Thermoresponsive interaction between cyclodextrin and amphiphilic biopolymers'

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Thermo-responsive interaction between
?-cyclodextrin and amphiphilic biopolymers.
Here we will discuss how the cosolute
?-cyclodextrin and temperature affect the
interactions in aqueous solutions of a
hydrophobically modified polymer.
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?-Cyclodextrin (?-CD)
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?-Cyclodextrin (?-CD)

• ?-cyclodextrin (?-CD) is a cyclic starch oligomer
  consisting of 7 (?-1,4)-linked ?-D-glucopyranose
  units.

• The apolar nature of ?-CD cavities allow CDs to
  act as hosts for both nonpolar and polar guests.

• Inclusion of hydrophobic moieties (C8 groups)
  that is adapted to the cavity size.

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Structure of alginate and HM-alginate
The chemical structure units of alginate (M
mannuronic acid and G guluronic acid)
Molecular weight 150 000
An anionic copolymer, comprised of
?-D-mannuronic acid (M-block) and
(1?4)-linked ?-L-guluronic acid (G-block)
units arranged in non-regular blockwise
pattern of varying proportion of
GG, MG and MM blocks.
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Hydrophobic modification
The synthesis of C8 hydrophobically modified
alginate was carried out using an aqueous
carbodiimide reaction. The investigated sample
contains 31 mol of C8 groups. The length of the
hydrophobic tails is about 7 Å.
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Turbidity of ? -CD
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Temperature-induced crystallization of ?-CD

• In aqueous solutions of ?-CD, a temperature
  decrease leads to the formation of crystallites
  and the solution becomes turbid.
• During the crystallization process that occurs at
  low temperatures, CD molecules assume a
  herringbone-like arrangement where the cavity of
  one molecule is blocked on both sides by
  adjacent, symmetry-related ?-CD molecules (cage
  structure).
• The crystalline structure is stabilized by
  hydrogen bonds and van der Waals interactions.

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Turbidity of alginate and HM-alginate solutions
in the presence of ?-CD
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Turbidity of alginate solutions in the presence
of ?-CD

• The increase of cloudiness in solutions of
  alginate with decreasing temperature and
  increasing ?-CD concentration indicates
  interaction between alginate and ?-CD.
• The rather modest change of turbidity is due to
  the fact that ?-CD clusters are distributed over
  the alginate network and serve as cross-linkers.

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Turbidity of HM-alginate solutions in the
presence of ?-CD

• The more drastic increase of the turbidity in
  solutions of HM-alginate with decreasing
  temperature and increasing ?-CD concentration is
  because of the formation of crystallites in the
  bulk.
• Due to steric hidrance, ?-CD clusters are not
  active in the cross-linking of the network.
• The decrease of the turbidity at the highest
  temperature is due to the encapsulation of
  hydrophobic tails.

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Comparison of the relative turbidity for the
alginate (2 wt )/?-CD/D2O and HM-alginate (2 wt
)/?-CD/D2O systems
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Effects of ?-CD concentration and temperature on
the shear rate dependence of the relative
viscosity
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Effects of ?-CD concentration and temperature on
the relative viscosity for alginate solutions

• For the alginate/?-CD system we observe a
  gradually more pronounced upturn of the relative
  viscosity at low shear rates as the temperature
  is lowered and the ?-CD concentration is
  increased.
• The cosolutes forms clusters or crystallites that
  act as cross-linker of the alginate chains.
• The junction zones formed through the interaction
  with ?-CD clusters, are disrupted at high shear
  rates.

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Effects of ?-CD concentration and temperature on
the relative viscosity for HM-alginate solutions

• For the HM-alginate/?-CD system we observe a
  strong decrease of the relative viscosity at low
  shear rates as the temperature is raised and the
  ?-CD concentration is increased.
• The ?-CD molecules encapsulate the hydrophobic
  tails and thereby suppress the associations.
• The HM-alginate network is disrupted at high
  shear rates.

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Effects of ?-CD concentration and temperature on
the relative zero-shear viscosity
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Effects of ?-CD concentration and temperature in
alginate solutions

• No viscosity enhancement is observed at ?-CD
  concentrations up to 8 mmolal, because the
  aggregates are too small to cross-link the
  polymer chains.
• At higher ?-CD levels and low temperatures, the
  cosolute aggregates grow and are sufficiently
  large to cross-link the chains and a strong
  viscosity enhancement is observed.

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Effects of ?-CD concentration and temperature in
HM-alginate solutions

• In the absence of ?-CD or low ?-CD concentrations
  the viscosity rises moderately with increasing
  temperature, because the increased mobility of
  the chains activate several hydrophobic groups
  for intermolecular associations.
• Steric hindrance prevent cross-linking of the
  network.
• At high concentration of ?-CD and elevated
  temperature, the hydrophobic tails are
  encapsulated and the hydrophobic associations are
  suppressed and this results in low viscosity.

