Thermo-responsive interaction between -cyclodextrin and amphiphilic biopolymers.

Here we will discuss how the cosolute -cyclodextrin and temperature affect the interactions in aqueoussolutions of a hydrophobically modified polymer.

-Cyclodextrin (-CD)

Chemical structure

15,4 Å

6,9 Å

7,9 Å

The toroidal shape

-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.

-Cyclodextrin (-CD)

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 (14)-linked -L-guluronic acid (G-block) units arranged in non-regular blockwise pattern of varying proportion of GG, MG and MM blocks.

Hydrophobic modification:

OO

COH

HO

O

HN

HM-Alginate

C8 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.

Turbidity of -CD:

0 5 10 15 2010

20

30

40

50

0 10 20 30 40 5010-3

10-2

10-1

100

101

CP

(oC

)

Conc -CD (mm)

T

urbi

dity

(cm

-1)

Temperature (oC)

2 mm -CD 4 mm -CD 8 mm -CD12 mm -CD16 mm -CD

0.05 oC/min

= (-1/L)ln(It/I

0)

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.

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

0 2 4 6 8 10 12 14 160.30.4

0.5

0.60.70.80.91.0

1.11.2

1.3

Concentration of -cyclodextrin (mmolal)

2 wt % Alginate

Tu

rbid

ity (

cm-1)

5 oC

10 oC

15 oC

20 oC

30 oC

40 oC

= 500 nm

0 2 4 6 8 10 12 14 160.30.40.50.60.70.80.91.01.11.21.3

Tu

rbid

ity (

cm-1)

Concentration of -cyclodextrin (mmolal)

2 wt % HM-alginate

= 500 nm

5 oC

10 oC

15 oC

20 oC

30 oC

40 oC

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.

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.

Comparison of the relative turbidity for the alginate (2 wt %)/-CD/D2O and HM-

alginate (2 wt %)/-CD/D2O systems

0 10 20 30 400.5

1.0

1.5

2.0

2.5

3.0

Temperature (oC)

/40

0 mm -CD

Alginate HM-alginate

0 10 20 30 400.5

1.0

1.5

2.0

2.5

3.0

/ 40

Temperature (oC)

8 mm -CD

0 10 20 30 400.5

1.0

1.5

2.0

2.5

3.0

/40

Temperature (oC)

12 mm -CD

0 10 20 30 400.5

1.0

1.5

2.0

2.5

3.0 /

40

Temperature (oC)

16 mm -CD

Effects of -CD concentration and temperature on the shear rate

dependence of the relative viscosity

10-5 10-3 10-1 101 103101

102

103

104

105

106

/ D

2O (

Pas

)

Shear rate (s-1)

40 oC

Alginate

0 mm 2 mm 4 mm 8 mm12 mm 16 mm

10-5 10-3 10-1 101 103101

102

103

104

105

106

/

D2O

(P

as)

Shear rate (s-1)

40 oC

HM-alginate

10-5 10-3 10-1 101 103101

102

103

104

105

106

Shear rate (s-1)

/ D

2O (

Pas

)

5 oC

Alginate

10-5 10-3 10-1 101 103101

102

103

104

105

106

Shear rate (s-1)

/ D

2O (

Pas

)

25 oC

Alginate

10-5 10-3 10-1 101 103101

102

103

104

105

106

Shear rate (s-1)

/

D2O

(P

as)

25 oC

HM-alginate

10-5 10-3 10-1 101 103101

102

103

104

105

106

Shear rate (s-1)

/

D2O

(P

as)

5 oC

HM-alginate

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.

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.

Effects of -CD concentration and temperature on the relative zero-shear viscosity

0 5 10 15 20 25 30 35 40 45102

103

104

105

106

Temperature (oC)

0/D

2O (

Pas

)

Alginate

-CD 0 mm

4 mm

8 mm

12 mm

16 mm

0 5 10 15 20 25 30 35 40 45102

103

104

105

106

0/D

2O (

Pas

)

Temperature (oC)

-CD 0 mm

HM-alginate

4 mm

12 mm

16 mm

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.

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.

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).

