Biomedical Engineering Reference
In-Depth Information
dustrial usage. Nevertheless, using cellulose has got also its limitations
too since cellulose has a strong sensitivity to water and moisture, when
it is dried it may form aggregates and it display poor compatibility with
the hydrophobic polymeric matrices [28]. However, if a well-dispersed,
homogenous system with good interfacial interactions is created, cellulose
can show its benefits.
In the literature there have been already used microcrystalline cellu-
lose [29-31] and microfibrillated cellulose (MFC) [32, 33] to improve
the properties of the PLA. Nanosized cellulose like cellulose nanofibers
(CNF) [34, 35] and cellulose nanowhiskers (CNW) [35-42] could offer
further improvements. Cellulose nanocrystals are mostly prepared in wa-
ter media with acidic hydrolysis. To remove the media the process is usu-
ally followed by lyophilization. However, freeze drying typically results
in agglomeration/aggregation of the CNC thus a fine dispersed system is
difficult to achieve especially in case of melt processed composites. In or-
der to enhance the dispersibility of the CNC in PLA matrix solvent based
composite producing process could be applied or a solvent based master-
batch preparation before the melt process. However, using large amount
of solvent is incompatible with the environmentally friendly nature of poly
(lactic acid). Surface modification of the CNC with anionic surfactants
can be a solution to improve the interfacial interactions between cellulose
and PLA as [38, 40, 42] reported. Oksman et al. [37] found that the pres-
ence of poly (ethylene glycol) (PEG1500) increased the dispersibility of
CNC in PLA matrix too (which was an effective plasticizer for the PLA as
well). Qu et al. [34] reported that in case of chemo-mechanical produced
CNF PEG1000 seemed to be a good choice in increasing the interactions
between the PLA and the cellulose.
According to [30] poly(ethylene glycol) (PEG) can improve the in-
terfacial interactions between PLA and cellulose since the C-O-C and the
O-H of the PEG can form H-bonding or dipolar interactions between ma-
trix and reinforcing material. PEG cannot only act as compatibilizer but as
plasticizer as well thus reducing the brittleness of the PLA [2, 43, 44]. In
general, the lower the molecular weight the higher the plasticizing effect
[2, 43, 45] due to PEGs with lower molecular weights have larger number
of hydroxyl groups (which can develop hydrogen bonds between the poly-
mer and the plasticizer replacing the polymer-polymer interactions.) per
mole compared to the PEGs with higher molecular weights. Low molecu-
lar weight PEGs has further advantages namely they are nontoxic, often
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