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Effects of ?-CD concentration and temperature on
the deactivation of polymer hydrophobic sites
Model (Karlson et al. Carbohydrate polymers
2002, 50, 219.)

• Based on the Langmuir adsorption model.

• The ?-CD molecules bind to the hydrophobic tails
  of the polymer chains with a complex formation
  constant K.

• The viscosity enhancement is considered to
  originate from associations via the polymer
  hydrophobic moieties (effect of entanglement is
  neglected).

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?0 and ?? are the zero-shear viscosity without
?-CD and in excess of ?-CD ? is the fraction of
occupied binding sites B is the concentration of
polymer hydrophobic tails c?-CD is the total
concentration of ?-CD
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Effects of ?-CD concentration and temperature on
the fraction of occupied binding sites (?)
High levels of ?-CD addition and elevated
temperature promote the decoupling of hydrophobic
interactions.
More efficient complex formation between the
hydrophobic tails and ?-CD at higher
temperatures.
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Deactivation of hydrophobic groups is promoted by
higher temperature and increasing ?-CD
concentration.
Cross-linking of alginate chains at low
temperatures and high ?-CD concentration.
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Schematic illustrations of alginate/?-CD and
HM-alginate/?-CD interactions
Alginate/?-CD interactions and formation of
crystallites
In HM-alginate solutions, the large amount of
hydrophobic groups prevent cross-linking due to
steric hindrance.
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HM-alginate/?-CD interactions and deactivation of
hydrophobic tails.
Low temperature
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Interactions between poly(?-cyclodextrin) and
HM-alginate.
(C. Amiel et al. Macromolecules 2005, 38, 5243)
Properties of poly(?-cyclodextrin).

• Poly(?-cyclodextrin) is a copolymer synthesized
  by polycondensation with epichlorohydrin (EP) and
  this induces the formation of poly-tails and
  poly-bridges.

• The polymer has a branched architecture where
  ?-CD molecules are modified by poly(2-hydroxypropy
  l)ether sequences of different lengths,
  possessing a free end or acting as a bridge
  between several CDs.

• A compact structure with Mw160 000 RG 55 Å
  and Mw/Mn 1.9. ?-CD content is 59 wt .

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Chemical structure of poly(?-CD) and a
schematic illustration of the compact structure
A branched and compact structure, which can form
bridges between different polymer chains
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Rheological results
Dilute mixtures of HM-alginate (0.5 wt ) and
poly(?-cyclodextrin) at a fixed temperature
Zero-shear viscosity of the HM-alginate solution
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Formation of interbridges between HM-alginate
chains and poly(?-cyclodextrin)

• Dilute mixtures of HM-alginate (0.5 wt ) and
  poly(?-cyclodextrin) at a fixed temperature.
• Addition of poly(?-cyclodextrin) to HM-alginate
  solutions generates interpolymer bridges and the
  viscosity increases.
• Optimal strength is achieved when all hydrophobic
  sites for interpolymer bridging have been
  occupied.

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Effect of temperature on the viscosity
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Effect of temperature on the viscosity of
HM-alginate/poly(?-cyclodextrin)

• A temperature increase promotes enhanced mobility
  of the polymer chains, and this reduces the
  tendency to form interpolymer connections with
  poly(?-CD).
• Due to the rather weak interpolymer associations,
  shear-thinning and disruption of the network
  occurs at fairly low shear rates.

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Effect of temperature on the viscosity
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Effect of temperature on shear-thinning and
shear-thickening in HM-alginate/poly(?-CD) mixtur
es.

• The general trend of the viscosity curves is
  shear thinning at low and high shear rates, and
  the shear-thickening behavior (peak) at
  intermediate shear rates.
• The viscosity peak is more pronounced as the
  temperature rises.
• The reason for this is that augmented motions of
  polymer chains and cosolute molecules facilitate
  the shear-induced orientation and extension of
  the chains in the bridging process.

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Schematic illustration of the HM-alginate-poly(?-c
yclodextrin) interaction
Addition of poly(?-CD) leads to the formation of
bridges between HM-alginate chains and this
process continues until these sites have been
occupied.
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Conclusions

• In ?-CD solutions without polymer, the cloud
  point increases with the ?-CD concentration.

• High level of ?-CD and a low temperature promote
  the formation of large-scale aggregates or
  crystallites in solutions of HM-alginate.

• Cross-linking of alginate chains at high
  concentrations of ?-CD and low temperatures.

• In HM-alginate solutions, elevated temperature
  and high levels of ?-CD addition favor
  deactivation of hydrophobic tails.

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• The ?-CD concentration and temperature effects on
  the viscosity could be rationalized in a simple
  model, based on the Langmuir adsorption approach.

• Addition of poly(?-CD) to dilute solutions of
  HM-alginate leads to association through bridging
  of polymer chains.

• A temperature increase gives rise to a lower
  viscosity and debridging of polymer chains.