B

Bc4

K

1cB

2

K1cB

11

21

CD-

2

CD-CD-

0

/

/

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

Effects of -CD concentration and temperature on the fraction of occupied

binding sites ()

0 2 4 6 8 10 12 14 16

0.0

0.2

0.4

0.6

0.8

1.0 15 oC

25 oC

40 oC

Concentration of -cyclodextrin (mmolal)

10 20 30 4002468

10

10 20 30 400.1

1

10

B (

mm

olal

) B and K are fitted parameters

K (m

molal -1)

Temperature (oC)

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.

0 5 10 1510-1

100

101

102

103

-cyclodextrin (mmolal) Zer

o sh

ear

visc

osity

(P

as)

5 oC

2 wt % Alginate 2 wt % HM-alginate

0 5 10 1510-1

100

101

102

103

Z

ero

shea

r vi

scos

ity (

Pas

)

-cyclodextrin (mmolal)

30 oC

0 5 10 1510-1

100

101

102

103

-cyclodextrin (mmolal)

15 oC

0 5 10 1510-1

100

101

102

103

-cyclodextrin (mmolal)

40 oC

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.

Schematic illustrations of alginate/-CD and HM-alginate/-CD interactions

Alginate/-CD interactions and formation of crystallites

Low temperature

Formation of crystallites

High conc. of -CD

In HM-alginate solutions, the large amount of hydrophobic groups prevent cross-linking due to steric hindrance.

HM-alginate/-CD interactions and deactivation of hydrophobic tails.

Elevated temperature:

Low temperature:

+

Interactions between poly(-cyclodextrin) and HM-alginate.

Properties of poly(-cyclodextrin). (C. Amiel et al. Macromolecules 2005, 38, 5243)

• 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-hydroxypropyl)ether sequences of different lengths, possessing a free end or acting as a bridge between several CDs.

• A compact structure with Mw=160 000; RG = 55 Å and Mw/Mn = 1.9. -CD content is 59 wt %.

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

Rheological results

Dilute mixtures of HM-alginate (0.5 wt %) and poly(-cyclodextrin) at a fixed temperature

10-3 10-2 10-1 100 101 102 10310-3

10-2

10-1

Vis

cosi

ty (

Pas

)

Shear rate (s-1)

2.0 wt % poly--CD

25 oC

HM-alginate (0.5 wt %)/Poly--CD

0.1 wt % poly--CD

0.5 wt % HM-alginate0 wt % poly--CD

0.5 wt % poly--CD

1.0 wt % poly--CD

1.5 wt % poly--CD

2.0 wt % poly--CD

3.0 wt % poly--CD 0 1 2 3

10-2

10-1

100

0 (P

as)

Poly--CD (wt %)

Zero-shear viscosity of the HM-alginate solution

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.

Effect of temperature on the viscosity

10-3 10-2 10-1 100 101 102 103 1045x10-3

10-2

10-1

100

2x100

Vis

cosi

ty (

Pas

)

Shear rate (s-1)

0 wt % poly--cyclodextrin, 25 oC

0.5 wt % HM-Alginate

1.5 wt % poly--cyclodextrin, 40 oC

1.5 wt % poly--cyclodextrin, 25 oC

1.5 wt % poly--cyclodextrin, 5 oC

0 10 20 30 400.0

0.2

0.4

0.6

0.8

1.0

0 (P

as)

Temperature (oC)

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.

Effect of temperature on the viscosity

10-5 10-3 10-1 101 10310-1

100

101

102 V

isco

sity

(P

as)

Shear rate (s-1)

5 oC

1.0 wt % HM-alginate1.5 wt % poly--cyclodextrin

15 oC

25 oC

35 oC

45 oC

Effect of temperature on shear-thinning and shear-thickening in HM-alginate/poly(-CD)

mixtures.

•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.

Schematic illustration of the HM-alginate-poly(-cyclodextrin) interaction

Addition of poly(-CD)

Increased conc. of poly(-CD)

Addition of poly(-CD) leads to the formation of bridges between HM-alginate chains and this process continues until these sites have been occupied.

Schemtic illustration of the effects of -CD or poly(-CD) addition to semidilute

solutions of HM-alginate

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.

• 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.