, m = 7) the copolymerization proceeded apparently transparently and azeotropically to afford a highly branched graft copolymer of poly(styrene-graft-PEO), which in water formed a “unimolecular particle” with 15 nm radius and 7.44 × 106 apparent molecular weight. A plausible mechanism for the present organized copolymerization behavior is presented and discussed.

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ABSTRACT:Solution properties of regular amphiphilic comb-shaped polymers have been studied in water by means of fluorescence technique using pyrene as a probe, dynamic light scattering, 1 H NMR, and scanning force microscopy (SFM). They consist of a hydrophobic poly(p-alkylstyrene) as a main chain and hydrophilic poly(ethylene oxide) (PEO) chains as a graft chain. Two types of the comb-shaped polymers have been explored, which were prepared by micellar, organized homopolymerization of amphiphilic PEO macromonomers (1) (C 1 -EO n -(CH 2 ) m -S, with a p-alkylstyrene ((CH 2 ) m -S) end group, with m = 1 and 7, and a PEO chain with n = 18, 46, 115, and 469) and by copolymerization of C 1 -EO 18 -(CH 2 ) 7 -S with styrene in water, respectively. Partitioning, I 1 /I 3 , and excimer formation of pyrene were studied for their dilute aqueous solutions, and rationalized with the local concentration and structure of the comb-shaped polymers. Hydrophobic pyrene molecules were principally dissolved into the vicinity of the hydrophobic p-heptylstyrene main chain of the comb-shaped polymers. The nature of the hydrophobic main chain of poly(styrene-co-C 1 -EO 18 -(CH 2 ) 7 -S) was found to be essentially similar to that of a series of poly(C 1 -EO n -(CH 2 ) m -S). Direct observation of single macromolecule of poly(C 1 -EO 46 -(CH 2 ) 7 -S) by SFM revealed a high molecular weight, cylindrical brush-like conformation with ca. 12.8 nm in thickness. In contrast, small fine particles were observed for poly(styrene-co-C 1 -EO 18 -(CH 2 ) 7 -S), suggesting the formation of a unimolecular micelle in water.KEY WORDS Poly(ethylene oxide) Macromonomer / Amphiphilic Graft Copolymer / Combshaped Polymer / Scanning Force Microscopy / Branched Polymer / Pyrene / Fluorescence Spectroscopy / Unimolecular Micelle / Polymacromonomer / Micellization of amphiphilic polymers in a selective solvent has been of continuing interest over the past two decades. This is not only because of their biological relevance but also because of their usefulness for such industrial applications as paints, coatings, cosmetics, inks, and oil recovery.