Multiphase star-like copolymers containing lignin: synthesis, properties and applications
Multiphase star-like copolymers containing lignin have been synthesized and characterized. All copolymers contained hydroxypropyl lignin (HPL) as the central core. Polycaprolactone (PCL), cellulose propionate (CP) or polystyrene (PS), served as radiating blocks attached to the lignin core in star-like manner. These materials were studied in relation to their structure, morphology, effect on crystallization behavior and application in polymer blends.
Three series of semi-crystalline (PCL)n — HPL have been synthesized with HPL segments of 2,100, 3,500 and 6,400 molecular weight, respectively, and polycaprolactone blocks of varying size. Copolymers were produced by either copolymerizing ɛ-caprolactone or grafting preformed PCL segments onto HPL. The thermal and optical properties of these copolymers were investigated by DSC, DMTA and optical microscopy. The copolymers exhibited variable thermal behavior in relation to composition. The crystallization of PCL blocks was mainly governed by the nature of the HPL phase. PCL block length was another variable that affected crystallinity. The longer the segment, the higher the degree of crystallinity. The compatibility, morphology and mechanical properties of (PCL)n - HPL copolymers blended with poly(vinyl chloride) were also investigated. Methods used in this study included DSC, DMTA, SEM, TEM and stress-strain testing. The blends were shown to be compatible in all proportions.
Mono-hydroxyl terminated cellulose propionate oligomers were synthesized by degradation with hydrogen bromide of a fully substituted, high molecular weight cellulose propionate molecule. Evidence of strict monofunctionality was demonstrated by H-NMR spectroscopy. Thermal analysis results indicated that the oligomers were semi-crystalline and their melting points were functions of molecular weight. (CP)n — HPL copolymers were synthesized by grafting oligomeric CP segments onto HPL via a coupling agent. The thermal and morphological properties of the copolymers were characterized by DMTA, DSC and TEM. Analysis by thermal methods and by electron microscopy showed strong evidence for microphase separation between HPL and CP segments. Cellulose propionate chains crystallize even at a low degree of polymerization, such as DP 5. The copolymer morphologies exhibit a broad variety of features. They vary from dispersed fibrils to spheres like and alternate lamella type patterns according to composition and molecular weights. The interfacial activity of the copolymers in blends of CP and HPL prepared in the melt state was also investigated. The tensile properties of the ternary blends were altered slightly by the presence of the copolymer. Melt blended cellulose propionate and HPL with low degree of propoxylation forms a miscible system with up to 40% HPL component. The incorporation of 5% of the (CP)n — HPL copolymer reduces the tensile strength by about 10%. Thermal behavior of melt blended cellulose propionate and HPL with high degree of propoxylation indicates the formation of an incompatible system at any composition. Before the addition of the copolymer the blend exhibits higher toughness, elongation up to 160%, and a Young's modulus of 23 ksi. The copolymer-modified blend shows a decrease in toughness and an increase in tensile strength by about 10%.
The synthesis and characterization of (PS)n — HPL copolymers was accomplished in an analogous manner. When added to blends of PS and HPL, (PS)n -- HPL produced improved mechanical properties of the blends. Scanning electron microscopy of fracture surfaces demonstrated that the addition of copolymer to the PS/HPL blends improved the adhesion of the two phases. The addition of (PS)n — HPL copolymer to the 90 PS/10 HPL blend system strongly reduced, by about 10 fold, the particle size of the unmodified blend. No significant difference was observed in the morphology of the 80 PS/20 HPL system. The phases exhibited poor adhesion before and after the addition of copolymer.